EFFECTS OF METASOMATISM AND HOST ROCK FUSION ON THE CHEMISTRY OF EARLY JURASSIC PALISADES DIABASE IN THE NEWARK BASIN

In general the ITi basalt population of early Jurassic CAMP continental flood basalt is remarkably homogenous. This is particularly true of extrusive basalt flows that are less susceptible to hydrothermal alteration and contact metasomatism during crystallization than co-magmatic intrusions. Important new data indicate that significant contamination of ITi magma has occurred due to two different processes (metasomatism and fusion). Metasomatism near the contacts of a major ITi intrusion (the Palisades sill) has resulted in K₂O enrichment and Na₂O depletion of the chill zone wherever it has been analyzed (Gorring and Naslund, 1995; Gottfried, et al. 1991; Walker, 1969; and Shirley, 1987) compared with extrusive co-magmatic ITi basalt (Orange Mountain basalt) at equal TiO₂ levels wherever it has been analyzed (Puffer, 1992; Tollo and Gottfried, 1992). Palisades chill zone K₂O levels range from 0.6% to 0.9% compared with a range of 0.4% to 0.5% for the Orange Mountain basalt. Palisades chill zone Na₂O levels range from 1.7% to 2.1% compared with range of 2.3% to 2.7% for the Orange Mountain basalt. These data agree with typically depleted K₂O (~0.5 %) and enriched Na₂O (~7.0 %) in the hornfels of the sedimentary country rock compared with country rock that has not been metasomatized (~3-6 % K₂O and 6-7 % Na₂O), (Benimoff and Puffer 2005). Fusion of xenoliths of country rock together with some magma mixing best explains the chemistry of some diabase layers within the Palisades sill that have been interpreted as the result of extreme fractionation. When the chemical composition of host rock hornfels is compared with these diabase layers similar compositions are seen, particularly CaO (1-4 %), MgO (1-4%), and Zr (330-520 ppm). The chemical composition of these layers contrast with adjacent diabase layers that present difficult fractionation modeling problems. Fusion of xenolith slabs is offered as a simplified mechanism in explaining the composition of these anomalous layers.