Silicate-Oxide Equilibria in the Wilson Lake Terrane, Labrador - Evidence for a Pre- Metamorphic Oxidizing Event
Abstract
The presence of Fe3+ and Ti in silicates and their presumed equilibration with Fe2+-Fe3+-Ti oxide minerals has long been recognized as an important factor in metamorphic phase equilibria. The Red Wine Mountains massif is a granulite facies unit in the Wilson Lake terrane of central Labrador, where this equilibration is especially important for estimating both temperature and fO2 during peak metamorphism. Peak assemblages are sapphirine + quartz, and orthopyroxene + sillimanite + quartz. The coexisting oxides, which are largely responsible for the pronounced aeromagnetic high of the massif, consist of nearly pure magnetite and an exsolved titanohematite. Estimates of fO2 based on magnetite + integrated titanohematite compositions are slightly below that defined by the pure magnetite-hematite buffer. This assemblage is also responsible for the magnetic signature of metagabbro and metanorite dikes, a fact which challenges the conventional wisdom that the high Fe3+ content of the host paragneisses was inherited from a highly oxidized ferruginous shale. We suggest here that prior to granulite facies metamorphism, an oxidizing hydrothermal event either coeval or following the emplacement of mafic dikes into the paragneiss host was responsible for the highly oxidized nature of the massif as a whole. Subsequent metamorphism then produced the observed assemblages. This scenario is supported by recent U-Pb zircon and monazite ages of ca. 1626 ± 10 Ma, which indicate that both metagabbro dikes and host paragneiss were metamorphosed at the same time. Dike emplacement and the oxidizing event must have preceded 1626 Ma. The implications of this pre-metamorphic oxidizing event is that Fe3+ becomes an inherent and fixed component in the chemical system during metamorphism. Phase relationships, preliminary thermodynamic modeling, and geothermobarometric constraints indicate that peak temperatures are lower than those previously determined for Fe3+-absent systems. More appropriate modeling of these rocks would benefit from a sapphirine mixing model involving Fe3+.
- Publication:
-
AGU Spring Meeting Abstracts
- Pub Date:
- May 2006
- Bibcode:
- 2006AGUSM.U21A..04K
- Keywords:
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- 3612 Reactions and phase equilibria (1012;
- 8412);
- 3617 Alteration and weathering processes (1039);
- 3660 Metamorphic petrology;
- 8102 Continental contractional orogenic belts and inversion tectonics;
- 9350 North America