The Emplacement Mechanism of the Gol-e-Zard Granodiorite Pluton, North of Aligoudarz, West of Iran, by Using of AMS Method

The Gol-e-Zard granitoidic pluton with an age of ~ 165±5 Ma is located in the north of Aligoudarz city, Lorestan province, in the Sanandaj-Sirjan structural zone. The pluton intruded into the late Triassic-early Jurassic slates, phyllites and micaschists. The granodiorites, dominant rock types of the pluton, are cut by the leucogranites, aplitic veins, pegmatites and gabbro-dioritic dykes. The S-type nature of the Gol-e-Zard granite is evidenced by the presence of metaplitic enclaves (surmicaceous and andalusite-sillimanite hornfels), silica enclaves, andalusite and garnet xenocrysts and silica veins relicts. The anisotropy of magnetic susceptibility (AMS) method was used to investigate the mode of emplacement. The total numbers of 868 oriented core samples were analyzed for magnetic susceptibility. The obtained mean susceptibility (Km) in µЅІ was 227, 57 and 585 for granodiorites, leucogranites and gabbrodiorite dykes, respectively. The low Km values (<500 µЅІ) is characteristics of biotite bearing granites (paramagnetic granite), which biotite is the main carrier of magnetic property of the rock. The leucogranites have the lowest Km due to the low concentration of biotites in these rocks. The main identified microstructural types of the studied pluton are magmatic, moderately deformed, and solid-state deformed at high temperature, which among magmatic type is predominant. The various magnetic data (magnetic lineation and foliation maps, Km, P% and T parameters) complemented by the field and microstructural observations revealed that the pluton originated the identified feeder zones in an extensional space related to the dextral shear zone. We assumed that the succession of intrusion began with the granodiorites as a large dyke structure, succeeded by the leucogranites as small body (apophyse) and cutting veins. The intrusion ended by mafic to intermediate dykes along the secondary fracturing occurred after the pluton emplacement.