The Geochemical Characteristics And Geological Significance Of The Late Paleozoic Mafic Rocks In Rongma Area,Qiangtang,Northern Tibet

The late Paleozoic basite of Qiangtang Rongma area in northern Tibet is located in the southern margin of Longmu Tso-Shuanghu Suture Zone and the Central Uplift Belt. They belong to the part of Qiangtang central dike group. They are discovered in metamorphosed rock series that they characterized by Carboniferous- early Permian rift type deposits. They consist of sillite, gabbro, diabase gabbro and basite. The zircons of sillites U-Pb dating using LA-ICP-MS obtain a weighted average age of its value(334.7 ± 3.6) Ma. Mafic intrusive rocks and basalts are both spatially close symbiosis. They have many similar geochemical characteristics such as rich Ca O, Mg O, Al2O3, poor alkali and low P2O5, Ti O2. These mafic rocks are characterized by high LILE(Rb, Th, Uand Pb) and obvious depletion of HFSE(Nb、Ta、Ti and P). The Nb, Ta depletion of basalts are greatly more than mafic intrusive rocks. It may suffer crustal contamination during the magma rising. Fractionation between LREE and HREE is obvious, and the amount of LREE is more higher than the amount of HREE. Tectonic environment discrimination diagrams from the trace element show which is formed within the continental plate inland rift environment. Their contact relationships with surrounding rocks and their geochemical characteristics reveal they should come from the same source region. Trace elements Nb/Zr diagram and Y/Zr diagram shows the basic magma comes from the enriched mantle source composed by the garnet + spinel lherzolite. Its melting degree ranges from 10% to 20%. The mafic rocks in research area should be the product of the northern margin of Gondwana because of its cracking. Its surrounding rock is a set of sedimentary strata which has already begun to accept rift type deposit as early as the early Carboniferous Datang period.