Petrology of the Bouein Zahra-Naein plutonic complexes, Urumieh-Dokhtar belt, Iran: with special reference to granitoids of the Saveh plutonic complex

摘要

The Bouein Zahra-Naein plutonic complexes are major plutonic bodies of the Central Iranian Volcanic Belt or Urumieh-Dokhtar Magmatic Assemblage (UDMA), Iran. They are composed of a chain of cogenetic plutonic bodies with calc-alkaline and metaluminous affinity which were intruded in Tertiary time. These shallow-level plutonic complexes consist of various intermediate to felsic rocks which are surrounded by volcanic and pyroclastic rocks, such as andesites, dacites and felsic tuffs. Plutonic bodies of the Saveh region, UDMA, can be divided into two distinct assemblages including: (1) An older assemblage of relatively large bodies which are composed of diorite, quartz-diorite, quartz-monzodiorite, tonalite, granodiorite and monzogranite. These rocks contain cognate mafic microgranular enclaves in some places. (2) A younger assemblage of smaller intrusions that consist of leu-cocratic syenogranite and minor alkali feldspar granite. Rocks of these assemblages vary from Mg-rich to Fe-rich and calc-alkalic to calcic, and they are commonly metaluminous, diopside-normative and I-type. The younger assemblage has higher contents of SiO_2, K_2O, K_2O+Na_2O, K_2O/Na_2O, FeO~(total)/MgO, Zr, Rb, Ba, Nb and Th, but lower contents of A1_2O_3, MgO, CaO, FeO~(total) and Sr. Rocks of the younger assemblage have the same chemical affinities as the older rocks, and, therefore, they can be considered as highly fractionated products of the same mafic-intermediate mantle-derived parent magma. The tectonic setting for the plutonic assemblages is a possible ancient subduction zone (continental arc or ensialic island arc).