The Queensborough mafic-ultramafic complex occurs as a fault-bounded block, up to 10 km wide and having an area > 220 km2. It lies in the Grimsthorpe Domain of the Bancroft-Elzevir-Mazinaw-Sharbot Lake Terrane in the Central Metasedimentary Belt of the Grenville Province. It has been suggested, without adequate supporting data, that the complex may represent oceanic crust, a fragment of an ophiolite, or even a metavolcanic sequence made up of basaltic and komatiitic flows. The geological and tectonic significance of the complex is assessed using field relationships, petrography and geochemistry. Structurally the lowest part of the complex comprises a series of ultramafic rocks characterized by metre-scale compositional layering, represented by several different metamorphic assemblages of talc, chlorite, carbonate, anthophyllite, and actinolite-tremolite. These assemblages indicate that the original rocks were cumulate peridotites and pyroxenites. The ultramafic rocks are overlain structurally by a series of mafic rocks, predominantly massive to highly sheared gabbros. The gabbros are penetrated by a series of mafic dykes and include a few small enclaves of pillowed mafic volcanics. Major- and trace-element chemistry shows that the mafic rocks represent a fractionally crystallized sequence of tholeiitic gabbros, lavas, and mafic dykes and that the ultramafic cumulates are co-genetic. The regional geological setting, and the trace-element signatures of the mafic rocks suggest that they were formed in a back-arc basin. Comparison of the Queensborough Complex with Proterozoic and Phanerozoic igneous complexes suggests that it represents a partially preserved crustal section of a Mesoproterozoic ophiolite. In addition, the rocks of the Queensborough Complex are petrographically and geochemically similar to those of the Vavilov Basin which occurs in the deepest part of the Tyrrhenian Sea. By analogy with this Neogene back-arc basin we suggest that the Central Metasedimentary Belt, which contains the Queensborough Complex, formed by attenuation of the continental crust. The complex is preserved along a suture and is interpreted as having formed during the collision of a continental island arc with the continental margin of Laurentia. The characteristics and preservation of the Queensborough mafic-ultramafic complex are very similar to those of many Phanerozoic ophiolites and suggest that Mesoproterozoic thermotectonic processes are also similar to those presently operating.
