Abstract

U–Pb analyses of zircon and titanite were carried out on eight rocks from the Belmont Lake Metavolcanic Complex, a volcano-sedimentary sequence in the Central Metasedimentary Belt of the Grenville Province of southeastern Ontario. The ages of concordant supracrustal rocks within the complex do not accord with stratigraphic position.The youngest volcanic age is graphic from a rhyolite near the base of the sequence. The oldest age, graphic, is from a dacite in the middle. This is overlain by a rhyolite graphic in age. A rhyolite at the top of the sequence appears to contain zircon inherited from a source about 1870 Ma old. The complex is therefore interpreted as comprising a lithotectonic sequence composed of structurally interleaved segments of contrasting age. Tectonic emplacement of these segments was most likely along previously unrecognized thrust faults.The supracrustal rocks were subjected to at least two major deformational events as well as a late metamorphism. The earliest event was probably associated with thrusting. The age of the youngest volcanic unit, graphic, is an upper age estimate for this event. A lower estimate is probably given by the age of the Cordova Gabbro, 1242 ± 3 Ma. The later event, including peak regional metamorphism, should be younger than graphic, the age of a sheared, recrystallized felsic sill intruded into the supracrustal rocks, and older than graphic, the age of the undeformed and unmetamorphosed Belmont Granite.Titanite fractions in the Belmont Granite and a volcanic andesite both give an age of 1071 ± 5 Ma. The age from titanite in the Belmont Granite may be due to thermal resetting during slow cooling. Titanite in the andesite is secondary and may have grown as a result of late metamorphic reactions.The youngest age measured, 1039 ± 5 Ma, is from a concordant analysis of a single zircon grain found within the oldest rhyolite. This may be an example of zircon growth from low-temperature, late-metamorphic fluids.

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