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Petrology, Geochemistry and Mineral Chemistry of Extrusive Alkalic Rocks of the Southern Caspian Sea Ophiolite, Northern Alborz, Iran: Evidence of Alkaline Magmatism in Southern Eurasia

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The alkalic basalts ofthe SCO ophiolite are made up of olivine, clinopyroxene (salite), plagioclase and Fe-Ti oxides. They show a narrow range of Sif), (45.2-48.85 wt. %) and MgO (3.59-4.85 wt. %) and are relatively enriched in Ti02 (3.13-3.82 wt. °Al). The rocks are enriched in incompatible trace elements such as Zr, Nb and Y. There is no evidence of significant crustal contamination; this may be related to the rapid ascent of the parental magma. Normalized trace element patterns and diagnostic elemental ratio are very similar to those of modern Ocean-Island Basalts (OIB) a feature which suggests that the mantle source region was the asthenosphere. Comparison with the different types of OIB indicates that the basalts may be derived from a high U/Pb (HIMU) source with slightly elevated K and Ba contents. The overall chemical characteristics suggest that the alkali basalts of the SCO were derived from a fertile mantle source and suggest that the magma was produced by a small-degree partial melting of a garnet lherzolite source. As inferred from geochemical and tectonic data, alkali rocks of the SCO were generated from a plume in a local extension regime.

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