Geochemistry and petrogenesis of the diabase dykes from the Boğazkale region (Çorum, Central Anatolia)

Balcı, Uğur
The Izmir-Ankara-Erzincan (IAE) Suture Belt contains oceanic and continental fragments derived from the northern branch of Neotethys and associated continental entities. In the Boğazkale region (Çorum, Central Anatolia), the pieces of Neotethyan oceanic lithosphere are present in a chaotic manner, and form an ophiolitic mélange. Diabase dykes are observed in the mélange, which cross-cut various rock types of oceanic lithosphere, such as pillow basalts, gabbros and ultramafics. This study aims to investigate the geochemical and petrogenetic features of the diabase dykes, and their importance in the Neotethyan framework. The investigated diabases are mainly made up of plagioclase and a mafic phase, which is clinopyroxene and/or hornblende. According to predominating mafic mineral phase, the diabases can be subdivided into two types as clinopyroxene- and hornblende-diabase. Sub-ophitic textures are common in all clinopyroxene-diabase samples and poikilitic texture in almost all hornblende-diabase samples. Both types are variably modified by low-grade hydrothermal alteration as reflected by the presence of secondary minerals, such as chlorite, epidote, prehnite and actinolite. vi The immobile trace element systematics reveals two chemical types, which seems consistent with the petrographical grouping. Both chemical types display sub-alkaline basalt characteristics. The relationship of MgO with some major and trace elements suggest that fractional crystallization was important in the magmatic evolution of the dykes. One of the major differences between two types is that while Type 2 diabases possess negative Nb anomalies, this feature is not observed in Type 1. In addition, Type 1 diabases are characterized by slight LREE enrichment, whereas Type 2 diabases display flat to slightly LREE-depleted patterns. Type 2 appears to have derived from a depleted asthenospheric mantle source metasomatized by slab-derived fluids/melts. Geochemical signatures of Type 1, on the other hand, require relatively enriched source region and/or small degrees of partial melting. Trace element systematics of Type 2 is consistent with an origin in an oceanic back-arc basin, whereas Type 1 diabases appear to have been generated in a mid-ocean ridge or an oceanic back-arc basin. 40Ar-39Ar radiometric dating on the amphiboles from two Type 2 samples reveals ages of 176.30 ± 0.52 Ma (Toarcian) and 178.82 ± 0.80 Ma (Toarcian), which suggests the presence of an intra-oceanic subduction zone within IAE Ocean during the Early Jurassic.
Citation Formats
U. Balcı, “Geochemistry and petrogenesis of the diabase dykes from the Boğazkale region (Çorum, Central Anatolia),” M.S. - Master of Science, Middle East Technical University, 2018.