重新认识西藏林周盆地基性岩石的地球动力学含义

Rethinking the geodynamical implications of the basic rocks from Linzhou Basin,Tibet

拉萨地体南部大面积分布的林子宗火山岩的地球动力学意义迄今尚未得到很好的约束,对于这个问题的深入理解有助于揭示雅鲁藏布特提斯洋岩石圈的俯冲和随后印度-欧亚大陆的陆陆碰撞过程。要合理认识林子宗火山岩所蕴含的地球动力学含义,在一定程度上还有赖于对林子宗火山岩中基性岩石的岩浆起源和性质的深入剖析。本文结合已有资料,报道和总结了位于林子宗火山岩命名地———林周盆地中基性岩石样品(56~53Ma)的全岩地球化学、Sr-Nd同位素数据。林周盆地基性岩石为高钾钙碱性-钾玄岩系列,富集大离子亲石元素(LILEs)、亏损高场强元素(HFSEs),初始87Sr/86Sr比值为0.7048~0.7054,εNd(t)值为+0.5^+1.8;同时还具有高Zr含量、Zr/Y比值以及高的Ta/Hf、Th/Hf比值。林周盆地基性岩石既显示岛弧玄武岩浆又显示板内玄武岩浆的双重地球化学特征,除了来源于受俯冲流体交代的岩石圈地幔的部分熔融外,同时可能还存在深部软流圈地幔物质的贡献。本文提出其成因很可能与雅鲁藏布特提斯洋壳北向俯冲板片在约52Ma发生的板片断离有关,指示印度与亚洲大陆的初始碰撞至少发生在60Ma以前。

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 will 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/S6 Sr = 0. 7048 -0. 7054 and eNd（t） = ＋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-Zangho Tethyan oceanic lithosphere at ca. 52Ma, indicating that the initial India-Asia collision should have occurred before 60Ma.