Subduction-modified mantle-derived Triassic high-Mg andesites in the Sanjiang Tethys, eastern Tibet

Journal Pre-proofs Subduction-modified mantle-derived Triassic high-Mg andesites in the Sanjiang Tethys, eastern Tibet Hong-Peng Fan, Bo Li, Jia-Xi Zhou, Li-Juan Du, Hai-Rui Sun, Zhi-Long Huang, Tao Wu PII: DOI: Reference:

S1367-9120(19)30568-1 https://doi.org/10.1016/j.jseaes.2019.104216 JAES 104216

To appear in:

Journal of Asian Earth Sciences

Received Date: Revised Date: Accepted Date:

9 May 2019 24 December 2019 24 December 2019

Please cite this article as: Fan, H-P., Li, B., Zhou, J-X., Du, L-J., Sun, H-R., Huang, Z-L., Wu, T., Subductionmodified mantle-derived Triassic high-Mg andesites in the Sanjiang Tethys, eastern Tibet, Journal of Asian Earth Sciences (2019), doi: https://doi.org/10.1016/j.jseaes.2019.104216

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Subduction-modified mantle-derived Triassic high-Mg andesites in the Sanjiang Tethys, eastern Tibet

Hong-Peng Fana, Bo Lib, Jia-Xi Zhoua,c,*, Li-Juan Dud, Hai-Rui Sune, Zhi-Long Huanga,*, Tao Wuf

a

State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China

b

Faculty of Land Resources Engineering, Kunming University of Science and Technology, Kunming 650093, China

c

School of Earth Sciences, Yunnan University, Kunming 650500, China

d

College of Resource and Environment Engineering, Guizhou University, Guiyang 550025,

China e

Development and Research Center, China Geological Survey, Beijing 100037, China

f

Ocean College, Zhejiang University, Zhoushan 316021, China.

*Corresponding author, Emails: [email protected] (J.-X. Zhou); [email protected] (Z. Huang)

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Abstract Andesites in the Yangla district of the Sanjiang Tethys, eastern Tibet, are characterized by relatively high MgO and with rich large-ion lithophile elements (LILEs), indicating similarities to typical high-Mg andesites formed in continental collisional zones. Zircon U-Pb age of 232±1 Ma for these andesites shows that they occurred synchronously with the regionally widespread late Middle Triassic granitoids and bimodal volcanic rocks. Primitive mantle-normalized spidergrams for these andesites show depletion in high field-strength elements (HFSEs), with Sr-Nd-Pb isotopic compositions that comparable to both arc- and MORB-like high-Mg rocks generated from subduction-modified sources. Modelling results suggest that an input of ~20% sediment-derived melts into mantle-derived melts could be possibly responsible for the Sr-Nd isotopic characteristic of these andesites. MORB- and arclike compositions likely indicate a metasomatized mantle wedge modified by subduction processes during the closure of the Paleo-Tethys Ocean and subsequent continental collision. It is speculated that the late Middle to Late Triassic intra-continental re-activation associated with regional tectonic collapse and thermal relaxation triggered the melting of this previously subduction-modified mantle, leading to the formation of the high-Mg andesites in the Yangla district. Keywords: High-Mg andesites; Subduction-modified mantle; the Yangla Cu ore district; Sanjiang Tethys

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1. Introduction High-Mg andesites with high Mg numbers and LILEs contents were originally considered to be generated by partial melting of subducted oceanic crust and subsequent interaction of magma with mantle peridotites prior to the emplacement (e.g., Defant and Drummond, 1990). These were also suggested to be produced by differentiation of basaltic magma (Castillo et al., 1999), melting of hydrous peridotites (e.g., Shirey and Hanson, 1984; Stern and Hanson, 1991) or mantle peridotites modified by continental crustal-derived melts (e.g., Kay and Kay, 1993; Xu et al., 2002; Chung et al., 2003; Huang et al., 2008). Therefore, the presence of high-Mg andesites in continental collisional zones could be the key for understanding crust–mantle interaction and continental crust recycling processes. The Sanjiang Tethys in southwestern China comprises as a significant segment of the great Tethyan-Himalayan tectonic domain (Mo et al., 1993; Li et al., 1999; Hou et al., 2003; Li and Jiang, 2003) (Fig. 1). The Jinshajiang suture zone is tectonically located in the eastern part of the Sanjiang Tethys, representing a remnant of a branch of the Paleo-Tethys Ocean and playing an important role in the reconstruction of Gondwana (e.g., Wang et al., 2000; Zi et al., 2012b). The tectonic evolution of this suture zone was mainly featured by the accretion of continental fragments during the Mesozoic and late Paleozoic (e.g., Wang et al., 2000; Xiao et al., 2008; Zi et al., 2013). As part of the Paleo-Tethys Ocean, the Jinshajiang Ocean opened in Late Devonian or Early Carboniferous (Wang et al., 2000; Metcalfe, 2002; Jian et al., 2008; Zi et al., 2012b), and closed probably before Late Triassic (e.g., Mo and Pan, 2006). The late Middle to Late Triassic bimodal volcanic rocks, granitoids, and felsic volcanic rocks exposed in the Jinshajiang suture zone were suggested to be products of the last episode of orogenic magmatism, after which the region entered into intraplate phase of

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evolution (Peng et al., 2006; Zhu et al., 2011; Zi et al., 2012a, 2013). Recent studies of the Mesozoic granitoids and bimodal volcanic rocks provide essential clues of the evolution history of the Jinshajiang suture zone (e.g., Zi et al., 2013 and references therein). However, andesites in the Jinshajiang suture zone are far from being well studied and the significance of these rocks are not known. In this paper, we firstly identified high-Mg andesites in the Triassic strata of the Yangla Cu ore district, and present robust ages, elemental and Sr-Nd-Pb isotopic compositions of the high-Mg andesites in Yangla. Our geochemical and geochronological data provide new insight into understanding the subduction-related magmatic processes, crust-mantle interaction and nature of the lithospheric mantle source during the evolution of the Sanjiang Tethys.

2. Geological background and petrography The Jinshajiang suture zone in the eastern Tibet is connected to the Ailaoshan suture in SW Yunnan and the Songma suture in Vietnam (Mo et al., 1993; Metcalfe, 2002; Hou et al., 2003; Zi et al., 2013; Fig. 1). This suture zone is marked by ophiolites and was once a back-arc basin or a branch of the Paleo-Tethys Ocean in Late Devonian or Early Carboniferous (e.g., Wang et al., 2000; Zi et al., 2013). Four units in the Jinshajiang suture zone were generally identified: the Gajinxueshan and Zhongxinrong Groups, the Eaqing Complex, and the Jinshajiang Ophiolitic Melange (references?). The Late Devonian to Carboniferous Gajinxueshan Group is composed of clastic sedimentary rocks and carbonates, and is overlain by the Late Triassic Zhongxinrong Group of flysch and molasse sequences (Wang et al., 2000). The Eaqing Complex consists of epidote–amphibolite to amphibolite facies rocks. The Jinshajiang Ophiolitic Melange exhibits characteristics of a typical variably disrupted

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ophiolitic suite. The opening of the Jinshajiang Ocean basin is suggested to have occurred in Early Carboniferous or Late Devonian and separated the Changdu-Simao micro-continent from the Yangtze Block (references?). The closure and regional collisional events occurred probably before Late Triassic as indicated by the widespread limestone and an unconformity between middle and Late Triassic (e.g., Mo et al., 1993; Metcalfe, 2002; Mo and Pan, 2006; Jian et al., 2009; Zi et al., 2012b). Silurian schists and metasandstone, Devonian marble and sandstone, Carboniferous basalts, and Triassic slate, sandstone, limestone, and volcanic rocks are presented in Yangla, between the Jinshajiang and Yangla faults (Fig. 2). The ca. 230 Ma granitoids, including biotite quartz monzonite, I-type quartz diorite, and granodiorite, intruded the Silurian to Devonian strata (e.g., Zhu et al., 2011) (Fig. 2). The Yangla Cu deposit hosted in Triassic granitoids has molybdenite Re-Os age of ca. 232 Ma. (e.g., Yang et al., 2012a, b; Zhu et al., 2015). Andesites in Yangla that were assigned as part of the Triassic strata are recently exposed due to ore exploration. The andesitic rocks show porphyritic texture (Fig. 3), and are mainly composed of plagioclase, pyroxene and chlorite microcrystals with andesite glass. Phenocrysts are mainly composed of plagioclase, amphibole, and pyroxene (Fig. 3).

3. Analytical methods Representative samples for this study were collected from outcrops near the Lunong, Linong, and Jiangbian ore sections of the Yangla Cu ore district (Fig. 2). Among them, the twenty-one least-altered samples were used for the following analysis. Two samples Y034 and LN12-06 with each about 15 kilogram in weight were collected. By using of magnetic techniques and conventional heavy liquid, zircon crystals were separated from these two samples. The clean grains were handpicked

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under a microscope and then mounted in epoxy. To identify grains for dating, zircon grains were photographed in reflected and transmitted light as well as cathodoluminescence (CL) after they having been polished. Finally, they were coated with gold to be prepared for analyze. U-Pb isotopic ratios of the chosen grains were analyzed using the LA-ICPMS (a GeoLas Pro 193 nm ArF excimer laser plus an Agilent 7500x ICPMS) at the State Key Lab of Ore Deposit Geochemistry (SKLODG), Institute of Geochemistry (IG), Chinese Academy of Sciences (CAS). Zircon 91500 was used as external standard for U-Pb dating. Those measured isotope ratios were reduced offline using ICPMS DataCal (Liu et al., 2010). The weighted mean ages were given in Table 1. Whole-rock major elemental contents were determined by wavelength-dispersive X-ray fluorescence spectrometers at ALS Chemex Co Ltd, Guangzhou with an analytical precision of ±5%. Whole-rock trace elemental compositions, including Rare Earth Elements (REEs), were analyzed using a Perkin-Elmer Sciex ELAN DRC-e ICPMS at the SKLODG, IGCAS after closed-beaker digestion with a mixture of HF and HNO3 in Teflon bombs (Qi et al., 2000). Rh was used as an internal standard, and OU-6, GBPG-1, and GSR-1 and -3 were used as reference materials. The precision was estimated to be ±10%. Samples for whole-rock Sr and Nd isotope analysis were firstly dissolved with a mixture of HF and HNO3 in high pressure Teflon bombs. The isotopic ratios were analyzed on a Neptune plus MC-ICPMS at the State Key Laboratory for Mineral Deposits Research, Nanjing University and a Triton TIMS at the SKLODG, IGCAS. 146Nd/144Nd=0.7219 86Sr/88Sr

and 86Sr/88Sr=0.1194 were used for mass fractionation correction.

values for the reference material NBS987 and BCR-2 were measured as

0.710263 ± 0.000009 (2σ) by MC-ICPMS and 0.710238 ± 0.000007 (2σ) by TIMS,

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respectively. 146Nd/144Nd values for BCR-2 were 0.512513 ± 0.000007 (2σ) measured by TIMS and for JNdi-1 were 0.512094 ± 0.000004 (2σ) measured by MC-ICPMS. Whole-rock Pb isotope analysis was performed on an IsoProbe-T TIMS at Beijing Research Institute of Uranium Geology. 204Pb/206Pb, 207Pb/206Pb and 208Pb/206Pb values for the reference material NBS981 were 0.059042 ± 0.000037, 0.91464 ± 0.00033 and 2.1681 ± 0.0008 (2σ), respectively.

4. Analytical results 4.1. U-Pb Zircon geochronology Samples Y034 (N 28°53'21.2", E 99°05'09.1") and LN12-06 (N 28°55'15.5", E 99°04'05.9") were collected from Lunong and Linong, respectively (Fig. 2). The LAICPMS Zircon U-Pb data are presented in Table 1. The zircon grains in samples Y034 and LN12-06 are generally euhedral and show oscillatory zoning in the CL images (Fig. 4a, b). Their lengths vary from 80 to 200 µm, and the ratios of length to width range from 1:1 to 2:1 (Fig. 4a). During a single analytical session, twenty-five analyses were conducted on twenty-five grains, and the results show that the thorium (Th) and uranium (U) contents were 58-406 and 156-1413 ppm, respectively, with Th/U ratios ranging from 0.21 to 0.47 (Table 1). All twentyfive measured results are concordant with a weighted average 206Pb/238U age of 232±1 Ma (95% confidence interval, MSWD = 1.2) (Fig. 5) which is, therefore, taken as the optimum estimation of the eruption age for andesites in Lunong and Linong.

4.2. Whole-rock elemental and Sr-Nd-Pb isotopic compositions 4.2.1. Major and trace elemental data Table 2 provides the data of whole-rock elemental compositions for the andesites

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in Yangla. In this study, the contents of major elements were used by being recalculated to 100% volatile free. The major and trace elemental compositions of the andesites in each locality are similar, and exhibit relatively evolved features with various Mg numbers (43.3-65.2). These andesites are characterized by higher MgO contents (3.276.78 wt.%) than those of normal andesites (Table 2; Fig. 6b, c). The samples collected from Jiangbian are less evolved as they have higher MgO contents (6.55-6.78 wt.%) than those from Linong and Lunong (Table 2). Moreover, the major elements all show restricted range in contents, suggesting that they might not undergo significant fractionation process (Table 2). On the diagram of Nb/Y vs. SiO2, these samples all plot in the subalkaline andesite field (Fig. 6a). These samples show higher MgO contents and Mg-numbers than those of normal andesites with comparable SiO2 contents, and thus plot away from normal arc-like magmatic rocks but into the field of high-Mg andesites in the SiO2 vs. Mg-number and SiO2 vs. MgO diagrams (Figs. 6b, c). Samples from Lunong, Linong and Jiangbian show REE patterns with depletion of heavy rare earth elements [(La/Yb)chondrite-normalized = 3.9-7.9] [chondrite normalization values are taken from Boynton (1984)] and enrichment of light rare earth elements (Fig. 7). All samples collected from the Yangla andesites exhibit “humped” patterns with varying levels of enrichment for all incompatible elements in the NMORB-normalized trace element spider diagram (Fig. 7). Among them, Eu anomalies are absent from all samples, while obvious negative Nb and P anomalies are shown (Fig. 7).

4.2.2. Sr-Nd-Pb isotopes Sr-Nd isotopic compositions of the Yangla andesites are variable (Table 3). The 87Sr/86Sr

I

ratios range from 0.7052 to 0.7084 (Table 3), with the 143Nd/144NdI ratios of

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0.512059-0.512213, and the εNd(T) values between -5.5 and -2.5. A negative correlation between 87Sr/86SrI and εNd(T) are shown (Fig. 8). Whole-rock 206Pb/204PbI, 207Pb/204PbI and

208Pb/204Pb

I

ratios are respectively of 18.22-18.38, 15.59-15.63 and 38.42-38.64,

which are higher than both lower crust and depleted mantle, but are similar to those of the ca. 230 Ma granitoids in Yangla (Zhu et al., 2011).

5. Discussion 5.1. Late Triassic eruption of the Yangla high-Mg andesite The zircons from the Lunong and Linong andesites are relatively high in Th/U ratios (Table 1) with oscillatory zoning shown by the CL images (Fig. 4), suggesting magmatic origins. Therefore, based on the mean weighted average

206Pb/238U

age of

zircons (232±1 Ma), we interpret that the Lunong and Linong high-Mg andesites were produced by a Triassic magmatism. The Jiangbian andesites have similar N-MORBnormalized trace elements and chondrite-normalized REE patterns to those of the andesites from the other two locations (Fig. 7). Considering their same Triassic stratum origin, we suggest that the andesites in Yangla were likely generated by synchronous magmatic events occurred at ~230 Ma. The Mesozoic magmatism associated with the evolution of the Jinshajiang suture zone was well constrained by the ca. 230-210 Ma granitoids and bimodal volcanic rocks exposed in the studied region (BGMRY, 1984; Wang et al., 2000; Zhu et al., 2011; Zi et al., 2013). For instance, the granitoids in Yangla have consistent zircon U-Pb age of ca. 230 Ma, the monzogranite, granodiorite, biotite monzogranite, and biotite granodiorite in Ludian have zircon U-Pb ages of ca. 230-220 Ma, and the bimodal volcanic rocks in Lanping and Deqin are suggested to have been emplaced at ca. 230210 Ma (Wang et al., 2000; Zhu et al., 2011; Zi et al., 2013). These ca.230-210 Ma

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magmatic rocks are all suggested to have been generated in a decompressed environment attributed to the late or post-collisional event that occurred in the Jinshajiang suture zone during late Middle to Late Triassic. Moreover, fieldwork and geological correlations revealed that some mafic intrusions that intruded the Triassic volcano-sedimentary rocks were closely coeval with the late Middle Triassic granitoids in the Jinshajiang suture zone, and also exhibited intraplate geochemical characteristics (BGMRY, 1984; Zhang et al., 1992). Therefore, these mafic and felsic rocks define a relatively consistent age range that is broadly contemporaneous with regional tectonic collapse and thermal relaxation (see Zi et al., 2013 and references therein). It is noted that the intermediate volcanic rock in the Jinshajiang suture zone are still not well studied so far. The ca. 230 Ma andesites reported in this study are therefore the first high-Mg andesitic volcanic rocks that generated during the late- or post-collision of the Jinshajiang suture zone. The high-Mg andesites in Yangla are of a similar age to the widespread diorite, monzonite, and granodiorite in this region (Gao et al., 2010; Zhu et al., 2011), which unequivocally indicates that the andesites in Yangla could have been generated by a common magmatic event that produced these granitoids and resulted in Cu mineralization (molybdenite Re-Os age: ca. 232 Ma; Zhu et al., 2015) in this region.

5.2. Effects of secondary alteration The samples collected from the Yangla andesites did not experience any metamorphism, however, their variable LOI values (1.5-12.3 wt.%) may caused by various degrees of alteration as identified from the microscopic photographs (Fig. 3d, e). Therefore, both the trace elements and major oxides of these samples should be double-checked to evaluate the effects of alteration. Zirconium is one of the most immobile elements during alteration and has been

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used to test the effect of alteration on other elements (e.g., Polat et al., 2002). Based on their different geochemical behaviors, elements contained in the Yangla samples, such as Y, La, Nb, Th, Nd, and V are selected to plot against Zr to elucidate whether they have been affected by alteration (Fig. 9). All HFSEs, including Th, Y, Nb and Pb, REEs including La and Nd, siderophile element V, and LILE Sr, are strongly correlated with Zr for samples collected from each location, suggesting that they were not significantly affected by secondary alteration, and should by taken into account for further discussions. The major oxides in the samples collected from each location are well correlated with MgO in the fenner diagrams (not shown), indicating that the effects of alteration on the major oxides in the Yangla samples were insignificant. But in this study, the contents of major elements are still used by being recalculated to 100% volatile free. The Sr, Nd and Pb isotopic data of the Yangla samples have not been significantly disturbed by alteration, as indicated by the relatively uniform Sr, Nd and Pb isotopic contents and the correlation between εNd(T) and 87Sr/86SrI (Fig. 8; Table 3).

5.3. Origin of the Yangla high-Mg andesites The Yangla andesites have higher MgO and LILE contents than normal andesites, and resembled typical high-Mg andesites (HMAs; Table 2; Fig. 6b, c) (e.g., Kelemen 1995) and are, therefore, defined as high-Mg andesite here. The HMAs have been divided into four types as adakitic, bajaitic, boninitic, and sanukitic (Kamei et al., 2004). The Yangla andesite compositionally resemble sanukitic-type HMA, as shown in the subdivision diagrams (Fig. 6c-f). The TiO2 contents of the Yangla andesites are not consistent with those of typical boninitic HMA which is characterized by low contents of TiO2. Besides, in the Mg-number vs. SiO2 diagrams, the amount of SiO2 in the Yangla andesites decreases dramatically with decreasing Mg-number, which, together

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with their abundant plagioclase phenocrysts (Fig. 3e), suggesting a distinctive origin with the boninite volcanic rocks from the NE Japan Arc (Fig. 6b, c). These rocks are also distinct from other HMAs due to their high Yb and Y contents and low Sr/Y ratios (Fig. 6). Additionally, the Cr, Ni and Eu contents of the Yangla andesites are much lower than those of typical sanukitic HMA with subduction-related geochemical characteristics (Kamei et al., 2004). The Yangla andesites differed from typical volcanic rocks generated by the depleted upper mantle. For example, highly incompatible elements such as Th and La are not depleted, while Nb shows significant depletion in the N-MORB-normalized trace element spider diagrams (Fig. 7). On the other hand, they seem to share some common characteristics with volcanic rocks generated in island arc. For instance, they have similar N-MORB-normalized trace element patterns, with enriched Th and light rare earth elements (LREEs) and depleted Nb, Ta and Ti, which are characteristics of volcanic arc rocks. However, the absence of negative Zr-Hf anomalies is contradictive to typical arc volcanic rocks. Additionally, the Ti/V ratios of these samples range between 23.1 and 54.6, and are noticeably higher than those of typical volcanic arc rocks which have Ti/V ratios lower than 20. They also exhibit higher MgO, Cr, Ni, and Mg numbers, but with less SiO2 than normal volcanic rocks generated in island arc (Table 2; Fig. 6a). It has been proposed that high-Mg andesites can be generated by the partial melting of thickened or delaminated lower continental crust mixing with the melts from mantle peridotites (e.g., Kay and Kay, 1993; Xu et al., 2002; Chung et al., 2003; Huang et al., 2008; Rapp et al., 2010). However, the high MgO, Cr and Ni contents, and La/Nd ratios and insignificant Zr–Hf anomalies, along with the obviously higher Nb/Ta ratios (12.4-20.1) than those of the lower crust (8.3; Sun and McDonough, 1989), dispute the significant involvement of the lower continental crust. Moreover,

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modelling calculations involving lower crustal and mantle-derived components suggest that lower crustal materials proportion of over 60% would have been required to reconcile the isotopic and elemental characteristics of the Yangla high-Mg andesites (Fig. 8). It is impossible to achieve the high-Mg andesitic features and parallel trace elemental patterns from mixing with such high proportions of crustal contamination (Table 2; Fig. 8). The radiogenic whole-rock Pb isotopic ratios of the Yangla high-Mg andesites (average

206Pb/204Pb

I

= 18.32,

207Pb/204Pb

I

= 15.62 and

208Pb/204Pb

I

= 38.52,

Table 3) are higher than those of both the depleted mantle and lower crust (Rollinson, 1993), which again do not support the occurrence of significant crustal assimilation. The Yangla andesites have higher Mg, Ni, and Cr contents, and 87Sr/86SrI, La/Nb and Th/Nb ratios than normal products of primary mantle-derived melts, but have similar contents and ratios to volcanic rocks produced by recycled sediment/slabderived component modified mantle (e.g. Wyllie and Sekine 1982; Zhang et al. 2012). The arc-like trace elemental signatures such as highly fractionated REE, depleted HFSE and enriched LILEs and the Sr-Nd isotopic characteristics are similar to those of the high-Mg rocks, which have both arc- and MORB-like geochemical affinity generated by a subduction-modified peridotite source (e.g., Taylor and Martinez 2003). The radiogenic whole-rock Pb isotopic ratios of the Yangla andesites are higher than those of both the lower crust and depleted mantle, suggesting significant involvement of sediment-derived components as they are relatively more radiogenetic. Moreover, the Yangla andesites exhibit low Ta/La ratios, but high Hf/Sm ratios, which can also be generated by metasomatic enrichment related to subduction (e.g. La Flèche et al., 1998). Therefore, their MORB- and arc-like signatures were probably caused by recent or ancient metasomatism attributed to the subduction-derived components. The components that are possible to be involved into the source of these rocks

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could be slab-derived and/or sediment-derived (e.g., fluid and melt). Nevertheless, the Yangla samples are rich in Th, suggesting the re-fertilization of the mantle wedge with subducted sediment-derived materials, rather than slab-derived components (Fig. 10) (e.g., Klimm et al. 2008). This evidence, together with the low U/Th and Nb/Pb ratios, and high Th/Sm, Th/Nb, and Th/Nd ratios (Fig. 10), strongly indicates the significant involvement of sediment-derived components into mantle wedge as Th and Nb are dominated by sediment-derived melt, and Pb is commonly controlled by sedimentderived fluid (e.g., Petrone and Ferrari, 2008; Zhao and Asimow, 2014). The sediment-derived components that contributed to the genesis of the andesites and granitoids in the Jinshajiang suture zone could be newly, or either previously input. The Upper Triassic sequence in the Jinshajiang suture zone is characterized by a basal molasse-conglomerate layer, which probably suggests a period of tectonic collapse and thermal relaxation (Dewey et al., 1988). It has been proposed that the final continental collision likely finished before Late Triassic (e.g., Mo and Pan, 2006). Moreover, the ca. 234-214 Ma bimodal volcanic rocks in Lanping and Deqin, and the granitoids and their coeval ultramafic-mafic intrusions in Yangla and Ludian were proposed to have been generated by the late- or post-collisional magmatic activity occurring during regional tectonic extension (e.g., Wang et al., 2000; Zhu et al., 2011; Zi et al., 2013). Among them, the ca. 230 Ma granitoids in Yangla and Ludian exhibit close affinity to the typical high-K calc-alkaline granitoids generated under late- or post-collisional settings due to plate movement along the shear zone (Liegeois et al., 1998). Therefore, a late-or post- orogenic extensional regime may take over the Jinshajiang suture zone rather than a subduction setting during the emplacement of the 230 Ma Yangla andesites. This sedimentary and bimodal magmatic evidence strongly argue against a new slab subduction during late Middle to Late Triassic. Therefore, we herein propose an

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alternative scenario that the mantle source was previously metasomatized by sedimentderived components prior to Late Triassic. The closure of the Sanjiang Paleo-Tethys Ocean marked by the subduction of the oceanic lithosphere and subsequent collisions resulted in the emplacement of numerous Late Permian to Early Triassic arc-related magmatic rocks (e.g., Zi et al., 2013). For example, several studies have demonstrated the presence of subduction-related magmatic-sedimentary sequences are composed of arc volcanic-sedimentary rocks and I-type granitoids along the eastern margin of the Changdu-Simao micro-continental Block during Late Permian to Early Triassic (e.g., Li et al., 2002; Zi et al., 2012a). It is noted that these sediments were deposited from marine to terrestrial environments (Zi et al., 2013). Therefore, it is speculated that there was a subduction process occurred prior to collision occurred before Late Triassic, which could have transported the sediment-derived components into the source of the Yangla andesites. Based on the above synthesis of available data, the Yangla high-Mg andesites were genetically linked to an ancient (Late Permian to Early Triassic) subduction, rather than a new subduction. They were derived from a heterogeneous mantle wedge previously metasomatized by sediment-derived melt. Therefore, a two-component mixing model involved mantle-derived melts and recycled sediment-derived components is proposed to interpret the variations of Sr-Nd isotopes and elements for these studied samples (Fig. 8). The mantle source component is modelled using the Jinshajiang MORB based on data for the Jinshajiang ophiolites (Wei et al., 2003; Xu and Castillo, 2004), which is chemically similar to the most primitive samples from the ca. 350 Ma basalts identified in Yangla (Zhu, 2012). The ranges for the amphibolites of the Yangtze LCC and sediments of the Yangtze UC represented by the metasediments from the Xiongsong Group are described in Zhu et al. (2011) and Zi et al. (2013). Our two-component

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mixing modelling results suggest that input of approximately 20% sediment-derived melts into the metasomatized mantle by ancient subduction can provide the Sr-Nd isotopic compositions of the Yangla andesites (Fig. 8). The above discussions allow us to propose the following genetic model, as shown in Figure 12. Subducted processes due to the closure of the Paleo-Tethys Ocean and subsequent continent collision transported convective upper mantle (probably the asthenosphere) along with recycled sediment-derived melts upwards to form a Triassic metasomatized mantle wedge (Fig. 11a). The metasomatized mantle wedge was then stably preserved beneath the Jinshajiang suture zone until the late- or post-collisional magmatic activity occurring during regional tectonic collapse and thermal relaxation in late middle to Late Triassic (Fig. 11b). This regional tectonic collapse and thermal relaxation elevated the thermal boundary of the lithosphere and then induced the partial melting of previously (Late Permian to Early Triassic) subduction-modified mantle wedge to generate the high-Mg andesites in Yangla (Fig. 11c).

6. Conclusions (1) The Yangla high-Mg andesites with a weighted mean zircon U-Pb age of 232±1 Ma were generated during the late- or post-collision of the Jinshajiang suture zone. (2) The Yangla high-Mg andesites are suggested to be produced by extensioninduced partial melting of a previously subduction-modified mantle wedge.

Acknowledgements We appreciate F. Long, Y.D. Liu, C. Luo and Z.F. Yan for the field assistance. The paper has benefited from English polishing and constructive comments of the

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Editor-in-Chief Prof. M.-F. Zhou, the reviewer Prof. J.-W. Zi and two anonymous reviewers. This work is jointly supported by the National Science Foundation of China (41402072, 41430315, 41862007 and 41402072), the 12th Five-Year Plan Project of the State Key Laboratory of Ore Deposit Geochemistry, Chinese Academy of Sciences (SKLODG-ZY125-02), the Analysis and Testing Foundation of Kunming University of Science and Technology (2017T20160006), the Key Scientific and Technological Project of the Yunnan Copper Group (20110103), the Yunnan Ten Thousand Talents Plan Young & Elite Talents Project (YNWR-QNBJ-2018-093), the Research Startup Project (YJRC4201804) and the Cultivation Project (2018YDJQ009) of Yunnan University to J.-X. Zhou.

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along the Jiangnan domain and their tectonic implications. Precambrian Research 220–221, 139–157. Zhao, J.H., Asimow, P.D., 2014. Neoproterozoic boninite-series rocks in South China: a depleted mantle source modified by sediment-derived melt. Chemical Geology 388, 98–111. Zhu, J.J., Hu, R.Z., Bi, X.W., Zhong, H., Chen, H., 2011. Zircon U–Pb ages, Hf–O isotopes and whole–rock Sr–Nd–Pb isotopic geochemistry of granitoids in the Jinshajiang suture zone, SW China: Constraints on petrogenesis and tectonic evolution of the Paleo-Tethys Ocean. Lithos 126, 248–264. Zhu, J.J., 2012. The geological setting and metallogenesis of the Yangla copper deposit, SW Yunnan. Unpublished PhD thesis, The Gradute School of the Chinese Academy of Sciences, China, 179 pp (in Chinese with English abstract). Zhu, J.J., Hu, R.Z., Richards, J.P., Bi, X.W., Zhong, H., 2015. Genesis and magmatichydrothermal evolution of the Yangla skarn Cu deposit, Southwest China. Economic Geology 110, 631–652. Zi, J.W., Cawood, P.A., Fan, W.M., Tohver, E., Wang, Y.J., McCuaig, T.C., 2012a. Generation of Early Indosinian enriched mantle-derived granitoid pluton in the Sanjiang Orogen (SW China) in response to closure of the Paleo-Tethys. Lithos 140–141, 166–182. Zi, J.W., Cawood, P.A., Fan, W.M., Tohver, E., Wang, Y.J., McCuaig, T.C., Peng, T.P., 2013. Late Permian-Triassic magmatic evolution in the Jinshajiang orogenic belt, SW China and implications for orogenic processes following closure of the PaleoTethys. American Journal of Science, 313, 81–112. Zi, J.W., Cawood, P. A., Fan, W.M., Wang, Y.J., Tohver, E., 2012b. Contrasting rift and subduction-related plagiogranites in the Jinshajiang ophiolitic mélange,

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southwest China, and implications for the Paleo-Tethys. Tectonics 31, TC2012.

Figure captions Fig. 1. a) Simplified tectonic map showing the study area in relation to China’s major tectonic units; b) tectonic framework and c) distribution of magmatic rocks in the Jinshajiang suture zone and adjacent areas. Modified after Hou et al. (2003).

Fig. 2. Simplified geological map of the Yangla Cu ore district, and the distribution of high-Mg andesites and granitoids in Yangla.

Fig. 3. Field photographs, hand specimen and representative microscopic photographs of the Yangla high-Mg andesites in Linong. a) Andesite overlain by the Triassic slate; b) and c) hand specimen and their d) and e) microphotography showing porphyritic texture. Pl = plagioclase.

Fig. 4. Transmitted CL images of representative zircons from sample a) Y034 (N 28°53'21.2", E 99°04'09.1") and b) LN12-06 (N 28°55'15.5", E 99°04'05.9"). 206Pb/238U

ages for the spots on representative grains are shown in Ma with 1σ

errors.

Fig. 5. Zircon U-Pb concordia diagram for the Yangla high-Mg andesites.

Fig. 6. a) Nb/Y vs. SiO2 diagram (Winchester and Floyd, 1977), b) plot of SiO2 vs. Mgnumber, c) plot of SiO2 vs. MgO, d) plot of TiO2 vs. MgO/(FeOT+MgO), e) plot of (La/Yb)cn vs. Ybcn, and f) plot of Sr/Y vs. Y for the Yangla high-Mg andesites.

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Fig. 7. N-MORB-normalized trace elements spider diagram of the andesites in a) Jiangbian, c) Linong, and e) Lunong, with normalizing values from Sun and McDonough (1989). Chondrite-normalized REE patterns of the mafic dykes in b) Jiangbian, d) Linong, and f) Lunong, with normalizing values from Boynton (1984). Field/curves for Setouchi HMAs, Bonin HMA average and continental crust after Tatsumi (2006).

Fig. 8. a) Plot of La/Nb vs. εNd(T), and b) plot of εNd(T) vs. initial

87Sr/86Sr

for the

Yangla high-Mg andesites. The mantle component is modelled using the Jinshajiang MORB based on data for the Jinshajiang ophiolites (Wei et al., 2003; Xu and Castillo, 2004). The rang for the amphibolites of the Yangtze LCC and sediments of the Yangtze UC are from Zhu et al. (2011) and Zi et al. (2013). The ca. 350 Ma Yangla basalts were taken from Zhu (2012). The ca. 230 Ma Yangla and Ludian granitoids were taken from Zhu et al. (2011) and Zi et al. (2013), respectively.

Fig. 9. Diagrams of Y, La, Nb, Th, Nd, V, Sr and Pb vs. Zr to evaluate the mobility of these elements during alteration.

Fig. 10. Plots of a) Th/Sm vs. Th/Ce, b) Th/Nb vs. Nb/Y, c) Nd/Pb vs. Th/Nd, and d) Th/Nb vs. U/Th for the Yangla high-Mg andesites.

Fig. 11. Schematic images illustrating the petrogenesis of the Yangla high-Mg andesites and tectonic evolution of the Jinshajiang suture zone during the Late Permian to

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Triassic. a) Palaeo-Tethys ocean subduction and subsequent block assemblage induced convective upper mantle (probably the asthenosphere) along with the recycled sediment-derived components transported into the overlying refractory mantle to form a metasomatized mantle wedge during Late Permian to Early Triassic; b) the metasomatized mantle wedge was stably preserved beneath the Jinshajiang suture zone during the Middle Triassic due to the regional decompression; and c) at late Middle to Late Triassic, regional tectonic collapse and thermal relaxation caused the elevation of the lithosphere thermal boundary and induced the partial melting of the previously (Late Permian to Early Triassic) subduction-modified mantle wedge to generate the high-Mg andesites in Yangla.

Table captions: Table 1. LA-ICPMS zircon U-Pb isotope analyses for the Yangla high-Mg andesites.

Table 2. Major (wt. %) and trace (ppm) elemental data of the Yangla high-Mg andesites.

Table 3. Sr-Nd-Pb isotopic compositions of the Yangla high-Mg andesites.

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Table 1. LA-ICP-MS zircon U-Pb isotopic analyses for the Yangla high-Mg andesites Sa Age/M Isotopic ratio mp U Pb a Th T le (p (p 207Pb 207Pb* 206Pb*/ (pp h/ 206Pb Sp p p m) U */238 ±1σ */235 ±1σ /206Pb ±1σ 238U±1 ot m) m) U U * σ # Y034 (N 28°53'21.2", E 99°04'09.1") 80 24 22 0. 0.037 0.0 0.263 0.0 0.0507 0.0 23 1 5 1 .0 3 0.036 63 005 14 039 2 006 3 34 14 9. 0. 0.0 0.267 0.0 0.0535 0.0 8 ±23 2 0 3 6 3 9. 5 58. 3 67 4 0.037 26 005 45 053 1 010 3 27 7. 0. 0.0 0.257 0.0 0.0503 0.0 0 ±23 3 8 1 0 2 1 4 0 65 2 13 005 48 066 0 012 5 ± 3 66 14 16 0. 0.036 0.0 0.265 0.0 0.0528 0.0 23 4 1 5 4 6 9 9 22 .9 0. 2 0.036 44 005 61 046 6 008 3 83 31 0.0 0.264 0.0 0.0523 0.0 1 ±23 5 2 5 5 0 8 3 .4 3 73 005 84 041 0 006 3 ± 3 73 22 18 0. 0.037 0.0 0.263 0.0 0.0506 0.0 23 6 7 2 3 9 9 1 18 .2 0. 3 0.036 70 006 21 055 1 009 4 72 17 0.0 0.262 0.0 0.0521 0.0 9 ±23 7 0 1 8 7 8 4 23 .3 0. 2 0.036 53 005 93 050 9 008 3 85 24 0.0 0.258 0.0 0.0507 0.0 1 ±23 8 4 5 0 7 8 6 .3 2 95 005 32 058 2 010 4 ± 3 15 73. 4. 0. 0.036 0.0 0.263 0.0 0.0517 0.0 23 9 9 5 6 9 6 2 23 72 0. 4 0.037 93 005 58 070 7 013 3 84 29 0.0 0.265 0.0 0.0514 0.0 4 ±23 10 7 3 2 9 2 4 .7 3 48 005 61 044 0 007 7 ± 3 50 11 14 0. 0.036 0.0 0.254 0.0 0.0503 0.0 23 11 5 2 3 6 3 5 .4 2 72 005 70 046 0 008 2 ± 3 LN12–06 (N 28°55'15.5", E3 99°04'05.9") 0 7 6 10 30 40 0. 0.036 0.0 0.264 0.0 0.0516 0.0 233 ± 1 1 91 3 26 .9 0. 2 0.036 85 002 12 043 2 008 70 18 0.0 0.255 0.0 0.0505 0.0 232 ± 2 2 8 3 8 8 6 0 29 .2 0. 2 0.036 67 002 51 064 3 013 79 20 0.0 0.256 0.0 0.0506 0.0 3 233 ±1 5 4 7 2 2 6 42 .3 0. 2 0.036 78 002 96 051 8 010 11 33 0.0 0.262 0.0 0.0513 0.0 233 ± 1 4 6 1 4 5 22 7 52 .2 0. 3 0.036 79 002 18 038 4 007 14 39 0.0 0.261 0.0 0.0513 0.0 233 ± 1 5 0 2 1 4 13 4 24 .9 0. 2 0.036 81 002 96 035 3 007 64 21 0.0 0.265 0.0 0.0521 0.0 233 ± 2 6 8 2 7 2 9 3 49 .9 0. 3 0.036 84 003 22 052 2 010 12 40 0.0 0.258 0.0 0.0507 0.0 233 ± 1 7 3 0 4 6 91 6 33 .7 0. 3 0.036 75 002 76 038 3 007 89 24 0.0 0.263 0.0 0.0516 0.0 234 ± 1 8 1 1 1 8 7 4 32 .5 0. 2 0.036 91 002 95 039 3 008 85 24 0.0 0.263 0.0 0.0519 0.0 233 ± 1 9 7 4 8 1 0 3 27 .0 0. 2 85 002 79 055 2 011 74 17 0.036 0.0 0.255 0.0 0.0507 0.0 231 ± 1 10 9 3 6 4 3 5 41 .9 0. 2 0.036 54 002 69 055 5 011 10 27 0.0 0.258 0.0 0.0515 0.0 230 ± 1 11 4 1 8 5 45 9 39 .8 0. 2 40 002 52 070 0 014 10 25 0.036 0.0 0.263 0.0 0.0522 0.0 232 ± 1 12 7 4 0 4 49 0 39 .6 0. 2 0.036 58 002 65 056 8 011 10 31 0.0 0.257 0.0 0.0514 0.0 230 ± 1 13 4 2 7 7 56 1 29 .9 0. 2 0.036 26 002 05 049 1 010 77 22 0.0 0.257 0.0 0.0506 0.0 233 ± 2 14 9 0 0 2 9 4 .5 2 80 002 17 064 8 013 9 6 4 2

Table 2. Major (wt.%) and trace (ppm) elemental data of the Yangla high-Mg andesites Sample No. Location SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI

4903-24 4903-25 4903-26 4903-27 4903-28 4903-29 Lunong mining area 50.08 49.76 49.53 49.05 50.02 51.04 0.77 0.80 0.79 0.83 0.75 0.77 17.01 17.10 17.43 17.19 16.90 16.89 8.36 8.45 6.50 6.61 8.05 8.00 0.12 0.11 0.12 0.12 0.12 0.13 3.75 3.70 2.87 2.93 3.30 3.34 6.19 6.41 7.60 7.80 6.32 7.62 2.17 2.03 0.66 1.49 2.06 2.27 1.83 1.90 1.88 1.47 1.16 0.96 0.12 0.11 0.13 0.13 0.12 0.13 9.04 9.64 12.30 11.70 10.50 8.38 57

Y034 48.04 0.78 16.65 8.47 0.14 3.26 7.43 1.35 2.03 0.13 11.55

N13ZK2-03 N13ZK2-04 N13ZK2-05 Jiangbian mining area 53.45 52.02 53.57 0.56 0.54 0.54 15.03 14.63 14.85 6.54 6.61 6.61 0.10 0.11 0.11 6.18 6.15 6.08 4.70 5.20 5.06 2.31 2.31 3.68 3.08 2.87 1.99 0.13 0.13 0.13 7.78 8.17 7.39

Total Mg#

99.44 47.04

100.01 46.44

99.81 46.65

99.32 46.75

99.30 44.81

99.53 45.26

99.83 43.25

99.86 65.17

98.74 64.82

100.01 64.56

Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

22.8 161 15.2 37.6 14.1 7.60 94.1 16.6 79.6 152 19.0 80.3 5.02 30.8 348 12.1 23.6 3.03 12.4 2.80 0.82 2.29 0.49 3.19 0.75 2.02 0.29 1.93 0.30 2.18 0.38 2.94 2.89 0.99

22.8 162 13.0 32.2 12.4 6.40 76.6 16.1 83.5 156 18.4 79.4 4.88 34.0 475 11.5 22.0 2.93 11.8 2.77 0.68 2.27 0.49 3.03 0.75 1.91 0.29 1.88 0.27 2.21 0.36 2.76 2.76 0.96

21.8 157 10.9 29.1 14.1 6.77 74.6 16.5 95.0 82.8 19.0 88.5 5.50 31.2 246 12.2 23.9 3.03 12.6 2.91 0.79 2.32 0.51 3.24 0.75 2.05 0.31 2.10 0.30 2.31 0.38 3.44 3.09 1.06

22.7 153 11.7 32.3 15.5 9.14 88.6 16.5 82.5 113 21.0 89.1 5.40 24.5 269 13.1 26.0 3.32 13.9 3.14 0.85 2.41 0.52 3.36 0.79 2.12 0.32 2.14 0.31 2.35 0.40 4.15 3.02 1.07

23.1 152 12.2 27.7 14.0 8.00 82.4 16.1 60.9 184 20.3 88.6 5.31 25.4 360 12.8 24.6 3.17 13.0 2.93 0.84 2.42 0.53 3.25 0.76 2.09 0.32 2.04 0.30 2.24 0.38 3.37 3.00 1.05

23.3 151 12.0 35.7 14.0 7.70 77.6 16.0 45.8 282 19.8 85.8 5.24 17.4 405 12.4 23.9 3.10 12.6 2.97 0.85 2.30 0.51 3.22 0.80 2.12 0.30 2.12 0.29 2.29 0.37 3.76 2.92 1.01

22.8 157 10.6 25.1 13.5 29.90 73.0 15.7 113 93.3 20.6 86.4 5.34 34.2 123 12.8 24.8 3.26 13.6 3.16 0.79 2.42 0.54 3.44 0.80 2.09 0.31 2.09 0.31 2.30 0.36 2.05 3.04 1.05

26.4 133 339 36.9 80.1 45.1 83.8 16.7 141 262 23.1 135 13.1 8.86 486 26.7 53.0 5.59 20.4 3.98 0.80 3.73 0.67 3.96 0.84 2.38 0.38 2.29 0.36 3.71 0.65 93.3 17.3 8.63

24.6 128 294 35.8 75.0 47.1 82.9 16.8 143 217 22.6 128 12.4 12.2 519 24.2 50.3 5.48 19.7 3.89 1.01 3.85 0.67 3.83 0.83 2.38 0.38 2.22 0.35 3.83 0.64 104 17.0 8.05

25.2 128 296 36.3 75.3 48.0 83.0 16.9 143 214 22.4 131 12.6 12.3 520 24.7 50.8 5.35 19.7 4.06 1.04 4.06 0.66 3.86 0.79 2.37 0.37 2.24 0.36 3.80 0.65 104 17.4 8.13

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Table 2 (Continued) Sample No.

LIN-01

Location SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 LOI Total Mg#

54.7 0.74 16.61 8.21 0.12 4.54 7.57 2.66 2.10 0.14 2.23 99.62 52.27

Sc V Cr Co Ni Cu Zn Ga Rb Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Th U

28.3 188 38.3 47.4 15.9 26.3 104 18.2 91.2 560.0 19.9 107 6.67 5.72 525 17.8 35.9 4.07 15.9 3.49 1.02 3.54 0.57 3.50 0.74 2.10 0.34 2.03 0.32 2.72 0.52 8.36 5.88 2.35

LIN-02 54.91 0.750 16.62 8.47 0.12 4.62 7.39 2.41 1.66 0.14 2.28 99.37 51.93 27.8 187 39.2 50.9 16.7 29.7 147 17.4 67.2 540 19.8 106 6.70 4.41 443 17.8 36.0 4.05 15.5 3.44 0.94 3.40 0.59 3.35 0.77 2.11 0.32 2.07 0.31 2.92 0.54 9.76 5.79 2.24

LIN-03 54.95 0.760 17.32 8.11 0.12 4.49 6.43 2.54 2.35 0.14 2.17 99.38 52.30 26.0 188 30.9 52.1 14.7 63.9 94.6 17.3 104 518 19.6 107 6.56 7.74 518 17.1 35.1 3.96 15.2 3.13 1.00 3.28 0.57 3.46 0.74 2.03 0.31 1.93 0.31 2.71 0.54 9.50 5.65 2.14

LIN-04 55.32 0.740 16.71 8.44 0.13 4.93 7.25 2.29 2.11 0.14 1.73 99.79 53.64 27.4 181 40.6 52.2 16.8 30.0 103 17.2 77.2 501 19.3 104 6.42 9.07 512 17.2 34.7 3.93 15.5 3.44 0.95 3.43 0.56 3.26 0.72 1.96 0.33 1.92 0.30 2.55 0.51 9.93 5.66 2.14

LIN-05

LIN-06

LIN-08

Linong mining area 54.71 55.30 51.46 0.750 0.760 0.960 16.72 17.52 18.04 8.41 8.19 9.54 0.13 0.13 0.12 4.58 4.39 3.88 7.26 6.85 7.78 2.66 2.31 2.52 1.55 1.68 2.14 0.14 0.14 0.18 2.54 2.53 2.43 99.45 99.80 99.05 51.89 51.49 44.61 27.5 188 37.8 42.9 15.3 28.3 100 17.3 61.9 550 19.3 107 6.46 7.21 376 17.9 36.1 4.05 15.8 3.30 0.94 3.62 0.56 3.32 0.73 2.02 0.32 1.99 0.29 2.87 0.49 10.4 5.77 2.24

26.2 187 29.7 47.1 14.0 28.2 97.9 17.7 77.6 509 19.3 105 6.35 7.72 396 17.1 35.2 3.89 15.1 3.23 0.97 3.34 0.59 3.38 0.73 2.01 0.30 1.86 0.31 2.66 0.50 7.19 5.67 2.04

24.4 138 8.6 36.4 5.10 8.00 97.4 19.2 93.4 346 25.5 112 7.46 21.30 377 18.3 37.0 4.39 17.9 3.94 1.19 4.31 0.73 4.43 0.97 2.70 0.42 2.71 0.44 2.98 0.58 6.40 5.11 1.34

LIN-09 54.66 0.740 16.71 8.40 0.12 4.95 7.08 2.58 1.91 0.14 1.78 99.07 53.85 28.7 192 41.2 50.8 16.9 12.1 91.9 17.0 87.8 383 19.2 103 6.36 9.80 423 16.5 34.5 3.85 15.2 3.21 0.88 3.22 0.54 3.35 0.74 2.00 0.32 1.97 0.30 2.66 0.49 9.07 5.44 2.09

LIN-10 54.27 0.740 16.76 8.46 0.13 4.80 7.08 2.71 1.87 0.14 1.70 98.66 52.91 28.6 193 37.7 53.3 16.4 16.4 86.2 17.7 90.2 422 19.6 105 6.38 9.31 461 16.9 34.9 3.89 15.6 3.31 0.89 3.43 0.57 3.35 0.73 2.08 0.31 1.94 0.31 2.70 0.49 8.82 5.54 2.10

LIN-11 54.53 0.730 16.62 8.10 0.10 4.92 7.75 2.86 1.86 0.13 1.71 99.31 54.61 29.4 187 42.0 65.9 17.0 10.0 80.8 17.9 90.8 397 19.3 103 6.32 7.63 372 16.5 34.4 3.94 14.7 3.08 0.87 3.19 0.55 3.23 0.72 2.07 0.30 1.89 0.30 2.66 0.50 8.16 5.54 2.11

Mg# (Mg-number) = 100*molar MgO/(Mg + FeOT), assuming FeOT = 0.9*Fe2O3, Total iron as FeOT. LOI = loss on ignition.

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LN12-06 54.50 0.730 16.81 8.31 0.10 4.71 7.55 2.88 1.76 0.14 1.50 98.99 52.89 28.5 194 36.9 49.8 16.5 21.9 77.0 17.4 88.4 401 19.6 104 6.51 6.79 420 16.6 34.4 3.82 15.9 3.35 0.87 3.31 0.57 3.37 0.74 2.02 0.31 1.80 0.30 2.61 0.52 6.92 5.37 2.02

Table 3. Sr-Nd-Pb isotopic compositions of the Yangla high-Mg andesites

Sample

4903-24a 4903-26b 4903-29a Y034b

Rb

Sr

(ppm) (ppm)

87Sr/ 86Sr

2σ

87Sr/ 86Sr

I

Sm

Nd

(ppm

(ppm

)

)

143Nd/ 144Nd

2σ

143Nd/ 144Nd I

εNd(T)

204Pb

207Pb

/ 204Pb

208Pb

/ 204Pb

206Pb/

207Pb/

204Pb

204Pb

I

I

208Pb

/ 204Pb

I

79.6 95.0

152 0.711466 9 82.8 0.718253 4

0.70651 0.70739

2.80 2.91

12.4 12.6

0.512303 14 0.512099 -4.7 0.512341 2 0.512132 -4.1

19.13 15.67 39.27 18.34

15.63 38.51

45.8

282

0.708682 6

0.70714

2.97

12.6

0.512406

4

0.512193 -2.9

18.93 15.65 39.07 18.30

15.62 38.48

93.3 0.718122 4

0.70519

3.16

13.6

0.512424

4

0.512213 -2.5

19.44 15.69 39.56 18.22

15.63 38.42

18.91 15.65 39.09 18.38

15.63 38.64

113

LIN-02b

67.2

540

0.709342 8

0.70816

3.44

15.5

0.512317

2

0.512116 -4.4

LIN-08b

93.4

346

0.710418 6

0.70786

3.94

17.9

0.512306

4

0.512107 -4.6

LN12-06b

206Pb/

88

401

0.709730 4

0.70764

3.82

15.9

0.512317

2

0.512099 -4.7

N13ZK2-04b

143

217

0.714491 8

0.70825

3.89

19.7

0.512251

4

0.512072 -5.3

N13ZK2-05b

143

214

0.714689 4

0.70836

4.06

19.7

0.512246

2

0.512059 -5.5

19.06 15.62 39.16

18.37 15.59 38.56

Initial isotopic ratios are calculated to 230 Ma. Chondrite uniform reservoir (CHUR) values (87Rb/86Sr = 0.0847, 87Sr/86Sr = 0.7045, 147Sm/144Nd = 0.1967, 143Nd/144Nd = 0.512638) are used for the calculation. λRb = 1.42×10−11year−1 (Steiger and Jäger, 1977); λSm = 6.54×10-12year-1 (Lugmair and Harti, 1978). Sr-Nd isotopic compositions for samples with a and b were analyzed by MC-ICP-MS and TIMS, respectively. Rb, Sr, Sm and Nd contents were determined by ICP-MS. Initial Pb isotopic ratios are calculated using whole-rock U, Th and Pb contents by ICP-MS.

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Graphical abstract

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Highlights

- High-Mg andesites in the Jinshajiang suture zone formed at ~230 Ma. - The andesites were generated by partial melting of a previously subduction-modified mantle wedge. - The Jinshajiang suture zone had experienced a period of regional tectonic collapse and thermal relaxation before Late Triassic.

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Credit Author Statement Hong-Peng Fan: Conceptualization, Methodology, Investigation, Validation, Formal analysis, Resources, Writing - Original Draft, Visualization Bo Li: Resources, Writing - Review & Editing, Project administration Jia-Xi Zhou: Resources, Writing - Review & Editing, Project administration Li-Juan Du: Formal analysis, Resources Hai-Rui Sun: Formal analysis, Resources Zhi-Long Huang: Conceptualization, Writing - Review & Editing, Project administration, Supervision Tao Wu: Writing - Review & Editing

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Declaration of Interest Statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work. There is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in or the review of the manuscript entitled.

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