Crystallization History and Textures of the Rearing Pond Gabbro,Northwestern Wisconsin

American Mineralogist, Volume 64, pages 844-855, 1979

Crystallization history and textures of the Rearing Pond gabbro,northwestern Wisconsin

JauEsF. Or-rrrsreo Department of Earth Sciences State University College of Arts and Science Plausburgh, New York 12901

Abstract

The Rearing Pond intrusion is a small basicbody that displaysa rangeof compositionand textures illustrating its cooling history. Modal and mineralogical data indicate that it is a singlediflerentiated unit or a cyclic unit in a larger poorly exposedintrusion. Fractionation has resultedin a successionthat varies from olivine cumulatesthrough olivine-plagioclase-clinopyroxenecumulates to plagioclaseclinopyroxene cumulates. Composi- tional variation of phasesis small, with iron enrichment about l0 mole percentin the mafic phasesand with plagioclasevarying from An 80 to An 60. Somezoning of plagioclaseis apparent,becoming stronger in the later units. Primary clinopyroxene flrst appears as large oikocrysts in the olivine-plagioclase cumulates,and later changesto a granular habit in the olivine-free rocks. This is interpretedas a changein the ratio ofgrowth to nucleation ratesofthe pyroxenecorresponding to the cessa- tion of olivine crystallizationas the magma compositionleaves the olivine field. This occurs at the quaternary invariant point, Fo-An-En-Di-Liquid in the systemAn,Di,Fo,SiOz. The habit changeoccurs at this point as a result ofthe changein degreeofsupersaturation as the liquid fractionatespast the invariant point as olivine is consumed.This is either a kinetic effect or a compositionalone due to the diflerencesin competition for componentson opposite sidesofthe invariant point. Evidencesuggests that both nucleationand growth acceleratedin the latestrocks, as grain sizeis diminishedand zoning becomesmore pronounced.

Introduction with fractionation of a basaltic magma. In this paper and related textural The textures of basic igneous rocks have received the changes in cumulus minerals compared with the considerable attention through the years. Some no- changes have been charted and by the petro- table examples have dealt wittr the ophitic texture mag|a composition' as determined to explain (Krokstrom, 1932; Walker, 1957; Wagir, 196l; op- g-raRhic analysis of the rocks' in an effort in that have oc- penheim, 1964). Others (Wager et al., 1960; He;s, the textural types and changes type is a small Keweenawan ieOO; W"g"r, 1963) studied features of igneous sedi- curred' The example used intrusion which displays a textural change mentation and the processes responsibie for them, gabbro granular that appears related to and Jackson (1967) developed a classification based from ophitic to composition resulting from fractionation on these processes.More recently a number of change in. magma' Mineral compositions deter- imental studies based on classical theory of crystal"*f"r- of. a basaltic end-member compositions' nucleation and growth have provided new insigtrt min-ed are approaching model provided by Osborn and Tait is an into the relationship between irystal habit, igneJus so that the excellent starting point for development of an hy- textufes and cooling history (Kirtpatrick , D7Z; Lof- to magma composition' It is gren, 1974;Swanson, 1977;Fenn, 1977). pothesis relating texture that a starting magma composition could It appears that textural relationships observed in unfortunate not be determined for the intrusion' these iock types are not only a function of rate of cooling but may also be controlled by the composi- Geologic setting tion ofthe magma, which in turn is often the result of the degree of fractionation. The quaternary system The Rearing Pond intrustion is one of several that anorthite-forsterite-diopside-silica (Osborn and lie along the southern boundary of the Keweenawan, Tait, 1952) illustrates the phase changes that occur Lake Superior syncline. The most cornmon intrusive w3 -w4x /7 9/ 0708-08,14$02.oo OLMSTED: REARING POND GABBRO 845

EXPLANATION

REARINGPOND INTRUSION ADJACENTUNITS euarrzGabbro [I,.T-.TI Granite ffi Troctorite& Gabbro E"l l- Peridotite #lHI"- F:r\sil Wisconsin d Basart = IntrusiveContact (inferred) -- -_ - GradationalContact lgneousLayering Index Map mi Scale 10 1.0

Fig. l. Geological map of Rearing Pond intrusion. Rock units south of the intrusion are part of the Mineral Lake intrusion. rocks are gabbro, anorthositic and troctottic gabbro, called the Mineral Lake intrusion (Olmsted, 1968), and micrographic granites. Other related intrusive which is about 5000 m thick and 18 kn long. rocks, such as peridotite and occasional intermediate The Rearing Pond intrusion occupies a con- rocks,are found, but they are rare. The detailsofthe spicuous embayment in the top of the Mineral Lake structure and stratigraphy of the Keweenawan and intrusion. Intrusive relationships between the two older Precambrian units of the region have been de- units are not clear, as the contact is poody exposed. scribed by Aldrich (1929) and Leighton (1954), and Both units, however, intrude the Lower Keweenawan severalmore recent reviews have been of value in un- volcanics, which in some caseshave been so engulfed derstanding both local and regional problems (Halls, that they are now little more than thin screensbe- 1966;White, 1966;Weiblen and Morey, 1975).These tween intrusives. Figure I shows the Rearing Pond and other studies (Olnsted, 1968; Hubbard, 1968, intrusion as a lens-shapedbody tapering to the east 1975;Books et al., 1966) suggestthat the intrusive between other igneous units and to the west in the units are tabular bodies emplaced along surfaces volcanics. The orientation of ig.eous laminations such as faults or unconformities and are of Middle and layering within the intrusion is shown in Figure Keweenawanage. Along its south limb the Keweena- 2, and suggeststhat the form of the body is that of an wan units of the Lake Superior syncline dip north- elongate funnel tilted to the northwest. westerlywith moderateto steepangles, exposing the Figure 2 shows that when the regional dip is re- units in cross section. The entire volcanic section. in- moved the dips of the internal structures all converge cluding the intrusives in this area, is on the order of to a point beneath the center of the intrusion. The 8000 to 10000m thick. In the Mellen area the intru- distribution of rock types also seems to indicate a sions have been emplaced into the Lower Keweena- tilted funnel or cone-like structure. The lowermost wan lavas and have nearly engulfed them. The major unit is a feldspathic peridotite which forms a layer or intrusion west of Mellen is a large anorthositic unit envelope about the centrally-situated main mass of 846 OLMSTED: REARING POND GABBRO

are shown in Table I and Figure 3. The earliest unit in a fractionation sequenceis the peridotite (an olivine cumulate) that forms the envelope around the central unit. Next are the troctolitic rocks (olivine- plagioclase cumulate, with and without pyroxene oikocrysts), which form much of the central part of the intrusion and account for about eighty percent of its exposed area. The uppermost unit is quartz gabbro (plagioclase-clinopyroxene cumulate) found in the northwestern corner of the intrusion. Actual con-

Table l' Modal analyses pERrDorrrE

w-204

O11v1ne* 72.4

PlaSioc lase 24.3 23.2

Diotite o.7

0rEhopyroxene 0.2 7.7

Cllnopyroxene 2.2 4.7

Opsque I.I 4,2

Nwber of counts 3500 3245

Leuco- l,laflc- Spotted TloctoliEe Troctolite Gabbro Noricic w-125L W-I63 !l-39 w-46 Fig. 2. Structural intcrpretation of igneous layering data and Ollvlne* 23,4 39.7 13,8 3. 1 rock type distribution. Upper: Measured orientation of layering Plsgioclase 72.4 52,a 54.6 60.1 shown at tails of arrows. Renoval of regional dip by.1661ie1 Biocite - ht about horizontal axis, which is shown as dashed line. results in the OrEhopyroxeu€ orientations shown at heads of arrows. The 44o average dip .2.1 2.2 5.4 20.9 located in the southwest corner of the intrusion, the 20o dip at the Clinopyroxene 0.5 5.1 25.6 9.4 northwestern end, and the 40" dip in the east c€trtral area are APstite - 0.1 shown in Fig. l. The result is the new strikes all roughly parallel Secondary the perimeter of the body with dips toward the center, as shown in Chlorite or - 0.6 - 1.4 trornblende th€ cetrter diagram. This 60" rotation of all values is justifed by a 60o average regional dip of the Keweenawan sedinentary and l,lagnerite 0.6 0.6 0.6 3.1 volcanic units. The lower diagram is an interpreted north-south Nudber of couts 1400 1555 9800 1600 crosssection, rcstored to the original orientation.

w-6 w-I26 w-19 w-I64 w-166 troctolitic rocks. The uppennost unit is a quartz gab- Plagloclase 55.6 48.7 61.7 52.7 55.0 bro which is clearly the product of fractionation of a Cllnopyroxene 13.2 30.6 14.l 25.5 34.O more basic magma. In general, layering is not wide, orthopyroxene L5.7 3.9 12.9 7.6 8.9 spread, but in the higher parts of the intrusion some olivine* 0.5 15.8 2.1 rhythmic layering, both graded and unimodal, has Biottte 0.I - 0.8 3.2 been observed.The implication of this lack of small- lfagn€tice 0.3 0.9 0.5 4.3 0.8 scale layering in the lower units is that convection Apatite Tr Tr Tr o.4 Tr was either very slow or not periodic until the later Aophibole 74.6 - 7.9 1.0 1U stageof the igneous period. cooling quartz 3.0 Petrographic descriptions Alkeli Feldspar Zircon Tr The Rearing Pond intrusion has been divided into Nuniber of counts 1400 1800 1548 L6t2 three units that are petrographically distinct and *Serpentlne was always co@ted as olivine. In sme exanples olivine was readily recognized in the field. Mineral compositions as much as 90% serpencinized, OLMSTED: REARING POND GABBRO 847 tacts between the units are rare, and field studies have confirmed that most changesfrom one type to anothertake placeover a narrow transition zone. Peridotite The peridotite unit is an olivine cumulate that occurs as a75- to lfi)-meter-thick layer-like envelope. It appearsto overlie a narrow mass of olivine gabbro to the eastthat is either a keel-shapedfeeder conduit or has been separatedfrom the similar central troctolitic rocks by faulting. Internal structures within the peridotite indicate that it is dipping west at a low angle. The rocks of this unit are dark gray to black and weather to a rusty brown color. Serpentinizedcu- mulus olivine and minor magnetite are enclosed in iarge irregular pyroxene and plagioclaseoikocrysts that appear to be entirely post-cumulus.Olivine grains averageabout 1.2mm in diameterand are euhedral when enclosedentirely by plagioclaseor pyroxene. When they are in contact with other olivine grains,they show the typical curving boundariesand 120" triple contact points between three grains. A typical texture is shownin Figure 4d. Modal compositionof the peridotite variesconsid- CD erably, as shown by two extremeexamples in Table Fig. a. (A) Labradorite gabbro showing granular texture l. Compositionsof olivine have beenobtainedby 2V resulting from cumulus nature of pyroxene; bar : 2 mm. (B) determinationsand are on the order of Fo 80. Com- Large poikilitic pyroxene in bytownite gabbro shown enclosing positionsof pyroxenesare also given in Table 2 and euhedral plagioclase and with concentrically oriented plagioclase Figure 5 and plagioclasein Figure 6. Note that the about the circumference of the poikilocryst; bar = 5 mm. (C) showing large amounts of serpentinizcd plagioclase is all intercumulus and ranges in compo- Bytownite troctolite olivine and resulting tension fractures in the plagioclase; bar : 5 sition from An 80 to An 65. This zoning is probably mm. (D) Peridotite or olivine cumulate; trote overgrowth of due to the fact that the intercumulus phasescrystal- olivine about embeddedeuhedral magnetite; bar: I mm. lized from a trapped liquid of limited volume. The texture indicates that this unit was formed phasesand were nucleating abundantly. Plagioclase while olivine and magnetite were the only solid and pyroxene are postcumulus,filling small inter- Pl. cumulus voids, and were thus not stable above the floor of the magma chamber.The large grain size of the pyroxene and plagioclaseindicates the small degreeofsupersaturation ofthese phaseseven in the intercumulus volume. The rocks are interpreted to be mesocumulateswhose formation involves a considerable period of olivine adcumulus growth. At some point the intercumulusliquid becametrapped, termi- nating the equilibrium with the overlying liquid. ?t. 0t. Troctolite Fig. 3. Partial modal compositionsof three major rock types:4 This plagioclase-olivine-rich rock comprises the olivine cumulates (peridotite); 21 bytownite troctolites and large central part of the intrusive. It overlies and is gabbros; 8 labradorite quartz gabbros. The separation between the olivine-bearing gabbros and olivine-free rocks is made on the surroundedlargely by the peridotite and gradesup- basis of texture as well as mineral content. ward into the quartz gabbro to the west. All of the 84E OLMSTED: REARING POND GABBRO

Table 2. Optical properties and compositions of minerals troctolites are plagioclase-olivine cumulat€s, and

Orthopvroxenes they contain varying amounts of poikilitic pyroxene. so the S€mple Number Rock lype 2V B lndex Compositioo Plagioclase and olivine are anhedral, that texture is best described as allotriomorphic and in- UICra@ fic 76" - En. 80

w-74 Ult!a@flc 78o - En. 82 dicates considerable adcumulus growth. Grain size is

W-2O4 Ultramafic 75o - En. 79 about 2 mm exoept for the pyroxene, which is from

W-9 Troctollte 75o - En. 79 1.5cm to 3.0 in diameter.Figures 4b, c and Figure 7 these textures at di-fferent scales. Both tl-39 Troctollte 75o I.684 En. 79 illustrate plagioclase and olivine are extensively altered, par- W-I15 Troctolite 7oo - En, 76 ticularly olivine, which may be up to 80 percent ser- W-2O6 TroccoliEe Etr.80 pentinized. Plagioclase contains fractures localized W-J GADDTO 6oo En. 68 between olivine grains. This illustrates how serpenti- W-6 cabbro 620 En.70 niz4tisl placed considerable stress on the minerals W-23 60" cabbro En. 68 surroundingthe olivine (Fig.,1c). w-roo GDoro 630 1,688 8n.72-76 Composition of the olivine is similar to that in the

Cllnopvroxenes peridotite, ranging betweenFo 75 and Fo 80. Plagro-

w-t4 ulEraMtlc 57o r.678 En. 49 tlo. 45 Fs. 5 clase compositions obtained by the 5-axis universal

W-39 Troctolite +g" 1.686 En, 46 lto. 40 F6. 14 stagemethod of Noble (1965)are shown in Figure 6.

W-115 Troctollre The inset showing compositions of cores and rims in-

W-125L TrocEollEe 5Io dicates compositional zoning in individual grains is w-166 cebbro 1..684 En. 46 wo. 44 Fs. 10 moderate, generally less than l0 mole percent. As in the case of the peridotite, the anhedral textures and 01lvine

W-6I UltraMflc 87o Fo. 79

I'-F74 lJltraMfic 9oo Fo. 85

W-204 UltraMflc 88o Fo. 81

w-9 froctollte 87o Fo. 79

W-2O6 Troccolite B5o Fo. ?5

W-125 Troctolite 89o Fo. 83

w-115 Troctolite 84o ro. 73

C' e // "'Ij { / 's; x 6o .yr' t ol ,oi o -tf- a /

20 30 40 Fig. 5. Plot ofpyroxene compositions, based largely on optical data. The inset shows clinopyroxene data including a compilation far2l1 of several electron rnicroprobe analyses of the oikocrysts from Fig. 6. Plagioclasc compositions determined by the method of sample W-39. One of these is given in Table 3. The arrow Noble (1965). The parameters measured are angles between indicates the variation in composition. The average of rim indicatrix and crystallographic directions. Compositions of analyses(head ofthe arrows) is slightly richer in iron and calcium plagioclase are shown along the curves. The dashed curve is for than the centers ofthe oikocrysts (ail ofanows), but the variation plutonic plagioclase. Open squares are from the peridotitc, solid is small and one reversal of this trend was observed. In tle main circles from the troctolite and olivine gabbro, and open circles diagram the line on the left is sample W-74, aa ultramafic; the frorn the quartz gabbro. The iriset shows the plutonic curve center one W-39, a troctolite with large oikocrysts of clino- between An 60 and An 90. The connected points are from cores pyroxene; that on the right is sample W-166 which is gabbro with and rims of plagioclase crystals demonstrating the extetrt of gratrular texture. zoning. Zoning is always normal so cores are always more calcic. OLMSTED: REARING POND GABBRO slight compositional zsning suggestslow adcumulus Table 3. Electron microprobe, partial analysesof the core portion growth of these phases. Pyroxene compositions are of a poikilitic p].roxene specimen W-39 (same rock as shown in Fig. '{c) shown in Figure 5 and an electron microprobe analysis in Table 3. Again they are very magnesian, and Oxlde t{eight Percent zoning is limited. The compositional range of a single sio2 52 oikocryst determined by electron microprobe analy- "93 sis is shown by the arrow in the inset. This range is T1O2 0.59 approximately the error of the analysis and is not A I2Or 2.24 consideredto be significant. Cao 20,48 Figure 3 and the modes of Table I illustrate that pyroxene content ranges from nil to nearly forty per- L7.70 cent in this unit. A seriesof modes taken acrossa py- Fe0 7"L3 roxene-richlayer (Fig. 8) showsthat the plagioclase- to-olivine ratio is nearly constantwith a value of 2.3, Relative Numbers of Ions Based on 6 o)

o.797

En 48.59

Fs 10.98

Di 40,4I

crysts. These features, combined with the observation that the oikocrysts are occasionally found as layers, provide evidence as to their origin. Figure 7b shows only a portion of an oikocryst, and illustrates how the included crystals are rarely in contact with one an-

Fig. 8. Variation of modal composition betweena pyroxene-rich and pyroxene-poor layer. The layers are actually quite distinct Fig. 7. Upper: photo of weathered surface of troctolite showing rather than gradational as shown. These data were collected poikilitic pyroxenes. Lower: detail of oikocryst showing modal analyses of 15 thin sections. The constant plagioclase/ relationship between enclosed crystals and enclosing pyroxene; olivine ratio demonstrates that the only variable is pyroxene width of photo, I cm. content (seetext for interpretation). OLMSTED: REARING POND GABBRO other. This texture is similar to that shown by Wager of the points and the impression that they may clus- and Brown (1967,p.258, Fig. l4l) as an exampleof ter to the left of the curve. This may indicate that re- a primocryst. All these textural features combine to vision of the curve is nec€ssary. indicate that the oikocrysts grew for the most part The open squares are values taken from the post- above the floor of the intrusion and settled to their cumulus plagioclase in the peridotite. Their spread present position. The difference between the habit of along the plutonic curve is shown. Compositions are the plagioclase and olivine and the pyroxene oiko- obtained by dropping a perpendicular to the curve. crysts is seen as a function of their respectivegrowth Note that the peridotite values range from An 80 to rates. An 65. As much of the plagioclasein the peridotite is altered, a relatively small number of values were ob- quartzgabbro Gabbroand tained. As the plagioclase in the peridotite is poiki- Near the western end of the intrusion, the troctolitic and crystal boundaries were not observed, it is lite gradesrather abruptly into gabbro and ultimately possiblethat zoning could be greater. quartz gabbro. This change is the result of the loss of Solid circles represent values from the troctolitic modal olivine, and is attended with a change in the zone. Values range from about An 85 to An 65, but clinopyroxene from poikilitic to a more granular zoning observed in a given specimen is rarely more habit. This results in a change in the texture of the than about l0 mole percent, as shown in the upper rock to what is best described as subophitic or granu- left inset. The plagioclase in these rocks is generally lar. Table I gives modes of three of these rocks, and anhedral and the 26ning is of a continuous nature, showsthe range ofvariation from rocks that appear suggestingextensive adcumulus growth. just saturated to a qaartz gabbro in which inter- Contrasted with the troctolitic plagioclase, that cumulus granophyric material is common. Grain size from the granular gabbros is euhedral, zoning is is in the range of 0.75 mm to 1.0mm, which is some- more apparent, and they are less calcic. Composi- what smaller than in the other units. Plagioclase is tions range from An 80 in the cores to as low as An often oriented to form a fluxion texture. [n contrast 45 on the rims. to the other units, the plagioclasesare euhedral and displAy zoning over a range of An 80 in the cores to Olivine An 45 in the rims. The smaller grain size and zoning Olivine compositions have been determined by combine to suggest a greater degree of super- measurementsof 2 Z with the universal stage and the saturation of all the phasesin this unit. An exception survesof Deer et al. (1962,p.22). Only valuesresult- to this is postcumulus orthopyroxene, which often ing from direct measurement between optic axes occursas large poikilitic crystals. were accepted.In every case several measurements were made on the same crystal to ensureprecision. Mineral compositions Becauseof the high valuesof 2V it is difficult to ob- Plagioclase tain more than a few values from each rock-type, and variation existing in similar examples is dis- Plagioclase compositions and their thermal states conc€rting. When all of the values of Table 2 are were determined by the 5-axis universal stage noted it is apparent that limited iron enrichmenthas method of Noble (1965). The values are plotted in taken place. Figure 6. Each point representsthe averageofseveral As noted earlier and illustrated in Figures 4b,c,d, determinations obtained from a particular specimen. the olivine has been largely serpentinized.It is a cu- For the points in the main part of the diagram, no mulus phase throughout its crystallization period, particular effort was made to focus attention on any which ends approxinately with the appearance of part of the crystal; extinctions that seemedto repre- the granular gabbro. This coincides with the change sentthe majority of the observedarea were used.The of clinopyroxene habit from poikilitic to subophitic values presentedin the inset plot the sameoptical pa- and granular. rameters, but only the plutonic (dashed) curve is shown. These values represent determinations of cores and rims from several troctolitic specinens. Pyroxene The core values are always more calcic than the rims. Clinopyroxene compositions have been obtained Evident in both parts of the diagram are the spread by measurement of the beta index, and 2V with the OLMSTED: REARING POND GABBRO

Si-An

Fig. 9. The systemFo-Di-An-SiO2 after Osborn and Tait (1952). The inset showsphase details ofplagioclase saturation surfac€. universal stage,using the curves of Hess (1949). hence show only the lamellar exsolution labeled as Again only limited iron enrichment is evident. Bushveldtype (Hess,1960). The development of spectacular oikocrysts in the troctolite and the abrupt change to granular habit in Composition trend basedon mineralog5r the gabbro provide an opportunity to study the oph- The order of appearanceof the major phasesof the itic and related textures. The oikocrysts appear to Rearing Pond intrusion has been estabtshed by this have a complex history. In their centers the included petrographic study to be olivine, plagioclase,clinopy- crystals do not touch (Fig. 4b, 7b), while they are roxene, and orthopyroxene. This sequenceis inter- much more tightly packed on the rins (Fig. 4b). This preted from the four major rock types: peridotite, suggests that the cores formed as primocrysts, as troctolite, olivine gabbro,and gabbro.This order can noted earlier, and the rims might represent contin- be describedin the quaternary system Fo-An-Di- ued, post-cumulus growth after the composite primo- SiOr, which provides some information on the com- crysts of pyroxene and included crystals settled to the position of the initial magma as compared with floor. If this representsthe correct course of events, magmacompositions that have beenproposed for the the oikocrysts would be expectedto be composition- Keweenawan, Lake Superior province. The phase ally zoned. The results of an electron microprobe diagram in this system is also helpful in explaining study are shown in the inset of Frgure 8. Slight en- the observedtextural changes. richment in iron is apparent in most samples, in- Particular attention is drawn to the di-fferencesin dicating some magma evolution during grofih of the clinopyroxene habit between the earlier units, in oikocrysts. This combined with textural evidence is which it is poikilitic, and the quartz gabbro, in which strongly suggestiveofa dual history in the pyroxenes. it is intergranular. This major change is correlated An analysis of one of the oikocryst cores is given in with the termination of olivine crystallization, and is Table 3. This analysis is typical of diopsidic augites due to an important change in the kinetics of clino- from mafic intrusions. pyroxene nucleation and growth at that point. Orthopyroxenes are all interpreted as being post- Frgure 9 illustrates the system Fo-An-Di-SiO, af- cumulus. They usually rim olivine in the peridotite ter Osborn and Tait (1952),and Figure l0 represents and troctolite and are poikilitic in the quartz gabbro. a surface of "anorthite saturation" within that sys- Compositionshave been determinedby 2V measure- tem. The surface is located at a composition of ap- ments on the universal stageand in two casesby beta proximately 50 weight percent anorthite and is index determinations, by using the curves of Deer el roughly parallel with the opposite side of the teta- aI. (1963,p. 28). Orthopyroxenesalso show someen- hedron (seeCoombs, 1963).The arrow in Figure l0 richment in iron in the gabbro, but none are suffi- shows a path of liquid evolution proposed for the ciently iron-rich to have inverted from pigeonite and Rearing Pond intrusion. The arrow implies the order OLMSTED: REARING POND GABBRO

Di.+lr. richment that typically parallels this path is not observed in the Rearing Pond intrusion. This is evidently the result of early crystallizationof magnetite, as observedin the peridotite.

Interpretation of the textures One of the purposesof this paper is to describeand explain the textural variation that appears to be strongly related to the habit of clinopyroxene. The linking of this changein texture with cessationof olivine crystallization relatesit to the peritectic point c to.+An. Si02+lr. of Figure 10. This point representsan abrupt change Fig. 10. Surface ofanorthite saturation from the quaternary sys- in the relative rates of growth and nucleation in tem in Fig. 9. Temperatures have been omitted but the detail im- clinopyroxene. This may be a manifestation of a proved over that shown in Fig. 9 from a numbcr of sourccs, the change in the kinslls5 of crystallization of clinopy- major difference being the division pyroxene of the field in two roxene, i.e. crystellization of olivine * plagioclase * parts. The spinel field has been omitted. The open arrow shows the cours€ of fractionation. Anorthite always accompanies the clinopyroxene is somehow di-fferentfrom crystalliza- phase shown in each section of the surface. a to b represents tion of plagioclase + clinopyroxene + ortho- troctolite, b to c representstroctolitic or olivine-rich gabbro con- pyroxene.Alternatively it may sirnply be a matter of taining clinopyroxene oikocrysts, and c to d is a gabbro containiry the composition of the fiquid relative to the field clinopyroxene of granular quaternary habit. d is the invariant boundaries and the point at which a silica polymorph begins to crystallize. degree of compositional supersaturation that occurs under varying circumstances. These alternatives are not independent of one an- of crystallization of olivine and plagioclase, clinopy- other and in fact are probably closely related. roxene, orthopyroxene, silica. Different segmentsof If this is 3a important textural relationship,it must the arrow representthe different mineral assemblages have been observedelsewhere in similar caseswhere on the liquidus. fractionation of olivine has moved the liquid from Path a-b representsthe period during which only olivine-normative to olivine-free composition. A fur- olivine and plagioclase are in equilibrium with the ther generalization might be that the textural change liquid. Prior to the arrival of the liquid composition may involve either calcium-rich or calcium-poor py- on the plane olivine crystallizedalone, as is shownby roxene.No observationswere made to support or re- the peridotite. Path a-b, then, representsthe trocto- fute such a generalization.The following descriptions lite, in which olivine and plagioclase are cr mulus. At are included as a sampling of other studiesin which point b clinopyroxene joins plagioclase and olivine, similar observationshave beenmade. and the path changesdirection toward the invariant Krokstrdm (1932) studied several Swedish dole- point at c. At that point the peritectic reaction rites and basalts, including the Breven dike, and noted that the ophitic texture was well developed forsterite + liquid : clinopyroxene * orthopyroxene only in olivine-bearing rocks. Similarly, Walker eliminates olivine as an equilibrium phase (Kushiro, (1957) reported, in a study of a large number of ba- 1972).This also representsthe point in the intrusion saltic intrusive rocks from the Karroo series,the Pali- where clinopyroxenechanges habit from poikilitic to sades sill, and Scotland, that the olivine-bearing equigranular. rocks are rrore often ophitic while those poor in or Path c-d is the region in which plagioclase, clino- lacking olivine (tholeiitic) are usually subophitic. pyroxene, and orthopyroxene coexist with the liquid, Walker (1969) describedthe lowermost dolerites in and at invariant point d a silica phase joins them. the Palisades sill as subophitic to intergranular and From point c onward the path representsthe granu- lacking olivine. Contrastingwith this, the'\aloside- lar gabbros, including ttLeqaartz gabbro of the west- rite dolerite" of the well-known olivine layer is de- ern part of the intrusion. While this path has not scribed as gabbroic and poikililic in texture, while been documented with rock analyses,the mineralogy the cessationof olivine marking the top of the layer is requires that point d was attained at least in the in- accompanied by a corresponding change back to the tercumulus liquid of the gabbro. The strong iron en- subophitic texture. The grain size of plagioclase and - OLMSTED: REARING POND GABBRO E53 pyroxene is described as about the same in these Clovth( Rotc rocks. The higher rocks in the intrusive are olivine- 6nvth Rate free and subophitic. iVuhationRctr Contrasted with the subophitic and intergranular lrTt rocks of the Palisadesare the chilled marginal rocks I of the Skaergaardintrusion (Wager, 196l), which contain modal olivine and poikilitic pyroxene. De- tailed descriptions of the Skaergaard textures show ti that while olivine is a stable phase (cumulus min- UNDERCOOLING eral), the pyroxenes are poikilitic (ophitic texture) Fig. ll. Nucleatioa and growth rates vs. undercooling' The growth to nucleation rate ratio as sug- without regard for the distance from the border of dashed lines represent the gestedby Shaw (1965). Whether the dashedcurve continues to rise (Wager Brown, 1967, p. 68-71, the intrusive and or falls at small undercoolings depends on whether growth or nu- 106-122).On the other hand, in the layered seriesthe cleation go€sto zero first as o146p66ling decreases'See discussion middle zone (MZ\ is characterized by and distin- in text. guishedfrom the lower zone (LZ)by (l) the absence ofolivine and (2) the abundantly nucleated pyroxene rate of sooling i5 unimportant. Clearly the examples (Wager and Brown, 1967,p. 68-74). Further, these of the Skaergaard chilled marginal rocks and the authors note (p. 7l) that clinopyroxeneis cumulus in dikes of Gray (1970) illustrate well the effect of the the lower zone(LZ\ but with poikilitic habit. Indeed m1sef cosling on grain sizeof the samerock compo- the boundary between ttLe LZ and MZ is based on sition. It is interesting that in the chilled margin of the cessationof olivine crystallization, and this is ap- the Skaergaardintrusion olivine was most affectedby proximately the point where pyroxene appearsto be- the change in cooling rate, actually Sscoping large gin to nucleate abundantly. The observation in the enough to poikilitically enclose much of the plagio- Skaergaard intrusion is that the chilled marginal clase(Carmichael et al.,1974,p. 168). rocks are olivine-bearing and ophitic, as are the pre- A number of workers have studied the implica- sumably more slowly cooled olivine-bearing rocks of tions of the theories of nucleation and growth rates the lower layered zone, while the equally slowly with cooling on the textures of igneous rocks (Shaw, cooled olivine-free rocks of the niddle layered zone 1965; Glay, 1970; Carmichael et al., 1974,p. 147- are of intergranular texture. 168; Dowty et al., 1974;Kirkpatrick, 1976). While In a detailed study of two contrastingdikes, Gray there are some experimental data on nucleation and (1970) found that a quartz tholeiite dike in Grenville crystal growth in silicate systems(Kirkpatrick,1974; Township, Quebec displayed intergranular texture at Lofgren, 1974;Donaldson, 1976;Fenn' 1977),most the edge and was still subophitic l0 m from the edge. arguments are based on the application of theory to On the other hand, the texture of an olivine gabbro observationsof natural systems.Shaw (1965) has dis- dike at Kigaviarluk, Baffin Island changed from cussed such applications, emphasizing the impor- granular at the edge to ophitic l0 m inward from the tance of the ratio of the growth rate to nucleation edge. Gray noted this relationship as well as that rate, particularly at small undersoslings.Plots of nu- noted above by Walker (1957)as indicating that sil- cleation and growth vs. undercooling are shown in ica saturationplayed an important role in 66atlelling Figure ll. Theseare only qualitative, so they are de- texturesof basicrocks. liberately drawn with maxima of equal heigbt. The These observations support the notion that the dashedcurve is the growth to nucleation rate ratio, as presenoeor absenceof olivine somehow controls the suggestedby Shaw. While olivine is crystallizing with nucleation and growth rate of the pyroxene. Most of clinopyroxene (the condition illustrated by curve BC the analysesoftextures in the geological literature re- of Fig. l0), the supersaturationin pyroxenemight be lated nucleation rate to the degree of undercooling kept small, i.e.,left of point A in Figure I l; when oli- (Wager, 196l; Oppenheim,1964; Gray, 1970;Carmi- vine is no longer crystallizing the supersaturation of chaelet al., 1974,p. 149-156;Kirkpatrick, 1976),but pyroxene may increase,to point B or more. The ex- in the examples cited here the change in nucleation perimental studies cited by Carmichael et al. (1974, rate, while related to cooling rate, is more closely de- p. 164), suggestthe di.fferencebetween the peaks in pendent on the changes in phase relationships be- terms of temperaturecoutd be as small as 25oC. Such tween olivine and pyroxene. That is not to say that a change in supersaturation could be sufficient to OLMSTED: REARING POND GABBRO

change the texture from ophitic to subophitic or even while olivine-free rocks may be of granular texture. intergranular. This is true in the caseof the Rearing Pond intrusion, The habit change of clinopyroxene in the Rearing and has also been described in the Skaergaardintru- Pond intrusion appearsto representsuch a sfuangein sion and the Palisadessill in addition to several mi- the growth to nucleation ratio in responseto chang- nor intrusions. ing phase relationships. While the liquid composition As the textural changes related to olivine o@ur- is on the curve joining clinopyroxene and olivine, rence seem to be more closely associatedwith clinopyroxene nucleates slowly but grows relatively changesin phaserelationships than with chilling, it is rapidly. Several mechanismsmay operate with equal proposed 1[4 this mechanism may be as important or varying degreesof effectivenessto slow the nucle- as oversaturation by chilling. In the chilled margin of ation of the last phase to become stable. If the rela- the Skaergaard intrusion, the texture is ophitic even tionship is of a cotectic nature the olivine and pyrox- at the contact. On the other hand, most olivine-free ene will compete for components, which in efect dikes are granular even tens of meters from the con- reduces the amount of undersaturation that can oc- tact. cur. Data from Donaldson(1975) indicate that basal- The Rearing Pond intrusion illustrates the crystal- tic magma in the vicinity of the crystallizingolivine is lization history which provided excellent conditions less saturated in pyroxene oomponents than at a for the observation of this inportant textural change. greater distance from the olivine. This suggeststhat While it was not possible to determine the initial liq- such a mechanism could decreasethe supersaturation uid composition, the trends were made apparent of pyroxene,which is the oppositeof what might be through modal and mineralogical studies. The path expectedfrom study of a phase diagram in which the began in the phase volume of olivine and a magne- liquid might overshoot a cotectic. tite-rich spinef moved into the plane of olivine and In contrast to the above situation, the nucleation plagioclase, then to the four-phase boundary where rate of clinoplnoxene becomes greater and growth clinopyroxene joined olivine and plagioclase. The rate apparently lower after the peritectic point (c of slow nucleation rate of pyroxene resulted in the de- Fig. l0) is reached,when olivine is no longer stable. velopment of the spectacularpoikilitic 'spots,' which This results in greater supersaturation in pyroxene, were crystallizing directly from the primary magma, so that the ratio of growth to nucleation rate becomes not from a trapped intercumulus liquid. The texture smaller. In every example considered, this mecha- and lack of zoning support this. Finally, as the nism would explain the sinultaneous cessationof oli- five-phase point where olivine reacts to form two py- vine and change to the subophitic or granular tex- roxenes is reached, the degree of saturation of clino- ture. p)noxene became great enough to allow abundant Calcium-poor pyroxene appears to be entirely nucleation, and the texture sfuengedto subophitic or postcumulus and occurs both with and without oli- granular. The mineral composition of these granular vine, so no change in habit is recorded. It might be gabbros suggests that the liquid composition pro- concluded that calcium-poor pyroxene never be- ceeded beyond the invariant point along the bound- oomessufficiently supersaturatedto nucleate rapidly ary joining the fields of plagioclase,clinop5noxene, as the liquid composition passesfrom point c to d. and orthopyroxene. The presenceofabundant inter- The large spectacular clinopyroxene oikocrysts of cumulus granophyric material is the strongest evi- the Rearing Pond intrusion produce an extreme ex- dence in this regard. The relative increase in the de- ample of the ophitic texture cornmon in many basic gree of zoning in the plagioclase of these rocks may intrusive rocks. It has been shown that the ophitic indicate an increased rate of crystallization of the texture is best developedin olivine-bearing rocks that feldspar as well. The general decreasein grain size have cooled slowly. This has been explained with suggests accelerated nucleation as well as growth classical theory relating nucleation rates and degree rates but with markedly different effect on the pyrox- of undercooling. Typically the degree of under- ene than on the plagioclase. cooling or supersaturation has been related to cooling rate (Carmichael et aI., 1974,p. 168). The de- Acknowledgments crease in grain size associatedwith the margins of This field work was accomplished with a grant from the Wis- intrusive bodies demonstratesthis. Alternatively it is consin Natural History and Geological Survey. The microprobe shown here that olivine-bearing rocks are ophitic, analysis of the pyroxene was made by W. McClain of McGill Ud- OLMSTED: REARING POND GABBRO 855 versity. Earlier versionsof the manuscript were read by A. R. Phil- Krokstrom, T. (1932) On the ophitic texture and the order of crys- pot6, B. Bonnichsen, and L. B. Gillett, and their suggestionspro- tallization in basaltic magmas. Geol. Inst. Upsala Bull., 24, I97- vided many improvements. The comments of an anonymous 216. reviewer greatly improved the manuscript, and these efforts are Kushiro, l. (1972) Determination of liquidus relations in synthetic appreciated. silicate systemswith electron probe analysis: the systcm forsterite-diopside-silica at one atmosphere.Am. Mineral., 57, 12ffi- References r271. Leighton" M. W. (1954) Petrogenesis of a gabbro-granophyre Aldrich, H. R. (1929) The geology of the Gogebic lron Range of complex in northern Wisconsin. Bull. Geol. Soc.Am., 65, &l- Wisconsin.llis. Geol. Nat. Hrsr.Sum. Bull.71. 42. Books, K. G., W. S. White and M. E. Beck (1966)Magnetization Lofgren, G. (1974) An experimental study of plagioclasc crystal of Keweenawan gabbro in northern Wisconsin and its relation morphology: isothermal crystallization. Atn J. ScL, 274,243- to time of intrusion. U.S. Geol. Sun. Prof. Pdp.,550-D,Dll7- 273. D124. Noble, D. C. (1965) Deternination of composition and structural Carmichael,I. S. E., F. J. Turner and J. Verhoogen(1974) Igreous state of plagioclase with the five-axis universal stage.Am. Min- Petrologlt. McGraw-Hi[ New York. eral., 50,367-38l. Coombs,D. S. (1963)Trends and affinitiesof basalticmagmas and Olmsted, J. F. (1968) Petrology of the Mineral Lake Intrusion, pyroxenes as illustrated on the diopside-olivine-silica diagram. Northwestern Wisconsin. In Y. W. Isachsen, H., Oigin of Mineral. Soc.Am. Spec.Pap., 1,227-250. Anorthosite and Relaled Rocks,p. 149-162.N.Y. Stat€ Mus. and Deer, W. A., R. A. Howie and J. Zussman (1962\ Rock-Forming Sci. Serv.Mem. lE. Minerals. Vol. I, Ortho- and Ring Silicates. Wiley, New York. Oppenheim, M. J. (1964) Basalt textures from the southeastern lower Galilee, lsrael. Geol.Mag, lil,548-557. Chain Silicates.Wiley, New York. Osborn, E. F. and D. B. Tait (1952) The system diopside-forste- Donaldson, C. H. 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(1970) Crystal growth and nucleation in two large - (1963) The mechanism of adcumulus growth ia the layered diabasedikes. Can. f. Earth Sci.,7,36-375. seriesof the SkaergaardIntrvsion'. Mineral. Soc.Am. Spec.Pap., Hall$ H. C. (1966) A review of the Keweenawan geology of the r, t-9. Lake Superiorregion. Am. Geophys.Union Monogr., 10,3-27. - atrd G. M. Brown (1967) Layered Igneous Rocks.Freeman, Hess,H. H. (1949) Chemical composition and optical properties of San Francisco. common clinopyroxene,l. Am. Mineral.,i4,621466. - (1960) Stillwat€r igneous complex, Montana, a quan- cumulates. J. Petrol., 1, 73-85. titative mineralogicalstudy. Geol. Soc.Am. Mem.80. Walker, F. (1957) Ophitic texture and basaltic crystallization. "/. Hubbard, H. A. (1968) Stratigraphic relationships of some Ke- Geol.,65,l-14. woenawan rocks of Michigan and Wisconsin. Abs. 14th Ann Walker, K. R. (1969) The PalisadesSill, New Jersey:a reinvestiga- Inst. on Lake SuperiorGeol.,35-16. tior. Geol. Soc.Am. Spec.Pap. 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