RESEARCH Petrogenesis of Early Paleozoic High Sr/Y Intrusive Rocks

RESEARCH

Petrogenesis of Early Paleozoic high Sr/Y intrusive rocks from the North Qilian orogen: Implication for diachronous continental collision

He Yang1,2,3,*, Hongfei Zhang2, Wenjiao Xiao1,4, Biji Luo2, Zhong Gao2, Lu Tao2, Liqi Zhang2, and Liang Guo2 1XINJIANG RESEARCH CENTER FOR MINERAL RESOURCES, XINJIANG INSTITUTE OF ECOLOGY AND GEOGRAPHY, CHINESE ACADEMY OF SCIENCES, URUMQI 830011, CHINA 2STATE KEY LABORATORY OF GEOLOGICAL PROCESSES AND MINERAL RESOURCES AND SCHOOL OF EARTH SCIENCES, CHINA UNIVERSITY OF GEOSCIENCES, WUHAN 430074, CHINA 3STATE KEY LABORATORY OF DESERT AND OASIS ECOLOGY, XINJIANG INSTITUTE OF ECOLOGY AND GEOGRAPHY, CHINESE ACADEMY OF SCIENCES, URUMQI 830011, CHINA 4STATE KEY LABORATORY OF LITHOSPHERIC EVOLUTION, INSTITUTE OF GEOLOGY AND GEOPHYSICS, CHINESE ACADEMY OF SCIENCES, BEIJING 100029, CHINA

ABSTRACT

A combination of U-Pb zircon ages and geochemical and Sr-Nd-Hf isotopic data are presented for the Early Paleozoic granodiorites from the Haoquangou and Baimawa plutons in order to probe the crustal thickness variation of the eastern North Qilian and the diachronous evolution of the North Qilian orogen. The granodiorites formed at 436–435 Ma and have high Sr/Y ratios (63–117). Elemental and isotopic data combined with geochemical modeling and comparisons with experimental data suggest that they were produced from the melting of relatively juvenile mafic rocks in the thickened lower crust. Together with other petrological and geochemical data and the calculation of variation in crustal thickness, this indicates that the eastern North Qilian experienced clear crustal thickening and thinning from the Late Ordovician to Late Silurian. Based on available data, we suggest that diachronous collision from east to west, which probably resulted in the distinct intensity of orogenesis between eastern and western North Qilian, can well account for the differential distribution of Early Paleozoic high Sr/Y magmatism and other geological differences between the eastern and western parts of the North Qilian. Our study also implies that diachronous collision may lead to, apart from distinct metamorphic, structural and sedimentary responses, the large differences in magmatism and deep crustal processes along the orogenic strike.

LITHOSPHERE; v. 12; no. 1; p. 53–73; GSA Data Repository Item 2020066 | Published online 19 December 2019 https://doi .org /10 .1130 /L1129 .1

INTRODUCTION stabilize garnet and/or amphibole (Defant and that, based on recent studies on experimental Drummond, 1990; Castillo, 2012). High Sr/Y petrology and geochemical modeling, high Sr/Y Magmatism is abundant in continental rocks were originally considered to be products rocks can form from crustal melting at pressures orogenic belts and it well records thermal of young (< 25 Ma) slab melting in arc settings as low as 1.0 GPa and the overthickened crust evolution of lithosphere (Wilson, 1989). Its (Defant and Drummond, 1990), but later studies may not be necessary (Moyen, 2009; Qian and genesis and temporal-spatial distribution can suggested that high Sr/Y rocks can also be Hermann, 2013; Ma et al., 2015). Even so, the provide important insights into the tectonic produced through other petrogenetic processes occurrence of widespread continental high evolution and deep geodynamic processes of in both arc or non-arc settings (Atherton and Sr/Y granitoids is often regarded as a sign of a orogenic belts, such as diachronous collision Petford, 1993; Castillo et al., 1999; Chung et collision or post-collision process (Chung et al., or post-collision processes (Richards, 2015; Hu al., 2003; Martin et al., 2005; Macpherson et 2003; Schwartz et al., 2011; Yu et al., 2019a). et al., 2016). High Sr/Y (or adakitic) rocks, as al., 2006). In particular, many high Sr/Y rocks Moreover, Sr/Y and La/Yb ratios in intermediate a special type of magmatic rocks, have been were inferred to be derived from the continental magmatic rocks have been used to quantify widely studied in terms of their petrogenesis lower crust (Atherton and Petford, 1993; Chung crustal thickness over time in magmatic arcs and tectonic settings, their relationship with et al., 2003; Wang et al., 2006b; Yu et al., 2019b), or continental collisional belts (Chapman et Cu-Au metallization, and their implications for but whether crustal thickening is necessary al., 2015; Profeta et al., 2015; Hu et al., 2017; the growth of early continental crust (Defant for their formation is still controversial. For DePaolo et al., 2019). Thus, whether high Sr/Y and Drummond, 1990; Martin et al., 2005; example, while many high Sr/Y rocks were granitoids can indicate crustal thickening, which Wang et al., 2006b; Moyen, 2009; Schwartz et suggested to result from deep melting of is important for the understanding of orogenic al., 2011; Castillo, 2012). High Sr/Y rocks are basaltic sources at pressures equivalent to a evolution, needs further evaluation. characterized by low HREE contents and high crustal thickness of >40–50 km and crustal Early Paleozoic high Sr/Y plutons are widely Sr/Y (> 40) and La/Yb ratios, suggesting that thickening was thought to be needed for their distributed in the eastern part of the North Qilian they were generated at pressures high enough to generation (Atherton and Petford, 1993; Rapp belt and many are genetically associated with and Watson, 1995; Chung et al., 2003; Wang Cu-Au deposits (Wang et al., 2006a; Tseng *Corresponding author: [email protected] et al., 2006b), some researchers have argued et al., 2009; Chen et al., 2016; Zhang et al.,

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2017b). However, their petrogenesis and mainly formed in the Late Mesoproterozoic to were suggested to be the products of the cold tectonic settings are still debated. In particular, Middle Neoproterozoic (Wan et al., 2001; Yan subduction and later exhumation of the North pressure conditions for their generation have et al., 2015), corresponding to assembly and Qilian oceanic slab (Zhang et al., 2007; Song et not been well constrained. On the other hand, breakup of the Rodinia supercontinent. This, al., 2009). The eclogite-facies rocks have U-Pb unlike that in the eastern part of the North combined with Pb-Nd isotopic data, suggests zircon ages of 489–463 Ma (Song et al., 2004; Qilian, exposure of Early Paleozoic high that the Central Qilian block has an affinity with Zhang et al., 2007), and blueschist-facies rocks Sr/Y rocks in the western part of the North the Yangtze block (Wan et al., 2001; Zhang et have 40Ar-39Ar ages of ~460–410 Ma (Zhang Qilian belt is very rare. The reason for this al., 2006). Early Paleozoic intrusive rocks are et al., 1997; Liu et al., 2006). Late Ordovician– differential distribution is not clear yet. Here, abundant in the Central Qilian, dominated by Silurian magmatic rocks, including high Sr/Y, we report zircon U-Pb ages and Hf isotopic granitic rocks (including high Sr/Y, I-, S- and I- and A-type granitoids, have been identified data and whole-rock geochemical and isotopic transitional I–S type granitic rocks), with less in the North Qilian and their magma generation compositions for the Early Paleozoic high Sr/Y diorites and mafic-ultramafic intrusive rocks was linked to late-stage oceanic subduction, rocks in the Baiyin area of the eastern North (Yang et al., 2018, and references therein) (Fig. continental collision, or post-collision processes Qilian belt. We use these data to discuss their 1B). They commonly intrude the Precambrian (Chen et al., 2012; Zhao et al., 2014; Yu et al., petrogenesis, the variation of crustal thickness, basements. These intrusive rocks, less exposed 2015; Zhang et al., 2017b). and the diachronous continental collision of the volcanic rocks, and related metamorphic There are many Early Paleozoic granitic plutons North Qilian orogen during the Early Paleozoic. rocks in the Central Qilian were suggested (e.g., Haoquangou, Baimawa, Heishishangou, to be the products of subduction of the South and Fangfuya plutons) in the Heishishan area of GEOLOGICAL BACKGROUND Qilian oceanic slab and subsequent continental the eastern North Qilian (Fig. 2) that are closely collision and post-collision processes during the related to regional Cu-Au mineralization (Wang et The Qilian orogen, located in the Early Paleozoic period (Bian et al., 2001; Xia al., 2005b). These small intrusions occur as stocks, northeastern margin of the Tibetan Plateau, is et al., 2016; Li et al., 2018). apophyses, and dykes, and intrude Cambrian– a NWW-trending linear belt lying between the The North Qilian belt is characterized by Ordovician arc volcanic rocks. The dominant rock Alax block and the North Qaidam–West Qinling an outcrop of Early Paleozoic ophiolites, high- types are granodiorite and trondhjemite (Wang et belts (Figs. 1A and 1B). It is separated from the pressure/low-temperature metamorphic rocks al., 2005b). The granodiorite samples used in Ordos block to the east and the Tarim Craton (e.g., eclogites and blueschists), and subduction/ this study were collected from the Haoquangou to the west by the Tongxin-Guyuan fault and collision-related intrusive and volcanic rocks and Baimawa plutons (Figs. 2, 3A, and 3B). The the Altyn Tagh fault, respectively (Feng and He, (Song et al., 2013; Xia et al., 2016). A typical Haoquangou pluton is located ~3 km north of the 1996). It formed by Early Paleozoic convergence trench-arc-basin system developed in the North Baiyin City, with an outcrop area of ~0.03 km2 of the Alax, Central Qilian, and Qaidam blocks Qilian belt during the Early Paleozoic period. (Figs. 1B and 2). It hosts Au deposits and comprises (Yang et al., 2012; Song et al., 2013; Xia et Recent study on volcanic-sedimentary formation medium-grained and undeformed rocks, ranging al., 2016). Tectonically, the Qilian orogen can of the Zhulongguan Group suggested that the in composition from trondhjemite to granodiorite be divided into three units from south to north: North Qilian Ocean could open after an ~600– (Wang et al., 2005b). Some small granite-porphyry South Qilian belt, Central Qilian block, and 580 Ma rifting event induced by a mantle plume dykes associated with the Haoquangou pluton North Qilian belt (Fig. 1B). during the breakup of Rodinia supercontinent have also been reported (Wang et al., 2005b). The South Qilian mainly comprises (Xu et al., 2015). The ophiolite sequences in the The Baimawa pluton lies ~1 km northeast of the Cambrian–Ordovician volcanic-sedimentary southern margin of the North Qilian represent Haoquangou pluton and has an outcrop area of rocks (lava flows, pyroclastic rocks, and abyssal seafloor spreading of the North Qilian Ocean. ~0.24 km2 (Fig. 2). It mainly consists of medium- and bathyal deposits), Silurian flysch sediments, They have zircon U-Pb ages of ~550–500 Ma grained granodiorite. The granodiorites from the Late Devonian molasses, and Late Caledonian (Song et al., 2013). Sr-Nd-Pb isotopes and trace Haoquangou and Baimawa plutons are mainly granitoids (Xu et al., 2006) (Fig. 1B). The elemental geochemistry of the basaltic rocks composed of quartz (~20–25 vol.%), plagioclase Cambrian–Ordovician volcanic-sedimentary from the Yushigou and Dongcaohe ophiolites (~50–55 vol.%), K-feldspar (~10 vol.%), biotite strata are mainly distributed in the northern suggested that the North Qilian Ocean has a (~10–15 vol.%), and secondary muscovite that part of the South Qilian. Geochemical data tectonic affinity with the Proto-Tethyan Ocean most likely resulted from decomposition of indicate that the Ordovician volcanic rocks were (Hou et al., 2006; Tseng et al., 2007). Studies plagioclase or transformation from other minerals probably generated in a subduction setting (Zhao on Early Cambrian volcanic-intrusive rocks such as biotite (Figs. 3C–3F). Polysynthetic et al., 2004). Early Paleozoic granitoids are show that northward subduction of the North twinning and zonal structure in the plagioclase widespread in the South Qilian and commonly Qilian oceanic slab initiated at ~520 Ma (Xia and tartan and Carlsbad twinning in the K-feldspar occur as batholiths or stocks. Geochronological et al., 2012; Chen et al., 2014), although some have been observed. Zircon, apatite, and Fe-Ti data suggest that they mainly formed at 473– researchers also inferred a southward subduction oxides are common accessory minerals. 433 Ma (Wu et al., 2006; Wang et al., 2013), for the North Qilian Ocean (Peng et al., 2017a, probably recording oceanic subduction and 2017b). Geochronological and geochemical ANALYTICAL METHODS closure of the South Qilian Ocean. data of the Jiugequan ophiolite imply that the The Central Qilian block mainly comprises extension of the back-arc basin in the North Fresh rock samples were crushed to powders Precambrian metamorphic basements, Early Qilian initially occurred at ~490 Ma (Xia and of 200 mesh in an agate mill. Major element Paleozoic magmatic rocks (especially Song, 2010). The high pressure metamorphic compositions were determined by X-ray fluo- intrusive rocks), and Paleozoic to Mesozoic rocks in the North Qilian are predominantly rescence (XRF) at the State Key Laboratory of sedimentary strata (Feng and He, 1996) (Fig. composed of eclogites, blueschists, and Geological Processes and Mineral Resources 1B). Geochronological data imply that the eclogite/blueschist-facies marble, pelite, chert, (GPMR), China University of Geosciences, Precambrian basement rocks in the Central Qilian and greywacke (Song et al., 2009), which Wuhan. The detailed procedures for XRF

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V V V V V 94°E V 97°E XLG: 454±2 Ma 100°E 103°E 106°E V V V JTS:V 430±2 Ma V V V V V XLG: 418±2 Ma YQDS: 427±4 Ma B Altyn Tagh fault V V V AYG: 438±3 Ma V Alxa Block V SRS: 422±3 Ma V V JFS: 428-424Ma MYYG: 463±5 Ma MJDW: 426±3 Ma V V V V CDN:V 516-505Ma MJDW: 443±4 Ma N

CMX: 423±6 Ma V V V V NXS: 435±4 Ma V V V XKG: 419-414 Ma V YNT: 460±3 Ma V V V V South Qilian Belt V V V V LHSH: 441±4 Ma V Central QilianV Block V 0 80km V V V Jin Chang MZS: 424±4 Ma V V

V V NorthV Qilian Belt MZS: 431±2 Ma 38°N V V Wuwei V LGS: 453±6 Ma V BJS: 434±3 Ma V V

KKL: 512-501 Ma V LHS: 424±3 Ma V XGL: 457±2 Ma V V V

V V V JZC: 464±15 Ma V QWS: 430±3 Ma V V Jingtai V V V V V V V V V QWS:Tongxin-Guyuan 446±3 fault Ma 37°N V North Qaidam Belt V V V BMW: 435±2 Ma V V V V V V Wulan V Fig. 2 Ordos V Baiyin Xi’ning Haiyuan Block HQG: 436±2 Ma Qaidam Block V V QWS: 430±2 Ma LanzhouV

36°N V West Qinling Belt SMT: 430±6 Ma

35°N V V V Tianshui V V Wushan V V Baoji

70°E 80°E 90°E 100°E 110°E 130°E A Proterozoic basement V V V Cambrian volcanics Fig. 1B Cambrian sediments V V V Ordovician volcanics

West Kunlun 40°E Orogen Tarim Block Ordovician sediments V V V Silurian volcanics Qilian Orogen Qaidam Block North China Craton Mafic rocks East Kunlun Orogen Sulu Orogen Ultramafic rocks Tibetan Plateau Songpan-Ganzi Orogen 30°E Dike-like ultramafic-mafic Qinling-Dabie Orogen intrusions Mafic dikes

Yangtze Block Early Paleozoic granotoids 436±2 U-Pb ages of high Sr/Y rocks Cathysia Block

20°E 419 U-Pb age of A-type granite 424±4 U-Pb ages of other magmatic rocks

Figure 1. (A) Geological sketch map showing the major tectonic units of China (modified from Yang et al., 2009). (B) Simplified geological map of the Qilian orogen, showing distribution of the Precambrian basement and the Early Paleozoic rocks (modified after Ma et al., 2002). Data sources for zircon U-Pb ages in B are as follows: Leigongshan (LGS) tonalite and Shenmutou (SMT) quartz monzonite—Tseng et al. (2009); Quwushan (QWS) granodiorite and granite—Yu et al. (2015), Chen et al. (2016); Xigela (XGL) granite—Yu et al. (2015); Maozangsi (MZS) granodiorites—Xiong et al. (2012), Yu et al. (2015); Laohushan (LHS) quartz diorite—Qian et al. (1998); Jingzichuan (JZC) quartz diorite—Wu et al. (2004); Baojishan (BJS) granodiorite—Chen et al. (2015); Mengjiadawan (MJDW) granodiorite and quartz diorite, Lianhuashan (LHSH) granodiorite, Yangqiandashan (YQDS) granodiorite, Shenrongsi (SRS) alkali-feldspar granite and Xinkaigou (XKG) porphyritic and fine-grained granites—Zhang et al. (2017b); Minyueyaogou (MYYG) granodiorite and Niuxinshan (NXS) quartz diorite—Wu et al. (2011); Kekeli (KKL) plagiogranite—Wu et al. (2010); Chaidanuo (CDN) granite—Chen et al. (2014); Jinfosi (JFS) dioritic-granitic rocks—Wu et al. (2010), Huang et al. (2017); Aoyougou (AYG) trondhjemite—Chen et al. (2012); Xiaoliugou monzogranite and granodiorite—Zhao et al. (2014); Jingtieshan (JTS) granite—Li et al. (2019); Yeniutan (YNT) granodiorite—Mao et al. (2000); Changmaxi (CMX) granite— Gehrels et al. (2003). The eastern North Qilian is roughly separated from the western North Qilian by the dashed line shown in B, for convenience of discussion on the diachronous evolution in this study.

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104°08′E 104°13′E Middle Cambrian volcanics/sediments Pre-Silurian strata

36°37′N Subspilite-porphyrite Subkeratophyre Fangfuya Heishishan Mountain Subquartz-keratophyre Upper Triassic

Heishishangou Granodiorite- Dike-like granitic plagiogranite intrusions

Haoquangou Dike-like intermediate- Q1214 Quaternary Baimawa mafic intrusions 36°35′N Q1209 36°35′N Q1215 Q1217 N Q1211 0 2 km Fault Sample locations

Figure 2. Geological map of the Haoquangou and Baimawa plutons in the eastern North Qilian.

A B

Haoquangou pluton Baimawa pluton

Two-mica granite

C Q1211 Qz D Q1211

Pl Qz Bt Bt Qz Ms Pl Kfs

Pl Bt

E Q1214 F Q1215

Qz Qz Qz Pl Qz Kfs Bt Pl Pl

Figure 3. Field photographs of the (A) Haoquangou pluton and (B) Baimawa pluton. Cross-polarized light photomicrographs of representative samples from the Haoquangou and Baimawa plutons: (C–D) Haoqu- angou granodiorite (Q1211), and (E–F) Baimawa granodiorite (Q1214 and Q1215). Mineral abbreviations (Whitney and Evans, 2010): Qz—quartz; Pl—plagioclase; Kfs—K-feldspar; Bt—biotite; Ms—muscovite.

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analyses are the same as those described by value of 0.512620 ± 2 (2s); these were identical In situ Lu-Hf isotopic analyses were Ma et al. (2012). Data quality was monitored within error to the previously reported values conducted using a Neptune Plus MC-ICP-MS at by simultaneous analyses of repeated samples (Thirlwall, 1991; Weis et al., 2006). GPMR. Analytical spots were located close to or (one in ten samples) and the standard samples Zircons were separated from whole-rock on the top of spots of U-Pb dating or in the same GBW07104 (GSA Data Repository Table DR11), samples by conventional heavy liquid and growth domain with dating spots as inferred from and analytical uncertainties were generally magnetic techniques, and then were selected CL images. Instrumental conditions and detailed less than 5%. under a binocular microscope. The selected analytical procedures were reported in Hu et al. Whole-rock trace elements (including rare zircon grains were mounted in epoxy resin (2012). Analyses were performed with a spot size earth elements) were analyzed by ICP-MS and then polished to about half of their of 44 mm, and zircons 91500, GJ-1 and Temora at GPMR. For the detailed procedures for thickness. Transmitted and reflected light were used as reference standards. The acquired ICP-MS analyses, see the description by Liu photomicrographs and cathodoluminescence data were processed using ICPMSDataCal (Liu et al. (2008). During analysis, compositions of (CL) images were taken to reveal the et al., 2010). During analysis, the standards USGS reference materials AGV-2, BHVO-2, morphology and internal texture of zircons 91500, GJ-1 and Temora gave 176Hf/177Hf ratios of BCR-2, GSP-2, and RGM-1 were also measured and guide the selection of in situ analysis 0.282300 ± 7 (n = 14, MSWD = 0.60), 0.282007 to monitor data quality (Data Repository Table spots. CL imaging was carried out at the State ± 6 (n = 12, MSWD = 0.73), and 0.282679 ± 11 (n DR2). Analytical uncertainties were less than Key Laboratory of Continental Dynamics, = 4, MSWD = 1.1), respectively (Data Repository 5% for most trace elements. Northwest University, Xi’an, China. Zircon Table DR4), which agree with the recommended/ Whole-rock Sr-Nd isotopic compositions U-Pb dating and trace element analyses were reported values within error (Woodhead et al.,

were acquired using a Triton thermal ionization conducted using LA-ICP-MS at GPMR. The 2004; Elhlou et al., 2006). Calculations of eHf(t)

mass spectrometer at GPMR. Details for Sr-Nd detailed analytical procedures are the same values and Hf model ages (TDM(Hf)) were similar isotopic analyses are similar to those described as described by Liu et al. (2010). Spot sizes to those in Yang et al. (2015). by Zhang et al. (2017a). Measured Sr and Nd adopted in this study were 32 mm. Zircon isotopic ratios were normalized to 86Sr/88Sr = standard 91500 and NIST SRM610 glass were RESULTS 0.1194 and 146Nd/144Nd = 0.7219, respectively, used as external standards for the calibration for mass fractionation corrections. During the of Pb/U ratios and concentrations, respectively. Zircon U-Pb Geochronology analyses, the NBS987 standard yielded an Offline selection and integration of background average 87Sr/86Sr value of 0.710239 ± 10 (2s) and and analyte signals and time-drift correction In this study, sample Q1211 from the the BCR-2 standard gave an average 143Nd/144Nd and quantitative calibration were performed Haoquangou pluton and sample Q1214 from the using ICPMSDataCal (Liu et al., 2010). Data Baimawa pluton were selected for zircon U-Pb correction and processing followed methods dating. Zircons from samples Q1211 and Q1214 1GSA Data Repository Item 2020066, data for standard reference materials used during our analyses (Tables similar to those in Yang et al. (2015). Zircon are mostly euhedral to subhedral. They show DR1–DR4), zircon U-Pb and Lu-Hf isotopic data for this standard GJ-1 was used as a reference standard. long to short prismatic crystals, 70–200 mm in study (Tables DR5 and DR6), partition coefficients used During the analyses, it gave a weighted average length and length-to-width ratios of 2:1–4:1. In for trace elemental modeling (Table DR7), and data sub- 206Pb/238U age of 608 ± 5 Ma (n = 7, MSWD CL images, they display weak oscillatory zoning sets used to calculate Moho depth and the calculated = 0.57) (Data Repository Table DR3), which (Figs. 4A and 4B). Fifteen zircon analyses Moho depth for the eastern North Qilian (Table DR8), is available at http://www.geosociety.org/datarepository is consistent with published LA-ICP-MS ages were obtained from sample Q1211 (Data /2020, or on request from [email protected]. for GJ-1 (Elhlou et al., 2006). Repository Table DR5), with U of 629–1231

0.074 0.074 439±5 Ma 460 460 Q1211 A 440±5 Ma Q1214 B 435±4 Ma 433±4 Ma +7.5 +7.1 450 +7.8 +7.7 450 0.072 0.072

100 μm U 440 U 100 μm 440 238 238 / 431±4 Ma / 431±5 Ma b 0.070 b 0.070 P P 206 206 430 430 +7.3

0.068 0.068 420 420 Mean = 436 ± 2 Ma Mean = 435 ± 2 Ma MSWD = 1.07, n = 15 MSWD = 1.13, n = 14 0.066 0.066 0.46 0.50 0.54 0.58 0.62 0.66 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 207Pb/235U 207Pb/235U Figure 4. Cathodoluminescence (CL) images of representative zircon grains and U-Pb concordia diagrams for (A) Haoquangou granodiorite (Q1211) and (B) Baimawa granodiorite (Q1214). The smaller white continuous circles show LA-ICP-MS dating spots and corresponding

U-Pb ages, and the larger white dashed circles show locations of Lu-Hf isotope analysis and corresponding eHf(t) values.

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ppm and Th of 310–564 ppm, and Th/U ratios (Data Repository Table DR5). They yield a to the Haoquangou granodiorite-trondhjemites of 0.37–0.59. All the analyses are concordant or weighted mean 206Pb/238U age of 435 ± 2 Ma and associated granite-porphyries reported by nearly concordant and yield a weighted mean (MSWD = 1.13) (Fig. 4B), representing the Wang et al. (2005b) in chemical composition 206Pb/238U age of 436 ± 2 Ma (MSWD = 1.07) crystallization age of the Baimawa pluton. (Figs. 5A–5D). The granodiorites in this study

(Fig. 4A), which is interpreted as the magma are subalkalic and characterized by high SiO2 crystallization age of the Haoquangou pluton. Major and Trace Elements (68.12–70.10 wt%) and CaO (2.07–3.51 wt%) Fourteen zircon analyses from sample Q1214 and low MgO (0.95–1.17 wt%) contents (Fig. display U and Th contents ranging from 666 The granodiorites from the Haoquangou and 5B), with Mg# of 46–49 (Table 1). They plot in to 1212 ppm and 292–654 ppm, respectively, Baimawa plutons share similar geochemical the granodiorite field in the total alkalis versus

with relatively uniform Th/U ratios of 0.41–0.54 characteristics (Table 1). They are also similar SiO2 classification diagram (Fig. 5A). All the

TABLE 1. MAJOR ELEMENT AND TRACE ELEMENT DATA Sample Q1210 Q1211 Q1213 Q1214 Q1215 Q1216 Q1217 Q1218 Pluton HQG HQG HQG BMW BMW BMW BMW BMW HQG* Rock type GD GD GD GD GD GD GD GD TR+GD+GP

Major elements (wt.%)

SiO2 69.37 69.19 70.10 68.12 69.25 68.67 69.49 68.97 69.32–72.38

TiO2 0.26 0.28 0.27 0.30 0.29 0.26 0.29 0.27 0.20–0.32

Al2O3 14.99 15.27 14.55 15.76 15.55 15.61 15.21 15.75 14.71–15.78 T Fe2O3 2.18 2.28 2.29 2.41 2.35 2.13 2.47 2.18 1.90–2.67 MnO 0.03 0.04 0.03 0.04 0.04 0.04 0.05 0.04 0.03–0.04 MgO 0.99 1.03 1.04 1.06 1.04 1.03 1.17 0.95 0.77–1.31 CaO 2.54 2.64 2.41 3.51 3.25 2.87 2.07 2.54 2.54–3.40

Na2O 4.01 4.18 3.85 4.09 4.15 4.47 3.85 4.29 4.17–4.50

K2O 2.16 2.13 2.05 1.71 2.02 1.79 2.65 2.11 1.70–2.16

P2O5 0.09 0.09 0.09 0.10 0.09 0.08 0.09 0.08 0.07–0.10 LOI 2.53 0.74 2.72 1.85 1.14 1.64 2.09 1.93 0.81–1.58 A/CNK 1.11 1.09 1.12 1.05 1.04 1.08 1.17 1.13 1.02–1.08 Mg# 47 47 47 47 47 49 48 46 45–50

K2O/Na2O 0.54 0.51 0.53 0.42 0.49 0.40 0.69 0.49 0.38–0.51 Trace elements (ppm) Cr 3.33 3.30 3.58 3.97 3.76 3.42 3.71 3.01 8.00–23.94 Co 80.4 83.4 88.2 101 82.7 89.7 65.2 88.6 143–188 Ni 3.96 3.45 3.91 4.35 3.59 3.27 3.92 3.72 4.80–12.6 Rb 68.9 65.2 69.9 53.4 66.2 53.5 77.8 63.8 45.7–68.9 Sr 369 435 376 493 471 536 343 485 438–540 Y 4.29 4.67 4.44 4.29 5.88 4.60 5.40 4.93 4.45–6.69 Zr 91.9 99.3 93.4 96.9 102 97.0 95.8 101 75.2–96.5 Nb 5.39 5.90 7.47 6.06 5.98 5.46 6.19 5.81 4.34–6.03 Cs 2.23 3.43 2.83 1.68 1.68 1.93 2.00 1.75 2.27–5.31 Ba 652 633 640 672 643 709 1062 847 796–1004 La 14.7 15.9 15.8 14.9 21.5 18.3 23.7 17.4 14.8–19.7 Ce 25.6 27.9 27.4 26.2 37.0 31.3 40.5 30.3 23.7–38.9 Pr 2.58 2.82 2.76 2.64 3.71 3.07 3.90 3.04 2.77–3.43 Nd 8.63 9.36 9.22 8.92 12.2 10.6 12.8 9.71 9.30–11.8 Sm 1.58 1.63 1.54 1.52 1.94 1.69 1.87 1.66 1.45–2.12 Eu 0.46 0.44 0.43 0.48 0.52 0.49 0.55 0.47 0.54–0.67 Gd 1.14 1.17 1.04 1.03 1.30 1.21 1.35 1.13 1.29–1.98 Tb 0.16 0.16 0.15 0.15 0.20 0.16 0.19 0.16 0.16–0.26 Dy 0.76 0.86 0.72 0.75 0.99 0.84 0.90 0.82 0.83–1.31 Ho 0.15 0.15 0.14 0.14 0.19 0.15 0.16 0.16 0.15–0.25 Er 0.36 0.40 0.33 0.33 0.48 0.39 0.44 0.44 0.38–0.62 Tm 0.061 0.062 0.059 0.057 0.074 0.054 0.059 0.064 0.056–0.09 Yb 0.37 0.41 0.32 0.35 0.48 0.38 0.43 0.41 0.39–0.58 Lu 0.050 0.058 0.042 0.049 0.064 0.053 0.055 0.065 0.065–0.088 Hf 2.38 2.53 2.34 2.44 2.58 2.48 2.40 2.53 2.56–3.20 Ta 0.62 0.59 1.43 0.61 0.80 0.53 0.95 0.64 0.49–0.70 Pb 12.9 16.9 12.3 13.3 17.0 16.2 20.5 16.5 Th 7.53 8.28 8.39 8.51 10.5 8.06 9.91 8.82 8.14–10.35 U 2.18 8.46 2.65 1.34 1.43 1.12 2.71 1.50 Sr/Y 86 93 85 115 80 117 63 98 70–121 Eu/Eu* 1.01 0.94 0.99 1.10 0.94 0.99 1.02 0.99 0.98–1.19

(La/Yb)N 28.69 27.66 34.90 30.28 32.21 34.36 39.94 30.20 18.92–32.00

T T T T Note: Fe2O3 = All Fe calculated as Fe2O3; Mg# = [100MgO/(MgO+FeO )]molar (FeO = 0.8998 * Fe2O3 ). HQG—Haoquangou pluton, BMW—Baimawa pluton, GD—Granodiorite, TR—Trondhjemite, GP—Granite-porphyry. *Data from Wang et al. (2005b).

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4.0 18 A B 3.5 15 3.0 Syenite 12 2.5 Quartz Monzonite

(wt.%) Monzonite

(wt.%) 2.0

O 9 2

Monzo- O K diorite g

M 1.5 O + 2 6 Monzo- gabbro Granite N a Diorite 1.0 3 Gabbroic Peridot- Granodiorite Experimental melts from mafic gabbro diorite Quartzolite 0.5 Gabbro lower crust (1–1.5 GPa) 0 0.0 30 40 50 60 70 80 90 57 62 67 72 77

SiO2 (wt.%) SiO2 (wt.%) 7 1.2 C Haoquangou granodiorite D Baimawa granodiorite 6 Haoquangou granodiorite-trondhjemite 1.0 and associated granite-porphyry from Wang et al. (2005b) 5 Shoshonite series 0.8 O 2 4 a N /

O 0.6 2 (wt.%)

3 K O 2 High-K calc-alkaline series 0.4 K 2

0.2 1 Calc-alkaline series Low-K (tholeiite) series 0.0 0 57 62 67 72 77 40 45 50 55 60 65 70 75 80

SiO2 (wt.%) SiO2 (wt.%)

Figure 5. Major element variation diagrams for the granodiorite samples from the Haoquangou and Baimawa plutons. (A) Total

alkali versus silica diagram (Middlemost, 1994); (B) MgO versus SiO2 plot; (C) K2O/Na2O versus SiO2 plot; (D) K2O versus SiO2 diagram (Peccerillo and Taylor, 1976). Data for experimental melts of lower continental crust are from Qian and Hermann (2013), and they have been normalized to 100% anhydrous totals.

samples possess high Na2O (3.85–4.47 wt%) [(La/Yb)N = 27.66–39.94] (Fig. 6B), with weak granodiorites (Table 2). Zircons from sample 176 177 and low K2O (1.71–2.65 wt%) contents and negative to weak positive Eu anomalies (Eu/Eu* Q1214 give Hf/ Hf ratios of 0.282693–

low K2O/Na2O ratios (0.40–0.69) (Figs. 5C = 0.94–1.10). 0.282751, with calculated eHf(t) values of +6.5

and 5D), and belong to a high-K calc-alkaline to +8.5 and TDM(Hf) of 0.71–0.79 Ga (Fig. 7; series (Fig. 5D). These rocks show a limited Sr-Nd-Hf Isotopes Data Repository Table DR6), which are also

variation of Al2O3 contents (14.55–15.76 wt%), similar to those of the zircons from Haoquangou

with relatively variable A/CNK values [molar The Haoquangou granodiorites have ISr granodiorite sample Q1211.

Al2O3/(CaO + Na2O + K2O)] ranging from 1.04 values of 0.7052–0.7054 and eNd(t) values of to 1.17. Therefore, these granodiorites exhibit –1.3 to +0.7, with depleted mantle Nd model ages DISCUSSION

peraluminous characteristics. [TDM(Nd)] of 1.02–1.23 Ga (Table 2). Zircons The granodiorite samples have low Cr (3.01– from sample Q1211 show relatively uniform Hf Petrogenesis 3.97 ppm) and Ni (3.27–4.35 ppm) contents. isotopic compositions, with 176Hf/177Hf ratios

Their Sr contents range from 343 to 536 ppm ranging from 0.282674 to 0.282750, eHf(t) Magmatic Processes and Magma Source and Y contents from 4.29 to 5.88 ppm, resulting values from +5.9 to +8.5, and depleted mantle The Haoquangou granitic rocks (including

in high Sr/Y ratios (63–117). In the primitive Hf model ages [TDM(Hf)] from 0.71 to 0.82 Ga the granodiorites in this study and the mantle-normalized trace element patterns (Fig. 7; Data Repository Table DR6). granodiorite-trondhjemites and associated (Fig. 6A), they show obvious enrichment of The Baimawa granodiorites show Sr-Nd granite-porphyries reported by Wang et al.,

U, K, Pb, and Sr and depletion of Nb, Ta, P, isotopic compositions (ISr = 0.7051–0.7058, 2005b) and the Baimawa granodiorites are all

and Ti, with slightly positive Zr-Hf anomalies. eNd(t) = +0.5 to +1.1, TDM(Nd) = 0.93–1.00 characterized by high Sr (343–540 ppm) and They have strongly fractionated REE patterns Ga) similar to those of the Haoquangou LREE (e.g., La = 14.7–23.7 ppm) and low

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10000 10000 Haoquangou granodiorite A High Sr/Y rocks derived from thickened lower B e

l Baimawa granodiorite

t crust (under 15-18.5 kbar) in the North Qaidam n 1000 1000 Haoquangou granodiorite- a e t i M

r trondhjemite and associated

d e granite-porphyry from n

v 100 100 i o t

i Wang et al. (2005b) h m C i / r

10 k 10 P / c k o c R o 1 1 R Early Paleozoic high Sr/Y rocks 0.1 from the eastern North Qilian 0.1

0.01 0.01 Rb Th Nb K Ce Pr P Zr Sm Ti Tb Y Er Yb La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er TmYb Lu Ba U Ta La Pb Sr Nd Hf Eu Gd Dy Ho Tm Lu Figure 6. (A) Primitive mantle-normalized trace element spider diagrams and (B) chondrite-normalized REE patterns. Chondrite and primitive mantle-normalized values are from Sun and McDonough (1989). For comparison, fields of thickened lower crust- derived high Sr/Y rocks from the North Qaidam and high Sr/Y rocks from other areas of the eastern North Qilian are also shown. Data sources: North Qaidam—Yu et al. (2012, 2019a, 2019b); eastern North Qilian—Wang et al. (2005b, 2006a, 2008), Tseng et al. (2009), Li (2012), Chen et al. (2015, 2016), Yu et al. (2015), and Zhang et al. (2017b).

TABLE 2. WHOLE-ROCK Sr-Nd ISOTOPIC DATA

87 86 87 86 147 144 143 144 Sample Rb/ Sr Sr/ Sr ±2σ ISr Sm/ Nd Nd/ Nd ±2σ εNd(t) TDM(Nd) (Ga) Haoquangou pluton (t = 436 Ma) Q1210 0.541 0.708535 4 0.7052 0.111 0.512326 11 -1.3 1.23 Q1211 0.434 0.708085 4 0.7054 0.105 Q1213 0.539 0.708676 5 0.7053 0.101 0.512399 11 0.7 1.02 Baimawa pluton (t = 435 Ma) Q1214 0.313 0.707025 5 0.7051 0.103 Q1216 0.289 0.707331 6 0.7055 0.096 0.512376 11 0.5 1.00 Q1217 0.658 0.709854 6 0.7058 0.088 0.512385 13 1.1 0.93

87 86 147 144 Note: Rb/ Sr and Sm/ Nd ratios are calculated from Rb, Sr, Sm and Nd contents, measured by ICP-MS (Table 1); εNd(t) values are 147 144 143 144 calculated based on present-day ( Sm/ Nd)CHUR = 0.1967 and ( Nd/ Nd)CHUR = 0.512638; TDM(Nd) values are calculated based on present- 147 144 143 144 day ( Sm/ Nd)DM = 0.2137 and ( Nd/ Nd)DM = 0.51315.

20 Haoquangou granodiorite Baimawa granodiorite DM 15 Lower crust-derived high Sr/Y rocks in the eastern North Qilian 0.5 Ga Lower crust-derived high Sr/Y rocks in the western North Qilian 10 I-type granites in the North Qilian 1.0 Ga inherited zircons Transitional I-S type granites in the North Qilian 5 A-type granites in the North Qilian

) 1.5 Ga CHUR Slab-derived high Sr/Y rocks in the Central Qilian

( t 0 f H ε -5 2.0 Ga Figure 7. Plots of eHf(t) versus zircon U-Pb ages for the -10 studied samples. CHUR—chondritic uniform reservoir; DM—depleted mantle. Data sources: high Sr/Y rocks— 2.5 Ga -15 177 Yang et al. (2015), Yu et al. (2015), Zhang et al. (2017b), Li 176Lu/ Hf = 0.015 Mean crust et al. (2019); I-type granites—Zhao et al. (2014), Zhang -20 et al. (2017b); transitional I-S type granites—Chen et al. (2014); A-type granites—Zhang et al. (2017b). -25 350 450 550 650 750 850 950 Age (Ma)

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Y (4.29–6.69 ppm) and Yb (0.32–0.58 ppm) sedimentary rocks (pelites or greywackes) can have also been identified in the North Qilian contents, with high Sr/Y and La/Yb ratios, produce high Sr/Y melts. Although the Haoqu- (Chen et al., 2014; Zhang et al., 2017b). How- similar to those of adakite and Archean high-Al angou and Baimawa high Sr/Y rocks contain ever, the Haoquangou and Baimawa high Sr/Y TTG (Figs. 8A and 8B) (Martin, 1986; Defant (secondary) muscovite as Al-rich mineral and rocks differ significantly from those in their Sr-

and Drummond, 1990). High Sr/Y rocks can be the four of them have A/CNK [Molar Al2O3/ Nd-Hf isotopic compositions (Figs. 7 and 9A).

produced by the following mechanisms: partial (CaO + Na2O + K2O)] > 1.1, they cannot be Therefore, the metasedimentary rocks and felsic melting of subducted oceanic slab (Defant and explained as S-type granites generated by materials in the continental crust are not suit- Drummond, 1990) or mantle peridotites that anatexis of metasedimentary rocks (Chappell able sources for the generation of the high Sr/Y were metasomatized by slab melts (Martin et and White, 1974). First, they have much more rocks in this study. al., 2005), partial melting of continental lower depleted Sr–Nd isotopic compositions than The Haoquangou and Baimawa high Sr/Y

crust (Atherton and Petford, 1993; Wang et al., the typical Early Paleozoic S-type rocks in the rocks have higher SiO2 contents and lower Mg#,

2006b), fractional crystallization of basaltic North Qilian (Fig. 9A). Second, as depicted in Cr, and Ni contents (Table 1) than the low-SiO2 magmas with or without crustal assimilation Figures 10A–10D, their chemical compositions high Sr/Y rocks (Martin et al., 2005), which (Castillo et al., 1999; Macpherson et al., 2006), are different from those of experimental melts does not support their derivation by partial or mixing of mantle-derived mafic and crust- obtained from partial melting of greywacke melting of peridotitic mantle modified by slab- derived felsic magmas (Chen et al., 2013). and pelitic sources. The transitional I–S type derived melts. The major elements and Sr–Hf Based on geochemical modeling, Moyen and A-type granites that were interpreted to be isotopic compositions of the Haoquangou and (2009) suggested that the melting of meta- dominantly derived from felsic crustal materials Baimawa high Sr/Y rocks all fall within a small

500 150 A Haoquangou granodiorite B Baimawa granodiorite 400 Adakite Haoquangou granodiorite-trondhjemite and associated granite-porphyry from Wang et al. (2005b) 100 Adakite

Late Ordovician–Silurian (~460–418 Ma) N 300 ) magmatic rocks in the North Qilian b Y Y / r / a

S Eastern segment 200 Western segment ( L Aoyougou slab-derived trondhjemites 50 in the western segment 100 Typical arc rocks Typical arc rocks

0 0 0 10 20 30 40 0 5 10 15 20 25

Y (Yb)N 200 140 C Delaminated lower crust- D SQM delaminated lower 180 Subducted derived high Sr/Y rocks crust-derived high Sr/Y oceanic 120 rocks in the North Qilian 160 crust- LDQX thickened lower 140 derived 100 crust-derived high Sr/Y adakites rocks in the North Qilian 120 80 100 i (ppm) N C r (ppm) 60 80 Thickened lower 60 crust-derived 40 high Sr/Y rocks 40 20 20 0 0 50 55 60 65 70 75 50 55 60 65 70

SiO2 (wt.%) SiO2 (wt.%)

Figure 8. (A) Sr/Y versus Y diagram; (B) (La/Yb)N versus YbN diagram; (C–D) Cr and Ni versus SiO2 diagrams. In A and B, fields of adakite and arc rocks are from Martin (1986) and Drummond and Defant (1990). Data sources: eastern segment—Qian et al. (1998), Wu et al. (2004), Wang et al. (2005b, 2006a, 2008), Tseng et al. (2009), Li (2012), Xiong et al. (2012), Chen et al. (2015, 2016), Yu et al. (2015), Zhang et al. (2017b); western segment—Mao et al. (2000), Chen (2009), Wu et al. (2010, 2011), Zhao et al. (2014), Huang et al. (2017), Li et al. (2019); Aoyougou—Chen et al. (2012). In C and D, fields of subducted oceanic crust–derived adakites, thickened lower crust–derived high Sr/Y rocks, and delaminated lower crust–derived high Sr/Y rocks after Wang et al. (2006b). Data for SQM (Shenmutou, Quwushan and Maozangsi) and LDQX (Leigongshan, Daiqiangou, Quwushan and Xigela) high Sr/Y rocks are from Tseng et al. (2009) and Yu et al. (2015).

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12 14 Early Paleozoic A Haoquangou granodiorite B Thickened juvenile lower crust-derived ophiolites (NQ) high Sr/Y rocks in the North Qaidam Slab-derived high Baimawa granodiorite 8 12 Sr/Y rocks (NQ) Curve 1 Curve 2 Thickened lower crust- Curve 3 Curve 4 derived high Sr/Y rocks 10 4 and their source and residues (NQD) Slab-derived high Sr/Y rocks (CQ) 8 Zircon 4 5 0 Ma)

Baiyin mafic arc volcanic rocks (ENQ) )

= 0 ( t

( t Lower crust-derived high Sr/Y rocks (ENQ) Hf d ε

N Lianhuashan lower crust- Typical I-type granites (NQ) 6 ε I-S transitional derived high Sr/Y rocks in Lower crust-derived Slab-derived high Sr/Y -4 granites (NQ) the eastern North Qilian high Sr/Y rocks (WNQ) 4 rocks in the Central Qilian A-type granites (NQ)

-8 2 Orthogneisses (Q) Terrestrial array Paragneiss (Q) Typical S-type granite (NQ) -12 0 0.70 0.71 0.72 0.73 0.74 0.75 -4 -2 0 2 4 6

ISr εNd(t)

Figure 9. (A) eNd(t) versus ISr diagram. Data sources: Early Paleozoic ophiolites in the North Qilian—Qian and Zhang (2001), Hou et al. (2006); slab-derived high Sr/Y rocks—Chen et al. (2012), Yang et al. (2015); lower crust–derived high Sr/Y rocks in the North Qilian—Tseng et al. (2009), Yu et al. (2015), Zhang et al. (2017b), Li et al. (2019); I-type granites—Wu et al. (2011), Zhang et al. (2017b); S-type granites—Wu et al. (2011); transitional I-S type granites—Chen et al. (2014); A-type granites—Zhang et al. (2017b); thickened lower crust–derived high Sr/Y rocks and their source and residues in the North Qaidam—Yu et al. (2019a); Baiyin mafic arc volcanic rocks—Wang et al. (2005a); orthogneiss and paragneiss (the Huangyuan Group)—Zhang et al. (2006). Also shown are isotopic mixing curves with the dots along the four curves that link up the end-members at 5% intervals. Curves 1–4 show the results of contamination of slab-derived melt by crustal

materials. For the slab-derived melt end-member, the ISr and eNd(t) values of the average composition of the MORB samples are adopted for curve 1, and the Sr and Nd contents are taken from the average composition of the Aoyougou high-silica adakites (Chen et al., 2012). The elemental and initial isotopic data of crustal end-members in curves 1–4 are adopted from the average composition of the orthogneisses,

composition of the orthogneiss sample with the lowest ISr, average composition of the paragneiss, and composition of the paragneiss

sample with the lowest ISr of the Huangyuan Group (Qilian Precambrian basement) (Zhang et al., 2006), respectively. NQ—North Qilian;

ENQ—eastern North Qilian; WNQ—western North Qilian; CQ—Central Qilian; NQD—North Qaidam; Q—Qilian. (B) εHf(t) versus eNd(t). Data sources: Lianshuashan high Sr/Y rocks—Zhang et al. (2017b); slab-derived rocks in the Central Qilian—Yang et al. (2015); high Sr/Y rocks in the North Qaidam—Yu et al. (2012, 2019a, 2019b). Global terrestrial array is from Vervoort et al. (1999).

range of variations (Figs. 5A, 5B, 5D, 7, and Paleozoic ophiolites and slab-derived high Sr/Y As shown in Figure 9A, the contamination of 9A) and many elements (e.g., MgO, Cr and rocks in the North Qilian and Central Qilian slab-derived melt by ~10%–20% felsic crustal Ni) for each type of rock do not display well- (Fig. 9A). However, derivation from subducted materials represented by Qilian Precambrian

defined correlations with SiO2 (Figs. 5B and oceanic slab cannot be readily precluded basements with different trace elemental and 8C–8D). These characteristics, combined with considering the possible involvement of felsic isotopic compositions can roughly reproduce

their high SiO2 and low MgO contents and the crustal materials in their genesis. Here, three the isotopic data of this study. However, for the lack of mafic endmember within the pluton and scenarios, including partial melting of MORB Haoquangou and Baimawa high Sr/Y rocks, the

contemporaneous mafic rocks in the study area, and subducted sediment, contamination of slab- sample with the lowest eNd(t) value also shows

suggest that fractional crystallization of mantle- derived melts by crustal materials, and mixing a lower ISr value than the samples with higher

derived magmas or mixing of mantle- and crust- between slab-derived and crust-derived melts, eNd(t) values (Table 2; Figs. 11B and 11C), derived magmas cannot well account for their were evaluated based on isotopic modeling and inconsistent with the gradual contamination of genesis. As shown in Figures 10A–10D, the other geological and geochemical evidence. The felsic crustal materials. Moreover, involvement major element compositions of the Haoquangou roughly negative correlation between the Th/Ce of felsic crustal materials with evolved isotopic

and Baimawa high Sr/Y rocks are similar to and ISr of the Haoquangou and Baimawa high compositions into the slab-derived melts would

the experimental melts obtained from partial Sr/Y rocks is in conflict with involvement of result in the decrease of both eNd(t) and eHf(t) melting of meta-basaltic rocks; thus they were subducted sediment (Fig. 11A) (Hawkesworth values for the generated melts. However, most likely derived from mafic lower crust or et al., 1997). In addition, the involvement of a compared with the Early Paleozoic slab- subducted oceanic slab. sediment component would result in decreased derived high Sr/Y rocks in the Central Qilian

The LILE (e.g., Rb = 45.7–77.8 ppm; Mg#, Sr/Y and eNd(t) values for oceanic slab- (Yang et al., 2015), which have Sr–Nd isotopic

Th = 7.53–10.5 ppm) contents of the Haoquangou derived high Sr/Y rocks. However, Sr/Y ratios for compositions (ISr = 0.7041–0.7069, eNd(t) = +2.8 and Baimawa high Sr/Y rocks are distinctly the Haoquangou and Baimawa high Sr/Y rocks, to +3.8) similar to those of the slab-derived

higher than those of the predicted MORB- with Mg# and eNd(t) decreasing, show a slight Aoyougou high Sr/Y rocks in the North Qilian derived melts by geochemical modeling (Martin increase instead of a systematic decrease (Fig. (Fig. 9A), the Haoquangou and Baimawa high et al., 2014). Moreover, the Haoquangou and 11F; Tables 1 and 2). Thus, melting of MORB Sr/Y rocks show similar Hf isotopes (Figs. 7 Baimawa high Sr/Y rocks possess more evolved and subducted sediment cannot account for the and 9B), though with clearly more evolved Sr-Nd isotopic compositions than the Early generation of high Sr/Y rocks in this study. Sr-Nd isotopic compositions (Figs. 9A and

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14 Haoquangou granodiorite A 1.4 B )

2 Baimawa granodiorite

O 12 i Haoquangou granodiorite- T ) 1.2 2 + trondhjemite and associated O O 10 i g

granite-porphyry from T + M Wang et al. (2005b) 1.0 + O T 8 g O Experimental melts M

e 0.8 +

Amphibolite sources T ( F O ) / 6 Greywacke sources e O 0.6 2 ( F /

K Pelite sources + 4 O O 0.4 2 C a a

( N 2 0.2

0 0.0 4 9 14 19 0 5 10 15 20 25 30 Na O+K O+FeOT+MgO+TiO T 2 2 2 CaO+FeO +MgO+TiO2 0.6 100 C D 0.5 O 0.4 C a 10 ) / 3 O O 2 2 l K A /

0.3 + O O 2 a C a

0.2 ( N 1

0.1

0.0 0 10 15 20 25 30 35 4 6 8 10 12 14 CaO+Al O 2 3 Na2O+K2O+CaO

T T T Figure 10. Plots of (A) (Na2O+K2O)/(FeO +MgO+TiO2) versus Na2O+K2O+FeO +MgO+TiO2, (B) CaO/(FeO +MgO+TiO2) versus T CaO+FeO +MgO+TiO2, (C) CaO/Al2O3 versus CaO+Al2O3, and (D) (Na2O+K2O)/CaO versus Na2O+K2O+CaO. Data sources: experimental pelite–derived melts—Vielzeuf and Holloway (1988), Patiño Douce and Johnston (1991), Patiño Douce and Harris (1998), and Patiño Douce and McCarthy (1998); experimental greywacke–derived melts—Skjerlie and Johnston (1993), Patiño Douce and Beard (1995, 1996), Montel and Vielzeuf (1997), and Patiño Douce and McCarthy (1998); experimental amphibolite–derived melts from Beard and Lofgren (1991), Rapp et al. (1991), Rushmer (1991), Sen and Dunn (1994), Wolf and Wyllie (1994), and Patiño Douce and Beard (1995). All the experimental data have been normalized to 100%, anhydrous totals.

9B), conflicting with the model of mixing oceanic crustal melting with different degrees of eHf(t) at given eNd(t), showing decoupling of the slab-derived melts with crust-derived melts. involvement of subducted sediment, overlying Lu-Hf and Sm-Nd isotopic systems (Fig. 9B). Furthermore, there is no significant positive mantle, and crustal materials (Figs. 8C–8D), This Nd-Hf decoupling was not observed in the

correlation between Sr/Y with Mg# or eNd(t) indicate that the Haoquangou and Baimawa Early Paleozoic slab-derived high Sr/Y rocks in for the Haoquangou and Baimawa high Sr/Y high Sr/Y rocks could not be generated by the Central Qilian (Fig. 9B), but was identified rocks (Fig. 11F; Tables 1 and 2), which also slab melting. in the Silurian newly emplaced (arc-related) does not support the model of involvement of As shown in Figures 5B and 5C, the major mafic lower crust-derived Dulan TTG-like crustal melts into slab-derived melts. Combined element compositions of the Haoquangou and high Sr/Y rocks in the North Qaidam adjacent with their limited variation of major and trace Baimawa high Sr/Y rocks resemble those of the to the Qilian and the newly underplated juvenile element and Sr-Hf isotopic compositions (Figs. melts formed from melting experiments on the basaltic lower crust-derived Lianhuashan high 5, 6, 7, and 9A; Tables 1 and 2), we suggest lower continental crust. Also, their Sr-Nd-Hf Sr/Y rocks in the eastern North Qilian. And, that the Haoquangou and Baimawa high Sr/Y isotopes lie in the range of the Early Paleozoic the decoupling was linked to the subduction- rocks could not result from mixing of slab- granitoids that were suggested to be derived related metasomatism in the mantle source of derived and crust-derived melts. The arguments from mafic lower crust in the North Qilian the arc-related magma sources of the high Sr/Y above, together with the relatively low Cr and (Figs. 7 and 9A). On the other hand, the Nd rocks (Yu et al., 2012; Zhang et al., 2017b; Yu et Ni contents of the Haoquangou and Baimawa and Hf isotopic data of the Haoquangou and al., 2019a, 2019b). Moreover, the Haoquangou high Sr/Y rocks as compared with those of Baimawa high Sr/Y samples plot above the and Baimawa high Sr/Y rocks also share typical high Sr/Y rocks that were derived by global terrestrial array and deviate toward higher similar Sr-Nd isotopic compositions with the

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0.708 5 0.7059 A B 4 C 0.7057 0.707 3 0.7055 0.706 2 1 ) r r 0.7053 ( t S S 0.705 d 0 I I N 0.7051 ε -1 0.704 0.7049 -2 0.703 -3 0.7047 -4 0.7045 0.702 -5 0.15 0.20 0.25 0.30 0.35 0.40 67 68 69 70 71 72 73 67 68 69 70 71 72 73 Th/Ce SiO (wt.%) SiO (wt.%) 140 60 2 130 2 130 D 55 E 120 F 120 50 110 110

b 45 100 Y Y Y / /

100 / r r a

S 40 S 90

90 L 35 80 80 70 30 70 60 25 60 50 20 50 67 68 69 70 71 72 73 67 68 69 70 71 72 73 43 45 47 49 51

SiO2 (wt.%) SiO2 (wt.%) Mg# Haoquangou granodiorite Baimawa granodiorite Haoquangou granodiorite-trondhjemite and associated granite- porphyry from Wang et al. (2005b)

Figure 11. Plots of (A) ISr versus Th/Ce; (B) ISr versus SiO2; (C) eNd(t) versus SiO2; (D) Sr/Y versus SiO2; (E) La/Yb versus SiO2; and (F) Sr/Y versus Mg#.

Early Ordovician mafic arc volcanic rocks in from melting pressure, source composition Yb and Sr/Y ratios of high Sr/Y rocks derived the Baiyin area (Wang et al., 2005a; Fig. 9A). also plays an important role in the generation by partial melting of mafic rocks are commonly Therefore, we suggest that the Haoquangou and of high Sr/Y signatures for high Sr/Y rocks attributed to the existence of garnet or Baimawa high Sr/Y rocks resulted from partial (Moyen, 2009; Qian and Hermann, 2013; Ma amphibole in their sources. Here, we used trace melting of relatively juvenile mafic rocks in et al., 2015), which means that the higher the element geochemical modeling to constrain the the lower crust that could be the products of Sr/Y ratios of the sources, the lower the required proportions of residual phases during the partial early arc magmatism. It should be noted that the pressures for the generation of high Sr/Y melting processes responsible for generation of high Sr/Y rocks (including those in this study) signatures. Thus, the continental crust can melt the Haoquangou and Baimawa high Sr/Y rocks. in the North Qilian, which were suggested to to produce high Sr/Y rocks at pressures lower The modal batch melting equation was used for be derived from lower crust, show relatively than those for MORB-like rocks. For example, modeling, and the compositions of the lower variable Nd-Hf isotopic compositions (Figs. 7 mafic continental crust can generate high Sr/Y continental crust of Rudnick and Gao (2003) and 9). Thus, the Early Paleozoic lower crust melts through partial melting at a pressure of 10 were assumed as the compositions of the source in the North Qilian could be inhomogeneous kbar (equivalent to crustal thicknesses of ~33 rock. The results show that residues of amphibole- in composition and probably contained both km), and the metasedimentary sources (pelites and plagioclase-dominated mineral assemblage juvenile and mature materials. or greywackes) may produce high Sr/Y melts without garnet in the source cannot explain the at pressures as low as 5–10 kbar (Moyen, 2009; low HREE contents and positive Sr anomalies of Constraints on Pressure Conditions for Qian and Hermann, 2013; Ma et al., 2015). the Haoquangou and Baimawa high Sr/Y rocks, Magma Generation Therefore, the occurrence of continental high whereas modeled melts in equilibrium with the Previous experimental studies have Sr/Y rocks may not necessarily indicate crustal residual assemblage containing ~30% garnet suggested that magma generation of high Sr/Y thickening, and detailed petrogenetic analysis and minor plagioclase (~10%) display trace rocks by melting of mafic rocks requires high should be done for high Sr/Y rocks before using element characteristics similar to those of the pressures (Rapp et al., 1991; Sen and Dunn, them to indicate whether or not the continental Haoquangou and Baimawa high Sr/Y samples 1994; Rapp and Watson, 1995). Thus, many crust was thickened. (Figs. 12A and 12B). Compared with the results researchers linked the generation of high Sr/Y Although metasedimentary rocks are able to of melting experiments on mafic lower crust with rocks to crustal thickening or lithospheric generate high Sr/Y melts at low pressures (5–10 major elements similar to those of our assumed delamination (Xu et al., 2002; Chung et al., kbar), leaving garnet as a restite phase (Moyen, source in the geochemical modeling (Qian and 2003; Wang et al., 2006b). However, previous 2009), the geochemical data suggest that, as Hermann, 2013), this coexisting garnet-bearing experiments were mainly conducted on MORB- discussed previously, the Haoquangou and residual assemblage indicates high pressure like sources with low Sr/Y ratios. Recent Baimawa high Sr/Y rocks were derived from (> 12.5 kbar, equivalent to crustal thicknesses of experiments on mafic continental crust and mafic lower crust rather than metasedimentary > 42 km) partial melting. Also, the Haoquangou geochemical modeling have argued that, apart sources. The low HREE contents and high La/ and Baimawa high Sr/Y rocks show lower HREE

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1000 1000 A F = 0.1 B Haoquangou granodiorite F = 0.2 Baimawa granodiorite F = 0.3 Haoquangou granodiorite-trondhjemite e e l l t t 100 F = 0.4 100 and associated granite-porphyry from n n a a Wang et al. (2005b) M M Lower continental crust

e e v

v (Rudnick and Gao (2003) i i t t i i 10 10 m m i i r r P P / / Residue: Residue: k k c c Amp 43% Gt 30% o o 1 1 R R Cpx 10% Amp 35% Opx 10% Cpx 25% Pl 37% Pl 10% 0.1 0.1 RbBaThNbTa La Ce SrNd Zr HfSmEu Ti GdTbDy Y Er Yb Lu RbBaThNbTa La Ce Sr Nd Zr HfSmEu Ti GdTbDy Y Er Yb Lu 1000 1000 C 10 kbar, 800°C D 13.5 kbar, 900°C e e 10 kbar, 900°C

l 15 kbar, 800°C l t t 10 kbar, 1000°C 100 100 15 kbar, 900°C M a n M a n 12.5 kbar, 800°C 15 kbar, 1000°C e e v v i i 12.5 kbar, 900°C 15 kbar, 1050°C t t

12.5 kbar, 1000°C m i m i 10 10 P r i P r i / / c k c k R o R o 1 1

0.1 0.1 RbBa Nb La Ce Pr Sr Nd Zr Hf SmEu Ti GdDy Y Er Yb Lu RbBa Nb La Ce Pr Sr Nd Zr Hf SmEu Ti GdDy Y Er Yb Lu 5 Experimental melts from E mafic sources coexisting F with garnet-bearing 26

) 4 residues at 10–32 kbar: O

2 10–12 kbar K O 2 + 12.5 kbar 21 a O 2 13.5–15.7 kbar N /

a 3 ) 16–18 kbar 3 ( N O 2 ) /

l 16

2 20–22 kbar A O i 25–27 kbar + T T 2 O + 29.5–32 kbar T e 11 O F ( e

( F 1 6

0 1 50 55 60 65 70 75 80 50 55 60 65 70 75 80

SiO2 SiO2 40 G 14 H

T 35 12 O e

F 10 + T O O

a 30 e C

F 8 + 3 O 2 l 25 6 A

4 20 2

15 0 50 55 60 65 70 75 80 0 2 4 6 8 10

SiO2 CaO

Figure 12. (A–D) Primitive mantle-normalized trace element spider diagrams for the Haoquangou and Baimawa high Sr/Y rocks and modeled melts from mafic lower crust. In A and B, primitive mantle normalized values are from Sun and McDonough (1989); compositions of lower continental crust are from Rudnick and Gao (2003); proportions of the residual mineral phases refer to the estimation by Qian and Hermann (2013); the partition coefficients used for modeling calculations are listed in Data Repository Table DR7. In C and D, the modeled lower continental crust–derived melts at 10–15 kbar T T using partition coefficients obtained by melting experiments are from Qian and Hermann (2013). (E–G) (FeO +TiO2)/(Na2O+K2O), (FeO +Al2O3)/Na2O and T T Al2O3+CaO+FeO versus SiO2. (H) FeO versus CaO plots. In E–H, data for experimental melts coexisting with garnet are from Rapp et al. (1991), Sen and Dunn (1994), Wolf and Wyllie (1994), Patiño Douce and Beard (1995), Rapp (1995), Rapp and Watson (1995), Skjerlie and Douce (1995, 2002), Winther (1996), Springer and Seck (1997), Lopez and Castro (2001), Prouteau et al. (2001), Pertermann and Hirschmann (2003), Xiong et al. (2005), Zhou et al. (2005), Clemens et al. (2006), Klimm et al. (2008), Xiong et al. (2009), Adam et al. (2012), Qian and Hermann (2013), and Zhang et al. (2013a), and only

melts with SiO2>53% were shown for comparison. All the experimental data have been normalized to 100%, anhydrous totals.

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and higher La/Yb than the lower continental (Figs. 7 and 9A). In particular, their Hf isotopic the association of low-Mg high Sr/Y, high-Mg crust–derived melts at 10–12.5 kbar modeled compositions, like those of high Sr/Y rocks high Sr/Y, low Sr/Y and A-type granitoids by Qian and Hermann (2013) using mineral in this study, clearly plot below the depleted with decreasing magma crystallization ages partition coefficients obtained by melting mantle evolution curve (Fig. 7), suggesting in the eastern North Qilian well record crustal experiments (Figs. 12C–12D), suggesting that that continental crustal materials were also thickening, delamination, and thinning during they were generated at higher pressures. On significantly involved in their genesis. the Late Ordovician to Silurian period. the other hand, the stability of garnet is in part The 436–435 Ma Haoquangou and Baimawa On the other hand, we use the Sr/Y and

controlled by the bulk composition of source high Sr/Y rocks with high SiO2 (68.12–72.38%) La/Yb ratios of the intermediate magmatic rocks and it can be stable over a large range of and low Mg# (< 50) are suggested to be most rocks to calculate the variation of crustal pressures (≥ 10 kbar) during partial melting of likely derived from thickened lower crust as thickness in the eastern North Qilian during mafic rocks (Wolf and Wyllie, 1994; Rapp and mentioned previously. As shown in Figure 14A, the Late Ordovician to Silurian according to Watson, 1995). If the mafic sources are relatively other 457–435 Ma high Sr/Y rocks in the eastern the method proposed by Hu et al. (2017). The enriched in Fe, Al, or Ca, garnet can occur as a North Qilian also show relatively low Mg# and results show that the eastern North Qilian residual phase at relatively low pressures (10–12 have been suggested to be the products of melting experienced crustal thickening beginning at kbar), but in this situation, the coexisting melts of thickened lower crust without interaction with ~460 Ma and crustal thinning after ~435–430 are also characterized by relatively high Fe, Al, mantle materials (Wang et al., 2005b; Tseng et Ma (Fig. 14F), consistent with the conclusion or Ca contents (Wolf and Wyllie, 1994; Rapp, al., 2009; Yu et al., 2015; Zhang et al., 2017b). In constrained by the petrological and geochemical 1995; Skjerlie and Douce, 1995, 2002). As this regard, the wide distribution of the 457–435 data mentioned above. Moreover, the Middle– shown in Figures 12E–12H, the major element Ma low-Mg high Sr/Y magmatism in the eastern Upper Silurian strata are absent in the eastern compositions of the Haoquangou and Baimawa North Qilian implies that the continental crust North Qilian (Xu et al., 2013), in line with strong high Sr/Y rocks that were generated in the garnet in the eastern North Qilian had been thickened uplift associated with lithospheric extension and stability field (Fig. 12A–12B) are different from during this period. mantle upwelling after the Early Silurian. those of experimental melts in equilibrium with However, from 434 to 430 Ma, the Mg# of garnet-bearing assemblages during melting of the high Sr/Y rocks in the eastern North Qilian Implications for Early Paleozoic mafic rocks at 10–12 kbar, but are similar to increased significantly as compared with those Diachronous Ocean Basin Closure and those of melts coexisting with garnet at higher of the 457–435 Ma high Sr/Y rocks and the Continental Collision from East to West

pressures (12.5–32 kbar). Moreover, the trace SiO2 of the 434–430 Ma high Sr/Y rocks also in the North Qilian Belt elemental compositions of the Haoquangou decreased (Figs. 14A–14B), indicating more and Baimawa high Sr/Y rocks all fall into the contributions from mantle materials in their The subduction polarity of the North Qilian range of the newly identified Silurian high Sr/Y formation. Considering their relationship with Ocean is debated, and the proposed models leucosomes and tonalites in the adjacent North the related low-Mg high Sr/Y rocks and the include southward subduction (Peng et al., Qaidam (Fig. 6), which coexisted with their petrologic and geochemical constraints, most of 2017a, 2017b), northward subduction (Xu et al., metagabbroic sources and melting residues them were suggested to be produced by partial 2010; Yang et al., 2012; Song et al., 2013; Xia et and were constrained to form in the thickened melting of delaminated lower continental crust al., 2016), and bidirectional subduction (Wu et lower crust at 15–18.5 kbar (Yu et al., 2012; Yu followed by interaction with mantle peridotite al., 2011). However, considering the distribution et al., 2019a, 2019b). Therefore, we suggest that (Tseng et al., 2009; Yu et al., 2015). Also, as of Early Paleozoic mid-ocean-ridge type the Haoquangou and Baimawa high Sr/Y rocks shown in Figures 14C–14E, compared with ophiolites, high pressure metamorphic rocks, were derived from thickened crust. those of older low-Mg high Sr/Y rocks in the arc magmatic rocks, and back-arc ophiolites in eastern North Qilian, the overall higher Y and the North Qilian from south to north (Liu et al.,

Secular Crustal Evolution in the Eastern Yb contents and lower SiO2/Yb, Sr/Y, and ISr 2006; Tseng et al., 2007; Song et al., 2013; Xia North Qilian during the Late Ordovician values of the younger high-Mg high Sr/Y rocks et al., 2016), and the related sedimentary and to Silurian in the eastern North Qilian are also consistent structural characteristics, northward subduction with the interaction of lower crust–derived of the North Qilian Ocean during the Ordovician The variations of Sr/Y and La/Yb ratios of high Sr/Y melts with mantle materials. Thus, was supported (Xu et al., 2010; Yang et al., 2012; granitoids have been linked to crustal thickness delamination of thickened lower crust probably Song et al., 2013; Xia et al., 2016). Nevertheless, changes in magmatic arcs and continental occurred after thickening of the continental crust the closure time of the North Qilian Ocean is collisional belts (Chung et al., 2009; Schwartz in the eastern North Qilian. still controversial, with inferred ages dominantly et al., 2011; Chapman et al., 2015; Hu et al., After 430 Ma, the Sr/Y and La/Yb ratios varying from the Late Ordovician to Late 2017), although they can also be controlled of magmatic rocks in the eastern North Devonian (Bian et al., 2001; Liu et al., 2006; by source compositions apart from melting Qilian clearly decreased (Figs. 13A and 13C). Xiao et al., 2009; Yang et al., 2012; Song et al., pressure. As shown in the Sr/Y and La/Yb Especially the 419–414 Ma A-type granites and 2013; Xia et al., 2016). It should be noted that versus age diagrams (Figs. 13A and 13C), the their differentiates have been identified in the most tectonic models proposed by previous Sr/Y and La/Yb ratios of the magmatic rocks in eastern North Qilian (Zhang et al., 2017b). As researchers were based on the assumption the eastern North Qilian are obviously high at pointed out by Zhang et al. (2017b) and Xiong that the evolution of the western and eastern ~457–430 Ma as compared with those of older et al. (2012), most of these low Sr/Y granitoids parts of the North Qilian belt are synchronous, or younger rocks, and many high Sr/Y plutons in the eastern North Qilian could be generated and thus neglect the differences between them were generated during this period (Fig. 1B). by partial melting of relatively shallow crustal along the orogenic strike. However, the results Compared with the Haoquangou and Baimawa materials in an extensional setting. Therefore, of this study combined with previous work high Sr/Y granodiorites, they show similar or strong crustal thinning could occur after show that distribution of Early Paleozoic high more evolved Sr-Hf isotopic compositions delamination of thickened lower crust. Thus, Sr/Y rocks is inhomogeneous along the North

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200 A Late Ordovician–Silurian 200 B Late Ordovician–Silurian magmatic rocks in the magmatic rocks in the eastern North Qilian western North Qilian 150 150 Aoyougou slab-derived trondhjemites Y Y / / r r

S 100 S 100

Late Ordovician–Silurian 50 50 magmatic rocks in the eastern North Qilian

0 0 415 425 435 445 455 465 415 425 435 445 455 465 Age (Ma) Age (Ma) 100 100 90 C 90 D 80 80 Late Ordovician–Silurian 70 70 magmatic rocks in the eastern North Qilian

b 60 b 60 Y Y / /

a 50 a 50 L L 40 40 30 30 20 20 10 10 0 0 415 425 435 445 455 465 415 425 435 445 455 465 Age (Ma) Age (Ma) 75 465 E F 65 455 55 #

445 g 45 M Age (Ma) 435 35

425 25

415 15 96°E 98°E 100°E 102°E 104°E 106°E 96°E 98°E 100°E 102°E 104°E 106°E Longitude Longitude 100 250 G 90 H 80 200 70

b 60 Y Y / /

r 150

a 50 S L 40 100 30 50 20 10 0 0 96°E 98°E 100°E 102°E 104°E 106°E 96°E 98°E 100°E 102°E 104°E 106°E Longitude Longitude

Figure 13. Plots of (A–B) Sr/Y versus age, (C–D) La/Yb versus age, and (E–H) age, Mg#, Sr/Y, and La/Yb versus longitude for the Late Ordovician to Silurian magmatic rocks in the eastern and western North Qilian. In B and D, the magmatic rocks from eastern North Qilian are indicated by light gray circles for comparison. Data sources are the same as in Figure 8.

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70 65 A 73 B Early Paleozoic high Sr/Y rocks 60 in the eastern North Qilian 55 68 50 (wt%) g # 45 2 M

O 63 i

40 S 35 58 30 25 20 53 415 425 435 445 455 465 415 425 435 445 455 465 Age (Ma) 1.3 Age (Ma) 200 C D 1.1

150 0.9 Y

r / 0.7 S 100 b (ppm) Y

0.5 1.1 b Y 50 0.3

0.3 I (t) 0 0.1 0.705 Sr 0.708 30 35 40 45 50 55 60 65 30 35 40 45 50 55 60 65 Mg# Mg#

) 100 250 16 4 E F Eastern North Qilian (Y*10 / 2

O Present Moho 200 i 80 S depth of the )

4 eastern North 6 I (t) Sr Qilian 150 0.705 0.708 60 b*10 ( Y / 2

O 40 i 100 S

20 50 Sr/Y-based data Current global average

(La/Yb)N-based data thickness of continental crust 0 0 Crustal thickness (or Moho depth) (km) 30 35 40 45 50 55 60 65 415 425 435 445 455 465 Mg# Age (Ma)

4 Figure 14. (A–B) plots of Mg# and SiO2 versus age, (C–E) plots of Sr/Y, Yb, and SiO2/(Yb*10 ) versus Mg# for the Early Paleozoic high Sr/Y rocks

in eastern North Qilian, and (F) changes in calculated crustal thickness from Sr/Y and (La/Yb)N over time for the eastern North Qilian. Insets in 87 86 4 D and E are plots of Yb versus ISr(t) (initial Sr/ Sr calculated at 450 Ma) and SiO2/(Y*10 ) versus ISr(t), respectively. Data sources in A–E: Wang et al. (2005b, 2006a, 2008), Tseng et al. (2009), Li (2012), Chen et al. (2015, 2016), Yu et al. (2015), Zhang et al. (2017b), and this study. Compiled and plotted data in F are included in Data Repository Table DR8. Present Moho depth of the eastern North Qilian is from Zhang et al. (2013b). Current global average thickness of continental crust is from Rudnick and Gao (2003). It should be noted that the present structure of continental crust in the Qilian orogen has been significantly affected by Cenozoic Indo-Eurasia collision. Especially the continental crust of the Qilian orogen has been obviously shortened and thickened due to the compression from the Indian plate. The light gray line in F shows the polynomial regression of calculated crustal thickness for all data.

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Qilian belt: Early Paleozoic high Sr/Y rocks zircon geochronological data and the regional North Qilian during the Early Paleozoic period. were lacking in the western North Qilian (Figs. angular unconformity, indicate that the initial On the other hand, the occurrence of detrital 8A–8B, 13A–13D, and 13G–13H), except for collision in the eastern North Qilian occurred zircons of 550–500 Ma corresponding with the Aoyougou trondhjemites (~438 Ma) that in the Middle Ordovician (~470–460 Ma), the age of the North Qilian ophiolites within probably derived from the exhumed oceanic corresponding to the Gulang movement (Xu et the Early–Middle Devonian sandstones in the slab and the newly reported small Jingtieshan al., 2010). As discussed above, the eastern North eastern North Qilian (Xu et al., 2013) and the

granitic intrusion (~430 Ma) (Figs. 8A–8B, Qilian experienced clear crustal thickening from distinctly higher Fe2O3, MgO, Cr, Ni, Sc, and 9A, and 13; Chen et al., 2012; Li et al., 2019), ~460 Ma (Fig. 14F), which could be the response V, lower Zr and Hf contents, and higher Eu*/ while high Sr/Y plutons are abundant in the to the initial collision in the eastern North Qilian. Eu ratios of the Early–Middle Devonian clastic eastern North Qilian (Figs. 1B, 8A–8B, 13A– In contrast, the Late Ordovician in the western sedimentary rocks in the eastern North Qilian 13D, and 13G–13H), which shows relatively part of the Hexi-Corridor Basin is dominated and more metamorphic fragments within them enriched Sr-Nd isotopic compositions with by carbonate sediments (Feng and He, 1996), as compared with those of the time equivalent a larger range of magma crystallization ages indicative of an open ocean environment. For strata in the western North Qilian (Hou et al., from ~457 to 430 Ma (Figs. 9A and 14A–14B). the western Hexi-Corridor Basin, the initial 2018) indicate that the eastern North Qilian This implies that evolution of the eastern and mixing of the Central Qilian Block and North could have experienced stronger uplift and western parts of the North Qilian belt could China Craton detritus was identified in the erosion than the western North Qilian, which be asynchronous. This inference is supported Early Silurian sandstones (Yang et al., 2009, could account for the fewer occurrences of by the following facts. First, Early Paleozoic 2012), and thus the collision in the western Early Paleozoic ophiolites, high pressure ophiolites are mainly distributed in the western North Qilian probably commenced at the end metamorphic rocks, and arc-related magmatic part of the North Qilian and have been suggested of the Ordovician to Early Silurian. Moreover, rocks in the eastern North Qilian. to be the products of North Qilian Ocean in the western North Qilian, the subduction- Overall, we suggest that the Early Paleozoic seafloor spreading and development of back- induced high pressure metamorphism lasted collision in the North Qilian was diachronous arc basin at ~550–500 Ma and ~490–480 Ma, from ~489 to 446 Ma, as indicated by reliable from east to west (Fig. 15), and the eastern North respectively (Tseng et al., 2007; Xia and Song, geochronological data for the eclogite- and Qilian experienced stronger collision-related 2010; Song et al., 2013), while in the eastern blueschist-facies rocks (Song et al., 2004; processes (e.g., crustal thickening and post- North Qilian, the ophiolitic fragments were Liu et al., 2006; Zhang et al., 2007), also collisional extension) than the western North only reported in the Laohushan area (Qian and suggesting that the collision in the west of the Qilian where subduction-accretionary processes Zhang, 2001). Second, Early Paleozoic high- North Qilian occurred no earlier than the end predominated. This diachronous collision could pressure/low-temperature metamorphic rocks of the Ordovician. Therefore, the diachronous have resulted in the inhomogeneous intensity of (e.g., blueschist- to eclogite-facies metamorphic collision from east to west could have occurred orogenesis and thus inhomogeneous uplift and rocks) were only exposed in the western part in the North Qilian during the Early Paleozoic erosion between the eastern and western North of the North Qilian, the protoliths of which are and could be responsible for the differential Qilian. The stronger crustal thickening in the dominantly subduction-accretionary oceanic distribution of Early Paleozoic high Sr/Y eastern North Qilian resulted in the widespread basalts, gabbros, greywackes, olistostrome, magmatism (Figs. 13A–13D and 13G–13H) occurrence of Early Paleozoic high Sr/Y rocks. and pelagic/semi-pelagic sediments (Song et al., and other geological differences between the Our results also suggest that diachronous 2009, 2013). Finally, there are a large number eastern and western North Qilian. collision may lead to the large difference in of arc-volcanic and intrusive rocks represented High Sr/Y magmatism in the western North magmatism and deep crustal processes along by the Zhamashi (Alaska-type) mafic-ultramafic Qilian occurred much later than in the eastern the strike of the orogenic belts. intrusions (~513 Ma), the Chaidanuo granitic North Qilian (Fig. 13A and 13B). Also, except batholith (516–505 Ma), and the Dacha-Daba for the Aoyougou trondhjemites derived by CONCLUSIONS boninite complex (505–487 Ma) in the western decompression melting of exhumed eclogite North Qilian (Xia et al., 2012; Song et al., 2013; (meta-oceanic crust) (Figs. 8A–8B, 9A, 13B, The Haoquangou and Baimawa granodiorites Chen et al., 2014), while arc-volcanic rocks are and 13G) and the newly reported Jingtieshan from the eastern North Qilian have magma less exposed (Fig. 1B) and subduction-related granites (Chen et al., 2012; Li et al., 2019), the crystallization ages of 436 and 435 Ma, (or Alaska-type) mafic-ultramafic intrusions are other magmatic rocks generated in the western respectively. They have high Sr/Y ratios, coupled lacking in the eastern North Qilian. North Qilian during the Late Ordovician to with low Y and HREE contents and relatively Recently, the Middle Ordovician–Early Silurian all feature low Sr/Y ratios (Figs. 8A, enriched isotopic compositions. These high Silurian sedimentary sequences in the Hexi- 13B, and 13G). Moreover, unlike in the eastern Sr/Y rocks were generated by partial melting of Corridor Basin, which was converted from a North Qilian, the La/Yb ratios of the granitoids relatively juvenile mafic rocks in a thickened lower retro-arc basin to a foreland basin due to the in the western North Qilian do not show crust in a post-collisional setting. Petrological collision between the Central Qilian block and obvious variation from the Late Ordovician to and geochemical data, in combination with the North China Craton, were studied in terms Silurian (Figs. 13D and 13H). These, combined the calculation of crustal thickness variation, of their composition, detrital-zircon provenance, with the duration time and volumes of the high show that the crust in the eastern North Qilian and formation environment (Yang et al., 2009; Sr/Y magmatism, the absence of the extension- experienced clear thickening and thinning from Xu et al., 2010; Yang et al., 2012). The mixing of related magmatism (e.g., A-type granite), and the Late Ordovician to Late Silurian. Combined detritus from both the Central Qilian Block and the fact that the Lower to Upper Silurian is with previous work, we suggest that diachronous North China Craton has been detected within completely preserved in the western North collision proceeding from east to west in the North Middle–Late Ordovician clastic sedimentary Qilian (Xu et al., 2013), suggest that crustal Qilian, which probably resulted in stronger crustal rocks in the eastern Hexi-Corridor Basin (Xu thickening, thinning, and uplift in the western thickening and post-collisional extension in the et al., 2010). These, combined with detrital North Qilian were not as strong as in the eastern eastern North Qilian, was responsible for the

Geological Society of America | LITHOSPHERE | Volume 12 | Number 1 | www.gsapubs.org 69

North China Craton

A A′ A′

North Qilian Arc A Central Qilian Block Early–Middle Ordovician Eastern North Qilian Western North Qilian B

North China Craton

B(E) B(E)′ B(W) B(W)′ B ′ (W) B(E)′

North Qilian Arc B B (W) (E) Central Qilian Block Late Ordovician

Early stage of the Early Silurian

C C(E) C(E)′ C(W) C(W)′ North China Craton

C(W)′ C(E)′

Late stage of the Early Silurian North Qilian Arc C C ′ C(W) C(E) C(E)′ (W) (W) C(E) Central Qilian Block Early Silurian

Figure 15. Schematic diagram showing Ordovician–Silurian diachronous collision between the Central Qilian Block and the North China Craton (Alax Block). Figures on the left are modified from Xu et al. (2010).

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