SR-ND-PB ISOTOPIC CONSTRAINTS ON THE ORIGIN AND TECTONOMAGMATIC EVOLUTION OF THE KARAJ DAM BASEMENT SILL: IMPLICATION OF MIDDLE TO LATE EOCENE LITHOSPHERIC THINING IN SOUTH-CENTRAL ALBORZ

During the Middle to Late Eocene, the Karaj Dam basement sill (KDBS), located within the Alborz mountain belt in northern Iran, intruded the middle and upper tuff units of the Karaj Formation as a saucer shaped sill. The well-exposed KDBS is made up of gabbro, monzogabbro, monzodiorite and monzonite with a shoshonitic affinity. The samples are enriched in LREE and LILE, depleted in HREE and HFSE and show significant negative Nb-Ti anomalies. Their low I_Sr = 0.70453–0.70535, moderate ƐNd (37.2 Ma) = 1.54–2.44 and T_DM ages ranging from 0.65 to 0.86 Ga, are consistent with a young and enriched lithospheric mantle source metasomatized by earlier activity of fluids derived from the subducted slab or sediments. They are identical with EM2 type end-member, which is generally interpreted as the recycled product of terrigeneous sediments from an earlier subducting setting. These interpretations are consistence with the ratios of ²⁰⁶Pb/²⁰⁴Pb = 18.43–18.67, ²⁰⁷Pb/²⁰⁴Pb = 15.59 and ²⁰⁸Pb/²⁰⁴Pb = 38.42–38.71 indicating an important role of chemically enriched mantle source and involvement of sediment derived component prior to partial melting. WR geochemical studies suggest a low degree partial melting of phlogopite bearing spinel peridotite source material for the KDBS parental magma. Rapid depletion in compatible elements vs. enrichment in incompatible elements and constant ƐNd values with increasing SiO₂ contents indicate that the melt has been evolved by means of fractional crystallization. I_Sr ratios show the role of crustal assimilation in their genesis as well. KDBS shoshonitic samples fall within the field of continental arcs in the tectonomagmatic discrimination diagrams indicating the evolution in a subduction-related setting. In summary, the pluton is considered to be emplaced at Middle to Late Eocene in an extensional arc enviroment following the lower Eocene slab rollback. The extension triggered asthenospheric upwelling and consequent thermal anomaly resulted in partial fusion of metasomatised and enriched subcontinental lithospheric mantle, produced by the ancient and prolonged subduction beneath the region. Then, the shoshonitic melt, which subsequently underwent fractional crystallization, ascent to shallower crustal levels to generate a rock series ranging from gabbro to monzonite.