Chemical composition, provenance and early diagenetic processes of playa lake deposits from the Boda Siltstone Formation (Upper Permian), SW Hungary

Abstract

In this paper, results of a bulk-rock geochemical study of silty and albitic claystone samples selected from the Upper Permian Boda Siltstone Formation (BSF) in the western part of the Mecsek Mountains (Tisza Mega-unit, Hungary) are presented. The high Na2O and P2O5 contents, relative to the post-Archean Australian average shale (PAAS) and the average Russian Paleozoic shale compositions, are the most striking features of the geochemistry of the Boda sediments. The samples studied are depleted in SiO2, TiO2 and Al2O3, and they are enriched in Fe2O3, MgO, CaO and K2O relative to the PAAS. The major element relations clearly show that the geochemistry of the BSF is strongly affected by post-depositional modification, corresponding to large-scale dispersal or addition of components. On the other hand, relatively high La content, low concentrations of V, Cr, Cu and Ni, and the result of the TiO2 versus Ni plot reflect a relatively felsic provenance of the BSF. By comparison with detrital mineralogy of the heteropic Cserdi alluvial fan system deposits, the authors assume that the Boda playa lake deposits had a similar immature primary composition consisting of quartz, plagioclase, K-feldspar, muscovite, biotite, chlorite and clay minerals. During weathering and transport in a semi-arid to arid climate, detrital mafic minerals were altered to yield chlorite and clay minerals plus Fe and Mg, and trace metals in solution. Mg was built into the structure of chlorite and of secondary carbonates such as dolomite and Mg-rich calcite. Based on previous geochemical studies, the BSF contains particularly sodic sedimentary rocks (up to 8 wt%) which may represent an addition of sodium in authigenic silicates from alkaline brine and/or evaporites in the playa deposits. In claystone, after albite formed, Na-depleted, Ca, K-enriched waters reacted with the clay minerals such as smectite and kaolinite to yield K-rich illitic sediments, Ca precipitated in calcite cement. P2O5 is enriched in some samples due to phosphorus mobilization during diagenesis.