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Application of garnet chemistry in thermodynamic studies of Dehnow Tonalite (Northwest of Mashhad)
Abstract
[PUBLISHED IN: IRANIAN JOURNAL OF CRYSTALLOGRAPHY AND MINERALOGY] ****** Hornblende biotite bearing tonalite from the west of Dehnow comprises of quartz, calcic plagioclase (andesine-labradorite), garnet (mostly almandine), biotite (annite to siderophyllite), calcic amphibole (mainly ferrohornblende) and accessory minerals of chlorite, epidote, calcite and ilmenite. According to thermobarometry of amphiboles, plagioclases and garnets chemistry that have CaO content of about 4.91-5.48 wt% and MnO content of about 1.89-2.40 wt%, these garnets have crystallized in temperature and pressure ranges of 696 to 950 o C and 6.4 to 12 kbar, and in a greater depth in contrast to the amphiboles and plagioclases.
... It is thought that the Silurian opening of the Palaeo-Tethys in northern Iran followed northward subduction beneath the Turan plate (the southern part of Eurasia) in the Late Devonian and the collision between the Iranian microcontinent and the Turan plate in Late Triassic time (Alavi 1991;Natalin and Sengör 2005). Mineralogical and geochemical works by Samadi (2009) and Samadi et al. (2012aSamadi et al. ( , b, 2014, together with the isotopic data of Mirnejad et al. (2013) confirm that the Dehnow DTG suite is metaluminous to mildly peraluminous with a calc-alkaline I-type character, and formed after the subduction of the Palaeo-Tethys oceanic slab beneath the Turan block along the Alpine-Himalayan suture zone. Karimpour et al. (2010) suggested that remnants of the Palaeo-Tethys crust (meta-ophiolite and meta-flysch) were intruded by granitic plutons in the Triassic, based on zircon U-Pb dating (215 ± 4 Ma for the DTG pluton). ...
The Triassic Dehnow pluton of NE Iran is a garnet-bearing I-type calc-alkaline metaluminous diorite-tonalite-granodiorite intrusion. The parental magma formed as the result of partial melting of intermediate to felsic rocks in the lower crust. Petrological and geochemical evidence which favors of a magmatic origin for the garnet includes: large size (~10 to 20 mm) of crystals, absence of reaction rims, a distinct composition from garnet in adjacent metapelitic rocks, and similarity in the composition of mineral inclusions (biotite, hornblende) in the garnet and the same minerals in the matrix. Absence of garnet-bearing enclaves in the pluton and lack of sillimanite (fibrolite) and cordierite inclusions in magmatic garnet suggests that the garnet was not produced by assimilation of meta-sedimentary country rocks. Also, the δ18O values of garnet in the pluton (8.3 to 8.7‰) are significantly lower than δ18O values of garnet in the metapelitic rocks (12.5 to 13.1‰). Amphibole-plagioclase and garnet-biotite thermometers indicate crystallization temperatures of 708 and 790 °C, respectively. A temperature of 692 °C obtained by quartz-garnet oxygen isotope thermometry points to a closure temperature for oxygen diffusion in garnet. The composition of epidote (Xep) and garnet (Xadr) indicate ~800 °C for the crystallization temperature of these minerals. Elevated andradite-content in the rims of garnet suggest that oxygen fugacity increased during crystallization.
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