SINGLE ZIRCON AGE CONSTRAINTS ON THE TECTONIC JUXTAPOSITION OF THE ARCHEAN ABITIBI GREENSTONE-BELT AND PONTIAC SUBPROVINCE, QUEBEC, CANADA

Zircons from metasediments and granitoids in the high-grade Lacorne block within the low-grade Archean Abitibi greenstone belt have been dated by the single zircon Pb-evaporation technique, yielding Pb-207/Pb-206 minimum ages. Detrital zircons in the mature clastic metasediments of the Lacorne block display a range of ages from 2691 +/- 8 Ma to 3042 +/- 6 Ma. The younger zircon ages (2691 +/- 8, 2695 +/- 4, 2715 +/- 16, and 2719 +/- 4 Ma) thus impose an upper limit for deposition and indicate that the high-grade Lacorne block is not basement to the Abitibi supracrustal sequence (2747-2680 Ma). Existence of abundant (69%) older detrital zircons (> 2750 Ma) suggests in turn that the Abitibi supracrustal rocks are not the source of the Lacorne sediments. Two generations of granitoids occur in the Lacorne block, an early monzodiorite-monzonite-granodiorite-syenite series (MMGS, 2671 +/- 4 to 2675 +/- 24 Ma) and a younger S-type garnet-muscovite granite series (GMG, 2630 +/- 20 to 2655 +/- 8 Ma). This contrasts with granitoid magmatism in the Abitibi greenstone belt which ended at approximately 2675 Ma. The Pontiac subprovince to the south of the Abitibi greenstone belt shares all of the above features of the Lacorne block, including detrital zircon ages as well as the composition and timing of granitoid magmatism. This is interpreted to indicate that the Lacorne block was originally part of the same tectonic terrane as the Pontiac subprovince. After development of the MMGS magmatism (21670-2680 Ma), the Pontiac subprovince locally underthrust the Abitibi greenstone belt, and crustal thickening promoted partial melting of underthrust Pontiac metasediments to form the approximately 2644 +/- 13 Ma S-type granites. Later, differential uplift exposed the Lacorne block as a high-grade tectonic window in the Abitibi greenstone belt.