GEOCHEMICAL MODELS FOR THE DIFFERENTIATION OF THE PALISADES SILL, USA, BY SLURRY INJECTION, ROOF TO FLOOR THERMAL TRANSFER, IN-SITU CRYSTALLIZATION, AND INTERSTITIAL MELT MIGRATION

The 275-300m thick Palisades sill has been examined along a traverse in Fort Lee, NJ. 355 samples were analyzed for major & trace elements by DCP-AES & ICP-MS. Aphanitic chilled margins at the roof & floor are high-Ti quartz tholeiite (Mg#=56). The sill is strongly differentiated with an S-shaped Mg# profile (max=73, min=17). Breaks in the Mg# trend occur at ~10, ~27, ~48, & ~95m. The break at 10m is the well-known olivine horizon that represents the injection of one or more post emplacement crystal-liquid slurries with olivine or olivine + augite phenocrysts. The breaks at 27m & 48m are small recharge horizons; the break at 95m is a fault.

Pearce Element Ratio Analysis (PER) indicates that the upper ~25% of the sill is a roof sequence dominated by samples that represent chill - crystals and the lower 75% of the sill is a floor sequence dominated by samples that represent chill + crystals. The roof sequence has an average fractionation assemblage of 53.5% Plag + 29% CPX + 17.5% OPX (or Pig) with ~An63 & Mg#75, and the floor sequence has an average accumulation assemblage of 38% Plag + 29.5% CPX + 32.5% OPX (or Pig) with ~An71 & Mg#78. Summation and Least Squares Fit models indicate little change in Si, Co, or Sc; a strong decrease in Cr, Mg, Ca, Eu, & Ni; a strong increase in Fe, Ti, K, Ba, V, Zr, Hf, Nb, U, Th, & REEs (except Eu); and a slight increase then decrease in Al, Sr, & Na during differentiation. Early fractionation (initial magma to 60%) of 40% Plag + 33.5% CPX + 27.5% OPX (or Pig.) is similar to the PER analysis of the floor, and late fractionation (initial magma to final 10%) of 52% Plag + 30% CPX + 18% OPX (or Pig.) is similar to the PER analysis of the roof. The roof sequence appears to have been injected with lenses of late stage magma. The final liquid represents ~68% fractionation of the initial magma.

Conclusions: The Palisades sill differentiated by in-situ crystal fractionation dominated by Plag>CPX>OPX (or Pig.). The An% & Mg# decreased, and Plag/CPX & CPX/OPX increased, with differentiation. The roof sequence, in general, represents magma from which crystals have been removed, and the floor sequence, in general, represents magma to which crystals have been added. Heat loss at the roof was effectively transferred to the floor indicating a convection process. Upward migration of interstitial magma was an important aspect of the differentiation.