Probing the depths of Oliverian magmas: Implications for Paleozoic tectonics in the northeastern United States
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چکیده
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. Data Repository item 9970 contains additional material related to this article. Figure 1. Generalized geologic map of central New England after Tucker and Robinson (1990), showing geographic disposition of Oliverian Magma Series gneisses and major lithotectonic units. BM and GMM = Berkshire and Green Mountain massifs (Proterozoic),TA = Taconian allochthons (Ordovician), KD = Keene dome, and AD = Alstead dome. Cover to domes consists of Ordovician Ammonoosuc Volcanics and Partridge Formation, and overlying Silurian-Devonian units (Clough, Fitch, and Littleton Formations). Small box shows location of inset; inset shows sample locations. M er rim ac k S yn cl in or iu m
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