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Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints
Accepted Manuscript Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints
Yang Li, Sanzhong Li, Wentian Liang, Rukui Lu, Youjun Zhang, Xiyao Li, Pengcheng Wang, Yongjiang Liu, Ian Somerville, Guowei Zhang PII: DOI: Reference:
S1342-937X(19)30097-8 https://doi.org/10.1016/j.gr.2019.04.001 GR 2127
To appear in:
Gondwana Research
Received date: Revised date: Accepted date:
16 November 2018 4 April 2019 5 April 2019
Please cite this article as: Y. Li, S. Li, W. Liang, et al., Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints, Gondwana Research, https://doi.org/10.1016/ j.gr.2019.04.001
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ACCEPTED MANUSCRIPT Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints
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Yang Li a, b, Sanzhong Li a, b, *, Wentian Liang c,, Rukui Lu c, Youjun Zhang d, Xiyao Li a, b,
Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, Institute for
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a
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Pengcheng Wang a, b, Yongjiang Liu a, b, Ian Somerville e, Guowei Zhang a, b, c
Advanced Ocean Study, College of Marine Geosciences, Ocean University of China,
Laboratory for Marine Geology and Environment, Qingdao National Laboratory for Marine
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b
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Qingdao 266100, China
Science and Technology, Qingdao 266237, China
State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069,
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c
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Tianjin North China Geological Exploration Bureau, Tianjin 300170, China
e
UCD School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland
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d
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*Corresponding author:
Sanzhong Li (College of Marine Geosciences, Ocean University of China, No. 238, Songling Road, Qingdao 266100, China; [email protected]; +86 053266781971)
Abstract Granitoids are important components of major orogenic belts, and provide important information about the regional geodynamic evolution. The emplacement mechanism of granite plutons and its relationship with regional tectonics has long been discussed, although 1
ACCEPTED MANUSCRIPT it still remains debated. The Qinling Orogen within the Central China Orogen was marked by the emplacement of numerous Late Triassic granitic plutons. Although the petrology, geochemistry and geochronology of these intrusions have been addressed in various studies, their tectonic setting remains controversial, particularly since the structural aspects not been evaluated in detail.
In this study, we attempt to reconstruct the emplacement process of the
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Late Triassic Dongjiangkou pluton in the South Qinling Belt. Field observations show
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extensive syn-plutonic deformations both in the pluton and its contact zones. Microstructural
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observations demonstrate that fabrics in the pluton were mainly acquired during submagmatic flow to high-T solid-state deformation. Zircon U–Pb ages reveal that the
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pluton is a composite intrusion which is composed of two juxtaposed small plutons with distinct ages (~210 Ma and ~200 Ma). Al-in-hornblende thermobarometer indicates that the
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pluton was formed at depths ranging from 4.7 km to 8.8 km, with an increasing depth trend from the inner unit to the outer unit. Distribution of the internal fabrics shows two concentric
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patterns which are concordant with pluton margins at the pluton scale and were probably
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induced by the regional sinistral transpression. Integrating these analyses, an incremental emplacement model is proposed for the syn-tectonic pluton. This model not only solves the
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‘room problem’ but also accounts for the zoned petrological features of the pluton. Combined with previous studies, we suggest that the Late Triassic granite plutons in the
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Qinling Orogen were emplaced under a syn-collisional convergence setting, and that the granite magmatism was probably controlled by regional tectonics. Additionally, the incremental emplacement model may be a common mechanism for the Late Triassic plutons.
Keywords Zircon U–Pb geochronology; Structural analysis; Incremental emplacement; Late Triassic granites; Qinling Orogen
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ACCEPTED MANUSCRIPT 1. Introduction Granites (sensu lato) constitute important components of major orogenic belts and carry important information on magma generation, segregation, transport and emplacement (Brown, 2013). Therefore, studies on granitoids are fundamental to understanding the magmatic and tectonic evolution of the continental crust (Li et al., 2013; Wang et al., 2017).
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During the past century, with the development of fabric measurements (e.g., Bouchez et al.,
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1997; Launeau and Robin, 2005), numerical simulation and analogue modeling (e.g.,
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Kavanagh et al., 2006; Menand, 2011; Petford et al., 1993), many studies have focused on the processes by which granite plutons are emplaced, although the emplacement mechanisms
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remain equivocal (e.g., Annen, 2011; Castro, 1987; Menand, 2011; Paterson and Fowler, 1993), as also the relationship between magmatism and tectonics (e.g., Brown, 2013;
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Paterson and Schmidt, 1999; Petford et al., 2000; Vigneresse, 1999; Weinberg et al., 2004). Traditionally, the emplacement of a granite pluton is considered to involve stopping, thermal
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ballooning, and diapirism, among other factors. In spite of the discrepancy on the nature of
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intrusion (Castro, 1987; Wang et al., 2017), most of the mechanisms emphasize a single magma intrusion that was emplaced in the crust quasi-instantaneously, which means that a
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granite pluton is in situ crystallized and differentiated from one single large magma body. However, none of the models can solve ‘space problem’ or ‘room problem’ (e.g., Menand,
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2011; Tikoff and Teyssier, 1992) for the emplacement of granites. Recent studies suggest that granite plutons (especially zoned plutons) probably formed through multiple and discrete magmatic pulses with distinct ages (Annen, 2009, 2011; Clemens and Stevens, 2016; Coleman et al., 2004; de Saint-Blanquat et al., 2011; Kavanagh et al., 2006; Menand, 2008, 2011; Miller et al., 2011; Paterson et al., 2011; Li et al., 2013) based on the following lines of evidence. (1) Field observations show that most plutons are tabular or wedge-shaped and emplaced incrementally by sheeted sills. (2) Geochronological
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ACCEPTED MANUSCRIPT studies reveal that many granite plutons formed in a time scale over millions of years. The proposal of incremental emplacement model provides a new perspective to address some of the issues, such as fractional crystallization in granite magmas (Clemens and Stevens, 2016; Coleman et al., 2004; Glazner et al., 2004), thermal aureole dimensions (Annen, 2011) and granite-related metallogenesis. However, the detailed mechanisms are not well-known.
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In the Qinling Orogen of central China, a large number of granite plutons were formed
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during ~225–200 Ma (Li et al., 2015; Wang et al., 2013; Zhang et al., 2008). These plutons
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(mostly the zoned plutons) are the keys to understand the tectonic evolution of the Late Triassic (Indosinian) orogeny. In spite of the petrological, geochronological and geochemical
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studies, the tectonic setting of these Late Triassic granite plutons remain controversial, with proposals of syn-subduction, syn-collision, and post-collision (e.g., Dong et al., 2012; Jiang
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et al., 2010; Li et al., 2015; Wang et al., 2013; Zhang et al., 2008). Moreover, the structures within these plutons have not been analyzed in detail to the same extent as the
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geochronological and geochemical studies.
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In this contribution, we present an integrated study of the Late Triassic Dongjiangkou pluton in the South Qinling Belt. This pluton is characterized by concentric zoning of
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petrological units and a large timescale of emplacement process (>10 Ma) (Cui et al., 1999; Gong et al., 2009; Hu et al., 2017; Qin et al., 2010). We present results from a
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multidisciplinary study including field and microstructural observations, zircon U–Pb geochronology, Al-in-hornblende thermobarometry, AMS (anisotropy of magnetic susceptibility) and SPO (shape preferred orientation) analysis, and derive information on the syn-plutonic deformations, crystallization ages, emplacement depths and internal fabrics. Based on the results, we propose an integrated emplacement model for the pluton and its implications.
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2. Geological Setting 2.1. Regional tectonic framework The Qinling Orogen is located between the North China Block (NCB) and the South China Block (SCB), constituting an important segment of the Central China Orogen. It
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extends more than 1500 km from the Dabie Mountains to the Qilian and Kunlun Mountains.
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This orogen is bounded by the Lingbao–Lushan–Wuyang thrust fault (LLWF) to the north
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and the Mianxian–Bashan–Xiangfan arc-shaped thrust fault (MBXF) to the south (Zhang et al., 2001) (Fig. 1). Previous studies have identified two ophiolitic mélange corresponding to
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two orogenies (Caledonian orogeny and Indosinian orogeny) that occurred in the Devonian (the Shangdan Suture) and Triassic (the Mianlue Suture) (Dong et al., 2016; Li et al., 2017a,
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2017b; Pei et al., 2005; Zhang et al., 2001, 2015). On the basis of these two sutures and the lithospheric-scale Luonan–Luanchuan Fault (LLF) (Zhang et al., 2001), the Qinling Orogen
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can be divided into four WNW-ESE trending tectonic belts (from north to south): Southern
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North China Block (S-NCB), North Qinling Belt, South Qinling Belt and Northern South China Block (N-SCB) (Dong et al., 2016) (Fig. 1).
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According to previous studies, the Qinling Orogen is a composite orogenic belt with a complex history of rifting, oceanic subduction, continental collision and intracontinental
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orogeny (Dong and Santosh, 2016; Li et al., 2007; Zhang et al., 2001). The Middle Paleozoic subduction to collision along the Shangdan Suture accreted the SCB to the NCB, leading to extensive metamorphism, magmatism and deformation. Contemporaneous rifting occurred along the southern margin of the South Qinling Belt followed by the opening of the Mianlue Ocean (a branch of the Paleo-Tethyan Ocean) (Zhang 2015). This breakup event resulted in the separation of the Qinling microblock from the SCB. Subsequently, the Qinling Orogen experienced the Triassic (Indosinian) collisional orogeny. The intense collision along the
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ACCEPTED MANUSCRIPT Mianlue Suture associated with the amalgamation of the NCB and SCB caused extensive thin-skinned fold-and-thrust deformation, extrusion, uplift and granitoid emplacement throughout the Qinling Orogen (Li et al., 2007; Zhang, 2015). During the subsequent phase of intracontinental orogeny, the boundary faults (LLWF and MBXF) of the orogen were formed.
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The Triassic orogeny played an important role in the long-term evolution of the Qinling
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Orogen, in that it not only formed the entire framework of the orogenic belt, but also
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represented an important transition from the Paleozoic plate tectonics to the Mesozoic-Cenozoic intracontinental tectonic regime (Dong et al., 2016; Zhang et al., 2001).
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However, there is still debate concerning the culmination of the Triassic orogeny. According to the integrated tectono-sedimentary studies, the collisional event took place during the
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Mid-Late Triassic (e.g., Zhang et al., 2001). More recently, this viewpoint has been challenged based on studies of the Late Triassic granites (~225–200 Ma) which are
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widespread in the orogen. A variety of tectonic settings including syn-subduction,
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syn-collision and post-collision were proposed and debated. For example, on the basis of geochemical and Sr–Nd–Hf isotopic studies of the Cuihuashan, Caoping, Dongjiangkou,
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Laocheng and Yanzhiba plutons (Fig. 1), Jiang et al. (2010) proposed that the Carnian (227–218 Ma) plutons represent the products of syn-subduction, whereas the Norian (211
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Ma) pluton emplaced in a syn-collisional setting. An integrated review by Wang et al. (2013) argued that all of the Late Triassic magmatism occurred in a post-collisional setting. Li et al. (2015) argued that the Triassic granitoids are the products of an active continental margin rather than syn-collision or/and post-collision settings.
2.2. Dongjiangkou pluton The Dongjiangkou granite pluton in the SQB is exposed over an area of 540 km2 (Gong
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ACCEPTED MANUSCRIPT et al., 2009). The surrounding country rocks of the pluton belong to the Middle to Upper Devonian Liuling Group (Fig. 2). The Liuling Group is mainly composed of low-grade (greenschist-facies) metasediments that were metamorphosed from a suite of pelagic turbidites (Dong and Santosh, 2016). In this Group, a series of folds, axial planar cleavages and thrust faults were developed at different scales and levels in response to the Triassic
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collisional orogeny, indicating uniformly a NE-SW convergence (Li et al., 2007; Zhang et al.,
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2001). Fig. 2 shows that the pluton was emplaced into the syncline of the Devonian Liuling
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Group, demonstrating that the magma emplacement is later than regional folding which is considered to have occurred during Early to Middle Triassic (Xu et al., 2005; Zhang et al.,
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2001). Moreover, a syn-tectonic granitic vein in the Liuling Group has been dated by Li et al. (2017c). The formation age (201 Ma) provides the precise limits on the timing of regional
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deformation.
The pluton is in direct contact with the Shagou Shear Zone (SSZ) and the
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Shanyang–Fengzhen Fault (SFF) (Fig. 2). To the north of the pluton, the SSZ is an E–W to
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ENE striking ductile deformation zone which is mainly composed of granitic mylonites (low-amphibolite grade metamorphism) (Fig. 2). This crustal-scale ductile shear zone shows
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kinematics of sinistral transpression and its formation age has been estimated as Late Triassic (Li et al., 2017c; Reischmann et al., 1990; Zhang et al., 2001). To the south of the
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pluton, the SFF is a WNW-ESE striking crustal-scale fault which is characterized by multi-stage activities, and its major deformation with sinistral transpressional kinematics occurred in the Late Triassic (Wu, 2013; Zhang et al., 2001). The Dongjiangkou pluton is composed of five petrographic units with different mineralogy and textures, showing zoned pattern (Cui et al., 1999; Gong et al., 2009; Hu et al., 2017; Qin et al., 2010). From the center to the margin, these are the Meihuazhuang unit, Shaluozhang unit, Yaowangtang unit, Yingpan unit and Xiaochuan unit (Fig. 2). On the basis
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ACCEPTED MANUSCRIPT of field-mapping (Cui, 1999) and related studies (e.g., Gong et al., 2009; Qin et al., 2010), the petrographic composition of these five units can be described as follows. The Meihuazhuang unit mainly consists of coarse-grained porphyritic monzogranites and granodiorites. The Shaluozhang unit is mainly composed of hornblende-bearing porphyritic monzogranites and tonalites. The Yaowangtang unit comprises medium- to coarse-grained
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monzogranites and tonalites. The Yingpan unit is mainly made of medium- grained
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porphyritic biotite monzogranites, tonalites and granodiorites. The Xiaochuan unit mainly
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consists of medium- to fine-grained granodiorites and quartz monzonites. Moreover, abundant mafic magmatic enclaves (MMEs) characterized by fine-grained hypidiomorphic
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granular texture are present in the pluton.
Except for the Xiaochuan unit, the other units were dated by zircon U–Pb
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geochronology, and their formation ages range from 222 to 210 Ma (Gong et al., 2009; Li et al., 2012; Liu et al., 2011; Qin et al., 2010). Liu et al. (2011) identified two-stage magmatic
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events in the Yingpan unit: the older magmatic zircons (~219 Ma) correspond to an
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earlier-stage and the younger age of magmatic zircons (~209 Ma) represents the final crystallization age of this unit. In addition, a biotite 40Ar/39Ar age of 198 Ma also has been
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reported by Zhang et al. (2006). The discrepancy in the formation ages of the pluton
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indicates that the emplacement history is probably far more complex.
3. Macroscopic and microstructural observations 3.1. Deformation records in the pluton Macroscopic structures are rare within the Dongjiangkou pluton due to the weak shape preferred orientations of minerals in granites. Nevertheless, the following two strain markers can reflect the syn-plutonic deformations.
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ACCEPTED MANUSCRIPT 3.1.1. Mafic magmatic enclaves (MMEs) Mafic magmatic enclaves (MMEs) can be observed all over the pluton, ranging in shape from round to elongated ellipse with high aspect ratio (Fig. 3a, b). Most of the MMEs are monzodiorites or diorites, having fine-grained textures with clustering of the mafic minerals. Zircon U–Pb ages show that the MMEs crystallized at the same time as the host granites
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(Gong et al., 2009; Li et al., 2012; Liu et al., 2011; Qin et al., 2010), which is also be attested
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by field observations. (1) MMEs and granites are enwrapped with each other (Fig. 3a). (2)
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Some of the MMEs contain plagioclase phenocrysts and scarce K-feldspar phenocrysts or megacrysts from the host granites (Fig. 3c, d). Compared with the host granites, the MMEs
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in the pluton commonly have relatively lower SiO2, higher MgO, K2O content, Rb/Sr and (La/Yb)N ratios (Gong et al., 2009), illustrating that these MMEs are products of mafic
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magmas with a source different from the host granites, rather than the residuum of the source rocks after partial melting.
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The deformation of the MMEs can provide important information about the
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syn-plutonic regional tectonics. Usually, shape preferred orientations of MMEs can be used to reflect the information of magma flows or deformation kinematics. However, it should be
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noted that each of the MMEs only represent one section (2-D ellipse) of the 3-D ellipsoid, and thus the single stretching shape can’t simply be regarded as the direction of magma flow.
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According to field observations, the MMEs with elongated shape are dominant in the Dongjiangkou pluton. Some of the MMEs show obvious offset without any mineral preferred orientation (Fig. 3c, d), indicating that the offset occurred by melt-assisted grain boundary sliding and stopped before final crystallization (Paterson, et al., 1998). This kind of structure is one of the most cogent evidences that can support the existence of melt-present deformation or syn-plutonic deformation. The same phenomenon can also be observed in other regions of the Dongjiangkou pluton that contains a large number of MMEs. In addition,
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ACCEPTED MANUSCRIPT the strike (~100°) and kinematics (sinistral movement) of the deformation within the MMEs is in accord with the structural records of the country rocks and the SSZ (Li et al., 2017c). .
3.1.2. Dykes . In the Dongjiangkou pluton, dykes are mainly granitic with a few mafic ones. The
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granitic dykes range from aplite to pegmatite with different textures and degree of
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crystallization. Sometimes, these dykes mutually intrude each other (composite dykes),
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indicating multi-stage magmatism (Fig. 4a). According to previous dating results, these granitic dykes are slightly younger than the MMEs and their host granites (Qin, 2010).
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Furthermore, no younger magmatism has been recognized in our study area. Therefore, we believe that the dykes in the Dongjiangkou pluton probably formed during the late cooling
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stage of pluton.
It is well established that dykes are important markers of magma infilled fractures,
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usually representing the channel of magma ascend or migration (e.g., Brown, 2013; Ernst et
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al., 2001; Petford et al., 2000). The geometry of a dyke can reflect the paleostress field and the kinematics of regional deformation by which the fractures were formed and the magma
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was transferred (Glazner et al., 1999; Mège and Korme, 2004). On the basis of field observations, dykes in the Dongjiangkou pluton are almost uniform in all directions,
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indicating that they are not formed by just one mechanism. These dykes are interpreted to be related to local stresses at the late stage of magma crystallization and regional tectonics. Furthermore, a series of segmented granitic dykes can be observed in the Dongjiangkou pluton (Fig. 4b-d). This kind of dyke is supposed to be formed by shear deformation, and thus can reflect the kinematics of regional deformation (Oberc-Dziedzic et al., 2013). In Figs. 4c, d, the segmented dykes striking 125° show a sinistral shear sense, and the segmented dykes striking 165° show dextral shear deformation (Fig. 4d). On the basis of their geometry,
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ACCEPTED MANUSCRIPT we infer that the maximum paleostress strikes 35°–75°. It should be noted that the principal stress axes may have been rotated during upward propagation to form the segmented dykes. Nevertheless, the syn-kinematic deformations of the segmented dykes can illustrate that regional shear deformation existed during the late stage of pluton construction.
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3.2. Thermal aureole structures
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Although the Dongjiangkou pluton and its country rocks have a long contact boundary,
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the contact relationship is observed only at a few locations due to the limited outcrop and the high degree of weathering. In the biotite-bearing country rocks of the pluton, syn-plutonic
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deformation is commonly observed (Fig. 5). Overall, the contact relationship can be characterized by the following features. (1) The pluton or syn-plutonic dykes emplaced into
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the rocks of the Liuling Group cut the previous foliation (mainly axial-plane cleavages) (Fig. 5a, b), indicating that the granite magmatism is later than the formation of the cleavage; (2)
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Around the contact there are little or almost no xenoliths of the country rocks can be found
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in the pluton, suggesting that the emplacement was not dominated by stoping which requires incorporation of large volumes of wallrock. (3) Sub-horizontal sills (the building blocks of
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the larger pluton) at the contact of the pluton were formed at the flat of the thrust fault (Fig. 5a, b), supporting that magmas can ascend and emplace along compressional structures
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(thrusts) (de Saint-Blanquat et al., 1998; Ferré et al., 2012); (4) Country rocks at the thermal aureole show relatively high-grade metamorphism (amphibolite facies, different from the lower greenschist facies far away from the pluton) and ductile deformations with sinistral shear kinematics (Fig. 5c-f), indicating that the ductile deformation resulted from the combination of regional deformation and the heat caused by pluton emplacement.
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deformation reflected by the thermal aureole structures is coeval with the D2 deformation described by Li et al. (2007).
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3.3. Microstructural observations Microstructural observation is fundamental to estimate the degree of deformation and to determine the physical state of fabric formation (Vernon, 2004). Microstructures were examined in 60 thin sections collected throughout the Dongjiangkou pluton. In general, the
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microstructures in the pluton can be described as the following groups. (1) Euhedral
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amphiboles (Fig. 6a), plagioclase with oscillatory zoning (Fig. 6b) and K-feldspar with
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simple twin show no evidence of crystal-plastic deformation and recrystallization, representative of primary magma-flow structures. (2) In some thin sections, submagmatic
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flow that is evidenced by transgranular fractures (formed by syn-plutonic deformation) filled with later melt can be locally observed (Fig. 6c), supporting the existence of melt-present
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deformation (Vernon, 2004); (3) In most of thin sections, quartz grains commonly show high-T solid-state deformation, such as the grain boundary migration recrystallization with
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irregular lobate boundaries of the quartz grains (Fig. 6d) and a chessboard pattern (Fig. 6e)
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which is characterized by square subgrains of quartz with boundaries parallel to both the prism and the basal planes (Kruhl, 1996). Moreover, myrmekite lobes on the margins of the
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K-feldspar can be locally observed, indicating subsolidus deformation or high-T deformation (Vernon, 2004). (4) A few samples located at the margin, especially the sites close to the
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Shagou Shear Zone and Shanyang–Fengzhen Fault, show occasional features of low-T solid-state deformation (which is characterized by brittle-ductile to brittle deformation), such as kinks in biotite grains (Fig. 6f) and deformation twinning in feldspars. However, these low-T solid-state deformations are too few that can be neglected In general, the microstructures described above are neither purely magmatic flow nor low-T solid-state (after the complete crystallization of the magma) products, demonstrating that the internal fabrics in the pluton were acquired mainly during submagmatic flow
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ACCEPTED MANUSCRIPT (melt-present deformation) to high-T solid-state deformation. Each section preserves some of the former magma-flow microstructures and the later low-T solid-state deformations. The result of microstructural observations is in accord with field-observed syn-plutonic deformations that were reflected by the granite dykes and the MMEs, supporting that
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regional deformation imposed on the process of pluton construction.
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4. Emplacement ages and depths 4.1. Analytical methods
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In order to better understand the emplacement succession of the entire Dongjiangkou pluton, five granite samples were collected from different units (Fig. 2). Zircon grains were
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separated by crushing and sieving followed by magnetic and heavy liquid separation, and handpicked under a binocular microscope. Cathodoluminescence (CL) images for zircon
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patterns were collected by a Mono CL3+ microprobe to determine target sites for U–Pb
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dating. Zircon U–Pb analyses were conducted by LA–ICPMS at the State Key Laboratory of Continental Dynamics, Northwest University, China. The laser–ablation system comprises a
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GeoLas 200 M equipped with a 193 nm ArF excimer laser (MicroLas, Göttingen, Germany). Laser spot diameter is 30 μm. Analyses were performed on the ELAN 6100 ICP–MS from
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Perkin Elmer/SCIEX (Canada) with a dynamic reaction cell (DRC). Detailed descriptions for analytical procedures refer to Yuan et al. (2004). 207Pb/206Pb, 207Pb/235U and 206Pb/238U ratios were calculated by the GLITTER 4.0 program (Macquarie University). Zircon 91500 and NIST610 were used as the external calibration and the reference standard for zircon U–Pb dating and trace element analyses. Additionally, on the basis of the positive correlation between the total aluminium content
of
magmatic
hornblende
and
crystallization
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pressure
(Al-in-hornblende
ACCEPTED MANUSCRIPT thermobarometer) (Anderson et al., 2008), the emplacement depths of 16 samples were estimated. The rims of hornblende in the matrix can represent the final-stage and are therefore appropriate for the calculation of the emplacement depth. Electron microprobe analysis was performed at the State Key Laboratory of Continental Dynamics, Northwest
(1992): r2＝0.99
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P (±0.6 Kbar) = -3.01+4.76 Altot
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University, China. Emplacement pressures were estimated by the equation of Schmidt
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Altot is the total aluminium content of the hornblende in atoms per formula unit.
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4.2. Zircon U–Pb ages
The zircon U–Pb dating results of 5 samples from the pluton are shown in Appendix
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Table S1. Zircons of Sample D114 from the Meihuazhuang unit (porphyritic biotite monzogranite, E108°48.392’; N33°40.164’) are mainly euhedral prismatic, from 100 to 300
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μm in length, with aspect ratios in the range of 1.5–3.0. CL images show that zircon grains
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have clear oscillatory zoning and some grains have inherited cores. Thirty dating sites located at the rim of zircon grains yielded concordant
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Pb/238U ages, ranging from 199.5
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Ma to 202.3 Ma, with a weighted mean age of 200.6±1 Ma (MSWD=0.032) (Fig. 7a). Th/U ratios of these dating sites vary from 0.5 to 1.13, indicative of a magmatic origin (Hoskin
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and Schaltegger, 2003). Therefore, the age of 200.6±1 Ma represents the crystallization age of Sample D114. The remaining six spots are located at the inherited core of zircons give various U–Pb ages ranging from 357.7 Ma to 1698.9 Ma. These ages represent the formation time of inherited zircons, reflecting multistage magmatic events within this area. Zircons of Sample D128 (porphyritic biotite monzogranite), collected from the Shaluozhang unit (E108°52.795’; N33°40.375’), show a similar grain size to Sample D114. A total of 36 zircon spots were analyzed, of which 30 spots show concordant ages. Only one
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ACCEPTED MANUSCRIPT spot with an age of 267 Ma is located in the zircon core. The remaining 29 spots are located at zircon rims. Although CL images show that the analyzed zircons are similar in shapes and internal patterns, the dating results show two contrasting groups of 206Pb/238U ages (Fig. 7b): (1) The first group of 16 spots shows relatively younger ages (ranging from 199.4 Ma to 201.3 Ma) with a weighted mean age of 200.2±1.4 Ma (MSWD=0.032); (2) The second relatively older ages (ranging from 220.5 Ma to 225 Ma) with a
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group of 13 spots displays
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weighted mean age of 221.9±1.8 Ma (MSWD=0.098), ~22 Myr older than the first group.
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Both groups show Th/U ratios are higher than 0.4 (0.45–0.82), indicative of a magmatic origin.
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Sample D215 (medium- to fine-grained granodiorite) was collected from the Xiaochuan unit (E108°46.186’; N33°35.321’). Zircons are characterized by hypidiomorphic to euhedral
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granular texture, 100–350 μm in length and 50–100 μm in width. CL images show that the majority of zircon grains have clear oscillatory zoning. A total of 38 zircon spots were
D
analyzed, of which 26 spots show concordant ages. Three spots located on inherited cores
PT E
show ages of 526.2 Ma, 743.3 Ma and 846 Ma, corresponding to the formation age of inherited zircons. Spots at zircon rims also show two contrasting groups of
206
Pb/238U ages
CE
(Fig. 7c). (1) The first group of 10 spots shows relatively younger ages (ranging from 199.3 Ma to 200.9 Ma) with a weighted mean age of 200.1±1.5 Ma (MSWD=0.037). (2) The
AC
second group of 13 spots has ages ranging from 220 Ma to 222.5 Ma with a weighted mean age of 221.1±1.6 Ma (MSWD=0.108), ~21 Myr older than the first group. Both groups show Th/U ratios higher than 0.4 (0.62–1.34), indicative of a magmatic origin. Sample D222 (medium- to coarse-grained granodiorite) was collected from the Xiaochuan unit (E108°46.643’; N33°35.501’). Zircon shapes and textures are similar to those of Sample D215. A total of 36 zircon spots were analyzed, of which 25 spots show concordant ages. One spot located at the inherited core show an age of 245.5 Ma,
15
ACCEPTED MANUSCRIPT representing the formation age of the inherited zircon. The remaining 24 spots on zircon rims also show two contrasting groups of 206Pb/238U ages (Fig. 7d): (1) The first group of 13 spots acquired relatively younger ages (ranging from 198.2 Ma to 200.7 Ma) with weighted mean age of 199.2±1.4 Ma (MSWD=0.110); (2) The second group of 11 spots has ages ranging from 219.2 Ma to 222.8 Ma with a weighted mean age of 221.1±1.8 Ma (MSWD=0.120),
PT
~22 Ma older than the first group. Th/U ratios are higher than 0.4 (0.59–2.22), indicative of a
RI
magmatic origin.
SC
Sample D297 (coarse- to medium- grained monzogranite) was collected from the Yingpan unit (E109°01.924’; N33°46.677’). Zircons are characterized by euhedral granular
NU
texture, 100–300 μm in length and 50–150 μm in width. CL images show that zircons are very similar to each other. The majority of zircon grains have clear oscillatory zoning, and
MA
some grains have inherited cores. A total of 37 zircon spots were analyzed, of which 28 spots show concordant ages. Seven spots located on inherited cores show ages of 454.1 Ma, 455.4
D
Ma, 400.3 Ma, 358.5 Ma, 350.0 Ma, 256.9 Ma and 231.9 Ma, indicating multi-stage
groups of
206
PT E
magmatic events. The remaining 21 spots located on zircon rims also show two contrasting Pb/238U ages (Fig. 7e): (1) The first group of 14 spots has relatively younger
CE
ages (mainly ranging from 210.0 Ma to 211.0 Ma, except for one spot which shows an age of 206.9 Ma), with a weighted mean age of 210.4±1.5 Ma (MSWD=0. 079; (2) The second
AC
group of 7 spots has ages ranging from 221.2 Ma to 224.8 Ma, with a weighted mean age of 222.3±2.1 Ma (MSWD=0.2), ~11 Ma older than the first group. Both groups show Th/U ratios higher than 0.4 (0.41–1.27), indicative of a magmatic origin. In summary, except one Sample (D114) in the pluton showing a single group of ages (200.6±1 Ma), all the other four samples show two contrasting groups of
206
Pb/238U ages
(Table 1) for zircon rims. Consequently, at least three stages of magma events can be recognized for the growth of the Dongjiangkou pluton: 222–221 Ma, ~210 Ma and 199–201
16
ACCEPTED MANUSCRIPT Ma (Fig. 7f).
4.3. Emplacement depths Due to the notable effect of the calculation results of pressure, we have to select the samples with XFe (XFe=Fe/(Fe+Mg)) values of 0.4–0.65 to calculate the emplacement
PT
pressures (Anderson and Smith, 1995). For the Dongjiangkou pluton, the XFe values of all
RI
samples are within the interval of 0.4–0.54 (Table 2). The estimated crystallization pressures
SC
range from 1.67 to 3.14 kbar, with a relatively high range. Furthermore, the density of country rocks (the Liuling Group) is about 2.75–2.8 g/cm3, which is approximately similar to
NU
the average density of the upper crust (2.8 g/cm3). Consequently, we transform the crystallization pressures to emplacement depths by 2.8 km/kbar. As a result, emplacement
MA
depths of the pluton range from 4.7 to 8.8 km (Table 2).
The emplacement depths of granites vary greatly in the pluton (Fig. 8a). We have to
D
consider other interpretations in that the error of the formula (±0.6 KPa) cannot account for
PT E
such a big variation in depths (up to 4.1 km) alone. After removing the elevation effect by adding the altitude, we obtained the emplacement depths of the same level (0 m). The
CE
corrected emplacement depths were then used to produce a contour map (Fig. 8b) which shows a similar zoned pattern with the petrological zonation, indicating that the
AC
emplacement depths of the pluton increase from the inner unit to the outer unit. Therefore, this study confirms that the different units of the pluton were emplaced at different depths.
5. Internal fabrics of the pluton 5.1. Methods and sampling It is widely acknowledged that fabrics (i.e., foliation and lineation) represent the most important structures in granites. AMS measurement is considered as the most efficient
17
ACCEPTED MANUSCRIPT method to obtain the petrofabrics, especially in rocks that are visually isotropic (e.g., Bolle et al., 2018; Liang et al., 2015b). Mathematically, the magnetic susceptibility can be defined by a second-rank tensor with three orthogonal eigenvectors, corresponding to three eigenvalues, K1, K2 and K3 (K1≥K2≥K3). K1 represents the magnetic lineation, and K3 is perpendicular to the magnetic foliation (Tarling and Hrouda, 1993). Furthermore, in order to better
PT
understand the AMS ellipsoid scalar parameters including Km (bulk susceptibility), PJ
RI
(corrected anisotropy degree) and T (shape parameter) can be calculated by the following
SC
formulas (Jelinek, 1981): (1) Km=1/3(K1+K2+K3); (2) PJ exp 2 ln Ki ln Km ; (3) 3
2
i 1
NU
T=2(lnK2 - lnK3)/(lnK1 - lnK3)-1, 0
MA
Oriented samples for AMS measurements were collected from 223 sites that are distributed throughout the Dongjiangkou pluton (Appendix Fig. S1). At each site, samples
D
were drilled and cut into cylindrical specimens, 2.5 cm in diameter and 2–2.2 cm in height.
PT E
At least 5 specimens for each site and a total of 1396 standard oriented specimens were collected for AMS measurements. Low field AMS measurements (at room temperature) were
CE
performed by an AGICO KLY–4S Kappabridge (sensitivity of 110-8 SI) at the State Key Laboratory of Continental Dynamics, Northwest University, China. Finally, scalar
AC
parameters were calculated by ANISOFT 4.2. Analysis results see Appendix Table S2. Additionally, we performed the shape preferred orientation (SPO) analysis on the mafic silicate minerals (mainly biotite) to constrain the reliability of the magnetic fabric. SPO measurements involve the measurement of 2-D sectional fabric ellipses using image analysis (IA) and the construction of a 3-D best-fit ellipsoid. High-resolution digital images for IA were directly taken on nonparallel sections (N≥3) at each site. By using the software of Photoshop cs6 and ImageJ, the dark mafic minerals were isolated from the quartzes and the
18
ACCEPTED MANUSCRIPT feldspars. We then use the program SPO2003 (Launeau and Robin, 2005) to obtain the long axis orientation and shape ratio of the sectional fabric ellipses. Finally, the three sectional ellipses were combined to construct a best-fit ellipsoid by using the program Best-Fit Ellipsoid–v2.0 (Mookerjee and Nickleach, 2011).
PT
5.2. Bulk susceptibility
RI
Bulk susceptibility (Km) of the Dongjiangkou pluton has a broad variation range, from
SC
73 μSI to 14900 μSI (average of 2252 μSI), and mainly distribute within the interval of 185–7000 μSI (Fig. 9a). Samples with Km values less than 500 μSI (n=60) mainly distribute
NU
in the northwest (Jiangkou). Samples with highest Km values mainly distribute in the south (Xiaochuan) and northeast of the pluton (Yingpan). Such a large variation in Km values
MA
indicates variable mineral contributions to the susceptibility. AMS in granites represent the combined contribution of all rock-forming minerals,
D
including the diamagnetic minerals (e.g., feldspar and quartz), the paramagnetic minerals
PT E
(e.g., biotite and amphibole) and the ferromagnetic minerals (e.g., magnetite) (Tarling and Hrouda, 1993). In granites with low bulk magnetic susceptibility (< 500 μSI), AMS are
CE
mainly related to the magnetocrystalline anisotropy of paramagnetic silicates. In granites with high magnetic susceptibility (>500 μSI), AMS are mainly contributed by the shape
AC
anisotropy of the ferromagnetic minerals. Although quartz and feldspars are the most important components of granites, they only have little effect on bulk susceptibility in rocks, with Km values higher than 102 μSI in that the magnetic susceptibility of such diamagnetic minerals is very low (usually on the order of -10 SI). Therefore, on the basis of the Km values and the composition of rocks we have a preliminary conclusion that the AMS of samples in the Jiangkou area (with low Km values) are mostly attributed to the biotites, and the AMS of samples with relatively high Km values in other parts of the pluton are dominated by some
19
ACCEPTED MANUSCRIPT ferromagnetic minerals. Moreover, microscopic observations of samples with high Km values show that the long axes of magnetite grains are parallel with biotites, and magnetite grains are preferentially distributed along the cleavages or adjacent to the periphery of biotite (Fig. 9b, c). The grain sizes of most magnetite grains are larger than 5 μm, indicative of MD magnetite grains. This
PT
phenomenon is very common in granites, since Fe oxides (such as magnetite grains)
RI
probably either crystallize aligned with the earlier formed mafics or grow/rotate along the
SC
pre-existing grain shape fabric during deformation process (Mamtani and Vishnu, 2012). Under the circumstances, the subfabrics of magnetites are commonly coaxial with the
NU
biotites in granites (e.g., Liang et al., 2015b; Stevenson et al., 2007). Therefore, the measured AMS fabrics in the Dongjiangkou pluton correspond mainly to the biotite subfabrics despite
D
5.3. AMS scalar parameters
MA
different Km values.
PT E
The anisotropy degree (PJ) is the intensity indicator of anisotropy (Bouchez et al., 1997). In general, PJ values of the samples in the Dongjiangkou pluton vary gently from 1.01 to
CE
1.36 (average=1.08), indicating weakly to moderately anisotropic. Except for three samples with relative high PJ values, all other samples show PJ values lower than 1.2 (Fig. 9d).
AC
Meanwhile, the shape parameter (T) values of the pluton display a broad range of variation (from -0.89 to 0.92), and the majority of samples show T>0 (Fig. 9e). T values can be used to determine the type of the deformation regime: an oblate ellipsoid (T>0) is related to flattening whereas a prolate ellipsoid is linked to stretching (T<0). Therefore, magnetic fabrics of the Dongjiangkou pluton were mainly induced by a flattening regime. Additionally, the PJ-Km plot shows a positive correlation, especially in the range of 0–2500 μSI (Fig. 9d), indicating that the anisotropy degree is influenced by the susceptibility.
20
ACCEPTED MANUSCRIPT In contrast, no clear correlations can be identified between T and PJ or T and Km (Fig. 9e, f). Similar correlation between PJ and Km has been documented in many other case studies (e.g., Bolle et al., 2018; Li et al., 2017c), and a variety of explanations have been proposed, such as distribution anisotropy of magnetite grains, growth characteristics of magnetite, and shape change of the magnetite grains due to the plastic deformation (Archanjo et al., 1995; Ferré et
PT
al., 2003). Investigating the mechanism of this positive correlation is beyond the scope of
RI
our research. Yet, the relationship can illustrate that the AMS of the samples in the pluton are
SC
associated with magnetite.
NU
5.4. AMS directional data
Magnetic fabrics, i.e. magnetic foliation and magnetic lineation, are defined by K1-K2
MA
and K1 of the AMS ellipsoid. In the Dongjiangkou pluton, magnetic fabrics are considered to be well defined, in that the majority of sites show confidence level (α95) smaller than 25°
D
(Appendix Table S2). At the pluton scale, the distribution of magnetic foliations clearly
PT E
shows two concentric patterns, Zone Ⅰ and Zone Ⅱ (Fig. 10), which are concordant with the pluton margins. Magnetic fabrics in the inner core of Zone Ⅰ are characterized by low dip
CE
angles and no regulation of inclinations. Moreover, it is worth noting that the concentric pattern of Zone Ⅰ (Fig. 10b) is very similar to the petrological pattern the pluton (Fig. 2). In
AC
contrast, magnetic fabrics in the inner core of Zone Ⅱ are sub-vertical. Furthermore, the distribution of magnetic lineations shows the same pattern as magnetic foliations. Magnetic lineations in the inside core of the pluton show relative low plunge angles and irregular directions, indicating chaotic magma flow directions.
5.5. SPO analysis Results of SPO analysis of 14 sites for the pluton are shown in Appendix Table S3.
21
ACCEPTED MANUSCRIPT Apparently, except for three samples (D037, D045 and D291) most sites show a rather small dihedral angle between the principal axes of the AMS and SPO ellipsoids (Fig. 11). Shapes of AMS and SPO ellipsoids also match well. The minor error probably resulted from the analytical precision that the photos for SPO analysis were mostly captured at the area of ca. 1m×1m and the cylindrical specimens for AMS measurements were drilled and cut from the
SPO
and T
SPO),
equivalent to those of AMS, were used to take a
RI
of the SPO ellipsoid (PJ
PT
samples smaller than 15×15×15 cm3. Moreover, the anisotropy degree and shape parameters
SC
comparison. In general, PJ values calculated by SPO analysis are higher than that of AMS ellipsoid, which is probably due to the fact that PJ values of AMS reflect the combined
NU
magnetic anisotropy degree from all rock-forming minerals, whereas PJ values of SPO record just the biotite fabric. T values of most samples for SPO analysis show the same
MA
regime (mainly flattening regime) as the AMS for the formation of the fabrics. In summary, SPO analysis of the biotites supports the use of AMS as a proxy for the orientation of the
6. Discussion
CE
PT E
the Dongjiangkou pluton.
D
petrofabric, since the measured AMS fabrics probably correspond to the biotite subfabrics in
AC
6.1. Incremental emplacement of the pluton Previous studies preferred to use the thermal ballooning model and in situ fractional crystallization to account for the five zoned petrographic units of the Dongjiangkou pluton (e.g., Cui et al., 1999). However, this model is difficult to explain the ‘room problem’, since very little compression deformation can be observed at the contacts of the pluton. Moreover, xenoliths of country rocks can rarely be observed in any petrographic unit of the pluton, leading us to exclude the stoping model (Glazner and Bartley, 2006). Therefore, a new
22
ACCEPTED MANUSCRIPT emplacement mechanism or growth model is needed to understand the construction process of the pluton. Our zircon U–Pb geochronology data show that at least three-stagemagma events contributed to the growth of the Dongjiangkou pluton (Table 1; Fig. 7f). Previous work by Liu et al. (2011), has shown that the Yingpan unit was formed by two-stage magma events.
PT
Similar situations have been documented in many other case studies in the Qinling Orogen,
RI
as follows. (1) Hu et al. (2017) obtained two groups of crystallization ages (216 Ma and 200
SC
Ma) for the monzogranite collected from the Zhashui pluton (~12 km east of the Dongjiangkou pluton, Fig. 2). (2) Zhang et al. (2009) obtained two groups of zircon ages for
NU
the host granites (210 Ma and 197 Ma), MMEs (197 Ma and 188 Ma) and the diorite dykes (230 Ma and 210 Ma) collected from the Shahewan pluton. (3) Duan et al. (2016) reported
MA
two groups of crystallization ages (212 Ma and 225 Ma) for the granites collected from Lvjing pluton in the West Qinling Orogen. Thus, two groups of zircon ages existing in one
D
sample is a common phenomenon in the Late Triassic granite plutons of the Qinling Orogen.
PT E
Two explanations have been proposed for this: (1) zircon xenocrysts existing in the granites; (2) pre-existing zircons that crystallized in the same magmatic system prior to emplacement.
CE
The former is more applicable for the Dongjiangkou pluton since the age discrepancy is higher than 10 Ma and granite magma crystallization can hardly be sustained for such a long
AC
time. Low magma temperature (~771°C, Liu et al., 2013) and fast crystallization process are probably the best reasons that can account for a large number of magmatic zircon xenocrysts preserving in late-stage magmas (Miller et al., 2003). Therefore, we interpret the 199–201 Ma age as the final crystallization age of the samples D114, D128, D215 and D222, and zircons of 222–221 Ma may represent xenocrysts from a previous magma batch. Similarly, ~210 Ma represents the crystallization age of sample D297 and 222 Ma represents the formation age of zircons xenocrysts.
23
ACCEPTED MANUSCRIPT We thus infer that the construction of the pluton was completed at ~200 Ma on the basis of zircon U–Pb ages. Compared with previous dating results, our new data show younger ages which could be more authentic. For example, dating work by Qin (2010) reported 220±2 Ma, 214±2 Ma and 222±2 Ma for different units of the pluton. However, the single zircon ages in their work have a broad distribution range in the same sample, from 212 Ma to
PT
226 Ma, 208 Ma to 222 Ma and 217 Ma to 236 Ma, respectively. Previous studies have
RI
demonstrated that granite magmatism is a relatively rapid process with a time scale less than
SC
100, 000 yrs (Petford et al., 2000). Therefore, such a big discrepancy in the crystallization ages of the Dongjiangkou pluton is impossible in one magma process. It is important to
NU
make a distinction between the crystallized zircons and the zircon xenocrysts (or captured zircons) (e.g., Brown, 2013).
MA
Additionally, the fabric patterns reveal that the northeastern part of the pluton (Zone Ⅱ) is markedly different from the other parts and probably crystallized at ~210 Ma (Liu et al.,
D
2011, and this study). This indicates that the pluton is a composite intrusion which is
PT E
composed of two juxtaposed small plutons. Meanwhile, it should be noted that the majority part of the pluton (Zone I) was constructed during 199.2 Ma (outer unit) to 200.6 Ma (inner
CE
unit). This indicates an inversely zoned pluton although the age gap is small (1.4 Ma). Combined with the five-unit zoned feature, we infer that Zone I of the pluton was
AC
incrementally formed by a series of magma batches. The incremental emplacement is also supported by the study of emplacement depth. According to the calculation of Al-in-hornblende barometer, different units of the pluton are thought to have been crystallized at depths ranging from 4.7 km to 8.8 km. After eliminating the elevation effect, the contour map shows a clear zoned pattern with the emplacement depths increasing from the inner unit to the outer unit (Fig. 8b), indicating that different units of the pluton were emplaced at different depths. The increasing emplacement depths of the
24
ACCEPTED MANUSCRIPT pluton from the inner unit to the outer unit are in accordance with the syn-collisional convergence setting (as discussed later), supporting an inversely zoned pluton as indicated by zircon U–Pb ages. Therefore, combining the zoned pattern, the zircon U–Pb geochronology and the emplacement depth, we argue that the construction of the major part of the pluton (Zone I) is related to the model of incremental emplacement. During the
PT
emplacement process (200.6–199.2 Ma) the emplacement pressures (depths) have an
RI
increasing trend.
SC
Consequently, we propose an integrated growth model for the whole Dongjiangkou pluton (Fig. 12). It should be noted that pre-existing structures including the channels or
NU
conduits of magma ascent and the emplaced structures are so fragmentary that the real emplacement process of the pluton could be far more complicated. Thus, utilizing the limited
MA
data, we infer the emplacement process as follows.
(1) At first, the pluton was laterally amalgamated by two small plutons with different
D
formation ages. Zircon U–Pb geochronology demonstrates that the northeastern part of
PT E
the pluton (Zone II, Fig. 10b) formed initially at ~210 Ma with ~222 Ma zircon xenocrysts, and the majority part of the pluton (Zone I) formed during 200.6–199.2 Ma
CE
with 222–221 Ma zircon xenocrysts. The inner cores of the two zoned plutons represent the most important magma feed zones. Although the deep geometry of the pluton is not
AC
clear due to the absence of high-resolution gravity data, the major feed dykes are inferred to strike NE-SW according to the regional convergent direction. (2) Subsequently, the pluton was vertically accreted by five successive units with different emplacement depths. The inner unit (200.6 Ma) is relatively older than the outer unit (199.2 Ma). In this case, the petrological zonation is called an inverse style or outward-building manner which has been documented in other case studies, such as the Guposhan pluton in the Nanling Range of the SCB (Feng et al., 2012). This type of
25
ACCEPTED MANUSCRIPT emplacement sequence is also equivalent to the under-accretion, i.e. repeated amalgamation of magma with younger age adds under the older igneous body (Annen, 2011; Menand, 2008, 2011). Moreover, the rigidity contrast between the pre-existing unit and the underlying strata (i.e. an upper, more rigid layer and a lower, less rigid layer) is propitious to trap the ascending magmas (Kavanagh et al., 2006; Menand, 2008).
PT
Consequently, the emplacement process of the pluton at this time is in the order of:
RI
~200.6 Ma, the inner Meihuazhuang unit (MZ) was emplaced at a depth of ca. 5–6 km;
SC
~200.2 Ma, the Shaluozhang unit (SL) accreted to the bottom of the former unit and laterally propagated; the Yaowangtang unit (YW) and the Yingpan unit (YP) accreted to
NU
the pluton by the same way; during 200.1 Ma–199.2 Ma, the Xiaochuan unit (XC) was finally emplaced at a depth of ca. 7–8 km, forming the outer ring of the pluton. This
MA
completes the entire emplacement process of the pluton with progressive crustal
D
thickening.
PT E
The incremental emplacement model alleviates the ‘space problem’ of the Dongjiangkou pluton. Compared with traditional emplacement mechanisms (e.g., stoping
CE
and thermal ballooning model), incremental emplacement does not need disposable room for quasi-instantaneous or rapid emplacement of the entire large pluton body (de Saint-Blanquat
AC
et al., 2011). It also does not need the rapid strain rate of the structural expansion for the space, since the pluton is accumulated by multiple magma batches. Furthermore, this model can account for the phenomenon that width of contacts is much smaller than previously presumed for such a big pluton. This proposed model is in contrast to the previous hypothesis that the pluton was emplaced by the thermal ballooning model, and thus with crystallization ages increasing from the center to the margin (Cui et al., 1999). Our conclusion that five units of
26
ACCEPTED MANUSCRIPT Dongjiangkou pluton were resulted from different magmatic pulses can be confirmed by geochemical and isotopic studies. Isotope studies show that the Dongjiangkou (high-Mg) granites were formed by magma mixing between melts derived from partial melting of the Neoproterozoic basement rocks of the SQB and mantle-derived mafic melts (Hu et al., 2017; Gong et al., 2009; Qin et al., 2010). The magma sources of the Dongjiangkou granites are
PT
heterogeneous in isotopic compositions with different proportions of magma mixing (Hu et
RI
al., 2017) which resulted in the different geochemical characteristics of each unit (Qin et al.,
SC
2010). Moreover, previous studies have demonstrated that large-scale crystallization differentiation in a granite magma chamber may not have existed, due to the low temperature
NU
and the high viscosity (e.g., Clemens and Stevens, 2016; Coleman et al., 2004; Glazner et al., 2004; Matzel et al., 2006). Consequently, low magma temperature (~771°C, Liu et al., 2013)
MA
of the Dongjiangkou pluton could go against large-scale crystallization differentiation. It should be noted that fractional crystallization may be locally existed during magma ascent
D
and emplacement of each unit (Qin et al., 2010), but it is not the major mechanism that
PT E
formed the zoned Dongjiangkou pluton. Therefore, we believe that the zoned Dongjiangkou pluton probably resulted from multiple magma batches with different properties (from the
CE
source), rather than induced by crystallization evolution after magma emplacement, i.e. the
magma.
AC
zoning is controlled by compositional heterogeneity in the ancient country rock of the
The incremental emplacement of the pluton also leads to new challenges when interpreting field observations and relating them to the processes involved during pluton construction. This model can account for many zoned patterns of petrology and geochemistry that were characterized by compositional and textural heterogeneity, especially in a pluton with a large body and long period of construction. In the Qinling Orogen, the same features exist in many Late Triassic plutons. For example, the Zhashui pluton were
27
ACCEPTED MANUSCRIPT built by the Laoansi, Xichuanjie and Dongergou units (Cui et al., 1999); the Laochen pluton was formed by the Muhe, Shibangou and Leigutai units, and Yanzhiba pluton was constructed by Tianwan and Yingzuishi units (Tao, 2014); the Wenquan pluton can be divided into five units (Cao et al., 2011); the Wulong pluton consists of three units (Qin, 2010); the Caoping pluton was formed by two different units (Hu et al., 2016). For plutons
PT
with similar features, the incremental emplacement model probably is the common
SC
RI
mechanism.
6.2. Syn-tectonic deformation
NU
In this study, both the field structures and microstructures reveal that the pluton was syntectonically emplaced. In the pluton, the MMEs and the segmented dykes show
MA
syn-plutonic deformations, which are in accordance with the kinematics reflected in the SSZ (Li et al., 2017c) and the SFF (Zhang et al., 2001). At the contact, thermal aureole structures
D
also display syn-plutonic ductile deformation. Under the microscope, the microstructures
PT E
indicate that the fabrics in the pluton were mainly formed during submagmatic flow (melt-present deformation) to High-T solid-state deformation, attesting that the pluton was
CE
constructed under the regional tectonics. In addition, the emplacement of the pluton is coeval with regional deformations that are constrained by the SSZ (219–210 Ma, Li et al., 2017c;
AC
Zhang et al., 2001), the SFF (~201 Ma, Wu, 2013) and the syn-tectonic granitic vein in the Liuling Group (~201 Ma, Li et al., 2017c) (Fig. 12). All these lines of evidence confirm that the Dongjiangkou pluton was syntectonically emplaced and therefore experienced a syn-tectonic deformation. Our conclusion is in agreement with other case studies in the Qinling Orogen, such as the Mishuling pluton (Liang et al., 2015a) and the Baliping pluton (Li et al., 2017c). Based on this understanding, we can then interpret the internal fabrics of the pluton. But before this interpretation, we still have to figure out the origin, source and the
28
ACCEPTED MANUSCRIPT reliability of magnetic fabrics. Bulk susceptibility analysis shows that AMS of the samples with low Km values are mostly attributed to the biotites and the AMS of samples with relatively high Km values are probably dominated by magnetite. Some rock-magnetic experiments have been conducted for the Late Triassic granites in the Qinling Orogen, such as the Laocheng and Yanzhiba
PT
pluton (Tao, 2014), the Minshuling pluton (Liang et al., 2015b) and the Baliping pluton (Li
RI
et al., 2017c). Hysteresis data of these granites reveal two contrasting types of hysteresis
SC
loops, i.e. hysteresis loops of samples with low susceptibility display straight lines, while hysteresis loops of samples with high susceptibility show significant paramagnetic
NU
contribution. χ-T curves and IRM acquisition curves show that ferromagnetic minerals in samples are mainly soft magnetic components with low coercivity (magnetite). Day plots
MA
demonstrate that most samples are located in the multidomain (MD) area. Therefore, the adjoining and coeval Dongjiangkou pluton may have the same rock magnetism, and the
D
magnetic fabrics in the pluton are induced by the subfabrics of biotites or magnetites.
PT E
Moreover, microscopic observations of the Dongjiangkou pluton attest that magnetites prefer to distribute along the cleavages or near the periphery of biotites (Fig. 9b), indicating that the
CE
subfabrics of magnetite are always in line with the biotite grains. Meanwhile, SPO analysis demonstrates that the directions of biotite subfabrics are consistent with the magnetic fabrics
AC
in most analytical sites. Therefore, these evidences approve that AMS can be used as a proxy for the orientation of the petrofabric in the Dongjiangkou pluton. At the pluton scale, the fabric patterns show two concentric zones that are concordant with the pluton margins, which is very similar to the petrological pattern (Fig. 10). In the core of Zone I, the present-day outcrop is interpreted as the top level of the unit, since the magnetic fabrics in the inner core are characterized by low dip angles and no regulation of inclinations. When ascending magmas were stopped by the overlying country rocks, mussy
29
ACCEPTED MANUSCRIPT fabrics with low dip angles formed (Závada et al., 2009). In contrast, magnetic fabrics in the inner core of Zone Ⅱ are sub-vertical, indicating a deep erosion level. In most cases, concentric zoned patterns of magnetic fabrics have been considered as magma feed zones or supply channels (e.g., Castro, 1987; Cruden et al., 1995; Hutton, 1988). Therefore, we infer that there existed two major magma channels (feeders) for the pluton. This kind of fabric
PT
pattern also suggests a syn-tectonic emplacement of the pluton (Castro, 1987; de
RI
Saint-Blanquat et al., 2001).
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Similar zoned or concentric fabric patterns can be found in other case studies, such as the Late Triassic Mishuling pluton in West Qinling Orogen (Liang et al., 2015a), the
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Laocheng pluton and the Yanzhiba pluton in the South Qinling Belt (Tao, 2014), and the Papoose pluton in California (de Saint-Blanquat et al., 2001). All these studies underlined
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the role of regional deformation in the formation of such internal fabric patterns. Combining the coeval regional (transpressional) deformation, we infer that the internal fabric pattern of
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the Dongjiangkou pluton was mainly induced by the deformation-controlled magma flow.
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During the emplacement of the pluton, the sustained emplacement of the subsequent unit may deliver the latent heat, preventing the former units from rapid cooling (to lower than the
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solidus) (Annen, 2011). Meanwhile, a magma vortex may have been induced in response to the deformation partitioning of the regional sinistral transpression (Vigneresse and Tikoff,
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1999), which can develop the zoned pattern of the fabrics. The progressive NE-SW folding may have assisted the formation of the concentric fabric patterns.
6.3. Tectonic Implications One of the important aims of this study is to constrain the tectonic setting in which the Late Triassic granites were emplaced. As mentioned above, syn-subduction, syn-collision, and post-collision have been proposed.
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ACCEPTED MANUSCRIPT In this study, we have distinguished the syn-plutonic deformations both in the pluton and its country rocks, demonstrating that the Qingling Orogen was still under an oblique convergence setting during the construction of the pluton (i.e. Late Triassic period). Moreover, our data indicate that the pluton was formed under the progressive crustal thickening which can be related to the collisional event. Therefore, the tectonic setting
convergence
setting.
Our
conclusion
is
supported
by
other
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syn-collisional
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during which the Late Triassic plutons were emplaced can be summarized as a
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tectono-sedimentary records and tectonic chronology in the Qinling Orogen. It has been shown that several tectonic events occurred during Middle-Late Triassic in 40
Ar/39Ar dating
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the Qinling Orogen, especially during the Late Triassic period. Using
method, Xu et al. (1986) reported 232±5 Ma and 216±7 Ma ages respectively, for the
reported 226.8±2.2 Ma
40
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phengites and riebeckites that were collected from the nappe tectonics in the SQB. Li (2008) Ar/39Ar age for the muscovites of the granitic mylonites that were 40
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collected from the Wushan area, West Qinling Orogen. Chen (2010) obtained the
Ar/39Ar
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age of 223±2 Ma for muscovites that were collected from the ductile shear zone with sinistral transpressional kinematics in the Mianlue suture zone. Hu (2011) obtained 40
Ar/39Ar ages for the sericites of seven mylonitized siltstones (Lower
CE
224.6–212.3 Ma
Silurian) from the footwall of the Fangxian–Qingfeng Fault (South Qinling Belt). Li et al.
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(2013) obtained 222.4–198.2 Ma
40
Ar/39Ar ages for the biotites of the mylonites that were
from the hanging wall of the Hanwang thrust. Wu (2013) reported 200.9±1.9 Ma
40
Ar/39Ar
age for the altered biotites in the Xiaogoucao thrust fault (north of the SFF). Meanwhile, the ages of syn-tectonic granite dykes can also constrain the time of the tectonic event. For example, the activity time of the Ningshan Fault with sinistral kinematics was estimated to 214.4–212.8 Ma by two syn-tectonic granite dykes (Li et al., 2015); syn-tectonic granite dykes developed in the Liuling Group adjacent to the SSZ also provide an age of 200.7±1.3
31
ACCEPTED MANUSCRIPT Ma (Li et al., 2017c). Tectonic chronology is in accord with the ages of the Late Triassic granites, suggesting that the Late Triassic magmatism and the tectonics are probably coupled. Previous tectono-sedimentary studies also suggest that the Qinling Orogen was still under a syn-collisional convergence setting during the Late Triassic. The youngest
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sedimentary strata involved in fold-and-thrust deformations are the middle Triassic (Dong et
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al., 2016; Zhang et al., 2001), such as the T1-2 Jinjiling Formation in the Shanyang–Fengzhen
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thrust-nappe. Therefore, the major collision is supposed to be not earlier than Middle Triassic. Furthermore, the Upper Triassic molasse formation was extensively deposited in the
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Dabasha area, such as the Upper Triassic Xujiahe Formation (T3x) (Dong, 1997; He et al., 1997). The palaeo-flow direction (SSW) of the molasse-type conglomerates suggests that the
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South Qinling Belt uplifted notably during the Late Triassic, i.e. the Qinling Orogen was still under the syn-collisional uplift stage. Following the collision, sedimentary rocks in the J1-2
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graben basins, including the Jiangluo Basin, the Ciba Basin and the Mianxian Basin, were
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unconformably deposited on the sedimentary strata that were involved in intense fold-and-thrust deformations (e.g., Dong et al., 2016; Zhang et al., 2001). These graben
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basins are the products of post-collisional collapse, suggesting that the collision happened prior to the Early Jurassic period.
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Combining our case studies and previous tectono-sedimentary records and tectonic chronology studies, we suggest that the Qinling Orogen was still under the syn-collisional convergence setting during the Late Triassic. Since the Late Triassic granites developed within the Qinling Orogen were under syn-collisional convergence setting, the dynamic mechanisms (e.g., the delamination) deduced from the post-collision extension or collapse for the Late Triassic plutons should be excluded. Furthermore, the distribution map of the Late Triassic plutons (Fig. 1) clearly shows
32
ACCEPTED MANUSCRIPT that they are exposed in a narrow segment and mainly bounded by crustal-scale fault zones (e.g., the Shangdan Suture, the Ningshan Fault and the Mianlue Suture). The close relationship between tectonics and granite magmatism has been
acknowledged in many
studies (e.g., Brown, 2013; Hutton, 1988; Liang et al., 2015a; Petford et al., 2000; Tikoff and Teyssier, 1992; Vigneresse, 1999; Weinberg et al., 2004), although a
few geologists have
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denied the linkage (e.g., Paterson and Fowler, 1993, Paterson and Schmidt, 1999; Schmidt
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and Paterson, 2000) and argued that the spatial and temporal coupling between the structures
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and the magma productions are the prerequisites.
In this study, the spatial and temporal relationships between the pluton and its intrusive
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structures can be indicated by the following observations. (1) The pluton is directly in contact with the south boundary fault of the SSZ and the SFF. (2) Both the pluton and its
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country rocks recorded syn-kinematic deformations in response to the Late Triassic orogeny. (3) At the contact, thermal aureole structures commonly show syn-plutonic ductile
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deformations, resulting from the combination of regional tectonics and pluton emplacement.
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(4) Regional thrust faults facilitate magma transport and emplacement. (5) Regional deformations imposed on magma flow, forming the submagmatic fabrics, high-T solid-state
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fabrics and concentric fabric patterns.
Other structural studies of the Late Triassic granite plutons in the Qinling Orogen also
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show good examples of tectonic control on the granite magmatism. An integrated study on the Laocheng pluton and the Yanzhiba pluton (221–200 Ma) by Tao (2014) shows that the internal syn-plutonic structures have a concordant kinematics with the regional shear deformation, demonstrating a syn-tectonic emplacement and the tectonic control of the Ningshan Fault. Another structural study on the Mishuling pluton (213–212 Ma) shows that the Huangzhuguan-Miaopin Fault controlled the ascent and emplacement of the magmas and resulted in syn-kinematic deformations during the construction of the pluton (Liang et al.,
33
ACCEPTED MANUSCRIPT 2015a). Hence, tectonic-controlled magma ascent and emplacement in our study area is not an exceptional example, and can be extended to the magmatism in the whole Qinling Orogen.
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7. Conclusions
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Based on integrated studies, including field observations, microstructural observations,
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zircon U–Pb geochronology, emplacement depth and the internal fabric analysis, we draw the following conclusions.
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(1) The Dongjiangkou pluton is a composite pluton which is laterally composed of two juxtaposed small plutons with different ages (~210 Ma and ~200 Ma). Vertically, the
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younger pluton is characterized by under-accretion incremental growth. The pluton was syntectonically emplaced at depths increasing from 4.7 km to 8.8 km. The concentric
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petrologic zoning of the pluton is interpreted as having been inherited from the
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heterogeneous magma source before emplacement rather than in situ magma differentiation. (2) The Dongjiangkou pluton experienced syn-tectonic deformation during its
CE
construction, which is evidenced by extensive syn-plutonic deformations in the pluton and its contact zones. Microstructural observations show that the fabrics in the pluton were
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mainly acquired during submagmatic flow to high-T solid-state deformation. The distribution of internal fabrics is characterized by two concentric patterns that were probably induced by the sinistral transpression. (3) The Qinling Orogen was still under a syn-collisional convergence setting during the Late Triassic period. Regional tectonics controlled the Late Triassic granite magmatism in the Qinling Orogen, at least in the aspect of magma ascent and emplacement. Moreover, incremental emplacement may be a common mechanism for the formation of the zoned
34
ACCEPTED MANUSCRIPT plutons in the Qinling Orogen.
Acknowledgments This work was supported by the National Natural Science Foundation of China [Grant
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numbers 41802212, 41672200 and 41421002]. We thank Prof. Santosh for carefully
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polishing our revised manuscript. We thank Prof. Laura Webb, Dr. Shan Li and one
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anonymous reviewer for their insightful comments. The authors would like to thank Yazhou Ran, Qi Shen, Chuanzhi Li and Jiao Yao for their help in field and laboratory work. The
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authors also benefitted from the discussions with Prof. Yunpeng Dong, Prof. Anlin Guo and
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Prof. Xianzhi Pei.
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Mishuling in the Qinling Orogen (central China). Lithos 112, 259-276. Qin, J.F., Lai, S.C., Grapes, R., Diwu, C.R., Ju, Y.J., Li, Y.F., 2010. Origin of Late Triassic
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high-Mg adakitic granitoid rocks from the Dongjiangkou area, Qinling Orogen, central China, implications for subduction of continental crust. Lithos 120, 347-367.
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Reischmann, T., Altenberger, U., Kroner, A., Zhang, G.W., Sun, Y., Yu, Z.P., 1990.
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Mechanism and time of deformation and metamorphism of mylonitic orthogneisses from the Shagou shear zone, Qinling belt, China. Tectonophysics 185, 91-109.
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Schmidt, K.L., Paterson, S.R., 2000. Analyses fail to find coupling between deformation and magmatism. Eos Transactions American Geophysical Union 81(197), 202-203.
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Schmidt, M.W., 1992. Amphibole composition in tonalite as a function of pressure: an experimental calibration of the Al-in-hornblende barometer. Contributions to Mineralogy and Petrology 110(2), 304-310. Stevenson, C.T.E., Owens, W.H., Hutton, D.H.W., 2007. Flow lobes in granite: The determination of magma flow direction in the Trawenagh Bay Granite, north-western Ireland, using anisotropy of magnetic susceptibility. Geological Society of America Bulletin 119, 1368-1386
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ACCEPTED MANUSCRIPT Tao, W., 2014. Emplacement Mechanism and Tectonic Significance of the Late Triassic Laocheng and Yanzhiba Granitic Pluton in South Qinling. Master's Thesis, Northwest University (in Chinese with English abstract). Tarling, D.H., Hrouda, F., 1993. The Magnetic Anisotropy of Rocks. London: Chapman and Hall, pp. 1-212.
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Tikoff, B, Teyssier, C., 1992. Crustal scale, en-echelon ‘P-shear’ tensional bridges: a possible
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solution to the batholithic room problem. Geology 20, 927-930.
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Vernon, R.H., 2004. A practical guide to rock microstructure. Cambridge: Cambridge University Press, pp. 1-578.
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Vigneresse, J.L., Tikoff, B., 1999. Strain partitioning during partial melting and crystallizing felsic magmas. Tectonophysics 312(2-4), 117-132.
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Vigneresse, J.L., 1999. Should felsic magma be considered as tectonic object, just like faults or folds? Journal of Structural Geology 21, 1125-1130.
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Wang, T., Wang, X.X., Guo, L., Zhang, L., Tong, Y., Li, S., Huang, H., Zhang, J.J., 2017.
English abstract).
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Granitoid and tectonics. Acta Petrologica Sinica 33(5), 1459-1478 (in Chinese with
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Wang, X.X., Wang, T., Zhang, C.L., 2013. Neoproterozoic, Paleozoic, and Mesozoic granitoid magmatism in the Qinling Orogen, China: Constraints on Orogenic process.
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Journal of Asian Earth Sciences 72, 129-151. Weinberg, R.F., Sial, A.N., Mariano, G., 2004. Close spatial relationship between plutons and shear zones. Geology 32(5), 377-380. Wu, F.F., 2013. Research on the magmatite and its metallogenic tectonic setting in the Shanyang-Zhashui area, Middle Qinling Orogenic Belt. Ph.D. thesis, Chinese Academy of Geological Sciences (in Chinese with English abstract). Xiong, X., Zhu, L.M. Zhang, G.W., Li, B., Qi, L., Stevenson, D., Yang, T., Wang, F., Zheng,
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ACCEPTED MANUSCRIPT J., Jiang, H., Guo, A.L., 2016. Geology and geochemistry of the Triassic Wenquan Mo deposit and Mo-mineralized granite in the Western Qinling Orogen, China. Gondwana Research 30(8), 159-178. Xu, Z.Q., Lu, Y.L., Tang, Y.Q., Mattauer, M., Matte, Ph., Malavieille, J., Tapponnier, P., Maluski, H., 1986. Deformation characteristics and tectonic evolution of the eastern
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Qingling Orogenic belt. Acta Geologica Sinica 3, 237-247 (in Chinese with English
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abstract).
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Xu, Y.J., Yang, K.G., Ma, C.Q., 2005. Deformation Characteristics and the ESR Dating of Chengkou-Fangxian Fault Zone in the Qinling area. Geoscience 19, 127-132 (in
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Chinese with English abstract).
Yuan, H.L., Gao, S., Liu, X.M., Li, H.M., Gunther, D., Wu, F.Y., 2004. Accurate U–Pb age
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and trace element determinations of zircon by laser ablation inductively coupled plasma mass spectrometry. Geostandards and Geoanalytical Research 28, 353-370.
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Závada, P., Kratinová, Z., Kusbach, V., Schulmann, K., 2009. Internal fabric development in
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complex lava domes. Tectonophysics 466, 101-113. Zhang, C.L., Wang, X.X., Wang, T., Dai, M.N., 2009. Origin of Shahewan granite intrusion
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in Eastern Qinling: evidences from zircon U–Pb dating and Hf isotopes. Journal of Northwest University (Natural Science Edition) 39(3), 453-465 (in Chinese with
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English abstract).
Zhang, C.L., Wang, T., Wang, X.X., 2008. Origin and tectonic setting of the early Mesozoic granitoids in Qinling Orogenic belt. Geological Journal of China Universities 13, 304-316 (in Chinese with English abstract). Zhang, Z.Q., Zhang, G.W., Liu, D.Y., Wang, Z.Q., Tang, S.H., Wang, J.H., 2006. Isotopic geochronology and geochemistry of ophiolites, granites and clastic sedimentary rocks in the Qinling Orogenic belt. Geological Publishing House, Beijing, pp. 1-348 (in
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ACCEPTED MANUSCRIPT Chinese). Zhang, G.W., Zhang, B.R., Yuan, X.C., Xiao, Q.H., 2001. Qinling Orogenic belt and continental dynamics. Science Press, Beijing, pp. 1-855 (in Chinese). Zhang, G.W. 2015. The Mianlue tectonic zone of the Qinling Orogen and China continental
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tectonics. Science Press, Beijing, pp. 1-855 (in Chinese).
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ACCEPTED MANUSCRIPT Figure 1. Tectonic framework of the Qinling Orogen. Phanerozoic granites were extensively developed in response to the subduction, collision and intra-continental orogeny. Note that almost all the Late Triassic granites are exposed in the narrower segment and mainly bounded by the crustal-scale fault zones. This indicates a correlation between magmatism and collisional tectonics. The orange colored zone in the inset
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map is the Central China Orogenic Belt (CCOB).
Figure 2. Simplified geological map of the Dongjiangkou pluton and its country rocks. Note that the The profile shows that the pluton emplaced into the syncline
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pluton consists of five petrographic units.
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of the Devonian Liuling Group, indicating that granite magmatism happened later than regional folding.
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Figure 3. MMEs within the Dongjiangkou pluton. (a) MMEs and granites enwrapped with each other, indicating simultaneous crystallization. (b) Different shapes of the MMEs, from round to elongated shape
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with high aspect ratio. (c)-(d) Typical melt-present deformation (submagmatic flow) of the MME. The offset of ~1 m in MME occurs without any mineral preferred orientation being developed along a magmatic fault, indicating that the offset occurred by melt-assisted grain boundary sliding and stopped
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photos are taken in vertical view)
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before final crystallization. The circle shows a feldspar phenocryst that came from the host granite. (All
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Figure 4. Granite dykes in the pluton. (a) Multi-stage (composite) dykes reflect multiple magma events. (b)-(d) Segmented dykes and their kinematics. Note that (d) is a sketch of field photograph (c) and shows
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the geometry of segmented dykes and the crosscuting relationship between the MMEs and dykes (dykes cutting MMEs). (All field photos taken in vertical view)
Figure 5. Thermal aureole structures of the Dongjiangkou pluton. (a)-(b) Sub-horizontal granitic sill developed at the north contact of the pluton, showing magma migration and emplacement along thrust (front view). Note that the thrust fault shows intense compressional damage zone and imbricated shear bands. (c)-(d) Structures including asymmetric folds and rigid porphyroclasts showing sinistral transpressional kinematics (vertical view). (e)-(f) Microstructures of the mica-quartzose schists are characterized by dynamic recrystallization and undulatory extinction of quartz grains and preferentially 48
ACCEPTED MANUSCRIPT aligned biotite crystals, indicating sinistral ductile shear deformation (crossed-polarized and plane-polarized light, respectively). Symbols: P–porphyroclast; Qz–quartz.
Figure 6. Characteristic microstructures of the Dongjiangkou pluton. (a) Euhedral amphibole. (b) Oscillatory zoning in plagioclases. (c) Transgranular fracture filled by later melt, indicating melt-present
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deformation (submagmatic flow). (d) Dynamic recrystallization of quartz by grain boundary migration indicating High-T deformation. (e) Chessboard pattern in quartz indicating High-T deformation. (f)
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Biotite kinking showing Low-T deformation. (a)-(e) were taken under crossed-polarised light; (f) was taken under plane-polarized light. Symbols: Amp–amphibole; Pl–plagioclase; Kfs–K-feldspar; Qz–quartz;
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Bt–biotite.
Figure 7. Zircon U–Pb ages of granites from the Dongjiangkou pluton. (a)-(e) U–Pb diagrams of 206Pb/238U
ages for analysed zircons of samples D114, D128, D215, D222
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concordia and weighted mean
and D297, respectively (data-point error ellipses and box heights are 2 sigma). Note that except for sample D114, all the other four samples show two contrasting groups of ages for zircon rims. (f) Histogram shows
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three peaks of zircon U–Pb ages in the pluton.
Figure 8. (a) Sampling sites and their corresponding emplacement depths. (b) Contour map shows that
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granite emplacement depths of the pluton increase from the inner unit to the outer unit.
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Figure 9. Parameters of AMS analysis in the Dongjiangkou pluton. (a) Distribution of the bulk susceptibility. (b) Microscopic observations show that long axes of magnetites mainly distribute along the cleavages or adjacent to the periphery of biotite. (c) Energy spectrum shows that magnetic minerals in the sample with relatively high Km values are mainly magnetites. (d), (e) and (f) are plots of PJ-Km, T-Km and T-PJ showing their correlations.
Figure 10. Magnetic fabric distributions within the Dongjiangkou pluton. (a) Magnetic foliations (dashed lines are the boundaries of each unit). (b) Simplified distribution map of the magnetic foliation. (c) Magnetic lineation (dashed lines are the trajectories of magnetic foliations). 49
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Figure 11. SPO vs. AMS of the Dongjiangkou pluton. The image analysis study supports the use of AMS as a proxy for the orientation of the petrofabric.
Figure 12. Simplified conceptual model for the regional structures and the construction of the
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Dongjiangkou pluton (See Figure 2 for explanation of letter code for units). The zoned patterns of fabrics were mainly induced by regional transpression, and the progressive regional folding would assist the
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formation of the fabric distribution patterns.
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ACCEPTED MANUSCRIPT Table 1.
206Pb/238U
ages of zircons from the Dongjiangkou pluton.
Pb/238U Num Mean age age ber E108°48.392’ porphyritic biotite 199.5–202. 200.6±1. I 30 N33°40.164’ monzogranite 3 Ma 0 Ma E108°52.795’ porphyritic biotite 199.4–201. 200.2±1. I 16 N33°40.375’ monzogranite 3 Ma 4 Ma 220.5–225. 221.9±1. Ⅱ 13 0 Ma 8 Ma D21 E108°46.186’ medium- to fine-grained 199.3–200. 200.1±1. I 10 5 N33°35.321’ granodiorite 9 Ma 5 Ma 220.0–222. 221.1±1. Ⅱ 13 5 Ma 6 Ma D22 E108°46.643’ medium- to coarse-grained 198.2–200. 199.2±1. I 13 2 N33°35.501’ granodiorite 7 Ma 4 Ma 219.2–222. 221.1±1. Ⅱ 11 8 Ma 8 Ma D29 E109°01.924’ coarse- to medium- grained 210.0–211. 210.4±1. I 14 7 N33°46.677’ monzogranite 0 Ma 5 Ma 221.2–224. 222.3±2. Ⅱ 7 8 Ma 1 Ma Zircon type I–magmatic zircons crystallized from the late-stage magmas; Zircon type Ⅱ–zircon Zircon type
Rock type
206
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Location
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xenocrysts.
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Sam ple D11 4 D12 8
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Table 2.
Electron microprobe results of hornblendes and the emplacement depths of the Dongjiangkou pluton.
1128
D114
1021
D128
1121
D199
876
D204
1111
D222
845
D232
1343
D247
1364
D269
1096
D299
1100
D303
960
D310
1139
D339
1333
6.57 6.13 6.23 7.31 5.75 6.87 6.39 5.99 6.42 6.88 6.97 6.56 6.61
Ca O 11. 60 11. 51 11. 65 11. 88 11. 21 11. 14 11. 88 11. 56 10. 96 11. 63 12. 28 11. 73 11. 44 11. 70 11. 94 11. 71
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Na2 O 0.6 3 0.5 8 0.6 5 0.5 7 0.5 8 0.6 5 0.3 4 0.4 5 0.6 2 0.5 7 0.3 2 0.5 2 0.7 2 0.6 0 0.5 2 0.6 8
K2 O 0.7 4 0.6 3 0.7 1 0.6 3 0.6 8 0.5 4 0.7 2 0.5 6 0.6 7 0.6 1 0.4 0 0.6 1 0.6 7 0.8 1 0.7 5 0.6 9
Tot al 95. 48 95. 95 96. 76 94. 54 95. 62 95. 85 94. 82 96. 71 96. 36 96. 19 96. 15 95. 95 95. 12 94. 95 96. 21 96. 15
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D113
6.83
Mg O 13. 54 14. 20 13. 40 13. 93 13. 70 13. 64 12. 66 14. 15 14. 89 14. 31 13. 12 13. 97 13. 93 12. 45 13. 86 13. 75
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1320
6.24
Mn O 0.3 7 0.3 4 0.4 1 0.2 5 0.3 3 0.4 2 0.3 2 0.3 9 0.2 1 0.3 6 0.3 3 0.2 4 0.3 4 0.3 6 0.2 9 0.3 4
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D097
6.62
Fe O 13. 50 13. 29 14. 20 12. 42 14. 07 13. 86 14. 38 13. 76 12. 98 13. 35 14. 26 13. 21 12. 86 14. 68 13. 82 13. 91
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993
Al2 O3
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D092
47. 29 48. 16 47. 87 47. 18 47. 84 48. 44 46. 42 48. 93 48. 16 48. 07 49. 05 48. 18 47. 14 46. 22 47. 38 47. 31
Ti O2 0.9 6 0.9 5 0.9 2 0.9 7 0.8 7 0.8 3 0.6 4 1.0 6 0.7 9 0.6 6 0.2 0 0.9 3 1.0 2 1.0 5 0.9 9 0.9 7
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1412
2
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D073
SiO
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Altitude (m)
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Sam ple
XF e 0.5 0 0.4 8 0.5 1 0.4 7 0.5 1 0.5 0 0.5 3 0.4 9 0.4 7 0.4 8 0.5 2 0.4 9 0.4 8 0.5 4 0.5 0 0.5 0
Pres sure (kba r)
Dep th (Km )
2.50
7.0
2.11
5.9
2.60
7.3
2.50
7.0
2.06
5.8
2.11
5.9
3.14
8.8
1.67
4.7
2.55
7.1
2.23
6.2
1.95
5.5
2.28
6.4
2.70
7.6
2.87
8.0
2.41
6.7
2.45
6.9
ACCEPTED MANUSCRIPT Highlights The Dongjiangkou pluton was built by an under-accretion incremental growth model The pluton was emplaced at depths increasing from 4.7 km to 8.8 km Syn-plutonic deformations were observed both in the pluton and its contact zones
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The Qinling Orogen was still under a convergence setting during the Late Triassic
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Figure 1
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Yang Li, Sanzhong Li, Wentian Liang, Rukui Lu, Youjun Zhang, Xiyao Li, Pengcheng Wang, Yongjiang Liu, Ian Somerville, Guowei Zhang PII: DOI: Reference:
S1342-937X(19)30097-8 https://doi.org/10.1016/j.gr.2019.04.001 GR 2127
To appear in:
Gondwana Research
Received date: Revised date: Accepted date:
16 November 2018 4 April 2019 5 April 2019
Please cite this article as: Y. Li, S. Li, W. Liang, et al., Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints, Gondwana Research, https://doi.org/10.1016/ j.gr.2019.04.001
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ACCEPTED MANUSCRIPT Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints
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Yang Li a, b, Sanzhong Li a, b, *, Wentian Liang c,, Rukui Lu c, Youjun Zhang d, Xiyao Li a, b,
Key Lab of Submarine Geosciences and Prospecting Techniques, MOE, Institute for
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a
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Pengcheng Wang a, b, Yongjiang Liu a, b, Ian Somerville e, Guowei Zhang a, b, c
Advanced Ocean Study, College of Marine Geosciences, Ocean University of China,
Laboratory for Marine Geology and Environment, Qingdao National Laboratory for Marine
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b
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Qingdao 266100, China
Science and Technology, Qingdao 266237, China
State Key Laboratory of Continental Dynamics, Northwest University, Xi’an 710069,
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c
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Tianjin North China Geological Exploration Bureau, Tianjin 300170, China
e
UCD School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland
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d
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*Corresponding author:
Sanzhong Li (College of Marine Geosciences, Ocean University of China, No. 238, Songling Road, Qingdao 266100, China; [email protected]; +86 053266781971)
Abstract Granitoids are important components of major orogenic belts, and provide important information about the regional geodynamic evolution. The emplacement mechanism of granite plutons and its relationship with regional tectonics has long been discussed, although 1
ACCEPTED MANUSCRIPT it still remains debated. The Qinling Orogen within the Central China Orogen was marked by the emplacement of numerous Late Triassic granitic plutons. Although the petrology, geochemistry and geochronology of these intrusions have been addressed in various studies, their tectonic setting remains controversial, particularly since the structural aspects not been evaluated in detail.
In this study, we attempt to reconstruct the emplacement process of the
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Late Triassic Dongjiangkou pluton in the South Qinling Belt. Field observations show
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extensive syn-plutonic deformations both in the pluton and its contact zones. Microstructural
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observations demonstrate that fabrics in the pluton were mainly acquired during submagmatic flow to high-T solid-state deformation. Zircon U–Pb ages reveal that the
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pluton is a composite intrusion which is composed of two juxtaposed small plutons with distinct ages (~210 Ma and ~200 Ma). Al-in-hornblende thermobarometer indicates that the
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pluton was formed at depths ranging from 4.7 km to 8.8 km, with an increasing depth trend from the inner unit to the outer unit. Distribution of the internal fabrics shows two concentric
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patterns which are concordant with pluton margins at the pluton scale and were probably
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induced by the regional sinistral transpression. Integrating these analyses, an incremental emplacement model is proposed for the syn-tectonic pluton. This model not only solves the
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‘room problem’ but also accounts for the zoned petrological features of the pluton. Combined with previous studies, we suggest that the Late Triassic granite plutons in the
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Qinling Orogen were emplaced under a syn-collisional convergence setting, and that the granite magmatism was probably controlled by regional tectonics. Additionally, the incremental emplacement model may be a common mechanism for the Late Triassic plutons.
Keywords Zircon U–Pb geochronology; Structural analysis; Incremental emplacement; Late Triassic granites; Qinling Orogen
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ACCEPTED MANUSCRIPT 1. Introduction Granites (sensu lato) constitute important components of major orogenic belts and carry important information on magma generation, segregation, transport and emplacement (Brown, 2013). Therefore, studies on granitoids are fundamental to understanding the magmatic and tectonic evolution of the continental crust (Li et al., 2013; Wang et al., 2017).
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During the past century, with the development of fabric measurements (e.g., Bouchez et al.,
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1997; Launeau and Robin, 2005), numerical simulation and analogue modeling (e.g.,
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Kavanagh et al., 2006; Menand, 2011; Petford et al., 1993), many studies have focused on the processes by which granite plutons are emplaced, although the emplacement mechanisms
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remain equivocal (e.g., Annen, 2011; Castro, 1987; Menand, 2011; Paterson and Fowler, 1993), as also the relationship between magmatism and tectonics (e.g., Brown, 2013;
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Paterson and Schmidt, 1999; Petford et al., 2000; Vigneresse, 1999; Weinberg et al., 2004). Traditionally, the emplacement of a granite pluton is considered to involve stopping, thermal
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ballooning, and diapirism, among other factors. In spite of the discrepancy on the nature of
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intrusion (Castro, 1987; Wang et al., 2017), most of the mechanisms emphasize a single magma intrusion that was emplaced in the crust quasi-instantaneously, which means that a
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granite pluton is in situ crystallized and differentiated from one single large magma body. However, none of the models can solve ‘space problem’ or ‘room problem’ (e.g., Menand,
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2011; Tikoff and Teyssier, 1992) for the emplacement of granites. Recent studies suggest that granite plutons (especially zoned plutons) probably formed through multiple and discrete magmatic pulses with distinct ages (Annen, 2009, 2011; Clemens and Stevens, 2016; Coleman et al., 2004; de Saint-Blanquat et al., 2011; Kavanagh et al., 2006; Menand, 2008, 2011; Miller et al., 2011; Paterson et al., 2011; Li et al., 2013) based on the following lines of evidence. (1) Field observations show that most plutons are tabular or wedge-shaped and emplaced incrementally by sheeted sills. (2) Geochronological
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ACCEPTED MANUSCRIPT studies reveal that many granite plutons formed in a time scale over millions of years. The proposal of incremental emplacement model provides a new perspective to address some of the issues, such as fractional crystallization in granite magmas (Clemens and Stevens, 2016; Coleman et al., 2004; Glazner et al., 2004), thermal aureole dimensions (Annen, 2011) and granite-related metallogenesis. However, the detailed mechanisms are not well-known.
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In the Qinling Orogen of central China, a large number of granite plutons were formed
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during ~225–200 Ma (Li et al., 2015; Wang et al., 2013; Zhang et al., 2008). These plutons
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(mostly the zoned plutons) are the keys to understand the tectonic evolution of the Late Triassic (Indosinian) orogeny. In spite of the petrological, geochronological and geochemical
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studies, the tectonic setting of these Late Triassic granite plutons remain controversial, with proposals of syn-subduction, syn-collision, and post-collision (e.g., Dong et al., 2012; Jiang
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et al., 2010; Li et al., 2015; Wang et al., 2013; Zhang et al., 2008). Moreover, the structures within these plutons have not been analyzed in detail to the same extent as the
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geochronological and geochemical studies.
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In this contribution, we present an integrated study of the Late Triassic Dongjiangkou pluton in the South Qinling Belt. This pluton is characterized by concentric zoning of
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petrological units and a large timescale of emplacement process (>10 Ma) (Cui et al., 1999; Gong et al., 2009; Hu et al., 2017; Qin et al., 2010). We present results from a
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multidisciplinary study including field and microstructural observations, zircon U–Pb geochronology, Al-in-hornblende thermobarometry, AMS (anisotropy of magnetic susceptibility) and SPO (shape preferred orientation) analysis, and derive information on the syn-plutonic deformations, crystallization ages, emplacement depths and internal fabrics. Based on the results, we propose an integrated emplacement model for the pluton and its implications.
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2. Geological Setting 2.1. Regional tectonic framework The Qinling Orogen is located between the North China Block (NCB) and the South China Block (SCB), constituting an important segment of the Central China Orogen. It
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extends more than 1500 km from the Dabie Mountains to the Qilian and Kunlun Mountains.
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This orogen is bounded by the Lingbao–Lushan–Wuyang thrust fault (LLWF) to the north
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and the Mianxian–Bashan–Xiangfan arc-shaped thrust fault (MBXF) to the south (Zhang et al., 2001) (Fig. 1). Previous studies have identified two ophiolitic mélange corresponding to
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two orogenies (Caledonian orogeny and Indosinian orogeny) that occurred in the Devonian (the Shangdan Suture) and Triassic (the Mianlue Suture) (Dong et al., 2016; Li et al., 2017a,
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2017b; Pei et al., 2005; Zhang et al., 2001, 2015). On the basis of these two sutures and the lithospheric-scale Luonan–Luanchuan Fault (LLF) (Zhang et al., 2001), the Qinling Orogen
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can be divided into four WNW-ESE trending tectonic belts (from north to south): Southern
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North China Block (S-NCB), North Qinling Belt, South Qinling Belt and Northern South China Block (N-SCB) (Dong et al., 2016) (Fig. 1).
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According to previous studies, the Qinling Orogen is a composite orogenic belt with a complex history of rifting, oceanic subduction, continental collision and intracontinental
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orogeny (Dong and Santosh, 2016; Li et al., 2007; Zhang et al., 2001). The Middle Paleozoic subduction to collision along the Shangdan Suture accreted the SCB to the NCB, leading to extensive metamorphism, magmatism and deformation. Contemporaneous rifting occurred along the southern margin of the South Qinling Belt followed by the opening of the Mianlue Ocean (a branch of the Paleo-Tethyan Ocean) (Zhang 2015). This breakup event resulted in the separation of the Qinling microblock from the SCB. Subsequently, the Qinling Orogen experienced the Triassic (Indosinian) collisional orogeny. The intense collision along the
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ACCEPTED MANUSCRIPT Mianlue Suture associated with the amalgamation of the NCB and SCB caused extensive thin-skinned fold-and-thrust deformation, extrusion, uplift and granitoid emplacement throughout the Qinling Orogen (Li et al., 2007; Zhang, 2015). During the subsequent phase of intracontinental orogeny, the boundary faults (LLWF and MBXF) of the orogen were formed.
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The Triassic orogeny played an important role in the long-term evolution of the Qinling
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Orogen, in that it not only formed the entire framework of the orogenic belt, but also
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represented an important transition from the Paleozoic plate tectonics to the Mesozoic-Cenozoic intracontinental tectonic regime (Dong et al., 2016; Zhang et al., 2001).
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However, there is still debate concerning the culmination of the Triassic orogeny. According to the integrated tectono-sedimentary studies, the collisional event took place during the
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Mid-Late Triassic (e.g., Zhang et al., 2001). More recently, this viewpoint has been challenged based on studies of the Late Triassic granites (~225–200 Ma) which are
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widespread in the orogen. A variety of tectonic settings including syn-subduction,
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syn-collision and post-collision were proposed and debated. For example, on the basis of geochemical and Sr–Nd–Hf isotopic studies of the Cuihuashan, Caoping, Dongjiangkou,
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Laocheng and Yanzhiba plutons (Fig. 1), Jiang et al. (2010) proposed that the Carnian (227–218 Ma) plutons represent the products of syn-subduction, whereas the Norian (211
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Ma) pluton emplaced in a syn-collisional setting. An integrated review by Wang et al. (2013) argued that all of the Late Triassic magmatism occurred in a post-collisional setting. Li et al. (2015) argued that the Triassic granitoids are the products of an active continental margin rather than syn-collision or/and post-collision settings.
2.2. Dongjiangkou pluton The Dongjiangkou granite pluton in the SQB is exposed over an area of 540 km2 (Gong
6
ACCEPTED MANUSCRIPT et al., 2009). The surrounding country rocks of the pluton belong to the Middle to Upper Devonian Liuling Group (Fig. 2). The Liuling Group is mainly composed of low-grade (greenschist-facies) metasediments that were metamorphosed from a suite of pelagic turbidites (Dong and Santosh, 2016). In this Group, a series of folds, axial planar cleavages and thrust faults were developed at different scales and levels in response to the Triassic
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collisional orogeny, indicating uniformly a NE-SW convergence (Li et al., 2007; Zhang et al.,
RI
2001). Fig. 2 shows that the pluton was emplaced into the syncline of the Devonian Liuling
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Group, demonstrating that the magma emplacement is later than regional folding which is considered to have occurred during Early to Middle Triassic (Xu et al., 2005; Zhang et al.,
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2001). Moreover, a syn-tectonic granitic vein in the Liuling Group has been dated by Li et al. (2017c). The formation age (201 Ma) provides the precise limits on the timing of regional
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deformation.
The pluton is in direct contact with the Shagou Shear Zone (SSZ) and the
D
Shanyang–Fengzhen Fault (SFF) (Fig. 2). To the north of the pluton, the SSZ is an E–W to
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ENE striking ductile deformation zone which is mainly composed of granitic mylonites (low-amphibolite grade metamorphism) (Fig. 2). This crustal-scale ductile shear zone shows
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kinematics of sinistral transpression and its formation age has been estimated as Late Triassic (Li et al., 2017c; Reischmann et al., 1990; Zhang et al., 2001). To the south of the
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pluton, the SFF is a WNW-ESE striking crustal-scale fault which is characterized by multi-stage activities, and its major deformation with sinistral transpressional kinematics occurred in the Late Triassic (Wu, 2013; Zhang et al., 2001). The Dongjiangkou pluton is composed of five petrographic units with different mineralogy and textures, showing zoned pattern (Cui et al., 1999; Gong et al., 2009; Hu et al., 2017; Qin et al., 2010). From the center to the margin, these are the Meihuazhuang unit, Shaluozhang unit, Yaowangtang unit, Yingpan unit and Xiaochuan unit (Fig. 2). On the basis
7
ACCEPTED MANUSCRIPT of field-mapping (Cui, 1999) and related studies (e.g., Gong et al., 2009; Qin et al., 2010), the petrographic composition of these five units can be described as follows. The Meihuazhuang unit mainly consists of coarse-grained porphyritic monzogranites and granodiorites. The Shaluozhang unit is mainly composed of hornblende-bearing porphyritic monzogranites and tonalites. The Yaowangtang unit comprises medium- to coarse-grained
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monzogranites and tonalites. The Yingpan unit is mainly made of medium- grained
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porphyritic biotite monzogranites, tonalites and granodiorites. The Xiaochuan unit mainly
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consists of medium- to fine-grained granodiorites and quartz monzonites. Moreover, abundant mafic magmatic enclaves (MMEs) characterized by fine-grained hypidiomorphic
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granular texture are present in the pluton.
Except for the Xiaochuan unit, the other units were dated by zircon U–Pb
MA
geochronology, and their formation ages range from 222 to 210 Ma (Gong et al., 2009; Li et al., 2012; Liu et al., 2011; Qin et al., 2010). Liu et al. (2011) identified two-stage magmatic
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events in the Yingpan unit: the older magmatic zircons (~219 Ma) correspond to an
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earlier-stage and the younger age of magmatic zircons (~209 Ma) represents the final crystallization age of this unit. In addition, a biotite 40Ar/39Ar age of 198 Ma also has been
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reported by Zhang et al. (2006). The discrepancy in the formation ages of the pluton
AC
indicates that the emplacement history is probably far more complex.
3. Macroscopic and microstructural observations 3.1. Deformation records in the pluton Macroscopic structures are rare within the Dongjiangkou pluton due to the weak shape preferred orientations of minerals in granites. Nevertheless, the following two strain markers can reflect the syn-plutonic deformations.
8
ACCEPTED MANUSCRIPT 3.1.1. Mafic magmatic enclaves (MMEs) Mafic magmatic enclaves (MMEs) can be observed all over the pluton, ranging in shape from round to elongated ellipse with high aspect ratio (Fig. 3a, b). Most of the MMEs are monzodiorites or diorites, having fine-grained textures with clustering of the mafic minerals. Zircon U–Pb ages show that the MMEs crystallized at the same time as the host granites
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(Gong et al., 2009; Li et al., 2012; Liu et al., 2011; Qin et al., 2010), which is also be attested
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by field observations. (1) MMEs and granites are enwrapped with each other (Fig. 3a). (2)
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Some of the MMEs contain plagioclase phenocrysts and scarce K-feldspar phenocrysts or megacrysts from the host granites (Fig. 3c, d). Compared with the host granites, the MMEs
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in the pluton commonly have relatively lower SiO2, higher MgO, K2O content, Rb/Sr and (La/Yb)N ratios (Gong et al., 2009), illustrating that these MMEs are products of mafic
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magmas with a source different from the host granites, rather than the residuum of the source rocks after partial melting.
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The deformation of the MMEs can provide important information about the
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syn-plutonic regional tectonics. Usually, shape preferred orientations of MMEs can be used to reflect the information of magma flows or deformation kinematics. However, it should be
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noted that each of the MMEs only represent one section (2-D ellipse) of the 3-D ellipsoid, and thus the single stretching shape can’t simply be regarded as the direction of magma flow.
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According to field observations, the MMEs with elongated shape are dominant in the Dongjiangkou pluton. Some of the MMEs show obvious offset without any mineral preferred orientation (Fig. 3c, d), indicating that the offset occurred by melt-assisted grain boundary sliding and stopped before final crystallization (Paterson, et al., 1998). This kind of structure is one of the most cogent evidences that can support the existence of melt-present deformation or syn-plutonic deformation. The same phenomenon can also be observed in other regions of the Dongjiangkou pluton that contains a large number of MMEs. In addition,
9
ACCEPTED MANUSCRIPT the strike (~100°) and kinematics (sinistral movement) of the deformation within the MMEs is in accord with the structural records of the country rocks and the SSZ (Li et al., 2017c). .
3.1.2. Dykes . In the Dongjiangkou pluton, dykes are mainly granitic with a few mafic ones. The
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granitic dykes range from aplite to pegmatite with different textures and degree of
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crystallization. Sometimes, these dykes mutually intrude each other (composite dykes),
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indicating multi-stage magmatism (Fig. 4a). According to previous dating results, these granitic dykes are slightly younger than the MMEs and their host granites (Qin, 2010).
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Furthermore, no younger magmatism has been recognized in our study area. Therefore, we believe that the dykes in the Dongjiangkou pluton probably formed during the late cooling
MA
stage of pluton.
It is well established that dykes are important markers of magma infilled fractures,
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usually representing the channel of magma ascend or migration (e.g., Brown, 2013; Ernst et
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al., 2001; Petford et al., 2000). The geometry of a dyke can reflect the paleostress field and the kinematics of regional deformation by which the fractures were formed and the magma
CE
was transferred (Glazner et al., 1999; Mège and Korme, 2004). On the basis of field observations, dykes in the Dongjiangkou pluton are almost uniform in all directions,
AC
indicating that they are not formed by just one mechanism. These dykes are interpreted to be related to local stresses at the late stage of magma crystallization and regional tectonics. Furthermore, a series of segmented granitic dykes can be observed in the Dongjiangkou pluton (Fig. 4b-d). This kind of dyke is supposed to be formed by shear deformation, and thus can reflect the kinematics of regional deformation (Oberc-Dziedzic et al., 2013). In Figs. 4c, d, the segmented dykes striking 125° show a sinistral shear sense, and the segmented dykes striking 165° show dextral shear deformation (Fig. 4d). On the basis of their geometry,
10
ACCEPTED MANUSCRIPT we infer that the maximum paleostress strikes 35°–75°. It should be noted that the principal stress axes may have been rotated during upward propagation to form the segmented dykes. Nevertheless, the syn-kinematic deformations of the segmented dykes can illustrate that regional shear deformation existed during the late stage of pluton construction.
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3.2. Thermal aureole structures
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Although the Dongjiangkou pluton and its country rocks have a long contact boundary,
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the contact relationship is observed only at a few locations due to the limited outcrop and the high degree of weathering. In the biotite-bearing country rocks of the pluton, syn-plutonic
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deformation is commonly observed (Fig. 5). Overall, the contact relationship can be characterized by the following features. (1) The pluton or syn-plutonic dykes emplaced into
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the rocks of the Liuling Group cut the previous foliation (mainly axial-plane cleavages) (Fig. 5a, b), indicating that the granite magmatism is later than the formation of the cleavage; (2)
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Around the contact there are little or almost no xenoliths of the country rocks can be found
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in the pluton, suggesting that the emplacement was not dominated by stoping which requires incorporation of large volumes of wallrock. (3) Sub-horizontal sills (the building blocks of
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the larger pluton) at the contact of the pluton were formed at the flat of the thrust fault (Fig. 5a, b), supporting that magmas can ascend and emplace along compressional structures
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(thrusts) (de Saint-Blanquat et al., 1998; Ferré et al., 2012); (4) Country rocks at the thermal aureole show relatively high-grade metamorphism (amphibolite facies, different from the lower greenschist facies far away from the pluton) and ductile deformations with sinistral shear kinematics (Fig. 5c-f), indicating that the ductile deformation resulted from the combination of regional deformation and the heat caused by pluton emplacement.
The
deformation reflected by the thermal aureole structures is coeval with the D2 deformation described by Li et al. (2007).
11
ACCEPTED MANUSCRIPT
3.3. Microstructural observations Microstructural observation is fundamental to estimate the degree of deformation and to determine the physical state of fabric formation (Vernon, 2004). Microstructures were examined in 60 thin sections collected throughout the Dongjiangkou pluton. In general, the
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microstructures in the pluton can be described as the following groups. (1) Euhedral
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amphiboles (Fig. 6a), plagioclase with oscillatory zoning (Fig. 6b) and K-feldspar with
SC
simple twin show no evidence of crystal-plastic deformation and recrystallization, representative of primary magma-flow structures. (2) In some thin sections, submagmatic
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flow that is evidenced by transgranular fractures (formed by syn-plutonic deformation) filled with later melt can be locally observed (Fig. 6c), supporting the existence of melt-present
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deformation (Vernon, 2004); (3) In most of thin sections, quartz grains commonly show high-T solid-state deformation, such as the grain boundary migration recrystallization with
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irregular lobate boundaries of the quartz grains (Fig. 6d) and a chessboard pattern (Fig. 6e)
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which is characterized by square subgrains of quartz with boundaries parallel to both the prism and the basal planes (Kruhl, 1996). Moreover, myrmekite lobes on the margins of the
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K-feldspar can be locally observed, indicating subsolidus deformation or high-T deformation (Vernon, 2004). (4) A few samples located at the margin, especially the sites close to the
AC
Shagou Shear Zone and Shanyang–Fengzhen Fault, show occasional features of low-T solid-state deformation (which is characterized by brittle-ductile to brittle deformation), such as kinks in biotite grains (Fig. 6f) and deformation twinning in feldspars. However, these low-T solid-state deformations are too few that can be neglected In general, the microstructures described above are neither purely magmatic flow nor low-T solid-state (after the complete crystallization of the magma) products, demonstrating that the internal fabrics in the pluton were acquired mainly during submagmatic flow
12
ACCEPTED MANUSCRIPT (melt-present deformation) to high-T solid-state deformation. Each section preserves some of the former magma-flow microstructures and the later low-T solid-state deformations. The result of microstructural observations is in accord with field-observed syn-plutonic deformations that were reflected by the granite dykes and the MMEs, supporting that
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regional deformation imposed on the process of pluton construction.
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4. Emplacement ages and depths 4.1. Analytical methods
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In order to better understand the emplacement succession of the entire Dongjiangkou pluton, five granite samples were collected from different units (Fig. 2). Zircon grains were
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separated by crushing and sieving followed by magnetic and heavy liquid separation, and handpicked under a binocular microscope. Cathodoluminescence (CL) images for zircon
D
patterns were collected by a Mono CL3+ microprobe to determine target sites for U–Pb
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dating. Zircon U–Pb analyses were conducted by LA–ICPMS at the State Key Laboratory of Continental Dynamics, Northwest University, China. The laser–ablation system comprises a
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GeoLas 200 M equipped with a 193 nm ArF excimer laser (MicroLas, Göttingen, Germany). Laser spot diameter is 30 μm. Analyses were performed on the ELAN 6100 ICP–MS from
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Perkin Elmer/SCIEX (Canada) with a dynamic reaction cell (DRC). Detailed descriptions for analytical procedures refer to Yuan et al. (2004). 207Pb/206Pb, 207Pb/235U and 206Pb/238U ratios were calculated by the GLITTER 4.0 program (Macquarie University). Zircon 91500 and NIST610 were used as the external calibration and the reference standard for zircon U–Pb dating and trace element analyses. Additionally, on the basis of the positive correlation between the total aluminium content
of
magmatic
hornblende
and
crystallization
13
pressure
(Al-in-hornblende
ACCEPTED MANUSCRIPT thermobarometer) (Anderson et al., 2008), the emplacement depths of 16 samples were estimated. The rims of hornblende in the matrix can represent the final-stage and are therefore appropriate for the calculation of the emplacement depth. Electron microprobe analysis was performed at the State Key Laboratory of Continental Dynamics, Northwest
(1992): r2＝0.99
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P (±0.6 Kbar) = -3.01+4.76 Altot
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University, China. Emplacement pressures were estimated by the equation of Schmidt
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Altot is the total aluminium content of the hornblende in atoms per formula unit.
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4.2. Zircon U–Pb ages
The zircon U–Pb dating results of 5 samples from the pluton are shown in Appendix
MA
Table S1. Zircons of Sample D114 from the Meihuazhuang unit (porphyritic biotite monzogranite, E108°48.392’; N33°40.164’) are mainly euhedral prismatic, from 100 to 300
D
μm in length, with aspect ratios in the range of 1.5–3.0. CL images show that zircon grains
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have clear oscillatory zoning and some grains have inherited cores. Thirty dating sites located at the rim of zircon grains yielded concordant
206
Pb/238U ages, ranging from 199.5
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Ma to 202.3 Ma, with a weighted mean age of 200.6±1 Ma (MSWD=0.032) (Fig. 7a). Th/U ratios of these dating sites vary from 0.5 to 1.13, indicative of a magmatic origin (Hoskin
AC
and Schaltegger, 2003). Therefore, the age of 200.6±1 Ma represents the crystallization age of Sample D114. The remaining six spots are located at the inherited core of zircons give various U–Pb ages ranging from 357.7 Ma to 1698.9 Ma. These ages represent the formation time of inherited zircons, reflecting multistage magmatic events within this area. Zircons of Sample D128 (porphyritic biotite monzogranite), collected from the Shaluozhang unit (E108°52.795’; N33°40.375’), show a similar grain size to Sample D114. A total of 36 zircon spots were analyzed, of which 30 spots show concordant ages. Only one
14
ACCEPTED MANUSCRIPT spot with an age of 267 Ma is located in the zircon core. The remaining 29 spots are located at zircon rims. Although CL images show that the analyzed zircons are similar in shapes and internal patterns, the dating results show two contrasting groups of 206Pb/238U ages (Fig. 7b): (1) The first group of 16 spots shows relatively younger ages (ranging from 199.4 Ma to 201.3 Ma) with a weighted mean age of 200.2±1.4 Ma (MSWD=0.032); (2) The second relatively older ages (ranging from 220.5 Ma to 225 Ma) with a
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group of 13 spots displays
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weighted mean age of 221.9±1.8 Ma (MSWD=0.098), ~22 Myr older than the first group.
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Both groups show Th/U ratios are higher than 0.4 (0.45–0.82), indicative of a magmatic origin.
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Sample D215 (medium- to fine-grained granodiorite) was collected from the Xiaochuan unit (E108°46.186’; N33°35.321’). Zircons are characterized by hypidiomorphic to euhedral
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granular texture, 100–350 μm in length and 50–100 μm in width. CL images show that the majority of zircon grains have clear oscillatory zoning. A total of 38 zircon spots were
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analyzed, of which 26 spots show concordant ages. Three spots located on inherited cores
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show ages of 526.2 Ma, 743.3 Ma and 846 Ma, corresponding to the formation age of inherited zircons. Spots at zircon rims also show two contrasting groups of
206
Pb/238U ages
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(Fig. 7c). (1) The first group of 10 spots shows relatively younger ages (ranging from 199.3 Ma to 200.9 Ma) with a weighted mean age of 200.1±1.5 Ma (MSWD=0.037). (2) The
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second group of 13 spots has ages ranging from 220 Ma to 222.5 Ma with a weighted mean age of 221.1±1.6 Ma (MSWD=0.108), ~21 Myr older than the first group. Both groups show Th/U ratios higher than 0.4 (0.62–1.34), indicative of a magmatic origin. Sample D222 (medium- to coarse-grained granodiorite) was collected from the Xiaochuan unit (E108°46.643’; N33°35.501’). Zircon shapes and textures are similar to those of Sample D215. A total of 36 zircon spots were analyzed, of which 25 spots show concordant ages. One spot located at the inherited core show an age of 245.5 Ma,
15
ACCEPTED MANUSCRIPT representing the formation age of the inherited zircon. The remaining 24 spots on zircon rims also show two contrasting groups of 206Pb/238U ages (Fig. 7d): (1) The first group of 13 spots acquired relatively younger ages (ranging from 198.2 Ma to 200.7 Ma) with weighted mean age of 199.2±1.4 Ma (MSWD=0.110); (2) The second group of 11 spots has ages ranging from 219.2 Ma to 222.8 Ma with a weighted mean age of 221.1±1.8 Ma (MSWD=0.120),
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~22 Ma older than the first group. Th/U ratios are higher than 0.4 (0.59–2.22), indicative of a
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magmatic origin.
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Sample D297 (coarse- to medium- grained monzogranite) was collected from the Yingpan unit (E109°01.924’; N33°46.677’). Zircons are characterized by euhedral granular
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texture, 100–300 μm in length and 50–150 μm in width. CL images show that zircons are very similar to each other. The majority of zircon grains have clear oscillatory zoning, and
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some grains have inherited cores. A total of 37 zircon spots were analyzed, of which 28 spots show concordant ages. Seven spots located on inherited cores show ages of 454.1 Ma, 455.4
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Ma, 400.3 Ma, 358.5 Ma, 350.0 Ma, 256.9 Ma and 231.9 Ma, indicating multi-stage
groups of
206
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magmatic events. The remaining 21 spots located on zircon rims also show two contrasting Pb/238U ages (Fig. 7e): (1) The first group of 14 spots has relatively younger
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ages (mainly ranging from 210.0 Ma to 211.0 Ma, except for one spot which shows an age of 206.9 Ma), with a weighted mean age of 210.4±1.5 Ma (MSWD=0. 079; (2) The second
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group of 7 spots has ages ranging from 221.2 Ma to 224.8 Ma, with a weighted mean age of 222.3±2.1 Ma (MSWD=0.2), ~11 Ma older than the first group. Both groups show Th/U ratios higher than 0.4 (0.41–1.27), indicative of a magmatic origin. In summary, except one Sample (D114) in the pluton showing a single group of ages (200.6±1 Ma), all the other four samples show two contrasting groups of
206
Pb/238U ages
(Table 1) for zircon rims. Consequently, at least three stages of magma events can be recognized for the growth of the Dongjiangkou pluton: 222–221 Ma, ~210 Ma and 199–201
16
ACCEPTED MANUSCRIPT Ma (Fig. 7f).
4.3. Emplacement depths Due to the notable effect of the calculation results of pressure, we have to select the samples with XFe (XFe=Fe/(Fe+Mg)) values of 0.4–0.65 to calculate the emplacement
PT
pressures (Anderson and Smith, 1995). For the Dongjiangkou pluton, the XFe values of all
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samples are within the interval of 0.4–0.54 (Table 2). The estimated crystallization pressures
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range from 1.67 to 3.14 kbar, with a relatively high range. Furthermore, the density of country rocks (the Liuling Group) is about 2.75–2.8 g/cm3, which is approximately similar to
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the average density of the upper crust (2.8 g/cm3). Consequently, we transform the crystallization pressures to emplacement depths by 2.8 km/kbar. As a result, emplacement
MA
depths of the pluton range from 4.7 to 8.8 km (Table 2).
The emplacement depths of granites vary greatly in the pluton (Fig. 8a). We have to
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consider other interpretations in that the error of the formula (±0.6 KPa) cannot account for
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such a big variation in depths (up to 4.1 km) alone. After removing the elevation effect by adding the altitude, we obtained the emplacement depths of the same level (0 m). The
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corrected emplacement depths were then used to produce a contour map (Fig. 8b) which shows a similar zoned pattern with the petrological zonation, indicating that the
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emplacement depths of the pluton increase from the inner unit to the outer unit. Therefore, this study confirms that the different units of the pluton were emplaced at different depths.
5. Internal fabrics of the pluton 5.1. Methods and sampling It is widely acknowledged that fabrics (i.e., foliation and lineation) represent the most important structures in granites. AMS measurement is considered as the most efficient
17
ACCEPTED MANUSCRIPT method to obtain the petrofabrics, especially in rocks that are visually isotropic (e.g., Bolle et al., 2018; Liang et al., 2015b). Mathematically, the magnetic susceptibility can be defined by a second-rank tensor with three orthogonal eigenvectors, corresponding to three eigenvalues, K1, K2 and K3 (K1≥K2≥K3). K1 represents the magnetic lineation, and K3 is perpendicular to the magnetic foliation (Tarling and Hrouda, 1993). Furthermore, in order to better
PT
understand the AMS ellipsoid scalar parameters including Km (bulk susceptibility), PJ
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(corrected anisotropy degree) and T (shape parameter) can be calculated by the following
SC
formulas (Jelinek, 1981): (1) Km=1/3(K1+K2+K3); (2) PJ exp 2 ln Ki ln Km ; (3) 3
2
i 1
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T=2(lnK2 - lnK3)/(lnK1 - lnK3)-1, 0
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Oriented samples for AMS measurements were collected from 223 sites that are distributed throughout the Dongjiangkou pluton (Appendix Fig. S1). At each site, samples
D
were drilled and cut into cylindrical specimens, 2.5 cm in diameter and 2–2.2 cm in height.
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At least 5 specimens for each site and a total of 1396 standard oriented specimens were collected for AMS measurements. Low field AMS measurements (at room temperature) were
CE
performed by an AGICO KLY–4S Kappabridge (sensitivity of 110-8 SI) at the State Key Laboratory of Continental Dynamics, Northwest University, China. Finally, scalar
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parameters were calculated by ANISOFT 4.2. Analysis results see Appendix Table S2. Additionally, we performed the shape preferred orientation (SPO) analysis on the mafic silicate minerals (mainly biotite) to constrain the reliability of the magnetic fabric. SPO measurements involve the measurement of 2-D sectional fabric ellipses using image analysis (IA) and the construction of a 3-D best-fit ellipsoid. High-resolution digital images for IA were directly taken on nonparallel sections (N≥3) at each site. By using the software of Photoshop cs6 and ImageJ, the dark mafic minerals were isolated from the quartzes and the
18
ACCEPTED MANUSCRIPT feldspars. We then use the program SPO2003 (Launeau and Robin, 2005) to obtain the long axis orientation and shape ratio of the sectional fabric ellipses. Finally, the three sectional ellipses were combined to construct a best-fit ellipsoid by using the program Best-Fit Ellipsoid–v2.0 (Mookerjee and Nickleach, 2011).
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5.2. Bulk susceptibility
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Bulk susceptibility (Km) of the Dongjiangkou pluton has a broad variation range, from
SC
73 μSI to 14900 μSI (average of 2252 μSI), and mainly distribute within the interval of 185–7000 μSI (Fig. 9a). Samples with Km values less than 500 μSI (n=60) mainly distribute
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in the northwest (Jiangkou). Samples with highest Km values mainly distribute in the south (Xiaochuan) and northeast of the pluton (Yingpan). Such a large variation in Km values
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indicates variable mineral contributions to the susceptibility. AMS in granites represent the combined contribution of all rock-forming minerals,
D
including the diamagnetic minerals (e.g., feldspar and quartz), the paramagnetic minerals
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(e.g., biotite and amphibole) and the ferromagnetic minerals (e.g., magnetite) (Tarling and Hrouda, 1993). In granites with low bulk magnetic susceptibility (< 500 μSI), AMS are
CE
mainly related to the magnetocrystalline anisotropy of paramagnetic silicates. In granites with high magnetic susceptibility (>500 μSI), AMS are mainly contributed by the shape
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anisotropy of the ferromagnetic minerals. Although quartz and feldspars are the most important components of granites, they only have little effect on bulk susceptibility in rocks, with Km values higher than 102 μSI in that the magnetic susceptibility of such diamagnetic minerals is very low (usually on the order of -10 SI). Therefore, on the basis of the Km values and the composition of rocks we have a preliminary conclusion that the AMS of samples in the Jiangkou area (with low Km values) are mostly attributed to the biotites, and the AMS of samples with relatively high Km values in other parts of the pluton are dominated by some
19
ACCEPTED MANUSCRIPT ferromagnetic minerals. Moreover, microscopic observations of samples with high Km values show that the long axes of magnetite grains are parallel with biotites, and magnetite grains are preferentially distributed along the cleavages or adjacent to the periphery of biotite (Fig. 9b, c). The grain sizes of most magnetite grains are larger than 5 μm, indicative of MD magnetite grains. This
PT
phenomenon is very common in granites, since Fe oxides (such as magnetite grains)
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probably either crystallize aligned with the earlier formed mafics or grow/rotate along the
SC
pre-existing grain shape fabric during deformation process (Mamtani and Vishnu, 2012). Under the circumstances, the subfabrics of magnetites are commonly coaxial with the
NU
biotites in granites (e.g., Liang et al., 2015b; Stevenson et al., 2007). Therefore, the measured AMS fabrics in the Dongjiangkou pluton correspond mainly to the biotite subfabrics despite
D
5.3. AMS scalar parameters
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different Km values.
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The anisotropy degree (PJ) is the intensity indicator of anisotropy (Bouchez et al., 1997). In general, PJ values of the samples in the Dongjiangkou pluton vary gently from 1.01 to
CE
1.36 (average=1.08), indicating weakly to moderately anisotropic. Except for three samples with relative high PJ values, all other samples show PJ values lower than 1.2 (Fig. 9d).
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Meanwhile, the shape parameter (T) values of the pluton display a broad range of variation (from -0.89 to 0.92), and the majority of samples show T>0 (Fig. 9e). T values can be used to determine the type of the deformation regime: an oblate ellipsoid (T>0) is related to flattening whereas a prolate ellipsoid is linked to stretching (T<0). Therefore, magnetic fabrics of the Dongjiangkou pluton were mainly induced by a flattening regime. Additionally, the PJ-Km plot shows a positive correlation, especially in the range of 0–2500 μSI (Fig. 9d), indicating that the anisotropy degree is influenced by the susceptibility.
20
ACCEPTED MANUSCRIPT In contrast, no clear correlations can be identified between T and PJ or T and Km (Fig. 9e, f). Similar correlation between PJ and Km has been documented in many other case studies (e.g., Bolle et al., 2018; Li et al., 2017c), and a variety of explanations have been proposed, such as distribution anisotropy of magnetite grains, growth characteristics of magnetite, and shape change of the magnetite grains due to the plastic deformation (Archanjo et al., 1995; Ferré et
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al., 2003). Investigating the mechanism of this positive correlation is beyond the scope of
RI
our research. Yet, the relationship can illustrate that the AMS of the samples in the pluton are
SC
associated with magnetite.
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5.4. AMS directional data
Magnetic fabrics, i.e. magnetic foliation and magnetic lineation, are defined by K1-K2
MA
and K1 of the AMS ellipsoid. In the Dongjiangkou pluton, magnetic fabrics are considered to be well defined, in that the majority of sites show confidence level (α95) smaller than 25°
D
(Appendix Table S2). At the pluton scale, the distribution of magnetic foliations clearly
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shows two concentric patterns, Zone Ⅰ and Zone Ⅱ (Fig. 10), which are concordant with the pluton margins. Magnetic fabrics in the inner core of Zone Ⅰ are characterized by low dip
CE
angles and no regulation of inclinations. Moreover, it is worth noting that the concentric pattern of Zone Ⅰ (Fig. 10b) is very similar to the petrological pattern the pluton (Fig. 2). In
AC
contrast, magnetic fabrics in the inner core of Zone Ⅱ are sub-vertical. Furthermore, the distribution of magnetic lineations shows the same pattern as magnetic foliations. Magnetic lineations in the inside core of the pluton show relative low plunge angles and irregular directions, indicating chaotic magma flow directions.
5.5. SPO analysis Results of SPO analysis of 14 sites for the pluton are shown in Appendix Table S3.
21
ACCEPTED MANUSCRIPT Apparently, except for three samples (D037, D045 and D291) most sites show a rather small dihedral angle between the principal axes of the AMS and SPO ellipsoids (Fig. 11). Shapes of AMS and SPO ellipsoids also match well. The minor error probably resulted from the analytical precision that the photos for SPO analysis were mostly captured at the area of ca. 1m×1m and the cylindrical specimens for AMS measurements were drilled and cut from the
SPO
and T
SPO),
equivalent to those of AMS, were used to take a
RI
of the SPO ellipsoid (PJ
PT
samples smaller than 15×15×15 cm3. Moreover, the anisotropy degree and shape parameters
SC
comparison. In general, PJ values calculated by SPO analysis are higher than that of AMS ellipsoid, which is probably due to the fact that PJ values of AMS reflect the combined
NU
magnetic anisotropy degree from all rock-forming minerals, whereas PJ values of SPO record just the biotite fabric. T values of most samples for SPO analysis show the same
MA
regime (mainly flattening regime) as the AMS for the formation of the fabrics. In summary, SPO analysis of the biotites supports the use of AMS as a proxy for the orientation of the
6. Discussion
CE
PT E
the Dongjiangkou pluton.
D
petrofabric, since the measured AMS fabrics probably correspond to the biotite subfabrics in
AC
6.1. Incremental emplacement of the pluton Previous studies preferred to use the thermal ballooning model and in situ fractional crystallization to account for the five zoned petrographic units of the Dongjiangkou pluton (e.g., Cui et al., 1999). However, this model is difficult to explain the ‘room problem’, since very little compression deformation can be observed at the contacts of the pluton. Moreover, xenoliths of country rocks can rarely be observed in any petrographic unit of the pluton, leading us to exclude the stoping model (Glazner and Bartley, 2006). Therefore, a new
22
ACCEPTED MANUSCRIPT emplacement mechanism or growth model is needed to understand the construction process of the pluton. Our zircon U–Pb geochronology data show that at least three-stagemagma events contributed to the growth of the Dongjiangkou pluton (Table 1; Fig. 7f). Previous work by Liu et al. (2011), has shown that the Yingpan unit was formed by two-stage magma events.
PT
Similar situations have been documented in many other case studies in the Qinling Orogen,
RI
as follows. (1) Hu et al. (2017) obtained two groups of crystallization ages (216 Ma and 200
SC
Ma) for the monzogranite collected from the Zhashui pluton (~12 km east of the Dongjiangkou pluton, Fig. 2). (2) Zhang et al. (2009) obtained two groups of zircon ages for
NU
the host granites (210 Ma and 197 Ma), MMEs (197 Ma and 188 Ma) and the diorite dykes (230 Ma and 210 Ma) collected from the Shahewan pluton. (3) Duan et al. (2016) reported
MA
two groups of crystallization ages (212 Ma and 225 Ma) for the granites collected from Lvjing pluton in the West Qinling Orogen. Thus, two groups of zircon ages existing in one
D
sample is a common phenomenon in the Late Triassic granite plutons of the Qinling Orogen.
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Two explanations have been proposed for this: (1) zircon xenocrysts existing in the granites; (2) pre-existing zircons that crystallized in the same magmatic system prior to emplacement.
CE
The former is more applicable for the Dongjiangkou pluton since the age discrepancy is higher than 10 Ma and granite magma crystallization can hardly be sustained for such a long
AC
time. Low magma temperature (~771°C, Liu et al., 2013) and fast crystallization process are probably the best reasons that can account for a large number of magmatic zircon xenocrysts preserving in late-stage magmas (Miller et al., 2003). Therefore, we interpret the 199–201 Ma age as the final crystallization age of the samples D114, D128, D215 and D222, and zircons of 222–221 Ma may represent xenocrysts from a previous magma batch. Similarly, ~210 Ma represents the crystallization age of sample D297 and 222 Ma represents the formation age of zircons xenocrysts.
23
ACCEPTED MANUSCRIPT We thus infer that the construction of the pluton was completed at ~200 Ma on the basis of zircon U–Pb ages. Compared with previous dating results, our new data show younger ages which could be more authentic. For example, dating work by Qin (2010) reported 220±2 Ma, 214±2 Ma and 222±2 Ma for different units of the pluton. However, the single zircon ages in their work have a broad distribution range in the same sample, from 212 Ma to
PT
226 Ma, 208 Ma to 222 Ma and 217 Ma to 236 Ma, respectively. Previous studies have
RI
demonstrated that granite magmatism is a relatively rapid process with a time scale less than
SC
100, 000 yrs (Petford et al., 2000). Therefore, such a big discrepancy in the crystallization ages of the Dongjiangkou pluton is impossible in one magma process. It is important to
NU
make a distinction between the crystallized zircons and the zircon xenocrysts (or captured zircons) (e.g., Brown, 2013).
MA
Additionally, the fabric patterns reveal that the northeastern part of the pluton (Zone Ⅱ) is markedly different from the other parts and probably crystallized at ~210 Ma (Liu et al.,
D
2011, and this study). This indicates that the pluton is a composite intrusion which is
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composed of two juxtaposed small plutons. Meanwhile, it should be noted that the majority part of the pluton (Zone I) was constructed during 199.2 Ma (outer unit) to 200.6 Ma (inner
CE
unit). This indicates an inversely zoned pluton although the age gap is small (1.4 Ma). Combined with the five-unit zoned feature, we infer that Zone I of the pluton was
AC
incrementally formed by a series of magma batches. The incremental emplacement is also supported by the study of emplacement depth. According to the calculation of Al-in-hornblende barometer, different units of the pluton are thought to have been crystallized at depths ranging from 4.7 km to 8.8 km. After eliminating the elevation effect, the contour map shows a clear zoned pattern with the emplacement depths increasing from the inner unit to the outer unit (Fig. 8b), indicating that different units of the pluton were emplaced at different depths. The increasing emplacement depths of the
24
ACCEPTED MANUSCRIPT pluton from the inner unit to the outer unit are in accordance with the syn-collisional convergence setting (as discussed later), supporting an inversely zoned pluton as indicated by zircon U–Pb ages. Therefore, combining the zoned pattern, the zircon U–Pb geochronology and the emplacement depth, we argue that the construction of the major part of the pluton (Zone I) is related to the model of incremental emplacement. During the
PT
emplacement process (200.6–199.2 Ma) the emplacement pressures (depths) have an
RI
increasing trend.
SC
Consequently, we propose an integrated growth model for the whole Dongjiangkou pluton (Fig. 12). It should be noted that pre-existing structures including the channels or
NU
conduits of magma ascent and the emplaced structures are so fragmentary that the real emplacement process of the pluton could be far more complicated. Thus, utilizing the limited
MA
data, we infer the emplacement process as follows.
(1) At first, the pluton was laterally amalgamated by two small plutons with different
D
formation ages. Zircon U–Pb geochronology demonstrates that the northeastern part of
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the pluton (Zone II, Fig. 10b) formed initially at ~210 Ma with ~222 Ma zircon xenocrysts, and the majority part of the pluton (Zone I) formed during 200.6–199.2 Ma
CE
with 222–221 Ma zircon xenocrysts. The inner cores of the two zoned plutons represent the most important magma feed zones. Although the deep geometry of the pluton is not
AC
clear due to the absence of high-resolution gravity data, the major feed dykes are inferred to strike NE-SW according to the regional convergent direction. (2) Subsequently, the pluton was vertically accreted by five successive units with different emplacement depths. The inner unit (200.6 Ma) is relatively older than the outer unit (199.2 Ma). In this case, the petrological zonation is called an inverse style or outward-building manner which has been documented in other case studies, such as the Guposhan pluton in the Nanling Range of the SCB (Feng et al., 2012). This type of
25
ACCEPTED MANUSCRIPT emplacement sequence is also equivalent to the under-accretion, i.e. repeated amalgamation of magma with younger age adds under the older igneous body (Annen, 2011; Menand, 2008, 2011). Moreover, the rigidity contrast between the pre-existing unit and the underlying strata (i.e. an upper, more rigid layer and a lower, less rigid layer) is propitious to trap the ascending magmas (Kavanagh et al., 2006; Menand, 2008).
PT
Consequently, the emplacement process of the pluton at this time is in the order of:
RI
~200.6 Ma, the inner Meihuazhuang unit (MZ) was emplaced at a depth of ca. 5–6 km;
SC
~200.2 Ma, the Shaluozhang unit (SL) accreted to the bottom of the former unit and laterally propagated; the Yaowangtang unit (YW) and the Yingpan unit (YP) accreted to
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the pluton by the same way; during 200.1 Ma–199.2 Ma, the Xiaochuan unit (XC) was finally emplaced at a depth of ca. 7–8 km, forming the outer ring of the pluton. This
MA
completes the entire emplacement process of the pluton with progressive crustal
D
thickening.
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The incremental emplacement model alleviates the ‘space problem’ of the Dongjiangkou pluton. Compared with traditional emplacement mechanisms (e.g., stoping
CE
and thermal ballooning model), incremental emplacement does not need disposable room for quasi-instantaneous or rapid emplacement of the entire large pluton body (de Saint-Blanquat
AC
et al., 2011). It also does not need the rapid strain rate of the structural expansion for the space, since the pluton is accumulated by multiple magma batches. Furthermore, this model can account for the phenomenon that width of contacts is much smaller than previously presumed for such a big pluton. This proposed model is in contrast to the previous hypothesis that the pluton was emplaced by the thermal ballooning model, and thus with crystallization ages increasing from the center to the margin (Cui et al., 1999). Our conclusion that five units of
26
ACCEPTED MANUSCRIPT Dongjiangkou pluton were resulted from different magmatic pulses can be confirmed by geochemical and isotopic studies. Isotope studies show that the Dongjiangkou (high-Mg) granites were formed by magma mixing between melts derived from partial melting of the Neoproterozoic basement rocks of the SQB and mantle-derived mafic melts (Hu et al., 2017; Gong et al., 2009; Qin et al., 2010). The magma sources of the Dongjiangkou granites are
PT
heterogeneous in isotopic compositions with different proportions of magma mixing (Hu et
RI
al., 2017) which resulted in the different geochemical characteristics of each unit (Qin et al.,
SC
2010). Moreover, previous studies have demonstrated that large-scale crystallization differentiation in a granite magma chamber may not have existed, due to the low temperature
NU
and the high viscosity (e.g., Clemens and Stevens, 2016; Coleman et al., 2004; Glazner et al., 2004; Matzel et al., 2006). Consequently, low magma temperature (~771°C, Liu et al., 2013)
MA
of the Dongjiangkou pluton could go against large-scale crystallization differentiation. It should be noted that fractional crystallization may be locally existed during magma ascent
D
and emplacement of each unit (Qin et al., 2010), but it is not the major mechanism that
PT E
formed the zoned Dongjiangkou pluton. Therefore, we believe that the zoned Dongjiangkou pluton probably resulted from multiple magma batches with different properties (from the
CE
source), rather than induced by crystallization evolution after magma emplacement, i.e. the
magma.
AC
zoning is controlled by compositional heterogeneity in the ancient country rock of the
The incremental emplacement of the pluton also leads to new challenges when interpreting field observations and relating them to the processes involved during pluton construction. This model can account for many zoned patterns of petrology and geochemistry that were characterized by compositional and textural heterogeneity, especially in a pluton with a large body and long period of construction. In the Qinling Orogen, the same features exist in many Late Triassic plutons. For example, the Zhashui pluton were
27
ACCEPTED MANUSCRIPT built by the Laoansi, Xichuanjie and Dongergou units (Cui et al., 1999); the Laochen pluton was formed by the Muhe, Shibangou and Leigutai units, and Yanzhiba pluton was constructed by Tianwan and Yingzuishi units (Tao, 2014); the Wenquan pluton can be divided into five units (Cao et al., 2011); the Wulong pluton consists of three units (Qin, 2010); the Caoping pluton was formed by two different units (Hu et al., 2016). For plutons
PT
with similar features, the incremental emplacement model probably is the common
SC
RI
mechanism.
6.2. Syn-tectonic deformation
NU
In this study, both the field structures and microstructures reveal that the pluton was syntectonically emplaced. In the pluton, the MMEs and the segmented dykes show
MA
syn-plutonic deformations, which are in accordance with the kinematics reflected in the SSZ (Li et al., 2017c) and the SFF (Zhang et al., 2001). At the contact, thermal aureole structures
D
also display syn-plutonic ductile deformation. Under the microscope, the microstructures
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indicate that the fabrics in the pluton were mainly formed during submagmatic flow (melt-present deformation) to High-T solid-state deformation, attesting that the pluton was
CE
constructed under the regional tectonics. In addition, the emplacement of the pluton is coeval with regional deformations that are constrained by the SSZ (219–210 Ma, Li et al., 2017c;
AC
Zhang et al., 2001), the SFF (~201 Ma, Wu, 2013) and the syn-tectonic granitic vein in the Liuling Group (~201 Ma, Li et al., 2017c) (Fig. 12). All these lines of evidence confirm that the Dongjiangkou pluton was syntectonically emplaced and therefore experienced a syn-tectonic deformation. Our conclusion is in agreement with other case studies in the Qinling Orogen, such as the Mishuling pluton (Liang et al., 2015a) and the Baliping pluton (Li et al., 2017c). Based on this understanding, we can then interpret the internal fabrics of the pluton. But before this interpretation, we still have to figure out the origin, source and the
28
ACCEPTED MANUSCRIPT reliability of magnetic fabrics. Bulk susceptibility analysis shows that AMS of the samples with low Km values are mostly attributed to the biotites and the AMS of samples with relatively high Km values are probably dominated by magnetite. Some rock-magnetic experiments have been conducted for the Late Triassic granites in the Qinling Orogen, such as the Laocheng and Yanzhiba
PT
pluton (Tao, 2014), the Minshuling pluton (Liang et al., 2015b) and the Baliping pluton (Li
RI
et al., 2017c). Hysteresis data of these granites reveal two contrasting types of hysteresis
SC
loops, i.e. hysteresis loops of samples with low susceptibility display straight lines, while hysteresis loops of samples with high susceptibility show significant paramagnetic
NU
contribution. χ-T curves and IRM acquisition curves show that ferromagnetic minerals in samples are mainly soft magnetic components with low coercivity (magnetite). Day plots
MA
demonstrate that most samples are located in the multidomain (MD) area. Therefore, the adjoining and coeval Dongjiangkou pluton may have the same rock magnetism, and the
D
magnetic fabrics in the pluton are induced by the subfabrics of biotites or magnetites.
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Moreover, microscopic observations of the Dongjiangkou pluton attest that magnetites prefer to distribute along the cleavages or near the periphery of biotites (Fig. 9b), indicating that the
CE
subfabrics of magnetite are always in line with the biotite grains. Meanwhile, SPO analysis demonstrates that the directions of biotite subfabrics are consistent with the magnetic fabrics
AC
in most analytical sites. Therefore, these evidences approve that AMS can be used as a proxy for the orientation of the petrofabric in the Dongjiangkou pluton. At the pluton scale, the fabric patterns show two concentric zones that are concordant with the pluton margins, which is very similar to the petrological pattern (Fig. 10). In the core of Zone I, the present-day outcrop is interpreted as the top level of the unit, since the magnetic fabrics in the inner core are characterized by low dip angles and no regulation of inclinations. When ascending magmas were stopped by the overlying country rocks, mussy
29
ACCEPTED MANUSCRIPT fabrics with low dip angles formed (Závada et al., 2009). In contrast, magnetic fabrics in the inner core of Zone Ⅱ are sub-vertical, indicating a deep erosion level. In most cases, concentric zoned patterns of magnetic fabrics have been considered as magma feed zones or supply channels (e.g., Castro, 1987; Cruden et al., 1995; Hutton, 1988). Therefore, we infer that there existed two major magma channels (feeders) for the pluton. This kind of fabric
PT
pattern also suggests a syn-tectonic emplacement of the pluton (Castro, 1987; de
RI
Saint-Blanquat et al., 2001).
SC
Similar zoned or concentric fabric patterns can be found in other case studies, such as the Late Triassic Mishuling pluton in West Qinling Orogen (Liang et al., 2015a), the
NU
Laocheng pluton and the Yanzhiba pluton in the South Qinling Belt (Tao, 2014), and the Papoose pluton in California (de Saint-Blanquat et al., 2001). All these studies underlined
MA
the role of regional deformation in the formation of such internal fabric patterns. Combining the coeval regional (transpressional) deformation, we infer that the internal fabric pattern of
D
the Dongjiangkou pluton was mainly induced by the deformation-controlled magma flow.
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During the emplacement of the pluton, the sustained emplacement of the subsequent unit may deliver the latent heat, preventing the former units from rapid cooling (to lower than the
CE
solidus) (Annen, 2011). Meanwhile, a magma vortex may have been induced in response to the deformation partitioning of the regional sinistral transpression (Vigneresse and Tikoff,
AC
1999), which can develop the zoned pattern of the fabrics. The progressive NE-SW folding may have assisted the formation of the concentric fabric patterns.
6.3. Tectonic Implications One of the important aims of this study is to constrain the tectonic setting in which the Late Triassic granites were emplaced. As mentioned above, syn-subduction, syn-collision, and post-collision have been proposed.
30
ACCEPTED MANUSCRIPT In this study, we have distinguished the syn-plutonic deformations both in the pluton and its country rocks, demonstrating that the Qingling Orogen was still under an oblique convergence setting during the construction of the pluton (i.e. Late Triassic period). Moreover, our data indicate that the pluton was formed under the progressive crustal thickening which can be related to the collisional event. Therefore, the tectonic setting
convergence
setting.
Our
conclusion
is
supported
by
other
RI
syn-collisional
PT
during which the Late Triassic plutons were emplaced can be summarized as a
SC
tectono-sedimentary records and tectonic chronology in the Qinling Orogen. It has been shown that several tectonic events occurred during Middle-Late Triassic in 40
Ar/39Ar dating
NU
the Qinling Orogen, especially during the Late Triassic period. Using
method, Xu et al. (1986) reported 232±5 Ma and 216±7 Ma ages respectively, for the
reported 226.8±2.2 Ma
40
MA
phengites and riebeckites that were collected from the nappe tectonics in the SQB. Li (2008) Ar/39Ar age for the muscovites of the granitic mylonites that were 40
D
collected from the Wushan area, West Qinling Orogen. Chen (2010) obtained the
Ar/39Ar
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age of 223±2 Ma for muscovites that were collected from the ductile shear zone with sinistral transpressional kinematics in the Mianlue suture zone. Hu (2011) obtained 40
Ar/39Ar ages for the sericites of seven mylonitized siltstones (Lower
CE
224.6–212.3 Ma
Silurian) from the footwall of the Fangxian–Qingfeng Fault (South Qinling Belt). Li et al.
AC
(2013) obtained 222.4–198.2 Ma
40
Ar/39Ar ages for the biotites of the mylonites that were
from the hanging wall of the Hanwang thrust. Wu (2013) reported 200.9±1.9 Ma
40
Ar/39Ar
age for the altered biotites in the Xiaogoucao thrust fault (north of the SFF). Meanwhile, the ages of syn-tectonic granite dykes can also constrain the time of the tectonic event. For example, the activity time of the Ningshan Fault with sinistral kinematics was estimated to 214.4–212.8 Ma by two syn-tectonic granite dykes (Li et al., 2015); syn-tectonic granite dykes developed in the Liuling Group adjacent to the SSZ also provide an age of 200.7±1.3
31
ACCEPTED MANUSCRIPT Ma (Li et al., 2017c). Tectonic chronology is in accord with the ages of the Late Triassic granites, suggesting that the Late Triassic magmatism and the tectonics are probably coupled. Previous tectono-sedimentary studies also suggest that the Qinling Orogen was still under a syn-collisional convergence setting during the Late Triassic. The youngest
PT
sedimentary strata involved in fold-and-thrust deformations are the middle Triassic (Dong et
RI
al., 2016; Zhang et al., 2001), such as the T1-2 Jinjiling Formation in the Shanyang–Fengzhen
SC
thrust-nappe. Therefore, the major collision is supposed to be not earlier than Middle Triassic. Furthermore, the Upper Triassic molasse formation was extensively deposited in the
NU
Dabasha area, such as the Upper Triassic Xujiahe Formation (T3x) (Dong, 1997; He et al., 1997). The palaeo-flow direction (SSW) of the molasse-type conglomerates suggests that the
MA
South Qinling Belt uplifted notably during the Late Triassic, i.e. the Qinling Orogen was still under the syn-collisional uplift stage. Following the collision, sedimentary rocks in the J1-2
D
graben basins, including the Jiangluo Basin, the Ciba Basin and the Mianxian Basin, were
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unconformably deposited on the sedimentary strata that were involved in intense fold-and-thrust deformations (e.g., Dong et al., 2016; Zhang et al., 2001). These graben
CE
basins are the products of post-collisional collapse, suggesting that the collision happened prior to the Early Jurassic period.
AC
Combining our case studies and previous tectono-sedimentary records and tectonic chronology studies, we suggest that the Qinling Orogen was still under the syn-collisional convergence setting during the Late Triassic. Since the Late Triassic granites developed within the Qinling Orogen were under syn-collisional convergence setting, the dynamic mechanisms (e.g., the delamination) deduced from the post-collision extension or collapse for the Late Triassic plutons should be excluded. Furthermore, the distribution map of the Late Triassic plutons (Fig. 1) clearly shows
32
ACCEPTED MANUSCRIPT that they are exposed in a narrow segment and mainly bounded by crustal-scale fault zones (e.g., the Shangdan Suture, the Ningshan Fault and the Mianlue Suture). The close relationship between tectonics and granite magmatism has been
acknowledged in many
studies (e.g., Brown, 2013; Hutton, 1988; Liang et al., 2015a; Petford et al., 2000; Tikoff and Teyssier, 1992; Vigneresse, 1999; Weinberg et al., 2004), although a
few geologists have
PT
denied the linkage (e.g., Paterson and Fowler, 1993, Paterson and Schmidt, 1999; Schmidt
RI
and Paterson, 2000) and argued that the spatial and temporal coupling between the structures
SC
and the magma productions are the prerequisites.
In this study, the spatial and temporal relationships between the pluton and its intrusive
NU
structures can be indicated by the following observations. (1) The pluton is directly in contact with the south boundary fault of the SSZ and the SFF. (2) Both the pluton and its
MA
country rocks recorded syn-kinematic deformations in response to the Late Triassic orogeny. (3) At the contact, thermal aureole structures commonly show syn-plutonic ductile
D
deformations, resulting from the combination of regional tectonics and pluton emplacement.
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(4) Regional thrust faults facilitate magma transport and emplacement. (5) Regional deformations imposed on magma flow, forming the submagmatic fabrics, high-T solid-state
CE
fabrics and concentric fabric patterns.
Other structural studies of the Late Triassic granite plutons in the Qinling Orogen also
AC
show good examples of tectonic control on the granite magmatism. An integrated study on the Laocheng pluton and the Yanzhiba pluton (221–200 Ma) by Tao (2014) shows that the internal syn-plutonic structures have a concordant kinematics with the regional shear deformation, demonstrating a syn-tectonic emplacement and the tectonic control of the Ningshan Fault. Another structural study on the Mishuling pluton (213–212 Ma) shows that the Huangzhuguan-Miaopin Fault controlled the ascent and emplacement of the magmas and resulted in syn-kinematic deformations during the construction of the pluton (Liang et al.,
33
ACCEPTED MANUSCRIPT 2015a). Hence, tectonic-controlled magma ascent and emplacement in our study area is not an exceptional example, and can be extended to the magmatism in the whole Qinling Orogen.
PT
7. Conclusions
RI
Based on integrated studies, including field observations, microstructural observations,
SC
zircon U–Pb geochronology, emplacement depth and the internal fabric analysis, we draw the following conclusions.
NU
(1) The Dongjiangkou pluton is a composite pluton which is laterally composed of two juxtaposed small plutons with different ages (~210 Ma and ~200 Ma). Vertically, the
MA
younger pluton is characterized by under-accretion incremental growth. The pluton was syntectonically emplaced at depths increasing from 4.7 km to 8.8 km. The concentric
D
petrologic zoning of the pluton is interpreted as having been inherited from the
PT E
heterogeneous magma source before emplacement rather than in situ magma differentiation. (2) The Dongjiangkou pluton experienced syn-tectonic deformation during its
CE
construction, which is evidenced by extensive syn-plutonic deformations in the pluton and its contact zones. Microstructural observations show that the fabrics in the pluton were
AC
mainly acquired during submagmatic flow to high-T solid-state deformation. The distribution of internal fabrics is characterized by two concentric patterns that were probably induced by the sinistral transpression. (3) The Qinling Orogen was still under a syn-collisional convergence setting during the Late Triassic period. Regional tectonics controlled the Late Triassic granite magmatism in the Qinling Orogen, at least in the aspect of magma ascent and emplacement. Moreover, incremental emplacement may be a common mechanism for the formation of the zoned
34
ACCEPTED MANUSCRIPT plutons in the Qinling Orogen.
Acknowledgments This work was supported by the National Natural Science Foundation of China [Grant
PT
numbers 41802212, 41672200 and 41421002]. We thank Prof. Santosh for carefully
RI
polishing our revised manuscript. We thank Prof. Laura Webb, Dr. Shan Li and one
SC
anonymous reviewer for their insightful comments. The authors would like to thank Yazhou Ran, Qi Shen, Chuanzhi Li and Jiao Yao for their help in field and laboratory work. The
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authors also benefitted from the discussions with Prof. Yunpeng Dong, Prof. Anlin Guo and
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Prof. Xianzhi Pei.
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ACCEPTED MANUSCRIPT Figure 1. Tectonic framework of the Qinling Orogen. Phanerozoic granites were extensively developed in response to the subduction, collision and intra-continental orogeny. Note that almost all the Late Triassic granites are exposed in the narrower segment and mainly bounded by the crustal-scale fault zones. This indicates a correlation between magmatism and collisional tectonics. The orange colored zone in the inset
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map is the Central China Orogenic Belt (CCOB).
Figure 2. Simplified geological map of the Dongjiangkou pluton and its country rocks. Note that the The profile shows that the pluton emplaced into the syncline
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pluton consists of five petrographic units.
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of the Devonian Liuling Group, indicating that granite magmatism happened later than regional folding.
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Figure 3. MMEs within the Dongjiangkou pluton. (a) MMEs and granites enwrapped with each other, indicating simultaneous crystallization. (b) Different shapes of the MMEs, from round to elongated shape
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with high aspect ratio. (c)-(d) Typical melt-present deformation (submagmatic flow) of the MME. The offset of ~1 m in MME occurs without any mineral preferred orientation being developed along a magmatic fault, indicating that the offset occurred by melt-assisted grain boundary sliding and stopped
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photos are taken in vertical view)
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before final crystallization. The circle shows a feldspar phenocryst that came from the host granite. (All
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Figure 4. Granite dykes in the pluton. (a) Multi-stage (composite) dykes reflect multiple magma events. (b)-(d) Segmented dykes and their kinematics. Note that (d) is a sketch of field photograph (c) and shows
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the geometry of segmented dykes and the crosscuting relationship between the MMEs and dykes (dykes cutting MMEs). (All field photos taken in vertical view)
Figure 5. Thermal aureole structures of the Dongjiangkou pluton. (a)-(b) Sub-horizontal granitic sill developed at the north contact of the pluton, showing magma migration and emplacement along thrust (front view). Note that the thrust fault shows intense compressional damage zone and imbricated shear bands. (c)-(d) Structures including asymmetric folds and rigid porphyroclasts showing sinistral transpressional kinematics (vertical view). (e)-(f) Microstructures of the mica-quartzose schists are characterized by dynamic recrystallization and undulatory extinction of quartz grains and preferentially 48
ACCEPTED MANUSCRIPT aligned biotite crystals, indicating sinistral ductile shear deformation (crossed-polarized and plane-polarized light, respectively). Symbols: P–porphyroclast; Qz–quartz.
Figure 6. Characteristic microstructures of the Dongjiangkou pluton. (a) Euhedral amphibole. (b) Oscillatory zoning in plagioclases. (c) Transgranular fracture filled by later melt, indicating melt-present
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deformation (submagmatic flow). (d) Dynamic recrystallization of quartz by grain boundary migration indicating High-T deformation. (e) Chessboard pattern in quartz indicating High-T deformation. (f)
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Biotite kinking showing Low-T deformation. (a)-(e) were taken under crossed-polarised light; (f) was taken under plane-polarized light. Symbols: Amp–amphibole; Pl–plagioclase; Kfs–K-feldspar; Qz–quartz;
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Bt–biotite.
Figure 7. Zircon U–Pb ages of granites from the Dongjiangkou pluton. (a)-(e) U–Pb diagrams of 206Pb/238U
ages for analysed zircons of samples D114, D128, D215, D222
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concordia and weighted mean
and D297, respectively (data-point error ellipses and box heights are 2 sigma). Note that except for sample D114, all the other four samples show two contrasting groups of ages for zircon rims. (f) Histogram shows
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three peaks of zircon U–Pb ages in the pluton.
Figure 8. (a) Sampling sites and their corresponding emplacement depths. (b) Contour map shows that
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granite emplacement depths of the pluton increase from the inner unit to the outer unit.
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Figure 9. Parameters of AMS analysis in the Dongjiangkou pluton. (a) Distribution of the bulk susceptibility. (b) Microscopic observations show that long axes of magnetites mainly distribute along the cleavages or adjacent to the periphery of biotite. (c) Energy spectrum shows that magnetic minerals in the sample with relatively high Km values are mainly magnetites. (d), (e) and (f) are plots of PJ-Km, T-Km and T-PJ showing their correlations.
Figure 10. Magnetic fabric distributions within the Dongjiangkou pluton. (a) Magnetic foliations (dashed lines are the boundaries of each unit). (b) Simplified distribution map of the magnetic foliation. (c) Magnetic lineation (dashed lines are the trajectories of magnetic foliations). 49
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Figure 11. SPO vs. AMS of the Dongjiangkou pluton. The image analysis study supports the use of AMS as a proxy for the orientation of the petrofabric.
Figure 12. Simplified conceptual model for the regional structures and the construction of the
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Dongjiangkou pluton (See Figure 2 for explanation of letter code for units). The zoned patterns of fabrics were mainly induced by regional transpression, and the progressive regional folding would assist the
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formation of the fabric distribution patterns.
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ACCEPTED MANUSCRIPT Table 1.
206Pb/238U
ages of zircons from the Dongjiangkou pluton.
Pb/238U Num Mean age age ber E108°48.392’ porphyritic biotite 199.5–202. 200.6±1. I 30 N33°40.164’ monzogranite 3 Ma 0 Ma E108°52.795’ porphyritic biotite 199.4–201. 200.2±1. I 16 N33°40.375’ monzogranite 3 Ma 4 Ma 220.5–225. 221.9±1. Ⅱ 13 0 Ma 8 Ma D21 E108°46.186’ medium- to fine-grained 199.3–200. 200.1±1. I 10 5 N33°35.321’ granodiorite 9 Ma 5 Ma 220.0–222. 221.1±1. Ⅱ 13 5 Ma 6 Ma D22 E108°46.643’ medium- to coarse-grained 198.2–200. 199.2±1. I 13 2 N33°35.501’ granodiorite 7 Ma 4 Ma 219.2–222. 221.1±1. Ⅱ 11 8 Ma 8 Ma D29 E109°01.924’ coarse- to medium- grained 210.0–211. 210.4±1. I 14 7 N33°46.677’ monzogranite 0 Ma 5 Ma 221.2–224. 222.3±2. Ⅱ 7 8 Ma 1 Ma Zircon type I–magmatic zircons crystallized from the late-stage magmas; Zircon type Ⅱ–zircon Zircon type
Rock type
206
NU
SC
RI
PT
Location
CE
PT E
D
MA
xenocrysts.
AC
Sam ple D11 4 D12 8
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ACCEPTED MANUSCRIPT
Table 2.
Electron microprobe results of hornblendes and the emplacement depths of the Dongjiangkou pluton.
1128
D114
1021
D128
1121
D199
876
D204
1111
D222
845
D232
1343
D247
1364
D269
1096
D299
1100
D303
960
D310
1139
D339
1333
6.57 6.13 6.23 7.31 5.75 6.87 6.39 5.99 6.42 6.88 6.97 6.56 6.61
Ca O 11. 60 11. 51 11. 65 11. 88 11. 21 11. 14 11. 88 11. 56 10. 96 11. 63 12. 28 11. 73 11. 44 11. 70 11. 94 11. 71
52
Na2 O 0.6 3 0.5 8 0.6 5 0.5 7 0.5 8 0.6 5 0.3 4 0.4 5 0.6 2 0.5 7 0.3 2 0.5 2 0.7 2 0.6 0 0.5 2 0.6 8
K2 O 0.7 4 0.6 3 0.7 1 0.6 3 0.6 8 0.5 4 0.7 2 0.5 6 0.6 7 0.6 1 0.4 0 0.6 1 0.6 7 0.8 1 0.7 5 0.6 9
Tot al 95. 48 95. 95 96. 76 94. 54 95. 62 95. 85 94. 82 96. 71 96. 36 96. 19 96. 15 95. 95 95. 12 94. 95 96. 21 96. 15
PT
D113
6.83
Mg O 13. 54 14. 20 13. 40 13. 93 13. 70 13. 64 12. 66 14. 15 14. 89 14. 31 13. 12 13. 97 13. 93 12. 45 13. 86 13. 75
RI
1320
6.24
Mn O 0.3 7 0.3 4 0.4 1 0.2 5 0.3 3 0.4 2 0.3 2 0.3 9 0.2 1 0.3 6 0.3 3 0.2 4 0.3 4 0.3 6 0.2 9 0.3 4
SC
D097
6.62
Fe O 13. 50 13. 29 14. 20 12. 42 14. 07 13. 86 14. 38 13. 76 12. 98 13. 35 14. 26 13. 21 12. 86 14. 68 13. 82 13. 91
NU
993
Al2 O3
MA
D092
47. 29 48. 16 47. 87 47. 18 47. 84 48. 44 46. 42 48. 93 48. 16 48. 07 49. 05 48. 18 47. 14 46. 22 47. 38 47. 31
Ti O2 0.9 6 0.9 5 0.9 2 0.9 7 0.8 7 0.8 3 0.6 4 1.0 6 0.7 9 0.6 6 0.2 0 0.9 3 1.0 2 1.0 5 0.9 9 0.9 7
D
1412
2
PT E
D073
SiO
CE
Altitude (m)
AC
Sam ple
XF e 0.5 0 0.4 8 0.5 1 0.4 7 0.5 1 0.5 0 0.5 3 0.4 9 0.4 7 0.4 8 0.5 2 0.4 9 0.4 8 0.5 4 0.5 0 0.5 0
Pres sure (kba r)
Dep th (Km )
2.50
7.0
2.11
5.9
2.60
7.3
2.50
7.0
2.06
5.8
2.11
5.9
3.14
8.8
1.67
4.7
2.55
7.1
2.23
6.2
1.95
5.5
2.28
6.4
2.70
7.6
2.87
8.0
2.41
6.7
2.45
6.9
ACCEPTED MANUSCRIPT Highlights The Dongjiangkou pluton was built by an under-accretion incremental growth model The pluton was emplaced at depths increasing from 4.7 km to 8.8 km Syn-plutonic deformations were observed both in the pluton and its contact zones
AC
CE
PT E
D
MA
NU
SC
RI
PT
The Qinling Orogen was still under a convergence setting during the Late Triassic
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Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Figure 6
Figure 7
Figure 8
Figure 9
Figure 10
Figure 11
Figure 12