Progressive Metamorphism of Amphibolite, Northwest Adirondack Mountains, New York

Abstract

The progressive metamorphism of amphibolite interlayers in the major para-gneiss of the northwest Adirondacks has been investigated throughout a belt 35 miles long. Maximum temperatures of metamorphism appear to have increased uniformly from about 525° C at one end of the belt to 625° C at the other. This gradient in temperature is calculated from the several geothermometers used to calibrate the metamorphism of the enclosing paragneiss (Engel and Engel, 1958). Depths of metamorphism, based upon geologic reconstructions, are at least 5 to 7 miles but may be considerably deeper. The chemical composition of the amphibolites changes, with increasing T and P of metamorphism, from that of typical saturated basalt to olivine basalt or pyroxene granulite. Mineralogical changes as a function of increasing temperature are marked and systematic. Average amphibolite metamorphosed at 525° C consists of (volume per cent) quartz 10, andesine 20, hornblende 68, and ilmenite and pyrite 2; at ~ 540° C green clinopyroxene appears; at ~ 575 ° C pink orthopyroxene appears; at 625° G the average amphibolite contains calcic andesine 39, hornblende 22, clinopyroxene 21, orthopyroxene 14, and ilmenite and pyrite 2. Principal changes i n chemical composition with increasing T are decreases in Si, K, H₂O, F, CI, and Fe⁺⁺⁺ . Amounts of Ca, Mg, and probably Alincrease. Along the same gradient the enveloping paragneiss also is distinctly depleted in Si, K, H₂O, F, CI, and Fe⁺⁺⁺ and enriched in total Fe, Ca, and Mg. Associated siliceous marble is partially decarbonated. Accordingly, during Adirondack orogenesis the metamorphism of rocks now exposed produced large volumes of “hydrothermal” fluids and, at highest temperatures, granite and possibly ore-forming elements. Threshold conditions for degranitization were about 550° C for paragneiss (metagraywacke ?) and about 600° C for basaltic amphibolite. This progressive metamorphism verging on, or resulting in, partialmelting of a gneiss-amphibolite complex is rarely exposed for study in other parts of the sial, principally because belts of regionally metamorphosed rocks reconstituted at T and P in excess of the amphibolite facies have been disrupted by faulting. Hence the rocks of the Colton type that commonly appear in other granulite terranes are tectonically disassociated from less highly reconstituted parental types. Very possibly many (most ?) rocks of the granulite facies are more mafic than their initial parent rocks. Metamorphic reconstitution o f metabasic rocks at some T and P higher than that deduced for the Adirondack metamorphism could, logically, lead to the formation of eclogites; the minimum T would be 700° C, the minimum depth some 15 miles. Eclogites may, therefore, form from metabasaltic rocks at the base of, or under, continental crusts. But they would seem to be most improbable products of reconstitution at the base of the much thinner oceanic crusts. The origin of the Emeryville-Colt on amphibolites remains conjectural. Most of them seem to have formed by simple recrystallization of basaltic sills or flows. This interpretation is not entirely convincing, however, lor the very thin-layered amphibolites and these, or all of the amphibolites, may have formed by metamorphic differentiation or by replacement of sheared beds of paragneiss.