扬子地台西缘淌塘花岗岩的地球化学特征及锆石U-Pb定年

Geochemistry and zircon U-Pb dating of Tangtang granite in the western margin of the Yangtze Platform

淌塘花岗岩岩体位于扬子地台西缘,岩石类型为钾长花岗岩。为了揭示其成因意义,本文对淌塘花岗岩体进行了SHRIMP锆石U-Pb定年和地球化学研究。地球化学及岩石学研究的结果显示淌塘花岗岩具有高硅（SiO2=72.70%~77.95%）、高碱（ALK=5.68%~7.32%）、过铝质（ASI=1.41~1.78）的特征,K2O/Na2O平均值为1.18,CaO、MgO含量偏低,其岩石系列为高钾钙碱性系列,属于强过铝质S型花岗岩,其稀土元素含量偏低且轻重稀土元素差异较大（LREE/HREE=2.71~4.09）,部分微量元素的含量显著区别于地壳平均值。锆石SHRIMP U-Pb定年结果获得其成岩年龄为1 063.2±6.9 Ma,属于格林威尔造山期运动产物。地球化学研究表明该花岗岩是由地壳中的泥质岩经过部分熔融形成的,其双峰式的岩浆岩组合模式和Al2O3-SiO2构造环境判别图的投图结果均显示该花岗岩的形成与裂谷环境密切相关,这与1.0 Ga时扬子地台西缘的大地构造背景相呼应,其形成也对应了Rodinia超大陆的汇聚事件。

The Tangtang granite occurs in the western margin of the Yangtze platform and mainly consists of plagioclase granite.In order to reveal the significance of its petrogenesis,the authors studied the SHRIMP U-Pb dating of zircon and geochemistry of the granite.Geochemical and petrologic data show that the Tangtang granite is characterized by high silicon（SiO2=72.70%～77.95%）,high alkali（ALK=5.68%～7.32%） and peraluminous nature（ASI=1.41～1.78）,the average of K2O/Na2O ratios is 1.18,and the content of CaO and MgO is very low.Thus,the granite is a typical high-K calc-alkaline S-type granite.It has low REE concentrations and obvious difference between LREE and HREE（LREE/HREE=2.71～4.09）,and its trace elements are different from those of the crust.SHRIMP U-Pb dating of the magmatic zircon yielded the age of 1 063.2±6.9 Ma,which should be regarded as the formation age of the granite,and hence the granite was probably related to the Grenville orogenic event.Based on these geochemical characteristics,this paper deals with the petrogenesis of Tangtang granite.It is suggested that the granite was formed by partial melting of argillaceous rocks in the crust.The pattern of bimodel magmatic rocks and the Al2O3-SiO2 diagram both indicate that the formation of Tangtang granite was closely related to the rift environment.The result is consistent with the tectonic setting of 1.0 Ga in the western margin of the Yangtze platform and also corresponds to the Rodinia supercontinent convergence.