云南武定左所辉绿岩铂族元素地球化学、Pb同位素及其地质意义

摘要

古元古代晚期至中元古代早期,西南康滇地区发生过一次强烈的构造、岩浆及成矿事件,即昆阳裂谷,以广泛出露1.8～1.5 Ga基性岩浆岩及铁铜矿床为重要标志.文章报道了滇中武定地区左所辉绿岩的岩石化学、微量元素、Pb同位素及铂族元素地球化学特征,通过铂族元素地球化学对昆阳裂谷期的岩石圈演化、壳幔相互作用进行了探讨.左所辉绿岩相对富碱(ω(Na2O+ K2O)平均为3.55％),低ω(SiO2),较高的ω(TiO2)(均值为3.30％)、ω(P2O5)(平均值0.35％),CaO/TiO2比值(平均值为2.4)及Al2O3/TiO2比值(平均值为3.4)低,具有典型地幔热柱玄武岩的岩石化学特征.左所辉绿岩微量元素中相对富集大离子半径元素,高场强元素弱亏损,稀土元素中出现弱的Eu负异常,反映了斜长石的轻度结晶分异.辉绿岩具有较高的铂族元素含量,PPGE和IPGE明显分异,PGE原始地幔标准化配分曲线呈较明显的左陡倾型.辉绿岩的Cu/Pd比值均低于原始地幔值,表明原始岩浆S不饱和,亦未发生硫化物的熔离过程,较高的PGE含量可能与初始岩浆本身的S含量较高有关.高的Pd/Ir比值可能与IPGE在部分熔融过程中更倾向于残留相以及岩浆演化早期铬铁矿和橄榄石的结晶分异有关.低程度的地幔部分熔融、Pb同位素特征暗示左所地区辉绿岩的形成过程:陆内裂谷环境下,富硫的原始岩浆在上升过程中受到较少下地壳物质的混染,侵位至浅表并较快冷却,形成辉绿岩.%Late Paleoproterozoic and Early Neoproterozoic Kunyang rift was an intense tectonic,magmatic and metallogenic event,which was characterized by the formation of plenty of 1.8 ～ 1.5 Ga basic rocks and Fe-Cu de-posits in Kangdian area,Southwest China.Much research work has been carried out on the petrogeochemistry and geochemistry of mineral deposits for investigating geological background and mineralization of Kunyan rift.In this paper,the characteristics of petrochemistry,trace elements,Pb isotope and platinum group elements (PGE) of diabases at Zuosuo in central Kangdian area are reported to discuss the lithospheric evolution and interaction of mantle and crust through PGE geochemistry.The diabases have the typical chemical characteristics of mantle plume basalts,such as rich alkali (the average of ω(Na2O + K2O) is 3.55 ％),low ω (SiO2),high ω(TiO2) (averagely 3.30％) and ω (P2O5) (averagely 0.35％),and low ratios of CaO/TiO2 (2.4％ on average) and Al2O3/TiO2(3.4 on average).The Zuosuo diabases are relatively enriched in large ion radius elements,with weak depletion of high field strength elements and weak negative Eu anomalies in REEs.The diabases have high content of platinum group elements with obvious differentiation of PPGE (Pt,Pd and Rh) and IPGE (Os,Ir and Ru),and the PGE primitive mantle-normalized patterns exhibit obvious left steep curve.Cu/Pd ratios of diabases are lower than those of primary mantle,indicating that the primary magma was S-unsaturated magma without PGE-enriehed sulfide separation.The high Pd/Ir ratio of diabases may be related to that IPGEs are more likely to be hosted in residual phase in partial melting,and crystallization differentiation of chromite and olivine for the primary magma in the early stage.Low partial melting of upper mantle and Pb isotopes indicate the formation process of the basite in Zuosuo:In the environment of inner continental rift,original basic S-rich magma was contaminated by lower crust materials during the rising process,and was then emplaced near the surface of the crust,where it became cool quickly,thus forming the diabases.