Petrology,geochronology and geochemistry of intermediate-acidic intrusions in Baogutu area,West Junggar,Xinjiang

Abstract

The intermediate-acidic intrusions in the Baogutu area( West Junggar,Xinjiang) are mainly granodiorite,quartz diorite and diorite,with SHRIMP ages of 310 ~ 319 Ma. They are sub-alkaline,enriched in sodium and poor in potassium,with average Mg#of 0. 48. Two magma types are identified: intrusion I,Ⅱ and Ⅴ belong to magnetite type,which has higher oxygen fugacity( LogfO2=- 22 ~- 16); while intrusion Ⅲ belongs to ilmenite type,with lower oxygen fugacity. These intrusions share similar trace element geochemical characteristics: Cs,Rb,Ba,Th,U and LREE enriched,while Nb,Ta and Ti depleted,without distinct Eu anomaly.They show a narrow range of initial Sr isotopic ratios( 0. 7036 ~ 0. 7038),relatively high eNd( t)( 6. 64 ~ 8. 30) and young Nd model ages( tDM1= 435. 4 ~ 587. 7Ma). These geochemical data indicates that the Baogutu intrusions might be produced by highly fractional crystallization of basaltic magma originated from metasomatised depleted mantle( and mixed with juvenile crustal melts) with no signs for old continental crust contamination.