Multi-stage volcanic activities and geodynamic evolution of the Lhasa terrane during the Cretaceous: Insights from the Xigaze forearc basin

摘要

The history of volcanic activity of the Gangdese arc in southern Tibet during the Cretaceous remains poorly known due to the intense erosion of the arc. Here we present zircon U-Pb ages, trace element and Hf isotopic data of tuffs and volcanic conglomerates from the Chongdoi and the Ngamring Formation in the Xigaze forearc basin. Three tuff samples from the Chongdoi Formation yield zircon U-Pb ages around 112 Ma. Such ages could be taken as their depositional ages, indicating that detrital clasts of the Chongdoi Formation were deposited at that time. One andesitic conglomerate sample from the Ngamring Formation was dated at Ca. 105 Ma, the other rhyolitic conglomerate sample and one tuff sample were dated to be ca. 95 Ma and 91 Ma, respectively, suggesting that this formation was formed during the late Albian-late Turonian. All zircons illustrate I-type granitoid characteristics and possess low Ti-in-zircon temperatures (<800 degrees C). Zircon epsilon(Hf)(t) values of tuffs from the Chongdoi Formation define two groups: (1) the first group of 117-110 Ma displays large positive epsilon(Hf)(t) values (+12.1 to +17.1), larger than those of the tuff from the Ngamring Formation (+6.0 to +10.2); (2) the second group of 119-111 Ma yields negative to small positive epsilon(Hf)(t) values (-4.5 to +1.1). All above observations indicate that their host rocks were derived from the juvenile materials with significant input of fluids. Combined with extant data, the first group and the tuff, volcanic conglomerates from the Ngamring Formation are mostly likely derived from the eroded Cretaceous volcanic rocks in the Gangdese arc, while the second group is likely sourced from the late Early Cretaceous volcanic rocks in the central Lhasa terrane. Our data confirm the presence of Cretaceous volcanism in the Gangdese arc, suggesting that the Neo-Tethyan Ocean lithosphere was most likely subducted northerly in a normal angle rather than in a manner of low-angle subduction. (C) 2015 Elsevier B.V. All rights reserved.