A LAMPROPHYRE DISTRICT IN THE MESOZOIC HARTFORD BASIN OF CONNECTICUT

We revisited several known lamprophyre dikes exposed along the traprock ridges in the Mesozoic Hartford Basin and discovered additional dikes, extensions, and areas where dikes are suspected to be buried beneath thin glacial till, all in a limited area in central Connecticut. The dikes range from 10 cm to no more than 2 m wide. En echelon segments can be traced for 10s of meters whereas the overall dike lengths are generally several hundred meters, but range from 10 m to ~3600 m.

The lamprophyre district is roughly 15 by 5 km in dimension, stretching across Middlefield, Durham, Wallingford, and North Haven-Guilford areas on the south and is centered on the north-south town boundaries. Within this area, 10 distinct dikes have been mapped or described and three areas discovered with concentrations of lamprophyre cobbles in limited, linear areas that lead us to suspect the existence of dikes covered by glacial till. All dikes are steeply dipping to vertical with an average strike of 010-190°.

The dikes are composed of porphyritic camptonite. Phenocrysts include phlogopite, augite, titanaugite, kaersutite, and magnetite. Megacrysts (> 2cm diameter) of euhedral phlogopite-biotite and broken, anhedral amphibole are also present in some dikes. The groundmass is composed of augite, kaersutitic amphibole and magnetite with Na- and K-feldspar. Many dikes are vesicular, and a few contain ocelli interpreted to be immiscible liquid droplets. Several dikes contain metamorphic foliated and unfoliated granitic xenoliths up to 8 cm in length. Contacts of the xenoliths against the lamprophyre are sharp and unreacted. This and the vesicles suggest that the xenoliths were captured at a relatively shallow depth below the Hartford Basin. These dikes are exposed where they cut Late Triassic to Early Jurassic (~200 Ma) basalt flows but are Middle to Late Jurassic in age (152-169 Ma, Armstrong and Stump, 1970). We report four new analyses of amphibole (n=2) and biotite (n=2) from dikes that all overlap in age at 161.7 + 0.9 to 163.4 + 0.9 Ma. These ages confirm that the dikes are significantly younger than CAMP magmatism and suggest that the lithospheric fractures that brought them to the surface were active throughout the Jurassic, but that the source of asthenospheric liquids evolved over this 40 m.y interval.