The Sullivan Island Carbonatite Complex and Its Relationship to Late Alkalic Magmatism in the Central Metasedimentary Belt boundary tectonic zone, Grenville Orogen, Southeastern Ontario

English Abstract:

The little studied Sullivan Island carbonatite complex is exposed in and along the Ottawa River 90 km northwest of Ottawa, and is hosted in deformed upper amphibolite facies gneisses of the boundary zone (CMBbtz) that separates the Central Metasedimentary and Central Gneiss belts of the Grenville Orogen. The complex consists of fenitized country rock and a breccia composed of a matrix of calcitic silicocarbonatite (~20% SiO₂) rich in strontium (5955 ppm) and a variety of variably fenitized country-rock fragments. REE-bearing phases in the complex are predominantly fluorapatite and allanite, with the latter forming large crystals. Small grains (<20µ) of synchsite, bastnasite and barian-strontianite are also present. Abundant scapolite within the silicocarbonatite may indicate the presence of Cl- and F-rich fluids. Radioactive mineralization in the complex is limited, but, 2 U-Th airborne gamma-ray spectrometric highs, 1 assayed at 927 ppm U and 2717 ppm Th by scintillometer, occur immediately south of the complex along the north-trending Ross fault, which may have controlled emplacement of the complex.

An unpublished U-Pb zircon age of circa 1050 Ma, obtained in the early 1980s from the siliciocarbonate, is likely an emplacement age, as it is consistent with the presence of zircon grains in the silicocarbonatite matrix, and with a minimum age for the complex based on a cross-cutting mafic dike of the 590 Ma Grenville dike swarm. Furthermore, volumetrically significant alkaline magmatism occurred along the length of the CMBbtz in Ontario between 1055 and 1030 Ma, resulting in the emplacement of nepheline syenite and syenite intrusions and calcite-diopside±scapolite±apatite vein dikes (pseudocarbonatites of Mitchell 2005), and the development of REE-bearing metasomatized marbles characterized by pink-orange calcite. Thus, the Sullivan Island carbonatite complex is likely related to this Grenvillian alkaline magmatic event, and its location in the younger Ottawa-Bonnechere graben may be simply fortuitous.