Geochemical And Zircon U-Pb Dating Studies Of The Volcanics Of Tanjianshan Group In The North Qaidam

The North Qaidam is a narrow orogenic belt which extends from southeast to northwest bounded by the Qaidam Massif in the south and the Qilian Massif in the north. It has been recently recognized as an HP-UHP metamorphic belt. This thesis focuses on the studies of petrography, geochemistry and geochronology of the volcanics of Tanjianshan group which distribute to the east of Yukahe aera and closely associated in space with the HP-UHP metamorphic rocks, and the gabbro exposure in the Tanjianshan aera, west of Yukahe aera. These studies can not only provide accurate constrains for the formation age and tectonic environment of the Tanjianshan volcanics, but provide important constraints for the tectonic setting and metamorphic age of the the North Qaidam HP-UHP metamorphic belt. The main achieves of the thesis can sum up in the following aspects:(1) Geochemical studies indicate that the volcanics of Tanjianshan group which were closely associated in space with the HP-UHP metamorphic rocks were formed mainly under oceanic event in different geological setting. Wanggaxiu gabbro complex were formed in a setting of an island arc; The basalt in Tuomoerrite aera shows features of N-MORB, which was formed in the mid-ocean ridge setting; There are two types of rocks in the Dapinggou aera. The basic rock has the characters of both N-MORB and IAB while the intermediate rock has features of boninite, indicating they were formed in a forearc basin setting. Further more, it has been found that the plagiogranite occur in the igneous rocks was resulted from the magma remelting from cumulate gabbros. The gabbro in the Tanjianshan aera exhibit the characteristics of E-MORB. Meanwhile, obvious continental contamination has been found. Therefore, it should be formed under continental rift setting.(2) LA-ICP-MS U-Th-Pb isotopic analyses for zircons yield the crystallization age of the volcanics of Tanjianshan group is 468-534Ma. The age of gabbro from Wanggaxiu and basalt from Tuomoerrite are 468±2Ma and 480±1.3Ma respectively. Basalt from Xitieshan aera is 517±5.7Ma. Volcanics and plagiogranite from Dapinggou aera are 534.5±2Ma and 495±4.2Ma. And the gabbro exposure in the Tanjianshan aera is 837±2.8Ma. (3) Integrated studies of petrography, geochemical and geochronology, we can learn that a muturing ocean basin had been developed in the North Qaidam in early Paleozoic(534Ma), and there still remains slab subduction in mid-Ordovician(468Ma), indicating the volcanics of Tanjianshan group were formed mainly under oceanic event in different geological setting in early Palaeozoic. They constitute an ophiolitic melange belt. And the age of the continental deep subduction of the North Qaidam should not be earlier than 468Ma.(4) The determination of the gabbro which was formed under continental rift setting at 837Ma in Tanjianshan aera, combines with the north Qaidam eclogites which are often derived from incipient oceanic basin or continental flood basalt protolith, and 730-850Ma protolith age of the eclogites, we could know the north Qaidam eclogites are probably formed in a continental rift or incipient oceanic basin setting during the dispersal of Rodinia super-continent, which had undergone the continental deep subduction with supracrustal sequence in Paleozoic.