2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 13
Presentation Time: 4:30 PM

Monazite as a Monitor of Melting, Garnet Growth, and Feldspar Recrystallization in Continental Lower Crust: Athabasca Granulite Terrane, Western Canadian Shield


DUMOND, Gregory1, WILLIAMS, Michael2, GONCALVES, Philippe3 and JERCINOVIC, Michael1, (1)Geosciences, Univ of Massachusetts, 611 N. Pleasant St, Amherst, MA 01003, (2)Department of Geosciences, University of Massachusetts, Amherst, MA 01003-9297, (3)Département des Géosciences, Université de Franche-Comté, 16 route de Gray, Besançon, 25030, France, gdumond@geo.umass.edu

Monazite is a common accessory phase in kyanite- and orthopyroxene-bearing felsic ribbon mylonite granulites exposed in the southern Upper Deck domain of the Athabasca granulite terrane, western Canadian Shield. Felsic granulites in the ~10 km-thick Upper Deck represent the restitic product of ultrahigh-temperature melting of deep continental crust, driven by emplacement of a mafic underplate. In situ high-resolution x-ray mapping and precise trace element electron probe microanalysis of monazite reveal first-order links between composition, microstructure, Th-U-total Pb geochronology, and evolution of continental lower crust.

Monazite inclusions in garnet are depleted in Y, Sm, and Gd and linked to melt-enhanced flow of continental lower crust during fluid-absent breakdown of biotite + plagioclase + quartz at ca. 1.0-1.1 GPa. Circa 2.62-2.58 Ga Y-depleted, Th-rich domains represent peritectic growth of monazite in the presence of melt, garnet, ternary K-feldspar, and orthopyroxene at T >900°C. Low-Th rims depleted in Ca and enriched in Eu are linked to melt-absent growth of grossular-rich garnet at the expense of dynamically recrystallized orthopyroxene and plagioclase during crustal thickening at ca. 2.58-2.55 Ga to >1.5 GPa. Greater than 650 m.y. of isobaric-cooling and strengthening was followed by syn-kinematic growth of LREE-rich monazite (concurrent with dynamic recrystallization of ternary feldspar) during intra-continental dextral transpressive shear strain at ca. 1.92-1.9 Ga.

Monazite grains in this study are marked by positive Eu-anomalies relative to CI chondrite. A direct link is implied between Y, Sm, Eu, and Gd in monazite and two major phases in continental lower crust: garnet and plagioclase. Europium-anomalies in lower crustal monazite associated with modally-abundant Grt +/- Pl appear directly related to depletions of Y, Sm, and Gd during garnet-growth and loss (or removal) of plagioclase. These links permit tight constraints on the evolution of continental lower crust at depth during melting, crustal flow, isobaric-cooling, and strain-partitioning.