A MAJOR EARLIEST SILURIAN LOW-ANGLE DETACHMENT IN THE BRONSON HILL MAGMATIC ARC, MASSACHUSETTS AND SW NEW HAMPSHIRE

The 1983 Massachusetts Bedrock Map showed Ordovician to possibly late Proterozoic intrusives (“OZ”) overlain, with potential unconformity, by Ordovician Ammonoosuc Volcanics and Partridge Formation, marked by Moosehorn conglomerate at their contact. 1990 U-Pb zircon geochronology proved unconformity untenable, with overlapping ages 454 to 442 Ma in the intrusives, 453 to 449 Ma in the cover, supplemented recently by ages to ~470 Ma in New Hampshire lower Ammonoosuc. Also, Prescott Complex Cooleyville Gneiss, previously shown Devonian, cutting Silurian Clough and Devonian Littleton, yielded a 449 Ma U-Pb zircon age, contrary to supposed field relations. In 2016, field data showed the NW “intrusive” contact of Cooleyville is a Mesozoic normal fault, and major- and trace-element geochemistry of Cooleyville indicated it is identical, except grain size, with Monson and Fourmile Gneisses of the deeper intrusives. It is a hypabyssal equivalent intruding Partridge and Ammonoosuc.

In a regional scenario, the deep intrusives lay in the most active part of the arc; the Ammonoosuc and overlying Partridge black shales, containing geochemically studied volcanics, including unusual picritic basalts, lay in or toward the back-arc region, where intruded by Cooleyville. The earliest Silurian detachment, named for the Moosehorn locality, then carried the eastern sequence west and down against the deep intrusives, in process cutting away an intermediate section of cover rocks with intrusions. Intense Early and Late Devonian, and Pennsylvanian ductile deformations, and early Silurian and younger erosion, make the original detachment geometry hard to deduce. However, Ammonoosuc Volcanics of the upper sequence in the W of the region are cut out to E, bringing Partridge direct against Monson. The Pelham Dome, where Partridge usually rests on Fourmile, may expose another detachment,. The detachment, or detachments, occurred after 442 Ma intrusions, before deposition of Clough ~435 Ma, locally unconformable on Fourmile in the Pelham Dome. Grand-scale extension was described for an equivalent part of the Popelogan Arc, N Maine and New Brunswick, where collision with Laurentia was tied to the Taconian Orogeny. Similar large magmatic-arc extension in the Aegean occurred during active magmatism, not after, as suggested here.