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Flow Differentiation, Phenocryst Alignment, and Compositional Trends Within a Dolerite Dike at Rockport, Massachusetts

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Flow Differentiation, Phenocryst Alignment, and Compositional Trends Within a Dolerite Dike at Rockport, Massachusetts Flow differentiation, phenocryst alignment, and compositional trends within a dolerite dike at Rockport, Massachusetts MARTIN E. ROI3S Department of Earth Sciences, Northeastern University, 360 Huntington Avenue, Boston, Massachusetts 02115 ABSTRACT Systematic variations in the volume per- cent, size, and orientation of plagioclase phe- nocrysts up to 12.0 cm long occur across a 5.6-m-thick porphyritic, alkaline dolerite dike in Rockport, Massachusetts. Field measure- ments indicate that phenocryst concentra- tions increase froim nearly zero at the dike margin to 46.0 vol. % at its center. Average phenocryst size increases inward from 4.1 x 2.2 mm at the dike margin to 19.2 * 7.9 mm at the center. The increase in size and abun- dance 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 strike, 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 I he dike. Average pheno- cryst strike deviations from dike strike in- crease inward 21.8° from the dike margin to Figure 1. Map showing location of the porphyritic dolerite dike exposed as en echelon its midpoint. Phenocryst dip-angle deviations segments (B, C, and D) along the shoreline of Rockport, Massachusetts. Locality B is the from dike dip increase inward by 18.8°. This Headlands segment, subject of this investigation. Locality A is a very similar and related dike more pronounced Clow alignment of pheno- referred to in text. Dotted line extensions of the dike segments are inferred. Possible faults crysts nearer the dike margins is interpreted (dashed lines) are from Dennen (1976). as being a function of the more extreme veloc- ity gradients (and resulting shear due to flow) Si02, KzO, FeO, TiOz, MgO, and MnO de- melts increasingly depleted of silica and in- within the marginal zones of the magma than crease. This inward decrease in certain ox- compatible elements; tapping of a zoned within its interior. ides, together with an inward (within-dike) magma chamber; and greater crustal contam- At least the outer few centimetres of decrease in the anorthite content of the cores ination of marginal liquids. Superposed on phenocryst-free chilled dike margins may of plagioclase phenocrysts, cannot be attrib- the chemical trends, the inward decrease in form primarily by rapid quenching of pheno- uted to flow differentiation. The inward- plagioclase anorthite content resulted from cryst-free magma rather than by flow differ- decreasing chemical trends are believed to fractional crystallization. entiation. have resulted from one or a combination of Whole-rock, major- and minor-element several of the following processes: the con- INTRODUCTION trends across this dike may be produced, in centration of silica and incompatible elements part, by flow differentiation. A1203, CaO, in glass (now devitrified) within chilled dike A coarsely plagioclase-phyric, alkaline, al- P2O5, and Na20 increase inward with plagi- margins compared to interiors; plagioclase tered dolerite dike is exposed as en echelon ¡seg- oclase phenocryst concentrations, whereas fractionation; continuous tapping of partial ments intruding the Cape Ann Complex (Oido- Geological Society of America Bulletin, v. 97, p. 232-240,9 figs., 2 tables, February 1986. 232 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/2/232/3434773/i0016-7606-97-2-232.pdf by guest on 28 September 2021 FLOW DIFFERENTIATION, DOLERITE DIKE, MASSACHUSETTS 233 vician or Silurian to Devonian; Zartman and Marvin, 1971; Zartman, 1977) along 2.3 km of the coastline of Rockport, Massachusetts (Fig. 1). This dike is readily recognized in the field because of its abundant, large (as long as 120 mm) plagioclase phenocrysts and meg- acrysts; the latter term is arbitrarily restricted to grains longer than 30 mm. The dike's northwest trend is in marked contrast to the predominant northeast trend of Mesozoic dikes in New Eng- land. It has a K-Ar biotite age of 351 ± 13 m.y. (Weston Geophysical, 1977); at the Headlands, it is cut by a thin, northeast-trending tholeiitic dolerite of probable Mesozoic age. The dike was mapped initially by Shaler (1889) and later by Dennen (1976); more recently it has been in- cluded in an ongoing field, petrographic, and geochemical investigation of the mafic dikes of eastern Massachusetts (Ross, 1981a, 1981b, 1984a, 1984b, 1985; Ross and Reidel, 1982, 1983). This dike was selected for further detailed study to quantitatively examine the possible ef- Figure 2. View north slightly oblique to the strike of the dike cutting Cape Ann Granite at the fects of flow differentiation on the distribution Headlands (Fig. 1, locality B). Erosional chasm and rubble within it has removed and covered and flow alignment of plagioclase phenocrysts the west (left) margin of the dike. The dike is 5.57 m thick here (contacts shown by arrows). and megacrysts across its width and to deter- The joint surface forming east (right) wall of chasm contains large, equant phenocrysts referred mine the degree to which plagioclase and whole- to in text. rock chemical compositions are affected. The study locality is at the promontory known as "the Headlands" immediately east of the en- trance to Rockport Harbor (Fig. 1, point B). This locality provides excellent exposure of the dike in both plan view and cross sections (trans- verse and longitudinal) produced by ocean- wave erosion (Figs. 2-4). A deep chasm eroded into the western part of the dike prevented sampling and measurements within ~50 cm of the west contact (Fig. 2). The dike is 5.6 m thick and strikes N7°-14°W and dips 88°E. The dike consists of plagioclase (An3&_67), augite, biotite, magnetite, and ilmenite; apatite, quartz, and zircon are minor accessories. Deu- teric and, perhaps, hydrothermal alteration in- cludes moderate sericitization and saussuritiza- tion of plagioclase, thorough uralitization of most augite grains, chloritization of biotite and augite, and minor interstitial "chlorite." To the north, across Sandy Bay (Fig. 1, point C), the dike thickens to 8.2 m, strikes N17°W and dips 74°E. A 1-m-thick dike of similar tex- tural, mineralogic, and major-element chemical properties is exposed in a roadcut -12 km southwest of the Headlands (Fig. 1, point A). It strikes N37°W and dips 88°E and probably was comagmatic with the dike under study. Detailed field analyses of the variations in Figure 3. View north along dike strike, showing inward (right to left) increase in phenocryst phenocryst size and abundance across the dike size and abundance on nearly horizontal outcrop surface in east-central part of dike. Subtle and measurements of their orientations relative alignment of elongate phenocrysts with dike strike is also shown. Joint intersection at left to that of the dike were made. Twelve samples (west) is 239 cm from east dike contact. The shorter increments on the lower half of the scale collected at intervals inward from the east con- are centimetres; inches are shown on upper half (scale is resting against a pebble). Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/97/2/232/3434773/i0016-7606-97-2-232.pdf by guest on 28 September 2021 234 M. E. ROSS Figure 4. View of north-facing, subvertical outcrop surface oriented at high angle to dike trend. Phenocryst alignment shown represents phenocryst "dips" subparallel to that of the dike. The scale (centimetres on left, inches on right) is parallel to the 88° dip of the dike iind is located -185 cm from the east (left) contact. Possible origins of this westward skew of grain sizes will be presented below. PHENOCRYST ORIENTATIONS RELATIVE TO DIKE ATTITUDE A weak alignment of elongate phenccrysts with the strike of the dike can be seen on nearly horizontal outcrop surfaces (Fig. 3). A more subtle alignment of elongate phenocrysts with the dip of the dike can be seen on nearly vertical tact were examined in thin section, and eight In thin section, the center of the dike contains outcrop surfaces that cross the dike at high angle were analyzed for major-element bulk-chemical 53.1 vol. % phenocrysts compared to 6.7% phe- to its strike (Fig. 4). composition. nocrysts and microphenocrysts (< 1 mm) at the An attempt to quantify these trends was done east contact. in the field by measuring the strikes and dips of VARIATIONS IN PHENOCRYST the phenocrysts with a Brunton compass. The ABUNDANCE VARIATIONS IN PHENOCRYST SIZE minimum grain-cross-sectional length for which reasonably accurate measurements could be Variations in the volume percent of plagio- The lengths and widths of 1,100 phenocrysts made was determined by trial and error as being clase phenocrysts (and megacrysts) were deter- (50/600-cm2 unit area) were measured across 5.0 mm. The strikes of all elongate phenocrysts mined in the field by counting 861 points within the dike in the field. A representative range in at least 5.0 mm in length were measured in each each of 20 unit areas (200 cm2 per unit area) sizes was selected for measurement in each area; of the 600-cm2 unit areas for which grain sizes using a 0.5 * 0.5 cm grid drafted on a transpar- smallest, intermediate, and largest phenocrysts were measured. A total of 1,031 strikes was ent plastic sheet. Unit counting areas were were selected in approximate proportion to their measured (23-107 grains per unit area). spaced an average of 6.0 cm apart across the modal abundance. The larger unit areas Similarly, phenocryst dips were measured for dike on nearly horizontal outcrop surfaces.
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