Crystal-Mush Compaction and the Origin of Pegmatitic Segregation Sheets in a Thick Flood-Basalt Flow in the Mesozoic Hartford Basin, Connecticut

Where the Holyoke flood-basalt flow in the Mesozoic Hartford Basin in Connecticut is thick and contains coarse-grained, horizontal segregation sheets in its central part, the lower part of the flow is strongly depleted in incompatible elements; where the flow is thin and contains no segregation sheets it is homogeneous throughout. This chemical variation can be explained only through compaction of the partly crystallized basalt. The composition of the segregation sheets shows that they separated from the basalt following only 33% crystallization. The segregation sheets, however, are clearly intrusive into the basalt, which must therefore have already formed a crystal mush with finite strength at this low degree of crystallinity. The incompatible element concentrations indicate that the partly crystallized basalt underwent as much as 28% compaction in the lowest 60 m of the flow. Between 60 and 130 m above the base of the flow, the crystal mush became dilated, and eventually ruptured with formation of the segregation sheets. No segregation sheet has a composition indicating separation after more than 33% crystallization of the basalt. This is interpreted to indicate that compaction ceased at this stage because of the increasing strength of the mush and the increasing density of the fractionating interstitial liquid