some characteristics of mafic dykes in gondwanan land from south of gorgan, northeastern iran: implication to petrogenesis and paleotectonic

the igneous dykes are a small mafic rock unit exposed in the south gorgan of eastern alborz, north iran. these mafic dykes are intruded in the middle–upper paleozoic sedimentary and metamorphic rocks units representing a part of the north gondwana province. the studied samples were collected from various locations along the exposed bodies. thin sections of these rocks show medium to coarse grained sizes and the texture varies from ophitic to intergranular under polarized microscope. the mineralogical composition of mafic dykes is dominated by large crystals of clinopyroxene, orthopyroxene, plagioclase and hornblende. these rocks can be classified as gabbroic rock. geochemical studies show that these rocks have low to medium k2o contents. the trace element data shows low la/nb ratios (1.2–3.2) and lree enrichment (la/yb 10–21.4). the high lree/hree ratios and low y content corresponding high ti/y ratios of the gabbros suggest that they could be derived from melt fractions of a garnet stable source. according to present data, it could be suggested that these rocks have been formed in a rift setting from partial melting of an asthenospheric mantle. this tectonic setting could be explained by the initiation of the opening of the paleothetys in the northern part of gondwana during the late ordovician.