NEW GEOCHRONOLOGY AND BEDROCK MAPPING OF THE SOUTHERN PORTION OF THE 7.5 MINUTE MT. CRESCENT QUADRANGLE, NORTHERN NEW HAMPSHIRE

New bedrock 1:24,000 mapping and complimentary zircon geochronology in the southern half of the Pliny East 7.5’ quadrangle in northern NH along the Bronson Hill Anticlinorium and within the Jefferson Dome was completed as part of the USGS/NHGS StateMap program. The mapping reveals the following: 1) a variety of gray to pink, variably foliated Ordovician Oliverian granites; 2) several narrow, highly sheared lenses of Ordovician Ammonoosuc Volcanics within the Oilverian suite; 3) a previously undiscovered metasedimentary xenolith; 4) Ordovician weakly to non-foliated syenite; and 5) the crescentic-shaped Jurassic Mt. Crescent ring dike. Regions of silicified zones marking late brittle faults were also found and correlate to those found in the SW adjacent Mt. Dartmouth 7.5’ quadrangle. Rare Jurassic mafic and rhyolitic dikes were also mapped and correlate to those found in the nearby Jefferson 7.5' quadrangle.

Crystallization and detrital zircon U-Th-Pb ages were determined for five samples from the study area. These five samples included three from the Oliverian granites, one from the granite porphyry of the Mt. Crescent ring dike and one from the metasedimentary xenolith found in the southwest corner of the quadrangle. Two of the three samples believed to be part of the Oliverian yielded concordant zircon ages of 440.1 +/- 2.6 Ma and 447.2 +/- 2.5 Ma, supporting their inclusion as part of the Ordovician Jefferson Dome. The third Oliverian sample yielded a concordant zircon age of 334.0 +/- 2.2 Ma, surprisingly indicating a Carboniferous age of intrusion. This sample also contained older zircons that were Ordovician in age, suggesting that it inherited them from the surrounding Oliverian Jefferson Dome rocks during intrusion. The sample of the Mt. Crescent ring dike yielded a concordant zircon age of 178.4 +/- 1.1 Ma, supporting its previous age designation. The metasedimentary xenolith yielded a maximum depositional age of 429.3 +/- 7.0 Ma, suggesting that it is Silurian and further that the enveloping coarse granite, previously designated as Ordovician, is no older than Silurian in age.