滇西北羊拉铜矿区路农岩体矿物学、地球化学特征及其地质意义

路农岩体位于滇西北羊拉铜矿区南部，岩体与围岩接触带上广泛发育层状矽卡岩型矿体，而斑岩型矿化较弱。其斑岩型铜矿化的潜力如何是羊拉铜矿床研究与勘查工作的难点之一，路农岩体岩石成因及源区认识存在不足。为此，本文选取路农岩体中新发现的暗色包体和寄主岩石花岗闪长岩进行了岩石地球化学及矿物学研究。结果表明：路农岩体暗色包体显示高镁闪长岩特征（SiO₂=53.83%~63.91%、MgO=2.30%~4.46%），寄主岩石属于I型花岗岩，属于准铝质-高钾钙碱性系列。二者具经典弧岩浆岩特征，富集Rb、K、Pb、Th、U等大离子亲石元素，亏损Nb、Ta、Ti、P等高场强元素，与里农岩体具有相似的稀土元素配分模式，均为右倾型，但二者稀土元素总量差异较大（∑REE=114.8×10^-6~261.6×10^-6），寄主岩石轻重稀土分馏更为明显（LREE/HREE=3.81~13.96），而暗色包体显示更强的负Eu异常（δEu=0.31~0.47）特征。结合岩体侵位时间（238~230Ma，锆石U-Pb年龄），认为路农岩体可能形成于后碰撞或碰撞晚期的背景下，板片断离造成软流圈物质上涌，携带少量幔源熔浆底侵于下地壳，诱发部分熔融形成长英质熔浆，二者在深部岩浆房混合形成混合熔浆，向上侵位至浅部地壳，经过结晶分异形成路农花岗闪长岩，包裹的幔源镁铁质岩浆残余体低度分异形成暗色包体。角闪石和黑云母的矿物化学特征指示路农岩体侵位较深（4.35~7.24km）、压力较大（121~178MPa）、岩浆温度（777~836℃）较高，氧逸度（logf_O2=-13.84~-12.77）相对较低，不利于形成大规模斑岩型铜矿化，因此路农矿段斑岩型矿化不发育。

Mineralogical and geochemical characteristics of the Lunong intrusion from the Yangla ore district in Northwest Yunnan Province and their geological implications

The Lunong intrusion is located in the southern part of Yangla copper ore district in Northwest Yunnan Province. The layered skarn ore bodies are widely distributed in the contact zone between the Lunong granodirite and wall rocks, however, the porphyry type mineralization is extremely weak. The origin and evolution of the Lunong intrusion is still unclear, at the same time, the potential porphyry mineralization in Yangla desposit is also a key issue for the further mineral resources exploration. In this paper, we collected the samples from Lunong granodiorite and the newly discovered mafic microgranular enclaves (MMEs) to analyze their petrology, mineralogy and geochemistry. The testing results indicate that the MMEs share the same characteristics of high Mg diorite (SiO2=53.83%~63.91%, MgO=2.30%~4.46%), and the granodiorite belongs to metaluminous Ⅰ-type granite. The granodirite and its MMEs are rich in large ion lithophile elements such as Rb, K, Pb, Th, U and strongly depleted of of high field strength elements such as Nb, Ta, Ti, and P; both of them belong to high-K calc-alkaline series and classic arc magmatic rock. The Lunong granodiorite has similar rare earth element pattern to other intrusions in the Yangla ore district (e.g. Linong intrusion), but the Lunong granodiorite and MMEs display some significantly different features, including the total REE contents, LREE/HREE ratio and δEu values, indicating their different origin. Based on the granodiorite emplacement age (238~230Ma, zircon U-Pb dating) and the corresponding tectonic setting (a post-collisional or late collisional setting), we propose that the Lunong granodiorite were generated by fractional crystallization of the mixed magmas, which formed by mixing mafic mantle-derived magmas and felsic crustal magmas in deep magma chamber, as a result of slab breaking off; and the MMEs were formed by the residual mantle-derived magmas. The mineral chemical compositions of hornblende and biotite indicate that the Lunong intrusion was emplaced under the physicochemical condition of deep depth (4.35~7.24km), high pressure (121~178MPa), high temperature (777~836℃) and relatively low oxygen fugacity (logfO2=-13.84~-12.77), which is not conducive to the large-scale porphyry mineralization in the Lunong ore block of Yangla copper deposit.