Jurassic acidic magmatism in a back-arc setting, eastern Sakarya Zone, Turkey: Geochemical constraints and an evolutionary model

The Early Jurassic in the Sakarya Zone, Turkey, was dominated by basic magmatism. The products of this magmatic episode are found as basaltic lavas and pyroclastic rocks in the Şenköy Formation, which was deposited in an extensional basin, or as intrusives cutting Variscan crystalline basement rocks. The Olur (Erzurum) region in the easternmost part of the Sakarya Zone is one of the few locations where contemporaneous acidic rocks crop out, exposed in an area of ~50 km 2 . The acidic rocks in the eastern Sakarya Zone consist of dacitic lavas, rare pyroclastic rocks, and very rare pebble dikes. Porphyries and trondhjemites are their intrusive equivalents and crop out as dikes and small stocks within the underlying Variscan basement. These Early Jurassic rocks have modal K-feldspar contents of <10% and are trondhjemitic in character. Laser ablation–inductively coupled plasma–mass spectrometry U-Pb dating of zircons from two samples yielded formation ages for these rocks of 185.3 ± 1.0 and 184.1 ± 1.1 Ma (±2σ). Geochemically, the rocks are strongly peraluminous and display a medium- to low-K affinity and sodic compositions (K 2 O/Na 2 O = 0.03–0.49). Enrichment in large-ion lithophile elements and depletion in Nb, Ta, P, and Ti are regarded as signatures of a subduction-related setting. Moderately radiogenic I Sr ratios (0.70508–0.70877) and depleted mantle-like Ɛ Nd (t) values (−0.6 to +3.0) and Pb(t) isotopes suggest that the acidic rocks were derived from juvenile lower crust. Geochemical and isotope data imply that the partial melting of juvenile mafic lithologies of the lower crust produced K-poor tonalitic melts that later fractionated to trondhjemitic melts while ascending to the surface. Considering the available data, we suggest that the Early Jurassic bimodal magmatic rocks of the eastern Sakarya Zone were formed in a back-arc setting during the southward subduction of the Paleotethys oceanic lithosphere. These rocks are a prime example of the formation of shallow subduction products following Late Triassic ridge subduction.