First report of microdiamond in kyanite-garnet schist and coesite in eclogite from the Central Rhodope Massif, Bulgaria

In this study we have investigated the melange units (Variegated Formation) of the Chepelare region of the Central Rhodope Massif in Bulgaria. These units consist of amphibolites, marbles, eclogites, metaperidotites and kyanite-garnet schists, all incorporated in Carboniferous gneisses (Arda Unit). The sequence closely resembles the microdiamond bearing Kimi Complex of the Greek Rhodopes, but no study has proven a common origin for these two formations. Protoliths of individual rock units have been determined via use of petrography, bulk rock elemental analysis and Sr and Nd isotope datasets, and the major element chemistry of individual crystals has been measured via EPMA. In the kyanite-garnet schist, garnets are almandine rich (Alm₇₂Gross₄Prp₂₀), with inclusions of kyanite, biotite, muscovite, apatite, quartz and rutile, and preserve evidence of UHP metamorphism. A 5 μm carbon inclusion has been confirmed as a diamond via Laser Raman Spectroscopy, with an intense band at 1333 cm^-1, comparable to microdiamonds from other UHP metamorphic provinces. In addition, a quartz inclusion surrounded by radial fracturing in a garnet grain from an eclogite sample has been interpreted as a pseudomorph after coesite. UHP conditions are further reinforced by geothermobarometry. For the diamond bearing kyanite-garnet schist samples the Zr in rutile thermometer yields temperatures of ~770 °C, and the Ti in phengite barometer yields pressures of ~3 GPa. Our new results compliment the well documented evidence for UHP conditions in the Greek (southernmost) part of the Rhodope Massif, and new data from kyanite and coesite bearing eclogites from the Pirin and Ograzden Mountains to the west of the Rhodope Massif. Together these new findings, and the wide spread nature of these melange units suggest that UHP conditions in SE Europe are much more widespread than previously believed.