信江盆地石炭纪硅质岩地球化学特征及沉积环境意义

在信江盆地中存在数层和石炭纪海相火山岩及其海底块状硫化物矿层相伴生,与石炭纪地层整合产出的层状硅质岩。由对硅质岩常量元素、微量元素、稀土元素、硅和氧同位素等地球化学特征研究表明,本区硅质岩具有一定的热水沉积硅质岩地球化学特征。在Al-Fe-Mn和Fe-Mn-(Ni+Co+Cu)三角图上,本区硅质岩属热水沉积硅质岩。由硅质岩MnO/TiO₂比值、δCe值和δ³⁰Si值分析表明,信江盆地石炭纪硅质岩的沉积环境主要为浅海。

THE GEOCHEMICAL CHARACTERISTICS OF CHERTS IN THE CARBONIFEROUS PERIOD AND THEIR SENDIMENTARY ENVIRONMENT IMPLICATIONS IN XINJIANG BASIN

There are bedded cherts associated with sea volcanic rocks and massive sulfide orebeds in sea floor of Carboniferous period in Xinjiang Basin.These bedded cherts are conformity with the Carboniferous system and they have a same occurrence.The volcanic rocks are mainly rhyolotes,dacites and volcanic tuffs.All the volcanic rocks have been argillized.The straum of carbonate rocks has a clear sedimentary stratification.The cherts are chiefly scattered the neighbouring of the Dongxiang-Yongping fault which is the boundary fault in Xinjiang basin.The thickness of chert sequence varies from several to 20m.The colors of cherts are grey brown and black.There are sedomentary stratifications in these bedded cherts.Quartz and flint are the main mincrals.The contents of quartz and flint are over 85%.The cherts have comparatively high contents of SiO₂,TiO₂,Al₂O₃,Fe₂O₃,FeO,MnO,MgO,CaO,Na₂O,K₂O and P₂O5.They are typical hydrothermal cherts in the Al Fe Mn triangle diagram.And twenty projection spots of cherts in this area are all situated in the area of hydrothermal cherts.The cherts have high contents of As,Sb,Bi,Ga,U.And the most U/Th values of cherts are over 1.They have trace element characteristics of hydrothermal sediment geochemistry.The cherts in this area are also typical hydrothermal ones in the Fe-Mn-(Ni+Co+Cu) trangle diagram.These cherts can also be identified as hydrothermal ones by the following REEgeochemical evidence: First,they have low total contents of REE,ranging from 10.88μg/g to 25.96μg/g.Second,the LREE/HREE ratioes range from 1.07 to 1.66 with comparatively enrichment in HREE.Third,there is slight negative δCe anomaly,and the δCe values range from 0.75 to 0.94 and the averge value is 0.84.Meanwhile,the cherts in this area have the curves in which Ce is a slight negative anomaly and HREE tilts silightly left in the REE distribution pattern for normalized cherts by the North America shale.The δ³⁰Si values of cherts in this area range from 0.5‰ to 0.8‰.The δ¹⁸O values range from 12.5‰ to 21.4‰.And the δ¹⁸O of the cherts shows that the temperatures for chert deposition changed from 82℃ to 170℃.They have all ofcertain characteristics of hydrothermal sediment cherts.Finally,the ratioes of MnO/TiO₂,the values of δCe and δ³⁰Si show that the sedimentary environment of the cherts in Xinjiang basin was chiefly the epeiric sea in continental margin.