Geochemical constraints on the Cenozoic, OIB-type alkaline volcanic rocks of NW Turkey: Implications for mantle sources and melting processes

Abstract

The volcanic province of North-West Turkey contains a number of intra-continental alkaline volcanic eruption sequences formed along the localized extensional basins developed in relation with the Late Cenozoic extensional processes. The volcanic suite comprises the extracted melt products of adiabatic decompression melting of the mantle that are represented by small-volume intra-continental plate volcanic rocks of alkaline olivine basalts and basanites with compositions representative of mantle-derived, primary (or near-primary) melts. The volcanic rocks have near-uniform 87Sr / 86Sr (0.70316–70353) and 143Nd / 144Nd (0.51291–0.51297; ɛ Nd = 5.08–6.32) ratios and are characterized by Ocean Island Basalt (OIB)-type trace element patterns with significant enrichment in LILE, HFSE and L-MREE, and a slight depletion in HREE, relative to N-MORB. Trace element variations of individual basaltic eruption sequences indicate that each lava sequence shows remarkably similar temporal–compositional trends that are characterized by an increase in incompatible elements and MgO, with decreasing SiO 2, as melt production proceeds. Systematic change in incompatible trace element concentrations (and ratios) of the lavas is not reflected by the isotopic ratios, leading to the suggestion that the temporal–compositional trends are not caused by source variations. The variations in melt chemistry with time does not reflect fractional crystallization nor can it be explained by variable proportions of mixing between melts produced by different degrees of partial melting of two (or more) compositionally distinct sources in the mantle. Instead, the observed trends are consistent with a progressive decrease in degree of melting from early-formed alkali olivine basalts to later basanites and systematic mixing between increments of melt derived from the same source but probably at different depths. Quantitative trace element modeling of fractionation-corrected data indicates that the mafic alkaline magmas originated from mixing of melts produced by variable degrees (∼2% to 8%) of incremental partial melting of a compositionally uniform, volatile-bearing mantle domain that is enriched in all incompatible elements (e.g. LILE, HFSE and L-MREE) relative to hypothetical Depleted MORB Mantle (DMM) and/or Primitive Mantle (PM) compositions.