Petrography, geochemistry, and provenance of Jurassic sandstones from theSakarya Zone, NW Turkey

摘要

The Jurassic sandstones exposed in the western part of the Sakarya Zone are yellowish to brown, moderate to well sorted,medium to coarse-grained, grain-supported, and cemented by calcareous and minor silica. Sandstones are mainly classified as litharenite,subarkose, and arkose according to the relative proportions of quartz, feldspar and rock fragments, and major element ratios.On the SiO2 variation diagram, Na2O and K2O show slight positive correlations, which could reflect abundance of sodic plagioclaseand K-feldspar with quartz within high Si-content sandstones. Sandstones generally exhibit slight LREE enrichment with respect toHREEs. The REE plots are characteristic for sedimentary rocks derived from the upper continental crust. Combining the results ofseveral provenance discrimination diagrams, elemental ratios (Th/Sc, Zr/Sc, La/Th), and REE contents in sandstones, they reveal thatthe sandstones originated from intermediate to felsic source rocks. Accordingly, the probable source of the Jurassic sandstones is thecrystalline basement of the Sakarya Zone consisting of Devonian, Carboniferous, and Permian granitoids, and high-grade metamorphicbasement rocks. The Jurassic sandstones mostly have characteristic features of passive continental margin basins. It can be noted that thesandstones may have been deposited in a marginal basin that began to open in Early Jurassic time. CIA and CIW values for the Jurassicsandstones suggest that the source area was subject to low to moderate chemical weathering under semihumid to semiarid climaticconditions.