Petrological Assmblage And Geological Implications Of The Indosinian Igneous Rocks In Southeastern Guangxi Province

Southeastern Guangxi is located at the southwestern part of the famous Qinhang tectonic fault zone in South China,and belongs to the famous Shiwandashan–Darongshan tectonic–magmatic belt.It is an important research area for the collision orogenic process between the Yangtze and Cathaysia Blocks.Based on the field geological survey,this paper was focused on the granite in the region from petrology,geochemistry,zircon U-Pb chronology and zircon Hf isotope,and discussed the geological significance in combination with igneous rock assemblage.Our results suggest that the three granites,i.e.,Napeng,Darongshan,and Pubei in southeastern Guangxi contain Al-rich minerals(tourmaline,garnet,cordierite)and belong to S type granite.All of them belong to high potassium calc-alkaline series rocks.The analysis of rare earth elements show that these granites are enriched in Rb,Th,K,U,and Pb,and depleted in Ba,Sr,Nb,Ta,and Ti.The eare earth element partition curveare generally right inclined,and show markedly negative Eu anomalies(?Eu=0.11?0.94).These results reflect the partial melting of magmatic source area for magma producing,and its evolution has experienced a certain degree of fractional crystallization.Zircon U-Pb dating results show that is the granites formed between(257.8±1.2)?(250.1±1.6)Ma,corresponding to the late Permian to the early Triassic,and they arethe products of the early Indosinian magmatic activity.Hf isotopic analysis present the?Hf(t)values have a range of-18.22?-3.43,mainly concentrated in-11?-7.The zircon Hf two-stage model ages(T_DM2)are 1.5?2.4 Ga,suggesting that the source material of granites in southeastern Guangxi were the results of remelting of ancient crust.Therefore,the igneous rock assemblages TTG are the product of magmatic consolidation of the magnesium andesite series(MA),which is the interaction result between the magma produced by partial melting of the subduction basalt oceanic crust and and the overlying mantle peridotite.The normal andesite series granitic magma is resulted from the intruding of magnesium andesite series(MA)magma into intruded lower crust,and caused by partial melting of heated lower crust rock.The rock assemblage of Napeng,Darongshan and Pubei granites in southeastern Guangxi is trondhjemite+granodiorite+monzogranite,in which monzogranite is dominantly exposed in this area.It can be considered that TTG(tonalite+trondhjemite+granodiorite)petrotectonio assmblage had developed here.This area had stayed in a tectonic pattern of“trench-arc-basin”system at the early Indosinian(P₃-T₁),and granites in this area formed in the island-arc environment of oceanic subduction and orogenic stage,which further suggests that the remaining ocean basin in Qinfang had not closed at P₃-T₁.