THE NORTHFIELD INTRUSIVE COMPLEX IN CENTRAL VERMONT: POST-TACONIC TRONDHJEMITIC EXTENSIONAL MAGMATISM

A complex of intrusions has been mapped and studied in central Vermont within the Ordovician Moretown Formation. It consists of a trondjhemitic dike swarm striking at ~N20E, slightly clockwise to the regional fabric and locally warped by the younger tonalitic Fernandez pluton, which is associated with two other small plutons, the Edgerton and the un-exposed Maloney tonalites. The belt of dikes is typically 20m wide with up to 10 dikes across strike and has been traced more than 20km. Both the dikes and plutons are peraluminous leucocratic rocks dominated by stubby plagioclase; the dike rocks include abundant phenocrysts of plagioclase, with lesser amounts of muscovite and quartz. Within the intrusive complex, geochemistry varies minimally with high SiO₂ ~70 wt% and Al₂O₃ >15 wt%, and ~3:1 Na:K ratios, much like that of the early Silurian intrusions in the Braintree Complex, in sharp contrast with the ~1:1 ratios of Acadian felsic rocks of Vermont. Trace element and rare-earth-element (REE) patterns show distinct contrast with the Acadian granites, while exhibiting oversaturation of LILE and HFSE relative to both NMORB and primitive mantle, and undersaturation of REE relative to NMORB. A weak en echelon pattern suggests emplacement in a dextral transtensional stress regime. Metamorphic host rocks locally preserve bedding with tight, isoclinal folds having axial planes parallel to the bedding, all cut obliquely by the dikes which have slightly more easterly strikes. The whole intrusive complex appears essentially undeformed except in thin section. Minor post-emplacement deformation and metamorphism includes fractured quartz grains with wisp-like ribbons of muscovite localized in the fractures and feathering into the rock matrix, plagioclase grains that are locally recrystallized to muscovite, and biotite that is locally retrograded to chlorite. This evidence suggests that the dikes are pre-Acadian, perhaps correlating with the 423 Ma ages reported for the Newport tonalite and Braintree complex, and taken together, supports an initial hypothesis that the intrusions of the NIC may be part of a tonalite-trondhjemite-dacite suite generated by slab melt during opening of the Siluro-Devonian Connecticut Valley Gaspé Trough to the east.