Geochemical evidence for deep mantle melting and lithospheric delamination as the origin of the inland Damavand volcanic rocks of northern Iran

Abstract

The intraplate Damavand volcanic field that overlies the Alborz Mountains in northern Iran is dominantly comprised of trachyandesite and alkali olivine basalt Alkali olivine basalt is the most primitive rock type among Damavand lavas and exhibits a number of geochemical characteristics (e.g., high MgO, Ni and Cr abundances) that preclude substantial differentiation and crustal contamination. The lavas are light rare earth element enriched and have small negative Nb-Ta-Ti and positive Ba and Pb anomalies. The initial Nd and Sr isotope compositions of these lavas plot within the mantle array and cluster near bulk silicate Earth-values, while Pb isotope ratios are typical of modern upper crust. Oxygen isotope ratios range from + 6.5 to + 6.9‰. Although previous studies suggest that the mantle source of the Damavand volcanic rocks was metasomatized by the release of fluids during Tertiary subduction of Neo-Tethys oceanic crust under the Iranian Plate, the geochemistry of the alkali olivine basalts (e.g., BSE-like Sr and Nd isotopic compositions; the position of samples on Ti/V vs. Zr/Nb, Ce/Pb vs. Ce, and Th-Ta-Hf diagrams) is not reconcilable with derivation of the parental magma from such a mantle source. In addition, the Mid-Ocean Ridge Basalt (MORB) normalized trace element patterns of the Damavand alkaline olivine basalts most resemble those of Ocean Island Basalt (OIB) that implies the role of deep mantle source in their genesis. It seems that local upwelling of deep mantle materials underneath the Alborz Mountains resulted in the eruption of intraplate Damavand lavas. Compressional stress exerted on the Iranian plate following Neo-Thetys closure was the likely cause of sub-continental lithosphere elamination which led to such local mantle upwelling.

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