Zircon U-Pb Geochronology,Geochemistry and Sr-Nd-Hf Isotopes of the Neoproterozoic Granites on the Southeastern Margin of the Yangtze Block:Constraint on Crustal Growth

Wang Yan; Ma Changqian; Wang Lianxun; Liu Yuanyuan

Abstract：The Jiugongshan gneissic granites exposed in the southeastern margin of the Yangtze Block,and intruded into the Shuangqiaoshan Group of Early Neoproterozoic.Using the methods of cathodoluminescent images analyzing and LA-ICP-MS zircon U-Pb dating,we obtained a magma crystallization age of 830±8 Ma for the Jiugongshan intrusion.Four spots of inheri-ted zircons yield a weighted mean at 873±7 Ma.This age might record the magmatic activity related to the collision between the Yangtze and Cathaysia blocks during the period of Early Neoproterozoic.The Jiugongshan granites are characterized by high SiO2(71.83%―74.20%),high K2O+Na2O(7.06%―7.90%),low contents of mafic components(∑TiO2+FeOT+MgO=2.64%―4.00%)and high K2O/Na2O ratios(1.25―1.64).They mainly belong to weakly peraluminous granites with A/CNK ratios(1.03―1.27).They share similar distribution patterns of REE and trace elements,i.e.,enrichment in LREE and large-ion lithophile elements of K,Rb,Th and U,depletion in Sr,P and high-field-strength elements of Nb,Ta and Ti,and pro-nounced negative Eu anomalies(Eu/Eu*=0.35―0.47).On the other hand,they have neutral whole-rockεNd(t)values of―1.49 to +5.24 and positiveεHf(t)values of 3.5±1.0 to 11.0±1.1 and 5.1±0.9 to 12.9±1.1 respectively for the coeval mag-matic zircon and inherited zircon.This indicates a magmatic source with significant components of the juvenile crust.The Jiugongshan intrusion might be formed in the environment of intracontinent rift and generated by partial melting of metamor-phic psammitic and volcanic source.Regional comparison shows that the Nd-Hf isotopic composition of the Middle Neoprotero-zoic granites along the Jiangnan orogeny has the characteristics of decreased gradually from east to west,and suggests that the effect of the Early Neoproterozoic juvenile crust on granitic magmatism decreased from east to west.This might be caused by the decreasing of arc magmatic components from east to west among the orogenetic process.