藏南马扎拉地区玄武岩地球化学特征、成因及其地质意义

Basalts from the Mazhala area in southern Xizang: Geochemistry, petrogenesis and geological implications

关键词:  玄武岩  地球化学  被动大陆边缘  地幔热柱  马扎拉  藏南

Key words:  basalt  geochemistry  passive continental margin  mantle plume  Mazhala  southern Xizang

摘要:

西藏东南部特提斯喜马拉雅带东段的遮拉组(J₂z)中出露有大量的玄武岩,其地球化学特征对了解该地区中侏罗世的构造背景具有重要意义。本文在详细的野外工作基础上,对发现于马扎拉地区遮拉组的玄武岩进行了地球化学研究。全岩主量元素及微量元素地球化学特征显示,玄武岩属钙碱性系列,SiO₂含量为47.50%~50.61%,具有高钾(K₂O平均为0.61%)、高钛(TiO₂平均为4.02%)、高磷(P₂O₅平均为0.52%)等特点;轻、重稀土元素分馏较为明显(∑LREE/∑HREE=7.30~7.58),(La/Yb)_N比值平均为7.94,富集大离子亲石元素(LILE)如Ba、Th和高场强元素(HFSE)如Nb、Ta、Zr、Hf,呈现出典型洋岛玄武岩(OIB)的地球化学特性。岩浆源于主要由石榴石橄榄岩组成的富集地幔发生10%左右部分熔融的产物,岩浆上升过程中未受到明显的地壳混染。结合区域地质资料,初步认为马扎拉玄武岩应为被动大陆边缘地幔柱或热点的产物。

Abstract:

The basalts from the Mazhala area are widespread in the Middle Jurassic Zhela Formation(J₂z) in the eastern part of the Tethyan Himalayas in southern Xizang. The present paper focuses on the geochemical signatures of the basalts aiming at improving our knowledge of tectonic settings of the study area during the Jurassic. The geochemical signatures of major and trace elements have disclosed that the basalts are characterized by SiO₂ contents ranging between 47.50% and 50.61%, high K(an average 0.61% for K₂O), high Ti((an average 4.02% for TiO 2, high P(an average 0.52% for P₂O₅, highly differentiated light/heavy REE ratios(ΣLREE/ΣHREE=7.30-7.58), and an average 7.94 for(La/Yb)_N ratios, enriched large-ion lithophile elements such as Ba and Th and high field strength elements Nb, Ta, Zr and Hf, indicating the geochemical signatures of the oceanic-island basalts. The basaltic magmas were originated from the partial(10%) melting of the garnet peridotite-dominated enriched mantle, and were not subjected to noticeable crustal contamination during the ascending of the magmas. The Mazhala basalts are interpreted to be generated on the Himalayan passive continental margin or derived from the mantle plume.