Geochemistry of intrusive rocks on the Prescott Peninsula, central Massachusetts, USA: Implications for late detachment faulting within the Ordovician Taconian volcanic arc

Abstract

The Prescott Peninsula contains two intrusive bodies: the Cooleyville Gneiss (449 Ma) and the Packard Gabbro (407 Ma), which cuts the Cooleyville. These are centrally located in the Bronson Hill Anticlinorium, a complex structural belt in western New England that includes metamorphosed rocks of the Ordovician Taconian arc plutonic complex (TAPC), and associated metamorphosed volcanic rocks. TPAC gneisses (locally 454–442 Ma) are overlain by stratified units including the Ordovician Ammonoosuc Volcanics (locally 453 Ma), Partridge Formation (locally 449 Ma), and several Silurian and Devonian units.

The Cooleyville Gneiss and Packard Gabbro are both calc-alkaline and have similar, arc-associated geochemical anomalies expressed in multi-element diagrams. These units are chemically distinct from several regionally exposed Devonian plutons, but are almost identical to plutonic rocks of the TAPC. The Cooleyville Gneiss cuts the Ammonoosuc Volcanics and Partridge Formation, but the TAPC does not, at least over extensive regions. Additionally, the Ammonoosuc Volcanics and Partridge Formation are each in contact with the TAPC in different areas: Partridge Formation locally to the east and west, and Ammonoosuc Volcanics in a central belt.

A multi-part model explains these relationships: 1) The Cooleyville Gneiss is part of the TAPC, having intruded the Ammonoosuc Volcanics and Partridge Formation east of the main arc axis. 2) Following arc collision with Laurentia, extensive detachment faulting decapitated the TAPC and Cooleyville pluton, sliding them and adjacent units west and down. 3) A second detachment carried part of the Ammonoosuc and Partridge farther west, completing the observed contact geographic pattern. 4) Following isostatic readjustment, erosion, and cooling, Silurian and Devonian units were deposited, in part unconformably on the TAPC. 5) Early Acadian (Devonian) subduction beneath composite Laurentia produced extensive central New England magmatism, and reactivated remnant Taconian lithospheric mantle beneath the TAPC, producing the Packard Gabbro and age-equivalent volcanics in the nearby Erving Formation.

Detachment faulting is common in arc and collisional settings. This model of late Taconian detachment faulting appears to solve a long-standing problem understanding the TAPC contact with overlying Ordovician units.