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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.

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