塔里木南缘元古代变质基性火山岩地球化学特征——古塔里木板块中元古代裂解的证据

超大陆裂解是当今地学研究最前沿课题.通过研究塔里木南缘铁克里克构造带前寒武纪变质火山岩的岩石化学特征、微量元素特征、稀土元素特征及Sm-Nd同位素特征, 探讨古塔里木板块的中元古代大陆裂解构造事件.该套变质火山岩组合下部以变质酸性火山岩(流纹岩夹部分晶屑凝灰岩)为主, 夹少量玄武岩, 上部为玄武岩夹少量条带状大理岩.基性火山岩w(SiO₂)为42.34%~49.55%, w(Na₂O+K₂O)为2.63%~8.81%.下部玄武岩w(Na₂O)/w(K₂O)值为0.54~1.5, 为钾玄岩系列, 稀土总量高, 且轻稀土富集, 平均w(La)_n/w(Yb)_n值为3.45, δ(Eu)为0.69~0.91, 具弱至中等的Eu负异常, 微量元素具有碱性板内玄武岩特征; 上部玄武岩w(Na₂O)/w(K₂O)为5.11~17.89, 为拉斑系列, 轻重稀土没有明显分异, δ(Eu)在0.81~1.13之间, 基本没有Eu异常, w(La)_n/w(Yb)_n平均为1.69, 配分曲线与大洋拉斑玄武岩一致, 微量元素显示LILE(如Rb、Ba、Th)富集, HFSE与大洋拉斑玄武岩一致.岩石地球化学反映从早到晚由碱性-钙碱系列逐渐向拉斑系列演化.在上部玄武岩中获得(1 200± 82.3)Ma的Sm-Nd等时线年龄.认为该套变质火山岩记录了古塔里木板块在中元古代大陆裂解的构造过程. 

Geochemistry Characteristics of the Proterozoic Meta-Basalt in Southern Tarim Plate:Evidence for the Meso-Proterozoic Breakup of Paleo-Tarim Plate

The study of supercontinent-breakup is a modern frontier project of earth science. Based on the lithochemistry, the trace element, REE and Sm-Nd isotope characteristics of the Precambrian metamorphic volcanic rocks of Tiekelike belt in Southern Tarim plate, this paper studies the tectonic events for the Meso-Proterozoic breakup of paleo-Tarim plate. The meta-volcanic rocks were composed of meta-rhyolite (the lower part) and meta-basalt interspersed by thin-layered marble (the upper part). The basalt has low w(SiO₂) arranging from 43.8% to 49.26% and averaging 47.29%, it has high alkaline and w(Na₂O+K₂O) is between 2.6% and 4.9%. The lower basalt has w(Na₂O)/w(K₂O) arranging from 0.54 to 1.50 and belongs to shoshinite which has high ∑REE and LREE, its w(La)_n/w(Yb)_n averages 3.45 and δ(Eu) is between 0.69 and 0.91 which shows middle negative anomality. The upper basalt has w(Na₂O)/w(K₂O) arranging from 5.11 to 17.89 and belongs to tholeiite, it has low ∑REE, δ (Eu) is between 0.82 and 1.13, its REE spider pattern (normalized by C1 chondrite) is similar with that of the oceanic tholeiite, its trace elements shows characteristics of oceanic tholeiite and the lower basalt is similar with that of intro-plate basalt. Sm-Nd isochron of the upper basalt is (1 200±82.3) Ma. Concluded on above studies, we put forward that the Precambrian metamorphic volcanic rocks recorded the whole breakup tectonic course in Meso-Proterozoic of the southern paleo-Tarim plate.