中国东部A型花岗岩形成时代及物质来源的Nd-Sr-O同位素地球化学制约  

Nd-Sr-O isotopic geochemistry constraints on the age and origin of the A-type granites in eastern China.

作　　者：魏春生[1] 郑永飞[1] 赵子福[1] 

机构地区：[1]中国科学技术大学地球与空间科学系化学地球动力学研究实验室,合肥230026

出　　处：《岩石学报》2001年第1期95-111,共17页Acta Petrologica Sinica

基　　金：中国科学院知识创新工程项目 ( KZCX2 -10 7);国家自然科学基金 ( 4 96 0 30 43和 40 0 330 10 );国土资源部壳幔物质组成;相互作用及动力学演化开放研究实验室联合资助成果

摘　　要：对中国东部 A型花岗岩有代表性的碾子山、山海关、崂山、苏州以及魁岐岩体的 Nd- Sr- O同位素地球化学组成进行了系统研究。经过筛选的全岩 Rb- Sr等时线拟合结果表明 ,基本未遭受岩浆期后大气降水交换的苏州和魁岐 Rb- Sr等时线年龄分别是 10 8± 10 Ma和 10 9± 5 Ma,它们代表岩浆冷却结晶时代。其它遭受热液蚀变较为显著的碾子山、山海关以及崂山岩体 Rb- Sr等时线年龄的地质意义不明确。全岩 εNd(t)、(87Sr/ 86Sr) 0 以及锆石 δ1 8O综合示踪研究表明 ,中国东部 A型花岗岩可能起源于曾经历过高温海水热液交换的再循环下部俯冲洋壳的部分熔融。模型计算结果表明 ,碾子山、山海关以及崂山等岩体所表现出的 εNd(t) - δ1 8O脱耦变化与大洋沉积物析出流体与下覆辉长岩洋壳之间不同程度的交代有关。在此基础上 ,提出了中国东部 A型花岗岩统一的成因模式 。Nd-Sr-O isotope studies are systematically carried out for five representative massifs of A-type granites in eastern China. From the north to the south, they are Nianzishan, Shanhaiguan, Laoshan, Suzhou and Kuiqi pluton, respectively. Fitting of whole-rock Rb-Sr isochrons for the Suzhou and Kuiqi plutons proceeds by screened isotopic data, obtaining the ages of 108+/-10Ma and 109+/-5Ma, respectively. It appears that the isochron ages were not significantly affected by post-magmatic meteoric-hydrothermal alteration and thus represent the closure time of Rb-Sr system in the granites. However, whole-rock Rb-Sr isochrons for the Nianzishan, Shanhaiguan and Laoshan plutons were significantly disturbed by hydrothermal alteration, resulting in geologically meaningfulless ages. A combined study of whole-rock epsilon (Nd)(t), (Sr-87/Sr-86)(o) and zircon delta O-18 suggests that the a-type granites in eastern China would be derived from partial melting of hydrothermally altered oceanic crust which would correspond to gabbro layer in lower oceanic crust and thus underwent oxygen isotope exchange with seawater at high temperatures prior to subduction of oceanic plate. A calculation of binary mixing model shows that the decoupling of the epsilon (Nd)(t) and delta O-18 values for the Nianzishan, Shanhaiguan and Laoshan granites is probably caused by metasomatic processes of subducted sediment-derived fluid to the underlying oceanic crust of gabbro. An unified model is proposed to account for petrogenesis of the A-type granites in eastern China and its geodynamic implications.

关 键 词：A型花岗岩 同位素地球化学 形成时代 物质来源 交代作用 大气降水 成因模式 岩浆 

分 类 号：P588.121[天文地球—岩石学] P597[天文地球—地质学]