Ages and Sr isotope systematics of Archean basement rocks from the south-central Beartooth Mountains

Journal Article · Sat Jul 01 04:00:00 EDT 1989 · Mountain Geologist; (USA)

OSTI ID:6831171

Montgomery, C W ^[1]; Gray, B A

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1. Northern Illinois Univ., DeKalb (USA)

The Precambrian basement rocks of the Beartooth Mountains north of Cooke City, Montana are dominated by granitic gneisses. Consistent intrusive relationships indicate that the pink gneiss, of granitic-to-adamellitic composition, is the younger. It yields a well-defined Rb-Sr whole-rock ages of 2,745 plus/minus 37 Ma with primitive initial {sup 87}Sr/{sup 86}Sr = .7005 plus/minus .0006 (all errors 2 standard deviations). The low initial Sr ratio is consistent with either a mantle source or a granulitic lower-crustal source. The older, tonalitic-to-granitic grey gneiss cannot be dated precisely by Rb-Sr methods, but does contain systematically more radiogenic Sr for given Rb/Sr ratio than does the pink gneiss. These data, combined with previously published zircon data suggest two alternative interpretations for the age of the grey gneiss: either it is up to several hundred million years older than the pink gneiss; or the two are nearly contemporaneous, but the grey gneiss has inherited some radiogenic Sr from assimilated pre-existing crustal material. No evidence has been found for earlier Archean rocks in this region, in contrast to portions of the eastern Beartooth Plateau block, where 3.4-Ga granulites and possible 3.9-Ga detrital zircons have been identified.

• OSTI ID: 6831171
• Journal Information: Mountain Geologist; (USA), Journal Name: Mountain Geologist; (USA) Vol. 26:3; ISSN MOGEA; ISSN 0027-254X
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
AGE ESTIMATION
ALKALI METAL ISOTOPES
ALKALINE EARTH ISOTOPES
BASEMENT ROCK
BETA DECAY RADIOISOTOPES
ELECTRON CAPTURE RADIOISOTOPES
EVEN-EVEN NUCLEI
EVEN-ODD NUCLEI
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GNEISSES
HOURS LIVING RADIOISOTOPES
INTERMEDIATE MASS NUCLEI
ISOMERIC TRANSITION ISOTOPES
ISOTOPE DATING
ISOTOPE RATIO
ISOTOPES
METAMORPHIC ROCKS
MINERALS
MONTANA
MOUNTAINS
NORTH AMERICA
NUCLEI
OXYGEN COMPOUNDS
PRECAMBRIAN ERA
RADIOISOTOPES
ROCKS
RUBIDIUM ISOTOPES
SILICATE MINERALS
SILICATES
SILICON COMPOUNDS
STABLE ISOTOPES
STRONTIUM 86
STRONTIUM 87
STRONTIUM ISOTOPES
TRANSITION ELEMENT COMPOUNDS
USA
ZIRCON
ZIRCONIUM COMPOUNDS
ZIRCONIUM SILICATES