40 Mesozoic Magmatism, Postrift Tectonics, and Passive Margin Development of Northeastern North America (Posters)

The northeastern region experienced protracted Mesozoic (Triassic-Cretaceous) magmatism associated with continental rifting and subsequent passive margin development. We welcome contributions related to the occurrence, tectonics, or driving mechanisms of these syn- and postrift events.

Tuesday, 24 March 2026
8:00 AM - 12:00 PM
CCC, Ballroom C

Geochemical Discrimination of the Four Eruptive Events in the Hartford Basin, Connecticut

Paper number: 40-2
Program: Technical Sessions
Time: 8:00 AM – 8:00 AM

Authors

Abstract

This study presents whole-rock geochemical data from Late Triassic to Late Jurassic igneous rocks of the Hartford Basin to characterize and distinguish among the three basalt flows and their associated feeder dikes and sills. The flows and associated feeder dikes are from oldest to youngest the Talcott/Higganum, Holyoke/Buttress, and Hampden/Bridgeport, respectively. Major, minor, trace, and rare earth element concentrations were determined by XRF and ICP-MS for 71 samples of diabase, basalt, and lamprophyre. Thin sections were prepared for all samples to assess mineralogy, textures, and degree of alteration. Earlier studies using major element analyses identified these three eruption events in the basin with TiO₂ and MgO the strongest discriminators. However, compositions of some samples of the oldest and youngest events overlap on these plots, which show discrimination of these units using major-element chemistry alone is not satisfactory. In contrast, variations in concentrations of rare earth elements have uniquely identified the three basalt events and their associated feeder dikes and sills. Light rare earth element enrichment expressed as (La+Ce)/(Dy+Gd) decreases with age of the flows from Talcott to the Holyoke to the Hampden from factors of approximately 4.5 to 3.0 to 2.0, respectively. The same elements in samples of the feeder dikes correlate very well with these three flows. In addition, samples of West Rock sill share the same composition as the Talcott and Bridgeport feeder dikes and are thus part of that event. A 30-m thick sill was identified in the subsurface below Hartford. The sill overlies 10m of East Berlin Formation which overlays the Holyoke basalt. This sill shares similar ratios of rare earth elements with the Holyoke Basalt but higher Al2O3 concentrations and lower rare earth element concentrations suggesting that it represents a less differentiated parental melt derived from the same mantle source. We have also identified cross-cutting lamprophyre dikes, that are ~162 Ma based on new 40Ar/39Ar data. Unique chemical characteristics of these dikes indicate a deeper mantle source than the earlier events. These results place new constraints on the magmatic evolution of igneous rocks in the Hartford Basin and provide a geochemical basis for improved correlation of basalt flows and associated feeder dikes. 

Geological Society of America Abstracts with Programs. Vol. 58, No. 2

doi: 10.1130/abs/2026NE-411182

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