陕西洛南县蓟县系洛南群层控玉化硅质岩特征、成因及意义*

华北板块南缘陕西洛南县北部中元古界蓟县系洛南群巡检司组顶部层控玉化硅质岩中,发育似层状硅质玉髓及透镜状燧石,这些燧石及硅质玉髓已经玉化,质细色艳,达到高品级石英岩质玉石级别,已申报为国家宝玉石矿种,并在宝玉石市场崭露头角,显示出巨大的经济开发潜力。作者以岩石学及地球化学分析为手段,研究该层控玉化硅质岩特征、成因及意义。研究表明: (1)层控玉化硅质岩赋存于中元古界蓟县系洛南群巡检司组顶部的紫红色、灰绿色薄层状砂泥质板岩层中,层厚一般0.5~4 m;似层状玉髓多呈紫红色、灰绿色、烟灰色和无色等,透镜状燧石多呈紫红色及灰绿色似玛瑙纹圈层状。(2)硅质玉髓主要成分为石英,隐晶、微晶结构,块状构造,质地细腻、坚硬,玻璃光泽,半透明—微透明;其Al/(Al+Fe+Mn)均值为0.62,Zr/Hf均值为34.83,Sr/Ba均值除1个异常点外为0.50,V/Ni均值为0.37,ΣREE均值为5.11,δCe均值为1.02,δEu均值为0.86,La_N/Ce_N均值为0.85,总体显示淡水沉积环境,硅质来源主要为陆源来源,也有后期热液作用来源的部分加入。(3)似层状硅质玉髓层是由于快速海退引起的淡水淋滤硅化作用形成的海底硬底构造或硅质壳;似玛瑙纹透镜状燧石及彩色似层状硅质玉髓层是由于沉积期及沉积期后成岩作用氧化还原条件变化,及后期热液侵入烘烤引起的紫色与绿色致色矿物或致色元素转变形成;角砾状燧石是由于后期秦岭强烈构造作用,引起似层状硅质玉髓上下岩层因软硬差异而产生的变形破裂作用形成的构造角砾岩。

Characteristics,origin and significance of strata-bound chalcedonization siliceous rocks of the Jixianian Luonan Group in Luonan Country,Shaanxi Province

Quasi-lamellar siliceous chalcedonies and lenticular flints developed at the top of the Xunjiansi Formation of Mesoproterozoic Luonan Group of the Jixian System,in northern Luonan of Shaanxi Province,southern margin of the North China Plate. The siliceous chalcedonies and flints have become jade of gay colors and fine texture,with high-grade quartzite jade level,which are claimed as the national gemstone minerals,budding in the gemstone market,with great economic development potential. At present,no research has been conducted on the characteristics,origins and diversity of jade in this siliceous layer. This paper used petrological and geochemical analyses to study the siliceous layer and grained the following findings. (1)The quasi-lamellar siliceous chalcedonies and lenticular flints were found in the fuchsia and sage green lamellar arenaceous pelitic slate at the bottom of the Xunjiansi Formation,with a thickness of 0.5~4 m;the quasi-lamellar chalcedonies are mostly fuchsia,sage green,smoke gray and colorless;most of the lenticular flints are ring-shaped with agate lines in fuchsia and sage green,and part of lenticular flints are brecciated and in sage green. (2)The main components of the silica chalcedonies are quartz;they have crypto-to microcrystalline or blocky structures,fine and hard texture,glassy luster,translucent-slightly transparent. Based on the geochemical analysis of Al/(Al+Fe+Mn)(0.62 on average),Zr/Hf(34.83 on average),Sr/Ba(0.50 on average,except an abnormal sample),V/Ni(0.37 on average),ΣREE(5.11 on average),δCe(1.02 on average),δEu(0.86 on average),and La_N/Ce_N(0.85 on average),the results suggest that the depositional environment is freshwater settings,and the silicon is mainly terrigenous origin. (3)The quasi-lamellar siliceous chalcedony layer in the Luoguan Group of the Jixian System is a hardgroud surface of seabeds or a siliceous crust formed by fresh-water leaching and siliceous metasomatism caused by rapid regression. Lenticular flints with agate lines and colorful quasi-lamellar siliceous chalcedonies formed during diagenesis under changes of redox conditions during deposition and post-deposition. Transformation of purple and green coloring trace element was caused by late hydrothermal invasion and baking. Brecciated flints resulted from tectonic brecciation of structural breakup zone formed by the soft and hard difference and crack deformation between the upper and lower strata of the quasi-lamellar siliceous chalcedony layer due to the strong tectonic movements of the Qinling Mountains.