Abstract
Within the bounds of the Marcy anorthosite massif, fresh and extensive rock exposures in the valley of Roaring Brook display a great variety of rock types ranging from hyper-sthenite and anorthosite to charnockite and leptynite. With the exception of xenolithic metasediments all rocks are considered members of one differentiation series. Consanguinity is indicated by transitional field relations between rock types, and by the complete modal and chemical gradation between members of the suite.
Transgressive relationships demonstrate that anorthosite and hypersthenite are the older, and the K-feldspar and quartz-rich rocks the younger members of the series. Textures indicate that hypersthenite, anorthosite, and leuconorite are orthocumulates. Orthopyroxene in hypersthenite, and plagioclase in anorthosite and leuconorite are the principal cumulus minerals. Igneous lamination is a common structure in leuconorite; rhythmic layering is observed in jotunite. Hypersthenite, which occurs in a dike intersecting the igneous lamination, apparently intruded leuconorite when both rocks were in the state of a crystal mush.
Although all rocks have undergone orogenic deformation and granulite-facies metamorphism, the rocks poor in K-feldspar and quartz are little affected. Foliation developed only in part of the area, in jotunitic, farsunditic and charnockitic rocks. The development of garnet and metamorphic recrystallization occurred in rocks with an Fe:Mg ratio above a value of about 1.3.
