CHEMICAL VARIATION IN THE TALCOTT FLOOD BASALT, CONNECTICUT

The Talcott basalt is the oldest of three flood basalts of the Central Atlantic Magmatic Province (~202Ma) that outcrop in the Connecticut Valley. Three dikes are thought to have acted as the feeders for the flows. The Westrock, Eastrock, and Sleeping Giant intrusives are also of the Talcott magmatic phase. We analyzed 26 samples from the Talcott flow and the Higganum and Fairhaven feeder dikes. Samples from the Holyoke basalt, the middle flood basalt unit, were studied for comparison. Talcott basalt samples are aphyric with abundant olivine and clinopyroxene; in contrast, the Holyoke samples are porphyritic with lath-shaped plagioclase phenocrysts. Major and trace elements were analyzed by XRF at Wesleyan University, and samples were analyzed by ICP-MS for a complete set of trace elements (SGS, Toronto). Loss on ignition was used to weed out samples suspect of alteration.

The most mafic Talcott basalt has 10.4 wt.% MgO and >700 ppm Cr, and the least primitive sample had 7.6 wt.% MgO and 300 ppm Cr. Positive correlations occur between Fe and Mg, and Ca and Mg, and negative correlations were found between Mg and Al. The LIL elements show variable enrichment with respect to magnesium. Most trace element spider diagrams show a negative Nb and Ti anomaly, similar to many arc rocks. A few samples show pronounced positive U and Th anomalies. All Talcott samples are enriched in LREE with chondrite-normalized values ~ 30-40. Most samples show a mild negative Eu anomaly, which is surprising for rocks devoid of phenocrystic plagioclase. The HREE show a gently downward sloping pattern. The chemistry of the Talcott flow shows strong similarities to the Higganum and Fairhaven dikes. The Holyoke flow is distinct from the Talcott with its lower HFSE, flat HREE pattern, and enrichment in Sc. Ongoing work involves evaluating fractional crystallization, crustal assimilation, and potential source differences between the Talcott and Holyoke basalts.