4 m-thick megacryst- and xenolith-rich camptonite dyke briefly exposed during excavation of a subway station at Porter Square in Cambridge, Massachusetts, was sampled for petrological study and radiometric dating to constrain its age.
This dyke does not outcrop at surface but was exposed at a depth of approximately 35 m in the 1981 excavation for the Porter Square subway station in Cambridge, Massachusetts, and hence it is hereafter referred to as the Porter Square camptonite.
The age of the Porter Square camptonite has hOt been established with certainty.
The Porter Square camptonite was exposed briefly in 1981 during excavation for the Porter Square subway station in Cambridge, Massachusetts, within the Boston Basin.
Thin serpentinite and calcite veins cut the camptonite in some thin sections.
Some individual, rounded and embayed grains of albite are fractured with groundmass infilling fractures but in other instances feldspar aggregates appear to have separated slightly with gaps infilled by the camptonite groundmass (Fig.
This Early Triassic age for the Porter Square camptonite is substantially older than the Late Triassic through Early Cretaceous K/Ar ages reported by McHone (1978) for ENA lamprophyre dykes in Vermont, New Hampshire, and Maine and the Middle Jurassic ages reported by Hermes et al.
3Ma age for the Porter Square camptonite reported here indicates that this apparent southward progression of CNE magmatism during the Mesozoic did not continue into coastal Massachusetts.
In the Montreal area such rocks are mostly alkaline syenite, monzonite, basanite or alkali olivine basalt, and lamprophyre, especially camptonite (Hodgson, 1968; Eby, 1985).
A very fine-grained rock from one dike was interpreted to be a nepheline microsyenite or micromonzosyenite, and a medium-grained, porphyritic rock was classified as a camptonite (D.
The rock is probably camptonite, which commonly contains phenocrysts of an olivine group mineral.
Serpentine is a common alteration product of olivine in lamprophyric rocks such as camptonite.