Trond Slagstad

The Muskoka and Shawanaga domains in the Central Gneiss Belt, southwestern Grenville Province, Ontario comprise ca. 1500-1350 Ma migmatitic orthogneisses and volumetrically minor paragneisses that were deformed and metamorphosed under upper amphibolite-to granulite-facies conditions during the Grenvillian orogeny between ca. 1090 and 1050 Ma. The objectives of this study are: i) to determine the tectonic setting and petrogenesis of the protoliths to the high-grade gneisses; and ii) to investigate the petrogenesis of the migmatites and its possible relationship to deformation. The new field, geochemical, petrographic, and geochronological data is used to test and further constrain current models for the evolution of the Laurentian margin between ca. 1500 and 1000 Ma.

The geochemical and geochronological data show that volumetrically dominant calc-alkaline rocks in the Muskoka domain, shown earlier to range in age from ca. 1480 to 1450 Ma, are spatially and temporally associated with A-type granite and charnockite. The calc-alkaline rocks probably formed in a juvenile continental arc at the southeastern (present-day coordinates) Laurentian margin, and the association with A-type rocks is interpreted to reflect intra-arc extension. Granulite-facies metamorphism in the Central Gneiss Belt was coeval with or slightly post-dated emplacement of the calc-alkaline and A-type rocks. The high-grade metamorphism could reflect increased heat flow resulting from arc/back-arc extension or the waning stages of voluminous arc magmatism, or from arc-continent collision, for which there is little direct evidence.

Petrographic observations from two types of migmatite in the Muskoka domain suggest that the large proportion of leucosome (up to 40-50 vol.%) is unlikely to have formed by dehydration melting alone, suggesting that partial melting took place in response to influx of externally derived fluids. The two types of migmatite contain leucosomes that, based on whole-rock geochemical data, are interpreted to represent disequilibrium and equilibrium melts. The disequilibrium melts formed in strongly migmatitic, granodioritic host rocks that display evidence of syn-melting deformation, whereas the equilibrium melts formed in less migmatitic, dioritic host rocks that lack evidence of syn-melting deformation. Based on field observations, geochemical data, and experimental studies by others, the disequilibrium melts are interpreted to have formed as a result of rapid, deformation-enhanced melt segregation. The less migmatitic dioritic host rocks were stronger, effectively shielding the melts from deformation, and the resulting slow melt segregation rates resulted in melts with equilibrium compositions.

Leucosome in the Muskoka and Shawanaga domains, dated at ca. 1068 and 1055 Ma, respectively, is typically associated with southeast-directed, normal-sense shear bands. The geochronological data, however, suggest that the orogen was undergoing northwest-directed convergence at those times. One interpretation is that the Muskoka and Shawanaga domains represent mid-crustal zones of channel flow in which ductile material was transported from the core toward the exterior of the orogen.

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Supervisor: Rebecca A. Jamieson