1.5 GA PYROCLASTIC VOLCANISM IN THE WAKEHAM GROUP, EASTERN GRENVILLE PROVINCE, QUEBEC

The depositional age of the Wakeham Group, the largest Early Mesoproterozoic supracrustal belt of eastern Laurentia, has recently been reassessed as older than 1.5 Ga (age of cross-cutting plutons) and younger than 1.6 Ga (minimum detrital zircon age of the sediments) while its metamorphism is Grenvillian and took place between 1.03 and 1.0 Ga. Discovery of intermediate to felsic lapillistones and lapilli tufs in two south-easterly extensions of the group (e.g., Musquaro lake) and further south among gneisses in the LaRomaine area provides conclusive evidence of volcanism in this mostly arenaceous detrital sequence and an opportunity to precisely date it. This finding is particularly significant as previously inferred felsic volcanic assemblages in the group have recently been proven to be hypabyssal porphyries at least 300 my younger than the host metasediments. In the pyroclastic rocks, the presence of mafic lapilli and of elongate wispy lapilli with quartz mosaic moulding angular felsic fragments and rounded lapilli suggest that felsic magmas were vesicular, coeval with mafic volcanism and able to sample some of their conduit. These rocks are intercalated with massive or laminated, pink and white granitic gneiss, some akin to metamorphosed flow-banded rhyolite, and are crosscut by quartz-feldspar porphyries. A lapillistone and a porphyritic granite display complex U-Pb zircon age populations clustering around 1.5 to 1.6 Ga, but an inferred flow-banded rhyolite displays an upper intercept U-Pb zircon age of 1503 +11/-9 Ma. SRHIMP work to refine these ages is in progress. The adjacent metasedimentary pile hosts concordant exhalite-like units (garnetite, coticule, orange garnet-hornblendite, and magnetite-rich units), and is in contact with granites that grade into large area of gneisses. Both the volcanic rocks and the granites share monazite U/Pb metamorphic ages of 1.01 Ga, providing a link with data acquired in the western and northeastern segments of the Wakeham Group. The granites contain enclaves of Ba-rich garnetite with peach-tan to pinkish garnet and amphibolite with up to 40% coarse garnets. This rock assemblage may unveil a paleotectonic setting where volcanic and subvolcanic intrusions nurtured and sustained hydrothermal cells and mineral (e.g., Cu, Pb, Zn and Au) precipitation.