BEDROCK MAPPING ACROSS THE BRONSON HILL BELT, THE CENTRAL MAINE BELT AND THE WHITE MOUNTAIN PLUTONIC-VOLCANIC SERIES OF NORTHERN NEW HAMPSHIRE

The bedrock geology of the Jefferson, Mt. Crescent, Berlin, and Shelburne NH 7.5-minute quadrangles was mapped at 1:24,000 scale from 2016 to 2023 with support by the New Hampshire Geological Survey through USGS StateMap. The region intersects three major lithotectonic features: The Ordovician Bronson Hill Belt; the Silurian-Devonian Central Maine Belt; and the Mesozoic White Mountain Plutonic-Volcanics Series. New U-Pb zircon dates from 27 samples (12 detrital zircon and 15 crystallization zircon ages) constrained the timing of magmatism, sedimentation, metamorphism, and deformation.

The Bronson Hill Belt includes: the Cambrian Albee Formation, a pin-striped gneiss with maximum depositional ages (MDA) of 545 to 522 Ma; the Ordovician Ammonoosuc Volcanics, a mylonitic amphibolite, rare felsic tuff (451 ± 2 Ma), and quartzite (MDA 460 ± 3 Ma); unnamed syenites (450 ± 3 Ma); the Ordovician Oliverian granitic gneisses of the Jefferson Dome (448 to 440 Ma); and the ~443 Ma Lost Nation meta-gabbro of the Highlandcroft Series.

The Central Maine Belt includes: the Rangeley Formation, a rusty and/or non-rusty schist and quartzite, granofels (MDA 443 to 420 Ma), calc-silicate pods, and rare conglomerate; the Perry Mountain Formation, a quartzite (MDA 424 ± 3 Ma); the Smalls Falls Formation, a rusty weathering schist and quartzite; the Madrid Formation, a granofels (MDA 442 ± 4 Ma); the Littleton Formation, a gray schist and quartzite (MDA 444 ± 3 Ma); a folded metasedimentary xenolith (MDA 440 ± 2 Ma); and the Terrace Mountain roof pendants (MDA 415-406 Ma).

Devonian and Carboniferous intrusions of the New Hampshire Plutonic Series intrude these rocks and include granite enveloping the metasedimentary xenolith (417 ± 2 Ma), quartz diorite (374 ± 4 Ma), and two mica granite (347-325 Ma).

The White Mountain Plutonic-Volcanic Series intrude the Bronson Hill Belt and include the Jurassic Pliny and Mt. Crescent Complexes. The Pliny Complex consists of, from oldest to youngest: diorite; porphyritic quartz monzodiorite; hornblende quartz syenite; quartz monzodiorite; hastingite-riebeckite granite; granite porphyry; pink biotite granite (188 ± 1 Ma); Conway granite stocks (187 ± 1 Ma); and flow banded and spherulitic rhyolite dikes (185 ± 2 Ma). The Mt Crescent cone sheet consist of a granite porphyry (178 ± 1 Ma).