عنوان مقاله [English]
IntroductionNorth Razi basalts in Ardabil province are located in the longitude range of º52 º47 to 505 º48 and latitude ˝00 ´00 º39 to ́50 ° 38. North of Razi magmatic region extends to the city of Kalibar (Sudi and Moazzen., 2014) in the southern part of the Caucasus, which has Lower and Middle Eocene basaltic magmas from northeast to southwest. Also it has olivine gabbroic dykes infiltrated in the eastern part of the Middle Eocene to Oligocene. The studied magmatism is restricted by the southern basin of the Caspian Sea on the east and the Allahyarlu ophiolite belt and Aras fault on the west. Allahyarloo melange ophiolite emerges in the southwestern of Lahroud magmatic zones in Cretaceous and pre-Cretaceous. In this research the genesis and tectono-magmatic environment of the Middle and Late Eocene North Razi basalts and their relationship with the the subductional back arc environment, have been investigated.MetodologyAfter field studies, 45 rock samples were selected for the study based on field relationships and petrographic evidence. Then thirty thin sections of the samples were prepared for petrographic studies. To investigate the geochemistry of major and minor elements, 20 samples were sent to the Amdel Laboratory of the University of New South Wales in Australia for chemical analysis by XRF and ICP.DiscussionIn macroscopic studies, North Razi basalts are black in color and melanocrate in terms of color index. Microscopic studies show Plagioclase and clinopyroxene phenocrysts are the main minerals while olivine and titanomagnetite are the lowest manufacturers of these rocks and show glomeroporphiritic and hyalomicrolitic porphiritic texture. The matrix of these rocks is mostly composed of glass with plagioclase microcrystals, small pyroxene granules, small amounts of olivine and opaque minerals. The parent magma of these rocks have alkaline nature regarding geochemical data. Multi-element diagrams indicate the enrichment of LREEs in comparison with HREEs suggesting magma involvement in enrichment of the mantle. Major oxide variation diagrams versus SiO2 confirm the role of normal magma differentiation. Petrological and geochemical evidences suggest 1- 5% partial melting of spinel garnet lherzolite metasomatic mantle as a result of sub ducted slabs and their sedimentary melting. ResultsIn northwestern Iran, north of the Razi city, a series of prismatic basalts are exposed which according to the stratigraphic studies, are related to the Eocene period of the northern part of Talesh zone. Based on geochemistry, the studied rocks have alkaline to shoshonitic nature and have been formed by subtraction of primary magma at relatively high oxygen fugacity. The order of crystallization of minerals was olivine and then the simultaneous crystallization of plagioclase and clinopyroxene occurred, respectively. Examining the trend of changes in major and minor elements indicates a genetic relationship with a basic magma and the effect of the subtraction process. Also, the study of petrogenesis indicates metasomatism of origins by submerged ocean fragments and associated fluids. This is obtained by unbalanced melting of garnet with spinel lerzolitic origin. These olivine basalts were formed in a tensile back arc basin, after collision along the Allahyarlu suture zone, beyond the Cretaceous magmatic arc.