The effects of grain-scale melt migration process on metagranite at eclogite facies,  Bohemian Massif

Abstract

Augen to banded metagranite from the Snieznik dome have been modified locally to have stromatic, schlieren, nebulitic and granite-looking textures typical of migmatites. Former presence, and increasing role of melt in transformation towards nebulite is inferred from interstitial phases along grain boundaries in the dynamically recrystallized monomineralic feldspar and quartz aggregates, and from textures of fine-grained plagioclase and quartz replacing K-feldspar. These features are interpreted as resulting from dissolution-reprecipitation along grain boundaries due to grain-scale melt migration, being pervasive at the grain-scale, but localized at hand-specimen to outcrop scales. The new minerals crystallized from melt are in textural equilibrium with phengite. All the rock types have the same mineral assemblage of Grt−Ph−Bt−Ttn−Kfs−Pl−Qz±Rt±Ilm, with similar garnet, phengite and biotite composition, leading to modelled equilibration conditions of 15−17 kbar and 690–740 °C. Because the mineral compositions in the assemblage of interest are independent of the amount of melt, the modelling did not allow to estimate melt quantities in individual rock types. However, migmatite textures suggest that increasing degree of melt-rock interaction occurred from the banded to the schlieren and nebulitic types. The initiation of melt migration is related to gently dipping structures related to continental subduction to eclogite-facies conditions, and more pronounced melt migration is related with vertical fabrics leading to exhumation of the continental subduction wedge from eclogite-facies to mid-crustal conditions. The effects of melt migration had impact on partial recrystallization of zircon. Zircon in augen to banded types shows oscillatory zoning and gives Cambro-Ordovician age of the protolith. In schlieren to nebulite types, zircon shows domains of blurred oscillatory zoning to structure-less textures. These metamorphic domains are located along grain boundaries, form embayments, form straight or curved linear structures cutting through the oscillatory zoned domains, or are affecting the whole grains. The domains with sharp oscillatory zoning tend to give Cambro-Oridovician ages, while the metamorphic domains tend to give Carboniferous age. Zircon shows numerous apparent “inclusions” of phengite, K-feldspar, quartz, plagioclase, rare garnet, rutile and biotite. However, the “inclusions” of phengite, garnet and rutile are located in the metamorphic domains of the zircon grains. In places, the inclusions are aligned, and these structures are interpreted as former cracks, along which the metamorphic phases crystallized and zircon (re)crystallized. As the assemblage of phengite-garnet-rutile is compatible with previously inferred eclogite-facies conditions, we interpret the Carboniferous zircon (re)crystallization as dating the eclogite-facies grain-scale melt migration process.