The Occurrrence of Garnet in the Granulite-Facies Terrane of the Addirondack Highlands and Elsewhere, an Amplification and a Reply

Elaborating on the theme of a previous paper (1965) and in response to Buddington‘s criticism (1966) the present discussion is primarily concerned with the high-grade meta-morphic reactions and regional-metamorphic zoning in the Adirondacks. The hypersthene isograd, delineated by the first appearance of orthopyroxene as a product of Pw-T-controlted reactions involving hornblende and biotite in quartz-buartz-bearing rocks. separates the almandite-amphibolite-facies terrane of the lowlands from the granulite-facies region of the highlands. In the almandite-amphibolite facies of the lowlands at least one isograd, the orthoclase-garnet isograd, (Ruddington's lst garnet isograd), can be recognized which is delineated by the Pw-T-controlled appearance of these minerals at the expense of muscovite and part of the biotite and quartz. The granulite-facies terrane of the highlands is characterized by the predominance of assemblages of the hornblende-granulite facies in which orthopyroxene cocxists with hydrous minerals such as biotite and hornblende in apparent equilibrium. In the Adlrondacks there is no sizeable, regional development of the pyroxene-granulite facies where hydrous minerals are virtually absent. Within the hornblende-granulite-facies terrane two isograds can be recognized. The garent-clinopyroxene isograd (Buddington's 3rd garnet isograd) delineates the regional development of garnet, clinopyroxene, and quartz by P1-T-controlled reaction between orthopyroxene and plagioclase. The isograd defines in the eastern Adirondack a region of the hornblende-clinopyroxene-almandite subfacies of the honblende-granulite facies. In the western Adirondacks, near the hypersthene isograd, a yet uncharted cordierite isograd may define the areal extent of the biotite-cordierite-almandite subfacies of the hornblende-granulite facies. The central portion of the Adirondacks, where cordierite is absent and garnet and clinopyroxene do not coexist except in silica-deficient rocks, is the region of the hornblende-orthopyroxene-plagioclase subfacies of the hornblende-granulite facies.