Geochemistry and mineralogy of the Silurian Akkas Formation, Iraqi western desert: implications for palaeoweathering, provenance and tectonic setting

Abstract

The mineralogical and geochemical composition of the Silurian Akkas Formation consisting of interbedded shales, sandstones, siltstones and rare marls from the western desert of Iraq was investigated based on geochemical and petrographic data to constrain their weathering condition, palaeoclimate, provenance and tectonic setting. Based on petrographic and SEM analysis, sandstones are dominated by quartz with rare rock fragments, mica and feldspar, whereas clay minerals assemblage is dominated by illite and kaolinite with a lesser amount of chlorite in shales and chlorite with a minor amount of illite, kaolinite and mixed layer illite/smectite in sandstones. The chemical index of alteration (high CIA), the index of compositional variability (ICV < 1), plagioclase index of alteration (high PIA) values and A–CN–K plot suggest mainly intensely chemically weathered source area. The elemental ratios critical for palaeoclimate (Sr/Cu and Ga/Rb) and Ga/Rb vs Sr/Cu plot in addition to clay mineral evidence indicate fluctuating climate between humid and arid conditions, which was affected by the extensive Late Ordovician–Early Silurian glaciation (Hirnantian glaciation event) during the deposition of the Akkas sediments. The geochemical data from major, trace and REE elements, as well as bivariate discrimination provenance diagrams, suggest mainly felsic–intermediate igneous source rocks. The mineralogical composition of sandstones (high quartz content, rare feldspar and rock fragments) and the high geochemical maturity of shales and sandstones indicate that they were derived from stable cratonic source and recycled orogeny with the possible contribution from quartz-rich crystalline provenance. These sources were likely the plutonic–metamorphic complexes of the Arabian Shield located to the southwest of the Akkas basin. The detrital mode tectonic provenance diagram, discriminant function-based major element diagrams and REE concentrations suggest continental margin/intracratonic depositional setting mixed with an older collisional source, which is consistent with the general geology of the study area.