Abstract

The contact aureoles of the Mistastin batholith and the Taylor Brook gabbro complex contain cordierite + spinel-bearing assemblages derived from quartz-, K-feldspar-, sillimanite-, garnet-, and biotite-bearing pelitic gneiss. Andalusite occurs locally. As a result of continuous, cordierite-forming reactions, garnet and biotite have recrystallized to relatively Fe-rich compositions compared with their counterparts outside the aureoles.Mosaics of fine-grained cordierite and spinel replace sillimanite and biotite throughout the Mistastin aureole. Similar assemblages and textures are preserved in metapelite in the outer part of the Taylor Brook aureole, where garnet adjacent to sillimanite is replaced by cordierite + spinel. The formation of spinel-free cordierite porphyroblasts by garnet-, sillimanite-, biotite-, and quartz-consuming reactions depleted metapelites in both aureoles in quartz, permitting the formation of the undersaturated assemblages. The undersaturated cordierite + spinel assemblages formed on a domainal scale (individually, a few hundred cubic millimetres in volume) and coexist metastably with saturated assemblages elsewhere in the same rock. Peak temperatures (700–750 °C) determined for both aureoles were insufficient to stabilize quartz + spinel parageneses owing to the low gahnite content of the oxide (ZnO < 2 wt.%).Metapelites in the innermost part of the Taylor Brook aureole appear to have been desilicified by the formation and fractionation of anatectic melt, yielding migmatitic rocks virtually devoid of quartz and K-feldspar. In contrast, migmatite in the inner part of the Mistastin aureole retains a quartz + two-feldspar mineralogy, so, as in the nonmigmatitic paragneiss, undersaturated assemblages are present only on a domainal scale.These aureole rocks demonstrate that undersaturated assemblages are not only characteristic of thermally- recrystallized low-grade pelites but also can occur in contact aureoles developed in granulite-facies paragneiss. Aluminous rocks can become undersaturated by subsolidus, quartz-consuming reactions yielding cordierite, or by the formation of anatectic liquids.

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