Petrology and isotopic geochemistry of the Archaean basement lithologies near Gardiner, Montana

Conference · Tue Jan 01 04:00:00 EST 1985 · Geol. Soc. Am., Abstr. Programs; (United States)

OSTI ID:6576443

Guy, R E; Sinha, A K

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-
• Conference Information: Journal Name: Geol. Soc. Am., Abstr. Programs; (United States) Journal Volume: 17
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580400* -- Geochemistry-- (-1989)
AGE ESTIMATION
BASEMENT ROCK
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
CHEMISTRY
FEDERAL REGION VIII
GEOCHEMISTRY
GEOLOGIC AGES
GEOLOGY
GNEISSES
IGNEOUS ROCKS
ISOTOPE DATING
ISOTOPE RATIO
LITHOLOGY
METAMORPHIC ROCKS
MINERALOGY
MONTANA
NORTH AMERICA
PETROGENESIS
PETROLOGY
PRECAMBRIAN ERA
QUANTITATIVE CHEMICAL ANALYSIS
RHYOLITES
ROCKS
USA
VOLCANIC ROCKS