摘要
晚前寒武纪在扬子板块东南大陆边缘发育厚度较大的层状硅质岩层 ,岩石化学成分较单一 ,硅质矿物含量 90 %以上 ,岩石富Fe、Mn ,相对贫Al、Ti、Mg。岩石贫微量元素 ,但富Ba、As、Sb、Ag、U元素。稀土元素总量低 ,Ce呈明显的负异常 ,轻稀土含量大于重稀土含量。δ3 0 Si值变化范围为 0 .0‰～ 0 .7‰ ,δ18O值变化范围为2 0 .1‰～ 2 3.6‰。
Late Precambrian bedded cherts are widely distributed on the southeast continentel margin of the Yangtze Plate. Its thickness ranges from 20m to 150m and chemical composition of them are very pure. A majorarity of them are composed of micritic quartz. The contents of siliceous mineral range from 93.85 percect to 97.08 percent. The average values of siliceous mineral contents exceed 95 percent coupled with relatively high contents of Fe、Mn and low contents of Al、Ti and Mg. The total contents of the trace elements change greatly. The contents of a large part of the trace elements are low compared with crust clarke values of these elements. The cherts are very rich in the trace elements Ba、As、Sb、Ag and U. In order the average contents of these trace elements are differently as 17,5,47,18,3.6 times as these elements corresponding crust clarke values. All these geochemical characteristices state that the siliceous mineral forming the cherts was derived from hot water. The cherts can also be identified as hydrothermal chert by the values of Fe/Ti、(Fe+Mn)/Ti、Al/(Al+Fe+Mn) and U/Th. In sequence these values differently range from 46.1 to 96.4,50.4 to 104,0.24 to 0.32,1.8 to 33.8, with average values distinctly 70.1,75.7,0.29,5.1. In the Al-Fe-Mn,Fe-Mn-(Cu+Ni+Co)×10 triangle diagrams, all samples in the study area fall in the hydrothermal sediment field. The cherts are characterized by low total REE abundance, varying from 5.46×10\+\{-6\} to 52.79×10\+\{-6\}, the average content 19.19×10\+\{-6\}, the content of LREE being higher that of HREE, clearly negative Ce anomaly and positive Eu anomaly. All these characteristics show these cherts were mainly the products of hydrothermal processes. δ\+\{30\}Si values of the cherts are 0.0‰—0.7‰ and δ\+\{18\}O 20.2‰—23.6‰. δ\+\{30\}Si values of the cherts belong to the range of δ\+\{30\}Si values of hydrothermal quartz and are near to δ\+\{30\}Si values of the dissolved silicon from hot springs at Yellowstone, Wyoming and Mammoth, California. δ\+\{18\}O values of the cherts also belong to the range of δ\+\{18\}O values of quartz from hot sinters. Calculation with the oxygen isotope equilibrium fractionation equation of chert-seawater shows formation temperature 66.0℃—89.7℃. All these temperaturs are clearly above to the temperature of plaeoocean when the cherts were formed. According to the values of silicon and oxygen isotopes and formation temperature of the cherts, it is believed that the silica forming quartz was derived from hot water.
出处
《沉积学报》
CAS
CSCD
北大核心
2000年第1期107-113,共7页
Acta Sedimentologica Sinica
基金
国家自然科学基金!资助项目 (批准号 :48970 0 115 )
关键词
硅质岩
地球化学
热水沉积
晚前寒武纪
扬子板块
chert geochemistry hot water sedimentation Late Precambrian Yangtze plate
