Geochemistry of Tsetserleg terrane (Mongolia) sandstones and mudrocks : Evaluation of grain size effect in immature volcanolithic arc sediments

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  • モンゴルの上部古生界(ツェツァレグ・テレーン)における未成熟砕屑物の分級効果による組成改変過程

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Abstract

Hydrodynamic sorting has been shown to have considerable effect on the geochemical compositions of quartzose and quartzo-feldspathic sediments, but less is known of the variations in texturally and mineralogically immature volcanolithic suites. Major and trace element abundances in a suite of 94 Devonian-Carboniferous sedimentary rocks from the Tsetserleg terrane of Mongolia have been examined to determine the effects of sorting. Some elements tend to broadly decrease (e.g. SiO2, Na2O, CaO, Sr) or increase (e.g. Al2O3, K2O, Rb) with decrease in grain size from coarse-grained sandstone to mudstone. However, variation in elemental abundances within any size grade is generally large, and consequently systematic trends with grain size are lacking. Values for commonly used provenance indices (e.g. SiO2/Al2O3, K2O/Na2O, Basicity Index, Th/Sc) and for paleoweathering proxies (e.g. CIA, CIW) also show almost total overlap between size grades. Lack of sorting effect is attributed to the volcanolithic nature of the sediments and their relatively low quartz contents. However, compositions are a little more uniform in the finer-grained sediments (very fine-grained sandstones, siltstones and mudstones). Sorting effects should also be minimal in similar volcanolithic terranes elsewhere, including those in arc environments such as Japan. In these cases all size grades of sediments can be used to evaluate provenance and paleoweathering, or for establishing baseline values for mineral exploration or environmental monitoring.

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