Geothermometry and geobarometry applied to early proterozoic “S-type” granitoid plutons, Wopmay Orogen, Northwest Territories, Canada

Abstract

Many of the “S-type” granitoid plutons that comprise Hepburn and Wentzel Batholiths of the Early Proterozoic Wopmay Orogen contain garnet, biotite and rarely cordierite and sillimanite. The garnet, cordierite and sillimanite are interpreted to be relict crystals brought up from the depth of origin of the magmas.

Two methods of geothermobarometry were applied to ten samples from the two batholiths: the garnet-biotite Fe-Mg exchange equilibrium (Ferry and Spear 1978) and the garnet-plagioclase-sillimanite-quartz equilibrium (Ghent 1976). The intersection of the two displaced equilibrium curves on a P-T plot provides an estimate of theP-T conditions of equilibration of the minerals.

P-T estimates in eight of ten samples ranged between 800 and 1,100°C, and 5 and l0 kbars. These values are in distinct contrast to the typical 650 °C — 3 kb results obtained from the immediately adjacent metapelites, to which the same geothermobarometers were applied.

Numerous theoretical and practical problems are encountered both in the application of the methods and in the interpretation of the results. Two of the important theoretical problems are (1) how the anatexis of certain minerals in the equilibria influences the elemental ratios of other minerals that remain as solids, and (2) how far the elemental ratios of the minerals re-equilibrated below the maximalP-T conditions. The most serious practical problem was chloritization of biotite, which generated spuriously high temperatures on the garnet-biotite geobarometer.

The significant difference in results between the batholith rocks and the metamorphic rocks indicates that the minerals did not re-equilibrate to sub-magmatic conditions. Using theP-T results as a guide to the conditions of origin of the plutons, it appears that several of the plutons were generated at depths between 21 and 29 km, and rose between 8 and 18 km to the level of emplacement.

The core-to-rim zonation of the garnets produces aP-T trend that is towards lower pressure, temperature or both. Such a pattern would be expected in a pluton that rose to the less extreme conditions at higher levels in the crust.

The geothermobarometers successfully distinguish between granitoid rocks that were generated at depth and those that were formed by anatexis of country rocks near the level of emplacement.