TIMING OF CRYSTAL-MUSH COMPACTION IN THE PALISADES SILL, NEW JERSEY

Regarded as a classic example of a vertically differentiated intrusion, the ~300-m thick Palisades sill of New Jersey has a thick floor zone and a thin roof zone, and the rocks range in composition from Mg-rich diabase to Fe-rich diorite to granophyre. Thick cumulate piles of crystal mush in the floor zone are thought to have undergone compaction, expelling residual liquid upward to form coarse-grained segregation sheets with elevated concentrations of incompatible elements at high levels in the sill. The degree of compaction has been estimated based on the chemistry of the rocks (18.6%) and by directly measuring the deformation of the plagioclase-chain network present in the rocks (3.8 – 5.2%). Bent and broken crystals provide physical evidence of compaction of crystal mush. Broken pyroxene crystals in a sample collected at 64 meters above the base of the sill have faulted augite cores that are encased with inverted pigeonite rims. The augite core shows displacement along a crack in the crystal. The timing of compaction can be modeled using the chemical composition of broken and faulted pyroxene crystals and the whole-rock chemical analyses. The chemical analyses, used with the MELTS software package (Ghiorso and Sack, 1995), a computer program that uses thermodynamic equations to model phase equilibria in magma over a given range of temperature, will show the density and mass of solids and liquids, magma viscosity, and composition of mineral phases at the time that the pyroxene crystal was faulted. The timing of compaction determined by analysis of broken and faulted pyroxene crystals will be presented.