Petrogenesis and tectonic setting of the Bazman Batholith, Southeast Iran

Abstract

The Bazman batholith is located in the south of Lut block, southeastern Iran. The intrusive rocks include alkali granite, intermediate granitoid (quartz-monzonite, quartz-monzodiorite and granodiorite), gabbro and some leucocratic dykes (aplitic) which have been intruded into upper Paleozoic sedimentary rocks and causing a contact metamorphic aureole. In the granitoids, A/NK>1 and A/CNK<1 ratios are indicative of metaluminous nature. Initial 87Sr/86Sr ratios for the Bazman samples (<0.706), are consistent with I-type magmatic sources. Furthermore, based on chemical characteristics and mineralogy, the granitoids are I-type. In the Harker diagrams major and trace elements variations are not consistent with generation of granitoids from gabbros via magmatic differentiation process which is also confirmed by the normalized REEs and multi-element diagrams. In the normalized multi-element diagrams, light ion lithophile elements (LILE) such as K and Rb display positive anomaly whereas high field strength element (HFSE) such as Ti, Nb and Ta represent negative anomaly. Based on major elements geochemistry including low (Na2O+K2O)/(MgO+FeO+TiO2), K2O/Na2O and Al2O3/(MgO+FeO+TiO2) ratios and high CaO/Al2O3 and CaO+FeO+MgO+TiO2 contents, intermediate granitoids are similar to those ofpartial melts of amphibolite/metabasalt sources. In addition, trace elements modeling show that the intermediate granitoids can be produced by dehydration partial melting (10 to 30%) from an amphibolitic source and restite minerals assemblage of plagioclase, clinopyroxene, orthopyroxene and Fe-Ti oxides. Based on major element geochemistry, the intermediate granitoids are comparable with the partial melts originating from a metabasaltic/amphibolitic source. Geochemical characteristics suggest that the Bazman batholith has been formed in a volcanic arc setting (active continental margin). Therefore, it seems that during upper-Cretaceous, the southern margin of the Lut block was an active continental margin due to the subduction of a Neo-Tethyan slab beneath southern margin of Eurasia. Here we proposed a new model in which the oceanic basin of southern Lut was probably different from the ocean which is now subducting beneath the Makran. Instead it was a closed oceanic basin beneath Jazmurian depression.