GARNET-PYROXENE SKARNS IN THE ADIRONDACK HIGHLANDS: GEOCHEMISTRY AND TECTONIC IMPLICATIONS

Skarns consisting of grandite garnet and calcic pyroxene, with or without wollastonite and other calcsilicates, are scattered throughout much of the Adirondack Highlands of New York. Geochemistry, including diagnostic rare earth element profiles, indicates three overlapping modes of origin for these skarns: Si and Fe metasomatism of carbonate rocks; concentration of contact metamorphic garnet and pyroxene by dissolution of calcite; and local Ca metasomatism of intrusive rocks. Garnet-pyroxene skarns are rare in Precambrian rocks, becoming increasingly common in younger rocks where they have been intensively studied because of their common association with economic metal deposits. Mapping, phase equilibria, fluid inclusion, and oxygen isotope studies by numerous investigators indicate that such skarns are ordinarily formed by contact metamorphism and metasomatism of carbonate rocks near shallow igneous intrusions at T ~ 350-650^oC and fluid pressures < 300 MPa, in the presence of abundant aqueous fluids (X_CO2< 0.3). Garnet in unmetamorphosed skarns is commonly strongly zoned. Adirondack skarns are hosted by, and have been deformed with, Mesoproterozoic granulite facies (700-800^oC, 650-800 MPa) metamorphic rocks. Some skarn bodies appear to be megaboudins. Zoning in garnet is largely absent, probably due to intra-grain homogenization during metamorphism. Relict zoning in high field strength elements (Y, Ti, Sn) is locally present. These observations indicate a premetamorphic origin for the skarns, consistent with the widely accepted but still controversial model of widespread, shallow, anorogenic or postorogenic AMCG magmatism in the Adirondack Highlands prior to the Ottawan (terminal Grenville) tectonometamorphic event.