NEWLY DISCOVERED ALBEE FORMATION IN NORTHERN NH: DETRITAL ZIRCONS, STRUCTURE, AND TECTONICS

New mapping and detrital zircon geochronology in the northern part of the Jefferson 7.5’ Quadrangle have revealed a previously unknown region of Cambrian Albee Formation. The region, near Lancaster, NH, contains two Appalachian lithotectonic units: 1) a section of the Bronson Hill Anticlinorium including the Ordovician Ammonoosuc and Cambrian Albee Formations and intrusive rocks of the Ordovician Oliverian Dome and Lost Nation Pluton; and 2) Jurassic igneous cone sheets of the Pliny Range Caldera Complex.

Detrital zircon U/Pb data were generated for four samples of thinly bedded to pin-stripped quartzites. Two samples came from lower elevations near Tug Mtn. with one previously mapped as Albee and the other as Ammonoosuc (Lyons et al., 1997). The other two samples were roof pendants in a Jurassic granite from the higher peaks of Terrace Mtn. previously mapped as Albee (Chapman, 1942).

Both lower elevation samples yielded youngest zircon ages of circa 500 Ma., supporting their designation as Albee and therefore expanding the mapped region of that unit. Population density plots show similarities to other Cambrian Ganderian units (e.g. Dead River, Ellsworth, and Moretown). By contrast, the roof pendants yielded youngest zircon ages of circa 400 Ma., suggesting a Devonian age and possible correlations to the 3rd Acadian DZ cycle, outboard-derived, Tarratine Formation (Bradley and O'Sullivan, 2016).

The Albee is likely in unconformable (Penobscottian?) contact with the Ammonoosuc and is significantly more deformed showing classic pin-striping and transposition. Newly discovered quartzite units in the Ammonoosuc are interlayered and gradational with the more typical mafic Ammonoosuc amphibolites. Both the Ammonoosuc and Albee units are complexly folded by NNE plunging reclined folds of Acadian (?) age and then deformed by the Oliverian Jefferson Dome.

 Lyons et al., (1997) show the Ammonoosuc Fault juxtaposing the Lost Nation and a sliver of Albee against the Ammonoosuc then continuing east through the Pliny Caldera Complex. Our mapping shows instead a chilled intrusive contact between the complex mafic intrusive rocks of the Lost Nation Pluton and the Albee that extends south to become folded with the Ammonoosuc. No Ammonoosuc fault was recognized there or in the Pliny Complex.