SODIUM CONTAMINATION OF NEWARK BASIN BASALTS

Major portions of very thick Hook Mountain, Preakness, and Orange Mountain basalt flows exposed throughout northern New Jersey have been albitized and are characterized by Na₂O levels within a 3.0 to 5.0 percent range that exceeds co-magmatic Palisades sill levels by up to 3.0 weight percent. These sodic levels are not accompanied by evidence of fractionation beyond average unalbitized flow portions or evidence of significant sediment assimilation. We propose that the albitized portions of these flows reacted with basin brine released from underlying sediments during loading caused by rapidly accumulating volcanism and sedimentation. During dissolution of calcic plagioclase and precipitation of albite most excess aluminum re-precipitated as chlorite whereas most calcium re-precipitated as carbonates and sulfates in vesicles at lower temperatures. Albitization was accompanied by highly erratic behavior of Ca, K, Ba, Sr, Cu, and Zn although most elements typically were not affected. Secondary albite also grew quickly in glass generating fine grains that resemble primary quench plagioclase. A similar process, at a much smaller scale, is occurring in active geothermal systems such as those of Iceland where high temperature geothermal waters are undersaturated in calcic-plagioclase but are close to equilibrium with albite-oligoclase (Stefansson and Arnorsson, 2000). The Newark basin abitization process was highly selective, typically confined to upper flow vesicular and lower flow microvesicular layers. However, some albitized non-vesicular and mid-flow exceptions are problematic and may be the result of deuteric processes or contaminated inflation pulses. The lower Orange Mountain flow, in particular, seems to have been locally injected with brackish superheated steam that contaminated the downstream lower interior of a rapidly flowing lobe.