HR: 1340h
AN: V23C-1551    [Abstracts]
TI: LA-ICP-MS dating of Archean Metamorphic Zircons From the Kapuskasing Structural Zone, Ontario: Evidence for the Geodynamic Origin and Evolution of the Abitibi–Opatica Terrane
AU: * Benn, K
EM: kbenn@uottawa.ca
AF: Ottawa-Carleton Geoscience Centre and Department of Earth Sciences, University of Ottawa, Department of Earth Sciences, University of Ottawa, Ottawa, ON K1N6N5, Canada
AU: Kamber, B
EM: bkamber@laurentian.ca
AF: Department of Earth Sciences, Laurentian University, Department of Earth Sciences, Laurentian University, Sudbury, ON P3E2C6, Canada
AB: The Late Archean Abitibi greenstone belt, in SE Superior Province, is one of the largest contiguous greenstone belts on Earth. Geochronological data from upper crustal rocks indicate it was formed by volcanism and plutonism that spanned 2760 Ma through 2660 Ma. The classical model for the Abitibi belt is one of tectonic accretion and thrust stacking of magmatic arcs, and possibly plateau fragments, resulting in a thick crust, with a granulitic base. A Proterozoic uplift, the Kapuskasing structural zone, on the western side of the Abitibi belt, is believed to represent the lower to middle crust of the Abitibi. Previous U-Pb zircon dates from the Kapuskasing uplift correspond to a period of granitic plutonism at the end of the Abitibi magmatic history (ca. 2660 Ma), consistent with underthrusting and underplating, during and following tectonic collision. An alternative model for the Abitibi belt is that it represents part of a single, large oceanic plateau, modified by subduction and slab-window magmatism. The Abitibi greenstone belt and the Opatica gneiss belt, to the north, would then represent a single tectonic terrane, called the Abitibi–Opatica terrane. That model requires the existence of a thick crust prior to tectonic accretion, rather than thickening of crust as the result of collision. The plateau model can be tested by dating metamorphic minerals from lower to middle crustal rocks in the Kapuskasing uplift. We have used laser ablation (LA) -ICP-MS, to carry out "in-situ" dating of zircons from a mafic clinopyroxene- garnet granulite and from two hornblende-biotite-garnet metatonalites. Zircons in the rocks are less than 100 microns in size, mostly 40 microns or less. BSE and CL images of the zircons reveal that some are unzoned, some are patchwork zoned, and others are sector zoned. The crystals are devoid of oscillatory zoning and their morphologies are mainly anhedral to subhedral; they are interpreted to be of metamorphic in origin. That interpretation is supported by REE elements patterns that show characteristic HREE depletion, suggesting equilibrium of zircons with garnet. Metamorphic zircons dated 3190 Ma, 2850 Ma, and 2800 Ma, all record pre-Abitibi (pre-2760 Ma) metamorphic events. Thus the amphibolite- to granulite-grade rocks of the Kapuskasing zone contain components of an older high-grade terrane. Younger zircon dates record thermal events in the lower to middle crust that accompanied three known Abitibi magmatic events, including tonalite (2748 Ma), granodiorite (2700 – 2690 Ma), syenite and sanukitoid (2685 – 2675 Ma), and granite (2660 Ma) magmatism. It is interpreted that the metamorphic zircons formed as the result of thermal events that accompanied the magmatic events. The results show that in-situ analyses of zircons, using LA-ICP-MS, is a powerful method for detailed investigations of metamorphic histories. The results also indicate the existence of high-grade metamorphic rocks in a crust 20 km thick (or more), that recorded thermal events before, as well as during, collisional tectonics, supporting the oceanic plateau model for the Abitibi–Opatica terrane. Finally, the results also indicate that the Abitibi–Opatica oceanic plateau was initiated during rifting of an older continental margin, from which the pre- 2760 Ma zircon components were derived.
DE: 1120 Isotopic disequilibrium dating
DE: 8103 Continental cratons
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting