GEOCHEMICAL AND ISOTOPIC COMPOSITION OF BASALTS FROM THE EARLY MESOZOIC POMPERAUG BASIN, WESTERN CONNECTICUT

The Early Mesozoic Pomperaug Basin of western Connecticut consists of an east-dipping half-graben filled with siliciclastic sedimentary strata intercalated with three chemically distinct tholeiitic basalt lavas referred to as the East Hill (lower), Orenaug (middle), and South Brook (upper) Basalts. The lavas have been tentatively correlated to flows and feeder dikes identified within and adjacent to the much larger Hartford Basin which is exposed 20 km to the east (Talcott-Higganum (lower), Holyoke-Buttress (middle), Hampden-Bridgeport (upper)) and flows from the Newark Basin (Orange Mtn. (lower), Preakness (middle), Hook Mtn. (upper)) exposed 60 km to the southwest. The chemical connection between the Orenaug, Holyoke, and Preakness basalts has been well established by previous studies but extensive secondary alteration of the East Hill and South Brook Basalts in the Pomperaug have hampered chemical comparisons with basalts in other basins. In particular, published analyses of the South Brook Basalt suffer from extensive hydration and alkali enrichment, probably resulting from their proximity to faulting along the eastern border of the basin. The alteration is so extensive that some recent work has questioned whether the outcrops identified as “South Brook Basalt” may in fact be altered sections of Orenaug and East Hill Basalt that are repeated by faulting.

A new suite of samples, which includes less-altered representatives from each of the lava units in the Pomperaug Basin, has been analyzed for major and trace elements, and Sr-Nd isotopes. These are compared directly to a new complimentary data set for flows and dikes associated with the southern Hartford Basin. The chemical data, in particular variations in the high-field-strength trace elements, establish the unique identity of each of the three Pomperaug lavas and provide clear genetic links between these units and the three basalt units in the Hartford Basin. Initial ⁸⁷Sr/⁸⁶Sr (0.7053-0.7087) and ¹⁴³Nd/¹⁴⁴Nd (ε = 0.5 to -1.7) ratios of basalts in the Pomperaug Basin are also consistent with chemically correlated units in the Hartford Basin. The new data support co-magmatic relationships between correlated volcanic units in the Pomperaug, Hartford, and Newark Basins which provide important time-stratigraphic markers for interbasinal correlation.