The escoumins supracrustal belt: a record of Mesoproterozoic (Pinwarian) evolution of the Laurentian margin and Grenvillian magmatism, central Grenville Province, Quebec, Canada
2019
The Mesoproterozoic, Pinwarian-age (1.52 – 1.46 Ga) Escoumins supracrustal belt
(southern central Grenville Province, Quebec) consists of a volcano-sedimentary
sequence (Saint-Simeon Group) and its structural basement (Tadoussac Intrusive Suite),
both deformed and metamorphosed to the amphibolite-facies during the Grenvillian
orogeny (1.09 – 0.98 Ga). It represents preserved parts of an island arc built on crustal
slivers rifted away from the southeastern Laurentian margin during a major Geon 15
extensional event. Change in arc dynamics led to the closure of the previously opened
seaway and back-arc basins and to the final accretion of the island arc at about 1.39 Ga,
as marked by the emplacement of a major 1.43 – 1.37 Ga arc-related plutonic belt. The
Saint-Simeon Group is an oceanic island arc, rifted-arc and back-arc volcano-sedimentary
succession that despite deformation and metamorphism, preserves well original
characteristics of the rocks. Its middle member (Moulin-a-Baude Formation) consists of
highly evolved alkaline felsic to intermediate volcanic rocks and ferrobasalt together with
island arc tholeiite, back-arc basalt and arc basaltic andesite interpreted to reflect the
rifting of the arc. Some rocks were extensively altered by synvolcanic hydrothermal
fluids and show mineral assemblages typical of metamorphosed VMS-type deposits. The
supracrustal belt represents higher crustal levels of the orogenic infrastructure in the
Grenville and was intruded by diverse types of syn-to late-orogenic plutons: (1) ca. 1.08
Ga crust-derived magnesian felsic magmatism at the onset of the continental collision, (2)
ca. 1.06 – 1.04 Ga ferroan and Ba – Sr enriched intermediate to felsic plutons fractionated
from a mafic magma derived from partial melting of a Geon 14 subduction-modified
subcontinental lithospheric mantle coeval with amphibolite-facies metamorphism in the supracrustal rocks, and (3) ca. 0.99 – 0.98 Ga crust-derived peraluminous two-mica
leucogranite bodies and pegmatite coeval with titanite growth and deformation. Despite
the deformation and the metamorphic overprint, primary features are still preserved
providing key information about the evolution of the southeastern Laurentian margin
before the final continental collision. This study represents one of the first complete
investigations of a polydeformed and metamorphosed volcano-sedimentary sequence and
of its hydrothermal alteration assemblages within the Grenville Province.
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