Middle Permian calc-alkaline basalts and ferroan rhyolites in the Istanbul Zone, NW Turkey: Evidence for Permo-Triassic subduction

Middle Permian bimodal volcanic rocks exposed in the Kocaeli Peninsula represent the first igneous event in the entire Paleozoic record of the Istanbul Zone together with coeval acidic intrusions reported from other parts of the zone. These volcanic rocks crop out as intercalations at the lower horizons of Permian-Earliest Triassic fluvial sedimentary rocks and mainly include basalts and rhyolites with subordinate andesites and rhyolitic tuffs. The basalts were derived from 1-3% partial melting of spinel peridotite in the lithospheric mantle; their high Mg-numbers (Mg# = 63-68) along with Ni (85-136 ppm) and Cr (198-240 ppm) concentrations point to derivation from near-primary mantle melts with minor fractionation. These rocks did not undergo low-pressure plagioclase crystallization based on the lack of a Eu anomaly (Eu/Eu* = 0.95-0.99). Their vesicles are filled by secondary calcite, epidote, pumpellyite, albite and chlorite due to hydrothermal alteration under subgreenschist facies conditions whereby temperatures ranged between 250-300°C. The rhyolites are ferroan [FeO*/(FeO*+MgO) = 0.87-0.96], characterized by high Zr concentrations (279-464 ppm) and compositionally similar to A₂-type granitic magmas. Incompatible trace element ratios, rare earth element patterns, initial εNd isotopic data along with temperatures of the rhyolitic melts and absence of inherited zircons in the rhyolites collectively suggest that the rhyolites were derived from fractional crystallization of some basaltic melts in a crustal magma chamber with plagioclase fractionation and minor crustal contamination while the basalts were directly derived from the lithospheric mantle and reached the surface with negligible fractionation. Both volcanic rocks display diagnostic features of subduction-zone melts such as (i) medium- and high-K calc-alkaline affinity and (ii) enrichment in large-ion lithophile elements (LILE) but depletion in high-field strength elements (HFSE) (e.g., Nb-Ta troughs). U-Pb dating of zircon grains extracted from one rhyolite sample yielded a concordia age of 262.7 ± 0.7 Ma (2σ) (Capitanian). The observation that the rhyolites occur near the base of the associated sedimentary rocks places a tight constraint on the age of deposition of these deposits. The bimodal nature of the volcanic rocks, A₂-type signature of the rhyolites, local stratigraphic record and data from regional geology (e.g., possible correlation with Late Permian-Early Triassic A-type rift-related granites in Carpathians and Balkans) all indicate an extensional event in the region which started in Middle Permian and resulted in the deposition of Early Triassic quartz sandstones. This extension seems to have taken place above a subduction zone developed in response to a Late Paleozoic-Triassic ocean floor (Paleo-Tethys) dipping northward beneath Laurasia, as evidenced by Permo-Triassic accretionary melanges restricted to Sakarya Zone. In conclusion, geochronological, geochemical and regional data provide additional evidence that the Paleo-Tethys Ocean was subducting northward beneath Laurasia during Permian time.