THE GROWTH OF EPIDOTE AND TITANITE DURING UPPER AMPHIBOLITE TO LOWER GRANULITE FACIES METAMORPHISM IN STENSJOESTRAND, SOUTHWESTERN SWEDEN

Titanite and epidote are common in lower-grade mafic metamorphic rocks but tend to be consumed by amphibole forming reactions during higher-grade metamorphism. In the Stensjöstrand area southwestern Sweden these minerals formed under upper amphibolite to lower granulite facies conditions (T ~ 860^oC, P ~ 1.0 GaP) at the boundary between calcsilicate enclaves and their amphibolite host rocks. Textures in traverses across 3 calcsilicate enclaves into amphibolite were examined by SEM at Hope College while minerals were analyzed by microprobe at the GeoForschungZentrum. The calc-silicate enclaves have epidote and epidote-garnet rich cores surrounded by diopside-quartz-plagioclase-titanite-oxide rims. Anorthite contents in plagioclase and hedenbergite contents in diopside increase across these rims towards the core indicating an increase in the chemical potentials of Ca and Fe. Titanite occurs in embayments within Fe-Ti oxides in the rims. Fe-Ti oxides in the calcsilicate rims and cores have lower Ti concentrations than oxides in adjacent amphibolite. In some cases a Ti-poor oxide occurs together with the Fe-Ti oxide in the Calc-silicate rims. Small scattered grains of REE-enriched epidote (Ce+La+Nd up to 13oxide wt%) occur in the amphibolite and outer edge of the calcsilicate rims. Overgrowths of REE-poor epidote (Ce+La+Nd < 0.1 oxide wt% ) appear on these grains towards the middle of the calcsilicate rims. In the calcsilicate cores small, complexly zoned REE-enriched zones (0.5 < Ce+La+Nd < 5 oxide wt%) are surrounded by large masses of REE-poor epidote. The textural and chemical relationships suggest that titanite and REE-poor epidote formed due to elevated chemical potentials of Ca and Fe developed by diffusional exchange between the amphibolite and a calcareous Fe- rich rock, with titanite forming by reaction with Fe-Ti oxides and the new epidote nucleating on pre-existing REE-rich epidote grains.