西藏冈底斯带洛巴堆组火山岩地球化学及构造意义

Volcanic rock geochemistry and tectonic implication of the Luobadui Formation on the Gangdese zone,Xizang(Tibet).

冈底斯带中-新生代火山岩的成因和地质演化是青藏高原地质研究的热点领域,但是对晚古生代火山岩的性质及其形成的构造背景仍缺乏研究。冈底斯带二叠系显示缓慢的海退过程,并伴随基性和中酸性两期火山活动,具有活动大陆边缘特征。笔者对墨竹工卡县唐家乡和林周县勒青拉地区二叠纪洛巴堆组火山岩进行了常量、微量元素和Sr、Nd、Pb同位素地球化学测试。研究认为二叠纪是冈底斯岛弧的形成和演化时期。洛巴堆组玄武岩从下部(可能为早二叠世)到上部(中二叠世),MgO含量从7.43%降到3.99%,Al_2O_3含量从15.54%上升到17.57%,后者类似于岛弧高铝玄武岩;稀土总量∑REE从54.12(10^(-6))上升到108.82(10^(-6)),LREE/HREE比值从3.00上升到5.40。洛巴堆组玄武岩均具有明显的Nb、Ta负异常,但中二叠世更显著。岩石地球化学示踪表明,冈底斯洛巴堆组玄武岩代表二叠纪初始岛弧环境。早期玄武岩形成于略亏损地幔和下地壳成分的混合源区,地壳相对较薄,源区或岩浆的壳源成分比例较低。到中二叠世地壳逐渐加厚,壳源成分在源区或岩浆中所占比例增大,代表向陆缘弧的演化过程。冈底斯带二叠纪岛弧是古特提斯洋向南的俯冲、消减的产物。

The Mesozoic and Cenozoic volcanic rocks in the Gangdese zone is an important field of Tibetan geologic studies. The Late-Paleozoic volcanic rocks and their tectonic settings however,still remain unstudied.The Permian stratigraphy represented a slow regression process with two volcanic events,characterizing an active margin.We measured key geologic sections at Tangjia and Leqingla of the Luobadui Fm.and analyzed volcanic rock samples for major and trace elements and Sr,Nd and Pb isotopes.These suggest that Permian was a period of arc formation in the Gangdese zone.From lower(possible early Permian)to upper part(middle Permian)of the Luobadui basalts,the MgO contents drop from 7.43% to 3.99% and Al_2O_3 contents increase from 15.54% to 17.57%.The later resembles high-Al basalts of island arc.Meanwhile,the∑REE values increase from 54.12×10^(-6)to 108.82×10^(-6)and LREE/HREE ratioes from 3.00 to 5.40.The basaltic rocks of the Luobadui Fro.show significant Nb and Ta negative anormallies.Geochemical studies suggest that basahs in the Luobadui Fm.represent a Permian initial arc.Early Permian basalts derived from a mixed source of slightly-depleted mantle and lower crustal materials at thin crust.Up to middle Permian,more crust components were involved in source regions or primitive magma at thicker crust,and suggest evolution to a continental arc. The Gangdese Permian arc was resulted from the southern subduction of the paleo-Tethys.