CHEMICAL COMPOSITION OF INTRUSIVE ROCKS FROM THE FAIR HAVEN HEIGHTS AND FOXON REGIONS, HARTFORD BASIN, CONNECTICUT

The Early Mesozoic Hartford Basin of central Connecticut consists of an east-dipping half-graben filled with sedimentary strata intercalated with three chemically distinct tholeiitic basalt lavas referred to as the Talcott (lower), Holyoke (middle), and Hampden (upper) basalts. Each of the basalt lavas have a distinct chemistry and have been chemically correlated to geographically distinct feeder dikes and sills identified within and adjacent to the Hartford Basin (from west to east: Bridgeport-Pelham, Buttress-Ware, and Fairhaven-Higganum dike systems). One region where physical relationships among the intrusive rocks is ambiguous is the area of Fair Haven Heights and Foxon. In this region, the state bedrock map compiled by Rogers (1985) suggests that there are two independent and cross-cutting intrusive systems which were interpreted largely based on outcrop geometry. One, inferred older, predominantly sill-like set of intrusions is suggested to be associated with the West Rock intrusion located in the central part of the basin. The other, more dike-like intrusions which trend predominantly N-NE, are inferred to have affinities with the Buttress dike system.

A new suite of 22 samples from the Fair Haven-Foxon intrusions, collected from an area extending from Black Rock to Peter’s Rock, have been analyzed for major and trace elements using XRF and HR-ICP-MS. Traditionally, Mesozoic igneous rocks from the Hartford Basin have been distinguished by variations in MgO and TiO₂ compositions, however this can lead to ambiguities for individual rock samples and localities when alteration is significant. Our new 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 identify and correlate the flows and intrusive rocks with greater reliability. The new chemical data establish that all the Fair Haven-Foxon samples represent a single magmatic lineage related predominantly by fractional crystallization processes. The new analyses are also compared directly to complementary data for other units in the southern Hartford Basin. The data provide clear genetic links between the Fair Haven-Foxon, East Rock, West Rock, Pine Rock intrusions, the Higganum dikes, and the Talcott lavas.