A COMPARATIVE FLUID INCLUSION STUDY OF THE WATERVILLE AND SANGERVILLE(VASSALBORO) FORMATIONS, SOUTH-CENTRAL MAINE

Petrologic and oxygen isotope data indicate that water-rich fluids inf iltrated metasedimentary rocks of the Waterville and Sangerville (form ally Vassalboro) Formations, south-central Maine, during peak metamorp hism, and depleted Sangerville rocks in alkalis but not equivalent Wat erville rocks. Fluid inclusion data from two outcrops, similar to 1 km apart, one of the Waterville and the other of the Sangerville Formati ons, suggest a cause for the geochemical difference between the two un its. Postulated peak metamorphic inclusions, the texturally earliest o f aqueous inclusions in the metasediments, approximate the water-rich compositions of peak fluids predicted by mineral-fluid equilibria, and have average salinity in the Sangerville Formation similar to three t imes that of equivalent Waterville inclusions. The higher salinity in the Sangerville fluids could explain the greater alkali depletion in t hese rocks. Probable pre-peak or prograde inclusions are preserved in metasediments as the texturally earliest carbonic inclusions which con tain CO2, CH4, N-2 +/- H2O, as determined by microthermometry and Rama n spectremetry. They may have formed by breakdown of organic matter. P robable retrograde inclusions occur as texturally late aqueous inclusi ons in healed fractures with salinity ranges indistinguishable between the two formations. Synmetamorphic granitic dikes present in the two outcrops were ruled out as a source for fluids in metasediments becaus e composition and density ranges of inclusions in dikes and metasedime nts are fundamentally different, and because there is no correlation b etween the abundance or composition of inclusions in a sample and prox imity to dikes. Isochores for many of the inclusions in both metasedim ents and dikes are not consistent with the inferred P - T conditions o f their trapping, but intersect at similar to 300 degrees to 400 degre es C and 1 to 2 kbar. The intersections probably resulted because incl usion densities continued to equilibrate during uplift and cooling unt il quartz became rigid. The present densities are those at the last eq uilibration, not the time of trapping. In contrast, the clear distinct ions in inclusion compositions between dikes and between dike and coun try rock show that the original compositional differences generally ha ve been preserved.