Middle-Late Jurassic magmatism in the west central Lhasa subterrane, Tibet: Petrology, zircon chronology, elemental and Sr-Nd-Pb-Hf-Mg isotopic geochemistry

Tong, Xin; Yan, Jingjing; Zhao, Zhidan; Niu, Yaoling; Qi, Ningyuan; Shi, Qingshang; Liu, Dong; Wang, Qing; Zhang, Liang-Liang; Dong, Guochen; Zhu, Di-Cheng

Abstract

Mesozoic magmatic rocks are widespread in the Lhasa terrane, but most of them are of cretaceous age. Because Jurassic rocks are relatively rare and our knowledge on such earlier magmatism in the context of the tectonic setting and evolution is limited. In this study, we focus on the mid-late Jurassic granitoids that occur in the west central Lhasa subterrane. We present the results of a systematic study of these granitoid rocks of tonalite composition together with the hosted mafic magmatic enclaves. We dated 4 representative tonalite samples and 2 enclaves using zircon U-Pb method that gives the age range of 167–154 Ma. All these samples have Sr-Nd-Pb isotopic compositions (87Sr/86Sr = 0.713941–0.718417, εNd(t) = −14 to −9.8, 206Pb/204Pb = 18.806–18.936, 207Pb/204Pb = 15.739–15.764, 208Pb/204Pb = 39.257–39.798) similar to the composition of gneisses from the basement of the Lhasa terrane, suggesting that magmas parental to these mid-late Jurassic granitoids of tonalitic composition are of largely crustal origin, which is also supported by the petrographic observations. Both of the tonalite samples and the mafic enclaves have a wide span of zircon εHf(t) of −15.9 to −0.2 and − 13.8 to −7.4, respectively. These samples also show varying δ26Mg ranging from −0.40 to −0.18, with an average δ26Mg = −0.27 ± 0.06‰ (2SD) that is best understood as representing the Mg composition of the continental crust of the Lhasa terrane. All these observations allow us to conclude that basaltic melts derived from metasomatized mantle were involved in the petrogenesis of these granitoids. Specifically, southward subduction of the Bangong-Nujiang Ocean lithosphere and subsequent slab rollback metasomatized the mantle wedge and the lithospheric mantle above, whose melting produced basaltic magmas. Underplating and intrusion of these basaltic magmas caused crustal melting and generation of granitoid magmas parental to the tonalite and enclaves we study.

Citation

Tong, X., Yan, J., Zhao, Z., Niu, Y., Qi, N., Shi, Q., Liu, D., Wang, Q., Zhang, L.-L., Dong, G., & Zhu, D.-C. (2022). Middle-Late Jurassic magmatism in the west central Lhasa subterrane, Tibet: Petrology, zircon chronology, elemental and Sr-Nd-Pb-Hf-Mg isotopic geochemistry. Lithos, 408-409, Article 106549. https://doi.org/10.1016/j.lithos.2021.106549

• Journal Article Type: Article
• Acceptance Date: Nov 23, 2021
• Online Publication Date: Nov 27, 2021
• Publication Date: 2022-01
• Deposit Date: Dec 1, 2021
• Publicly Available Date: Nov 27, 2022
• Journal: Lithos
• Print ISSN: 0024-4937
• Publisher: Elsevier
• Peer Reviewed: Peer Reviewed
• Volume: 408-409
• Article Number: 106549
• DOI: https://doi.org/10.1016/j.lithos.2021.106549
• Public URL: https://durham-repository.worktribe.com/output/1220178