Metamorphic evolution of a critical segment in the North Dobrogea orogenic belt, Romania

Abstract

The North Dobrogea orogen in eastern Europe, located between the Scythian-East European craton and the Moesian Platform in the vicinity of the Carpathian Mountains and the Black Sea, represents a key area for understanding the geodynamical evolution of Hercynian and Cimmerian orogeneses. Although the tectonic evolution of the North Dobrogea orogen is generally thought to be the result of a large number of orogenic cycles, metamorphic and pressure-temperature evolution associated with burial and exhumation is less understood. We have performed a detailed structural and petrological study focused on the Orliga and Megina units, where the medium to high grade metamorphics of the North Dobrogea orogen are largely cropping out. A structural field research was conducted by measuring ductile kinematics, including folds and stretching lineations with shear-sense criteria, and sampling for petrological analyses, with particular focus on tectonic contacts and relationships with the felsic intrusions and dykes. A high-resolution geothermobarometry study was furthermore performed to obtain pressure-temperature constraints for the metamorphic evolution associated with burial and exhumation. The results demonstrate that the Orliga unit was affected by multiple shortening events, while the Megina unit recorded only the last stage of burial by nappe stacking that pervasively affected the entire studied area. The top-NE shearing associated with orogenic burial, ultimately created the peak metamorphic burial conditions characterised by up to ~650 MPa pressures and 650 °C temperatures. The high metamorphic temperatures were likely enhanced by the widely observed syn- and post-kinematic magmatism. These high temperatures resulted in further partial melting, migmatisation, and syn-kinematic emplacement of felsic plutons, dykes, and sills, including large invasive bodies of secondary quartz, affected by various stages of deformation. The exhumation started already at peak metamorphic conditions, was syn- to post-kinematic with respect to the top-NE direction of nappe stacking, and was driven by an orogen-parallel top-SE extension that ultimately created the observed greenschist-facies overprint, characterised by temperatures of 250-450 °C. Given the metamorphic conditions, it is likely that the last top-NE nappe stacking event took place during the latest Paleozoic (i.e. Hercynian) – Triassic continental collision, which was coeval with the northward subduction of the Paleotethys ocean in the Carboniferous-Permian. The earlier stages of shortening observed only in the Orliga unit are likely associated with deformation in an upper continental plate during oceanic subduction before the early Carboniferous. The absence of these earlier deformation events and petrological characteristics demonstrate that the Megina high grade unit represents a collisional suture zone. The orogen-parallel extensional exhumation, which started during the last stages of collision, ended before the deposition of the widely observed Late Cretaceous post-tectonic cover. The extension could have been coeval or postdated by the Triassic continental extension recorded by the neighbouring Niculiţel unit and by the subsequent Jurassic - Early Cretaceous Cimmerian shortening. All these results demonstrate for the first time that the dominant metamorphism observed in the Măcin unit of the North Dobrogea orogen is the result of a single orogenic cycle affecting the inherited upper continental plate and its suture zone.