Metamorphism, Fluid Flow and Partial Melting in Pelitic Rocks from the Onawa Contact Aureole, Central Maine, USA

Field, petrologic and geochemical data were used to characterizefluid infiltration and partial melting during metamorphism ofpelitic rocks in the contact aureole of the Onawa pluton, centralMaine, USA. Mineral assemblages delineate five metamorphic zoneswithin the contact aureole: chlorite zone, andalusite–cordierite(a–c) zone, alkali feldspar zone, sillimanite zone andleucocratic-vein (l–v) zone. The sequence of observedmineral assemblages and mineral–fluid reactions calculatedby mass balance is similar to those observed in other contactaureoles. Pressure of contact metamorphism is 3 kbar, on thebasis of optimum geothermobarometry calculations. Metamorphictemperatures vary from 500C in the andalusite–cordieritezone to 65OC in the leucocratic-vein zone. Data from fieldobservations, mineral textures, observed reaction stoichiometry,geothermometry and major-element geochemistry suggest that theleucocratic veins of the l-v zone represent crystallized, partialmelts. Two overall calculated mineral reactions are responsiblefor vein formation: which can be modeled as combinations of two NKFMTASH meltingreactions: Progress of (M1) and (M2) was measured in eight samples, andreaction (M1) is the dominant melt-forming reaction in all samples.Partial melting (and vein formation) was therefore driven byinfiltration of the l-v zone by H²O-rich fluids. Calculatedtime-integrated fluid fluxes for l-v zone samples range from09 10⁴ to 31 10⁴ mol/cm², and flow was in the directionof increasing temperature. KEY WORDS: pelites; contact metamorphism; fluid infiltration; partial melting; Onawa Pluton; Maine; USA ^*Corresponding author. Telephone:(516) 632–8192. Fax (516)632–8240 e-mail: gsymmes{at}ccmail.sunysb.edu