Mineralogical and bulk compositional records of an advective fluid flux event near the Variscan HT–LP metamorphic climax. Catalonian Coastal Ranges, NE Iberian Peninsula

Abstract

Metamorphic rocks of the Osor complex (NE Iberian peninsula) show quartz-rich gneissic semi-pelitic, and quartz-absent, mica-rich, schistose domains that have been related to the effects of infiltration of a magmatic fluid evolved during cooling of syntectonic peraluminous leucogranites and pegmatites. The infiltrating fluid became progressively alkali-rich through leaching of these elements from the enclosing metamorphic rocks as it became progressively focused in developing micaceous, high-strain domains subparallel to the main crenulation cleavage. The flux took place around the peak of the HT–LP metamorphism during nearly isobaric heating followed by initially nearly isobaric cooling evolving towards decompressional cooling. Channels of focused flow became the loci of late- to post-tectonic Na-rich plagioclase, muscovite and biotite growth. The growth of the alkali-rich phases promoted further weakening of the domains that continued to be zones of high-strain during a late extension coeval with cooling and decompression.