扬子东南缘新元古代花岗岩的锆石U-Pb年代学、地球化学和Sr-Nd-Hf同位素:对地壳生长的约束

出露于扬子板块东南缘的九宫山片麻状花岗岩侵位于早新元古代双桥山群.通过阴极发光图像分析和LA-ICP-MS锆石U-Pb年龄测试表明，九宫山岩体的岩浆结晶年龄为830±8 Ma；4个继承锆石分析点给出加权平均值为873±7 Ma的年龄值，可能记录了新元古代早期扬子与华夏板块碰撞产生的岩浆活动.九宫山岩体具有高SiO₂（71.83%~74.20%）、高K₂O+Na₂O（7.06%~7.90%）、低基性组分（∑TiO₂+FeOT+MgO=2.64%~4.00%）的含量特征，高K₂O/Na₂O比值（1.25~1.64），铝饱和指数（A/CNK）为1.03~1.27，主要为弱过铝质花岗岩.岩石具有轻稀土富集的右倾模式和明显的Eu负异常（Eu/Eu*=0.35~0.47），K、Rb、Th、U等大离子亲石元素富集，Sr、P、Nb、Ta、Ti等相对亏损.全岩ε_Nd（t）值为-1.49~+5.24，同岩浆锆石和继承锆石的ε_Hf（t）值分别为3.5±1.0~11.0±1.1、5.1±0.9~12.9±1.1，指示岩浆源区含有显著的新生地壳物质.九宫山岩体可能形成于陆内裂谷环境，由类似双桥山群的变质砂屑岩和火山岩部分熔融形成.区域对比表明，江南造山带新元古代中期花岗岩的Nd-Hf同位素组成具有从东向西逐渐降低的特征，表明该区早新元古代时期新生地壳物质对花岗岩的影响由东向西呈减弱趋势，这可能是扬子与华夏板块碰撞拼合过程中岛弧岩浆作用由东向西逐渐减弱而使新生地壳物质减少所引起. 

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

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 inherited 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 pronounced negative Eu anomalies (Eu/Eu*=0.35-0.47).On the other hand, they have neutral whole-rock ε_Nd(t) values of -1.49 to +5.24 and positive ε_Hf(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 magmatic 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 metamorphic psammitic and volcanic source. Regional comparison shows that the Nd-Hf isotopic composition of the Middle Neoproterozoic 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.