Very high temperature, moderate pressure metamorphism in the New Russia gneiss complex, northeastern Adirondack Highlands, metamorphic aureole to the Marcy anorthosite: Discussion

Abstract

Alcock and Muller (1999) assert four inferences regarding Grenville gneisses near New Russia, N.Y. (named “New Russia gneiss complex” by them) and the adjacent ca. 1130 Ma Marcy anorthosite massif. These four inferences follow: (1) metamorphism and anatexis of the New Russia complex were due to contact effects in the thermal aureole of the ca.1130 Ma Marcy anorthosite; (2) pressure estimates in the gneiss suggest anorthosite emplacement pressures of ∼750 MPa; (3) heat from intrusion of the Marcy anorthosite and coeval anorthosite–mangerite–charnockite–granite (AMCG) plutons caused regional granulite-facies metamorphism of the Adirondack Highlands; and (4) the Marcy anorthosite massif, which commonly is only mildly deformed in its core, is a postkinematic intrusion, thus implying that the Adirondacks experienced no post-Marcy, Ottawan orogenesis at ca. 1090–1030 Ma. We consider all four assertions to be seriously flawed, and we address these and related issues, beginning with inference (4), above, which is critical to their entire argument. Alcock and Muller ignore or misrepresent two critical pieces of widely reported data based upon field relations and U–Pb zircon geochronology. In addition, their own most recent geochronological results contradict the model that they propose. These critical issues are discussed below under three separate headings. 1. Alcock and Muller cite the existence of only “younger undeformed or mildly deformed granites” following the ca. 1150–1130 Ma AMCG plutonism of the Adirondack Highlands. This ignores the existence of the well-established Hawkeye granitic suite of the Highlands that is closely related to Buddington’s (1939) “younger hornblende granite” and has been dated (six samples from across the region), by U/Pb zircon, at 1095 ± 13 Ma (Chiarenzelli and McLelland 1991; McLelland et al. 1996, in press). The Hawkeye suite commonly exhibits intense ductile strain (Fig. D1) and, therefore, predates high grade, granulite-facies ( T ≈ 700–850°C) metamorphism and penetrative deformation of the Ottawan Orogeny (ca. 1090–1030 Ma) the age of which has …