秦岭沙河湾造山带型环斑花岗岩地球化学及构造属性讨论  

Geochemistry and tectonic affinity of Shahewan orogenic rapakivi from Qinling.

作　　者：周滨[1] 汪方跃[2,3] 孙勇[1] 孙卫东[2] 丁兴[2,3] 胡艳华[2,3] 凌明星[2,3] 

机构地区：[1]西北大学地质学系,大陆动力学国家重点实验室,西安710069 [2]中国科学院广州地球化学研究所,同位素年代学和地球化学重点实验室,广州510640 [3]中国科学院研究生院,北京100049

出　　处：《岩石学报》2008年第6期1261-1272,共12页Acta Petrologica Sinica

基　　金：科技部973项目(编号:2006CB403505)资助

摘　　要：对沙河湾岩体的主量元素、微量元素地球化学进行了研究,结果显示沙河湾岩体与附近同一时期的曹坪岩体,及南秦岭花岗岩带的米坝、光头山、张家坝等岩体的地球化学特征很相近。SiO_2为64.99%～69.69%,具有高的 Al_2O_3(14.53%～16.2%),富集轻稀土和大离子亲石元素,尤其是沙河湾岩体没有明显 Eu 的负异常,表现出与典型的板内奥长环斑花岗岩明显不同的微量元素地球化学特点。因此,它可能是一种造山带环斑花岗岩,而不是典型的奥长环斑花岗岩。沙河湾环斑花岗岩的年龄曾被看作秦岭造山带主造山过程结束、后造山陆内演化开始的证据。本文研究结果表明沙河湾岩体与曹坪和其它南秦岭同碰撞型花岗岩形成于相同的地质背景,所以不能以其环斑结构来限制秦岭造山带主造山过程结束时代,该岩体也很可能不是沿商丹断裂带的早期碰撞的产物。The Shahewan granite pluton is located near Shangzhou, in Qingling orogenic belt, central China. It was regarded as rapakivi based on petrographic studies, which is taken as part of a Triassic ＂rapakivi belt＂ in the north Qinling mountains. Although the formation of Rapakivi is still not well understood, it is usually formed more than 100Ma after collision, and thus the rapakivi belt in the Qinling orogen was used to argue for that the main collision between the South and North China blocks occurred much earlier than the final collision in Triassic. Therefore, detailed study on the geochemcial characteristics of the Shahewan granite is important to understanding its tectonic affinity and, the evolution of Qinling orogenic belt. In this study, we analyzed the major and trace element compositions of samples from Shahewan and nearby syn-collisional granites. Our results show that the Shahewan granite has the same geochemical characteristics as those of the nearby Caoping granites and other syn-collisional granites all along the South Qinling granite belt, but dramatically different from typical rapakivi. It has high Al2O3 content, ranging from 14. 53% to 16.2% , and is enriched in LREE and LILE, with SiO2content of 64. 99% -69. 69%. Remarkably, there is no obvious （Eu/Eu^＊ ）N anomaly （0. 83 - 1. 10） in the Shahewan granite, which is distinctively different from typical rapakivi. Therefore, Shahewan granite is more likely to be an orogenic rapakivi, which cannot represent the end of the collision between the North and South China Blocks along the Qinling orogenic belt, nor resulted from an early collision along the Shang-Dan fault. Instead, it was formed in a geological setting similar to Caoping and other South Qinling granite plutons.

关 键 词：沙河湾 奥长环斑花岗岩 地球化学 同碰撞 秦岭造山带 

分 类 号：P588.121[天文地球—岩石学]