The Baiyin Group,which is the ore-hosted strata in Baiyin ore field,is located at the southeast of North Qilian. The research of isotope chronology,petrology and whole-rock geochemistry was carried out on the bimodal volcanic rocks to reveal their ages,petrogenesis and forming environment. LA-ICP-MS zircon U-Pb dating results show that the group age of the rhyolites are 473.0±1.7 Ma and 473.9±2.1 Ma (206Pb/238U ages),and it can be confirmed that the rhyolite suite formed during the Early Ordovician. The basalt samples,which are regarded as the low-K sub-alkaline tholeiitic series,have geochemical characteristics including high Al2O3,low K2O,TiO2 and P2O5. The rhyolitic rocks (SiO2>70％),which belong to cold-wet-oxidized rhyolite,exhibit low FeOT/MgO values (2.44-2.80) and total REE content. With enrichment in incompatible element (Ba,Th,U),obvious negative anomalies of Nb,Ta,Ti and no obvious Eu anomalies (δEu=0.76-0.92),the bimodal volcanic rocks show the geochemical features of island arc magma. The rhyolite zircon εHf(t) values range from-4.14 to 14.78 with Hf Model ages (tDM2) varying from 1707 Ma to 505 Ma. These features can be concluded that the basalt derived from partial melting of depleted mantle under the effect of subduction fluids and crustal contamination. Meanwhile,the rhyolite may be derived from partial melting of crustal material with a small amount of mantle-derived magma,therefore they are not comagma. The petrology and geochemical information of bimodal volcanic rocks from Baiyin Group indicate they formed in transitional environment from island arc to back-arc basin when northward subduction happened in Early Ordovician.
维普
万方数据
