The metamorphism and migmatization of the Xanthe-Echinos metamorphic complex, Central Rhodope, Greece

Abstract

The Xanthe-Echinos metamorphic complex in the central Hellenic Rhodope records early Tertiary convergent tectonics on the Eurasian continental margin of Neotethys. It consists principally of layered gneiss and migmatite with minor amphibolite and calc-silicate rocks. Three metamorphic events are recognised: a high-pressure eclogite event (?Cretaceous); medium-pressure, Barrovian-type metamorphism (Eocene); and later low-pressure, greenschist facies metamorphism. Petrographic composition and electron microprobe analysis of minerals in forty four samples of gneiss and migmatite shows that the Eocene Barrovian event is characterized by replacement of clinopyroxene by pargasitic hornblende in clinopyroxene-garnet and clinopyroxene-edenitic hornblende assemblages. The compositional variation and textures of the amphiboles suggest initial growth of edenitic hornblende zoned to hornblende and development of syn- to late-tectonic pargasitic hornblende under higher temperature conditions associated with migmatization. The textures, zoning and paragenetic relationships of sodic-calcic amphiboles are consistent with uplift accompanied by synmetamorphic deformation. Partial melting in the migmatites involved plagioclase+K-feldspar+quartz. The solidification products of the produced melt are present in the form of quartz monzonitic leucosome layers and is also represented in syntectonic plutons. Metamorphic pressures of 7.5–8.5 kbar (at T = 600° C) were estimated from garnet-clinopyroxene pairs in gneisses adjacent to the migmatites. Amphibole-plagioclase thermometry of migmatite samples yielded temperatures of 728°–746°C (at P = 8.5 kbar). The activity of H 2O was probably less than 1.0 in the partially melted rocks. The geothermobarometric results indicate an increase of the geothermal gradient during uplift, probably related to intrusion of basic magma into the crust and/or changes in geothermal gradient associated with crustal thickening.