The Geochemical Evolution of the Granitoid Rocks in the South Qinling Belt: Insights from the Dongjiangkou and Zhashui Intrusions, Central China

Lithos 278–281 (2017) 195–214

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The geochemical evolution of the granitoid rocks in the South Qinling Belt: Insights from the Dongjiangkou and Zhashui intrusions, central China

Fangyang Hu a,ShuwenLiua,⁎, Mihai N. Ducea b,c, Wanyi Zhang d, Zhengbin Deng a a Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education, Peking University, Beijing 100871, PR China b Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA c Faculty of Geology and Geophysics, University of Bucharest, Bucharest, Romania d Development Research Center, China Geological Survey, Beijing 100037, PR China article info abstract

Article history: Widespread Late Triassic granitoid rocks in the South Qinling Belt (SQB) represent excellent subjects to study the Received 6 October 2016 geochemical and geodynamic evolution of magmatic rocks during the collision between the North China Craton Accepted 29 January 2017 and South China Block. In this study, we report new geological, geochemical, zircon U–Pb geochronology and Available online 08 February 2017 zircon Hf isotopic data for the Dongjiangkou and Zhashui intrusions, two large igneous complexes typical of the SQB. We also summarize published geochemical data of similar granitoid rocks. Our results show that Keywords: the Dongjiangkou intrusion is composed of ca. 223–214 Ma quartz diorites, tonalites and granodiorites with South Qinling Belt ﬁ – Dongjiangkou and Zhashui intrusions abundant coeval ma c magmatic enclaves (MME), and the Zhashui intrusion consists of ca. 203 197 Ma Zircon geochronology and Hf isotopes monzogranites and K-feldspar granites with rare MME. The Dongjiangkou granitoid rocks are characterized by Petrogenesis high K2O, MgO, Rb, and Mg# values, but low TiO2, Sc, and A/CNK (molar Al2O3/(CaO + Na2O+K2O)) values,

Geochemical evolution with εHf(t) values of −18.1 to −0.4 and TDM2(Hf) values of 1141–2117 Ma. We suggest that the Dongjiangkou granitoid rocks are formed by magma mixing between 70%–90% melts derived from partial melting of the Neoproterozoic basement rocks of the SQB and 10%–30% mantle-derived maﬁc melts with minor involvement of Archean to Paleoproterozoic old basement materials of the SQB. The Zhashui granitoid rocks exhibit high

K2O, Rb, A/CNK, AMF(Al2O3/(MgO + FeOT)), Al2O3/TiO2 and Rb/Sr values, but low MgO, TiO2, Sc, CaO/Na2O values, with εHf(t) values of −3.5 to +2.3 and TDM2(Hf) values of 979–1300 Ma, suggesting they are mainly produced by partial melting of Neoproterozoic metamorphic greywackes mixed with minor amounts of mantle-

derived melts. The MME are characterized by low SiO2, Sr/Y ratio, but high MgO, K2O, and Rb contents, with εHf(t) values of −0.8 to +4.3 and TDM(Hf) values of 732–905 Ma, suggesting they are produced by partial melting of metasomatized continental lithospheric mantle. A ﬂare-up event is recognized during 225–205 Ma

and the lower SiO2 and Sr/Y of the granitoid rocks during this period indicate prominent involvement of

mantle-derived melts mixed with crustal melts. The granitoid rocks with higher SiO2 and Sr/Y during 225–205 Ma, however, suggest they are formed under a greater pressure than those of previous period

(235–225 Ma) granitoid rocks. Few granitoid intrusions are formed after 205 Ma, and they display high SiO2 with low Sr/Y reﬂecting that they may be produced by low pressure partial melting of crustal materials or may represent highly fractionated magmas in the upper crust. These geochemical variations show a regular change with their formation ages, reﬂecting the possible variation in petrogenesis and crustal thickness. We propose that the granitoid rocks may have been formed in a transitional setting from subduction to collision during ca. 235–225 Ma, and a slab break-off event followed during ca. 225–205 Ma. Slab detachment was completed during ca. 205–190 Ma. We propose that no intensive delamination/convective removal of lower crust occurred in the SQB. © 2017 Elsevier B.V. All rights reserved.

1. Introduction

Geochemical characteristics of granitoid rocks were used for decades to constrain their tectonic evolution (Atherton and Ghani, 2002; ⁎ Corresponding author at: School of Earth and Space Sciences, Peking University, Beijing 100871, PR China. Barbarin, 1999; Batchelor and Bowden, 1985; Maniar and Piccoli, E-mail address: [email protected] (S. Liu). 1989; Pearce et al., 1984; Pitcher, 1983). Some recent studies proposed

that chemical indices, such as Ce/Y, Sr/Y and (La/Yb)N, could be served upper crust (Dong and Santosh, 2016; Dong et al., 2012a; Yang et al., as proxies for crustal thickness in magmatic arcs and further to evaluate 2012a; Zhang et al., 2008). Overall, the tectonic evolution of the QOB the tectonic evolution of orogenic belts (Chapman et al., 2015; remains controversial and the magmatic rocks of the region provide Chiaradia, 2015; Ducea et al., 2015; Mantle and Collins, 2008; Profeta the most continuous geologic archive available for its resolution. et al., 2015). However, these studies focused mainly on subduction- In this study, we study two sizable intrusive suites (Dongjiangkou related magmatism, and only few studies applied these indices to the and Zhashui intrusions), which formed during the second half of the continental collisional magmatism (e.g. Chung et al., 2009; Hou et al., Triassic, and show common characteristics with other coeval previously 2012). Here, we apply these geochemical indices to granitoid rocks studied intrusions (Gong et al., 2009a, 2009b; Qin et al., 2010a; Yang that formed during transition from subduction to continental collision et al., 2009). We report new geological, petrological, whole-rock geo- in an ancient orogen and examine their implications for geodynamic chemical, and zircon U–Pb and Lu–Hf isotopic data on these intrusions, evolution. and compile previously published geochemical data of other granitoid The Qinling Orogenic Belt (QOB) is one of the most important intrusions in the SQB. We conclude that the Dongjiangkou granitoid continental collisional orogens in east continental Asia. It connects the rocks are formed by magma mixing between 10–30% melts from metaso- Qilian-Kunlun Orogens to the west and Dabie–Sulu Orogens to the matic lithospheric mantle and 70–90% melts from Neoproterozoic base- east, and it is located between the North China Craton (NCC) and ments, whereas the Zhashui granitoid rocks are mainly formed by South China Block (SCB) (Diwu et al., 2012; Wu and Zheng, 2013). partial melting of Neoproterozoic crustal materials. The systematic Widespread Mesozoic granitoid intrusions in the QOB, especially Early changes between ages and geochemical characteristics of granitoids are Mesozoic granitoids in the South Qinling Belt (SQB), provide targets to also observed and it can reflect geochemical and geodynamic evolution unravel the geodynamic evolution of the orogen by using petrologic in the QOB. and geochemical data (e.g. Dong and Santosh, 2016; Jiang et al., 2010; N. Li et al., 2015; Wang et al., 2015; Yang et al., 2012a). A plethora of 2. Geological setting and sampling recently published chronological data from this region reveals that granitoids were intruded during 248–190 Ma time interval; the 2.1. The South Qinling Belt petrogenesis of these granitoid intrusions is still debated, especially for granitoids formed during the ~225–190 Ma interval, which may The QOB is separated from the NCC by the Lingbao–Lushan–Wuyang hinder the use of geochemical indices (N. Li et al., 2015, and the fault (LLWF) in the north, and from the YZC by the Mianlue–Bashan– references therein; Wang et al., 2015; Yang et al., 2012a). Scholars of Xiangguang fault (MBXF) in the south (Fig. 1; Diwu et al., 2012; Dong this area usually considered that intrusions older than 235 Ma might and Santosh, 2016). There are three well-documented sutures in the be derived from partial melting of the subducted slab or mantle area: the Kuanping suture, Shangdan suture and Mianlue suture from wedge during subduction (Li et al., 2013; Yang et al., 2014). As for the the north to south (Dong and Santosh, 2016; Dong et al., 2014). There- ~225–205 Ma granitoid rocks, it is generally believed that most of fore, the QOB is subdivided into four tectonic domains: the southern them were formed by mixing between felsic and mafic magmas, margin of the NCC, North Qinling Belt (NQB), South Qinling Belt although the sources of felsic magmas are debated. Some suggested (SQB), and northern margin of the YZC, respectively (Fig. 1B; Diwu that the felsic magma is derived from partial melting of subducted et al., 2013; Dong and Santosh, 2016; Dong et al., 2014). Yangtze continental crust (Qin et al., 2010a, 2013) or subducted The SQB, bounded by the Shangdan suture zone to the north and sediments (Jiang et al., 2010; Zeng et al., 2014). In contrast, other the MBXF to the south, comprises some Precambrian basements researchers considered that felsic rocks formed by partial melting of which were unconformably overlain by Proterozoic (Sinian) carbonates, lower crust or basements of the SQB (Bao et al., 2015; Dong and Cambrian–Ordovician limestones, Silurian shales, Devonian– Santosh, 2016; Gong et al., 2009a; Hu et al., 2016a; Wang et al., 2011). Carboniferous limestones, and Permian–Triassic sandstones (Dong and Granitoid rocks that were emplaced during ~205–190 Ma are either Santosh, 2016; Yang et al., 2012b). These sedimentary sequences, highly fractionated I-type granites or may have been legitimate S-Type together with the Precambrian basements, were intruded by Mesozoic intrusions derived from partial melting of metasediments (Dong et al., granitoid intrusions (Deng et al., 2016; Dong and Santosh, 2016; Hu 2012a; Lu et al., 2016; Xiao et al., 2014; Yang et al., 2012a). et al., 2016a; Yang et al., 2012b). Early Paleozoic ophiolitic mélanges Most researchers accept that the closure of Mianlue ocean and and subduction-related volcanic and sedimentary rocks in the collision between the NCC and SCB occurred during the Middle to Late Shangdan suture zone also named as Danfeng Group, represent the Triassic based on the paleomagnetic data and the age of ultrahigh- remnants of Proto-Tethyan oceanic crust (Dong et al., 2011; Y. Li et al., pressure metamorphism in the Dabie–Sulu Orogenic Belt (Ames et al., 2015). The Mianlue suture zone consists chiefly of Paleozoic to Triassic 1996; Dong and Santosh, 2016; Dong et al., 2011; Lai et al., 2008; Li ophiolites and arc-related volcanic rocks, which marks a Triassic et al., 1993; Meng and Zhang, 1999; Wu and Zheng, 2013; Zhang collision event (Lai et al., 2008; Li et al., 2007). The middle Triassic and et al., 2006; Zhao and Coe, 1987; Zhu et al., 1998), whereas the evolution pre-Triassic strata in the Mianlue suture underwent intense deforma- of the QOB during Late Triassic to Early Jurassic is still unclear (Dong and tion, which is dominated by folds with brittle thrusting, suggesting the Santosh, 2016; Ratschbacher et al., 2003; Sun et al., 2002; Wang et al., collision began at Late Triassic (Li et al., 2007). The ~223 Ma ductile- 2009). In contrast, N. Li et al. (2015) proposed oceanic subduction brittle left-lateral strike-slip shear deformation and ~227–219 Ma operated during the Triassic and into the Middle Jurassic. Some others metamorphic ages of ophiolite mélanges indicated the collision should argued that granitoid intrusions emplaced during 248–190 Ma in the be occurred during 230–220 Ma (Chen et al., 2010; Li et al., 1999). SQB formed continuously from subduction to collisional environments Recently, some discrete Neoproterozoic volcanic rocks were also (Bao et al., 2015; Deng et al., 2016; Dong and Santosh, 2016; Jiang found in the Mianlue suture zone, which are the products of another et al., 2010; Liu et al., 2011a, 2011b; Wang et al., 2015; Yang et al., subduction process (Dong and Santosh, 2016; Lin et al., 2013). 2012a). The main Precambrian basement blocks in the SQB comprise the Recently, a new study proposed a slab tear tectonic model during Foping block, Douling Group, Wuguan Group, Yaolinghe Group, and ~225–205 Ma based on regional structural and sedimentary geology, Wudangshan Group (Shi et al., 2013; Zhang et al., 2001). Except for geochronology, petrochemistry and paleomagnetism (Hu et al., the Neoarchean TTG assemblages exposed in the Douping Group (Wu 2016a). Furthermore, it has been suggested that magmatism occurring et al., 2014), most other Precambrian basement blocks are composed during the ~200–190 Ma period took place under a post-collisional mainly of Meso- to Neoproterozoic greenschist facies metamorphosed setting, probably concomitant with a delamination event, because of volcano-sedimentary rocks (Dong et al., 2011, 2012b; Ling et al., 2008; the coeval deposition of redbeds and no deformation features in the Shi et al., 2013). A recent study reported a series of Neoproterozoic F. Hu et al. / Lithos 278–281 (2017) 195–214 197

Fig. 1. (A) Schematic diagram showing the North China Craton, South China Craton and Tarim Craton. The location of Qinling–Dabie orogenic belt is shown by the rectangle (after Wang et al., 2012). (B) Simplified geological map showing the distribution of tectonic units, sedimentary sequences, and the Early Mesozoic granitoid rocks in the Qinling Orogenic Belt, modified after Dong et al. (2011). The study area is indicated by the dashed rectangle. Abbreviations for intrusions (from west to east): ZC, Zhongchuan; WQ, Wenquan; MSL, Mishuling; BJ, Baoji; MB, Miba; XY, Xinyuan; JJP, Jiangjiaping; ZJB, Zhangjiaba; HJZ, Hejiazhuang; GTS, Guangtoushan; EDHX, Erdaohexiang; HSD, Huoshaodian; LB, Liuba; GQP, Gaoqiaopu; YB, Yangba; DA, Da'an; XB, Xiba; HY, Huayang; TB, Taibai; XCH, Xichahe; LCP, Longcaoping; WL, Wulong; LC, Laocheng; YZB, Yanzhiba; DJK, Dongjiangkou; CHS, Cuihuashan; ZS, Zhashui; CP, Caoping; SHW, Shahewan; LNS, Laoniushan. basement blocks containing mafic to felsic intrusive rocks in the SQB (Figs. 2 and 3A). Abundant mafic magmatic enclaves (MME) are (Hu et al., 2016b). preserved in the Dongjiangkou intrusion. They are round or elliptical Early Mesozoic granitoid intrusions are widely exposed in the SQB ranging from centimeter to meter in long dimension. The MME are (Fig. 1B). They include the Zhongchuan–Wenquan–Hejiazhuang– characterized by fine-grained hypidiomorphic granular texture and Mishuling intrusions and the Guangtoushan granitoid suite in the west- massive structure (Fig. 3C). Some of the MME contain plagioclase ern segment of SQB, as well as the Huayang–Wulong and Dongjiangkou phenocrysts and scarce K-feldspar phenocrysts, suggesting possibly granitoid suites in the eastern SQB (Fig. 1B; e.g. Deng et al., 2016; Dong phenocryst transfer from host granitoid rocks to the MME (Fig. 3C; et al., 2011; Liu et al., 2011a, 2011b; Yang et al., 2012a). Triassic granit- Barbarin, 2005; Pietranik and Koepke, 2014). oid intrusions also outcrop in the NQB (e.g. Baoji–Taibai–Cuihuashan We collected 21 samples from Dongjiangkou intrusion, including intrusions) and south of the Mianlue suture zone (e.g. Yangba-Da'an 6mafic magmatic enclaves (MME), and 2 tonalites, 12 granodiorites intrusion) (Dong and Santosh, 2016; Liu et al., 2011a). Geochronological and 1 monzogranite (Table 1). The granodiorite consists of quartz data indicate that these granitoid intrusions were emplaced during (24–27%), K-feldspar (24–27%), plagioclase with partially showing ca. 248–190 Ma, mainly ca. 225–200 Ma (Deng et al., 2016; Dong and zoned texture (33–37%), hornblende (5–10%), biotite (4–8%), with Santosh, 2016; Hu et al., 2016a; Liu et al., 2011a, 2011b; Qin et al., accessory mineral association of titanite, magnetite, apatite and zircon 2010a, 2013; Wang et al., 2015; Yang et al., 2011, 2012a, 2014; Zhang (Fig. 3G and H). The monzogranite sample contains more quartz (29%) et al., 2011, 2012). The N235 Ma granitoids are mainly subduction- and K-feldspar (30%), but less plagioclase (29%) and hornblende (4%). related andesites (~246–234 Ma) and adakites (245–238 Ma) (Li Tonalites show relatively less amount of quartz (21–23%) and et al., 2013; Qin et al., 2008; Yang et al., 2014). The 235–225 Ma K-feldspar (6–8%), but have more plagioclase (42–45%) and hornblende granitoids show significant ductile deformation and gneissic textures (13–15%). (Deng et al., 2016; Qin et al., 2013), whereas the b225 Ma granitoids The MME are mainly composed of hornblende (23–29%), biotite display mainly massive structures, and limited to no gneissic textures (18–31%), plagioclase (33–36%), K-feldspar (2–10%), and quartz especially for rocks formed b220 Ma (Deng et al., 2016; Dong and (3–11%), with accessory minerals of magnetite, apatite, titanite, and Santosh, 2016; Hu et al., 2016a; Qin et al., 2013). Subsequently, Late zircon (Fig. 3I and J). Some apatites in the MME have an acicular Mesozoic granitoid stocks, formed during ca. 160–140 Ma distributed shape, suggesting quenching of the mafic melt globules trapped in sporadically in the eastern SQB, which are thought to be formed by granitoid magma (Fig. 3J; Kocak et al., 2011). partial melting of lower basaltic crust (Yan et al., 2014).

2.2. Dongjiangkou intrusion 2.3. Zhashui intrusion

The Dongjiangkou intrusion, ca. 60 km south of the Xi'an city, The Zhashui intrusion is located to the east of the Dongjiangkou is mainly composed of porphyritic and coarse to medium-grained intrusion (Fig. 2), and intruded into felsic gneisses of the Qinling tonalites, granodiorites, and monzogranites, showing weak gneissic to Group in the north and Devonian Liuling Group in the south, with an massive structures (Figs. 2 and 3A–C; Gong et al., 2009a; Qin et al., outcrop area of ca. 260 km2 (Fig. 2; Gong et al., 2009b). The Zhashui 2010a). The intrusion emplaced into the sandy slates of the Devonian intrusion consists mainly of porphyritic and coarse to medium- Liuling Group in the south, with exposed area of ca. 400 km2 (Fig. 2; grained monzogranites and K-feldspar granites, with massive structures Qin et al., 2010a). Locally, the granitoid intrusion shows EW-trending (Figs. 2 and 3D–F; Gong et al., 2009b). The Zhashui intrusion also shows gneissic foliation, which is consistent with the NEE–SWW-trending of the similar trending to the long axis of the Dongjiangkou intrusion the long axis of the intrusion and to the regional structural trending (Fig. 2). 198 F. Hu et al. / Lithos 278–281 (2017) 195–214

Fig. 2. Simpliﬁed geological map of the Dongjiangkou and Zhashui intrusions.

We collected 12 samples from Zhashui intrusion, including 8 fluorescence (XRF) spectrometry at the Key Laboratory of Orogenic monzogranites and 4 K-feldspar granite (Table 1). The monzogranite Belts and Crustal Evolution, Ministry of Education, School of Earth and samples are composed of quartz (30–33%), K-feldspar (27–31%), plagio- Space Sciences, Peking University. The analytical precision is at 0.5% clase (22–27%), hornblende (4–6%), biotite (7–11%), together with for major element oxides, and detailed analytical procedures were titanite, magnetite, zircon and apatite as accessory minerals (Fig. 3K). given by Liu et al. (2004). The K-feldspar granite samples contain more quartz (34–36%) and The trace elements, including rare earth elements, were analyzed at K-feldspar (30–34%), but less plagioclase (12–18%) and hornblende the Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of (2–3%). Education, School of Earth and Space Sciences, Peking University. Pre- The MME are extremely rare in the Zhashui intrusion and much treatment of the sample powders was performed at Peking University, smaller relative to those of Dongjiangkou intrusion (Fig. 3D and F). with procedures described as follows (Guo et al., 2015). Firstly, powders The MME have round or elliptical shapes, and are a few centimeters to were weighed (25 mg) into Savillex Teflon beakers loaded into high- more than ten centimeters in dimension, and are characterized by pressure bombs, with HF and HNO3 mixing at 1:1 added. The beaker fine-grained hypidiomorphic granular texture (Fig. 3D). Some of the was heated for 24 h at 80 °C, and evaporated. Then, 1.5 ml HNO3 MME contain either plagioclase or K-feldspar phenocrysts, suggesting and 1.5 ml HF accompanied by 0.5 ml HClO4 were added after the possibly phenocryst transfer from host granitoid rocks to MME evaporation, and the beakers were capped for digestion within a high- (Fig. 3F; Barbarin, 2005; Pietranik and Koepke, 2014). temperature oven at 180 °C for 48 h or longer, until powders were

completely digested. Finally, the residue was diluted with 1% HNO3 to 3. Analytical methods 50 ml for determination. Trace elements, including rare earth elements (REEs), were measured using an Agilent 7500 ICP–MS at the Key Labo- Chemical analyses of major and trace elements were conducted ratory of Orogenic Belts and Crustal Evolution, Ministry of Education, upon a total of thirty-three representative whole-rock samples School of Earth and Space Sciences, Peking University. The international including six MME, ﬁfteen porphyritic and coarse to medium-grained standards GSR-1 (granite), GSR-2 (andesite) and GSR-9 (granodiorite) granitoid rocks that were collected from the Dongjiangkou intrusion, were used for analytical control. The measurement precision of trace and twelve porphyritic and coarse to medium-grained granitoid rocks elements is better than 0.5‰. from the Zhashui intrusion (Table 1). Among these samples, zircon U–Th–Pb and Lu–Hf isotopic analyses were performed on one MME, 3.2. Zircon U–Pb and Lu–Hf analyses three porphyritic and coarse to medium-grained granitoid rocks. Four representative samples, including one MME sample (14QL28–1 3.1.Majorandtraceelements from Dongjiangkou intrusion), one sample of granodiorite (14QL38–1 from Dongjiangkou intrusion), and two samples of monzogranites Whole-rock samples were washed and trimmed in order to remove (14QL39–1 and 15QL12–1 from Zhashui intrusion), were selected for weathered surfaces. The fresh portions were then pulverized and in-situ zircon U–Th–Pb and zircon Lu–Hf isotopic analyses. Zircon grains powdered in an agate mill to about 200 mesh for major and trace ele- were separated from the samples using conventional heavy liquid and ments analyses. Volatile contents (e.g., CO2 and H2O) were determined magnetic techniques and the grains were handpicked under a binocular by measuring the weight loss after heating the samples at 1050 °C for microscope to ensure purity. The sample zircons, together with a zircon 30 min. Major oxides were determined using an automatic X-ray U–Pb isotopic standard Qinghu, were cast in an epoxy mount, and were F. Hu et al. / Lithos 278–281 (2017) 195–214 199

Fig. 3. Outcrop photographs and photomicrographs of Dongjiangkou and Zhashui intrusions showing (A) coarse to medium-grained tonalite showing gneissosity from Dongjiangkou intrusion; (B) porphyritic to coarse to medium-grained granodiorite from Dongjiangkou intrusion; (C) elliptical MME in granodiorite containing K-feldspar phenocrysts from Dongjiangkou intrusion; (D) porphyritic to coarse to medium-grained monzogranite from Zhashui intrusion; (E) hand specimen of monzogranite from Zhashui intrusion; (F) minor elliptical MME in monzogranite containing K-feldspar phenocrysts from Zhashui intrusion. Photomicrographs of representative samples from Dongjiangkou and Zhashui intrusions showing (G) granodiorite having zoned plagioclase from Dongjiangkou intrusion; (H) titanite in granodiorite from Dongjiangkou intrusion; (I) ﬁne-grained MME containing mainly plagioclase and hornblende from Dongjiangkou intrusion; (J) acicular apatite in quartz from Dongjiangkou intrusion MME; (K) K-feldspar, titanite and biotite in monzogranite from Zhashui intrusion. Mineral abbreviations: Hb-hornblende, Bi-biotite, Ttn-titanite, Ap-apatite, Pl-plagioclase, Kf-potassic feldspar, Q-quartz. then polished to approximately a half of the average zircon grain 20–40 μm. Harvard zircon 91500 was used as the external standard and thickness. Prior to analysis, cathodoluminescence (CL) images were NIST610 as the reference standard for zircon U–Pb geochronological obtained using a FEL Quanta 200 FEG environmental scanning electron and trace element (U and Th) analyses. 207Pb/206Pb, 207Pb/235U and microscope (ESEM) at the Electron Microscopy Laboratory of Peking 206Pb/238U ratios were calculated using the GLITTER program (van University, based on which internal structures were characterize Achterbergh et al., 2001) and common Pb was corrected using the meth- and potential target sites for U–Pb dating and Lu–Hf analyses were od proposed by Anderson (2002). Age calculations and concordia plots determined. were made using Isoplot (ver 3.0) (Ludwig, 2003). The 91500 standard U–Th–Pb elements and isotopes analyses for zircons were conducted zircon has been dated at 1061.9 ± 3.8 Ma (2σ, n = 26; 206Pb/238Uage) on quadruple-based inductively coupled plasma mass spectrometry in this study (the recommended 206Pb/238U age is 1065.4 ± 0.6 Ma (ICP–QMS), equipped with a 193 nm laser. All four samples (14QL28–1, (2σ); Wiedenbeck et al., 1995). 14QL38–1, 14QL39–1 and 15QL12–1) at the Key Laboratory of Orogenic In-situ Lu–Hf analyses for zircons were carried out using a Neptune Belts and Crustal Evolution, Ministry of Education, Peking University. MC–ICPMS, equipped with a 193 nm laser. All four samples (14QL28–1, Detailed descriptions for analytic procedures were documented by 14QL38–1, 14QL39–1 and 15QL12–1) were analyzed at the State Key Liu et al. (2012). The laser beam is of 32 μm and ablated depth is Laboratory of Geological Process and Mineral Resources (GPMR), 200

Table 1 The sample location and petrographic features of MME and granitoids from Dongjiangkou and Zhashui intrusions.

Intrusion Sample no. Latitude Longitude Lithology Texture Mineral association

Dongjiangkou 14QL27–6 33°44′37″N 108°58′23″E MME Fine-grained Hb (29%) + Bt (23%) + Pl (36%) + Kf (4%) + Qrz (4%) + Mag + Ap + Ttn + Zr 14QL27–8 33°45′11″N 108°58′23″E MME Fine-grained Hb (25%) + Bt (24%) + Pl (34%) + Kf (7%) + Qrz (8%) + Mag + Ap + Ttn + Zr 14QL28–1 33°45′59″N 109°00′57″E MME Fine-grained Hb (23%) + Bt (18%) + Pl (33%) + Kf (10%) + Qrz (11%) + Mag + Ap + Ttn + Zr 14QL38–6 33°36′40″N 108°45′54″E MME Fine-grained Hb (28%) + Bt (31%) + Pl (33%) + Kf (2%) + Qrz (3%) + Mag + Ap + Ttn + Zr ZS03–03 33°47′02″N 109°01′53″E MME Fine-grained Hb (25%) + Bt (25%) + Pl (33%) + Kf (6%) + Qrz (7%) + Mag + Ap + Ttn + Zr ZS03–04 33°47′02″N 109°01′53″E MME Fine-grained Hb (24%) + Bt (27%) + Pl (34%) + Kf (5%) + Qrz (6%) + Mag + Ap + Ttn + Zr 14QL27–1 33°41′32″N 108°52′20″E Granodiorite Coarse to medium-grained Qtz (25%) + Kf (25%) + Pl (35%) + Hb (7%) + Bi (5%) + Ttn + Ap + Zr + Mag 14QL27–4 33°41′56″N 108°56′44″E Granodiorite Coarse to medium-grained Qtz (27%) + Kf (27%) + Pl (33%) + Hb (7%) + Bi (4%) + Ttn + Ap + Zr + Mag .H ta./Lto 278 Lithos / al. et Hu F. 14QL27–5 33°44′25″N 108°58′23″ E Monzogranite Coarse to medium-grained Qtz (29%) + Kf (30%) + Pl (29%) + Hb (4%) + Bi (4%) + Ttn + Ap + Zr + Mag 14QL27–7 33°44′37″N 108°58′23″E Granodiorite Porphyritic Phenocryst: Kf (4%) + Pl (2%); Matrix: Qtz (26%) + Kf (21%) + Pl (32%) + Hb (6%) + Bi (6%) + Ttn + Ap + Zr + Mag 14QL27–9 33°45′31″N 109°00′09″E Granodiorite Coarse to medium-grained Qtz (24%) + Kf (26%) + Pl (36%) + Hb (8%) + Bi (4%) + Ttn + Ap + Zr + Mag 14QL28–3 33°45′59″N 109°00′57″E Granodiorite Coarse to medium-grained Qtz (27%) + Kf (26%) + Pl (33%) + Hb (7%) + Bi (5%) + Ttn + Ap + Zr + Mag 14QL38–1 33°38′24″N 108°46′16″E Granodiorite Coarse to medium-grained Qtz (26%) + Kf (25%) + Pl (34%) + Hb (8%) + Bi (5%) + Ttn + Ap + Zr + Mag 14QL38–3 33°37′59″N 108°46′13″E Granodiorite Porphyritic Phenocryst: Kf (4%) + Pl (3%); Matrix: Qtz (25%) + Kf (20%) + Pl (32%) + Hb (7%) + Bi (6%) + Ttn + Ap + Zr + Mag 14QL38–4 33°37′44″N 108°46′01″E Granodiorite Porphyritic Phenocryst: Kf (5%) + Pl (2%); Matrix: Qtz (26%) + Kf (18%) + Pl (34%) + Hb (8%) + Bi (6%) + Ttn + Ap + Zr + Mag

– ′ ″ ′ ″ – 14QL38 5 33°37 25 N 108°45 55 E Granodiorite Coarse to medium-grained Qtz (25%) + Kf (26%) + Pl (35%) + Hb (6%) + Bi (6%) + Ttn + Ap + Zr + Mag 195 (2017) 281 14QL38–7 33°36′40″N 108°45′54″E Granodiorite Coarse to medium-grained Qtz (24%) + Kf (24%) + Pl (37%) + Hb (8%) + Bi (5%) + Ttn + Ap + Zr + Mag ZS03–01 33°47′02″N 109°01′53″E Granodiorite Coarse to medium-grained Qtz (25%) + Kf (25%) + Pl (35%) + Hb (7%) + Bi (5%) + Ttn + Ap + Zr + Mag ZS04–01 33°46′37″N 109°02′19″E Tonalite Coarse to medium-grained Qtz (23%) + Kf (8%) + Pl (42%) + Hb (13%) + Bi (10%) + Ttn + Ap + Zr + Mag ZS05–01 33°46′13″N 109°02′54″E Granodiorite Porphyritic Phenocryst: Kf (4%) + Pl (3%); Matrix: Qtz (25%) + Kf (20%) + Pl (32%) + Hb (8%) + Bi (6%) + Ttn + Ap + Zr + Mag – ′ ″ ′ ″

ZS06 01 33°45 47 N 109°03 11 E Tonalite Coarse to medium-grained Qtz (21%) + Kf (6%) + Pl (45%) + Hb (15%) + Bi (9%) + Ttn + Ap + Zr + Mag – Zhashui 14QL39–1 33°41′57″N 109°10′45″E Monzogranite Coarse to medium-grained Qtz (32%) + Kf (30%) + Pl (22%) + Hb (5%) + Bi (8%) + Ttn + Ap + Zr + Mag 214 14QL39–2 33°41′33″N 109°10′22″E Monzogranite Porphyritic Phenocryst: Kf (5%) + Pl (1%); Matrix: Qtz (33%) + Kf (24%) + Pl (21%) + Hb (4%) + Bi (10%) + Ttn + Ap + Zr + Mag 14QL39–3 33°41′10″N 109°10′18″E K-feldspar granite Porphyritic Phenocryst: Kf (5%) + Pl (1%); Matrix: Qtz (35%) + Kf (26%) + Pl (16%) + Hb (3%) + Bi (11%) + Ttn + Ap + Zr + Mag 14QL39–4 33°40′38″N 109°09′31″E K-feldspar granite Coarse to medium-grained Qtz (34%) + Kf (30%) + Pl (18%) + Hb (3%) + Bi (10%) + Ttn + Ap + Zr + Mag 14QL39–5 33°40′08″N 109°09′08″E K-feldspar granite Coarse to medium-grained Qtz (36%) + Kf (34%) + Pl (12%) + Hb (2%) + Bi (11%) + Ttn + Ap + Zr + Mag 15QL12–1 33°43′32″N 109°20′24″E Monzogranite Coarse to medium-grained Qtz (31%) + Kf (28%) + Pl (24%) + Hb (5%) + Bi (10%) + Ttn + Ap + Zr + Mag 15QL12–2 33°43′32″N 109°20′24″E Monzogranite Coarse to medium-grained Qtz (31%) + Kf (29%) + Pl (23%) + Hb (4%) + Bi (11%) + Ttn + Ap + Zr + Mag 15QL12–3 33°43′32″N 109°20′24″E Monzogranite Coarse to medium-grained Qtz (32%) + Kf (31%) + Pl (22%) + Hb (4%) + Bi (9%) + Ttn + Ap + Zr + Mag 15QL12–5 33°43′32″N 109°20′24″E Monzogranite Porphyritic Phenocryst: Kf (5%); Matrix: Qtz (31%) + Kf (23%) + Pl (23%) + Hb (5%) + Bi (9%) + Ttn + Ap + Zr + Mag 15QL12–7 33°43′32″N 109°20′24″E K-feldspar granite Coarse to medium-grained Qtz (35%) + Kf (32%) + Pl (18%) + Hb (3%) + Bi (10%) + Ttn + Ap + Zr + Mag ZS01–01 33°39′26″N 109°08′15″E Monzogranite Coarse to medium-grained Qtz (30%) + Kf (27%) + Pl (27%) + Hb (6%) + Bi (7%) + Ttn + Ap + Zr + Mag ZS02–01 33°39′32″N 109°06′41″E Monzogranite Coarse to medium-grained Qtz (32%) + Kf (29%) + Pl (24%) + Hb (4%) + Bi (9%) + Ttn + Ap + Zr + Mag

Abbreviation: Qtz, quartz; Kf, K-feldspar; Pl, plagioclase; Hb, hornblende; Bi, biotite; Ttn, titanite; Ap, apatite; Zr, zircon; Mag, magnetite. F. Hu et al. / Lithos 278–281 (2017) 195–214 201

China University of Geosciences, Wuhan. The instrumental conditions 218 Ma, with TDM(Hf) values of 732–905 Ma (Appendix Table S2; and data acquisition were described by Hu et al. (2012). The analyses Fig. 5). were conducted with a spot size of 44 μm, a hit rate of 8 Hz and laser Thirty zircon spots are analyzed for the monzogranite sample energy density of 5.3 J/cm2. Harvard zircon 91500, TEM and GJ-1 were 14QL39–1 from Zhashui intrusion. Sixteen analyses yield an older used as references during analysis, and Harvard zircon 91500 was weighted mean 206Pb/238U ages of 216 ± 1 Ma (MSWD = 0.65) used as the external standards for analyses. Off-line selection and inte- (Fig. 4F). The remaining fourteen analyses yield a weighted mean gration analytic signals, as well as isobaric interference and mass frac- 206Pb/238Uageof200±1Ma(MSWD=0.32)(Fig. 4F), which is consid- tionation correction of Lu–Hf isotopic ratios were performed by ered to represent the magmatic crystallization age of this rock. Eight 176 177 ICPMS–DataCal (Liu et al., 2010). The Hf/ Hf isotopic ratios of analyses with the weighted mean age of 216 Ma show εHf(t) values of standard zircons are 0.282308 ± 5 for 91500 (1σ, n = 22; the recom- −0.6 to +1.7, and TDM2(Hf) values of 1027–1151 Ma (Appendix mended value is 0.282308 ± 3; Blichert-Toft, 2008). Table S2; Fig. 5). Seven analyses corrected to their magmatic crystalliza-

tion age of 200 Ma exhibit εHf(t) values of −3.5 to +2.3, with TDM2(Hf) values of 979–1300 Ma (Appendix Table S2; Fig. 5). 4. Results A total of thirty zircon spots were analyzed for a monzogranite sample 15QL12–1 from Zhashui intrusion, and seven analyses fall 4.1. Zircon geochronology and Lu–Hf isotopes below the concordia curve, implying possible Pb loss (Appendix Table S1; Fig. 4H). Four analyses yield apparent 206Pb/238U ages of Four representative samples, comprising one MME sample 932 ± 15 Ma, 231 ± 4 Ma, 224 ± 4 Ma, and 191 ± 3 Ma, respectively, (14QL28–1 from Dongjiangkou intrusion), one granodiorite sample and six analyses yield a weighted mean 206Pb/238U age of 216 ± 2 Ma (14QL38–1 from the Dongjiangkou intrusion), and two samples of (MSWD =0.19) (Fig. 4H). The remaining thirteen analyses yield a monzogranites (14QL39–1and15QL12–1 from the Zhashui intrusion), weighted mean 206Pb/238U age of 201 ± 1 Ma (MSWD =0.48) were chosen for zircon U–Pb and Lu–Hf isotopic analyses (Figs. 4 (Fig. 4H), which is taken as the crystallization age of this sample. One and 5). The analytical results of the U–Pb isotopes and calculated ages analysis with the apparent age of 932 Ma show εHf(t) values of +12.5, are listed in Appendix Table S1, and the Lu–Hf isotopes and calculated with TDM(Hf) values of 978 Ma, and five analyses with ages of 231 Ma relevant parameters are listed in Appendix Table S2. Analyses that fall and 216 Ma have the εHf(t) values of +0.8 to +1.7, with TDM2(Hf) below the concordia curve, which could be the results of Pb loss influ- values of 1024–1074 Ma. Nine Lu–Hf isotopic analyses are corrected to enced by later thermal events, are not shown in the figures entirely their magmatic crystallization age of 201 Ma, and give the εHf(t) values but are listed in Appendix Table S1. of +0.3 to +2.3, with TDM2(Hf) values of 981–1091 Ma (Appendix As shown in representative CL images (Fig. 4), zircon grains from Table S2; Fig. 5). these four samples exhibit similar morphological characteristics. They are generally round to long-prismatic shapes, with lengths between 4.2. Whole-rock geochemistry 100 and 350 μm and length/width ratios of 1:1 to 3:1 (Fig. 5). CL images of zircon grains from the MME samples show mainly chaotic textures 4.2.1. Dongjiangkou intrusion and poorly-developed oscillatory zoning (Fig. 3A; Corfu et al., 2003). The major and trace element data of fifteen host granitoid rock CL images of zircon grains from granitoid rocks show clear oscillatory samples and six MME samples from the Dongjiangkou intrusion are zoning textures (Fig. 3C, E and G; Corfu et al., 2003). These analyses of shown in Appendix Table S3. The host granitoid rocks have intermedi- zircons from granitoid rocks exhibit Th and U contents range from 72 ate SiO2 (64.0–69.4 wt.%), and mainly plot in the quartz monzonite, to 2075 ppm and 69 to 2679 ppm, respectively, with Th/U ratios of tonalite and granodiorite fields and in the sub-alkaline field in the

0.19–2.18 (Appendix Table S1). Zircons grains from MME sample SiO2 vs. (Na2O+K2O) diagram (Fig. 6A). In the SiO2 vs. K2O diagram, exhibit higher Th (578–10,569 ppm) and U (737–5762 ppm) contents, they plot in the high-K calc-alkaline rock series (Fig. 6B). These samples with Th/U ratios of 0.39–2.34 (Appendix Table S2). Synthesis of their display high MgO (2.19–3.69 wt.%), Mg# (58–64) and Sr/Y (38.48– morphologies, inner structures and Th/U ratios, indicate their magmatic 78.94 ppm) values, with metaluminous to weakly peraluminous origin (Corfu et al., 2003; Rubatto, 2002). features (A/CNK = 0.87–1.05), (Appendix Table S3; Fig. 6). As shown

A total of thirty zircon spots were analyzed for a granodiorite sample in Fig. 6, they deﬁne a negative correlation between SiO2 and MgO, 14QL38–1 from Dongjiangkou intrusion (Appendix Table S1). Among CaO, P2O5,TiO2, and Sc contents, but positive correlations between these, two analyses (spot # 22 and 26) fall below the concordia curve, SiO2 and K2O, Rb and Th concentrations. They are characterized by indicating probably Pb loss. Four analyses yield older apparent steep chondrite-normalized REE patterns with high (La/Yb)N, (La/ 206 238 Pb/ U ages of 697 ± 14 Ma, 453 ± 9 Ma, 349 ± 7 Ma, and 232 ± Sm)N, and (Gd/Yb)N ratios of 12.43–21.45, 3.81–6.57, 1.98–2.33, respec- 5Ma(Fig. 4B). The remaining twenty-four analyses yield a weighed tively, and show weak to no negative Eu anomalies (δEu = 0.79–1.01) mean 206Pb/238U age of 217 ± 1 Ma (MSWD =0.35) (Fig. 6A), which (Fig. 7C, Appendix Table S3). In the primitive mantle-normalized is taken as the magmatic crystallization age of this sample. Fifteen multi-element patterns (Fig. 7D), these samples show obviously nega- isotopic analyses are corrected to their magmatic crystallization age of tive Nb–Ta anomalies, and moderately negative Ti anomalies.

217 Ma, and give εHf(t) values of −18.1 to −0.4, with TDM2(Hf) values The MME samples have lower SiO2 contents of 55.5 to 61.7 wt.% and of 1141–2117 Ma (Appendix Table S2; Fig. 5). higher K2O contents of 3.45 to 4.89 wt.% relative to host granitoid rocks, A total of thirty zircon grains from a MME sample 14QL28–1 from falling in the monzonites ﬁeld in the SiO2 vs. (Na2O+K2O) classiﬁcation Dongjiangkou intrusion were selected for the analysis (Appendix diagram (Fig. 6A), belonging to high-K calc-alkaline to shoshonite series

Table S1). Due to the possibly Pb loss, one analysis falls below the in the SiO2 vs. K2O diagram (Fig. 6B). These MME samples also show concordia curve and eight analyses yield an older weighted mean high concentrations of MgO (4.90–7.36 wt.%), CaO (4.45–6.75 wt.%), 206 238 Pb/ Uageof235±2Ma(MSWD=0.08)(Fig. 4D). The remaining P2O5 (0.22–0.38 wt.%), TiO2 (0.62–0.81 wt.%), Sc (13.3–22 ppm), and twenty-one analyses deﬁne a weighted mean 206Pb/238U age of 218 ± Rb (122–185 ppm), but low Th contents (3.64–14.4 ppm) and Sr/Y 1 Ma (MSWD = 0.24) (Fig. 4D), and this is considered to be the (18.89–50.11) ratios (Appendix Table S3; Fig. 6). In the chondrite- magmatic crystallization age of this sample. Five analyses given normalized REE diagrams (Fig. 7A), these samples are enriched in 206 238 the weighted mean Pb/ U age of 235 Ma show their εHf(t) values LREEs and depleted in HREEs, which are similar to those of the granitoid of +0.5 to +3.3, with TDM(Hf) values of 770–865 Ma (Appendix rocks, with mainly negative Eu anomalies ((La/Yb)N = 4.91–21.05, Table S2; Fig. 5). Ten analyses exhibit εHf(t)valuesof−0.8 to +4.3 (La/Sm)N =1.76–5.28, (Gd/Yb)N =1.89–2.73; δEu = 0.75–0.93). In while they are corrected to their magmatic crystallization age of the primitive mantle-normalized multi-element patterns (Fig. 7B), all 202 F. Hu et al. / Lithos 278–281 (2017) 195–214 F. Hu et al. / Lithos 278–281 (2017) 195–214 203

Fig. 5. Plots of zircon εHf(t) values versus crystallization ages for four dated samples. The old basements of the SQB is defined based on initial Hf isotope ratios from magmatic zircons in the Douling Archean TTG gneisses and detrital zircons from Wudangshan and Yaolinghe Groups (after Wang et al., 2013; Wu et al., 2014) at assumed 176Lu/177Hf ratios of 0.015. The Neoproterozoic basements of the SQB is defined based on initial Hf isotope ratios from detrital zircons in the Wudangshan and Yaolinghe Groups and newly found blocks in the SQB 176 177 (after Hu et al., 2016b; Wang et al., 2013) at assumed Lu/ Hf ratios of 0.015. Zircon εHf(t) values of previous studies on the Dongjiangkou and Zhashui intrusions are constructed using data from the following: Dongjiangkou intrusion (Gong et al., 2009a; Jiang et al., 2010, 2012; Ping et al., 2013; Qin et al., 2010a); Zhashui intrusion (Gong et al., 2009b; Ping et al., 2013). Reference lines representing meteoritic Hf evolution (CHUR) and the depleted mantle (DM) are from Blichert-Toft and Albarede (1997) and Griffinetal.(2000), respectively. The inset is histogram of εHf(t) values of Dongjiangkou granitoid rocks, MME and Zhashui granitoid rocks, including data of previous studies and our data. Numbers are equivalent to spot analyses given in Appendix Table S2.

MME samples show enrichment in large ion lithospheric elements 5. Discussion (LILEs, such as Ba and Rb) and are characterized by negative Nb–Ta anomalies and slightly negative Ti anomalies without P anomalies, 5.1. Ages of the Dongjiangkou and Zhashui intrusions which are similar to the trace element distribution patterns of host granitoid rocks. The granitoid rocks of the Dongjiangkou intrusion preserve inherited zircons with ages of ~697 Ma, ~453 Ma, ~349 Ma, and ~232 (Fig. 4B), and the granitoid rocks of Zhashui intrusion contain inherited 4.2.2. Zhashui intrusion zircons with ages of ~932 Ma, ~231 Ma, ~224 Ma, and ~216 Ma (Fig. 4F

The granitoid rocks from Zhashui intrusion exhibit higher SiO2 and H), whereas the MME sample from Dongjiangkou intrusion also contents than the other intrusion (68.38 to 72.2 wt.%), and plot into preserves some inherited zircons with the age of ~235 Ma (Fig. 6Band granodiorite and granite ﬁeld in the TAS classiﬁcation diagram, mostly E). The ~932 Ma and ~697 Ma ages from the inherited populations belonging to high-K calc-alkaline rock series (Fig. 6A and B). They exhib- may be connected to the emplacement ages of the Neoproterozoic it relatively high A/CNK ratios of 0.97 to 1.10, most of them belonging to magmatism in the SQB (e.g. Hu et al., 2016b). The zircon ages of peraluminous. These samples also display low MgO (0.51–1.42 wt.%), ~453 Ma and ~349 Ma may be captured from Paleozoic crustal materials

CaO (1.65–2.61 wt.%), P2O5 (0.07–0.16 wt.%), TiO2 (0.25–0.46 wt.%), Sc during magma transfer and emplacement (Deng et al., 2016; Qin et al., (3.88–5.55 ppm) concentration and Sr/Y (18.89–40.64) ratios, but 2010a). The ~232–231 Ma ages of the inherited zircons are consistent high K2O (3.40–5.60 wt.%), Rb (102–182 ppm) and Th (11.6–27) con- with the emplacement ages of the Triassic Carnian magmatism in the tents, with wide range of Mg# (34–55) values. In the chondrite- SQB (Jiang et al., 2010; Qin et al., 2013). The inherited zircon ages of normalized REE diagrams (Fig. 7E), these samples are characterized by ~224 Ma and ~216 Ma from Zhashui intrusion are consistent with wide- steep REE patterns with weakly negative to positive Eu anomalies spread Triassic Norian magmatism in the SQB (N. Li et al., 2015; Wang

((La/Yb)N = 14.45–24.45, (La/Sm)N =4.47–7.57, (Gd/Yb)N = et al., 2015). Therefore, these older zircons are considered to be 1.83–2.42; δEu = 0.62–1.11). In the primitive mantle-normalized inherited from either the source regions or captured from the wall- multi-element patterns, these samples show negative Nb–Ta anomalies, rocks of the intrusions and ascending channels of the magmas. and prominently negative P and Ti anomalies, and enrichment of Ba, Rb, The magmatic zircons from the Dongjiangkou granitoid rocks and Th, and K (Fig. 7F). MME show similar ages of 217 ± 1 and 218 ± 1 Ma. Previous studies

Fig. 4. Cathodoluminescence images and U–Pb concordia diagram of representative samples. (A and B) Dongjiangkou porphyritic to coarse to medium-grained granodiorite (14QL38–1); (C and D) Dongjiangkou ﬁne-grained MME (14QL28–1); (E and F) Zhashui porphyritic to coarse to medium-grained monzogranite (14QL39–1); (G and H) Zhashui porphyritic to coarse to medium-grained monzogranite (15QL12–1). The yellow and white dashed circles indicate zircon in-situ U–Pb and Lu–Hf isotope analyses with beam diameters of 32 μmand44μm, respectively. Apparent ages and εHf(t) values are also shown. Numbers are equivalent to spot analyses given in Appendix Table S1. 204 F. Hu et al. / Lithos 278–281 (2017) 195–214

Fig. 6. Petrochemical classiﬁcations for the samples from Dongjiangkou and Zhashui intrusions and their geochemical characteristics. (A) Total alkalis versus silica diagram (TAS, after

Middlemost, 1994). (B) K2OversusSiO2 classiﬁcation diagram (after Rollinson, 1993). (C) MgO versus SiO2 diagram (after Martin et al., 2005). (D) A/NK (molar Al2O3/(Na2O+K2O)) versus A/CNK (molar Al2O3/(CaO + Na2O+K2O)), (after Maniar and Piccoli, 1989). (E–I) Covariation diagrams of SiO2 versus selected whole-rock major oxides, trace elements and trace element ratios. The published MME data are from Hu et al. (2016a), Jiang et al. (2012), Qin et al. (2010a), and Wang et al. (2011). Abbreviation: LSA: low-SiO2 adakites; HAS: high-SiO2 adakites; PMB: melts obtained by experimental melting of basalts or amphibolites. Symbols: porphyritic to coarse to medium grained granitoid rocks from Dongjiangkou intrusion, blue diamond; MME from Dongjiangkou intrusion, blue empty diamond; porphyritic to coarse to medium grained granitoid rocks from Zhashui intrusion, red circle.

obtained the zircon 238U/206Pb ages of 223 Ma, 222 Ma, 220 Ma, 214 Ma, 5.2. Petrogenesis 211 Ma and 209 Ma for these granitoid rocks, and zircon 238U/206Pb ages of 222 Ma and 219 Ma for the MME from Dongjiangkou intrusion, which 5.2.1. MME are basically consistent with our new zircon U–Pb chronological data Mafic enclaves are present in virtually all intermediate volcanic (Gong et al., 2009b; Jiang et al., 2010; Liu et al., 2011a). Whereas the rocks and granitoid batholiths formed in classic Andean subduction sys- biotite 40Ar/39Ar age of 198 Ma was also obtained by previous study, tems such as those along the western margins of the Americas (Ducea which may be related to the later thermal event (Zhang et al., 2006). et al., 2015). They usually represent a signature feature of subduction re- Therefore, we suggest the Dongjiangkou granitoid rocks were mainly lated magmas in the Phanerozoic and late Proterozoic. The petrogenesis emplaced during 223–214 Ma, which were approximately coeval with of the MME in the granitoid rocks has long been debated, and three mafic magmas (MME). Our new zircon U–Pb chronological data reveal models were proposed as: (1) cogenetic cumulates or early segregation that the Zhashui intrusion mainly emplaced at ~200 Ma. Previous (autolith) from the host magma (Chen et al., 2016; Dahlquist, 2002; studies obtained a biotite 40Ar/39Ar age of 197 Ma and the zircon Donaire et al., 2005); (2) residual material from the partial melting of 238U/206Pb ages of 225 Ma, 213 Ma, 203 Ma, and 199 Ma from the the source rocks (restite) (Chappell et al., 1987, 2000); and (3) products granitoid rocks in the Zhashui intrusion, which were similar to our of magma mingling, i.e. globules of mafic magma that were injected into data except for the ages of 225 Ma and 213 Ma that may stand for a felsic magma chamber (Barbarin, 2005; Hu et al., 2016a; Jiang et al., early magmatisms (Gong et al., 2009b; Hu et al., 2004; Liu et al., 2012; Laumonier et al., 2015; Vernon, 1984). The magma mingling 2011a, 2013; Zhang et al., 2006). Therefore, we suggest the Zhashui model is widely accepted as the origin of enclaves in North America granitoid rocks were mainly emplaced during 203–197 Ma. Cordilleran geology (Ducea et al., 2015). F. Hu et al. / Lithos 278–281 (2017) 195–214 205

Fig. 7. Chondrite-normalized REE patterns and primitive mantle-normalized multi-element spider diagrams for samples from MME from Dongjiangkou intrusion (A and B); porphyritic to coarse to medium grained granitoid rocks from Dongjiangkou intrusion (C and D); porphyritic to coarse to medium grained granitoid rocks from Zhashui intrusion (E and F). Normalization values are after Sun and McDonough (1989). The published MME data are from Hu et al. (2016a), Jiang et al. (2012), Qin et al. (2010a),andWang et al. (2011). Legends as for Fig. 6.

The MME studied here contain acicular shape apatites and K- (e.g. ZS03–04), reflecting the MME are not altered by magma mixing feldspar phenocrysts, suggesting that the MME were formed by mixing significantly (Appendix Table S3; Fig. 8B and C). In spite of some sam- between mafic magma and felsic magma (Fig. 3). Moreover, their mag- ples are affected by magma mixing process, the MME samples display matic textures further indicate that they were formed by magma mixing relatively uniform geochemical features with low SiO2, Th and Sr/Y but not cumulates or restites (Fig. 3). Their relative wide range of zircon but high MgO, K2O, Sc, and Rb contents, and distinct evolution trend εHf(t) values (−17.7 to +13.3) also indicate magma mixing/mingling relative to host granitoid rocks (e.g. Rb vs. SiO2), indicating these process (Fig. 5). Some elements (e.g. K2O and Rb) of the MME from geochemical features of the MME are not modified and could be used the Dongjiangkou intrusion displaying nonlinear trends with host gran- to trace their source. Zircons are early crystallization phase during itoid rocks also argue against a cumulate or restite origin (Fig. 6; Chen magmatism and could retain their original geochemical information et al., 2016). Therefore, we suggest that the MME represent injected (Hoskin and Ireland, 2000). The zircons from the MME in this study magmatic globules of a mantle-derived magma into the felsic host exhibit higher εHf(t) values (−0.8 to +4.3) than the host granitoid magma. rocks (−18.1 to −0.4) and their relative consistent εHf(t) values further The MME from the Dongjiangkou show mainly low SiO2 (55.48– indicate these MME are not significantly altered by magma mixing 61.74 wt.%) and Th (3.64–14.4 wt.%) contents, but high MgO (4.90– (Fig. 5). Previous studies obtained wide range of zircon εHf(t) values 7.36 wt.%), TiO2 (0.62–0.81 wt.%), Sc (13.3–22 ppm) contents, and (−17.7 to +13.3) suggesting part of their samples are altered by Mg# values (62.18–69.27), indicating they are derived from a mantle mixing and it is obvious that the majority of zircons from the MME source (Figs. 6 and 8; Chen et al., 2009). However, because of the show chondritic-like εHf(t) values (Fig. 5). Therefore, we suggest that magma mixing process, the geochemical characteristics of the MME our samples are not significantly altered by magma mixing processes are inevitably modified by felsic magmas (Blundy and Sparks, 1992). and their geochemical features could be used to trace their source.

Compared with published MME data of Dongjiangkou intrusion and The somewhat surprisingly higher K2O and Rb concentrations of the adjacent Caoping and Shahewan intrusions, our MME samples from MME than those of their host granitoid rocks indicate that these MME the Dongjiangkou intrusion show basically the same geochemical are not greatly affected by felsic magma, but come from a distinct characteristics but have slightly higher SiO2 relative to the most maﬁc mantle source. Similarly, the MME in the adjacent intrusions also MME sample (Figs. 6 and 7). Additionally, MME show mainly lower show higher K2O and Rb contents, e.g. MME in the Caoping and Sr/Y ratios (average 37.43) relative to the host granitoid rocks (average Shahewan intrusions (Hu et al., 2016a; Jiang et al., 2012; Wang et al.,

53.51), although few samples with higher SiO2 display higher Sr/Y ratios 2011). Therefore, such distinct feature may not be from their alterations 206 F. Hu et al. / Lithos 278–281 (2017) 195–214

Fig. 8. Petrogenetic discrimination diagrams for MME and granitoid rocks from the Dongjiangkou and Zhashui intrusions. (A) Mg# versus SiO2 diagram (modiﬁed after Stern and Killian, 1996). It also shows the ﬁelds of pure crustal partial melts obtained in experimental studies by dehydration melting of amphibiotic rocks and eclogite (Rapp and Watson, 1995; Rapp et al.,

1999; Sisson et al., 2005; Smithies, 2000). The mixing trend illustrate mixing between MME and granitoid rocks starting with the highest SiO2 sample of the Zhashui granitoid rock and the lowest SiO2 MME sample published by Jiang et al. (2012). (B) Sr/Y versus Y diagram for the granitoid rocks from the Dongjiangkou and Zhashui intrusions (modiﬁed after Drummond and

Defant, 1990). (C) Chondrite-normalized (La/Yb)N versus YbN for the granitoid rocks from the Dongjiangkou and Zhashui intrusions (modified after Drummond and Defant, 1990). The fractionation trends are after Castillo (2012). (D) Eu/Eu* versus Sr diagram. Legends as for Fig. 6. during the magmatic mixing but are likely to be inherited from their et al., 1987, 2000; Figs. 3H and 6F). The abundance of the MME in the source. The MME show high Rb/Sr ratios (0.12–0.30) and low Ba/Rb Dongjiangkou intrusion suggest mantle-derived melts are involved ratios (5.29–12.89) that conform to melts in equilibrium with phlogo- during the formation of the granitoid rocks. They show higher MgO pite, which are expected to exhibit high K2O, Rb/Sr (N0.1) and low Ba/ and Mg# values than the experimental melts that are derived from Rb (b20) features (Furman and Graham, 1999). Although the MME the partial melting of pure crustal materials or basalts and amphibolites, have wide range of εHf(t) values suggesting magma mixing or crustal and the granitoid rocks plot along the trajectory of magma mixing contamination, most MME show relatively concentrated range with process in the Th/La versus Th, Th versus Th/Sc, and 1/Sc versus Th/Sc near-chondritic εHf(t) values and their Neoproterozoic TDM(Hf) ages diagrams, all of these indicate the these granitoid rocks were formed imply that they were mainly formed from a metasomatized continental by a magma mixing between mafic and felsic melts (Figs. 6C, 8A and lithospheric mantle (Fig. 5; Hu et al., 2016a). The MME also display 9A–C; Martin et al., 2005; Rapp and Watson, 1995; Schiano et al., similar Sr–Nd isotopic compositions to the lamprophyres and mafic 2010). The granitoid rocks show similar evolutionary trends to the dikes from the QOB, which were thought to be formed by partial fractional crystallization of plagioclase in Sr/Y versus Y and (La/Yb)N melting of a metasomatized mantle source (Qin et al., 2010a; Wang versus YbN diagrams, suggesting they may underwent plagioclase et al., 2011). Therefore, we suggest that the MME are formed by partial fractionation after the initial granitoid magma was formed (Fig. 8B–C; melting of phlogopite-bearing metasomatized continental lithospheric Castillo, 2012). However, these granitoid rocks don't show positive cor- mantle and contaminated by or mixed with felsic melts or crustal relation between Eu/Eu* and Sr indicating the fractional crystallization material subsequently. of plagioclase is not a major process in their petrogenesis (Fig. 8D). Previous studies focused in the petrogenesis of the Dongjiangkou 5.2.2. Granitoid rocks of the Dongjiangkou intrusion granitoid rocks and proposed four models (1) interaction between The granitoid rocks from the Dongjiangkou intrusion display subducted Yangtze continental crust and the overlying mantle wedge medium to high SiO2, and high K2O, MgO, Rb, and Th contents, and (Qin et al., 2010a); (2) partial melting of subducted sediments with sub- low TiO2,P2O5, Sc contents, and A/CNK values (Appendix Table S3; sequent melts interacting with the overlying mantle wedge (Jiang et al., Fig. 6). These granitoid rocks are typical I-type granites (Chappell 2010); (3) the magma mixing between depleted mantle melts and F. Hu et al. / Lithos 278–281 (2017) 195–214 207

Fig. 9. Petrogenetic discrimination diagrams for porphyritic to coarse to medium-grained granitoid rocks from the Dongjiangkou and Zhashui intrusions. (A) Th/La versus Th diagram for Dongjiangkou and Zhashui granitoid rocks. (B) Th versus Th/Sc diagram for Dongjiangkou granitoid rocks. (C) 1/Sc versus Rb/Sc diagram Dongjiangkou granitoid rocks. (D) Th versus Th/Sc diagram Zhashui granitoid rocks. The inset in A is a schematic CH versus CH/CM diagram (CH, highly incompatible element concentration; CM, moderately incompatible element concentration). The inset in B is a schematic Cl/CC versus Cl diagram (Cl, incompatible element concentration; CC, compatible element concentration). The inset in C is a schematic Cl/CC versus 1/Cl diagram. The curves are calculated melt compositions produced by partial melting, magma mixing, and fractional crystallization processes (after Schiano et al., 2010). Legends as for Fig. 6. crustal melts that were derived from partial melting of Meso- to and Yaolinghe Groups in the SQB. On the other hand, the inherited

Neoproterozoic lower crust materials (Gong et al., 2009a); and (4) the zircons with positive εHf(t) values plot into the evolutionary region of magma mixing between metasomatized mantle melts and Proterozoic the Neoproterozoic basement blocks of the SQB, defined by the newly mafic lower crust melts (Bao et al., 2015; Zeng et al., 2014). Because found Neoproterozoic rocks and detrital zircons from the Wudangshan the MME represent products of mantle-sourced melts and inject into and Yaolinghe Groups (Fig. 5). In this case, the magmatic sources of the felsic magma chamber, contrasting to the situation of felsic melts the Dongjiangkou granitoid rocks may be heterogeneous in isotopic that are derived from subducted continental crust and interacted with compositions. However, the involvement of the old basement is limited overlying mantle wedge. Additionally, the subducted sediments usually because only few zircons exhibit εHf(t) values that are lower than −15. show high radiogenic Sr composition (generally 87Sr/86Sr N 0.71000; Thus, the Neoproterozoic basement blocks of the SQB is the major GLOSS: 87Sr/86Sr = 0.71730; Plank and Langmuir, 1998), which are magmatic sources of the Dongjiangkou granitoid rocks. quite different with Dongjiangkou granitoid rocks according to previous To further understand magmatic sources of the granitoid rocks, we studies (87Sr/86Sr = 0.70411–0.70672; Jiang et al., 2010; Liu et al., use Neoproterozoic hornblende gabbro samples as the source rock to 2011b; Qin et al., 2010a; Zhang et al., 2006). Taking into consideration model partial melting process and obtain the partial melts (Appendix their mainly metaluminous compositions and absence of coeval Table S4; Fig. 10). The residual mineral assemblages and the melting subduction-related adakites and related volcanic rocks, subducted degrees are employed from the experimental results of Qian and sediments are unlikely the source of the felsic melts (Hermann and Hermann (2013). As shown in the Fig. 10A and B, the trace element Spandler, 2008; Stern et al., 2006). patterns of all the modelling melts are extremely similar to those of Previous studies have found out the presence of the Proterozoic Dongjiangkou granitoid rocks. Moreover, according to our modeling inherited zircons and these inherited zircons show a wide εHf(t) range results, the compositions of the source rocks are the primary control from −24.1 to +12.5 (Fig. 5; Gong et al., 2009a; Jiang et al., 2010; factor for the granitoid melts, which is consistent with previous study Ping et al., 2013; Qin et al., 2010a). As shown in the Fig. 5, the inherited (Fiannacca et al., 2015). For instance, the Th is an easily mobile element, zircons with negative εHf(t) values plot into the evolutionary region of especially for those Precambrian rocks, hence the model results of Th the old basements of the SQB, which is defined by the Douling display large uncertainty due to the different compositions of the source Neoarchean TTG gneisses and detrital zircons from the Wudangshan rocks. We further calculate the mixing results of these pure partial melts 208 F. Hu et al. / Lithos 278–281 (2017) 195–214

Fig. 10. Trace element modeling for the petrogenesis of Dongjiangkou granitoid rocks. (A) The calculated melts with degrees of melting from 10% to 40% as source rocks is 08LY2-14. (B) The calculated melts with degrees of melting from 10% to 40% as source rocks is 08LY2-9. (C) Mixing of 10% MME with 90% calculated melts as source rock is 08LY2-14. (D) Mixing of 30% MME with 70% calculated melts as source rock is 08LY2-14. Detailed parameters and calculations of models see Appendix Table S4. The MME is the lowest SiO2 MME sample published by Jiang et al. (2012).

Fig. 11. Petrogenetic discrimination diagrams for porphyritic to coarse to medium-grained granitoid rocks from the Zhashui intrusion. (A) Molar Al2O3/(MgO + FeOT) (AFM) versus CaO/

(MgO + FeOT) (CFM) diagram showing the source composition for the granitoid rocks from the Zhashui intrusion, modiﬁed after Altherr et al. (2000). (B) CaO/(FeOT + MgO + TiO2) versus CaO + FeOT + MgO + TiO2 diagram, modiﬁed after Patiño Douce (1999). (C) CaO/Na2O versus Al2O3/TiO2 diagram, after Sylvester (1998). (D) Rb/Ba versus Rb/Sr diagram, after Sylvester (1998). Legends as for Fig. 6. F. Hu et al. / Lithos 278–281 (2017) 195–214 209

with a MME sample with SiO2 of 50.02 wt.% reported by Jiang et al. versus Y and (La/Yb)N versus YbN diagrams, indicating they might (2012) (Fig. 10 C and D). The results show that 10% to 30% MME melts undergo plagioclase fractionation (Fig. 8B–C; Castillo, 2012). Some sam- may mix with the felsic melts and the mixed melts also display similar ples also show negative Eu anomalies in the chondrite-normalized REE trace element patterns to those of Dongjiangkou granitoid rocks. Addi- diagrams but no clear positive correlation between Eu/Eu* and Sr, tionally, the mixing of 10% to 30% MME melts also enable to acquire reflecting that the negative Eu anomalies are due to formed by partial the similar Mg# of the Dongjiangkou granitoid rocks (Fig. 8A). There- melting process with plagioclase as one of the residue minerals fore, we suggest that the Dongjiangkou granitoid rocks were formed (Figs. 7Eand8D). by magma mixing of mainly two sources: 70%–90% melts derived Previous researchers studied the petrogenesis of the Zhashui from the Neoproterozoic basement rocks of the SQB and 10%–30% granitoid rocks and they proposed that the Zhashui granitoid rocks mantle-derived mafic melts, with minor involvement of older basement were formed by mixing between crustal-derived melts and mantle- materials (as old as Archean) of the SQB. derived melts (Gong et al., 2009b; Liu et al., 2013). Gong et al. (2009b) also suggest that the felsic end member might be derived from 5.2.3. Granitoid rocks of the Zhashui intrusion Neoproterozoic or older mafic lower crustal material. However, we The granitoid rocks of the Zhashui intrusion, showing quite dif- propose here that magma mixing is not a major petrogenetic process, ferent geochemical characteristics from the granitoid rocks of the and chemical diversity of these granitoid rocks is mainly due to partial

Dongjiangkou intrusion, have higher SiO2,K2O, Rb contents, and melting process of a heterogeneous source. On the basis of the zircon A/CNK values, but lower MgO, TiO2,P2O5, Sc contents, Mg# and Sr/Y εHf(t) values that plot within the range of Neoproterozoic basements values (Appendix Table S3; Fig. 6). These granitoid rocks are also of the SQB, we suggest that the Neoproterozoic basement blocks of the I-type granites (Figs. 3K and 6F; Chappell et al., 1987). Nevertheless, SQB may be their main source (Fig. 5). Also, the Neoproterozoic the rare MME in the Zhashui intrusion and all samples plot within the inherited zircons from the Zhashui intrusion also exhibit similar region of the experimental melts that derived from the partial melting εHf(t) values to those of the zircons from the Neoproterozoic rocks, of pure crustal materials or basalts and amphibolites in the MgO versus which further supports that the granitic magma may be derived from

SiO2 diagram, suggesting the involvement of mantle-derived melts are partial melting of the Neoproterozoic crustal materials (Fig. 5). insigniﬁcant (Fig. 6C; Martin et al., 2005; Rapp and Watson, 1995). Additionally, the zircon εHf(t) values of the Zhashui granitoid rocks According to our modeling calculation, less than 10% MME melts may display less variations than those of the Dongjiangkou granitoid rocks, be involved into the crustal melts (Fig. 8A). The geochemical modeling suggesting their source rocks are relative homogeneous in isotopic also reveals that the granitoid rocks array along the trend line of partial compositions, which are mainly Neoproterozoic basements (Fig. 5). melting in the Th/La versus Th and Th versus Th/Sc diagrams, indicating The Zhashui granitoid rocks plot within the ﬁeld of partial melting of the these granitoid rocks were formed by a partial melting process metamorphic greywackes in the AMF (Al2O3/(MgO + FeOT)) versus (Fig. 9A and D; Schiano et al., 2010). The granitoid rocks show similar CMF (CaO/(MgO + FeOT)) diagram, indicating that metamorphic evolution trends to the fractional crystallization of plagioclase in Sr/Y greywackes may be the major source materials of these granitoid

Fig. 12. Plotting of compiled data of granitoid rocks from the middle SQB. (A) MgO versus SiO2 diagram of unfiltered data showing differentiation trend. (B) Sr/Y versus SiO2 diagram of unfiltered data reflecting variations of Sr/Y. (C) Rb/Sr versus SiO2 diagram of unfiltered data reflecting the influence of fractional crystallization on Rb/Sr. The grey area shows the limits to data filters used for Sr/Y analysis in this study. (D) Sr/Y versus SiO2 diagram of filtered data. The triangles represent the experimental results of partial melting of basaltic rocks at different pressures, after Qian and Hermann (2013). 210 F. Hu et al. / Lithos 278–281 (2017) 195–214 rocks (Fig. 11A; Altherr et al., 2000). Similarly, these granitoid rocks plot within the range of the partial melting of metamorphic greywackes and amphibolites in the CaO/(FeOT + MgO + TiO2) versus CaO + FeOT + MgO + TiO2 diagram (Fig. 11B; Patiño Douce, 1999). These rocks also plot within the field of strongly peraluminous granites in the CaO/Na2OversusAl2O3/TiO2 diagram, and show similar Rb/Ba and Rb/Sr ratios to the greywackes (Fig. 11CandD;Sylvester, 1998). Never- theless, these rocks do not belong to strongly peraluminous granites, because of their relatively low A/CNK value (b1.1) and the absent of primary highly-aluminous minerals (e.g. muscovite, cordierite, garnet) (Sylvester, 1998). On the other hand, Zhashui granites show I-type trend in P2O5 versus SiO2 diagram (Fig. 6F; Wu et al., 2003). Metamor- phic greywackes also has been proposed for a potential source of I-type granite on the basis of experimental and regional studies (Altherr et al., 2000; Kemp et al., 2007; Patiño Douce, 1999). However, it should be noted that these samples also exhibit similar signatures of melts derived from partial melting of basaltic rocks, e.g. low AMF values and high (CaO + FeOT + MgO + TiO2) values, which may be influenced by MME melts as evidenced by their elevated Mg# values. In view of their zircon Hf isotopic feature, the metamorphic greywackes may be mainly Neoproterozoic volcanic rocks (Figs. 5 and 7F). Therefore, in consideration of geochemical characteristics, we suggest that granitoid magmas of the Zhashui granitoid rocks may be mainly derived from partial melting of Neoproterozoic metamorphic greywackes with some additions of mantle-derived mafic melts.

5.3. Geochemical evolution in the SQB

To investigate the geochemical evolution in the SQB, we compiled the geochemical and chronological data of ca. 230–190 Ma granitoid rocks and associated MME in the SQB (Appendix Tables S5–S7;

Figs. 12–14). The SiO2 content could somewhat reﬂect the extent of the involvement of crustal melts and mantle-derived melts, and could further indicate the melting temperature of the crust because less mantle-derived melts will lead to lower partial melting temperature. Fig. 13. Geochemical evolution in the middle SQB. (A) Plot of changes in median SiO2 in ﬂ granitoid rocks and MME through time. (B) Plot of changes in median Sr/Y in granitoid The Sr/Y ratio could re ect the depth of partial melting because of the rocks and MME through time. Compiled and plotted data are included in Appendix different geochemical character of Sr and Y as many studies have proved Tables S5 and S6. (Chapman et al., 2015; Drummond and Defant, 1990; Lee and Morton, 2015; Lu et al., 2015). According to our compilation, all granitoid rocks show differentiation trends in MgO–SiO2 diagram (Fig. 12A). However, the Sr will materials at shallow level after ca. 205 Ma due to lack of heat from decline in highly fractionated magmas, which may cause the decrease mantle-derived melts. of Sr/Y ratios (Fig. 12B; Lee and Morton, 2015). We used elevated Rb/ There are also some variations in the same period. The Sr/Y ratios of

Sr ratio as an indicator of highly fractionated magmas because Rb will samples with Rb/Sr b 0.3 exhibit positive correlations with their SiO2 increase in highly fractionated magmas (Fig. 12; Lee and Morton, contents during 225–205 Ma (Fig. 14). We consider that those high 2015). The samples with Rb/Sr ratios higher than 0.3 were discarded. Sr/Y granitoid rocks are relatively pure crustal melts that are formed Additionally, to obtain more reliable results, we further filtered the at a deeper crustal level and the low Sr/Y of some granitoid rocks data by SiO2 (58–71 wt.%), but we did not filter MME samples due to are results of the mixing with low Sr/Y MME (Figs. 13 and 14). By their low SiO2 (Appendix Tables S6–S7). combining with previous studies, we consider those high SiO2 and Rb/ Based on our compilation, the most conspicuous feature is that a dis- Sr (N0.3) but low Sr/Y rocks were derived from partial melting of tinct magmatic flare-up event was happened during ca. 225–205 Ma metasedimentary rocks at relatively shallow crustal level or highly and abundant MME were associated with coeval granitoid rocks that fractionated magmas (Dong et al., 2012a; Meng et al., 2013; Yang some of them show very high Sr/Y ratios (N100) with variable SiO2 et al., 2012a). (Fig. 13). According to the experimental studies of partial melting, the pressure would be higher than 1.35 GPa to obtain the melts with such 5.4. Implications for geodynamic evolution high Sr/Y (Fig. 12D; Qian and Hermann, 2013; Rapp et al., 1999). As for the granitoid rocks formed during ca. 235–225 Ma, they are accom- A consensus has been achieved that the QOB was developed as a panied with some MME but rare in outcrop, and show mainly lower result of consumption of an ocean and ultimate collision between the

SiO2 and Sr/Y than those of ca. 225–205 Ma granitoid rocks (Deng SCB and NCC during Triassic era (Dong and Santosh, 2016). Paleomag- et al., 2016; Qin et al., 2013). The granitoid rocks after ca. 205 Ma that netic and sedimentary studies suggest that the initial collision between are associated with no or rare MME, show consistent high SiO2 content the NCC and YZC occurred at the location of the Dabie Orogenic Belt in with low Sr/Y ratios. Roughly, such variations reﬂect that the 225 to the Early Triassic or Late Permian and a clockwise rotation of the SCB 205 Ma granites may be formed at the time that crust reached to to the northwest was followed (Zhao and Coe, 1987; Zhu et al., 1998). the thickest level along with a great quantity of mantle-derived melts. During the entire collision process, extensive magmatism was produced The crust may not become thinner after 205 Ma although no high Sr/Y in the SQB from ca. 248 Ma to 190 Ma, and mostly between ~225– granitoid rocks are formed. We suggest that partial melting of crustal 205 Ma (N. Li et al., 2015, and the references therein). F. Hu et al. / Lithos 278–281 (2017) 195–214 211

Fig. 14. Sr/Y versus SiO2 showing geochemical variations of granitoid rocks and MME in the SQB. The MME display low SiO2 contents with low Sr/Y values. The granitoid rocks with Rb/Sr higher than 0.3 show high SiO2 contents with low Sr/Y ratios, suggesting they are high fractionated magmas or formed by partial melting of metasedimentary rocks at shallower depth.The granitoid rocks with Rb/Sr lower than 0.3 display positive correlation between SiO2 and Sr/Y, and more MME are formed from 235 to 225 Ma to 225–205 Ma, suggesting the increase of the partial melting depth and increase of involvement of mantle-derived magmas. The ca. 205–190 Ma granitoid rocks with Rb/Sr lower than 0.3 show high SiO2 contents and low Sr/Y ratios with no coeval MME, indicating the decrease of partial melting depth and less involvement of mantle-derived magmas. Compiled and plotted data are included in Appendix Table S7.

Prior to ca. 235 Ma magmatism in the SQB was formed in a Ghani, 2002; Davies and von Blanckenburg, 1995; Zhu et al., 2015). subduction-related setting (Li et al., 2013; Liu et al., 2011a, 2011b; Qin Slab break-off may lead to rapid uplift of the orogenic belt, which was et al., 2008; Yang et al., 2014). Flysch deposition lasted until the Carnian recognized in the SQB during ca. 225–205 Ma, based on the cooling and nonmarine molasses deposition (post-tectonic) started in the history of granitoid rocks and absence of upper Triassic deposits Norian in the foreland basin (Liu et al., 2005). Ca. 235–225 Ma granitoid (Dong and Santosh, 2016; Wang et al., 2007, 2014). In the meanwhile, rocks were only exposed in the middle and western SQB (Wulong and the inferred crustal thickness reached to the peak during this period Guangtoushan intrusions), perhaps suggesting a transitional period according to Sr/Y values of granitoids (median value N100) (Figs. 13B from subduction to the later collisional magmatic flare-up event and 14). Therefore, we suggest that the prominent mantle-derived (Deng et al., 2016; Hu et al., 2016a; Qin et al., 2013). The ~224 Ma magma during this period was formed by partial melting of lithospheric outer zone of Guangtoushan granitoid complex was emplaced into the mantle, induced by slab break-off. Then the mantle-derived mafic Mianlue suture zone, which could be regarded as stitching intrusion magmas heated the lower continental crust resulting in partial melting (Deng et al., 2016). Whereas ~230 Ma Xinyuan, Zhangjiaba granitoid of continental crust and subsequent magma mixing. intrusions and outer zone of Wulong granitoid intrusion all situated in After ca. 205 Ma, the magmatism becomes much less volumetrically the north of the Mianlue suture zone (Deng et al., 2016; Qin et al., significant and MME are extremely rare in these rocks (Dong et al.,

2013). Therefore, combined with regional geology (see geological 2012a). Granitoid rocks show high SiO2, low Sr/Y ratios (median value setting), the collision in the western SQB should have started before b40) and significant negative Eu anomalies indicating their source is ~225 Ma. Based on the Sr/Y (median value b70) of granitoid in this mainly the crustal materials, primarily metasedimentary rocks, in a period, the crust was still not greatly thickened. The 235–225 Ma relatively low pressure setting (Dong et al., 2012a). Cooling rates of granitoids are mainly intermediate rocks with subduction-related geo- granitoid rocks in the SQB decreased significantly after ca. 200 Ma, chemical features (Deng et al., 2016; Qin et al., 2013), but such andesitic suggesting a tardiness uplift event was followed afterwards (Wang rocks could be formed just after the initial collision (Zhu et al. (2015). et al., 2007, 2014). Therefore, we proposed that the slab break-off was Therefore, we suggest that the SQB is under a subduction setting prior finished in this period resulting in lacking heat and limited melts to 235 Ma and transitional setting from subduction to collision during derived from the continental lithospheric mantle. Because of the low ca. 235–225 Ma. melting point, partial melting of the metasedimentary rocks where

A distinct magmatic flare-up event took place within the 225– situated at higher crustal level is plausible, which leads to high SiO2 205 Ma time window in the SQB. A recent study suggested that a melts with low Sr/Y. Some researchers proposed that the delamination westward dipping slab-tearing process took place during this period of thickened lower crust is responsible for the formation of granitoid (Hu et al., 2016a); this conclusion is based mainly on the westward rocks in this period (Deng et al., 2016; Dong et al., 2012a; Yang et al., younging ages of intrusions and associated mantle-derived mafic 2012a). However, no massive high-Mg adakitic rocks are exposed in rocks. The magmatism during period is characterized by voluminous the SQB during ca. 205–190 Ma, which are quite common production granitoid intrusions with abundant MME, reflecting intense mantle– of intensive delamination of thickened lower crust in the southern crust interaction. Slab break-off is a process associated with continental Tibet and eastern NCC (Chung et al., 2009; Gao et al., 2004). Therefore, collision and could happen over a period of several million years after intensive delamination is not appropriate for explaining the fact of the initial collision and could generate massive magmas (Atherton and observation and geochemical evidence, but regional small-scale 212 F. Hu et al. / Lithos 278–281 (2017) 195–214 delamination may happen. From the above, we proposed that the slab Barbarin, B., 1999. A review of the relationships between granitoid types, their origins and – – their geodynamic environments. Lithos 46, 605 626. detachment is completed in the SQB during ca. 205 190 Ma and no in- Barbarin, B., 2005. Mafic magmatic enclaves and mafic rocks associated with some gran- tensive delamination of lower crust is occurred. itoids of the central Sierra Nevada batholith, California: nature, origin, and relations with the hosts. Lithos 80, 155–177. 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