Age, compositional, and isotopic evidence for crustal recycling in a Late Archean arc, Beartooth Mountains
Late Archean rocks of the eastern Beartooth Mountains range in composition from basaltic andesite to granite and were emplaced 2.73-2.80 Ga ago in a middle to early Archean terrane as indicated by U-Pb zircon studies. Although trace element abundances are extremely variable for this group of rocks, their initial Pb, Sr, and Nd isotopic compositions are remarkably homogenous. A composite Rb-Sr isochron (>30 samples) yield an age of 2.79/plus minus/0.04 Ga with an initial ratio of 0.7022/plus minus/2 while /epsilon/Nd 2.78 Ga ago ranges from -1.5 to -3.1 (av. -2.2). Whole-rock Pb data for these rocks scatter about a 2.75 Ga isochron and feldspar Pb data suggest initial 206/204 = 13.88, 207/204 = 14.96, and 208/204 = 34.3. These values lie well above values for average crustal leads 2.78 Ga ago as modeled by Stacey and Kramer (1975) and would require development in a reservior with /mu/= 12 from 3.7-2.8 Ga (/mu/= 7.2, 4.5-3.7 Ga). The marked differences between these values and those of the late Archean mantle require that an early to middle Archean crust played a role in the genesis of these rocks. The compositional variety and isotopic homogeneity may have developed as the result of crust-mantle mixing similar to that observed in modern volcanic-plutonic arcs along continental margins where crustal materials can be subducted, and fluids derived from these materials added to the overlying mantle wedge and lower crust. During this period, contaminated mantle may have been generated on a regional scale as evidenced by the isotopic systematics of young mafic volcanics from the northwestern U.S. (e.g. Snake River Plain, Yellowstone, Columbia River).
- Research Organization:
- Geological Survey, Menlo Park, CA (USA)
- OSTI ID:
- 5811466
- 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, 28-31 Oct 1985
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ANDESITES
ISOTOPE DATING
PETROGENESIS
QUANTITATIVE CHEMICAL ANALYSIS
GRANITES
MONTANA
CHEMICAL COMPOSITION
EARTH MANTLE
FELDSPARS
GEOCHEMISTRY
GEOLOGIC HISTORY
LEAD ISOTOPES
MINERALOGY
NEODYMIUM ISOTOPES
PRECAMBRIAN ERA
RUBIDIUM ISOTOPES
STRONTIUM ISOTOPES
TRACE AMOUNTS
AGE ESTIMATION
ALKALI METAL ISOTOPES
ALKALINE EARTH ISOTOPES
CHEMICAL ANALYSIS
CHEMISTRY
FEDERAL REGION VIII
GEOLOGIC AGES
IGNEOUS ROCKS
ISOTOPES
MINERALS
NORTH AMERICA
PLUTONIC ROCKS
ROCKS
USA
VOLCANIC ROCKS
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580400 - Geochemistry- (-1989)