Petrology of flecked gneisses in the northern Wet Mountains, Fremont County, Colorado

Flecks consisting of centimetre-sized quartz-K-feldspar-plagioclase segregations with cores of magnetite and/or hornblende occur locally in discrete zones within sillimanite-grade, migmatitic biotite gneiss and amphibolite in the Wet Mountains, south-central Colorado. The close field association of flecked gneisses with migmatites in this and other reported occurrences suggests a genetic relationship. This study evaluates the possibility that flecks formed by anatexis of the host rocks.

Peak metamorphic conditions are estimated as 600-700 °C and 3-5 kbar from biotite-garnet-plagioclase-sillimanite equilibria, using garnet-core compositions. Plagioclase-K-feldspar and magnetite-ilmenite equilibria give apparent retrograde temperatures of 500-550°C.

Although the metamorphic conditions may have permitted anatexis in the gneisses, the flecks show no tendency to cluster at a minimum melt field on a plot of An- Ab-Or-Q, and the REE patterns of separated flecks do not show the LREE enrichment relative to the matrix which would be expected in partial melting. It is suggested that the flecks formed by metamorphic segregation during diffusion- controlled dehydration reactions of biotite or hornblende. Mass-balance calculations indicate significant transport of Fe, Mg, and Ca between flecks and matrix; Si and A1 are constant; Na and K show slight and inconsistent changes.