Canadiutt III ineralogist Vol. 19,pp.35-46 (1981)
PETROLOGYOF CORDIERITE.AND ALMANDINE.BEARINGGRANITOID PLUTONS OF THESOUTHEBN APPALACHTAN PIEDMONT, U.S.A.
J. ALEXANDER SPEER Orogenic Studies Laboratory, Department of Geological Sciences, Vlrginia Polytechnic Institate and State University, Blacksburg, Virginia 24061 , US.A.
ABsrRAcr phase hdtive importante dans le pluton Clouds Creek, a 6t6 d6truite par r€action.Une suite d'in- The most distinctive peraluminous granitoid plu- clusionsi grenat (Almao.rPy,o:Spn.rcr4.?)+ biotite tons in the southern Appalachians are: (1) the Late -t. :L cordi€rite (Stumpy Point) et d'autolithesi an- Paleozoic Clouds Creek pluton (313 2 Ma\ in dalousite (Clouds Creek) illustrent l'6volution du South Carolina, a composite body consisting of bio- milieu magmatique.Compar€ aux intrusiqnsgrani- tite and cordierite-biotite monzogranite and grano- tiques calco-alcalinespostm€tamorphiques typiques diorite, and (2) the Precambrian (?) cordierite- du Pal6ozoiquesupdrieur de la r6gion. le massifde biotite granite pluton of Stumpy Point. North Clouds Creek montre les effets d'une interaction Carolina. The cordierite in both plutons is mag- plus pouss6eavec un socleenrichi en Al et B. matic, with Na2O contents of up to 1.5 wt. Vo and Fel(Fe*Mg) - 0.424,52 in the Clouds Creek (Traduit par la R6daction) pluton and 0.56-.0.81 pluton. in the Stumpy Point gre- The biotites Mots-dAst graniteshyperalumineux, cordiErite, are siderophyllites with Fe/(Fe*Mg) (Caroline - 0.65-0.78. Tourmaline, monazite and ilmenite are nat almandin. Clouds Creek du sud). StumDvPoint (Carolinedu nord). characteristic accessoryminerals. Magnetite. a prom- inent early phase in the Clouds Creek pluton, was consumed in a subsequent reaction. An inclu- INTRoDUCTIoN sion assemblage of almandine (AImeo.rPVro.zSpr." Speer er a/. (1980) reported that the Clouds Grr.") biotite :! cordierite in the Stumpy Point * pluton, granite and andalusite-bearing autoliths in the Creek South Carolina, contains cordi- Clouds Creek pluton record changing conditions in erite, but the extent and nature of the cordierite- the magmas. Compared with the tvpical Late Pale- bearing granitoid rocks were unknown. Subse- ozoic postmetamorphic calc-alkaline eranitoid quent study of the field relations, petrography plutons of the region. the Clouds Creek has had and mineral chemistry of the Clouds Creek plu- more interaction with older crustal rocks rich in ton, described in this paper, revealed a promi- aluminum and boron. nent facies containing magmatic cordierite. A K eywords : peraluminous granites, cordierite. alman- few other Late Paleozoic, postmetamorphic, dine. Clouds Creek (South Carolina). Stumpy coarse grained granitoid plutons of the southern Point (North Carolina). Appalachians are peraluminous, but the Clouds Creek is conspicuousin its distinctive mineralogy SoMMATRE and mineral chemistry. Samples recovered from' a drillhole near Les massifs granitiques hyperalumineux des Ap- Stumpy Point, Dare County, North Carolina, palaches m6ridionales les plus distinctifs au point part the de vue min6ralogique sont les plutons de Clouds examined as of a study of crystalline Creek (en Caroline du sud) et de Stumpv Point basement underlying the Atlantic Coastal Plain, (Caroline du Nord). Le premier, d'6ge Pal6ozoique were found to be almandine * cordierite + bio- sup6rieur (313 -f 2 Ma\ et d'aspect composite, tite granite. Texturally, the rock appears to be contient des unit6s granodioritiques et monzograni- postmetamorphic, but a whole-rock, model Rb- tiques i biotite et d biotite a cordi6rite: le second, Sr isotopic age of 924 +- 24 Ma has d'dge Pr6cambrien(?) consiste en granite d cordi€- been reported (Daniels & Zietz 1978). rite + biotite. Dans les deux cas. la cordi6rite est In this article, these two cordierite-bearing primaire, magmatique; posslde elle iusqu'i 1.5% granitic plutons in the southern Appalachians de Na2O (en poids). Son rapport Fe/(Fe Mc) + (l) varie de 0.42 iL 0.52 (!r Clouds Creek) et de 0.56 are described, although the available n 0.81 (A Stumpy Point). Dans les biotites (plus data suggest that the rocks at Stumpy Point pr6cis6ment. des sid6rophyllites), ce raDDort varie are not the product Late Paleozoic magmatism, de 0.65 b 0.78. Tourmaline. monazite et ilm6nite and (2) its geological setting is hidden under sont les accessoires caract6ristiques. La maqn6tite, 1.8 km of Coastal Plain sediments. 35 36 THE CANADIAN MINERALOGIST
Clouds Creek Pluton, .u....i4 South Corolino ,sl'"Flu''* $ ,.!1.:lo''l ,cordierirE-biofiregroniroid rocks o - ffi .",*:";j:*rr:
0 o . o 9 ;."":' I (J ;:::,';::;:,::"'' AH :1ff#il:i:"1%,r*",*" ffi zft;A
h.''.I hornfels of the fffir*K,EV) tiJ Corolino Slote Belt :.; ryftzqgY-"
North Slumpy
. Clouds Carollna
FIc. 1. Map of the Clouds Creek pluton, Saluda County, South Carolina, showing distribution of rock facies (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.
Petnocnepuy The Clouds Creek pluton intruded the Caro- lina slate (Fig. Cloucls Creek pluton belt 1). Country rocks adjacent to the pluton are metamudstones,metasiltstones The Clouds Creek pluton crops out in an and metagreywackesof the Cambrian(?) Richtex elongate, elliptical area of approximately 50 Formation. The intrusive contacts are sharp and km2 in Saluda County, South Carolina (Fig. generally parallel the regional foliation of the 1). It was first mapped as a dissrete pluton by country rocks, but are locally discordant. On its Overstreet& Bell (1965a, b). Field relations and southern border, the pluton is bounded by augen the geological setting were reported by Secor & gneissesof the Kiokee belt, which are separated Snoke (1978). Earlier petrographicdescriptions from the Clouds Creek by the Modoc fault are given in Sloan (1908), Watson (1910) and zone. Wagener (1977). Fullagar & Butler (1979) rc- The Carolina slate-belt phyllites have under- -+ ported a Rb-Sr isotopic age of 313 2 Ma gone contact metamorphism within 0.5 km of 87Sr/88Sr with an initial ratio of 0.7099 = the contact. The most evident change is textural. 0.0001. An interpretation of the aeroradioactivi- With increasingproximity to the pluton, the rock ty and aeromagnetic survey of the area is given changes from fine grained phyllite to fine by Daniels (1971). grained granoblastic hornfels spotted with ag- CORDIERITE-BEARING GRANITES OF THE PIEDMONT 37 gregatesof muscovite up to 2 mm in diameter. complex either are pseudomorphs of earlier The regional metamorphic assemblage of the pyroxenes or contain clinopyroxene cores, which Carolina slate-belt phyllites is magnetite + epi- indicates that the rocks were originally gab- dote + biotite + phingitic muscovite + albite broic. Consanguinity and the age relationship of + quartz. The contact metamorphic assemblage the two rock types are problematical becauseof -r is ilmenite + magnetite * biotite chlorite * the limited exposure. Only the granitoid rocks muscovite + plagioclase (Anso) + quartz. are discussed here. The megacrystic granitoid Locally, the hornfels is dominated by body has three mappable facies: (l) massive, muscovite + quartz. Th9 texture, mineral as- biotite-bearing granitoid rocks, (2) massive,cor- semblage and mineral chemistries of the horn- 'dierite-biotite-bearing granitoid rocks and (3) felses are comparable to the hornfels produced foliated biotite- or cordierite-biotite-bearing in phyllites of the Carolina slate belt in contact granitoid rocks (Fig. l). aureoles of other postmetamorphic granitic plu- The massive,biotite-b€aring rocks occur along tons of the southern Appalachians. The abund- thg western border of the pluton.- The area ance of muscovite-rich rocks in the aureole covered by this facies ranges up to 2 km wide suggestswidespread metasomatism of the horn' and narrows to less than 0.5 km on the north' fels, presumably by the granite. ern contact. The massive, cordierite-biotite The Clouds Creek is a composite pluton, granitoid facies occupies the core of the north- which Secor & Snoke (1978) have divided into ern half of the pluton. The contact between two parts: (1) a megacrystic granitoid body the biotite and cordierite-biotite granitoid facies ranging from massiveto gneissicand (2) a heter- is gradational. Both facies have the same general ogeneous mafic complex of biotite-amphibole appearance in that they are subhedral-inequi- quartz diorite on the eastern edge of the pluton granular, coarse grained, blue-grey rocks with (Fie. 1). Many of the amphibolesin the mafic blocky, subhedral megacrystsof grey alkali feld-
O Stumpy Poinf,NC o CloudsCreek, SC
K-feldspor 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 Igneous Rocks classification. 38 THE CANADIAN MINERALOGIST
spar up to 4 cm long. Modal analysesof stained Only two rock analyseshave been published slabs and thin sections show that they are mon- for the Clouds Creek pluton. One is a.'biotite zogranites and granodiorites with a color index granitoid rock from the Praetor quarry (locality between 9 and 15 (Fig. 2). Near the contacts 63, Fig. 1; Sloan 1908); the other is an aver- of the pluton, the massive granitoid facies con- age of four rock analyses for which locations tains fewer and smaller alkali feldspar mega- are not given (Fullagar & Butler 1979). The crysts. This gives the rock a more equigrannlar rocks are corundum-normative,with 3.1 and 3.6 appearance. Biotite occurs as subhedral flakes wt. Vo corundum, and the molar ratio AlzOr/ 2 mm or less in size. Commonly, biotite is re- (CaO+Na,O+K'O) is 1.41 and 1.51, respect- pl-aced or rimmed by a finer grained aggregate ively. The Fe/(Fe+Mg) ratios are 0.71 and of recrystallized biotite. Cordierites of the ior- 0.68. dierite-biotite granitoid facies are waxy, blue- Several types of enclavescan be distinguished green, sector-twinned prisms up to 3 mm in the Clouds Creek pluton. Xenoliths of rocks long and comprise between I and 5 modal /o from the adjacent Carolina slate belt are the of the rock. Accessory epidote, allanite and ti- most readily identifiable. They are subangular tanite are largely confined to the biotite. grani- to rounded and range up to 0.5 m in diameter. toid rocks. Imenite is the major opaque phase, They consist of laminated biotite 1 muscovite but magnetite is widespread. Much of the origi- -l- plagioclase+ quartz hornfels. These enclaves nal magnetite has been consumed in subsequent rqsemble the hornfelses of the aureole except reactions. The former abundance of magnetite that they are coarser grained and contain more is indicated bv skeletonsof the exsolvedilmE- hiotite. Less common in the Clouds Creek plu- nite. Other primary accessory minerals include ton are xenoliths of amphibolite and vein quartz. apatite, tourmaline, zircon, pyrite, chalcopyrite A second type of enclave consistsof melano- and monazite. Secondary minerals inslude chlo- cratic, microgranular cordierite-biotite grano- rite, muscovite, kaolinite, rutile, epidote, cal- diorite that contains subhedral phenocrysts of cite and titanite as rims on ilmenite. quartz and compositionally and texturally zoned Rocks of the foliated granitoid facies occupy plagioclaseup to 2 cm in length..These are the southeastern third of the pluton adjacent rounded enclaves up to 0.25 m in diameter. to the Modoc fault zone. They are diverse in The groundmass consists of grains up to 2 rnm appearance,varying from gneissicgranitic rock, across of biotite + cordierite f plagioclase prqtomvlonite, or porphyroclastic mylonite to a (An'0) + alkali feldspar + quartz. The cor- lineated blastomylonite. The foliation is defined dierite is generally subhedral and rimmed either by oriented biotite and elongated ovoid feld- by quartz or by a graphic intergrowth of quartz spar and quartz porphyroclasts. The feldspars * cordierite. These composite grains range up are broken in the less-deformedrocks. The de- to 0.5 cm in diameter. They resemble the gra- forrnational effects gradually increase, though phic intergrowths of cordierite and quartz de- not uniformly, towards the Modoc fault. As scribed by .shibata (1936) and are similarly shown in many outcrops, the deformation was believed to result from simultaneous crystalliza- heterogeneons, because augen and lineated tion rather than replacementof an earlier phase, gneissesare associatedwith macroscopically un- as describedby Henry 0974) and van Reenen deformed megacrystic granitoid rocks. The ex- and dtr Toit (1978) for vermicular quartz r- treme southeasterncorner of the pluton is com- cordierite symplectites. The groundmass of the posed of relatively undeformed cordierite-biotite enclaves is dominated by unoriented laths of granite (locality 38, Fig. l). The primary min- plagioclaseand biotite, which gives the ground- eralogy of the foliated granitoid facies includes mass a bostonitic texture. Accessory minerals biotite, cordierite, plagioclase, alkali feldspar include muscovite, magnetite, ilmenite, apatite and quartz, with accessoryilmenite, magnetite, and monazite. An additional, scarce poikilitic apatite, zircon and pyrite. Secondary minerals mineral pseudomorphedby kaolinite has a habit contemporaneouswith the deformation include that suggestsandalusite. Unlike xenoliths derived rare to abundant epidote and muscovite and from the Carolina slate belt, the mineralogy and subordinate chlorite, titanite" barite and allanite. mineral compositions of these microgranular en- The primary biotite is commonly recrystallized, claves are similar to those of the enclosing with a differing composition, as dissussed be- granitoid rocks; they differ only in texture and low. Whether or not cordierite was originally modal abundances. These microgranular en- present cannot be determined in every case be- claves could have several origins: (l) autoliths cause of the difficulty in recognizing deformed of an early crystallizing facies, (2) xenoliths and pinitized cordierites. that have been subjected to a higher grade of CORDIERITE-BEARINC GRANITES OF THE PIEDMONT 39 metamorphism and have equilibrated with the enelosingm&gma; or (3) restitesfrom the site so3tonlte of magma generation. *StumpyPoint, NC o0louds Stumpy Point pluton = Creek,SC w/cd 3 o0louds . z 2.8 CreelqSC v/ocd cordierite-biotite granite from Stumpy The = Point, Dare County, North Carolina (Fig. l), f is the crystalline basementencountered in the J Cities Service Westvaco well no. 1A completed in September1971. The drillhole is located at J 35o39'36'N, 75"46'4O-W (Coffey 1977). Base- G (61 (f 2.4 ment was encounteredat 1862.9 m 12 ft) lrl and total depth is 1916.6m (6288 ft). A con- I tinuous core sample 10 cm in diameter was E, taken between 19O0.7(6236 ft) and 1907.7 m F 2.2 Irl (6259 ft). This petrological study is based on F six sections of core, each 5 cm long, taken from depths of 6236, 6243,6248, 6253, 6256 and 6257 feet, which are also the sample num- K2Mg6Al2S|6O29(OH)4 KzFe6A12Si602O(oHh bers. ghlogopifo onnilo The Stumpy Point granite is grey-green with Fe /(Fe+ Mg) grains an anhedral-inequigranular texture. Most Frc. 3. Biotite compositions from the Clouds Creek are smaller than 7 mm, but subhedral mega- and Stumpy Point plutons (Table l) projected crysts of quartz up to I cmo plagioclase up to onto the phlogoirite-annite--eastonite-siderophyl- 2.5 cm and alkali feldspar up to 1.5 cm occur lite field. Solid circles refer to biotites coexist- locally. A composite modal analysis of 1462 ing with cordierite (w/cd); open circles refer to points, located on a 5 x 5 mm grid on stained biotites in rocks without cordierite (w/o cd).
TABLE 13. SIJI.II,IARYOF I'IICROPROBEAMLYSES OF THE FERROI4AcTESIATIMINERALS! CLOUDSCREEK PLUTON, 5. C., ANDSTIJI'IPY POINT GRANITE' II. C.
------clouds ctek blotltes------)> st@py Polnt <-'---- grallte
<------Clouds CEk cordlerltes-----> -<------Stmpy Point granlte----> nlcrcgfanular cd.-bt. granlte enclave cord l erlte garnet Sl0r 47.74(0.6r) 48.19(0.20) 46.49(1,07) 37,57 roi o.0l(0,02) 0.00(0) 0.00(o) 0.00 (0.58) 20.52 AlrOq 32.48(0.61) '10.33(0.48)31.82(0.29) 3r.r r Fe0*' 9,78(0.?) 14.340.20) 35.29 Mno 0.50(0.07) 0.51(0.06) 0.36(0.06) 2.'t0 llS0 6.32(0.34) 6.23(0.38) 3.39(1.07) 2.51 ca0 0.00(0) o.02(0.02) 0.ol(0.01 ) I .60 ila20 0.69(0.22) 0.66(0.35) 1,12(0.27) 0.00 Kzo 0.00(0) o.o1(0.01 ) 0.0r(0.02) 0.02 Tdbl q.E E6S-
Fe/(Fe+l'lg) 0.477 0.494 0.709 0.887 Nuder of analyses 16 8 8 2
* Total lrcn erpressedas Feo. H Calculated to give 24(0,F,Cl,ClH),** Nmber ln parenthesesls the standard devlatlon, 40 THE CANADIAN MINERALOGIST
slabs of all samples, shows that the rock is a ceous biotites, those containing the most tetra- Ieucocratic syenogranite (Fig. 2) with quartz hedral aluminum, occur in rocks that contain : 30.8Vo,alkali feldspar = 429%, plagioclase cordierite, namely, - the cordierite-biotite granite 22.2% and a color index of 4.1. Mafic min- and microgranular enclaves(Fig. 3, Table 13). erals are biotite, cordierite and almandine gar- The biotites in the biotite granites are more net. Cordierite, which occurs as sector-twinned silica-rich, but the most silica-rich are the re- euhedral to subhedral prisms up to 2 mm long, crystallized biotites. Another compositional dis- is extensively altered to chlorite * muscovite. tinction of the recrystallized biotites is their Garnet oscurs as anhedral grains 0.1 mm or low titanium content. The higher titanium con- less in diameter included in some of the larger tents of the original biotites produce the inter- plagioclase crystals. Biotite occurs as an in- grown ilmenite and rutile. cluded phase in plagioclase as well as in the The biotites of the Stumpy Point granite are matrix. Accessory minerals include apatite, fluo- iron-rich; Fe,;*orl(Fe*'*Mg) is 0.76. The range rite, ilmenite, monazite, sphalerite, tourmaline, of tetrahedral aluminum contents is between titanite, uraninite and zircon. Secondarv min- 2.48 and 2.73 per 24(O,OH,CI,F) and produces erals include calcite, chlorite, epidote, muscovite a vertical trend in the biotite quadrilateral (Fig. and rutile. The density of the rock is 2.65 o 3). cm-t. The biotites in the Clouds Creek and Stumpy Point plutons are more aluminous and more MlNsneLocy iron-rich than are biotites of the postmetamor- phic, coarse grained granitoid rocks of the Biotite southeasternUnited States (Fig. 3). Biotites without coexisting cordierite in the Clouds The biotites in both plutons are pleochroic Creek pluton overlap the biotite compositional tan to brown and, where altered, are intergrown field of these other granitoid rocks. A similar with chlorite, muscovite, rutile and alkali feld- iron and aluminum compositionaldistinction of spar. Results of microprobe analyses (Table I ) biotites coexisting with aluminous minerals has show that the biotites lie in the siderophyllite has been noted by de Albuquerque (1973) in quadrant of the phlogopite-annite-eastonite- sillimanite- and andalusite-bearing granites of siderophyllite projection (Fig. 3). Tables 1 to northern Portugal and by Clarke et al. (1976) 12, in which are reported results of microprobe in the South Mountain batholith. Nova Scotia. analyses, have been sent to the Depository of The aluminous nature of these biotites would Unpublished Data, CISTI, National Research be anticipated from the coexisting cordierite or Council of Canada,Ottawa, Ontario KIA 0S2. aluminum silicatesand the presumedaluminum- A summary of average compositions for the rich nature of the melt from which they crystal- ferromagnesian phases, biotite, cordierite and lized. In the caseof the Clouds Creek pluton, the garnet, is given in Table 13. iron-rich nature of the biotite may be a result of In Table 13, averages of the Clouds Creek reactionsthat consumedmost of the magnetite. biotite analyses are grouped into biotites from the cordierite-biotite granites, biotite granites Cordierite and microgranular enclaves. The rocks in the Clouds Creek pluton locally have undergone In the Clouds Creek and Stumpy Point plu- postconsolidation recrystallization as a result of tons, cordierite constitutes between I and 5 subsolidus reactions as well as deformation in modal 7o of the rocks. It forms subhedral. the foliated granitic facies. This recrystallization sector-twinned prisms up to 3 mm long that are has produced secondary biotite, which forms generally free of inclusions but locally contain unoriented, polycrystalline aggregatesintergrown zircon, biotite or opaque minerals. The cor- with ilmenite and rutile rimming or pseudomor- dierite is commonly replaced by a bright green, phing the original biotite grain. The composi- fine grained aggregateof chlorite f muscovite. tions of thesesecondary biotites are distinguished Lesscommonly it is replacedby intergrown mus- from the qriginal biotites in Table 13. For the covite and biotite grains that are nearly the size Clouds Creek biotites, the average Fero,/ of the original cordierite crystal. In rocks with (Febhr+Mg) is 0.68. The biotites have com- fresh cordierite, the cordierite is partly replaced positions uniformly near 0.68 but vary in tetra- by a brown, nearly amorphous material with a hedral cation composition,which resulisin verti- composition suggestiveof a chlorite f musco- cal trends in the phlogopite-annite-eastonite- vite mixture. This brown material is probably siderophylliteprojection (Fig. 3). The least sili- a product of weathering.Results of microprobe CORDIERITE-BEARING GRANITES OF THE PIEDMONT 41
analysesof the Clouds Creek cordierite (Tables values of Orr or less in plagioclasesof the coarse- 2, 13) show .an .averageFe...r/(Fe,o,u,*Mg) grained granitoid rocks of the southern Appala- value of 0.48, with a variation of. 0,42 to 0.52. chians (Speeret al. 198O).The plagioclasesare Less than 3 mole Vo of. the Mn end-member is Iocally altered to white mica * epidote f present. The Stumpy Point cordierite is sekani- quartz -F calcite. naite (Tables 2, l3), with an average Fe.,.r/ (Feoar*Mg) value of O.71 but a range of 0.56 Garnet to 0.81. Manganesecontent is less than 2 mole Vo of the Mn end-member. Unlike the cordi- The garnets of:the Stumpy Point granite occur erites of the Clouds Creek pluton, compositions as clusters of 2 to 4 grains in the plagioclase. of the unaltered sekaninaitesfrom the Stumpy They are anhedral and range up to 0.25 mm in Point granite vary greatly within a single.thin- in diameter. Results of microprobe analyses section. No evidence of systematic zoning was (Tables 4, 13) show that they are almandine: found. Almeo.rPyto..Spa.eGra.z.These garnets differ from The analysesof cordierites have low lotals of those in the other coarse grained granitoid plu- between 96 and 98% (Tables 2, l3), which tons of the southern Appalachians, which are suggest lhe presence of unanalyzed elements. spessartine-almandine solid solutions in which Most of the addiiional weight is probably con- the spessartinecomponent exceeds 5O% (Speer tributed by water in the channel sites. The cor- et al. l98O); however, the Stone Mountain plu- dierites of the Clouds Creek and Stumpy point ton in Georgia is reported to contain an alman- plutons contain up to 1.5 wt. /p NarO, or 0.3 dine-rich garnet (Whitney e/ al. L976). Na atoms per 18 oxygens. The sodium content of cordierites from quartz veins, pegmatites and Accessory minerals granites ranges between 0.61 and ,3.53 wt. Vo (Leake 1960, Flood & Shaw 1975). These Ilmenite is at presentthe most abundantoxide values are higher than those of metamorphic mineral in the Clouds Creek and Stumpy Point cordierite, reported to have as high as 0.78 wt. rocks.'The ilmenites are ilmenite-pyrophanite /a NazO but which have generally less than 0.5 solid solutionswith a small amount of the hema- (Leake 1960). The sum Fe + Mg * Mn is tite component (Table 5). Ilmenites in the Iow, suggestingthat vacancies in sites occupied Clor"rdsCreek granitoid facies range in composi- by Fe * Mg + Mn balance the Na in the tion from (Mno.rtrFe2+o.gt)(Fe8+o.ouJio.ers)Os to channel sites rather than the coupled substitu- (Mno.rurFe'*0.,")(Fet*o.oroTin.*o)O*. Ilmenites in tion Na * Be : Al suggestedby Schreyerer a/. the microgranular enclaves are less manganifer- (1e79). ons, with Mno.oz,as are the ilmenites in the Stum- py Point granite. Titanites in the Clouds Creek Feldspars pluton have variable compositions (Table 6), with substitution for TiO, ranging from 7.1 wt. The alkali feldspars in the Clouds Creek and o/o FetOt to 5.2 wt. /6 AlzOs.Fluorine content Stumpy Point plutons are microcline microper- is high: 1.4 to 2.3 wt. % F. Anhedral, pleo- thites. Results of microprobe analyses (Table chroic blue-brown tourmaline occurs in the aplite 3) show that the exsolved phasesof the alkali dykes and cordierite-bearing granitoid rocks of feldspars are nearly end-member compositions, the Clouds Creek pluton. Anhedral tourmaline, at least as sodic and potassic as Abre and Orso, color-zoned brown to green and attaining 1.5 respectively. The perthites in the Clouds Creek mm in diameter, occurs in the Stumpy Point rocks are uniformly distributed string perthites granite. Results of microprobe analyses (Table up to 0.02 mm long and 0.001 mm wide. They 7) show the tourmalines to be intermediate are smaller than the perthites in other coarse members of the series hydroxyl dravite-schorl, grained granitoid plutons in the southeastern with Fe.',r/(Feror*MB) between 0.5 and 0.6 United States and permit a better estimate of and with Cal(Ca*Na) less than 0.06. These the bulk composition of the alkali feldspar plutons and the Stone Mountain pluton in Geor- megacrysts: Orzr.eAbr".aAno.rCno.g. gia are the only postmetamorphic granitoid plu- The plagioclases of the Clouds Creek and tons in the southern Appalachians known to Stumpy Point plutons have normal oscillatory contain tourmaline as an accessoryphase. Color- zoning of Ansa to Anrr, with locally developed less apatites are ubiquitous in the Clouds Creek discontinuous rims of Anr. The sodic andesine and Stumpy Point plutons. Microprobe results and oligoclase have potassium feldspar contents (Table 8) show them to be fluorapatites with of Or,-u (Table 3), which is higher than the less than 0.2 wt. /p substitution by Cl. A Ca- 42 THE CANADIAN MINERALOGIST bearing thorium phosphate (brabantite or brock- growths in recrystallized biotites. Kaolinite(?) ite?) was found in the Clouds Creek pluton. replacesa mineral thought to have been andalu- site in the microgranular enclaves of the Clouds Other secondary minerals Creek pluton. Compositions of the kaolinite(?) are given in Table 12. Calcite occurs as a prod- Secondary minerals in the Clouds Creek and uct of plagioclase saussuritization and is inter- Stumpy Point plutons include (in addition to the grown with biotite and epidote. recrystallized biotite) chlorite, epidote, musco- vite, rutile, ilmenite, kaolinite and calcite. The DrscusstoN chlorites occur as alteration products of biotite and cordierite. Results of microprobe analyses The assemblagecordierite f biotite in the of the Clouds Creek chlorites (Table 9) show Clouds Oreek and Stumpy Point plutons is them to be ripidolites and brunsvigites according believed to be magmatic and to have crystal- to the classificationof Hey (1954), with Fe.,"'/ lized at the conditions of final emplacement. (Fe..r*MB) between0.51 and 0.66. Chlorites This conclusion is based on the euhedral shapes in the Stumpy Point granite have a wider com- of the cordierites, their sodium contents and positionalrange: Fe.rurl(Fe**r*Mg) is between the aluminous compositions of the coexisting 0.37 and 0.71 and tetrahedral silicon between biotites, as compared with biotites in cordierite- 2.7 and 3.5 (Table 9). free granitoid rocks both in the Clouds Creek Epidote occurs in all rock types of the Clouds and other postmetamorphicplutons of the south- Creek and Stumpy Point plutons as a product ern Appalachians. In the Clouds Creek pluton, of saussuritization of plagioclase or in inter- the widespread occurrence of cordierite-bearing growths with biotite. Epidote is rarer in the cor- granitoid rocks isolated within a border zone of dierite-bearing rocks, but locally euhedral crys- biotite granitoid rocks makes it unlikely that tals form in the chlorite + muscovite replace- cordierite xenocrysts or aluminous hornfels ments of cordierite. Microprobe results for the xenoliths from the adjacent Carolina slate belt Clouds Creek epidote (Table l0) show a range were incorporated to produce the cordierite. A in composition between Pszs.rlnd Psr+.e,with border zone lacking cordierite also suggeststhat a manganesecontent between Pto.osand Ptr.e. the pltrton has not become peraluminous by Epidotes are normally colorless to pale green. alkali loss to the immediate country rocks. Brown epidotes, which occur either as zones in Crystallization of the Clouds Creek and otherwise colorless grains or as separatecrystals, Stumpy Point magmas was completed on the are solid solutions toward allanite, with up to cordierite-biotite liquidus boundary. The cor- 25% of the Ca replaced by rare-earth elements. dierite *. biotite crystallization assemblage is Muscovite occurs as anhedral flakes, 1 mm consistent with conditions for any of the regions or smaller, intergrown with biotite, in saussurit- below ng3 (Fig. 4a) using the liquidus topology ized plagioclase,in altered cordierite and as vein of Abbott & Clarke (1979). The intermediate fillings in fractured feldspars. Results of micro- to iron-rich composition of the AFM assemblage probe analyses(Table 1l ) show that the Clouds suggests conditions in the lower part of this Creek white micas are phengitic, with an aver- pressure interval. Because neither garnet nor age Si content. of 6.2O (range 5.70 to 6.45) aluminum silicate was encountered during pro- atoms per 24(O,OH,F). Average Fe and Mg gressive crystallization, the biotite--cordierite contentsare 0.31 (range0.10 to 0.57) and 0.16 crystallization assemblagesuggests the topology (range 0.10 to 0.38) atoms. F content is less of region cel (Fie. 4a). But the compositions than 0.17. The rnuscovites are relatively sodic. of coexisting biotite-cordierite pairs (Fig. 4b) with Na/(Na*K) ranging between 0.02 and do not show sufficient chemical evolution to 0.11 and with an averageof 0.O7.Compositions limit conditions of crystallization to the cef of the Stumpy Point muscovites (Table 1 I ) are topology. The inclusions of almandine and bio- similar to those of the Clouds Creek white tite in the Stumpy Point plagioclases suggest micas, with Si contents between 6.24 and 6.3O, that garnet 1 biotite was the crystallization as- Fe contents between 0.14 and 0.17 and Mg semblageduring an earlier period in the history contents between 0.1I and 0.15. F content is of the magma. Evolution from an almandine * less than 0.09, and Na/(Na*K) is between biotite I cordierite assemblageto a more mag- 0.09and 0.11. nesian cordierite * biotite assemblagecan be Additional secondary minerals include rutile, achieved by a change in the AFM topology, which occurs as oriented rods in chloritized indicating a drop in temperature, pressure, or biotites" and ilmenite" which occurs as inter- both (Fig. 4a). Crystallization would have oc- CORDIERITE.BEARING GRANITES OF THE PIEDMONT 43
curred somewhere along the path labeled Stum- py Point in Figure 4a. If the kaolinite in the microgranular enclaves of the Clouds Creek pluton is interpreted as a pseudomorph of anda- lusite, these enclaves would have crystallized with the assemblageandalusite + cordierite + biotite. This assemblagerepresents P-T condi- tions of regions emng or ecim in Figure 4a, and the microgranular enclaves represent possible equilibria at pressures greater than the final level of emplacement, The almandine garnets in the Stumpy Point 4 c granite have compositions comparable to the 0 2-6 mol grossular 2-10 mol spes- o /a and /o Y sartine garnets in the silicic volcanic and plu- E3 Stumpy tonic rocks of easternAustralia (Green 1977). Based on experimental work, Green concluded that thesegarnets crystallized from a silicic mag- ma at pressuresof 5-7 kbar and are not stable at pressures of less than 4 kbar for alumino- silicate-freecompositions. 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 the Stumpy Point granite, not encounteredas in- 600 700 800 clusions in plagioclase, indicates that the peri- T(.C) tectic reaction garnet - cordierite * liquid (Abbott & Clarke 1979) consumed the matrix garnet during ascent and cooling of the magma (Fig. 4a). Preservationof garnetsin the Australian rocks resulted from rapid quenching in the volcanic rocks and developmentof spes- sartine-rich rims on garnets in the plutonic rocks (Green 1977, Flood & Shaw 1977). Final crystallization of the Clouds Creek and Stumpy Point magmas at conditions of region cef of Figure 4a would have several implica- tions: (t) the plutons were emplacedat a relat- ively shallow level in the crust, probably between I and 2.5 kbar (Fig. 4a). This interval is a minimum estimateof emplacementpressure be- cause the granites violate some of the assump- tions used to construct Figure 4a: they have intermediate compositions of iron-magnesium phases,and the cordierite may behave as a hy- drous phase (Newton & Wood 1979, Kurepin 1979). (2) These magmas would have had a F M Frc. 4a. Regions of distinct AFM liquidus topol- suggestscrystallization somewhere along the P-T ogies in P-T space, from Abbott & Clarke path labeled Stumpy Point. The andalusite(?) (1979). The Clouds Creek and Stumpy Point -1- cordierite 1 biotite assemblage in enclaves plutons completed their crystallization on the cor- of the Clouds Creek pluton suggests the Clouds dierite-biotite liquidus boundary without encoun- Creek path. 4b. AFM projection from alkali tering either garnet or an aluminum silicate. An feldspar for biotite, cordierite and garnet in the inclusion assemblageof almandine + biotite with Clouds Creek pluton, S.C. (solid circles) and or without cordierite in the Stumpy Point granite Stumpy Point, N.C. (open squares). 44 THE CANADIAN MINERALOGIST
high partial pressure of water. This is reflected hibit characteristics of the l-type or magnetite- in the more extensive deuteric alteration of the series granites (Chappell & White 1974, Ishi- Clouds Creek and Stumpy Point rocks com- hara 1977). They commonly contain hornblende pared with other postmetamorphic granitoid and, more rarely, clinopyroxene. The biotites plutons in the southern Appalachians. In addi- have intermediateFel(Fe*Mg) values (Speer tion, fluid expelled from the crystallizing Clouds et al. 198O). Prominent accessory minerals in- Creek magma could account for the probable clude magnetite, titanite and allanite. A vein- retrogression in its contact aureole. (3) Figure disseminated,molybdenum-copper-type mineral- 4a suggeststhat the muscovite in .these cordi- ization is evident in severalplutons (Speer 1978, erite-bearing rocks is not a magmatic phase, in Speerer a/. 1980). The initial 875r/865rratios are accordance with the textural observations indi- less than 0.706, with a range af. 0.7O24 to cating that the muscovite is secondary.However, 0.7052 (Fullagar & Butler 1979). Values of the rare biotite + muscovite pseudomorphs of 8'8Oshow a range of 5 to 9%orelative to SMOW cordierite may have crystallized at the four- (Wenner et al. 1977, 1978). phase peritectic reaction cordierite * liquid = In contrast, some of the Late Paleozoic gran- biotite * muscovite suggested by Abbott & itic plutons are composed entirely of peralum- Clarke (1979). inons rocks, such as at Clouds Creek and Stone The Clouds Creek and Stumpy Point plutons Mountain (Georgia). These plutons, unlike the are mineralogically the most distinctive granitoid others, show characteristicsof S-type or ilmenite- rocks in the southern Appalachians because of series granites. They contain either muscovite, their cordierite. The Stumpy Point granite oc- cordierite or garnet. The biotite is more alum- curs in the region of a large, negative gravity inous and iron-rich than the biotites in I-type anomaly where the rocks encountered beneath granites. Common accessoryminerals are mon- the Atlantic Coastal Plain are granites, biotite azite. ilmenite and tourmaline. The initial 8?Sr/ gneisses and schists (Denison et al. 1967, 865rratios are greater than 0.706: Clouds Creek Daniels & Zietz 1978). It is included in the pluton 0.7099(1) (Fullagar & Butler 1979), Hatteras belt of Daniels & Zietz, The Stumpy Stone Mountain 0.7250(5) (Whitney er a/. Point granite has a model whole-rock Rb-Sr 1976). The Stone Mountain pluton is enriched -r isotopic age of 924 4O Ma (Daniels & Zietz in r8O" with 8180values greater than 11 (Wen- 1978). Isotopic agesof other rocks in the Hat- ner et al. 1978). teras belt range from Precambrian to Paleozoic The features of the I-type granites suggest a (granites); the metamorphic rocks are Paleozoic magmatic source containing an insignificant con- (Denison et al. 1967" Daniels & Zietz 1978). tribution from older cntstal material; the source The Stumpy Point granite does not appear to region may be the upper mantle or a crustal have undergone deformation. source formed not much before generation of The Late Paleozoic magmatic event, of which the melt. The features of the Clouds Creek and the Clouds Creek pluton was a part, produced Stone Mountain plutons suggestthat the source chiefly rather uniform biotite or amphibole- region consisted of older, more aluminum- and biotite granites, the petrography, mineralogy and boron-rich crustal rocks as compared with the chemistry of which are typically calc-alkaline sourceregion for the I-type, Late Paleozoic gran- (Speer et al. 1980, Fullagar & Butler 1979). ites. The contrasting types of granites indicate Locallv the rocks can be slightly peraluminous, the heterogeneity of the source regions of the containing an aluminous biotite as the only mafic Late Paleozoic magmatic event. mineral. Many plutons evolved more aluminous compositions, distinguished by the presence of AcKNowLEDGEMENTS aluminous biotite * muscovite + spessartine garnets.These more strongly peraluminous rocks This paper summarizes part of an investiga- are volumetrically minor facies of compositeplu- tion of low-temperature, geothermal resources tons. Late-stage,finer grained biotite + musco- supported by the U. S. Department of Energy vite -r spessartinegranitoid dykes and stocks in- (contract DE-ACO5-78ET27001) awarded J. K. truding earlier biotite and amphibole + biotite Costain and Lynn Glover III, vdth supplemental granites provide the most common occurrences support from the Los Alamos Scientific Labora- of the strongly peraluminous rocks in these post- tory (contract N28-77506-l to A. K. Sinha, J. metamorphic plutons. Less common is a garnet- K. Costain and L. Glover III) and the U. S. bearing facies such as that on the northwestern Geological Survey ( contract 14-08-0001-G-685 corner of the Siloam pluton, Georgia (Speer to L. Glover III and J. K. Costain). Samples et al. 1980). These Late Paleozoic granites ex- from the Stumpy Point drillcore were obtained CORDIERITE-BEARING GRANITES OF THE PIEDMONT 45
'Garuet from the sample library of the North Carolina GnrEN, T.H. (1977): in silicic liquids and Geological Survey through James Sampair. its possible us€ as a P-T indicator. Contr. Min- erul. Petrology 65, 59-67. REpEnENcns HENRy, I. (1974): Garnet-cordierite gneisses near ABBorr, R.N. JR. & CLARKE,D.B. (1979): Hypo- the Egersund-Ogna anorthositic intrusion, south- thetical liquidus relationships in the subsystem western Norway. Lithos 7, 207-216, AlrOe-FeO-MgO projected from quartz, alkali feldspar and plagioclase for a(H2O) a L Can. Hrv, M.H. (1954): A new review of the chlorites. Mineral, 17, 549-5@. Minerul. Mag. ,iJ'0,277-292.
CHArIELL, B.W. & Wrnrr, A.J.R. (1974): Two con- ISHIHARA,S. (1977): The magnetite-seriesand il- trasting granite types. Pac. Geol.8, 173-174. menite-seriesgranitic rocks. M:ning Geol. (Kozan chishitsu\ 27, 293-305. CLARKE,D.B., MCKENNE, C.8., MUECKE,G.K. & RlcsanosoN, S.W. (1976): Magmatic andalusite KunrrrN, V.A. ( 1979): Thermodynamicsof hydrous from the South Mountain batholith, Nova Scotia. cordierite, and mineral equilibria involving it. Contr. Mineral. Petrology 56, 279-287. Geoclrent. Int. l6(L), 34-44.
CrrusNs, J.D. & Wer.l, V.J. (1981): Origin and Lrers, B.E. (1960): Compilation of chemical anal- crystallization of some peraluminous $type gran- yses and physical constants of natural cordierites. itic magmas. Can. Mineral. 19, lll-131. Amer. Mitreral. 45, 282-298, (1977): CoFFEy, I.C. Exploratory oil wells of NEwroN, R.C. & Wooo, B.J. (1979): Thermody- North Carolina 1925-1976. N, Car, Geol. Mineral namics of water in cordierite and some petrologic Resoarces Inf. Circ, 22. consequences of cordierite as a hydrous phase. Contr. Minerel. Petrology 68, 391-405. DANTELS,D.L. (1971): Geologic interpretation of geophysical maps, central Savannah River area, Ovrnsmerr, W.C. & Berl, H., III (1965a): Geol- South Carolina and Georgia. U. S. Geol. Surv. ogic map of the crystalline rocks of South Caro- Geophys. Invest. Map GP-89$. lina. U. S. Geol. Surv. Misc. Geol, Invest, Map I-413. & Zrnrz, I. (1978): Geologic interpretation of aeromagnetic maps of the Coastal Plain region & -. ( l96jb) : The crystalline rocks of South Carolina and parts of North Carolina of South Carolina. U. S. Geol. Surv. Bull. 1188. and Georgia. U, S. Geol. Surv. Rep. Open File Ser. 78-261' ScHREvER,W., Gonptlr-o, C.E. & WERDINc, G. (1979): A new sodian-beryllian cordierite from or Arruqurnqur, C.A.R. (1973): Geochemistry of Soto, Argentina, and the relationship between biotites from granitic rocks, northern Portugal. distortion index, Be content, and state of hydra- Geochim. Cosmochim. Acta 37, 1779-1802. tion. Contr. Mineral. Petrology 70, 421-428. DrNrsoN, R.8., RAvELTNG,H.P. & Rouss, J.T. (1978): (1967): Age and descriptions of subsurface base- SEcon, D.T., Jn. & SNors, A.W. Stratigra- phy, plutonism the central South ment rocks, Pamlico and Albermarle Sound areas, structure, and in Investigations North Carolina. Amer. Assoc. Petroleum Geol, Carolina Piedmont. /z Geological Bull. 61, 268-272. of the Eastern Piedmont, Southern Appalachians (A.W. Snoke, ed.). Corolina Geol. Soc. Field Trip Guidebook 1978. Froor, R.H. & Ssew, S.E. (1975): A cordierite- bearing granite suite from the New England batholith, N.S.W., Australia. Contr. Mineral. Sune're, H. (1936): Graphic intergrowth of cor- Petrology 52, 157-164. dierite and quartz in pegmatites from Sasago and Dosi, Province of Kai,. lapan. Jap. l. Geol. Geog. & _ (1977): Two ..$type" granite L3,205-227. suites with low initial 8?Sr/EoSrratios from the New England batholith, Australia. Contr. Mineral. SI-oeN,E. (1908): Catalogue of the mineral locali- Petrology 61, 163-173. ties of South Carolina. S. Car. Geol. Surv. BulI. 2 (Ser.4), FuLrecen, P.D. & Burr"rn, J.R. (1979): 325 to 265 m.y.-old granitic plutons in the Piedmont of Smn, J. A. (1978): Molybdenum mineralization in the soqtheastern Appalachians. Amer. J. Sci. 279, the Liberty Hill and Winnsboro plutons, South 161-185. Carofina. Econ. Geol. 73. 558-561. 46 THE CANADTAN MINERALOGIST
BEcKER,S.W. & FARRAR,S.S. (1980): Field WENNER,D.8., Wrurxsv, I.A. & SronvEn" J.C., Jn. relations and petrology of the postmetamorphic, (1977): Oxygen isotopic evidence for the genesis coarse-grained granitoids and associated rocks of of two distinctive suites of 300 m.y. granitic the southern Appalachian Piedmont. /r The Cale- plutons from the southern Appalachian province. donides in the USA (D.R. Wones, ed.). Vir- Geol. Soc, Amer, Abstr. Programs'9, 1221. ginia Polytechnic Inst. State Univ., Dep. Geol. Sci. Mem. 2, 137-148. studies of post-metamorphic granitic plutons from vAN DU (1978): REENEN,D.D. & Torr, M.C. The the Piedmont province of Georgia. Geol. Soc. garnet quartz - reaction + cordierite * hyper- Amer. Abstr. Programs 10, 201. sthene in granulites of the Limpopo, metamorphic complex in northern Transvaal. fn Mineralization WHrrNEy, J.A., JoNrs, L.M. & WALKER, R.L. (W.f. in Metamorphic Terranes Verwoerd, ed.). (1976): Age and origin of the Stone Mountain Geol. Soc. S. Alr. Spec. Publ. 4, 149-177. granite, Lithonia district, Georgia. Geol. Soc. Wecrrrn, H,D, (1977): Granitic stone resources Amer. Bull. 87, 1067-1077. of South Carolina. S. Car. State Dev. Board, Geol. Surv. Mineral ResourcesSer. 5.
WArsoN, T.L. (1910): Granites of the southeastern Received May 1980, revised manuscript accepted Atlantic states. U. S. Geol. Surv. Bull. 426. Noventber 1980.