PETROGENESIS AND PALEOSTRESS ANALYSIS OF THE MESOZOIC MILL BROOK DIKE ZONE IN THE MOUNT DARTMOUTH 7.5' QUADRANGLE, NEW HAMPSHIRE

Field mapping linked with petrogenetic and brittle structural studies of Mesozoic basalt dikes was performed in the Mount Dartmouth 7.5' Quadrangle, adjacent to the Presidential Range, NH. This was done in hopes of creating a better understanding of the intrusion setting and brittle deformational history of the basalt dikes, particularly in relation to the regional tectonic context. Field work done during the summer of 2014 revealed a 100 meter wide, 7.8 km long zone containing multiple mafic dikes that are exceptionally well exposed at the junction of NH Rt. 115 and the Cherry Mountain road. The largest dike in the zone is approximately 20 meters in width, comparable in size to the largest dikes mapped in the Northeast (Christmas Cove dike, Higganum dike, etc.). The newly named Mill Brook Dike Zone was previously discovered and described by Katherine Fowler-Billings in the 1940's. The orientation of the Dike Zone is N-S and steeply dipping. Within the zone, the strike and dip of dikes ranges from 207, 63°SE to 178, 79°W to 5, 69°E. The chill margins were approximately 3-5 cm in width, while the core of the dike contained 1-3 cm subhedral plagioclase grains, creating a porphyritic texture. When viewed in thin section, the plagioclase grains show significant core to rim magmatic zoning. Additionally, there is notable alteration of the other minerals, primarily sericite and chlorite alteration. In some samples there is cataclasis of the grains, likely due to hydrothermal alteration during faulting. The Mill Brook dike cuts through the Jurassic Cherry Mountain syenite pluton along the western edge of the quadrangle, but was not observed to crosscut the Jurassic Conway granite in the south of the quadrangle. A regional, dike-parallel N-S, steeply dipping joint set was also discovered. Previous work in the adjacent Mount Washington region found mafic intrusions and associated joints striking (from oldest to youngest) NE-SW and E-W, but found only joints and no dikes striking N-S related to the youngest joint set. The Mill Brook Dike Zone and its joints are likely younger than the Late Cretaceous New England-Quebec igneous province and record a significant period of basalt magmatism during E-W extension in the Cretaceous.