BIVERGENT SUBDUCTION DURING THE GRENVILLIAN OROGENY? GEOCHEMICAL AND ISOTOPIC EVIDENCE FROM THE CENTRAL GRENVILLE PROVINCE IN QUEBEC

Subduction relationships during the Grenvillian Orogeny are controversial, in part because widespread ~1.5-1.3 Ga calc-alkaline continental-arc rocks (now gneisses) in the SE Grenville Province imply an upper plate setting for Laurentia prior to the orogeny, whereas deep-crustal seismic data illustrate SE-directed subduction of Laurentian crust under the northern Grenville Orogen.

Data for three late-Grenvillian igneous suites, two mafic and one ultrapotassic, from an area near the Manicouagan Reservoir in central Quebec provide new information relevant to this issue. Geochemical and isotopic fingerprinting of their mantle sources, combined with published data, indicate changes over a period of ~20 M.y. from enriched Proterozoic sub-continental lithospheric mantle (SCLM), to asthenosphere, to enriched Archean (Superior) SCLM. We interpret these changes to imply: (i) NW-directed subduction under the SE Laurentian margin during the early-Ottawan phase of the Grenvillian Orogeny; (ii) mid- to late-Ottawan delamination of overthickened Proterozoic SCLM and its replacement by rising asthenosphere (resulting in mafic magmatism); followed by (iii) SE-directed subduction of Laurentia, with the Superior Province at its leading edge (resulting in ultrapotassic magmatism), during the Rigolet phase of the Grenvillian Orogeny. This preliminary orogenic model of sequential bivergent subduction separated by an episode of delamination not only reconciles the geochemical and seismic interpretations noted above, but is also compatible with the empirical division of the Grenvillian Orogeny into Ottawan and Rigolet phases, which can now be related to NW- and SE-directed subduction, respectively. Moreover, it has clear parallels with the Himalaya-Tibet Orogen, where continental collision initiated with NW-directed subduction of the Indian plate under the Eurasian plate, but following delamination subsequently evolved to a bivergent subduction setting involving SE-directed subduction of the Eurasian margin.