Mineralogy and Petrology of the Dutchmans Creek Gabbroic

American Mineralogist, Volume64, pages 531-545,1979

Mineralogyand petrology of the DutchmansCreek gabbroic intrusion, South Carolina

Hnnny Y. McSwrnN,Jn. Departmentof GeologicalSciences, The Uniuersityof Tennessee Knoxuille, Tennessee3 79 I 6

nNnPe,ul G. Nvsrnou, Jn. South CarolinaGeological Suruey Columbia,South Carolina 29210

Abstract

The DutchmansCreek gabbro is a differentiatedpluton consisting of olivinegabbro, anorthositicgabbro, pyroxene gabbro, and hornblende-pyroxene gabbro. This sequence of rock typesrepresents advancing differentiation, as indicated by progressiveincreaie in Fel (Fe+Mg) ratiosof coexistingferromagnesian minerals and decreasein the anorthitecom- ponentof plagioclase.Modal olivinedecreases systematically in the sequence.The parent magmacrystallized under conditions of highPHrO and./Cr, which increased with fractiona- tion,as suggested by modalincrease in hydrousminerals and changes in thecompositions of opaqueminerals. Late-magmatic reactions occurred between many mineral phases and pro- duceda varietyof symplecticor replacementtextures. Petrologic differences between the DutchmansCreek gabbro and other neighboring plutons include early precipitation of Fe-Ti oxides,absence of olivinereaction relationships, and systematic areal distribution of olivine withinthe pluton. These differences may be due to thefact that much of thepluton has barely beenunroofed by erosion, and only the upper regions of themagma chamber are visible. Early cumulaterocks are exposedin the pluton interiordue to deepererosion, whereas late differentiatesoutcrop at highertopographic levels at plutonmargins. Water migration to the top of the chambermay have resulted in higherprevailing.lO, and subsequent lowering of liquidustemperatures in the upper volatile-rich portions of thechamber.

Introduction petrologicexamination of onelarge, elongate pluton, the DutchmansCreek gabbro in centralSouth Caro- More than 30 post-metamorphic(late paleozoic) lina (McSween,1972). gabbro plutons occur in an arcuatechain extending The few detailedstudies of neighboringpiedmont for at least 500 km in the southern Appalachian plutonssimilar to the Dutchmans-Creekgabbro have Piedmontareas of North Carolina, South Carolina, indicatedthat systematicrelationships between pet- and Georgia. Detailed petrographicdescriptions of rographicproperties, mineralogy and mineral chLm- only two of thesebodies, the Mecklenburg,North istry, and location within the pluton are poorly de- Carolina pluton (Hermes,1968, 1970), and the Buf- fined or nonexistent(Hermes, 1968;Medlin it al., falo, South Carolinapluton (Medlin et al.,1972) are 1972).Moreover, macroscopic layering phenomena availablein the literature.Limited petrographicde- are generallynot observed,und litit" if any differenti- scriptionshave beenpublished for severalother plu- ation has occurred. This paper demonitratesthat tons(McCauley,l96l; Butler,1966;Chalcraft, 1968; subtlebut clearlydiscernible differentiation trends, Mathews, 1969; Medlin, 1969a;McSween, 1970; reflectedin systematicvariations in modal mineral- Waskom and Butler, l97l; Hadley, 1973);other de- ogy, mineral chemistry,and texture, are presentin scriptivestudies have been submitted as unpublished rocks of the DutchmansCreek pluton. T-heunique theses(Morgan, 1963;Mathews, 1967; Myers, 1968; featuresof this pluton resultfrom crystallizationun- Medlin, 1969b;Cabaup, 1969).This paper presents der conditions of high water content and oxygen and interpretsresults of a detailedmineralogic and fugacity,and an unusualerosional exposure paft;;. o003-w4x/ 79 / 0s0G053I $02.00 531 532 MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION

Field relationships streams,and distinctivebrownish-gray gabbro sap- A geologic map and preliminary petrologic de- rolite exposuresare common in road cuts and road scriptionof the DutchmansCreek gabbro were pub- ditches.Both residualboulder outcrops and bedrock lishedby McSween(1972). The gabbrowas intruded exposuresare most abundantnear the marginsof the nearthe contactbetween the Charlotteand Carolina pluton or in the vicinity of the roof pendants,but slate belts in southeasternFairfield County, South there are large tracts of gabbro terrain where there Carolina, and is situated along regionalstrike be- areno outcrops.No layeringwas observed in anyof tween two large circular post-metamorphicbatho- the exposures.Pyroxene hornfels was found at sev- liths, the Winnsboro qvartzmonzonite to the south- eral localitiesalong the pluton margins. westand the Liberty Hill graniteto the northeast. The pluton is crosscutby numeroussmall leuco- The enveloperocks are a sequenceof interlayered cratic dikes consistingof quartz, muscovite,ortho- quartz-muscoviteschists, amphibolites, and leuco- clase, plagioclase,and chlorite. The planar dikes cratic gneissesregionally metamorphosed to the am- range from severalcentimeters up to 3 metersthick phibolite facies(Wagener, 1970). Two stratigraphic and vary from fine-to coarse-grained.They occur in units can be distinguishedin the country rocks.A variousorientations from near-verticalto almosthor- metavolcanicgroup, originally an interbeddedse- izontal,and appearto be confinedto the gabbro. quenceof volcanicand pyroclasticmaterial, includes The gabbro marginsclosely correspond with the felsic,intermediate, and mafic rocks with prominent outline of a pronouncedlocal gravity high depicted foliations, and is noted as "fine-grainedgneiss and on the map by Popenoeand Bell(1975), as shown in amphibolite"in Figure1. The leucocraticgneiss unit Figure 2. The delineationof this gravity high in- is a metasedimentaryunit comprised mainly of dicatesthat at depth the gabbro is elongateand of quartzo-feldspathicrocks with massiveor porphy- roughly the samesize, shape, and orientationas sug- roblastictextures. gestedby its outcrop areaat the surface. Preparationof a refined map of the Dutchmans Creek pluton showingmore detailsof the gabbro Mineralogyand petrography contractswas undertakenas part of this study (Fig. l). The irregularly-shaped,elongate intrusion oc- Modal and textural uariations cupies a distinct, topographicallylow area along Gabbro samplesconsist largely of plagioclase,oli- Dutchman'sCreek, for which the pluton is named. vine, clinopyroxene,and orthopyroxenein varying The margin of the gabbroas mappedfrom outcrops proportions, with lesseramounts of biotite, horn- and distinctivesoil changeswas found to correspond blende,and opaqueminerals. Rock typesbased on with a prominentbreak in slope,easily determined on the proportionsof thesemajor phasescan be classi- topographic maps and stereoscopicpairs of aerial fiedas: photographs.The marginsof this gabbro,and other (l) oliuine gabbro, charactertzedby significant Piedmont gabbros,can thereforebe delineateddi- quantitiesof euhedralto subhedralolivine ()20 per- rectlyfrom the localtopography with someaccuracy. cent) with plagioclase,augite, bronzite, and biotite; Steep-slopedprominences occurring within the plu- (2) anorthositicgabbro, containing )60 percent ton are largeroof pendantsor inclusions,and their plagioclase,with augite,hypersthene, opaque miner- lithologiesare the same as the country-rock strata als,and fairly abundantolivine (-15 percent); which that portion of the pluton intrudes(Fig. l). (3) pyroxenegabbro, in which augite(16-30 per- Topographicelevations for contactsof the gabbro cent) and hypersthene(7-14 percent)are the pre- with surroundingcountry rocks and with the large dominant mafic minerals,with plagioclaseand usu- inclusionsare the same,suggesting that the inclusions ally abundantbiotite (5-15 percent); and areroof pendantsand that the pluton hasbarely been (4) hornblende-pyroxenegabbro, composed of )15 unroofedby erosion. percentamphibole with augite,hypersthene, plagio- Exposuresof gabbroare massive,spheroidal, and clase,and fairly largeconcentrations of Fe-Ti oxides havedistinctive pitted surfacesfrom the differential (usually9-10 percent). weatheringof componentminerals. The msst com- Typical modal analysesare presentedin Table l. mon outcropsare large residual boulders from oneto The locationsof thesesamples are shown in Figure2. severalmeters in diameter.Though theseboulders Figure 3 illustratesvariations in the modesof these are not bedrockin the strict sense,they do occur "in rock types;the samplesin Figure 3 are plottedfrom place." Bedrock exposuresoccur in a number of top to bottomin theorder of increasingFel(Fe+Mg) MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION 533

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2- 3= ::=-t'4

--7:4 /J, i? ia --:: '-.lo\

GREEl{SCHISTFACIES

RIOGCWAY

MAP UNITS 'ii consml prAtl sEDtMEr{Ts I !l l ourcHmANscREEK GABBRo :"""q MAP il.il r-eucocRATrc AREA cNErss 72 rne-cRATNEDGNErss AND t(tot(EE PLU tOt{ S ATIPHIBOLITE BELT atil crrnlttc o50 l?l Gabbrolc t(t

Fig. I . Geologicmap of the DutchmansCreek gabbro. Inset at lower left showsthe positionof the studyarea in the SouthCarolina Piedmont. MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION

OUTCROP'YIAP tous accessoryminerals, but their abundanceis low- estin olivinegabbro and increasesin the sequenceto hornblende-pyroxenegabbro, which contains9-10 percent.Traces of hematiteare presentin pyroxene gabbro and significant quantities occur in hornblende-pyroxenegabbro. Apatite generallyoccurs only in pyroxenegabbro and hornblende-pyroxene gabbro,along with minor epidoteand sericite. Systematictextural variations occur among the various rock types. Hypidiomorphic granular or cumulate textures charucterizeolivine gabbro and anorthositicgabbro samples(Fig. 4,A.,B). In pyroxene gabbro and hornblende-pyroxenegabbro, large pyroxeneand amphibolegrains poikilitically enclose GRAVITY MAP olivine,plagioclase, and sometimesopaque minerals (Fig. 4C, D). A few pyroxeneoikocrysts occur in olivine gabbro and anorthositicgabbro, indicating that the abundanceof pyroxene,especially clinopyroxene,determines whether the textureis predomi- natelygranular or poikilitic. Superimposedon theseprimary texturalvariations are late-magmatic alterations. Hydrous mineral phasespartially replacemafic mineralsin somesam- ples,as in Figure4D wherepoikilitic pyroxenegrains havebeen largely replaced by hornblende.As hydra- tion and replacementbecome more pronounced,pri mary igneoustextures are somewhatobscured by a myriad of reactionrelationships among various minerals (Fig. 4E). Extensivedeuteric alteration pro- duces altered gabbro, readily recognizedin hand specimenby coarse-grainedtextures and the dis- tinctive green (hornblende) and purple to gray (plagioclase)crystals. Both prismaticand fibrousva- Fig. 2. Sketchmaps illustrating the outcroplocations of gabbro rietiesof amphiboleoccur. Pyroxenes are partially or (prefix samplesdescribed in the text DC is omitted) and the completelyreplaced by amphiboleand biotite, and correspondencebetween the surfaceoutcrop pattern of the pluton and the positivegravity anomalymeasured by Popenoeand Bell olivine is absent. Plagioclaselaths are partially ( 1975).The verticalline at theleft of thegravity map is thewestern sericitized.Common accessoryminerals are calcite, limit of Popenoeand Bell'sstudy area. pyrite, hematite,and sphene. Small xenolithsin the gabbrohave sharp contacts in ferromagnesianminerals (arrow), which is alsothe with the hostrocks, often delineatedby reddishreac- order of decreasingmodal olivine.As will be demon- tion rims suggestingoxidation of Fe (McSween, strated,this sequenceof samples,and the sequenceof 1972).These rocks havefine-grained granular (horn- rock types delineatedin the precedingparagraph, felsic) textures(Fig. 4F), and consist primarily of representincreasing differentiation. The outcrop dis- olivine, clinopyroxene,plagioclase, and magnetite. tribution of theserock types will also be discussed They are relativelyfree ofhydrous phases,except lor later. small quantitiesof biotite. Accessoryminerals vary systematicallyin the sequence of rock types. Sulfide minerals, especially Mineral chemistry pyrrhotite,are abundantin olivinegabbro (Table I ), Mineralsin ten thin sectionswere analyzed using a but occur in only minor quantitiesin other members MAC Model400 electronmicroprobe with appropri- of the sequence.Pyrite is the predominantsulfide in ate syntheticand natural standards.Data were re- alteredgabbro. Magnetite and ilmenite are ubiqui- duced using an iterative routine that correctsfor MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION 535

Table 1. Modal analyses(volume percent)

Mineral DC8 DC9 DC10 DC2L DC13 DC11 DC5 DC6 DC1 DC 3x

Olivine 29.2 28.9 22.8 L4.3 ro.q 4.L 1.1* L.2* 0.9* 12.4

Clinopyroxene t2.3 18.8 I.d 9a 7.6 27.2 19.5 16.t 19.8 40.2

Orthopyroxene 6.7 2.7 8.6 o, o1 LJ.I I.J 7.7

Plagioclase 40.6 39.8 50.8 60.5 62.8 qt Q 50.2 53.8 43.t 39.5

Biotite d.J t.h 7.8 0.6 4.5 7.0 15.3 1.9 0.1

la a llornblende 0.8 0.7 0.5 l.o 0.L 4.0 3.5

Magnetite 0.3 0.2 0.5 2.0 Z.J I.J 3.4 7.8

Ilmenite 0.3 0.5 0.7 2.I 1.4 1.5 2.r 1.2 o.u

Spinel ET ET n2 ET ET ET

Sulfldes 1.5 0.8 0.5 fr 0,1 0.1 0.1 0.3

Apatite

No. Polnts L547 L523 1547 L576 1500 I5I2 L5L2 L525 1560 1000

*olivine + iddingsite in partings

+ilmenite + hmatite lamellae

DC 8, DC 9, DC 10 are olivine gabbroi DC 21, DC 13 are anorLhosLtic gabbro; DC 11, DC 5' DC 6 are

pyroxene gabbro; DC I is hornblende pyroxene gabbro; DC 3x ls a granular xenolith. background,atomic number, absorption,and fluo- in Fe contentsoccur between the ferromagnesiansili- rescenceeffects. catesof different samples.Ferromagnesian silicates Mafic mineralsin the DutchmansCreek gabbro coexisting in the same analyzedthin section (repre- exhibita remarkablecompositional uniformity in any sentedby filledsymbols) are connected by solidlines particular hand specimen,but systematicvariations in Figure5. Coexistingminerals in a granularxenolith

sAA,tPt E YARIAI'ONS DC8 t? DC9 DC ( ""':lr'i'r"- DC2l c""T" DC13 DCII DC5 DC6 DCI

VOLUME PERCENI Fig. 3. Modal variationsamong analyzed gabbro samples (from Table l). The samplesare ordered from top to bottom by increasing Fel(Fe+Mg) in the ferromagnesianminerals (advancing differentiation). DC 8, DC 9, DC l0 are olivine gabbro;DC 21, DC l3 are anorthositicgabbro; DC I l, DC 5, DC 6 are pyroxenegabbro; DC I is hornblende-pyroxenegabbro. Sample locations are shown in Fig. 2' 536 MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION

Fig.4. Texturalvariations among gabbro samples. All photomicrographsin plane-polarizedlight to samescale as in (A), except(F). (A) Hypidiomorphic-granulartexture in olivinegabbro (DC 8). (B) Cumulatetexture formed of largeeuhedral olivines in olivinegabbro (DC 9). (C) Plagioclaseand minor olivineenclosed poikilitically in large,optically-continuous clinopyroxene crystal in pyroxenegabbro (DC ll). (D) Cloudedplagioclase, opaque minerals, and alteredolivine poikilitically enclosed by hornblendein hornblende-pyroxene gabbro(DC l). The amphiboleis secondary,as evidencedby relict areasof clinopyroxene(arrows). (E) Late magmaticalteration of pyroxenegabbro (DC 5) in the form of biotite growth,replacement of orthopyroxene(Im) in anorthositicgabbro; scale as in (A) (C) Rims of red-brownbiotite around magnetite/ilmenitein contactwith plagioclasein pyroxenegabbro. (D) Vermicularintergrowths of magnetitewith orthopyroxenehost in pyroxenegabbro(reflected light). (E) Symplectiteofaluminous orthopyroxene and plagioclasein pyroxenegabbro (DC 5) in the form of biotite growth,replacement of orthopyroxeneby magnetite,and saussuritizationof plagioclase. (F) Granular hornfelsictexture in xenolith(DC 3X) consistingof clinopyroxene,olivine, plagioclase, and magnetite(crossed nicols).

are connectedby a dashedline. Completeanalyses the augitefield nearthe saliteboundary on the pyrox- for pyroxenesand olivinesare presentedin Table 2 enequadrilateral (Fig. 5). Clinopyroxenesin the ana- and for hydrousminerals in Table3. lyzed hornfelsicxenolith (DC 3x) are more Mg-rich Clinopyroxene and lesser amounts of ortho- and Ca-poor than the correspondingphase in the pyroxeneare the dominant mafic mineralsof most enclosinggabbro (Fig. 5). gabbro samples.Clinopyroxene typically occurs as Orthopyroxeneoccurs generallyas discretesub- large oikocrysts,although this phasealso forms dis- hedralto anhedralcrystals, or lesscommonly as large cretegrains in somegranular rocks. Clinopyroxenes oikocrysts enclosingplagioclase and olivine. This havea very limited compositionalrange, and plot in mineralis slightlypleochroic from palepink to pale MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION 53'l

MAFICMINERALS

Pyroxenes En Fo Olivines

Micos

Amphiboles Fig. 5. Compositionsof ferromagnesiansilicates in gabbrosamples. Minerals coexisting in the samesample (filled symbols) are connectedby solid tie-lines.Coexisting phases in a granularxenolith are connectedby a dashedline. The tie-linesrepresent the following samplesfrom left to right: DC3x, DC 8, both DC 9 and DC 10, DC 21, DC 13, DC ll, both DC 5 and DC 6, DC L greenand oftencontains exsolution lamellae of clino- Olivine occursas largeeuhedral crystals in cumu- pyroxeneon (100).Orthopyroxenes also demonstrate late rocks (Fig. 48), as subhedralto anhedral a limited Fe-enrichmenttrend (Fig. 5), althoughhy- chadacrystsenclosed by pyroxenein poikiliticrocks persthenevariations are greaterthan thosein clino- (Fig.4C), or asirregular interstitial grains in granular pyroxene. samples(Fig. aA). It is characteristicallyfresh and The partitioningof Fe and Mg betweencoexisting unalteredin granular and cumulaterocks, but re- orthopyroxeneand clinopyroxenediffers from most placementby magnetiteand iddingsitealong internal stratified gabbroic intrusions. KD (Xfr'XrtE*/ partingsoccurs in pyroxenegabbro and hornblende- Xtf,E.Xffi-)values average 0.97, which is significantly pyroxenegabbro. The olivine compositionalrange differentfrom the Kp valueof 1.4reported for most measuredin all gabbroic rocks is Fo66-s6,and Fe- other bodies,but is very similar to the 0.94-1.09 enrichmentin olivinesvaries directly with Fe-enrich- range reportedfor the Jabal Sha'l' intrusion (Cole- ment in coexistingpyroxenes (Fig. 5). No composi- man et al., 1977). tional zoningin olivineswas observed. Although the Minor elementsin pyroxenesalso show regular distributionof Fe and Mg betweencoexisting olivine variationswith increasingfractionation [i.e. increas- and orthopyroxeneis relativelyinsensitive to temper- ing Fel(Fef Mg)1.The trendsfor Ti, Al, Cr, and Mn ature, such data can be used to assessequilibrium in pyroxenesare illustratedin Figure 6. Ti and Mn betweenthe two phases.The averagef" (FelMg increaseand Cr decreaseswith increasingFe content oliv/Fe/Mg opx) for DutchmansCreek samplesis of the pyroxenes(arrows). Clinopyroxenes have sub- 1.15,in closeagreement with the experimentally-de- stantiallylower Mn contentsthan coexistingortho- termined distribution coefficientof l.l2 (Medaris, pyroxenes. 1969).Olivine in the hornfelsicxenolith (DC 3x) is 538 MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRT]SION

Table 2. Chemical analysesof pyroxenesand olivine (weight contentof the amphiboleincreases as the Fe content percent oxidesby microprobe) of otherminerals (Fig. 5). A plot of (Na+K) us.Si in

Orthopyroxene Clinopyroxene olivine amphibolesindicates that all are hornblendesinter- DC8 DCll DC8 DClI DC8 DC11 mediatein composition betweenthe pargasiteand Meo 28.4 26.0 15.4 I4.7 37.1 34.9 edeniteseries (Fig. 7). Ti/ Al ratios in hornblendes o1203 1.98 1.59 3.8r 2.76 .34 .00 apparentlyincrease with Fel(Fe*Mg), as illustrated si02 53.3 54.3 5r.3 51.3 40.3 37.9 in Figure7, lower diagram. CaO L.82 1.34 20.4 20.9 .00 .00 Plagioclaseoccurs as euhedral laths enclosed poiki- Tio2 .38 .39 1.49 .77 .07 .04 litically by pyroxeneor as largermore equantgrains Ct .18 .03 .07 .03 .00 .00 2O3 in granular or cumulate rocks. plagioclase Mno .28 .48 .26 .29 .35 .48 Many FeO I3.4 16.5 1,54 8.95 2r.9 26.3 crystalsare cloudedwith abundantoriented needles Total 99.74 r00.65 IOO.27 99.10 100.06 99,62 of opaqueinclusions. Individual grainsare normally zoned,and plagioclasein hornblende-pyroxenegab-

Atoms/6 oxygens Atoms/6 Oxygens Atofrs/4 Oxygens bro is partiallyaltered to sericite,epidote, and calcite. Mg 7.52 1.40 .85 .82 1.43 1.38 The measuredcompositional range for plagioclasein A1 .08 .o7 .17 ,r2 all gabbroicrocks analyzed is An.,-r., but plagioclase I.92 1,96 1 .89 L92 1.04 1.01 in any one specimenhas a more restrictedcomposi- Ca .o7 .05 .81 .84 tional range which varies with Fe content of the Ti .01 .01 .04 .02 coexistingmafic minerals.For example,olivine gab- Cr bro sample DC 8, which has the most magnesian Mo .01 .o2 .01 .10 .01 .01 mafic phasesobserved in Figure 5, containsCa-rich Fe .40 ,50 .23 ,28 .47 .58 plagioclaseof An.,-r.; hornblende-pyroxenegabbro TotaI 4.OL 4.Ol 4.00 4.01 2.95 2.98 DC l, containingthe most Fe-rich mafic phasesin Figure 5, has plagioclasecompositions of An.r_u, Forr-rr, apparentlyin equilibriumwith the coexisting magnesianclinopyroxene, but not in equilibrium Table 3. Chemicalanalyses of hydrous mineral phases(weight percentoxides by microprobo) with the adjacenthost anorthositicgabbro (DC 2l), which containsolivine of For.-rr. Anph lbo I e Biot lte Biotite forms large irregular sheathsoften inter- DC8 DC1 DC8 DC1 grown with opaque minerals.This phaseis highly NarO 2,14 r.32 .8s . 00 pleochroic 1.38 ,94 9.01 9.75 from cream or light tan to deepreddish- "20 brown. The total Fel(Fe+Mg) ratios of biotitesvary CaO 11.8 11.8 .01 . o0 sympatheticallywith the Fe contentsof other ferro- Mso 74.6 12.3 18.7 14.6 FeO 9.99 L3.7 8.92 magnesianminerals (Fig. 5). Whetherthis variation 15.0 l4n0 .09 10 .03 .06 in total Fe reflectsprogressive changein the oxida- Ti02 .45 3. 50 5.39 5.L9 tion stateof Fe is not known. Assumingfull octahe- .11 .03 -0t .o4 "tr0, dral-siteoccupancy, charge balance constraints from A1203 r2.5 10.9 16.0 t4.6 the reactionTi'+ + E e 2Fe2+indicate the presence sio- 43.5 44,2 39.2 38.7 of someFes+ in all analyzedbiotites, but no inference Total 97.76 98.88 98,r2 97.94 can be madefrom availablemicroprobe data on the Feo/Feo+Mgo .4L .53 1' qr relativeproportions of Fe2+and Fe8+.Biotites have high TiO, contents(4.2-5.0 weight percent, Table 3). Atons/24 (0,0H,F) Atoms/12(0,0H,F) Na .18 .12 .00 The Na contentof biotitesdecreases with increasing K .26 .18 .81 ol Fel(Fe+Mg) (Table3); Na in more highly differenti- 1.85 r .84 .00 .00 ated rocks is incorporated into hornblende and Mg 3,20 2.68 t.96 I .58 plagioclaserather than biotite. Fe L.23 L.61 .9L Amphiboleoccurs in mostgabbro samples as small Mn .0r .o2 .00 .00 brownish-greencrystals in reactionrelationship with Ti .05 .38 .29 .28 clinopyroxene.In more highly differentiatedsamples .01 "00 .00 .00 it has greater modal abundance,sometimes almost A1 2.L7 t .88 r.32 t.25 completelyreplacing clinopyroxene. Typical amphi- Si 5.47 6.46 2.75 2.81 L5 -97 7.77 bole analysesare presentedin Table 3. The total Fe 7.74 MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION

NON- QUADRILATERAL COMPONENTS lN PYROXENES(MOLE o/o) . OPX oCPX

o.5Ti O.5Cr

o. o.4 Mn o.2

0.20 o.2s o.30 o.35 Fe Fe+Mg Fig. 6. Minor elementvariations in orthopyroxenesand clinopyroxenes.Top: Cr decreasesand Ti increaseswith increasing Fel(Fe+Mg) (arrow).Bottom: Mn variessympathetically with Fel(Fe+Mg); orthopyroxeneshave consistently higher Mn contentsthan clinopyroxenes.

(Fig. 8). The KrO contentof plagioclase,expressed as 9A). Minor amountsof greenhercynitic spinel occur mole percent orthoclase,is very low but increases in associationwith Fe-Ti oxides in all rock types slightlywith increasingalbite component(Fig. 8). except hornblende-pyroxenegabbro. The green Fe-Ti oxides are presentin all gabbroic rocks, spinel,which commonlyoccurs at the boundarybe- either as discretecrystals or connectedgrains with tweenmagnetite and ilmenite(Fig. 98), hascomposi- sharp,straight boundaries (Fig. 9,{). The magnetite- tions which vary from about (Mgo..oFeo.rr)Al2O.in ilmenite pairs analyzed (MtrrUsp, and Ilmr.-r. olivine gabbro and anorthositic gabbro to Hmu-r.) have undergonesubsolidus reequilibration (Mgo.uuF%..uXFeo.rAlr.e)O.in pyroxene gabbro. and do not definean oxygenfugacity or temperature Sulfideminerals occur as roundedblebs in all rock of crystallization.Most ilmenitegrains contain con- types excepthornblende-pyroxene gabbro, but are siderableMg, with formulae approximatelycorre- especiallyabundant in olivinegabbro. The blebscon- spondingto Fq.rMgo.,TiOg.Both oxide phasesoccur sist predominantlyof pyrrhotite with lesseramounts as individual homogeneousgrains in olivine gabbro of pyrite, chalcopyrite,and pentlandite. and anorthositicgabbro. Minor oxidationof ilmenite Apatite forms large euhedralcrystals in pyroxene to hematiteoccurs in pyroxenegabbro samples,and gabbro and hornblende-pyroxenegabbro. Other ac- thick, subparallellamellae of hematite are present cessoryminerals in thesetwo rock typesinclude epi- within ilmenitein hornblende-pyroxenegabbro (Fig. dote and sericiteas alteration products ofplagioclase. 540 MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION

Edenile Porqosile enclosingof olivine,plagioclase, and opaqueminerals by pyroxenes,etc. f o8 /t'/ presence o -/ oO ./- The of roundedsulfide blebs of uniform ./ o. ,a mineralogyin early-crystallizedrocks suggeststhat E -/ tyu o /./ theserepresent immiscible globules suspended in the 4 /o-' t- crystallizingmagma, analogous to sulfideblebs ob- Y ,'/t at - + o. servedin Hawaiian tholeiiticlava lakes(Skinner and ,r/ --- z /.- Peck, 1969).These dense globules may have settled 8 78 76 7.4 7.2 7.O 6.8 66 64 62 6 with cumulate phasesin early differentiates.The Cummingfonile Tschermokife Si/ Formulo Unil roundedshapes of someblebs have been modified by growingsilicate crystals abutting or impingingon the a globules. a a This sequenceof primary crystallizationis com- lo a plicated olt= by a complexassortment of late-magmatic Ll+ 0 to a reaction relationshipsbetween coexisting minerals. lL Among different samplesthe pervasivenessof these mineralreactions increases with increasingfractiona- o or ort,rol't o4 o5 tion, i.e. increasingFel(Fe*Mg) ratios of the ferromagnesiansilicates. Examples of the following reac- Fig. 7. Compositionalvariations of amphibolesbased on 24 tion relationshipscan be found in almost (O,OH,F). Top: hornblendes all of the fall betweenpargasite and edenite gabbroic series.Bottom: TilAl ratio increaseswith Fe/(Fe+Mg). rocks,but they areespecially well developed in pyroxenegabbro and hornblende-pyroxenegabbro. Most of the mineralreactions observed are simi- Mineral reactions and the crystallization sequence lar to those describedin other Piedmont gabbros (Hermes,1968; Medlin et al., 1972\with onenotable The order of crystallizationfor most mineralscan exception:complex reaction coronas of pyroxenes be determinedfrom a synthesisof modal, textural, and hornblendearound olivine grainsin rocks from and chemicaldata. The progressivesequence of Fe- other plutonsare absentin DutchmansCreek sam- enrichment in coexistingferromagnesian minerals ples. presumably represents increasing differentiation. Biotiteis almostinvariably associated with opaque Modal mineralogy,and consequentlyrock type,vary minerals,characteristically forming rims around the systematicallywith increasingFel(Fe+Mg) ratiosin magnetiteand ilmenite,especially where these are in the silicates.Modal variationsin the rock samples contactwith plagioclase(Fig. 9C). Spectacularreac- orderedby increasingferromagnesian silicate Fe con- tion relationshipsbetween orthopyroxene and mag- tents(Fig. 3) suggestthe following primary crystalli- netite are illustrated by vermicular intergrowthsof zalion model:early crystallizationand accumulation magnetitein hypersthene(Fig. 9D). Goode (1974) of olivine, followed closely by Fe-Ti oxides and plagioclase(olivine gabbro); steady depletion of olivine with increasingcrystallization of magnetite/il- meniteand plagioclase(anorthositic gabbro); contin- ued precipitationof opaque minerals,with further reductionof plagioclasecrystallization and near ex- haustionof olivine, formation of large quantitiesof pyroxene(pyroxene gabbro); continuedcrystallization of magnetite/ilmenite,pyroxenes, and some plagioclase,stabilization of amphibole relative to 6l-76 clinopyroxene(hornblende-pyroxene gabbro). This 41 proposedsequence is consistentwith the progressive Fig. 8. Plagioclase compositions fall within black area of the changein plagioclasetoward more sodic composi- triangular inset. Analyses from all samples indicate a range of An.r-r". However, tions in the series.It is also supportedby textural each individual sample has a more restricted range which varies with compositions of the mafic minerals. constraints, Sam- such as the euhedralshapes of olivine ple DC 8 contains plagioclase in the range An.,_r. (upper bar); and opaque minerals in early cumulate rocks, the plagioclase in DC I has An.r-r, (lower bar). MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION 541

;;::.:i

Fig. 9. Mineral reactionsin gabbrosamples. (A) Euhedralmagnetite crystal (center) surroundecl by ilmenitegrains in hornblende- pyroxenegabbro (reflected light). White subparallellamellae in ilmeniteare hematiteformed by subsolidusoxidation. (B) Intergrowth of greenspinel (Sp) with magnetite(Mt) and ilmenite(Im) in anorthositicgabbro; scale as in (A). (C) Rimsof red-brownbiotite around magnetite/ilmenitein contactwith plagioclasein pyroxenegabbro. (D) Vermicularintergrowths of magnetitewith orthopyroxenehost in pyroxenegabbro (reflectedlight). (E) Symplectiteof aluminousorthopyroxene and plagioclasein pyroxenegabbro (arrows).The intergrowthabuts amphibole (Am) and orthopyroxene(Opx) beingreplaced by magnetite;scale as in (A). (F) Olivinegrain (lower right) in pyroxenegabbro replaced by iddingsitealong internal partings. The olivineis enclosedpoikilitically by pyroxene,which has reacted to form magnetiteand minor amphibole(dark spotsperpendicular to Bushveld-typeto exsolutionlamellae). Scale as in (A). has explainedsimilar orthopyroxene-magnetite sym- more Ca-rich(An") than adjacentplagioclase grains plectitesas resulting from subsolidusoxidation of (-Anuo). Theseintergrowths may representearly-set- olivinein the compositionalrange Fo6s-?u. This cor- tling glomerocryststhat have partially reacted.Al- respondsto the range of olivine compositions though'reactionrims of pyroxeneand hornblende (Forr-ru) in which symplecticintergrowths occur in around olivine are not observed,the rare olivine the Dutchmans Creek samples.Symplectic inter- grainsin pyroxenegabbro and hornblende-pyroxene growths of aluminous orthopyroxene and calcic gabbroare replaced along partings by iddingsiteandl plagioclase(Fig. 9E) are also commonin pyroxene or magnetite(Fig. 9F). Hornblendecommonly re- gabbro samples.These symplecticpyroxenes have places clinopyroxenealong grain boundaries. In Fel(Fe+Mg) ratios identical to other ortho- hornblende-pyroxene gabbro, hornblende com- pyroxenesin the host rock, but higherAlzOs contents pletelyreplaces large poikilitic clinopyroxenecrystals (3.4-3.8weight percent).Associated plagioclase is exceptfor a few small relict areas,with only minor MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION

alterationof the texturesof the enclosedplagioclase phic propertiesor mineral chemistrythat correlated andmagnetite (Fig. 4D). Fewolivine grains or relicts with locationwithin the plutons.Hermes did report are containedin theseoikocrysts, indicating that oli- that rocks with poikilitic textures occurred near vine was practicallyexhausted as a liquidusphase Mecklenburgpluton margins,while granularrocks beforethis melt crystallized. containingmore hydrous phases were common in the The mineral reactionsdescribed in the preceding interior portions.The DutchmansCreek gabbro is an paragraph are apparently late-magmaticchanges irregularly-shaped,elongate intrusion (Fig. l), in producedby local equilibriumbetween early crystal- contrastto the more circularoutlines of the other lizingphases and residualvolatile-rich melt. A sum- plutons.This irregularityin shapemakes a cleardefi- mary of the combinedcrystallization and reaction nition of arealvariations within the body extremely sequenceinferred from petrographicobservation of difficult.The locationsof the gabbrosamples ana- DutchmansCreek rocks is as follows: lyzed in detail (Fig. 2) suggestthat rocks from the (l) early magmatic-separationof immisciblesul- pluton interior(olivine gabbro, anorthositic gabbro) fide blebsfrom the silicatemelt; olivine,plagioclase, havecumulate or granulartextures and are lessdif- and primary magnetiteand ilmenitecrystallize from ferentiatedthan the poikilitic rocksnear pluton mar- the melt; settlingof early-crystallizingphases to pro- gins (pyroxenegabbro, hornblende-pyroxenegab- ducesome cumulate rocks and fractionatedliquid; bro), but sucha conclusionis tenuousbecause of the (2) intermediatemagmatic-crystallization of or- limited numberof samples.However, the analyzed thopyroxeneand clinopyroxene,enclosing pre-exist- samplesindicate one readily-observableindex of dif- ing grains poikilitically; crystallizationof apa- ferentiationin theserocks: the modal contentof oli- tite and continuedcrystallization of plagioclaseand vine. The proportion of olivine decreasesregularly opaqueminerals; with increasingdifferentiation, as is clearlyindicated (3) late magmatic-reactions between residual in Figure3. We haveestimated olivine contents in a melt and (a) orthopyroxeneto form magnetite,(b) representativenumber (43) of thin sectionsfrom magnetiteand ilmenite to stabilizebiotite, (c) clino- other partsof the pluton, and find that rocksbearing pyroxeneto form hornblende;recrystallization of appreciablequantities of olivine(> l0 percent)ap- opaque minerals,reaction betweenmgnetite and il- pear to be concentratedonly in the pluton interior meniteto form greenspinel, and subsolidusoxidation (Fig. l0). We thereforesuggest that the Dutchmans of ilmeniteto form hematite. Creek pluton exhibits a regular areal variation in This third (late-magmatic)stage of reactionis well rock types(and therefore mineral chemistry, since the developedonly in more highly differentiatedrocks, two are linked) causedby differentiation. such as pyroxenegabbro and hornblende-pyroxene At face value, this areal variation in rock types gabbro. All samples,however, exhibit some late-mag- suggeststhat fractionalcrystallization has proceeded matic mineralreactions. The proposedcrystallization from the interior outwards to the pluton margins. model is similar to the sequenceadvocated by Her- However,this appearancemay resultfrom erosional mes(1968) for theMecklenburg gabbro, although the exposureof the pluton to differentlevels. The interior early magmaticcrystallization of Fe-Ti oxidesand portions of the pluton representlower topographic the lack of late-magmaticdevelopment of olivine co- levelsthan the pluton marginsbecause of erosionto ronasdistinguish the DutchmansCreek rocks. Simi- deeperlevels. Olivine gabbro and anorthositicgab- larly, thereis little evidencethat significantquantities bro, both of which are probably cumulates,have of opaque mineralsever precipitateddirectly from been collectedfrom topographic levelsof 230-300 the melt at any time during the crystallizationof the feet, whereaspyroxene gabbro and hornblende-py- Buffalo pluton (Medlin et al., 1972).and olivine co- roxenegabbro have been collected from the 270-400- ronasare well developedin theserocks aswell. Med- foot contour interval.Therefore. it is more likelv that lin et al. alsoindicated that clinopyroxene,along with the plutonmay havecrystallized from differentbatch- olivine and plagioclase,were early-crystallizing min- es of magma successivelyinvading the chamber erals. minerals, and the higher portions of the chamber werepredominantly the residualliquid. Butler and Areal variationswithin the pluton Ragland(1969) indicated that layeringin othergab- The two previously-publisheddetailed studies of bro plutons in the Piedmont is rarely well-defined, Piedmontgabbros (Hermes,1968; Medlin et al., and the few occurrencesnoted were suggestedto 1972) found few systematicvariations in petrogra- result from flowagerather than gravitationalcrystal MCSWEEN AND NYSTROM: DL]TCHMANS CREEK GABBROIC INTRUSION 543

tite and hornblendebecome stable phases (Best and Mercy, 1967). The early crystallizationof some magnetitewith magnesianolivine in the DutchmansCreek pluton is likely a resultof relativelyhigh watercontent and the high concomitant.fOz values(Kennedy, 1955; Os- born, 1962;Hamiltonet al.,1964).During fractional crystallizationat high .fO2, magnetite and olivine coprecipitateand the residual liquid is enrichedin MODATOIIVINE silica(the Bowentrend), whereasat low../02,Fe-rich * )t0% olivinecrystallizes without magnetiteand the residual . (10% melt is more Fe-rich (the Fennertrend). The leuco- cratic dikeswhich crosscutthe pluton are especially abundant in more highly differentiatedmarginal )10 percentand (10 Fig. 10.Distribution of rockscontaining rocks and may representa late-stage,silica-enriched percentmodal olivine in the DutchmansCreek gabbro. Rocks containingappreciable olivine outcropin the interior portionsof melt.The observedearly precipitation of magnetitein the pluton. the DutchmansCreek magma conflicts with the inter- pretationof Butlerand Ragland(1969) that opaque mineralsin Piedmontgabbros were late in the crystalsettling.It is possible,however, that the scaleof lizationsequence because ofsolidification under con- layering in theseplutons is too great to be seenin ditionsof lower./C2.However, it shouldbe notedthat bouldersor smalloutcrops. although there is somecumulate magnetite in early The possibility that different stratigraphiclevels differentiatesof the DutchmansCreek pluton, much of the pluton may have crystallizedfrom different of the magnetiteappears later in the crystallization batchesof magmasuccessively invading the chamber sequence(Fig. 3). Euhedralcumulate olivine has not (Colemanet al., 1977)cannot be excluded.However, beenreported in other post-metamorphicgabbros of sucha complexmodel does not seemto be necessary the Piedmont(Hermes, 1968; Medlin et al., 1972). for this pluton. The water was probably dissolvedin the magma prior to emplacement,rather than beingabsorbed by The influenceof water diffusion from the surroundingcountry rocks after From the abundanceof hydratedphases in all sam- intrusion.Shaw (1974) has argued that hydrationofa ples of the DutchmansCreek gabbro, it is evident magma by diffusion is probably limited to within that this magmacontained an appreciablequantity of severaltens of metersof the pluton contacts,and water, although a quantitativestatement cannot be isotope systematicsindicate that water circulating made.The high dissolvedwater content is responsible near the margins of intrusions is mostly absorbed for many of the petrographicfeatures observed in only after significant crystallizationhas occurred theserocks. Hermes (1968) noted that the Mecklen- (Taylor, 1974;Taylor and Forester,l9T l, 1973;Shep- burg gabbro also crystallizedat high PHzO, and the pard et al., 1977). following discussionmay apply to other Piedmont plutonsas well. Comparisonwith other Plutons Ca-rich pyroxenesin the DutchmansCreek gab- problem bro, as well as the Buffalo and the Mecklenburg The gabbros, exhibit a very restricted Fe-enrichment The DutchmansCreek pluton is similar to other trend (Fig. 5) similar to pyroxenesfrom other hy- post-metamorphicgabbro intrusions in the southern drous gabbroic intrusions,such as the Marangudzi Appalachian Piedmont, such as the Buffalo and ring complex,Rhodesia (Hossain, 1977),the Guada- Mecklenburggabbros. However, a detailedcom- lupecomplex, California (Best and Mercy, 1967), and parison suggeststhat the DutchmansCreek gabbro the Jabal Sha'l' gabbro,Saudi Arabia (Colemanet has beendifferentiated, and systematic,albeit subtle, al., 1977).Fe-enrichment in clinopyroxenescrystal- variationsoccur, whereasthe other two plutons are lized from hydrous magmasmay be preventedbe- largely undifferentiated,and variations are unsyste- causeclinopyroxene is a stable liquidus phase for matic (Hermes,1968; Medlin et al., 1972).The distri- only a short crystallizationinterval, after which bio- bution of modal olivine, which varies regularly 544 MCSIryEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION

enoughto be an indexof differentiationin the Dutch- thesemineral segregationsmay representearly-crys- mansCreek pluton, is randomin otherplutons. This tallizedmagma, possibly rafted upward from below major difference,together with the presenceof early- during magmaintrusion. These xenoliths crystallized crystallizingFe-Ti oxidesand absenceof olivine co- at lower PHrO (evidencedby generallack of hydrous ronasin DutchmansCreek rocks, suggests that the minerals)and.liC, (suggested by red oxidizedreaction crystallizationhistory of this pluton was somehow rims on the xenoliths)than the enclosinggabbro. differentfrom other plutons with which it has been Alternatively,the xenolithscould be formed by sink- comparedin this study. ing of piecesof the upperchilled margin which crystallized beforesignificant quantities of water could A possiblesolution migrateupward. Medlin et al. (1972)described simi- It was previouslynoted that the closecorrespon- lar granularxenoliths in the Buffalopluton as"segre- dencebetween topography and the pluton margins gatedmineral clusters," and thesemay haveorigi- and includedroof pendantsindicates that much of natedin a similarmanner. the DutchmansCreek pluton has beenbarely un- Conclusions roofed by erosion.An increasein temperatureand confiningpressure with depth in a magmachamber The DutchmansCreek intrusion crystallized under could result in a tendencyfor dissolvedwater to conditionsof relativelyhigh PHrO and.lOr.Although concentratein the uppermostportions of the cham- similar in many respectsto other post-metamorphic ber (Kennedy,1955), especially if it separatesas a gabbrosin the Piedmont,the DutchmansCreek plu- distinctaqueous phase (Burnham, 1967). This con- ton has severalunique characteristicswhich may re- centration of water would result in higher oxygen sult from the fact that the intrusionhas barely been fugacitiesduring crystallization.This differencemay unroofedby erosionand only the uppermostregions explainthe early crystallizationof Fe-Ti oxides,and of the magmachamber are visible.Topographically later subsolidusoxidation of ilmeniteto hematite. lower portions of the pluton appearto have crystal- Other bodies,such as the Mecklenburgand Buffalo lized beforemore differentiatedroof rocks,possibly complexes,may representdeeper levels exposed by as a resultof crystalsettling and water migration to erosion.It is possiblethat the upperportions of these the top of the chamberwith subsequentlowering of magma chamberswere similar to the Dutchmans liquidus temperaturesin the volatile-richportions of Creekpluton asnow exposed,and that deeperlevels the chamber. Early crystallizationof olivine and of the DutchmansCreek pluton arecharacterized by magnetiteprevented significant Fe-enrichment trends lower water contentsand oxygenfugacities. The in- in ferromagnesianminerals. The highdissolved water creasedwater content in magma at the roof of the content facilitateddiffusion and augmenteda series chambermay alsohave resulted in (l) lowerliquidus of late-magmaticmineral reactions, which areespe- temperaturesand thereforelater crystallization,and cially well developedin more highly differentiated (2) more pervasivelate-magmatic mineral reactions. marginalsamples. Verticalinhomogeneity of the pluton in termsof water content is more likely than horizontal varia- Acknowledgments tion. Migrationof waterto thecontacts of thepluton We have greatly benefitedfrom discussionswith Arthur W. (the Soret effect)is minimal becauseof the nega- Snoke on field interpretations,and from critical reviewsby J. Robert Butler and Robert This work partially tive temperature-dependenceof water solubility G. Coleman. was in supportedby the SouthCarolina Geological Survey (N. K. Olsen, magmaand slow diffusionrate of wateror its ionic StateGeologist). products(Burnham, 1967).However, a separated aqueousphase could rise due to gravity effects. References Such a verticalstructure in Piedmontgabbro plu- Best,M. (1967) tonsmay alsobe suggestedby the presenceof horn- G. and E. L. P. Mercy Compositionand crystallization of mafic mineralsin the Guadalupeigneous complex, felsic, largely anhydrousxenoliths. Xenoliths in the California.Am Mineral.,52, 436-474. DutchmansCreek pluton contain highly magnesian Burnham, C. W. (1967) Hydrothermalfluids at the magmatic olivinesthat are largerthan the other granularmin- stage.ln H. L. Barnes,Ed., Geochemistyof HydrothermalOre eral grains and may be partially resorbed Deposits,p. 34-76.Holt, Rinehart,and Winston,New York. Butler, (1966) phenocrysts.The mineralchemistries of the xenoliths J. R. Geology and mineral resourcesof York County, SouthCarolina. Div. of Geol., S.C. StateDevel. Bd., are obviouslymore primitive (lessFe-rich) than the Bull.33. enclosinggabbro (DC 3x, Fig. 5). We suggestthat - and P. C. Ragland(1969) A petrochemicalsurvey of plu- MCSWEEN AND NYSTROM: DUTCHMANS CREEK GABBROIC INTRUSION 545

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