HR: 17:45h
AN: V34C-08    [Abstracts]
TI: Tracking the Petrogenetic and Rheological Evolution of Continental Lower Crust Through Time With Monazite
AU: * Dumond, G
EM: gdumond@geo.umass.edu
AF: Department of Geosciences, University of Massachusetts 611 N Pleasant ST, Amherst, MA 01003, United States
AU: Williams, M L
EM: mlw@geo.umass.edu
AF: Department of Geosciences, University of Massachusetts 611 N Pleasant ST, Amherst, MA 01003, United States
AU: Mahan, K H
EM: Kevin.Mahan@colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309, United States
AU: Jercinovic, M J
EM: mjj@geo.umass.edu
AF: Department of Geosciences, University of Massachusetts 611 N Pleasant ST, Amherst, MA 01003, United States
AU: Flowers, R M
EM: Rebecca.Flowers@colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309, United States
AB: Monazite (Mnz) is a common accessory phase in one of Earth's largest exposures of exhumed continental lower crust: the Athabasca granulite terrane, western Canadian Shield. The terrane includes three shear zone-bounded domains in northern Saskatchewan interpreted to have experienced lower crustal residence near the Moho for at least 650 m.y. prior to exhumation. In situ high-resolution X-ray mapping and precise trace element electron probe microanalysis of Mnz reveal fundamental links between Mnz composition, texture, Th-U-total Pb geochronology, and evolution of continental lower crust. Monazite is used to contrast two variations of weak, lower crustal flow in the Neoarchean with a Paleoproterozoic record of partitioned sub-vertical fabric development and strain-hardening. Early, sub-horizontal fabrics in granodioritic orthogneisses contain Ca-rich Grt that preferentially nucleated on the Na-rich, dynamically- recrystallized mantles of calcic Pl-porphyroclasts. Syn-kinematic Mnz domains are depleted in Y, Sm, and Gd relative to early Y-rich domains and are interpreted to directly date the onset of Grt-growth concurrent with solid- state lower crustal flow at ca. 2.6-2.55. Melt-enhanced lower crustal flow is implicated elsewhere in felsic granulites that record peritectic Grt-growth. In general, sealed Mnz inclusions in Grt consist of two domains: high- Th cores depleted in Y and Sm that are linked to crystallization in the presence of melt + Grt at ca. 2.62-2.6 Ga and low-Th rims depleted in Ca and enriched in Eu that are linked to melt-absent growth of Grs-rich Grt and Pl-loss during crustal thickening at ca. 2.58-2.55 Ga. Following a protracted period of isobaric-cooling and strengthening of lower crust, syn-kinematic Mnz domains throughout the terrane constrain the onset of sub-horizontal shortening and intra-continental dextral shear strain at ca. 1.9 Ga, immediately prior to the onset of terrane exhumation. Nearly all Mnz grains analyzed in this study are marked by positive Eu-anomalies relative to CI chondrite (1.2 to 14.5), in stark contrast to virtually all published examples. A direct link is implied between Y, Sm, Eu, and Gd in Mnz and two major phases in continental lower crust: garnet and plagioclase. Eu-anomalies in lower crustal Mnz associated with modally-abundant Grt + Pl appear directly related to depletions of Y, Sm, and Gd during Grt- growth, and loss (or removal) of Pl. These links permit tight constraints on the evolution of continental lower crust at depth, e.g. during melting, crustal flow, isobaric-cooling, and strain-partitioning.
DE: 1020 Composition of the continental crust
DE: 1065 Major and trace element geochemistry
DE: 1100 GEOCHRONOLOGY
DE: 8031 Rheology: crust and lithosphere (8159)
DE: 8100 TECTONOPHYSICS
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting