GEOCHEMICAL ANALYSIS OF SILURO-DEVONIAN MAFIC DIKES IN EAST-CENTRAL VERMONT

In the Woodsville and Newbury quadrangles in east-central Vermont and west-central New Hampshire, Siluro-Devonian mafic dikes intrude metasedimentary rocks of the Vermont and New Hampshire Sequences. The dikes are thought to be related to a diorite-gabbro complex, dated at ~420Ma. In this study, we focus on four previously unstudied outcrops where mafic dikes intrude the Albee Formation. In particular, petrography and geochemistry, including major and trace elements, are used to interpret the magmatic and tectonic origin of the dikes. Preliminary geochemical data indicate the dikes have basaltic compositions. Furthermore, linear trends between MgO vs Ni, SiO₂ vs Al₂O₃ and Fe₂O₃/MgO vs TiO₂ suggest that the dikes have retained original igneous signatures despite metamorphism associated with the Acadian Orogeny. Dikes from the four localities are all related geochemically. Moreover, there are well-defined chemical trends among the dikes that can be accounted for by differing amounts of crystal fractionation from the same parent. Trace element tectonic discriminant diagrams, e.g. Ti-Zr-Y, Cr-Y and Fe-Mg-Al, indicate the dikes may have formed in a continental extensional environment. The dikes are also geochemically similar to dikes in the Comerford Dike Complex, about 40 km northeast. Thus, they appear to be part of a regional swarm of dikes that formed by partial melting of sub-continental, asthenospheric mantle and then intruded the crust in an extensional tectonic environment between the time of Taconic and Acadian orogenies.