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Evidence from a Gabbro-Diorite Complex in the Gangdese Magmatic

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Evidence from a Gabbro-Diorite Complex in the Gangdese Magmatic Research Paper GEOSPHERE Identification of a new source for the Triassic Langjiexue Group: Evidence from a gabbro-diorite complex in the Gangdese magmatic GEOSPHERE, v. 16, no. 1 belt and zircon microstructures from sandstones in the Tethyan https://doi.org/10.1130/GES02154.1 Himalaya, southern Tibet 16 figures; 1 set of supplemental files Xuxuan Ma1,2, Zhiqin Xu3, Zhongbao Zhao1, and Zhiyu Yi1 1 CORRESPONDENCE: [email protected] Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China 2Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA 3State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China CITATION: Ma, X.X., Xu, Z.Q., Zhao, Z.B., and Yi, Z.Y., 2020, Identification of a new source for the Triassic Langjiexue Group: Evidence from a gabbro- diorite complex in the Gangdese magmatic belt and zircon ABSTRACT began no later than the Middle Triassic. Arc-affin- During the past decades, progress has been microstructures from sandstones in the Tethyan Hima- laya, southern Tibet: Geosphere, v. 16, no. 1, p. 407– ity magmatic rocks supplied some materials to the achieved on understanding the formation of the 434, https://doi.org/10.1130/GES02154.1. Considerable debate persists as to the Triassic Langjiexue Group. This scenario sheds new light Himalayan-Tibetan orogen. However, many basic paleogeographic framework of the Neotethys and on the provenance of the Langjiexue Group and the questions remain open to debate. This study Science Editor: Shanaka de Silva the origin of the Late Triassic Langjiexue Group in Triassic paleogeography of the Neotethyan realm. focuses on the following issues: (1) the timing for Associate Editor: Christopher J. Spencer the Tethyan Himalaya. Triassic magmatic rocks in initial subduction of the Neotethyan oceanic lith- the Gangdese belt and Late Triassic Langjiexue sed- osphere; and (2) the tectonic setting of the Late Received 7 May 2019 Revision received 10 September 2019 iments play a pivotal role in addressing these issues. ■ INTRODUCTION Triassic Langjiexue Group in the Tethyan Himalaya, Accepted 2 December 2019 Geochronological, petrological, and geochemical in other words, the provenance for the sediments analyses have been performed on the Middle Tri- An ongoing continent-continent collisional oro- of the Langjiexue Group. Published online 19 December 2019 assic gabbro-diorite complex (with crystallization gen, the Himalayan-Tibetan orogen, has attracted Recent studies have revealed that voluminous ages of ca. 244–238 Ma) from the Gangdese belt. much attention among the geological community calc-alkaline igneous rocks are exposed in the These plutonic rocks are characterized by relatively (Fig. 1; Yin and Harrison, 2000; Spencer et al., 2012). Gangdese magmatic belt, with ages ranging from low MgO and high Al2O3 contents, calc-alkaline The Indo-Asian collision took place at ca. 60–50 Ma, Middle Triassic to Late Cretaceous (Ma et al., 2018a; trends, and depletion of Nb, Ta, and Ti, resem- triggering the uplift of the Tibetan Plateau (Ding et al., Wang et al., 2016a). The Middle Triassic to Jurassic bling low-MgO high-alumina basalts or basaltic 2016; Hu et al., 2015; Jin et al., 2018; Sun et al., 2016; magmatic rocks are ascribed to southward sub- andesites. These plutonic rocks exhibit depleted Zhu et al., 2015). However, the pre-plateau history of duction of the Bangong-Nujiang Tethyan oceanic whole-rock εNd(t) values of ~+5 and zircon εHf(t) values the Lhasa terrane, especially the evolutionary history lithosphere beneath the Lhasa terrane (Zhu et al., peaking at ~+14. These features resemble those of of the Neotethyan Ocean, remains enigmatic (Li et al., 2013; Yang et al., 2017), or to the northward sub- rocks in a subduction-related arc setting. 2010; Zhu et al., 2010). The Gangdese magmatic belt, duction of the Neotethyan oceanic slab beneath the We also completed detrital zircon U-Pb dating located in the southern margin of the Lhasa terrane, Lhasa terrane (Guo et al., 2013b; Kang et al., 2014; and microstructure analysis for the sandstones of documents voluminous Middle Triassic to Late Cre- Ma et al., 2018a; Wang et al., 2016a). Whether the the Langjiexue Group in the Tethyan Himalaya. Zir- taceous subduction-related igneous activity (Ji et al., southward or northward model is correct, these con grains with ages >300 Ma are dominated by 2009; Meng et al., 2016a, 2019a; Mo et al., 2005a; results suggest that these magmatic rocks were preweathered and weathered surfaces as well as Wang et al., 2016a), indicating that the Gangdese generated in a convergent margin setting either fairly rounded to completely rounded scales, indi- magmatic belt experienced a protracted history prior as an active continental margin or intra-oceanic cating a high degree of polycyclicity. In contrast, to the Indo-Asian collision. Thus, study of the mag- arc. About 40% of the modern convergent margin 300–200 Ma ones are characterized by fresh sur- matic rocks in the Gangdese magmatic belt is very around the globe is interpreted as intra-oceanic faces and completely unrounded to poorly rounded important for deciphering the subduction-accretion subduction zones (Larter and Leat, 2003). This scales, indicating nearby sources. Collectively, our orogeny of the Gangdese magmatic belt and the raises the questions of whether an intra-oceanic This paper is published under the terms of the data, combined with published results, support framework of the Neotethyan realm before the final subduction system developed within the eastern CC-BY-NC license. that the subduction initiation of the Neotethys collision and formation of the Tibetan Plateau. Neotethyan realm, and whether some intra-oceanic © 2019 The Authors GEOSPHERE | Volume 16 | Number 1 Ma et al. | New source for the Triassic Langjiexue Group Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/16/1/407/4925462/407.pdf 407 by guest on 29 September 2021 Research Paper N 75°E 80°E 85°E 90°E 95°E 100°E 105°E LGR: Longgar rift ATF: Altyn Tagh fault Normal fault NTR: Nyima-Tingri rift HYF: Haiyuan fault Strike-slip fault XDR: Xainza-Dingjye rift JLF: Jiali fault Thrust fault elt YGR: Yadong-Gulu rift KF: Karakoram fault Suture zone enic b LST: Longmen Shan thrust rog MBT: Main Boundary thrust Eclogite zone sian o BNS: Bangong-Nujiang suture ral A XSF: Xianshuihe fault Cent AKS: Anymaqen-Kunlun suture IYS: Indus–Yarlung Tsangpo suture 40°N North China JSS: Jinsha suture LS: Longmu Co–Shuanghu suture SQS: South Qilian suture North Pamir Tarim block block SS: Shyok suture Central Pamir Qilian terrane TS: Tanymas suture Q K F im Qa South Pamir AT an T ida HY F K agh m b SQ F m te unl as S a rr un t in S or an erra T ak e AKS ne ar Kohistan K Songpan-Gan ze flysch complex 35°N SS Ladakh North Qiangtang J H B So SS i S NS uth Q m hi iangt a qu ang l an Gaize Shuanghu Amdo LS ay he a X T n f SF S o L ld Lhasa terrane -t IY Sumdo 30°N hr S JLF us R t b G L Linzhi e R Xigaze Lhasa lt T R R N G India M D B Y South China T X block 0 500 km Figure 2 Figure 1. Tectonic map of the Tibetan Plateau showing the study location (modified after Kapp and Guynn [2004] and Yin and Harrison [2000]). arc rocks are preserved in the Gangdese mag- configuration for the eastern Neotethyan realm, as the eastern Neotethyan realm, especially the Cim- matic belt. well as the possible source for the Langjiexue Group. meride and the northern Gondwana landmasses. The Late Triassic Langjiexue Group, exposed in The foregoing issues are closely related to the In this study, we discuss new results from the the Tethyan Himalaya belt, plays a pivotal role in opening of the Neotethyan Ocean. Based on the ca. 240 Ma gabbro-diorite complex in the Gang- reconstructing the framework of the Neotethyan paleogeographic reconstruction of the Pangea dese magmatic belt and microstructures of detrital realm. However, its tectonic affinity has been hotly supercontinent and the Neotethyan realm, the Neo- zircon grains of sandstones from the Late Triassic debated for decades. Models proposed to explain tethys has been suggested to have opened in the Langjiexue Group in the Tethyan Himalaya. Based the formation of the Langjiexue Group include basin- early Permian (Angiolini et al., 2003; Garzanti et al., on our combined analyses of regional geology, we fill during the initial rifting between the Indian and 1996; Kroner et al., 2016). However, as remnants propose the existence of another possible source Lhasa blocks (Dai et al., 2008; Webb et al., 2012), of the Neotethyan oceanic lithosphere, the Indus– in the Gangdese magmatic belt, which partly pro- forearc basin deposition due to the northward Yarlung Tsangpo ophiolites, whose formation is vided some source materials for the Langjiexue subduction of the Neotethyan oceanic lithosphere attributed to forearc extension, mainly fall into an sandstones. beneath the Lhasa terrane or an intra-oceanic arc age range of 130–120 Ma (Wu et al., 2014; Liu et al., (Li et al., 2010), passive continental margin deposi- 2016; Maffione et al., 2015; Xiong et al., 2017). Fur- tion along the northern or northwestern margin of thermore, the opening of the Neotethys has been ■ GEOLOGICAL SETTING the Gondwana landmass (Cai et al., 2016; Cao et al., proposed to have been a byproduct of the south- 2018; Fang et al., 2018; Wang et al., 2016b; Meng et al., ward subduction of the Bangong-Nujiang Tethys Tectonic Framework 2019b), and a multi-source model within the Neo- during the Late Triassic (Zhu et al., 2013; Yang et al., tethys (Li et al., 2016; Zhang et al., 2017).
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