滇西小龙潭矿区始新世岩浆岩的成因及其地质意义

云南省宾川县小龙潭矿区是一个大型斑岩型铜多金属矿床,铜矿体主要产于斑岩体及其外围接触带,矿化与斑岩体密切相关。对矿区主要侵入岩开展全岩地球化学、锆石LA-ICPMS U-Pb年代学和Hf同位素研究,获得含矿二长斑岩的~(206)Pb/~(238)U加权平均年龄为(35.98±0.16)Ma,属于始新世岩浆活动的产物。斑岩体SiO_2质量分数变化较大(64.81%~69.6%),高钾(4.36%~10.95%)和碱(Na_2O+K_2O8%),A/CNK为0.99~1.36,属高钾的碱性—过碱性、准铝质—过铝质斑岩;富集大离子亲石元素(K、Rb、Ba、Sr等)、亏损高场强元素(Ta、Nb、P、Ti等),富集轻稀土元素、亏损重稀土元素;虽被划入富碱侵入岩及A型花岗岩,但具C型埃达克岩地球化学特征。斑岩体锆石ε_(Hf)(t)值为-26.93~1.66,地壳模式年龄为1 009~4141 Ma,显示斑岩体来源于下地壳物质的部分熔融,并有幔源物质的加入。结合区域演化特征,认为小龙潭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弛,岩浆沿断裂及次级断裂上侵,进而形成铜多金属矿床。

Petrogenesis and geological significance of the Eocene porphyry in the Xiaolongtan mining area of the western Yunnan,China

The copper mineralization of Xiaolongtan porphyry type copper-polymetallic deposit in Midu County of Yunnan Province is closely related to the porphyry and the ore-bodies mainly occur in the porphyry and its outer hornfels zone.Study of bulk geochemistry,zircon U-Pb dating,and Hf isotopic composition of the porphyry in the mining area reveals that the zircon U-Pb age of porphyry samples is (35.98±0.16) Ma,representing a product of the Eocene magmatic activity.Geochemical characters of the porphyry show large variation in SiO2 (64.81％～69.6％),high in K2O (4.36％ ～ 10.95％) and Na2O+K2O (＞8％),with the ratio of A/CNK being 0.99～1.36,belonging to K-rich alkaline-peralkaline and metaluminous-peraluminous porphyry series.Trace element analysis shows that the porphyry is deplete in Ta,Nb,P,Ti and rich in K,Rb,Ba,Sr,similar to the features of C-adakite.The eHf (t) from the zircon is-26.93～ 1.66 and the model age is 1 009 ～ 4 141 Ma,suggesting that the Eocene porphyry are derived from the partial melting of the lower crust material,and associated with the addition of mantle source material.On the basis of regional geological evolution,it is considered that the porphyry is formed in an extensional setting of post collision.After the compression of continental collision,the stress relaxation leads to the magma intrusion along the faults and secondary faults,and forms the copper polymetallic deposits.