Abstract The migmatites with Kfs-poor leucosomes from the Svecofennian domain of southern Finland are characterized by a suite of anastomosing leucosome veins and patches characterized by low and varying abundance of K-feldspar with or without cordierite. These leucosomes are generated by in situ melting reactions at different P-T-a H 2 O conditions. Microtextural analysis in conjunction with thermocalc calculations and geothermobarometry show that these rocks were metamorphosed under granulite facies conditions at 700–750°C, 4–5 kbar and a H 2 O = 0.4−0.7. The formation of cordierite coronas around garnet and the late crystallization of andalusite suggest that the final stage of the P-T history was characterized by decompression and cooling to within the andalusite stability field, estimated at 500–650°C and 3–4 kbar. U-Pb and Sm-Nd conventional analyses of monazite and garnet, respectively, from different parts of these migmatites (mesosomes and leucosomes) indicate that, within error limits, all leucosomes were formed at about 1878 Ma during a single tectonometamorphic event. Since there is no evidence of an earlier high-grade metamorphic event in this area, it is assumed that this is the approximate age of peak metamorphism and partial melting.

The Precambrian migmatites in the central Front Range were formed during high-grade regional metamorphism, prior to the emplacement of the major granitic bodies. The leucosomes of the migmatites consist essentially of varying proportions of microcline and plagioclase together with 30 to 50 percent quartz. The concentrations of biotite along the margins of the leucosomes suggest that the migmatite formed by local segregation. This interpretation is supported by data indicating equilibrium in the concentrations of alkali and alkaline earth elements between the leucosomes and the melanosomes. Strontium isotopic compositions are also in equilibrium between adjacent leucosomes and melanosomes. All the parameters considered in this study are consistent with the migmatites having formed by segregation of metamorphic rocks; the process apparently did not involve partial melting.