Литосфера (Jun 2017)
Specific composition features of Kodinka Formation mudstones (Upper Devonian, the Middle Urals eastern slope)
Abstract
Main subjects for study are the phase-mineral and geochemical composition specific features of Kodinka Formation mudstones. Mudstones build up laminas, beds, pack of beds and strata (up to 9 m). It`s expected that sedimentation occurred in shallow-marine and deltaic (including delta front and prodelta) environments. Mudstone (31 samples) was comprehensive research by using the data of ICP-MS, XRD, XRF, thermal and petrographic analysis. In addition, the lithic fragments of sandstones are used. Samples collected from the most representative Kodinka Formation section (Iset’ River near Kamensk-Uralskiy town). As a result, the following features of mudstones were revealed. 1. Main rockforming minerals are chlorites, illite, illite-smectite mixed-layered minerals (MLM), fine micas, quartz and feldspars, and their content is subject to facial control. 2. In the major element-based discriminant function diagram of Roser and Korsch, the samples of the Kodinka Formation plotted on fields of mafic, intermediate igneous and quartzose sedimentary provenance (mostly). 3. Compositions are enriched in trace elements typical of mafic igneous rocks and depleted in trace elements typical of felsic and alkali igneous rocks. 4. Some evidence of recycling is received, for example in Zr/Sc-Th/Sc diagram the samples depart from the compositional trend. 5. CIA (chemical index of alteration) value vary generally within 61-63 with maximum at 66-77 for calcareous mudstones, CIW (chemical index of weathering) value range within 73-83 and ΣCe/ΣY ((La-Eu)/(Gd-Lu, Y)) value vary within 1.86-3.66 (average 2.7). Specific features of Kodinka Formation mudstones composition are indicative of their terrigenous origin (not first cycle rocks). There were some diverse rock massifs in source area, including cherts, igneous (mafic, intermediate, felsic), metamorphic (quartzite, serpentine rock and shale) units. Source rocks undergone weathering at intermediate semi-arid/semi-humid climate conditions and passive tectonic regime. Very likely that were the block of continental crust (surface of the microcontinent?) which were intensively eroded. Degree of mudstone alteration is different.
