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

摘要

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 defi ned 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 ～56 %). There are high positive correlations between SiO_2 Al_2O_3, and TiO_2, and negative correlations between SiO_2 and K_2O. Geochemically, the sandstones are classifi ed 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 fi nal deposition. The ratios of Th/Sc (on average ～0.42), La/Sc (～1.81), Co/Th (～5.05) and Zr/Hf (～39.1) of sandstone samples in the study area indicate a mafi c source. Furthermore, high light rare earth elements (LREE)/heavy rare earth elements (HREE) ratios (on average ～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_2O_3 (3.2 –7.9) and K_2O/Na_2O (0.3 –1) ratios, together with the Th (on average ～3.3 ppm), Zr (on average ～91.9 ppm), Co (on average ～15 ppm), and Sc (on average ～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.