Differentiation and Compaction in the Palisades Sill, New Jersey

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Abstract
The Palisades Sill was studied chemically and petrographically in order to elucidate its differentiation mechanism. Neutron activation analysis was performed on 53 whole rock samples from a 342-m thick section located near the George Washington bridge. Data for 31 elements in these samples are presented here. The study revealed that at least three and probably four pulses of magma were intruded into the sill. Following the final pulse the remaining two-thirds of the thickness of the sill differentiated primarily through inefficient fractional crystallization in which cumulate material contained 40–50 per cent interstitial magma. In addition to differentiation by fractional crystallization, postcumulus transport of interstitial magma in the Palisades cumulates played a significant role. Crystallization of the last of the main body of magma produced a peak in incompatible clement concentrations at a sandwich horizon located at the 300 m level, where the floor and roof of the sill converged. At the time this enriched zone crystallized, the underlying 150 m of cumulus material still contained interstitial melt Subsequent buoyancy driven upward transport of interstitial melt in this zone concentrated incompatible elements 45 m below the main peak.

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