Geochemical reversals within the lower 100 m of the Palisades sill, New Jersey

Abstract

Transects through the lower part of the Palisades sill were made at Fort Lee and Alpine, New Jersey in order to characterize the petrologic signature of previously proposed “reversals” in the normal, tholeiitic differentiation trend. Petrographic and geochemical data include: (1) modal and grain size analyses, (2) bulk rock major and trace element concentrations by DCP-AES, and (3) augite, orthopyroxene, magnetite, and olivine compositions by electron microprobe analysis. Anomalous horizons, defined by increased bulk rock Mg#, Cr, Ni, and Co concentrations and abrupt modal and grain-size changes, occur at 10 m (the well known olivine zone), 27 m, 45 m, and 95m above the basal contact. Thermal models coupled with estimates of the emplacement rate and total magma volume indicate that the olivine zone (OZ) is an early-stage feature, related to the emplacement of initial magma into the Palisades chamber. Stoke's Law calculations indicate that the settling velocity of average-sized olivine crystals in a high-titanium, quartz-normative (HTQ) magma is too slow for significant gravity settling to have occurred prior to the solidification of the basal 20 m of the sill. It is suggested that the OZ resulted from the emplacement of a heterogeneous initial magma from a compositionally stratified, sub-Palisades storage chamber located within the upper crust; however, heterogeneity may have been derived directly from the mantle or during rapid ascent. Geochemical models indicate that the OZ contains accumulated olivine that is not in cotectic (or constant) proportions with the other cumulus phases, suggesting a mechanical sorting process. Magma chamber recharge is proposed to have occurred at the 27 m and 45 m levels, when a slightly more-primitive HTQ magma was injected into the Palisades sill chamber. Zones of elevated Mg# and Cr, 6 to 10 m thick, at these two horizons may indicate the thickness of the hybrid magma formed by the mixing of these two compositions. Geochemical models indicate that the rocks at these levels have accumulated excess orthopyroxene relative to samples from the rest of the sill. Normal faulting in the Fort Lee area at the 95 m level has caused repetition of the stratigraphic section, and hence, the sharp reversal observed at this level.