Newly discovered Late Triassic Baqing eclogite in central Tibet indicates an anticlockwise West–East Qiangtang collision

Abstract

The Triassic eclogite-bearing central Qiangtang metamorphic belt (CQMB) in the northern Tibetan Plateau has been debated whether it is a metamorphic core complex underthrust from the Jinsha Paleo-Tethys or an in-situ Shuanghu suture. The CQMB is thus a key issue to elucidate the crustal architecture of the northern Tibetan Plateau, the tectonics of the eastern Tethys, and the petrogenesis of Cenozoic high-K magmatism. We here report the newly discovered Baqing eclogite along the eastern extension of the CQMB near the Baqing town, central Tibet. These eclogites are characterized by the garnet + omphacite + rutile + phengite + quartz assemblages. Primary eclogite-facies metamorphic pressure–temperature estimates yield consistent minimum pressure of 25 ± 1 kbar at 730 ± 60 °C. U–Pb dating on zircons that contain inclusions (garnet + omphacite + rutile + phengite) gave eclogite-facies metamorphic ages of 223 Ma. The geochemical continental crustal signature and the presence of Paleozoic cores in the zircons indicate that the Baqing eclogite formed by continental subduction and marks an eastward-younging anticlockwise West–East Qiangtang collision along the Shuanghu suture from the Middle to Late Triassic.

Mesozoic multiple magmatic events in Bangor area: Constraints on the tectonic evolution of Bangong–Nujiang Tethys Ocean

“…Combined with previous data on the age of magmatic rocks in the Bangor area, we suggest that the BNO should have closed later than 116 Ma (e.g., Dong et al, 2013; Gao et al, 2011, 2019; Teng, 2019; Wang et al, 2012). This conclusion is also supported by the following evidences: (a) sedimentologically, the Qushenla Formation (107–100 Ma), which is widely distributed in the BNSZ and presents lacustrine stage, is considered to be evidence for the closure of the BNO (Chen et al, 2017; Fan et al, 2014; Wu, Li, Xie, Wang, & Hu, 2013; Xu, Dilek, et al, 2015; Zhang et al, 2018), and the discovery of the argillaceous siliceous rocks in the Shiquanhe area, all formed in active continental margin environments with an age of ~108 Ma, also suggests that the closure time for the BNO should be the late period of Early Cretaceous (Liu, Wang, et al, 2018); (b) lithologically, zircon U–Pb geochronology of Early Cretaceous ophiolites (e.g., the Zhonggang Oceanic Island basalt at 116 Ma, the Tarenbon Oceanic Island basalt at 108 Ma, and the Pengcuo/Juowong cumulate gabbro at 120 Ma) also indicates that the BNO was not completely closed during the Cretaceous (Chen et al, 2020; Fan et al, 2014; Xu, Li et al, 2015 ; Zhu et al, 2006); (c) paleomagnetically, there is a paleo‐latitude difference of 7.2° ± 5.5° between the southern edge of the South Qiangtang Terrane and the northern edge of the northern Lhasa Terrane during the 115–120 Ma period in the Gaize area, and the paleolatitude difference between the two was reduced to 3.2° ± 3.1° during the period of 100–110 Ma (within the error range indicating coincidence). This result implies that the closure of the BNO in the central–western section should have occurred in the late period of Early Cretaceous (Cao et al, 2019).…”

Late Triassic granites (ca. 230 Ma) from Cairi area in eastern Qiangtang subterrane, central Tibet: Product of a mid‐oceanic ridge subduction

“…Zhang, Cai, et al, 2006; Y. X. Zhang et al, 2018). Thus, the Cairi granites (229.6 Ma) are probably related to the subduction of Shuanghu Palaeo-Tethys Ocean rather than the slab break-off or delamination after the continent subduction.…”

Newly recognized retrograde eclogites overprinted by high‐temperature metamorphism in the Amdo microcontinent, Central Tibet: Implications for subduction erosion during continental subduction

“…These metamorphic rocks were suggested to experience peak HP granulite facies or amphibolite facies metamorphism with variable metamorphic P–T conditions 8–17 kbar at 700–920°C (Guynn et al, 2013; Zhang et al, 2010, 2014). Uncertainty of protolith properties, peak metamorphic conditions, and spatial relationships of Amdo metamorphic rocks lead to multiple controversial tectonic evolution models for the Amdo microcontinent, including the following: (1) Middle Jurassic closure of back‐arc Basin resulted in high‐grade metamorphism of Amdo basement (Guynn et al, 2006), (2) Amdo high‐grade metamorphic rocks formed by the subduction of the Amdo microcontinent beneath the Qiangtang terrane followed the Tethys Ocean crust subduction (Zhang et al, 2014), (3) Amdo high‐grade metamorphic rocks formed after the intra‐oceanic subduction of the Tethys Ocean (Chen, Shi, Zou, Huang, & Wu, 2015; Zhang, Bader, et al, 2018, Zhang, Jin, et al, 2018). Besides, the possibility of Amdo high‐pressure granulite or amphibolite derived from the middle and lower crust as a result of crustal thickening in a contractional setting (Guynn et al, 2006) can also not be excluded.…”

“…Prior to the Cenozoic India‐Asia collision, the subduction of the Bangong–Nujiang Tethyan Ocean and collision between the Lhasa and Qiangtang terranes contributed significantly to the growth of the Tibetan Plateau, drastic climate changes, and the formation of multiple mineral resources (e.g., Airaghi et al, 2018; Deng, Wang, Li, Li, & Wang, 2014; Richards, 2015). Different from the Longmu Tso–Shuanghu Tethys Ocean (LSO) (Zhang, Bader, et al, 2018) in the North and the Sumdo Palaeo–Tethys Ocean in the south (Zhang, Jin, et al, 2018), fewer eclogite outcrops were discovered within the BNSZ. The Amdo microcontinent is located within the Bangong–Nujiang Tethyan Ocean，and is a key to understanding the tectonic evolution of Central Tibet.…”