39AR-40AR DATING OF GRANITIC AND METAMORPHIC ROCKS OF THE THETFORD-MINES OPHIOLITE: IMPLICATIONS FOR THE THERMAL AND EMPLACEMENT HISTORY OF THE SOUTHERN QUÉBEC OPHIOLITE BELT DURING THE TACONIAN OROGENY

New laser step-heating 40Ar/39Ar dating has been performed on oceanic plagiogranites, peridotite-hosted granites (PHG) and infraophiolitic metamorphic rocks of the Thetford-Mines ophiolite in order to better constrain the thermo-tectonic evolution and the exhumation of both the ophiolite and the Laurentian margin during the Taconian orogeny. Oceanic plagiogranites and PHG are interpreted as the products of fractionation of basaltic magma and of melting of the Laurentian margin, and yielded U-Pb zircon ages of 480-478 and 470-469 Ma, respectively (Whitehead et al., 2000). Existing 40Ar/39Ar ages for the metamorphic sole are a hornblende isochron age of 477 ± 5 Ma for amphibolite facies rocks (Whitehead et al., 1995) and muscovite plateau ages of 469 ± 1 and 462 ± 2.4 Ma for the associated micaschists (Castonguay et al., 2001). The same sites as those used for previous U-Pb and 39Ar-40Ar analyses have been sampled. Amphiboles from plagiogranites yield 39Ar-40Ar plateau ages of 470-467 Ma, whereas muscovites from PHG yield ages of 466-465 Ma for an undeformed facies, and ca. 460 Ma for both a granitic mylonite and crosscutting pegmatite facies. The metamorphic sole yields 39Ar-40Ar plateau ages of 471-470 Ma in amphiboles and high-temperature ages of ca. 466 Ma in muscovites, with evidence for recrystallization as young as 435 Ma. Muscovites from Laurentian metasediments exposed in Acadian-related inliers and from micaschists clasts in ophiolitic debris flows yield pseudo-plateau and high-temperature ages of 462-460 Ma. The U-Pb and 39Ar-40Ar age data suggest that the entire obduction history of the peri-Laurentian oceanic crust lasted ~20 m.y. Approximately 10 m.y. after its formation, the obducting ophiolite was still at a temperature of ca. 550-500oC, indicating that it likely supplied, by downward-directed conductive heat transfer and shear heating, the thermal energy necessary for the formation of both the PHG and the infraophiolitic metamorphic rocks. Muscovite ages of PHG record cooling below ca. 450oC at ca. 465 Ma, but 39Ar-40Ar ages from a mylonitized PHG facies and Laurentian metasedimentary clasts indicate that, although the emplacement of the oceanic crust over Laurentia was still going on around 460 Ma, it was likely vanishing due to foreland-directed downward thrust propagation and related exhumation.