High-pressure metamorphism in mafic schist from northern Vermont
- Creators
- Laird, Jo
- Albee, Arden L.
Abstract
The only confirmed occurrence of glaucophane and omphacite in the Appalachian Mountains of New England is in mafic schist from Tillotson Peak (44°48', 72°33'), north-central Vermont. Mineral assemblages observed, all with epidote + sphene ± magnetite ± phengitic muscovite ± carbonate (calcite or dolomite) ± apatite ± sulfide, are: (1) actinolite + glaucophane + chlorite + garnet + quartz, (2) actinolite + glaucophane + chlorite + albite + garnet, (3) actinolite + glaucophane + omphacite + garnet, (4) glaucophane + chlorite + garnet+ quartz, (5) glaucophane + omphacite + quartz, (6) albite + glaucophane + chlorite + quartz, and (7) albite + paragonite + chlorite + quartz. Electron microprobe investigations show that plagioclase, chlorite, sphene, white mica, carbonate, and magnetite grains are relatively homogeneous. However, amphibole, pyroxene, epidote, and garnet grains are zoned, indicating incomplete equilibration and changing physical conditions during metamorphism. Glaucophane in assemblage (6) has the general formula (rim composition): □ (Na_(1.9^(M4)Ca_(0.1)) (Mg_(1.8Fe_(1.2)^(2+)Al_(1.0)^(VI)Fe_(0.3)^(3+)) (Si_(7.9)Al_(0.1)^(IV)) O_(22) (OH)_(2•) Glaucophane in assemblages (1) to (5) is somewhat poorer in the glaucophane endmember and richer in actinolite. Ca-rich amphibole grains are zoned extensively with barroisite cores and actinolite rims. Omphacite grains in assemblages (3) and (5) have the general formula (rim composition) (Na_(0.5)Ca_(0.5)) (Mg_(0.4)Fe_(0.1)^(2+)Al_(0.4)^(VI)Fe_(0.1)^(3+)) Si_2O_6. In spite of the significant zoning, the partitioning of elements among the minerals at their grain margins suggests that all seven assemblages above may coexist at the same physical conditions of metamorphism in mafic rocks with a relatively small range in bulk composition. Correlation of mineral compositions and assemblages with mineral stability data suggests high-pressure facies series metamorphism at about 9 ± 2 kb and 450 ± 100°C with P_(h2O) ≈ P_(Total). It is proposed that the metamorphism occurred during the Ordovician.
Additional Information
© 1981 American Journal of Science. Contribution 3042, Publications of the Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125. We thank A. H. Chidester for providing several samples, including one of those studied in detail (A-Bl-100), and W. M. Cady for sending copies of field notes, field maps, and thin section descriptions made by Cady, Albee, and Chidester in their study of Tillotson Peak. Data collection for this project was aided by the fact that A. A. Chodos keeps the Caltech electron microprobe in top running condition. This paper has certainly benefitted from the reviews of P. Robinson, A. B. Thompson, and W. E. Trzcienski, Jr. We gratefully acknowledge financial support from National Science Foundation Grants DES 69-0064 and EAR 75-03416 to A.L.A. and Geological Society of America Penrose Bequest Research Grants 1554-71 and 1670-72 to J.L. The senior author also wishes to thank the Institut für Kristallographie und Petrographie, Eidgenossische Technische Hochschule Zürich, for support during the writing of later drafts of this manuscript.Additional details
- Eprint ID
- 44911
- DOI
- 10.2475/ajs.281.2.97
- Resolver ID
- CaltechAUTHORS:20140414-093459837
- DES 69-0064
- NSF
- EAR 75-03416
- NSF
- 1554-71
- Geological Society of America Penrose Bequest Research Grants
- 1670-72
- Geological Society of America Penrose Bequest Research Grants
- Created
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2014-04-14Created from EPrint's datestamp field
- Updated
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2021-11-10Created from EPrint's last_modified field
- Caltech groups
- Division of Geological and Planetary Sciences
- Other Numbering System Name
- Caltech Division of Geological and Planetary Sciences
- Other Numbering System Identifier
- 3042