阿尔金构造带西段阿尔金杂岩变质基性火山岩的地球化学特征及其构造意义

阿尔金杂岩变质基性火山岩主要分布于阿尔金构造带西段的中阿尔金地块中,主要由斜长角闪岩类、(含)石榴黑云斜长辉石角闪岩、榴闪岩、石榴斜长辉石岩类等岩石组成.通过岩石化学、微量元素和稀土元素地球化学研究表明,中阿尔金地块北部和中南部的变质基性火山岩形成的构造环境存在明显差异:地块北部变质基性火山岩形成的构造环境具大洋(洋脊)和大陆岛弧两种,其实质上为构造蛇绿混杂岩带(英格里克构造蛇绿混杂岩带)的组成部分,而地块中南部变质基性火山岩形成的构造环境为大陆岛弧环境.由此表明,在中元古代以前中阿尔金地块可能是一个古岛弧,在古岛弧的北侧还存在有一个古大洋盆地(或古弧后盆地).

Geochemical Characteristics and Tectonic Significance of the Metabasic Volcanic Rocks of Altyn Tagh Complexes in the Western Segment of Altyn Tagh Tectonic Belt

Metabasic volcanic rocks in Altyn Tagh complexes are mainly distributed in the central Altyn Tagh block of the western segment of Altyn Tagh tectonic belt.They are composed mainly of plagioclase-amphibolite,garnet-biotite-pyroxene-amphibolite,garnet-amphibolite,garnet-plagioclase-pyroxenite and so on.The study of petrology and geochemistry indicates that the structural setting of metabasic volcanic rocks in the northern margin of the central Altyn Tagh block is different from those in the middle-south of the block: part of the metabasic volcanic rocks of the northern margin of the block has protolith feature resembling E-type MORB(mid-ocean ridge basalt)which may represent the tectonics block of mid-ocean ridge basalt of the Yinggelike tectonic ophiolitic complex belt;another has protolith feature resembling continent(or island arc) tholeiite,possibly showing that the tectonic block of igneous rocks comes from the old oceanic island(sea mountain) in the Yinggelike tectonic ophiolitic complex belt.But metabasic volcanic rocks in the south of the central Altyn Tagh originality composition have protolith feature resembling continent tholeiite,likely from the environment of island-arc volcanic.It is concluded that there is a former island arc in central Altyn Tagh and an ocean basin in the north of the old island arc before Mesoproterozoic Era.