Mineral chemistry and alteration evolutions of peridotite clasts in conglomerate from the Cretaceous Piranshahr basin, NW Iran

The study area forms part of the Zagros Zone. It is located on the NW of Piranshahr town, NW Iran. The study of altered peridotite clasts of the Creteceous conglomerate is the main subject of this research. They have polygenetic origins having calcite as the constituent cement. The size of clast is ranging from mm to cm including sedimentary and metamorphic rocks fragments, altered peridotites and basaltic fragments. The peridotite clasts have been altered and carbonitized intensely. On the basis of spinel compositions, origin of the altered peridotite clasts are determined as the Alpine type ophiolite, comparable with the associated serpentinized peridotites of the Piranshahr ophiolitic complex. Presence of rare red-brown spinels as well as talc and serpentine support peridotites as the protolith composition of the intensive altered clasts. Calcite is the main secondary phase in the altered peridotite clasts of the conglomerates from the Piranshahr ophiolitic complex. Calcite in association with quartz and/or itself completely replaced magnesian silicate hydrous phases (serpentine) of the altered peridotite clasts. The secondary garnet having composition as andradite and grossular occurs as scattered grains in the clasts. Alteration processes of the peridotite clasts and formations of the secondary quartz and Ca2, CO32- rich mineral phases related to MgO-CaO-CO2 metasomatic fluid phases in the study area have been discussed in this research. Occurrence of the secondary quartz as well as high modal amounts of calcite in the altered peridotite calsts are evidence of carbonization and Ca metasomatism. The depositional basin can be probably supposed as appropriate and sufficient source of CO2 reservation which has been caused intensive carbonatization of previously serpentinized peridotite clasts with high carbonate formations. So it can be concluded that carbonitization of the formerly serpentinized peridotite clasts have been occurred after obduction of ophiolitic rocks and subsequent weathering and transportation of the serpentinized peridotites and conglomerate formation into the Piranshahr Cretaceous basin.