PETROGENESIS OF LAMPROPHYRES IN THE WALLINGFORD QUADRANGLE

Lamprophyres are uncommon, subvolcanic rock types that are characteristically silica undersaturated and contain abundant, exceptionally large, mafic phenocrysts and vesicles, which intrude into the upper to mid crust as dikes. A previously undocumented, 500 m long and 0.5-1.5 m wide lamprophyre dike that contains large phenocrysts was discovered in Wallingford, CT. The dike crosscuts a Talcott-aged diabase intrusion that formed during the first stage of the breakup of Pangea and is roughly parallel to the N-S regional tectonic joints. Previously, lamprophyres have only been found as float in CT, with the exception of one outcrop lacking the characteristic phenocrysts, located on Mt. Higby, 10 km to the north. In order to determine the petrogenesis of the lamprophyre, thin section analysis was performed to investigate the mineral textures of samples collected from the northern and southern ends of the dike, examining both the contacts and center of the dike at each locale. The lamprophyre primarily contains crystals of augite, titanaugite, biotite, and magnetite set in a groundmass of feldspar and glass. Augite is euhedral, and crystallized as single crystals or glomerocrysts and is more abundant than titanaugite. The first crystals to form were magnetite, indicated by the mineral’s presence on augite crystal growth edges. Augite, titanaugie and biotite all exhibit core to rim zoning, indicating that crystallization began inside a deeper magma chamber and then finished during dike emplacement. In contrast, the Talcott basalt is composed of augite, plagioclase, and lesser olivine and magnetite. Although it does contain augite glomerocrysts, it does not contain megacrysts. These petrological observations suggest that the phenocrysts and megacrysts in the dike are in equilibrium with the lamprophyre melt. Furthermore, we suggest the megacrysts are not zenoliths from a partially crystalline Talcott-basalt magma chamber nor inherited from the lower crust. Preliminary lamprophyre whole rock major and trace element compositions portray a silica undersaturated melt, 42.16 wt% SiO2, that relative to Hartford basin basalts, is enriched in K but not Na, Zr and Ti but not Y, and also Sr, P, Nb, and Ce.