Petrography and geochemistry of the Ab-e-Haji Formation in central Iran: implications for provenance and tectonic setting in the southern part of the Tabas block

Abstract

Sandstone petrography and shale geochemistry from the lower Jurassic Ab-e-Haji Formation, in the southern part of Tabas block, were used to constrain provenance, tectonic setting and weathering conditions. The sandstones consist mainly of quartz and sedimentary and low grade metamorphic lithic fragments and therefore, show quartzolithic nature (Qm₃₈-F₂-Lt₆₀, Qt5₈-F₂-L₄₀). However, modal analysis as well as highly labile lithics in Ab-e-Haji sandstones point to short transport of sandstone components from a recycled source of a fold thrust belt to its nearby foreland basin. Discrimination diagrams based on major and trace elemen content point to a role of recycled sources for the deposition of Ab-e-Haji Formation, which at the upper part of the section were probably mixed with a minor felsic source. Negative Eu anomalies, similar to those displayed by Post-Archean Australian Shale (PAAS), along with depleted Ca, Na, Cs, Ba and Rb, and low K₂O/Al₂O₃ ratios in studied shales, suggest low abundance of feldspar in the source terrane. Depletion of transition metals (Cu, Sc, Ni, Cr, and V) can be explained by derivation from a more silicic and fractionated source than the PAAS. Moreover, the geochemical results from La–Th–Sc diagram as well as La/Sc, Th/Cr, and Th/Sc ratios of Ab-e-Haji sediments are within the range of fine-grained sediments derived from silicic sources. The chondrite-normalized rare earth elements (REE) patterns of samples are similar to those of PAAS, with light REE enrichment, a negative Eu anomaly, and almost flat heavy REE pattern, similar to those of a source rock with felsic and (meta) sedimentary components. Most probably, displacement of intrabasinal faults, such as the active Kuh-Banan basement fault, and exposure of supracrustal successions (fold thrust belt) provided a mixed source area that supplied the sediments for the Ab-e-Haji foreland basin. This tectonic activity could have been related to the Eo-Cimmerain orogeny in central Iran during the Late Triassic to Jurassic. Furthermore, the point counting data from Ab-e-Haji sandstones imply a semi humid climatic condition, which is supported by the CIA (chemical index of alteration) values for the shales of this formation, which indicate moderate to intense weathering of the parent rocks in the source area.