四川理塘地区花岗闪长岩特征及其增生楔弧岩浆活动

Characteristics of granodiorite in the Litang area of Sichuan and its volcanic arc magmatism accretionary wedge

【研究目的】通过查明理塘地区拉扎嘎山花岗闪长岩的年龄、地球化学特征,探讨花岗闪长岩形成的时代、成因及构造背景,为研究甘孜—理塘洋盆俯冲增生构造演化过程提供依据。【研究方法】选取甘孜—理塘蛇绿混杂岩带俯冲增生楔内花岗闪长岩,系统开展岩相学、LA-ICP-MS锆石U-Pb年代学和岩石地球化学研究。【研究结果】花岗闪长岩含有大量的角闪石、黑云母等铁镁矿物,局部见大量的闪长质包体和围岩捕掳体。岩体形成于晚三叠世((207.2±1.5)Ma),岩石属I型钙碱性准铝质花岗岩类,具富集大离子亲石元素Rb、Ba、K、Th、U,亏损高场强元素Nb、Ta、P、Zr、Ti,显示轻稀土富集、重稀土亏损的右倾式配分模式,具有Eu的负异常,是典型的火山弧型花岗岩。【结论】结合区域地质资料及本文研究成果,认为四川理塘地区拉扎嘎山花岗闪长岩与甘孜—理塘洋向西俯冲致使中咱地块东缘增生楔不断扩大密切相关,是增生楔杂岩熔融成不同类型岩浆混合的产物。

[Objective] By finding out the age and geochemical characteristics of granodiorites in Lazhagashan area, this paper discusses the age, genesis and tectonic background of granodiorites, so as to provide a basis for the study of subduction-accretion processes in the Ganzi-Litang Ocean Basin. [Methods] Our project carried out systematically study the petrography, LA-MC-ICP-MS zircon UPb chronology and rock geochemistry from the granodiorites in subduction accretion complex of Ganzi-Litang ophiolite melange belt. [Results] The granodiorites contain a large amount of femic mineral such as amphibole, biotite, and a large number of dioritic enclaves and surrounding rock xenoliths. The intrusive rocks were formed in the Late Triassic((207.2±1.5)Ma), and are I-type calcalkaline quasi-aluminous granites with enriched in large-ion-lithophile elements(LILE), such as Rb, Ba, K, Th and U, depleted in high-field-strength elements(HFSE), such as Nb, Ta, P, Zr and Ti. It shows a right-leaning pattern of enrichment of light rare earth and depletion of heavy rare earth. It is a typical volcanic arc granite showing strong enrichment of LREE and depletion of HREE,with negative Eu anomaly. [Conclusions]Combined with the regional geological data and the research results of this paper, it is considered that the Lazhagashan granodiorite in Litang area is closely related to the westward subduction of the Ganzi-Litang Ocean resulted in the expansion of the accretionary complex in the eastern margin of the Zhongza block, which is the product of melting accretionary complex into different types of magma mixing.