Origin and tectonic significance of the metamorphic sole and isolated dykes of the Divrigi ophiolite (Sivas, Turkey): Evidence for slab break-off prior to ophiolite emplacement

Abstract

The Late Cretaceous Divrigi ophiolite of east-central Anatolia comprises. from bottom to top. an ophiolitic melange, metamorphic sole and remnants of oceanic lithosphere. The ophiolitic melange has been thrust onto the Lower Carboniferous-Campanian Munzur Limestone (Tauride platform), and is in turn tectonically overlain by the metamorphic sole. The meta morphic-sole rocks are represented by amphibolite, plagioclase amphibolite, plagioclase-amphibole schist, plagioclase-epidote-amphibole schist and calc-schist. The oceanic-lithosphere remnant exhibits a complete section, excluding volcanic rocks, comprising mantle tectonites, ultramafic to mafic cumulates, isotropic gabbros and sheeted dykes. Isolated dykes intrude the metamorphic sole and mantle tectonites at different structural levels. The metamorphic-sole rocks beneath the Divrigi ophiolite can be divided into two groups with distinct geochemical features. The first group is tholeiitic (Nb/Y=0.07-0.18). whereas the second group is alkaline (Nb/Y=1.77-3.48) in chemistry. Chondrite-normalized REE patterns, N-MORB-normalized spider diagrams and tectonic discrimination diagrams suggest that the protolith of the first group was similar to island-arc tholeiitic basalts, whereas the protolith of the second group was more akin to within-plate alkali basalts. The isolated dykes cutting the metamorphic sole and the mantle tectonites exhibit alkaline (Nb/Y=0.68-2.11) character and are geochemically similar to within-plate alkaline basalts. The geochemical evidence suggests that the Late Cretaceous Divrigi ophiolite formed in a suprasubduction zone tectonic setting to the north of the Tauride platform. During intraoceanic subduction/thrusting, the IAT type and seamount-type alkaline basalts were metamorphosed and accreted to the base of the Divrigi ophiolite. The alkaline isolated dykes were probably the result of late-stage magmatism fed by melts that originated within an asthenospheric window due to slab break-off, shortly before the emplacement of the Divrigi ophiolite onto the Tauride platform to the south.