Provenance of the Lower Cretaceous clastic rocks from the Gadvan Formation (Zagros Basin, Iran): Evidences from whole-rock geochemistry and petrography

Abstract

The petrography and geochemistry of clastic rocks from the Gadvan Formation in the Abadan Plain (southwest Iran) were analysed to infer their weathering intensity, compositional maturity, provenance, and tectonic setting. The Index of Compositional Variability (0.47–0.71) indicates high compositional and mineralogical maturity. The Chemical Index of Alteration and the Plagioclase Index of Alteration suggest high intensity of chemical weathering in the source area. In addition, a remarkable high content of REE and LREE/HREE and Th/U ratios, as well as high C-value (1.7) suggest high chemical weathering in the source area. Rounded zircon grains, mineral homogeneity, and a lack of feldspar grains could be related to high weathering and the effect of recycling. Elemental ratios (La/Sc, La/Co, Th/Sc, Th/Co, GdN/YbN, Cr/Ni, Co/Th, La/Yb, and La/Th), bivariate diagrams (La/Sc vs. Co/Th, La/Sc vs. Th/Co, Cr/Th vs. Th/Sc, Th/Yb vs. Ta/Yb, and La/ Yb vs. La/Th), and an enrichment of Nb, Zr, Th, La, Cr, Ni contents imply felsic to intermediate parent rocks, which are similar to the massive granitoids of the Arabian Shield. This could be supported by the dominance of zircon grains, as well as inclusions of rutile and tourmaline in quartz grains in the Gadvan sandstones. Moreover, further petrographical evidences, such as undulatory quartz grains, rare biotite and a small orientation of grains is also related to low-grade metamorphism in a felsic source rock. Finally, the Mudrock Maturity Index and tectonic discrimination diagrams reveal a convergence process in a collisional setting, in which the Zagros Mountains originated.