Age and origin of the Cannon Point syenite, Essex County, New York: Southernmost expression of Monteregian Hills magmatism?

Abstract

Two syenite sills intrude the local Paleozoic strata of eastern New York State and are exposed along the western shore of Lake Champlain. The sills are fine-grained alkali feldspar syenites and quartz syenites, with phenocrysts of sanidine and albite. The two sills are compositionally distinct, with crossing REE profiles and different incompatible element ratios, prohibiting a simple petrogenetic relationship. Zircon extracted from the upper sill yield a U-Pb age of 131.1 +/- 1.7 Ma, making the sills the youngest known igneous rocks in New York State. This age is similar to the earliest intrusions in the Monteregian Hills of Quebec, over 100 km to the north. Sr and Nd radiogenic isotope ratios are also similar to those observed in some of the syenitic rocks of the eastern Monteregian Hills. The Cannon Point syenites have compositions typical of A-type, within-plate granitoids. They exhibit unusually high Ta and Nb concentrations, resulting in distinct trace element signatures that are similar to silicic rocks of the Valles Caldera, a large, rift-related magmatic system. We suggest that the Cannon Point syenites were melts derived primarily by anatexis of old, primitive, lower crustal material in response to Mesozoic rifting and to the intrusion of mantle-derived magmas. The sills indicate that the effects of continental rifting were spatially and temporally extensive, resulting in the reactivation of basement faults in the Lake Champlain valley hundreds of kilometers west of the active rift boundary, and crustal melting over 50 Ma after the initiation of rifting.