Geochemical comparison of Padeha Formation sandstones in the north of Tabas and Yazd blocks: Implication for paleogeography

This research is a part of the studies of sedimentology and stratigraphy of deposits associated with the Devonian located in Central Iran, which is known as the Padeha Formation (Lower-Middle Devonian age). In this research, the Padeha Formation in north of Tabas (Ozbak-Kuh) and Yazd (Doruneh) blocks were selected and studied. These succession is measured in Ozbak-Kuh, 492 meters, which is often composed of siliciclastic, dolomitic and evaporite rocks, while in the Doruneh section, this formation consists of 310 meters of siliciclastic, calcareous and dolomitic rocks. The aim of this study is to focus on the petrography and geochemistry aspects for provenance studies.

Twenty one sandstone samples from Ozbak-Kuh and fifteen sandstone samples from Doruneh section were selectedand studied with Polarized microscope. In each thin section, about 300 points were counted using the Gazzi-Dickinson method. According to the point counting, the major and accessory components of this sandstones are identified and quartz, feldspar and rock fragments modes are utilized for naming the sandstones according to the Folk classification (Folk, 1980). 16 samples of medium grain sandstones with the lowest amount of carbonate cement were selected for geochemical analysis (ICP-MS method).

Petrographic studies of sandstone in Ozbak-Kuh section are mainly indicative of quartzarenite and sublitharenite (subchertarenite) petrofacies, however, quartzarenite and subarkose petrofacies have been identified in the Doruneh section. Compared to the Upper Continental Crust (UCC) (Taylor & McLennan, 1985), sandstones of the Padeha Formation in the Ozbak-Kuh section are somewhat enriched in some elements, such as zirconium and hafnium, but in the Doruneh section, zirconium and hafnium are slightly depleted compared to the Upper Continental Crust. All sandstone samples in both sections are strongly depleted compared to UCC. In both the Ozbak-Kuh and Doruneh sections, vanadium, cobalt, nickel and scandium exhibit much depletion compared to the UCC.

Geochemical diagram of Zr/Sc against Th/Sc (McLennan et al., 1993), along with plotting the data of Padeha Formation on this diagram shows that the samples of Doruneh section are plotted near the UCC. This shows that Padeha Formation sandstones in the Doruneh section are derived from intermediate to acidic igneous rocks, and recycling have not been affected. But the Ozbak-Kuh samples often reflect the granitic parent rock, which is located in the range of recycling. Additionally, the La/Sc versus Co/Th (Gu et al., 2002) is also a confirmation of the felsic to granitic parent rocks for Padeha Formation in both sections. For determination of tectonic setting, in this reseach, triangular diagrams based on trace elements (Bhatia, 1985) have been used. According to these diagrams, Ozbak-Kuh samples, mostly plotted in the passive continental margin field and Doruneh section sandstones are plotted in passive and active continental margin fields. The present study, using the results of petrography and geochemistry and the paleogeographic investigations, shows that the deposits of the Lower-Middle Devonian Padeha Formation can be formed in the transition between the transformations of rifted margin to passive continental margin of Paleo-tethys.