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Chemical analyses, radiometric data, petrographic data, and physical features define three different rock types (and associated provinces) in the dikes of southeastern New Hampshire: (1) an alkali basalt group (coastal New England) that ranges in thickness from 2 cm to 5 m, is common throughout the area and tends to follow structural trends of the metamorphic host rock; (2) a tholeiitic dike (eastern North America) classified as high-TiO2 quartz-normative (HTQ) that ranges in thickness from 18 to 30 m, cuts across the trend of the foliation of the host rock, and can be traced along a north-northeast line for 24 km; and (3) a felsite group (New England–Quebec) that includes andesite, dacite, and rhyolite. Whereas most of the felsitic rocks occur in dikes oriented N45°E to east-west, the rhyolite crops out in a massive stock or laccolith(?) hundreds of meters across. K-Ar whole-rock age determinations yield 222 Ma for an alkali basalt, 177 Ma for a tholeiitic basalt, and 118 Ma for a rhyolite. Although all three rock types traditionally have been considered part of the White Mountain Magma Series in southeastern New Hampshire, the 100 m.y. spread in their emplacement ages suggests that the three rock types are products of different and unrelated igneous events. The mafic rocks were emplaced before or during rifting events that opened the North Atlantic Ocean. The HTQ tholeiitic Onway dike is most likely a northeastern extension of the Higganum dike in Connecticut and Massachusetts. The felsite group appears to be associated with hotspot activity that produced a series of plutons extending from the Monteregian Hills in Quebec, through southeastern New Hampshire, to the New England Seamounts.

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