BEDROCK GEOLOGY AND GEOCHRONOLOGY OF THE MT. CRESCENT, NH 7.5' QUADRANGLE

The bedrock geology of the Mt. Crescent, NH 7.5' quadrangle has been remapped at a scale of 1:24,000. The quadrangle lies along the along the eastern edge of the Bronson Hill Anticlinorium and is dominated by two principal rock units, the Ordovician Oliverian Jefferson Dome and the Jurassic Mt. Crescent ring complex. The bedrock units from oldest to youngest are: 1) highly sheared lenses of Ordovician Ammonoosuc Volcanics; 2) Ordovician weakly to non-foliated coarse and fine syenite; 3) a variety of gray to pink, variably foliated and sheared, and rarely porphyritic Ordovician Oliverian granitic gneisses; 4) a Silurian folded metasedimentary quartzite and schist xenolith; 5) a Devonian to Silurian (?) coarse granite surrounding the xenolith; 6) a Carboniferous fine grained granite that is variably sheared; and 7) the crescentic-shaped Jurassic Mt. Crescent monozogranite porphyry, probably a cone sheet. Regions of Triassic (?) silicified zones marking late brittle faults were also found. Regions of Carboniferous (?) mylonitic fabrics with S-C mylonites and rotated porphyroblasts were found in two distinct shear zones. Crystallization and detrital zircon U-Th-Pb ages were determined for three samples from the Oliverian granites, one from the granite porphyry of the Mt. Crescent cone sheet, and one from the metasedimentary xenolith. 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 cone sheet 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. An additional detrital zircon age for this unit and the enveloping pluton will be presented at the meeting!