昌都地块小苏莽一带二叠纪变玄武岩地球化学特征及其构造环境

摘要

宁多岩群作为北羌塘昌都地块的变质岩系之一,其形成时代的有效限定便显得至关重要.通过高精度的LA-ICP-MS(激光剥蚀等离子体质谱)锆石微区原位U-Pb同位素测年,在青海省玉树县小苏莽乡宁多村北原划为中新元古代宁多岩群的变玄武岩中获得原岩形成年龄为(272.88±0.48) Ma,将该套变玄武岩从宁多岩群中解体出来,划归中二叠统.变玄武岩具有高Ti、富Fe、偏碱的特征,呈现无Eu异常、轻稀土富集的右倾型稀土元素分布模式,微量元素分布模式为部分大离子亲石元素和高场强元素不同程度富集的大隆起型曲线.综合判断,该变玄武岩形成于大陆裂谷环境,可能与地幔柱活动有关,是羌塘地区二叠纪大规模陆内裂谷作用的地质记录.%It is a key issue to confine the formation period of the Ningduo Group,which is one of metamorphic rock series of Changdu Block in North Qiangtang area.This group was previously classified as Meso-and Neo-Proterozoic Era.Meta-basalt samples were collected from the Ningduo Group in Ningduo village,Xiaosumang Township,Yushu County,Qinghai Province.High precision LA-ICP-MS zircon U-Pb dating yielded an age of (272.88±0.48) Ma,which indicates that this meta-basalt should be separated from Ningduo Group,and should be classified into Middle Permian.The meta-basalt is high titanium,iron rich,alkaline; its REE distribution pattern is a right inclining curve without Eu anomaly.The trace element distribution patterns show different degrees of enrichments of large ion and high field strength elements.We propose the meta-basalt formed in a continental rift,which may be related to mantle plume,and is the geological record of Permian large-scale intracontinental rifting in Qiangtang Area.