湘南骑田岭岩体菜岭超单元花岗岩侵位年龄和物质来源研究

On the Emplacement Age and Material Sources for the Granites of Cailing Superunit, Qitianling Pluton, South Hunan Province

湘南骑田岭岩体菜岭超单元花岗岩以角闪石黑云母二长花岗岩为主,其全岩Rb-Sr等时线年龄为159±1.2Ma,单颗粒锆石熔融法U-Pb年龄为161±2Ma,Sr同位素初始比值偏低(0.70854),Nd同位素初始比值偏高(-5.4～-5.8),Nd模式年龄偏低(0.94～1.49Ga)。在岩石化学和微量元素方面,属钙碱系列,准铝,富含K、Rb等大离子亲石元素和Th、REE、Nb等高场强元素。常见较多具有岩浆混合特征的微花岗岩类暗色包体。这些同位素、岩石学和微量元素特征,总体上反映了菜岭超单元花岗岩的成岩组成中有明显的地幔物质的参与。该花岗岩可能是在燕山早期华南地壳开始拉张减薄的构造背景下定位的,壳幔相互作用对本区大陆地壳物质的重熔和花岗岩浆的形成和演化起了重要的作用。

The granitic rocks of the Cailing superunit in the Qitianling pluton are mainly horblende-biotite monzonitic granite. Its whole rock Rb-Sr isochron age is 159±1. 2 Ma. The single-crystal melting U-Pb method yields an age of 161±2 Ma. The initial Sr ratio is relatively low (0. 70854 ), the initial Nd ratios are relatively high ( - 5. 4- - 5. 8) , and the Nd model ages are relatively young (0. 94-1. 49 Ga). In terms of petrochemistry and trace elements, the Cailing granites are calc-alkaline, metaluminous, enriched in LIL elements, such as K and Rb, and HFS elements, such as Th, REE and Nb. Mafic microgranitoid enclaves with mixing signature are very common. These isotopic, petrologic and trace element features suggest that there is obvious involvement of mantle materials in the granite constituents, and the Cailing granites were probably emplaced during the Early Yanshanian period under an extensional geotectonic environment. Mantle-crust interaction plays an important role in the melting of continental crust and in the course of generation and evolution of the Cailing granitic magma.