地幔流体在滇西富碱斑岩成岩成矿过程中的作用——地质年代学和同位素地球化学制约

滇西金沙江哀牢山断裂以东广泛发育富碱斑岩型多金属矿床,其富碱斑岩体中有多处产出深源岩石包体。对包体岩石和寄主富碱斑岩及其成矿石英脉的地质年代学研究显示,深源岩石包体的成岩年龄大于寄主富碱斑岩,而富碱斑岩的成岩与成矿是基本同时的。结合铅硅锶钕同位素地球化学研究表明,在富碱岩浆的成岩过程中,伴随富硅成矿流体对围岩和岩体的交代蚀变,并与地壳岩石一定程度混染而实现成矿作用。这种富硅成矿流体作用实质上是地幔流体作用在地壳中成矿作用的延续。据此,从地幔流体交代矿物的结晶年龄(116.0 Ma)到富硅成矿流体年龄(51.2 Ma),揭示地幔流体作用贯穿于富碱岩浆成岩成矿的全过程。正是这一地幔流体作用过程,导致Si、Al、Na、K及其它硅不相容元素和成矿元素富集,进而导致其Sr-Nd同位素特征由亏损地幔向富集地幔过渡,并引起从岩体→围岩对应、从高温→低温的系列成矿效应。也正是这种流体作用,构成滇西新生代广泛成矿的内在统一制约因素和大型-超大型矿床形成的重要地球化学背景。

Mantle Fluid's Action in the Processes of Petrogenesis of Alkalic Porphyry and Associated Mineralizations,Western Yunnan,China-Constraint of Geochronology and Isotope Geochemistry

There are well-developed alkali-rich porphyry polymetallic deposits in the eastern Jinshajiang-Ailaoshan fault,western Yunnan.The alkali-rich porphyries in the region have been disco-(vered) containing deep xenoliths.The studies on the geochronology for deep xenoliths,their host alkalic porphyries and mineralizing quartz viens show that the diagenetic age of the deep xenoliths are older than that of their host alkalic porphyry,and the alkalic porphyry's diagenesis and associated mineralization are almost same in age.Combined with the studies of Pb-Si-Sr-Nd(isotopic) geochemistry,it is indicated that during the period of alkalic magma diagenisis the alkalic porphyry and its country rocks were suffered metasomatic alteration by the silicon-rich mineralizing fluids which were contaminated with crustal rocks,meanwhile,the ore deposits were formed.The mineralizing process of the silicon-rich fluids is essentially a continuation of the mantle fluid metasomatism during mineralizing processes in the crust.The mineral crystalline age(116.0 Ma) of the mantle fluid metasomatism and the age(51.2 Ma) of the silicon-rich mineralizing fluid suggest that the mantle fluid had been active through the whole processe of the alkalic magma diagenism and associated mineralization.The mantle fluid resulted in the concentrations of Si,Al,Na,K,Si-incompatible elements and ore elements,and the transition of Sr-Nd(isotopic) geochemistry from depleted mantle to enriched mantle,and the high temperature mineralization in the porphyry and the low temperature mineralization in the country rocks.The action of the mantle fluid is not only an internal constraint to wide spread Cenozoic mineralization but also an important geochemical background for the formation of large and super-large deposits in the western Yunnan,China.