Geochemistry of shales of the Qadir Member (Nayband Formation, Upper Triassic), East Central Iran (Tabas Block): implications for provenance and palaeogeography

摘要

The Upper Triassic shale of the Qadir Member of the Nayband Formation, East Central Iran has been analysed geochemically to evaluate provenance and palaeo geography. The Qadir Member in the Parvadeh Coal Mine section is 450 metres thick, and includes sandstone, shale, coal, siltstone, and fossiliferous limestone. XRD analysis of shale samples from the Qadir Member largely indicated the presence of illite and chlorite, with small amounts of kaolinite and mont morillonite. On binary and triangular diagrams the data suggests an intermedi ate igneous source rock for these shales. Plotting the geochemical data on binary diagrams also indicates the tectonic setting of an active continental margin, perhaps reflecting the Early Cimmerian tectonic event with Neothetys subduction under the Iran Plate, and collision of the Iran Plate with Turan during the Late Trias sic. The Chemical Index of Alteration (CIA) and Plagioclase Index of Alteration (PIA) values for shale from the Qadir Member of Nayband Formation vary from 74.04 to 80.54 (average 78.02) and 84.31 to 91.85 (average 87.81), respectively, indicating moderate to high chemical alteration in the source area and suggesting a semi-humid climate during deposition. The geochemical data and palaeogeographical models indicate that the Qadir Member shale was deposited on an active margin in a shoreline to transitional-marine setting.