Diabase dykes from Boğazkale (Çorum), Central Anatolia: Geochemical insights into the geodynamical evolution of the northern branch of Neotethys

Abstract

In the Boğazkale region (Çorum, Central Anatolia), pieces of Neotethyan oceanic lithosphere are found in an ophiolitic mélange. Within the mélange, there are occurrences of isolated diabase dykes that intrude serpentinized and carbonatized ultramafics. The diabases are phaneritic, holocrystalline and equigranular, and consist of plagioclase and hornblende as the main primary mineral phases. They have all undergone low-grade alteration, to varying extents, as reflected by the presence of secondary minerals, including chlorite, epidote, prehnite and actinolite. Immobile trace element systematics suggest that all diabases are of subalkaline basalt composition and display negative Nb anomalies in the N-MORB-normalized plots. They also display flat to slightly LREE-depleted patterns in chondrite-normalized diagrams. The relative Th (and La) enrichment coupled with N-MORB-like HFSE composition of the diabases suggests their derivation from a depleted mantle source metasomatized by slab-derived fluids/melts. Overall trace element characteristics are consistent with an oceanic back-arc origin. 40Ar-39Ar radiometric dating on the hornblende grains extracted from the two diabase dykes yield ages of 176.30 ± 0.52 Ma and 178.82 ± 0.80 Ma (Toarcian, Early Jurassic). This suggests the existence of an intra-oceanic subduction zone within the Izmir-Ankara-Erzincan (IAE) Ocean during the Early Jurassic, which can also be traced along the western (Vardar Zone) and the eastern (Sevan/Akera ophiolites) extensions of the Northern Neotethys. This result requires the presence of a Northern Neotethyan oceanic crust of pre-Early Jurassic age, which in turn may suggest that oceanization of the IAE Ocean have occurred during the Late Triassic or earlier.