The Talcott, oldest of the three basalts in the Mesozoic Hartford basin of Connecticut, contains abundant euhedral olivine and plagioclase phenocrysts and less common rounded augite-plagioclase aggregates and orthopyroxene phenocrysts rimmed by augite and olivine. The orthopyroxene, which has a remarkably constant composition (En84) and is Cr-rich, is believed to be a refractory residue from an upper-mantle source region. The Holyoke basalt, the middle unit, is essentially aphyric, with only minor plagioclase and olivine phenocrysts. The Hampden, youngest of the three basalts, contains abundant plagioclase and minor olivine and augite phenocrysts.

Materials-balance calculations using phenocryst compositions from the Talcott basalt indicate that the Holyoke can be derived from a Talcott magma by addition of 7.8% orthopyroxene and removal of 7.9% olivine, 15% clinopyroxene, and 13.3% plagioclase. The Hampden basalt cannot be derived from a Holyoke magma by any reasonable fractionation scheme, but it can be derived from a Talcott magma by removal of 4.4% olivine, 12.0% clinopyroxene, and 14.3% plagioclase.

One-atmosphere melting experiments under controlled oxygen fugacities indicate successively lower liquidus temperatures for Talcott, Holyoke, and Hampden basalts. Both the Talcott and the Holyoke have olivine and plagioclase on the liquidus simultaneously, with augite and pigeonite appearing ∼15 °C below the liquidus. The Hampden has plagioclase alone on the liquidus; olivine and augite appear only 5 °C below the liquidus.

Orthopyroxene is not present in the low-pressure experiments but is thought to have been stable on the liquidus at depths corresponding to pressures exceeding 8 kbar. Assimilation of orthopyroxene and crystallization of other phases necessary to produce the Holyoke basalt from a Talcott magma must have taken place at depth; however, because the low-pressure phases obtained in the experiments are those required to produce the Hampden from the Talcott magma, this fractionation likely occurred in a near-surface magma chamber.

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