Late Cretaceous UHP metamorphism recorded in kyanite–garnet schists from the Central Rhodope Mountains, Bulgaria

Abstract

In this study, we report the first discovery of microdiamond inclusions in kyanite-garnet schists from the Central Rhodope Mountains in Bulgaria. These inclusions occur in garnets from metapelites that are part of a metaigneous and meta-sedimentary mélange hosted by Variscan (Hercynian) orthogneiss. Ultra-high-pressure (UHP) conditions are further supported by the presence of exsolved needles of quartz and rutile in the garnet and by geothermobarometry estimates that suggest peak metamorphic temperatures of 750-800°C and pressures in excess of 4 GPa. The discovery of UHP conditions in the Central Rhodopes of Bulgaria compliments the well-documented evidence for such conditions in the southernmost (Greek) part of the Rhodope Massif. Dating of garnets from these UHP metapelites (Chepelare Shear Zone) using Sm-Nd geochronology indicates a Late Cretaceous age (70.5-92.7 Ma) for the UHP metamorphic event. This is significantly younger than previously reported ages and suggests that the UHP conditions are associated with the Late Mesozoic subduction of the Vardar Ocean northward beneath the Moesian platform (Europe). The present-day structure of the RM is the result of a series of subduction-exhumation events that span the Cenozoic, alongside subsequent post-orogenic extension and metamorphic core complex formation.

The Age, Origin, and Emplacement of the Tsiknias Ophiolite, Tinos, Greece

“…We believe this event records the final closure of the southern Vardar Ocean to the NE of the Cyclades during the Late Cretaceous‐Eocene associated with the transition in Nubia's motion from transcurrent (E‐W) to convergent (N‐S; Vissers & Meijer, ). This interpretation agrees with geological evidence from mainland Greece (Collings et al, ; Sharp & Robertson, ) where it is suggested that the northern part of Vardar Ocean remained as an open seaway until at least the Late Cretaceous time before NE directed subduction under Eurasia, subsequent (U)–HP metamorphism, and finally continental collision. Previous models of continuous slab rollback and extension in the Aegean since 70 Ma (Jolivet & Brun, ; Jolivet et al, ) should therefore be reexamined considering these new data.…”

“…Tectonics beneath Eurasia at~92-71 Ma (Collings et al, 2016). This is interpreted to record the closure of the Northern Vardar Ocean and precisely correlates with the timing of sole formation on Tinos.…”

“…High‐pressure metamorphism occurred ~17 Myr after the formation and initial detachment of the Semail Ophiolite (Searle et al, ). It suggested the closure of the Northern Vardar Ocean as recent as the Late Cretaceous‐Paleogene (Sharp & Robertson, ) and 92–71 Ma (U)–HP metamorphism associated with attempted NE dipping continental subduction under Eurasia (Collings et al, ). In this scenario, the same Late Cretaceous to Eocene subduction zone to that recorded on Tinos continued along the entire western margin of the Vardar Ocean (striking NNE to SSW today) and was responsible for the collision with Eurasia to the NE and Cyclades (Adria‐Apulia) to the SW producing what has been termed the Aegean Orogeny (Lamont et al, ).…”

Metamorphic conditions and structural evolution of the Kesebir-Kardamos dome: Rhodope metamorphic complex (Greece-Bulgaria)

“…15). If this younging trend of isotopic ages is considered, then the Late Cretaceous and younger metamorphic ages obtained (e.g., ca 70-92Ma for the Chepelare shear zone; Collings et al 2016) cannot be explained by metamorphism in a subduction channel at great depths as their authors have contended. We propose that the post-Jurassic metamorphic ages recorded in the Rhodope represent crustal rather than deep subduction geodynamic processes.…”

“…The nappe system was deformed in dome-and-basin structures during syn-to post-convergent exhumation (Burg et al 1996). High-pressure rocks showing various degrees of retrogression occur in the intermediate sheets of the nappe system (see Burg 2012 for grouping of units), which contains eclogites (Liati and Mposkos 1990;Liati and Seidel 1996;Moulas et al 2013;Mposkos et al 2013) and microdiamond-bearing paragneisses (Mposkos and Kostopoulos 2001;Perraki et al 2006;Schmidt et al 2010;Collings et al 2016). These rocks crop out in the flanks of several, kilometer-wide, gneiss domes such as the Kesebir-Kardamos dome (KKD) in southern Bulgaria and northern Greece Jahn-Awe et al 2012).…”