SUPRA-SUBDUCTION MAGMATISM IN THE MORETOWN TERRANE, A FRAGMENT OF GONDWANA IN THE WESTERN APPALACHIANS

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. e_Nd 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.