An oxygen isotope study of the Loon Lake pluton and the Apsley gneiss, Ontario

The Loon Lake pluton in the Grenville province of Southeastern Ontario consists of a quartz monzonite rim surrounding a monzonite core containing inclusions of gneiss, gabbro and diorite. The pluton was emplaced in amphibolite facies Apsley gneiss, amphibolite and marble. Abnormally high δ18O values are observed in all igneous rock types: quartz monzonite (8.9–13.9‰), monzonite (8.9–9.7‰), diorite-gabbro (8.0–9.3‰). High 18O contents are attributable to interaction between pluton and country rocks, through either isotopic exchange or direct mixing of mobilizate anatectically produced in the contact aureole of the pluton.The Apsley gneiss displays a δ18O range from 8.3 to 16.9‰. There is no difference in δ18O distribution between rocks inside and outside the contact aureole, although intermineral isotopic fractionations in the aureole are smaller than those outside. A chemical composition discriminant function that distinguishes rocks of igneous origin (DF>0) from sedimentary (DF<0) is inversely correlated with δ18O of the gneisses, indicating that low δ18O values are inherited from a silicic volcanic protolith. Al2O3/Na2O, an index of maturity of sediments, increases with δ18O for the DF<0 group but is almost constant for the DF>0 group over a δ18O range from 8.3 to 13.4‰. The DF<0 group is inferred to have formed from a series of clastic sediments of varying degree of weathering or maturity; the DF>0 group formed either from tuffs partially altered to zeolites, or from hydrated volcanic flows or ignimbrites.