T130 Monazite: The Ultimate Geologic Record

Monday, 10 October 2011
1:30 PM - 5:30 PM
Room 102BC

MONAZITE AS A MONITOR OF MELTING IN CONTINENTAL CRUST

Paper number: T130-5
Program: Technical Sessions
Time: 2:40 PM – 2:55 PM

Authors

Abstract

Monazite is sensitive to fluid- and melt-rock interactions, and thus may be used to monitor the spatially- and temporally-variable impact of melting in continental crust. Understanding the behavior of monazite in partially molten systems is essential for interpreting dates from complexly-zoned grains, constraining the P-T-t-D evolution of HT-terranes, understanding the mechanisms of U and Th mobility and the formation of ore deposits. Monazite + melt relationships have been studied in two well-exposed middle to lower crustal orogens in the Canadian Shield: the CAGE district (Torngat Orogen) and the Upper Deck domain (Athabasca granulite terrane).

Melting in the CAGE district was associated with kilometer-scale S-C-C’ structure development. Melt and late fluids were preferentially transferred into sub-vertical C- and C’-type shear zones. Garnet-bearing migmatites in the S-structures contain abundant, large monazite with complex zoning. Circa 1.85-1.83 Ga dates are interpreted as the age of partial melting. Identification of layer-specific textures, dates and zoning in monazite suggests that the volume of rock interacting with monazite during growth and dissolution is less than the cm3-scale. In the C structures, Grt-Sil-Crd paragneiss is interpreted as restitite related to melt loss during decompression with rare small euhedral and homogeneous monazite grains that yield a consistent date of 1.81 Ga. Thermodynamic modeling of monazite-melt solubility suggests that monazite dissolution was nearly complete during the decompression and melt loss associated with C structure development.

Fluid-absent biotite melting in the Upper Deck domain was associated with sub-horizontal fabric development in the lower crust (>925C, >1.3 GPa). Pre-melting monazite domains (>2.61 Ga) are typically resorbed, Y-rich cores. Syn-melting domains are Y-depleted due to growth in the presence of peritectic garnet and Th-rich due to crystallization of monazite from anatectic melt at 2.61-2.55 Ga. Low-Th overgrowths with positive Eu-anomalies at ca. 2.54 Ga occurred during melt loss and crustal thickening, concurrent with growth of HP-Grt at the expense of plagioclase.

Our research emphasizes the important role of monazite in monitoring the effects of partial melting, melt loss, and melt crystallization during orogeny.