Silurian anorogenic basic and acidic magmatism in Northwest Turkey: Implications for the opening of the Paleo-Tethys

The Sakarya Zone (northern Turkey) is a Gondwana-derived continental block accreted to northern Laurussia during the Carboniferous, and is regarded as the eastward extension of Armorica. Timing of its detachment from the northern margin of Gondwana, thus opening of the Paleo-Tethys, is poorly known. Here, we report on metagranite and amphibolite with Silurian igneous crystallization ages from the Early Carboniferous high-temperature/middle to low-pressure amphibolite-facies metamorphic rocks of the Sancakaya Massif within the Sakarya Zone (NW Turkey). The metagranite-amphibolite complex is exposed mainly along the southern margin of the Sancakaya Massif over an area of ca. 12 km by 15 km. The metagranite contains preserved domains of porphyric texture, indicative of derivation from a former granite porphyry. The amphibolite is devoid of any relict igneous texture. Both the metagranite and amphibolite are crosscut by late up to 50 cm thick felsic veins. Uranium-Pb dating on igneous zircons from both metagranite and amphibolite yielded Silurian ages of ca. 419 +/- 6 to 434 +/- 7 Ma (2 sigma), and on those from a felsic vein an age of 319 +/- 5 Ma (2 sigma) (Late Carboniferous). Geochemically, amphibolite displays anorogenic transitional tholeiitic to alkaline signatures. Initial cHf values of the igneous zircons from both metagranite and amphibolite show a large variation with medial values of -16 to -9 and + 3 to +6, respectively. Thus, the protoliths of amphibolite were derived from melts of depleted mantle, and those of the metagranite, on the other hand, from melts of reworked crustal material. We suggest that the Silurian anorogenic magmatism is related to a rifting event at the northern margin of Gondwana leading to the detachment of the Sakarya Zone and hence placing an age on the initial opening of the Paleo-Tethys. This interpretation is based on (i) the presence of Late Silurian to Devonian deep-sea sedimentary blocks in the PaleoTethyan accretionary complexes, and (ii) the resemblance of the U-Pb age spectra of detrital zircons in the metadastic sequence of the Sancakaya Massif to those of Cambro-Ordovician sandstones in Jordan (Gondwana), and (iii) the local occurrence of anorogenic A-type granites of Late Ordovician-Silurian age in the AnatolideTauride Block, a continental block which rifted from Gondwana during the Early Triassic. Wholly anorogenic nature of the Late Ordovician to Silurian igneous rocks in the Sancakaya Massif and reported in literature does not support the opening of the Paleo-Tethys as back-arc ocean, as suggested in most paleogeographic reconstructions. (C) 2019 Elsevier B.V. All rights reserved