High- 18O granitic plutons from the Frontenac Axis, Grenville province of ontario, Canada

Abstract

18O 16O ratios have been determined for whole rock as well as coexisting minerals from nine syn- to post-tectonic monzonite and quartz monzonite plutons in the Westport-Gananoque area covering approximately 2500 km 2. The plutons fall in two distinct isotopic groups: 1. (I) Low- 18O group ( δ = 8.0–11.2): Rideau Lake, Westport and Wolfe Lake plutons and 2. (II) High- 18O group ( δ = 12.7–16.2): Lyndhurst, Perth Road, Battersea, Gananoque, South Lake and Crow Lake plutons. The 18O 16O ratios of granitic rocks from the latter group are among the highest ever reported in the literature. The isotopic compositions of samples within each pluton bear no relationship to the sample distance from the intrusive contacts nor are they affected by the types of country rocks (marbles vs. pelitic gneisses) into which they are emplaced. Oxygen isotope fractionations among coexisting quartz, feldspar, biotite and magnetite all show normal plutonic values. Thus, all lines of evidence suggest that the monzonite and quartz monzonite plutons did not acquire their characteristic 18O 16O ratios at the present level of exposure, either by isotopic exchange with the surrounding metasedimentary country rocks or by any post-crystallization low-temperature alteration processes. The distinct 18 O/ 16 O ratios observed in the high- and low- 18O groups probably reflect the difference in the isotopic compositions of the source materials. The most plausible source for the high- 18O group plutons is the Grenville Supergroup metasedimentary rocks and that for the low- 18O group plutons the Grenville basement gneisses mixed with a small amount of Grenville Supergroup metasedimentary rocks.