Compositional Variations in the Pyroxenes of the Differentiated Palisades Sill, New Jersey

The Palisades Sill is a multiple intrusion of oversaturated tholeiitic magma which differentiated largely through fractional crystallization. This differentiation followed a normal course of strong iron enrichment and slight alkali enrichment, then fayalite granophyre formed as the trend changed to one of alkali and silica enrichment in the very late fractionation stages.

The ferromagnesian mineral series of this fractionation sequence is augite/mauve-brown ferroaugite (Wo₃₈ En₅₁ Fs₁₁ to Wo₃₈ En₁₀ Fs₅₂), pale green ferroaugite (Wo₄₄ En₂₇ Fs₂₉ to Wo₄₄ En₁₀ Fs₄₆), pigeonite (Wo₈ En₅₄ Fs₃₈ to Wo₁₂ En₃₄ Fs₅₄), bronzite/hypersthene (Wo_4.5 En₇₅ Fs_20.5 to Wo₃ En₅₈ Fs₃₉), and ferrohypersthene (Wo₃ En₃₄ Fs₆₃ to Wo₄ En₂₆ Fs₇₀). Associated with these pyroxenes are olivine (Fo₇₇ to Fo₅₅ and Fo₁₃ to Fo_4.5), biotite whose Fe/Mg goes from 1 to 23, and hornblende whose Fe/Mg is between 5 and 10.

The compositional trends of the pyroxenes in the pyroxene quadrilateral are more complete than any previously recorded from a single intrusion of Fe-rich tholeiitic magma. The normal augite/mauve-brown ferroaugite trend shows only moderate variation in Ca content (Wo₃₃ to Wo₃₈) without any inflection in the trend at the limit of the two-pyroxene field. The bronzite/hypersthene and pigeonite series are fairly normal, except that the pigeonite inversion is nearer En₆₀ than the En₇₀ found in most other tholeiitic intrusions. The most interesting aspects of pyroxene behavior are in late-middle and early-late stages of fractionation, where transformation of the mauve-brown ferroaugite in its final stages of crystallization led to the formation of a separate more Ca-rich ferroaugite series with a trend nearer the Di/Hd join than the mauve-brown ferroaugite trend, and where reactions in the Ca-poor series continued as cooling proceeded into the subsolidus, so that in the early-late stages ferriferous pigeonite inverted to ferrohypersthene while ferriferous orthopyroxene was crystallizing directly from the magma. This orthopyroxene then became unstable in the late stages, and the reaction clinopyroxene + orthopyroxene + liquid → clinopyroxene + olivine + quartz + liquid took place. With further iron enrichment in the magma, ferrohortonolite changed to fayalite, and the coexisting ferroaugites moved toward ferrohedenbergitic compositions. In the final stages, where silica and alkali enrichment became more pronounced, hornblende largely replaced the ferroaugites in the granophyric dolerite.