Comparative geothermometry based on major-element and oxygen isotope distributions in Precambrian metamorphic rocks from southwestern Montana

Journal Article · Thu Nov 01 00:00:00 EST 1979 · Am. Mineral.; (United States)

OSTI ID:5704326

Dahl, P S

Mineral assemblages in the Kelly and Carter Creek areas of the Ruby Range, southwestern Montana, indicate orthopyroxene-zone and sillimanite-K felspar-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/sup 0/C. For the Carter Creek area, garnet-clinopyroxene, garnet-hornblende, garnet-biotite, and K fledspar-plagioclase methods plus assemblage data yield 675 +- 45/sup 0/c. Coexisting quartz and magnetite in metamorphic iron-formations from the two areas yield anomalously low temperature estimates of 530 +- 15/sup 0/C and 475 +- 25/sup 0/C, respectively, based on oxygen isotopic 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/sup 0/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/sup 0/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.

• Research Organization: Kent State Univ., OH
• OSTI ID: 5704326
• Journal Information: Am. Mineral.; (United States), Vol. 64:11-12
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
54 ENVIRONMENTAL SCIENCES
GEOTHERMOMETRY
ISOTOPE APPLICATIONS
METAMORPHIC ROCKS
MONTANA
OXYGEN ISOTOPES
ISOTOPE RATIO
EXPERIMENTAL DATA
ISOLATED VALUES
DATA
DATA FORMS
INFORMATION
NORTH AMERICA
NUMERICAL DATA
ROCKS
ROCKY MOUNTAIN REGION
USA
580203* - Geophysics- Geophysical Survey Methods- (1980-1989)
510101 - Environment
Terrestrial- Basic Studies- Radiometric Techniques- (-1989)