滇西分水岭矿区始新世岩浆岩成因及其构造指示:地球化学、锆石U-Pb年代学及Hf同位素制约

云南省永胜县分水岭矿区是近年发现的斑岩型铜多金属矿床。铜矿化多产于斑岩体中部外侧(喇叭箐断裂两侧)及部分岩枝的节理裂隙中,矿化与斑岩体密切相关。矿区主要侵入岩锆石LA-ICP-MS U-Pb年代学研究获得含矿石英二长斑岩的206Pb/238U加权平均年龄为36.0±0.2Ma,表明分水岭矿区斑岩体为古近纪始新世岩浆活动的产物。全岩地球化学研究显示,斑岩体Si O2含量变化较大(53.7%~71.06%),高钾(1.67%~6.1%)和碱(Na2O+K2O平均值8%),A/CNK为0.35~1.03,属高钾的钙碱性-碱性、准铝质-过铝质斑岩;富集大离子亲石元素(Rb、K、La、Nd、Hf)、亏损高场强元素(Ta、Nb、P、Ti),富集轻稀土元素,亏损重稀土元素,具A2型花岗岩特征。斑岩体锆石Hf同位素研究,得到εHf(t)值为-8.70~+4.67,地壳模式年龄(tDMC)为533.7Ma~1062.8Ma,显示矿区始新世斑岩体来源于壳幔岩浆的混合。结合区域演化特征,认为分水岭斑岩体形成于造山期后的拉张环境,陆陆碰撞挤压后应力松驰,岩浆沿断裂及次级断裂上侵,进而形成铜多金属矿床。

Petrogenesis and tectonic indicators of the Eocene porphyry in the Fenshuiling mining area, western Yunnan Province: Constraints from bulk geochemistry, zircon U-Pb geochronology and Hf isotopes

The Fenshuiling mine is a large porphyry type copper-polymetallic deposit in Yongsheng County of Yunnan Province. The copper mineral-ization of this district is closely related to the porphyry, and orebodies mainly occur in the porphyry and outer hornfels zone. This work studied the bulk ge-ochemistry, zircon U-Pb dating and Hf isotopes of the porphyry from this mining area. The zircon U-Pb age of porphyry samples is 36. 0 ± 0. 2 Ma, in-dicating that the porphyry of this mine area is the product of the Eocene magmatic activity. Geochemical characters of the porphyry show large variation in SiO2(53.7% ~71.06%), high in K2O(1.67% ~6.1%) and Na2O+K2O( >8%), and the ratio of A/CNK is 0.35~1.03, and the porphyry be-longs to the K-rich calcined alkaline-alkaline and metaluminous-peraluminous alkalic series. Although the porphyry deposits are classified into alkali-rich intrusive rocks and A-type granite, microelements show that the porphyry is depleted in Ta, Nb, P, Ti and rich in Rb, K, La, Nd, revealing some C-adakite features. Thevalues ofεHf(t)from the zircon change from -8.70 to +4.67 and the values of tDM Cvary from 533Ma to 1062Ma,sug-gesting the Eocene porphyries were derived from the partial melting of the lower crust and associated with the addition of mantle source material. Combined with the regional geological evolution history, we suggest that the porphyry was formed in an extensional setting of the post-collision period. After the compression of continental collision, the stress relaxation led to the magma intrusion along faults and secondary faults,concomitantly forming copper poly-metallic deposits.