PRE-GRENVILLIAN TECTONIC SIGNIFICANCE OF HIGH-GRADE GNEISSES IN THE GRENVILLE PROVINCE, ONTARIO

Late Mesoproterozoic Grenvillian deformation and metamorphism led to extensive reworking of older crust along the SE margin of Laurentia. Here we present geochemical data from two allochthonous domains of Laurentian affinity in the CGB, Ontario, that shed some light on the Mesoproterozoic geological evolution of this part of Laurentia.

The Muskoka domain is dominated by variably migmatized orthogneisses of granodioritic to dioritic compositions, with continental arc affinities. Plutonic ages range from ~1460-1430 Ma. Small bodies of granitic gneiss have transitional arc-within plate (WP) affinities, and one such body has been dated at ~1390 Ma. Small bodies of a characteristic green, pyroxene-bearing granitic to syenitic gneiss with WP affinities have been identified in several places, but have not yet been dated. Disrupted mafic enclaves, believed to represent former dikes, are common in the Muskoka domain, and have continental affinities.

The Sand Bay gneiss association (g.a.) in the Shawanaga domain comprises pink, rhyolitic gneisses, associated with pelites, calc-silicate, quartzite, and amphibolitic enclaves or layers. The rhyolites have geochemical characteristics similar to the granitic gneisses in the Muskoka domain, with transitional arc-WP affinities, but are more evolved. The amphibolites are compositionally similar to those found in the Muskoka domain. The depositional age of this supracrustal assemblage is believed to be ~1360 Ma.

We interpret the Muskoka domain to represent continental arc magmatism at the margin of Laurentia between ~1460-1430 Ma, succeeded by arc-extension (and rifting?) starting at ~1390 Ma, as suggested by the granitic and pyroxene-bearing granitic/syenitic gneisses, and the mafic enclaves. The rhyolites and associated supracrustal rocks of the Sand Bay g.a. may have been deposited in an intra-arc or back-arc basin, outboard of the then inactive Muskoka arc, inboard of a new magmatic arc formed at the distal margin of Laurentia. Evidence of arc magmatism at this time can be found in the Parry Sound domain, overlying the Sand Bay g.a., as well as in the La Bostonnais complex to the NE.