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Petrology of a Georgia Blue Ridge Amphibolite Unit with Hornblende* Gedrite * Kyanite * Staurolite Tnonrl.Ss

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Petrology of a Georgia Blue Ridge Amphibolite Unit with Hornblende* Gedrite * Kyanite * Staurolite Tnonrl.Ss American Mineralogist, Volume 72, pages 1086-1096, 1987 Petrology of a Georgia Blue Ridge amphibolite unit with hornblende* gedrite * kyanite * staurolite 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 composedof subsetsof the assemblagehornblende + chlorite + garnet * 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 amphibolites 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+ epidote + 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-biotite geothermometry indicates a temperature of 540-625 "C for both adjacent pelitic schists 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 amphibole * 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 amphiboles 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 sillimanite 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 pelite 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 mineral 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 pargasite(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
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