Diagenetic history and provenance of Devonian terrestrial sandstones at the margin of Gondwana: Padeha Formation, Eastern Alborz, Iran

摘要

The northern margin of Gondwana in Iran rifted in the Devonian, forming a passive continental margin to the Paleotethys Ocean, the details of which are obscured by later collision. A record of the early phase of opening of Paleotethys is preserved in the 600-m thick Padeha Formation, a Middle Devonian terrestrial rift basin succession. The sandstones are quartz arenite, sub-arkose and arkose and contain minimal detrital matrix (-1%). Bulk rock geochemistry, according to major elements, suggests a source to the sandstones that was dominated by I type granite, but also, based on trace elements, included small amounts of intermediate or mafic rocks. Low Zr and HREE contents suggest little or no sandstone in the source area and thus low abundance of fine-grained heavy minerals (1%). The sandstones were sourced by the widespread granitic gneisses of central and northern Iran that formed at the northern margin of Gondwana in the late Neoproterozoic-Ediacarian. Diagenesis of the sandstones is dominated by compaction and fracturing of framework quartz without widespread stylolites in the sandstones, and cementation by silica, carbonates and Fe-oxides. Fractures within quartz grains are filled mostly by silica, but locally by carbonate and authigenic K-feldspar. Albitization of plagioclase, K-feldspar overgrowths and partial dissolution of detrital K-feldspar post-date silica cements and suggest temperatures between 70 and 120 ?C. The particular diagenetic assemblage of altered K-feldspar and limited quartz stylolites was a consequence of a high geothermal gradient of -70 degrees C/km at a burial depth of 1.5 km. Further mesodiagenesis was terminated by the Eocimmerian collision.