Petrographic and Geochemical Fingerprints of Sub-Volcanic Dykes and their Host Harzburgites from the Ulaş Ultramafics (Sivas, Turkey)
Abstract
In the southern Ulaş region (Sivas, mid-Anatolia), the ophiolitic fragments of the Neotethys oceanic lithosphere are widespread. The study area consists of commonly harzburgite, with minor dunite and chromitite pods and also pyroxenites. The latter are sub-volcanic gabbro and diabase dykes. In this study, the petrographic and geochemical importance of the mantle harzburgites and the sub-volcanic dykes cutting them were evaluated, within these ophiolitic units. The textural properties of the harzburgites were generally anhedral, granular and partially poikilitic. They consisted of olivine, orthopyroxene and minor amount of clinopyroxene and chromian spinel. On the other hand, the textural and mineralogical characteristics of sub-volcanic dykes, consisted of plagioclase, pyroxene, hornblende, opaque minerals and their secondary products, reflected semi-depth characters. The major, trace and rare earth element compositions indicated that the harzburgites were similar to that of both abyssal and supra-subduction (SSZ) peridotites with depleted character. Furthermore, the sub-volcanic dykes that formed the focus of this study in the mantle harzburgites exhibited N-MORB (Normal Mid-Ocean Ridge Basalt) type patterns of rare earth (REE) and trace elements. Especially, negative Nb anomaly showed the presence of a subduction component with the late stage modification of subduction-derived fluids/melts. All results indicated that the dykes characterized as N-MORB and late recorded the characteristics of Ocean Island Basalts (OIB). Consequently, geochemical features of the harzburgites and sub-volcanic dykes that cut them showed that peridotites were affected by two different tectonic setting conditions such as MORB and SSZ.
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