Petrology and isotopic geochemistry of the Archaean basement lithologies near Gardiner, Montana
In an attempt to recognize potential source rocks for some of the rhyolites of the Yellowstone Rhyolite Plateau, four major exposures of Precambrian rocks have been analyzed for major and trace elements and isotopic composition. The terrain is characterized by granitic gneisses with subordinant mica schist, quartzite, amphibolite, and two-mica granite. The gneiss units from the northern (Yankee Jim Canyon) and eastern (Lamar Canyon) outcrops are characterized by k-feldspar augen in a gneissic groundmass of two-feldspar--quartz--mica--epidote. The feldspar compositions are Or/sub 95/ and An/sub 5-15/ indicating metamorphic re-equilibration. Mafic phases are iron-rich with Fe:Mg of 1.0 in epidote, 0.7 in pyroxene, and 0.5 in biotite. Sr isotopic analyses yield present day values of 0.7201-0.7519 for Lamar Canyon, 0.7157-0.7385 for Yankee Jam Canyon, and 0.7200-0.7679 for mica schist from the western and northern outcrops. Rb-Sr whole-rock data indicate a complicated isotopic history with ages ranging from 2800 to 3600 my. The 2800 my ages are consistent with ages for the Tobacco Root and Ruby Mountains to the NW (James and Hedge, 1980) and the Beartooth Range to the NE (Nunes and Tilton, 1971) while the 3600 my age may be related to the formation of the protolith. The rhyolites of the northern Yellowstone Rhyolite Plateau (Sr/sub I/=0.7100) cannot be derived from the exposed Archaean rocks based on Sr isotopic and whole-rock chemistry, and must be derived from lithologies not exposed in the area. This study shows that care must be taken when using surface lithologies to model potential sources materials for volcanic rocks in an associated terrain.
- Research Organization:
- Dept. of Geological Sciences, Blacksburg, VA (USA)
- OSTI ID:
- 6576443
- Report Number(s):
- CONF-8510489-
- Journal Information:
- Geol. Soc. Am., Abstr. Programs; (United States), Vol. 17; Conference: 98. annual meeting of the Geological Society of America, Orlando, FL, USA, 28 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
GNEISSES
CHEMICAL COMPOSITION
MINERALOGY
PETROGENESIS
PETROLOGY
MONTANA
RHYOLITES
BASEMENT ROCK
GEOCHEMISTRY
ISOTOPE DATING
ISOTOPE RATIO
LITHOLOGY
PRECAMBRIAN ERA
QUANTITATIVE CHEMICAL ANALYSIS
AGE ESTIMATION
CHEMICAL ANALYSIS
CHEMISTRY
FEDERAL REGION VIII
GEOLOGIC AGES
GEOLOGY
IGNEOUS ROCKS
METAMORPHIC ROCKS
NORTH AMERICA
ROCKS
USA
VOLCANIC ROCKS
580400* - Geochemistry- (-1989)