重新认识西藏林周盆地基性岩石的地球动力学含义

The geodynamical implications of the Linzizong volcanic rocks, which widely exposed in the southern Lhasa Terrane, Tibet, remain poorly understood so far. A better understanding on such issue wiil help us to explore the subduction history of the Yarlung-Zangbo Tethyan oceanic lithosphere and subsequent India-Asia collisional process. Key to explore this issue is to constrain the magmatic origin and nature of the basic rocks from the Linzizong volcanic rocks. This paper reports new data and summarizes the existing data in the literature (including whole-rock major and trace element, Sr-Nd isotopic data) of the basic rocks from Linzhou Basin where the Linzizong volcanic rocks named. The basic rocks from Linzhou Basin are high-K calc-alkaline-shoshonite , enriched in large-ion lithophile elements ( LILEs) , depleted in high field strength elements ( HFSEs) with initial ⁸⁷ Sr/⁸⁶Sr = 0. 7048 ∼ 0. 7054 and £Nd(0 = +0. 5 ∼ + 1. 8. These rocks display high Zr abundances , high Zr/Y , Ta/Hf , and Th/Hf ratios. These signatures indicate that the basic rocks from Linzhou Basin have geochemical affinities with arc-type and within-plate basalts. Thus they are most likely derived from partial melting of an enriched lithospheric mantle that was metasomatized by subduction-related components , accompanied with significant contributions from sub-slab asthenospheric mantle. In combination with the coeval zonal magmatic flare-up , this paper suggests that the basic rocks from Linzhou Basin are indicative of the slab break-off of the northward subduction of the Yarlung-Zangbo Tethyan oceanic lithosphere at ca. 52Ma , indicating that the initial India-Asia collision should have occurred before 60Ma.