--> --> Early Carboniferous Magmatism and Rift Tectonics in the Western Sector of Paleotethysas Evidenced by the Misho Mafic Complex (NW Iran)

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Early Carboniferous Magmatism and Rift Tectonics in the Western Sector of Paleotethysas Evidenced by the Misho Mafic Complex (NW Iran)

Abstract

The Misho mafic complex (MMC) in NW Iran represents represents the product of late Paleozoic magmatism and rifting associated with the opening of Paleotethys, and provides significant geochronological and tectonic constraints for is paleogeography and evolution. It consists of gabbro, anorthosite, troctolite, pyroxenite, olivine gabbro and diorite that are crosscut by numerous basaltic dykes. It is intrusive into the Kahar Infracambrian rocks representing the northern margin of Gondwana, and is overlain by Permian sedimentary rocks. U-Pb zircon dating of a leucogabbro dyke reveals the igneous emplacement age of the MMC as the early Carboniferous. Gabbros and basaltic dykes in MMC are represented by (1) a suite showing normal mid-ocean ridge basalt (N-MORB) affinity, and (2) another suite displaying plume-type MORB (P-MORB) affinity. The N-MORB rocks have almost flat N-MORB normalized incompatible element patterns, low La/Yb, Sm/Yb, Th/Yb, Ta/Yb Zr/Y ratios, and high Zr/Nb ratios. The P-MORB rocks show characteristic OIB-type trace element signatures, such as enrichments in Th, Ta, Nb and light rare earth elements (LREE) with respect to N-MORB, high La/Yb, Sm/Yb, Th/Yb, Ta/Yb Zr/Y ratios, and low Zr/Nb ratios. The N-MORB suite of the MMC was generated by ca 13% partial melting of a depleted MORB mantle (DMM) source, whereas the P-MORB suite was generated by ca 4–6% partial melting of a DMM source, which was metasomatized by variable proportions of OIB-type enriched components. The mantle melting appears to have started initially deep in the garnet-stability field of the mantle, and then shifted to shallow levels in the spinel-field with higher degrees of melting. Thus the MMC displays a record of melt evolution involving depleted MORB-type asthenosphere and plume-type mantle sources. Its mafic-ultramafic rocks represent an early Carboniferous plume-related magmatic event developed during the continental break-up of the northern edge of Gondwana that led to the opening of Paleotethys. This model is consistent with the well-documented late Devonian-early Carboniferous mantle plume activity along the Paleotethyan margins in central-eastern Asia to the east, and suggests that the initial rift-drift tectonics of Paleotethys was strongly affected by plume-related magmatism and associated lithospheric weakening at a regional scale.