Study On Carboniferous Adakite In The West Alxa And Its Geological Significance

The study area is located in Dashankou,Alxa district,geotectonicly belongs to Longshou mountain-Heli mountain continent margin,the southwest edge of Alxa block.Which is bounded to Alkin,Jingta-Dingxin(Ruoshui)rift in the west and is adjacent to Tarim plate,and also connect Longshoushan rift and North Qilian in the south.Based on the field research and references in this region,distribution characteristics,nature,age and geochemical characteristics of Neopaleozoic adakite in Shazaoquan,Alxa have been studied using comprehensive analysis of the petrology,geochemistry,isotope geochronology and structural geology etc.in order to discuss the genesis and diagenetic tectonic setting of the Neopaleozoic adakite in Alax,and to provide a reliable petrologic,isotope chronological evidence to build Neopaleozoic crustal evolution and tectonic framework of Alxa block.From the analysis result,the LA-ICP-MS zircon U-Pb age of Shazaoquan granite is ranging from 295±5Ma to 307±8Ma with weighted average of 302±9.2Ma which is late Late Carboniferous,representing the rock forming time.The SiO2 content are stable,which is ranging from 60.66%to 61.37%with average value of 61.03%,high alumina content(Al2O3 = 17.98%-18.44%,average of 18.28%)and alkaline(Na2O + K2O = 4.96%-5.22%),rich in calcium(CaO = 6.08-6.59%),relative high magnesium(MgO = 2.13-2.51%,Mg#= 0.48-0.51),rich sodium but poor potassium(Na2O =4.20%-4.46%,K2O =0.67%-0.92%,K2O/Na2O = 0.16-0.22),peraluminous(A/NKC=1.13-1.17),but there is no standard mineral corundum.The rock enriches incompatible elements(Rb?Th?K?U?Pb,etc.)for the rare element geochemistry feature,highly delepeted HFSE(Nb?Ta?Ti)and HREE,high Sr(582x10-6-620×10-6)low Y(11.4× 10-6-13.7× 1 0-6)?Yb(1.02× 10-6-1.51 × 1 0-6),high Sr/Y(35.15-53.25),Y/Yb(11.00-11.23),La/Yb(7.28-15.10),Rb/Sr(0.03-0.05)?La/Ce(0.42-0.50),Nb/U(4.70-8.93,one sample reaches to 14.3),the rock features listed above are similar to the Pacific Rim newly born adakite.Therefore the relative low compatible elements content of Ni(10.3-11.6×10-6)?Cr(13.4-16.8×10-6 indicates the metasomatism is not sufficient between plate melt and mantle peridotite.The total rare earth elements are 9.43×10-6-89.71×10-6 with average value of 85.04×10-6,LREE/HREE is between 5.57 and 8.97,LREE is medium enrichment that(La/Yb)N = 5.0-10.0,there is no Eu anomaly and slightly Eu positive anomaly(?Eu=0.97-1.18,the average value is 1.096),HREE appears flat type.The rock rare earth elements geochemistry features suggest the original rocks are partial melted in a highly degree,and the presence of fractional crystallization of plagioclase during magma ascent or no residual plagioclase in the source,should be amphibole,pyroxene and garnet.Combining with the regional tectonic background studies,it suggests that Shazaoquan adakite is the block formed by young(<25Ma),high heat flow value basaltic oceanic crust of late Late Carboniferous Epoch in Chaganchulu back-arc basin subducted to Alxa continent plate which leads to the plate patial melting and rapidly ascent.The Chaganchulu back-are basin closed right after the formation of Shazao block which also suggests the Engger Panthalassic ocean closes at the same time.Also the late Carboniferous Shazao block should form Cu,Au deposit by the metasomatism of subduction plate partial melt and mantle wedge,and based on the field research,the edge and the surrounding areas of the block show a good metallogenic prospects.So the found the late Carboniferous adakite points out a new direction for the prospecting exploration in this area.