GEOCHEMICAL COMPOSITION OF THE SOUTH BROOK BASALT 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 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 basalts 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 which is exposed 60 km to the southwest [Orange Mtn. (lower), Preakness (middle), Hook Mtn. (upper)]. The chemical association 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, which is exposed at only one locality, suffer from extensive hydration and alkali enrichment most likely as a result of their proximity to faulting along the eastern border of the basin. This alteration causes significant ambiguity when attempting to evaluate the identity of samples using the classic SiO₂ versus TiO₂ diagram commonly used to discriminate among basalts in these basins.

A new suite of 13 samples from the South Brook Basalt have been analyzed for major and trace element concentrations using XRF and HR-ICP-MS. These analyses include a number of high-field strength elements (REE, Y, Zr, Hf, Nb, Ta, U, Th) that are more resistant to alteration and preserve the original magmatic elemental ratios that can be used to uniquely identify and correlate the flows. The new analyses are compared directly to complimentary data for other lava units in the Pomperaug and southern Hartford Basins. The chemical data establish the unique chemical identity of the South Brook Basalt within the Pomperaug Basin and provide clear genetic links with the Hampton Basalt in the Hartford Basin. This supports the existence of co-magmatic relationships between correlated volcanic units in the Pomperaug, Hartford, and Newark Basins which provide important time-stratigraphic markers for interbasinal correlation.