AGE AND COMPOSITION OF THE CANNON POINT SYENITE, ESSEX, NY: WESTERNMOST EXPRESSION OF WHITE MOUNTAIN MAGMATISM

There were two major episodes of igneous activity in northern New England and Quebec during the Mesozoic: a period between 200-155 Ma, and a later period between 130 and 100 Ma. The older activity was centered around central and northern New Hampshire, and was dominated by alkali syenites, quartz syenites, and granites (Eby, 2004). The younger activity was more broadly distributed geographically, and was dominated by mafic alkaline intrusions.

The Cannon Point syenite is exposed in two nearly horizontal sills (each at least 5 m thick) that intrude the Ordovician Canajoharie shale along the western shore of Lake Champlain. Both sills are porphyritic, with large phenocrysts of sanidine and sparse hornblende (pseudomorphs). The two sills are variably altered, with some samples being extensively sericitized and carbonatized. Despite the alteration, the two sills preserve distinct chemical signatures, with the upper sill being a syenite with ~ 75 wt. % SiO₂ and a La/Yb_CN of ~3.5 and the lower sill being an alkali feldspar syenite with ~70 wt. SiO₂ and a La/Yb_CN of ~15. Both have compositions characteristic of A-Type, within-plate, granitoids. The Cannon Point syenites are compositionally similar to the syenitic rocks of central New Hampshire that make up the largest portion of the White Mountain igneous province.

Previous attempts to date the Cannon Point syenite by whole-rock Rb-Sr yielded a result of “less than 140 Ma” (Fisher, 1968). Considering the extent of alteration of these rocks, this result was, at best, a rough approximation of the emplacement age. Over 50 zircons were recently extracted from a sample of the Cannon Point syenite and were analyzed by LA-ICPMS for U-Th-Pb isotopes, and yielded a concordant crystallization age of 131 +/- 1.8 Ma. This age is consistent with the age of other igneous rocks in western Vermont and southern Quebec, which are almost exclusively mafic alkaline intrusions. The Cannon Point syenite is the only young syenitic intrusion west of central New Hampshire, and probably was generated by melting of the lower crust due to the intrusion of alkaline mantle melts. Emplacement of these crustal melts into the upper crust was most likely facilitated by extension across the Lake Champlain graben.