Probing the depths of Oliverian magmas: Implications for Paleozoic tectonics in the northeastern United States

Oliverian Magma Series gneisses (442-454 Ma) in the cores of domes in south western New Hampshire have an igneous mineralogy that crystallized at press ures (P) of 9-11 kbar and temperatures (T) of 650-775 degrees C, whereas th e cover (Ammonoosuc Volcanics) has the same formation age (453 +/- 2 Ma) ye t is clearly extrusive, A significant crustal section, similar to 30 km thi ck, must have originally separated the two units, a recognition that demand s tectonic reinterpretation of this classic orogen. Acadian age (ca, 400 Ma ) metamorphism at P similar to 4-6 kbar, T greater than or equal to 625 deg rees C is well documented in the cover, but the absence of expected diffusi onal reequilibration in Oliverian garnets indicates that they never exceede d similar to 550 degrees C after formation, Juxtaposition of the Oliverian Magma Series gneisses and their cover must have occurred after the cover ha d cooled below 550 degrees C, and was likely via either (neo-)Acadian or Al leghanian thrusting.