Northeastern Section - 50th Annual Meeting (23–25 March 2015)

Paper No. 2
Presentation Time: 8:00 AM-12:00 PM


COISH, Raymond, Geology Department, Middlebury College, Middlebury, VT 05753, KIM, Jonathan, Vermont Geological Survey, 1 National Life Drive, Davis 2, Montpelier, VT 05620-3902, RYAN-DAVIS, Juliet, Geology, Middlebury College, Bicentennial Hall, Middlebury, VT 05753, PIERCE, Natashia, Geology, University of Cincinnati, 500 Geology-Physics, University of Cincinnati, Cincinnati, OH 45221 and DIETSCH, Craig, Department of Geology, University of Cincinnati, PO Box 0013, Cincinnati, OH 45221-0013,

Meta-sandstones of the Moretown terrane in western New England contain detrital zircons that reveal an affinity with the Gondwanan (eastern) rather than the Laurentian (western) side of the Cambrian-Ordovician Iapetus Ocean. In the Vermont Appalachians, metamorphosed mafic rocks, in the form of dikes and sills, cut this Moretown terrane. The meta-mafic rocks have geochemical characteristics similar to modern-day supra-subduction volcanics. Geochemistry suggests that the meta-mafic rocks geochemically were formed as mostly basalts or basaltic andesites. They 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. eNd values for two samples are +3 and +5. The chemistry, taken together, suggests protoliths of the meta-mafic rocks may have formed in an extensional marginal basin(s), perhaps near a volcanic arc(s). The meta-mafic rocks of this study are similar in chemistry to the pre-Silurian Mount Norris Intrusive Suite (MNIS) of north-central Vermont, and also to the Silurian-aged Comerford Intrusive Suite (CIS) of northeastern Vermont. If the meta-mafic rocks of the Moretown are correlated with the MNIS, then the supra-subduction extensional environment could have formed in response to lithospheric delamination, following collision of the Gondwanan Moretown terrane with either a Laurentian microcontinent or Laurentia itself in the Ordovician. If , on the other hand, they are correlated with the CIS, then the extensional environment may have formed by lithospheric delamination following collision of a Ganderia fragment with Laurentia in the Silurian. In either case, it is likely that the magmatism occurred after the peri-Gondwanan Moretown terrane became part of the Laurentian plate.
  • Coish_NEGSA_2015_Poster1.pdf (3.5 MB)