广西崇左三叠系北泗组火山岩锆石U-Pb年龄、Hf同位素和地球化学特征OA

The Chongzuo Liutang area exposes the northernmost rhyolite and dacite of the contemporaneous Beisi Formation volcanic rocks.We obtained high-precision LA-ICP-MS zircon U-Pb ages of(247.8±1.4)Ma and(248.0±1.4)Ma for these rocks.Statistical analysis indicates that volcanic rocks of the Beisi and Banba Formations in southwestern Guangxi Province were formed at 240-248 Ma and 248-250 Ma,respectively,which were the products of the Early-Middle Triassic tectonic movement.The geochemical analysis shows that the volcanic rocks of the Beisi Formation comprise high SiO2 and alkali(Na2O+K2O)contents and extremely low MgO,MnO,and CaO contents.The rhyolite and dacite REE contents both exhibit right-inclined LREE enrichment patterns,with δEu values of 0.14-0.16 and 0.16-0.18.Furthermore,they are enriched in Rb,K,Th,U,and Nd contents,and are depleted in Ba,Sr,P,Nb,Ta,and Ti,etc.The Hf isotope study shows that the rhyolites have εHf(t)values ranging from-14.1 to-4.4 and TDM2 values ranging from 1 553 Ma to 2 169 Ma,while the dacites have εHf(t)values ranging from-16.2 to-7.8 and TDM2 values ranging from 1 774 Ma to 2 296 Ma.It can be concluded that the volcanic rocks in this study originated from the partial melting of Paleoproterozoic metasedimentary rocks.Based on a comprehensive study,the Early-Middle Triassic volcanic rocks in southwestern Guangxi are interpreted to have formed in a continental margin arc setting associated with the subduction of the Paleo-Tethys Ocean.These rocks,together with the Early-Middle Triassic island arc volcanic rocks in the Funing-Napo-Chongzuo-Pingxiang area and the Babu-Cao Bang ophiolitic melange belt,constitute an oceanic crust subduction-accretion complex-arc system.Thus,the Liutang area volcanic rocks constrain the spatial extent and temporal limits of Paleo-Tethys oceanic subduction in southwestern Guangxi.