Dokuz, Abdurrahman; Uysal, Ibrahim; Dilek, Yildirim; Karsli, Orhan; Meisel, Thomas; Kandemir, Raif

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 Beyçam harzburgite (BH) contains low Al2O3 (0.51-1.88 anhydrous wt.%) and high MgO (41.35-42.34 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 Beyçam harzburgite are highly depleted, whereas its Os, Ir, 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 lherzolite (PL) to the south and the absence of an isochronous relationship between its 187Os/188Os and 187Re/188Os show that the trace element distribution and the isotope ratios of the Beyçam 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 lherzolite also contains a secondary magmatic phase in the form of networks of clinopyroxene veins and channels, which are interpreted as an evidence for solid state melt-rock reactions between the lherzolite and a percolating basaltic melt above a subduction zone. This clinopyroxene addition resulted in the formation of variable concentrations of Al2O3 (2.47-4.33wt.%) and MgO (29.76-40.10wt.%) 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 187Os/188Os 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 Beyçam area represent the partial melting product of the moderately depleted Beyçam harzburgite and the basaltic parental melt from which the clinopyroxene precipitated. The covariation between the 187Re/188Os and 187Os/188Os of these basaltic rocks defines an isochron age of 377±8Ma (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.

Uysal, Ibrahim; Ersoy, E. Yalcin; Karsli, Orhan; Dilek, Yildirim; Sadiklar, M. Burhan; Ottley, Chris J.; Tiepolo, Massimo; Meisel, Thomas

We present new, whole-rock major and trace element chemistry, including rare earth elements (REE), platinum-group elements (PGE), and Re-Os isotope data from the upper mantle peridotites of a Cretaceous Neo-Tethyan ophiolite in the Mugla area in SW Turkey. We also report extensive mineral chemistry data for these peridotites in order to better constrain their petrogenesis and tectonic environment of formation. The Mugla peridotites consist mainly of cpx-harzburgite, depleted harzburgite, and dunite. Cpx-harzburgites are characterized by their higher average CaO (2.27 wt.%), Al2O3 (2.07 wt.%), REE (53 ppb), and Os-187/Os-188((i)) ratios varying between 0.12497 and 0.12858. They contain Al-rich pyroxene with lower Cr content of coexisting spinel (Cr# = 13-22). In contrast, the depleted harzburgites and dunites are characterized by their lower average CaO (0.58 wt.%), Al2O3 (0.42 wt.%), and REE (1.24 ppb) values. Their clinopyroxenes are Al-poor and coexist with high-Cr spinel (Cr# = 33-83). The Os-187/Os-188((i)) ratios are in the range of 0.12078-0.12588 and are more unradiogenic compared to those of the cpx-harzburgites. Mineral chemistry and whole rock trace and PGE data indicate that formation of the Mugla peridotites cannot be explained by a single stage melting event; at least two-stages of melting and refertilization processes are needed to explain their geochemical characteristics. Trace element compositions of the cpx-harzburgites can be modeled by up to similar to 10-16% closed-system dynamic melting of a primitive mantle source, whereas those of the depleted harzburgites and dunites can be reproduced by similar to 10-16% open-system melting of an already depleted (similar to 16%) mantle. These models indicate that the cpx-harzburgites are the products of first-stage melting and low-degrees of melt-rock interaction that occurred in a mid-ocean ridge (MOR) environment. However, the depleted harzburgites and dunites are the product of second-stage melting and related refertilization which took place in a supra subduction zone (SSZ) environment. The Re-Os isotope systematics of the Mugla peridotites gives model age clusters of similar to 250 Ma, similar to 400 Ma and similar to 750 Ma that may record major tectonic events associated with the geodynamic evolution of the Neo-Tethyan. Rheic, and Proto-Tethyan oceans, respectively. Furthermore, >1000 Ma model ages can be interpreted as a result of an ancient melting event before the Proto-Tethys evolution. (C) 2011 Elsevier B.V. All rights reserved.