Tectonic and palaeogeographic implications of compositional variations within the siliciclastic Ab-Haji Formation (Lower Jurassic, east-central Iran)

摘要

The Lower Jurassic Ab-Haji Formation which is widespread across east-central Iran but best exposed on the Tabas Block consists of sandstones, siltstones and mudstones deposited in fluviodeltaic and marginal marine settings. Provenance studies of this formation record the geodynamic history of Central Iran during and in the aftermath of the main Cimmerian event (Triassic-Jurassic boundary to Liassic) and are crucial for the reconstruction of the tectonic setting and palaeogeography of this region. Sandstone provenance analysis of six selected outcrop sections using modal analysis suggests that the predominantly quartzose and quartzolithic sandstones of the Ab-Haji Formation were largely derived from plutonic, quartzose sedimentary and low-grade metamorphic rocks, attributed to a recycled-orogen and cratonic sources. Sandstone and shale geochemistry suggests that most major and trace element patterns are near upper continental crust values, indicating felsic and (meta-) sedimentary sources. Key elemental ratios are consistent with a mixed recycled source under humid climatic conditions and moderate to intensive chemical weathering. Weathering indices indicate that recycling processes and/or humid climatic conditions contributed to the high compositional maturity of the sandstones such that they plot in craton-interior and passive-margin fields on tectonic QtFL and QmFLt ternary diagrams. Paleogeographic reconstructions and geologic mapping indicate that the erosion of uplifted pre-Jurassic rocks exposed in tilted fault blocks within an extensional continental retro-arc basin provided the principal sources. Strata of the Ab-Haji Formation in the western and southern Tabas Block were predominantly derived from low-grade metamorphic and sedimentary rocks exposed on the adjacent Yazd Block, while those in the eastern Tabas and western Lut blocks were derived from the erosion of mature sedimentary strata of the uplifted Shotori Swell, i.e., from the eastern margin of the tilted Tabas Block.