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Metagabbro complexes in a portion of the Grenville Province lying northwest of Ottawa occur as sheets, cylinders, and irregular bodies within a medium-to high-grade marble-gneiss -amphibolite terrane. The largest bodies (0.5 -10 krn in greatest dimension) consist principally of felsic metagabbro, mafic metagabbro, and minor metapyroxenite. Major-element and select trace-element analyses show that different complexes contain distinctly different amounts of K and other elements resulting in different interelement trends.Microstructure and microprobe mineral analyses provide evidence that the following metamorphic changes have occurred: (i) recrystallization of Ca pyroxene, orthopyroxene, and plagioclase; (ii) reaction of Mg-rich olivine with plagioclase to produce reaction zones consisting of orthopyroxene and a hornblende-spinel intergrowth; (iii) reaction of Mg-Fe olivine with plagioclase to produce garnet and hornblende; (iv) production of anthophyllite and hornblende from orthopyroxene and plagioclase; (v) production of hornblende (locally as rims about Ca pyroxene) from Ca pyroxene and plagioclase; (vi) crystallization of biotite, possibly by reaction between orthopyroxene and K-feldspar; and (vii) crystallization of small inclusions of spinel and ilmenite in Ca pyroxene and of spinel and biotite in plagioclase.With regard to the reaction olivine + plagioclase = orthopyroxene + hornblende + spinel, the anorthite and locally the forsterite components were extracted preferentially from plagioclase and olivine; K and Ti (for hornblende) and Zn (for spinel) were evidently obtained from the surrounding minerals; and H, F, and C1 (for hornblende) were obtained from beyond the gabbro bobies. Locally the reaction occurred within large crystals of Ca pyroxene where embedded olivine and plagioclase crystals were in contact.The production of hornblende rims about Ca pyroxene evidently involved plagioclase as a reactant, but the rims formed regardless of the contacting minerals. For example, rims were locally produced where Ca-pyroxene crystals were embedded in large crystals of orthopyroxene.Application of five geothermometers to crystals of both igneous and metamorphic origin yield temperatures of about 700°C, similar to temperatures recorded for the enclosing marble and gneiss.Les complexes de mktagabbro dans une partie de la province du Grenville, au nord-ouest d'ottawa, se prCsentent sous forme de nappes, cylindres et corps irrkguliers B I'intCrieur d'un terrane composC de marbre-gneiss-amphibolite de degrC moyen a ClevC. Les corps les plus grands (la plus grande dimension : 0,s -10 km) sont formCs principalement de mktagabbro felsitique, mktagabbro mafique et en moindre abondance de mCtapyroxCnite. Les analyses des ClCments majeurs et d'C1Cments traces sklectionnCs montrent que les divers complexes sont caractCrisCs par des teneurs diffkrentes en K et autres Cltments, crCant ainsi des tendances inter-ClCments diffkrentes.La microstructure et les analyses de minkraux h la microsonde indiquent les changements mCtamorp...

The Grenville tholeiitic dikes of Late Proterozoic or early Paleozoic age cut marbles and gneisses of the Grenville Province within and adjacent to the Ottawa rift structure. Where traversed the swarm consists of about 40 large dikes (up to 100 m wide) representing a crustal extension of 1 km. The principal minerals are olivine, augite, pigeonite, plagioclase, magnetite, ilmenite, K-feldspar, and quartz. Crystals of pigeonite around olivine and complex augite–pigeonite composite grains suggest that the reactions olivine + melt → pigeonite and pigeonite + melt → augite have taken place. Conspicuous zoning and grain to grain variation in the composition of augite are consistent with fractional crystallization, but crystal – melt equilibrium during crystal growth is indicated by a restricted range in pyroxene paleotemperatures of 1180–1060 °C. Rock textures (subophitic, combined ophitic–subophitic, and equigranular) in dike centres are related to dike width and are determined principally by the influence of cooling rate on the nucleation of augite. The K2O content of the gabbro (centres of 15 dikes, 3–100 mm wide) ranges from 0.2 to 1.2% and is closely correlated with other elements, positively with Ti, Na, P, Rb, Sr, Y, Zr, and Ba, and negatively with Mg, Ca, and Cr. Relatively minor within-dike variation exists in the form of local (centimetre-scale) inhomogeneity, slight enrichment in K and Na, and depletion in Ca and Mg in the centre of one large dike and complex rhythmic variation in K, Na, Ca, and Fe across one small dike. Centre–margin comparisons in several large dikes indicate minor residence con tamination (Ca, Mg) in the margins of some dikes cutting marble. Of the various possible causes for variation in composition, those favoured at present are Fractional crystallization at depth (separation of augite and plagioclase), the partial preservation of compositional heterogeneity during intrusion, and the gravitational sinking of early formed crystals (with the upward displacement of K-enriched melt) in the central regions of large dikes.

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