Stable isotope systematics of medium-grade pelitic rocks of southwestern Maine: Implications of postmetamorphic exchange

Conference · Sun Jan 31 23:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:6138756

Colucci, M T ^[1]; Dyar, M D ^[2]; Gregory, R T; Holdaway, M J ^[3]; Guidotti, C V ^[4]

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1. Southern Methodist Univ., Dallas, TX (United States). Stable Isotope Lab.
2. Univ. of Oregon, Eugene, OR (United States). Dept. Geological Science
3. Southern Methodist Univ., Dallas, TX (United States). Dept. Geological Science
4. Univ. of Maine, Orono, ME (United States). Dept. Geological Science

Oxygen and hydrogen isotope ratios were determined on coexisting biotite-muscovite-quartz-plagioclase mineral separates from 61 Siluro-Devonian pelitic schists of the Rangeley, Oquossoc, and Bryant Pond quadrangles of western Maine. Contact metamorphic zones range from garnet grade (450 C) through K feldspar-sillimanite grade (650 C) at 3-4Kb. Muscovite and biotite [sup 18]O-fractionations are remarkably constant and independent of metamorphic grade, recording apparent temperatures of 430 [plus minus] 70 C (Javoy, 1977). Multi-phase oxygen isotope isotherms likewise yield low (320 [plus minus] 60 C) temperatures that bear no systematic relation with metamorphic grade. The thermal history of intruding plutons as inferred from [sup 40]Ar/[sup 39]Ar studies (De Yoreo et al., 1989) indicates a 50--100 Ma period of postmetamorphic ambient conditions at 350--400 C. The observed oxygen isotope fractionations across the entire metamorphic complex therefore resulted from prolonged residence at postmetamorphic crustal conditions, obscuring details of isotopic exchange during the earlier polymetamorphism. Apparent equilibrium deuterium fractionations between coexisting mica minerals locally vary (8 < [Delta]D[sub ms-bt] < 25) but bear no systematic relation with mineral chemistry or metamorphic grade. The measured deuterium fractionation factors are generally lower and more variable than those predicted from previous experimental studies. By analogy with shallow-level hydrothermal systems, [delta]D vs. [delta][sup 18]O relations and fluid-rock isotope exchange modeling indicate low integrated fluid-rock ratios (0.05--0.15) during post-peakmetamorphic isotope exchange.

• OSTI ID: 6138756
• Report Number(s): CONF-9303212--
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Journal Name: Geological Society of America, Abstracts with Programs; (United States) Vol. 25:1; ISSN GAAPBC; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
580000* -- Geosciences
ANORTHOSITES
BIOTITE
CHEMICAL COMPOSITION
DEVELOPED COUNTRIES
DEVONIAN PERIOD
GABBROS
GEOLOGIC AGES
GEOLOGIC DEPOSITS
GEOLOGIC FORMATIONS
GEOLOGIC HISTORY
GEOLOGY
GEOTHERMOMETRY
HYDROGEN ISOTOPES
HYDROTHERMAL ALTERATION
IGNEOUS ROCKS
ISOTOPE RATIO
ISOTOPES
ISOTOPIC EXCHANGE
MAINE
METAMORPHIC ROCKS
METAMORPHISM
MICA
MINERALS
MUSCOVITE
NORTH AMERICA
OXIDE MINERALS
OXYGEN ISOTOPES
PALEOZOIC ERA
PETROCHEMISTRY
PETROGENESIS
PETROLOGY
PLUTONIC ROCKS
QUARTZ
ROCK-FLUID INTERACTIONS
ROCKS
SCHISTS
SILICATE MINERALS
SILURIAN PERIOD
USA