Mineralogy of Amphibolite Interlayers in the Gneiss Complex, Northwest Adirondack Mountains, New York

Abstract

The progressive metamorphism of amphibolite interlayers in the Adirondack gneiss complex is reflected in the complete transition between the following mineral assemblages: (1) green hornblende-andesine-quartz-ilmenite, stable at an estimated 525° C. at Emeryville, New York, and (2) brown hornblende-labradorite-clinopyroxene-orthopyroxene-ilmenite, stable at approximately 625° C. at Colton, New York. The hornblende in the Emeryville assemblage is bluish-green, and averages about 70 volume per cent of the amphibolite rock. With increasing grade of metamorphism, the color, density, and composition of the hornblende changes toward a hydroxyl-deficient, brownish-green, denser variety, relatively enriched in Ti, Na, K, F, Cr, V, Sc, Co, and Ni, and depleted in Mn, Zn, and OH. Both Fe₂O₃/FeO, and Fe/Mg decrease with increasing grade of metamorphism. The coexisting plagioclase increases in An content from to with increasing grade of metamorphism. This compositional change is accompanied by an increase in grain size, and abundance, but the orthoclase molecule in the plagioclase decreases slightly with rising temperatures and pressures of crystallization. Ilmenite constitutes 75-95 per cent of the opaque mineral constituent in the amphibolites, ranges up to 9 volume per cent, and averages approximately 2 per cent at both Emeryville and Colton. Analyses of two ilmenites from Emeryville and two from Colton indicate little variation in composition from (in weight per cent) TiO₂ 52, total Fe (as FeO) 44.5, MgO 1.2, and MnO 1.4. A pale-green clinopyroxene first appears in the amphibolites formed at an estimated temperature of 550° C. At Colton clinopyroxene composes from 15 to 52 per cent of the rock, averaging about 20 per cent. Orthopyroxene first appears in the Russell area where temperatures of metamorphism are estimated at 575° C. At Colton orthopyroxene averages about 15 per cent of the rock but constitutes over 30 per cent of a few "amphibolites." The distribution coefficients of Mg/(Mg + Fe⁺⁺) in coexisting pyroxene pairs at Colton are about 0.52, similar to those of pyroxene pairs found in mafic granulites in India, Africa, Scandinavia, and Great Britain. This partitioning of Fe and Mg, along with Ca, is distinctive for metamorphic pyroxenes, and may be employed as both a geologic thermometer and to distinguish metamorphic from igneous mineral assemblages. The gross distribution patterns of elements between the coexisting minerals of the Colton assemblages reflect (1) the concentration of Al, K, Na, Ba, Ga, Sr, and Pb in the plagioclase and hornblende relative to the pyroxenes, and (2) the greater variety, and complex substitutions of the common ions in hornblende and Ca-rich pyroxene compared to the Ca-poor pyroxene and plagioclase. In any one area in the belt of progressive metamorphism, the partitioning of specific elements and their distribution coefficients are remarkably uniform. This is indicative of essential equilibrium between the coexisting phases. The sequence of chemical changes within the associated minerals in the amphibolites metamorphosed between Emeryville and Colton appears to be typical of those in basic rocks reconstituted in the deeply eroded parts of continental crusts, under P-T conditions that have induced partial melting in the enveloping, more felsic, paragneiss.