Metabasic rocks from the Chepelare variegated complex, Central Rhodope massif, Bulgaria – preliminary studies

Metabasic rocks in the Chepelare area occur in two different tectonic settings. Lences of garnet amphibolites are part of Chepelare mélange embedded in migmatic gneisses of Arda 1 tectonic unit. They reach length up to 15 m and in the variegated complex closely associate whit garnet-kyanite schists, impure marbles and granitoid migmatic gneisses. Whereas numerous small bodies of retrogressed eclogites trace out the ductile shear zone between Arda 1 and Arda 2 tectonic units.
   Garnet, amphibole, plagioclase, ± diopside, ± quartz constitute the main minerals in garnet amphibolites from the Chepelare mélange. Accessory minerals are rutile, titanite, ilmenite ± apatite. Garnet occurs as lobate and resorbed porphyroblasts, up to 5 mm in diameter, containing inclusions of amphibole, plagioclase, epidote, quartz, titanite and abundant rutile. Many porphyroblasts have overcrowded by undistinguishable small inclusions core, often surrounded by inclusion-free rim. In finegrained samples garnets rarely include amphibole or quartz. It is almandine-rich (Alm 41-58, Grs 23-34, Pyr 18-30, Sps 1-3 mol%) with weak prograde zonation and almost lacking retrograde alternation to the rim. Porphyroblastic garnet is commonly surrounded by corona-like symplectites of sodic plagioclase (An_27-30) and pargasitic amphibole, indicating retrograde metamorphic reactions at expense of garnet and omphacitic clinopyroxene. Amphibole inclusions in garnet have higher Al and Ti content and are mainly tschermakites. In some samples pseudomorph replacement of amphibole by K-feldspar, chlorite and andesitic plagioclase close to garnet porphyroblasts suggests further decompression reactions at active fluid regime. Pale green diopsidic clinopyroxene (Na₂O = 0.7-2 wt%) in the matrix associates with oligoclase and is partly resorbed and enveloped by amphibole. In samples where abundant leucocratic material is present and close to almost completely resorbed garnet it includes unoriented small idiomorphic amphiboles. Incomplete replacement of rutile by ilmenite and titanite in matrix reflects the decompression path. The assemblage without the presence of Opx should reflect metamorphism in the HP granulite facies.
   P-T estimates using Fe-Mg exchange equilibrium between garnet and clinopyroxene or amphibole and Al-in-amphibole and Grt-Hbl-Pl barometers indicate that the amphibolites reached at least pressures of 12-14 kbars and temperatures of 700-750°C for garnet inclusions and 750-800°C for the matrix assemblage. These new P-T data are consistent with previously reported for the garnet-kyanite gneisses from the Chepelare mélange.
   Preliminary major and trace elements geochemistry plotted on discrimination diagrams suggests MORB affinity for the studied garnet amphibolites. Enrichment in Zr, Y, Nb, Ta, TiO₂, LREE and more pronounced Eu anomaly of two samples from the southernmost outcrops do not precludes the possibility of incorporation in Chepelare mélange of metabasics with different protoliths or stronger interaction with the host migmatic gneisses of granite composition. The later is supported also by high variability in LREE patterns. Additional geochemical studies are planned to reveal the possible connection with retrogressed eclogites from the ductile shear zone to the north, which according to the previous publications also show MORB-type geochemistry.
   Petrological observations and P-T data support the metamorphism at least in HP granulite facies for the rock of variegated complex. We do not refer these new estimations as peak metamorphic conditions, as the HP/UHP metamorphic records could be completely erased by observed late high-temperature metamorphic overprint involving hydration reactions during the exhumation.
   Acknowledgements: This study was supported by National Science Fund - Bulgaria, VU-NZ- 05/ 2005, DO02-363/2008.