EARLY CRETACEOUS MAGMATISM IN WESTERN VERMONT AND EASTERN NEW YORK: TIMING, GEOCHEMISTRY, AND ROLE IN THE POSTRIFT EVOLUTION OF THE EASTERN NORTH AMERICAN MARGIN

Renewed interest in postrift magmatism along the eastern North American margin stems in part from the recognition of a coincidence between regions of Mesozoic and Cenozoic magmatic activity and modern-day low velocity anomalies in the uppermost mantle. While recent work by Mazza et al. (2017) has focused on the intrusions in the vicinity of the VA/WV anomaly, less attention has been paid to the New England-Quebec igneous province (NEQ) which also coincides with a low velocity anomaly. The western margin of the Early Cretaceous NEQ consists of small plutons and sheet intrusions that can provide information on the timing of magmatism and paleostress conditions which can be linked to passive margin development and driving mechanisms for igneous activity. The results provided here are part of a broader investigation to assess the origin of the western margin of the NEQ in western Vermont and eastern New York.

Since much of the published data for the region relies on K-Ar ages, we are refining the geochronology to identify patterns tied to geography and intrusion orientation. Our U-Pb LA-ICP-MS age of 103 ± 0.5 Ma for the Cuttingsville complex in southern VT is consistent with previously reported K-Ar ages and supports a pattern of older magmatism to the north (Armstrong and Stump, 1971; Eby and McHone, 1997). The Cannon Point syenite, located north of Cuttingsville, is significantly older based on a U-Pb LA-ICP-MS date of 131 ± 1.7 Ma (Bailey et al., 2017). Barber Hill, a small pluton east of Cannon Point, is at least 111 ± 2 Ma based on an Armstrong and Stump (1971) K-Ar age. Barber Hill may be older based on a McHone and Corneille (1980) Rb-Sr age of 126 ± 5 Ma (recalculated with an updated decay constant) for trachyte dikes that may be genetically linked to Barber Hill. We are awaiting ⁴⁰Ar/³⁹Ar ages for lamprophyre dikes.

Representative samples of Early Cretaceous dikes and small plutons were analyzed for major and trace element geochemistry by XRF and ICP-MS. The REEs display a steep negative slope and are consistent with intraplate alkaline magmatism and a metasomatized mantle source. These NEQ intrusions also share some traits with the Late Jurassic and Eocene intrusions of VA/WV. Trace element patterns for the trachytes in the vicinity of Barber Hill are consistent with Cannon Point, supporting a genetic link between these features.