M. Rahgoshay; J. Mehdipour Ghazi; H. Shafaii Moghadam
Abstract
Peridotites (Cpx-bearing harzburgites, lherzolites, wherlites and dunites) associated with pegmatite gabbros, gabbro impregnations, gabbroic-diabasic dikes, pyroxenites and websterites are the main constituents of the mantle sequence in the Nain ophioliote assemblage. Petrographic and geochemical studies ...
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Peridotites (Cpx-bearing harzburgites, lherzolites, wherlites and dunites) associated with pegmatite gabbros, gabbro impregnations, gabbroic-diabasic dikes, pyroxenites and websterites are the main constituents of the mantle sequence in the Nain ophioliote assemblage. Petrographic and geochemical studies indicate that peridotites in the Nain ophiolites are Cpx-bearing harzburgites with Al-rich spinels (Cr#=16.12-41.55) equilibrated at about 1000˚C. Pegmatite gabbros are found as small pockets within peridotites showing deformed clinopyroxenes and plagioclases. Gabbro-diabase dikes, mostly rodingitized, injected into the peridotites without chilled margins. These dikes are geochemically co-genetic with pillow lavas and micro-gabbros of the crustal sequence, but display variable degrees of partial melting. Based on the geochemistry of the mafic rocks and the peridotites, we suggest that Nain ophiolites were developed in a back arc spreading basin.
H. Shafaii Moghadam; M. Rahgoshay; H. Whitechurch; I. Monsef; A. Soltan Mohamadi
Abstract
Basalts, gabbros, tuffs, diabasic dikes, agglomerates and breccias and the rock fragments of breccias are the main mafic constituents of the Balvard-Baft ophiolites. Geochemical signatures of these rocks reveal depletion in HFSE and enrichment in LILE and simultaneous occurrence of island-arc tholeiitic ...
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Basalts, gabbros, tuffs, diabasic dikes, agglomerates and breccias and the rock fragments of breccias are the main mafic constituents of the Balvard-Baft ophiolites. Geochemical signatures of these rocks reveal depletion in HFSE and enrichment in LILE and simultaneous occurrence of island-arc tholeiitic and calc-alkalic volcanisms. These geochemical behaviors are the evidences suggesting that these mafic rocks are of supra-subduction zone affinity. These special signatures associated with the position of the Nain-Baft ophiolitic belt behind the Mesozoic magmatic arc of the Sanandaj-Sirjan zone, suggest back-arc spreading (in the active margin of the Central Iranian block), during middle Cretaceous due to the oblique subduction of Neo-Tethys beneath the Central Iranian block.
M. Rahgoshay; I. Monsef; H. Shafaii Moghadam; M. Mohajjel
Abstract
Petrofabric, structural and geochemical study of the ultramafic tectonites in the Khoy ophiolitic complex suggest that these tectonites including low-temperature NW-SE shear zones cut the high-temperature to medium-temperature NE-SW mantle flow direction. Microstructures in these tectonites, record a ...
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Petrofabric, structural and geochemical study of the ultramafic tectonites in the Khoy ophiolitic complex suggest that these tectonites including low-temperature NW-SE shear zones cut the high-temperature to medium-temperature NE-SW mantle flow direction. Microstructures in these tectonites, record a fabric transition from oblate porphyroclastic and oblong porphyroclastic textures (related to the high- and medium-T deformations in mantle sections) to mylonitic textures (with low-T deformation in the shear zones). The study of olivine LPO patterns in high- and medium-T deformation samples of mantle shows slip on the (010) [100] high-temperature-low stress and (0kl) [100] moderate-temperature systems (up to 1000o) while olivine LPO patterns in the low-T deformation samples within the shear zones indicate gliding along (001) [100] low-temperature slip system (800-900o). Spinels in these peridotites show high variations in Cr number (10 to 90) and Mg number (50 to 90). Cpxs rich in Cr suggest a low degree of partial melting in these peridotites. The very variable composition of the spinels may be the result of partial melting process and recrystallization of these minerals in the mantle lithosphere during the detachment phase and the development of the shear zones.