Canadiutt III ineralogistVol. 19, pp.35-46 (1981)

PETROLOGY OF CORDIERITE. AND ALMANDINE.BEARING GRANITOID PLUTONSOF THE SOUTHEBN APPALACHTAN PIEDMONT, U.S.A.

J. ALEXANDER SPEER

Orogenic Studies Laboratory, Department of Geological Sciences, Vlrginia PolytechnicInstitate and State University, Blacksburg, Virginia 24061 , US.A.

ABsrRAcr

The most distinctive peraluminous granitoid plu-tons in the southern Appalachians are: (1) the LatePaleozoic Clouds Creek pluton (313 -t. 2 Ma\ inSouth Carolina, a composite body consisting of bio-tite and cordierite-biotite monzogranite and grano-diorite, and (2) the Precambrian (?) cordierite-biotite granite pluton of Stumpy Point. NorthCarolina. The cordierite in both plutons is mag-matic, with Na2O contents of up to 1.5 wt. Vo andFel(Fe*Mg) - 0.424,52 in the Clouds Creekpluton and 0.56-.0.81 in the Stumpy Point pluton.The biotites are siderophyllites with Fe/(Fe*Mg)- 0.65-0.78. Tourmaline, monazite and ilmenite arecharacteristic accessory minerals. Magnetite. a prom-inent early phase in the Clouds Creek pluton,was consumed in a subsequent reaction. An inclu-sion assemblage of almandine (AImeo.rPVro.zSpr."Grr.") * biotite :! cordierite in the Stumpy Pointgranite and andalusite-bearing autoliths in theClouds Creek pluton record changing conditions inthe magmas. Compared with the tvpical Late Pale-ozoic postmetamorphic calc-alkaline eranitoidplutons of the region. the Clouds Creek has hadmore interaction with older crustal rocks rich inaluminum and boron.

K ey words : peraluminous granites, cordierite. alman-dine. Clouds Creek (South Carolina). StumpyPoint (North Carolina).

SoMMATRE

Les massifs granitiques hyperalumineux des Ap-palaches m6ridionales les plus distinctifs au pointde vue min6ralogique sont les plutons de CloudsCreek (en Caroline du sud) et de Stumpv Point(Caroline du Nord). Le premier, d'6ge Pal6ozoiquesup6rieur (313 -f 2 Ma\ et d'aspect composite,contient des unit6s granodioritiques et monzograni-tiques i biotite et d biotite a cordi6rite: le second,d'dge Pr6cambrien(?) consiste en granite d cordi€-rite + biotite. Dans les deux cas. la cordi6rite estprimaire, magmatique; elle posslde iusqu'i 1.5%de Na2O (en poids). Son rapport Fe/(Fe + Mc)varie de 0.42 iL 0.52 (!r Clouds Creek) et de 0.56n 0.81 (A Stumpy Point). Dans les biotites (pluspr6cis6ment. des sid6rophyllites), ce raDDort variede 0.65 b 0.78. Tourmaline. monazite et ilm6nitesont les accessoires caract6ristiques. La maqn6tite,

phase hdtive importante dans le pluton CloudsCreek, a 6t6 d6truite par r€action. Une suite d'in-clusions i grenat (Almao.rPy,o:Spn.rcr4.?) + biotite:L cordi€rite (Stumpy Point) et d'autolithes i an-dalousite (Clouds Creek) illustrent l'6volution dumilieu magmatique. Compar€ aux intrusiqns grani-tiques calco-alcalines postm€tamorphiques typiquesdu Pal6ozoique supdrieur de la r6gion. le massif deClouds Creek montre les effets d'une interactionplus pouss6e avec un socle enrichi en Al et B.

(Traduit par la R6daction)

Mots-dAst granites hyperalumineux, cordiErite, gre-nat almandin. Clouds Creek (Caroline du sud).StumDv Point (Caroline du nord).

INTRoDUCTIoN

Speer er a/. (1980) reported that the CloudsCreek pluton, South Carolina, contains cordi-erite, but the extent and nature of the cordierite-bearing granitoid rocks were unknown. Subse-quent study of the field relations, petrographyand mineral chemistry of the Clouds Creek plu-ton, described in this paper, revealed a promi-nent facies containing magmatic cordierite. Afew other Late Paleozoic, postmetamorphic,coarse grained granitoid plutons of the southernAppalachians are peraluminous, but the CloudsCreek is conspicuous in its distinctive mineralogyand mineral chemistry.

Samples recovered from' a drillhole nearStumpy Point, Dare County, North Carolina,examined as part of a study of the crystallinebasement underlying the Atlantic Coastal Plain,were found to be almandine * cordierite + bio-tite granite. Texturally, the rock appears to bepostmetamorphic, but a whole-rock, model Rb-Sr isotopic age of 924 +- 24 Ma hasbeen reported (Daniels & Zietz 1978).In this article, these two cordierite-bearinggranitic plutons in the southern Appalachiansare described, although (l) the availabledata suggest that the rocks at Stumpy Pointare not the product Late Paleozoic magmatism,and (2) its geological setting is hidden under1.8 km of Coastal Plain sediments.

35

36 THE CANADIAN MINERALOGIST

o

-0oo9

I(J

Clouds Creek Pluton, .u....i4South Corolino ,sl '"Flu' '*

$ ,.!1.:lo''l,cordierirE-biofire groniroid rocks

ffi .",*:";j:*rr:

H ;:::,';::;:,::"'' ffi ; ."":' . zft;AA :1ff#il:i:"1%,r*",*"

fffir*K,EV)

FIc. 1. Map of the Clouds Creek pluton, Saluda County, South Carolina, showing distribution of rockfacies (modified from Secor & Snoke 1978, Plate 2) and localities of samples used for microprobe studies,presented in Tables 1 to 12. The index map includes the location of the Stumpy Point almandine + cor-dierite -f biotite granite encountered in the Cities Service Westvaco well 1A, Dare County, North Caro-lina.

h.''. I hornfels of thetiJ Corolino Slote Belt :.;

Petnocnepuy

Cloucls Creek pluton

The Clouds Creek pluton crops out in anelongate, elliptical area of approximately 50km2 in Saluda County, South Carolina (Fig.1). It was first mapped as a dissrete pluton byOverstreet & Bell (1965a, b). Field relations andthe geological setting were reported by Secor &Snoke (1978). Earlier petrographic descriptionsare given in Sloan (1908), Watson (1910) andWagener (1977). Fullagar & Butler (1979) rc-ported a Rb-Sr isotopic age of 313 -+ 2 Mawith an initial 87Sr/88Sr ratio of 0.7099 =0.0001. An interpretation of the aeroradioactivi-ty and aeromagnetic survey of the area is givenby Daniels (1971).

ryftzqgY-"

The Clouds Creek pluton intruded the Caro-lina slate belt (Fig. 1). Country rocks adjacentto the pluton are metamudstones, metasiltstonesand metagreywackes of the Cambrian(?) RichtexFormation. The intrusive contacts are sharp andgenerally parallel the regional foliation of thecountry rocks, but are locally discordant. On itssouthern border, the pluton is bounded by augengneisses of the Kiokee belt, which are separatedfrom the Clouds Creek by the Modoc faultzone.

The Carolina slate-belt phyllites have under-gone contact metamorphism within 0.5 km ofthe contact. The most evident change is textural.With increasing proximity to the pluton, the rockchanges from fine grained phyllite to finegrained granoblastic hornfels spotted with ag-

North Slumpy

. CloudsCarollna

CORDIERITE-BEARING GRANITES OF THE PIEDMONT 37

gregates of muscovite up to 2 mm in diameter.The regional metamorphic assemblage of theCarolina slate-belt phyllites is magnetite + epi-dote + biotite + phingitic muscovite + albite+ quartz. The contact metamorphic assemblageis ilmenite + magnetite * biotite -r chlorite *muscovite + plagioclase (Anso) + quartz.Locally, the hornfels is dominated bymuscovite + quartz. Th9 texture, mineral as-semblage and mineral chemistries of the horn-felses are comparable to the hornfels producedin phyllites of the Carolina slate belt in contactaureoles of other postmetamorphic granitic plu-tons of the southern Appalachians. The abund-ance of muscovite-rich rocks in the aureolesuggests widespread metasomatism of the horn'fels, presumably by the granite.

The Clouds Creek is a composite pluton,which Secor & Snoke (1978) have divided intotwo parts: (1) a megacrystic granitoid bodyranging from massive to gneissic and (2) a heter-ogeneous mafic complex of biotite-amphibolequartz diorite on the eastern edge of the pluton(Fie. 1). Many of the amphiboles in the mafic

complex either are pseudomorphs of earlierpyroxenes or contain clinopyroxene cores, whichindicates that the rocks were originally gab-broic. Consanguinity and the age relationship ofthe two rock types are problematical because ofthe limited exposure. Only the granitoid rocksare discussed here. The megacrystic granitoidbody has three mappable facies: (l) massive,biotite-bearing granitoid rocks, (2) massive, cor-'dierite-biotite-bearing granitoid rocks and (3)foliated biotite- or cordierite-biotite-bearinggranitoid rocks (Fig. l).

The massive, biotite-b€aring rocks occur alongthg western border of the pluton.- The areacovered by this facies ranges up to 2 km wideand narrows to less than 0.5 km on the north'ern contact. The massive, cordierite-biotitegranitoid facies occupies the core of the north-ern half of the pluton. The contact betweenthe biotite and cordierite-biotite granitoid faciesis gradational. Both facies have the same generalappearance in that they are subhedral-inequi-granular, coarse grained, blue-grey rocks withblocky, subhedral megacrysts of grey alkali feld-

K-feldspor

O Stumpy Poinf, NCo Clouds Creek, SC

Plogioclose

Frc. 2. Modal distribution of quartz. alkali feldspar and plagioclase in the

Clouds Creek and Stumpy Point plutons. Rock names are those recom-mended by the I.U.G.S. sttbcommission on the Systematics of IgneousRocks classification.

38 THE CANADIAN MINERALOGIST

spar up to 4 cm long. Modal analyses of stainedslabs and thin sections show that they are mon-zogranites and granodiorites with a color indexbetween 9 and 15 (Fig. 2). Near the contactsof the pluton, the massive granitoid facies con-tains fewer and smaller alkali feldspar mega-crysts. This gives the rock a more equigrannlarappearance. Biotite occurs as subhedral flakes2 mm or less in size. Commonly, biotite is re-pl-aced or rimmed by a finer grained aggregateof recrystallized biotite. Cordierites of the ior-dierite-biotite granitoid facies are waxy, blue-green, sector-twinned prisms up to 3 mmlong and comprise between I and 5 modal /oof the rock. Accessory epidote, allanite and ti-tanite are largely confined to the biotite. grani-toid rocks. Imenite is the major opaque phase,but magnetite is widespread. Much of the origi-nal magnetite has been consumed in subsequentreactions. The former abundance of magnetiteis indicated bv skeletons of the exsolved ilmE-nite. Other primary accessory minerals includeapatite, tourmaline, zircon, pyrite, chalcopyriteand monazite. Secondary minerals inslude chlo-rite, muscovite, kaolinite, rutile, epidote, cal-cite and titanite as rims on ilmenite.

Rocks of the foliated granitoid facies occupythe southeastern third of the pluton adjacentto the Modoc fault zone. They are diverse inappearance, varying from gneissic granitic rock,prqtomvlonite, or porphyroclastic mylonite to alineated blastomylonite. The foliation is definedby oriented biotite and elongated ovoid feld-spar and quartz porphyroclasts. The feldsparsare broken in the less-deformed rocks. The de-forrnational effects gradually increase, thoughnot uniformly, towards the Modoc fault. Asshown in many outcrops, the deformation washeterogeneons, because augen and lineatedgneisses are associated with macroscopically un-deformed megacrystic granitoid rocks. The ex-treme southeastern corner of the pluton is com-posed of relatively undeformed cordierite-biotitegranite (locality 38, Fig. l). The primary min-eralogy of the foliated granitoid facies includesbiotite, cordierite, plagioclase, alkali feldsparand quartz, with accessory ilmenite, magnetite,apatite, zircon and pyrite. Secondary mineralscontemporaneous with the deformation includerare to abundant epidote and muscovite andsubordinate chlorite, titanite" barite and allanite.The primary biotite is commonly recrystallized,with a differing composition, as dissussed be-low. Whether or not cordierite was originallypresent cannot be determined in every case be-cause of the difficulty in recognizing deformedand pinitized cordierites.

Only two rock analyses have been publishedfor the Clouds Creek pluton. One is a.'biotitegranitoid rock from the Praetor quarry (locality63, Fig. 1; Sloan 1908); the other is an aver-age of four rock analyses for which locationsare not given (Fullagar & Butler 1979). Therocks are corundum-normative, with 3.1 and 3.6wt. Vo corundum, and the molar ratio AlzOr/(CaO+Na,O+K'O) is 1.41 and 1.51, respect-ively. The Fe/(Fe+Mg) ratios are 0.71 and0.68.

Several types of enclaves can be distinguishedin the Clouds Creek pluton. Xenoliths of rocksfrom the adjacent Carolina slate belt are themost readily identifiable. They are subangularto rounded and range up to 0.5 m in diameter.They consist of laminated biotite 1 muscovite-l- plagioclase + quartz hornfels. These enclavesrqsemble the hornfelses of the aureole exceptthat they are coarser grained and contain morehiotite. Less common in the Clouds Creek plu-ton are xenoliths of amphibolite and vein quartz.

A second type of enclave consists of melano-cratic, microgranular cordierite-biotite grano-diorite that contains subhedral phenocrysts ofquartz and compositionally and texturally zonedplagioclase up to 2 cm in length..These arerounded enclaves up to 0.25 m in diameter.The groundmass consists of grains up to 2 rnmacross of biotite + cordierite f plagioclase(An'0) + alkali feldspar + quartz. The cor-dierite is generally subhedral and rimmed eitherby quartz or by a graphic intergrowth of quartz* cordierite. These composite grains range upto 0.5 cm in diameter. They resemble the gra-phic intergrowths of cordierite and quartz de-scribed by .shibata (1936) and are similarlybelieved to result from simultaneous crystalliza-tion rather than replacement of an earlier phase,as described by Henry 0974) and van Reenenand dtr Toit (1978) for vermicular quartz r-cordierite symplectites. The groundmass of theenclaves is dominated by unoriented laths ofplagioclase and biotite, which gives the ground-mass a bostonitic texture. Accessory mineralsinclude muscovite, magnetite, ilmenite, apatiteand monazite. An additional, scarce poikiliticmineral pseudomorphed by kaolinite has a habitthat suggests andalusite. Unlike xenoliths derivedfrom the Carolina slate belt, the mineralogy andmineral compositions of these microgranular en-claves are similar to those of the enclosinggranitoid rocks; they differ only in texture andmodal abundances. These microgranular en-claves could have several origins: (l) autolithsof an early crystallizing facies, (2) xenolithsthat have been subjected to a higher grade of

metamorphism and have equilibrated withenelosing m&gma; or (3) restites from theof magma generation.

Stumpy Point pluton

The cordierite-biotite granite from StumpyPoint, Dare County, North Carolina (Fig. l),is the crystalline basement encountered in theCities Service Westvaco well no. 1A completedin September 1971. The drillhole is located at35o39'36'N, 75" 46'4O-W (Coffey 1977). Base-ment was encountered at 1862.9 m (61 12 ft)and total depth is 1916.6 m (6288 ft). A con-tinuous core sample 10 cm in diameter wastaken between 19O0.7 (6236 ft) and 1907.7 m(6259 ft). This petrological study is based onsix sections of core, each 5 cm long, takenfrom depths of 6236, 6243,6248, 6253, 6256and 6257 feet, which are also the sample num-bers.

The Stumpy Point granite is grey-green withan anhedral-inequigranular texture. Most grainsare smaller than 7 mm, but subhedral mega-crysts of quartz up to I cmo plagioclase up to2.5 cm and alkali feldspar up to 1.5 cm occurlocally. A composite modal analysis of 1462points, located on a 5 x 5 mm grid on stained

CORDIERITE-BEARINC GRANITES OF THE PIEDMONT

so3tonlte

K2Mg6Al2S|6O29(OH)4ghlogopifo

TABLE 13. SIJI.II,IARY OF I'IICROPROBE AMLYSES OF THE FERROI4AcTESIATI MINERALS!

CLOUDS CREEK PLUTON, 5. C., AND STIJI'IPY POINT GRANITE' II. C.

F e / ( F e + M g )

Frc. 3. Biotite compositions from the Clouds Creekand Stumpy Point plutons (Table l) projectedonto the phlogoirite-annite--eastonite-siderophyl-lite field. Solid circles refer to biotites coexist-ing with cordierite (w/cd); open circles refer tobiotites in rocks without cordierite (w/o cd).

thesite

39

KzFe6A12Si602O(oHhonnilo

=3z 2.8=fJ

J

G(f 2.4lrlI

E,F 2.2IrlF

*Stumpy Point, NCo0louds Creek, SC w/cdo0louds Creelq SC v/ocd .

<-'---- ------clouds ctek blotltes------ -----)> st@py Polnt

<qodlerltgblotlte granJtes> <---biotlte granJtes----> mlcrcgranular grallte

origlnal r4rystsll lzed o|lglnal Erystal l lzed enclaves blotlte

34.01 (1 .31 )2.29(0,991

20.080.50)23.80(1 . I0 )0 .17(0 .04)4 .38(0 .23)0 .06(0 .04)0 .22(O,21)o . l2 (0 .05)8,86(0.36)0.i l (0.i l )0 .04(0 .01 )

-rf.66'

st 0,fi0iA l r 0 ?Fed*'Irln0llS0Ca0Ba0Nar0K?0tctHlo*T6ta l0 : FiCltola I

Fe,/ ( F*ilS )Nuber ofanalyses

34.33(0.96)*3.53(0.95)

r6 .97(0 .4r )3.53(0.95)

r6 .97(0 .4r )24,3611 .2610,30(0 .05)6.48(0,45)o .0r (0 ,01 )0 .18(0 . r7 )0 . r0 (0 .05)9 ,72(0 .36)0 .33(0 ,23)0 . r3 (0 .07)

35.32(0.66)1 .88(0 .52)

16.87(0.33)25,42(1.44)0 .30(0 .07)6 .e4(0 .58)0.02(0.02)0 .21 (0 .23)0.04(0.03)9 .69(0 .45)0 .27(0 . r2 )0 . r3 (0 .08)

34,38(0,41 )2 .73(0 .3e)

r6 .76(0 .26)25.80(0 .76)0 .33(0 .04)6 .62(0 .35)0 .0s(0 .03)0 .27(0 , i l )0 .05(0 .0r )9 .49(0 .29)o.3l (0.0s)0 . r7 (0 .04)

]llo3z0 . 1 7

100.450.689(0.005)

l0

35.53(0 .61)r .67(0 .62)

r7.05(0.44)25.37(1 .10)0.33(0.05)6 .5e(0 .53)0.04(0.03)0 ,20(0 .18)0 .05(0 .02)9 .66(0 .26)0 . 1 6 ( 0 , 1 6 )o . l0 (0 ,07)3.75

T00:500.09

100.410.686(0.007)

34.6S(0.68)3.54(0.56)

t7 .44(0 .66)23.51(0 .77)0 .22(0 .05)6 .49(0 .46)0.03(0.03)0 . r3 (0 .14)o .09(0 ,03)

0 .45(0 . r3 )0 .16(0 .02)3 .60

T00.o{TOtrTd0 . 1 7

99.92

0.681 (o.oo4)

Totr5q0 . 1 4'100.55

0.67S(0.005)

.10

0.22 0 .0699.82 97.81

0.672(0.007) 0.755(o.oo2)

43

<-------Clouds CEk cordlerltes----->

1 6 1 2

-<------Stmpy Point granlte---->

Sl0rroiAlrOqFe0*'MnollS0ca0ila20KzoTdbl

cord l erlte

4 6 . 4 9 ( 1 , 0 7 )0 . 0 0 ( o )

3r . r r (0 .58)14 .340.20)0 .36(0 .06)3 .39(1 .07)0.ol (0.01 )1,12(0.27)0 .0r (0 .02)

E6S-

0.709

8

cd.-bt. granlte

47.74(0.6r )o.0l (0,02)

32.48(0.61 )9 ,78(0 .?)0 .50(0 .07)6 .32(0 .34)0.00(0)0.69(0.22)0 .00(0)

nlcrcgfanularenclave

48.19(0 .20)0 .00(0)

31 .82(0 .29)'10 .33(0 .48)0 .51 (0 .06)6.23(0.38)o .02(0 .02)0 .66(0 .35)o .o1 (0 .01 )q . E

garnet

3 7 , 5 70 . 0 0

20.5235.292. ' t02 . 5 1I .600 . 0 00 . 0 2

0.887

2

Fe/(Fe+l ' lg) 0.477 0.494Nuder ofanalyses 16 8

* Total l rcn erpressed as Feo. H Calculated to give 24(0,F,Cl,ClH), ** Nmber ln parentheses ls thestandard devlatlon,

40 THE CANADIAN MINERALOGIST

ceous biotites, those containing the most tetra-hedral aluminum, occur in rocks that containcordierite, namely, the cordierite-biotite graniteand microgranular enclaves (Fig. 3, Table 13).The biotites in the biotite granites are moresilica-rich, but the most silica-rich are the re-crystallized biotites. Another compositional dis-tinction of the recrystallized biotites is theirlow titanium content. The higher titanium con-tents of the original biotites produce the inter-grown ilmenite and rutile.

The biotites of the Stumpy Point granite areiron-rich; Fe,;*orl (Fe*'*Mg) is 0.76. The rangeof tetrahedral aluminum contents is between2.48 and 2.73 per 24(O,OH,CI,F) and producesa vertical trend in the biotite quadrilateral (Fig.3 ) .

The biotites in the Clouds Creek and StumpyPoint plutons are more aluminous and moreiron-rich than are biotites of the postmetamor-phic, coarse grained granitoid rocks of thesoutheastern United States (Fig. 3). Biotiteswithout coexisting cordierite in the CloudsCreek pluton overlap the biotite compositionalfield of these other granitoid rocks. A similariron and aluminum compositional distinction ofbiotites coexisting with aluminous minerals hashas been noted by de Albuquerque (1973) insillimanite- and andalusite-bearing granites ofnorthern Portugal and by Clarke et al. (1976)in the South Mountain batholith. Nova Scotia.The aluminous nature of these biotites wouldbe anticipated from the coexisting cordierite oraluminum silicates and the presumed aluminum-rich nature of the melt from which they crystal-lized. In the case of the Clouds Creek pluton, theiron-rich nature of the biotite may be a result ofreactions that consumed most of the magnetite.

Cordierite

In the Clouds Creek and Stumpy Point plu-tons, cordierite constitutes between I and 5modal 7o of the rocks. It forms subhedral.sector-twinned prisms up to 3 mm long that aregenerally free of inclusions but locally containzircon, biotite or opaque minerals. The cor-dierite is commonly replaced by a bright green,fine grained aggregate of chlorite f muscovite.Less commonly it is replaced by intergrown mus-covite and biotite grains that are nearly the sizeof the original cordierite crystal. In rocks withfresh cordierite, the cordierite is partly replacedby a brown, nearly amorphous material with acomposition suggestive of a chlorite f musco-vite mixture. This brown material is probablya product of weathering. Results of microprobe

slabs of all samples, shows that the rock is aIeucocratic syenogranite (Fig. 2) with quartz: 30.8Vo, alkali feldspar = 429%, plagioclase- 22.2% and a color index of 4.1. Mafic min-erals are biotite, cordierite and almandine gar-net. Cordierite, which occurs as sector-twinnedeuhedral to subhedral prisms up to 2 mm long,is extensively altered to chlorite * muscovite.Garnet oscurs as anhedral grains 0.1 mm orless in diameter included in some of the largerplagioclase crystals. Biotite occurs as an in-cluded phase in plagioclase as well as in thematrix. Accessory minerals include apatite, fluo-rite, ilmenite, monazite, sphalerite, tourmaline,titanite, uraninite and zircon. Secondarv min-erals include calcite, chlorite, epidote, muscoviteand rutile. The density of the rock is 2.65 ocm- t .

MlNsneLocy

Biotite

The biotites in both plutons are pleochroictan to brown and, where altered, are intergrownwith chlorite, muscovite, rutile and alkali feld-spar. Results of microprobe analyses (Table I )show that the biotites lie in the siderophyllitequadrant of the phlogopite-annite-eastonite-siderophyllite projection (Fig. 3). Tables 1 to12, in which are reported results of microprobeanalyses, have been sent to the Depository ofUnpublished Data, CISTI, National ResearchCouncil of Canada, Ottawa, Ontario KIA 0S2.A summary of average compositions for theferromagnesian phases, biotite, cordierite andgarnet, is given in Table 13.

In Table 13, averages of the Clouds Creekbiotite analyses are grouped into biotites fromthe cordierite-biotite granites, biotite granitesand microgranular enclaves. The rocks in theClouds Creek pluton locally have undergonepostconsolidation recrystallization as a result ofsubsolidus reactions as well as deformation inthe foliated granitic facies. This recrystallizationhas produced secondary biotite, which formsunoriented, polycrystalline aggregates intergrownwith ilmenite and rutile rimming or pseudomor-phing the original biotite grain. The composi-tions of these secondary biotites are distinguishedfrom the qriginal biotites in Table 13. For theClouds Creek biotites, the average Fero,/(Febhr+Mg) is 0.68. The biotites have com-positions uniformly near 0.68 but vary in tetra-hedral cation composition, which resulis in verti-cal trends in the phlogopite-annite-eastonite-siderophyllite projection (Fig. 3). The least sili-

CORDIERITE-BEARING GRANITES OF THE PIEDMONT 41

analyses of the Clouds Creek cordierite (Tables2, 13) show .an .average Fe...r/(Fe,o,u,*Mg)value of 0.48, with a variation of. 0,42 to 0.52.Less than 3 mole Vo of. the Mn end-member ispresent. The Stumpy Point cordierite is sekani-naite (Tables 2, l3), with an average Fe.,.r/(Feoar*Mg) value of O.71 but a range of 0.56to 0.81. Manganese content is less than 2 moleVo of the Mn end-member. Unlike the cordi-erites of the Clouds Creek pluton, compositionsof the unaltered sekaninaites from the StumpyPoint granite vary greatly within a single.thin-section. No evidence of systematic zoning wasfound.

The analyses of cordierites have low lotals ofbetween 96 and 98% (Tables 2, l3), whichsuggest lhe presence of unanalyzed elements.Most of the addiiional weight is probably con-tributed by water in the channel sites. The cor-dierites of the Clouds Creek and Stumpy pointplutons contain up to 1.5 wt. /p NarO, or 0.3Na atoms per 18 oxygens. The sodium contentof cordierites from quartz veins, pegmatites andgranites ranges between 0.61 and ,3.53 wt. Vo(Leake 1960, Flood & Shaw 1975). Thesevalues are higher than those of metamorphiccordierite, reported to have as high as 0.78 wt./a NazO but which have generally less than 0.5(Leake 1960). The sum Fe + Mg * Mn isIow, suggesting that vacancies in sites occupiedby Fe * Mg + Mn balance the Na in thechannel sites rather than the coupled substitu-tion Na * Be : Al suggested by Schreyer er a/.(1e79).

Feldspars

The alkali feldspars in the Clouds Creek andStumpy Point plutons are microcline microper-thites. Results of microprobe analyses (Table3) show that the exsolved phases of the alkalifeldspars are nearly end-member compositions,at least as sodic and potassic as Abre and Orso,respectively. The perthites in the Clouds Creekrocks are uniformly distributed string perthitesup to 0.02 mm long and 0.001 mm wide. Theyare smaller than the perthites in other coarsegrained granitoid plutons in the southeasternUnited States and permit a better estimate ofthe bulk composition of the alkali feldsparmegacrysts: Orzr.eAbr".aAno.rCno.g.

The plagioclases of the Clouds Creek andStumpy Point plutons have normal oscillatoryzoning of Ansa to Anrr, with locally developeddiscontinuous rims of Anr. The sodic andesineand oligoclase have potassium feldspar contentsof Or,-u (Table 3), which is higher than the

values of Orr or less in plagioclases of the coarse-grained granitoid rocks of the southern Appala-chians (Speer et al. 198O). The plagioclases areIocally altered to white mica * epidote fquartz -F calcite.

Garnet

The garnets of:the Stumpy Point granite occuras clusters of 2 to 4 grains in the plagioclase.They are anhedral and range up to 0.25 mm inin diameter. Results of microprobe analyses(Tables 4, 13) show that they are almandine:Almeo.rPyto..Spa.eGra.z. These garnets differ fromthose in the other coarse grained granitoid plu-tons of the southern Appalachians, which arespessartine-almandine solid solutions in whichthe spessartine component exceeds 5O% (Speeret al. l98O); however, the Stone Mountain plu-ton in Georgia is reported to contain an alman-dine-rich garnet (Whitney e/ al. L976).

Accessory minerals

Ilmenite is at present the most abundant oxidemineral in the Clouds Creek and Stumpy Pointrocks.'The ilmenites are ilmenite-pyrophanitesolid solutions with a small amount of the hema-tite component (Table 5). Ilmenites in theClor"rds Creek granitoid facies range in composi-tion from (Mno.rtrFe2+o.gt) (Fe8+o.ouJio.ers)Os to(Mno.rurFe'*0.,") (Fet*o.oroTin.*o)O*. Ilmenites inthe microgranular enclaves are less manganifer-ons, with Mno.oz, as are the ilmenites in the Stum-py Point granite. Titanites in the Clouds Creekpluton have variable compositions (Table 6),with substitution for TiO, ranging from 7.1 wt.o/o FetOt to 5.2 wt. /6 AlzOs. Fluorine contentis high: 1.4 to 2.3 wt. % F. Anhedral, pleo-chroic blue-brown tourmaline occurs in the aplitedykes and cordierite-bearing granitoid rocks ofthe Clouds Creek pluton. Anhedral tourmaline,color-zoned brown to green and attaining 1.5mm in diameter, occurs in the Stumpy Pointgranite. Results of microprobe analyses (Table7) show the tourmalines to be intermediatemembers of the series hydroxyl dravite-schorl,with Fe.',r/(Feror*MB) between 0.5 and 0.6and with Cal(Ca*Na) less than 0.06. Theseplutons and the Stone Mountain pluton in Geor-gia are the only postmetamorphic granitoid plu-tons in the southern Appalachians known tocontain tourmaline as an accessory phase. Color-less apatites are ubiquitous in the Clouds Creekand Stumpy Point plutons. Microprobe results(Table 8) show them to be fluorapatites withless than 0.2 wt. /p substitution by Cl. A Ca-

42 THE CANADIAN MINERALOGIST

bearing thorium phosphate (brabantite or brock-ite?) was found in the Clouds Creek pluton.

Other secondary minerals

Secondary minerals in the Clouds Creek andStumpy Point plutons include (in addition to therecrystallized biotite) chlorite, epidote, musco-vite, rutile, ilmenite, kaolinite and calcite. Thechlorites occur as alteration products of biotiteand cordierite. Results of microprobe analysesof the Clouds Creek chlorites (Table 9) showthem to be ripidolites and brunsvigites accordingto the classification of Hey (1954), with Fe.,"'/(Fe..r*MB) between 0.51 and 0.66. Chloritesin the Stumpy Point granite have a wider com-positional range: Fe.rurl (Fe**r*Mg) is between0.37 and 0.71 and tetrahedral silicon between2.7 and 3.5 (Table 9).

Epidote occurs in all rock types of the CloudsCreek and Stumpy Point plutons as a productof saussuritization of plagioclase or in inter-growths with biotite. Epidote is rarer in the cor-dierite-bearing rocks, but locally euhedral crys-tals form in the chlorite + muscovite replace-ments of cordierite. Microprobe results for theClouds Creek epidote (Table l0) show a rangein composition between Pszs.r lnd Psr+.e, witha manganese content between Pto.os and Ptr.e.Epidotes are normally colorless to pale green.Brown epidotes, which occur either as zones inotherwise colorless grains or as separate crystals,are solid solutions toward allanite, with up to25% of the Ca replaced by rare-earth elements.

Muscovite occurs as anhedral flakes, 1 mmor smaller, intergrown with biotite, in saussurit-ized plagioclase, in altered cordierite and as veinfillings in fractured feldspars. Results of micro-probe analyses (Table 1l ) show that the CloudsCreek white micas are phengitic, with an aver-age Si content . of 6.2O (range 5.70 to 6.45)atoms per 24(O,OH,F). Average Fe and Mgcontents are 0.31 (range 0.10 to 0.57) and 0.16(range 0.10 to 0.38) atoms. F content is lessthan 0.17. The rnuscovites are relatively sodic.with Na/(Na*K) ranging between 0.02 and0.11 and with an average of 0.O7. Compositionsof the Stumpy Point muscovites (Table 1 I ) aresimilar to those of the Clouds Creek whitemicas, with Si contents between 6.24 and 6.3O,Fe contents between 0.14 and 0.17 and Mgcontents between 0.1 I and 0.15. F content isless than 0.09, and Na/(Na*K) is between0.09 and 0.11.

Additional secondary minerals include rutile,which occurs as oriented rods in chloritizedbiotites" and ilmenite" which occurs as inter-

growths in recrystallized biotites. Kaolinite(?)replaces a mineral thought to have been andalu-site in the microgranular enclaves of the CloudsCreek pluton. Compositions of the kaolinite(?)are given in Table 12. Calcite occurs as a prod-uct of plagioclase saussuritization and is inter-grown with biotite and epidote.

DrscusstoN

The assemblage cordierite f biotite in theClouds Oreek and Stumpy Point plutons isbelieved to be magmatic and to have crystal-lized at the conditions of final emplacement.This conclusion is based on the euhedral shapesof the cordierites, their sodium contents andthe aluminous compositions of the coexistingbiotites, as compared with biotites in cordierite-free granitoid rocks both in the Clouds Creekand other postmetamorphic plutons of the south-ern Appalachians. In the Clouds Creek pluton,the widespread occurrence of cordierite-bearinggranitoid rocks isolated within a border zone ofbiotite granitoid rocks makes it unlikely thatcordierite xenocrysts or aluminous hornfelsxenoliths from the adjacent Carolina slate beltwere incorporated to produce the cordierite. Aborder zone lacking cordierite also suggests thatthe pltrton has not become peraluminous byalkali loss to the immediate country rocks.

Crystallization of the Clouds Creek andStumpy Point magmas was completed on thecordierite-biotite liquidus boundary. The cor-dierite *. biotite crystallization assemblage isconsistent with conditions for any of the regionsbelow ng3 (Fig. 4a) using the liquidus topologyof Abbott & Clarke (1979). The intermediateto iron-rich composition of the AFM assemblagesuggests conditions in the lower part of thispressure interval. Because neither garnet noraluminum silicate was encountered during pro-gressive crystallization, the biotite--cordieritecrystallization assemblage suggests the topologyof region cel (Fie. 4a). But the compositionsof coexisting biotite-cordierite pairs (Fig. 4b)do not show sufficient chemical evolution tolimit conditions of crystallization to the ceftopology. The inclusions of almandine and bio-tite in the Stumpy Point plagioclases suggestthat garnet 1 biotite was the crystallization as-semblage during an earlier period in the historyof the magma. Evolution from an almandine *biotite I cordierite assemblage to a more mag-nesian cordierite * biotite assemblage can beachieved by a change in the AFM topology,indicating a drop in temperature, pressure, orboth (Fig. 4a). Crystallization would have oc-

CORDIERITE.BEARING GRANITES OF THE PIEDMONT 43

4c0oYE3

600 700T(.C)

800

FFrc. 4a. Regions of distinct AFM liquidus topol-

ogies in P-T space, from Abbott & Clarke(1979). The Clouds Creek and Stumpy Pointplutons completed their crystallization on the cor-dierite-biotite liquidus boundary without encoun-tering either garnet or an aluminum silicate. Aninclusion assemblage of almandine + biotite withor without cordierite in the Stumpy Point granite

curred somewhere along the path labeled Stum-py Point in Figure 4a. If the kaolinite in themicrogranular enclaves of the Clouds Creekpluton is interpreted as a pseudomorph of anda-lusite, these enclaves would have crystallizedwith the assemblage andalusite + cordierite +biotite. This assemblage represents P-T condi-tions of regions emng or ecim in Figure 4a, andthe microgranular enclaves represent possibleequilibria at pressures greater than the finallevel of emplacement,

The almandine garnets in the Stumpy Pointgranite have compositions comparable to the2-6 mol /a grossular and 2-10 mol /o spes-sartine garnets in the silicic volcanic and plu-tonic rocks of eastern Australia (Green 1977).Based on experimental work, Green concludedthat these garnets crystallized from a silicic mag-ma at pressures of 5-7 kbar and are not stableat pressures of less than 4 kbar for alumino-silicate-free compositions. However, Clemens &Wall (1981) found that in their experiments,almandine garnet is stable to much lower pres-sures in a granitic melt. Lack of garnet in theStumpy Point granite, not encountered as in-clusions in plagioclase, indicates that the peri-tectic reaction garnet - cordierite * liquid(Abbott & Clarke 1979) consumed thematrix garnet during ascent and cooling of themagma (Fig. 4a). Preservation of garnets in theAustralian rocks resulted from rapid quenchingin the volcanic rocks and development of spes-sartine-rich rims on garnets in the plutonic rocks(Green 1977, Flood & Shaw 1977).

Final crystallization of the Clouds Creek andStumpy Point magmas at conditions of regioncef of Figure 4a would have several implica-tions: (t) the plutons were emplaced at a relat-ively shallow level in the crust, probably betweenI and 2.5 kbar (Fig. 4a). This interval is aminimum estimate of emplacement pressure be-cause the granites violate some of the assump-tions used to construct Figure 4a: they haveintermediate compositions of iron-magnesiumphases, and the cordierite may behave as a hy-drous phase (Newton & Wood 1979, Kurepin1979). (2) These magmas would have had a

suggests crystallization somewhere along the P-Tpath labeled Stumpy Point. The andalusite(?)-1- cordierite 1 biotite assemblage in enclavesof the Clouds Creek pluton suggests the CloudsCreek path. 4b. AFM projection from alkalifeldspar for biotite, cordierite and garnet in theClouds Creek pluton, S.C. (solid circles) andStumpy Point, N.C. (open squares).

M

Stumpy

44 THE CANADIAN MINERALOGIST

high partial pressure of water. This is reflectedin the more extensive deuteric alteration of theClouds Creek and Stumpy Point rocks com-pared with other postmetamorphic granitoidplutons in the southern Appalachians. In addi-tion, fluid expelled from the crystallizing CloudsCreek magma could account for the probableretrogression in its contact aureole. (3) Figure4a suggests that the muscovite in .these cordi-erite-bearing rocks is not a magmatic phase, inaccordance with the textural observations indi-cating that the muscovite is secondary. However,the rare biotite + muscovite pseudomorphs ofcordierite may have crystallized at the four-phase peritectic reaction cordierite * liquid =biotite * muscovite suggested by Abbott &Clarke (1979).

The Clouds Creek and Stumpy Point plutonsare mineralogically the most distinctive granitoidrocks in the southern Appalachians because oftheir cordierite. The Stumpy Point granite oc-curs in the region of a large, negative gravityanomaly where the rocks encountered beneaththe Atlantic Coastal Plain are granites, biotitegneisses and schists (Denison et al. 1967,Daniels & Zietz 1978). It is included in theHatteras belt of Daniels & Zietz, The StumpyPoint granite has a model whole-rock Rb-Srisotopic age of 924 -r 4O Ma (Daniels & Zietz1978). Isotopic ages of other rocks in the Hat-teras belt range from Precambrian to Paleozoic(granites); the metamorphic rocks are Paleozoic(Denison et al. 1967" Daniels & Zietz 1978).The Stumpy Point granite does not appear tohave undergone deformation.

The Late Paleozoic magmatic event, of whichthe Clouds Creek pluton was a part, producedchiefly rather uniform biotite or amphibole-biotite granites, the petrography, mineralogy andchemistry of which are typically calc-alkaline(Speer et al. 1980, Fullagar & Butler 1979).Locallv the rocks can be slightly peraluminous,containing an aluminous biotite as the only maficmineral. Many plutons evolved more aluminouscompositions, distinguished by the presence ofaluminous biotite * muscovite + spessartinegarnets. These more strongly peraluminous rocksare volumetrically minor facies of composite plu-tons. Late-stage, finer grained biotite + musco-vite -r spessartine granitoid dykes and stocks in-truding earlier biotite and amphibole + biotitegranites provide the most common occurrencesof the strongly peraluminous rocks in these post-metamorphic plutons. Less common is a garnet-bearing facies such as that on the northwesterncorner of the Siloam pluton, Georgia (Speeret al. 1980). These Late Paleozoic granites ex-

hibit characteristics of the l-type or magnetite-series granites (Chappell & White 1974, Ishi-hara 1977). They commonly contain hornblendeand, more rarely, clinopyroxene. The biotiteshave intermediate Fel(Fe*Mg) values (Speeret al. 198O). Prominent accessory minerals in-clude magnetite, titanite and allanite. A vein-disseminated, molybdenum-copper-type mineral-ization is evident in several plutons (Speer 1978,Speer er a/. 1980). The initial 875r/865r ratios areless than 0.706, with a range af. 0.7O24 to0.7052 (Fullagar & Butler 1979). Values of8'8O show a range of 5 to 9%o relative to SMOW(Wenner et al. 1977, 1978).

In contrast, some of the Late Paleozoic gran-itic plutons are composed entirely of peralum-inons rocks, such as at Clouds Creek and StoneMountain (Georgia). These plutons, unlike theothers, show characteristics of S-type or ilmenite-series granites. They contain either muscovite,cordierite or garnet. The biotite is more alum-inous and iron-rich than the biotites in I-typegranites. Common accessory minerals are mon-azite. ilmenite and tourmaline. The initial 8?Sr/865r ratios are greater than 0.706: Clouds Creekpluton 0.7099(1) (Fullagar & Butler 1979),Stone Mountain 0.7250(5) (Whitney er a/.1976). The Stone Mountain pluton is enrichedin r8O" with 8180 values greater than 11 (Wen-ner et al. 1978).

The features of the I-type granites suggest amagmatic source containing an insignificant con-tribution from older cntstal material; the sourceregion may be the upper mantle or a crustalsource formed not much before generation ofthe melt. The features of the Clouds Creek andStone Mountain plutons suggest that the sourceregion consisted of older, more aluminum- andboron-rich crustal rocks as compared with thesource region for the I-type, Late Paleozoic gran-ites. The contrasting types of granites indicatethe heterogeneity of the source regions of theLate Paleozoic magmatic event.

AcKNowLEDGEMENTS

This paper summarizes part of an investiga-tion of low-temperature, geothermal resourcessupported by the U. S. Department of Energy(contract DE-ACO5-78ET27001) awarded J. K.Costain and Lynn Glover III, vdth supplementalsupport from the Los Alamos Scientific Labora-tory (contract N28-77506-l to A. K. Sinha, J.K. Costain and L. Glover III) and the U. S.Geological Survey ( contract 1 4-08-0001-G-685to L. Glover III and J. K. Costain). Samplesfrom the Stumpy Point drillcore were obtained

CORDIERITE-BEARING GRANITES OF THE PIEDMONT 45

from the sample library of the North CarolinaGeological Survey through James Sampair.

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46 THE CANADTAN MINERALOGIST

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Received May 1980, revised manuscript acceptedNoventber 1980.