Abstract

Oliverian Magma Series gneisses (442–454 Ma) in the cores of domes in southwestern New Hampshire have an igneous mineralogy that crystallized at pressures (P) of 9–11 kbar and temperatures (T) of 650–775 °C, whereas the cover (Ammonoosuc Volcanics) has the same formation age (453 ± 2 Ma) yet is clearly extrusive. A significant crustal section, ∼30 km thick, must have originally separated the two units, a recognition that demands tectonic reinterpretation of this classic orogen. Acadian age (ca. 400 Ma) metamorphism at P ∼4–6 kbar, T ≥ 625 °C is well documented in the cover, but the absence of expected diffusional reequilibration in Oliverian garnets indicates that they never exceeded ∼550 °C after formation. Juxtaposition of the Oliverian Magma Series gneisses and their cover must have occurred after the cover had cooled below 550 °C, and was likely via either (neo-)Acadian or Alleghanian thrusting.

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