ANALYZING THE COMPLEX TEXTURE OF A SIMPLE DIABASE: PALISADES SILL, NEW JERSEY

Diabase is commonly thought to have a simple ophitic texture, but careful study of the Palisades Sill, New Jersey, indicates that this diabase has a complicated texture that reflects the processes responsible for the differentiation of the sill. Textural and chemical variability between the upper chilled margin and lower portions of the sill can be used to show that partly crystallized material was transported from the roof to the floor during solidification of the magma sheet.

The texture of the upper chilled margin consists of ophitically intergrown pyroxene and plagioclase clusters. Long, tapered plagioclase crystals radiate from the center of each cluster and become less calcic toward the rim. The pyroxene, a sub-calcic augite zoned from a Mg-rich core to more Fe-rich rim, have compositions falling within the pyroxene solvus, indicating disequilibrium crystallization. Sheets of ophitically intergrown clusters, with radiating crystal laths, are visible in thin section and resemble tiny feather boas or tinsel. Diabase in the central part of the sill is intergranular and consists of patches of small granules of augite rimmed by long, euhedral crystals of inverted pigeonite. Plagioclase-crystal chains surround the pyroxene patches, forming a branching 3-D network.

We propose that crystal mush in an upper solidification front sank to the floor where ophitic clusters recrystallized to form the intergranular diabase in the lower portion of the sill. Sinking plumes of coherent sheets of pyroxene-plagioclase clusters and trapped liquid provide a mechanism for transporting negatively buoyant plagioclase crystals to the floor. Some of this diabase exhibits cm-scale modal layering, chemical analyses of which, when processed with MELTS, indicate liquidus temperatures that oscillate around the plagioclase-augite cotectic within a few degrees. The layered diabase has a 10° higher liquidus temperature than the upper chilled margin, which could be explained by the removal of some interstitial liquid through compaction of crystal mush. Compaction at this level of the sill is estimated to be ~15%. Although the mechanism producing the layering is unknown, compaction may have played a role by dissolving and redistributing material during recrystallization, in a manner similar to that suggested by Boudreau (2000).