Abstract
Mineral assemblages in the Kelly and Carter Creek areas of the Ruby Range, southwestern Montana, indicate orthopyroxene-zone and sillimanite–K feldspar-zone metamorphism, respectively. Because the two areas are small and contain diverse lithologies, they are ideal for a comparative study of mineral-pair geothermometers and grade indicators. For the Kelly area, various major-element methods (clinopyroxene–orthopyroxene, garnet–orthopyroxene, garnet–hornblende, garnet–biotite, and K feldspar–plagioclase) yield a consensus peak temperature estimate of 745±50°C. For the Carter Creek area, garnet–clinopyroxene, garnet–hornblende, gamet–biotite, and K feldspar–plagioclase methods plus assemblage data yield 675±45°C.
Coexisting quartz and magnetite in metamorphic iron-formations from the two areas yield anomalously low temperature estimates of 530±15° and 475±25°C, respectively, based on oxygen isotopie fractionations of 8.41±0.30 per mil and 9.92±0.20 per mil. Similarly, Mg distributions between calcite and dolomite in metadolomites from the Kelly area yield estimates of ca. 545°C. These lower temperatures are probably the result of (1) solid-state diffusion of chemical species between minerals or (2) recrystallization of quartz, magnetite, and carbonates (i.e., the non-refractory minerals) during retrograde or subsequent low-grade metamorphism. Another low estimate (545°C) is obtained from rim compositions of coexisting garnet and cordierite in one magnesian meta-pelite sample from the Kelly area. This estimate is consistent with the observation of cordierite coronas on garnet, a texture that suggests retrograde origin for the cordierite.
These data show it cannot be assumed a priori that a given geothermometer will record peak metamorphic temperature. Instead, the bimodal discordance of estimates presented here reflects the varied response of different mineral pairs to a given metamorphic and tectonic history.
