DEGREE OF CRYSTAL-MUSH COMPACTION IN THE HOLYOKE BASALT, CT AS INFERRED FROM OVERGROWTHS ON PLAGIOCLASE PHENOCRYSTS

The thickness of marginal zones on plagioclase phenocrysts in the 200-m-thick Holyoke flood-basalt flow in North Branford, CT is a function of both the cooling rate and the amount of residual liquid that remained as a result of compaction or dilation of the crystal mush during solidification.

Plagioclase phenocrysts in this basalt have three prominent parts, a relatively homogeneous oval core, surrounded by a zone with prominent compositional oscillations, and a normally zoned margin. Measurements of the radii of these three components were made perpendicular to (010) on crystals with (010) perpendicular to the section (sharp Carlsbad albite twins). Core radii remain relatively constant throughout the flow at 160 ± 50 µm. The radii of the oscillatory zones fluctuate more (75 ± 50 µm), but show no systematic variation with height in the flow. The radii of the margins, however, change significantly and in a systematic manner with height in the flow. The marginal zones in the lower chilled margin and entablature are ~17 µm; between 15 and 50 m, the radii decrease slightly, but significantly, from 32 to 25 µm; above 50 m, the radii increase sharply to a maximum of ~80 µm at 90 m and then decrease to 17 µm at 120 m, the colonnade/entablature boundary.

The lack of systematic variation in core radii and oscillatory zones with height in the flow suggests that these dimensions were determined prior to emplacement of the lava. The narrow width of the marginal zones in the entablature and chilled margin is a result of rapid quenching. The sigmoidal variation in radii of the marginal zones between 15 and 120 m is remarkably similar to incompatible element profiles, that is, where these elements are low, margins are thin, and where high, margins are thick. The incompatible element concentrations have been interpreted to reflect the distribution of residual liquid that was expelled upward from the lower part of the flow by compaction. In the zone of compaction, where the amount of residual liquid is small, the overgrowth on the phenocrysts is small. Higher up (zone of dilation), the flow is enriched in residual liquid and overgrowth widths increase in proportion to the amount of liquid added. Marginal widths of phenocrysts therefore provide a measure of the degree of compaction and dilation in the Holyoke basalt.