MAGMAS WITHOUT BORDERS

Metamorphosed igneous rocks in northern Vermont can be directly linked to equivalents in Québec and provide important constraints on tectonic models for the New England-Québec Appalachians. Three magmatic suites in west-central Vermont can be correlated across the international border: 1) late Proterozoic rift volcanics; 2) Early Paleozoic suprasubduction ophiolitic rocks; 3) Early Paleozoic suprasubduction arc-backarc dikes and volcanics. Late Proterozoic rift volcanics are represented by greenstones in the Camels Hump Group and the Stowe Formation of Vermont. These greenstones were alkali to tholeiitic basalts with compositions ranging from high-Ti, light rare earth element enrichment to moderate-Ti, light rare earth element depletion. They have been interpreted as magmatism associated with the opening of Iapetus Ocean, from earliest to latest stages. Equivalents in Québec include meta-volcanics in the Caldwell Formation and Macquereau Group. Cambrian-Ordovician suprasubduction ophiolitic rocks are preserved in Vermont as partially serpentinized ultramafic bodies (e.g., East Dover, Ludlow) and as meta-volcanics and dikes in the Warner Hill Complex of northern Vermont. High Cr/Al chromites within olivine grains in serpentinites and boninitic-like geochemistry in some of the meta-volcanic rocks indicate formation in a fore-arc setting. These may be correlated to the 504 Ma Mont-Orford or Lac-Brompton ophiolites in Québec. Early Ordovician? suprasubduction zone volcanic remnants are found throughout the Moretown and Cram Hill formations in Vermont. These include dikes of the Mount Norris Igneous Suite, the extent of which has recently been expanded, and flows of the Coburn Hill volcanics. Their chemistry, particularly distinct negative Nb anomalies, indicates they were formed in a marginal basin near a volcanic arc. These may be correlatives of the Bolton volcanics in Quebec. The three volcanic groups can be explained, respectively, as remnants of 1) continental rift magmatism leading up to formation of Iapetus, 2) fore-arc ophiolites formed above an east-facing subduction zone that also formed the Shelburne Falls arc, and 3) magmatism associated with lithospheric delamination associated with collision of Laurentia with the Shelburne Falls arc.