Geochemical characterization of the metabasic rocks in the lower karakaya complex (Western pontides)

Abstract

The Karakaya Complex covers extensive areas in the Sakarya Zone and is subdivided into two units such as the Lower Karakaya and Upper Karakaya complexes. The Upper Karakaya Complex is a strongly deformed, unmetamorphosed chaotic unit made up mainly of basic volcanic and volcanoclastic rocks, arkosic sandstone and limestone. The Lower Karakaya Complex comprises greenschist- to albite-epidote-amphibolite–facies metabasite, phyllite, marble, and minor serpentinite and metachert with locally preserved slices of blueschist and eclogite. There are mainly two opposite views on the formation setting of the Karakaya Complex such as (i) a back-arc rift basin that formed due to the southward subduction of the Paleotethys beneath the northern margin of Gondwana during the Late Triassic, or (ii) a subduction-accretion complex that formed to the southern margin of Eurasia by northward subduction of the Paleotethys during Late Triassic. The purpose of this study is twofold. The first purpose is to map the part of the Lower Karakaya Complex cropping out between Bilecik and Bozüyük towns (the Bozüyük Metamorphic Rocks) at the scale of 1/25.000 and to determine the rock assemblage and their mineral parageneses. The second purpose is to constrain the geochemical characteristics of the metabasic rocks in the Lower Karakaya Complex cropping out in areas such as Bandırma, Bursa, and Bilecik to understand their geodynamic formation setting. The Bozüyük Metamorphic Rocks comprise blueschist- and greenschist-facies rock assemblages involving metabasite, phyllite, marble and, minor serpentinite. The greenschist-facies metamorphism is retrograde with respect to epidote blueschist-facies, suggesting formation in a subduction zone. The age of the metamorphism is constrained as ≥ 200 Ma (Late Triassic) by 40Ar-39Ar stepwise dating of phengite. This age value in conjunction with Ar-Ar phengite data in the literature (201-215 Ma) suggests that a significant part of the Lower Karakaya Complex in the western Pontides, 30 km by 230 km, extending from Bandırma to the north of Eskişehir formed during the latest Triassic. About 80 vol.% of the metabasic rock in the Lower Karakaya Complex cropping out between the Bandırma to the north of Eskişehir show anorogenic alkaline to tholeiitic affinity similar to ocean island basalts (OIBs). About 10 vol.% of the metabasic rocks within the serpentinite slivers, interpreted as ophiolite fragments, are characterized by enriched mid-ocean ridge basalt (E-MORB) signature. Nearly 5 vol.% of the metabasite samples show normal mid-ocean ridge basalt (N-MORB) affinity, and the remaining metabasites are island-arc tholeiite (IAT). All these geochemical features conclusively demonstrate that the overwhelming part of accreted material originated in seamounts and the remaining part from related oceanic lithosphere (enriched and normal MORBs) and supra-subduction zone oceanic setting.