江南古陆西北部塔下岩体年代学及成因研究

Geochronology and Genesis of Taxia Granodiorite Intrusion in the Northwest Jiangnan Proterozoic Terrane

塔下岩体位于东至地区南部,是除戴村岩体之外该区出露的第2个中酸性侵入体。通过对塔下岩体进行了地质特征、年代学和地球化学研究,结果显示:塔下岩体形成于早白垩世(～142 Ma),为江南古陆燕山期第一阶段(154～138 Ma)岩浆活动的产物。塔下花岗闪长岩体主量元素显示,其SiO2质量分数在68.12%～71.75%之间,K_2O+Na_2O质量分数在6.15%～7.88%之间,铝过饱和度(A/CNK值)在0.98～1.39之间,指示该岩体具有富硅、弱过铝质、钙碱性的I型花岗岩特点;塔下花岗闪长岩的微量元素Rb、Th、U、K富集,Ba和Sr亏损;稀土总量(∑REE)在109.59×10-6～135.49×10-6之间,轻重稀土比值(LREE/HREE)在17.37～20.73之间,轻重稀土分异明显,具有与壳源一致的右倾特征,Eu负异常不明显。综合地质和地球化学分析,塔下花岗闪长岩体的初始岩浆是在古太平洋西向俯冲背景下,由幔源物质混入的下地壳源区发生部分熔融作用产生,上升过程中发生了显著的围岩混染作用。

Taxia granodiorite intrusion is located in the southern Dongzhi district,western Jiangnan Proterozoic Terrane(JPT),which is another known granitoid intrusion after Daicun intrusion in this area. This study systematically researched the geological,geochronological and geochemical characteristics of Taxia granodiorite intrusion. The results show that Taxia granodiorite intrusion was intruded at early Cretaceous(~142 Ma), corresponding to the first stage(154~138 Ma)of Yanshanian magmatism in the JPT area.The SiO2 content of the Taxia granodiorite samples are between 68.12% and 71.75%,the K2O+Na2O value is 6.15% to 7.88%,and the A/CNK ratios is 0.98 to 1.39,which suggest that the lithology of Taxia granodiorite intrusion is high silica, peraluminous and calc-alkaline Ⅰ-type granite.Trace elements are enriched with Rb,Th,U,K and depleted with Ba and Sr elements.The ΣREE varies from 109.59×10^-6 to 135.49×10^-6,and enriched with LREE and depleted with HREE(LREE/HREE are from 17.37 to 20.73),the visible right-trending REE patter is corresponding to the crust-derived magmatism.Combining geological,geochronological and geochemical characteristics,Taxia granodiorite intrusion was formed in the Yanshanian paleo-Pacific Plate subduction background,its melt was probably formed from partial melting of lower crust with mantle materials addedtion which experienced strong wall rock contamination during its emplacement.