墨脱县幅地质调查新成果及主要进展

New results and major progress in regional geological survey of the Mêdog County Sheet

在传统的冈底斯岩浆弧中新发现一条时代为晚三叠世的迫龙藏布蛇绿混杂岩带,并查明中侏罗世杂色砾岩角度不整合覆盖在该蛇绿混杂岩带两侧的地层之上,表明冈底斯岩浆弧中有一条印支晚期叠合的缝合带。查证了雅鲁藏布江蛇绿混杂岩带的空间展布和物质组成。雅鲁藏布江缝合带在研究区主要表现为蛇绿混杂岩,呈向NE凸的倒U字型连续分布于南迦巴瓦变质岩系和冈底斯岩浆弧之间,主要由变质的超镁铁质侵入岩、辉绿-辉长岩、玄武岩和硅质岩组成。该岩石经历了强烈的剪切变形和糜棱岩化作用,主期韧性剪切的性质为伸展拆离,变玄武岩的地球化学特征同日喀则地区蛇绿岩中的玄武岩基本相同。将南迦巴瓦岩群解体,厘定了该岩群原岩及变质峰期的时代。

Relics of the Late Triassic Parlung Zangbo ophiolitic mélange belt are found in the traditional Gangdise magma arc and it has been demonstrated that the Middle Jurassic mottled conglomerate unconformably overlies the strata on both sides of this ophiolitic mélange belt. This indicates that there is a Yanshanian suture zone in the Gangdise magma arc. The spatial distribution and composition of the Yarlung Zangbo ophiolitic mélange belt have been checked and verified. In the study area the Yarlung Zangbo suture zone is manifested by ophiolitic mélanges, which occur continuously as an upside-down U letter convex toward the NE between the Namjagbarwa metamorphic series and Gangdise magma arc and are composed predominantly of meta-ultramafic intrusive rocks, gabbro-diabase, basalt and cherts. The ophiolitic mélanges experienced strong shear deformation and mylonitization. The main-stage ductile shear is of extensional detachment nature. The geochemical characteristics of meta-basalt are similar to basalt in ophiolites in the Xigazê. The Namjagbarwa Group-complex is disintegrated, and on the basis of the isotope chronological data the ages of the protoliths and metamorphic peak of this group-complex have been defined.