南秦岭富钠长石岩石的岩石学和地球化学研究

钠长石岩是秦岭造山带中的一类特色岩石,主要分布于南秦岭凤-太、山-柞和镇-旬盆地的泥盆纪和志留纪地层中,与秦岭众多大中型铅锌矿、金矿成矿关系密切。钠长石岩呈层状、似层状产于碎屑岩或碳酸盐岩岩系中,与区域地层整合产出,渐变过渡。具有条带状、块状、角砾状构造;XRF分析数据显示钠长石岩化学成分以富Na2O、Al2O3、SiO2等常量元素及Pb、Zn、Au、Ag、Cu等微量元素为特征;富含元素Ba和Fe-Mn-(Co+Cu+Ni)×10分布图投影表明钠长石岩具有热水沉积的特征。而TiO2与Al2O3关系图解又反映出钠长石岩中有陆源碎屑物混入的特点,其Ti、Al可能来自水成沉积物中的粘土;ICP-MS分析所得稀土元素数据显示,钠长石岩与地层稀土元素在稀土总量、轻重稀土量和配分模式均十分相似,其较高∑REE就是成岩过程中水成沉积物混入造成的。富钠长石岩石在形成时间上发生在沉积和早期成岩阶段,与正常沉积岩同时形成。兼具有正常沉积岩石和热水沉积岩石的特点,是一种有别于二者的混合型非典型热水沉积岩石。研究认为钠长石岩是由一种富钠和金属元素的热水通过对地层元素的萃取并与海底未固结沉积物混合而成的混合热液经沉积成岩作用形成的。

Petrology and geochemistry of albite-rich rocks in South Qinling

Albitite, a special kind of sedimentary rock in South Qinling, is mainly distributed in Devonian and Silurian strata of Fengxian-Taibai, Shanyang-Zhashui and Zhen''an-Xunyang basins. It is closely related to many large-middle lead-zinc and gold deposits in the South Qinling orogenic belt. Occurring in clastic rocks or carbonate rocks in the stratified or stratoid form, albitite shows a conformable contact and gradual relationship with the strata and has thick-bedded, massive and brecciated structures. XRF dating of albitite shows that it is enriched in primary elements such as Na2O, A12O3 and SiO2 and trace elements such as Pb, Zn, Au, Ag and Cu. The enrichment of element Ba and the Fe-Mn-(Co+Cu+Ni)×10 projection indicate a hydrothermal sedimentary origin. However, the diagram of TiO2-Al2O3 suggests it also has some characteristics of terrigenous deposit. Ti and Al probably came from the clay matter of clastic sediments. The REE data by ICP-MS show that albitite and strata have similar total REE, HREE and LREE. The chondrite-normalized REE patterns of albitite and regional strata are quite similar, caused probably by the mixture of hydrogenous sediments. Albitite was formed at the sedimentary or early diagenetic stage, simultaneity with the formation of normal sedimentary rocks. Therefore it is a special kind of hydrothermal sedimentary rock, having characteristics of both marine sedimentary rock and typical hydrothermal sedimentary rock. The albitite is of sedimentary lithogenesis of mixed hydrothermal solution with enriched Na and metallic elements by extracting ore-forming elements from the strata and mixing with non-consolidated sedimentary concretions at the seabed.