Abstract

Geometric data from more than 1,100 localities and 13 radiometric ages have been compiled and measured for post-metamorphic dikes in eastern New York, Vermont, New Hampshire, western Maine, and southern Quebec. Alkalic lamprophyres (monchiquite and camptonite) in Vermont and Quebec form three westward-trending lobate swarms. The northernmost lobe envelops the Monteregian Hills of Quebec, the central lobe crosses the central Lake Champlain Valley into the eastern Adirondacks, and the southernmost lobe crosses the northern Taconic Mountains into New York. These lobes extend from central New England, where numerous alkalic lamprophyre, spessartite, and diabase dikes were intruded in several episodes and orientations during Mesozoic time.

Most of the monchiquite dikes are found in the Monteregian and Champlain areas, and with associated camptonite dikes intruded west-northwest– to east-trending regional fractures during Early Cretaceous time. Alkalic lamprophyres of this episode can also be found in New Hampshire and Maine. Most diabase dikes are found east of central Vermont, where they intruded northeast-trending fractures during Late Triassic to Early Jurassic time. Many camptonite and some spessartite dikes in eastern New England and the Taconics lobe have northeast trends and may also be of Early Jurassic age.

Extensional stress directions in the New England crust shifted from northwest to north or north-northeast between Early Jurassic and Early Cretaceous time, as inferred from dike trends. The dikes may delineate extensional fracture zones that also controlled the emplacement of Mesozoic plutons in Quebec and New England.

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