Calc-Silicate Enclaves in High-Grade Gneisses from Halland, Southwest Sweden
Dr. Edward Hansen, Hope College
The Stensjöstrand association crops out in a 5 kilometer strip along the coast north of Steninge, Halland Province, Southwest Sweden. It consists of amphibolites, sillimanite-bearing quartzofeldspathic gneisses and sillimanite-free quartzofeldspathic gneisses interbanded on scales ranging from 10’s of centimeters to 100’s of meters. On the north and south margins of the coastal strip, the Stensjöstrand association is bordered by granitic to granodioritic orthogneisses. Partial melting has led to the widespread development of metatexites with tonalitic leucosomes in the amphibolites and granitic leucosomes in the sillimanite-bearing and sillimanite-free quartzofeldspathic gneisses. Calc-silicate lenses ranging from 10’s of centimeters to several meters in length occur sporadically in both the amphibolites and sillimanite-free quartzofeldspathic gneisses. These lenses tend to occur in linear arrays suggesting that they may have been continuous layers boudinaged and disrupted during deformation. The core of these lenses contains epidote with variable amounts of plagioclase and garnet. Electron microprobe analyses of samples from 3 lenses give epidote compositions with 9-13 mole % clinozoisite and piemontite concentrations < 2.5 mole %, an average plagioclase concentration of 87% anorthite and calcic garnet compositions that range from 31 to 41 mole % grossular, 33 to 60 mole% andradite, and lesser amounts of almandine (7 to 15 mole %,) and spessartine (1 – 13 mole %). In lenses within amphibolites, the epidote/garnet core is surrounded by a clinopyroxene-plagioclase zone. Microprobe transects across this zone towards the epidote/garnet core show a continuous increase from 50% to 85% in the anorthite content of plagioclase accompanied by an increase from 0.25 to 0.5 in the Fe/(Fe+Mg) ratio of clinopyroxene. The current mineralogy and mineral chemistry of the calc-silicate lenses appear to have developed by diffusion metasomatism during high-grade metamorphism between an iron-rich, calcareous, possibly sedimentary protolith and a mafic, possibly volcanic, rock.
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