Island arc volcanic rocks in the north Qaidam UHP belt, northern Tibet plateau: Evidence for ocean–continent subduction preceding continent–continent subduction

A suite of Early Paleozoic island arc volcanic rocks, consisting mainly of basaltic lavas and some intermediate-silicic rocks, crops out in the North Qaidam basin, northeastern Tibet Plateau. These rocks have been extensively metamorphosed under greenschist facies conditions, and hence differ from the older Proterozoic intermediate-mafic volcanic rocks, which have generally experienced amphibolite facies metamorphism. The Early Paleozoic volcanic rocks are divided into three groups based on their geochemical characteristics: (1) VTG-I, consists of island arc tholeiitic basalts, basaltic andesites and andesites that crop out in Jilusu and Shuangkoushan; (2) VTG-II, which are high-Al, alkaline and sub-alkaline basalts in Jilusu; and (3) VTG-III, an assemblage of highly depleted, MORB-type basalts, andesites and dacites that occurs mostly in Shuangkoushan. Groups VTG-I and -II are considered to be the products of mature island arcs produced in two stages. In the first stage, following initiation of ocean–continent subduction, the island arc tholeiites were generated by partial melting of the subsiding oceanic crust and mantle wedge beneath the island arc. As subduction progressed and accelerated, calc-alkaline and high-Al basalts were formed and the crust beneath the island arc was thickened. The highly depleted MORB-type lavas (VTG-III) have boninitic affinities and are believed to have been generated from depleted mantle material modified by subduction zone fluids, possibly in the fore-arc region. The island arc tholeiites are dated at 514.2±8.5 Ma whereas UHP eclogite in the North Qaidam Belt has been dated at 494 Ma. These ages suggest that ocean–continent subduction took place in the North Qaidam Basin in the Early Paleozoic and was followed shortly by continent–continent collision and subduction, which produced the UHP rocks.