藏南白垩纪桑日群麻木下组埃达克岩的地球化学特征及其成因

Geochemistry and origin of Cretaceous adakites in Mamuxia Formation,Sangri Group,South Tibet

桑日群火山岩位于冈底斯南缘火山-岩浆弧的中、东段,紧邻雅鲁藏布江缝合带的北侧,包括下部麻木下组与上部比马组中的火山岩,长期以来被认为是雅鲁藏布江新特提斯洋沿冈底斯南缘向北俯冲消减的产物,但是这些火山岩还未见系统的地球化学数据,对其成因也缺乏深入的讨论。通过对桑日群火山岩详细的地球化学研究,识别出麻木下组火山岩具有典型的埃达克岩的地球化学特征:高Sr(平均为977μg/g),低Y(平均为10.4μg/g)、Yb(平均为0.942μg/g)含量,轻重稀土强烈分异(La/Yb平均为24.8);另外岩石还具有相对低Sr和高Nd的同位素组成(87Sr/86Sr)i=0.703506～0.704369,(143Nd/144Nd)i=0.512728～0.512778,εNd(t)平均为5.58,显示出与雅鲁藏布江蛇绿岩的MORB相似的同位素特征,表明其可能来源于新特提斯雅鲁藏布江洋岩石圈的部分熔融。而相对高的MgO(平均为4.38%)、Cr(平均为198μg/g)和Ni(平均为117μg/g)含量暗示原始的埃达克质熔体在其上升过程中与上覆地幔楔发生了反应。综合考虑新特提斯洋以及冈底斯造山带的演化历史,提出麻木下组埃达克岩可能是在早白垩世时雅鲁藏布江新特提斯洋北向俯冲消减的初始阶段由洋壳部分熔融所形成的认识。

The early Cretaceous Sangri Group arc-volcanic rocks, including Mamuxia Formation and Bima Formation, are distributed on the south side of Gangdese arc belt. They are generally thought to be typical products of the Neo-Tethys＇ northward subduction along the south margin of the Gangdese arc belt, but it is still argued about their origin because of lacking reliable geochemical data. Based on detailed geochemical research, this paper has recognized that volcanic rocks in the Mamuxia Formation show typical chemical characteristics of adakite, such as high Sr （977 ug/g in average）, low Y （10.4 ug/g in average）, Yb （0. 942 ug/g in average） and strongly fractionation between LREE and HREE （La/Yb =24.8 in average）. Meanwhile they have relatively high MgO （4.38% in average）, Cr （198 ug/g in average） and Ni （117 ug/g in average）, indicating that the original adakitic melts from subducted Neo-Tethys oceanic crust were probably interacted with a peridotite when they went up through the overlying mantle wedge. In addition, they have relatively low ^87Sr/^86Sr （0. 703506 -0. 704369） and high ^143Nd/^144Nd （0. 512728 -0. 512778）; the average εNd（t） is 5. 58, similar to those of the Neo-Tethys＇ MORB,hinting that their magmatic source was the lithosphere of the Neo-Tethys. Gombmmg me geologlcal evolution history of the Neo-Tethys in South Tibet, we suggest that adakites of Mamuxia Formation were likely produced by slab melting of the subducted oceanic crust at the initial stage of the Neo-Tethys＇ northward subduction during Early Cretaceous.