Strontium Isotope Composition And Control Factors Of Cambrian Marine Carbonate, Xiushan, Chongqing

The key point of this thesis is the Strontium Isotope Composition and Control Factors of Cambrian Marine Carbonate, Upper Yangtze Platform, which the samples were collected from Xiushan section in Chongqing.The section of the Cambrian in Xiushan of Chongqing has been well re-appraised over again by studying its stratigraphy, sedimentary petrology, sedimentary facies and environment in this thesis. Characteristics of its petrology reveal rock types mainly involve sandstone, mudstone, carbonate rock and their transitional types. The clastic rocks distribute mainly in the Lower Cambrian and the carbonate rocks in the upper part of the Lower Cambrian and middle and Upper Cambrian. The sedimentary environment of carbonate is platform facies.Evaluation on the diagenetic alteration of marine carbonate and its preservation for information of sea water have been paid more attention recently and become an important premise of the research on Palaeo-oceanography, whose development is very important for the improvement of the research level in Palaeo-oceanography and relative other subjects. Based on the test of cathodolummescence and the analyses of Mn, Sr, Fe and Mg of the marine carbonate samples collected from the sedimentary section of Cambrian in Xiushan of Chongqing, Southwest China, this paper assesses the preservation of different structural components for the information of Cambrian sea water. The main control factors of carbonate of preservation for information of seawater are: a) the components of original minerals; b) the primary porosity of rock; c) the content of terrigenous elastics of carbonate rocks. Among the discrimination criterion controlling the alteration of carbonate, neither cathodoluminescence of the carbonate minerals nor the content of Sr can act solely as discrimination criterion for controlling the alteration of marine carbonate. The Mn/Sr value considers the Mn content for control factors of cathodoluminescence and the relative content of the marine versus terrigenous elements. So the Mn/Sr ratio is the relatively comprehensive discrimination criterion for controlling the alteration of marine carbonate. By the analysis for Cambrian stratum in Xiushan of Chongqing, according to the Mn/Sr value of the marine carbonate of different structural components, the 28% samples thatcan't stand for the original seawater in different structural components of the marine carbonate had strongly undergone diagenetic alteration. This paper emphasizes the effect of the dissolution of aluminosilicate during diagenesis may provide more radiogenic strontium which would result in a high content of strontium in carbonate rocks. So we should avoid the vein of carbonate and select the samples with less terrigenous clastic, less grain content and lacking calcite cementation collected as analysis samples. The microcrystalline limestone and micro- to fine-crystalline dolomite are rather fairly whole rock samples.The experimental process of the strontium isotope measurement was discussed in brief here.This paper also discusses the strontium isotope evolution curve published over the world, present strontium isotope results of the section of Cambrian in Xiushan of Chongqing. Samples studied here were collected from the sedimentary section of Cambrian in Xiushan of Chongqing, Southwest China. Samples for strontium isotopic screened were selected by way of the test of cathodoluminescence, observation of thin sections, and analysis of Sr and Mn. The Mn/Sr value of all samples presented in this paper are less than 2, and the cathodoluminescence is dull, however, the samples from the Cambrian underwent diagenetic alteration to a certain degree, and the plotted points of the thickness and 87Sr/86Sr value are somewhat discrete. The strontium isotope evolution curve drawn from the data with relatively low 87Sr/86Sr value are consistent with other curves based on samples from different areas of the world, supporting the notion of global consistency of strontium isotope composition of marine carbonates. The stronti...