Zircon U-Pb dating shows that tonalites in the Xiaoxinancha Au-Cu ore deposit from NE China were formed at (112±1) Ma. They are geochemically similar to adaktic rocks with high Na2O, Al2O3, Sr (Sr = 369-774 μg/g) and LREE and low Y, Yb and HREE (e.g., La/YbCN = 7.5-17.5) concentrations, and are characterized by moderate radiogenic Sr ((87Sr/86Sr)i = 0.7044-0.7048), Nd (εNd(t) = +0.5- +1.7) and Hf (εHf(t) = +5.4- +8.6, ΔεHf = 2.3-4.2), but radiogenic Pb ((206Pb/204Pb)i = 18.40-18.60; (207Pb/204Pb)i = 15.56-18.60; (208Pb/204Pb)i = 38.22- 38.36) isotopes. Different from adakitic magmas that were produced through partial melting of subducted oceanic slab and/or through crustal assimilation and fractional crystallization (AFC) of basaltic magma, the tonalitic magmas are interpreted as melts derived from partial melting of underplated basaltic crust. The moderate Sr/Y ratios (23-76) and weak HREE fractionation (e.g., (Dy/Yb)CN = 1.1-1.4) suggest that magma derived from the transition of garnet amphibole to amphibole residual phases. The 40Ar-39Ar age of (110.1±1.5) Ma for separated hornblende from Au-Cu mineralization associated with Late-Paleozoic high-Mg diorites is consistent with the zircon U-Pb age of the tonalites, indicating that the regional Au-Cu mineralization was genetically related to the Early Cretaceous magmatism.%锆石U-Pb测年结果表明小西南岔富金铜矿床矿区内的英云闪长岩形成于(112_±1) Ma.该英云闪长岩富Na20和A1203,高Sr、LREE(轻稀土元素),低Y、Yb和HREE(重稀土元素)(如Sr=369～774 μg/g,(La/Yb)cN=7.5～17.5),具有类似于埃达克质岩石的元素地球化学特征.在同位素组成上具中等放射成因Sr(87Sr/86Sr)i=0.7044～0.7048)、Nd (εNd(t)=+0.5～+1.7)和Hf (εHf(t)=+5.4～ +8.6,△εHf=2.3～4.2)同位素,高放射成因Pb (206pb/204pb)i=18.40～18.60; (207pb/204pb)i=15.56～18.60; (208pb/204pb)i=38.22～38.36)同位素的特点.结合区域构造背景以及古太平洋板块俯冲历史,笔者倾向认为该岩体可能是玄武质岩浆底侵下地壳部分熔融形成的,而非板片熔融形成的埃达克质岩浆,岩浆源区为石榴子石角闪岩相向角闪岩相过渡的岩石.该岩体侵位的晚古生代((257±3)Ma)镁铁质围岩的角闪石Ar-Ar坪年龄为(110.1±1.5) Ma,与英云闪长岩的侵位时间一致,暗示晚古生代镁铁质侵入岩经历的热液事件与英云闪长岩的侵位作用存在成因联系,区域金铜矿化可能为早白垩世岩浆热液作用下成矿物质富集而形成.
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