WHOLE-ROCK AND MINERAL CHEMISTRY OF CALCSILICATE LENSES WITHIN MAFIC GNEISSES IN A HIGH-GRADE METAMORPHIC TERRAIN, HALLAND PROVINCE SWEDEN

Linear arrays of epidote-rich, andradite-grossular bearing calcsilicate lenses occur sporadically in upper amphibolite mafic and felsic gneisses of the Eastern Segment of the Sveconorwegian Orogen in Halland Province, Sweden. We used electron microprobe and LA-ICP-MS mineral analyses and whole rock analyses to study calcsilicate lenses within hornblende-plagioclase mafic gneisses. The lenses are separated from the host by a transition zone approximately 5 cm wide in which, moving towards the calcsilicate, diopside appears, hornblende disappears, quartz abundance increases, titanite appears, and epidote abundance increases. Fe/(Fe+Mg) ratios of diopside increase from 0.25 to 0.5 while anorthite contents of plagioclase increase from 0.5 to 0.9 . Compared to the host, calcsilicates are enriched in CaO (20 vs 10 wt%) and Sr (3,000 vs 1200 ppm) and depleted in MgO (0.6 vs 6.4 wt%), Na₂O (0.3 vs 2.4 wt%), K₂O (0.1 vs 0.5 wt%) and Ba (65 vs 500 ppm). Concentrations in the transition zone are between the two. Whole-rock REE patterns are similar in hosts and lenses with moderate LREE enrichment (La/Yb ~9), while the transition zone has lower concentrations of HREE (La/Yb ~20). Epidote grains near the host have large BSE-bright zones with REE contents (LA-ICP-MS analyses) up to 10 wt% and irregular BSE- dark patches with REE contents ranging down to 0.1 wt%. Near calcsilicate lenses BSE-bright patches in epidote (total REE up to 5 wt%) are surrounded by BSE-dark rims (total REE ranging down to 0.001 wt%). In the transition zone La/Yb in epidote ranges from 0.01 to 630 increasing linearly with total REE. In calcsilicate lenses small BSE-bright patches (total REE concentrations up to 3 wt%) are embedded in masses of BSE-dark epidote (total REE contents ranging down to 0.001 wt %). La/Yb ratios range from 0.03 to 10,000 and increase linearly with total REE, although with a shallower slope than the transition zone. The transition zone appears to have formed by metasomatic reactions between a mafic and a calcareous rock. The REE patterns within epidote probably reflect a tendency for LREE to be concentrated in grains during the early stages of growth leaving later epidotes relatively depleted in LREE