云南永胜分水岭矿区富碱斑岩地球化学、锆石U-Pb年龄及其地质意义

云南永胜分水岭矿区富碱斑岩体位于扬子地块西缘宾川-程海断裂与箐河断裂交汇部位,属于金沙江–哀牢山富碱斑岩带组成部分,主要由黑云母花岗闪长斑岩(BGDP)和花岗闪长斑岩(GDP)组成。两类岩石均具有富碱(BGDP的K_2O+Na_2O均值为8.22%、GDP的K_2O+Na_2O均值为8.13%)、富钾(BGDP的K_2O/Na_2O均值为1.08,GDP的K_2O/Na_2O均值为1.2)和准铝质(BGDP的A/CNK均值为0.95、GDP的A/CNK均值为0.94)特征,表明二者均属准铝质高钾钙碱性系列的富碱斑岩。两类岩石均富集大离子亲石元素(Rb、Ba、Pb、Sr)、Th、U和LREE,亏损高场强元素(Ta、Nb和Ti),二者稀土元素配分模式均为右倾平滑型,且Eu异常不显著(δEu=0.85~1.12)。地球化学特征表明它们属同源岩浆演化的产物,具有相似的源区特征和成岩动力学背景。花岗闪长斑岩LA-ICP-MS锆石U-Pb年龄为34.5±0.3 Ma(MSWD=1.5,n=19),落在金沙江–哀牢山富碱斑岩带岩浆活动的高峰期内(45~30 Ma)。综合岩石学、地球化学和成岩年代学资料,认为分水岭矿区富碱斑岩形成的动力学条件和源区特征与金沙江–哀牢山富碱斑岩带一致,即形成于印度–欧亚板块碰撞期后动力学性质转换背景下,是加厚下地壳和上地幔部分熔融的产物,与金沙江–哀牢山走滑断裂构造活动密切相关。

Geochemistry and Zircon U-Pb Age of the Alkali-rich Porphyry in the Fenshuiling Ore District,Yongsheng City,Yunnan Province,and its Geological Implications

The alkali-rich porphyry in the Fenshuiling ore district, Yongsheng city, Yunnan province, which located in the intersection zone of the Binchuan-Chenghai fault and Qinghe fault, is an important part of the Jinshajiang- Ailaoshan alkali-rich porphyry belt. The porphyry consists of biotite granodiorite porphyry (BGDP) and granodiorite porphyry (GDP). They are enriched in alkali (the average K2O+Na2O values of BGDP and GDP are 8.22% and 8.13%, respectively) and potassium (the average K2O/Na2O ratios for BGDP and GDP are 1.08 and 1.2, respectively), and metaluminous (the average A/CNK values for BGDP and GDP are 0.95 and 0.94, respectively). These results show they belong to the metaluminous and high-k calc-alkaline porphyry, and the products of the same magma. BGDP and GDP are enriched in LILE (Rb, Ba, Pb, Sr), Th, U and LREE, and depleted in HFSE (Ta, Nb and Ti), with insignificant Eu anomalies (δEu=0.85–1.12). These features of the Fenshuiling porphyry are similar to those of A-type granite. LA-ICP-MS dating of zircon grains from GDP yields a U-Pb age of 34.5±0.3 Ma. It is in the peak ages (45–30 Ma) of the Jinshajiang-Ailaoshan alkali-rich porphyry. The petrological, geochemistry and geochronological results suggest that the origin of the alkali-rich porphyry in the Fenshuiling ore district is similar to that of the Jinshajiang-Ailaoshan alkali-rich porphyry. The porphyries in the belt were likely derived from partial melting of the over-thickened lower crust and the upper mantle under post-collisional setting subsequent to the India-Eurasian plate collision, and are closely related to the activity of the Jinshajiang-Ailaoshan strike-slip fault.