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On the basis of geological, geophysical, and geochronological data, the Grenville Province has been divided into three first-order longitudinal belts, the Parautochthonous Belt (PB), Allochthonous Polycyclic Belt (APB), and Allochthonous Monocyclic Belt (AMB). These are set apart by three first-order tectonic boundaries, the Grenville Front (GF), Allochthon Boundary Thrust (ABT), and Monocyclic Belt Boundary Zone (MBBZ). The belts are subdivided into terranes based on internal lithological character. The GF separates the Archcan to Proterozoic foreland northwest of the orogen from reworked equivalents to the southeast. Continuous at the scale of the orogen, its main characteristic is that of a crustal-scale contraction fault. The PB, although less clearly identified along the length of the orogen, in most places represents upgraded and tectonically reworked rocks of the adjacent foreland. The boundary between the PB and the APB to the southeast, the ABT, is most clearly delineated in the eastern half of the province. It is the locus of major crustal delamination along which high-grade, mostly middle Proterozoic, polycyclic terranes were tectonically transported northwest toward and onto the PB. The AMB comprises two separate areas underlain by the Wakeham Supergroup and what is currently known as the Grenville Supergroup, respectively; its basal contact, the MBBZ, is a d•collement zone of variable kinematic significance between older polycyclic rocks Copyright 1989 by the American Geophysical Union. Paper number 88TC03358. 0278-7407/89/88TC-03358510.00 and tectonically overlying monocyclic rocks. This first-order zonation implies a tectonic polarity to the Grenville Province, superimposed on which are second-order features evident from contrasting tectonic styles and radiometric ages. These characteristics are consistent with a diachronous or oblique collisional model for the Grenville orogen. Rivers et al.' Tectonic Divisions of the Grenville Province c• 80øW SUPE R I •' SOUTHERN --

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Granitic rocks related to 1.18 to 1.13 Ga anorthosite-mangerite-charnockitegranite plutonism stitch three terranes in the southwestern Grenville Province (Adirondack Highlands-Morin terrane, Frontenac terrane, Elzevir terrane). Because of the refractory nature of zircon (Zrn), analysis of oxygen-isotope ratios of dated igneous zircon from these rocks allows calculation of δ δ 18 O values of original magmas even if the rocks were subjected to late magmatic assimilation, postmagmatic alteration, or metamorphism. Documented variability in δ δ 18 O(Zrn) for these granitic rocks corresponds to their geographic location. Seven plutons from the central Frontenac terrane (Ontario) have a high average δ δ 18 O(Zrn) = 11.8 ± ± 1.0‰, which corresponds to δ δ 18 O magma values of 12.4-14.3‰. In contrast, twenty-seven other plutons and dikes of this suite (New York, Ontario, and Québec) average δ δ 18 O(Zrn) = 8.2 ± ± 0.6‰, with a typical igneous range of 8.6 to 10.3‰ for δ δ 18 O magma values. High δ δ 18 O values in the Frontenac terrane are some of the highest magmatic oxygen-isotope ratios recognized worldwide, but these plutons are not unusual with respect to whole-rock chemistry or radiogenic isotope compositions. Such high δ δ 18 O values can result from mixing between paragneiss (δ δ 18 O ≈ ≈ 15‰) and hydrothermally altered basalts and/or oceanic sediments (δ δ 18 O ≈ ≈ 12‰) in the source region. We propose that high-δ δ 18 O, hydrothermally altered basalts and sediments were subducted or underthrust to the base of the Frontenac terrane during closure of an ocean basin between the Frontenac terrane and the Adirondack Highlands at or prior to 1.2 Ga.