sign in sign up
<<
Home , Hornblende, Petrogenetic grid, Staurolite

American Mineralogist, Volume 72, pages 1086-1096, 1987

Petrology of a Blue Ridge unit with hornblende* * * TnoNrl.sS. Hnr,rvrs.Hl.RRy Y. McSwnnn.Jn.. Tnrooonn C. L,c.sorKA Department of Geological Sciences,University of Tennessee,Knoxville, Tennessee37996, U.S.A. EucrNn Jlnosrwrcn National Museum of Natural History, Smithsonian Institution, Washington, D.C. 20560, U.S.A.

Ansrn-q.cr The Laurel Creek amphibolite is composed of subsetsof the assemblagehornblende + chlorite + * kyanite * staurolite * orthoamphibole + plagioclase(Anor-Annr) + qwrtz + ilmenite * rutile + pyrrhotite. Low-variance assemblagesprovide a relatively complete view of a rare amphibolite facies type. These rare assemblagesare believed to be the result of high-pressuremetamorphism at moderate temperatures,not of unusual bulk compositions. Laurel Creek containing kyanite, staurolite, and gedrite have Mg-rich, olivine-normative basaltic compositions. Mass-balancecalculations indi- cate that the bulk compositions of these rocks can produce common amphibolite assem- blagesunder appropriate conditions. The assemblagehornblende + staurolite (or + kya- nite) has previously been demonstrated to be in reaction relation with the assemblage hornblende + plagioclase+ + chlorite (andlor garnet),the common amphibolite assemblagein medium-pressure metamorphic terranes. The assemblageshornblende * chlorite + epidote + plagioclaseand hornblende + garnet + epidote * plagioclaseare graphically demonstratedto be compatible at the sameconditions that hornblende coexists with aluminous phasesfor Mg-rich and Fe-rich bulk compositions, respectively.However, the common low-variance assemblagehornblende * chlorite + garnet + epidote * pla- gioclaseis precluded by the coexistenceof hornblende * kyanite. We also demonstrate that the latter assemblageis in reaction relation with gedrite + anorthite, hornblende + gedrite + kyanite, hornblende * corundum, and hornblende + anorthite. Garnet- geothermometry indicates a temperature of 540-625 "C for both adjacent pelitic and the Laurel Creek amphibolite. Garnet + kyanite + qvafiz + ilmenite + rutile equi- librium in the Laurel Creek amphibolite indicates a minimum pressureof 7.7 kbar. The coexistenceof hornblende * kyanite + staurolite + gedrite * plagioclase(Anr*-Anrr) in mafic rocks indicatesunusual metamorphic conditions. Geothermobarometryfrom Laurel Creek, as well as other field and experimental data, indicates that amphibolites bearing kyanite-staurolite + hornblende result from a pressurehigher than that appropriate for the common amphibolite assemblage.

INrnooucttoN mental work (cooper, 1980; ward, 1984; Hellman and Amphibolites of the Laurel Creek Mafic-Ultramafic Green, 1979), corroborate their suggestion. Complex in northeasternGeorgia display a rare amphib- This study was undertaken to determine whether Lau- olite facies type, composed of (hornblende * quartzl rel Creek amphibolite assemblagesresulted from unusual bearing subsetsof the assemblageorthoamphibole + pla- bulk rock compositions or from unusual physical condi- gioclase* staurolite + kyanite + garnet + chlorite. These tions. The significanceofhornblende * kyanite + stau- unusual amphibolites have olivine-normative basaltic rolite stability is important to the understanding of the compositions(Table 1). Selverstoneet al. (1984) suggested evolution of the Blue Ridge metamorphic province of the that rocks of basaltic composition stabilize hornblende* Southern Appalachians. The coexistenceof hornblende staurolite (and/or kyanite) at pressureshigher than those with aluminous silicates (or corundum) is relatively rare appropriate for the stability of the common amphibolite in most barrowian metamorphic terranes. Its occurrence assemblagecalcic * plagioclase* epidote * is, however, relatively widespread in mafic-ultramafic chlorite + gamet (Laird, 1980; Thompson et al., 1982); complexesin the portion of the Blue Ridge province east results presentedhere, as well as other field and experi- of the Hayesvitle fault, along the Georgia-North Carolina 0003-o04x/87l1I 12-1086$02.00 1086 HELMS ET AL.: GEORGIA BLUE RIDGE AMPHIBOLITE 1087 border (McElhaney and McSween, 7984; K. Walter, in Tneue1. Bulk compositionsof LaurelCreek amphibolitesand prep.; Pratt and Lewis, 1905).The stability of the unusual norm calculations assemblagecan place a tight constraint on the tectonic L-112-D L-112-N L-100 L-79 environment in which thesemafic-ultramafic bodies were emplaced. sio, 48.76 47.75 44.43 47.27 Tio, 0.51 0.43 1.94 0.66 Al,03 13.41 15.62 16.54 13.81 Gpor,ocrc sETTTNG FerO. 2.72 2.85 2.56 2.24 The FeO 9.68 7.95 12.59 10.77 Laurel Creek Mafic-Ultramafic Complex, Rabun Mgo 13.42 12.41 9.28 12.54 County, Georgia Blue Ridge, is located approximately 5 MnO o.23 o.22 0.26 o.23 km northwest ofthe Brevard zone and 15 km northeast CaO 8.99 9.57 9.37 9.42 Naro 1.35 1.53 1.64 1.55 of the Tallulah Fallsdome (Hatcher,l97l;McSween and KrO 0.11 0.13 0.18 0.15 Hatcher, 1985).The complex is one of severalultramafic Pro. 0.07 0.06 0.23 0.10 and mafic-ultramafic complexes forming a continuous Total 99.25 98.52 s--g-o- 98.59 northeast-striking belt in the eastern Blue Ridge (Misra CIPWnorm Orth 0.65 0.77 1.06 0.89 and Keller, 1978).The unit is tensof metersin width and Ab 1142 12.95 13.88 13.12 extends approximately 5 km along strike (Fig. 1). The An 30.21 35.37 37.24 30.28 unit was thrust over the Tallulah Falls Formation Di 10.73 9.42 6.34 12.83 along Hp 35.00 26.18 16.92 22.67 a premetamorphic fault and may be ophiolitic in origin ol 6.04 8.75 15.64 14.23 (Petty, 1982). The Tallulah Falls Formation (late Pre- tl 097 0.82 3.68 1.25 Mt 3.94 4.13 3.71 3.25 cambrian?), composed dominantly of metagralrvackes, Ap 0.39 0.14 0.53 o.23 amphibolites, and subordinate amounts of pelitic schists (Hatcher, 197l), may be time-equivalent with the Ocoee Supergroup(Hatcher, 19721,Hadley,1972). This forma- suming -890/oFeO. The amount of error involved in this tion unconformably overlies 1200-Ma Grenville base- assumption for other is uncertain. ment around the flanks of the Tallulah Falls dome. Bulk chemical analyseswere obtained with an oRrEC The southern Blue Ridge is a barrovian metamorphic rc&c tube excited X-ray fluorescenceanalyzer using U.S. terrane (Carpenter, 1970),and Laurel Creek is located in GeologicalSurvey rock standards(Flanagan, 1976). Sam- the kyanite zone, characterizedby the pelitic assemblage ples were pulverized in a tungstencarbide shatterbox and kyanite + garnet + biotite. Staurolite is stable only in pressedinto pellets.FeO and FeO,o,u,were determined by the absenceof muscovite. This zone is flanked 3.5 km to a standard titrimetric process.FeO,"*1 was within 20loof the northwest by a isograd, to the southeast the value given by xnr analysis. by the Brevard fault, and farther to the south by a garnet and chlorite zone(Hatcher, 197l). Assnlrnr,.tcEs AND MTNERALCHEMISTRy A diversity of mafic bulk compositions resulted in a Mrrrroo oF sruDy variety of amphibolite assemblages.Important low-vari- More than 100 thin sectionswere prepared from var- anceassemblages include Hbl + An , + Ky + St + Ged + ious rock types from the Laurel Creek area. Phasesfrom Chl + Bt + Qtz + Rut + Po (sampleL-ll2-N), Hbl + I I amphibolite and 5 sampleswere analyzedwith An no* Grt + Ged + Chl + Qtz + Rt + Po (L-l l2-D), an automated rraac4005 electron microprobe, using nat- Hbl + Anno+ Grt + St + Chl + Qtz + Ilm + Rt + Po ural and synthetic standardsoperating at an accelerating (L-100), Hbl + Ged + Grt + St + Chl + Qtz + Rt + voltageof 15 kV. A beam currentof0.03 pA and a beam Po (L-79), Hbl + An uu_,o* Ky + Chl + Bt + Qtz + Rt -10 diameter of pm were used to analyze amphibole, (L-75), An,,_,o+ Grt + St + Ky + Bt + Qtz (L-36). biotite, and plagioclase.All other phaseswere analyzed Locations of samplesdiscussed in the text are shown in with a beam current of 0.05 pA and a beam diameter of Figure l. Of special note is the coexistenceof Hbl + I pm. Correction procedures were those of Bence and Ged + Ky + St, a associationnot mentioned in Albee (1968)but utilized the data of Albee and Ray (1970). current literature dealing with similar low-variance am- Silicateswere normalized on the basis of the appropri- phibolites (seeRobinson et al., 1982). Laurel Creek am- ate number of oxygens. All Fe in garnet, chlorite, or- phibolite is dominantly composedof higher-variancesub- thoamphibole, staurolite, and biotite was assumedto be sets of the assemblageHbl + Pl + Grt + Qtz + Rut + Fe2*.Staurolite compositions were normalized to 23 oxy- Ilm + Po. gensconsistent with a half unit cell of 22 oxygensplus 2 Both calcic and orthoamphiboles are present in the (OH-) groups (Griffin and Ribbe, 1973; Pigage and Laurel Creek amphibolite. Amphibole microprobe anal- Greenwood, 1982). FeO total and FerO, for hornblende yses from important low-variance assemblagesare pre- L-75 and L-79 were analyzedby a titrimetric procedure. sentedin Table 2. Calcic amphibole compositions range The averageFe contents ofhornblende measuredby the from magnesio-hornblendeand tschermakitic horn- microprobe varied less than 2o/o(relative) from the value blende to ferroan (Leake, 1978). Figures 2 and determined by wet chemistry. In accordancewith wet- 3 illustrate major amphibole substitutions.Al contentsof chemical data, Fe3*of hornblende was calculated by as- amphiboles correlate strongly with the presenceor ab- r088 HELMS ET AL.: GEORGIA BLUE RIDGE AMPHIBOLITE

NNNLourel Creek Amphibolite ffi Lourel Creek Ultromofic Body W TollulohFolls Amphibolite TollulohFollr Fm. E Undif ferentioted

N Scale/meters 0 ,00 r0o0 I

Fig. l. Location and a generalizedgeologic map (Petty, 1982) of the Laurel Creek Mafic-Ultrarnafic Complex. senceofplagioclase. Figure 3 is a plot ofA-site occupancy Increase in anorthite component correlates with an in- relative to charge excessin the octahedral sites in or- creasein the Mg/Fe ratio (Fig. 2) and a decreasein A-site thoamphiboles and calcic amphiboles. Na of amphiboles. Plagioclaseof composition Anr*n, is Several studies report the presenceof a solvus in or- confined to low-variance assemblagescontaining staurol- thoamphiboles in both the tschermakitic and edenitic ex- ite, kyanite, or gedrite. changes(Robinson et al., 197l; Stout, I 972; Spear,I 980). Staurolite is presentin severalsamples from the Laurel No samplesfrom Laurel Creek contain orthoamphibole Creek amphibolite. The M/(Mg + Fe) ratio rangesfrom pairs or visible (a00 x ) exsolution lamellae. Data in Fig- 0.21 to 0.39 in various assemblages(Table 4). Staurolite ure 2 imply that a small gap in the amount of total Al grains are fairly homogeneous;however, grains of stau- exists.When all analysesare considered,there is no dis- rolite included in various phasesare more Mg-rich than continuity in the amount of A-site occupancy (edenitic matrix grains. exchange)and octahedral charge excess (tschermakitic Garnet rim compositions contain approximately 55- exchangeapproximately) (Fig. 3). Robinsonet al. (1971) 600/oalmandi ne, 22-3 5o/opyrope, I G-l 30/ogrossular, and noted fine exsolution lamellae of gedrite within anthoph- 2-30lospessartine components (Table 4). from yllite with widths less than 1.0 pm. It is possible that amphibolites and adjacent are fairly homoge- extremely fine exsolution lamellae do exist, but have been neous. However, in amphibolite sample L-100, they are averagedby a broadly focused electron-microprobe beam. zoned with FelMg decreasingtoward the rims. There is Alternatively, the solvus was never encounteredat Laurel no preferred enrichment pattern, however, with respect Creek. to Fe and Mg in garnetsfrom different amphibolite sam- Plagioclase,ranging in composition from An.' to An' ples. (Table 3), is presentin all but the most Mg-rich samples. Chlorite is presentin both progradeand retrogradeas- HELMS ET AL.: GEORGIA BLUE RIDGE AMPHIBOLITE 1089

TneLe2. Selected microprobe analysesof amphiboles

Calcic amphiboles Orthoamphiboles L-112-D L-79 L-79-A L-75 L-112-N L-100 L-112-D L-112-N L-79 sio, 45.02 43.36 44.29 45.04 44.63 44.77 43.45 54.72 49.79 48.46 49.27 Al,o3 15.32 16.53 15.99 15.85 15.78 15.84 15.55 3.24 10.64 12.63 11.50 Tio, 0.39 0.34 0.39 0.52 0.48 0.39 0.53 0.08 0.13 0.15 0.17 Mgo 12.77 12.2 12.61 13.09 13.00 12.77 11.13 21.41 18.66 18.67 17.83 FeO 12.03 12.48 12.27 10.93 10.76 11.39 14.28 17.70 17.17 15.84 16.78 MnO 0.11 0.04 0.07 0.14 0.22 0.28 0.11 0.53 0.47 0.60 0.36 CaO 10.93 10.75 10.64 10.2 10.37 10.59 10.2 0.56 0.62 0.67 0.58 Naro 1.3 1.23 1.08 1.78 1.63 1.22 1.66 0.34 1.15 0.76 0.99 KrO o.2 0.2 0.17 0.2 o.21 o.21 0.2 n.d. n.d. n.d. n.d. Total 9-6T7 s7l3- e-z5T CTTB 97.08 97.46 97.'t1 e8s8- 98-55 9tr78 TZ6 Cations normalizedto 23 oxygens Si 6.399 6240 6.329 6.391 6.379 6.383 6.302 7.703 7.028 6.8s8 7.011 rvAl 1.601 1.760 1.671 1.609 1.621 1.617 1.698 0.297 0.972 1.142 0.989 vrAl 0.966 1.045 1.023 1.042 1.038 1.046 0.960 0.241 0.799 0.966 0.941 Fec 0.257 0 268 0.263 0.235 0.233 0.245 0.305 n.c n.c. n.c. n.c. Ti 0.042 0.037 0.042 0.055 0.052 0.o42 0.058 0.008 0.014 0.016 0.018 Mg 2.705 2.617 2.686 2.768 2.769 2.713 2.406 4.492 3.926 3.938 3.781 Fe'?* 1.030 1.034 0.987 0.899 0.907 0.954 1.271 0.259 0.262 0.081 0.260 Fe2* 0.272 0.334 0.347 0.280 0.262 0.282 0.308 1.825 1.765 1.794 1.737 Mn 0.013 0.005 0.009 0.017 0.026 0.034 0.014 0.063 0.056 0.072 0.043 Ca 1.665 1.658 1.630 1.551 1.589 1.618 1.585 0.084 0.094 0.102 0.088 Na 0.050 0.003 0.014 0.152 0.122 0.067 0.093 0.027 0.085 0.032 0.131 Na 0.038 0.340 0.286 0.337 0.328 o.271 o.374 0.066 0.230 0.176 0.142 K 0.001 0.001 0.001 0.001 0.001 0.001 0.001 n.d. n.d. n.d. n.d. Total 15300 Tsit4T 157F' Tslt3B 15330 16fr2 15.375 15T66 15230- T5T76 1614. A/ote.'Fe3tcalculated; See text for method. n.d. : not detected. n.c. : not calculated. semblages.Retrograde chlorite occursin isolated patches All amphibolite assemblagescontain rutile andlor il- of randomly oriented books that are discordant to folia- menite. Rutile occurs in Mg-rich samples, whereas il- tion. Epidote, and less commonly actinolite, are associ- menite occurs in Fe-rich samples(Fig. 2). ated with retrogradechlorite. Progradechlorite has a flne FelMg enrichment follows the order staurolite x gar- bladed habit that is texturally conformable with adjacent net > orthoamphibole > hornblende > chlorite. In three . The prograde nature was verified by regularity out of four samples,Fe/Mg is greater in staurolite than in elementpartitioning among coexistingphases. Chlorite in coexistinggarnet. This is opposite to the rKf,"normally has Mg-rich compositions,with averageMg/(Mg + Fe) : observedin pelitic schists(Ganguly, 1972; Thompson, 0.80, and approximately 2.6 formula units of Al (Table 1976).The Kf is close to unity and the apparentreversal 5).

1 o

a

.. , aa

ca-amohibore

/l / .;o t a AI

V vw v 9 q - V- 9vv'-v

0.a o'8 1.2 1.6 Fig. 2. Al-Fe-Mg plot of amphibole compositions with tie lines connecting calcic amphiboles on the Al-Fe-Mg plane to R3'+2xli respectiveamounts of calculatedA-site Na and, Xo^ in plagio- Fig. 3. A-site occupancy vs. the amount ofexcess charge in clase.Also shown is the oxide that coexistswith a given the octahedral sites of amphibole. dots are calcic amphi- assemblage. bole; open triangles are orthoamphiboles. 1090 HELMS ET AL.: GEORGIA BLUE RIDGE AMPHIBOLITE

TaaLE3. Selected microprobe analysesof plagioclase

L-100 L-112-D L-36 L-112-N L-75 sio, 44.81 43.68 44.29 44.55 50.54 Atro3 35 34 35.73 36.06 35.27 32.15 Na.O 1.14 0.36 1.06 0.63 3.80 CaO 17.91 19.27 17.61 19.27 13.59 KrO n.a. n.a. 0.01 n.a. 0.03 Total 99.20 99.04 99.03 @i2 TOo:Ti Ged Cationsnormalized to 8 oxygens JI 2.078 2.037 2.055 2.063 2.321 AI 1 931 1.964 1 972 1.925 1.694 Na 0.102 0.033 0.095 0.056 0.334 0.890 0.961 0.875 0.9s6 0.661 K n.a. n.a 0.000 n.a. 0.001 Total STOi 4305 4397 t000 5.01-T 4. Diagnosticlow-variance assemblages shown on an : Fig. ivote: n.a no analvsis Al-Fe-Mg-Naanorthite (Anro) projection.

may reflectanalytical uncertainty. However, Ward (1984) comparison with mineralogically similar amphibolites and Schreyeret al. (1983) also reported a reversal in 1(B' (e.g.,Spear, 1971, 1978, 1982;Robinson et al., 1982). for Mg-rich rocks. Low-variance amphibolite assemblagesare divided into an anorthite-presentand an anorthite-absentgroup. The Pn,rsn RELATToNSoN THE Al-Fe-Mg lNl latter assemblagesare not amenableto projection on the Al-Na-(Fe + Mg) PLANES Al-Fe-Mg plane unlessamphiboles are usedas projection Laurel Creek amphibolites representa rare amphibo- points. In Figure 4, anorthite-present assemblagesare lite facies type. Following the assumptionsof Thompson projected through anorthite (Anno)into the Al-Fe-Mg-Na (1957),Harte and Graham (1915),Spear (1978), and Laird tetrahedron. The measuredrange in plagioclasecompo- (1980), amphibolites can be graphically treated in the sition is Anrr-nr. Although hornblende and gedrite are simplified system FeO-MgO-AlrOr-CaO-NarO. Ideally, plotted on the Al-Fe-Mg plane, they actually reside off reduction of a systemwith (3 -l n) or (4 + n) components the plane. Their positions relative to Na are shown on is accomplished by either projection from ubiquitous the Al-Na-(Fe + Mg) plane. For clarity, only tie lines phasesof fixed chemical composition or from phasecom- betweenthe two different types of amphiboles are shown. ponents having equivalent chemical potential for all as- The general regularity in cation partitioning argues semblagesconsidered (Thompson, 1957; Greenwood, against gross disequilibrium. AssemblagesL-l l2-D and 1975).Considering these guidelines, chlorite is the phase L- I 00 exhibit crossingtie lines that suggesteither p"ro or ofchoice for projection; however, this does not facilitate po, is not equivalent in the two assemblagesconsidered.

TneLe4. Selected microprobe analysesof garnets and staurolite

Staurolite L-112-D L-100 L-79 L-36 L-112-N L-100 L-36 sio, 39.20 38.80 38.30 38.20 27.60 28.40 28.05 Tio, 0.00 n a. n.a. n.a 0.54 0.46 0.69 Alro3 22.40 22.90 22.90 22.10 53.76 53.80 52.57 Cr,03 n.a. n.a. n.a. n.a. 0.29 0.00 0.04 Mgo 8.48 8.10 7.96 6.16 4.12 3.48 2.77 '11.70 FeO 26.20 26.20 27.20 29.10 12.80 14.03 MnO 1.71 1.07 1.42 1.95 0.22 0.00 0.15 CaO 3.27 4.03 2.98 3.46 n.a. n.a. n.a. ZnO n.a. n.a. n.a. n.a. 0.28 0.00 0.11 Total 101.26 101.10 100.76 100.97 98.51 e&%- 967T Cations normalizedto 12 oxygens Cations normalizedto 23 oxygens ai 2.988 2.959 2.947 2.967 3.786 3.875 3.881 Ti n.a. n.a. n.a n.a. 0.052 0.044 0.069 AI 2.008 2.053 2.075 2.026 8.687 8.655 8.570 n.a. n.a. n.a. n.a. 0.028 0.000 0.004 Mg 0.961 0.919 0.913 0.714 0.833 0.705 0.568 FE 1.668 '1.677 1.749 1.894 1.324 1.454 1.620 Mn 0.109 0.067 0.090 0.127 0 028 0.000 0.016 0.266 0.329 o.237 0.287 n.a. n.a. n.a. Zn n.a. n.a. n.a. n.a. 0.028 0.000 0.008 Total 8.000 6:00;[ 8.011 E-.OiE 1"{,766- Ii.frtg fi7rc Xrn 0.366 0.354 0.343 0.274 0.386 Note.'n.a.: no analysis. HELMS ET AL.: GEORGIA BLUE RIDGE AMPHIBOLITE 109l

TneLe5. Selectedmicroprobe analyses of chloritesand ilmenite idote and into the Fe-Mg-Na-Al tetrahedron. Hbl + Ky, Hbl + St, and Hbl * An do not necessarily prohibit the assemblagesHbl + Chl + Ep * An- or L-l 12-N L-11 2-D L-100 L-100 Hbl + Grt + Ep + An., at Mg-rich and Fe-rich com- sio, 27.37 26.12 27.27 positions, respectively.However, the assemblageHbl + Alros 22.90 23.92 22.47 0.16 FerO. 14.53 Chl + Grt * Anr, * Ep cannot occur with Ky + Hbl. Tio, 0.02 0.08 0.00 45.27 Coexistenceof Grt + Chl with Hbl + Anr, + Ep, though Mgo 24.77 23.46 23.67 0.97 not as common as higher-variancesubsets, is nonetheless FeO 11.83 13.33 14.03 38.77 MnO 0.05 0.09 0.14 0.31 reported in well-documented medium-pressure meta- Total 86.93 87.00 87.58 100.02 morphic terranes(Laird, 1980;Harte and Graham, 1975). Cations normalizedto The preceding results suggestthat the bulk-rock FelMg 't4 play presence 14 14 o ratio may a critical role in the or absence oxygens oxygens oxygens oxygens of staurolite- or kyanite-bearingamphibolites (as well as 5l 2.696 2.596 2.698 the common amphibolite assemblagebearing Grt + Chl). AI 2.659 2.803 2.621 0.005 Unless the bulk composition is appropriate, typical am- Fe3* 0.276 Ti 0.001 0.007 0.000 0.859 phibolite assemblages,such as Hbl + Chl * An., * Ep, Mg 3.637 3.475 3.491 0.037 can persist in the stability field of Hbl + Ky + St + An. Fe 0 974 1.108 1.161 0.818 Assemblagessuch as Hbl + Grt * Ano, are prevalent at Mn 0.001 0.007 0 000 0 005 Total 5369 9.996 9.972 2.000 Laurel Creek; however, the five-phase common assem- blage is not observed. A/ote:Fe3* calculated on the basis of stochiometryand chemistry Graphical demonstration of the reaction relation be- tween the common amphibolite assemblageand the Lau- Alternatively, combining Fe3*and Al may be an invalid rel Creek amphibolite is accomplished by projecting assumptionfor theseamphibolites. Low-variance assem- through chlorite (composition L-79) into the Fe-Na-Ca- blagesreported here contain 2-3o/oFerO,(Table l). Al tetrahedron(Fig. 5b). The composition of chlorite does not vary significantly.Tie lines crossingthe plane An35+ RracrroN RELATIoN BETwEENKyANITE Ep + Grt (+ ChD are numerous,indicating that the com- AMPHIBOLITES AND THE COMMON mon amphibolite assemblagecontaining Chl + Grt can- AMPHIBOLITE ASSEMBLAGE not occur at physical conditions appropriate to the sta- Selverstone et al. (1984) noted that staurolite- and bility of Hbl + St + Ky + An. kyanite-bearing amphibolites are related by reaction to Crossing tie-line relationships observed in Figure 5b the common amphibolite assemblagefound in medium- are demonstrated more clearly with reactions generated pressuremetamorphic terranes.A Na-Ca-Al-(Fe + Mg) by linear algebra (Braun and Stout, 1975; Greenwood, projection was used to demonstrate this relationship. In 1967),as presentedin Table 6. Mineral compositionsused this projection, Chl * Grt cannot coexist with Hbl + to balance reactions are also listed in Table 6. All low- Ep + Pl as a divariant assemblage.Therefore, combining variance assemblagesat Iaurel Creek may be derived from Fe and Mg obscuresa significant portion of amphibolite the common amphibolite assemblageby linear combi- composition space.Figure 5a is a projection through ep- nations ofreactions listed in Table 6. Thesereactions are

Fig. 5. Some reaction relations in amphibolite space.(a) Al-Fe-Mg-Na diagram; phasesare projected through epidote. (b) A1- Fe-Na-Ca diagram; generalizedLaurel Creek assemblagesare projected through the composition ofchlorite (L-79). 1092 HELMS ET AL,: GEORGIA BLUE RIDGE AMPHIBOLITE

TneLe6. Reactionsrelating the common amphiboliteassemblage to the LaurelCreek amphibo- lite assemblages

Common amphiboliteassemblage : Laurel Creek assemblages 4.16Ep+ 1.00An..+ 0.88chl + 0.49crt + 3.42Qtz: 6.87An + 1.31Hbl+ 4.30H,O (1) 1.65Ep+ 1.00Ans5+ 1.55chl + 0.85crt + 3.16otz : 2.82KV+ 2.29Hbl+ 4.72H2O (2) 1.58Ep+ 1.00An".+ 1.61chl + 1.34crt + 3.42Qtz: 0.76St + 2.29{bl + 4.17H2O (3) 4.40Ep+ 1.09Ans5+ 1.56chl + 1.00crt + 8.15otz : 9.59An + 1.73ced + 6.73H,O (4) 1.00An..+ 11.77C1+ 7.45crt + 36.16Ot2:11.14Ky+ 1.00Hb1+ 12.29ced+ 33.77H2O (5) 1.00An,,+ 8.88chl + 6.94crt + 22.60Qt2: 2.2751+ 1.00Hbl+ 8.88ced + 23.35H,O (6) 0 45An35+ 7.55chl + 4.83crt + 28.73Qt2: 10.62Ky+ 1.00An+ 8.38ced + 21.88H,O (7) 2.91An35+ 4.51chl + 2.48crt + 1.00Qtz+ 4.79Ep: 8.21Crn+ 6.68Hbl+ 13.74H2O (8)

Hornblende Nao3escar 62oM92,4Fe, 421A12 zn4si6 *6O2(OH), Chlorite Mgs s$Fe, @7A12ssrsi, 71sO1o(OH)6 Staurolite Fer 32dMgoBA16 MSi3 rsor2(OH), Anorthite Caoe5Naoo5Al1 $Si2 600 Garnet Fe, e$Mgo &Cao 247A12@7Si2 sO j2 Gedrite Nao 22ecao,@M93rsFe2 @1A11 1ssi6 eTOz(OH), Epidote CarAl35i30,r(OH) Oligoclase CaorNaosAll 65i12Os consistent with observed high modal volumes of horn- tably aluminous, and gedrite-bearingsamples are not low blende (approximately 65-900/o)and low modal volumes in Ca. Compositions of Laurel Creek amphibolites are of chlorite (<<2o/o),garnet (<< 100/o),and epidote (00/o). compared on an (Al * Fe*3)rOr-CaO-(Fe+ Mg)O dia- The preceding results are consistent with the sugges- gram (Fig. 6) with compositions of amphibolites from tions of Selverstoneet al. (1984);also recognizedis the equivalent significanceof Ged + An, Hbl + An, Hbl + st + Ged, Hbl + Ky + Ged, and Hbl + crn. The lasr TneLe7. Modes(vol%) of LaurelCreek amphibolite and least- assemblagehas been noted in several other localities in squaresmass-balance calculations the Blue Ridge (Pratt and Lewis, 1905; McElhaney and L-100 L-112-N L-112-D L-79 McSween. 1984) is with kvanite. and often associated Modal volumes in Laurel Creek amohibolites staurolite, and anorthite. Hbl 78.9 80.9 66.1 81.3 Ged 0.0 0.5 13.4 5.5 Bur,r-nocr coMPosrrroNs chl 0.4 0.3 1.0 0.8 An 5.9 tr. 0.2 0.0 xRF and wet-chemical analysis of four samples con- Grt 7.3 0.0 10.2 4.2 taining kyanite, staurolite, or gedrite (or all three) reveal St 3.0 2.3 0.1 3.2 Ky Ir. 29 0.0 0.0 Mg-rich olivine-normative basaltic compositions (Table Otz 2.2 130 8.0 4.2 l). Samplescontaining kyanite and staurolite are not no- llm 0.6 00 0.0 0.0 Rt 1.4 0.1 1.0 0.6 Po 0.3 0.0 0.0 o.2 Total 100 100 T00- 100 Common amphibolitephases fit to bulk compositionsin Table 1 Hbl 472 60.9 52.3 61.8 chl 165 15.8 23.5 19.1 Ah* 8.2 10.9 6.4 7.2 Ep 14.5 4.4 5.J 1.3 Spn 0.0 1.0 1.3 1.6 Qtz 7.7 5.1 10.4 5.8 llm 3.7 0.0 0.0 0.0 Hem 0.0 2.9 2.7 2.2 Mag a7 0.0 0.0 0.0 Total 10rV 101.1 102.1 990 (2 e1'e 0.000 0.000 0.645 0.370 Note:r: residual.

Hornblende(L-100) Nao s(Car sFeo,XFe, 6Mg, ,AlXSi6 sAll ,)Orr(OH), Chlorite(L-100) FeM g4Al(Si3Al)O,,(OH)6 Hornblende(L-1 1 2-N) Nao,(Ca, ,)(FeMg3AlXSi68Al1 ,)Orr(OH), "Feo Chlorite(L-112-N) Fe, reMg3s6Allrs(Sirr5A11 rs)Orr(OH)g Hornblende(L-1 1 2-D) Nao o(Ca, eFeo,XFe*Mg3Alo 6)- (si6sAl1r)orr(oH), Chlorite(L-112-D) Fe1sM$ 5Al(Si3Al)O,o(OH)s Fig. 6. Plot of measuredbulk compositionsof unusuall,aurel Hornblende(L-79) Nao 4(Ca1eFeo rXFe, .Mg2 sAlos)- (si6dt'rorr(oH), Creek amphibolites on an (Al + Fe)rO.-CaG{Fe + Mg)O dia- Chlorite (L-79) Fe1$Mg3 45Al(Si3Al)O,o(OH)o gram; also plotted are compositions of common amphibolites from Vermont and of low-Ca amphibolites that bear orthoam- /Votej Plagioclaseis always Anro,epidote is the pure Al end member; sphene and all other phases are pure end-membercompositions. ohibole. HELMS ET AL.: GEORGIA BLUE RIDGE AMPHIBOLITE 1093

TeeLe8. Garnet-biotitethermometry TeeLe9. Geobarometryfrom sample L-l00

Garnet + rutile + kyanite + ilmenite + quartz Mg/ Mg/ T x"' xw Xn, X"o Sample Fe X"" X." Fe Xtu K,^, Xn rc) 0.600 o.292 0.091 0.027 0.594 L-73 0.274 0.017 0.129 1.728 0.328 0.09s 0.024 540 X' Xs* X". 4,*' P (kbar) L-77 0.232 0.112 0.076 1.2s5 0.364 0.130 0.038 601 0.767 0.093 0.142 0.642 7.7 L-63A 0.279 0.075 0.064 1.310 0.339 0.164 0.029 62s 'a (1981). L-83 0.238 0.116 0.061 1.263 0.372 0.126 0.028 604 calculatedfrom Newtonand Haselton L-47 0.341 0.056 0.117 1.780 0.300 0.135 0.024 610 " a calculatedfrom Andersen and Lindsley(1981).

Vermont that contain the common assemblage(Laird, mately isochemical with the common amphibolite as- 1980)and compositionsof "low-Ca amphibolites" (Til- semblage.The amounts of normative in these Iey, 1935;Lal and Moorhouse,1969). All but threeof the calculated common assemblagesare consistent with re- "low-Ca amphibolites" fall on the (Al * Fe*3)rO.{Fe * actions presentedin Table 6. Hornblende occursin small- Mg)O join. The "low-Ca amphibolites" all lack horn- er normative amounts in the calculatedcommon assem- blende, whereasgedrite-bearing Laurel Creek samplesal- blage than in Laurel Creek rocks; chlorite, plagioclase, ways contain high modal quantities of hornblende (-65- epidote, and spheneare presentin greateramounts in the 900/o).Compositions of Laurel Creek amphibolites bear- calculatedcommon assemblagethan in the rocks. ing Ky + St + Ged + An fall within, or very close to, the common amphibolite field. Bulk compositions of four Laurel Creek amphibolites CoNlrrroNs oF have been fitted by least-squarescalculations to the com- Temperatures estimated with the garnet-biotite geo- mon amphibolite assemblageHbl + Chl + Anro + Ep + thermometer calibrated by Ferry and Spear (1978) vary Spn (or Ilm) + Mag (or Hem) + quartz (Table 7). Al- from 540 to 653 "C (Table 8). The expectedrange in error though thesesolutions are nonunique, they strengthenthe reportedby Ferry and Spear(1978) is +50 "C. Garnet- hypothesis that Laurel Creek amphibolites are approxi- biotite pairs from the Laurel Creek amphibolite (sample

o 4 J A

T"C Fig. 7. A portion ofthe petrogeneticgrid ofSpear and Rumble (1986). t094 HELMS ET AL.: GEORGIA BLUE RIDGE AMPHIBOLITE

L-47) and adjacentpelitic schists(all other samples)were seeSpear, 1982) at amphibolite-faciestemperatures, Sel- used. verstone et al. (1984) suggestedthat it reflects pressures The assemblageGrt * Rut + Ky + Ilm + Qtz higher than those appropriate to the stability of the com- ('GRAIL'), found in amphibolite sampleL-100 (Table mon amphibolite assemblage.Data presentedhere are in 9), indicatesa minimum pressureof -7.7 kbar, at a tem- agreementwith their conclusions. perature of 600 'C. This value was inferred from the K*- Experimental studies corroborate this idea. Tholeiitic contouredP-T diagramof Bohlen et al. (1983a,1983b). basaltssubjected to 23kbar at 750'C (Hellman and Green, Sample L-100 contains one of kyanite; becauseit 1979), under hydrous conditions, crystallized high-Mg is almost entirely surrounded by hornblende, it is not staurolite(X*": 0.57), kyanite, orthoamphibole,clino- interpreted as part of the peak metamorphic assemblage. ,and garnet. The assemblageCpx + Ky + St is The assumption that kyanite has an activity of one yields the high temperature equivalent of Hbl + Ky + St. Fur- a minimum pressure.The activity of almandine in garnet ther experimental support for a high-pressureorigin for was calculatedaccording to the model presentedby New- kyanite-bearingamphibolites is given by the experimen- ton and Haselton(1981); the activity of FeTiO. in ilmen- tal and theoretical investigations of Schreyer(1968) and ite was calculatedusing the model presentedby Andersen Schreyerand Seifert(1969a, 1969b) in the systemMgO- and Lindsley(1981). There are other mixing modelsfrom AlrO3-SiOr-HrO, in which it was demonstratedthat stau- which the activity of almandine may be calculated that rolite (pure Mg end member), , and "magnesio- result in significantly different pressures.Ilmenite com- gedrite" are all important phasesat high pressures.It was positions used for pressurecalculation contain approxi- also predicted (Schreyerand Seifert, 1969b) that Ged + mately l4olo(formula units) FerOr,4olo MgTiO., and <

AcrNowlnocMENTS Leake,B.E. (1978) Nomenclatureof amphiboles.American Mineralogist, 63, 1023-1052. We expressthanks to Frank Spear and Donald Lindsley for helpful Lindsley, D.H (1973) Delimitation of the hematite-ilmenite miscibility reviewsofthis manuscript.This paperconstitutes part ofthe first author's gap GeologicalSociety ofAmerica Bulletin, 84,657-662. M.S thesis at U.T K. Researchwas funded in part by the professor's Miyashiro, A (1961) The evolution of metamorphic belts. Journal of honors fund, Department of GeologicalSciences, U T.K. Petrology,2, 277-311. McElhaney,M.S, and McSween,H Y, Jr. (1984)Perrology of the Chunky RnrnnrNcns crrED Gal Mountain mafic-ultramafic complex, North Carolina Geological Societyof America Bulletin, 94, 85 5-874 Albee, A.L., and Ray, L. (1970) Correction factors for electron probe McSween,H Y., Jr., and Hatcher, R.D , Jr. (1985) Ophiolires (?) of the microanalysisof silicates,oxides, carbonates, phosphates, and sulfates. southernAppalachian Blue Ridge. In N.B. Woodward, Ed., Field trips Analytical Chemistry, 42, | 408-1414. in the southem Appalachians.University ofTennesseeDepartment of Andersen,D.J., and Lindsley,D.H. (1981)A valid Margulesformulation GeologicalSciences Studies in Geology 9, 144-170. for asymmetric ternary solution: Revision of the olivine-ilmenite Misra, K.C, and Keller, F.B. (1978)Ultramafic bodiesin the southem geothermometer,with application Geochimicaet CosmochimicaActa, Appalachians:A review American Journal of Science,27 8, 389-418 45,847-853 Newon, R.C., and Haselton,H T. (1981) Thermodynamicsof the garnet- Bence,A.E., and Albee, A.L (1968) Empincal correlation factors for the plagioclase-AlrSiOr-quartzgeobarometer. In R C. Newton, A Navrot- electron microanalysisof silicatesand oxides Journal of Geology, 76, sky, and B J. Wood, Eds., Thermodynamis of minerals and melts, p. 382-403 l3l-147 . Springer-Verlag,New York. Bohlen,S.R., Wall, V.J, and Boettcher,A.L. (1983a)Experimental in- Petty, S.M. (1982)The geologyof the Laurel Creek mafic-ulrramaficcom- vestigationand geologicapplications of equilibira in the system FeO- plex in northeastGeorgia: Intrusive complex or ophiolite? M S. thesis, TiOr-AlrO3-SiOr-HrO.American Mineralogist, 68, 1049-l05g. - Florida StateUniversity, Tallahassee,147 p ( I 983b) Geobarometryin .In S.K. Saxena,Ed., Krnetics Pigage,L.C., and Greenwood,H.J. (1982) Intemally consistentestimates and equilibrium in mineral reactions,p. l4l-171. Springer-Verlag, New ofpressureand temperature:The stauroliteproblem. American York. Journal of Science.282. 943-969. Braun, G., and Stout, J.H. (1975) Some chemographicrelationships in Pratt, C F., and Lewis, A.P. (1905)Corundum and the peridotitesof wesr- n-component systems Geochimica et Cosmochimica Acta, 39, l2S7_ 1259 em Nonh Carolina North Carolina Geologic Suryey Report p. 464. Robinson,P., Ross,M., and Jaffe,H.W. (1971)Composition Carpenter,R.H. (1970) Metamorphic history of the Blue Ridge province of the an- thophyllite-gedriteseries, comparisons of gedrite and hornblende,and ofTennesseeand Nonh Carolina. GeologicalSociety ofAmerica Bul- letin, 81, 749-762 anthophyllite-gedritesolvus. American Mineralogist, 56, 1005-1039 Robinson,P., Spear,F.S., Schumacher, Cooper, A.F. (1980) Retrogradealteration ofchromian kyanite in meta- J.C, Laird, J., Klein, C, Evans, B W., and Doolan, B L. (1982) Phaserelations chert and amphibolite whiteschistsfrom the southernAlps, New Zea- of metamorphic am- phiboles: Natural occurrence and theory. Mineralogical land, with implications for uplifit on the Alpine fault. Contributions to Society of Mineralogy and Petrology,7 5, 153-164. America Reviews in Mineralogy, 9b, 1-211. (1968) Ferry, J.M., and Spear,FS. (1978) Experimental calibration of the par_ Schreyer,W A reconnaissancestudy of the system MgO-Al,Or- pressures -staurolite. titioning of Fe and Mg between biotite and gamet. Contributions to SiOr-HrO at between l0 and 25 kb Camegie Mineralogy and Petrology,66, ll3-117. Institution of Washington Year Book 66, 380-392. - (1973) Flanagan,F.J. (1976) Description and analysesofeight newU.S.G.S rock Whiteschists:A high pressurerock and its geologicsignif- standards:U S. Geologic Survey Professionalpaper 840. icance.Joumal of Geology,81, 735-7 39. Ganguly, J (1972) Staurolite stabiliry and related parageneses:Theory, Schreyer,W, and Seifert,F. (1969a)High-pressure phases in the system experimentsand applications.Joumal of petrology,13, 335-365. MgO-Al,Or-SiOr-HrO American Journal of Science,267-A, 407-443 - Gibson, G M. (1978) Staurolite in amphibolite and homblendite sheets (1969b) Compatibility relations ofthe aluminum silicatesin the from the upper SeaforthRiver, central Fiordland, New Zealand. Min- system MgO-Al,O3-SiOr-HrO and K,O-MgO-AlrO3-SiO,-HrO at high erafogicalMegazine, 42, 153-154. pressures.American Joumal of Science,267 , 37 l-388 Greenwood,H.J. (1967) The N-dimensional tie-line problem. Geochim- Schreyer,W, Horocks,P C, and Abraham,K. (1983)High-magnesium ica et CosmochimicaActa, 31,465-490. staurolitein a sapphirine-garnetrock from the Limpopo belt, Soulhern - (1975)Thermodynamically valid projectionsofextensive phase Africa. Contributions to Mineralogy and Petrology, 86, 200-201. relationships.American Mineralogist, 60, I -8 Selverstone,J., Spear, F.S., Franz, G., and Morteani, G. (1984) High Griffin, D T., and Ribbe, P H. (1973) The crystal chemistry of staurolite. pressuremetamorphism in the S. W Tauren Window, Austria: P-?" AmericanJoumal of Science,273-A, 479-495. paths from homblende-kyanite-stauroliteschists. Joumal of Petrology, Hadley, J.B. (1972) The Ocoee Series and its possible conelatives. In 25,501-533. G W. Fisher and others, Eds., Studies of Appalachian geology, central Spear,F.S (1977)Phase equilibria ofamphibolites from the Post Pond and southem, p.247-259 IntersciencePublishers, New york. Volcanics, Vermont. Carnegie Institution of Washington Year Book Harte, 8., and Graham, C M. (1975)The graphicalanalysis ofgreenschist 66,613-619 to amphibolite facies mineral assemblagesin metabasites.Journal of - (1978) Petrogeneticgrid for amphibolites from the Post Pond Petrology,16,347-370 Volcanics:Carnegie Institution ofWashington Year Book 67, 805-808. Hatcher,RD., Jr (1971)Geology of Rabun and HabershamCounties, - (1980) The gedrite-anthophillitesolvus and the composition lim- Georgia.Geological Survey ofGeorgia Bulletin 83, 48 p. its of orthoamphibole from the Post Pond Volcanics,Vermont. Amer- - (1972) Developmental model for the southern Appalachians: ican Mineralogist,65, I 103-l I 18. Mineral assemblagesin metabasitesJournal ofpetrology, I 6, 347-370. - (1982) Phaseequilibria ofamphibolites from the Post Pond Vol- Hellman, P L., and Green, T.H. (1979) The high pressureexperimental canics,Mt Cube Quadrangle,Vermont. Journal of Petrology, 23, 383- crystallization of staurolite in hydrous mafic compositions. Contribu- 426 tions to Mineralogy and Petrology, 68, 369-372. Spear,F.S, and Rumble, D III. (1986) Pressure,temperature and struc- Holdaway, M J. (l 97 l) Stability of andalusiteand the aluminum silicate tural evolution of the Orfordville belt, west-centralNew Hampshire: diagram.American Journal of Science,27 l. 97-l 3 l. Journal of Petrology,27, 107l-109 4. Lal, RK, and Moorhouse,W.W. (1969) Cordierite-gedriterocks and. Stout, J.H. (197f) Phasepetrology and mineral chemistry ofcoexisting associated ofFishtail Lake, Harcourt Township, Ontario. Ca- amphibolesfrom Telemark, Norway. Journal ofPetrology, 13, 99-145. nadianJournal ofEarth Sciences.6. 145-165. Thompson, A B. (1976) Mineral reactionsin pelitic rocks: I. Prediction Laird, (1980) J Phaseequilibria in mafic schistsfrom Vermont. Journal of P-Z-X(Mg-Fe) phaserelations. American Joumal of Science,276, of Petrology,21, l-37 . 401-424. 1096 HETMS ET AL.: GEORGIA BLUE RIDGE AMPHIBOLITE

green Thompson, J.B (1957) The graphicalanalysis ofmineral assemblagesin Ward, C.M. (1984) Magnesium staurolite and chromian staurolite pelitic schists.American Mineralogist, 42, 842-858. from Fiordland. New Zealand. American Mineralogist, 69,531-540- Thompson,J.B., Thompson, A.B., and Laird, J. (1982)Reactions ln am- phibolite, gresnschistand .Joumal of Petlology, 23, 1-27. Tilley, C.E. (1935) Metasomatismassociated with greenstone-hornfelsof Kenidjack and Botallack, Cornwall. Mineralogical Magazine, 24, l8l- M.qrruscrur'r RECETvEDSpprei"rsen 25, 1986 202 MeNuscnrpr AcCEPTEDJurv 2,1987