Detailed textural analysis of the Palisades sill, New Jersey

Date of Completion

January 2006






Regarded as a classic example of a vertically differentiated intrusion, the ∼300m thick Palisades sill of New Jersey has a thick floor zone and a thin roof zone and the rocks range in composition from Mg-rich diabase to Fe-rich diorite to granophyre. Differentiation of the sill is proposed to have occurred by two main igneous processes; transfer of crystal mush from the roof to the floor of the sill in the form of dense plumes and compaction of crystal mush in a thickening cumulate pile. ^ Rocks are pyroxene-phyric in the lower part of the sill, are of near-cotectic composition at the 100-m level, and are plagioclase-phyric in the upper part of the sill. This change in composition suggests a process of plume transfer of an originally pyroxene-saturated magma. Some neutrally buoyant plagioclase phenocrysts were lost from the earliest descending plumes only to be caught up in later descending plumes to form the plagioclase-rich rocks in the upper part of the sill. ^ Plumes of crystal mush that deposited on the floor generated a thick cumulate pile that underwent compaction, expelling residual liquid upward to form coarse-grained segregation sheets at high levels in the sill. Bent and broken crystals provide physical evidence of compaction of crystal mush, and the degree of compaction is estimated by directly measuring the deformation of the plagioclase-chain network present in the rocks. ^ Cm-scale modal layering observed at the 100-m level develops in rocks with a near-cotectic composition. Each couplet in the layering is defined as an overlying plagioclase-rich layer and an underlying pyroxene-rich layer. Detailed analysis shows that the modal layers are more complicated than variation in the abundance of pyroxene and plagioclase. Plagioclase-rich layers contain plagioclase crystals that are imbricated and specific types of pyroxene that are present in the pyroxene-rich layers are absent in the plagioclase-rich layers. Modal layering of the type in the Palisades ill is proposed to form from the redistribution of pyroxene in response to compact on along the base of deposited plumes of crystal mush. Pyroxene dissolves and diffuses downward to form a pyroxene-rich layer below and leaving a plagioclase-rich layer above. ^