MESOZOIC MAGMATISM IN NEW ENGLAND AND QUEBEC

Mesozoic magmatism in New England and Quebec begins with Triassic dikes emplaced at Cape Ann and other eastern Massachusetts locations. The oldest dike is a camptonite (245 Ma amphibole Ar-Ar) and other dikes have ages ranging from 240 to 210 Ma. The Jurassic is represented by large felsic intrusions in New Hampshire and Maine which have ages ranging from 207 to 170 Ma with most ages between 180-190 Ma. The Cretaceous is represented by geographically wide spread igneous activity in Quebec (Monteregian Hills) and New Hampshire. Most of this intrusive activity occurs between 121 and 124 Ma, with the exception of the Oka carbonatite complex at the western end of the Monteregian Hills which has a 10 million year emplacement history, and some mafic dikes in Vermont (131 – 138 Ma). The continental expression of Cretaceous magmatism ends with the emplacement of the Cuttingsville complex at ~103 Ma. A Cretaceous age (104 – 82 Ma) seamount chain extends into the Atlantic Ocean from coastal New England.

There is no isotopic data for the Triassic dikes, but on the Nb/Yb vs Th/Yb discriminant diagram the dikes plot in a field extending from OIB to EMORB. For the other periods of Mesozoic magmatism the data plot near OIB and extend outside the mantle array reflecting variable amounts of crustal contamination. Other discriminant diagrams show similar trends. Standard spider plots support an OIB-like source with subsequent fractional crystallization and crustal assimilation producing the more evolved magmas.

Sr, Nd, Pb, and some Hf isotopic data exist for many of the igneous intrusions that comprise the Mesozoic period of igneous activity. These data consistently plot within the depleted mantle field and/or on arrays trending towards potential crustal contaminants. Irrespective of age, the least contaminated samples consistently plot in the depleted mantle field suggesting that all these magmas were extracted from the same mantle source.

A general petrogenetic model involves partial melting of a depleted mantle source with subsequent fractional crystallization and variable crustal contamination producing the range of magma types (from silica undersaturated to silica oversaturated) observed throughout the New England – Quebec province. The data do not support a hotspot model but rather support episodic periods of crustal extension.