Geochronology, Radiogenic Isotope Geochemistry, and Petrogenesis of Sangbast Paleo-Tethys Monzogranite, Mashhad, Iran

The study area is located in northeastern Iran (south of Mashhad). Paleo-Tethys Ocean opened during Silurian time and subduction under Turan plate was started in Late Devonian. By Late Triassic (225 Ma) there was no Paleo-Tethys left on an Iranian transect, therefore Turan plate obducted over Iran Plate. Two stages of low grade regional metamorphism are exposed, that are related to Hercynian (Late Paleozoic) and Cimmerian (Jurassic) orogenies. The Paleo-Tethys remnants (meta-ophiolite and meta-flysch) were intruded by Sangbast monzogranite. Chemically, monzogranite is moderately peraluminous S-type granitoid. It has low values of magnetic susceptibility [(5 to 11) × 10-5 SI] therefore it is classified as belonging to the ilmenite-series of reduced type granitoids. Monzogranite is characterized by strong light rare earth element (LREE) enrichment and less low heavy REE (HREE). All samples have very small negative Eu anomalies (Eu/Eu* = 0.62 to 0.88). Total REE content of monzogranite is between 212-481 ppm. The result of U-Pb zircon age dating of monzogranite is 201.3 ± 3.6 Ma (Upper Triassic, Rhaetian time). The initial 87Sr/86Sr and 143Nd/144Nd ratios for monzogranite is (0.706776 and 0.512219) when recalculated to an age of 201 Ma, consistent with the new radiometric. The initial 87Sr/86Sr and 143Nd/144Nd ratios for slate is (0.720613 and 0.511601) respectively when recalculated to an age of 201 Ma, consistent with the new radiometric results. Initial εNd isotope values for monzogranite is -3.13 and the slate is -15.19. Based on radiogenic isotopic data and REE monzogranite magma originated either from lower continental crust which was very different from slate or it is originated from mantle and contaminated in continental crust during ascending.