Detrital zircon geochronology of Neoproterozoic to early Paleozoic sedimentary rocks in the North Qinling Orogenic Belt: Implications for the tectonic evolution of the Kuanping Ocean

Precambrian Research 279 (2016) 1–16

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Detrital zircon geochronology of Neoproterozoic to early Paleozoic sedimentary rocks in the North Qinling Orogenic Belt: Implications for the tectonic evolution of the Kuanping Ocean Huahua Cao a,b, Sanzhong Li a,b,⇑, Shujuan Zhao a,b, Shan Yu a,b, Xiyao Li a,b, I.D. Somerville c a b c

College of Marine Geosciences, Ocean University of China, Qingdao 266100, PR China The Key Lab of Seabed Resource and Exploration Techniques, Ministry of Education, Qingdao 266100, PR China UCD School of Earth Sciences, University College Dublin, Belfield, Dublin 4, Ireland

a r t i c l e

i n f o

Article history: Received 13 October 2015 Revised 30 March 2016 Accepted 2 April 2016 Available online 8 April 2016 Keywords: North Qinling Orogenic Belt Taowan Group Kuanping Group Detrital zircon U–Pb geochronology

a b s t r a c t This paper presents the results of U–Pb dating of detrital zircons from six metasedimentary rocks in the Taowan and Kuanping groups in the northern margin of the North Qinling Orogenic Belt (NQB), Central China, for constraining the tectonic affinity of the NQB, the existence time and tectonic features of the Kuanping Ocean and the accurate northern boundary of the NQB. Zircon U–Pb dating indicates that the protoliths of the two metasedimentary rocks from the upper and lower parts of the Taowan Group were deposited later than
1111 and
871 Ma respectively. However, the protoliths of the four metasedimentary rocks from different layers of the Kuanping Group were deposited later than
781,
668,
515 and
477 Ma, respectively, suggesting that the Taowan Group and the Kuanping Group both have an abnormal or non-Smithian stratigraphic sequence. The age populations and predominant peak ages of detrital zircon grains from the Mesoproterozoic–middle Neoproterozoic metasedimentary rocks in the Taowan and Kuanping groups indicate that their provenances were mainly attributed to the NQB and subordinately from the Yangtze Craton (YZC), suggesting that the two groups both belong to the NQB and the northern boundary of the NQB should be placed to the north of the Taowan Group, which can be called the Paleo-Luonan–Luanchuan Fault. However, the Nd–Pb whole-rock isotopes, zircon Hf isotopes and trace element compositions of the Precambrian basement rocks and the Neoproterozoic granitic magmatic events in the NQB are significantly different from those in the YZC, suggesting that the NQB was more likely to be an independent microcontinent adjacent to the YZC from the Mesoproterozoic to the Neoproterozoic, and experienced a unique geological history. On the other hand, together with the record of the oldest MORB-type basalt with an age of 1445 Ma in the Kuanping Group, the first late Mesoproterozoic sedimentary record (
1111 Ma) in the Taowan Group, to be reported using precise geochronological data in this study, it indicates that the Kuanping Ocean separated the NQB from the NCC that had already existed to the north of the NQB before the Mesoproterozoic, and was a major ocean basin which had lasted for a long time, rather than as a back-arc basin. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction The Qinling Orogenic Belt (QOB) (Fig. 1a), situated between the North China Craton (NCC) and the Yangtze Craton (YZC), is one of the major composite collisional orogens in East Asia (Zhang et al., 2001, 2015; Wu and Zheng, 2013; Zhao et al., 2015; Yu et al., 2015), and consists of the Paleozoic accretion-dominated North Qinling Orogenic Belt (NQB) and the early Mesozoic ⇑ Corresponding author at: College of Marine Geosciences, Ocean University of China, No. 238, Songling Road, 266100 Qingdao, PR China. Tel.: +86 0532 66781971. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.precamres.2016.04.001 0301-9268/Ó 2016 Elsevier B.V. All rights reserved.

collision-dominated South Qinling Orogenic Belt (SQB) separated by the Shangdan and Mianlue sutures (Li et al., 2007; Liu et al., 2008a, 2011, 2013a; Liu et al., 2012) (Fig. 1b). Previous researches proposed that the NQB was formed through the subduction/accretion and convergence between the NCC and the YZC for the closure of the Proto-Tethys Ocean during the Neoproterozoic to early Paleozoic interval, and this belt was amalgamated with the YZC along the Shangdan suture zone by the northward subduction of the Shangdan Ocean (an eastern branch of the Proto-Tethys Ocean, Ren and Xie, 1991; Li et al., 1995; Zhang et al., 1996a; Pan et al., 2009; Chen et al., 2014) in mid-late Paleozoic (Ratschbacher et al., 2003; Dong et al., 2011a,

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PLLF: Paleo-Luonan-Luanchuan Fault SDSZ: Shangnan-Danfeng suture zone MLSZ: Mianxian-Lueyang suture zone

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Fig. 1. Simplified geological map of the NQB and its neighboring area. The insets (a and b) shows the sketched tectonic map of the NQB and the location of the study area; the main map (c) shows the basic geological setting of the study area (simplified from HBGMR, 1993). S-NCC: southern North China Craton; NQB: North Qiling Orogenic Belt; SQB: South Qinling Orogenic Belt.

2011b, 2013, 2014; Liu et al., 2013a; Shi et al., 2013; Wang et al., 2013c; Wu and Zheng, 2013). However, the Neoproterozoic to early Paleozoic tectonic evolutionary history between the NQB and the NCC has not well been constrained – Two fundamental issues still remain controversial: ① the tectonic affinity of the NQB – is it part of the southern margin of the NCC? (Zhang et al., 1994a; Zhang et al., 1996a, 2001; Meng and Zhang, 1999, 2000; Dong et al., 2011a; Wu and Zheng, 2013) or the northern margin of the YZC? (Zhang et al., 1996b, 2002; Lu et al., 2003; Hacker et al., 2004; Liu et al., 2013a), or as an independent microcontinent intervening between the YZC and the NCC? (Ouyang and Zhang, 1996; Xu et al., 1997; Dong et al., 2003; Yang et al., 2010; Zhu et al., 2011; Diwu et al., 2010, 2012, 2014); ② the tectonic features of the ocean basin situated between the NQB and the NCC (in this study we refer to it as the Kuanping Ocean) – is it a major ocean basin, earlier than the Shangdan Ocean to the south (Xue et al., 1996a, 1996b; Faure et al., 2001; Ratschbacher et al., 2003, 2006; Liu et al., 2013a; Wu and Zheng, 2013), or a back-arc basin caused by the northward subduction of the Shangdan Ocean? (Zhang et al.,

1994a; Zhang et al., 1996a, 2001; Meng and Zhang, 1999, 2000; Lu et al., 2003, 2009; Dong et al., 2011a, 2011b, 2013). In addition, the boundary between the NQB and the NCC, which also represents the northern boundary of the Kuanping Ocean, is generally regarded as the Luonan–Luanchuan Fault (Fig. 1c), but some researchers suggest that it may have been in different places at different stages for the complex geological evolution of the NQB (Meng and Zhang, 1999; Wang et al., 2011a). Obviously, the above controversies arose from a lack of detailed study of the tectonic evolution of the Kuanping Ocean. Ophiolites, as fragments of oceanic lithosphere, provide important clues to identify the existence of the ancient ocean basin, and the continuous and lasting marine-facies sedimentary record also is one of the most effective evidences to evaluate the ocean basin. Meanwhile, the age distribution of detrital zircon populations in sedimentary successions can be used to yield both the provenance and maximum age of deposition, and thereby assessing the affinity of the sedimentary units and help in the reconstruction of the tectonic evolution of ancient basins (Carter and Steve, 1999; Fedo

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et al., 2003; Griffin et al., 2004; Moecher and Samson, 2006; Wu et al., 2007; Thomas, 2011). The Kuanping Group is a widely exposed sedimentary unit between the NQB and the NCC, and the protoliths of the group are mainly composed of the ophiolite mélange and marine deposits. Therefore, they are the key to understanding the evolution of the Kuanping Ocean and tectonic relationship between the NQB and the NCC. In addition, the Taowan Group, which is generally considered as the southernmost unit of the southern edge of the NCC (S-NCC) and separated from the Kuanping Group by the Luonan–Luanchuan Fault to the south, is mainly composed of the marine deposits, thus the sedimentary history of this group can provide a comparison with that of the Kuanping Group, with the purpose of determining the boundary between the NQB and the NCC. This paper presents the results of U–Pb dating of detrital zircons in a suite of metasedimentary rocks from the Kuanping and Taowan groups, combined with previous geochronological data from magmatic zircons from the ophiolite mélange in the Kuanping Group, for constraining the relationship between the NQB and the NCC, the existence time and feature of the Kuanping Ocean and the precise location of the northern boundary of the Kuanping Ocean.

2. Geological setting and sample description The NQB and the S-NCC are separated by the Luonan– Luanchuan Fault (Zhang et al., 1996a, 2001), so a brief introduction about the S-NCC and the NQB is given below.

2.1. Southern edge of the NCC The S-NCC adjacent to the NQB is mainly composed of the typical 2.8–2.5 Ga basement rocks of the Taihua Group (Kröner et al., 1988; Wu and Zheng, 2013), the Paleo-Mesoproterozoic sedimentary rocks of the Guandaokou and Luanchuan groups (Gao et al., 1996; Dong et al., 2011a; Zhu et al., 2011; Liu et al., 2013a) and the Mesoproterozoic volcanics of the Xiong’er Group (ca. 1.80–1.75 Ga, Peng et al., 2008; Zhao et al., 2009; Wang et al., 2010), with the rocks of the Taowan Group overlying those of the Guandaokou and Luanchuan groups (Peng and Tu, 1984; Han et al., 2009). U–Pb dating of detrital zircons from the Guandaokou Group yielded three age peaks at ca. 2.5, 2.3 and 1.85 Ga (Zhu et al., 2011), and that from the Luanchuan Group defined one major age peak at ca. 2.35 Ga (Liu et al., 2013a), which are widely distributed in the NCC. The Taowan Group, cropping out in the southernmost part of the S-NCC (Wang et al., 2009a), is distributed in east–west direction along the Luonan–Luanchuan Fault (Fig. 1c) and forms an unconformable contact with the southern Kuanping Group (Zhang and Li, 1989; Zhang et al., 1994a; Zhu et al., 2011). This group mainly consists of metamorphic sandstone, phyllite, slate, schist, meta-conglomerate and marble (Wang, 1985; Zhang et al., 1992; Wang et al., 2007), intruded by Neoproterozoic gabbros (830–826 Ma, Wang et al., 2011a). Based on palaeontological evidence, rock associations and comparisons with coeval sediments, this group is proposed to have formed in very different times, such as the Qingbaikou-Sinian System (Wang and Liu, 1982; Peng and Tu, 1984; Wang, 1985; Zhang et al., 1994a), the Early Cambrian (Wang et al., 1989), the Cambrian-Ordovician (Hu and Huang, 1987), the Ordovician (Zhang and Li, 1989; Wang et al., 2007) and the Ordovician–Silurian (Wang, 1983; Zhang, 1984). In addition, abundant metamorphic ages of ca. 400 Ma have also been reported for the Taowan Group, suggesting that it has

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experienced the early Paleozoic metamorphism widely developed in the NQB (Zhang et al., 1994a; Li and Sun, 1999; Sun et al., 2002). 2.2. North Qinling Orogenic Belt (NQB) The NQB is bounded by the Luonan–Luanchuan Fault to the north and the Shangdan Suture (SDSZ) to the south (Fig. 1b). It is predominately composed of Precambrian basement units, the Neoproterozoic and early Paleozoic ophiolites and volcanosedimentary assemblages which were overlain by CarboniferousPermian clastic sediments (Zhang et al., 1994a, 2001), forming five main tectonic units. They are, from north to south, the Kuanping Group, Erlangping Group, Qinling Group, Songshugou Ophiolite and Danfeng Group, all of which are separated from each other by thrusts or ductile shear zones (Dong et al., 2011a; Shi et al., 2013; Zhao et al., 2015; Fig. 1c). 2.2.1. The Kuanping Group The Kuanping Group, the northernmost unit of the NQB, is unconformably overlain by the Taowan Group to the north (Zhang et al., 1991; Zhang et al., 1994a; Zhu et al., 2011). This unit is mainly composed of greenschist, amphibolite, micaschist, gneiss, meta-clastic rock and marble, and the protolith of both greenschist and amphibolite were tholeiitic basalt or ophiolite (Zhang and Zhang, 1995), with later greenschist-facies to upper amphibolitefacies metamorphism overprinted at ca. 442–415 Ma (Zhai et al., 1998; Liu et al., 2011). It has been subdivided into different formations in the different areas (Fig. 2), e.g., from bottom to top, into the Sichakou and Guangdongping formations in the Luonan area (SBGMR, 1990), the Lower Kuanping and Upper Kuanping formations in the Luanchuan area (HBGMR, 1958), and the Xiewan, Sichakou, and Guangdongping formations in the Nanzhao area (HBGMR, 1990). The group was previously thought to have formed in the Proterozoic (Zhang et al., 1991; Zhang et al., 1994a, 2001; Zhang and Zhang, 1995; Li, 2002; He et al., 2007). Recently, some reliable geochronological data have been reported, with the metabasic rocks formed at 1445, 943 and 611 Ma (Yan et al., 2008; Diwu et al., 2010; Dong et al., 2014) and the protoliths of the metasedimentary rocks deposited after the late Neoproterozoic (600 Ma, Diwu et al., 2010; 640 Ma, Zhu et al., 2011; 580 Ma, Liu et al., 2013a; 570 Ma, 550 Ma, Shi et al., 2013) or early Paleozoic (500–400 Ma, Lu et al., 2009; 460 Ma, Shi et al., 2013), and the Ordovician fossils have also been found in the Kuanping Group at the Huxian area (Wang et al., 2009b). A number of previous results show that the metabasic rocks in the unit display the geochemical characteristics similar to N-MORB-, E-MORB- and T-MORB-types tholeiitic basalts (Zhang and Zhang, 1995; Yan et al., 2008; Diwu et al., 2010; Dong et al., 2014), which represent the remnants of oceanic crust. Based on the studies above, this lithological assemblage was considered to have formed as a marginal sea basin sequence in the passive continental margin of the S-NCC (Xue et al., 1996a, 1996b; Zhai et al., 1998; Wu and Zheng, 2013), or as a subduction-accretion complex (i.e. an accretionary wedge) deposited on the southern margin of S-NCC or the northern margin of the NQB related to the closure of the Kuanping Ocean (Zhang et al., 1991; Ratschbacher et al., 2003, 2006; Hacker et al., 2004; Wang et al., 2009a; Diwu et al., 2010; Liu et al., 2013a; Shi et al., 2013). 2.2.2. The Erlangping Group The Erlangping Group, regarded as a tectonic mélange unit, is distributed as lensoid unit between the Kuanping and Qinling groups extending from east to west. It was mainly composed of minor ultramafic rock, mafic to felsic metavolcanic rock, metaclastic rock and marble (Dong et al., 2011a, 2013; Liu et al., 2013a), intruded by numerous granitoid plutons with ages of

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Luanchuan Area

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Fig. 2. The stratigraphic sequences of the Taowan and Kuanping groups in the southern margin of the North China Craton and the North Qinling Orogenic Belt (modified by 1: 200,000 geologic maps).

490–400 Ma (Lerch et al., 1995; Wang et al., 2009c, 2013c), and underwent regional greenschist- to amphibolite-facies metamorphism at ca. 440 and 404–394 Ma (Sun et al., 1996; Zhai et al., 1998; Ratschbacher et al., 2003; Liu et al., 2011). Cambrian-Silurian fossils have also been found in the marble (Niu et al., 1993; Wang et al., 1995). Zircon U–Pb ages indicate that the mafic to felsic metavolcanic rocks in the group have formed at 490–467 Ma (Lerch et al., 1995; Xue et al., 1996a; Lu et al., 2003; Yan et al., 2007; Liu et al., 2013a) and 441–436 Ma (Liu et al., 2013a), and one metapelite in the Tongbai area was deposited after 454 Ma (Liu et al., 2013a). The metamafic volcanic rocks have trace element features similar to island arc or E-MORB rocks, implying that they were formed in an island arc (Xue et al., 1996a; Ratschbacher et al., 2003, 2006; Hacker et al., 2004; Liu et al., 2013a) or in a back-arc basin (Kröner et al., 1993; Sun et al., 1996; Zhang et al., 1996a, 2001; Meng and Zhang, 2000; Dong et al., 2011a, 2011b). 2.2.3. The Qinling Group The Qinling Group (or the Qinling Complex), cropping out as several lenticular units in the NQB, is separated from the Erlangping unit by the Zhuyangguan–Xiaguan Fault (Fig. 1c). This complex unit is mainly composed of gneiss, amphibolite, marble and subordinate metapelite (Xue et al., 1996a; Zhang et al., 2001), and all intruded by Neoproterozoic and Silurian-Devonian granitoids (Lu et al., 2003, 2009; Wang et al., 2003, 2005a,b, 2009b; Zhang et al., 2004; Chen et al., 2004a, 2006; Wang et al., 2013c). The geochronological data of gneisses (2267–2172 Ma) and meta-tholeiite amphibolite (1987 Ma) suggest that the oldest rocks of the Qinling Group were formed in the Paleoproterozoic

(Zhang et al., 1994a, 2001). Therefore, the Qinling Group was regarded as Precambrian crystalline basement and the inner core of the NQB (Hu et al., 1993; You et al., 1993; Zhang et al., 1996a; Meng and Zhang, 2000; Diwu et al., 2014), and named as the ‘‘Qinling Microcontinent” by some researchers (Zhu et al., 2011; Liu et al., 2013a). Moreover, some other gneisses in the Qinling Group have protolith ages of ca. 1000–800 Ma (Chen et al., 1992; Zhang et al., 2004; Lu et al., 2005; Shi et al., 2009, 2013; Yang et al., 2010), and the protoliths of partial metasedimentary rocks were also deposited after late Mesoproterozoic to early Neoproterozoic or even later in age (Lu et al., 2006; Wan et al., 2011a; Shi et al., 2013; Diwu et al., 2014), indicating the complexity of the ‘‘Qinling Group”. In addition, Neoproterozoic metamorphic ages (996–793 Ma) were also obtained from the Qinling Group through Rb–Sr, Ar–Ar and Pb–Pb isochron methods (Chen et al., 1991, 1992; Zhang et al., 1994a). Ultrahigh-pressure (UHP) eclogite and highpressure (HP) granulite also have been discovered from the northern side (Guanpo and Shuanghuaishu areas) and the southern side (Songshugou and Qinyouhe areas) of the unit, respectively (Kröner et al., 1993; Hu et al., 1995; Yang et al., 2003; Zhai et al., 1998; Liu et al., 2011; Wang et al., 2011b; Xiang et al., 2012), with their metamorphic ages were dated at 490–485 Ma (Cheng et al., 2012; Wang et al., 2011b). In the middle Paleozoic (450–400 Ma), the unit was intruded by arc magmatic rocks and many granitoid plutons (Wang et al., 2009a, 2013c) and underwent extensive medium-pressure granulite facies metamorphism and migmatization (Zhai et al., 1998; Sun et al., 2002; Faure et al., 2008; Liu et al., 2011; Wang et al., 2011b; Zhang et al., 2011; Xiang et al., 2012). Meanwhile, whether the Qinling unit belongs to the NCC, the

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YZC, or serves as an independent microcontinent remains an open issue (Lu et al., 2003; Hacker et al., 2004; Diwu et al., 2010, 2012, 2014; Yang et al., 2010; Zhu et al., 2011; Dong et al., 2011a; Liu et al., 2013a; Shi et al., 2013; Wu and Zheng, 2013). 2.2.4. The Songshugou Ophiolite The Songshugou Ophiolite occurs as a block exposed for a length of 27 km and width of 3 km at the southern margin of the Qingling Group and the northern side of the SDSZ (Fig. 1c), which was thrust over the southern Qinling Group, and is bounded by the Jieling ductile shear zone on the north and the Xigou Fault to the south. The ophiolite consists mainly of metamorphosed ultramafic and mafic rocks. The ultramafic rocks are enclosed within the mafic rocks, with the tectonic boundary showing intense ductile deformation and mylonitization (Liu et al., 2004; Dong et al., 2008). The geochronological data show that Songshugou ophiolite crystallized at ca. 1.03 Ga (Dong et al., 2008), and the available data from petrology and geochemistry suggest that the protolith of the Songshugou ophiolite has N-MORB and E-MORB affinities, indicative of a fragment of oceanic lithosphere (Li et al., 1991; Zhou et al., 1995; Liu et al., 2004; Dong et al., 2008). 2.2.5. The Danfeng Group The Danfeng Group is discontinuously exposed in the south of the NQB along the SDSZ (Fig. 1c), and consists mainly of gabbros, basalts, andesites, small amounts of ultramafic rocks and minor sedimentary rocks (Zhang et al., 1994b; Zhou et al., 1995). Regional geochronological data reveals that the group formed at ca. 520–420 Ma (Dong et al., 2011b, and references therein), consistent with radiolarian ages ranging from Ordovician to Silurian (Cui et al., 1995). Trace element geochemistry and isotope composition show that the group is composed of N-MORBs and E-MORBs as well as arc-related volcanic rocks (Zhang et al., 1994b; Ratschbacher et al., 2003, 2006; Dong et al., 2011b). Therefore, some researchers have suggested that the Danfeng Group, which is located in the Shangdan suture zone, represents remnants of Shangdan oceanic crust, along with part of an island-arc succession (Dong et al., 2011a, b). 3. Petrology and sample description Detrital zircons for U–Pb dating were separated from four schists, one quartzite and one sandstone of the Taowan or Kuanping groups. The sampling locations and sedimentary sequences are shown in Figs. 1c and 2. All these samples are described as follows. Sample HN077 is a sericite quartz schist from the Taowan Group that was collected
9 km northwest of the Taowan Town in the Luanchuan area (GPS coordinates: 111°20.0940 E, 33°52.5000 N; Fig. 1c). The rock has a clasto-lepidoblastic texture and a blasto-bedding structure, with local carbonation, and mainly consists of sericite (
45%), quartz (
40%), plagioclase (
5%) and magnetite (
8%; Fig. 3a), with accessory zircon and apatite. Sample HN079-2 is a muscovite quartz schist, at the lowerseated than the sample HN077 in the Taowan Group, and collected
7 km west of the Taowan Town in the Luanchuan area (GPS coordinates: 111°20.8050 E, 33°51.4960 N; Fig. 1c). It has clastolepidoblastic texture and a schistose structure, and consists of muscovite (
30%), biotite (
4%), quartz (
51%), plagioclase (
7%), and alkali feldspar (
3%), with accessory zircon and magnetite (Fig. 3b). Sample HN042-2 is a quartzite from the Xiewan Formation (Fig. 2) that was collected
4 km northeast of the Nanzhao County in the Nanzhao area (GPS coordinates: 112°27.5850 E, 33°31.7290 N; Fig. 1c). It has a sutured granoblastic texture and a massive

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structure, and is mainly composed of quartz (
95%), and accessory zircon, magnetite, and apatite (Fig. 3c). Sample HN048 is a metamorphic quartz sandstone from the Guangdongping Formation (Fig. 2) that was collected
2.5 km southeast of the Mashiping Town in the Nanzhao area (GPS coordinates: 112°15.5300 E, 33°32.7410 N; Fig. 1c). It has a blastopsammitic texture and a massive structure, and mainly consists of quartz (
85%), biotite (
5%) and accessory magnetite, zircon, and apatite (Fig. 3d). Sample HN135 is a biotite quartz schist from the Guangdongping Formation (Fig. 2) that was collected
1 km west of the Guanpo Town in the Luonan area (GPS coordinates: 110°42.7050 E, 33°53.2260 N; Fig. 1c). It has a lepidogranoblastic texture, a gneissic structure, and consists of fine biotite (
8%), quartz (
83%), and magnetite (
7%), with accessory zircon and apatite (Fig. 3e). Sample HN148-1 is a biotite quartz schist from the Sichakou Formation (Fig. 2) that was collected
1.5 km northwest of the Hongmenhe Town in the Luonan area (GPS coordinates: 109°46.1210 E, 33°59.7670 N; Fig. 1c). It has a lepidogranoblastic texture and a schistose structure, and consists of biotite (
35%), quartz (
83%), and accessory magnetite, zircon and apatite (Fig. 3f).

4. Analytical methods Zircons were separated using heavy liquids and a Frantz magnetic separator at the Langfang Regional Geological Survey, Hebei Province, China. Approximately 250 zircons were handpicked from each sample under a binocular microscope, and mounted in epoxy resin. The resin discs were then polished until most of the zircon grains were exposed at the grain centers in preparation for analysis by laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS). The standards are mounted in a separated epoxy resin. Images of the zircons were taken using an optical microscope in transmitted and reflected light, and the surfaces of the zircon grains were washed in dilute HNO3 and pure alcohol to remove any lead contamination, and then carbon is coated on the mounts. Cathodoluminescence (CL) images were obtained using a JEOL scanning electron microscope housed at the State Key Laboratory of Continental Dynamics, Northwestern University in Xi’an, China, to identify internal structures and select target sites for U–Pb analyses. Zircons were dated using a 193 nm Elan 6100 DRC ICPMS housed at the State Key Laboratory of Continental Dynamics, Northwest University in Xi’an, following the standard operating techniques described by Yuan et al. (2004). 207Pb/206Pb and 206 Pb/238U values were calculated using the GLITTER 4.0 program (Jackson et al., 2004), with Zircon 91500 used as an external standard (206Pb/238U age: 1065.4 ± 0.6 Ma; Wiedenbeck et al., 1995), and the standard silicate glass NIST used to optimize the ICPMS. The spot diameter was 30 lm, and ages were calculated using ISOPLOT 3 (Ludwig, 2003). Our measurements of standard sample GJ-01 yielded a weighted mean 206Pb/238U age of 602.2 ± 2.4 Ma (2r, MSWD = 1.15, n = 19; MSWD = mean square of weighted deviates; Yuan et al., 2004), in good agreement with the recommended ID-TIMS 206Pb/238U age of 598.5–602.7 Ma (Jackson et al., 2004). Finally, common-Pb corrections were made following the method proposed by Anderson (2002).

5. Results of U–Pb dating 5.1. Taowan Group The results of U–Pb analyses of zircons from the rocks of the Taowan Group are presented in Table S1.

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H. Cao et al. / Precambrian Research 279 (2016) 1–16

a

b Ser Q

Q

Q

Mu

Q Ser

Q

Q Mu

Ser

HN079-2

HN077

c

Q

d

Q

Bi

Q Q Q

Q

Bi

Q

Q HN048

HN042-2

e

Q

f Q

Q Q

Bi

Bi Q

Bi Q

Q

Bi

Q

HN135

Bi HN148-1

Fig. 3. Representative photomicrographs (all in cross-polarized light) of the metasedimentary rocks from the Taowan and Kuanping groups. (a) Sericitequartz schist (HN077); (b) Muscovite quartz schist (HN079-2); (c) Quartzite (HN042-2); (d) Metamorphic quartz sandstone (HN048); (e) Biotite quartz schist (HN135); (f) Biotite quartz schist (HN148-1). Q: Quartz; Ser: Sericite; Mu: Muscovite; Bi: Biotite.

Sample HN077: The majority of zircon grains separated from sample HN077 (a sericite quartz schist) are subhedral with a few rounded grains. These zircons display clear internal srtucture and fine-scale oscillatory growth zoning in the CL images (Fig. 4a), and with Th/U ratios from 0.17 to 1.57, indicating a magmatic origin (Koschek, 1993; Table S1). Fifty-nine zircons were analyzed and most of them yield concordant but variable ages, typical of detrital zircons in sedimentary rocks. The U–Pb data define a broad range of ages from 1019 Ma to 2589 Ma (Figs. 5a and 6a; Table S1), can be divided into four major populations of 1019–1166, 1222–1618, 1694–2029 and 2225–2589 Ma, with four major age peaks at ca. 1118, 1512, 1776 and 1859 Ma (Fig. 6a), and two subordinate age peaks at ca. 1300 and 2024 Ma. Nine youngest concordant zircons (concordance > 95%) yield a mean 206Pb/238U age of 1111 ± 48 Ma. Sample HN079-2: Most zircon grains from sample HN079-2 (muscovite quartz schist) are subhedral-subrounded. CL images of these zircons show oscillatory or planar compositional zoning (Fig. 4b) with Th/U values of 0.13–1.25, suggesting derivation from a magmatic protolith. Most of the sixty zircons show concordant ages with a wide range from 821 to 2895 Ma (Fig. 5b; Table S1). The analyzed zircons yield five major groups of 822–922, 1128–1279, 1402–1479, 1540–1667 and 1914–2057 Ma, and

display five dominant age peaks at ca. 861, 1207, 1440, 1624 and 1990 Ma (Fig. 6b). In addition, the remaining zircons yielded ages of 2154, 2500 and 2895 Ma. Seven youngest data gave a mean 206 Pb/238U age of 871 ± 33 Ma. 5.2. Kuanping Group The results of U–Pb analyses of zircons from the rocks of the Kuanping Group are presented in Table S2. Sample HN042-2: The zircons selected from the sample HN0422 (quartzite) are subhedral-subrounded, and display clear internal structure and oscillatory zoning in CL images (Fig. 4c). Their Th/U ratios generally range from 0.17 to 1.31, indicating a magmatic origin (Koschek, 1993). Among them, 58 near-concordant zircon analyses yielded an age range from 769 to 3077 Ma (Fig. 5c; Table S2). Four major age populations can be identified at ca. 769–983, 1002–1226, 1332–1580 and 1652–2456 Ma, with five peak ages at ca. 1053, 1391, 1572, 1893 and 1967 Ma (Fig. 6c). The two youngest 206Pb/238U ages are between 769 and 793 Ma, with a mean age of 781 ± 15 Ma. Sample HN148-1: The majority of zircon grains separated from sample HN148-1 (biotite quartz schist) are subhedralsubrounded, exhibit clear internal structure and fine-scale

7

H. Cao et al. / Precambrian Research 279 (2016) 1–16

100um

HN077

1773Ma 124

1

51

6

15

23

1073Ma

1166Ma

1280Ma

25

12

50

35

2029Ma 2589Ma

1873Ma

1509Ma

100um

HN079-2 1633Ma 44

36

19

40

865Ma

997Ma

1217Ma

51

2474Ma

2056Ma

1436Ma

39

58

9

2884Ma 100um

HN042T2 1355Ma

1759Ma

18

40

14

9

1151Ma

2

55

1

1332Ma

1960Ma

1522Ma

1434Ma

100um

HN148-1 5

64

86

68 29

670 Ma

60

814 Ma

907 Ma

981 Ma 989 Ma

1294 Ma 1511 Ma

1066 Ma

100um

HN048 57

8

25

28 513Ma

2 571Ma

840Ma

922Ma

63

14

1189Ma

948Ma

32

44

477Ma

2525Ma 100um

HN135

2

74

84

14

785Ma

42

966Ma

37

975Ma

54

1167Ma 1615Ma

39

1777Ma 2450Ma

Fig. 4. Cathodoluminescence (CL) images of selected zircons from the metasedimentary rocks of the Taowan and Kuanping groups. (a) Sericite quartz schist (HN077); (b) Muscovite quartz schist (HN079-2); (c) Quartzite (HN042-2); (d) Biotite quartz schist (HN148-1); (e) Metamorphic quartz sandstone (HN048); (f) Biotite quartz schist (HN135).

oscillatory growth zoning in the CL images (Fig. 4d), and have Th/U ratios in the range 0.05–1.66, indicating their magmatic origin (Pupin, 1980; Koschek, 1993). Meanwhile, some grains show narrow structureless rims (Fig. 4d), suggesting that they may have experienced late-stage metamorphism. Most of the U–Pb ages of eighty-nine zircons are concordant, and span from 666 to 3295 Ma (Fig. 5d; Table S2). Based on their ages, the zircons can be divided into five major populations of ca. 666–814, 907–1001, 1045–1534, 1751–2440 and 2538–3295 Ma with four major peaks at ca. 666, 913, 987 and 1609 Ma (Fig. 6d), showing a complex source. The three youngest data gave a mean 206Pb/238U age of 668 ± 13 Ma. Sample HN048-1: The majority of zircons separated from sample HN048-1 (metamorphic quartz sandstone) are euhedral–subhedral with clear internal structure (Fig. 4e), and with Th/U ratios from 0.17 to 1.41, indicative of magmatic origins (Koschek, 1993; Table S2). Sixty-one of the sixty-six zircons define concordant ages of around 513–3147 Ma (Fig. 5e; Table S2), and can be divided into four major populations of ca. 513–592, 812–1003, 1088–1496 and 1639–2548 Ma, with six major peaks at ca. 515, 581, 845, 944,

2442 and 2525 Ma (Fig. 6e). The two youngest 206Pb/238U ages gave a mean age of 515 ± 15 Ma. Sample HN135: Most zircons selected from the sample HN135 (biotite quartz schist) are subhedral-subrounded, and display clear internal structure in CL images (Fig. 4f). Some grains display a striped absorption, but others display an inherited core that is overgrown by a rim with fine-scale oscillatory zoning. Their Th/U ratios generally range from 0.10 to 1.98, indicating a magmatic origin (Pupin, 1980). Sixty-four zircons were analyzed and most of them yield concordant but variable ages. The U–Pb data define a broad range of ages between 477 and 2779 Ma (Fig. 5f; Table S2), and the largest population of zircons shows ages of 785–999 Ma and the subordinate populations display ages of 477–685, 1077–1181, 1457–1779 and 2430–2779 Ma (Fig. 6f). 6. Discussion The zircon U–Pb isotopic system is inherently stable and can survive multiple transportation episodes (Cherniak and Watson,

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H. Cao et al. / Precambrian Research 279 (2016) 1–16

a

0.55

b

HN077 ( Pinal Quartz Schist )

HN079-2 (Two-mica Schist) 3000

0.6

2600

2600

0.45

238

Pb/ U

2000

0.35

1800 1600

1300

1400

1400

1000

1500

1800

1400

0.25

1700

2200

0.4

206

1800

206

Pb/ 238U

2200

0.2

0.22

1100

1000

0.18

1200

0.15

900 0.14

0.18

1000

700

0.141

0.05 0

2

2

4

0.10 0.5

0.0

6

8 207

10

12

0

14

4

1.5

8

12 207

Pb/ 235U

16

20

Pb/ 235U

0.8

c

d

HN042-2 (Quartzite)

HN148-1 (Biotite Quartz Schist)

3400

3000

0.6

3000

0.6

2600 1900

Pb/ U

1800

238

1700

1500

2200

0.4

206

1500

206

Pb/ 238 U

2600 2200

0.4

1300

1800

1100

1300

1400

1400 0.2

1100

1000

900

0.18

0.2 900

0.12

0.14

700

700 0.10 0.5

0.0 0

4

1.5

0.08 0.5

0.0

8

12 207

16

20

24

0

4

8

1.5

12

16 207

Pb/ 235 U

20

24

28

Pb/ 235U

0.8

e

3400

3000 2600

2600

206

238

2200

0.4

Pb/ U

1000 900 800

1800

1100

1800 900

1400

700

1400

0.2 0.10

1000

600

700

1000 0.08

500

0.08

1300

2200

0.4

206

238

Pb/ U

HN135 (Biotite Quartz Schist)

0.6

3000

0.6

0.2

f

HN048 (Quartz Sandstone)

500

400 0.06 0.4

0.0 0

4

8

0.6

12 207

0.8

0.04 0.4

0.0 16

20

24

28

Pb/ 235U

0

4

0.8

8

12 207

16

20

Pb/ 235 U

Fig. 5. Zircon U–Pb concordia diagram for the metasedimentary rocks from the Taowan and Kuanping groups. (a) Sample HN077; (b) Sample HN079-2; (c) Sample HN042-2; (d) Sample HN148-1; (e) Sample HN048; (f) Sample HN135.

2000; Kosler and Sylvester, 2003; Wu and Zheng, 2004), meaning that zircon geochronology is an essential tool for tectonic reconstruction, yielding the ages of magmatic protolith with a precision of a few million years (Scherer et al., 2007). Modern techniques have enabled the identification of detrital zircon age distributions to be used to constrain both the maximum age and the provenance of deposition, thereby permitting the identification of the tectonic evolution of ancient orogenic belts and basins, a key factor for understanding continental dynamics (Meng et al., 2010; Wan et al., 2011b; Yao et al., 2011; Zhu et al., 2011; Pereira et al., 2012).

6.1. Deposition age of the sedimentary sequences 6.1.1. Taowan Group According to the paleontological evidence, rock associations and regional stratigraphic correlation, previous researches suggest that the Taowan Group were deposited in very different times (Wang and Liu, 1982; Wang, 1983, 1985; Peng and Tu, 1984; Zhang, 1984; Hu and Huang, 1987; Wang et al., 1989, 2007; Zhang and Li, 1989; Zhang et al., 1994a). However, no precise depositional ages determined by geochronological data for the

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H. Cao et al. / Precambrian Research 279 (2016) 1–16

Relative probability

10

a

~1859

HN077

8

~1512

~1776

6

~1118 ~1300

~2024

4

~2482 2

Relative probability

0

~861

~1440

6

~995

~1990

~1624

4

~2153

~2499

~2894

2 0 12

Relative probability

b

~1207

HN079-2

8

~1572

~1053

HN042-2

c

~1967

10

~1893

~1391

8 ~864

6 ~781 4

~2091

2 0 ~1069

Relative probability

d

HN148-1

25

~987

20

~913

15 ~666

10

~1292

~1514

~2791

~2436

~814

5

Relative probability

0 ~944

HN048

8

~581

e ~2442

~845

6

~2525

~515 4

~1718 ~1833

~1191

~2829

2

~3147

Relative probability

0 ~966

HN135

16

f

12 ~1849 8 ~1172

~1608-1654

4

~2648 ~2778

~1776

0

Relative probability

g

Southern NCC

160

~271 120

~2504 80

~1858

~427

40

0

Relative probability

80

h

~926

NQB ~402

60

40

~715

~1600

20

~1500

0

Northern YZC

Relative probability

400

I

~818

300

~2000

200

~2486 ~2652

~1855 100

0

0

400

800

1200

1600

2000

2400

2800

3200

3600

Age(Ma) Fig. 6. Zircon U–Pb age spectra of the studied metasedimentary rocks from the Taowan and Kuanping groups (a–f), and other rocks from the southern NCC (g), NQB (h) and YZC (i). Zircon ages of the studied samples with concordance range from 0.9 to 1.1 have been included in figures a–f. Other data sources: southern NCC (Yang et al., 2009, 2014; Li et al., 2010; Zhu et al., 2011; Diwu et al., 2012); the NQB (Lu et al., 2006; Shi et al., 2009; Yan et al., 2010; Wan et al., 2011a; Diwu et al., 2014); northern YZC (Zhang et al., 2006b; Liu et al., 2008b; Wang et al., 2012, 2013b; Chen et al., 2013; Wang et al., 2013a; Yin et al., 2013).

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H. Cao et al. / Precambrian Research 279 (2016) 1–16

Taowan group have been reported, which restricts our understanding of the depositional age of the Taowan Group. In this paper, LA-ICP-MS U–Pb zircon dating of two samples from the Taowan Group provides evidence for the maximum depositional age of the group. The youngest concordant detrital zircon ages can be used to constrain the maximum depositional age of the sediments (Meng et al., 2010; Wang et al., 2014). Hence, the youngest detrital zircon age (1111 ± 48 Ma) of Sample HN077 from the Taowan Group suggests that the protolith of Sample HN077 was deposited after the Jixianian System, and the detrital zircon age populations of Sample HN079-2 indicate a deposition age younger than the Qingbaikou System (871 ± 33 Ma), suggesting that the Taowan Group cannot represent a standard stratigraphic sequence combined with their sampling locations in the stratigraphy (Fig. 2) and the geological section of the Taowan Group from Wang et al. (2011a). In addition, the ages of late-stage intrusive rocks enable us to further constrain the timing of deposition of the sediments (Wang et al., 2014). The formation ages of the Neoproterozoic gabbros (830–826 Ma; Wang et al., 2011a), intruding into the Taowan Group in the Luanchuan area and not far away from the two studied samples, suggests that the depositional ages of the two studied samples can be better constrained between 1111 and 830 Ma, indicating they were deposited during the late Mesoproterozoic-early Neoproterozoic interval, in accordance with the paleontological evidence (after the Qingbaikou Period) for the same unit (Wang and Liu, 1982; Wang, 1983, 1985; Peng and Tu, 1984). 6.1.2. Kuanping Group In order to provide a further constraint on the depositional age of the Kuanping Group, four metasedimentary rocks (HN042-2, HN148-1, HN048-1 and HN135) collected from the Kuanping Group were dated. The youngest U–Pb ages of the four metasedimentary rocks indicate that their protoliths were deposited later than
781,
668,
515 and
477 Ma, respectively. Together with the recently published zircon U–Pb dating, samples from the Kuanping Group yield a wide range of maximum depositional ages from 785 to 460 Ma (Fig. 2), with all of them having different depositional ages (e.g., 600 Ma, Diwu et al., 2010; 640 Ma, Zhu et al., 2011; 580 Ma, Liu et al., 2013a; 570, 550 and 460 Ma, Shi et al., 2013), indicating that the Kuanping Group was composed of different ages of depositional units, which had later depositional ages than those from the Taowan Group. This is unexpected from a stratigraphical consideration (Zhang et al., 1991; Zhang et al., 1994a; Zhu et al., 2011), and further suggests a tectonic contact between the Kuanping and Taowan groups. On the other hand, previous researches indicate that the formation ages of the meta-mafic rocks in the Kuanping Group are 1445 Ma (Dong et al., 2014), 943 Ma (Diwu et al., 2010) or 611 Ma (Yan et al., 2008) (Fig. 2), which are markedly older than the depositional ages of the sedimentary rocks from the same unit. And these meta-basalts are characterized by N-MORB-, E-MORBand T-MORB-type tholeiitic basalts (Diwu et al., 2010; Dong et al., 2014), representing remnants of oceanic crust. Obviously, it is impossible for the MORB-type tholeiitic basalts and terrigenous clastic rocks formed in the same environment (Diwu et al., 2010), implying that the units in the Kuanping Group cannot represent a standard stratigraphic sequence. Zhang et al. (1991) and Diwu et al. (2010) suggested that the Kuanping Group is a nonSmithian stratigraphic group, and consists of many structural slices of different ages, further suggesting that the Kuanping Group is not a stratigraphic unit but a tectonic unit. Consequently, we support the idea of the Kuanping Group as a subduction-accretion complex (i.e. an accretionary wedge) with a

metamorphic MORB-terrigenous-carbonate rock association of different ages, rather than a continuous depositional sequence (Zhang et al., 1991; Ratschbacher et al., 2003, 2006; Hacker et al., 2004; Diwu et al., 2010; Liu et al., 2013a; Shi et al., 2013). 6.2. Provenance of the sedimentary sequences The U–Pb age spectrum (Fig. 6) outlined for detrital zircons of two metasedimentary rocks from the Taowan Group (Samples: HN077 and HN079-2) and one sample from the Kuanping Group (Sample HN042-2) show that their sediments obtained material mainly from Mesoproterozoic-early Neoproterozoic (1.62 Ga– 781 Ma) and middle Paleoproterozoic (2.15–1.78 Ga) sources, and few from the Neoarchean sources (
2.50 Ga); and the age peaks of detrital zircons of three metasedimentary rocks from the Kuanping Group (Samples: HN148, HN048 and HN135) are concentrated at 1.07 Ga–814 Ma, 666–476 Ma, 1.88–1.13 Ga, and 2.83–2.44 Ga, suggesting a mixed provenance of early-middle Neoproterozoic, late Neoproterozoic-early Ordovician, late Paleo proterozoic–Mesoproterozoic and a few Neoarchean materials. These analyzed metasedimentary rocks show low maturity, implying short-distance transportation. Thus, the possible provenances for the protolith of the metasedimentary rocks are the adjacent continents, e.g. the NCC, the NQB or the YZC. Recently, abundant Mesoproterozoic-early Ordovician isotopic ages have been identified in the NQB and the YZC, such as: ① the gneisses, meta-mafic rocks, amphibolites and volcanic rocks with formation ages of 1575–1545, 1429–1422, 1382–1310, 1249–1243, 975–843, 797–726, 686–623, 594–520 and 477 Ma from the Qinling and Danfeng groups in the NQB (Yang et al., 2003, 2010; Shi et al., 2009, 2013); ② voluminous Neoproterozoic metagranitoids and Cambrian-early Ordovician granitic rocks that intruded into the Qingling and Erlangping groups, and the northern YZC—represented by strongly deformed S-type granites at 979–911 Ma, weakly deformed I-type granites at 894–815 Ma, A-type granites at 759–711 Ma and I-type granites at 507– 470 Ma (Xue et al., 1996a; Lu et al., 2003; Zhang et al., 2004, 2006a; Wang et al., 2005b; Liu et al., 2006; Niu et al., 2006; Pei et al., 2007; Bao et al., 2008; Wang et al., 2009c, 2013c); ③ meta-basalts with formation ages at 1445, 943 and 611 Ma in the Kuanping Group (Yan et al., 2008; Diwu et al., 2010; Dong et al., 2014) and the gabbros emplaced in the Taowan Group at ca. 830 Ma (Wang et al., 2011a); and ④ eclogites and granulites with protolith ages of ca. 843–573 Ma in the Qinling Group (Chen et al., 2004b, 2006; Chen and Liu, 2011; Wang et al., 2011b; Liu et al., 2013b). Then, taking into account the absence of Mesoproterozoic-early Ordovician magmatic activities and tectonic-thermal events in the NCC (Fig. 6g, Yang et al., 2009; Diwu et al., 2012, 2014), we can reasonably infer that the geological units and bodies in the NQB and the YZC were probably the main source for the Mesoproterozoic-early Ordovician detrital zircons in the Taowan and Kuanping groups. Generally, the peak values of
2.51 and
1.78 Ga is similar to those of debris from the NCC, and since they have also been identified from the YZC (Fig. 6i, Douling Group, Kongling), the Qinling and the Danfeng groups (Hu et al., 2013; Liu et al., 2008a, 2013a; Shi et al., 2013; Wu and Zheng, 2013; Li et al., 2014), the 2.5–2.4 Ga and 1.8–1.7 Ga peaks cannot be uniquely considered as indicative of the NCC affinity. Moreover, the subordinate age populations of ca. 2.0 Ga zircons in the Mesoproterozoic-early Neoproterozoic metasedimentary rocks (samples: HN077, HN079-2, HN042-2 and HN148-1) from the two groups are widely recorded in the northern YZC (Fig. 6i, Zhang et al., 2006b; Liu et al., 2008b; Li et al., 2014), and only a few were found in the NQB. On the other hand, the paleomagnetic data indicate that the NCC and the YZC were located far away from each other during the

H. Cao et al. / Precambrian Research 279 (2016) 1–16

Meso-Neoproterozoic (Li et al., 2008), it was impossible that the sedimentary materials of the Taowan and Kuanping groups were derived from both the cratons at the same time. Therefore, we conclude that the material source for the Mesoproterozoic–middle Neoproterozoic sedimentary rocks (samples: HN077, HN079-2, HN042-2 and HN148-1) in the Taowan and Kuanping groups are mainly from the NQB and subordinately from the YZC (Liu et al., 2013a; Wu and Zheng, 2013), rather than the NCC. The 759–711 Ma and
2.0 Ga zircons are poor in the early Ordovician sedimentary rocks (samples: HN048 and HN135) from the Kuanping Group, which is inconsistent with the common occurrence of such zircons in the northern YZC, suggesting the absence of the YZC material in the source provenance of the early Ordovician sedimentary rocks. Combined with the limited presence of detrital materials of ca. 2.83–2.44 Ga in the samples, which are remarkably similar to the S-NCC basement, it is reasonable to suggest that the early Ordovician sedimentary rocks were largely produced from the NQB with minor contribution from S-NCC basement, rather than from the YZC. It is possible that some tectonic slices with an affinity of the S-NCC were involved in the mélange during collisional deformation. 6.3. Tectonic implications 6.3.1. The boundary of the NQB and the NCC The age populations and predominant peak ages of detrital zircon grains from the Mesoproterozoic–middle Neoproterozoic metasedimentary rocks in the Taowan and Kuanping groups indicate that their provenance were mainly attributed to the NQB and subordinately from the YZC, and limited from the NCC, suggesting that the protoliths of the two groups both came from the NQB and the NQB had been at some distance from the NCC before the formation of the Taowan and Kuanping groups. In addition, the Taowan Group was previously believed to be the southernmost lithostratigraphic unit of the S-NCC (Wang, 1985; Zhang et al., 1994a, 2001; Li and Sun, 1999; Sun et al., 2002; Wang et al., 2009a; Shi et al., 2013), and the Luonan–Luanchuan Fault between the Taowan and Kuanping groups was regarded as representing the boundary of the NCC and the NQB. Whereas, both the Taowan and Kuanping groups have already been confirmed as belonging to the NQB, rather than being the part of the S-NCC. Thus, combined with the fact that the Taowan Group is mainly composed of mixed terrigenous clastics and carbonates with different ages and also has a non-Smithian stratigraphic sequence, we propose that the present Taowan and Kuanping groups both represent the subductionaccretion complex in the northern part of the NQB formed in the southward subduction of the Kuanping Oceanic Plate beneath the NQB, and the boundary of the NQB and the NCC should be placed to the north of the Taowan Group (Han and Zhang, 1995), which can be referred to as the Paleo-Luonan–Luanchuan Fault (Fig. 1c). 6.3.2. The NQB – an independent microcontinent Based on the above scenario, it can be reasonably inferred that the NQB can’t be a southern sector of the NCC as advocated by some previous scholars (Zhang et al., 1994a; Zhang et al., 1996a, 2001; Meng and Zhang, 1999, 2000; Dong et al., 2011a; Wu and Zheng, 2013), but perhaps has a tectonic affinity with the YZC. However, the Nd–Pb whole-rock isotopes, zircon Hf isotopes and trace element compositions of the Precambrian basement rocks of the NQB, the YZC and the NCC are significantly different from each other (Ouyang and Zhang, 1996; Zhang et al., 1996b; Dong et al., 2003; Zhu et al., 2011), suggesting that the NQB was more likely to be an independent microcontinent. Recently, Diwu et al. (2012) obtained four age groups of 2600–2400, 1000–850, 450–350 and 250–170 Ma from the detrital zircons collected from modern rivers in the NQB, and this age spectrum is different from

11

those in the NCC and the YZC, furthering the notion of a discrete microcontinent. In addition, the Neoproterozoic granitoids in the N-YZC are characterized by the 894–815 Ma I-type and 759–711 Ma A-type granitoids, while the NQB mainly consists of the 979–911 Ma S-type and 889–844 Ma I-type granitic rocks (Wang et al., 2013c). Moreover, all of the early Neoproterozoic S-, I-, and A-type granites in the NQB and the N-YZC can be interpreted as the products of syn- and post-collisional processes. This could be closely tied to the Grenvillian Orogeny and a response to the assembly and breakup of the supercontinent Rodinia during the early Neoproterozoic (Lu et al., 2003; Wang et al., 2003, 2013c; Chen et al., 2006; Li et al., 2008; Liu et al., 2013a), thus indicating that the NQB and the YZC had experienced a similar but different evolution history from each other in the Neoproterozoic. All of these observations suggest that the NQB was an independent microcontinent, perhaps adjacent to the YZC from Mesoproterozoic to Neoproterozoic. 6.3.3. The Kuanping Ocean – not a back-arc basin The protolith of the Taowan Group, the northernmost geological unit of the NQB, mainly occur as mixed deposits of terrigenous clastics and carbonates, which were documented to represent deposition on a shallow-marine passive continental margin (Wang et al., 1989; Liu et al., 1993); and the large number of tholeiitic basalts in the Kuanping Group, which are characterized by N-MORB, T-MORB, or E-MORB (Zhang and Zhang, 1995; Yan et al., 2008; Diwu et al., 2010; Dong et al., 2014), represent the remnants of the Kuanping oceanic crust. Combined with the first late Mesoproterozoic sedimentary record (Sample HN079-2, 1111 Ma) in the Taowan Group, to be reported using precise geochronological data according to our study, and the oldest MORB-type basalt with age of 1445 Ma (Dong et al., 2014) in the Kuanping Group, it is obvious that the Kuanping Ocean had already existed to the north of the NQB in the Mesoproterozoic, much older than the occurrence of the Shangdan Ocean to the south, which was represented by the mafic and ultramafic rock assemblages (traditionally named as the Danfeng ophiolite or the Danfeng Group) with ages of ca. 534–422 Ma in the YuanyangzhenWushan, Guanzizhen, Tangzang, Yanwan, Xiaowangjian and Danfeng regions (Dong et al., 2011a,b). And it has been confirmed that the NQB was an independent microcontinent and had no affinity with the NCC from the Mesoproterozoic to Neoproterozoic, further suggesting that the Kuanping Ocean was a major ocean basin rather than a back-arc basin caused by the northward subduction of the Shangdan Ocean underneath the southern NCC, and thus most likely to be a part of the Panthalassic Ocean before the late Mesoproterozoic. 6.3.4. Tectonic evolution of the NQB during Mesoproterozoic to early Devonian The most striking feature of detrital zircon age populations in the NQB is 1.0–0.84 Ga with a peak at 926 Ma (Fig. 6h), which is in good agreement with the early Neoproterozoic magmatic rocks in the NQB, e.g., the widely developed 979–911 Ma S-type and 889–844 Ma I-type granitic rocks intruded into the southern part of the Qinling Group (Wang et al., 2013c), the Songshugou Ophiolite with a formation age of ca. 1.03 Ga (Li et al., 1991; Wang et al., 2003; Lu et al., 2005; Chen et al., 2006; Dong et al., 2008), and the MORB-type metabasalts in the Kuanping Group with a formation age of 0.93 Ga (Diwu et al., 2010). The 979–911 Ma S-type granites and the coeval metamorphism, which represent a collisional orogenic setting, just recognized in the southern part of the Qingling Group and were absent in the Taowan and Kuanping groups, suggesting the Grenville Orogeny only took place in the southern part of the NQB and caused the southern margin of the NQB to assemble to the Supercontinent Rodinia, while the northern

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H. Cao et al. / Precambrian Research 279 (2016) 1–16

(a) ~1111 Ma Qinling microcontinent

Uncertain paleo-continent

Taowan Group

QLM Panthalassic Ocean

Kuanping Ocean (part of the Panthalassic Ocean)

(b) ~1000-910 Ma Grenville orogen

Songshugou ophiolite Taowan Group

Uncertain paleo-continent

QLM

Super contin ent Rodini a

Kuanping Ocean

(c) ~844-726 Ma Superco ntinent Rodinia

Shangdan Ocean

Sendimentary rocks from the Kuanping Group Kuanping Ocean

(d) ~490-440 Ma Yangtze craton

Uncertain paleo-continent

QLM

Erlangping island arc Shangdan Ocean

the subduction-accretion complex of the Kuanping and Taowan Groups

QLM

North China Craton Kuanping Ocean

(e) ~440-400 Ma Qinling group

Yangtze craton

Erlangping group

Kuanping and Taowan Groups

North China Craton Shangdan Ocean

Fig. 7. Simplified cartoons showing the proposed tectonic evolution of the Kuanping Ocean from Mesoproterozoic to early Paleozoic.

margin of the NQB was facing toward the Kuanping Ocean attributed to the existence of the MORB-type metabasalts (ca. 0.93 Ga, Diwu et al., 2010) in the Kuanping Group. And the Songshugou ophiolite represents a fragment of oceanic lithosphere that once separated the North Qinling from the main part of the Supercontinent Rodinia before the Grenville Orogeny occurred between the southern margin of the NQB and the Supercontinent Rodinia, while the 844 Ma I-type granites and the only 726 Ma A-type granite (the Fangzhuang Pluton) outcropping in the Qinling Group and the 840 Ma gabbros intruded into the Taowan Group can be considered as the products of the breakup of the Supercontinent Rodinia in the global background. Additionally, the 611 Ma MORB-type basalts (Yan et al., 2008) and the continuous succession of late Neoproterozoic to Middle Ordovician sandstones (Lu et al., 2009; Diwu et al., 2010; Zhu et al., 2011; Liu et al., 2013a; Shi et al., 2013) with limestones in the Kuanping Group suggest the Kuanping Ocean in the north of the NQB had been long-lived. At the same time, the ocean basin became progressive narrower and the NQB moved closer and closer to the NCC from the late Neoproterozoic to Early Ordovician interval, which is supported by the increase of the NCC material and decrease of the YZC material in the contemporaneous sedimentary rocks from the NQB. The occurrence of I-type granites and the mafic to felsic metavolcanic rocks geochemically similar to island-arc rocks in the Erlangping Group at ca. 490– 440 Ma suggest the Kuanping Ocean had subducted to the NQB

at the same time, leading to the mixture of the Kuanping oceanic slices (MORB) and the clastic–carbonate assemblage and the formation of the Taowan and Kuanping subduction-accretion complex. Then the nearly identical metamorphic ages of ca. 440– 400 Ma obtained from the Kuanping, Erlangping and Qinling groups might indicate the final collision of the NQB with the S-NCC during the Early Silurian (Liu et al., 2013a). Accordingly, we can propose the tectonic evolution of the NQB during the Mesoproterozoic to early Devonian as follows: (1) In the Mesoproterozoic, the NQB as an independent microcontinent (which is called here ‘‘Qinling Microcontinent”) located in the southern side of the Kuanping Ocean—most likely to be part of the Panthalassic Ocean at this time, and the Taowan Group began the early deposition on the northern passive margin of NQB (Fig. 7a); (2) During the period of ca. 1000–910 Ma, the southern margin of the NQB assembled to the Supercontinent Rodinia during worldwide Grenville orogenic events, and the northern margin of the NQB was still facing towards the Kuanping Ocean (Fig. 7b). Meanwhile the clastic–carbonate assemblage of the Taowan Group continued to deposit on the passive northern margin of the NQB; (3) At ca. 844–726 Ma, the NQB was split from the Supercontinent Rodinia to form an independent microcontinent again (Fig. 7c), and the Shangdan Ocean was initiated between the main part of the Supercontinent Rodinia and the southern margin of the NQB. The sedimentary rocks from the Kuanping Group began to deposite

H. Cao et al. / Precambrian Research 279 (2016) 1–16

on the northern passive margin of the NQB due to the long-lasting spreading Kuanping Ocean; (4) During the period of ca. 490 to 440 Ma, the NQB had moved adjacent to the NCC in the north – separated by the Kuanping Ocean, and the YZC in the south – separated by the Shangdan Ocean; and meanwhile, the northern Kuanping Ocean and the southern Shangdan Ocean both started their subduction beneath the NQB (Liu et al., 2013a; Zhao et al., 2015; Yu et al., 2015), respectively, which triggered voluminous arc volcanic rocks and I-type granites in the NQB, leading to the conversion of the northern margin of the NQB to an active continental margin (the Erlangping island arc) and the formation of the present Taowan and Kuanping subduction-accretion complex (Fig. 7d); (5) At ca. 440–400 Ma, the NQB collided with the northern NCC and the southern YZC in turn (Fig. 7e), causing the regional metamorphism obtained in the Kuanping, Erlangping and Qinling groups might indicate the final collision of the NQB with the SNCC during the early Silurian. 7. Conclusions The following conclusions are based on analyses of the detrital zircon U–Pb geochronology of selected rocks from Neoproterozoic to early Paleozoic sedimentary strata of the NQB, Central China. (1) The Taowan and Kuanping groups both represent the subduction-accretion complex in the northern part of the NQB, which formed as a consequence of the late Cambrian-early Silurian southward subduction of the Kuanping Oceanic Plate beneath the NQB. (2) The provenance of the Mesoproterozoic–middle Neoproterozoic metasedimentary rocks in the Taowan and Kuanping groups were mainly attributed to the NQB and subordinately from the YZC, suggesting that the two groups both belong to the NQB and the boundary line between the NQB and the NCC should be placed to the north of the Taowan Group, called the Paleo-Luonan–Luanchuan Suture. (3) The NQB was an independent microcontinent from Mesoproterozoic to Neoproterozoic and had experienced a unique geological history. (4) The Kuanping Ocean was a major ocean basin that lasted for a long time, rather than a back-arc basin caused by the northward subduction of the Shangdan Ocean underneath the southern NCC, and was most likely to have been part of the Panthalassic Ocean before the late Mesoproterozoic. Acknowledgements We thank the staff of the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, for their advice and assistance during U–Pb zircon dating using LA-ICP-MS. We also thank two anonymous reviewers for their thorough and constructive comments that helped to improve the manuscript. This work was financially supported by the National Natural Science Foundation of China (Grants 41190072, 41195009, and 41190070), the National Natural Science Foundation for Young Scientists of China (Grants: 41502042), the Foundation for Outstanding Young Scientist in Shandong Province (Grants: BS2014HZ020), the China’s Post-doctoral Science Fund (Grants: 2014M551959), the Fundamental Research Funds for the Central Universities (Grants: 201413058), and the Taishan Scholorship to Prof. Sanzhong Li. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.precamres.2016. 04.001.

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