Garnet-pyroxene skarns were formed 90 m.y. B.P. in the Osgood Mountains at or near contacts of grandiorite with calcareous rocks of the Cambrian Preble Formation. The metasomatic replacement followed contact metamorphic recrystallization of the Preble. The sources, temperature, and variation in H2O/CO2 ratios of the metasomatic fluid are interpreted from 269 analyses of oxygen, carbon, hydrogen, and sulfur isotopes in whole rocks, minerals and inclusion fluids. Skarns formed in three mineralogical stages. Oxygen isotope data indicate that temperatures during the crystallization of garnet, pyroxene and wollastonite (Stage I) were least 550 ?? C, and that the metasomatic fluid had an {Mathematical expression} ??? 0.035 in the massive skarns, and ??? 0.12 in vein skarns up to 3 cm thick. Pore fluids in isotopic equilibrium with garnet in calc-silicate metamorphic rocks, on the other hand, had {Mathematical expression} ??? 0.15. The metasomatic fluids of Stage I were derived primarily from the crystallizing magma. The isotopic composition of magmatic water was ??18O =+9.0, ??D= -30 to -45. Oxygen isotope temperatures of greater than 620 ?? C were determined for the granodiorite. Isotopic and chemical equilibria between mineral surfaces and the metasomatic fluid were approached simultaneously in parts of the skarn several meters or more apart, while isotopic and chemical disequilibria (i.e. zoning) have been preserved between 20 to 40 ??m-thick zones in grandite garnet. More Fe-, or andradite-rich garnet crystallized in more H2O-rich C-O-H fluids ( {Mathematical expression} ??? 0.01) than present with grossularite-rich garnet ( {Mathematical expression}??? 0.035). Stage II was marked by the replacement of garnet and pyroxene by quartz, amphibole, plagioclase, epidote, magnetite, and calcite. Many of the replacement reactions took place over a relatively narrow range in temperature (480-550 ?? C), as indicated by 18O fractionations between quartz and amphibole. Meteoric water comprised 20 to 50% of the metasomatic fluid during Stage II. Calcite was formed along with pyrite, minor pyrrhotite, and chalcopyrite during Stage III, although the crystallization of pyrite and calcite had begun earlier, during Stages I and II, respectively. Carbon and sulfur isotope compositions of calcite and pyrite indicate a magmatic source for most of the C and S in the metasomatic fluids of Stage III. By the end of Stage III, meteoric water constituted as much as 100% of the metasomatic fluid. Minerals from grandiorite and skarn do not show large depletions in 18O because the oxygen isotope composition of the metasomatic fluid was buffered by the calcareous wall rocks and the grandiorite. Meteoric water in the vicinity of the Osgood Mountains during the Late Crectaceous (??18Ocale. ??? -14.0, ??D = - 107) was slightly enriched in 18O and D relative to present-day meteoric water (??18O = 15.9, ??D = - 117) ?? 1977 Springer-Verlag.