川东茅口组硅质岩地球化学特征及成因

The Origin and Geochemical Characteristics of Maokou Formation Silicalites in the Eastern Sichuan Basin

川东地区中二叠统茅口组硅质岩广泛分布。通过野外剖面的详细研究和硅质岩主量、稀土和同位素的分析,认为茅口组硅质岩为沉积于台盆相中的热水硅质岩,其化学成分以SiO2为主,含量为80.09%～97.91%,富集Fe、Mn等元素,Al/(Al+Fe+Mn)平均值为0.30;REE总量平均值为8.72×10-6,δCe值为0.39～0.81,为负异常,δEu值为0.35～5.85,从Eu负异常到Eu正异常;δ30Si值为0.2‰～1.2‰,与热水来源石英较为接近;根据δ18O计算出硅质岩形成时古海水的温度为34～89℃。热水硅质岩的形成与火山活动和断裂有关,下渗的海水与岩浆热液混合,并被加温,溶解了大量SiO2等有关元素;然后以热泉形式通过断裂喷出,使附近海水中SiO2含量极大提高并沉淀。

Silicalites are distributed widely in the Middle Permian Maokou Formation in the eastern Sichuan basin.Detailed observation in the field and the contents of major elements,REE and isotope of silicalites suggest that the Maokou Formation silicalites were hydrothermal origin and were deposited under platform-basin facies environments.The silicalites are characterized by relatively high contents （80.09%～97.91%） of SiO2 and enrichment of Fe and Mn,with a ratio of Al/（Al＋Fe＋Mn） being 0.30 and average total REE of 8.72×10-6.δCe values range from 0.39 to 0.81 with a negative anomly.δEu of 0.35～5.85,changes from no obvious anomaly to positive anomaly.δ30Si is from 0.2‰ to 1.2‰ that is very similar to that of hydrothermal quartz.δ18O calculation also indicates that the paleotemperatures of the sea water was 34～89℃ during the formation of the silicalites.The formation of hydrothermal silicalites is closely related to volcanic activities and faulting,the mixed fluid of seawater and magmatic fluid was heated and dissolved a large voluminous of SiO2,and other minerals,which erupted along faults in the form of spring and then deposited in adjacent areas.