Petrology and geochemistry of Ghoshchi batholith, NW Iran

The ghoshchi bathol ith, ~ 150 knr' in size is a granitoidic pluton, which intruded the permiancountry rocks, in sanandaj-sirjan zone, nw iran. this granitoidic pluton is covered by oligocene-miocenesedimentary rocks known as qom formation. the ghoshchi batholith comprises five plutons with followingcompositions: (a) gabbro-diorite (b) biot ite gran ite, (c) alkali granite, (d) syenites, and (e) aplit ic dikes .gabbro-diorites are the oldest intrusiv e unit and have interac tion zone with biotite gran ites. these rocks havewithin -plate tholeiitic nature. graphic, microgranophyric, and perthitic textures can be found in alkal ifeldspargranites, indicate their shallow emplacement depth and hypersolvus nature. alkali-feldspar gran itesgeochemically are high-k alkaline, metaluminous to mildly peralkaline. the alkali -feldspar granitic rockscontain lower a1203, cao, fe203, ti0 2, ba, rb, and sr but higher si0 2, na20, k20, nb, th, y and zr thanbiotite granite s samples. alkal i-felds par a-type within-plate granite s were presumably formed by highdegre e of fractional crystall izat ion of mantle derived mafic magmas. plagioclase and amphibole are twomain fractionated mineral s. the alkali-feldspar granites fall into the at group (mantle derived) suggestingan anorogenic tectonic setting. biotite granites and syenites are peraluminous and have crustal sourc e.