Metamorphism, Fluid Flow and Partial

Field, petrologic and geochemical data were used to characterize fluid infiltration and partial melting during metamorphism of pelitic rocks in the contact aureole of the Onawa pluton, central Maine, USA. Mineral assemblages delineate five metamorphic tones within the contact aureole: chlorite zone, andalusite— cordierite (a—c) zone, alkali feldspar zone, sillimanite zone and leucocratic-vein (l-v) zone. The sequence of observed mineral assemblages and mineral—fluid reactions calculated by mass balance is similar to those observed in other contact aureoles. Pressure of contact metamorphism is ~3 kbar, on the basis of optimum geothermobarometry calculations. Metamorphic tem-peratures vary from ~ 500 ° C in the andalusite-cordierite zone to ~650°C in the leucocratic-vein zone. Data from field observations, mineral textures, observed reaction stoichiometry, geothermometry and major-element geochemistry suggest that the leucocratic veins of the l-v zone represent crystallized, partial melts. Two overall calculated mineral reactions are responsible for vein formation