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.

Dokuz, Abdurrahman; Uysal, Ibrahim; Kaliwoda, Melanie; Karsli, Orhan; Ottley, Chris J.; Kandemir, Raif

The boundary between the Gondwana and Laurussia, or a terrane separated from it, is traced by a suture recording the closure of the Rheic ocean in the Pulur, Beyçam and Kurtoǧlu areas of the eastern Sakarya Zone of Turkey. Pulur lherzolite has a lower Cr# [100Cr/(Cr+Al)=5-37] of the spinel and slightly enriched flat medium-heavy rare earth element (M-HREE) whole-rock pattern typical for those of the fertile abyssal-type lherzolite. By contrast, the Beyçam harzburgite displays the mineralogical and geochemical features of a moderately depleted residue at a supra subduction zone (SSZ). Some of spinels from the Beyçam peridotites show TiO2 contents (0.15-0.48wt.%) identical to a later interaction with a melt of island-arc tholeiite (IAT) composition. Meta-basalt from the Beyçam area is the representative of such a melt and displays the whole-rock M-HREE pattern of a complementary product of the Beyçam harzburgite. Data presented in this study support the primarily northward directed closure of the Rheic ocean, but suggest that the Variscan basement in the eastern part of the Sakarya Zone consists of mantle peridotites formed in a south-vergent SSZ. This dual polarity of subduction in the Rheic ocean, at least in the second half of its closure, is also favored by the geological structure and lithological diversity of the Variscan rocks. These are: (1) spatial distribution of the peridotites, e.g., SSZ-type harzburgites to the north and abyssal-type lherzolites to the south, (2) lack of high-pressure metamorphic rocks, (3) decrease in the degree of Variscan metamorphism toward the north, and (4) emplacement of Variscan syn-collisional magmatism into the low grade metamorphic rocks to the north. Paleotethys was the other ocean of the region created in the Late Paleozoic. Northward subduction of the Rheic ocean from Early Devonian to Early Carboniferous appears to be the most plausible mechanism responsible for the opening of Paleotethys. In this scenario the Paleotethys is envisaged as a back-arc basin opened to the north in the wake of separated terranes from Laurussia. Onset of trench-Rheic mid-ocean ridge collision in the Early Devonian along the northern margin of Laurussia has been proposed to account for the initiation of SSZ-type subduction to the south. The short duration of this subduction appears insufficient to open a fully developed back-arc basin to the south because the elimination of Rheic mid-ocean ridge to the north accelerated the oceanic closure.

Uysal, Ibrahim; Kaliwoda, Melanie; Karsli, Orhan; Tarkian, Mahmud; Sadiklar, M. Burhan; Ottley, Chris J.

The bulk composition (major and trace elements) and the composition of minerals in peridotites of the Ortaca area, south-western Turkey, have been investigated to understand the processes controlling lithological and compositional variations in the dismembered mantle section, containing clinopyroxene-bearing harzburgite, harzburgite and dunite. The Ortaca peridotites vary from almost fertile clinopyroxene-bearing harzburgite (1.92-2.42 wt% Al2O3, 2.00-2.64% CaO) with a lower Cr# of the spinel phase (<24) to depleted harzburgite and dunite (0.17-1.60% Al2O3, 0.32-1.71% CaO) with a higher Cr# of the spinel phase (31-76). Systematic variations of major and trace elements of whole rocks and mineral phases are related to varying degrees of partial melting (up to 30%). The depleted harzburgite and dunite display LREE-enriched primitive-mantle-normalized patterns, suggesting mantle metasomatism, probably by fluid-bearing melt derived from the subducted slab, due to the melt-rock interaction process, whereas clinopyroxene-bearing harzburgite show HREE-enriched patterns. The Ortaca peridotites have low temperatures of equilibration relative to abyssal peridotites, because of H2O-assisted diffusional equilibration in the suprasubduction zone. They are characterized by oxygen fugacities Δlog FMQ between -1.61 and +0.74. Clearly, the clinopyroxene-bearing harzburgites define an increasing oxygen fugacity with almost constant Cr#, whereas the refractory harzburgite and dunite show a strong positive correlation between oxygen fugacity and Cr#. In view of the different measurements on minerals and whole rocks, and as a result of calculated temperatures and oxygen fugacities, we contend that the peridotites were subsequently affected by percolating hydrous boninitic melt, from which the high-Cr, low-Ti chromitites were formed, surrounded by a dunite envelope. It is likely that such an envelope of dunite resulted from interaction between melt and harzburgite, which caused the local removal of orthopyroxene within the mantle wedge above the suprasubduction zone.