湖南铜山岭矿区花岗闪长岩岩石成因:岩石地球化学、U-Pb年代学及Hf同位素制约

PETROGENESIS OF GRANODIORITES IN TONGSHANLING DEPOSIT OF HUNAN PROVINCE:CONSTRAINTS FROM PETROGEOCHEMISTRY,ZIRCON U-Pb CHRONOLOGY AND HF ISOTOPE

铜山岭矿区位于南岭西段,铜铅锌矿体主要围绕Ⅰ号岩体分布。通过岩石地球化学特征研究揭示岩体为准铝质弱过铝质高钾钙碱性花岗闪长岩系列,轻重稀土元素分异强烈,多数投点落入Ⅰ型花岗岩区,少数点落入S型区。微量元素判别图反映构造环境为火山弧和同碰撞环境。岩体La-ICPMS锆石U-Pb年龄(Ⅰ号成岩年龄为166.64 Ma±0.40Ma,Ⅲ号成岩年龄为148.30Ma±0.35 Ma)和Hf同位素特征(Ⅰ号岩体εHf(t)在-15.86～-22.67之间,Ⅲ号岩体εHf(t)在-10.09～-11.00之间);Ⅰ号岩体可能混入新生地壳物质,而Ⅲ号岩体源区仅是壳源。两个岩体的Hf同位素模式年龄也不相同,Ⅰ,Ⅲ号岩体两阶段模式年龄分别为2 220Ma～2 647Ma和1 841Ma～1 898Ma,反映Ⅰ号岩体源区物质时代为太古至早元古代,而Ⅲ号岩体源区时代为早元古代。综上所述,Ⅰ号岩体是以古老地壳物质为主体混入新生地壳物质形成岩浆源区而熔融形成的,形成时代是在166Ma;Ⅲ岩体,是由于150Ma前后早期挤压增厚的下地壳发生熔融而形成的。

Tongshanling Cu-Pb-Zn polymetallic deposit is located in the junction position of Yangtze plate and Cathaysia block,and its formation is related closely with shallow-seated granodiorite. There are three plutons described as Ⅰ , Ⅱ ,Ⅲ pluton in Tongshanling area, and these rocks are mainly biotite granodiorite. Ⅰ and Ⅱ plutons have been studied by previous workers,while there is no detailed study about the Ⅲ pluton as well as the relations among them. Based on above facts,the study of features of Ⅰ and Ⅲ pluton by petrogeochemistry and isotope geochemistry was carried out. Some fresh samples are collected from mining tunnel, the whole rock chemical composition test are obtained. Characteristics of common elements display that these plutons are can be classified as granodiorite. The trace element discrimination diagram shows that tectonic environment of these two plutons is volcanic arc and collision zone. Meanwhile zircon grains are collected from granite samples, and their U-Pb and Hf isotope composition are analyzed by La- ICPMS. Results of U-Pb dating show that the sample collected from I pluton has an age of 166.6 Ma＋0. 40 Ma,while the Ill pluton has an age of 148.3 Ma-F0.35 Ma. Combined them with other ages carried out by previous workers,it implies that there are multi-stage magmatic intrusive activity in this area. According to Hf isotope compositions,the two-stage model ages of the Ⅰ and Ⅲpluton are calculated for 2 220 Ma-2 647 Ma and 1 841 Ma-1 898 Ma,respectively,the values of - Hf（t） are -15.86--22.67 and -10.09--11.00 respectively and the values of - Hf （t） and their variation range are quite different. Combined with the two-stage model ages,it is implied that these two plutons had different source area within different epoch. In summary, Ⅰ pluton is formed by the melting of ancient crustal material mainly mixed with newborn mantle material in 166 Ma-while Ⅲpluton is formed by the melting thickened lower crust in 149 Ma under the extensional tectonic setting.