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Volume 19, Issue 2 (7-2011)
Abstract
Eocene- Oligocene volcanic and subvolcanic bodies of Masjed-Daghi Located in 35 km East of Jolfa close of Aras River. On the base of structural geology the study area is a part of the West Alborz-Azarbayejan zone. Volcanic rocks are rhyolite, dacite to trachyandesite and subvolcanic rocks are porphyritic monzonite to diorite. The Formation of these rocks related to subduction zone in an active margin continent that are described by enriched LREE and LILE elements rather than HREE and HFSE, negative anomaly Ti, Ta and Nb elements and high ratio Ba/Nb and Ba/Ta in diverting tectonic setting diagrams. In this area, chemical characteristics such as SiO2>57%, MgO<3%, low ratio Y and Yb (Y<13 and YB<1.4), Sr/Y>40 and La/Yb>20 are representing formation from full of SiO2 adakitic magma. These evidences with geochemical diagrams probably indicate that studied rocks product from melting ocean crust along component eclogite and garnet amphibolite.
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Volume 23, Issue 4 (1-2016)
Abstract
Ophiolitic belt of Northern Sabzevar is invaded by basic, intermediate and acidic post-ophiolitic magmtism as dome, dyke and cone. Microscopically, the texture of the studied samples is porphyric and trachytic. Phenocryst mostly consists of amphibole, feldspar and clinopyroxene. All researchers that have investigated basic to acidic post ophiolitic magmatism of northern Sabzevar have consensus about their subduction related genesis. According to geochemical characteristics, basic and intermediate samples are resemble to normal arc island magma whilst rhyolitic and dacitic samples display adakitic nature. According to the binary diagrams, adakitic characteristic of these samples are as a consequence of amphibole, titanite and zircon fractionation. The existence of amphibole phenocryst and titanite and zircon as accessory minerals confirming this process as well. According to the barometry results, amphibole fractionation from water-rich melt of subduction zone at depth of equivalent to the middle and lower continental crust led to creation of adakitic signatures in the ultimate dacitic and rhyolotic magma.
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Volume 26, Issue 2 (7-2018)
Abstract
Quchan- Esfarayen magmatic belt (north of Sabzevar) include Neogene adakitic domes with andesite to rhyolite in composition which is cut by Jurasic sedimentary rocks, Eocene volcano-sedimentary rocks, Miocene sedimentary rocks and even occasionally Peliocene conglomerate. The main minerals of these rocks are plagioclase and amphibole with various textures such as felsitic porphyry, microlitic porphyry, glomero porphyry, sieve and flow textures. The calc-alkaline and metaluminous to peraluminous nature, enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREE) and depletion in heavy rare earth elements (HREE) in primitive mantle and chondrite normalized spider diagrams, along with high contents of Na2O, Al2O3, Sr, high ratio of Sr/Y and low ratio of K2O/Na2O and depletion in HFSEs in these rocks are characteristics of the young arc volcanics. Furthermore, these rocks display initial 87Sr/86Sr ratios of 0.70390 to 0.70562 and εNd(t) values of -0.86 to +4.98 respectively, which show that they are originated from partial melting of oceanic slab with crustal contamination. Emplacement age of these rocks into the quchan-Esfarayen magmatic belt obtained with U–Pb dating on separated zircons of 17.83±0.24 to 8.50±0.34 Ma. The results show that parental magma of the rocks generated by partial melting of a garnet-amphibolitic to eclogitic subducted Sabzevar Neo-Thethyan oceanic slab underneath the Binaloud continental lithospher. Presence of continental metamorphic and sedimentary xenoliths, corrosion and chemical disequilibrium of the phenocrysts and their sieve textures in plagioclase along with Sr-Nd isotopic ratios in these rocks indicate the operation of differentiation crystallization, assimilation and crustal contamination (AFC) in the genesis and evolution of igneous rocks of this belt.