PROTEROZOIC ZIRCON GROWTH IN ARCHEAN LOWER CRUSTAL XENOLIFTS, SOUTHERN SUPERIOR CRATON - A CONSEQUENCE OF MATACHEWAN OCEAN OPENING

Granulite-grade, anorthositic and mafic xenoliths recovered from a Jur assic 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 el ectron microscopy of zircon from four individual xenoliths reveals a c omplex crystallization history, characterized by two main stages of zi rcon growth. The age of the two stages has been constrained by combini ng imaging results with isotope dilution U-Pb dating of grain fragment s and single grains. Minimum ages for the first crystallization stage in individual xenoliths are 2584 +/- 7 Ma, 2629 +/- 8 Ma, 2633 +/- 3 M a, whereas an approximate crystallization age for a fourth sample is 2 788 +/- 57 Ma. The second main stage of growth consists of chemically and isotopically distinct metamorphic zircon overgrowths. Times of sol id-state zircon growth are most broadly constrained in three samples t o 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 inter grown with get-net and clinopyroxene of the host granulite-facies asse mblage, and thus the Paleoproterozoic ages of the metamorphic overgrow ths are interpreted to reflect an interval of isobaric, granulite-grad e metamorphism of the lower crust beneath the greenstone belt approxim ately 150 million years after craton formation. This interval of metam orphism is broadly coeval with the intrusion of the Matachewan dyke sw arm across the southern Superior craton, and with mafic magmatism and deposition of Huronian rift-margin sediments 200 km to the south durin g the opening of the Matachewan ocean. It is proposed that a significa nt 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 mar gin apparently failed to produce a similar lower crustal response. The results have important implications for the structure of lithosphere beneath Archean continental crust.