Triassic trachytic volcanism in the Bangong–Nujiang Ocean: geochemical and geochronological constraints on a continental rifting event

Ming Zheng,Yang Song,Qing Zhang,Haifeng Li,Carl Guilmette,Faqiao Li,Juxing Tang,Zhibo Liu

doi:10.1017/s0016756821001114

Abstract

AbstractThe Bangong–Nujiang suture zone (BNSZ), which separates the Gondwana-derived Qiangtang and Lhasa terranes, preserves limited geological records of the Bangong–Nujiang Ocean (BNO). The timing of opening of this ocean has been hotly debated due to the rare and complicated rock records in the suture zones, which span over 100 Ma from Carboniferous–Permian to Early Jurassic time, based on geological, palaeontological and palaeomagnetic data. A combination of geochemical, geochronological and isotopic data are reported for the Riasairi trachytes, central BNSZ, northern Tibet, to constrain its petrogenesis and tectonic settings. Zircon U–Pb dating by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) yields mean ages of 236 Ma. Geochemically, these rocks are high-K calc-alkaline with moderate SiO2 (59.1–67.5 wt%) and high K2O + Na2O (8.1–11.6 wt%) contents. They are enriched in light rare earth elements with negative Eu anomalies, and show enrichments in high-field-strength elements with positive ‘Nb, Ta’ anomalies, similar to the intra-continental rift setting-related felsic lavas from the African Rift System. The high positive zircon ϵHf(t) and bulk ϵNd(t) values, as well as high initial Pb isotopes, imply a heterogeneous source involving both asthenospheric and subcontinental lithospheric mantle. The field and geochemical data jointly suggest that the Riasairi trachytes within the Mugagangri Group were formed in a continental rift setting. We interpret that the continental-rift-related Riaisairi trachytic lavas as derived from the southern margin of the Qiangtang terrane, implying that the BNO would have opened by Middle Triassic time, well after the commonly interpreted break-up of the Qiangtang terrane from Gondwana.

Highlights

The central Tibetan Plateau consists of the Gondwana-derived Qiangtang and Lhasa terranes that migrated N-wards and sequentially accreted during the Mesozoic Era
The N-wards transfer of these terranes was accommodated by the birth and destruction of major oceanic basins, the remnants of which are exposed in the Indus–Yarlung Zangbo and Bangong–Nujiang Suture Zone (Dewey & Burke, 1973; Metcalfe, 2011)
This suture zone contains remnants of the Bangong–Nujiang Ocean (BNO) (Allégre et al 1984; Yin & Harrison, 2000; Li et al 2006; Pan et al 2006; Zhu et al 2011), and this palaeo-ocean closed during Late Triassic–Early Cretaceous time when the Lhasa and Qiangtang terranes collided (Li et al 2016b, 2019c)

Summary

Introduction

Silicic volcanism is a key component of rift magmatism and is a common product across different rift zones (Hutchison et al 2018), such as the East African Rift System (EARS) (Debre Birhan: Feyissa et al 2017; Rungwe: Fontijn et al 2013; Axum-Adwa: Natali et al 2013) and the West Antarctic Rift System (WARS) (Marie Byrd Land: Panter et al 1997; LeMasurier et al 2011) Such volcanism has a distinct geochemical signature reflecting fractional crystallization from a basaltic parental melt derived from partial melting of asthenospheric depleted mantle, and can be dated using precise radiometric methods. We integrate our field observations, petrography, whole-rock geochemistry, zircon U–Pb ages, zircon Lu–Hf isotopes and whole-rock Sr–Nd–Pb isotopic data to evaluate the evolution of the nowvanished BNO to further understand the plate tectonic dance of the Lhasa and Qiangtang terranes, the two main constituents of the Tibetan Plateau

First report of trachytic lava in BNSZ

Analytical methods

Results

Discussion

Findings

Conclusions