Oxygen isotope studies of granite and migmatite, Grenville province of Ontario, Canada

O18/O16 ratios have been obtained for rocks and minerals along a 200 mile traverse from near the Grenville front north of Lake Nipissing to the Paleozoic contact to the east of Georgian Bay. Two types of oxygen isotopic relationships correlated with metamorphic grade and style of plutonic emplacement are observed: (A) from the Grenville fron to the Haliburton Highlands, upper amphibolite to granulite facies paragneisses, migmatites and concordantly emplaced granites all show relatively low and uniform δO18-values, e.g. δ (whole rock) range from 5·0 to 8·9 per mil (SMOW) and 80 per cent of the 44 samples analyzed lie between 6·5 and 8·4. We attribute this to extensive syn-anatectic oxygen isotope homogenization and exchange with a deep-seated mafic or ultramafic reservoir through a water-rich pore fluid. (B) To the southeast of the Harvey-Cardiff Arch, in the Chandos Lake-Silent Lake area, where metasediments of lower metamorphic grade are abundant and granites intrude at higher crustal level are common, we observed higher O18/O16 ratios in the granites (δ = 9−14) and paragneisses (δ = 12−17). The granites apparently have undergone various degrees of oxygen isotope exchange with the intruded metasediments. It thus appears that the granite-paragneiss assemblege as a whole has not been extensively open to some extraneous oxygen reservoir as in case (A).