ORTHOPYROXENE-BEARING MIGMATITES FROM THE SOUTHWEST SWEDISH GRANULITE REGION, HALLAND, SWEDEN

Opx-producing dehydration melting reactions are commonly invoked as a mechanism of granulite-facies metamorphism. Opx-bearing migmaties offer an opportunity to study these reactions. Opx megacrysts (2-5 cm in diameter) occur in neosomes in migmatitic amphibolites at the Stensjöstrand Naturreservat and Nösslinge, Halland, Southwest, Sweden. At Stensjöstrand, Opx is limited to neosomes in amphibolitic migmatites with relatively low proportions of leucosome while amphibolites with higher proportions of leucosme are Opx free. At Nösslinge small Opx crystals are scattered throughout the amphibolite. Leucosomes in both localities are tonalitic. At Stensjöstrand Plg in leucosomes are antiperthitic with average compositions 3 to 10 mole % more sodic than Plg in the adjacent amphibolitic host. Opx-megacrysts are surrounded by rims of Cpx partially altered to Hblnd, which is surrounded by a Qtz-Hblnd symplectite. Hblnd grains, surrounding Opx megacrysts, are lower in Na + K and Ti compared to grains in leucosomes or amphibolite hosts. Other compositional differences between minerals in Opx-bearing and non-Opx-bearing migmatites are small. The evidence is constituent with a model in which Opx megacrysts and associated leucosomes formed through dehydration melting in the presence of a fluid with low a_H2O. During crystallization of the melt pyroxene reacted with the liquid to form Hblnd rims, which armored the Opx in the center of the megacrysts from further reaction. The higher proportion of leucosome in the Opx-free migmatitic amphibolites suggests that in these rocks melting may have been fluxed by a fluid with higher a_H2O. However, the evidence is also consistent with a model in which all rocks underwent Opx-producing dehydration melting and in which some H₂O bearing melt migrated from amphibolites that now contain Opx, into units that are now Opx-free. In this model a relative deficiency in H₂O-bearing melt allowed some pyroxene to survive the crystallization of the melt in the leucosme-poor units. In order to distinguish between these two models we are carrying out a systematic program of whole-rock analyses.