Paleocene-Eocene migmatites in the Bulgarian Rhodope revisited

Migmatites of proved Paleocene-Eocene age are widespread in several tectonic units of the Rhodope metamorphic complex (RMC). Most of the migmatitic unit precursors consist of orthogneisses. These were predominately felsic rocks of granite to granodiorite and diorite composition of late-Paleozoic (Arda unit) and late-Jurassic protolith ages (Madan, Startsevo, and Chepinska unit). Zones of post-anatectic extension outline the unit boundaries obscuring the melt-in isograd in the RMC. The peak metamorphic conditions correspond to kyanite sillimanite transition at 650 - 700ºC / 0.6 – 0.8 GPa. The most common thermobarometric estimates are close to the water-saturated granite solidus in sillimanite stability field. The absence of clearly distinguished residuum and peritectic anhydrous minerals indicate fluidassisted melting of metagranitoid precursors. Field observations distinguish metatexite and diatexite structural types. The metatexites occur in all migmatitic tectonic units. The diatexites occupy large domains in the Arda and Chepinska unit suggesting advanced melting within cores of regional thermal antiforms flanked by metatexite. The common constituents of metatexite sections are concordant to the foliation in situ leucosome ± melanosome, and mesosome. Discordant leucosomes fill decimetric scale shears and form vein-network together with concordant leucosomes, marking a transition to structurally disrupted diatexites. The latter include subautochtonous lens-like bodies and sheets of inhomogeneous granite. The mesoscale interconnected structures indicate syn-deformation melt flow. Different mechanisms of melt transfer resulted in injection of melt batches into metatexite and subsolidus sections of the RMC.
   The rock-forming mineral assemblages comprise biotite, plagioclase, K-feldspar and quartz, plus amphibole in metagranodiorite and metadiorite mesosome, or muscovite in some diatexite (Arda unit). The leucosome is quartz-feldspar dominated and differs from mesosome and particularly from melanosome with lower mafic and accessory mineral proportions. Scarce amphibole-bearing leucosomes resulted from back-reactions between migrating felsic melts and refractory amphibole-bearing rocks. The normative mineral ratios span tonalite to granite field in mesosomes, and trondhemite to granite field in metatexite leucosomes, whereas granite bodies from diatexite domains correspond to low-temperature granite melts. The uniform accessory mineral assemblage includes: magnetite, zircon, allanite, apatite, ± titanite in metatexite; and magnetite, monazite, ± xenotime, ± garnet in diatexite. Inherited protolithic zircons dominate both metatexite and diatexite, while new Paleocene-Eocene zircon is scarce. The low temperature melts had little or no impact on dissolution and growth of zircon, however they were responsible for partial dissolution of allanite and apatite, and subsequent crystallization of Paleocene-Eocene monazite.
   The geochemical features corroborate fluid-assisted low-temperature melting of felsic minerals and limited solubility of accessory phases in the melt. The felsic products of migmatization are depleted in Fe, Mg, Ca, HFSE (Zr, Hf, Nb, Ta, Th, and U), Y, and REE. The contents of Zr and LREE cluster close to felsic peraluminous melts saturation at 650 - 750ºC. The REE patterns display general depletion, strongly variable LREE/HREE ratios, and Eu/Eu* ≥ 1 in leucosome and anatectic granite bodies. The systematic compositional changes in the succession concordant leucosomes - discordant leucosomes - subautochtonous granite bodies reflect anatectic melt fractionation and emphasize the most incompatible LILE enrichment during anatectic melt migration and crystallization: increasing LILE/HFSE, K/Ba, and Rb/Sr, and decreasing Ba/Rb ratios; positive correlation between K/Rb and Eu/Eu* values of anatectic granite bodies. The elements ratios between the HFSE remain relatively unchanged overlapping mesosome ratios variation. For that reason the products of migmatization and precursors plot on the same fields of the discrimination diagrams and illustrate this way inheritance of geochemical features related to HFSE mainly.
   Acknowledgments. This study was supported by the Bulgarian National Scientific Fund grants DO 02-327 and DO 02-363.