IMPLICATIONS FOR THE PRESERVATION OF MAGMATIC TEXTURES IN THE LYON MOUNTAIN GRANITE, ADIRONDACK MOUNTAINS, NEW YORK

The preservation of magmatic textures within the Lyon Mountain granite (LMG) confirms that the LMG never experienced high-grade Ottawan metamorphism or penetrative deformation. Perthitic feldspar and clinopyroxene contain magmatic exsolution lamellae. Perthitic feldspars from a sample of the fayalite-bearing LMG near Ausable Forks, NY, contain long, straight albite lamellae indicative of magmatic crystallization. The same sample also contains hedenbergite pyroxene with exsolution lamellae of probable pigeonite. Pigeonite is only stable above temperatures of 900º C and must be of igneous origin. The preservation of these textures and mineral assemblages implies these minerals were not recrystallized during metamorphism or deformation. Recrystallization would favor the growth of stable phases at temperatures well below those expected to produce pigeonite and would destroy the delicate feldspar perthite textures. In contrast to the LMG, feldspar grains in the penetratively deformed Hawkeye granite have few exsolution lamellae. Though chemically similar to the LMG, the Hawkeye granite typically contains two feldspars and primary orthopyroxene that is locally replaced by a metamorphic assemblage of hornblende and augite, indicative of granulite facies metamorphism. Within the LMG, the ubiquitous presence of low-Ti magnetite instead of ilmenite and orthopyroxene (Opx) also suggests a lower temperature and pressure history compared to other granitic rocks in the Adirondack Highlands. Though chemically similar to many of the Opx-bearing ferroan granitoids of the anorthosite-mangerite-charnockite-granite (AMCG) suite, the LMG contains no Opx. These observations are consistent with zircon textures and ages that indicate the LMG must be late Ottawan (1060-1045 Ma) and could not have experienced the main phase of the Ottawan orogeny apparent in older granitoids. It is likely that LMG magmas formed as a result of decompression melting of over-thickened crust via partial melting of Opx-bearing granitoids in the AMCG suite and older Grenville Supergroup lithologies.