RARE-EARTH ELEMENT MOBILITY DURING PROGRADE GRANULITE-FACIES METAMORPHISM - SIGNIFICANCE OF FLUORINE

Mafic gneisses occur as lenses or thin layers in spatial association w ith tonalitic leucosomes in a granulite zone of the Quetico subprovinc e of the Superior Province, Ontario, Canada, and exhibit concentric zo ning with a biotite-rich margin, orthopyroxene-rich outer zone, clinop yroxene-rich central zone, and, occasionally, patches of relict amphib olites within the clinopyroxene-rich zone. The granulites (biotite-, o rthopyroxene- and clinopyroxene-rich zones) in the mafic gneisses are characterized by significant amounts of rare earth element (REE)-beari ng fluorapatite (1-10 vol.%) and other REE-rich minerals (allanite, mo nazite and zircon). Fluorapatite shows an increase in modal abundance from the biotite- and orthopyroxene-rich zones to the clinopyroxene-ri ch zone, but is rare in the relict amphibolites. Textural evidence and element partitioning indicate that fluorapatite (and other REE-rich m inerals) was part of the peak metamorphic assemblages. Whole-rock geoc hemical analyses confirm that the granulites in the mafic gneisses con tain anomalously high contents of REE and high field strength elements (HFSE), whereas the relict amphibolites are geochemically typical of tholeiitic basalts. Mass-balance calculations reveal that REE and HFSE were introduced into the mafic gneisses during the prograde granulite facies metamorphism, pointing to REE mobility under granulite facies metamorphic conditions. The presence of high F contents in the REE-ric h minerals and their associated minerals (e.g. biotite and hornblende) suggests that REE and HFSE may have been transported as fluoride comp lexes during the granulite facies metamorphism. This conclusion is sup ported by previously published results of hydrothermal experiments on the partitioning of REE between fluorapatite and F-rich fluids at 700 degrees C and 2 kbar.