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 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 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 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 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 phenocryst strike deviations from dike strike increase 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 velocity 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

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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 megacrysts; 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 (transverse 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 includes 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 textural, 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).

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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. (600 cm2 compared to 200 cm2 for modes) 449 grains (40-100/600-cm2 unit area) at six Theoretical variances of these modal analyses were required to obtain sufficiently large popu- localities across the dike on the nearly vertical are 0.18 to 1.06, well below the upper limit of lations of phenocrysts for size (and orientation) outcrop surfaces. acceptability of 2.0 recommended by Solomon measurements. These larger unit areas were Grain strikes and dips were recorded sepa- (1963). centered on the smaller modal unit areas. As in rately for phenocrysts longer than 15 mm to de- The results are summarized in Figure 5. The the above modal trend, a pronounced gradual termine possible effects of grain size on degree of dike margin is generally phenocryst free (mega- increase in phenocryst size occurs inward from flow alignment. Strike and dip angles were sub- scopically) within -6.0 cm of the east contact the dike margins (Figs. 3 and 6). Mean cross- tracted from those of the dike (strikes N7°W, (the west contact hits eroded away) with only a sectional dimensions increase from 4.1 x 2.2 dips 88°E) to obtain deviations of phenocryst few, small (<5-mm) phenocrysts present locally. mm to 19.2 x 7.9 mm. As the longest megacryst orientations from dike attitude. Averages were Phenocryst abundance increases markedly at 6 (120 mm) observed did not occur along the calculated for each unit area and plotted against cm inward and averages 7.7 vol. % between 6 traverse, it is not shown in Figure 6. Grain position in the dike (Fig. 7). and 24 cm from the east contact (Fig. 5). Phe- lengths increase at a faster rate across the dike The average phenocryst strike deviation from nocryst abundance gradually increases to a max- than do grain widths (Fig. 6), the average dike strike increases inward from 19.1° and imum of 46.0% at the center of the dike. The length/width ratio increasing inward from 1.86 26.8° at the east and west margins, respectively, rate of increase is slightly lower in the central to 2.68. Average phenocryst dimensions are to 40.9° at the dike's midpoint; the total average zone of the dike, starting ~ 100 cm in from each 12.0 x 4.9 mm for the entire dike; those of the for all sizes is 25.4°. Phenocryst strike deviations contact (Fig. 5). This core represents -62% of west half are slightly coarser (12.9 x 5.2 mm) east and west of the dike strike average 25.2° the total dike width; its possible origin will be than those of the east half (10.6 x 4.4 mm). The and 25.1°, respectively. The average pheno:ryst discussed in a later section. The distribution is west half also contains a higher average fre- strike deviation is smaller for the east half symmetrical about the dike's center; its west half quency of phenocrysts longer than 15 mm (22.3 (20.1 °) than for the west half (31.3°) of the dike. averages 32.8% phenocrysts; the east half, 33.5%. per unit area compared to 15.6 for the east half). This closer alignment of phenocrysts and dike in

the east half is slightly more pronounced for grains longer than 15 mm (17.2° for east half, 30.6° for west half). VOLUME PERCENT The average deviations of phenocryst dips PHENOCRYSTS from dike dip also increase inward from 23.8° (38-72 cm from east contact) to 35.2° at the center of the dike (all sizes). Outcrop surfaces of suitable orientation and area for dip measure- I I I I 2I I ' ments closer to the east contact and in most of 0 120 160 200 240 J 240 200 160 0 the west half of the dike are absent (Fig. 7). EHST CENTIMETERS FROM CONTACTS WEST Phenocryst orientations (and sizes) were measured within a representative unit area on a Figure S. Variations in volume percent plagioclase phenocrysts across the Headlands dike nearly vertical joint face approximately parallel based on field modal analyses (see text for details). Dashed line indicates measurements not to the dike and 3.36 m from the east contact possible due to eroded or covered contacts or to the absence of suitable surfaces. Each data (Fig. 2). The most noteworthy feature of this point represents the mean value plotted at the center of a 200-cm2 unit area along the traverse. face is the presence of scattered, large, relatively equant phenocrysts (the four in the unit area measured ranged in size from 53 x 34 mm to 29 x 25 mm). Similar grains are extremely rare on joint faces and erosional surfaces oriented at high angle to the strike of the dike. There is, 16- furthermore, no obvious vertical alignment of elongate phenocrysts evident on this joint plane; MB AN 12- average phenocryst inclinations from the hori- PHENOCRYST DIAMETERS zontal are 55.9°N and 47.8°S (these are pheno- IN MM ( cryst "plunge angles" along the strike of the dike). The alignment of phenocrysts with dike attitude (documented above), exhibited on horizontal and vertical surfaces perpendicular to the dike, would not be evident on surfaces parallel

EAST CENTIMETERS FROM CONTACTS to the dike. The similar dike-parallel alignment of platey grains would result in their exhibiting Figure 6. Histogram showing the increase in average length (white bar) and width (black their equant cross sections on surfaces oriented bar) of phenocrysts with increasing distance from the margins of the dike at the Headlands. parallel to the dike, as is the case on the joint Numbers along top of histogram are the maximum individual phenocryst length (mm) mea- surface in question. 2 sured within each 600-cm unit area. Widths of bars reflect variations in widths of unit areas as The grain orientation data thus indicate that dictated by configuration of outcrop surfaces. phenocrysts are more nearly aligned with the dike nearer its margins than within its interior, especially in the east half of the dike (Figs. 6 and 7).

PETROGRAPHIC AND MINERALOGIC TRENDS

The chilled margin of the dike is holocrystal- line, aphanitic to microporphyritic, with microphenocrysts (1 mm or less) of plagioclase and scattered augite set in an aphanitic groundmass of plagioclase, augite, biotite, ilmenite, magnetite, apatite, and quartz. The groundmass coarsens noticeably within 2.0 cm of the dike contact. Interstitial chlorite may represent, at EAST CENTIMETERS FROM CONTACTS WEST least in part, altered palagonite. As documented above for plagioclase phenocrysts, groundmass Figure 7. Plot of variations in average phenocryst strike deviations (solid line) and average grain sizes increase gradually inward. The most dip deviations (dashed line) from the strike and dip of the dike. The smaller the angle, the more rapid increases are within ~ 10-20 cm of the east nearly aligned are the phenocrysts with the dike. Suitable outcrop surfaces were not present contact (west contact eroded). The groundmass closer to the dike margins than shown. of the dike interior is fine- to medium-grained

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phaneritic and locally (on scale of a thin section) ophitic and subophitic. Modal analyses (1,000 points each) of the center and margin of the dike are as follows (margin in parentheses): plagioclase phenocrysts, 51.8% (23.9%, including microphenocrysts); groundmass plagioclase, 22.0% (35.2%); augite, 12.9% (23.4%); biotite, 7.3% (11.0%); ilmenite and magnetite, 3.1% (3.9%); apatite, 2.8% (2.4%); quartz, 0.1% (0.2%); trace of zircon. Tex- tural evidence indicates the following paragene- sis: apatite + opaques + zircon followed by plagioclase phenociysts, then groundmass plagioclase + augite, then biotite + opaques followed by quartz. The most significant mineralogic trend is the decreasing plagioclase anorthite composition inward within the dike for the cores and edges of phenocrysts and groundmass grains (Table 1). Similar trends in plagioclase compositions occur in all mafic dikes that I have examined in eastern Figure 8. Plot of major-and minor-element variations across the dike. Data are from whole- Massachusetts, the Columbia River Plateau, and rock XRF analyses presented in anhydrous form normalized to 100%. Analyses were made the northern Bighorn Mountains of Wyoming with a Philips P.W. 1410 XRF spectrometer on fused glass beads by the Basalt Research (Ross and Heimlich, 1972; Ross, 1983; unpub. Laboratory of Washington State University. Data points shown as larger dots on Si02 plot are data). The origin of this trend is discussed below. olivine-normative; the rest are quartz-normative.

CHEMICAL TRENDS

TABLE 1. CROSS-DIKE VARIATIONS IN PLAGIOCLASE ANORTHITE CONTENT Whole-rock, major- and minor-element

chemical analyses of eight samples collected Position on Plagiodase anorthite contents across the dike along the same traverse as above, grain exhibit the following trends inward from the 0* 1 6 7 8 25 51 183 320 443 505

dike margins: SiO-, K20, FeO, Ti02, MgO, Phenocrysts and MnO decrease; AI2O3, P2O5, CaO, and Grain core 67 49 58 51 45 47 47 45 45 59 66 Grain edge 31 39 24 36 27 28 29 30 39 41 Na20 increase (Fig. 8, Table 2). The analyses Groundmass Grain core 53 39 34 30 38 33 3« 30 28 34 43 plot in the alkaline fields of the alkalies-silica Grain edge 26 27 24 23 24 26 30 diagram of Irvine and Baragar (1971) and the Sole: data determined by Michel-Levy method; dots indicate no suitable grains present. P205-Ti02 plot of Ridley and others (1974). 'Location of sample in centimetres (rounded off to nearest centimetre) from east contact of dike. Dike is 557 cm thick. The slight chemical variations across the dike (Fig. 8) are sufficient to change its normative composition from quartz-normative to slightly olivine-normative (Table 2). this summary, the relationships between the The Magnus effect arises from rotation in- trends described earlier and flow differentiation duced on individual grains by the motion of the DISCUSSION OF MODAL are considered. surrounding fluid. A resultant transverse force AND ALIGNMENT TRENDS At least three forces are involved in flow dif- produces an inward translation of the crystal ferentiation: the Bagnold effect (Bagnold, 1954; (Pao, 1961). Komar (1972a) considered the Previous workers have dealt primarily with Komar, 1972a), the Magnus effect, and the wall Magnus effect to be "one one-hundredth" that of the mechanics of flow differentiation without effect (Bhattachaiji, 1967; Pao, 1961). The the Bagnold effect within a 1-m-thick dike. considering petrologic implications in terms of Bagnold effect, produced when phenocrysts are The wall effect occurs within only about sev- cross-dike compositional trends (mineralogic sheared past one another in a flowing magma, is eral grain diameters of a dike contact ancl in- and chemical) or phenocryst flow alignment. believed to be by far the most important com- volves an "inward repulsion" arising from The higher concentration of increasingly larger ponent of flow differentiation (Komar, 1972a; mechanical interaction between the conduit wall phenocrysts toward the center of dikes or sills Barriere, 1976). The resulting grain-dispersive and adjacent crystals (Barriere, 1976). generally has been attributed to flow differentia- pressure is directed inward perpendicular to Komar (1972a, 1972b, 1976) concluded that tion (Bhattacharji, 1967; Komar, 1972a, 1972b, flow and proportional to the rate of shear the velocity profile across a 15.2-m-thick picritic 1976; Gibb, 1972; Barriere, 1976; refer to these (Komar, 1972b; Barriere, 1976). The effect re- dike would be plug-like with very steep velocity articles for detailed discussions of the process quires a phenocryst concentration in the flowing gradients (increasing inward) within the margi- which is summarized briefly below). Following magma to be greater than -5% (Komar, 1976). nal 0.5 m. This gradient would flatten slightly

TABLE 2. MAJOR-ELEMENT CHEMISTRY AND NORMATIVE COMPOSITION ACROSS THE ROCKPORT HEADLANDS DIKE

Centimetres from east dike contact (dike width = 557 cm)

0 5.1 25.4 50.8 182.9 320.0 435.0 533.0 2(7«

Si02 51.55 51.45 51.39 51.33 51.03 50.86 52.15 51.69 0.550 AI2O3 16.34 16.13 16.35 16.40 17.35 17.93 17.12 16.36 0.310 TiOj 2.59 2.66 2.56 2.54 2.46 2.41 2.36 2.63 0.050 Ft2°3 2.00 2.00 2.00 2.00 2.00 2.00 2.00 2.00 FeO 11.10 10.79 10.93 10.71 10.03 9.49 9.89 11.10 0.350 MnO 0.29 0.30 0.29 0.28 0.24 0.25 0.28 0.28 0.010 CaO 6.22 6.53 6.19 6.44 6.77 7.33 7.36 6.73 0.220 MgO 2.95 3.01 2.86 2.76 2.93 2.83 2.87 3.00 0.150 K2O 2.34 2.24 2.44 2.36 2.10 2.03 2.04 2.28 0.030 NajO 3.45 3.71 3.82 4.01 3.85 3.63 2.72 2.69 0.160 P205 1.17 1.19 1.18 1.16 1.24 1.23 1.20 1.23 0.014

Q 1.5 0.5 0.3 5.2 c Or 13.8 13.3 14.4 14.0 12.4 12.0 13.3 Ab 29.2 31.4 32.3 33.9 32.6 30.7 22.5 An 22.2 20.7 19.8 23.9 26.6 25.9 Ne f Wo 0.4 1.6 1.1 1.9 0.7 0.7 di- En 0.1 0.6 0.4 0.6 0.3 0.3 I Fs 0.3 1.3 0.3 0.3 [ En 7.2 6.9 4.7 6.5 hy- 6.8 7.5 * 14.7 13.2 12.6 9.8 11.7 11.8 14.8 Fo 1.1 0.4 Fa 2.5 0.8 Mt 2.9 2.9 2.9 2.9 2.9 3.0 n 4.9 5.1 4.8 4.7 4.6 5.0 Ap 2.8 2.8 2.8 2.8 2.9 2.9 2.8

Note.• analyses presented in anhydrous form normalized to 100 wt %. Fe^ assumed as 2.00%. Analyses were made with a Philips P.W. 1410 XRF spectrometer on glass beads at the Basalt Research Laboratory of Washington State University. 'Total analytical precision: represents total variation between analyses of triplicate beads prepared from each of 13 rock samples. It includes instrumental precision determined for about 150 analyses of each of 4 U.S. Geological Survey Columbia River Basalt standards.

inward to 3.8 m, beyond which a nearly con- duce phenocryst alignment parallel to a vertical groundmass microlites (viewed in thin section) stant maximum-flow velocity (flat profile) dike is unlikely. As shown in this paper, pheno- show pronounced contact-parallel alignment not would be maintained within the central plug, crysts are more nearly aligned with the assumed present at the center of the dike. Reasons why representing -50% of the total dike width flow direction (upward, parallel to dike) within similar alignment is absent in the marginal (Komar, 1976). the marginal zones of the dike (Fig. 7). This groundmass of the thicker dike of this investiga- Komar's velocity profile can be used as an corresponds to the zones of higher shear rate tion are considered below. approximation of the profile for the Rockport responsible for rotation of grains into orienta- Higher magma-flow velocity associated with dike (Fig. 9E). Changes in phenocryst abun- tions parallel and subparallel to flow. The fewer a more rapid rate of intrusion might be expected dance, phenocryst size, and degree of flow and smaller phenocrysts within these marginal to produce more extreme marginal-velocity gra- alignment should reflect this flow-velocity gra- zones, furthermore, would require lower shear dients and higher rates of shear, and, as a result, dient. Phenocryst size and abundance increase rates for rotation to occur than would the larger, more pronounced phenocryst alignment. Evi- inward at a slower rate within the central part more abundant phenocrysts within the core of dence for this is suggested by the very pro- of the dike (Figs. 5 and 6), corresponding to the dike. A probable result of this effect within nounced alignment of feldspar phenocrysts the predicted central zone of plug-like flow ve- the dike is the substantially poorer alignment of within the marginal zone of a xenolith-rich, por- locity (Fig. 9E). The marginal zones show the phenocrysts within the west half of the dike, phyritic camptonite dike in Cambridge, Massa- most rapid inward increases across the zones of where average phenocryst size and frequency of chusetts (Ross, 1981b; Ross and others, 1983). rapidly increasing flow velocity. As grain- grains longer than 15 mm are greater, compared The phenocrysts (as much as 4 cm long), which dispersive pressure is proportional to the rate of to those of the east half of the dike (Figs. 6 and occur locally within a narrow zone 9.0 to 20.0 shear (Bagnold, 1954) and the rate of shear de- 7). As a result of the interaction of the above cm inward from the contact of the 2.4-m-thick creases rapidly inward, approaching zero (Bar- factors, flow alignment is less pronounced dike, are strongly aligned with its margin. The riere, 1976), the observed inward increase in size within the central zone of plug-like flow veloc- freshness, angularity, and textural characteristics and abundance of phenocrysts across the dike is ity, where shear rate approaches zero, compared of the xenoliths and xenocrysts and the high readily explained. to the marginal zones. volatile content (4.85 wt. %) of the dike suggest A relationship between phenocryst alignment Further evidence of a more pronounced flow that it was emplaced very rapidly (Ross, 1981b; and flow differentiation within a dike has not alignment near dike margins than in their inte- Ross and others, 1983). been proposed previously. Gibb (1972) attrib- riors occurs in the 1-m thick dike (Fig. 1, local- Barriere (1976) calculated practical limits for uted contact-parallel alignment of tabular oli- ity A) mentioned earlier as being essentially the Bagnold effect and concluded that it would vine grains near the base of a thin, nearly identical to the dike under study. Within this increase in effectiveness inversely with dike horizontal, ultrabasic sheet, to crystal settling thinner dike, not only is alignment of marginal thickness. He also concluded that, for porphy- rather than magma flow. Crystal settling to pro- phenocrysts evident, but microphenocrysts and ritic intrusions with diameters > 100 m, the Bag-

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Figure 9. Schematic diagram illustrating model proposed for the development of textural and compositional trends observed across the Rockport dike. A. Aphyric (or microporphi 'ritic) magma (stippled) initially enters propagating or pre-existing fracture in country rock above zoned magma chamber. Phenoerysts and megacrysts (rectangles) continue to grow in lower zone and react with liquid to reduce their anorthite content compared to microphenocrysts of upper zone. B. Fracture propagation and i dilation continue with quenching of marginal liquids to form c? ' a 0 > 0 0 aphyric to microporphyritic chilled margins (black). Phenoeryst- free liquid ascends fracture in advance of phenocryst-rich liquid, D which can enter only lower in fissure where dilation is sufficient nh> to allow passage of large grains. C. Flow differentiation in full « progress as fissure dilation continues. Chilled dike margins of "0, ttj- MM els i s slightly different chemistry and more calcic plagioclase than in I *J the interior act as conduit walls no longer affected by flow differ- » 0 fiff Mil »111 entiation; large phenoerysts are unable to become embedded in them. The chilled margins thin with increasing depth in crust as temperature difference between magma and country rock be- L ! comes less. D. Dilation is complete as flow differentiation con- Umax tinues to concentrate phenoerysts of larger average dimensions

ft 0 o n^y'j^ ft 1.1 v « 0 n 0 toward the center of the dike. E. A typical magma flow velocity » c=3 <=• fl 1=3 ft s

nold effect is unimportant and would produce Factors unrelated to flow differentiation may and abundance trends. It would, however, be only 1 cm of inward translation of crystals of 1 also affect phenocryst distribution across a dike. the principal factor controlling the inward in- cm radius. He calculated that a 1-m inward A phenocryst-free chilled margin may result, at crease in groundmass coarseness (or general translation of a 1-cm-long crystal would occur least in part, from relative dimensions of pheno- grain size of aphyric dikes) characteristic of within a 10-m-thicl: dike (Barrière, 1976). These erysts, chilled margins, and the propagating fis- dikes in general, because of the postemplace- calculations, when applied to data for the Rock- sure. An initial, ascending magma obviously ment crystallization of groundmass grains. The port Headlands dike, show that inward pheno- cannot carry large phenoerysts and megacrysts importance of cooling rates would be much cryst translations exceed the width of the dike, into an incipient, narrow, propagating and/or greater in dikes with relatively small pheno- indicating that flow differentiation would have dilating fissure of smaller diameter than those of erysts, which require substantially less time for been more than adequate to have produced the the phenoerysts. The initial liquid, devoid of growth compared to coarser dikes. observed inward increases in phenocryst abun- larger phenoerysts, would penetrate the fissure The above mechanisms for forming at least dance and size. The inverse relationship of flow ahead of phenocryst-bearing magma, to be rap- the outer few centimetres of phenocryst-free differentiation with dike thickness proposed by idly quenched against the fracture walls, and be chilled dike margins might be more important Barrière (1976) may account for the alignment preserved as aphanitic chilled margins as dila- than the wall effect during the earliest stages of of elongate plagioclase microlites in the mar- tion and intrusion proceeded (Fig. 9). The mar- intrusion. After the formation of chilled margins ginal groundmass of the thinner dike (Fig. 1, ginal chilled zones of dikes are typically less than and further fissuredilation , the wall effect would locality A) and the absence of such alignment in a few centimetres wide and must have formed increase in importance. These mechanism; may the groundmass of the thicker dike of this inves- rapidly upon contact of the magma with colder have been more important than flow differentia- tigation. Average phenocryst size and concentra- country rock. It seems more likely that large tion in producing some of the observed chemical tion and the whole-rock major- and minor-ele- plagioclase phenoerysts, if present when margins and mineralogic cross-dike trends. ment chemistries of the two dikes are nearly solidified, would have been swept away by the The west half of the dike contains phenoerysts identical, as mentioned previously. Higher flow flowing magma rather than becoming em- that are, on the average, larger and more poorly velocities may also have occurred within the bedded in chilled margins far narrower than the aligned with the dike (Figs. 6 and 7) than those thinner dike to produce steeper marginal veloc- lengths of the phenoerysts themselves. Follow- of the east half. This may be a function of the ity gradients, higher shear rates, and more effec- ing further dilation, the slower cooling rate dike not being perfectly vertical (dips 88CE). If tive rotation of grains, including groundmass within the interior of the dike would allow more this were the dike's dip during its intrusion, then microlites. The shorter probable time interval grains to attain phenocryst dimensions and exist- the effective "base" of the dike would have been required for grain rotation would have enabled a ing phenoerysts to increase slightly in size during its west margin. This may have caused a slight greater degree of a lignment to develop within and after magma emplacement. The slower "downward" (westward) shift of the velocity the groundmass prior to the complete crystalliza- cooling rate within the dike interior would con- profile and possibly the development of the ob- tion of the marginal zone of the thinner dike. tribute slightly to the observed phenocryst size served asymmetry in phenocryst size and align-

ment trends. An equally plausible explanation Rockport dike does not entirely preclude glass phenocrysts were no longer available for further would be the local diversion of magma flow as a controlling factor of the Si02, Ti02, and lowering of their anorthite content by reaction slightly westward by irregularities in the conduit K2O trends (decrease inward) because it would with the remaining liquid, nor were they af- walls. be devitrified in a dike of this age (early Car- fected by flow differentiation. The larger pheno- boniferous). Interstitial chlorite in the ground- crysts, concentrated in the interior of the dike, DISCUSSION OF CHEMICAL mass of the chilled margins may represent crystallized from deeper, slower cooling liquid TRENDS altered palagonite, although in amounts proba- within the magma chamber prior to its ascent bly too minor to account entirely for the ob- into the fissure (Fig. 9A). As described above, The cross-dike chemical trends (Fig. 8, Table served trends. The inward increase in P2O5 may this phenocryst-rich liquid entered a fissureafte r 2) are in general agreement with trends across reflect the slightly higher modal content (0.4% the chilled marginal liquids and after sufficient other mafic dikes that I have studied (Ross and higher) of apatite within the interior of the dike. dilation had occurred to accommodate these Heimlich, 1972; Ross, 1983 and unpub. data) Much of the apatite occurs as inclusions in large phenocrysts and megacrysts (Fig. 9B). Be- but with these important differences: the inward plagioclase phenocrysts and megacrysts, and so sides their large size, further indication that increases in CaO and P2O5 and decrease in flow differentiation, rather than original glass the plagioclase phenocrysts crystallized prior to MgO are opposite, and the increase in Na2Ü is content, may be the indirect cause of this P2O5 intrusion rather than within the fissure is the opposite and/ or stronger than I have observed trend. presence of large, unzoned cores in many of the elsewhere. Heimlich and Manzer (1972) show If glass never formed in the chilled margins, larger phenocrysts and megacrysts. Reaction two analyses (one each from contact and inte- then other possible origins of the chemical trends would have been more complete during this rior) of a coarsely porphyritic "leopard rock" may involve sequential tapping of a zoned period of slow cooling and would have produced dike from Wyoming that suggest the same magma chamber undergoing plagioclase frac- cores of lower anorthite content compared to trends reported here for all oxides except K20 tionation. This would have produced residual the earlier, smaller phenocrysts emplaced at chilled margins. (opposite; TÍO2, P2O5, and MnO not deter- liquid enriched in Si02, FeO, MgO, Ti02, and mined). They attributed those trends (and an MnO and depleted in CaO and AI2O3 (Cox, Flow differentiation could not produce this inward decrease in plagioclase An content) en- 1980; Reidel, 1983). If this liquid, after accumu- systematic trend in plagioclase composition and tirely to flow differentiation. This would imply lating high in the magma chamber, had been would, in fact, reduce such a trend by mixing that, upon intrusion, the magma fractionated by tapped first, it would have formed the chilled early and later formed phenocrysts during their plagioclase phenocrysts being removed from its dike margins (Fig. 9). The dike interior would concentration in the dike interior. Two por- margins and concentrated axially by flow differ- have been emplaced slightly later (no internal phyritic dikes of the Columbia River Group entiation. If this were true, the bulk composition chilled margins) from a compositional (por- basalts show smaller inward decreases in plagio- of the chilled margins should correspond to that phyritic) zone deeper in the magma chamber clase anorthite content than two aphyric dikes of the groundmass of the dike interior. This does (Fig. 9). Flow differentiation during intrusion of that group, which probably reflects the effect not appear to be true for the Rockport dike would have further enriched dike interiors in of flow differentiation (Ross, 1983). If, as because its contact composition cannot be de- AI2O3, CaO, and Na20 by concentrating plagio- demonstrated by Barriere (1976), flow differen- rived by volumetrically removing the plagioclase phenocrysts. The marginal enrichment in tiation increases in effectiveness as dikes become clase phenocryst component from the interior MgO and MnO may have resulted from more thinner, the modification of plagioclase trends chemical analyses. The chilled dike margin clinopyroxene crystallizing early during intru- should be greater in thinner dikes. This appears compositions thus probably are not equal to that sion of the dike than later, as indicated by the to be the case when trends across the very of the groundmass within the dike interior and presence of 23.4 vol. % augite in the dike margin similar but thinner dike at locality A (Fig. 1) are were not derived from a residual liquid of plagi- groundmass compared to 12.9% at its center. compared to those across the dike of this investi- oclase phenocryst crystallization. The higher The same relationship occurs for biotite (11.0% gation. This thinner dike shows no significant anorthite content of plagioclase in the chilled margin, 7.3% center), which also would have decrease in anorthite content of phenocryst cores margin than in the dike's interior (Table 1) also contributed to the MgO and MnO decrease in- inward within it (An^ in to Ai^)- This thinner supports this conclusion and indicates that the ward within the dike. Other processes that may dike may also have had a shorter duration of chilled margin plagioclase crystallized earlier have contributed to the observed chemical intrusion and/or a slower rate of magma flow and at higher temperatures than in the interior of trends include continuous tapping of partial and thus tapped a narrower range of liquid and the dike. The origin of this trend is considered melts increasingly depleted in Si, Ti, and K phenocryst compositions than did the thicker below. or crustal contamination of the early, marginal dike. It cannot be assumed, however, that com- liquids (see Ross, 1983, for a more complete Si0 , A1 0 , CaO, and Na 0 should vary positional (mineralogic and chemical) trends 2 2 3 2 discussion). sympathetically with the increased concentra- across thinner dikes in general have lower mag- tion of plagioclase phenocrysts (and in ground- The anorthite content of the cores of plagio- nitudes than those across thicker dikes because no such correlation has been uniformly observed mass) toward the dike interior. A1203, CaO, clase phenocrysts are 22% higher at the chilled and Na20 do vary with plagioclase abundance, margin of the dike than in its interior (Table 1). in dikes from other areas (Ross, 1983; Ross and Heimlich, 1972). The duration of activity and but Si02 does not (Fig. 8), suggesting some con- This is interpreted as being a primary trend re- the total volume of magma transported may be trol on the bulk Si02 content other than plagio- lated to the crystallization history of the magma clase. It has been shown that SÍO2 (and and not flow differentiation (Ross, 1983). more important factors and may be largely in- incompatible elements, including Ti, K, and P) Phenocrysts and microphenocrysts in the chilled dependent of dike thickness. are concentrated in glass of glass-rich, chilled margin crystallized from a slightly earlier liquid CONCLUSIONS dike margins compared to the interiors of four than did phenocrysts in the interior. After be- Tertiary dikes of the Columbia River Basalt coming quenched in chilled margins, these more Average phenocryst abundance, size, and Group (Ross, 1983). The absence of glass in the calcic, although smaller, phenocrysts and micro- angular deviation from dike attitude (and flow

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direction) all incrisase inward from the margins Na 0, and, indirectly, P O with plagioclase Cox, K. G., 1980, A model for flood basalt vulcanism: Journal of Petrology, 2 2 s v. 21, p. 629-650. of the dike. These trends can be attributed to phenocryst concentrations (increase inward). Dennen, W. H., 1976, Plutonic series in the Cape Ann area, in Cameron. B.( ed., Geology of southeastern New England: New EngLtnd Inter- flow differentiation and the more extreme The slightly higher modal content of apatite in collegiate Geological Conference guidebook, p. 265-278. velocity gradients (increase inward) within the the dike interior is due, in part, to the presence of Gibb, F. G., 1972, A differentiated ultrabasic sheet on Sgurr DEI rg. Isle of Skye: Mineralogical Magazine, v. 38, p. 811-818. portions of the magma flowing near the walls of apatite inclusions in plagioclase phenocrysts and Heimlich, R. A., and Manzer, G. K., 1972, Flow differentiation within leopard rock dikes, Bighorn Mountains, Wyoming: Earth and Planetary Science the conduit (Fig. 9E). The plug-like velocity accounts for the inward increase in P2O5. SiC>2, Letters, v. 17, p. 350-356. profile across a dike produces higher strain rates MgO,and MnO trends (decrease inward) appear Irvine, T. N., and Baragar, W.R.A., 1971, A guide to the chemical classification of the common volcanic rocks: Canadian Journal of Earth Sciences, near its margins, but strain rates approach zero to be unrelated to flow differentiation and may v. 8, p. 523-548. Komar, P. D., 1972a, Mechanical interactions of phenocrysts anci the flow within its interior (Komar, 1976; Barriere, reflect differences in crystallization histories, in- differentiation of igneous dikes and sQls: Geological Society c f America 1976). This accounts for the more pronounced itial glass contents (now devitrified), incompati- Bulletin, v. 83, p. 973-988. 1972b, Flow differentiation in igneous dikes and sills: Profile! of veloc- flow alignment of phenocrysts within the ble element and SiC>2 contents of partial melts, ity and phenocryst concentration: Geological Society of An erica Bul- letin, v. 83, p. 3443-3448. marginal zones of the Rockport dike. The fewer or differences in degree of crustal contamination 1976, Phenocryst interactions and the velocity profile of magma flowing and smaller phenocrysts in the marginal zone across the dike. Those trends listed above as through dikes or sills: Geological Society of America Bulle in, v. 87, p. 1336-1342. would respond more readily (rotate) to the sympathetic with plagioclase phenocryst abun- Pao, R., 1961, Fluid mechanics: New York, John Wiley & Sons, p. 124-427. Reidel, S. P., 1983, Stratigraphy and petrogenesis of the Grande Rotide Basalt higher shear rates; than do the larger, more dance can also be explained by these other proc- from the deep canyon country of Washington, Oregon, aid Idaho: abundant phenocrysts farther inward, even if esses and merely have been accentuated by flow Geological Society of America Bulletin, v. 94, p. 519-542. Ridley, W. I , Rhodes, J. M„ Reid, A. M., Jakes, P., Shih, C„ and Bt ss, M. N„ shear rate did not decrease inward. differentiation. 1974, Basalts from Leg 6 of the Deep Sea Drilling Project: J ournal of Petrology, v. 15, p. 140-159. The inward decrease in plagioclase anorthite Flow differentiation clearly occurred during Ross, M. E., 1981a, Four petrographically distinct mafic dike types from eastern Massachusetts: Geological Society of America Abstracts with Pro- content resulted from compositional differences intrusion of this dike and controlled the size and grams, v. 13, p. 172-173. 1981b, Mafic dikes of northeastern Massachusetts, in Boothroyd, J. C., between marginal liquids tapped slightly earlier abundance distributions of phenocrysts and and Hermes, O. D., eds., Geologic field studies in Rhode liland and and emplaced ahead of phenocryst-rich magma. megacrysts, produced some of the major- and adjacent areas. New England Intercollegiate Geologic Conference Guidebook, p. 285-302. Large phenocrysts could not enter an initially minor-element chemical trends, and acted to re- 1983, Chemical and mineralogic variations within four diles of the southeastern Columbia Plateau: Geological Society of Amelia. Bulletin, narrow fissure until sufficient dilation had duce the cross-dike plagioclase anorthite trend. v. 94,p. 1117-1126. occurred, by which time chilled margins would As other processes also may have contributed to 1984a, Flowage differentiation and flow alignment of plagioclase phenocrysts in an alkaline dolerite dike at Cape Ann, Massichusetts: already have formed. The marginal, small these textural and compositional trends, care Geological Society of America Abstracts with Programs, v. It', p. 60. 1984b, Mafic dikes from Boston to Cape Ann, in Hanson, L S., ed., phenocrysts (and groundmass grains) expe- must be taken when attributing such trends in Geology of the Coastal Lowlands: Boston, Massachusetts to Kenne- rienced shorter grovrth periods due to rapid chill- other dikes entirely to flow differentiation. bunk, Maine, New England Intercollegiate Geologic Conference, p. 81-102. ing, and as a result, less reaction to lower their 1985, Mafic dike swarms of the Boston Platform, eastern Massachu- setts: International Conference on Mafic Dyke Swarms, Abstnicts, Uni- anorthite content occurred. Substantially more ACKNOWLEDGMENTS versity of Toronto, Ontario, Canada, p. 142-147. reaction occurred between the larger dike in- Ross, M. E., and Heimlich, R. A., 1972, Petrology of Precambrian mific dikes from the Bald Mountain area. Bighorn Mountains, Wyoming: Geologi- terior phenocrysts and the magma as they grew Critical review of the manuscript by S. P. cal Society of America Bulletin, v. 83, p. 1117-1124. Ross, M. E., and Reidel, S. P., 1982, Mafic dikes of northeastern Massichusetts: in the source body prior to entering the fissure. Reidel greatly improved it and is most appre- Geological Society of America Abstracts with Programs, v. 14, p. 78. The similar groundmass trend developed in 1983, Dolerite and tamprophyre dikes of northeastern Massichusetts: ciated. Critical review of a part of the manu- Geological Society of America Abstracts with Programs, v. 15, p. 674. much the same way but occurred primarily script by V. E. Camp is also gratefully Ross, M. E., Knowles, C. R„ and Chamness, J. S., 1983, Megacrysts, xenocrysts, and ultramafic xenoliths from a camptonite dike in Cambridge, within the dike during and after intrusion. Flow acknowledged. Funding for this study was pro- Massachusetts: Geological Society of America Abstracts with Programs, differentiation served only to reduce this trend v. 15, p. 174. vided by the Northwest College and University Shaler, N. S„ 1889, The geology of Cape Ann, Massachusetts: U.S. Gi »logical by mixing earlier- and later-formed phenocrysts Association for Science and the Basalt Waste Survey Ninth Annual Report, p. 537-610. Solomon, M., 1963, Counting and sampling errors in modal analysis by point within the dike inte rior. Isolation Project (Analogue Studies) under con- counter Journal of Petrology, v. 4, p. 367-382. Weston Geophysical, 1977, Geological and seismological invesnga ions for At least the outer portion of phenocryst-free tract DE-AM06-76-RL02225 to Rockwell In- Pilgrim Unit II of Boston Edison: U.S. Nuclear Regulatory Commission ternational, Hanford Operations. Document BE-5G7603, Appendix G-6, 19 p. chilled dike margins is formed by the early Zartman, R. E., 1977, Geochronology of some alkalic rock provinces in eastern quenching of initial liquids bearing fewer and and central United States: Annual Review of Earth and Flanetary Sciences, v. 5, p. 257-286. smaller phenocrysts than the liquid following up Zartman, R. E., and Marvin, R. F., 1971, Radiometric age (Late Ordovician) of REFERENCES CITED the Quincy, Cape Ann, and Peabody granites of eastern Massa :husetts: the fissure. The wall effect of flow differentiation Geological Society of America Bulletin, v. 82, p. 937-958. becomes more dominant as the system develops, Bagnold, R. A., 1954, Experiments on a gravity-free dispersion of Urge solid spheres in a Newtonian fluid under shear: Royal Society of London with the chilled margins acting as the walls. Proceedings, set. A, v. 225, p. 49-63. Barriere, M., 1976, Flowage differentiation: Limitation of the "Bagnold effect" The principal effect of flow differentiation on to the narrow intrusions: Contributions to Mineralogy and Petrology, the whole-rock chemical trends across this dike v. 55, p. 139-145. MANUSCRIPT RECEIVED ay THE SOCIETY MARCH 25, 1985 Bhattacharji, S., 1967, Mechanics of flow differentiation in ultramafic and REVISED MANUSCRIPT RECEIVED SEPTEMBER 23,1985 is the sympathetic variation of AI2O3, CaO, mafic sills: Journal of Geology, v. 75, p. 101-112. MANUSCRIPT ACCEPTED SEPTEMBER 26,1985

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