GARNET+CLINOPYROXENE-BEARING METABASITES: HIGH PRESSURE GRANULITES, UPPER AMPHIBOLITES, OR WHO CARES?

The presence of metamorphic orthopyroxene is universally taken to be indicative of granulite facies conditions. Numerous experimental studies have shown that the minimum temperatures for the formation of orthopyroxene from a hornblende-bearing protolith in metabasic rocks, or a biotite bearing protolith in arkosic rocks, is 800-850 °C under vapour-absent conditions. Thus, the presence of orthopyroxene alone appears to provide an important independent constraint on temperature estimation in granulites in which conventional thermometry (eg Fe-Mg exchange) is compromised by retrograde diffusion on cooling. In metabasic rocks, orthopyroxene is only stable at relatively low pressures (less than about 10 kbar). As pressure increases, orthopyroxene reacts with plagioclase through a compositionally mediated pressure interval to form garnet + clinopyroxene. To some authors, coexisting clinopyroxene + garnet + plagioclase is characteristic of high pressure (metabasic) granulites. If so, one might hope that its occurrence would be as indicative of high temperature granulite conditions as orthopyroxene is at lower pressures. Whereas experimental studies and most field localities seem to support this view (all experimental studies show that the minimum temperature of formation of garnet + clinopyroxene from a hornblende+plagioclase-bearing protolith under fluid-absent conditions is 800-850 °C), some field localities suggest that the assemblage can form at lower temperatures within the upper amphibolite facies. An example is the Mica Creek area (Ghent et al., 1983, CJES), in which the Grt+Cpx isograd in metabasites occurs between the Ky-Sil and Kfs+Sil isograds in interbedded metapelites, constraining the temperature of the Grt+Cpx isograd to 650-700 °C. Possible explanations include either or both of: (1) the reaction forming Grt+Cpx from Hbl+Pl is more strongly controlled by the bulk composition of the rock than is the equivalent reaction forming Opx at lower pressure, leading to a lower temperature of inception of the Grt+Cpx-producing reaction for certain bulk compositions (Fe-rich, Ca-rich, Ti-poor); (2) the vapour-absent experimental studies are somehow over-estimating the minimum temperature of formation of Grt+Cpx and perhaps Opx, possibly because of reaction overstepping.