扬子地台晚元古代以来硅岩地球化学特征及其成因

在扬子地台通过对晚震旦世-古生代地层中许多硅岩层的主元素和微量元素含量及组合关系的研究,总结了硅岩的地球化学特征,进而讨论硅质的来源和硅岩形成的地质背景及其有利的地球化学条件。

THE GEOCHEMICAL FEATURE OF SILICEOUS ROCKS SINCE LATE PROTEROZOIC IN THE YANGTZE PLATFORM AND THEIR GENESIS

Silicolite strata with different thickness and shapes are developed in most horizons of Sinian to Upper Palaeozoic in the Yangtze Platform.Most of the siliceous rocks are located in the anoxic melanocratic rock-series,while some of them are directly deposited between volcanic rocks and tuffs.The siliceous rocks mainly consist of tiny α-quartze.The main geochemical features of siliceous rocks in this area are as follows:l)Host elements are simple and concentric,beside SiO₂,only Al₂O₃,CaO,FeO,organic carbon,and K₂O of some samples can reach more than 1% in content;2) The ratio of Al/(Al + Fe+Mn) of all the samples are greater than 0.1,most of them are 0.2-0.6; They show obvious tendency change towards different times,different sections or different layers at the same section;3) Most of K₂O/Na₂O ratio are larger than 1,only same sections which related to the volcanic activity have the ratio less than 1;4) Mn contents are low generally, most of the MnO/TiO₂ ratio are smaller than 0.1,only a small amount of samples have the ratio greater than 1:5) Generally,the content of trace elements in siliceous rocks is low having a positive correlation to the content of clay and organic matter,and it has an obvious negative correlation to the crystallization degree of minerals; 6) P, Ba, V, Cr and Mn in siliceous rocks vary greatly in content,Ba,V have a positive correlation to Cr and a negative one to Mn; 7) Rich in organic matter;and 8) The mean δ ³⁰Si of siliceous rocks in this area ranges from +0.3‰to + 0.4‰.The formation of primary bedded siliceous rocks since Late Proterozoic in the Yangtze Platform is the products of particular sedimentary environment under particular geotectonic setting at definite geohistorical stage.The silicon in this area has a complicated source being accumulated by multiple manners.Long term surface weathering,erosion and halmyrolysis,including large-scale glacial erosion and miclobial action made a large amount of silicon freed into sea-water and become the principle source of silicon for forming silicolites. Locally,especially in active zone,large-scale volcanic eruption and hot spring up-welling may abruptly release a large amount of SiO₂, which will be the important source of silicon.In addition to the function of silicon-concentration,the abrupt increase of living beings and organic matter can constantly change the medium condition so thatthe medium become favourable for the concentration,preservation,saturation and deposition of silicon.Submarine volcanism is another important factor for silicolite formation.It not only can enlarge local silicon concentration rapidly,but also can change water temperature,increase nutrient for microbes,promote the reproduction of bacteria and microbes,raise the content of anions such as S^2- obviously,and constantly change local medium condition of sea-water to form local geochemical domain more favourable for silicolite formation.In one word,the formation process of silicolites in this area is very complicated.It can be concluded as follows:Under particular geological setting and on the basis of abundant material source,biochemical process can concentrate and preserve silicon,and constantly improve the medium condition favourable to the saturation and deposition of silicon so that silicon can deposit steadily.Then,after diagenesis and a series of epigenetic changes as well as reworking,the existing bedded silicolites are formed.