中国东部与玄武岩有关的锆石巨晶的宝石学及地球化学特征

Gemmological and Geochemical Characteristic of Zircon Megacryst Related to Cenozoic Alkaline Basalt from Eastern China

本文系统的统计了近年来中国东部与碱性玄武岩有关的锆石巨晶的宝石学、矿物学特征以及地球化学特征。锆石的宝石学特征表明其粒度一般在3~10mm之间,发育单形以四方柱和四方单锥为主,且常见熔蚀坑,颜色以红褐色为主,玻璃光泽—油脂光泽,荧光相对较弱,包裹体主要包括富CO_2气液包裹体、锆石等矿物包裹体以及色带等非物质型包裹体;紫外-可见光谱以及热处理结果表明,其颜色主要与色心和变价元素有关;拉曼光谱显示锆石巨晶为高型锆石,并且具有明显的异向性;通过详细的对比,尚未发现不同矿床之间锆石巨晶的宝石学特征差异。利用锆石Ti-in-zircon地质温度计,同时对TiO_2和SiO_2活度以及压力进行校正后,结果显示锆石巨晶的结晶温度应在700℃以上;结合单斜辉石巨晶以及玄武岩包体,锆石结晶深度为上地幔,其形成环境可能与二辉辉石岩和二辉橄榄岩接近;锆石Ce明显正异常、赤铁矿等包裹体等证据显示锆石结晶于氧化环境中;锆石的Hf-O同位素表明锆石结晶于地幔环境;结合锆石地球化学数据以及所处的大地构造背景,锆石结晶于富硅、富碱、富碳的熔体中,属于岩石圈减薄进入衰退期的产物,是岩石圈趋于稳定的标志之一,而西太平洋板块的俯冲是引起东部地幔岩石圈减薄、碳循环以及富硅熔体存在的主要动力。

Gemmological/mineralogical characteristics and geochemical characteristics of zircon megacrysts related to Cenozoic alkaline basalt from eastern China are systematically collected and reanalyzed.These zircon megacrysts those common contain CO_2-rich gas-liquid and mineral inclusions(zircon,apatite,etc)with flat ribbon are generally reddish brown,glassy-oily lustre,weak fluorescence and most of them are 3~10 mm.The tetragonal dipyramid and prism constitute the most developed crystal forms.The UV-Vis-NIR and the results of heating treatment show that the mechanism of coloration should be closely related tolattice defect caused by radioactive elements.Valence elements and roman spectral features show that these zircon megacrysts are high-type zircon.However,there is no differences of gemmological characteristic of these zircons from different deposits.According to Ti-in-zircon geothermometer after correcting the crystallization pressure and the activity of TiO_2 and SiO_2,the crystallization temperatures are above 700℃.Combination of formation conditions of clinopyroxene megacrysts and basaltic xenoliths,the zircon megacrysts should crystallize in upper mantle.Ce positive anomaly and hematite inclusions indicate that zircon megacrysts crystalize in the oxidizing environment.Hf-O isotope composition show that the crystallization of zircon megacrysts should be related to the mantle environment.Combined with the zircon geochemical data and the tectonic background,the zircon megacrysts should be resulted from retrogression of lithospheric thinning and crystallize from SiO_2-rich and CO_2-rich melt condition.The crystallization of zircon megacrysts should be considered as a symbol of the lithosphere tending to be stable,and the subduction of western Pacific plate is the mainly geodynamic of lithosphere thinning,carbon cycling and silica rich melt existing in lithosphere mantle of eastern China.