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Research Article Identification of Forearc Sediments in the Milin-Zedong Region and Their Constraints on Tectonomagmatic Evolution of the Gangdese Arc, Southern Tibet

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Research Article Identification of Forearc Sediments in the Milin-Zedong Region and Their Constraints on Tectonomagmatic Evolution of the Gangdese Arc, Southern Tibet GeoScienceWorld Lithosphere Volume 2020, Article ID 8835259, 20 pages https://doi.org/10.2113/2020/8835259 Research Article Identification of Forearc Sediments in the Milin-Zedong Region and Their Constraints on Tectonomagmatic Evolution of the Gangdese Arc, Southern Tibet 1,2 1 1 1 1 Shao-Hua Zhang, Wei-Qiang Ji , Hao Zhang, Guo-Hui Chen, Jian-Gang Wang, 1,2 1 Zhong-Yu Meng, and Fu-Yuan Wu 1State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China Correspondence should be addressed to Wei-Qiang Ji; [email protected] Received 12 May 2020; Accepted 19 September 2020; Published 2 November 2020 Academic Editor: Songjian Ao Copyright © 2020 Shao-Hua Zhang et al. Exclusive Licensee GeoScienceWorld. Distributed under a Creative Commons Attribution License (CC BY 4.0). The Xigaze forearc sediments revealed the part of the tectonomagmatic history of the Gangdese arc that the bedrocks did not record. However, the sediments’ development is restricted to the region around and west of Xigaze City. Whether the eastern segment of the arc had a corresponding forearc basin is yet to be resolved. In this study, a field-based stratigraphic study, detrital zircon U-Pb geochronology (15 samples), and Hf isotopic analyses (11 of the 15 samples) were carried out on four sections in the Milin-Zedong area, southeast Tibet. The analytical results revealed the existence of three distinct provenances. The lower sequence is characterized by fine-grained sandstone, interbedded mudstone, and some metamorphic rocks (e.g., gneiss and schist). The detrital zircon U-Pb age distribution of this sequence is analogous to those of the Carboniferous-Permian strata and metasediments of the Nyingtri group in the Lhasa terrane. The middle and upper sequences are predominantly composed of medium- to coarse-grained volcaniclastic/quartzose sandstones, which are generally interbedded with mudstone. The detrital zircon U-Pb ages and Hf isotope signatures indicate that the middle sequences are Jurassic to Early Cretaceous in ~ – ffi ε ðtÞ age ( 200 100 Ma) and show clear a nity with the Gangdese arc rocks, that is, positive Hf values. In contrast, the upper – ε ðtÞ sequences are characterized by Mesozoic detrital zircons (150 100 Ma) and negative Hf values, indicative of derivation from the central Lhasa terrane. The overall compositions of the detrital zircon U-Pb ages and Hf isotopes of the middle to upper sequences resemble those of the Xigaze forearc sediments, implying that related forearc sediments may have been developed in the eastern part of the Gangdese arc. It is possible that the forearc equivalents were eroded or destroyed during the later orogenesis. Additionally, the detrital zircons from these forearc sediments indicate that this segment of the Gangdese arc experienced more active and continuous magmatism from the Early Jurassic to Early Cretaceous than its bedrock records indicate. 1. Introduction of the Xigaze forearc sediments have been conducted. These studies have revealed its tectonic, erosional, and sedimentary Forearc basins develop along convergent plate margins and evolution and have effectively constrained the magmatic his- receive detritus from the adjacent magmatic arc, and thus, tory of the Gangdese arc [2, 3, 6, 7]. However, only restricted they are important for studying the evolution of arcs [1–5]. exposures of the forearc sediments have been documented The Xigaze forearc basin, located in the southern margin of along the huge Gangdese plutonic belt (>1500 km long) [8], the Lhasa terrane, southern Tibet, is one of the best exposed that is, mainly outcropping near and to the west of Xigaze. forearc basins in the world (e.g., [4, 6]). Numerous studies of In contrast, no forearc sediments have been reported in the the detrital zircon U-Pb ages and Hf isotopic characteristics eastern part of the Gangdese arc (e.g., [4, 5]). There are two Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/doi/10.2113/2020/8835259/5293362/8835259.pdf by guest on 29 September 2021 2 Lithosphere possible explanations for this: either forearc sediments did tectonic units (from north to south): the widely distributed not develop along this section of the arc or they developed Cretaceous to Miocene Gangdese batholith (100–16 Ma) but were destroyed during later orogenesis. (e.g., [9, 26, 27, 30, 31, 34–39]) and the Yarlung Zangpo In this study, sporadic occurrences of potential forearc mélange zone (Figure 1(a)). There are exposures of sedimen- sedimentary records were verified in the Milin-Zedong tary strata distributed sporadically along the Yarlung Zangpo region with a discontinuous distribution in the suture zone River near Milin-Baga, but they are distributed more contin- mélange (Figure 1 and Fig. S1). We conducted a detailed field uously near Nang-Dongga (Figures 1(c) and 1(d)). An addi- investigation of four exposures in this region (Figure 2) and tional, well-developed exposure of these sediments was also performed detrital zircon U-Pb geochronology and Hf found in Jinlu in the Zedong area (Figure 1(e)). isotope geochemistry on the samples collected from these In this study, a total of 23 samples were collected, of exposures. Our study emphasized provenance tracing and which 21 samples were collected from the four sections (see comparative analysis with the detrital zircons in the Xigaze Figure 2 for detailed lithology and sample positions), and forearc basin with the goal of further constraining the two samples were collected from Nang County magmatic evolution of the Gangdese arc. (Figures 1(d) and 3(m)–3(o)). Samples Ld01–07 were col- lected from section 1 in the Milin area. They are mainly com- 2. Geologic Setting and Samples posed of sandstones (Ld01–04 and Ld07), but a small proportion is composed of gneiss (Ld05–06). Samples The Tibetan Plateau is composed of several east–west- Bg01–04 were collected from section 2, which predominantly trending blocks, including the Himalayas, Lhasa, Qiangtang, comprises metamorphic rocks. Samples Dzg01–07 were col- Songpan-Ganzi, and Kunlun terranes from south to north lected from section 3, including five sandstone samples [10] (Figure 1(a)). The Lhasa terrane is separated from the (Dzg01–02 and Dzg04–06) and two marble samples (Dzg03 Himalayas by the Indus-Yarlung Zangpo suture zone to the and Dzg07). Samples Jl01–03 are medium- to coarse- south and is separated from the Qiangtang terrane by the grained volcaniclastic sandstones collected from section 4. Bangong-Nujiang suture to the north, which is interpreted to A relatively fresh sandstone (Lx01) and an intrusive vein be the southernmost part of the Asian Plate [9] (Figure 1(a)). sample (Lx02) were collected from Nang County. Their The Lhasa terrane consists of the locally developed Pre- detailed stratigraphy and sedimentology are described in cambrian metamorphic basement (i.e., the Nyainqentanglha the next section. In addition, eight samples were collected group and Nyingtri group), the Paleozoic to Cenozoic sedi- from several other sporadic exposures of the suture zone mentary strata, and various types of magmatic rocks [10]. mélange in Milin-Nang (see supplementary materials and The Luobadui-Milashan fault zone in the south and the Figures S1-S3 for more details). Shiquan River-Nam Tso ophiolitic mélange belt in the north separate the Lhasa terrane into its southern, central, and 3. Stratigraphy and Sedimentology northern parts [9] (Figure 1(b)). The south Lhasa terrane mainly consists of the juvenile Gangdese magmatic arc, Exposures and remnants of potential forearc sediments were including the Gangdese batholith and Mesozoic-Tertiary vol- observed and described as follows. Additionally, detailed field canic rocks such as the Middle-Lower Jurassic Yeba Forma- and petrographic observations of several other outcrops are tion [11, 12], the Upper Jurassic-Lower Cretaceous Sangri listed and illustrated in the Supplementary materials. group [13], and the Paleocene Linzizong group [14–16]. ° ′ The Gangdese batholith, which is an important part of the 3.1. Section 1. Section 1 (GPS location: 29 13.1177 N, ° Trans-Himalayan batholith, occurs as a huge plutonic belt 94 11.3320′ E) outcrops on a hillside near Lidi Village, (>1500 km) along the southern margin of the Lhasa terrane approximately 5 km northwest of Milin (Figure 1(c)), where [8], which extends from the Kailas in the west to the Namche a well-preserved sedimentary stratum is exposed Barwa in the east. It is widely considered to have been a (Figures 3(a)–3(d)). It is ~10 m thick but has good continuity typical Andean-type convergent continental margin prior to (>100 m). The lower part of the succession is characterized the Paleocene India-Eurasia collision [10, 17–20]. Studies of by medium-grained sandstone and interbedded mudstone. the Gangdese batholith have revealed its long magmatic his- Above this, the strata transition to relatively intact units of tory from the Middle Triassic to the Miocene, with peaks in medium-grained sandstone (~2 m thick), which overlies the activity at 109–80 Ma and 55–45 Ma [18, 21–29]. Specifically, lower sequence conformably (Figure 2(a)). Further up, the the Gangdese batholith has a juvenile isotopic composition, section is composed of ~2 m thick gneiss, with preferentially for example, positive εHf ðtÞ values [18, 19, 25, 26, 28, 30, 31]. oriented biotite and muscovite (Figure 4(a)). Thin-bedded The north Lhasa terrane consists of Early Cretaceous volca- sandstone and gneiss interbedded with weakly bedded mud- nic rocks and granites (130–100 Ma) [9, 32]. In contrast, stone occur near the upper part of the succession. There are the central Lhasa terrane consists of ancient crystalline base- no faults developed in this section area.
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  • Hartmann Dissertation 5.5.20
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