云南九顶山正长斑岩年代学、岩石地球化学及Sr-Nd-Hf同位素特征

Geochronological Dating, Geochemical and Sr-Nd-Hf Isotopes of Cenozoic Syenite Porphyries in Jiudingshan, Yunnan: Constrains on its Petrogenesis

九顶山复式岩体沿北西向金沙江–红河断裂与南北向程海断裂交汇处发育,处于南北地洼区与滇西地洼区接触带,是滇西地洼期斑岩成矿带中段的代表性富碱斑岩体之一。该岩体由呈岩株、岩脉、岩墙或岩床等产出的斑状花岗岩、正长斑岩、（二长）花岗斑岩、碱长花岗斑岩和煌斑岩等组成。本文着重对正长斑岩的形成年代、岩石地球化学和成因特征开展研究,结果表明：正长斑岩的LA-ICP-MS锆石U-Pb年龄为34.6±0.7 Ma,即岩浆侵位于始新世（E2）,属滇西地洼激烈期及新生代富碱岩浆活动高峰期（45~30 Ma）的产物;岩石具高硅（Si O2〉56%）、高钾（K2O=3.38%~8.92%,K2O/Na2O〉1）、富碱（ALK=8.15%~11.15%）和低Mg O（〈3%）的特征,属钾玄岩系列–高钾钙碱性系列过铝质（A/CNK=0.71~1.22）花岗岩;在微量元素组成上,岩石高Sr（〉400×10-6）、低Y（〈18×10-6）和Yb（〈1.9×10-6）,与陆内造山环境形成的＂C型＂钾质埃达克岩地球化学特征类似;全岩的Sr-Nd-Hf同位素组成特征显示岩浆源区是壳幔物质混合的＂EMII型＂富集地幔源。正长斑岩的形成应与印度–欧亚板块碰撞俯冲背景下,金沙江–红河断裂大规模左行走滑引起的热扰动和局部引张作用有关,在这种区域热–动力学条件下,地幔部分熔融与地壳物质发生混合作用,形成活化型壳幔混合源高钾富碱岩浆。

The Jiudingshan granite complex in the western Yunnan province is located at the intersection of the NW trending Jinshajiang-Red River fault and NS trending Chenghai fault, where the South-East China diwa region joins western Yunnan diwa region. The complex consists of porphyritic granite, syenite porphyry, granite porphyry, alkali-feldspar granite porphyry and lamprophyre, occurring as stock, dike, dyke or sill. To constrain its petrogenesis, we carried out geochronological and geochemical studies on the Jiudingshan complex. The results show the syenite granite was formed at 34.6±0.7 Ma, which is consistent with the Eocene diwa stage and Cenozoic alkali-rich magmatic activity（45–30 Ma） in West Yunnan. The syenite porphyry contains Si O256%, K2O=3.38%-8.92%, ALK= 8.15%–11.15%, and Mg O3%, showing characteristics of peraluminous（A/CNK=0.71–1.22）, high-K cal-alkaline to shoshonitic. The syenite has high Sr（400×10^-6）, and low Y（18×10^-6） and Yb（1.9×10^-6） contents, which is the similar to those of the ＂C-Type＂ adakite. The Sr-Nd-Hf isotopes indicate that the magma was resulted from crust-mantle（EMII-type） interaction. All the evidence listed above suggests that the left-slip shear extrusion of the Jinshajiang-Red River fault, promoted high felsic magma emplacement, as a result of the collision-subduction between India palte and Asia plate.