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Mineralogy and Petrology of the Dutchmans Creek Gabbroic

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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 shownin 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 -N-A . \r, I -ts.-:,tri2c,/ (l_-r. -:|ii:\,-?r_- :i'="i1:.:;'/ &l+t1 #i...-i, !i,,! t) i;,ii;)1;-1.'/ o\"i_;J lii!,.. il 'i :Trj r, I ,:i: l:);ild rn. 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 horn- blende-pyroxenegabbro. Apatite generallyoccurs only in pyroxenegabbro and hornblende-pyroxene gabbro,along with minor epidoteand sericite. Systematictextural variations occur
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