ORIGIN AND SUBSEQUENT MODIFICATION OF K-FELDSPAR MEGACRYSTS WITHIN THE GRAY'S SCHOOL BODY OF THE HERMON GNEISS, ADIRONDACK LOWLANDS, NEW YORK

A ca. 1300 to 1150 Ma K-feldspar megacrystic gneiss (the Gray's School body) occurs as an elongate (10 by 3km) lens within multiply-deformed amphibolite to granulite facies metasediments in the Adirondack Lowlands of New York. This visually striking rock is texturally complex and contains textural elements implying magmatic crystallization and metasomatism, as well as metamorphic recrystallization. The body is locally strongly deformed and possesses a foliation of varying intensity made up of discontinuous alternating felsic and mafic (mainly amphibole and biotite) laminae. Highest strain levels occur within the marginal zones where significant grain reduction has occurred. In these areas, megacrysts lie in the foliation plane and recrystallization tails are prominent. A few exceptions lie oblique to the foliation plane, seemingly truncating the gneissic foliation, suggesting the possibility of post-fabric growth. This fabric, therefore, may not be solely of tectonic origin but could represent a relict magmatic flow foliation, or some metasomatic growth of K-feldspar as was recently suggested by others. Isolated examples of myrmekite in these marginal zones may be strain induced. In lower strain areas, where megacrysts tend to be euhedral to subhedral, myrmekite replacement occurs within the megacrysts, possibly suggesting late-stage magmatic crystallization. Most megacrysts are mantled by a thin grain-reduced selvage and locally, in outcrop, a number of feldspars appear to be fragmented. Microperthites are common and a complex zoning resulting from variations in the concentration of exsolved plagioclase may reflect an original magmatic zonation (as suggested by previous workers).

Our initial study indicates that deformational effects dominate the textures in the Gray's School body, but that there are significant indications of an igneous protolith. Recent suggestions of a predominantly metasomatic origin for the K-feldspars in the Gray's School body have not been substantiated, although we do not exclude the possibility of minor alkali metasomatism.