40 Ar/39 Ar thermochronology of the SE Central Gneiss Belt, Grenville Province, Ontario

Abstract

As different minerals begin to retain Ar at distinct "closure temperatures", 40 Ar/39 Ar dating of suites of minerals can be used to reconstruct the cooling history of rocks in ancient orogenic belts. This information furthers our understanding of tectonic processes by placing constraints on the timing and relative rates of cooling, uplift and erosion. The Grenville Province, exposed from Georgian Bay, Ontario to southern Labrador, is generally accepted to represent the deeply eroded remains of a collisional mountain belt formed during theca 1.0-1.3 Ga Grenville Orogeny. During this event, a composite magmatic arc, represented in Ontario by the Central Metasedimentary Belt (CMB) was accreted to the pre-existing Laurentian craton. The Central Gneiss Belt ( CGB) represents the reworked Laurentian craton and is separated from the CMB by the Central Metasedimentary Belt boundary thrust zone (CMBbtz) a major crustal scale thrust belt. This study was designed to complement U/Pb thermochronology work in progress. Results are presented for 6 hornblende and 4 Keldspars from the Muskoka domain, McClintock domain, Kawagama shear zone which lie in the footwall of the CMBbtz. Five hornblende spectra yielded concordant 40 Ar/39 Ar age spectra, from which preferred (near-plateau) ages of 969 ± 9 Ma to 1001± Ma were interpreted. Excess argon is present in one sample. No correlation between age and structural position is evident from the present data set. More data are required to determine whether variations in hornblende ages across the study area reflect differences in cooling history or argon retentivity between the samples. Preferred ages for K-feldspar samples from the Muskoka domain (807 ± 5 Ma and 820 ± 5 Ma) are- 70 My younger than those from the McClintock subdomain. This may indicate that cooling rates in the McClintock domain were higher than in the Muskoka domain over the- 500-350°C interval. However, further modelling of the data are required. Approximate cooling rates for the Muskoka and McClintock domain were calculated based on the available data. Co-existing titanite and hornblende from one location in the Muskoka domain suggest more rapid cooling ( 5°C/My) followed by slower cooling ( 0. 7°C/My). Cooling rates calculated for the McClintock domain between hornblende and K-feldspar closure are on average faster than in the Muskoka domain, however they overlap within error.