THE STRONTIUM ISOTOPE COMPOSITION AND TIMING OF SECONDARY MINERALIZATION IN THE ORENAUG BASALT FROM THE EARLY MESOZOIC POMPERAUG BASIN, WESTERN CONNECTICUT

The Early Mesozoic (201 Ma) 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 Orenaug Basalt, which is ≈80 meters in thickness, consists of three distinct cooling units and is well known by mineral collectors for producing prize specimens of a variety of secondary minerals, in particular prehnite. Other important secondary minerals found in vug and fracture fillings include quartz, calcite, pumpellyite, datolite, apophyllite, laumontite, heulandite, and stilbite. The general assumption of prior work has been that this mineralization was produced primarily from constituents leached from the glassy basalt during hydrothermal conditions that existed shortly after emplacement of the lavas. In order to better understand the timing of secondary mineralization, preliminary Rb-Sr isotopic analyses have been conducted on several secondary minerals from the Orenaug Basalt that crops out in the O&G Quarry (Silliman Quarry) which straddles the Southbury/Woodbury town line. The basaltic host consists of plagioclase and pyroxene in a glassy mesostasis. Initial ⁸⁷Sr/⁸⁶Sr ratios of only slightly altered basalt generally fall in the range 0.707-0.709. A prehnite sample from the basalt has an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.709 which is permissibly in equilibrium with the basalts at the time of eruption. In contrast, a calcite sample has a much higher initial ⁸⁷Sr/⁸⁶Sr ratio of 0.714 which suggests that it was either precipitated at some time significantly after the host basalts were emplaced (allowing time for ⁸⁷Rb decay) or that it incorporated Sr that was derived from a more radiogenic source than the host lavas. The very high Rb/Sr ratios of apophyllite (Rb=245-290 ppm, Sr=9-12 ppm; ⁸⁷Rb/⁸⁶Sr = 65-90) crystals from the lavas allows for calculation of model ages that are relatively insensitive to assumptions of initial Sr isotope ratios. Model ages determined for four apophyllite samples range from 30 to 65 Ma. We interpret the ages to broadly reflect the time of mineral precipitation. The ages suggest that mineralization continued in the basalts well after their initial eruption.