摘要
广泛发育在西藏冈底斯岩浆岩带中的林子宗火山岩及其与下伏地层间的区域性不整合 ,提供了印度—亚洲大陆碰撞 (在西藏南部 )的构造火成岩新证据。岩石学、主元素、微量元素、稀土元素及Nd Sr Pb同位素地球化学研究表明 ,林子宗火山岩早期带有较多陆缘弧火山岩特征 ,中期开始出现标志陆内岩浆活动的钾玄岩 ,晚期更多地显示了加厚陆壳条件下火山岩的特点 ,记录了由新特提斯俯冲消减末期过渡到印度—亚洲大陆碰撞的信息。系统的40 Ar/3 9Ar同位素测年确定林子宗火山岩的年龄区间为 4 0 .84～ 6 4 .4 7Ma ;其底部年龄给出了林子宗火山岩与下伏地层间不整合形成时间的最晚时限 (～ 6 5Ma)。该不整合面上、下之地层在沉积相、变形样式与变形程度上均截然不同 ,反映形成环境的重大变化 ,标志着一次重大的地质事件。根据上述事实 ,结合青藏高原岩浆活动的区域时空分布 ,及西藏南部地层、古生物与沉积研究成果 ,笔者认为印度—亚洲大陆碰撞开始于 6 5Ma左右(K/T界限时间 )。
Linzizong volcanic rocks occurred in Gangdese magmatic belt in southern Tibet and regional unconformity between the volcanic strata and strata underlain provide new tectonomagmatic evidences for India-Asia continental collision. Based upon the studies of petrology, major, trace and rare earth element geochemistry, and Nd-Sr-Pb isotope systematics, it is revealed that Linzizong volcanic rocks recorded the information in the transitional process from the end of subduction of Neo-Tethyan oceanic plate to India-Asia continent collision. While volcanic rocks formed in early stage of Linzizong volcanism had fingerprints of continental margin-arc settings, those in middle and late stages reflected environments of intra-continental convergence and crust thickening. 40.84～64.47 Ma of formation ages of Linzizong volcanic rocks have been determined by ^(40)Ar/^(39)Ar dating. The formation time of the unconformity mentioned above was constrained by the ^(40)Ar/^(39)Ar age of the base of Linzizong volcanic strata (～65 Ma). The strata across the unconformity were tremendously different in sedimentary facies and structural deformation, implying a major tectonic event. In combination with evidence from the temporal evolution and spatial distribution of igneous rocks in Tibetan Plateau, and the stratigraphical and paleontological evidence in southern Tibet that documented dramatic change in sedimentary facies and microfauna content across the Cretaceous-Tertiary (K/T) boundary, it is concluded that the collision between India and Asia continents (in southern Tibet) was most likely initiated at ～K/T boundary time (～65 Ma).
出处
《地学前缘》
EI
CAS
CSCD
2003年第3期135-148,共14页
Earth Science Frontiers
基金
国家重点基础研究发展规划项目 (2 0 0 2CB412 60 0
G19980 40 80 0 )
国家自然科学基金资助项目 (4 0 172 0 2 5
40 10 3 0 0 3 )
国土资源部青藏专项计划项目 (2 0 0 10 10 2 0 40 1)
关键词
青藏高原
林子宗火山岩
印度—亚洲大陆碰撞
构造—岩浆作用
Qinghai-Xizang(Tibet) Plateau
Linzizong volcanic rocks
India-Asia continent collision
tectonomagmatism
