RECORD OF ARC DYNAMICS IN THE MESOPROTEROZOIC LAURENTIAN MARGIN, CENTRAL GRENVILLE PROVINCE (QUEBEC)

The Grenville Province is largely composed of remnants of pre-Grenvillian (~1.7 to 1.25 Ga) arc and back-arc systems developed at the SW margin of Laurentia, that experienced high-grade metamorphism and deformation during the Grenvillian orogeny (1.09 – 0.98 Ga) obscuring the original characteristics of the rocks. However, it is still possible to identify the primary characteristics and tectonic setting of many pre-Grenvillian rock associations. A good example is the Quebecia terrane, a unique 1.5 to 1.3 Ga composite arc belt in the central Grenville Province, separating crustal segments to the east and west that were parts of continental arc systems at that time. A major element of Quebecia is remnants of 1.50 to 1.45 Ga peri-Laurentian oceanic arcs to the south built on rifted crustal slivers. One of these arc remnants is represented by the Escoumins supracrustal belt; an association of marine sedimentary rocks overlain by an island arc/arc-rift and back-arc bimodal volcanic sequence with back-arc sediments on top. The supracrustal package tectonically overlies a ca. 1.5 Ga suite of tonalite, trondhjemite and granodiorite with geochemical signatures of a mature oceanic arc, crosscut by arc-rift related granite. Farther north, Quebecia contains >1.5 Ga extension-related metasedimentary sequences indicative of earlier rifting of the Laurentian margin, a major 1.4 to 1.3 Ga felsic plutonic belt that represent potential stitching plutons emplaced during the collision of the peri-cratonic island arcs with Laurentia, and the only 1.3 Ga anorthosite complexes in the Grenville, the latter implying mafic underplating at the base of the crust up to 100 My earlier. Differences between Quebecia and continental arcs elsewhere in the Grenville attest to lateral variations in subduction dynamics under Laurentia comparable to the modern-day Andean system.