Petrography and trace-element geochemistry of the Dillon Quartzofeldspathic Gneiss, Ruby Range, southwestern Montana
Conference
·
· Geological Society of America, Abstracts with Programs; (United States)
OSTI ID:5290756
- Kent State Univ., OH (United States). Dept. of Geology
- Ohio State Univ., Columbus, OH (United States). Nuclear Reactor Lab.
The Ruby Range is a Mid- to Late-Archean terrane dominated by quartzofeldspathic gneisses overlain by metasedimentary rocks that underwent upper-amphibolite- to lower-granulite- grade metamorphism during the Beartooth Orogeny (ca. 2,750 Ma). The Dillon Quartzofeldspathic Gneiss (DQG) is a thick sequence of conformable microcline-rich gneisses that occupy intermediate stratigraphic levels within the exposed section. Streckheisen (1976) diagrams reveal that 60 gneisses sampled from 3 transects across structural grain are granitic in composition. Minor mafic constituents among the gneisses include garnet, biotite, garnet+hornblende, garnet+biotite, and hornblende+biotite. Whole-rock INAA data (in ppm, [+-]1[sigma]) for 20 rocks provided. Most chondrite-normalized REE plots exhibit strong enrichment in LREE and slight enrichment in HREE; most also exhibit a significant negative europium anomaly. Mean chondrite-normalized La/Yb, Ce/Yb, and La/Lu ratios are 14.9, 14.2, and 10.0, respectively. These patterns: (1) resemble those observed in similar gneisses in the adjacent Blacktail Range (Clark, 1987) and (2) are typical of Late-Archean K-rich granitic rocks from other localities, whose protolith(s) (according to Taylor 1985) were probably derived from moderate partial melting of continental crust, with plagioclase as a residual phase.
- OSTI ID:
- 5290756
- Report Number(s):
- CONF-9305259--
- Conference Information:
- Journal Name: Geological Society of America, Abstracts with Programs; (United States) Journal Volume: 25:5
- Country of Publication:
- United States
- Language:
- English
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