Geochemical properties of early-middle jurassic sandtones and shales (Gümüşhane, Bayburt)

Early Jurassic clastic rocks of the Eastern Black Sea Region have the capability of supplying substantial information on the geochemical characteristics of their provenance. In this contex, totaly three Early Jurassic successions, two of which are from Gümüşhane region and one is from Bayburt region, were sampled particularly based on their sandstone and shale contents. Of these, Hur (Gümüşhane) succession is thought to be representative of horst area while Kösedağı (Gümüşhane) and Taht (Bayburt) successions to be representatives of the graben areas of the basins. Rock fragments, plagioclase, alkaline feldspar and quartz are the main components of the rocks. Pyroxene, amphibole, biotite, opaque and muscovite are included as subordinate amounts in some of these rocks. Rock fragments can also be divided into two sub-groups as acidic and basic fragments. Active arc volcanism during the early Jurassic in the region has led to the probability that particularly some of basic volcanic fragments, together with pyroxene and amphibole clasts, would have been derived from this arc volcanism which was syncronous with sedimentation. Sandstones are moderately weathered (CIA=60-70) indicating that the region tectonically was not much quiescent during their accumulation. Trace elements such as Th and U which represent the proportion of felsic components as well as Sc and Cr which measure the abundance of mafic components in clastic rocks display large variations in contents. This may suggest that the felsic rocks, as well as mafic rocks in significant proportions, in the source area were found. However, lacking of mafic rocks in general in the Paleozoic basement argue against this view and enhance the probability that an important portion of the mafic contribution has been derived from the syn-sedimentary basic volcanism (Early Jurassic). Whole-rock geochemistry of these rocks defines more than one tectonic environment, e.g., island arc, active continental margin and passive margin, rather than a specific tectonic environment during their accumulation. The clastic rocks that tend to define such a large interval in terms of tectonic environment may point to a specific tectonic environment which was not identified in these diagrams. Back-arc basin, which forms such an example, is one of the basins that have geographic location to be fed from either active volcanic arc and also from the continent. Taken into account that the study area was in a back arc position during the Early Jurassic, large intervals in terms of source area composition and tectonic environment for the clastic rocks studied seem to be reasonable.