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

Abstract

Systematic variations in the volume percent, size, and orientation of plagioclase phenocrysts up to 12.0 cm long occur across a 5.6-m-thick porphyritic, alkaline dolerite dike in Rockport, Massachusetts. Field measurements indicate that phenocryst concentrations increase from nearly zero at the dike margin to 46.0 vol. % at its center. Average phenocryst size increases inward from 4.1 × 2.2 mm at the dike margin to 19.2 × 7.9 mm at the center. The increase in size and abundance of phenocrysts toward the center of the dike is interpreted as resulting from flow differentiation. The magma-flow direction is assumed to have been upward and parallel to the dike margins (N7°W stiike, 88°E dip). The strikes and dips of all elongate phenocrysts (viewed in cross section) within one traverse across the dike were measured and compared to the dike attitude to determine the degree of flow alignment across the dike. Average phenocryst strike deviations from dike strike increase inward 21.8° from the dike margin to its midpoint. Phenocryst dip-angle deviations from dike dip increase inward by 18.8°. This more pronounced Clow alignment of phenocrysts nearer the dike margins is interpreted as being a function of the more extreme velocity gradients (and resulting shear due to flow) within the marginal zones of the magma than within its interior. At least the outer few centimetres of phenocryst-free chilled dike margins may form primarily by rapid quenching of phenocryst-free magma rather than by flow differentiation. Whole-rock, major- and minor-element trends across this dike may be produced, in part, by flow differentiation. A1 2 O 3 , CaO, P 2 O 5 , and Na 2 O increase inward with plagioclase phenocryst concentrations, whereas SiO 2 , K 2 O, FeO, TiO 2 , MgO, and MnO decrease. This inward decrease in certain oxides, together with an inward (within-dike) decrease in the anorthite content of the cores of plagioclase phenocrysts, cannot be attributed to flow differentiation. The inward-decreasing chemical trends are believed to have resulted from one or a combination of several of the following processes: the concentration of silica and incompatible elements in glass (now devitrified) within chilled dike margins compared to interiors; plagioclase fractionation; continuous tapping of partial melts increasingly depleted of silica and incompatible elements; tapping of a zoned magma chamber; and greater crustal contamination of marginal liquids. Superposed on the chemical trends, the inward decrease in plagioclase anorthite content resulted from fractional crystallization.