Geochemical Signatures for the Grain Size Variation in the Siliciclastics of Kaimur Group, Vindhyan Supergroup from Markundi Ghat, Sonbhadra District, (U.P.), India

摘要

The chemical composition of siliciclastics from Mesoproterozoic Kaimur Group, Vindhyan Supergroup have been investigated to determine and appraise the influence of the weathering, hydraulic sorting and recycling processes upon source rock signature. The studies pertaining to the relationship between major, trace and REE and the grain size variation were carried out along Markundi ghat section, Sonbhadra district, Son valley. Upper Kaimur Group has been divided into three principal formations-the uppermost-Dhandraul quartzites, Scarp sandstone and Bijaigarh shales. Petrographically, they range from quartz arenite, sublitharenite, subfeldspathic arenite to pyritiferous shale. The elemental enrichment, except for SiO2, is in the following order-Bijaigarh Shale > Scarp Sandstone Dhandraul Sandstone.They correspond to three distinct textural/grain size variations gradually, with their coarsening upward sequence. Chemical composition is closely dependent on the grain size, with Al2O3 increasing toward finer sediments and SiO2 towards sands. The other major elements behave like Al2O3. This strongly reflects min-eralogical composition, in which clay minerals dominate the fine fraction and quartz in the coarse fraction. HREE fractionation seems to have been controlled completely by zircon. The effect of quartz dilution has drasticaUy affected the concentration of all the elements though the elemental ratios in these three grain sizes such as Zr/Th, Zr/Sc, Zr/Yb, Th/Sc and Th/U have not been disturbed. The abundances of REE also varies depending upon the textural types due to quartz dilution effect, though the pattern remains the same. The shale normalized La/Yb and Gd/Yb ratios are affected by quartz dilution. Chondrite-normalized REE patterns and negative Eu anomaly size (Eu/Eu* = 0.5-0.6) of the studied rocks are similar to that of Post-Archean Australian shales (PAAS) indicating their differentiated silicic source.The CIA values (67-84) for shale and sandstone indicate that the source rocks had undergone moderate to very high extents of chemical weathering under humid tropical conditions. The similar CIA index in all the textural rock types indicates that the recycling processes were important enough to homogenize shale and sandstone compositions. The geochemistry of the sedimentary rocks of Kaimur Group have been affected by higher degree of source rock weathering and also by large scale fluvial processes of hydraulic sorting and mineral differentiation. In general, these rocks reflect a higher intensity of these processes than in the PAAS shales. The compositional maturity of analyzed sandstones is typical of cratonic environments and their La, Sc, Th and Zr contents reflect their passive continental margin setting.