Mineralogy and whole rock geochemistry of late Cretaceous sandstones from the eastern Pontides (NE Turkey)

Abstract

This study focuses on Upper Cretaceous sandstones of the Eastern Pontides to identify the source area, subaerial weathering level, and tectonic environment. Thirty four sandstone samples from six stratigraphic sections were analyzed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MC) and by petrographic investigation. The late Cretaceous sandstones are defined as lithic arenites and feldspathic wackes, and lithic wackes depending on mineralogical and matrix content. The sandstones have low to moderate SiO(2) contents (25-59 %; on average similar to 56 %). There are high positive correlations between SiO(2) Al(2)O(3), and TiO(2), and negative correlations between SiO(2) and K(2)O. Geochemically, the sandstones are classified mainly as greywackes and lithic arenites. Chemical index of alteration (CIA) and chemical index of weathering (CIW) values for Upper Cretaceous sandstones vary from 19 to 48 % and 20 to 49 % respectively. The combination of CIW/CIA values suggests little transportation until final deposition. The ratios of Th/Sc (on average similar to 0.42), La/Sc (similar to 1.81), Co/Th (similar to 5.05) and Zr/Hf (similar to 39.1) of sandstone samples in the study area indicate a mafic source. Furthermore, high light rare earth elements (LREE)/heavy rare earth elements (HREE) ratios (on average similar to 19.72) and negative Eu anomalies (except in the Dagbasi section) suggest a contribution from a felsic source. The lower trace element contents of the Upper Cretaceous sandstones in relation to the Post-Archean Australian Shale (PAAS), which are considered to be representative of the upper continental crust, show that the recycling processes in their source areas were probably less intense than those of the PAAS. SiO(2)/Al(2)O(3) (3.2-7.9) and K(2)O/Na(2)O (0.3-1) ratios, together with the Th (on average similar to 3.3 ppm), Zr (on average similar to 91.9 ppm), Co (on average similar to 15 ppm), and Sc (on average similar to 12.6 ppm) contents of the late Cretaceous sandstones of the eastern Pontides indicate a magmatic arc origin. Sandstones of the Trabzon region were probably deposited in fore-arc basins, and those from the Gumushane and Giresun regions in back-arc basins.