Paleomagnetism and magnetic mineralogy of Grenville metamorphic and igneous rocks, Adirondack Highlands, USA

Abstract

Paleomagnetic samples were collected from an east–west traverse across the Highlands region of the Adirondack Mountains, northern New York, which forms part of the southern most exposure of the Grenville Province of North America. Granulite facies metamorphism (T>650°C) at ∼1050Ma completely reset pre-existing magnetic directions in sampled microcline gneisses and metamorphosed anorthosites. Fourteen sites of microcline gneiss yield a mean direction of I=−62.8°, D=289.2° with α95=7.6° and a corresponding pole at 18.4°S and 151.1°E. Metamorphic anorthosites and associated rocks (N=14) show a direction of I=−67.3°, D=283.9° with α95=7.7° and a corresponding pole at 25.1°S and 149.0°E. Post-metamorphic fayalite granites possess a statistically different direction of I=−75.8° and D=297.0° (α95=3.9°, N=8 sites) and a pole of 28.4°S and 132.7°E. Both normal and reverse polarities are recorded in all units, with reverse sites occurring on the eastern and western ends of the traverse, and normal polarities restricted to the central part of the massif. The remanence is carried by ilmeno-hematite in the microcline gneisses. In the metamorphosed anorthosites, and the unmetamorphosed fayalite granites, hemo-ilmenite and magnetite occur, though magnetite is the predominant oxide. Using cooling curves established for the Highlands, and blocking temperatures determined for ilmeno-hematite, hemo-ilmenite and magnetite, the age of remanence is determined to be ∼990Ma for the magnetite-bearing Wanakena granite, ∼970Ma for the metamorphosed anorthosite and related rocks, and ∼960Ma for the ilmeno-hematite rich microcline gneiss. The pole data from the Adirondacks, as well as selected studies from other areas of the Grenville Province on units with similar mineralogy and some age control, helps define the southerly part of the Grenville loop of the apparent polar wander path. The three units from the Adirondacks indicate counter-clockwise motion of the APWP between 990 and 960Ma.