义敦岛弧措交玛岩体岩浆混合成因:镁铁质微粒包体的证据

义敦岛弧北部的措交玛岩基岩体主要由黑云母二长花岗岩和边部的花岗闪长岩组成。在黑云母二长花岗岩中存在有少量镁铁质微粒包体,其成分为闪长质,与寄主岩石接触关系从渐变到截然。在包体周围的寄主岩石中存在黑云母、角闪石自身的包含结构,角闪石包含黑云母,斜长石发育明显的溶蚀结构,核部斜长石被溶蚀成筛状,边部环带状斜长石溶蚀不明显,是基性岩浆注入到酸性岩浆中导致岩浆混合的结果。黑云母二长花岗岩具有更高的轻重稀土分异系数,闪长质包体轻重稀土分异系数较低,黑云母二长花岗岩和暗色闪长质微粒包体具有明显相似性的微量元素特征。寄主岩黑云母二长花岗岩锆石U-Pb年龄为236±1.9Ma,闪长质包体为235±3.9Ma,二者形成年代在误差范围内基本一致,可能为甘孜-理塘洋向西俯冲过程中,俯冲洋壳部分熔融形成的玄武质岩浆上涌底侵于壳-幔边界导致地壳的部分熔融形成酸性的黑云母二长花岗岩岩基。

Magma mixing of the Cuojiaoma batholith in the Yidun Arc:Evidence from mafic microgranular enclaves

In the northern Yidun Arc, Cuojiaoma batholith consists of biotite quartz monzonite granite and granodiorite. Some mafic microgranular enclaves (MMEs) were discovered in the biotite quartz monzonite granite, showing gradual or clear contacts with the hosted rocks. There were poikiloblastic textures of amphiboles and biotites, coexisting minerals and corrosion zone of plagioclases, as the consequence of the magma mixing of biotite quartz monzonite granite and MMEs. The hosted rocks had higher ratios of LREE/HREE (7.31~19.8) than MMEs (2.88~3.14). Zircons from the hosted rocks yielded the U-Pb age of 236±1.9Ma which is consist with the age of MME (235±3.9Ma) within the error. The Cuojiaoma batholith was originated from the early stage of the westward subduction of the Ganzi-Litang Ocean, a branch of the Paleo-Tethys, during the Early Triassic. The partial melting of the subducted oceanic slabs served as the magmatic source of the MMEs which ascended and resulted in the partial melting of the lower crust which was the magmatic source of the biotite quartz monzonite. Though the MMEs had higher Cu, Zn contents, MMEs had a little impact on the geochemical compositions of the hosted rocks.