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Evolution of Magmatic and Subsolidus AFM Mineral Assemblages In American Mineralogist, Volume 77, pages 821-833, 1992 Evolution of magmatic and subsolidusAFM mineral assemblagesin granitoid rocks: Biotite, muscovite,and garnet in the Cuffytown Creek pluton, South Carolina J. Ar.nx.qxrnn Spnnn Department of Marine, Earth, and Atmospheric Sciences,Box 8208, North Carolina State University, Raleigh,North Carolina27695-8208, U.S.A. SusaN W. Bncxnn l0l YeonasCircle SE,Vienna, Virginia 22180,U.S.A. Ansrnlcr The Cufttown Creek pluton is a leucocratic alkali feldspar granite with (F + Mn)-rich biotite, muscovite, garnet, and fluorite as varietal minerals. Bt + Ms is preserved as inclusion assemblagesin quartz; Bt + Ms + Grt and Ms + Grt are matrix assemblages. The inclusion assemblagesare interpreted as early magmatic, and the matrix assemblages as late magmatic with superimposedsubsolidus partial reequilibration. In sequence,the AFMMn crystallization reactions are liquid : Bt, liquid : Bt + Ms, liquid + Bt : Ms * Grt, and liquid : Grt + Ms. Inclusion biotite showsFeMg-, and Mn(Fe,Mg)-' exchanges, which may reflect both changein melt composition and cooling along an,fo, buffer. Inclu- sions of white mica are nearly end-member muscovite. Evolution from inclusion to matrix mica compositions shows narrowing of the compositional gap between the dioctahedral and trioctahedral micas, as well as OHF-, and MnMg-, exchangesin the biotite and (Fe2*,Mg,Mn)Sil6lAl'lolAl-' (celadonite),NaIL', OHF-r, and Ti enrichmentin the white micas. Garnets are spessartine-almandine. The liquid line of descentindicates that the melt becameincreasingly rich in Fel(Fe + Mg) and Mn/(Fe + Mn) with decreasingtemperatures, but each at differing rates at dif- fering stagesduring crystallization. Peraluminosity of the melt increasedrapidly at first, then decreasedslightly before increasing at a much slower rate. Final crystallization con- ditions are estimated at 650 "C and 2-3 kbar. Miarolitic cavities indicate that, at the end of crystallization, Po,,o: P,o,,and a separatefluid phase formed that persisted into the subsolidus.The fluid was dominantly HrO, with lesseramounts of F [ogffiro / f"r): 3.15- 3.s41. Subsolidus replacement textures, mineral compositions, and fluid buffers result from two reactions that are estimated to have occurred at 300-400'C: (l) plagioclase+ iron biotite * muscovite * iron chlorite + fluid : albite * potassium feldspar * magnesium biotite + phengite + magnesium chlorite + fluorite + qtrartzand (2) garnet + potassium feldspar + fluid: muscovite * rhodochrosite + quartz. INrnooucrroN relations should also provide some insight into the final granites Investigating sequencesof magmatic AFM mineral liquidus AFMMn crystallization reactions of in- garnet crystallization reactions in granites is difficult because, volving biotite, muscovite, and and discussedwith among other reasons,we are forced to rely on observa- varying amounts of agreement by Miller and Stoddard (l98la, tions from different samplesor even ditrering rock types (1980, l98l), Abbott l98lb, 1985), and znn of composite plutons. We must interpret the relationships (1988). among the samples or rock types and assumethat they representparts of a continuous liquid line of descent.In- Gnor,ocrc SETTTNG dividual samplesfrom the Cufttown Creek, South Car- The Cufiltown Creek pluton, which is 293 + 14 m.y. olina, pluton contain two texturally distinct AFMMn as- old, is exposedin an oval-shaped area of 20 km'z in the semblages:biotite + muscovite assemblagesincluded in Carolina slatebelt of Edgefieldand McCormick Counties, quartz and muscovite + garnet + biotite matrix assem- South Carolina (Fig. l). Speer and Becker (1990) de- blages.We investigatethese assemblages as a sequenceof scribedthe geology,petrography, and geochemistryofthe magmatic or subsolidus crystallization reactions that Cufttown Creek, as well as the previous work on it; the evolved within the same rock volume. Becausethe Cuf- description here is a summary of that work. fytown Creek pluton is an evolved granite, its mineral The pluton comprisesunfoliated, leucocratic,medium- 0003404x/ 92/0708-o8 2 I $02.00 82r 822 SPEER AND BECKER: ASSEMBLAGES IN GRANITOID ROCKS compared with calc-alkaline granitoids. Rock composi- r"A -_-:----_ cpf tions lie at the metaluminous-peraluminous boundary, /oro tsz-sr-:-t-- normative + Or + Ab: 95-98o/oand Rb/Sr: 45. A /' Q i, large gravity low is centered on the pluton, and we con- .,'t clude that the surfaceexposure is only the top of a much / --. .----' t '- . 87-21( Iarger body and that it crystallized from a highly frac- + 57-52 cez:i ti?l tionated, felsic melt in the cupola of a magma chamber. 'rr-u. The Cuffytown Creek pluton is emplaced in green- \ -EDl^cttn schist-grademetavolcanics of the Persimmon Fork For- ,\. 57 mation of late Precambrian(?)to Cambrian in age in the \\ Carolina slate belt. The wall rocks were metamorphosed l\ -.. ts7-5 '., \.'-: -s5 Cpf to the low-pressureamphibolite facies,with assemblages \\-'..s I I containing hornblende, andesine,and clinopyroxene. Lo- cpf ,-.. t\ cally, there are skarns containing abundant quartz and --j fluorite, with epidote, andradite, and manganiferoushed- .--/o )r2 I i g2o07'30" xm enbergite. Fig. |. Geologicand samplemap of the Curytown Creek MrNnn-rl RELATToNS : pluton, SouthCarolina (after Speer and Becker,1990). Cccg Compositionsof the mineralsin polishedthin sections granite, : CarboniferousCufiltown Creek Cpf CambrianPer- were determined on a nine-spectrometer ARL-SEMQ simmonFork formation,EDI : drill core. (Virginia Polytechnic Institute), using the analytical schemedescribed by Solbergand Speer(1982), and a Ca- (University grained alkali-feldspar granite, with a color index be- meca Camebax SX 50 of South Carolina). tween 2.2 and 5.3 and scarcephenocrysts of quartz and Analysesare reportedin Appendix l,'and selectedanal- yses given alkali feldspar up to 2 cm in length. Pegmatiteand aplite in Tables l-4. Mineral abbreviations in the veins are rare. Varietal minerals are (F + Mn)-rich bio- text, tables, and figures are those recommended by the (Kretz, tite, F-rich white mica, F-bearing spessartine-almandine American Mineralogist I 983). Color desigrrations (1980). garnet, chlorite, and fluorite. Accessoryminerals include are thoseof the Rock-Color Chart Committee allanite, apatite, fergusonite, carbonate, hematite, ilmen- Feldspars ite, magnetite, manganocolumbite, monazite, pyrite, niobian rutile, sphalerite, titanite, xenotime, and zircon. Macro- to microperthitic alkali feldspar exhibits both primary growth Bismuth lead and bismuth tellurium sulfides(cosalite and Carlsbad twins and microcline albite- tetradymile?)occur in veins. 'To a copy of Appendix 1, order Document AM-92- Major- and trace-element obtain analysesshow the Cufttown 500 from the BusinessOfrce, Mineralogical Societyof America, Creek granite to be low in Al, Ti, Ca, Mg, Ba, and Sr and I 130 SeventeenthStreet NW, Suite 330, Washington,DC 20036, high in Si, Fe/Mg, F, Ga, Nb, Rb, Th, Y, and U, as U.S.A. Pleaseremit $5.00 in advance for the microfiche. TlaLe 1. Selected biotite analyses for the Cuffytown Creek pluton, South Carolina sio, 38.84 38.07 38.42 38.39 37.86 37.42 39.18 39.24 38.07 38.52 Tio, 2.08 1.68 1.64 1.61 1.21 1.66 1.59 1.24 1.72 1.70 Alro3 12.76 1853 18.62 18.22 12.99 16.57 18.90 12.76 13.62 17.59 FeO' 18.80 18.58 18.74 18.30 18.68 18.66 19.00 14.75 18.44 18.28 MnO 2.23 3.37 4.32 2.19 3.03 2.87 4.41 1.77 2.64 3.66 Mgo 12.53 489 4.15 5.98 10.66 4.85 4.22 13.89 9.83 5.96 CaO 0.14 0.01 0.0 0.03 0.14 0.14 0.0 0.14 0.14 0.0 Naro 0.27 0.08 0.09 0.12 0.25 0.21 0.08 0.18 0.20 0.14 KrO 9.87 9.98 9.86 983 9.51 9.44 9.05 o77 9.70 9.01 BaO 0.23 o.21 0.21 0.21 0.20 F 3.97 4.18 2.72 4.26 4.11 cl 0.10 0.10 0.05 0.08 0.07 H,Ot* 2.09 3.89 3.92 3.92 1.84 2.47 3.98 1.89 1.90 3.92 -O:F+Cl 1.69 1.78 1.16 1.81 1.75 Total 102.22 99.00 99.76 98.59 98.88 96.11 100.86 98.37 98.89 98.78 /Vofe.'(1) CB7-15 : biotite includedin quartz. (21 CB7-15 : averageof eight matrix-biotiteanalyses. (3) S7-50a : biotite includedin quartz. (4) 57- 50a : averageof seven analyses,inclusion biotite reequilibratedwith the matrix. (5) 57-55 : averageof four biotite samplesincluded in quartz analyses. (6) 57-55 : matrix biotitewith muscovite.(7) ED1-872: averageof three analyses,inclusion biotite reequilibrated with the matrix. (8) ED1-877: biotite includedin quartz. (9) ED1-877: biotite includedin quartz. (10) ED1-926: average of three biotite samples includedin quartz analyses. All Fe reDortedas FeO. -- Calculatedon the basis of (OH + F + CD: 2124 anions. SPEERAND BECKER: ASSEMBLAGES IN GRANITOID ROCKS 823 TreLe 2. Selected white mica analyses for the Cuffytown Creek pluton, South Carolina 10 sio, 47.11 47.27 48.83 45 84 46 50 45.82 4635 46.13 45.01 49.42 Tio, 0.10 0.28 055 017 0.59 0.71 0.43 0.13 0.58 0.64 Alro3 32.81 27.48 30.31 35.86 27.11 26.33 30.70 34.95 27.49 29.05 FeO. 4.00 6.03 6.48 150 6.76 7.35 5.01 2.40 718 7.85 MnO 0.76 1.09 u./o 0.54 0.70 1.18 0.71 0.60 1.00 1.11 Mgo 0.37 1.72 2.08 0.03 2.41 2.14 0.67 0.05 1.84 2.03 CaO 0.12 0.14 0.0 0.03 0.01 0.12 0.13 0.0 0.08 0.0 Naro 041 0.43 o.22 0.49 0.27 o42 0.31 0.50 0.39 0.20 KrO 9.71 8.74 7.88 10.37 10.87 8.79 9.40 10.34 9.50 6 83 BaO o.21 0.19 0.19 0.18 F 0.73 1.23 0.54 1.14 1.96 0.97 0.42 1.18 0.03 003 0.02 0.02 0.04 0.04 0.01 0.01 Hro.' 4.10 3.72 4.53 4.21 3.79 3.31 3.89 4.27 3.68 4.53 -O:F+Cl 0.31 0.52 0.23 048 0.83 0.42 0.18 0.50 Total 100.1 5 97.83 101.64 99.37 99.69 97.53 98.37 99.64 97.44 101.66 Note: (1) CB7-15: muscovitein plagioclase.(2) CB7-15: white matrix mica.
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