MAGMATIC COMPACTION IN THE HOLYOKE FLOOD-BASALT FLOW, CT, AS INDICATED BY TEXTURAL ANISOTROPISM AND CHEMICAL VARIATION

The texture in the lower part of thick flood-basalt flows, which differs from that in the downward solidifying upper part, can be interpreted as resulting from recrystallization of material that sank from the roof zone where crystals initially grew rapidly with disequilibrium compositions. The texture consists of patches of granular pyroxene crystals with equilibrium compositions surrounded by chains of plagioclase crystals that form a 3-D network. Because the bulk density of this bottom-accumulating crystal mush is greater than that of the residual liquid, the mush undergoes compaction, with the residual liquid being expelled upward. Compaction depletes the mush in incompatible elements and simultaneously deforms the plagioclase chain network, leaving both a chemical and physical record of compaction.

Three independent methods of measuring the anisotropism of the plagioclase network in the lower part of the 200-m-thick Mesozoic Holyoke flood-basalt flow of Connecticut indicate a compaction profile that is remarkably similar to that indicated by the incompatible element distribution. From digitized tracings along center lines of plagioclase chains in oriented thin sections, the anisotropism of the network is determined by measuring: 1) the average intercept distance between chains along oriented traverses, 2) the orientation distribution of line segments forming the chains, and 3) the average ellipse fitted to polygons outlined by the chains. In the compaction zone, all methods indicate a compaction direction that is within a few degrees of the original vertical, as defined by the columnar joints. Compaction is first detectable ~20 m above the base of the flow and reaches a maximum of ~13% at a height of 43 m. It then decreases upward to the zone where chemical analyses indicate enrichment in residual liquid. Here the texture is still anisotropic, but the network becomes less regular and the compaction direction is inclined at significant angles to the vertical. Moreover, the principal directions of anisotropism indicated by method 3 can be significantly different from those indicated by the other two methods in this zone. In method 3, these directions can be strongly influenced by the presence of just a few large polygons, such as those outlining pipe-like patches of residual liquid found in this part of the flow.