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 meta-igneous 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 4GPa. 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 (70.5–92.7Ma) 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.
Highlights
It is widely accepted that in convergent margin settings, rocks can be subducted to depths exceeding 150 km and returned to the surface
All three examples of microdiamonds are found as inclusions in garnets from metapelitic units in Greece, leaving the full extent of UHP conditions in the remaining parts of the Rhodope Massif (RM) (N3/4 of which are in Bulgaria) as unknown
This study focuses on the metamorphic history of the Variegated Formation (VF) in the vicinity of the town of Chepelare, on the edge of the large Arda gneiss dome (25 × 25 km) in the Central Rhodope Mountains Here, the Variscan orthogneisses from the lower high-grade units can be divided into two distinct sub-units—“Arda 1” and “Arda 2”, with the boundary between these units interpreted as a high strain shear zone or synmetamorphic thrust fault (Ivanov et al, 1985; Ricou et al, 1998; Burg, 2012; this study, see Fig. 2)
Summary
It is widely accepted that in convergent margin settings, rocks can be subducted to depths exceeding 150 km and returned to the surface. Recognition of this deep subduction–exhumation cycle followed the discovery of coesite in rocks from the Dora Massif in the Western Alps (Chopin, 1984) and discovery of microdiamonds in the Kokchetav Massif of Kazakhstan (Sobolev and Shatsky, 1990). All three examples of microdiamonds are found as inclusions in garnets from metapelitic units in Greece, leaving the full extent of UHP conditions in the remaining parts of the RM (N3/4 of which are in Bulgaria) as unknown
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