PALISADES SILL MODAL LAYERING RESULTING FROM COMPACTION

The MELTS program (Ghiorso and Sack, 1995) indicates that the Palisades magma is pyroxene-saturated in the lower part of the sill and plagioclase-saturated in the upper part. Near 100 m above the base of the sill the magma is saturated in both pyroxene and plagioclase, and it is in this cotectic mixture that the rock develops rhythmic cm-scale modal layering. The layering consists of couplets with an overlying plagioclase-rich layer and an underlying pyroxene-rich layer. Although layers differ in the degree of modal differentiation, the sum of each couplet is equivalent to the homogeneous rock composition; that is, pyroxene-rich layers are enriched in pyroxene by the same amount that plagioclase-rich layers are depleted in pyroxene. Plagioclase crystals in plagioclase-rich layers show a foliation that dips westward at a slightly shallower angle than the dip of the sill, thus creating an imbrication toward the west (down dip). In contrast, plagioclase crystals in pyroxene-rich layers show no significant alignment. Crystal size distribution of plagioclase indicates that crystals in both layers are almost identical. The degree of crystal alignment indicates up to 20% compaction in the plagioclase-rich layers, where in addition, the incompatible element concentrations reach minimum values.

We propose that the modal layering of the type found in the Palisades sill is caused by the redistribution of phases in a near-cotectic plagioclase-pyroxene crystal mush in response to compaction forces. Redistribution of pyroxene in this mush allows flat plagioclase crystals in resulting plagioclase-rich layers to reorient more easily than they would in homogeneous crystal mush. Thus, compaction is made more efficient by segregating the phases into layers. The more differentiated the couplet, the stronger the plagioclase crystal alignment. Plagioclase crystals in the pyroxene-rich layers become cemented in place by pyroxene, thus preventing significant alignment in these layers. The slight imbrication of plagioclase crystals in plagioclase-rich layers is believed to result from compaction on a down-dip sloping surface.