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

Duocai ophiolite is a component part of melange 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/Hf0.15,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.