Document Type : Research Paper
Department of Geology, Mashhad Branch, Islamic Azad University, Mashhad, Iran
The Early to Middle Devonian Padeha Formation in the eastern Alborz, is a thick terrestrial syn-rift succession of Early to Middle Devonian age. Sandstones in the basal 115 m, mainly of submature, mature and supermature in terms of their grain angularity, sorting, and matrix content. Most of the studied sandstones are characterized by their quartzofeldspathic nature, and are classified as quartzarenite, subarkosic and arkosic composition. Also, based on petrographic results, a paragenetic sequence is proposed for the sandstones, including three general stages. The sandstones have been examined using scanning electron and hot-cathodoluminescence microscopy to study the albitized detrital feldspars. Detrital K-feldspar is the major feldspar type that has been partly to albitized throughout the study sandstone. The textural variation is considerable, with vein-like, blocky, and patchy forms. Probable sodium sources for albitization include smectite to illite or chlorite transition, diabase sills, detrital albite dissolution, and replacement of detrital sodium plagioclases by authigenic minerals. From comparison with other basins worldwide, a temperature estimate of 75–130°C for the mesodiagenetic albitization is inferred, suggesting a burial depth of 2000-5500 m based on thermal burial model for the basin. The model suggests that the temperature at the base of the Padeha Formation did not exceed 150°C, constraining conditions for albitization.