HR: 0830h
AN: GP31C-0760    [PDF]
TI: Paleomagnetism and Monazite Dating of Grenville Rocks, Adirondack Mountains, NY
AU: * Brown, L
EM: lbrown@geo.umass.edu
AF: Dept. Geosciences, Univ. Massachusetts, Amherst, MA 01003 United States
AU: McEnroe, S
EM: Suzanne.McEnroe@ngu.no
AF: Geological Sur.Norway, N-7491, Trondheim, N-7491 Norway
AU: Jercinovic, M
EM: mjj@geo.umass.edu
AF: Dept. Geosciences, Univ. Massachusetts, Amherst, MA 01003 United States
AB: Paleomagnetic studies on three rock units from the Adirondack Highlands, New York State yield stable magnetic directions. Electron microprobe monazite geochronology suggests a strong ca. 1050 Ma signature, corresponding to Ottawan granulite-facies metamorphism. Remnants of older (ca. 1130-1190 Ma) monazite, consistent with early-Grenville tectonomagmatic events are also documented. There is no evidence of younger ($<1050$ Ma) events with the exception of partial alteration (with Ca-enrichment) of some monazite. Sillimanite-microcline gneisses (gms) of the far-western Highlands, associated with negative aeromagnetic anomalies, exhibit strong stable magnetization dominated by titanohematite with abundant exsolutions of ilmenite, pyrophanite, rutile and spinel. Mean magnetic directions for 14 sites are I-62.8, D=289.2 and a-95=7.6. Sampled in the central Highlands is the post-orogenic fayalite ferro-hedenbergite Wanakena Granite. Samples contain magnetite with ilmenite oxy-exsolution, occurring as discrete grains and inclusions in silicates. Directions from the Wanakena are steeply negative with westerly declinations (I=-76.4, D=296.7, a-95=4.4, N=7). The Marcy meta-anorthosite was sampled in the central and eastern Highlands, although many of these sites proved unstable. Stable results were combined with unpublished data from Rob Hargraves for 13 sites (I=-64.4, D=286.2, a-95=9.1). Over half of the anorthosites and one gms site have normal directions; all Wanakena sites are reversed. Combined anorthosites and gms units give a pole position of 20S/151E; the Wanakena pole is at -29S/132E. Both poles fall in the southern extent of the Grenville loop. The thermodynamically constrained equilibrium phase diagram for ilm-hem predicts that very fine exsolution, most likely responsible for the stable magnetization of the gms rocks, starts to form around 390C, well below the conditions of granulite grade metamorphism. The abundant lamellae provide a stable NRM through the formation of lamellar magnetism acquired at a late stage in the orogen. Using monazite ages for an upper limit and published age data for the Adirondacks produces a cooling curve yielding 920-900 Ma as time of magnetization acquisition for gms.
DE: 1519 Magnetic mineralogy and petrology
DE: 1525 Paleomagnetism applied to tectonics (regional, global)
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2003 Fall Meeting