MINERALOGY OF UPPER CRUSTAL XENOLITHS FROM A LAMPROPHYRE DIKE

Connecticut’s Mesozoic Hartford Basin consists of igneous and sedimentary strata that formed during rifting 200 Ma. The youngest intrusive bodies, ~160 Ma, are a set of at least six lamprophyre dikes that outcrop between northern Guilford and Middlefield, Connecticut. A previously unmapped 0.35 m wide lamprophyre dike has been found near Black Pond in Middlefield. Based on mineral occurrences, it is classified as a camptonite as it contains phenocrysts, up to 4.5 cm long, of kaersutite amphibole, Ti-augite, phlogopite-biotite, and magnetite set in a matrix of glass, apatite, and the same mafic minerals as the phenocrysts. It also contains vesicles and feldspar-rich ocelli. Notably, it is the first lamprophyre dike in the region to be found containing abundant xenolith inclusions. The xenoliths are felsic gneisses composed of k-feldspar, plagioclase, quartz, and up to 15% amphibole and biotite. Zircons are present in the ground mass and as biotite inclusions. They are rounded, contain a minimum reaction rim (less than 1 mm in width) and exhibit little evidence of melting. The xenoliths range in size from just less than 1 cm to ~8 cm in length and make-up less than 3% of the dike volume. We conclude that the xenoliths were picked up during magmatic transport through the upper crust and are from the basement just beneath the rift basin. They are not from the lower crust or mantle. Emplacement and crystallization of the dike was rapid once the xenoliths were included, taking hours rather than days, based on the width of the reaction rim and the presence of unaltered quartz. The quartz-bearing mineralogy of the gneiss is not in equilibrium with the silica undersaturated composition of host camptonite and did not greatly contribute to the composition of the lamprophyre.