山西代县金红石矿床中锆石的内部特征和矿物包裹体研究及其地质意义

对山西代县洪塘矿区富含红色和黑色两种金红石的直闪岩进行岩相观察和矿物成分分析,并就其中锆石在阴极发光下的外部形态、内部结构及矿物包裹体进行研究.所分选出的锆石按其颗粒上的相对位置、形态、阴极发光图像、SHRIMP年龄值以及内部矿物包裹体划分为复杂锆石和单成因锆石,其中复杂锆石由核部、幔部和边部锆石三类锆石组成.第1类锆石无环带,部分边缘可见溶蚀痕迹,为继承锆石.内部包裹富锾矿物组合顽火辉石+金云母,与寄主岩石的组合明显不同,反映其可能是寄主岩石形成之前保留于锆石中的原岩矿物并源于地幔.第Ⅱ类锆石为半自形-自形晶,环带较宽且清晰,推测为高级变质锆石.其内矿物包裹体主要为直闪石,其次为钙铁辉石(?),反映其形成于角闪岩相,与寄主岩石的矿物组合可能一致.第Ⅲ类锆石主要位于增生边,阴极发光亮度高,推测为流体改造的变质锆石.第Ⅳ类单成因锆石多呈自形,阴极发光下较暗,生长环带规则且较窄.矿物包裹体有钠长石+石英+白云母,对应中压绿片岩相,其形成温度和压力低于寄主岩石,它与第Ⅲ类锆石可能是在同一期后期事件中形成.结合岩相学研究结果,认为该区金红石矿床之原岩可能是岩浆岩,后期经历了至少两期构造热事件,早期热事件可能导致了锆石内U-Th-Pb体系的重置.以上研究结果表明经历复杂变质作用的变质岩中锆石内部矿物包裹体的研究分析还是探讨寄主岩石成因及所经历地质事件的有效手段.

Study on internal characteristics and mineral inclusions of zircons from the Daixian ruffle deposit, Shanxi and its geological implication

Combined with petrographic observations and mineral compositions, morphologies, inner structures and mineral inclusions in zircons of ores from the Hongtang rutile deposit of Daixian, Shanxi. The zircons are subdivided into complicated zircons with core, mantle and rim and single zircons without overgrowth based on their relative locations, morphologies, CL images, SHRIMP ages and mineral inclusions. Type Ⅰ zircons are considered as inherited zircons, with rounded shape and no obvious zoning, which contain enstatite and phlogopite, showing different mineral association from that of its host rock. They are inferred to be previous minerals preserved in zircons before the formation of its host rock, and likely derived from mantle. Type Ⅱ zircons have broad and clear zoning, and contain anthophyllite and hedenbergite (?), in accordance with mineral associations of the host rock. They are speculated to be high grade metamorphic zircons. Type Ⅲ zircons are mainly rimed and speculated to be fluid-related zircons. Type Ⅳ zircons are mostly euhedral, with low-luminescence of CL and narrow growth zoning. They have albite, quartz and muscovite mineral inclusions, corresponding to middle-pressure green-schist facies. Type Ⅳ and type Ⅲ zircons might form in coeval thermal event. Combined with petrographic study results, it is inferred that the parent rock of this rutile ore deposit may be magmatic rock, and experience at least two thermal events, the earlier ones may result in the reset of system U-Th-Pb in zircons. The results of this investigation also show that study and analysis of mineral inclusions in zircons of high grade metamorphic rocks are an effective means of discussing the genesis and subsequent geological events of the host rock.