云南宁蒗光马山二长斑岩岩石地球化学及锆石U-Pb年龄

宁蒗光马山二长斑岩位于扬子陆块西缘、金沙江断裂带小金河断裂与宾川-程海断裂夹持部位。通过对宁蒗光马山二长斑岩进行锆石U-Pb定年,获得年龄为33.37±0.26Ma(MSWD=1.7,1σ),表明其形成时代为渐新世早期,与盐源-丽江富碱斑岩带主体侵位时代一致。岩石SiO_2含量为61.55%～69.13%,全碱(Na_2O+K_2O)含量为8.85%～10.92%,N/K值为0.88～1.07,A/NK值为1.54～1.84,A/CNK值为1.23～1.72,属过铝质碱性石英二长斑岩。同时富集轻稀土元素、亏损重稀土元素,高La/Yb值,低Y、Yb和镁值(Mg~#0.5),表明其具有大陆型或钾质C型埃达克岩的特征。该岩石可能形成于区内加厚地壳变泥质岩部分熔融,其起源演化受金沙江-红河走滑断裂系控制,是新生代印度-欧亚板块碰撞后伸展构造背景的产物。

Zircon U-Pb age and geochemistry of the Guangmashan monzonitic porphyry in Ninglang, Yunnan Province

The Guangmashan monzonitic porphyry in Ninglang is located between Xiaojinhe and Binchuan-Chenhai faults in western Yangtze block. In this paper, the authors carried out LA-ICP-MS zircon U-Pb dating for the monzonite porphyry and obtained a weighted mean age of 33.37±0.26Ma, suggesting Oligocene, which is consistent with the main formation age of Lijiang-Beiya alkali-rich porphyritic belt. The monzonite porphyry shows peraluminous alkaline affinities, with SiO₂ content of 61.55%~69.13%, (Na₂O+K₂O) content of 8.85%~10.92%, N/K ratio of 0.88~1.07,A/NK ratio of 1.54~1.84,and A/CNK ratio of 1.23~1.72. It is also enriched in LREE but depleted in HREE, with high ratio La/Yb, low Y and Yb content, and low values of Mg^#(<0.5). The porphyry shows continent-type potassium C-type adakite characteristics, and might have been derived by partial melting of the thickened lower crust caused by Jinshajiang-Red River strike-slip fault system, formed in an extensional setting at post-collisional stage, caused by the collision between the India and Eurasian blocks.