T170 Strengthening Links between Metamorphic Conditions and Time: New Advances in High-Temperature Geochronology and Tracing P-T-t Paths of Metamorphic Terranes

Sunday, 5 October 2008
1:30 PM - 4:45 PM

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

Paper number: T170-14
Program: Technical Sessions
Time: 4:30 PM – 4:45 PM

Authors

Abstract

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.