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

Matthew J. Kohn, Frank Spear

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10 Scopus citations

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.

Original languageEnglish
Pages (from-to)803-806
Number of pages4
JournalGeology
Volume27
Issue number9
DOIs
StatePublished - 1999

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