Abstract

The Brabant Lake area mostly comprises Early Proterozoic clastic metasediments of the MacLean Lake belt, eastern La Ronge Domain, Reindeer Zone. The belt underwent upper amphibolite facies metamorphism, with sillimanite occurring in both psammitic and pelitic gneisses. Biotite + garnet + sillimanite + cordierite + quartz + K-feldspar + plagioclase parageneses suggest peak metamorphic pressure–temperature (PT) conditions of 650–750 °C at 4–6 kbar (1 bar = 100 kPa) and graphic. Both psammitic and pelitic gneisses underwent anatexis, which may have caused a reduction in water pressure. PT conditions were also calculated by applying thermometry on the basis of garnet–biotite Fe–Mg exchange and barometry on the basis of garnet + plagioclase + sillimanite + quartz assemblages. The most precise garnet–biotite thermometer yielded temperatures of 608–758 °C (mean 690 °C). A more recent version, which incorporates the effects of minor components and nonideal mixing in both garnet and biotite, yielded a wider range of temperatures (565–826 °C), suggesting that imprecision in garnet–biotite thermometry may be due to disparate closing temperatures or disequilibrium. Application of several recent calibrations of the grossular + anorthite + sillimanite + quartz barometer produced a range of pressure estimates: three of these calibrations gave results compatible with documented mineral parageneses and experimental data, yielding 6.1 ± 0.5,6.1 ± 0.8, and 6.8 ± 0.9 kbar, respectively. Estimated mean pressures from another calibration are about 1–2 kbar higher. Peak metamorphic conditions at Brabant Lake are comparable with those experienced by metasediments of the Kisseynew Domain in Saskatchewan and Manitoba and other portions of the Reindeer Zone. Of 19 analyzed garnets from metapelites, 4 showed distinct chemical zonation. Garnet core growth appears coeval with syn-D2 peak metamorphic conditions; such cores yield the highest temperature estimates. Lower temperature estimates from garnet rims (range 610–660 °C, mean 630 °C) reflect retrogression under postpeak metamorphic conditions.

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