Geochemistry, Re-Os isotopes and highly siderophile element abundances in the Eastern Pontide peridotites (NE Turkey): Multiple episodes of melt extraction-depletion, melt-rock interaction and fertilization of the Rheic Ocean mantle

Abstract

We report on the structure, geochemistry, Re-Os isotopes and relative abundances of highly siderophile elements (HSEs) of the Paleozoic peridotite-basalt occurrences in the Eastern Pontide belt of northeastern Turkey. These peridotites and the associated basaltic rocks are the remnants of the Rheic oceanic lithosphere, incorporated into the Eurasian continental margin during the Variscan (Hercynian) orogeny. The peridotites display a complex record of multiple magmatic, metasomatic and metamorphic events in different tectonic settings during the evolution of the Rheic upper mantle. The Beycam harzburgite (BH) contains low Al2O3 (0.51-1.88 anhydrous wt.%) and high MgO (4135-4234 wt.%) contents, and its bulk-rock trace element compositions are less than 10% of the primitive upper mantle (PUM) values. The platinum, Pd and Re contents of the Beycam harzburgite are highly depleted, whereas its Os, It, and Ru contents are slightly enriched relative to the PUM values. Its Pd and Re contents that are higher than those of the fertile Pulur Iherzolite (PL) to the south and the absence of an isochronous relationship between its Os-187/Os-188 and Re-187/Os-188 show that the trace element distribution and the isotope ratios of the Beycam harzburgite were significantly modified after the first melt-extraction episode. The first melt extraction occurred beneath the Rheic mid-ocean ridge spreading center, whereas the second melt extraction occurred in a mantle wedge above a Rheic subduction zone. The primary magmatic phases of the Pulur lherzolite show the geochemical characteristics typical for fertile lherzolite, formed in the early stages of oceanic lithosphere generation subsequent to the continental break up. The Pulur Iherzolite also contains a secondary magmatic phase in the form of networks of dinopyroxene veins and channels, which are interpreted as an evidence for solid state melt-rock reactions between the Iherzolite and a percolating basaltic melt above a subduction zone. This dinopyroxene addition resulted in the formation of variable concentrations of Al2O3 (2.47-433 wt.%) and MgO (29.76-40.10 wt.%) in the lherzolite. The rhenium, Pd and Pt concentrations of the Pulur lherzolite are depleted relative to the PUM values, whereas the Os, Ir and Ru concentrations are in the range of the PUM values as commonly observed in peridotites with a melt depletion history. The high suprachondritic Os-187/Os-188 is however, inconsistent with a simple melt depletion history, and can be explained by the addition of radiogenic Os-bearing sulfide phases into the lherzolite as a result of melt-rock reactions. Basaltic rocks with an island arc tholeiitic composition from the Beycam area represent the partial melting product of the moderately depleted Beycam harzburgite and the basaltic parental melt from which the dinopyroxene precipitated. The covariation between the Re-187/Os-188 and Os-187/Os-188 of these basaltic rocks defines an isochron age of 377 +/- 8 Ma (late Devonian). The combined structural, geochemical and isotope data indicate a prolonged history of multiple episodes of melt extraction-depletion, and melt-rock interaction and fertilization of the mantle lithosphere of the Rheic Ocean. (C) 2013 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.