扬子板块东南大陆边缘上震旦统热水成因硅质岩的地球化学标志

GEOCHEMICAL CRITERIA OF THE UPPER SINIAN HYDROTHERMAL CHERT ON THE SOUTHEAST CONTINENTAL MARGIN OF THE YANGTZE PLATE

晚震旦世时,在扬子板块东南大陆边缘地带发育一套层状硅质岩建造,岩石化学成分纯净,SiO2含量在90%以上,其它化学成分含量均很低。但Fe和Mn相对富集,而Al,Ti,Mg相对贫乏。微量元素含量变化大,大部分微量元素含量相对地壳克拉克值偏低,仅U,As,Sb,Ag,Ba等富集。岩石的δ30Si为0.0‰～0.7‰,δ18O为20.2‰～23.6‰。

The Upper Sinian bedded chert is widely distributed on the southeast continental margin of the Yangtze plate. Its thickness ranges from 20 m to 150 m. The chemical compositions of the chert are very pure, and the contents of siliceous minerals exceed 90 percent with those of any other minerals very low. The total content of the trace elements change to much degree. The contents of a large part of the trace elements are low compared with crust clarke values of these elements. But the chert is very rich in the trace elements Ba, As, Sb, Ag and U coupled with relatively high contents of Fe, Mn and low contents of Al, Ti, Mg. It can be identified as hydrothermal chert by the values of Fe/Ti, (Fe+Mn)/Ti, Al/(Al+Fe+Mn) and U/Th. In the Al Fe Mn, Fe Mn (Cu+Ni+Co)×10 triangle diagrams and lg[U] lg[Th], Cr Zr diagrams, all samples in the study area fall in the hydrothermal sediment field. All these characteristics show the cherts are mainly the prodocts of hydrothermal processes. That δ 30 Si values of the chert range from 0.0‰ to 0.7‰ with δ 18 O from 20.1‰ to 23.6‰ clearly manifests that the submarine hydrothermal water is the main source of silica.