Geochronology and geochemistry of the Early Jurassic YebaFormation volcanic rocks in southern Tibet: Initiation of back-arc rifting andcrustal accretion in the southern Lhasa Terrane

作者：Youqing Wei, Zhidan Zhao* , Yaoling Niu, Di-Cheng Zhu,Dong Liu, Qing Wang, Zengqian Hou, Xuanxue Mo, Jiuchuan Wei

期刊：LithosVolumes 278–281, May 2017, Pages 477–490

摘要

Understandingthe geological history of the Lhasa Terrane prior to the India–Asia collision(~ 55 ± 10 Ma) is essential for improved models ofsyn-collisional and post-collisional processes in the southern Lhasa Terrane.The Miocene (~ 18–10 Ma) adakitic magmatism with economicallysignificant porphyry-type mineralization has been interpreted as resulting frompartial melting of the Jurassic juvenile crust, but how this juvenile crust wasaccreted remains poorly known. For this reason, we carried out a detailed studyon the volcanic rocks of the Yeba Formation (YF) with the results offeringinsights into the ways in which the juvenile crust may be accreted in thesouthern Lhasa Terrane in the Jurassic. The YF volcanic rocks arecompositionally bimodal, comprising basalt/basaltic andesite anddacite/rhyolite dated at 183–174 Ma. All these rocks have an arc-likesignature with enriched large ion lithophile elements (LILEs; e.g., Rb, Ba andU) and light rare earth elements (LREEs) and depleted high field strengthelements (HFSEs; e.g., Nb, Ta, Ti). They also have depleted whole-rock Sr–Ndand zircon Hf isotopic compositions, pointing to significant mantle isotopiccontributions. Modeling results of trace elements and isotopes are mostconsistent with the basalts being derived from a mantle source metasomatized byvarying enrichment of subduction components. The silicic volcanic rocks show thecharacteristics of transitional I–S type granites, and are best interpreted asresulting from re-melting of a mixed source of juvenile amphibole-rich lowercrust with reworked crustal materials resembling metagraywackes. Importantly,our results indicate northward Neo-Tethyan seafloor subduction beneath theLhasa Terrane with the YF volcanism being caused by the initiation of back-arcrifting. The back-arc setting is a likely site for juvenile crustal accretionin the southern Lhasa Terrane.

关键词：Geochemistry; Geochronology; Sr–Nd–Hf isotopes; Back-arc; Yeba Formation; Crustal accretion