1. Introduction
The Northwest Tibet area, also known as the West Kunlun orogenic belt (Xiao et al., Reference Xiao, Windley, Chen, Zhang and Li2002, Reference Xiao, Han, Windley, Yuan, Zhou and Li2003a, Reference Xiao, Windley, Hao and Zhai2003b; Wang et al., Reference Wang, Gao, Zhang, Yan, Xu, Zhou, Dong and Li2020), serves as crucial junction between the Central Asia and Tethys tectonic domains (Wang et al., Reference Wang, Liu, Korhonen, Yang, Cao, He, Zhu and Liang2016; Zhang et al., Reference Zhang, Zou, Ye and Chen2019a; Yin et al., Reference Yin, Xiao, Sun, Chen, Yuan, Zhang, Wang, Du, Wang and Xia2020). The abundant magmatic rocks in this area reveal the evolution of the Proto-, Palaeo- and Neo-Tethys Oceans (Pan, Reference Pan and Pan1996; Xiao et al., Reference Xiao, Han, Windley, Yuan, Zhou and Li2003a, Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Zhang et al., Reference Zhang, Zou, Ye and Chen2019a; Yin et al., Reference Yin, Xiao, Sun, Chen, Yuan, Zhang, Wang, Du, Wang and Xia2020). However, limited access and harsh natural conditions have hindered our understanding of the Precambrian magmatic record, and these factors also impeded the study of the basement in the Northwest Tibet area.
The Proto-Tethys Ocean is widely believed to have formed by the break-up of the supercontinent Rodinia, spanning from the Neo-Proterozoic until at least the Early Silurian (Ding et al., Reference Ding, Wang, Liu and Sun1996; Xiao et al., Reference Xiao, Han, Windley, Yuan, Zhou and Li2003a, Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Jia et al., Reference Jia, Jiang, Liu, Zhao and Zhou2013; Wang et al., Reference Wang, Hattori, Liu, Song, Gao and Zhang2017). Island arc magmatism related to this process may have occurred before the Early Cambrian (>533 Ma; Yin et al., Reference Yin, Xiao, Sun, Chen, Yuan, Zhang, Wang, Du, Wang and Xia2020) and continued until the Silurian (Zhang et al., Reference Zhang, Zou, Ye and Chen2018a) in present-day West Kunlun. The later subduction associated with the closure of the Palaeo-Tethys Ocean occurred from the Late Permian to Late Jurassic (Pan, Reference Pan and Pan1996; Matte et al., Reference Matte Ph, Arnaud, Bourjot, Avouac, Vidal Ph, Yusheng and Yi1996; Mattern and Schneider, Reference Mattern and Schneider2000; Xiao et al., Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Jiang et al., Reference Jiang, Jia, Liu, Liao, Zhao and Zhou2013; Liu et al., Reference Liu, Jiang, Jia, Zhao and Zhou2015). However, the source of the Precambrian to Late Jurassic granitoids remains unclear, and the basement nature and Precambrian tectonic evolution of the Northwest Tibet area, particularly in the Tashikuergan-Tianshuihai terrane, remains largely unknown. Zhang et al. (Reference Zhang, Ji, Ma, Gao, Sun, Hong and Lv2020) and Yin et al. (Reference Yin, Xiao, Sun, Chen, Yuan, Zhang, Wang, Du, Wang and Xia2020) proposed that the Cambrian andesites, dacites and granitoid plutons in the Tashikuergan-Tianshuihai terrane, Northwest Tibet area, were likely derived from partial melting of Palaeo-Proterozoic and Meso-Proterozoic basement rocks. Ji et al. (Reference Ji, Li, Chen, He, Zhao, Bian, Zhu, Cui and Ren2011) have reported 2481 ± 14 Ma zircons in foliated rhyolite from the Tashikuergan-Tianshuihai terrane. However, further systematic discussion of their petrogenesis and formation mechanism is still needed to understand the basement nature and tectonic evolution of the Tashikuergan-Tianshuihai terrane. Previous studies have primarily focused on Cambrian to Triassic granitoids (Yuan et al., Reference Yuan, Sun, Zhou, Zhou, Xiao and Li2002; Jiang et al., Reference Jiang, Jiang, Ling, Zhou, Rui and Yang2002, Reference Jiang, Jia, Liu, Liao, Zhao and Zhou2013; Jia et al., Reference Jia, Jiang, Liu, Zhao and Zhou2013; Liu et al., Reference Liu, Jiang, Jia, Zhao and Zhou2015; Zhang et al., Reference Zhang, Liu, Huang, Wu and Zhou2016, Reference Zhang, Zou, Ye and Chen2019a; Zhang et al., Reference Zhang, Ji, Ma, Gao, Sun, Hong and Lv2020; Yin et al., Reference Yin, Xiao, Sun, Chen, Yuan, Zhang, Wang, Du, Wang and Xia2020; Wang et al., Reference Wang, Gao, Wang, Yan, Wang, Huang and Qin2022), with limited investigation of Precambrian metamorphic igneous and volcaniclastic rocks. These rocks are of critical importance, as they likely represent the magmatic or volcanic products of the assembly and break-up of the supercontinent Rodinia and crucial information about basement formation and evolution. Despite the scarcity of Precambrian metamorphic igneous rocks, their significance warrants further study.
Our recent detailed field investigations in the Hongliutan area, northeastern Tashikuergan-Tianshuihai terrane, have led to the discovery of Precambrian to Early Cambrian metamorphic igneous and volcaniclastic rock strata. In this study, we integrate our field observations with new petrography, geochronology and geochemistry and combine these with previous results to (1) ascertain the source, petrogenesis and formation environment of metamorphic rocks in the Hongliutan area and (2) propose the basement nature and the Precambrian tectonic evolution of the Tashikuergan-Tianshuihai terrane in the Northwest Tibet. These findings have significant implications for delineating the geodynamic evolution of the Proto-Tethys Ocean and Tibetan Plateau.
2. Regional tectonic setting
The Northwest Tibet region is situated between the Tibetan Plateau to the south and the Tarim Basin to the north (Fig. 1a; Hu et al., Reference Hu, Wang, Huang, Tong, Mu and Qiu2016). It comprises of several tectonic units, including the southwestern Tarim block, West Kunlun orogenic belt, Hoh Xil-Bayan Har block, Tashikuergan-Tianshuihai terrane, Linjitang basin, Karakorum terrane, Qiangtang terrane, Lhasa terrane and Himalaya orogenic belt, arranged from north to south (Fig. 1b; Pan et al., Reference Pan, Zhu, Wang, Liao, Geng and Jiang2004; Li et al., Reference Li, Ji, Yang, Pan, Yu, Chen, Zhao, Yang, Meng, Luo, Li and Zhang2009a; Li et al., Reference Li, Xu, Borg, Albert Gilg, Dong, Fan, Zhou, Liu, Hong, Ke, Wu, Zhang and Li2019). These units accreted during the subduction and collision of the Proto- and Palaeo-Tethys Oceans (Mattern and Schneider, Reference Mattern and Schneider2000; Xiao et al., Reference Xiao, Han, Windley, Yuan, Zhou and Li2003a, Reference Xiao, Windley, Hao and Zhai2003b, Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Wang et al., Reference Wang, Gao, Zhang, Yan, Xu, Zhou, Dong and Li2020). Our study focuses on the Hongliutan metamorphic igneous rocks in the northeastern part of the Tashikuergan-Tianshuihai terrane. This terrane lies between the West Kunlun orogenic belt, the Hoh Xil-Bayan Har block, the Qiangtang terrane and the Karakorum terrane. The Kangxiwar suture, Jinshajiang suture, Altyn fault and Qiaoertianshan fault separate the Tashikuergan-Tianshuihai terrane from the surrounding units, respectively (Fig. 1b; Pan, Reference Pan1990; Jiang et al., Reference Jiang, Yang, Feng, Zhu, Zhao, Chai, Shi, Wang and Hu1992; Deng, Reference Deng1995; Yin and Harrison, Reference Yin and Harrison2000; Jiang et al., Reference Jiang, Jiang, Ling, Zhou, Rui and Yang2002; Chung et al., Reference Chung, Chu, Zhang, Xie, Lo, Lee, Lan, Li, Zhang and Wang2005; Li et al., Reference Li, Ji, Yang, Yu, Zhao, Chen, Meng, Pan, Shi, Zhang, Li and Luo2008a; Pan et al., Reference Pan, Wang, Li, Yuan, Ji, Yin, Zhang and Wang2012; Li et al., Reference Li, Xu, Borg, Albert Gilg, Dong, Fan, Zhou, Liu, Hong, Ke, Wu, Zhang and Li2019).
Figure 1. Location (a) and tectonic units (b) of the West Kunlun-Karakorum area (revised from Pan et al., Reference Pan, Zhu, Wang, Liao, Geng and Jiang2004; Li et al., Reference Li, Ji, Yang, Pan, Yu, Chen, Zhao, Yang, Meng, Luo, Li and Zhang2009a; Dong et al., Reference Dong, Xu, Fan, Qu, Li, Wan, An, Zhou, Li, Chen and Liu2015; Li et al., Reference Li, Xu, Borg, Albert Gilg, Dong, Fan, Zhou, Liu, Hong, Ke, Wu, Zhang and Li2019).
The Tashikuergan-Tianshuihai terrane is composed of Precambrian basement and overlying Phanerozoic strata (Wang et al., Reference Wang, Gao, Zhang, Yan, Xu, Zhou, Dong and Li2020). The basement consists of a Palaeo-Proterozoic medium- to high-grade metamorphic complex and Meso-Proterozoic metamorphic shallow-marine carbonate rocks, including the Tianshuihai, Bulunkuole and Saitula groups (Ding et al., Reference Ding, Wang, Liu and Sun1996; Chu, Reference Chu2008). The terrane cover comprises Palaeozoic strata, including Carboniferous and Permian clastic limestones and other carbonate rocks (Anhui Geological Survey, 2005a, 2005b). A Late Triassic granitoid plutonic belt trends west to east and extends through the Tashikuergan-Tianshuihai terrane and Hoh Xil-Bayan Har block (209.6 Ma and 217.0 to 216.0 Ma; Qiao et al., Reference Qiao, Zhang, Wu, Jin, Du, Zhao and Chen2015a; Wei et al., Reference Wei, Wang, Hu, Mu, Qiu, Yan and Li2017). In the southern part of the terrane, a Jurassic-Cretaceous granodioritic plutonic belt trends southeastward and is situated in the Tashikuergan-Tianshuihai, Karakorum, and Qiangtang terranes (Fig. 1b; Pan et al., Reference Pan, Zhu, Wang, Liao, Geng and Jiang2004, Reference Pan, Wang, Li, Yuan, Ji, Yin, Zhang and Wang2012; Li et al., Reference Li, Ji, Yang, Yu, Zhao, Chen, Meng, Pan, Shi, Zhang, Li and Luo2008a; Li et al., Reference Li, Xu, Borg, Albert Gilg, Dong, Fan, Zhou, Liu, Hong, Ke, Wu, Zhang and Li2019). The Jurassic-Cretaceous magmatic arc and the Linjitang back-arc basin are related to the northward-directed subduction of the Bangong-Nujiang oceanic plate beneath the Tashikuergan-Tianshuihai and Karakorum terranes (Fig. 1b; Pan et al., Reference Pan, Wang, Li, Yuan, Ji, Yin, Zhang and Wang2012; Li et al., Reference Li, Ji, Yang, Yu, Zhao, Chen, Meng, Pan, Shi, Zhang, Li and Luo2008a; Li et al., Reference Li, Xu, Borg, Albert Gilg, Dong, Fan, Zhou, Liu, Hong, Ke, Wu, Zhang and Li2019).
3. Geological setting of Hongliutan area
3.a. Lithology
The Tashikuergan-Tianshuihai terrane and Hoh Xil-Bayan Har block are separated by the Dahongliutan fault, which reactivates the western part of the Jinshajiang suture. The Hongliutan area is situated in the northeastern part of the Tashikuergan-Tianshuihai terrane and on the southwestern side of the Dahongliutan fault (Fig. 2). The area is characterized by Meso- to Palaeo-Proterozoic (c.a. 1.4 to 1.8 Ga, Tianshuihai Group), Neo-Proterozoic (ca. 0.8 to 1.0 Ga, Xiaoerke Group), lower Silurian (Wenquangou Group) and upper Carboniferous (Qiatier Group) sediments and metasediments. The Meso- to Palaeo-Proterozoic Tianshuihai Group comprises a suite of greenschist-facies, shallow-marine siliciclastic rocks (e.g. sericite-quartz schist, phyllite, and argillaceous slate), and limestone (Hu et al., Reference Hu, Wang, Huang, Tong, Mu and Qiu2016; Wang et al., Reference Wang, Gao, Zhang, Yan, Xu, Zhou, Dong and Li2020). The Neo-Proterozoic Xiaoerke Group features siliceous dolomite and limestone. The Lower Silurian Wenquangou Group mainly comprises thick layers of grey-green to light-grey feldspar-quartz slate, meta-sandstone and siltstone, interlayered with limestone and marble (Zhang et al., Reference Zhang, Zou, Ye and Chen2019a; Li et al., Reference Li, Xu, Borg, Albert Gilg, Dong, Fan, Zhou, Liu, Hong, Ke, Wu, Zhang and Li2019; Wang et al., Reference Wang, Gao, Zhang, Yan, Xu, Zhou, Dong and Li2020).
Figure 2. Geological map of the Hongliutan area.
In addition, an Ordovician monzonitic granitic pluton (484.5 ± 2.7 Ma) and a Cambrian granite porphyry (527.9 ± 2.8 Ma) located to the northeast intruded into the aforementioned sedimentary and metasedimentary sequences (Fig. 2; Qiao et al., Reference Qiao, Wang, Wu, Hao, Zhao, Chen, Lv and Du2015b; Hu et al., Reference Hu, Wang, Huang, Tong, Mu and Qiu2016). The Tashikuergan-Tianshuihai terrane is thought to have been an accretionary prism during the Palaeozoic-Mesozoic and contains thick, partly metamorphosed Triassic flysch sequences that record subduction-related orogenic processes resulting from the collision between the West Kunlun orogenic belt and Karakorum terranes (e.g. Matte et al., Reference Matte Ph, Arnaud, Bourjot, Avouac, Vidal Ph, Yusheng and Yi1996; Mattern and Schneider, Reference Mattern and Schneider2000; Yin and Harrison, Reference Yin and Harrison2000; Xiao et al., Reference Xiao, Windley, Fang, Zhou, Yuan, Wang, Hao, Hou and Li2001, Reference Xiao, Windley, Chen, Zhang and Li2002, Reference Xiao, Han, Windley, Yuan, Zhou and Li2003a, Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Wang et al., Reference Wang, Gao, Zhang, Yan, Xu, Zhou, Dong and Li2020).
3.b. Geological section in the Hongliutan area
A newly discovered metamorphic rock stratum has been found adjacent to the Cambrian leucogranite in the western region. It is exposed along a tightly folded syncline, with both limbs dipping to the northwest and a fold axis plunging to the northeast. This formation measures approximately 3.0 km in length and 0.5 km in width. The eastern part of this syncline exhibits an unconformable contact with the Cambrian leucogranite, with an NW-striking main dip angle of 50° to 60° (Fig. 3a). The stratum is mainly composed of foliated plagioclase schist (Fig. 3b), amphibolite (Fig. 3d), and quartz schist (Fig. 3c). This metamorphic rock stratum was intruded by Cambrian leucogranite and cut by multiple-stage faults (Figs. 3a and 3b). It underwent greenschist-facies to epidote-amphibolite facies metamorphism simultaneously, transforming into middle-low grade metamorphic rocks. The quartz schist, interlayered between layers of plagioclase schist less than 100 m wide, is found alongside intrusive bodies of amphibolite, which are themselves surrounded by the plagioclase schist (Fig. 3a). Detailed stratigraphic descriptions of the granitoid and metamorphic rocks are outlined below:
Figure 3. A simplified sketch of the geological section in the Hongliutan area (a). Field photos of leucogranite intrusion into plagioclase schist (b), schistosity in quartz schist (c) and amphibolite layers (d).
The leucogranite is a felsic intrusion located to the east of the metamorphic rock stratum (Figs. 3a and 3b). It mainly consists of euhedral to subhedral phenocrysts of 12–20 vol% plagioclase (100–700 μm), 5 vol% quartz (200–500 μm), and 3 vol% muscovite (20–150 μm), as well as an anhedral matrix of felsic minerals (plagioclase and quartz) with minor accessory minerals (apatite, zircon, and ilmenite). The leucogranite exhibits a porphyritic texture (Figs. 4a and 4b).
Figure 4. Photomicrographs of leucogranite (a, b), plagioclase schist (c, d), quartz schist (e, f) and amphibolite (g, h). Abbreviations: Am–amphibole, Bt–biotite, Chl–chlorite, Grt–garnet, Ms–muscovite, Pl–plagioclase, Py–pyrite, and Qtz–quartz.
The grey-dark grey plagioclase schist forms the main body of the metamorphic rock stratum (Figs. 3a and 3b). The foliation is clearly evident with an inclination of 58°, exhibiting a distinct banded structure with oriented minerals under the microscope (Figs. 4c and 4d). The garnet within the plagioclase-quartz-biotite assemblage exhibits a distinct counterclockwise rotation, indicating its original alignment. However, the development of lineation is not obvious. It contains 40 vol% subhedral to anhedral plagioclase (<200 μm) and 35 vol% quartz (<200 μm), euhedral to subhedral 10 vol% biotite (100–250 μm), and 8 vol% chlorite (80–1,200 μm), as well as subhedral 4 vol% garnet (250–500 μm).
The grey quartz schist forms the central part of the metamorphic igneous rock stratum, showing an inclination of 55° (Figs. 3a and 3c), and exhibits a banded structural texture under the microscope (Fig. 4). It mainly comprises subhedral to anhedral fine-grained 50 to 100 μm quartz (35–40 vol%) and plagioclase (30–35 vol%) phenocrysts, oriented euhedral 200 to 600 μm garnet (12–15 vol%), 20 to 500 μm biotite (8–10 vol%), and chlorite (8–10 vol%) phenocrysts (Figs. 4e and 4f). The accessory minerals include apatite, zircon, ilmenite, magnetite, and pyrite.
The grey amphibolites appear as additional intrusions, measuring 200 m in length and 50–80 m in width, within the plagioclase schist section (Figs. 3a and 3d). It mainly comprises subhedral plagioclase (35 vol%, 50–200 μm), quartz (25 vol%, 50–200 μm), biotite (4 vol%, 30–100 μm), and euhedral to subhedral hornblende (35 vol%, 50–700 μm). It is characterized by the curved lineation of hornblende (Figs. 4g and 4h). Therefore, the mineral paragenesis supports an amphibolite facies.
4. Sample selection and analytical techniques
This contribution employed two leucogranite samples (HLT01-2 and HLT01-3), three plagioclase schist samples (HLT02-2, HLT02-3, and HLT02-4), two quartz schist samples (HLT04-3 and HLT04-4), and two amphibolite samples (HLT03-3 and HLT03-4). These samples were selected after detailed petrographic examination as representative samples to evaluate the whole-rock geochemical composition. Major and trace element abundances were obtained by using X-ray fluorescence (XRF) spectrometry and inductively coupled plasma mass spectrometry (ICP-MS), respectively. The detailed preparation and analysis process were described in Hong et al. (Reference Hong, Xu, Gao, Peters, Zhang, Jielili, Xiang, Li, Wu, You, Liu and Ke2018) and Wu et al. (Reference Wu, Hong, Xu, Cao, Li, Zhang, You, Ke and Dong2018). The international standard samples, GSR-1 (granite) and GSR-3 (basalt), were used to monitor the preparation process and instrument status. Analytical uncertainties were 1–3% for major elements, 5% for rare earth elements (REEs), and 5–10% for trace elements. The whole-rock major and trace element data are listed in Supplementary Table DR1.
To evaluate the Rb–Sr and Sm–Nd isotopic compositions, we employed the leucogranite samples HLT01-2 and HLT01-3, plagioclase schist sample HLT02-2, quartz schist sample HLT04-3, and amphibolite sample HLT03-3, all corresponding to the bulk rock samples. Sr–Nd isotope analyses were performed by using the classical two-step ion-exchange chromatographic method and measured using a Thermo Fisher Triton Plus multi-collector thermal ionization mass spectrometer. The Sr and Nd isotopic ratios were respectively normalized to 86Sr/88Sr = 8.375209 and 146Nd/144Nd = 0.7219. During the data acquisition period, Sr standard NBS-987 gave 87Sr/86Sr = 0.710265 ± 0.000014 (n = 4, 2σ), and JNdi-1 Nd standard yielded 143Nd/144Nd = 0.512110 ± 0.000009 (n = 4, 2σ). Additionally, the leucogranite samples HLT01-2 and HLT01-3, plagioclase schist samples HLT02-2 and HLT02-3, quartz schist samples HLT04-3 and HLT04-4, and amphibolite samples HLT03-3 and HLT03-4 were analysed for Pb isotope analyses. Pb was separated and purified using the conventional cation-exchange technique, yielding procedural blanks with Pb concentrations of 200 pg. Repeated analysis of the NBS981 gave a value of 207Pb/206Pb = 0.91405 ± 0.00040 (2σ). The detailed preparation and analysis process for the above Sr–Nd–Pb isotopic tests followed procedures similar to those described by Li et al. (Reference Li, Chu, Guo, Li, Yang and Li2015, Reference Li, Wang, Guo, Chu and Feng2016). The results of the Sr–Nd–Pb isotopic analysis are summarized in Supplementary Table DR1.
Zircon grains from part of the samples were collected for U–Pb dating using laser ablation (LA) ICP-MS (sample HLT01-1, leucogranite, 14 grains) and secondary ion mass spectrometry (SIMS) (sample HLT04-2, quartz schist, 43 grains; sample HLT03-2, amphibolite, 14 grains). Conventional heavy fraction and magnetic techniques were used for sample separation. The freshest, unbroken, and representative zircon grains were selected and mounted in epoxy mounts. The morphology, internal structure, pellucidity, inclusions, and zonation were revealed in transmitted and reflected light photomicrographs and cathodoluminescence (CL) images. All epoxy mounts were polished to expose half-sections of the zircon grains and vacuum-coated with high-purity gold before CL imaging and LA-ICP-MS/SIMS analysis. Measurements of U, Th, and Pb were conducted using the Agilent 7500a Q-ICP-MS with a 193 nm excimer laser ablation system and the Cameca IMS-1280 SIMS. LA-ICP-MS followed operating and data processing procedures similar to those described by Xie et al. (Reference Xie, Zhang, Zhang, Sun and Wu2008), and a detailed description of the Cameca IMS-1280 SIMS analytical procedure is given in Li et al. (Reference Li, Liu, Li, Guo and Chamberlain2009b). Common-Pb corrections and data reductions were made using the method of Andersen (Reference Andersen2002) and the Isoplot/Ex version 2.49 programs of Ludwig (Reference Ludwig2001). Errors quoted as age dates are at the 2σ level. All zircon U–Pb data is presented in Supplementary Tables DR2 and DR3.
All of the above analyses were conducted at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), China.
5. Results
5.a. Major and trace element compositions
The analysed samples have low loss on ignition (LOI) values ranging from 0.48 wt% to 2.16 wt%, with no observed positive relationship between LOI and K2O or K2O + Na2O. The leucogranite and amphibolite are classified as igneous rocks. Accordingly, we utilize geochemical diagrams specifically designed for igneous rocks to identify their distinct types and characteristics. While the geochemical data for the plagioclase schist and quartz schist strongly suggest a volcaniclastic origin, it is important to note that other origins cannot be entirely ruled out, as the geochemical evidence remains insufficient. Therefore, geochemical diagrams typically used for sedimentary rocks were applied to assess their characteristics, while also considering igneous rock diagrams for comparison.
These samples exhibit a wide compositional variation, with SiO2 contents varying from 51.04 to 80.23 wt% and Zr/Ti ratios from 0.02 to 0.16, ranging from basalt (gabbro) to rhyolite (granite) compositions (Figs. 5a–c; Le Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986; Wilson, Reference Wilson1989; Winchester and Floyd, Reference Winchester and Floyd1977). All samples are peraluminous and belong to the calc-alkaline series, as shown in the A/CNK versus A/NK (Fig. 5d; Maniar and Piccoli, Reference Maniar and Piccoli1989) and AFM (Fig. 5e; Irvine and Baragar, Reference Irvine and Baragar1971) diagrams, respectively.
Figure 5. Geochemical diagrams for Hongliutan leucogranite and metamorphic rocks: (a, b) The TAS diagram (after Le Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986; Wilson, Reference Wilson1989), (c) Nb/Y versus Zr/Ti diagram (after, Winchester and Floyd, Reference Winchester and Floyd1977), (d) A/CNK versus A/NK diagram (after Maniar and Piccoli, Reference Maniar and Piccoli1989) and (e) AFM diagram (after Irvine and Baragar, Reference Irvine and Baragar1971).
The leucogranite samples (HLT01-2 and HLT01-3) exhibit relatively high SiO2 contents (79.92 wt% and 80.23 wt%) and Na2O (5.73 wt% and 5.45 wt%) contents, along with high Zr/Ti ratios of 0.16 and 0.15. These samples are plotted in the rhyolite field of the TAS diagram (Fig. 5a; Le Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986) and the Nb/Y versus Zr/Ti diagram (Fig. 5c; Winchester and Floyd, Reference Winchester and Floyd1977), and near the granite field in the TAS diagram for plutonic rocks (Fig. 5b; Wilson, Reference Wilson1989). These leucogranite samples are enriched in LREE (LREE/HREE ratios of 5.38 and 6.00, (La/Yb)N of 5.42 and 5.93) and exhibit flat HREE, and show significant negative Eu anomalies (Eu/Eu* ratios of 0.07 and 0.08; Fig. 6a; Boynton, Reference Boynton1984). In the primitive mantle-normalized diagrams, they exhibit relative enrichments in Rb, Th, Nd, Zr, and Hf, and depletions of Ba, Nb, Sr, P, Eu, and Ti (Fig. 6b; Sun and McDonough, Reference McDonough1989).
Figure 6. (a, c) Chondrite-normalized REE patterns (after Boynton, Reference Boynton1984) and (b, d) primary mantle-normalized incompatible trace element patterns of the Hongliutan leucogranite and metamorphic rocks (after Sun and McDonough, Reference McDonough1989).
The amphibolite samples (HLT03-3 and HLT03-4) exhibit relatively low contents of SiO2 (51.04 wt% and 53.13 wt%), with high contents of TFe2O3 (10.91 wt% and 10.36 wt%), MnO (0.11 wt% and 0.13 wt%), MgO (5.83 wt% and 5.16 wt%), and CaO (6.49 wt% and 7.56 wt%). These samples plot within the basaltic andesite, trachybasalt, and gabbro fields of the two TAS diagrams (Figs. 5a and 5b; Le Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986; Wilson, Reference Wilson1989) and in the basalt and sub-alkaline basalt fields of the Nb/Y versus Zr/Ti diagram (Fig. 5c; Winchester and Floyd, Reference Winchester and Floyd1977). The REE are weakly fractionated, with relatively low LREE/HREE ratios (3.84 and 3.49) and (La/Yb)N of 2.71 and 2.62, accompanied by negative Eu anomalies (Fig. 6a; Boynton, Reference Boynton1984). These samples exhibit relative enrichments in Rb, U, Pb, and Nd, as well as depletions of Ba, Sr, and Ti on primitive mantle-normalized diagrams, along with variable depletions of K (Fig. 6b; Sun and McDonough, Reference McDonough1989).
The plagioclase schist samples (HLT02-2, HLT02-3, and HLT02-4) exhibit elevated levels of Al2O3 (17.61 to 18.93 wt%) and K2O (3.02 to 4.65 wt%). In the Rb versus K2O diagram (Fig. 7a; Shen et al., Reference Shen, Shu, Xiang, Zhang and Wang2009), these samples plot in the felsic domain, and in the ΣREE versus La/Yb diagram (Fig. 7b; Liu, Reference Liu1991), they are classified within the granite field. In igneous rock geochemical discrimination diagrams, these samples plot in the trachy-andesite and andesite regions on the TAS diagram (Fig. 5a; Le Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986), and in the Nb/Y versus Zr/Ti diagram (Fig. 5b; Winchester and Floyd, Reference Winchester and Floyd1977). They also plot the syeno-diorite and diorite regions in the TAS diagram for plutonic rocks (Fig. 5b; Wilson, Reference Wilson1989). Total REE concentrations vary from 176.50 to 208.20 ppm with enrichment in LREE (LREE/HREE ratios from 7.42 to 11.68, (La/Yb)N from 8.62 to 19.19) (Fig. 6c; Boynton, Reference Boynton1984). They are enriched in Rb, Th, K, Pb, Nd, Zr, and Hf, but depleted in Nb, Ta, La, Ce, Sr, P, and Ti on primitive mantle-normalized diagrams (Fig. 6d; Sun and McDonough, Reference McDonough1989).
Figure 7. (a) The Rb versus K2O diagram (after Shen et al., Reference Shen, Shu, Xiang, Zhang and Wang2009) and (b) the ΣREE versus La/Yb diagram (after Liu, Reference Liu1991) of the Hongliutan plagioclase schist and quartz schist.
The quartz schist samples, HLT04-3 and HLT04-4, exhibit relatively high SiO2 contents (70.34 wt% and 71.77 wt%) and relatively low Al2O3 contents (11.62 wt% and 12.16 wt%), respectively. Their plots are located in the felsic domain and granite domain on the Rb versus K2O diagram (Fig. 7a; Shen et al., Reference Shen, Shu, Xiang, Zhang and Wang2009) and the ΣREE versus La/Yb diagram (Fig. 7b; Liu, Reference Liu1991), respectively. In the two TAS diagrams, they respectively plot within the dacite field and near the quartz diorite (granodiorite) field (Figs. 5a and 5b; Le Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986; Wilson, Reference Wilson1989). The samples show total REE concentrations of 335.54 ppm and 416.07 ppm and are characterized by enrichment in LREE, with (La/Yb)N of 8.42 and 10.29 (Fig. 6c; Boynton, Reference Boynton1984). In primitive mantle-normalized diagrams, the trace element concentrations show depletion in Ba, Nb, Sr, P, Eu, and Ti, and enrichment in Rb, Th, U, La, Ce, Nd, Zr, and Hf (Fig. 6d; Sun and McDonough, Reference McDonough1989).
5.b. Sr–Nd–Pb isotopes
The leucogranite samples (HLT01-2 and HLT01-3) exhibit (87Sr/86Sr)i ratios of 0.719779 and 0.721624, (143Nd/144Nd)i ratios of 0.512013 and 0.512026, εNd(t) values of +0.4 and +0.6, TDM(Nd) ages of 1245 Ma and 1181 Ma, and T2DM(Nd) ages of 1197 Ma and 1176 Ma. The amphibolite sample (HLT03-3) exhibits (87Sr/86Sr)i, (143Nd/144Nd)i, εNd(t) values, TDM(Nd) age, and T2DM(Nd) age of 0.715405, 0.511964, −0.6, 1329 Ma, and 1275 Ma, respectively.
The plagioclase schist sample (HLT02-2) has a mean (87Sr/86Sr)i ratio of 0.723142, (143Nd/144Nd)i ratio of 0.511418, εNd(t) value of −11.3, TDM(Nd) age of 2120 Ma, and T2DM(Nd) age of 2139 Ma. The quartz schist sample (HLT04-3) shows (87Sr/86Sr)i value of 0.714030, (143Nd/144Nd)i value of 0.511073, εNd(t) value of −6.0, TDM(Nd) age of 1741 Ma, and T2DM(Nd) age of 2115 Ma.
All samples show relatively high 208Pb/204Pb ratios ranging from 39.858 to 44.339, 207Pb/204Pb ratios ranging from 15.805 to 16.056, and 206Pb/204Pb ratios ranging from 19.237 to 22.951 (Supplementary Table DR1).
5.c. Geochronology
Zircons from the leucogranite sample (HLT01-1) and amphibolite sample (HLT03-2) display distinct oval (or sectored) cores and oscillatory zoning in CL images (Figs. 8a and 9a). They also show elevated Th/U ratios ranging from 0.04 to 1.17 with nearly 97% of measurements above 0.1 (Belousova et al., Reference Belousova, Griffin, O’Reilly and Fisher2002; Long et al., Reference Long, Sun, Yuan, Xiao, Lin, Wu, Xia and Cai2007, Supplementary Tables DR2 and DR3), similar to magmatic or regrowth magmatic zircons. Some zircons appear to be inherited from earlier zircon grains, which are mantled by zircon overgrowths and/or present absorption borders (Figs. 9a and 10a, e.g. Hong et al., Reference Hong, Klemd, Gao, Xiang, Xu, You, Wang, Wu, Li and Ke2017; Wu et al., Reference Wu, Hong, Xu, Wang and Dong2023). Additionally, they are not detrital zircons, despite being obtained from amphibolite. One common feature of magmatic zircon is its tetragonal shape, typically forming as doubly-terminated prismatic crystals (Corfu et al., Reference Corfu, Hanchar, Hoskin and Kinny2003). In contrast, detrital zircon is characterized by high grain roundness in morphology (e.g. Pietranik et al., Reference Pietranik, Hawkesworth, Storey, Kemp, Sircombe, Whitehouse and Bleeker2008; Rojas-Agramonte et al., Reference Rojas-Agramonte, Kröner, Demoux, Xia, Wang, Donskaya, Liu and Sun2011). Another typical feature of magmatic zircon is well-developed growth zoning (Corfu et al., Reference Corfu, Hanchar, Hoskin and Kinny2003). This feature is most evident in CL images, but in detrital zircon affected by physical and chemical processes, variations and disruption in zoning become apparent under microscopic examination. Therefore, the core and oscillatory zoning were chosen as the target areas for LA-ICP-MS and SIMS analyses, respectively, in this study.
Figure 8. (a) The CL photo and (b) LA-ICP-MS U–Pb concordia age diagram of zircons of the Hongliutan leucogranite. The solid ellipses in the CL photo represent the analytical location of U–Pb age. The numbers adjacent to the solid ellipses show the plot number (n).
Figure 9. (a) The CL photo and (b-d) SIMS U–Pb concordia age diagrams of zircons of the Hongliutan amphibolite. The solid ellipses in the CL photo represent the analytical location of U–Pb age. The numbers adjacent to the solid ellipses show the plot number (n) with 207Pb/206Pb age data.
Figure 10. (a) The CL photo and (b-d) SIMS U–Pb concordia age diagrams of zircons of the Hongliutan quartz schist. The solid ellipses in the CL photo represent the analytical location of U–Pb age. The numbers adjacent to the solid ellipses show the plot number (n) with 207Pb/206Pb age data.
Fourteen zircons from the leucogranite sample (HLT01-1) exhibit euhedral morphology with clear zoning, ranging in length from 200 to 300 μm and have elongation ratios from 2 to 4 (Fig. 8a). The LA-ICP-MS analytical plots yield a concordia age of 506.4 ± 8.0 Ma (MSWD = 0.73), with 206Pb/238U ages ranging from 496.9 ± 11.4 Ma to 515.6 ± 9.0 Ma, and Th/U ratios from 0.27 to 0.44 (Fig. 8b).
Zircons from the amphibolite sample (HLT03-2) vary in size from 50 to 120 μm with elongation ratios from 1 to 3 (Fig. 9a). These zircons can be divided into three groups (Fig. 9b). Group 1 zircons (7 grains of zircons) display clear oscillatory zoning and have Th/U ratios ranging from 0.40 to 1.17, which are greater than 0.4 as reported by Belousova et al. (Reference Belousova, Griffin, O’Reilly and Fisher2002), and are typical of magmatic zircons. Their plots yield a concordia age of 521.8 ± 9.4 Ma (MSWD = 0.18), with 206Pb/238U ages ranging from 507.3 ± 14.6 Ma to 530.8 ± 15.4 Ma, clearly indicating the emplacement time of the amphibolite (Fig. 9c). Group 2 and Group 3 zircons are inherited zircons with ellipsoid morphology and obvious absorption borders. The four zircons from Group 2 have Th/U ratios of 0.36 to 0.68 and yield a concordia age of 977.9 ± 28.0 Ma (MSWD = 1.60) (Fig. 9c). This age may represent the basement age or reflect a large-scale tectonic-magmatic activity. The four inherited zircons in Group 3, which are grains 2, 5, 6, and 7, have ages over 1.5 Ga, with 207Pb/206Pb ages ranging from 1566 ± 22.8 Ma to 3064 ± 29.0 Ma.
The CL images reveal that the majority of detrital zircons from the quartz schist sample (HLT04-2) are characterized by sharp-edged anhedral-to-subhedral crystals with distinct concentric oscillatory zoning (Fig. 10a). Their elevated Th/U ratios, with nearly 91% of the measurements exceeding 0.1 (Belousova et al., Reference Belousova, Griffin, O’Reilly and Fisher2002; Long et al., Reference Long, Sun, Yuan, Xiao, Lin, Wu, Xia and Cai2007, Supplementary Table DR3), suggest a proximal igneous source for the zircons. A subset of ancient detrital zircons displays low luminescence and nebulous zoning (Fig. 10a), but their high Th/U ratios are also indicative of an igneous derivation (Hanchar and Rudnick, Reference Hanchar and Rudnick1995). Additionally, some detrital zircons possess low luminescent rims with diminished Th/U ratios falling below 0.1 (as seen in Plots 18, 24, 39, and 40, Supplementary Table DR3), lacking the oscillatory zoning typical of the core. These rims are interpreted as evidence of metamorphic overgrowths (Corfu et al., Reference Corfu, Hanchar, Hoskin and Kinny2003; Long et al., Reference Long, Sun, Yuan, Xiao, Lin, Wu, Xia and Cai2007).
The quartz schist sample (HLT04-2) contains four groups of detrital zircons, each characterized by distinct features and age ranges. The zircons exhibit 207Pb/206Pb ages ranging from 901 ± 16.8 Ma to 3364 ± 12.8 Ma and yield a discordant line with a lower and upper intercept age of 1003 Ma and 2480 Ma, respectively (Fig. 10b). The Group 1 comprises zircons with magmatic characteristics, including low roundness with sharp edges, clear oscillatory zoning, and relatively high Th/U ratios ranging from 0.11 to 0.85. These features yield a concordia age of 973.7 ± 6.2 Ma (MSWD = 0.56) (Fig. 10c). The Group 2 contains three detrital zircons with a spherical morphology and unsharp corrosion rims, yielding 207Pb/206Pb ages from 1597 ± 12.4 Ma to 2082 ± 6.0 Ma. The Group 3 comprises seventeen zircons with 207Pb/206Pb ages ranging from 2423 ± 22.6 Ma to 2528 ± 13.6 Ma. Ten zircons in the Group 3 zircons have Th/U ratios ranging from 0.04 to 0.99, and yield a concordia age of 2468.1 ± 22.0 Ma (MSWD = 7) (Fig. 10d), potentially reflecting a Palaeoproterozoic source. The Group 4, previously described, includes eleven detrital zircons that indicate even older 207Pb/206Pb ages, ranging from 2609 ± 11.0 Ma to 3364 ± 12.8 Ma.
6. Discussion
6.a. The protoliths of plagioclase schist, quartz schist, and amphibolite
The plagioclase schist and quartz schist in the Hongliutan area are characterized by their foliated mineral assemblages. These rocks have undergone metamorphism ranging from greenschist-facies to amphibolite facies, resulting in the transformation of micas, albite, and chlorite (Figs. 4c and 4d). Additionally, the rotation and recrystallization of rounded plagioclase and quartz subgrains, coupled with garnet overgrowth on biotite and quartz, provide evidence that the interior sections of these rocks have experienced deformation under ductile conditions (Figs. 4e and 4f). The interlayered contact observed between the plagioclase schist and quartz schist (Figs. 3a and 3c), along with their similar mineral compositions, structural deformation, and alteration patterns, suggests that the protoliths of these rocks are contemporaneous volcaniclastic rocks.
The protoliths of the plagioclase schist and quartz schist are inferred to be andesitic tuff and dacitic tuff, respectively. Geochemical analysis places their plots within the felsic domain and granite domain on the Rb versus K2O diagram and the ΣREE versus La/Yb diagram (Fig. 7a and 7b; Shen et al., Reference Shen, Shu, Xiang, Zhang and Wang2009; Liu, Reference Liu1991), indicating an acidic magmatic source. Detrital zircons from the quartz schist samples display features consistent with a magmatic source, including oscillatory zoning, high Th/U ratios, and sharp-edged anhedral-to-subhedral morphologies (Fig. 10a). These features, combined with their limited transportation, suggest a proximal igneous source. Additionally, the geochemical signatures of these rocks align with those commonly observed in intermediate acid igneous rocks, further supporting an igneous origin (Figs. 5a-e) (e.g. Winter and John, Reference Winter and John2001; Le Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986; Wilson, Reference Wilson1989; Winchester and Floyd, Reference Winchester and Floyd1977; Maniar and Piccoli, Reference Maniar and Piccoli1989; Irvine and Baragar Reference Irvine and Baragar1971).
The lance-shaped amphibole and biotite within the amphibolite exhibit weak epidotization and chloritization, suggesting a metamorphic progression from greenschist-facies to epidote-amphibolite facies conditions (Figs. 4g and 4h). Geochemical analysis of the amphibolite samples indicates a relatively low SiO2 content, with values of 51.04 wt% and 53.13 wt%, consistent with the geochemical signatures of pyroxene diorite or gabbro (Figs. 5a-c; Le Bas et al., Reference Bas, Maitre, Streckeisen and Zanettin1986; Wilson, Reference Wilson1989; Winchester and Floyd, Reference Winchester and Floyd1977). Combined with the relationship between amphibolite and plagioclase schist (Fig. 3), these findings collectively suggest that the amphibolite represents a mafic intrusive phase that was emplaced after the formation of the volcaniclastic rocks.
The juxtaposition of mafic amphibolite with more felsic volcaniclastic rocks provides significant insights into the magmatic and metamorphic processes of the region. Petrogeochemistry analysis, with its capacity to clearly and intuitively reveal the petrogenesis of both volcaniclastic and igneous rocks, serves as an indispensable tool in deciphering the complex geological history of the area (e.g. Xu et al., Reference Xu, Li, Jiang, Li, Qu, Yang, Zhou and Dong2015a, Reference Xu, Jiang, Li, Wu, Qu, Zhou and Dong2015b; Wu et al., Reference Wu, Hong, Xu, Li, Ke, Li and Dong2020).
6.b. The tectonic setting for the formation of leucogranite and metamorphic rocks
Previous studies on the ca. 500–530 Ma rock association, including mafic and intermediate-acidic volcanogenic sediments and leucogranite, suggested that the Tianshuihai unit likely represents a Cambrian continental margin arc, associated with the southward subduction of the Proto-Tethys Ocean (Hu et al., Reference Hu, Wang, Mu, Wang and Hou2017; Wang et al., Reference Wang, Hattori, Liu, Song, Gao and Zhang2017; Zhang et al., Reference Zhang, Zou, Ye and Chen2018a, Reference Zhang, Zou, Ye and Chen2018b, Reference Zhang, Liu, Wu, Huang, Li and Zhou2019b). In this study, the leucogranite, amphibolite, plagioclase schist, and quartz schist exhibit continental arc geochemical characteristics, as supported by the following evidence:
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(1) All analysed samples display a calc-alkaline affinity with no indication of tholeiitic rocks, which are typically associated with oceanic island arcs or ocean ridge settings (Fig. 5e) (e.g. Winter and John Reference Winter and John2001; Xu et al., Reference Xu, Li, Jiang, Li, Qu, Yang, Zhou and Dong2015a; Wu et al., Reference Wu, Hong, Xu, Li, Ke, Li and Dong2020; Irvine and Baragar Reference Irvine and Baragar1971).
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(2) There is notable enrichment in Th, U, Zr, and Hf, coupled with the depletion of Nb, Ta, P, and Ti (Fig. 6b; Sun and McDonough, Reference McDonough1989), suggesting a significant crustal component.
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(3) The majority of the leucogranite and amphibolite samples are situated at the intersection of within-plate granites and volcanic arc granites on the Y + Nb versus Rb diagram (Fig. 11a; Pearce et al., Reference Pearce, Harris and Tindle1984), and within continental-related crust, arc, or island arc areas on the Ce versus Ce/Pb diagram (Fig. 11b; Hofmann, Reference Hofmann1988), the Ta/Yb versus Th/Yb diagram (Fig. 11c; Pearce, Reference Pearce1983), and the Sc/Ni versus La/Yb diagram (Fig. 11d; Bailey, Reference Bailey1981).
Figure 11. (a) The (Y + Nb) versus Rb diagram (after Pearce et al., Reference Pearce, Harris and Tindle1984), (b) Ce versus Ce/Pb diagram (after Hofmann, Reference Hofmann1988), (c) Ta/Yb versus Th/Yb diagram (after Pearce, Reference Pearce1983), and (d) Sc/Ni versus La/Yb diagram (after Bailey, Reference Bailey1981) of the Hongliutan leucogranite and metamorphic rocks. Abbreviations: MORB–middle ocean ridge basalts, OIB–oceanic island basalts, ORG–ocean ridge granites, syn-COLG–syn-collision granites, VAG–volcanic arc granites, WPB–within-plate basalts and WPG–within-plate granites.
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(4) The plagioclase schist samples plot within the continental island arc area on the La–Th–Sc (Fig. 12a), Th–Sc–Zr/10 (Fig. 12b), and Th–Hf–Co (Fig. 12c) diagrams (Bhatia and Crook, Reference Bhatia and Crook1986), while the quartz schist samples are positioned in the passive continental margin field.
Figure 12. (a) The La-Th-Sc, (b) Th-Sc-Zr/10 and (c) Th-Hf-Co diagrams of the Hongliutan plagioclase schist and quartz schist (after Bhatia and Crook, Reference Bhatia and Crook1986). Data points for the plagioclase schist (green solid squares) and quartz schist (orange solid squares) are plotted in the continental island arc and passive continental margin fields, respectively.
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(5) High La/Sm ratios, ranging from 5.43 to 6.69, indicate a clear involvement of crustal-derived materials (Lassiter and DePaolo Reference Lassiter and DePaolo1997; White Reference White2003).
The U–Pb microanalysis of magmatic zircons in this study has dated the formation of the leucogranite and amphibolite to 506.4 ± 8.0 Ma (Fig. 8b) and 521.8 ± 9.4 Ma (Fig. 9c), respectively. However, their TDM(Nd) ages, ranging from 1.18 to 1.33 Ga and T2DM(Nd) ages from 1.18 to 1.28 Ga, indicate a Mesoproterozoic source of material. These ages are nearly consistent with the 973.7 ± 6.2 Ma concordia age of detrital zircons with magmatic characteristics from the quartz schist (Fig. 10c) and the 977.9 ± 28.0 Ma concordia age of inherited zircons within the amphibolite (Fig. 9d). This suggests a significant magmatic-tectonic event during the Neoproterozoic, and it is plausible that this suggests a significant magmatic-tectonic event during the Neoproterozoic, and it is plausible that a Mesoproterozoic basement underlies the area.
The Sr–Nd isotopic compositions of the plagioclase schist and quartz schist, characterized by even older TDM(Nd) ages ranging from 1.74 to 2.12 Ga and T2DM(Nd) ages from 2.12 to 2.14 Ga, along with negative εNd(t) values ranging from −6.0 to −11.3 (Figs. 13a and 13b; Xu et al., Reference Xu, Jiang, Li, Qu, Yang, Mao, Wu, Zhang and Dong2013, Xu et al., Reference Xu, Jiang, Li, Wu, Qu, Zhou and Dong2015b), indicate the presence of an older basement. This interpretation is further supported by SIMS U–Pb microanalysis, which identified Archaean-Mesoproterozoic inherited zircons in the amphibolite (Fig. 9a; 1566–3064 Ma) and Archaean-Palaeoproterozoic detrital zircons in the quartz schist (Fig. 10a; 1597–3364 Ma). Notably, the concordia age of 2468.1 ± 22.0 Ma for detrital zircons from the quartz schist (Fig. 10d) suggests that the basement may be of Palaeoproterozoic age. These findings support that the Tianshuihai unit is underlain by a Palaeoproterozoic basement that underwent significant tectonic and magmatic reworking during the Neoproterozoic era.
Figure 13. (a) (87Sr/86Sr)i versus εNd(t) diagram (after Winter and John, Reference Winter and John2001; Wu et al., Reference Wu, Jahn, Wilde, Lo, Yui, Lin, Ge and Sun2003; Tang et al., Reference Tang, Wang, Wyman, Li, Zhao, Jia and Jiang2010a, Reference Tang, Wang, Wyman, Sun, Li, Zhao, Sun, Jia and Jiang2010b), (b) 206Pb/238U(Ma) versus εNd(t) diagram (after Hu et al., Reference Hu, Jahn, Zhang, Chen and Zhang2000; Winter and John, Reference Winter and John2001), (c) 147Sm/144Nd versus 143Nd/144Nd diagram (after Goldstein et al., Reference Goldstein, O’Nions and Hamilton1984; Nelson and Depaolo, Reference Nelson and Depaolo1985), and (d) 207Pb/204Pb versus 206Pb/204Pb diagram (after Hart, Reference Hart1984; White, Reference White2003; Tang et al., Reference Tang, Wang, Wyman, Li, Zhao, Jia and Jiang2010a, Reference Tang, Wang, Wyman, Sun, Li, Zhao, Sun, Jia and Jiang2010b) of the Hongliutan leucogranite and metamorphic rocks. Abbreviations: AA–Andean andesite, BSE–bulk silicate earth, CHUR–chondritic uniform reservoir, DM–depleted mantle, DML–depleted mantle line, Geochron for zero isochron line, GLOSS–global subducting sediment, HIMU–high U/Pb mantle, MORB–mid-ocean ridge basalt, NHRL–north hemisphere reference line, OIAA–oceanic island arc andesite, OIAB–oceanic island arc basalt, OIB–oceanic island basalt, and WPA–within-plate basalt. The data for marine sediments and global subducting sediment (GLOSS) are from Plank and Langmuir (Reference Plank and Langmuir1998) and Chauvel et al. (Reference Chauvel, Lewin, Carpentier, Arndt and Marini2008).
In summary, the metamorphic and igneous rocks within the Hongliutan area were generated in a continental arc setting, indicative of a tectonically active margin. The Tianshuihai unit is underpinned by a Precambrian continental crustal basement, dating back to the Palaeoproterozoic era. It is conceivable that this ancient basement of the Tianshuihai unit underwent significant tectonic reworking during the Neoproterozoic era. However, there is a certain possibility that the aged basement could be an amalgamation of continental crust fragments from various geological periods.
6.c. The petrogenesis of leucogranite and metamorphic rocks
The plagioclase schist exhibits an εNd(t) value of −11.3, a TDM(Nd) age of 2.12 Ga, and a T2DM(Nd) age of 2.14 Ga. In the (87Sr/86Sr)i versus εNd(t) diagram, it falls between the upper and lower continental crust line (Fig. 13a). In the U–Pb age versus εNd(t) diagram, it is close to the continental crust line (Fig. 13b). It aligns with the Scotland Archaean and Proterozoic field in the 147Sm/144Nd versus 143Nd/144Nd diagram (Fig. 13c) and falls within the upper continental crust field in the 207Pb/204Pb versus 206Pb/204Pb diagram (Fig. 13d). The low Sr-Nd isotopic composition and continental crust geochemical affinity (Fig. 10d) suggest that the protolith of plagioclase schist, specifically andesitic tuff, was primarily derived from the anatexis and recycling of intracrustal metaigneous and metasedimentary sources (e.g. Landenberger and Collins, Reference Landenberger and Collins1996; Jung et al., Reference Jung, Hoernes and Mezger2000; Jiang et al., Reference Jiang, Ling, Jiang, Fan, Shen and Ni2005; Jiang et al., Reference Jiang, Zhang, Zhou and Liu2009; Huang et al., Reference Huang, Li, Li and Li2011; Wu et al., Reference Wu, Hong, Xu, Wang and Dong2022).
The broad spectrum of 207Pb/206Pb ages in the quartz schist ranges from 901 ± 16.8 Ma to 3364 ± 12.8 Ma (Fig. 10a). This is complemented by the εNd(t) value of −6.0, and the TDM(Nd) age of 1.74 Ga, and T2DM(Nd) age of 2.12 Ga. These results collectively suggest a complex provenance for the protolith of the quartz schist, identified as dacitic tuff. It appears to have been formed mainly by the integration of a minor fraction of Neoproterozoic or Mesoproterozoic juvenile materials with more ancient crustal components dating from the Palaeoproterozoic to the Palaeoarchaean.
The amphibolite has an εNd(t) value of −0.6, a TDM(Nd) age of 1.33 Ga, and a T2DM(Nd) age of 1.28 Ga. The U–Pb age versus εNd(t) plot falls between the DM line and continental crust line (Fig. 13b), the 147Sm/144Nd versus 143Nd/144Nd plot is in the Andean andesite field (Fig. 13c), and the 207Pb/204Pb versus 206Pb/204Pb plot is in the upper continental crust field (Fig. 13d). These results, combined with a concordia age of 521.8 ± 9.4 Ma for magmatic zircons and inherited zircons ranging from 0.96 to 3.06 Ga (Fig. 9a), suggest that the amphibolite magma was likely derived from Early Cambrian juvenile material, mixed with old Neoproterozoic to Mesoarchaean crust or materials (e.g. Fourcade and Allègre, Reference Fourcade and Allegre1981; Kistler et al., Reference Kistler, Chappell, Peck and Bateman1986; Lassiter and DePaolo, Reference Lassiter and DePaolo1997; Clynne, Reference Clynne1999; White, Reference White2003; Xu et al., Reference Xu, Li, Jiang, Li, Qu, Yang, Zhou and Dong2015a).
The leucogranite has a concordia age of 506.4 ± 8.0 Ma, with εNd(t) value, TDM(Nd) age, and T2DM(Nd) age range from 0.4 to 0.6, 1.18 Ga to 1.45 Ga, and 1.18 Ga to 1.20 Ga. In geochemical diagrams, it aligns with the upper continental crust line in the (87Sr/86Sr)i versus εNd(t) diagram (Fig. 13a), falls between DM line and continental crust line in the U–Pb age versus εNd(t) diagram (Fig. 13b), and lies in the Andean andesite field in the 147Sm/144Nd versus 143Nd/144Nd diagram (Fig. 13c). The moderate depletion of Sr-Nd and its continental arc geochemical signature indicate that the leucogranite primarily originated from the mixture of Cambrian juvenile material and old crust or materials (Figs. 10b-d). Considering that amphibolites and leucogranites share similar trace element and isotopic compositions, it is plausible that leucogranites could be formed from the melting of amphibolites. However, there is a lack of additional petrological evidence and direct genetic contact relationships to substantiate this perspective.
6.d. Basement nature of Tashikuergan-Tianshuihai terrane
The nature of the Tashikuergan-Tianshuihai terrane has been subject to debate, with three primary models proposed: (1) a huge accretionary wedge (also known as Tianshuihai accretionary complexes) containing thick flysch sequences between the West Kunlun terrane (also called South Kunlun terrane) and Karakorum terrane, resulting from northward subduction of the Proto-Tethys Ocean from the Cambrian to Triassic periods (Mattern and Schneider, Reference Mattern and Schneider2000; Wang, Reference Wang2004; Xiao et al., Reference Xiao, Windley, Fang, Zhou, Yuan, Wang, Hao, Hou and Li2001, Reference Xiao, Windley, Chen, Zhang and Li2002, Reference Xiao, Han, Windley, Yuan, Zhou and Li2003a, Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Hu et al., Reference Hu, Wang, Mu, Wang and Hou2017); (2) a section of an arc-like magmatic belt (or a part of the West Kunlun terrane, e.g. Yuan et al., Reference Yuan, Sun, Zhou, Zhou, Xiao and Li2003) between the two coexisting Proto-Tethys Oceans (one located between the Karakorum terrane and West Kunlun terrane, and the other between the Tarim and West Kunlun terrane, e.g. Wang et al., Reference Wang, Hattori, Liu, Song, Gao and Zhang2017). The south- and north-dipping subduction of the Proto-Tethys Ocean beneath the northern and southern margin of the West Kunlun terrane produced this early Palaeozoic arc-like magmatic belt (West Kunlun terrane) (Wang et al., Reference Wang, Hattori, Liu, Song, Gao and Zhang2017); (3) an independent terrane or microcontinent block (Qiao et al., Reference Qiao, Wang, Wu, Hao, Zhao, Chen, Lv and Du2015b; Hu et al., Reference Hu, Wang, Huang, Tong, Mu and Qiu2016). Zhang et al. (Reference Zhang, Zou, Ye and Chen2018a, Reference Zhang, Zou, Ye and Chen2018b, Reference Zhang, Liu, Wu, Huang, Li and Zhou2019b) proposed that the Proto-Tethys Ocean only existed between the Tarim and southern Kunlun terrane. The Tashikuergan-Tianshuihai terrane on the southern side of the West Kunlun terrane could be considered as a back-arc microcontinent block.
Numerous studies have proposed that the Tashikuergan-Tianshuihai terrane may represent either an accretionary wedge or an arc-like magmatic belt, based on geochemical and isotopic analyses of Palaeozoic-Mesozoic granitoids. These environments are typically characterized by abundant oceanic crust components and pelagic sediments, and igneous rocks predominantly of tholeiitic composition, while synchronously formed volcanic rocks within accretionary wedges are uncommon (Xu et al., Reference Xu, Li, Jiang, Li, Qu, Yang, Zhou and Dong2015a; Wu et al., Reference Wu, Hong, Xu, Wang and Dong2022). However, in this study, all samples analysed belong to the calc-alkaline series and exhibit geochemical characteristics consistent with a continental arc. The presence of terrestrial-derived detrital zircons with limited transportation suggests a more ancient terrane history, contradicting the accretionary wedge or arc models.
Further supporting this interpretation, Ji et al. (Reference Ji, Li, Chen, He, Zhao, Bian, Zhu, Cui and Ren2011) documented the presence of inherited zircons dated at 2481 ± 14.0 Ma within Palaeoproterozoic foliated rhyolite in the western part of the Tashikuergan-Tianshuihai terrane. Additional studies have also reported zircon U–Pb ages or bulk rock Rb–Sr ages ranging from 2.13 to 2.77 Ga within the same rock sequence (Pan et al. Reference Pan, Wen and Sun2000; Sun et al., Reference Sun, Li, Wu, Wang, Qi, Chen, Liu and Gao2003; Wang, Reference Wang2008). In this research, a concordia age of 2468.1 ± 22.0 Ma for detrital zircons from quartz schist was identified in the northeastern part of the Tashikuergan-Tianshuihai terrane. These findings further illuminate the terrane’s ancient evolutionary history.
The West Kunlun terrane also has a Palaeoproterozoic basement (ca. 2.5 Ga) composed mainly of gneisses, migmatites, amphibolites, and schists (e.g. Xiao et al., Reference Xiao, Han, Windley, Yuan, Zhou and Li2003a, Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Yuan et al., Reference Yuan, Sun, Zhou, Xiao and Zhou2005; Zhang et al., Reference Zhang, Zou, Ye and Chen2018b; Wang et al., Reference Wang, Hattori, Liu, Song, Gao and Zhang2017; Zhang et al., Reference Zhang, Zou, Ye and Chen2019a; Zhang et al., Reference Zhang, Liu, Wu, Huang, Li and Zhou2019b; Yin et al., Reference Yin, Xiao, Sun, Chen, Yuan, Zhang, Wang, Du, Wang and Xia2020). It is separated from the Tashikuergan-Tianshuihai terrane by the Kangxiwa suture. Similarly, the Tashikuergan-Tianshuihai terrane is separated from the Hoh Xil-Bayan Har block, Qiangtang terrane, and Karakorum terrane by the Dahongliutan fault, Altyn fault, and Qiaoertianshan fault, respectively. These sutures, reactivated by successive faulting, are super-lithospheric faults that delineate distinct tectonic units. Consequently, we suggest that the Tashikuergan-Tianshuihai terrane is a relatively independent terrane with a Precambrian basement. However, the exact relationship, migration, and evolution between the basement of the Tashikuergan-Tianshuihai terrane and that of the West Kunlun terrane remain uncertain.
6.e. The tectonic evolution of Tashikuergan-Tianshuihai terrane
This study identified a NW to SE trend and continuous Palaeoproterozoic basement in the Tashikuergan-Tianshuihai terrane, similar to that located in the adjacent West Kunlun terrane. The Late Archaean-Palaeoproterozoic (ca. 2.7–2.5 Ga) is crucial for granitoid magmatism and the generation of juvenile continental crust in Earth’s history (Condie et al., Reference Condie, Belousova, Griffin and Sircombe2009a; Li et al., Reference Li, Li and Li2014). According to the age compilation of granitoid rocks, North China, India, and East Antarctica have shown major periods of granitoid production at ca. 2.5 Ga (Condie et al., Reference Condie, O’Neill and Aster2009b; Li et al., Reference Li, Li and Li2014). The terrane may also exhibit a geological similarity, such as the Gondwanan affinity observed between Tarim and the North Kunlun terrane of the West Kunlun. Therefore, the c.a. 2.48–2.47 Ga basement in the Tashikuergan-Tianshuihai terrane may be derived from the c.a. 2.5 Ga granitoids in North China, India, and East Antarctica, or from potential underlying Gondwanan crust, indicating that it is a Palaeoproterozoic crustal block.
It is worth noting that the Yangtze Block features tonalite-trondhjemite-granodiorites (TTGs) at ca. 2.1–2.0 Ga, while the rifting-related anorogenic magmatism of the supercontinent Columbia/Nuna in several continents, such as Australia, North China, Laurentia, and South America, is centred around ca. 1.8–1.6 Ga (Bickford and Boardman, Reference Bickford and Boardman1984; Page and Hoatson, Reference Page and Hoatson2000; Norcross et al., Reference Norcross, Davis, Spooner and Rust2000; Halls et al., Reference Halls, Campal, Davis and Bossi2001; Peng et al., Reference Peng, Zhai, Ernst, Guo, Liu and Hu2008; Li et al., Reference Li, Li and Li2014). Therefore, the 2.2–1.6 Ga inherited zircons in amphibolite and the 2.1–1.6 Ga detrital zircons in quartz schist in the Tashikuergan-Tianshuihai terrane may record the break-up of the Columbia/Nuna supercontinent (Fig. 14a). Although the Tashikuergan-Tianshuihai terrane underwent significant transformation and metamorphism due to the tectonic-magmatic activity of the Columbia/Nuna supercontinent, it may also have been amalgamated from younger metasediments (after 2.1–1.6 Ga) that acquired material from a distinct crustal section, not solely from the ancient Tashikuergan-Tianshuihai basement.
Figure 14. Simplified model illustrating the tectonic evolution of the Tashikuergan-Tianshuihai terrane. (a) The 2.1–1.6 Ga crustal extension stage, (b) the ca. 1.0 Ga passive continental margin stage and (c) the ca. 0.5 Ga continental arc stage. Modified after Zhang et al. (Reference Zhang, Zou, Ye and Chen2018c) and Wu et al. (Reference Wu, Hong, Xu, Cao, Li, Zhang, You, Ke and Dong2018).
The assembly of the supercontinent Rodinia is believed to have taken place around 1.1–0.9 Ga, followed by its disintegration (Li et al., Reference Li, Bogdanova, Collins, Davidson, De Waele, Ernst, Fitzsimons, Fuck, Gladkochub, Jacobs, Karlstrom, Lu, Natapov, Pease, Pisarevsky, Thrane and Vernikovsky2008b). The younger Group 1 detrital zircons from the quartz schist, showing magmatic characteristics, display a concordia 207Pb/206Pb age of 973.7 ± 6.2 Ma (Fig. 10b). This evidence suggests that the quartz schist protolith formed during the tectonic and magmatic activities related to the Rodinia. This interpretation matches the concordant age of inherited zircons from amphibolite, dated at 977.9 ± 28.0 Ma (Fig. 9d). The geochemical and isotopic signatures reveal that the plagioclase schist and quartz schist are rich in ancient crustal materials. Their protoliths were predominantly derived from the partial melting and recycling of intracrustal metaigneous and metasedimentary rocks, with minor Neoproterozoic or Mesoproterozoic juvenile contributions. They likely formed in a continental arc setting, potentially a rifted (passive) continental margin environment, as shown in Figs. 12a-12d. Subsequent tectonic activities, inducing rifting-related anorogenic magmatism within the Tashikuergan-Tianshuihai terrane, resulted in the formation of plagioclase schist and quartz schist. This event coincided with the initial stages of the Proto-Tethys Ocean’s development (Fig. 14b). The formation and disintegration evolution of similar Palaeo-Proterozoic ancient terrane have also been reported in the Wenquan area of West Tianshan, NW China (Central Asian Orogenic Belt, Wang et al., Reference Wang, Liu, Shu, Jahn, Chung, Zhai and Liu2014; Liu et al., Reference Liu, Wang, Shu, Jahn and Lizuka2014; Wu et al., Reference Wu, Hong, Xu, Wang and Dong2023).
The magmatic zircons with ages of 506.4 ± 8.0 Ma and 521.8 ± 9.4 Ma from leucogranite and amphibolite, respectively, record Cambrian continental arc-related magmatism, indicating that the Tashikuergan-Tianshuihai terrane experienced the subduction of the Proto-Tethys Ocean (Fig. 14c, Xiao et al., Reference Xiao, Windley, Chen, Zhang and Li2002, Reference Xiao, Han, Windley, Yuan, Zhou and Li2003a, Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Li et al., Reference Li, Ji, Zhao, Chen, Meng, Yu and Pan2007; Hu et al., Reference Hu, Wang, Mu, Wang and Hou2017; Wang et al., Reference Wang, Hattori, Liu, Song, Gao and Zhang2017; Yin et al., Reference Yin, Xiao, Sun, Chen, Yuan, Zhang, Wang, Du, Wang and Xia2020). This magmatism continued until the Silurian and the closing of the Proto-Tethys Ocean (Zhang et al., Reference Zhang, Zou, Ye and Chen2018a). Subsequently, this region entered a new tectonic evolution stage associated with the Palaeo- to Neo-Tethys Ocean (Mattern and Schneider, Reference Mattern and Schneider2000; Xiao et al., Reference Xiao, Windley, Liu, Jian, Liu, Yuan and Sun2005; Jiang et al., Reference Jiang, Jia, Liu, Liao, Zhao and Zhou2013; Liu et al., Reference Liu, Jiang, Jia, Zhao and Zhou2015; Yan et al., Reference Yan, Qiu, Wang, Wang, Wei, Li, Zhang, Li and Liu2018; Wang et al., Reference Wang, Gao, Zhang, Yan, Xu, Zhou, Dong and Li2020).
7. Conclusions
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(1) The leucogranite and amphibolite crystallized at 506.4 ± 8.0 Ma and 521.8 ± 9.4 Ma, respectively. The quartz schist is rich in detrital zircons with a broad range of 207Pb/206Pb ages extending from 901 to 3364 Ma.
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(2) The leucogranite and amphibolite, identified as Cambrian igneous rocks, display distinct geochemical features indicative of a continental arc origin, including calc-alkaline affinity, enrichment in Th, U, Pb, Zr and Hf and depletion in Ba, Nb, Sr and Ti, with εNd(t) values approaching zero.
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(3) The younger Group 1 detrital zircons from the quartz schist, exhibiting some magmatic zircon characteristics, have 207Pb/206Pb ages ranging from 901 ± 16.8 Ma to 989.9 ± 23.6 Ma, and a concordia age of 973.7 ± 6.2 Ma. This, combined with the 977.9 ± 28.0 Ma concordant age of inherited zircons from the amphibolite, indicates a Neoproterozoic magmatic activity.
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(4) The plagioclase schist and quartz schist are interpreted as Neoproterozoic volcaniclastic rocks, likely formed in a rifted (passive) continental margin setting.
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(5) The U–Pb zircon ages Sr–Nd–Pb isotopic results from quartz schist, amphibolite, leucogranite and metamorphic rocks collectively support the presence of a Palaeoproterozoic basement beneath the Tianshuihai unit that has experienced Neoproterozoic reworking.
Supplementary material
The supplementary material for this article can be found at https://doi.org/10.1017/S0016756824000360.
Data and material availability
All the data are available in Supplementary Table DR 1, 2 and 3.
Acknowledgements
This research was jointly supported by the National Natural Science Foundation of China (Grant Numbers 42302083, 42250202, 41672088 and 42272075) and the Project of Development and Research Center of China Geology Survey (Grant Numbers DD20221785 and DD20230600). We would like to express our gratitude to Dr. Qiang Ke, Dr. Hang Li and Dr. Lei Niu for their invaluable assistance with experimental analyses. Furthermore, the first author would like to extend a heartfelt thank you to his parents and family for their unwavering support and encouragement.
Competing interests
The authors declare that they have no competing interests.
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