Fluid-fluxed melting and melt loss in a syntectonic contact metamorphic aureole from the Variscan eastern Pyrenees

Abstract

Open-system behaviour through fluid influx and melt loss can produce a variety of migmatite morphologies and mineral assemblages from the same protolith composition. This is shown by different types of granulite facies migmatite from the contact aureole of the Ceret gabbro-diorite stock in the Roc de Frausa Massif (eastern Pyrenees). Patch, stromatic and schollen migmatites are identified in the inner contact aureole, whereas schollen migmatites and residual melanosomes are found as xeno- liths inside the gabbro-diorite. Patch and schollen migmatites record D1 and D2 structures in folded melanosome and mostly preserve the high-T D2 in granular or weakly foliated leucosome. Stromatic migmatites and residual melanosomes only preserve D2. The assemblage quartz-garnet-biotite- sillimanite-cordierite K-feldspar-plagioclase is present in patch and schollen migmatites, whereas stromatic migmatites and residual melanosomes contain a sub-assemblage with no sillimanite and/or K-feldspar. A decrease in XFe (molar Fe/(Fe + Mg)) in garnet, biotite and cordierite is observed from patch migmatites through schollen and stromatic migmatites to residual melanosomes. Whole-rock compositions of patch, schollen and stromatic migmatites are similar to those of non-migmatitic rocks from the surrounding area. These metasedimentary rocks are interpreted as the protoliths of the migmatites. A decrease in the silica content of migmatites from 63 to 40 wt% SiO2 is accompa- nied by an increase in Al2O3 and MgO+FeO and by a depletion in alkalis. Thermodynamic modelling in the NCKFMASHTO system for the different types of migmatite provides peak metamorphic conditions ~7-8 kbar and 840 °C. A nearly isothermal decompression history down to 5.5 kbar was followed by isobaric cooling from 840 °C through 690 °C to lower temperatures. The preservation of granulite facies assemblages and the variation in mineral assemblages and chemical composition can be modelled by ongoing H2O-fluxed melting accompanied by melt loss. The fluids were probably released by the crystallizing gabbro-diorite, infiltrating the metasedimentary rocks and fluxing melting. Release of fluids and melt loss were probably favoured by coeval deformation (D2). The amount of melt remaining in the system varied considerably among the different types of migmatite. The whole-rock compositions of the samples, the modelled compositions of melts at the solidus at 5.5 kbar and the residues show a good correlation.