مجله بلور شناسی و کانی شناسی ایران
سال هجدهم شماره 3 (پیاپی 41، پاییز 1389)

The structural of fresco pigments of period parti in kuh-e khwaja of Sistan were studied by XRD and Complementary EDX analysis. This study showed that Artist pigments were mainly used are inorganic.

Microstructural and petrographical features as well as field relations, show that formation of the metapelitic rocks of Bolandparchin and Poshtuk in northwestern Iran was poly-metamorphic. Mineral assemblages in the metapelites of two areas are different. The Bolandparchin schists are graphite-rich and have dark color. Peak metamorphic assemblage is garnet, biotite, staurolite, aluminosilicate minerals (andalusite/sillimanite), muscovite and quartz in these rocks, but Poshtuk metapelites are graphite-free and their mineral assemblages are garnet, staurolite, chloritoid, chlorite, muscovite and quartz, which usually form by contact metamorphism. The isograds are not compatible with Barrovian type, but reflect the first appearance of chloritoid in the aluminous bulk compositions. This study shows that in spite of same protolith and geothermal gradient in the both areas, existence of different mineral assemblages is due to presence or absence of graphite and consequently different aH2O, so that Poshtuk rocks are graphite-free and have aH2O ~ (0.8-1.0); therefore, chloritoid is formed. Bolandparchin rocks have aH2O ~ (0.5) and aluminosilicate minerals such as andalusite/kyanite/sillimanite are formed in these rocks.

Neogene high silica adakitic domes of south Quchan, cropped out in the northern part of the Quchan-Esfarayen Cenozoic magmatic arc (north of Sabzevar ophiolitic and metamorphic belt). In this volcanic belt, magmatic activities has been started since Eocene (about 40 Ma ago) and continued to Plio-Pleistocene (about 2 Ma ago). The ages of volcanic rocks range from Eocene to Plio-Pleistocene from south (in adjacent to the Sabzevar ophiolitic belt) to north (in south of Quchan) respectively. Northern part of this high silica adakitic arc is composed of pyroclastic units and several domes contain trachyandesites, trachytes, dacites and rhyodacites (2-12 Ma ago) which are usually overlain an olivine basaltic- basaltic basement of Eocene to Lower Miocene(19-20 Ma ago). Existence of Eocene volcanic enclaves and gneissic, siltstone, marl and pellitic enclaves, appearance and disappearance of some mineral phases, corrosions and chemical disequilibriums of some phenocrysts and sieve textures are some evidences of magmatic contamination. 87Sr/86Sr ratio ranges from 0.7041 to 0.7057 confirms this contamination. A clear positive anomaly in LREE and LILE and a negative anomaly in HREE found in the rocks of Neogene domes. Negative anomalies in HFSE (e.g. P, Nb, Ti) which is the indicator of arc settings, also found in these rocks. Calc-alkaline nature, continental arc subduction setting, presence of an eclogitic or garnet-amphibolitic source rock (resulted from metamorphism of Sabzevar subducted oceanic crust as a source of magma generation), high silica adakitic nature of magmatism (HAS) and the role of fractional crystallization, assimilation and magmatic contamination in the genesis and evolution of magma in these domes, indicated by the geochemical evidences. These adakitic magmas were the latest melts resulted from partial melting of young and hot Sabzevar Neotethyan subducted oceanic crust and its overlaying mantle wedge, which have been emplaced and manifested in the form of subvolcanic adakitic domes.

The chromite deposits of Aland (Barajok and Kochak villages) and Gheshlagh area in the Khoy ophiolite occur as layered, lenticular or irregular masses, surrounded by dunitie and harzburgites. These chromites are compositionally similar to alpine-type chromites, characterized by nodular, massive, disseminated and banded textures. EMPA data show that they vary widely in term of Cr-number [100Cr/Cr+Al]. On average, Cr# of chromites in harzburgites is 45.13, in disseminate Gheshlagh and Barajok chromites are 40.58 and 58.12 respectively but in Barajok and Kochak Chromitites are 66.7 and 73.43 respectively. The chromite composition in terms of Cr#, Mg#, Cr2O3, Al2O3, Fe2O3, MgO and TiO2 contents as well as correlation coefficients between different oxides, these chromits are comparable to podiform chromitites. The compositions of chromitites from Aland area with Cr#>66% and Gheshlagh area with Cr# about 40 wt% fall within high-Cr and high-Al types respectively. According to TiO2, Cr2O3 and Al2O3 content of the samples, it seems that the Aland chromites were formed from boninitic magmas in a supra-subduction zone although the Gheshlagh chromites were formed from tholeiitic magma in a geotectonic setting similar to the MORB.

In the mafic-ultramafic suite of Tazehkand, Kalaybar-NW Iran, of Early-Cenozoic age, pyroxenitic core is surrounded by gabbro and total complex is surrounded by Paleocene volcano clastic of Majid Abad Formation. The pyroxenite is composed chiefly of Cpx+Ol+Ore. The associated gabbro minerals consist of Pl + Cpx + Am + Phl. The pyroxene composition in the two rock types is diopside-rich and ranges between hedenbergite to Ca-tschermak. Plagioclase is anorthitic type and opaque mineral is mostly hematite. Some results of this investigation on the relationship between major phases activity and mineral chemical composition in these rocks are: Ca-tschermak activity in the pyroxenes is controlled with Al-content in T-site, Tschemakite activity in amphiboles has direct relation with (IV) Al, but reverse relations with (IV) Si, (VI) Al and (A) Na. Using several thermobarometric methods, temperature of 900 ºC for pyroxenites and pressure of 7-9 kbar on the same temperature for gabbro crystallization, are estimated. The temperature of observed ilmenite lamellae exsolution from hematite matrix during sub-solidus phenomena is detected about 470 ºC. Calculated Log fO2 for the source magma is -8 to -14. This quantity is higher for pyroxenites than gabbros.

The Mashhad ophiolitic and metamorphic complex was intruded by Mashhad granitoids during different episodes of magmatisms. Dehnow pluton with diorite-granodiorite composition was intruded at earliest stage of magmatism during Late Triassic (Norian, 215± 4My). This pluton is cut by the NW-SE trending ductile shear zones which are dominated by an intense ductile deformation. The rocks of Dehnow pluton along the shear zones are converted into protomylonite, mylonite and ultra-mylonite with steeply dipping mylonitic foliation and gently plunging stretch lineation on it. Kinematic analysis of shear sense indicators such as S-C fabrics, asymmetric folds, asymmetric porphyroclasts, mica fish and domino-type fragmented porphyroclasts reveal that the ductile deformation is related to right lateral reverse slip that may describe by a transpressional deformation regime. the minerals in the mylonitic rocks show variable microstructures such as patchy to crosshatch undulose extinction, shear fracture, deformation lamellae, subgrains formation, bulging dynamic recrystalization (BLG) and subgrain rotation recrystalization (SGR) in quartz grains, also subgrain formation, deformation twins, flame-shaped prethities, and dynamic recrystalization (BLG) in feldspars, these indicated that the ductile deformation have occured in 300-500 °C (upper green schist and lower amphibolites facies conditions). Age and cross cutting relations between various lithological units in the study area revealed that the ductile shear zones were formed during Cimmerian orogeny between Norian to Early Jurassic age.