Petrographic, structural, and whole-rock geochemical data from eight diabase sheets and one associated dike in the central Newark basin indicate a common petrogenesis. The petrogenetic model also can be applied to the Palisades sill in the northern Newark basin, that together with some of the central Newark basin diabases make up a single “megasheet” extending for ~150 km. Regional chill-margin compositions are extremely constant and are high-titanium, quartz-normative tholeiites (HTQ) typical of the Mesozoic eastern North America (ENA) magmatic province. On the basis of their compositional uniformity and their close similarity to many other ENA-HTQ basalts, the HTQ chills are believed to best approximate the typical diabase parental magma composition. A few coarse-grained samples may have been derived from an ENA low-titanium, quartz-normative tholeiite (LTQ) basalt. Mass-balance models show that the early fractionation of the HTQ parent was dominated by clinopyroxene with lesser amounts of orthopyroxene and plagioclase; accumulations of these phases in parental magmas produced MgO-rich compositions. Plagioclase fractionation became dominant after ~20%–25% crystallization; Fe-Ti oxide and apatite fractionation increased dramatically at the latter stages of differentiation. No in situ olivine fractionation is required throughout the entire diabase differentiation sequence. Residual granophyric compositions were produced by ~70%–80% total crystallization and they exhibit no geochemical evidence for being the result of large-scale crustal anatexis. However, a local, selective contamination by Triassic sedimentary wall rocks or xenoliths resulted in alkali enrichment and calcium depletion in a few diabase samples and the Quarry dike. Typically, diabase sheets with MgO-rich rocks lack abundant granophyres; conversely, granophyre-rich sheets usually lack any significant amounts of MgO-rich rocks. Granophyre-rich zones are consistently found at higher structural levels than exposures with MgO-rich units. This distribution is consistent with a dynamic fractionation model where low-density residual liquids are displaced both laterally and up-dip along a sheet. The injection of multiple magma pulses is recognized in at least one sheet exposure, and this process, along with cumulate compaction, could have been the driving force for the migration of residual liquids.

The Upper Nyack (UN) section of the Palisades sill is comprised principally of pigeonite-augite and augite diabases with an interstitial (remnant) glassy mesostasis. Major- and trace-metal analyses of 110 diabases, 20 being analyzed for rare-earth elements (REE), provide data regarding the source of chemical variations within the UN section and suggest a unifying theory, the propsed C-T-D Model, for differentiation within the Palisades sheet. Preliminary work establishes that the chemostratigraphic horizons present in the type section at Englewood Cliffs (ENG)are broadly continuous from south to north and that important petrographic facies have undergone substantial mineralogic reorganization. The three most significant S → N transitions include: (1) holocrystalline subophitic chilled diabase (ENG) → hyalosiderite chilled diabase (Piermont) → glomeroporphyritic to intersertal chilled diabase (UN); (2) olivine zone (ENG) → (augite diabase) [UN]) → bronzite diabase (Nyack Beach State Park); and (3) fayalite granophyre and related “sandwich” horizon facies (ENG) → altered (porphyritic) augite ferrodiabase (UN). Bulk compositional and trace-metal plots establish that south to north petrologic tiers comprise a single differentiation series. Differentiation series data are consistent with three-stage model that involves early olivine fractionation followed by pyroxene- and plagioclase-dominated sequences. The cumulus horizons are modeled principally in terms of the accumulation of pyroxene-dominated fractionation products by varying proportions of olivine fractionated or later liquid. It is proposed that crystals collect in convection cells or like environments at deep levels within the sheet are partially remelted, resorted, and/or recrystallized during epizonal emplacement due to: magma decompression, shallow level flow differentiation, and magma–wall rock interactions. The Cumulus-Transport-Decompression (C-T-D) Model, based on these observations, is advanced to integrate multiple differentiation factors within a single interpretive framework for Palisades sill evolution.

The Palisades Sill of New Jersey and New York states was emplaced in the Newark Formation in the Upper Triassic, and is about 1000 feet thick. Its form is typical of a hypabyssal sheet except for some dike-like features, which are mainly in the northern part of the intrusion. The sill is a multiple intrusion in which two magma phases of oversaturated tholeiite have been recognized. Olivine crystallized in appreciable amounts only after the emplacement of the second and larger phase. The accumulation of olivine enveloped in early plagioclase and augite determined the location of the famous hyalosiderite dolerite layer above the crystallized part of the first phase. The order in which the essential minerals began to crystallize in each magma phase was olivine, plagioclase, augite, and orthopyroxene. In the northern part of the intrusion, where the Mg-olivine layer is absent, the contact between the two magmas is marked by a reversal in the pyroxene fractionation trend. Equilibrium was established between the two magma phases early in the fractionation sequence. Fractional crystallization dominated the differentiation process, and progressive crystallization proceeded on a normal course to an advanced stage of iron-, silica-, and alkali-enrichment where fayalite granophyre and granophyric dolerite formed. The whole-rock analyses for the major and minor elements, and for B, Nd, Nb, Pb, and Sn, are presented. In addition, element partitioning and the behavior of elements with fractionation have been determined in the sill by studying the variations of Ca, Sr, Ba, Fe, Mn, Ti, Cr, V, Sc, Mg, Co, Ni, Y, Zr, Mo, La, Cu, and Ga in the rock and mineral series. The elements distribute and arrange themselves in crystal lattice sites during fractional crystallization according to their total chemical properties. It is not only the properties of the ion in the liquid, but also its properties within the crystal lattice, which determine its behavior, and the trace cations like the major cations show varying site preference according to their bonding characteristics in different lattice structures.