轻矿物物源分析研究进展

Development on provenance analysis of light minerals

文章对Dickinson图解、石英阴极发光、石英氧同位素在物源分析方面的研究进展进行了综述,探讨目前存在的问题和未来的发展趋势。Dickinson图解的使用必须建立在Gazzi-Dickinson计数法、三级流域采样、杂基含量<25%、没有遭受强烈风化作用、多图解配合使用等前提条件下,同时存在粒度分异、岩屑分类简单、成岩蚀变、不同构造环境的时空过渡与重叠、统计误差等一系列问题。除阴极发光为深红色的变质岩石英可区分外,火山岩与侵入岩石英、变质岩与岩浆岩石英、侵入岩与热液石英的阴极发光光谱均存在重叠,其结果并不可靠。与扫描电镜(SEM)结合,进行CL/SEM组构分析,并与传统显微光学特征对比,可识别不同成因类型的石英。通过石英氧同位素,可区分沉积自生石英与岩浆石英,而变质石英形成条件复杂,其氧同位素变化大,与岩浆石英和沉积自生石英的氧同位素均存在交集。碎屑岩成分受源岩、化学风化、机械磨蚀、水动力分选、自生矿物和成岩作用等因素的影响,反演源岩成分需要排除其它因素的干扰。利用现代环境进行各影响因素的正演分析,同时结合其它方法(如碎屑锆石U-Pb年龄、Nd同位素等)的优势,才能全面、准确的解释物源。

This article reviews provenance development on Dickinson diagrams,quartz cathodeluminescence,quartz δ18O,and discusses their existing problems and future tendency. Dickinson diagrams are used based on conditions including Gazzi-Dickinson point-counting method,sampling in third order drainage, 25% matrix,not strongly weathered,all four diagrams considered and so on. And some problems arise like grain size differentiation,less lithic types,diagenesis,source inheritance and transformation in different tectonic setting,statistical errors,thus much attention should be paid as using. As far as cathodeluminescence（ CL） image is concerned,metamorphic quartz with CL dark red is well recognized,nevertheless CL color overlaps between volcanic and plutonic quartz,metamorphic and igneous quartz,plutonic and hydrothermal quartz,leading to unreliable interpretation. Instead of CL color analysis,combination of CL-SEM（ scanning electron microscope） on textural features with optical microscopy allows reliable distinction of quartz varieties. The last is about oxygen isotope of quartz（ i. e. δ18O）. Though chert and igneous quartz could be distinguished from each other by δ18O,metamorphic quartz is formed so complicatedly that its δ18O shows great variation and shares some common values with both δ18O of igneous quartz and chert. Clastic composition is controlled by source rocks,modification of chemical weathering,mechanical disaggregation,hydrodynamic sorting,authigenic inputs and diagenesis,therefore source rocks recovery necessitates exclusion of other influencing factors which could be modelled on modern environment by a priori,and integration of other useful methods（ e. g. detrital zircon U-Pb ages,ε Nd etc.） for exact provenance interpretation.