Strontium isotope variation and whole-rock isochron studies, Grenville Province of Ontario

Abstract

Rb-Sr whole-rock isochrons were obtained for five granitic bodies and two metasedimentary horizons distributed over a distance of 100 miles in a region of typical Grenville metasediments in southeastern Ontario. Ages (in millions of years: m.y.) of plutonic rocks obtained and their geological significance are as follows: 1285 ± 41 m.y., a time of emplacement for a major pretectonic sill that was deformed on a region scale (the Blue Mountain nepheline syenite). 1103 ± 39 m.y., a maximum value for the time of intense metasomatism that accompanied the formation of the gneisses in the Burleigh paragneiss dome. 1096 ± 50 m.y., a time of emplacement for a major syenite mass that was deformed during regional deformation (the Gananoque syenite). 1035 ± 60 m.y., a minimum time for the formation of a major sill-like granitic body (the Kaladar gneissic granite). 1016 ± 39 m.y., a time of emplacement for a major posttectonic granite (the Westport pluton). Projected initial Sr87/Sr86 ratios for these rocks range from 0.704 to 0.7055, whereas measured values for gabbros and marbles are 0.703 and 0.705, respectively (with the exception of the Kaladar granite for which a less precise value of 0.711 was determined). The indicated age of 1230 ± 72 and 1056 ± 33 m.y. for the Morton sillimanite paragneiss and the metasediments adjacent to the Burleigh paragneiss dome may indicate the time of metamorphism of these rocks. The low initial strontium ratios (0.707 and 0.705) for these metasediments indicate a very short pre-Grenville history. Two granitic bodies in the Grenville province (1 and 9 miles south of the Grenville front) in the North Bay area gave somewhat inconsistent isochrons that approach the age of the Superior province to the northwest (2000–2700 m.y.). Isochron data for metasedimentary rocks in this geological setting are inconsistent. Their high initial strontium ratio (greater than 0.726 in some cases) is an indication of the pre-Grenville history of these rocks. It is therefore concluded that the rocks adjacent to the Grenville front in the areas studied are the metamorphic equivalents of the rocks to the northwest. The rocks studied in southeastern Ontario were deposited or emplaced between 1300 and 1000 m.y. ago and were not derived from an ancient sialic basement such as that found north of the Grenville front. We refer to this time of plutonism and metamorphism in this area as the ‘Grenville orogeny’ but acknowledge that a revision of ‘Grenville’ nomenclature is needed.