| The Emeishan Large Igneous Province is mainly distributed in Yunnan,Guizhou and Sichuan provinces of China,which is the only recognized large igneous province in China.Its formation has been attributed to be the cause of the mass extinction and climatic change during the Midlle-Late Permian.Therefore,the age of the Emeishan volcanism has been widely concerned in the world.Mineral compositions,whole-rock geochemistry and zircon trace elements,U-Pb and Hf isotopic data for the Late Permian tuffs at the Guangyuan area in Sichuan are analyzed to discuss their sources and implications for the age of the Emeishan volcanism.The Late Permian tuffs from the Shangsi section are featured by a large amount of clay minerals and a few amount of quartz.These mineralogical features are similar to the altered volcanic ashes.The Si O2/Al2O3 values(1.34~2.34)of the Late Permian tuff samples from Shangsi section are relatively low,reflecting the high degree of weathering and alteration in the tuff samples from the same source.The early Late Permian tuffs in Shangsi show higher Al2O3/Ti O2 values(24.1~45.1),Th/Sc values and Zr/Ti O2 values than Emeishan basalts,and have strong negative Eu anomalies(Eu/Eu* = 0.10-0.40),no negative anomalies of Nb,Ta and Ti.These geochemical characteristics are similar to the Emeishan felsic volcanic rocks and felsic ignimbrites from the uppermost part of the Emeishan volcanic succession at Binchuan,suggesting that the early Late Permian tuffs from Shangsi section are related to the felsic volcanism of the Emeishan Large Igneous Province.The late Late Permian tuffs from the Shangsi section show the enrichment in large ion lithophile elements and pronounced depletion in Nb,Ti and Ta,zircons have εHf(t)values(-5.7~+9.8)and high Th/Nb ratios(most >70).These geochemical characteristics are basically consistent with the late Late Permian tuffs in the eastern part of South China,implying that the genesis of the late Late Permian tuffs from Shangsi section may be magmatic island arc.The mineralogical and geochemical characteristics of the Late Permian tuffs from Mingyuexia section are similar to the early Late Permian tuffs from the Shangsi section,with high Al2O3/Ti O2 values and obvious Eu negative anomalies,and lack of negative anomalies for the high field strength elements(e.g.,Nb,Ta and Ti),suggesting that these tuffs have same similar sources.Most zircons of the tuffs from the Shangsi section have εHf(t)values > +6 and Th/Nb ratios <15,similar to those from the Mingyuexia section(εHf(t)=+7.8~+11.4,Th/Nb =5.4~14.9),but different from those of the claystones from the Wangpo bed.These results indicate that the tuffs of the early Late Permian from Shangsi section and the tuffs from Mingyuexia section have the same sources,which are different from soures of the vocanism related to the Wangpo Bed.Considering that these tuff beds are overlying the Wangpo Bed,we conclude that the felsic volcanism of Emeishan Large Igneous Province related to the Wangpo Bed was not terminated,and the age of the Emeishan volcanism may extend to the early period of the Late Permian. |
