Geochemistry, zircon U-Pb geochronology and Rb-Sr & Sm-Nd isotopes of Najmabad monzonitic rocks south of Ghonabad

Najmabad is located south of Ghonabad in the province of Khorasan Razavi. This area is situated in the northern part of Lut Block. Two types of granitoid, ilmenite series (Jurassic), and magnetite series (Tertiary) are recognized in the study area. The aim of this research is to find out the age and petrogenesis of granitoid (magnetite series). The result of U-Pb zircon age dating of monzonite is 39.9 Ma (Middle Eocene time). Chemically, Monzonitic rocks are meta-aluminous to moderately peraluminous. They are classified as the magnetite-series (I-Type granitoid). Based on enrichment of LREE relative to HREE, enrichment of LILE (K, Rb, Ba and Sr) and depletion of HFSE (Nb, Ti and Ta) monzonitic magma originated in subduction zone. The average of SiO2 (66.52%), Al2O3, (15.26%), MgO (1.13%), Na2O (4.7%), K2O/Na2O (0.31), as well as LREE enrichment, positive Eu, high Sr (492 ppm), high ratio of (La/Yb)N (>13.58), Y (<7.9 ppm) and Yb (<0.7 ppm) all are indicative of silica rich adakitic magma. The initial (143Nd/144Nd)i = 0.512851 and (87Sr/86Sr)i = 0.70512 could be considered as representative of oceanic slab subducted derived magmas. The monzonitic magma originated from partial melting of amphibolites having 10 to 25% garnet. Based on the geochemistry of trace-REE and radiogenic isotopes (Rb-Sr & Sm-Nd) all of the Tertiary granitoid within the Lut Block formed in subdction zone. The age of the granitoid rocks are between Middle Eocene to lower Oligocene. The age of granitoid within the Lut Block, decreases from north to south. Kaybar Kuh with 43.3 Ma located to the north and Chah Shaljami 33.3 Ma to the south. The (87Sr/86Sr)i also decreases from the north (Kaybar Kuh 0.707) to the south (Chah Shaljami 0.7047). In comparison with Tertiary granitoid rock within the Lut Block, Najmabad monzonitic rocks are high in Na and based on Nb<8 ppm and isotopic composition the magma show lower crustal contamination.