滇西碧罗雪山花岗岩体地球化学特征及其地质意义

碧罗雪山花岗岩体位于三江成矿带中段的左贡岩浆弧带内,由花岗闪长岩、二长花岗岩、钾长花岗岩和少量暗色包体组成。花岗岩类SiO_2含量67.10%~74.67%,富钾(K_2O/Na_2O=1.32~2.67)和低CaO(0.83%~2.6%),铝过饱和指数A/CNK=1.07~1.26,标准矿物中出现刚玉(1.06~3.38)。以上特征表明,该岩体为强过铝质高钾钙碱性S型花岗岩。稀土元素配分曲线呈右倾型,具弱至中等负Eu异常(δEu=0.38~0.97),相对亏损Nb、Ta、P、Ti等高场强元素,富集Cs、Rb、Th、U、K等大离子亲石元素。研究表明花岗岩为富含长石的砂屑岩部分熔融形成,形成于挤压环境向伸展环境转换的后碰撞阶段,为临沧花岗岩基的北延部分。

Geochemical characteristics of the Biluoxueshan granitoid pluton in western Yunnan and their geological implications

The Biluoxueshan granitoid pluton is located in the Zuogong magmatic arc of middle section of the Sanjiang metallogenic belt in Yunnan. The pluton consists mainly of granodiorites, monzonitic granites, and moyites, and contains a few dark enclaves. The granitoids in this pluton have 67.10%~74.67% SiO2, with high K2O(K2O/Na2O=1.32~2.67) and low CaO (0.83%~2.6%). The ASI (A/CNK) of the granitoids is from 1.07 to 1.26, with corindite numerator(1.06~3.38) appearing in the CIPW normative mineral. These geochemical signatures indicate that the pluton is high-K calc-alkaline peraluminous S-type granites. The granitoids show a LREE enrichment pattern and obviously weak to intermediate negative Eu anomalies. The trace element geochemistry is characterized evidently by negative anomalies of Ta, Nb, Ti, and P and positive anomalies of Cs, Rb, Th, U, and K. The granitoids originated from partial melting of arenite, which enriched in fledspar, in a post-collision stage and formed in a transitional environment from compressive environment to extensional environment, as a northward extension of Licang batholith.