The Jurassic-Early Cretaceous basalt-chert association in the ophiolites of the Ankara Melange, east of Ankara, Turkey: age and geochemistry

This study is focused on slide blocks including oceanic lavas associated with pelagic sediments within the eastern part of the Ankara Melange. A detailed petrological characterization of the volcanic rocks and a detailed biochronological investigation of the associated radiolarian cherts in eight sections (east of Ankara) was carried out. The volcanic rocks are largely represented by basalts and minor ferrobasalts and trachytes. They show different geochemical affinities and overlapping ages including: (a) Late Jurassic - Early Cretaceous garnet-influenced MORB (middle late Oxfordian to late Kimmeridgian-early Tithonian and early-early late Tithonian; late Valanginian-early Barremian); (b) Early Cretaceous enriched-MORB (middle late Barremian-early early Aptian; Valanginian to middle Aptian-early Albian); (c) Middle Jurassic plume-type MORB (early-middle Bajocian to late Bathonian-early Callovian); (d) Late Jurassic - Early Cretaceous alkaline basalts (middle-late Oxfordian to late Kimmeridgian-early Tithonian; late Valanginian to late Hauterivian). All rock types show a clear garnet signature, as testified to by their high MREE/HREE (middle rare earth element/heavy rare earth element) ratios. The coexistence of chemically different rock types from Middle Jurassic to Early Cretaceous times suggests that they were formed in a mid-ocean ridge setting from partial melting of a highly heterogeneous mantle characterized by the extensive occurrence of OIB-metasomatized portions, which were likely inherited from Triassic mantle plume activity associated with the continental rift and opening of the Neotethys branch.