Mineral chemistry of Tonalitic Stock of Mahirud complex, Sistan suture zone, East of Iran

The Mahirud volcano-plutonic complex (Cheshme-Ostad Group) in east of Iran is tectonically situated in the north-eastern part of the Sistan suture zone near to the boundary with the Afghan block. This tectonic zone includes the Retuk complex in the west and the Neh complex in the east. These two accretionary-prism-ophiolitic assemblages underlie the Sefidabeh forearc sedimentary basin. The Mahirud complex consists of the Upper Cretaceous volcano-sedimentary sequence which is structurally overlain by the Paleogene sediments. The Upper Cretaceous-Lower Tertiary stratigraphical formations in this area are subdivided into two distinct groups: 1-The upper group includes the Paleocene-Early Eocene volcano-sedimentary succession; 2- The lower group Includes the Cretaceous flysch-type sediments. These units construct the Lahno anticline. The most important igneous constituents of the Mahirud volcano-plutonic complex comprises in a younger part including the acidic intrusions and an earlier part including intermediate to basic extrusions. Acidic rocks injected as intrusive bodies contain tonalite, granodiorites, quartz diorite and rare leuco-plagioclase tonalite and tronhjemite. The basic to intermediate rocks appeared as pillow to flow volcanic and sub-volcanic rocks including of basalts, basaltic andesite, andesite, tuff, and dacitic dikes. Field observation clearly suggests that the tonalitic stocks injected after the extrusion of the lavas, and enclosed, assimilated, and altered the basaltic pillow lavas. The prepared thin-polished sections were studied by polarized microscope for petrographic descriptions in University of Sistan and Baluchistan. Electron probe micro analysis (EPMA) of minerals was conducted in wavelength-dispersion mode on a JEOL JXA-8600 super probe at Yamagata University of Japan with accelerating voltage of 15 kV, a beam current of 20 nA, a beam diameter of about 5 μm, detection limits of 0.05 wt. %, and maximum 40 s counting interval. According to the results of the previous research, volcanic rocks have the characteristics of the calc-alkaline to tholeiitic magmatic series and have been attributed to a rift tectonic setting. However, the results of recent work based on the spider diagram patterns normalized to N-MORB and Chondrite show the magmatic rocks are similar to the ones belong to the supra-subduction zone (SSZ) and especially recent Island arcs ( IAT) setting. The studying of the EPMA on the key minerals provides the same results. Plagioclase, hornblende, quartz, and biotite are the main minerals of the tonalitic stocks. According to EPMA, the chemical composition of plagioclase illustrates ranging from bytownitein the core to albite in the rim documenting a normal zoning resulted by a normal differential crystallization. Amphibole is a calcic magnesio-hornblende and could crystalize by a magma originated in a subduction zone. According to Aluminum content in amphibole, the crystallization characters such as pressure, temperature, and oxygen fugacity are respectively between 1 and 2 kb, 700 and750°C, and a high fO2. These results confirm the consequence of our other studies which assumed the Mahirud volcano-plutonic complex was formed in an intra oceanic subduction zone.