WENTWORTH DOME: NEWLY CONFIRMED GNEISS DOME OF THE BRONSON HILL ANTICLINORIUM, WEST-CENTRAL NEW HAMPSHIRE

Trondhjemitic biotite gneiss with xenoliths of amphibolite occurs in the core of a ~6 X 3 km northeast-trending asymmetric dome, east of the main axis of the Bronson Hill anticlinorium (BHA) in west-central New Hampshire. Amphibolite, pelitic schists, and quartzite locally overlie the trondhjemitic gneiss. The intrusive Devonian Bethlehem Granodiorite of the Mt. Clough pluton lies above the rocks of the dome.

The rocks in the dome were first mapped as volcanics of the Devonian Littleton Formation (Page, 1940; Billings, 1955), redefined as informal members of the proposed Silurian Church Farm formation (Dean, 1976; Malinconico, 1982), and most recently interpreted as Devonian trondhjemite intrusive into surrounding Littleton volcanics (Lyons and others, 1997).

Correlations of the trondhjemitic gneiss with the Ordovician Oliverian Plutonic Suite and of the overlying units with mantling strata of the BHA domes were considered nearly 30 years ago, but were not adopted because multi-grain TIMS analyses of zircons from the gneiss yielded a Devonian age. However, this result was questionable due to problems related to inheritance and Pb-loss in the zircons, and the adjacent metamorphosed mafic volcanics and pelitic schists remained assigned to the Littleton Formation. New SHRIMP analyses of well-preserved igneous zircon with metamorphic rims, however, yield weighted average ²⁰⁶Pb/²³⁸U ages of 457±7 Ma (cores) and 412±12 Ma (rims). The core results confirm correlation with the Late Ordovician Swanzey Gneiss and Monson Gneiss of the BHA in SW New Hampshire and N Massachusetts, respectively. Rims likely formed during intrusion of the Bethlehem Granodiorite.

Metamorphic rocks mantling the domes are probably correlative with the Ordovician Ammonoosuc Volcanics and Partridge Formation. The locally overlying, very thin units of quartzite, calc-silicate granofels, and garnet-sillimanite schist compare well with the Siluro-Devonian Clough, Fitch and Littleton Formations of the BHA stratigraphic sequence to the west. The Bethlehem Granodiorite may have intruded along a northern extension of the Brennan Hill thrust, along which rocks of the eastern Siluro-Devonian Rangeley sequence were emplaced tectonically over those of the dome.