Seven accretionary sutures, formed between 1.16 and 1.03 Ga, have been iden
tified by different authors in the Ontario-Quebec-Adirondack (OQA) segment
of the Mesoproterozoic Grenville orogen in Canada. With one exception, the
inferred accretionary terrane boundaries lie within, or at the margins of t
he Central Metasedimentary Belt (CMB), located between the Central Gneiss B
elt and the Adirondack Highlands (Central Granulite Terrane). However, geol
ogical, geochronological, and petrological data suggest that the Grenville
orogen on both sides of the proposed terrane boundaries (sutures) preserves
a common 1.4-1.03 Ga tectonomagmatic history, inconsistent with its origin
as a post-1.4 Ga collage of exotic tectonic blocks. Features which straddl
e the proposed 1.16-1.03 Ga 'sutures', from the Central Gneiss Belt, via th
e Adirondack Highlands, to the Mauricie area, include: (1) Mesoproterozoic
continental crust (1.5-1.4 Ga) forming the host and/or basement to younger
magmatic and supracrustal suites. (2) A 1.35-1.3 Ga continental are, remnan
ts of which occur from the CMB boundary zone (CMBBZ) in Ontario to the Appa
lachians in the United States, built on the 1.5-1.4 Ga continental crust. (
3) Intrusions of 1.17-1.13 Ga age in the Central Gneiss Belt (mafic suite),
and the Adirondack Highlands and their Quebec extension (AMCG suite, i.e.
anorthosite massifs and related granitoids). (4) Relies of 1.18-1.14 Ga sed
imentary basins in the northwestern CMB and the Mauricie area.
We propose that an alternative model can adequately account for the observe
d geology of this part of the Grenville orogen wherein, the rocks of the OQ
A segment were part of an Andean-type margin between 1.4 and 1.2 Ga. At 1.3
5-1.3 Ga, a continental magmatic are was built upon the southeastern margin
of Laurentia represented by the 1.5-1.4 Ga Mesoproterozoic continental cru
st. The are split at 1.3 Ga forming an ensialic back are basin, relies of w
hich now occur in the northwestern part of the CMB, and the back are basin
was flanked to the southeast by an active 1.28-1.25 Ga are. Collision betwe
en the Laurentian margin and another continent (Amazonia?) occurred at 1.2
Ga, resulting in closure of the back are basin and initiation of thrusting
along the CMBBZ. Post-collisional lithospheric shortening led to convective
removal of thickened subcontinental lithosphere, upper mantle melting, and
extension of the overlying crust, resulting in widespread magmatic activit
y at 1.17-1.13 Cia, including emplacement of the AMCG massifs. Crustal exte
nsion generated sedimentary basins now represented by the St Boniface sedim
ents in the Mauricie area (1.18 to between 1.15 and 1.09 Ga), and the penec
ontemporaneous Flinton Group in the northwestern CMB. Renewed, post-collisi
onal, granulite facies shortening commenced at 1.12 Ga, manifested as nappe
s in the Central Gneiss Belt, and thrusting in the. Mauricie area. Continue
d post-collisional shortening at. 1.08-1.05 Ga was more localised, resultin
g in reactivation of thrusting in the CMBBZ, and initiation of the kinemati
cally compatible Tawachiche shear zone along the eastern border of the Queb
ec extension of the Adirondack Highlands.
The characteristics of the OQA segment of the Grenville orogen can all be a
ccounted for in the context of. (1) a 1.4-1.2 Ga, southeast facing Andean-t
ype margin to a Laurentian upper plate, associated with northwest, dipping
subduction; (2) continental collision at 1.2 Ga; and (3) subsequent, contin
ued, post-collisional shortening, without invoking accretion of exotic terr
anes between 1.4 and 1.0 Ga. (C) 2000 Elsevier Science B.V. All rights rese
rved.