The protolith of the Aksu Precambrian blueschist and its tectonic setting
阿克苏前寒武纪蓝片岩原岩产出的大地构造背景

Aksu blueschist terrane is located in the northwest of Tarim Basin. It is presently considered to be one of the only two Precambrian blueshists with reliable evidence in the world, for the unconformity with overlying Sinian System on its southeast. Former researches have focused on the isotope ages of the high-pressure metamorphism, with some data showing the Neoproterozoic age of the subduction-exhumation process. Nevertheless, the protolith of the blueschist and its original geotectonic setting have rarely been discussed. A geochemical analysis on mafic schists has been made in this paper, indicating the ocean crust properties of the protolith. Si contents and some immobile element diagrams show that the protolith of all samples is tholeiite. All these samples are divided into two groups: Group A and Group B, due to the distinguishable geochemical differences. The REE distribution pattern curves, the primitive mantle normalized spider diagrams and some common basalt geotectonic discrimination diagrams show that samples in Group A and Group B have EMORB(enriched mid-ocean ridge basalt) and NMORB(normal mid-ocean ridge basalt) properties, respectively. The protolith of Aksu blueschist is part of the ocean crust adjacent to Tarim in Neoproterozoic, it experienced deep subduction and exhumination as part of accretionary wedge and finally became exposed to the surface.