Mohammadreza Naeeji, Iraj Maghfori-Moghaddam, Mahboubeh Hosseini-Barzi
Aug 27, 2021
Arabian Journal of Geosciences
The siliciclastic sediments of Lower Triassic-Middle Jurassic Shemshak Group is located in the Central Alborz Mountain Range along the southern Caspian Sea shore. The analysis (CIA, ICV, PIA, CIW, Th/U ratio) indicates a moderate to intense weathering. The graph of Qp/F+RF versus Qt/F+RF revealed semi-humid climate. The petrography data reveal mainly a recycled orogeny provenance. Point counting data reveal mean value of Q40.9:F2.3:RF56.8 (Emarat section) and Q58.6:F8.2:RF33 (Emamzadeh Hashem section) for the dominant siliciclastic grains. The Th-Sc-Zr/10 and La-Th-Sc ternary diagrams indicate continental island arc setting for the source area of recycled sediments. Similar results were reported from the Middle Paleozoic-Triassic of the Turan plate and the southern border of the Scythian plate. New discrimination functions revealed active margin setting. Ratios or concentration of La/Sc, Th/Sc, La/Co, Eu/Eu*, Th/Co ,Th/Cr, Cr, Ni, Rb and bivariate plots of La/Sc- Co/Th and La/Sc-Th/Co testify for acidic-felsic source rocks. Ratios of Al2O3/Na2O, K2O/Na2O, and Th/U as well as variable compositions of conglomerate particles documented the recycling. Quartz pebble-cobbles and fine-grained quartz indicate mainly medium-high-grade metamorphic source rocks which has some contradict with Gorgan Schist (GS) with low-grade metamorphic as source remnant. High-grade metamorphic rock fragments increase towards the top of the section which is consistent with un-roofing in the source area. The results are mostly in the favor of a northern source from the Turan and/or Scythian plates near the Caucasus Mountains.