Petrology of major granitic plutons of the northwestern part of the Sanandaj-Sirjan Metamorphic Belt, Zagros Orogen, Iran: with emphasis on A-type granitoids from the SE Saqqez area

摘要

Various types of granitic rocks occur in several plutonic complexes in the Sanandaj-Sirjan Metamoiphic Belt (SSMB), Zagros Orogen, Iran including I-type, M-type, S-type and H-type granitoids. A-type granites are less abundant; their outcrops are seen in some localities such as Muteh, Almogholagh and Saqqez in the northwest of the SSMB. Radiometric dating has resulted in upper Jurassic to Tertiary ages for major plutonic bodies, especially for granitic rocks of the SSMB. These bodies indicate a magmatic suite generated in a continental arc setting followed by collisional and post-collisional magmatism. In the Saqqez area, there are two contrasting granitoid complexes (G_1 and G_2). G_1 granitoids are peralkaline whereas G_2 granitoids are subalkaline (calc-alkaline) and metaluminous. G_1 granitoids are ferroan alkali and ferroan alkali-calcic but G_2 granitoids are magnesian and calcic. G_1 granitoids show agpaitic trends on Barker diagrams. They contain high concentrations of alkalis, Zr, Rb, Nb, Y, Ce, Th, high FeO/MgO ratios and low concentrations of Mg, Ca and Sr. According to tectonic discrimination diagrams, G_1granitoids plot into the field of within plate granites and G_2 granitoids into the field of granitic rocks of volcanic arc setting. Considering petro-graphic and geochemical characteristics it seems likely that G_1 granitoids are of A-type whereas G_2 granitoids have similar properties to I-type granites. G_1granitoids of SE Saqqez resemble the A_2 subtype of A-type granites, which are generated in a post-orogenic/post-collisional setting.