Flow differentiation, phenocryst alignment, and compositional trends within a dolerite dike at Rockport, Massachusetts

Across a 5.6 m-thick porphyritic, alkaline dolerite dyke, average phenocryst size, abundance and angular deviation from dyke attitude and flow direction all increase inwards from the margins. These trends can be attributed to flow differentiation and the more extreme velocity gradients (increasing inwards) within the portion of magma flowing near the walls of the conduit. The plug-like velocity profile across a dyke produces higher strain-rates near its margins reaching zero within its interior, resulting in observed phenocryst alignment. The effect of flow differentiation on the whole-rock chemistry is the sympathetic inward increase of Al (sub 2) O (sub 3) , CaO, Na (sub 2) O and, indirectly, P (sub 2) O (sub 5) with plagioclase phenocryst concentration. SiO (sub 2) , MgO and MnO trends (decreasing inwards) appear to be unrelated to flow differentiation and may reflect differences in crystallization histories, initial glass contents, incompatible elements and SiO (sub 2) contents of parent melts, or difference in degree of crustal contamination across the dyke.