Association of Permian gabbro and granite in the Langshan, southern Central Asian Orogenic Belt: Age, origin, and tectonic implications

Accepted Manuscript Association of Permian gabbro and granite in the Langshan, southern Central Asian Orogenic Belt: Age, origin, and tectonic implications

Rongguo Zheng, Jin Zhang, Wenjiao Xiao PII: DOI: Article Number: Reference:

S0024-4937(19)30333-0 https://doi.org/10.1016/j.lithos.2019.105174 105174 LITHOS 105174

To appear in:

LITHOS

Received date: Revised date: Accepted date:

3 March 2019 4 July 2019 13 August 2019

Please cite this article as: R. Zheng, J. Zhang and W. Xiao, Association of Permian gabbro and granite in the Langshan, southern Central Asian Orogenic Belt: Age, origin, and tectonic implications, LITHOS, https://doi.org/10.1016/j.lithos.2019.105174

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ACCEPTED MANUSCRIPT Association of Permian gabbro and granite in the Langshan, southern Central Asian Orogenic Belt: age, origin, and tectonic implications Rongguo Zheng†a, Jin Zhang a, Wenjiao Xiao

b,c,d

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a Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

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b Xinjiang Research Center for Mineral Resources, Xinjiang Institute of Ecology and Geography,

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Chinese Academy of Sciences, Urumqi 830011, China

Academy of Sciences, Beijing 100029, China

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c State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese

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d College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China

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†Corresponding author: Rongguo Zheng ([email protected])

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Abstract

The Permian magmatism in the Alxa region provides critical geological evidence to constrain the geodynamic processes in the southern Central Asian Orogenic Belt. The

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Shouji batholith is located in the southern Langshan, eastern Alxa region, and contains

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monzogranites, K-feldspar granites, and gabbros. New zircon U–Pb age data show that the

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Shouji batholith formed at 282–268 Ma. The Shouji granites have high (Na2 O+K 2O), Nb, and Zr contents, and high 10000*Ga/Al, FeO T /(FeOT +MgO), Yb/Nb, and Yb/Ta ratios,

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similar to the values of A2 -type granites. The Shouij granites exhibit enrichment in light

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rare earth elements (LREE) and large ion lithophile elements (LILE); depletion in Nb, Ti, and Sr; and enriched isotopic compositions. These Shouji granites were generated by

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partial melting of dehydrated tonalite–trondhjemite–granodiorite rocks of the Diebusige

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Complex in the lower crust. The Shouji gabbros are sodium- rich and low-K calc-alkaline rocks. They are enriched in LREE and LILE, depleted in high field strength elements, and

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have enriched isotopic compositions. These Shouji gabbros were generated by partial melting of a lithospheric mantle source metasomatized by melts derived from an ancient continental crust in an active continental margin. Roll-back of the Enger Us subducting slab accounts for generations of the Shouji A2 -type granites and gabbros. The zircon U–Pb ages of Carboniferous–Permian magmatism show a younging trend toward the northwest in the southern Alxa region. Evidence from late Paleozoic magmatism and arc-derived sediments indicates that the southern Alxa switched from an active continental margin to an intra-oceanic arc during the early Carboniferous–Late Permian as a result of retreat of the

ACCEPTED MANUSCRIPT subducting Enger Us slab.

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Key words: Central Asian Orogenic Belt, Alxa, A-type granite, slab roll-back

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1. Introduction

The Central Asian Orogenic Belt (CAOB) is one of the largest and long- lived accretionary orogenic collages in the world, which is situated between the Siberian and

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Baltica cratons to the north and the Tarim and North China cratons to the south (Fig. 1a,

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Sengör et al., 1993; Jahn et al., 2000; Kovalenko et al., 2004; Windley et al., 2007 ; Xiao et

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al., 2015, 2018). It is generally accepted that the CAOB grew by successive lateral

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accretion of arcs, accretionary complexes, and continental blocks southward from Siberia and southern Mongolia during the evolution of the Paleo-Asian Ocean (Windley et al.,

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2007; Xiao et al., 2015, 2018).The accretionary process may have lasted from the Neoproterozoic until the end of the Paleozoic (Mossakovsky et al., 1993; Jahn et al., 2000;

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Windley et al., 2007; Xiao et al., 2015, 2018). However, the accretionary processes during

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the late Paleozoic are still unclear, and details of the tectonic settings during the Carboniferous–Permian interval also remain controversial. Previous studies showed that

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Carboniferous–Permian subduction-related and almost coeval extensional events are widely recorded in the CAOB (Xiao et al., 2018), which make the Carboniferous–Permian tectonic setting more controversial. The Alxa Block and its vicinity (Alxa region for short) were involved in Paleozoic orogeny during evolution of the Paleo-Asian Ocean (PAO), which is key to understanding the architecture of the central segment of the southernmost CAOB (Fig. 1a). Late Paleozoic plutons are widely exposed in the Alxa region (Fig. 1b), and record important information on the evolution of the PAO. Zircon U–Pb data of plutons indicate that late Paleozoic

ACCEPTED MANUSCRIPT plutons in the northern margin of the Alxa Block, exhibit zircon U–Pb age peaks of 278 Ma, 270 Ma, and 248 Ma (Zheng et al., 2016). In the Alxa Youqi area, of the western Alxa region (Fig. 1b), late Paleozoic to Mesozoic plutons are widespread, including 418–239Ma granitoids and minor 328–249Ma gabbros–diorites (e.g., Liu et al., 2016 and references

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therein). The temporal–spatial pattern and petrogenesis of the late Paleozoic plutons

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provide an opportunity to elucidate the tectonic setting of the Alxa region.

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In this study, we obtained zircon U–Pb ages and whole-rock major- and trace-element, Sr–Nd isotopic, and zircon Lu–Hf isotopic compositions of granitoids and gabbros from the

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Shouji batholith in the southern Langshan, eastern Alxa. On the basis of our data, together

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with previously published data, we attempt to constrain the magma sources and petrogenetic processes of these rocks, and the geodynamic setting of late Paleozoic

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2. Geological background

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magmatism in the southern Langshan, southernmost of CAOB.

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The Alxa Block is located to the south of the CAOB (Fig. 1a). Late Archean and Paleoproterozoic metamorphic complexes and Neoproterozoic meta-supracrustal rocks are widely exposed in the Alxa Block (Geng et al., 2007). Some studies consider the Alxa Block to be the western extension of the North China Craton (NCC), whereas other researchers have proposed that the Alxa Block may have been an independent Paleoproterozoic terrane that became amalgamated with the NCC during the Phanerozoic (Zhang et al., 2012; Li et al., 2012; Yuan and Yang, 2015; Dan et al., 2016). Paleozoic strata and magmatic rocks are widespread in the Alxa region because of the

ACCEPTED MANUSCRIPT influence of the PAO. Three late Paleozoic ophiolitic mélanges have been reported in the northern margin of the Alxa Block: the Enger Us, Quagan Qulu, and Tepai ophiolitic mélanges (Wu and He, 1993; Wang et al., 1994; Zheng et al., 2014, 2018). The Enger Us ophiolitic mélange is located in the Enger Us fault belt. The tectonic blocks of the Enger Us

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ophiolitic mélange are mainly composed of ultramafic and mafic rocks, with a matrix of

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highly deformed Carboniferous clastic rocks and tuffs (Wu and He, 1993; Zheng et al.,

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2014). Massive and pillow basalts in the Enger Us ophiolitic mélange exhibit N–MORB geochemical affinities. A pillow lava sample yielded a zircon SHRIMP U–Pb age of 302 ±

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14 Ma (Zheng et al., 2014). Moreover, albaillellarians from cherts have been reported in the

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Enger Us ophiolitic mélange (Xie et al., 2014). The Quagan Qulu ophiolitic mélange is exposed in the Badain Jaran fault belt, in which the tectonic blocks mainly consist of

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lenticular and striped ultramafic rocks, gabbros, cherts, and rare basalts (Zheng et al., 2014).

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The Quagan Qulu ophiolite contains gabbro blocks with boninite-like geochemical characteristics. Zircons in a gabbro sample from the Quagan Qulu ophiolite yielded a SHRIMP zircon U–Pb age of 275 ± 3 Ma (Zheng et al., 2014). The Tepai ophiolitic

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mélange is situated in the northern Alxa imbricate thrust faults, northwest of the Alxa Youqi region. The tectonic blocks in the Tepai ophiolitic mélange are mainly composed of serpentinized peridotites, serpentinites, mylonitized gabbros, gabbros, basalts, and quartzites, with a matrix of highly deformed clastic rocks. A gabbro exhibits a zircon LA-ICP-MS U-Pb age of 278 ± 3 Ma (Zheng et al., 2018). The Tepai ophiolitic mélange is considered as the western continuation of the Quagan Qulu ophiolite because of the similarities of their zircon U–Pb ages and whole-rock geochemical compositions (Zheng et

ACCEPTED MANUSCRIPT al., 2018). The Alxa region is roughly divided into the northern and southern Alxa by the Enger Us ophiolitic belt. More importantly, on the basis of the different sedimentary sequences and magmatic events during the Paleozoic, the Alxa region can be divided into four

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transmeridional Paleozoic tectonic zones: the Yagan tectonic zone (YTZ), the

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Zhusileng–Hangwula tectonic zone (ZHTZ), the Zongnaishan–Shalazhashan tectonic zone

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(ZSTZ), and the Nuru–Langshan tectonic zone (NLTZ; Fig. 1b, Wu and He, 1993; Zheng et al., 2014). The NLTZ is characterized by extensive outcrops of Precambrian rocks and

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Paleozoic plutons (Wu and He, 1993; Zheng et al., 2014). Abundant late Paleozoic plutons

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were emplaced into the Precambrian strata in the NLTZ, implying that those Precambrian rocks were strongly reworked in the orogenic processes of the CAOB during the late

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Paleozoic (Geng and Zhou, 2012).

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The Langshan region is located in the eastern segment of the NLTZ (Fig. 1b). A high- grade metamorphic complex (the Diebusige Complex) and low-grade meta-volcanic and meta-sedimentary rocks (the Langshan Group) are widely distributed in the southern

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Langshan region (Fig. 2). The Diebusige Complex is mainly composed of mafic gneisses, paragneisses, tonalite–trondhjemite–granodiorite (TTG) rocks, amphibolites, quartzites, magnetite-quartzites, and minor marbles. The depositional ages of the protoliths of the Diebusige paragneisses are estimated to be between ca. 2.45 and 2.0 Ga (Dan et al., 2012). Protoliths of the Diebusige TTG exhibit Paleoproterozoic ages (2.41 Ga, our unpublished data). The Diebusige Complex was intruded by Paleoproterozoic granitoids (ca. 1.97–1.98 Ga), and affected by later metamorphic events at ca. 1.89 Ga and ca. 1.79 Ga (Dan et al.,

ACCEPTED MANUSCRIPT 2012). The Langshan Group is located to the northwest of the Diebusige Complex, and is mainly composed of meta-volcanic and meta-sedimentary rocks, such as marbles, quartzites, meta-sandstones, and mica-quartzite schists. The minimum detrital zircon U–Pb ages of meta-sedimentary rocks in the Langshan Group are 810–1187 Ma, indicating that

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these meta-sedimentary rocks were deposited during the Neoproterozoic (Hu et al., 2014).

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Neoproterozoic meta- volcanic rocks with zircon U–Pb ages of 804–816 Ma are also

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present in the Langshan Group (Hu et al., 2014).

Late Paleozoic sedimentary rocks are uncommon in the study area, and only some

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Carboniferous–Permian sediments are exposed in the west of the Langshan region (Fig. 1b).

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Paleozoic magmatic rocks are widely distributed in the Langshan region, including predominant plutons and minor volcanic rocks. Most plutons intruded into Precambrian

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units (Fig. 1), and yield Carboniferous–Permian zircon U–Pb ages. Zircon U–Pb data of

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plutons in the Langshan region indicate multi-periodic magmatism during the Paleozoic–Triassic (Wang et al., 2015 and references therein), of which the

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Carboniferous–Permian magmatiosm was most significant.

3. Sample description

The Shouji batholith is located in the southern Langshan, lies parallel to the strike of the mountain range, and has an area of more than 150 km2 . The Shouji batholith is mainly composed of granitoids and gabbros (Fig. 2). Granitoids can be observed to have intruded into gabbros in several outcrops (Fig. 2e). The Shouji batholith intruded into the Diebusige Complex southeastward, and intruded into the Langshan Group northwestward (Fig. 3a and

ACCEPTED MANUSCRIPT b). Granitoid samples were collected from the Shouji pluton (Fig. 2), including monzogranites (LZ04 and LZ06) and K- feldspar granites (Rg50 and L30). The K-feldspar granites are pinkish and coarse-grained (Fig. 3e and f). They are mainly composed of

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K-feldspar (50–60 vol.%), quartz (20–30 vol.%), and biotite (5–10 vol.%); the main

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accessory minerals are opaque oxides, apatite, and titanite (Fig. 3k). The monzogranites are

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gray and fined- grained (Fig. 3c and d). They mainly contain quartz (15–30 vol.%), alkali- feldspar (20–35 vol.%), plagioclase (20–35 vol.%), and biotite (5–10 vol.%) (Fig.

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3m).

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Five gabbro samples were collected from an elliptical pluton near Elesiting (Fig. 2), numbered Rg61-1, Rg61-2, Rg61-3, Rg61-4, and Rg61-5. Five gabbro samples were

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obtained from two elongated plutons (Fig. 2): samples of LZ07-1, LZ07-2, LZ07-3,

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LZ07-4, and LZ08-2. Pegmatite veins were observed in the outcrop (Fig. 3g). These gabbros are grayish-green, and display massive structures (Figs. 3e, g and h). They mainly contain olivine (5–10 vol.%), clinopyroxene (15–25 vol.%), plagioclase (40–55 vol.%), and

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hornblende (5–10 vol.%) with minor biotite and Fe–Ti oxides (Fig. 3i).

4. Analytical results

In this study, we analyzed zircon U–Pb ages, whole-rock major and trace element, Sr–Nd isotopes and in situ Lu–Hf isotopes in zircons for granitoids and gabbros from the Shouji batholith. Detailed analysis methods are presented in the Appendix Text 1. The U–Pb dating results, in situ Lu–Hf isotopes in zircons, whole-rock major and trace

ACCEPTED MANUSCRIPT elemental and Sr–Nd isotopic compositions are presented in the Appendix Table 1 to Table 4, respectively.

4.1 Zircon U-Pb ages and Hf isotopic compositions

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Represented CL images of zircon grains are presented in Fig. 4. The U–Pb dating

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results are presented as concordia diagrams (Fig. 5). Zircon Lu–Hf isotopes were also

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analyzed for dating samples from those plutons, with corresponding U–Pb dating on the same or similar domains (Fig. 6). The εHf(t) values and model ages are calculated using 206

Pb/238 U ages.

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their

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In general, the majority of zircon grains from granitoid samples (including L30–1, Rg50–2, LZ04–4 and LZ06– 2) have a rather simple morphology of magmatic origin. Those

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dated zircon grains are euhedral and prismatic, and most zircon grains display obvious oscillatory or planar zoning in the CL images (Fig. 4), indicating their magmatic origins.

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Zircon grains from samples of Rg61-3, LZ07-1 and LZ08-2 are subhedral, and exhibit weak concentric oscillatory zoning, also indicating their magmatic origins (Fig. 4).

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Zircons of all analyzed samples have variable Th and U concentrations (Appendix Table 1). The Th/U ratios of dating zircons vary between 0.10 and 1.47, and all of them are higher than 0.1 (Fig. 5h, Table 1). Thus, all these analyzed zircon grains have magmatic origins (Hoskin and Black, 2000).

Detailed descriptions of zircon U-Pb-Hf isotopic

compositions are listed in the Appendix Text 2, which are also summarized in a table as below (Table 1).

ACCEPTED MANUSCRIPT Table 1 Summary table of zircon U-Pb geochronological and Hf isotopic compositions for the Shouji batholith Sample

Th/U

No.

ratios

(176 Hf/177 Hf)i

Mean age

ε Hf (t) values

Two-stage Hf model ages (Ma)

L30-1

0.54–1.01

279.5 ± 1.9 Ma

0.282294–0.282457

From -5.0 to -10.9

1615–1979

0.282160–0.282465

0.11–0.76

268.4 ± 1.8 Ma

1601–2281

0.282208–0.282501

From -3.4 to -13.7

1514–2166

0.282263–0.282489

From -4.1 to -12.1

1547–2052

0.282151–0.282297

From -10.9 to -16.1

1976–2301

0.282244–0.282357

From -8.7 to -12.8

1842–2089

none

none

none

0.12–0.96

281.3 ± 2.7 Ma

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(MSWD = 0.019, n=21) LZ04-4

0.15–1.60

269.7 ± 2.8Ma

LZ07-1

0.10–0.55

269.8±2.0 Ma

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(MSWD = 0.092, n=17)

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(MSWD = 0.047, n = 19) LZ06-1

From -5.7 to -15.8,

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Rg50-2

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(MSWD = 0.60, n=26)

(MSWD = 0.118, n=27) 0.83–2.98

270.5±2.3 Ma

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LZ08-2

(MSWD = 0.042, n=29) 282 ± 2 Ma

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0.38–1.20

(MSWD = 0.53, n=15

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Rg61-3

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4.2 Whole-rock major and trace element compositions

4.2.1 Granite samples Those samples of the L30 series, the Rg50 series, the LZ04 series and the LZ06 series

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are monzogranites and syenogranites (Fig. 7a, Shouji granites for short), and display similar

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geochemical characteristics.

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The Shouji granites have relatively high SiO 2 (66.45–75.88 wt.%), total alkali (7.00–8.81 wt.%), and their K 2 O/Na2 O ratios vary from 1.11 to 1.74. They exhibit

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calc-alkaline characteristics with low Rittmann index values (σ=2.07–3.12), and display

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high-K calc-alkaline characteristics on the SiO 2 –K2O diagram (Fig. 7b). Except one sample, other samples display weakly peraluminous with the moderate A/CNK values (1.00–1.11)

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(Fig. 7c). These granite samples have relatively high TFe2 O 3 (1.15–4.81 wt.%) and high TFeO/(TFeO+MgO) ratios (>0.88), indicating a ferroan nature (Fig. 7d). They exhibit low

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CaO (0.76–3.37 wt.%), Al2 O3 (12.59–15.42 wt.%), MgO (0.17–0.69 wt.%), and P2 O5 (0.028–0.18 wt.%) contents.

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The Shouji granites have similar chondrite- normalized REE patterns (Fig. 8e). These granites display light REE enrichments relative to the medium REEs (MREEs) [(La/Sm)N = 3.61–5.20], and the heavy REEs (HREEs) [(La/Yb)N = 5.19–32.36], respectively (Fig. 7g). They have nearly flat heavy rare earth element (HREE) patterns [(Tb/Yb)N = 1.00–2.20], and obvious negative Eu anomalies (δEu = 0.22–0.67). In the N-MORB normalized spiderdiagrams (Fig. 8f), all the Shouji granite samples exhibit similar patterns. They are enriched in large ion lithophile elements (LILEs) relative to

ACCEPTED MANUSCRIPT high field strength elements (HFSEs), with marked negative Nb, Ta, P and Ti anomalies and no depletion of Hf and Zr. The negative Sr anomalies indicate some degree of plagioclase fractionation in generation of the granites. The Shouji granites have high initial

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Sr/86Sr isotopic ratios (0.708757–0.725943), and

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low εNd(t) values ranging from -10.8 to -13.1. Moreover, they have Paleoproterozoic

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two-stage Nd model ages (1910–2096 Ma).

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4.2.2 Gabbro samples

In the Q'-ANOR diagram (Fig. 7a), those samples of the Rg61 series are plotted in the

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gabbro field, and the LZ07 series and LZ08-2 samples are quartz diorite. They all have low

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SiOz contents, and exhibit similar geochemical compositions, then we discuss them together. Considering their mineral compositions and low SiO 2 contents (<53 wt.%), we take samples

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of the Rg61, LZ07 and LZ08 series as gabbros (Shouji gabbros for short). These Shouji

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gabbros exhibit high Al2 O3 (12.87–19.68 wt. %), CaO (6.57–9.47 wt.%), MgO (4.72–6.01 wt.%) and TiO 2 (1.07–2.05 wt.%) contents. They are sodium- rich with low K 2 O/Na2 O ratios (0.14–0.45) and low-K clac-alkaline (Fig. 7b).

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The Shouji gabbros display weak fractionated chondrite normalized REE patterns (Figs. 8a and c) with low (La/Sm)N (1.78–3.14) and (La/Yb)N ratios (4.17–9.55). They exhibit no obvious Eu anomalies (δEu=0.84–1.10). All these gabbros exhibit similar trace element compositions. In the primitive mantle-normalized spiderdiagram (Figs. 8b and d), they display enrichments in the LILEs (such as Rb, Ba and Th), and depletions in HFSEs (such as Nb, Ta, Zr and Hf). Their patterns are similar with that of the average continental and oceanic arc basalts (Figs. 8b and d).

ACCEPTED MANUSCRIPT The Shouji gabbros exhibit fertile whole–rock Sr–Nd isotopic compositions (Fig. 9a). They have high initial 87 Sr/86 Sr ratios (0.708420–0.720330) and low εNd(t) values (from -11.7 to -18.8). Moreover, they have Paleoproterozoic-Neoarchean two-stage Nd model ages

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(1987–2570 Ma).

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5. Discussion

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5.1 Temporal–spatial distribution of late Paleozoic plutons in the southern Alxa

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Most zircon grains from the Shouji granites are euhedral and prismatic, show obvious oscillatory or planar zoning in the CL images (Fig. 4), and have high Th/U ratios, all of which

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are typical features of magmatic zircons. Thus, the crystallization ages of Shouji granites range from 281 Ma to 268 Ma. In addition, zircon grains from the Shouji gabbros are

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subhedral, and exhibit weak concentric oscillatory zoning and high Th/U ratios, also implying

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a magmatic origin. Their crystallization ages are 282–270 Ma. Considering possible analytical error, we regard the Shouji granites and gabbros as contemporaneous plutons.

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A large quantity of zircon U–Pb age data has been obtained for plutons in the southern Alxa region (south of the Enger Us ophiolitic belt). These geochronological data show that large numbers of late Paleozoic plutons are distributed in the southern Alxa, mainly in the ZSTZ and NLTZ, indicating that large-scale magmatic events occurred during the late Paleozoic in the southern Alxa. To ascertain the temporal–spatial distribution of these late Paleozoic plutons, we collected geochronological and isotopic data of plutons in the southern Alxa region from recent studies (Appendix Table 5). The locations of representative plutons included in the data set are presented in the Fig. 10a. We also constructed age–frequency

ACCEPTED MANUSCRIPT distributions of late Paleozoic plutons in the southern Alxa area, including the NLTZ and ZSTZ (Fig. 10b). From the collected data, we found that most plutons in the NLTZ were formed during Permian (ca. 290–265 Ma), with peak ages at 270 Ma and 279 Ma (Fig. 10b). In addition,

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there are also some early Carboniferous plutons (e.g., the Dabashan pluton, Zheng et al., 2019)

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in the NLTZ, which are mainly distributed along the Bayanwulashan-Langshan fault zone

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(Fig. 10a). Moreover, most plutons in the NLTZ exhibit negative whole-rock εNd(t) and/or zircon εHf(t) values (Figs. 9a and 10c), and relatively old two-stage model ages, indicating an

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important role for ancient crust materials in their sources. In contrast, plutons in the ZSTZ

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have a wide range of zircon U–Pb ages, and most were formed at ca. 270–240 Ma, with a peak age of 249 Ma. Nearly all of these plutons have positive whole-rock εNd(t) and zircon

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εHf(t) values (Figs. 9a and 10c), and relatively young two-stage model ages, indicating larger

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amounts of juvenile and/or mantle-derived materials in their sources. In general, plutons in the NLTZ have relatively older zircon U–Pb ages and more enriched isotopic compositions (Fig. 10) than that of plutons in the ZSTZ. Spatially speaking, plutons in the southern Alxa

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area show a younging trend toward the northwest, and exhibit more depleted isotopic compositions from SE to NW (Fig. 10a). In addition, most plutons in the NLTZ intruded into Precambrian strata; however, plutons in the ZSTZ mostly intruded into the Paleozoic strata, such as the Amushan Formation. It is worth noting that Permian plutons in the southern Langshan exhibit zircon U–Pb ages ranging from 282 to 268 Ma, and have enriched isotopic compositions and old two-stage model ages; all of these characteristics are similar to those of plutons in the Bayan Nuru and

ACCEPTED MANUSCRIPT Yabulai regions (Fig. 10). Moreover, Precambrian strata are widely distributed in the southern Langshan (Fig. 2), and were intruded by these Permian plutons. Thus, we propose that the southern Langshan represents an eastern extension of the NLTZ rather than of the ZSTZ.

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5.2 Petrogenesis of Permian plutons in the southern Langshan

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Crustal contamination will cause a strong increase in LILE/HFSE ratios and ( 87 Sr/86 Sr)i

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ratios, but a decrease in ε Nd(t) values. In this study, all the Shouji granites and gabbros exhibit

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consistent whole-rock geochemical characteristics with relatively consistent isotopic compositions, such as εHf(t), εNd(t), and (87 Sr/86 Sr)i values. There are no obvious decreasing

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trends in the SiO 2 –εNd(t) diagrams (Fig. 9b), and no obvious correlations in some variation

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diagrams, such as SiO 2 –Nb/La. Furthermore, inherited zircons are rare in the analyzed samples. Therefore, we infer that crustal contamination was insignificant during the genesis

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5.2.1 Granite samples

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of the Shouji granites and gabbros analyzed in this study.

5.2.1.1 Classification of the Shouji granite In

general,

typical A-type

granites

have

higher

Na2 O+K 2O

values;

high

FeOT /(FeOT +MgO), Ga/Al, and HFSE (e.g. Zr, Y, Nb, and Ce) values; and lower CaO, Sr, and Eu levels than typical I-type varieties (Collins et al., 1982; Whalen et al., 1987; King et al., 1997; Bonin, 2007). In contrast to other types of granites, A-type granites are generated in some relatively high-temperature environments (King et al., 1997). The Shouji granites have high Na2 O+K 2O (7.00–8.81 wt. %), Nb (10.6–21.0 ppm), and Zr (147–446 ppm) contents,

ACCEPTED MANUSCRIPT and 10000*Ga/Al (2.50–3.33), and FeO T /(FeOT +MgO) (0.89–0.94) ratios, which are geochemical characteristics of typical A-type granites. In the Zr, Nb, and FeO T /(FeOT +MgO) vs. 10000*Ga/Al, and FeO T /(FeOT+MgO) vs. (Zr+Nb+Ce+Y) diagrams (Fig. 11a–d, Whalen et al., 1987), the Shouji granites are all plotted in the A-type field. The Shouji granites have

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relatively high zircon saturation temperature (TZr) values, ranging from 691 to 850 °C with an

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average value of 792 °C, which also confirm their A-type granite affinities. It is noteworthy

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that samples of the L30 series exhibit steeper REE patterns than those of other series, which indicate a deeper source with residual garnet ± amphibole.

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Eby (1992) divided A-type granite into the A1 and A2 chemical subgroups. A1 granites

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exhibit some geochemical similarities to oceanic-island basalts; in contrast, A2 granites display similar geochemical characteristics to those of rocks with continental crust or

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island-arc origins, and are thought to have formed from continental crust at convergent

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margins (Eby, 1992). On the basis of the geochemical subdivision of Eby (1992), the Shouji granites belong to the A2 -type granites on the Nb–Y–Ce triangular plot (Fig. 11e). They have higher Yb/Nb (1.37–3.06) and Yb/Ta (1.89–4.58) ratios than those of typical OIB,

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overlapping the A2 -type granite field and the Shouji gabbro field (Fig. 11f). 5.2.2.2 Petrogenesis of the Shouji A-type granite Several petrogenesis models have been proposed for generations of A-type magmas: (a) fractionation of mantle-derived alkaline basaltic magmas (Eby, 1992; Turner et al., 1992); (b) melting of meta-sedimentary rocks (Collins et al., 1982; Huang et al., 2011); (c) partial melting of anhydrous lower-crustal granulitic meta- igneous sources that had been previously depleted in a hydrous felsic melt (Landenberger and Collins, 1996; Jiang et al., 2005); (d)

ACCEPTED MANUSCRIPT hybridization of mantle-derived mafic magmas and crustal-derived granitic magmas; and (e) high-temperature partial melting of charnockite formed by dehydration of TTG without chemical depletion (Zhao et al., 2008). The Shouji A-type granites exhibit high SiO 2 and low MgO contents, indicating that they

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were not derived directly from the mantle. They exhibit high (87 Sr/86 Sr)i and negative εNd(t)

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and zircon εHf(t) values; these enriched Sr-Nd and zircon Hf isotopic compositions imply that

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crustal materials played an important role in their genesis. Moreover, the Shouji granites have high Zr, Nb, Ce, Y and Cr contents, inconsistent with fractionations of granitic magma

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(Gelman et al., 2014; Lee and Morton, 2015); thus, the fractionation was insignificant in their

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formation. There are positive correlations between the La/Sm ratios and La contents of the Shouji A-type granites, suggesting that their compositional variations are more likely to have

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been derived from the effects of partial melting and source composition from fractional

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crystallization (Allegre and Minster, 1978). Few coeval mafic rocks are observed in the southern Langshan, which also argues against an origin from fractionation of basaltic magmas. Therefore, these Shouji A-type granites could not have originated from the

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fractionation of mantle-derived mafic magmas. Some studies proposed that A-type magmas could be derived from partial melting of certain meta-sedimentary rocks (Huang et al., 2011; Sun et al., 2011); however, granitic magmas derived from meta-sedimentary sources usually have low (Na2 O+K2 O) contents, and display strongly peraluminous characteristics (Clemens and Stevens, 2012). The weakly peraluminous nature and highly alkaline signatures of the Shouji granites could rule out such a meta-sedimentary origin. Some studies have suggested that the residue after extraction of

ACCEPTED MANUSCRIPT felsic magmas is usually depleted in K 2 O and SiO 2 (Creaser et al., 1991; Frost and Frost, 1997), which is inconsistent with the high K 2 O, SiO 2 and K 2 O/Na2 O ratios in the Shouji A-type granites. In addition, the Shouji A-type granites have high (K 2 O+Na2 O)/Al2 O3 (0.54–0.66) and TiO 2 /MgO (0.66–1.66) ratios, which could not have been produced by

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remelting of granulitic residues (Patiño Douce, 1997 and references therein). Besides, some

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studies proposed that a residual granulitic source is too refractory to produce A-type granitic

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melts (Creaser et al., 1991). Thus, these lines of evidence rule out an origin from partial melting of anhydrous lower-crustal granulitic residues.

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The Shouji A-type granites could also not have been generated by mixing of mafic

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magmas and crustal-derived granitic magmas. The Shouji granites have relatively high SiO 2 and K2 O contents, and limited variations in the whole-rock major- and trace- element and

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Sr–Nd isotopic compositions. They have highly negative εNd(t) and zircon ε Hf(t) values, which argues against involvement of mafic magma in the source. There are no obvious

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positive trends between (87 Sr/86 Sr)i, εNd(t) values and SiO 2 contents, which would be expected in a binary mixing model (Huang et al., 2011). Plutons that originated from magma mixing

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usually host massive MMEs, which we did not detect in the Shouji A-type granite. Thus, the Shouji A-type granites could not have been generated by mixing of mafic and granitic magmas.

All the Shouji A-type granites exhibit high SiO 2 and K 2 O levels and high A/CNK and K2O/Na2O ratios (most >1), as well as low MgO, Cr, and Ni concentrations, indicating that they are more likely to have originated from meta-sedimentary or meta- igneous sources (Kalsbeek et al., 2001). Considering their weakly peraluminous nature and highly alkaline

ACCEPTED MANUSCRIPT signature, we favor a meta-igneous rock origin. More importantly, they have crust-like isotopic compositions, including fertile whole-rock Sr–Nd and zircon Hf isotopes, and old two-stage isotopic model ages. To be specific, the Shouji A-type granites have low εNd(t) values (−10.8 to −13.1), and old T2DM (1910–2096 Ma); in addition, they also exhibit low

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zircon εHf(t) values (−15.8 to −3.4), and old T2DM(1514–2166 Ma). These crust- like isotopic

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were derived from ancient continental crustal sources.

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compositions and old two-stage isotopic model ages imply that the Shouji A-type granites

Precambrian TTG have been proposed as the source rocks of A-type granite (Frost et al.,

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1998; Moyen et al., 2003; Almeida et al., 2007; Zhou et al., 2014). A-type granite could be

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generated by partial melting of tonalite and granodiorite at low pressures and high temperatures (Patiño Douce, 1997). Precambrian strata are widely distributed in the southern

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Langshan, which are divided into the Langshan Group and the Diebusige Complex (Fig. 2).

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The Langshan Group is mainly composed of Neoproterozoic meta-volcanic (804–816 Ma) and meta-sedimentary rocks (Hu et al., 2014). The Diebusige Complex is mainly composed of TTGs, mafic gneisses, paragneisses, amphibolites, quartzite, magnetite quartzite and minor

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marble. The protoliths of the Diebusige TTG yield Paleoproterozoic ages (2.41 Ga, our unpublished data). The metamorphic ages of the Diebusige Complex are ca. 1.89 and ca. 1.79 Ga (Dan et al., 2012). The two-stage Nd and zircon Hf isotopic model ages of the Shouji A-type granites are similar to the ages of the Diebusige TTGs, but much older than the ages of meta- volcanic rocks in the Langshan Complex. Thus, it is possible that Diebusige TTG was the main source rock of the Shouji A-type granites. The Shouji A-type granites exhibit high Y contents and flat HREE patterns (Fig. 8),

ACCEPTED MANUSCRIPT which indicate that garnet and amphibole were absent from their source region (Patiño Douce and Beard, 1995; Watkins et al., 2007). Their sources were also enriched in pyroxene, because

the Shouji A-type granites have high HREE contents (Mark, 1999). The Shouji A-type granites exhibit remarkable negative Sr and Eu anomalies and low Sr/Y ratios (1–9),

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indicating that plagioclase and K-feldspar were present in their source region. Plagioclase in

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equilibrium with magma requires a pressure of <1.5 GPa (0.8–1.0 GPa), equivalent to a depth

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of less than 30 km (Rapp et al., 1991; Xiong et al., 2007). Zircon saturation temperatures (TZr) for the Shouji A-type granites are relatively high (691–850 °C, average 792 °C; Watson and

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Harrison, 1983). As temperature can decrease with evolution of the parent magmas, the

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primitive-melt temperature would have been greater than 850 °C. In addition, the Shouji A-type granites exhibit relatively high P2 O5 contents (0.028–0.18 wt. %), also indicating high

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temperatures (Green and Pearson, 1986). High-temperature conditions suggest a refractory

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source. Moreover, the Shouji A-type granites have high HFSE and HREE contents, indicating a relatively dry source (Whalen et al., 1987; Zhao et al., 2008). Therefore, the primitive melt

conditions.

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of Shouji A-type granites was produced in high-temperature, low-pressure, anhydrous

On the basis of the physical conditions (T–P–H2 O) and source materials, we propose that the Shouji A-type granites were generated by partial melting of dehydrated Diebusige TTG rocks in the lower crust. The high calculated zircon saturation temperatures imply that those Diebusige TTG rocks were underplated and heated by mantle-derived mafic magmas. During this process, the TTG rocks could have been converted to charnockites without melt generation (Landenberger and Collins, 1996; Jiang et al., 2005; Zhao et al., 2008). In addition,

ACCEPTED MANUSCRIPT the Shouji A-type granites have relatively low Mg# values (21–48), which are similar to those of pure crustal partial melts (Rapp and Watson, 1995). Their low Mg# values, together with the lack of mafic enclaves in the pluton, indicate no significant additions of mafic magmas into the crustal melts during generations of the Shouji A-type granites. In addition, some

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Shouji granite samples exhibit large variations in zircon Hf isotope values, which may have

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resulted from magma mixing between relatively juvenile and ancient crustal materials. Thus,

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would have generated the Shouji A-type granites.

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partial melting of these charnockite (dehydrated but still fertile TTG rocks; Zhao et al., 2008)

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5.2.2 Gabbro samples

The Shouji gabbros have moderate SiO 2 and K 2 O concentrations and are classified as

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calc-alkaline rocks (Fig. 7b). In contrast to MORB, they yield relatively high Al2 O3 and MgO, and low TiO 2 concentrations. In addition, the Shouji gabbros are enriched in LREE and LILE,

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and depleted in HFSE, similarly to oceanic arc basalts (Figs. 8a and c). These gabbros display subduction-related geochemical features on some triangular diagrams: for instance, they plot

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exclusively in the field of volcanic arc basalts on the Hf–Th–Ta diagram (Fig. 12a), whereas they straddle the island-arc tholeiite and calc-alkaline basalt fields (Fig. 12b). The Shouji gabbros have enriched isotopic compositions with negative zircon ε Hf(t) and whole-rock εNd(t) values (Fig. 9a), which can result from partial melting of asthenospheric mantle with crustal contamination or enriched lithospheric mantle. Considering that there was insignificant crustal contamination during their genesis, we propose that the Shouji gabbros were derived from partial melting of enriched lithospheric mantle. We use HFSE/HREE

ACCEPTED MANUSCRIPT ratios to depict the composition and the enrichment or depletion history of the mantle wedge. The Zr/Yb vs. Nb/Yb diagram (Fig. 12c) is used to assess the degree of depletion of their sources (Pearce and Peate, 1995). All the Shouji gabbros cluster around the composition of enriched mid-ocean ridge basalt (E-MORB) within the mantle array defined by

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mantle-derived oceanic volcanic rocks in Fig. 12c, showing that they might have been

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derived from an enriched mantle source, which is consistent with their negative εNd(t) values

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(−11.7 to −18.8). Ba/Nb ratios can be used to constrain total subduction input, as they are independent of mantle source, partial melting, or fractional crystallization (Parce and Stern,

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2006). Ba is released over a wide range of temperatures during subduction, and therefore Ba

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enrichment (i.e., high Ba/Nb) can be used as a tracer of subductio n. In the Ba/Yb–Nb/Yb diagram (Fig. 12d), all the Shouji gabbros display upward drift from the mantle array,

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similarly to Mariana arc volcanic rocks, indicating substantial addition of subducted materials.

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In addition, the Shouji gabbros exhibit isotopic compositions (zircon Hf and whole-rock Sr–Nd isotopes) similar to those of the Shouji granites (Fig. 9), which derived from melting of Precambrian continental crust. This similarity in the isotopic compositions indicates

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addition of Precambrian lower crustal materials into the mantle sources of the Shouji gabbros. The Rg61 series have more enriched Nd isotopic composition than the LZ07 series, which may have resulted from varying degrees of hybridization of ancient continental crust-derived melts. Thus, the mantle source of the Shouji gabbros was modified by melts derived from ancient lower crust resulting from subduction erosion of old crust materials (Zhang et al., 2016). Considering all the evidence documented above, we propose that the Shouji gabbros were generated by partial melting of lithospheric mantle sources that had been metasomatized

ACCEPTED MANUSCRIPT by melts derived from an ancient continental crust in an active continental margin.

5.3 Tectonic implications

5.3.1 Permian tectonic setting of the southern Alxa

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The Permian geodynamic setting of the southern Alxa area remains controversial. Some

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studies have suggested a subduction-related setting during the Permian in the southern Alxa (Feng et al., 2013; Zhang et al., 2014; Zheng et al., 2014; Zhang et al., 2016), whereas others

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have argued for a post-collisional setting (Zhang, 2013). A previous study hypothesized that

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Permian magmatic rocks were probably formed in an extensional setting triggered by the

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adjacent ca. 280 Ma Tarim mantle plume (Dan et al., 2014). However, as shown in Fig. 10, these Permian magmatic rocks display spatial and temporal zonation along the northern

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margin of the Alxa Block, different from the expected distribution of magmatic rocks formed in a mantle plume setting. Magmatic activities associated with a mantle plume always exhibit

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a planar distribution, and are characterized by a sudden onset and short duration (Campbell and Griffiths, 1990); both features differ from those of Permian magmatic rocks in the

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southern Alxa. In addition, some studies regarded the Permian as a tectonic transition period in the southern Alxa (Shi et al., 2014; Liu et al., 2017). They proposed that a tectonic switch from subduction to post-collision tectonic regimes could have resulted in a marked shift in the isotopic compositions of magmatic rocks at 280–265 Ma in the Alxa region (Liu et al., 2017). Based on our collected data, such a shift in isotopic compositions does not occur within a tectonic unit; in contrast, magmatic rocks in the southern Alxa area display zonation in their isotopic compositions. In detail, magmatic rocks in the NLTZ have older and more

ACCEPTED MANUSCRIPT fertile isotopic compositions than magmatic rocks in the ZSTZ (Fig. 10). Thus, the shift in isotopic composition is more likely to have resulted from the different locations of magmatic rocks rather than a tectonic switch over the whole Alxa region. In addition, the Enger Us and Quagan Qulu ophiolitic belts are important boundaries for Paleozoic magmatic rocks, and

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divide two different magmatic zones (Fig. 10). In contrast, magmatic rocks formed in a

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post-collisional setting often cross the ophiolitic belt (regarded as the location of

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paleo-oceanic basin), and span the whole region, different from the distribution of late Paleozoic magmatic rocks in the southern Alxa. Besides, there is no definite evidence for

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closure of the paleo-ocean during the Permian in the southern Alxa. Thus, a post-collisional

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environment is not consistent with the origin of Permian magmatic rocks in this area. Subduction-related and almost coeval extensional magmatic rocks formed over a large

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area during the Permian; this observation is regarded as key to understanding the tectonic

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regime in the southern CAOB (Xiao et al., 2018). It is the case for the concomitant A2 -type granites and gabbros in the Shouji batholith, helping us to determine the Permian tectonic setting in the Alxa region. In this study, we identify contemporaneous gabbros and A2 -type

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granitoids in the Shouji batholith, northeastern Alxa. On the basis of their arc-like geochemical characteristics and enriched isotopic compositions (Sr–Nd–Hf), we suggest that the Shouji gabbros were derived from partial melting of lithospheric mantle sources that had been metasomatized by melts derived from an ancient continental crust in an active continental margin. In addition, the Shouji granites have high (Na2 O+K 2O) and HFSE contents, together with high Ga/Al and FeO T /(FeOT+MgO) (0.89–0.94) ratios, similar to typical A2 -type granites. A-type granites are indicators of a regional extensional environment

ACCEPTED MANUSCRIPT (Zhao et al., 2008); furthermore, it is increasingly well documented that A-type granites could form in various tectonic environments (Bonin, 2007), including some subduction-related environments (e.g. continental arcs and back-arc settings). Studies of ophiolitic mélanges, Permian strata and structural analysis all support a

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subduction setting in the southern Alxa during the Permian. As stated above, three late

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Paleozoic ophiolitic mélanges have been reported in the Alxa region (Wu and He, 1993;

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Zheng et al., 2014, 2018). Massive and pillow basalts in the Enger Us ophiolitic mélange exhibit N-MORB geochemical affinities, which indicate that it should be derived from a

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depleted mantle source in a mid-ocean ridge setting (Zheng et al., 2014). Tectonic blocks in

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the Enger Us ophiolitic mélange exhibit late Carboniferous zircon U–Pb ages, including 302 Ma (N–MORB-type pillow lave, Zheng et al., 2014) and 324 Ma (gabbro, our unpublished

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data). Moreover, a recent study found Albaillellarians from cherts in the Enger Us ophiolitic

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mélange, suggesting a deep-water setting during Middle Permian-early Late Permian (Xie et al., 2014). The Quagan Qulu and Tepai ophiolites both contain gabbro blocks with boninite- like geochemical characteristics, indicating the Permian (275 Ma and 278 Ma,

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respectively) subduction (Zheng et al., 2014, 2018) along the northern margin of the Alxa Block. Therefore, data from ophiolitic mélanges support a south-dipping paleo-ocean subduction in the southern Alxa (Zheng et al., 2014, 2018), which lasted until at least the Late Permian. In addition, Song et al. (2018) proposed that the Middle–Late Permian marine sedimentary successions in the southwestern Alxa were deposited in a continental-slope environment related to a convergent plate margin. Upper Permian fine-grained conglomerate and coarse- grained sandstone show consistently unimodal age spectra with single peaks at

ACCEPTED MANUSCRIPT ~261 and ~263 Ma, respectively, which could constrain the subduction process to during the Late Permian (Song et al., 2018). Similarly, geochemical data of volcaniclastic sandstones in the Amushan Formation near the Quagan Qulu ophiolitic mélange show a magmatic affinity to subduction-related tectonic settings, and a dacite lava in the Amushan Formation yielded a

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zircon U-Pb age of ~254 Ma (Shi et al., 2016), also consistent with a Late Permian

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subduction in the southern Alxa. Moreover, two deformation stages have been recognized in

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the Nuoergong Group around the Bayan Nuru region, including regional foliation striking NE–SW, and later dextral strike-slip shear along the strike of foliation (Wang et al., 2016).

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These deformation structures resulted from the SE-trending subduction of paleo-oceanic crust

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beneath the Alxa block during Carboniferous–Permian interval (Wang et al., 2016). In the Langshan region, a series of northeast–southwest-trending folds verging to the southeast

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developed in the Neoproterozoic Langshan Group, implying SE-trending subduction events.

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These folds involve not only the Late Devonian shear zone (muscovite 40 Ar/39 Ar plateau ages 379 and 356 Ma; Gong et al., 2017), but also affected the Permian gabbros and granites (our unpublished data).

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In summary, Permian SSZ-type ophiolites, Upper Permian arc-related sediments, and Permian subduction-related deformation all confirm a subduction-related tectonic setting in the southern Alxa during the Permian. In addition, the mantle plume and post-collisional settings are inconsistent with Permian magmatic activities. Furthermore, a subduction-related environment is the most likely setting for the origins of the associated Shouji gabbros and A2 -type granites. Thus, we propose that the Shouji gabbros and A2 -type granites were generated in an active continental margin that formed as a result of a south-dipping

ACCEPTED MANUSCRIPT subduction of the paleo-oceanic slab (the Enger Us oceanic slab). 5.3.2 Transition from an active continental margin to an intra -oceanic arc during the Early Carboniferous–Late Permian in the southern Alxa Our new zircon U–Pb ages, together with those from previous studies on the magmatic

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rocks, indicate three episodes of formation of late Paleozoic magmatic belt along the northern

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margin of the Alxa Block: early Carboniferous, Early Permian, and Late Permian-Early

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Triassic (Fig. 10b). The temporal–spatial distributions of these three magmatic belts indicate a northwestward younging trend in magmatic activity along the northern margin of the Alxa

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Block (Fig. 10a); this spatial difference reflects the NW-trending migration of magmatism

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from the early Carboniferous to Early Triassic. Migration of magmatic belts is usually regarded as a direct response to a change in the angle of slab subduction (Coney and

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Reynolds, 1977). Slab roll-back resulting from an increase in the angle of slab subduction

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would have led to ocean-ward retreat of the trench (Stern, 2002; Chai et al., 2018). Both spatial migration of magmatic rocks and temporal transitions in their isotopic compositions are consistent with roll-back of the subducted Enger Us oceanic slab. Considering the

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geochemical characteristics of the Shouji A2 -type granites, together with data for ophiolites and contemporaneous gabbros, we suggest that the Permian plutons in this study formed in a subduction-related extensional zone (intra-arc rift or back arc) resulting from roll-back of the subducted Enger Us oceanic slab. In the early Carboniferous, the Paleo-Asian Ocean crust (also referred to as the Enger Us ocean in the southern Alxa region) was subducted southeastward beneath the Alxa Block (present coordinates). Some early Carboniferous high Ba-Sr granites, tonalites, quartz diorites,

ACCEPTED MANUSCRIPT and olivine gabbros were generated in the southeastern Alxa during this period (Fig. 13a, Liu et al., 2016; Zheng et al., 2019). During the late Carboniferous–Middle Permian, the Enger Us slab subduction became steeper, resulting in slab roll-back, extension, and asthenospheric upwelling (Fig. 13b).

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During this process, lithosphere extension related to the slab roll-back, accompanying

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asthenosphere upwelling, may have triggered partial melting of a lithospheric mantle source

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that had been metasomatized by melt derived from ancient continental crust. These Shouji gabbros with arc- like geochemical affinities could have been generated during this process.

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The lithosphere extension resulted in upwelling of this mafic magma underplating in the

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lower crust. Underplating of contemporaneous mafic magmas would have provided heat and a high- temperature environment for generation of the Shouji A-type granites (e.g., Jiang et al.,

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2005; Zhao et al., 2008; Karsli et al., 2018). As mentioned above, mantle-derived mafic

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magmas were not significant in the generation of the Shouji A2 -type granites, thus underplating of mafic magma may have provided only the heat source for the formation of the Shouji A2 -type granites. Thus, partial melting of dehydrated Diebusige TTG rocks in the

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lower crust generated the Shouji A2 -type granites (Fig. 13b). Meanwhile, weak intra-oceanic subduction occurred in the ZSTZ, and some slab-derived adakites were derived from partial melting of the subducted oceanic crust (Shi et al., 2014). Some subduction-related plutons were also generated in the ZSTZ during this period (Fig. 13 b). Rare Late Permian–Early Triassic plutons have been reported in the NLTZ, although coeval plutons are widespread in the ZSTZ, and were considered to represent post-collisional magmatism (Shi et al., 2014; Liu et al., 2017). However, studies on Permian arc-related

ACCEPTED MANUSCRIPT sediments and Late Permian radiolarians constrained the final closure of the PAO in the southern Alxa to after the Late Permian (Xie et al., 2014; Song et al., 2018). Thus, we propose that continuous Late Permian magmatism in the ZSTZ would have occurred in a subduction-related setting. From their depleted isotopic compositions (Fig. 10c), together

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with the evidence of a subduction setting, these Late Permian plutons in the ZSTZ were

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generated in an intra-oceanic arc setting (Fig. 13c). Around the Late Permian, the active

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continental margin likely terminated along the NLTZ. As a result of retreat of the subducted Enger Us oceanic slab, the subduction process probably jumped northward as an oceanic arc

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away from the continental margin. During the Late Permian, an intra-oceanic subduction

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system developed along the ZSTZ, in which various Late Permian plutons with depleted isotopic compositions were generated. Therefore, from late Carboniferous–Middle Permian

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to the Late Permian an active continental margin switched to an intra-oceanic arc in the

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southern Alxa as a result of the northwestward retreat of the subducted Enger Us oceanic slab. Considering our new data, we propose that the final closure of the PAO in the southern Alxa

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area might have taken place after the Late Permian.

6. Conclusion

Our conclusions, based on analyses of the in situ zircon U–Pb–Hf isotopic, whole-rock major- and trace- element and Sr–Nd isotopic compositions of the Shouji batholith combined with regional geological data for the southern Langshan are listed as below. (1) Zircon LA–ICP–MS and SHRIMP U–Pb analyses indicate that the Shouji batholith in the southern Langshan formed at 282–268 Ma. Regional geochronological data show that

ACCEPTED MANUSCRIPT Permian magmatic rocks are widespread along the northern margin of the Alxa Block. (2) Geochemical data demonstrate that the Shouji granites are A2 -type granites, and were generated by partial melting of dehydrated Diebusige TTG rocks in the lower crust; the Shouji gabbros have arc- like geochemical characteristics, and were formed by partial melting

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of a lithospheric mantle source that had been metasomatized by melt derived from an ancient

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continental crust.

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(3) Roll-back of the Enger Us subducted slab was responsible for the association of the Shouji A2 -type granites and coeval gabbros with arc- like chemical characteristics in the

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southern Langshan.

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(4) Retreat of the subducted Enger Us oceanic slab resulted in transition from an active continental margin to an intra-oceanic arc in the southern Alxa region during the early

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Carboniferous–Late Permian.

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Acknowledgements

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We appreciate Prof. Jinyi Li for his help in preparing this manuscript. We also appreciate Beihang Zhang, Heng Zhao and Yiping Zhang for their help in the field work. This study was jointly supported by the National Key Research and Development Program of China from the Ministry of Science and Technology of China (No. 2017YFC0601301), the National Natural Science Foundation of China (41502214 and 41572206), the Outlay Research Fund of Institute of Geology, Chinese Academy of Geological Sciences (J1706), the CPSF-CAS Joint Foundation for Excellent Postdoctoral Fellows (Grant no.2015LH0049), the China Postdoctoral Foundation funded project (2016M590990). This is a contribution to IGCP 662.

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Figure caption

Fig. 1 (a) Geological sketch map of the Central Asian Orogenic Belt (modified after Şengör et al., 1993; Jahn et al., 2000). (b) Geological map of the Alxa region (modified after Zheng

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et al., 2014, 2018; YTZ–Yagan tectonic zone, ZHTZ–Zhusileng-Hangwula tectonic zone,

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ZSTZ–Zongnaishan-Shalazhashan tectonic zone, NLTZ–Nuoergong-Langshan tectonic

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zone);

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Fig. 2 Geological map of the southern Langshan region, showing sampling locations.

Fig. 3 Representative field and microscopic photos of Shouji batholith in the southern

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Langshan. a-the Shouji batholith intruded into the Langshan complex; b-the Shouji batholith

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intruded into the Diebusige complex; c and d-outcrop of monzogranites; e-K-feldspar granite intruded into gabbros; f-outcrop of K-feldspar granite; g and h-outcrop of gabbros; i- Mineral

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compositions and texture of gabbros (i and j), K-feldspar granites (k), and monzogranites (m). Pl, plagioclase; Qtz, quartz; Bt, biotite; Kf, K-feldspar; Cpx, clinopyroxene; Hbl, hornblende.

Fig. 4 Representative cathodoluminescence (CL) images of zircon grains from Shouji batholith in the southern Langshan (The circles with yellow solid lines for U–Pb analysis points and the circles with red dash lines for Hf isotope analysis points).

Fig. 5 U–Pb concordia diagrams of zircons from Shouji batholith in the southern Langshan

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Fig. 6 Zircon εHf(t) vs.

206

Pb/238 U age diagram of the Shouji batholith in the southern

Langshan. Data sources of late plutons in the ZSTZ, Bayan Nuru- Yabulai and Langshan are

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listed in the Table 5.

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Fig. 7 (a) Normative classification diagram, Q’=Q*100/(Q+Or+Ab+An), ANOR = diagram (after

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An*100/(Or+An) (Streckeisen and Le Maitre, 1979); (b) K 2 O vs. SiO 2

Peccerillo & Taylor, 1976); (c) A/NK vs. A/CNK diagram (after Maniar & Piccoli, 1989),

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A/CNK = mol Al2 O3 /(Na2 O+K2 O+CaO), A/NK = mol Al2 O3 /(Na2 O+K2 O); (d) TFeO/(TFeO

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+ MgO) vs. SiO 2 discrimination diagrams (after Frost et al., 2001) for Shouji batholith in the

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southern Langshan.

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Fig. 8 Chondrite normalized rare earth element patterns (left) and primitive mantle normalized trace element spider diagrams (right) of Shouji batholith in the southern Langshan. The values of Chondrite and primitive mantle are from Sun & Mcdonough (1989). Data for

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average continental arc basalt and oceanic arc basalts are from Kelemen et al., 2005.

Fig. 9 (a) The ε Nd(t) versus (87 Sr/86 Sr)i diagram of Shouji batholith in the southern Langshan. (b) The εNd(t) versus SiO 2 diagram of Shouji batholith in the southern Langshan. Sr-Nd isotopes for late Paleozoic plutons in the ZSTZ are from Zhang W. (2013) and our unpolished data. Data for late Paleozoic plutons in the Bayan Nuru- Yabulai are from Dan et al. (2014, 2015), Zhang et al. (2016) and Liu Q. et al. (2017a).

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Fig. 10 (a) Geological map of southern Alxa region, showing locations of geochronological data we collected. (b) Zircon U-Pb age frequency distributions of late Paleozoic plutons in the southern Alxa. (c) Zircon ε Hf(t) vs. 206 Pb/238 U age diagram of late Paleozoic plutons in the

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southern Alxa. Details of geochronological data are listed in the Appendix Table 5.

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Fig. 11 (a) TFeO/MgO vs. Zr+Nb+Ce+Y, (b) TFeO/MgO vs. 10000Ga/Al, (c) Nb vs.10000Ga/Al, and (d) Zr vs. 10000*Ga/Al discrimination diagrams (Whalen et al., 1987);

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e- the Nb-Y-Ce triangular plots, showing the fields for A1 and A2 subgroups of A-type

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granites (Eby, 1992); f- the Yb/Ta vs. Y/Nb diagram (Eby, 1992).

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Fig. 12 (a) Hf/3–Th–Ta diagram (Wood, 1980); (b) Ti/100-Zr–Y/3 diagram (Pearce & Cann, 1973); (c) Nb/Yb versus Zr/Yb diagram (the MORB-OIB array are from Green, 2006); (d) Nb/Yb versus Ba/Yb

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diagram (after Pearce & Stern, 2006).

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Fig. 13 A schematic illustration of a proposed genetic model for late Paleozoic magmatism in the southern Alxa, and also showing generations of Permian A2 -type granites and gabbros in the southern Langshan.

Appendix tables

Table 1 Zircon U–Pb analytical data of Shouji batholith in the southern Langshan.

ACCEPTED MANUSCRIPT Table 2 Zircon Hf isotopic compositions of Shouji batholith in the southern Langshan Table 3 Major and trace elements compositions of Shouji batholith in the southern Langshan Table 4 Whole rock Rb-Sr and Sm-Nd isotopic compositions of Shouji batholith in the southern Langshan

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Table 5 Summary of zircon U-Pb age data of ophiolites and plutons in the southern Alxa

ACCEPTED MANUSCRIPT Highlights: Permian association of A2 -type granites and gabbros are identified in the Langshan.

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Permian magmatism resulted from a slab rollback in an active continental margin.

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An active continental margin switched to an intra-oceanic arc in the southern Alxa.

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