Petrology of Eclogitic Rocks from the Fairbanks District, Alaska
Published:January 01, 1975
RICHARD C. SWAINBANK, ROBERT B. FORBES, 1975. "Petrology of Eclogitic Rocks from the Fairbanks District, Alaska", Contributions to the Geology of the Bering Sea Basin and Adjacent Regions, R. B. Forbes
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Eclogitic rocks occur in a restricted area, some 13 mi north of Fairbanks, as conformable bands and lenses intercalated with amphibolite, impure marble, and pelitic schist. The structural style of the eclogite-bearing terrane is characterized by northwest-trending, isoclinal recumbent folds that have been deformed by open or overturned folding along northeast-trending axes. Crystalline schist masses south of the eclogite-bearing terrane show only the northeast-trending folds and contain mineral assemblages of the greenschist facies. Mineral assemblages from the crystalline schists, which are intimately associated with the eclogitic rocks, are of the lower amphibolite facies.
No basic igneous analogs were found for the eclogitic rocks, and their bulk compositions are very different from those reported for other eclogites. On the ACF plot, the calcite-rich variants appear to have been derived from marls, and the calcite-free varieties are compositionally similar to subgraywackes rather than mafic igneous rocks.
The eclogitic rocks and associated amphibolites are characterized by the following essential mineral assemblages: (a) garnet-clinopyroxene (± calcite, quartz, sphene); (b) garnet-clinopyroxene-amphibole (± calcite, quartz, sphene); and (c) garnet-amphibole (± calcite, quartz, plagioclase, epidote, rutile). Mica and plagioclase feldspar also occur in some variants.
The garnets in these eclogites are compositionally similar to those from “group C” eclogites as defined by Coleman and others (1965), and many plot within the field defined by the above authors for garnets from eclogites within blueschist terrane.
The eclogitic clinopyroxenes are true omphacites, averaging 27 percent of the jadeite component. When shown on White’s (1964) diagram, these clinopyroxenes also plot in a field defined by other clinopyroxenes from group C eclogites. The group C affinity of these eclogitic rocks is also reinforced by the garnet-pyroxene tie-line relations in the MgO-CaO-FeO system, and even more conclusively by KD data on garnet-clinopyroxene pairs.
Based on experimental data from various authors, and thermodynamic considerations, these eclogitic mineral assemblages were probably crystallized at temperatures of 540° to 590°C at 5.5 to 7.5 kb.
K40/Ar40 mica and hornblende dates indicate that earlier deformation of the eclogite-bearing terrane occurred in Paleozoic time, while the later event was of Cretaceous age.
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Contributions to the Geology of the Bering Sea Basin and Adjacent Regions
- absolute age
- amphibole group
- chain silicates
- chemical composition
- East-Central Alaska
- Fairbanks Alaska
- metamorphic rocks
- mineral composition
- phase equilibria
- structural analysis
- United States