Geochemistry of the Duocai ophiolite in north Qiangtang Basin, Qinghai-Tibet plateau: environments for tectonics

Abstract：Duocai ophiolite is a component part of mélange ophiolite in Tongtian River area, and regionally belongs to the northward extension part of Xijinwulan-Jinsha River suture zone. Each Duocai ophiolite occurs in the form of tectonite and rock chips exposed as small and scattered outcrops, but has fairly complete kinds of rocks, mainly composed of diallage-peridotite, pyroxenite monoclinic pyroxinite, gabbro, basalt, diabase and siliceous rocks, with no mantle peridotite. Ophiolite components assume tectonic contact with wall rocks. Diallage-peridotite is characterized by rich MgO, poor TiO2 and P2O5, with no or only weak negative Eu anomaly. Diallage-peridotite is an important rock of SSZ type. Gabbro has high TiO2 and Al2O3 content; basalt and diabase have similar characteristics. Th/Ta of basalt has an average value of 1.78, Ta/Hf＞0.15, and Ta/Hf=0.15-0.2, and characteristics of these elements are similar to those of oceanic intraplate basalts. Al2O3/(Al2O3+Fe2O3) ratio of siliceous rocks is 0.9, (La/Ce)N is 1.81, showing characteristics of a land-based sea. Based on the above-mentioned characteristics in combination with the age of ophiolite as well as tectonic setting discrimination diagrams of basalt and diabase, the authors hold that Duocai ophiolite was formed in a subduction zone tectonic setting.