Petrology, mineral chemistry, and tectonomagmatic evolution of Late Cretaceous suprasubduction‐zone ophiolites in the İzmir–Ankara–Erzincan suture zone, Turkey

Abstract

The Late Cretaceous ophiolites sited along the İzmir–Ankara–Erzincan suture zone (İAESZ) in north‐central Turkey occur as south‐directed thrust sheets on the Anatolide‐Tauride platform. These ophiolites are remnants of the northern branch of the Neotethyan Ocean. The Orhaneli‐Harmancık (Bursa), Tavşanlı (Kütahya), and İnönü‐Dağküplü‐Mihalıçcık (Eskişehir) ophiolites crop out in the western part of the İAESZ (the İzmir‐Ankara segment). The Orhaneli ophiolite consists mainly of cumulate peridotites and minor layered gabbros. Serpentinized harzburgites of the depleted upper‐mantle tectonites are common in the Harmancık and Tavşanlı ophiolites. The Dağküplü ophiolite shows an almost intact suite of a Penrose‐type sequence. Mantle harzburgites are cut by isolated diabase dikes and pegmatitic gabbros. Ultramafic‐mafic cumulates (dunites, wehrlites, pyroxenites, layered gabbros) are less abundant than the mantle harzburgites. Isotropic gabbros and diabasic sheeted dikes occur as blocky, hard, massive rocks. Blocks of pillow lavas, radiolarites, and pelagic limestones are present within an ophiolitic mélange in the suture zone. Thin slivers of metamorphic sole rocks along the base of the ophiolites include garnet amphibolites, amphibolites, and albite‐epidote‐amphibole schists, which collectively yield an age of 103 Ma. Large chromium deposits occur in the host mantle dunites of the Mihalıçcık ophiolite. Isotropic gabbros and diabases have undergone ocean floor‐type metamorphism under low‐grade greenschist conditions, and have variable contents of Rb, Ba, K, and Sr (large‐ion lithophile elements), with the exception of Th. High field‐strength (Ti, Nb, Zr, Y, Yb) elements show negative anomalies relative to mid‐ocean ridge basalts. Nb/Y ratios of the gabbros and diabases are very low (<0.2). Normalized rare earth–element patterns and incompatible‐element contents of the mafic rocks indicate suprasubduction zone‐type affinities. Suprasubduction harzburgite‐type ophiolitic mantle peridotites (MgO% = 36–42) contain dispersed chromite crystals with Cr# (0.71–0.73) and Mg# (0.42–0.46). Massive chromitites have higher Cr# (0.78–0.83) and Mg# (0.59–0.68). The increased Cr contents of chromite crystals indicate boninite‐like magma development in a SSZ environment. Olivine in the peridotites is magnesium‐rich (Fo90–93), whereas orthopyroxenites are enstatite (En90–92). Clinopyroxene in the gabbros is rich in calcium and magnesium and poor in iron (En42–44Wo48–50Fs8–9), i.e. diopsite. Based on field relations and geochemical features, the Late Cretaceous ophiolites the in the western part of the İAESZ are interpreted to have formed in a forearc tectonic setting.