Geochronology And Geochemistry Characters On Some Magmatic Rocks In The West Of Xijir Ulan-Jinshajiang Suture Zone

In order to know the evolution history of the west of the Xijir Ulan-Jinshajiang suture zone, some rocks closely related with the Paleo-Tethys were studied in geochemistry and geochronology.On the base of field geological surveys, the magmatic rocks with the Paleo-Tethys, such as basalts, quartz diorites, gabbros from the ophiolites, and trachytes and granites formed by collision of India Plate and the Eurasian Plate, and so on, were collected. By the U-Pb SHRIMP dating and geochemical analyzing of the samples, the following conclusions were obtained:1. The gabbro of Nieqiaqu ophiolite has the specialties of MORB and the zircon from the rock gave a ²⁰⁶Pb/²³⁸U age of 268±11 Ma.2. The compages of Jizaneng basic-ultrabasic rocks in Zaduo County belongs to calc-alkaline series. The zircon in the gabbro from the compages gave a ²⁰⁶Pb/²³⁸U mean age of 252±6 Ma. The rock might be produced by the subduction of the oceanic crust of Paleo-Tethys in Late Permian contrasting with the contemporary adakite.3. The Indosinian granites at Yushu-Zhiduo area were dated as 209±4 Ma. The geochemical characters indicated that they might be formed in volcanic arc environment by the Plate subduction. So, it is believed that, along the Xijir Ulan-Jinshajiang suture zone, the subduction of the crustal remnants of the Paleo-Tethys, with concomitance of the plate collision, were still carried on until Late Trias.4. Telongsai volcanic rocks in Zaduo County, mainly belonging to the calc-alkaline series, were formed in epicontinental volcanic arc environment, possibly brought by the subduction of the residual oceanic crust of Paleo-Tethys to Zaduo micro-continents, and the U-Pb age of the zircon indicated that the volcano erupted at 217±9 Ma.5. The remnant zircons, being of Proterozoic ages, were found in the gabbro of Nieqiaqu ophiolites, which showed that the material of the basement might take part in the process of the formation of the ophiolites.6. After measuring zircon U-Pb age precisely, it was confirmed that the Shanglaxiu volcano eruption should be at 50±1 Ma, and the Baimahai granite was emplaced at 49±1 Ma. Both of them might be produced by the long-distance effects of the subduction collisions between the Indian Plate and Eurasian Plate.