Impact of sharp changes in source rocks on the geochemistry of Tanjero formation in Dokan district, northeast Iraq

Analysis of major, trace and rare earth elements, in 17 samples, some are composite of 3 samples, collected along a vertical section of Tanjero Formation (Upper Campanian – Maastrichtian) exposed at Dokan district in northeast Iraq, have shown that sharp changes in the geochemical characteristics of the formation occur at certain level in the section. These changes involved a sharp increase in the content of elements (Cr, Ni, Co, Sc and V) that are known to be concentrated in mafic and ultramafic rocks and an obvious decrease in content of elements (Y, Rb, Zr, Hf, Th, ..etc.) known to be relatively concentrated in felsic rocks. These changes reflect sharp changes in the nature of the source rocks supplying clastics to the basin of deposition of the formation. The changes involved the emplacement of island arc volcanics and ophiolites onto the ridge separating the foreland basin to the west and southwest from the Neo-Tethys to the east and northeast. The emplacement of these mafic and ultramafic rocks is undoubtedly related to the tectonic activity associated with the different stages of the collision of the Arabian Plate with Eurasian Plate. On the bases of these chemical and mineralogical characteristics, it is found necessary to divide the Tanjero Formation into lower and upper parts. It is even suggested that a new formation name should be given for the lower part, in support of previous suggestions by early geologists working on the formation