Petrology and geochemistry of Ghoshchi batholith, NW Iran

Petrology and geochemistry of Ghoshchi batholith, NW Iran

The Ghoshchi batholith, -150 km2 in size is a granitoidic pluton, which intruded the Permian country rocks, in Sanandaj-Sirjan Zone, NW Iran. This granitoidic pluton is covered by Oligocene-Miocene sedimentary rocks known as Qom Formation. The Ghoshchi batholith comprises five plutons with following compositions: (a) gabbro-diorite (b) biotite granite, (c) alkali granite, (d) syenites, and (e) aplitic dikes. Gabbro-diorites are the oldest intrusive unit and have interaction zone with biotite granites. These rocks have within-plate tholeiitic nature. Graphic, microgranophyric, and perthitic textures can be found in alkali- feldspar granites, indicate their shallow emplacement depth and hypersolvus nature. Alkali-feldspar granites geochemically are high-k alkaline, metaluminous to mildly peralkaline. The alkali-feldspar granitic rocks contain lower A1203, CaO, Fe203, Ti02, Ba, Rb, and Sr but higher Si02, Na2Q, K20, Nb, Th, Y and Zr than biotite granites samples. Alkali-feldspar A-type within-plate granites were presumably formed by high degree of fractional crystallization of mantle derived mafic magmas. Plagioclase and amphibole are two main fractionated minerals. The Alkali-feldspar granites fall into the Ai group (mantle derived) suggesting an anorogenic tectonic setting. Biotite granites and syenites are peraluminous and have crustal source.