Proterozoic zircon growth in Archean lower crustal xenoliths, southern Superior craton – a consequence of Matachewan ocean opening |
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| Authors: | D E Moser L M Heaman |
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| Institution: | (1) Jack Satterly Geochronology Laboratory, Royal Ontario Museum, Toronto, Ont., M5S 2C6, Canada, CA |
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| Abstract: | Granulite-grade, anorthositic and mafic xenoliths recovered from a Jurassic kimberlite pipe near Kirkland Lake, Ontario are
fragments of the lower crust that underlies the ca. 2.7 Ga Abitibi greenstone belt of the Superior craton. Cathodoluminescence
imaging and/or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization
history, characterized by two main stages of zircon growth. The age of the two stages has been constrained by combining imaging
results with isotope dilution U-Pb dating of grain fragments and single grains. Minimum ages for the first crystallization
stage in individual xeno liths are 2584 ± 7 Ma, 2629 ± 8 Ma, 2633 ± 3 Ma, whereas an approximate crystallization age for a
fourth sample is 2788 ± 57 Ma. The second main stage of growth consists of chemically and isotopically distinct metamorphic
zircon overgrowths. Times of solid-state zircon growth are most broadly constrained in three samples to the interval between
2.52 Ga to 2.40 Ga, and most precisely dated in a meta-anorthosite at 2416 ± 30 Ma. These complex zircons are intergrown with
garnet and clinopyroxene of the host granulite-facies assemblage, and thus the Paleoproterozoic ages of the metamorphic overgrowths
are interpreted to reflect an interval of isobaric, granulite-grade metamorphism of the lower crust beneath the greenstone
belt approximately 150 million years after craton formation. This interval of metamorphism is broadly coeval with the intrusion
of the Matachewan dyke swarm across the southern Superior craton, and with mafic magmatism and deposition of Huronian rift-margin
sediments 200 km to the south during the opening of the Matachewan ocean. It is proposed that a significant volume of magma
intruded the crust-mantle interface during rifting, promoting isobaric metamorphism and zircon growth in the deep levels of
the Superior craton. Subsequent major rifting events along this margin apparently failed to produce a similar lower crustal
response. The results have important implications for the structure of lithosphere beneath Archean continental crust.
Received: 3 October 1995 / Accepted: 11 February 1997 |
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