Mineralogy and geochemistry of the mafic dikes in the mantle sequence of ophiolitic complex from south of Soltan Abad (north east of Sabzevar)

Abstract

The occurrence of doleritic, microgabbroic, hornblende- gabbroic, pegmatoidic and gabbronoritic dikes is the conspicuous characteristics of mantle sequences of the ophiolitic complex from south Soltan Abad. Dikes trend is mostly western- eastern and because of deformation, some display boudin- domino structure. Pegmatoidic and doleritic dikes contain clinopyroxene and plagioclase minerals. Magmatic amphibole is observed in addition to clinopyroxene and plagioclase in the hornblende- gabbroic dikes. There are plagioclase, clinopyroxene and orthopyroxene in the gabbronoritic dikes. Microgabbroic dikes include plagioclase and secondary amphibole. Clinopyroxene is replacing by amphiboles in the most dikes. Occurrence of orthopyroxene and magmatic amphibole, pegmatoidic texture, pyroxene crystallization prior to plagioclase and anorthite rich composition of the plagioclase indicate water participation in the magma genesis. The pattern of the multi- element spider diagram of the most investigated samples shows noticeable resemblance to the suprasubduction ophiolites and Cenozoic fore arc basalts. Hornblende - gabbroic and doleritic dikes are alkali basalt, so derived from enriched lower mantle source rock. However, the others exhibit geochemical signatures of tholeiitic basalt with arc island affinity. Hence based on these documents, mafic- ultramafic complex of south Soltan Abad represents oceanic lithosphere of forearc from suprasubduction zone setting. According to the classification and tectonic setting discrimination diagrams, most dikes exhibit geochemical characteristics of tholeiitic basalt with boninitic and island arc affinity. Hornblende- gabbroic and doleritic dikes encompass alkali nature, so they have derived via melting of source rock from lower enriched mantle. The compositional sequence, probably represents incorporation of various mantle source rock during the evolutionary trend of the subduction zone at late Cretaceous.

Keywords

• dike
• ophiolitic complex
• suprasubduction zone
• soltan abad