Recent field work in the Pliny Range of northern New Hampshire established the existence of two syenite units which predate the igneous activity of the White Mountain magma series but postdate the formation of highly foliated units of the Oliverian Jefferson dome. A total of ten syenite samples were analyzed for Sr isotopes, major elements, and selected trace elements. The syenites describe a whole-rock Rb-Sr isochron of 441 ± 5 m.y. with initial 87Sr/86Sr of 0.70453 ± 0.00006. The syenites of the Jefferson Dome thus formed during the same Oliverian activity which produced the rocks of the domes extending along the Bronson Hill anticlinorium in New Hampshire. Furthermore, the structural position of the rocks establishes 441 m.y. as a minimum age for the Taconic deformation in northern New Hampshire.
The syenites have high total alkalis, high K2O/Na2O contents, and low MgO and CaO concentrations. They have extremely fractionated chondrite-normalized rare-earth element patterns with either small negative or no Eu anomalies. The magmas which produce the syenites were probably produced from partial melting of eclogite; however, the composition of the source material (either mid-ocean ridge basalt, MORB plus sediments, altered MORB, or island-arc tholeiite) prior to metamorphism within the eclogite facies is uncertain.