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Research ArticleArticles

Geochemistry and origin of metamorphosed mafic rocks from the Lower Paleozoic Moretown and Cram Hill Formations of North-Central Vermont: Delamination magmatism in the western New England appalachians

Raymond Coish, Jonathan Kim, Evan Twelker, Scott Zolkos and Gregory Walsh
American Journal of Science November 2015, 315 (9) 809-845; DOI: https://doi.org/10.2475/09.2015.02
Raymond Coish
* Geology Department, Middlebury College, Middlebury, Vermont 05753
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  • For correspondence: coish@middlebury.edu
Jonathan Kim
** Vermont Geological Survey, 1 National Life Drive, Main 2, Montpelier, Vermont 05620
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Evan Twelker
* Geology Department, Middlebury College, Middlebury, Vermont 05753
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Scott Zolkos
* Geology Department, Middlebury College, Middlebury, Vermont 05753
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Gregory Walsh
*** US Geological Survey, 87 State Street, Montpelier, Vermont 05602
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Abstract

The Moretown Formation, exposed as a north-trending unit that extends from northern Vermont to Connecticut, is located along a critical Appalachian litho-tectonic zone between the paleomargin of Laurentia and accreted oceanic terranes. Remnants of magmatic activity, in part preserved as metamorphosed mafic rocks in the Moretown Formation and the overlying Cram Hill Formation, are a key to further understanding the tectonic history of the northern Appalachians. Field relationships suggest that the metamorphosed mafic rocks might have formed during and after Taconian deformation, which occurred at ca. 470 to 460 Ma. Geochemistry indicates that the sampled metamorphosed mafic rocks were mostly basalts or basaltic andesites. The rocks have moderate TiO2 contents (1–2.5 wt %), are slightly enriched in the light-rare earth elements relative to the heavy rare earths, and have negative Nb-Ta anomalies in MORB-normalized extended rare earth element diagrams. Their chemistry is similar to compositions of basalts from western Pacific extensional basins near volcanic arcs. The metamorphosed mafic rocks of this study are similar in chemistry to both the pre-Silurian Mount Norris Intrusive Suite of northern Vermont, and also to some of Late Silurian rocks within the Lake Memphremagog Intrusive Suite, particularly the Comerford Intrusive Complex of Vermont and New Hampshire. Both suites may be represented among the samples of this study. The geochemistry of all samples indicates that parental magmas were generated in supra-subduction extensional environments during lithospheric delamination.

  • Appalachians
  • Vermont
  • delamination
  • geochemistry
  • mafic rocks
  • Moretown Formation
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American Journal of Science: 315 (9)
American Journal of Science
Vol. 315, Issue 9
1 Nov 2015
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Geochemistry and origin of metamorphosed mafic rocks from the Lower Paleozoic Moretown and Cram Hill Formations of North-Central Vermont: Delamination magmatism in the western New England appalachians
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Geochemistry and origin of metamorphosed mafic rocks from the Lower Paleozoic Moretown and Cram Hill Formations of North-Central Vermont: Delamination magmatism in the western New England appalachians
Raymond Coish, Jonathan Kim, Evan Twelker, Scott Zolkos, Gregory Walsh
American Journal of Science Nov 2015, 315 (9) 809-845; DOI: 10.2475/09.2015.02

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Geochemistry and origin of metamorphosed mafic rocks from the Lower Paleozoic Moretown and Cram Hill Formations of North-Central Vermont: Delamination magmatism in the western New England appalachians
Raymond Coish, Jonathan Kim, Evan Twelker, Scott Zolkos, Gregory Walsh
American Journal of Science Nov 2015, 315 (9) 809-845; DOI: 10.2475/09.2015.02
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  • Article
    • Abstract
    • INTRODUCTION
    • REGIONAL GEOLOGICAL CONTEXT
    • GEOLOGICAL SETTING AND STRATIGRAPHY
    • STRUCTURAL GEOLOGY AND METAMORPHISM
    • FIELD RELATIONSHIPS OF THE MAFIC ROCKS
    • AGE INFORMATION
    • PETROGRAPHY
    • GEOCHEMISTRY
    • ORIGIN OF THE MAFIC ROCKS
    • TECTONIC IMPLICATIONS
    • CONCLUSIONS
    • ACKNOWLEDGMENTS
    • Appendix
    • Footnotes
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Keywords

  • Appalachians
  • Vermont
  • delamination
  • Geochemistry
  • mafic rocks
  • Moretown Formation

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