Genesis And Tectonic Significance Of The Early Palaeozoic Magmatite In The Baoshan Block And Tengchong Block,Yunnan Province

The early Paleozoic puzzle of continental blocks of the Sanjiang region in SW China,such as the tectonic evolution and its geodynamic mechanism,has long been a subject of debate.Based on former researchers,most early Paleozoic magma emplacement happened in 500-448Ma in the Tengchong-Baoshan block.However,the geodynamic mechanism is not certain yet.One opinion believe that the oceanic crusts subduction made the magma before 490Ma,while the micro blocks collision and accretionary provided environment to form the magma after 490Ma.The other hold the opinion that the granite was formed by oceanic crusts subduction during 500-460Ma.Based on the data collected from Baoshan block and Tengchong block,this paper discussed the related puzzle.The samples collected from Gongyanghe Group,Baoshan block shows that the rocks are relatively enricher in incompatible elements?Th,U?and and depleted in LILEs?Ba,Rb,K?.The contents of Nb,Ta and the other elements are relatively stable.According to the data feature,we believed the samples are formed in a continental intraplate setting.The rock sample collected in the Gaoligongshan Group,Tengchong block was formed in⁴37Ma supported by the data analyzed through LA-ICP-MS zircon U-Pb.According to the major element data,these Early Silurian granites are high silicic,high alkali and peraluminous.Besides,the trace element shows the samples are relatively enricher in LREEs,LILEs?Rb,K?and Pb,and depleted in HFSEs?Nb,Ta,P,Zr,Ti?,Ba,Sr and Eu.Based on the varieties of mineral assemblage characteristics and geochemical features,the rock samples are S-type granites.The S-type granites are partial melting of metasediments dominated by pasmmite with residual plagioclase in the source area.?Hf?t?and t_DM2 of zircon also prove that the samples mainly derived from the ancient crustal metasedimentary,with little of mantle-derived components introduced into the melt.According to zircon saturation temperatures(T_Zr)and Ti-in-zircon thermometer,the temperature decreases from⁷94?to⁷54?during the process of partial melting to magma consolidation.The high temperature of the melt indicates the mantle may as a sourse for the extra heat requird for crustal anataxis.Previous research indicates in the Early Paleozoic Gondwana continental margin experienced proto-Tethyan subduction,accretionary orogenesis of microcontinental fragments and slab breakoff,lithospheric thickening,and lithospheric delamination.The samples from Baoshan block prove the progress of slab breakoff.And the Tengchong block may be produced through partial melting of the ancient crustal metasediments dominated by psammite,and mantle may supply some extra hear for the partial melting progress.