THE RIFTING AND REASSEMBLY OF QUEBECIA: A TALE OF ANDEAN STYLE 1.50–1.35 GA ARC DYNAMICS IN THE SOUTHEASTERN LAURENTIAN MARGIN (Invited Presentation)

Despite pervasive overprint during the Grenvillian orogeny (1.09 – 0.98 Ga), the Grenville Province in Canada preserves a varied record of the preceding evolution of the SE Laurentian margin (present day coordinates). Most of the Province consists of 1.50 – 1.35 Ga Mesoproterozoic rocks (with the 1.52–1.46 Ga age range also known as Pinwarian) that formed in two continental arc systems, separated by a composite arc belt (Quebecia) in the central Grenville. Remnants of the continental arc system are exposed in the Central Gneiss Belt (western Grenville) and in the Pinware and Wakeham terranes (Eastern Grenville).

The Quebecia terrane preserves record of rifting and re-amalgamation of parts of the Laurentian margin. Key constituents are: (a) ca. 1.5 Ga metasedimentary sequences indicative of earlier rifting of the margin; (b) remnants of 1.50 – 1.45 Ga peri-Laurentian island arcs and back-arcs to the south, built on rifted crustal slivers; (c) 1.45 – 1.42 Ga mafic dyke swarms to the northwest, linked to an asthenospheric window or back-arc opening; (d) a 1.43 –1.37 Ga felsic plutonic belt marking the time of accretion of the island arcs to Laurentia; and (e) 1.38 – 1.35 Ga AMCG suites suggesting slab-breakoff and mafic underplating of the lower crust during arc accretion. Evidence of Pinwarian-age metamorphism was only recognized in the northern edge of Quebecia.

The stratigraphy of the peri-Laurentian island arcs is well-preserved and provides an exceptional record of events because Grenvillian-age metamorphism only reached amphibolite facies. A key section shows an evolution from a 1.49 Ga mature transitional to calc-alkaline arc-related volcanism to a younger ca. 1.46 Ga higher-T tholeiitic and A-type bimodal volcanism typical of rifted-arcs and back-arcs. This sequence is deposited in a sub-aerial to shallow-water environment with evidence of exhalative hydrothermal mineralization of SEDEX-type at the edge of the basin and VMS-type in the volcanic sequence. Volcanism was followed by deposition of a thick sedimentary sequence with mafic sills, consistent with subsidence of the back-arc basin and migration of the magmatism. Differences between Quebecia and continental arcs of the same age on both sides attest to lateral variations in subduction dynamics under Laurentia comparable to the modern-day Andean system.