Triassic arc mafic magmatism in North Qiangtang: Implications for tectonic reconstruction and mineral exploration

Journal Pre-proof Triassic arc mafic magmatism in North Qiangtang: Implications for tectonic reconstruction and mineral exploration

Yanning Wang, Qingfei Wang, Jun Deng, Shengchao Xue, Chusi Li, Edward M. Ripley PII:

S1342-937X(20)30065-4

DOI:

https://doi.org/10.1016/j.gr.2020.01.013

Reference:

GR 2299

To appear in:

Gondwana Research

Received date:

29 September 2019

Revised date:

9 January 2020

Accepted date:

18 January 2020

Please cite this article as: Y. Wang, Q. Wang, J. Deng, et al., Triassic arc mafic magmatism in North Qiangtang: Implications for tectonic reconstruction and mineral exploration, Gondwana Research(2020), https://doi.org/10.1016/j.gr.2020.01.013

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© 2020 Published by Elsevier.

Journal Pre-proof

Triassic arc mafic magmatism in North Qiangtang: Implications for tectonic reconstruction and mineral exploration Yanning Wang1, Qingfei Wang1, Jun Deng1, Shengchao Xue1*, Chusi Li2, Edward M. Ripley2

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State Key Laboratory of Geological Processes and Mineral Resources, China University of

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USA

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Department of Earth and Atmospheric Sciences, Indiana University, Bloomington, IN 47405,

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Prepared for Gondwana Research

Corresponding author’s email: [email protected]

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Geosciences, Beijing 100083, China

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Journal Pre-proof ABSTRACT The North Qiangtang continental block in central Tibet is a critical piece of the Pangea puzzle. This paper uses integrated geochronological and geochemical data for selected mafic dykes and dioritic enclaves in this block to evaluate its tectonic evolution in the Triassic. Zircons from two mafic dykes and the dioritic enclaves of a large arc granodiorite pluton in eastern North

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Qiangtang yield indistinguishable U-Pb ages from 248 ± 2 to 251 ± 3 Ma, contemporaneous with

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widespread arc basaltic andesites and crust-derived rhyolites in the region. The mafic dykes and 87

Sr/86Sr = 0.707 to

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coeval arc basaltic andesites have almost identical Sr-Nd isotopes (initial

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0.708, εNd = -4.4 to -3.6), and are all characterized by light REE enrichments and pronounced negative Nb-Ta anomalies. The dioritic enclaves and the hosts have indistinguishable zircon U-

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Pb ages, almost identical Sr-Nd isotopes (initial 87Sr/86Sr = 0.709 to 0.711, εNd = -7.4 to -5.9), and

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similar zircon εHf (-13.7 to -5.7), but contrasting chondrite-normalized REE patterns due to

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hornblende fractionation. The Sr-Nd isotope data indicate that the dioritic enclaves formed

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from the hybrid melts produced by mixing at depth between the arc basaltic andesites and the crust-derived rhyolites. We propose that the Early Triassic arc igneous suites are related to the northward subduction of the southern Paleo-Tethys beneath the North Qiangtang block from Early to Middle Triassic. The occurrence of several Late Triassic porphyry Cu deposits plus a VMS Ag-Pb-Zn deposit in the Yidun arc, which is the product of the southward subduction of the northern Paleo-Tethys beneath the North Qiangtang block in the Late Triassic, indicates that the arc magmas generated during the subduction of the Paleo-Tethys are fertile in ore

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Journal Pre-proof metals. Therefore, exploration for Early–Middle Triassic porphyry Cu and VMS deposits in the southern part of the North Qiangtang block is warranted.

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Keywords: Mafic rocks; Subduction; North Qiangtang; Paleo-Tethys; Pangea

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Journal Pre-proof 1. Introduction Pangea, the youngest known supercontinent in Earth’s history, was assembled during the Permian–Triassic from Gondwana-derived and Laurasia-derived continental blocks (Metcalfe, 2013; Zhao et al., 2018). The configurations of major continental blocks such as Australia, Antarctica and India in Pangea are well established and widely accepted, owing to the

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existence of large amounts of reliable geological, paleomagnetic and paleontological data

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(e.g., Muttoni et al., 2003, 2009; Zhao et al., 2018). Less known is the tectonic evolution of

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some micro-continental blocks in the eastern part of Pangea, particularly the North Qiangtang block, which is situated in the northern part of the Tibetan Plateau. Widespread

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deformation in the region associated with the Cenozoic India-Asian continental collision that

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occurred along the Himalayan mountain range to the south, makes it very difficult to collect

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critical data for this type of study. With limited amount of data, various tectonic models have been proposed for the North Qiangtang block and its surrounding oceans. Kapp et al. (2000)

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and Pullen et al. (2008, 2011) suggested that there existed a low-angle southward subduction

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of the Songpan–Garze oceanic plate beneath the North Qiangtang block along the Zhiduo– Yushu suture in the Triassic and there was no Paleo-Tethys Ocean between the North and South Qiangtang blocks at that time. However, there are 355–467 Ma ophiolites and 221–237 Ma eclogites in the Longmu Co–Shuanghu suture zone that separates these two blocks (Fig. 1). These authors contended that under-thrusting and then uplift of the subducted flysch gave rise to the Longmu Co–Shuanghu ophiolite-eclogite belt, as these rocks are exposed in the footwalls of low-angle normal faults (Kapp et al., 2000). In contrast, many other researchers regarded the ophiolites in the suture zone as the remnants of the Paleo-Tethys 4

Journal Pre-proof Ocean (e.g., Li et al., 1995, 2008; Zhai et al., 2007, 2010, 2013b, 2016; Zhang et al., 2016) and the younger eclogites as the products of continental collision after the ocean was consumed by northward subduction (e.g., Li et al., 2006; Zhang et al., 2006, 2018; Zhai et al., 2011, 2017). This model is consistent with the presence of Triassic blueschists in the suture zone (e.g., Li, 1997; Zhai et al., 2009; Tang and Zhang, 2014), and abundant Late Permian–Middle

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Triassic arc-like volcanic rocks and coeval granitoids that occur north of the suture zone (e.g.,

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Yang et al., 2011, 2014; Wang et al., 2018b, 2018c).

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There is no disagreement on the existence of another, south-dipping, subduction zone along the northern margin of the North Qiangtang block until Late Triassic. This is consistent

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with the occurrences of 236–422 Ma ophiolites at Zhiduo, Yushu and Garze (Fig. 1b; Liu et al.,

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2016, 2019 and references therein), and the distribution of Late Triassic arc volcanic rocks

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and associated granites south of the suture (e.g., Reid et al., 2007; Gao et al., 2018). However, there is no consensus on the configuration of this suture east of Yushu (Fig. 1b), with one

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group of researchers (Yang et al., 2014) proposing a single west-dipping subduction zone

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along the eastern rim of the Yidun arc, and other groups of researchers adding another subduction zone, either east-dipping (Reid et al., 2005) or west-dipping (Zi et al., 2012a, 2012b, 2013; He et al., 2018a) approximately along the Jinsha River (or Jinshajiang) from Yushu to Gongka (Fig. 1b). The double subduction model places a branch of the Tethys Ocean between the Yidun arc and the North Qiangtang block. Currently the double subduction model seems to be more widely adopted, although the main reasons are not given (e.g., Deng et al., 2014, 2019; Wang et al., 2018a). Since the various models have significantly different implications not only for the reconstruction of Pangea involving the North Qiangtang block 5

Journal Pre-proof but also for the tectonic controls on the distribution of the Triassic porphyry Cu and VMS AgPb-Zn deposits in the region, we have carried out an independent study to evaluate the different models. We focus on two Triassic mafic dykes and the dioritic enclaves of a large granodiorite pluton in an area west of the Yidun arc terrane. The samples were analyzed for zircon U-Pb ages, zircon Hf isotopes, apatite-hornblende trace elements, whole-rock major

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and trace elements, and Sr-Nd isotopes. Our new data, together with the existing data for the widespread Triassic arc volcanic rocks and associated granite plutons in the region, are used

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to corroborate the existence of an Early–Middle Triassic arc magmatic suite in the North

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Qiangtang block. The most significant implication of the new finding is that the Jinshajiang

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suspect suture is not a Paleo-Tethyan tectonic suture along the Jinsha River as suggested

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previously. Based on the new results, we postulate that the North Qiangtang block has

the Early–Middle Triassic.

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2. Geological background

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potential for hosting porphyry Cu deposits associated with arc magmatism that took place in

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2.1. Tectonic units, igneous rocks and sutures in central Tibet and adjacent regions The North Qiangtang block is located in the northern part of the Tibetan Plateau (Fig. 1a, b). It is bounded by the Triassic Songpan-Garze basin to the north and by the South Qiangtang block to the south. The Tibetan Plateau was assembled by the southward accretion of the Songpan-Garze basin to the North Qiangtang block in the Late Triassic (Jian et al., 2019; Liu et al., 2019) and by northward accretion of several Gondwana-derived micro-continental blocks to the North Qiangtang block from the Triassic to the end of the Cretaceous (Burchfiel and 6

Journal Pre-proof Chen, 2012). The accretions were preceded by oceanic subduction of the Paleo-Tethys, MesoTethys and Neo-Tethys (Zhu et al., 2013; Kapp and DeCelles, 2019, and references therein). Major uplift of the plateau took place during the India-Tibet continental collision in the Cenozoic after oceanic subduction of the Neo-Tethys (Yin and Harrison, 2000). Significant crustal shortening, bending and southward extrusion in the eastern part of the plateau and

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the adjacent regions including western Yunnan, are the results of the continental collision

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(Tapponnier et al., 1990; Yin and Harrison, 2000; Pellegrino et al., 2018).

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Voluminous igneous rocks were produced during the assembly of central Tibet. These include abundant Late Permian–Late Triassic bimodal volcanic rocks and granitoids in the

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North Qiangtang block and the adjacent Simao block to the southeast (Fig. 1b). The bimodal

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volcanic rocks can be further divided into two age groups, a Late Permian-Middle Triassic

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group and a Late Triassic group. The former is widely distributed across the North Qiangtang

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block whereas the latter tends to be concentrated in the margins of this block (Fig. 1b). The exposed igneous rocks of the South Qiangtang block, an important Gondwana-

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derived microcontinent (Metcalfe, 2013), are dominated by widespread Jurassic–Cretaceous granitoids and Early Permian (~280–300Ma) mafic dykes in the western part of this block (Fig. 1b; Zhai et al., 2013a). Abundant continental flood basalts erupted at ~280–310 Ma, as indicated by the zircon U-Pb ages of associated dolerite dykes (Liu et al., 2020), are abundant in the Baoshan region (Fig. 1b). The Baoshan region is the northern part of another important Gondwana-derived microcontinent referred to as the Sibumasu block (Fig. 1b; Metcalfe, 2013). In the western part of the South China block a slightly younger continental flood basalt province occurs, referred to as the Emeishan province (~ 260 Ma, Fig. 1b; Li et al., 2016). 7

Journal Pre-proof The Pre-Cenozoic convergent tectonic processes in central Tibet produced three major, undisputed Tethyan sutures with ophiolite ± eclogite ± blueschist. From north to south they are the Triassic Zhiduo–Garze Paleo-Tethyan suture, the Triassic Longmu Co–Shuanghu PaleoTethyan suture and the Cretaceous Bangong Co–Nujiang Meso-Tethyan suture (Fig. 1b). In addition, some researchers have proposed another Triassic Paleo-Tethyan suture along the

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western segment of the Jinshajiang (Jinsha River) that runs through the eastern part of the North Qiangtang block, namely the Jinshajiang suture (Reid et al., 2005; Zi et al., 2012a,

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2012b, 2013). This suspect suture is inferred mainly based on the occurrence of a

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controversial ophiolite belt from Gongka to Gongbo (Fig. 2). The gabbros of the suggested

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ophiolites have zircon U-Pb ages varying from 267 to 344 Ma, which includes a sample from a

with

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small outcrop located ~15 km to the southeast of Jijiading that contains four zircon crystals 206

Pb/238U ages from 267 to 285 Ma (Fig. 2). Intermediate-felsic dykes in the region, or

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trondhjemite dykes as described by the proponents of the ophiolite belt, have zircon U-Pb

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ages varying from 238 to 347 Ma. The youngest ages are similar to the ages of some large

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granite plutons in the region, which do not support an ophiolitic origin for these intermediate-felsic dykes.

Early Permian warm-water fauna are present in the North Qiangtang block, the Yidun arc terrane, and the Indochina block to the south (Metcalfe, 1994). Paleomagnetic data indicate that the relative positions of the North Qiangtang and Indochina blocks have remained unchanged since the Early Triassic (Li et al., 2004). Based on these important constraints, many researchers proposed that these two blocks have been close to each other since Gondwana (Metcalfe, 2013; Zhao et al., 2018, and references therein). Accordingly, the 8

Journal Pre-proof Ailaoshan and Changning–Menglian sutures in Yunnan and the Zhiduo–Garze and Longmu Co–Shuanghu sutures in Tibet are considered to be comparable Paleo-Tethyan sutures (e.g., Wang et al., 2014a; Deng et al., 2017; Liu et al., 2020). Such an interpretation is further supported by the occurrence of abundant Late Permian–Triassic arc volcanic rocks and associated granites along these sutures in both Tibet and northern Yunnan (Fig. 1b).

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2.2. Songpan-Garze basin, Yidun arc and Zhongza massif

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The Songpan-Garze basin, located to the northeast of the Zhiduo–Garze suture with 234–

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422 Ma ophiolites (Fig. 1b; Liu et al., 2016, 2019 and references therein), was filled with thick

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Middle–Late Triassic flysch strata (Jian et al., 2019 and references therein) and intruded by

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Late Triassic granitoids (Zhan et al., 2018). Late Triassic granitoids are also present in the Yidun arc, but are less abundant than the Early–Middle Triassic granitoids in this region.

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Several porphyry Cu deposits associated with the Late Triassic granitic magmatism have been

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discovered in the southern part of the Yidun arc (Fig. 2; Li et al., 2017; Yang et al., 2018). Late Triassic volcanic rocks are rare in the Songpan-Garze basin but are abundant in the Yidun arc

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and the adjacent North Qiangtang block to the west (Fig. 1b). A VMS Ag-Pb-Zn ore deposit has been found to be associated with the Late Triassic volcanic rocks in the northern part of the Yidun arc (Fig. 1b; Hou et al., 2001). Late Triassic volcanic rocks are distributed across the northern segment of the Jinshajiang suspect suture (Fig. 1b) but are restricted to the east of the suspect suture in the southern segment (Fig. 2). The western part of the Yidun arc is also referred to as the Zhongza massif (Chen et al., 1987). The exposed rocks are dominated by Paleozoic marine sedimentary sequences and 9

Journal Pre-proof interbedded basalts in the upper parts. Based on stratigraphic relation, the basalts were interpreted to be erupted from Early to Late Permian in the original geological maps (BGMRS, 1977; BGMRY, 1982, 1985). Without providing any new constraint, some researchers (Song et al., 2004, Xiao et al., 2008, He et al. 2010) arbitrarily assigned a Late Permian age to these basalts, thereby they become contemporaneous with the Emeishan continental flood basalt

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province (~260 Ma) that is centered in the western part of the South China block (Fig. 1b). Based on the assumed temporal correlation and similar mantle-normalized incompatible

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trace element patterns between the different occurrences, these authors further suggested

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that the Permian basalts in the Zhongza massif are parts of the Emeishan continental flood

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basalt province.

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Paleozoic marine sedimentary rocks are not exposed in the eastern part of the Yidun arc

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but are present elsewhere in the North Qiangtang block. Triassic granitoids are rare within the massif but abundant on both sides of the massif. Mafic-ultramafic intrusions are rare

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within the massif but common to the west of the massif. Although no Proterozoic basement

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is exposed within the massif and the eastern part of the Yidun arc terrane, a Proterozoic continental sliver or ‘microcontinent’ is considered to underlie the region (e.g., Chang, 1997). This interpretation is mainly based on the geochemistry of the Triassic felsic volcanic rocks in the region that suggests a continental crustal source for these rocks (Hou, 1993). The apparent fossil and lithological similarities between the Paleozoic sedimentary rocks of the massif and those in the northwestern rim of the South China block (Fig. 1b) provide the basis for the suggestion that the Zhongza massif rifted from the northwestern margin of the South China block in the Paleozoic (Chen et al., 1987; Zhang et al., 1994, 1998). However, this 10

Journal Pre-proof interpretation is not unique, because the Paleozoic sedimentary and fossil records of the Zhongza massif are also similar to those in the North Qiangtang block to the west, as well as the Indochina block to the south (Metcalfe, 1994, 2013). 3. Sample descriptions The samples used in this study were collected from two mafic dykes situated ~10 km west

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of the Xiaruo village (Figs. 2 and 3a), and from the dioritic enclaves of the Triassic

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Baimaxueshan granodiorite-granite pluton situated near the Jijiading village (Figs. 2 and 3b),

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both occur in the eastern part of the North Qiangtang block and to the west of the Jinshajiang

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suspect suture. The two selected mafic dykes occur subparallel and ~100 m apart, and are

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named as Xiaruo mafic dyke-I and -II in this study (Fig. 3a). Their immediate country rocks are Early Triassic volcanic rocks (Fig. 3a). The Xiaruo mafic dyke-I is ~750 m in width and ~2 km in

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length (Fig. 3a). Eight samples, including a large one (~5 kg) for zircon separation, were

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collected from a road cut across the center of this dyke (Figs. 3a and 4a). The Xiaruo mafic dyke-II is ~850 m in width and ~2.5 km in length (Fig. 3a). Eight samples, including a large one

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(~5 kg) for zircon separation, were taken from a road cut across the center of this dyke (Figs. 3a and 4c). The samples from both dykes are all characterized by a medium to fine grained, sub-porphyritic texture and similar modal composition comprising 50–60 % plagioclase and 35–45 % clinopyroxene, plus minor quartz and Fe-Ti oxides, with the phenocrysts dominated by clinopyroxene (Fig. 4b) or plagioclase (Fig. 4d). Five dioritic enclave samples, including three large ones (~3 kg) that were suitable for mineral (zircon or apatite) separation, were collected from the northeastern margin of the 11

Journal Pre-proof Baimaxueshan granodiorite-granite pluton that intruded the Late Permian volcanic rocks (Fig. 3b). Previously, the dioritic enclaves were referred to as mafic enclaves (Zi et al., 2012a; He et al., 2019). The sizes of the dioritic enclaves used in this study vary from 20 cm to a half meter in diameter. Their contacts with the host rocks are sharp (Fig. 4e). The grain sizes of the dioritic enclaves are generally smaller than those of the host rocks (Fig. 4e). The selected

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dioritic enclaves are vari-textured, with variable crystals sizes and irregular contacts between different minerals. They contain 40–45 % plagioclase, 35–40 % hornblende, 5–15 % quartz,

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and 5–10 % biotite (Fig. 4f).

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4. Analytical methods

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Zircon and apatite grains were separated from crushed rocks using conventional magnetic and standard density methods, and then were handpicked under a binocular microscope. The

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selected zircon and apatite grains were mounted in epoxy discs, and then polished.

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Cathodoluminescence (CL) images were used to select the zircon crystals with simple, regular zoning patterns for U-Pb dating. Back-scatted electron images were used to select the apatite

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grains with homogeneous compositions for trace element measurement. U-Pb isotopes of the selected zircons were determined using a CAMECA IMS 1280-HR ion microprobe at the Research Institute of Uranium Geology in Beijing, following the procedures described in detail in Li et al. (2009). The ion beam size was ~25 μm. The Plešovice zircon standard with known U-Th-Pb abundances and isotopic ratios (Sláma et al., 2008) was used for calibration. The Qinghu zircon standard with known age (Li et al., 2013) was analyzed together with our samples to monitor instrument stability. Fifteen analyses on the Qinghu 12

Journal Pre-proof zircon yielded a Concordia age of 160.4 ± 1.5 Ma, which is indistinguishable with the recommended value of 159.5 ± 0.2 Ma (Li et al., 2013). The CIPS software and Isoplot 3.0 software of Ludwig (2012) were used for data processing and plotting. Zircon Lu-Hf isotopes were measured using a multiple collector-inductively coupled plasma-mass spectrometer (MC-ICP-MS) from Thermo Fisher Ltd (Neptune Plus), equipped

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with a femtosecond (λ = 343 nm) laser ablation system (Applied Spectra J-200), in the

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National Research Center of Geo-analysis in Beijing. The detailed operation conditions and

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analytical procedures are given in Zhou et al. (2018). The laser ablation crater was approximately 40 x 20 μm. Twenty-three analyses of the Plešovice zircon standard that were

Hf/177Hf = 0.282484 ± 0.000023, which is within error of the recommended value of

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176

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carried out shortly before and during the course of this study yielded a weighted mean of

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0.282483 ± 0.000013 (Sláma et al., 2008).

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Whole-rock major and trace element concentrations were analyzed in the Key Laboratory of Western China Mineral Resources and Geological Engineering, Chang’an University, Xi’an,

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China. The major element compositions were determined on the fused glass disks of the powdered samples using an X-Ray Fluorescence (XRF) analytical instrument (Shimadzu XRF1800). The abundances of trace elements were determined on the sample solutions using an ICP-MS (Inductively Coupled Plasma Mass Spectrometry) analytical instrument (Thermo Elemental X1). The solutions were prepared by digesting 50 mg of powdered samples with acid (HNO3+HF) in a heated (180 °C) high-pressure Teflon bomb for 48 hours. The analytical errors, estimated from the results of a national standard (GSR-4) and an international

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Journal Pre-proof standard (BCR-2) analyzed with our samples, are better than 5% for major elements and 10% for trace elements, respectively. Whole-rock Rb-Sr and Sm-Nd isotopes were measured using a Neptune Plus MC-ICP-MS in the Guangxi Key Laboratory of Hidden Metallic Ore Deposits Exploration, Guilin University of Technology, Guilin, China, following the operation conditions and analytical protocols

Sr/86Sr = 0.1194 and

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Nd/144Nd = 0.7219, respectively. Thirty analyses of the NBS-987 Sr

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given in Zou et al. (2018). The measured 87Sr/86Sr and 143Nd/144Nd ratios were normalized to

isotope standard and twenty-two analyses of the Shin Etsu JNDi-1 Nd isotope standard that

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Nd/144Nd of 0.512093 ± 1 (2σ), which are indistinguishable with the certified

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(2σ) and

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were carried out shortly before and during this study yielded mean 87Sr/86Sr of 0.710253 ± 3

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values. The BHVO-2 rock standard analyzed together with our samples yielded a

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Sr/86Sr

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ratio of 0.703482 ± 12 and a 143Nd/144Nd ratio of 0.512970 ± 4, which are in good agreement with the values from other laboratories in the world as compiled by GeoRem

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(http://georem.mpch-mainz.gwdg.de/).

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The major and minor element compositions of clinopyroxene, plagioclase and hornblende were determined using a JXA-8100 microprobe in the Key Laboratory of Western China Mineral Resources and Geological Engineering, Chang’an University, Xi’an, China. The operating conditions were 15 kV accelerating voltage, 10 nA beam current and 1 μm beam diameter. The trace elements of hornblende and apatite were determined using an ICP-MS machine (Agilent 7700X) equipped with a Geolas 193 nm laser ablation system in the State key Laboratory of Continental Dynamics, Northwest University, Xi'an, China. Helium was used as a carrier gas and the laser energy was 80 mJ at 6 Hz. The beam diameter for hornblende 14

Journal Pre-proof and apatite grains was 60 and 44 μm, respectively. The weighted mean of CaO contents in hornblende and apatite were used for internal calibration. The NIST 610 standard was used for external calibration. The instrument stability was monitored using the repeated analyses of the international standards (BHVO-2 and BCR-2). Data reduction was done using the Glitter 4.4 software.

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5. Results

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5.1. Zircon U-Pb ages and Hf isotopes

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The U-Pb data for the selected zircons from the Xiaruo mafic dykes and the Baimaxueshan

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dioritic enclaves are given as supplementary data (Table S1). The CL images of the analyzed

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zircon crystals with 206Pb/238U ages are illustrated in Fig. 5. The zircon grains from the Xiaruo mafic dyke-I (XR17-03) vary from 30 to 100 m in length (Fig. 5a). The Th and U contents in

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these zircons are from 1293 to 3374 ppm and from 2115 to 9016 ppm, respectively, with the

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Th/U ratios from 0.23 to 1.30. The zircon grains from the Xiaruo mafic dyke-II (XRL17-07) is

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also from 30 to 100 m in length (Fig. 5b). The Th and U contents in these zircon crystals vary from 608 to 3494 ppm and from 677 to 3519 ppm, respectively, with the Th/U ratios from 0.46 to 2.23. The zircon crystals from a large dioritic enclave (ML17-01) in the Baimaxueshan granodiorite-granite pluton vary from 100 to 200 m in length (Fig. 5c). They contain 87– 1715 ppm Th and 401–2201 ppm U, with the Th/U ratios from 0.21 to 0.78. The zircon grains from another large dioritic enclave (ML17-04) vary from 80 to 180 m in length (Fig. 5d). They contain 81–642 ppm Th and 296–1081 ppm U, with the Th/U ratios from 0.27 to 0.68. The zircon U-Pb isotope data of the different samples yield the Concordia ages of 251 ± 3 Ma (2σ, 15

Journal Pre-proof N = 8, MSWD = 0.87, probability = 0.35) for the Xiaruo mafic dyke-I (Fig. 6a), 248 ± 2 Ma (2σ, N = 12, MSWD = 0.08, probability = 0.78) for the Xiaruo mafic dyke-II (Fig. 6b), 248 ± 2 Ma (2σ, N = 16, MSWD = 11, probability = 0.001) for one dioritic enclave (ML17-01, Fig. 6c), and 251 ± 2 Ma (2σ, N = 19, MSWD = 0.91, probability = 0.34) for the other dioritic enclave (ML1704, Fig. 6d). The age range of the Xiaruo mafic dykes and the Baimaxueshan dioritic enclaves

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are identical. The ages of the Baimaxueshan dioritic enclaves from this study and a previous study (Zi et al., 2012a) are all from 248 to 251 Ma, which are the same as the ages of the host

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pluton (Zi et al., 2012a).

The Lu-Hf data for the dated magmatic zircon from the Xiaruo mafic dykes and the

Hf/177Hf ratios of the zircon crystals from the Xiaruo mafic dyke-I (XR17-03) are from

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Baimaxueshan dioritic enclaves are provided as supplementary data (Table S2). The

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0.282509 to 0.282614, with the calculated εHf(t) values (t = 251 Ma) varying from -3.9 to -0.3; the 176Hf/177Hf ratios of the zircon crystals from the Xiaruo mafic dyke-II (XRL17-07) are from

Hf/177Hf ratios of the zircon crystals from one (ML17-04) of the two Baimaxueshan dioritic

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0.282583 to 0.282695, with the calculated εHf(t) values (t = 248 Ma) from -1.3 to 2.6; the

enclave samples are from 0.282292 to 0.282411, with the calculated εHf(t) values (t = 251 Ma) varying from -11.7 to -7.3. 5.2. Major elements The major and trace element compositions of the Xiaruo mafic dykes and the Baimaxueshan dioritic enclaves are given as supplementary data (Table S3). The variations of major and minor oxide concentrations in these rocks on an LOI-free basis (LOI = loss on 16

Journal Pre-proof ignition) are illustrated in Figs. 7 and 8. The Xiaruo mafic dykes contain 47.7–54.8 wt.% SiO2, 1.4–3.4 wt.% Na2O and 0.4–2.3 wt.% K2O. Like the contemporaneous basaltic andesite (~250 Ma, Wang et al., 2014b) in the nearby Jijiading area (see Fig. 2 for location), the Xiaruo mafic dyke samples also plot in the sub-alkaline field (Fig. 7a) and mostly in the fields of calcalkaline and high-K calc-alkaline (Fig. 7b). The coeval Baimaxueshan dioritic enclaves have

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higher SiO2 contents (57.0–59.7 wt.%), Na2O contents (2.6–2.8 wt.%) and K2O contents (1.9– 3.4 wt.%) than the mafic dykes but plot in the same fields (Fig. 7a, b). With a few exceptions,

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samples from the host pluton (data from Zi et al., 2012a; He et al., 2018a) of the dioritic

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enclaves have higher SiO2 contents, but they also plot in the sub-alkaline field (Fig. 7a). The

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contents of K2O in the samples from the host pluton are more scattered and extend from the

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calc-alkaline and high-K calc-alkaline fields into the shoshonite field (Fig. 7b). The contemporaneous rhyolites (242–248 Ma, Zi et al., 2012a; Wang et al., 2014b) in the nearby

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Yezhi area (see Fig. 2 for location) have the highest SiO2 contents but also plot in the sub-

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alkaline field (Fig. 7a). Due to highly variable K2O contents, the Yezhi rhyolites plot across the

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different fields of magma series in the plot of K2O versus SiO2 contents (Fig. 7b). The phase controls on the compositional variations of the contemporaneous igneous rocks in the studied region are illustrated in the Harker diagrams (Fig. 8a–f) using the anhydrous compositions of whole rocks and the compositions of silicate minerals such as clinopyroxene and plagioclase in the mafic dykes and hornblende in the dioritic enclaves (Table S4). The strong negative correlation between SiO2 and MgO as well as CaO (Fig. 8a, b), coupled with the lack of correlation between SiO2 and Al2O3 (Fig. 8c) for the dioritic enclaves and the host rocks indicate fractional crystallization ofhornblende, and plagioclasefrom the 17

Journal Pre-proof parental magmas. In the plots of FeOtotal and TiO2 versus SiO2 (Fig. 8d, e), these intrusive rocks lie below the tie-line of the associated volcanic rocks, consistent with early removal of Fe-Ti oxides from the magmas. Scattering of the intrusive rocks across the tie-line of the associated volcanic rocks in the plot of P2O5 versus SiO2 (Fig. 8f) is consistent with variable amounts of cumulus apatite in the intrusive rocks.

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The contents of MgO (4.2–9.4 wt.%), CaO (7.0–13.7 wt.%), Al2O3 (14.4–22.9 wt.%), FeOtotal

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(6.1–11.9 wt.%) and Al2O3 (14.4–22.9 wt.%) in the mafic dykes overlap the compositions of

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the Jijiading basaltic andesites (Fig. 8a–d). The contents of TiO2 and P2O5 in the mafic dykes are lower than those in the basaltic andesites (Fig. 8e, f). Unlike the basaltic andesites, which

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have rather restricted major element compositions, the Xiaruo mafic dykes have highly

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variable major element compositions, showing a negative correlation between SiO2 and MgO

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(except two samples), CaO, and Al2O3 (Fig. 8a–c), and a positive correlation between SiO2 and FeOtotal, TiO2, and P2O5 (Fig. 8d–f). The compositions of the mafic dykes are mostly bracketed

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between the basaltic andesites and two major silicate minerals (clinopyroxene and

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plagioclase) in the dykes, consistent with the interpretation that these intrusive rocks formed from crystal mushes containing variable amounts of clinopyroxene and plagioclase and variable amount of liquids with compositions similar to those of the basaltic andesites. 5.3. Trace elements The chondrite-normalized rare earth elements (REE) and primitive mantle-normalized trace elements patterns for the Xiaruo mafic dykes and the Baimaxueshan dioritic enclaves are illustrated in Fig. 9. The Xiaruo mafic dykes show moderate light REE enrichment relative 18

Journal Pre-proof to heavy REE, with several samples with relatively low abundances of REE and other incompatible trace elements showing positive Eu-Sr anomalies (Fig. 9a, b), consistent with cumulus plagioclase in these rocks. The mafic dyke samples are all characterized by pronounced negative Nb-Ta anomalies and moderately negative Zr-Hf-Ti anomalies (Fig. 9b), similar to the coeval basaltic andesites in the nearby Jijiading area as well as typical arc

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basalts worldwide. The coeval Jijiading basaltic andesite have the similar patterns with the Xiaruo mafic dykes, except higher trace element concentrations and pronounced negative Eu-

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Sr anomalies (Fig. 9a, b).

The Baimaxueshan dioritic enclaves are all characterized by negative Eu-Sr anomalies to

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the degrees similar to the host rocks but less than the coeval rhyolites in the region (Fig. 9c,

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d). Negative Ti anomaly is present in these three different types of rocks. These three

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different types of rocks show similar degrees of negative Nb-Ta and Zr-Hf anomalies (Fig. 9d), but the dioritic enclaves can be distinguished from the rest by the presence of moderate La

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depletion and minor Ce depletion relative to Nd in the dioritic enclaves (Fig. 9c). In addition,

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the concentrations of heavy REE in the dioritic enclaves are higher than in the host granodiorites (Fig. 9c). A comparison with the trace element compositions of hornblende and apatite from the dioritic enclaves (data given in Table S5) indicates that such differences are mainly due to the higher abundance of cumulus hornblende in the dioritic enclaves than the host granodiorites, as this major phase contains higher amounts of heavy REE coupled by lower amounts of La and Ce (Fig. 9e, f). 5.4. Whole-rock Sr-Nd isotopes

19

Journal Pre-proof The Rb-Sr and Sm-Nd isotopes data for the Xiaruo mafic dykes and the Baimaxueshan dioritic enclaves are provided as supplementary data (Table S6). The calculated εNd (t) values and initial 87Sr/86Sr ratios for the Xiaruo mafic dykes are from -4.2 to -3.8 and from 0.7072 to 0.7080, respectively, which are almost identical to the isotope composition of the coeval basaltic andesite in the nearby Jijiading area (Fig. 10a). The Baimaxueshan dioritic enclaves have 87

Sr/86Sr ratios from 0.7101 to 0.7105,

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εNd (t = 250 Ma) values from -6.7 to -6.1 and initial

respectively, which are indistinguishable with that of the host granodiorites and are between

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the values for the associated basaltic andesites and rhyolites (Fig. 10b). The significance of such

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variations will be discussed below.

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6. Discussion

6.1.1. The Xiaruo mafic dykes

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6.1. Magma differentiation and generation

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Compared with the coeval basaltic andesites, the Xiaruo mafic dykes have higher MgO

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contents for some samples, higher CaO contents for all samples, positive Eu-Sr anomalies for some samples, lower TiO2 and P2O5 for all samples (Fig. 8), and lower abundances of incompatible trace elements but similar chondrite- and mantle-normalized incompatible trace element patterns (Fig. 9), plus similar Sr-Nd isotopes (Fig. 10). The results are consistent with the interpretation that the mafic dykes formed from the magmas with compositions similar to the basaltic andesites, which explains the observed similarities, plus higher amounts of cumulus minerals such as clinopyroxene and plagioclase in the intrusive rocks than the extrusive rocks,

20

Journal Pre-proof which explains the observed differences. In other words, the results support our view that the intrusive and extrusive rocks share a common parental magma. The Sr-Nd isotope compositions of the Early Triassic Xiaruo mafic dykes and the coeval Jijiading basaltic andesites are much more enriched than the Longmu Co–Shuanghu PaleoTethyan ophiolites (Fig. 10a; Zhai et al., 2013b). In the Sr-Nd isotope diagram (Fig. 10a), the

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Xiaruo mafic dykes and the Jijiading basaltic andesites plot between the mantle array of

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DePaolo and Wasserburg (1979) and the subducton-related sediment (GLOSS) reference of

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Plank and Langmuir (1998) as well as the North Qiantang crust (Tao et al., 2014; Peng et al., 2015). As shown in Fig. 10a, the isotope data are consistent with (1) contamination of an

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enriched mantle-derived melt with the upper crust (model-1), or (2) addition of GLOSS during

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partial melting of the mantle (model-2). The degree of contamination (model-1) or the input of

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the recycled sediment (model-2) is estimated to be ~12 wt.% and 16 wt%, respectively (Fig. 10a). The combination of whole-rock Nd isotopes and zircon Hf isotopes is also consistent with the

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new alternatives proposed by us (model-1 and model-2, Fig. 11a).

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6.1.2. The Baimaxueshan dioritic enclaves Enclaves with more dark-colored minerals in granitic plutons elsewhere in the world are commonly regarded as (1) residual or refractory materials from the source rocks (e.g., Chappell and White, 1991), (2) country rock xenoliths (e.g., Mass et al., 1997), or (3) cognate fragments of cumulate rocks from the host magma (e.g., Clemens, 2003). After considering these general genetic models, previous researchers who studied the dioritic enclaves in the Baimaxueshan pluton (Zi et al., 2012a; He et al., 2018a) developed two different genetic models specifically for 21

Journal Pre-proof the Baimaxueshan dioritic enclaves. Zi et al. (2012a) suggested that both the dioritic enclaves and the host granitic rocks formed from the arc magmas that underwent different degrees of fractional crystallization but originated from the same mantle source. In contrast, He et al. (2018a) proposed that both mantle-derived magma and a crust-derived melt were involved, and that the dioritic enclaves formed from hybrids dominated by the mantle component

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whereas the host granites formed from the hybrids dominated by the crustal melt. These two competing models are all consistent with the fact that the dioritic enclaves and the host rocks

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have the same ages, but the model of Zi et al. (2012a) cannot readily explain the fact that the

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dioritic enclaves are of cumulate rock compositions (see Fig. 9c–f and related explanation

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above) instead of pure liquid compositions as suggested by the model, whereas the model of

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He et al. (2018a) is at odds with the remarkably similar whole-rock Sr-Nd isotopes (Fig. 10b) and zircon Hf isotopes (Fig. 11b) between the dioritic enclaves and the host rocks. We reconcile

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these problems by suggesting that the Baimaxueshan dioritic enclaves and their host rocks

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share a common parental magma, with the former representing the fragments of the early-

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formed cumulate rocks from the roof and the hanging wall (the solidification fronts) of the magma chamber of the pluton. Our new model is supported by the geochronological, petrological and geochemical constraints described above, plus the restricted distribution of the dioritic enclaves in the margin of the pluton (Fig. 2). In our new model, the initial parental magma is thought to be produced by mixing between a crust-derived melt with composition similar to the coeval rhyolites and the enriched mantle-derived arc magma with isotope compositions similar to the coeval basaltic andesites and mafic dykes in the region (model-3, Fig. 11b). Our calculations based on Sr-Nd-Hf isotopes indicate that the mantle-derived and 22

Journal Pre-proof crust-derived melts in the hybrid magma are from 50 to 70 wt.% and from 30–50 wt.%, respectively (Figs. 10b and 11b). It is worth mentioning that contamination of the mantlederived magma with the North Qiangtang crust is an alternative to generate the parental magma for the Baimaxueshan dioritic enclaves, but this process would require >25 wt.% of crustal contamination (model-4, Fig. 10b), which may be too high as far as the heat budget is

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concerned. Therefore, we favor the magma mixing model (model-3, Figs. 10b and 11b). It is important to note that there is at least one feature in common for all the various genetic

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models for the Baimaxueshan dioritic enclaves proposed by us and the previous researchers (Zi

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et al., 2012a; He et al., 2018a), which is the critical role of arc magmatism.

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6.2. Arc tempos and crustal affinity

The arc tempos in the North Qiangtang block and the northern part (Simao) of a

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contemporaneous Gondwana-derived micro-continental block (Indochina), as defined by the

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ages of arc basaltic magmatism and associated granitoids (~270 to 210 Ma), plus eclogites (~220 Ma) in the southern margin of the North Qiangtang block are very similar, lasting from

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Late Permian to Middle Triassic (Fig. 12a, b). In contrast, the granitoids in the Yidun arc are younger, varying from 240 to 200 Ma (Fig. 12c). Such difference can be regarded as one line of evidence for the interpretation that subduction beneath the northern rim of the Yidun arc was terminated ~20 Ma after the termination of subduction beneath the southern margin of the North Qiangtang block. This interpretation is further supported by the distribution of the Late Permian–Middle Triassic arc volcanic rocks in the south or west and the Late Triassic arc volcanic rocks in the north or east across the North Qiangtang block, including the Yidun arc (Figs. 1b and 2). 23

Journal Pre-proof The basement of the Yidun arc and the Zhongza massif were considered to be derived from the Yangtze block in the Early Paleozoic instead of the integral parts of the North Qiangtang block since Gondwana (Chen et al., 1987; Zhang et al., 1994, 1998). As shown in Fig. 13, this pointis apparently inconsistent with existing detrital zircon data (Reid et al., 2007; Chen et al., 2018; Jian et al., 2019, and references therein). The characteristic age peaks at ~944 Ma and

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~558 Ma for the Yangtze block (Fig. 13a) are not present in the Triassic sedimentary rocks in the Yidun terrane (Fig. 13b). The zircon age patterns of the Yidun terrane and the rest of the North

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Qiangtang block (Fig. 13c) are remarkably similar, supporting our new interpretation that the

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Yidun terrane and the Zhongza massif are the integral parts of the North Qiangtang block since

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Gondwana.

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6.3. Implications for Pangea reconstruction and mineral exploration

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The analysis in the previous section implies that the Yidun terrane is an integral part of the

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North Qiangtang block and that the Jinshajiang suspect suture between these two units is not a Paleo-Tethyan tectonic suture or subduction zone but a Cenozoic shear/fault zone (Fig. 1b). This

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is apparently at odds with the interpretation of the bimodal intrusive rocks (gabbros and trondhjemites or granitoids) in the southern part of the Jinshajiang suspect suture from Gongbo to Gongka (Fig. 2) as the Paleo-Tethyan ophiolites, i.e., the remnants of the Paleo-Tethyan oceanic crust due to oceanic subduction (Jian et al., 2008, 2009a, 2009b). As shown in Fig. 2, the bimodal intrusive rocks were emplaced from 347 to 236 Ma, during which time the PaleoTethyan oceanic plate was subducted episodically beneath the southern margin of the North Qiangtang block. Evidence for this includes the distribution of Late Devonian–Early Carboniferous arc bimodal volcanic rocks along the southwestern margin of the North 24

Journal Pre-proof Qiangtang block between Longmu Co and Shuanghu (see Fig. 1b for location; Wang et al., 2017; Zhai et al., 2018), the occurrence of Carboniferous arc-type mafic-ultramafic complex in the southeastern part of this block in the Yezhi region (see Fig. 2 for location; Jian et al., 2009a, b), and the Late Permian–Middle Triassic arc volcanic rocks (Zi et al., 2012b; Yang et al., 2014) and associated intrusive rocks (Zi et al., 2012a; He et al., 2019; this study) in the southeastern part

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of the North Qiangtang block, including the Jinshajiang shear/fault zone (Fig. 2). Based on the

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temporal correlation and short distances (~50 km) to the southwestern margin of the North

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Qiangtang block, we propose that the 347–236 Ma bimodal intrusive rocks in the Cenozoic

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Jinshajiang shear/fault zone originally formed in an intra-arc rifting environment or a

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continental arc-backarc system, similar to the model of Burchfiel and Chen (2012). As pointed out by Metcalfe (1994), Early Permian warm-water fauna are present in the

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Indochina block as well as the North Qiangtang block including the Yidun arc terrane,

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supporting a popular notion that these two blocks were close to each other before and after

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Gondwana breakup (e.g., Metcalfe, 2013; Zhao et al., 2018, and references therein). During the Permian, faunas in the South Qiangtang and Sibumasu blocks changed from cool water periGondwanan Indoralian province faunas in the Early Permian to endemic Sibumasu province faunas in the late Early Permian, to warm water Cathaysian province faunas in the Late Permian (Metcalfe, 2013), indicating the northward drifting of these two blocks due to slab pull of the subducting Paleo-Tethyan oceanic plate beneath the southern margin of the North Qiangtang block. This model is further supported by the paleolatitude data that show northward

25

Journal Pre-proof subparallel drifting for these blocks since the Late Permian (Fig. 14, Li et al., 2004; Song et al., 2012). Metcalfe (2013, 2017) and Zhao et al. (2018) proposed similar models for the eastern part of Pangea in the Triassic. Since the model of Metcalfe (2013, 2017) is more accurate in paleogeography than that of Zhao et al. (2018), we have selected the model of Metcalfe (2013,

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2017) as a base and added the new information about arc magmatism and metallogeny during

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this period to the model. Based on the distribution of Late Permian–Middle Triassic arc igneous

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suites to the northeast of the Longmu Co–Shuanghu Paleo-Tethyan suture (Figs. 1b and 2), we have added a northeastward subduction zone for this period beneath the southwestern margin

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of the North Qiangtang block (Fig. 15a). Based on the distribution of Late Triassic arc igneous

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suites to the southwest of the Zhiduo–Garze Paleo-Tethyan suture (Figs. 1b and 2), we have

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added a southwestward subduction zone beneath the northeastern margin of the North Qiangtang block, including the Yidun arc (Fig. 15b). As shown in Fig. 15a, the Early Triassic mafic

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intrusions, such as the 248–251 Ma Xiaruo mafic dykes (this study), the coeval granitoids, such

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as the 248–253 Ma Baimaxueshan pluton (Zi et al., 2012a), and the coeval arc volcanic rocks, such as the 245–249 Ma basaltic andesites in the nearby Jijiading area (Wang et al., 2014b) are related to the northward subduction of the southern branch of the Paleo-Tethyan Ocean beneath the southern margin of the North Qiangtang block. This branch of the Ocean was finally consumed by subduction in the Late Triassic, as indicated by the formation of eclogites at 221–237 Ma in the suture (Zhai et al., 2011; Jin et al., 2019) and the emplacement of abundant post-collision granites at 219–220 Ma across the suture (Tao et al., 2014; Peng et al., 2015). As shown in Fig. 15b, the northern branch of the Paleo-Tethyan Ocean began to subduct 26

Journal Pre-proof southward beneath the Yidun arc terrane, as indicated by the eruption of arc volcanic rocks and the emplacement of coeval granitoids during this period in the region (Hou et al., 2004; Cao et al., 2016). The collision between the Yidun arc terrane and the South China block to the east likely took place at the end of the Triassic (e.g., Hou et al., 2004). In the Late Triassic another subduction zone was present along the southern margin of the South Qiangtang block to the

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Sibumasu block during this period (Searle et al., 2012).

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south (Fig. 15b), as indicated by the emplacement of I-type granitoids in the western part of the

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Richards and Sengor (2017) suggested that no discovery of Late Permian–Middle Triassic porphyry Cu deposits in central Tibet and northern Yunnan is due to an anoxic Paleo-Tethyan

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Ocean during this period, because without the input of oxidized sediment-derived melt into the

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mantle wedge above a subduction zone, arc magmas are likely to be less fertile in Cu. According

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to these authors, the Paleo-Tethyan Ocean was anoxic because it was closed off from the Paleo-Pacific Ocean by the Gondwana-derived micro continental blocks such as North

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Qiangtang and Indochina (Fig. 15c); an interpretation originally suggested by Sengor and

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Atayman (2009). The reconstructed model of Pangea by Sengor and Atayman (2009) is highly speculative, because critical constraints such as reliable paleomagnetic data for the North Qiangtang, Indochina and adjacent blocks, and useful paleoclimatic and tectonic-magmatic records for these blocks were not considered by the authors. The models of Metcalfe (2013) and Zhao et al. (2018) are far more accurate, because these models are based on holistic geological and geophysical constraints. The models of Metcalfe (2013, 2017) and Zhao et al. (2018) all show an open Paleo-Tethyan Ocean (Fig. 15a, b), implying that this Ocean was not anoxic. 27

Journal Pre-proof As shown in Fig. 15b, Late Triassic porphyry Cu deposits, thought to be related to arc magmatism due to the subduction of the Paleo-Tethyan oceanic lithospheric plate beneath the Yidun terrane, occur in the southern part of the Yidun terrane, such as the ~222 Ma Pulang deposit and the ~221 Ma Xuejiping deposit (Li et al., 2017; Yang et al., 2018). Since the PaleoTethyan Ocean in the Early to Middle Triassic was connected with the global ocean, similar to

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that in the Late Triassic (Fig. 15a, b), there is no reason to believe that the subduction of the Paleo-Tethyan oceanic lithospheric plate beneath the southwestern margin of the North

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Qiangtang block from Early to Middle Triassic would not produce any porphyry Cu deposit at all.

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No discovery of this type of deposit in North Qiangtang may be due to poor preservation or the

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lack of exploration effort. Since the arc-related volcanic rocks formed during this period are still

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preserved in many places within the North Qiangtang block (Fig. 1b), poor preservation due to

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7. Conclusions

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severe erosion appears not to be the problem.

The Xiaruo mafic dykes and the Baimaxueshan dioritic enclaves in the North Qiangtang

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block have indistinguishable zircon U-Pb ages from 248 to 251 Ma. The mafic dykes and the coeval basaltic andesites in the region are all characterized by typical arc incompatible trace element ratios and enriched Sr-Nd-Hf isotope compositions, consistent with their parental magmas being derived from an enriched mantle wedge plus subduction-related sediments. The Baimaxueshan dioritic enclaves and the host granodiorites have indistinguishable zircon U-Pb agesand share a common parental magma that was produced by mixing between arc magma and associated crustal melt at depth, and that the dioritic enclaves are the fragments of early cumulate rocks (enriched in cumulus hornblende). Our new results are used to 28

Journal Pre-proof modify the reconstruction of central Tibet including North Qiangtang and the surrounding Tethyan Ocean in the Triassic. In our new model, we show that the Paleo-Tethyan Ocean was open widely and could not have been as anoxic as thought previously by some researchers. We conclude that the apparent lack of porphyry Cu deposits associated with the Late Permian–Middle Triassic arc magmatism in the North Qiangtang block is due to poor

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exploration effort instead of the presence of an anoxic ocean.

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Acknowledgments

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We thank Drs. Sheng He and Ya-Nan Yang for their assistance in zircon dating, and Drs. XiJuan Tan, Jin-Hua Du and Hong-Xia Yu for their assistance in chemical analyses. This research

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was financially supported by the Major Research Project of National Natural Science

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Foundation of China (NSFC Project; Grant # 91855217), National Key Basic Research Development Program of China (973 Program; Grant # 2015CB452602 and 2015CB452606), and 111 Plan under the Ministry of Education of China and the State Administration of Foregin Experts Affairs, China (Grant # B07011). Constructive reviews from Prof. Ian Metcalfe and an anonymous reviewer and editorial guidance from the capable AE Dr. Ze-Ming Zhang are greatly appreciated.

29

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distribution of important mineral deposits in the Sanjiang region, SW China. Gondwana Res. 26, 419–437.

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Deng, J., Wang, Q., Li, G., 2017. Tectonic evolution, superimposed orogeny, and composite metallogenic system in China. Gondwana Res. 50, 216–266.

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Zi, J.-W., Cawood, P.A., Fan, W.-M., Tohver, E., Wang, Y.-J., McCuaig, T.C., 2012a. Generation of Early Indosinian enriched mantle-derived granitoid pluton in the Sanjiang Orogen (SW

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Triassic collision in the Paleo-Tethys Ocean constrained by volcanic activity in SW China.

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Lithos 144–145, 145–160.

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subduction-related plagiogranites in the Jinshajiang ophiolitic mélange, Southwest China, and implications for the Paleo-Tethys. Tectonics 31, TC2012. Zi, J.-W., Cawood, P.A., Fan, W.-M., Tohver, E., Wang, Y.-J., McCuaig, T.C., Peng, T.-P., 2013. Late Permian–Triassic magmatic evolution in the Jinshajiang orogenic belt, SW China and implications for orogenic processes following closure of the Paleo-Tethys. Am. J. Sci. 313, 81–112. Zou, J., Yu, H., Wang, B., Huang, F., Zeng, Y., Huang, W., Wen, Y., Zhang, Z., Fan, Z., Tan, R., 2018. Petrogenesis of the Early Jurassic granodiorites in Renqinze area, central part of southern Lhasa subterrane: Implications for subduction process of the Neo-Tethyan Ocean. Earth Sci. 43, 2795–2810 (in Chinese with English abstract). 42

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Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Fig. 1 (a) Continental blocks of SE Asia (modified from Metcalfe, 2013). (b) Simplified geological map of the eastern Tibet and Sanjiang region showing the temporal and spatial distribution of important magmatic rocks in the North Qiangtang block and surrounding blocks (modified from Wang et al., 2018a). Data sources: Emeishan basalt (Li et al., 2016), Permian–Triassic volcanics (Yang et al., 2012, 2014; Wang et al., 2018b), granitoids and basaltic rocks in the South Qiangtang block (Zhai et al., 2013a; Zhang and Zhang, 2017; Li et al., 2018a), Late Triassic volcanics in the Songpan-Garze basin (Zhan et al., 2018), granitoids and basaltic rocks in the Baoshan terrane (Liu et al., 2020 and references therein), ophiolite (Jian et al., 2009a, b; Wang et al., 2013, 2018a; Zhai et al., 2013b; Zhu et al., 2013; Liu et al., 2016, 2019, and references therein), eclogite (Zhai et al., 2011; Zhang et al., 2013; Wang et al., 2019; Jin et al., 2019), blueschist (Zhai et al., 2009), strike-slip faults (DeCelles et al., 2002; Jian et al., 2019). Abbreviations: SG = Songpan-Garze basin; YD = Yidun; NQ = North Qiangtang; SQ = South Qiangtang; WB = West Burma; LSS = Longmu Co-Shuanghu Suture; BNS = Bangong Co-Nujiang Suture; ALS = Ailaoshan Suture; CMS = Changning-Menglian Suture.

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Fig. 2 Simplified geological map of the southeastern part of the North Qiangtang block and surrounding area showing the distribution of volcanics, granitoids and ultramafic-mafic intrusive rocks (modified after BGMRS, 1977; BGMRY, 1982, 1985; Zi et al., 2012b; Yin et al., 2014). Data sources: ages of gabbro and trondhjemite (Jian et al., 2008, 2009b; Zi et al., 2012c), ages of Cu deposits (Leng et al., 2012; Zhu et al., 2015; Cao et al., 2019).

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Journal Pre-proof Fig. 3 Geological maps showing the Xiaruo mafic dykes (a) (modified after BGMRY, 1985; Zi et al., 2012b) and the Baimaxuehsan pluton (b) (modified after Zi et al., 2012a; Yin et al., 2014). The age data for the Baimaxueshan pluton are from Zi et al. (2012a).

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Fig. 4 Field photographs and photomicrographs (cross-polarized light) of the Xiaruo mafic dykes and the Baimaxueshan mafic enclaves. Mineral abbreviations: CPX = clinopyroxene; Pl = plagioclase; Hb = hornblende; Bt = biotite; Qtz = quartz; Fe-Ti = Fe-Ti oxides.

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Fig. 5 Cathodoluminescence (CL) images of the selected zircon crystals (with 206Pb/238U age, Ma) from the Xiaruo mafic dykes and the Baimaxueshan mafic enclaves. The circles in red and rectangles in yellow are targets for U-Pb dating and Hf isotope analysis, respectively.

Fig. 6 Zircon U-Pb concordia diagrams for the Xiaruo mafic dykes and the Baimaxueshan mafic enclaves.

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Fig. 7 (a) Na2O+K2O versus SiO2 diagram (Le Maitre, 2002), with magma series classification from Irvine and Baragar (1971) and (b) K2O versus SiO2 diagram (Middlemost, 1985) for the Xiaruo mafic dykes and the Baimaxueshan mafic enclaves. Data for the Baimaxueshan granodiorite host, Jijiading basaltic andesite and Yezhi rhyolite are from Zi et al. (2012a, 2012b), Wang et al. (2014b) and He et al. (2018a).

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Fig. 8 Harker diagrams for the Xiaruo mafic dykes and the Baimaxueshan mafic enclaves. Silicate mineral compostions for the Xiaruo mafic dykes and the Baimaxueshan mafic enclaves are also shown. The sources for other data are same as Fig. 7.

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Fig. 9 Chondrite-normalized rare earth elements and primitive mantle-normalized trace elements patterns for the Xiaruo mafic dykes (a, b) and the Baimaxueshan mafic enclaves (c, d). Chondrite-normalized rare earth elements patterns for amphibole and apatite grains from the Baimaxueshan mafic enclaves (e, f). Normalizing values are from Sun and McDonough (1989). The average values of continental arc basalt (CAB) are from Li et al. (2015a). The sources for other data are same as Fig. 7.

Fig. 10 Plots of εNd(t) versus (87Sr/86Sr)i for the Xiaruo mafic dykes (a) and the Baimaxueshan mafic enclaves (b). The mantle array is from DePaolo and Wasserburg (1979). The composition of the North Qiangtang crust is based on the compostion of S-type granites in the region (Tao et al., 2014; Peng et al., 2015). Data sources: global subducting sediment (GLOSS) (Plank and Langmuir, 1998); Longmu Co-Shuanghu ophiolite (Zhai et al., 2013b); Jijiading basaltic andesite 46

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(Wang et al., 2014b); Yezhi and Jijiading rhyolite (Zi et al., 2012b; Wang et al., 2014b). The parameters in the mixing calculations of line 1: mantle-derived melt, 12 ppm Nd, 170 ppm Sr, εNd(t) = -1, (87Sr/86Sr)i = 0.7045; crustal material, 36 ppm Nd, 140 ppm Sr, εNd(t) = -11, (87Sr/86Sr)i = 0.730. The parameters in the mixing calculations of line 2: mantle-derived melt, 12 ppm Nd, 170 ppm Sr, εNd(t) = -2, (87Sr/86Sr)i = 0.7045; GLOSS, 27 ppm Nd, 327 ppm Sr, εNd(t) = -8.9, (87Sr/86Sr)i = 0.717. The parameters used in the mixing calculations of line 3 and 4: basaltic melt, 30 ppm Nd, 280 ppm Sr, εNd(t) = -3.65, (87Sr/86Sr)I = 0.7078; felsic melt, 40 ppm Nd, 120 ppm Sr, εNd(t) = -10, (87Sr/86Sr)i = 0.718; crustal material, 36 ppm Nd, 140 ppm Sr, εNd(t) = -11, (87Sr/86Sr)i = 0.730.

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Fig. 11 Plots of εHf(t) versus εNd(t) values for the Xiaruo mafic dykes (a) and the Baimaxueshan mafic enclaves (b). Data sources: Mid-ocean ridge and ocean island & plateau basalts (Salters et al., 2011), Clays and biogenic muds (Chauvel et al., 2008), global subducting sediment (GLOSS) (Plank and Langmuir, 1998); North Qiangtang (NQ) crust (Tao et al., 2014; Peng et al., 2015), Yezhi and Jijiading rhyolite (Wang et al., 2014b). The parameters used in the mixing calculations of line 1: mantle-derived melt, 12 ppm Nd, 2.35 ppm Hf, εNd(t) = -1, εHf(t) = 0; crustal material, 36 ppm Nd, 5.3 ppm Hf, εNd(t) = -11, εHf(t) = -8. The parameters used in the mixing calculations of line 2: mantle-derived melt, 12 ppm Nd, 2.35 ppm Hf, εNd(t) = -2, εHf(t) = 0; GLOSS, 27 ppm Nd, 4.1 ppm Hf, εNd(t) = -8.9, εHf(t) = -5. The parameters used in the mixing calculation of line 3: basaltic melt, 30 ppm Nd, 5.6 ppm Hf, εNd(t) = -3.65, εHf(t) = -4.8; felsic melt, 40 ppm Nd, 10 ppm Hf, εNd(t) = -10, εHf(t) = -13.5.

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Journal Pre-proof Fig. 12 Secular variations of igneous activity in the North Qiangtang block (a), the Simao block (b) and the Yidun terrane (c). Data sources for magmatic rocks in the North Qiangtang block (Hu et al., 2014; Tao et al., 2014; Li et al., 2015b; Yang et al., 2014; Peng et al., 2015; Wang et al., 2018a, 2018c, and references therein), the Simao block (Wang et al., 2018a and references therein) and the Yidun terrane (Reid et al., 2007; Wang et al., 2014c; Wu et al., 2014; Cao et al., 2016; Gao et al., 2017, 2018, 2019, and references therein; Li et al., 2017; He et al., 2018a, 2018b, 2019; Yang et al., 2018).

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Fig. 13 Detrital zircon U-Pb age distribution comparision of Cambrian–Devonian sedimentary rocks in northwest Yangtze block (a) (Chen et al., 2018 and references therein), Middl–Late Triassic turbidites in the Yidun terrane and inherited zircons (b) (Reid et al., 2007; Jian et al., 2019) and Latest Triassic–Early Jurassic sanstones of Qamdo basin in the North Qiangtang block (c) (Shang, 2016).

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Fig. 14 Change of paleolatitudes with time for the North Qiangtang block (Song et al., 2017 and references therein), the South Qiangtang block (Li et al., 2004 and references therein; Song et al., 2012), the Indochina block (Li et al., 2004 and references therein), the Baoshan terrane (Li et al., 2004 and references therein; Ali et al., 2013; Zhao et al., 2015; Li et al., 2018b) and the Lhasa terrane (Song et al., 2017 and references therein).

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Fig. 15 Modified paleogeographic reconstructions (a, b) of this study (modified from Metcalfe, 2013, 2017) and the model (c) of Richards and Sengor (2017). Abbreviations: NC = North China; SC = South China; Y = Yangtze; C = Cathaysia; SG = Songpan-Garze; YD = Yidun; NQ = North Qiangtang; I = Indochina; SQ = South Qiangtang; WC = Western Cimmerian Continent; S = Sibumasu.

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Journal Pre-proof Highlights There are mafic dykes emplaced at ~250 Ma in the eastern part of the North Qiangtang block

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They are coeval with arc basaltic andesites and granodiorite-hosted mafic enclaves in the region

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The arc igneous suite provides evidence for the NE-ward subduction of the Paleo-Tethys Ocean

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The Paleo-Tethys Ocean in the Triassic was not closed and isolated and probably not anoxic

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