Nearly Pure Iron Staurolite in the Llano Uplift and Its Petrologic Significance

American Mineralogist, Volume 79, pages 154-160, 1994

Nearly pure iron staurolite in the Llano Uplift and its petrologic significance

Wrr,r,lclu D. C.Lnr-sox, JosnrH F. RBnsB Department of Geological Sciences,University of Texas at Austin, Austin, Texas 78712, U.S.A.

Ansrnlcr Staurolite very close in composition to the Fe end-member is an abundant component of two lithologic layers in an unusual metasedimentarysequence in the Proterozoic Rough Ridge Formation (PacksaddleGroup) along White Creek in the southeasternLlano Uplift, central Texas. The exceptional composition of this staurolite makes it an attractive can- didate for use in mineralogical, experimental, and thermochemical studies. The abundanceof this nearly pure iron staurolite at White Creek contrastswith nearby evidence for early crystallization and later elimination of staurolite with a typical Fe-Mg ratio. This is consistent with the hypothesis that staurolite spanning a range of Fe-Mg ratios grew during an early moderate-P to high-P dynamothermal episode,but that only Fe-rich staurolite survived later low-P static metamorphism. The White Creek rocks also record incomplete reaction of staurolite + quartz to produce almandine + sillimanite. This reaction might representthe peak of prograde crystallization during early dynamothermal metamorphism near 700'C and 7 kbar. Alternatively, the reaction might represent partial reequilibration to conditions near 550 'C at -2.75 kbar during the later static metamorphic event; this alternative presumesthat breakdown of unstable H-rich (high- P) staurolite yields a metastablealmandine + sillimanite assemblagebecause conversion to a stable low-H staurolite composition is kinetically impeded. Under either interpretation, these observations imply that the southeasternLlano Uplift shared the complex polymetamorphic history documentedpreviously for the northern and northwesternuplift.

INrnonucrroN high-pressuremetamorphism. Regional correspondence Unusually Fe-rich staurolite occurs in the southeastern of structural styles suggeststhat the southeasternuplift Llano Uplift of central Texas in a petrologic context that shared the dynamothermal metamorphism that affected shedslight on the regional metamorphic history. In over- the northern and northwestern uplift, and the overprint- view, that history is dominated by two kinematically dis- ing ofearly foliations by granoblastictextures recordsthe tinct thermal events during mid-Proterozoic metamor- influence of a later static thermal event. But until now, phism. The first event produced early syndeformational the southeasternuplift has lacked unequivocal petrologic metamorphism at moderate to high pressures(Wilkerson evidenceof a polythermal, polybaric metamorphichistory. et al., 1988),but becauseofthorough overprinting,the In this article we document the occurrence in the principal evidence for this event is restricted to small southeasternLlano Uplift of rocks containing a nearly bodies of mafic eclogite that preservehigh-pressure min- pure iron staurolite. From the staurolite's chemistry, in eral assemblagesand compositions. The second event, combination with textural observations,phase equilibria, which resulted in widespread static recrystallization and and evidence for early crystallization and later destruc- hydration at low pressures,was coincident in time with tion of ordinary iron-magnesium staurolite, we infer that voluminous granitic intrusion and is genetically linked the southeasternuplift has undergonea polymetamorphic by isotopic and petrologic evidence to pluton emplace- history similar to that revealed by eclogitic remnants in ment (Beboutand Carlson, 1986). the north and northwest. Although remnants of mafic eclogitesthat record high pressureshave been found in the northern and north- OccunnnNcE AND cHEMTCALcoMposrrroN westernparts of the Llano Uplift (Wilkersonet al., 1988; White Creek in the southeasternLlano Uplift (Fig. l) Schwarze,1990; Carlson and Johnson, l99l; Carlson, exposeswithin the Proterozoic Rough Ridge Formation 1992), eclogitic rocks have not been found in the south- (PacksaddleGroup) an uncommon suite of pelitic meta- easternuplift. Instead, mafic rocks in the southeastyield sedimentaryrocks. Although the predominant rock types only amphibolite faciesassemblages that lack garnet.This in the PacksaddleGroup are felsic schists,felsic gneisses, fact is consistent either with an exclusively low-pressure and amphibolites, the metasedimentarypackage at White history in the southeastor with complete reequilibration Creek (Universal Transverse Mercator coordinates there during late static recrystallization following early 14RNJ487775)includes interlayeredandalusite-rich 0003-004x/94l0I 02-0I 54$02.00 r54 CARLSON AND REESE:IRON STAUROLITE IN THE LLANO UPLIFT r55

TABLE1. Modes for staurolite rocks at White Creek

Sample WC91c WC9117 Pssgl PSsg2 SC919C2 Quartz 45 55 45 40 45 Staurolite 25 25 20 20 15 Biotite 10 15 5 5 10 Muscovite 3<1 10 15 20 Garnet o4 15155 Sillimanite 15 <1 550 llmenite 21 <1<14 Tourmaline 00 001 /Vote:modes are percentagesestimated visually in individualthin sections. Garnet is irregularlydistributed, and so its absenceor high concen- 0 15mi tration in a singlethin section in this table is not representativeof its modal abundance at an outcrop scale (-2-5% on average).All samples also contain trace amounts of apatite and zircon, and severalcrystals of monazite were identifiedin samplesWC91c and WC9117during microprobe analysis.

as 3.50/oof Fe,",(coexisting ilmenite possessesonly -0.250lo hematite component), and H was estimated by subtract- ing the total cation charge from 96 after normalizingto fix Si + Al - thLi * 2hTi + Fe3+at 25.55 ions pfu. contacl (including In the staurolite crystal from White Creek,components formationalcontacts ffiI=ffiEt in Packsddle Group) outside the systemFe-Al-Si-O-H are presentonly in min- PACKSADDI.!COALCREEK OTHEF GRNITES GRoUP SERPENTINIEMf,AMORPHIC uNrrs Paleozoiclault ute amounts. On an atomic basis,the most Mg-poor sam- UNITS / ple among the 3l analyzedspecimens of Holdaway et al. stEam @u6e PBECAMBFNN UNITS (1991)contains five times more Mg than is presentin the Fig. l. Locationmap f,or staurolite occurences in the south- White Creek samples. The specimen (from Znn, l98l) easternLlano Uplift, showinghighly generalized geology. Stau- used for thermochemical measurementsby Hemingway rolitehas been found only in the RoughRidge Formation of the and Robie (1984) contains nine times as much Mg. Ti PacksaddleGroup: nearlypure iron stauroliteis abundantat and Li are present in the White Creek staurolite crystal occursonly as in- White Creek,but iron magnesiumstaurolite at levelsjust over halfthe averagevalues in the tabulation clusionsin garnetat SandyCreek. of Holdaway et al. (1991);Zn and Mn are negligible.Al- though the estimated H contents are subject to lo uncer- quartzites and muscovite schists.In this sequence,stau- tainties of about +0.4 H pfu (Holdawayet al., 1991,p. rolite occurs abundantly in two distinct, discontinuous l9l7), the White Creek staurolitecrystals are composi- layers, each 25 cm thick, that are rich in Al, Si, and Fe tionally very close to the stoichiometric referencecom- but very poor in Na, Ca, and Mg. The mineralogy of both position HoFerrrAl,, nosi,uroos proposed by Holdaway et layers is dominated by quartz and staurolite, with sub- al. (1991).We areunaware of any staurolitemore Fe-rich sidiary biotite, garnet, sillimanite, and ilmenite (Table l). than those describedhere. Trace tourmaline, apatite, zircon, and monazite are also Staurolite at White Creek grew early in the sequenceof present. Some samplesof each layer contain appreciable deformational and metamorphic events. It displays a muscovite, but others bear only traces of it. Although moderate preferred orientation of elongate crystals par- plagioclaseand potassium feldspar were sought, they were allel to the rock's dominant foliation (Fig. 2), indicating not found. that it formed during the early dynamothermal meta- The data in Table 2 illustrate that staurolite, garnet, morphism. In many instancesstaurolite is included with- and biotite at White Creek are extreme compositional in almandine, or partially replacedby almandine,or both. variants, with atomic Fe/(Fe + Mg + Mn + Ca) of 0.99, Staurolite crystallization also predates the cessation of 0.99, and 0.93, respectively;for staurolite, atomic Fel(Fe almandine growth in the only other known occurrenceof + Mg + Mn + Zn + Li) is 0.96. Electron microprobe staurolite in the uplift, where it appears exclusively as analysis reveals no compositional zoning within stauro- inclusions in garnet that grew during dynamothermal lite crystals and no appreciable crystal to crystal varia- metamorphism(Carlson and Nelis, 1986). tions in composition. The staurolite compositions in Ta- One possible origin for the staurolite rocks at White ble 2 for samplesWC9lc and WC9l17 are averagesof Creek is metamorphism of intensely weatheredhorizons ten analysesofrandomly selectedcrystals from eachsam- in an argillaceoussedimentary sequence.This is suggest- ple. Li content was determined by secondary-ion mass ed by the close correspondencebetween their bulk com- spectrometry,courtesy of R. Hervig (Arizona State Uni- positions and those of some bauxites and metabauxites versity). Atomic proportions were calculated as recom- (cf. Table 2 of Bardossyet al., 1970)and by their intimate mendedby Holdaway et al. (1991):Fe3* was estimated associationwith highly aluminous andalusite-richquartz- 156 CARLSON AND REESE:IRON STAUROLITE IN THE LLANO UPLIFT

TABLE2. Analysesof staurolite,garnet, and biotitefrom White Creek and Sandy Creek

White Creek Sandy Creek Sample WC91c wc9117 wc9117 wc9117 GT-1 GT-1 GT.1 Mineral str stt Grt Bio stt Grt Bio Weight percent oxides sio, 26.26(18) 26.60(271 3614(19) 32 65(13) 26.51(6) 37.35(23) 35.13(20) Tio, 0.30(7) 0.30(7) 0.05(1) 1.14(9) 0.47(1) <0.01(1) 1.70(13) Al,03 54.03(51) s4.13(48) 20 33(9) 19.67(1 7) 54.01(30) 21.47(151 17.82(151 FeO,o, 16 93(19) 16.92(1 6) 42.37(13) 30.87(17) 13.14(8) 25.72(17) 19.66(18) Mgo 0.13(1) 0.18(2) 0.28(1) 1.40(3) 2.05(4) 2.67(5) 11.61(14) n.a. n.a. 0.25(2) <0.01(2) n.a. 1.66(3) n.a. MnO 0.01(1) <0.01(1) 0.32(1) <0.01(1) 0.74(11 11.16(10) 0.94(3) ZnO 0 01(1) 0.01(1) n.a. n.a. 0.74(1) n.a. n.a. Naro n.a. n.a. 0.43(1) n.a. n.a. 0.30(2) K.o n.a. n.a. n.a. 8.24(1) n.a. n.a. 8.35(4) Liro 0.0911) 0.1311) n.a. n.a. 0.20.. n.a. n.a. Total 97.76 98.28 99.76 94.39 97.86 100.03 95.51 Atomicproportions o 48 48 12 22 48 12 22 Si 7.4'l 7.47 3.00 5.31 7.48 3.00 5.19 Ti 0.06 0.06 0 0.14 0.10 0 0.11 AI 17.99 17.95 1.99 3.77 17.97 2.O3 3.44 FE'*T 0.14 0.14 0.01 0 0.11 0 0 Fe2* 3.86 3.84 2.93 4.20 2.99 1.73 2.53 Mg 0.0s 0.07 004 034 0.86 0.32 2.66 0 0 0.02 0 0 0.14 0 Mn 0 0 0.02 0 0.18 0.76 0.12 Zn 0 0 0 0 0.15 0 0 Na 0 0 0 0.14 0 0 0.15 K 0 0 0 1.71 0 0 1.71 Li 0.10 0.15 0 0 023 0 0 H+ 3.78 3.63 0 z 2.85 0 2 A/ote.rumbers in parenthesesare one standard error of the mean; n.a. : not analyzed. Data for staurolite in sample GT-1 are from Carlson and Nelis (1986). Electron microprobe data were acquired with wavelength-dispersivetechniques (60-s analyses, 15 kV, 15 nA on brass, 1o-pm beam diameter)using natural and synthetic silicate standards and employing (1970). ' the correction scheme of Albee and Ray lon-probeanalysis (R. Hervig,Arizona State University);average per '. of threeanalyses sample. Not analyzed.Value of 0.20 is assumed,following the recommendationof Holdawayet al. (1991). t Fep+is calculatedfrom the stoichiometryfor staurolite (see text) and garnet, but all Fe in biotite is reported as FeF+. + Atomic proportions for H are calculatedfrom the stoichiometryfor staurolite (see text) but are assigned arbitrarilyfor garnet and biotite. ites. The bulk chemistryand mineral texturesin the Lla- the Rougb Ridge Formation exposed in Sandy Creek, no rocks are distinctly different from those of modally roughly 4 km west of the White Creek locality (Fig. l). similar staurolitequartzites in New Mexico describedby The staurolite inclusions in garnet at Sandy Creek (Table Schreyerand Chinner (1966), for which a metasomatic 2, sampleGT-l) have ordinary Fe-Mg ratios and typical origin was proposed. minor-element contents. The matrix enclosing the garnets contains quartz, biotite, calcic oligoclase,and micro- MrNnnnlocrc AND pETRoI,ocrc srcNrFrcANCE cline but is entirely devoid of staurolite. Nearby units Staurolitecomprises -20 modalo/oof the layersin which bear large (up to lO-cm) postkinematic poikiloblasts of it occurs,shows minimal alterationin thin sections,and iron-magnesium cordierite that overgrow all metamor- is relatively free of inclusions. Combined with its excep- phic foliations (Nelis et al., 1989). tional composition, thesecharacteristics make the White These observations must be interpreted in light of the Creek staurolite an excellent subject for a variety of min- evidence summarized above for two kinematically dis- eralogic, crystallographic, experimental, and thermo- tinct thermal eventsin the Llano Uplift. Thermobarome- chemical investigations. (This occurrence is entirely on try on remnants of mafic eclogites in the northern and private property, however, and can be visited only with northwesternparts of the uplift (Wilkersonet al., 1988; prior permissionfrom the landowner.) Schwarze,1990; Carlson and Johnson, l99l; Carlson, 1992)demonstrates that early dynamothermal metamor- Polymetamorphic history inferred from contrasting phism in those areastook place at pressuresranging from staurolite occurrences 6 (?) to I I kbar and temperatures from 670 to 750 'C. When contrastedwith the regional scarcityof staurolite Later static metamorphism took place at lower temper- in the Llano Uplift, the Fe-rich occuffenceat White Creek aturesand substantiallylower pressures.Calcite-dolomite takeson petrologic significance.As noted above, only one thermometry, calc-silicatephase equilibria, garnet-biotite other occurrenceof staurolite in the uplift is known. Carl- thermometry, gamet-ilmenite thermometry, garnet-cor- son and Nelis (1986) report its presence,exclusively as dierite thermometry, O-isotope thermometry, and the inclusions in a large garnet, in a garnet-biotite gneissof stability of muscovite + quartz and aluminum silicate CARISON AND REESE:IRON STAUROLITE IN THE LLANO UPLIFT t57

assemblagesall indicate that 7"and P for the static event arein the range550 + 75 "C and 2.75 + 0.75 kbar across the entire uplift (Bebout and Carlson, 1986; Schwarze, 1990;Letargo and Lamb, 1992, 1993).Gradients in peak temperaturesfor the late static metamorphism spanning - 125 "C have been documented in proximity to some granitic intrusions(ktargo and Lamb, l99l). In the context of this regional perspective,we suggest that staurolite with a range of ordinary Fe-Mg ratios (like the inclusions within garnet at Sandy Creek) may have formed during the early dynamothermal event at moderate to high pressures,but that only nearly pure iron staurolite (like that at White Creek) survived later static metamorphism at low pressure.Although phaseequilibria involving staurolite and cordierite in the system Fe- (darkgray to black),and minor ilmenite(black). Mod- A1-Si-O-Hcontinue to defy preciseexperimental defini- biotite eratedimensional preferred orientation parallels dominant foli- tion (cf. Holdaway and Mukhopadhyay, 1993b), early ation, definedby compositionallayering and alignedbiotite. (e.g., experimentalresults Richardson,1968) indicate in SampleWC9l 17.Photomicrograph in plane-polarized light. l-ong at least a qualitative way that iron staurolite + quartz dimensionof fieldof viewis 4.1mm. should react to produce iron cordierite * aluminum silicate as pressuresdrop; the equilibrium boundary for Fe end-members probably passesslightly below 2 kbar at blages and from quantitative barometry where suitable temperatures near 550 oC. Becausecordierite preferen- assemblagesexist. Combined with the proximity of the tially incorporatesMg, "the low-pressuretermination [of two localities,this makes it unlikely that pressuresat the staurolite + quartz stability fieldl will be significantly Sandy Creek were markedly different from those at raised" for Mg-bearingstaurolite (Richardson, 1968, p. White Creek. 484). h is therefore likely that regional reequilibration at Thus regionalreequilibration to -550 "C and,-2.75 -550'C and -2.75 kbar could eliminate staurolitewith kbar during late static metamorphism in the southeastern ordinary Fe-Mg ratio while preserving nearly pure iron uplift would account for the elimination of iron-magne- staurolite. sium staurolite from the matrix of garnet-biotite gneisses Conditions of final equilibration at White Creek and at SandyCreek despite the persistenceofnearly pure iron Sandy Creek appear to match those found regionally. staurolite at White Creek. This requires only that the Thermometry for garnet rims and matrix biotite using magnesianbulk composition at Sandy Creek raises the Ferry and Spear's(1978) experimental calibration in con- minimum pressurefor stability of iron magnesiumstau- junction with Berman's(1990) activity model for garnet rolite above -2.75 kbar, a displacementof -l kbar or yields -530'C at White Creekand -570'C at Sandy less,relative to pure iron staurolite.The abundanceof Creek.(Representative analyses appear in Table 2.) As- iron-magnesium cordierite in some layersat SandyCreek semblagessuitable for quantitative barometry are absent, suggeststhat conditions offinal reequilibration there were but aluminum silicate phaserelations provide somebaro- indeed outside the field of stabilitv for staurolite of or- metric constraints at White Creek. The metasedimentary dinary Fe-Mg ratio. sequencethere containsboth andalusiteand sillimanite: - slightly ferruginous andalusite is spectacularlyabundant The reaction staurolite + quartz almandine in quartzites only a few metersaway from staurolite rocks * sillimanite bearing small amounts of sillimanite. It is conceivable Petrographic evidence for the incomplete reaction of that both polymorphs occur in equilibrium assemblages, staurolite + qvarlz to produce almandine + sillimanite but, we suggestbelow, that sillimanite is probably meta- is unmistakablein the rocks from White Creek,but it is stable,either a relic from the early dynamothermal event difficult to assessunambiguously the conditions under or the product ofa metastablereaction that took placeat which this reaction occurred. Figure 3a illustrates a re- low pressurein the later staticevent. Thus the aluminum action texture in which almandine has engulfedand re- silicatesat White Creek require pressuresbeneath or per- placed iron staurolite as garnet crystallized along stau- haps along the equilibrium boundary betweenandalusite rolite-quartz interfaces.Elsewhere in that specimen,small and sillimanite; at 530 "C, the inferred maximum pres- amounts of sillimanite have grown, after nucleating pref- sureis -3.3 kbar (Holdawayand Mukhopadhyay,1993a). erentially upon biotite. In other thin sectionsfrom this (The triple point of Bohlenet al., 1991,would yield pres- outcrop, sillimanite replacesstaurolite directly. Devel- suresnearly I kbar higher.)Although aluminum silicates opment of almandine + sillimaniteis limited and highly are not found at Sandy Creek,regionally uniform pres- localized; most contacts between staurolite and quartz suresfor the late static event acrossthe uplift are inferred show no evidenceof reaction (Fig. 2). Figure 3b docu- from the ubiquity of Buchan-seriesstatic mineral assem- ments the samereaction in a contrastingtexture. In that 158 CARLSON AND REESE:IRON STAUROLITE IN THE LLANO UPLIFT

stability. This explanation is somewhat problematic, however, becausethe temperature and pressurerequired for the reaction of staurolite with quartz depends upon the H content of the staurolite, and the White Creek staurolite crystals have high estimated H contents (on average,3.7 H pfu). The preliminary calculationsof Holda- way, Mukhopadhyay, and Dutrow (presentedin Holdaway and Mukhopadhyay, 1993b,their Fig. 5) indicate that the reaction between quartz and staurolite with 3.7 H pfu to produce almandine should occur at - 675 "C and I I kbar, within the kyanite stability field (Fig. 4). Although it is possible that fibrous sillimanite formed metastably in place of kyanite in this incomplete reaction, a stable reaction to produce sillimanite would be expectedonly for the breakdown of staurolite with a H content of - 3.3 H pfu or less.However, the lo uncertainty in the estimates of H content from electron microprobe data is on the order of +0.4 H pfu (Holdaway et al., 1991, p. l9l7), and so the Llano staurolite crystals might in fact contain as little as 3.3 H pfu. Furthermore, additional uncertainty must exist in the calculated locations of the contours of H content. Thus one cannot rule out the possibility that at White Creek, progrademetamorphism in the early dynamothermal event peaked in the vicinity of 700 "C at 7 kbar, triggering the partial reaction ofstaurolite + qvartz to almandine+ sillimanite. Reaction at high temperaturesduring the early dynamothermal event would be consistentwith the occurrence Fig. 3. (a) At right center, a basal section through a formerly in ultramafic tectonitesof the nearby Coal Creek Serpen- euhedral staurolite crystal (S) is engulfed and embayed by al- tinite (Garrison, 198l) of a relict metamorphic assem- mandine garnet (G) at the boundary along which staurolite and blage offorsteritic olivine * enstatitic orthopyroxene (+ quartz were formerly in contact. Garnet crystal at left center (G) anthophyllite, which may, however, have grown in the encompasseslarge (S, inclusion of staurolite arrows). Sample static event). For the magnesianend-members, this ul- WC9lc. Photomicrographin plane-potarizedlight. Long dimen- tramafic assemblagedemands minimum temperaturesof sion of field of view is 1.25 mm. (b) Cluster of euhedral garnets -700 between 2 and 7 kbar, given with staurolite inclusions (left) is surroundedby selvageoffelted "C unit activities of sillimanite needles(left center), within a quartz-rich, staurolite- HrO (Greenwood,1963; Day et al., 1985).The small Fe free halo (center). Small crystals of staurolite (right center and componentsof olivine (Fono)and orthopyroxene (Ennr)in right) appear only at greater distances from the almandine + the Coal Creek body would have a negligibleeffect on the sillimanite cores; thesecrystals are embayed and have irregular equilibrium conditions(Trommsdorff and Evans, 1972). margins, whereasstaurolite crystalsat still greaterdistances (be- Reduced activity of HrO might decreasesomewhat this yond this field ofview) are subhedralto euhedral.Sample PSsg2. estimateof minimum temperature,but high HrO activ- Photomicrograph in plane-polarized light. Long dimension of ities are indicatedby foliated amphibolitesthat result from field view of is 4.9 mm. thorough hydration during dynamothermal metamorphism of basaltic dikes crosscuttingthe ultramafic body (cf. Gillis, 1989,p. 16). Peak temperaturesbetween 670 specimen, euhedral garnet with numerous inclusions of 'C and 750 "C for dynamothermalmetamorphism in the staurolite is surrounded by a felted selvage of sillimanite; southeasternuplift are also indicated by the extent ofpar- both are encompassed within a quartz-rich, staurolite- tial homogenization of garnet gfowth zoning (Schwarze, absent halo. Although two interpretations of these tex- 1990;Carlson and Schwarze,1993). tures are offered below, both alternatives support the The localizedpersistence of almandine * sillimanite, hypothesis that the rocks at White Creek underwent despitereequilibration to 530 "C of the Fe-Mg exchange moderate- to high-P metamorphism during the early dy- between garnet rims and biotite during the late static namothermal event. metamorphism,must be rationalizedunder this interpre- Reaction at high pressures during dynamothermal tation as a consequenceof the slower kinetics of retro- metamorphism. One interpretation is that this reaction gradenet-transfer reactions in comparisonwith reactions represents a situation in which peak metamorphic con- involving only exchangeequilibria. Also, if this interpre- ditions during the early dynamothermal event simply ex- tation is valid, one might expectthe formation of silli- ceeded the high-temperature limit for staurolite + quartz manite needleswith a preferredorientation (not in un- CARLSON AND REESE: IRON STAUROLITE IN THE LLANO UPLIFT 159 orientedfelted masses), unless the thermalmaximum was 12 reachedafter deformationhad waned-a reasonablepos- sibility. Alm pressures + Reaction at low during static metamorphism. Kya A second possible interpretation arises from the likeli- hood that the high H contents of the White Creek staurolite crystals, ascribed to their origin at high pressures during the early dynamothermal metamorphism, would render them unstable during static metamorphism at -2.75 kbar. Becauserecrystallization to the stable staurolite composition with lower H content would require ^8 reaction to produce the exchangeof H* for Fe2+, it is (6 possible that these high-H staurolite crystals might react -o to a wholly metastable intermediate assemblageof al- 5 mandine + sillimanite instead of producing low-H stau- o_ rolite directly. 6 This interpretation readily explains both the unorient- ed fabric of the sillimanite and the fact that many of the White Creek almandine crystalsare euhedral,in contrast to the highly resorbed, anhedral character of nearly all other almandine-rich garnet found in the uplift (excepting only those occuffenceswith high spessartinecontents). It also accountseasily for the fact that garnet-biotite Fe-Mg exchange thermometry reflects equilibration at conditions matching those found throughout the uplift for the late static metamorphism. However, it requires accep- tance of the disconcertingnotion that a wholly metastable assemblagewas produced in this rock during a meta- 450 500 550 600 650 700 morphic event that had sufficient duration and intensity nearly to obliterate in most other rocks in the uplift those r('c) mineral assemblagesreflecting early metamorphism at Fig. 4. Comparison between inferred conditions for static pressure. production high Reaction kinetics that compel reequilibration in the southeasternLlano Uplift and P-T limits of a metastableintermediate assemblagein preferenceto of staurolite stability in the presenceof quartz. Stability field for a stable lower-H staurolite are difficult to reconcile with iron staurolite + qvartz is that calculated by Holdaway, Mu- an event that has largely rehydrated and recrystallized khopadhyay, and Dutrow (Fig. 5 of Holdaway and Mukhopa- eclogitic and amphibolitic masseson the scaleof several dhyay, 1993b)and is contouredin terms ofthe H contenl of hundred meters and that has completely eliminated ear- staurolite. Numbers in boxes label contours of H atoms pfu, ly-formed matrix staurolite from the nearby occurrence correspondingto iron staurolite in equilibrium with almandine at Sandy Creek. (dashedcontours) and with aluminum silicate (solid contours). The limit of staurolite + q:uartzstability is given by the locus of intersections of the contours, and is shown here by the bold Coxcr,usrox curves. Stability relations for aluminum silicatesare from Hol- : Staurolite at White Creek may representthe most Fe- daway and Mukhopadhyay (1993a).A andalusite,K and Kya : : : : rich occurrence of this mineral known. The occurrence kyanite, S and Sil sillimanite, Stt staurolite, Alm almandine, and : qtuartz. and textural relations ofstable nearly pure iron staurolite Qtz Darker cross locates conditions of final reequilibration for at White Creek,in conjunction with the evidenceat Sandy White Creek crystals(WC) at 530 + 30 "C and2.75 + 0.75 kbar. Creek for initial crystallization and later destruction of Lighter cross locates nearly equivalent conditions of final ree- iron-magnesium staurolite, imply that the southeastern quilibration for the Sandy Creek crystal (GT-l) at 570 + 30 Llano Uplift experienced a polythermal and polybaric "C and 2.75 + 0.75 kbar. The low-pressurelimit of stability of metamorphic history similar to, and possibly identical iron staurolite is not shown becauseof the uncertainty in irs with, that which has been more rigorously documented preciselocation, but it is likely to be just below 2 kbar for 7 near for the northern and northwestern parts ofthe uplift. 550'C. Becausestaurolite inclusions in garnetin GT-l contain appreciableMg, the low-pressurelimit of stability for staurolite at SandyCreek lies at significantlyhigher pressuresthan the limit AcxNowLnlcMENTS for the staurolite at White Creek, which is close in composition to the Fe end-member. Michael Holdaway and William lamb provided insightful and helpful formal reviews;in particular, the suggestionthat almandine + sillimanite may have formed at low pressureas metastableproducts of the breakdown of high-H staurolite originated with Holdaway's review. We are 160 CARLSON AND REESE:IRON STAUROLITE IN THE LLANO UPLIFT also especiallygrateful to Michael Holdaway for his assistancein assessing Gillis, GM. (1989) Polyphasedeformation and metamorphisrn of the electron microprobe data and staurolite stoichiometry during preparation Middle Proterozoic Coal Creek Serpentinite,Gillespie County, Texas, ofthe manuscript. Richard Hervig's analytical expertiseand his gracious 103 p. Master's thesis,University of Texas,Austin, Texas. assistancemade possiblethe analysesof Li by ion probe at Arizona State Greenwood, H.J. (1963) The synthesis and stability of anthophyllite. University. Carlotta Chernoffassistedin microprobe analysisofthe stau- Journal of Petrology,4, 317-351. rolite samplesand created a spread sheet for the calculation of garnet- Hemingway, B.S., and Robie, R.A. (1984) Heat capacity and thermody- biotite temperaturesusing the Berman activity model. Mark Helper pro- namic functions for gehlenite and staurolite: With comments on the vided a helpful informal review of an early version of the manuscript. Schottky anomaly in the heat capacity of staurolite. American Miner- Field work in the Llano Uplift has been supportedby the Geology Foun- alogist,69, 307-318. dation ofthe University ofTexas at Austin and by NSF grants EAR-88- Holdaway, M.J., and Mukhopadhyay, B. (1993a) A reevaluation of the 047 l7 and EAR-92- 19484to W. Carlson,S. Mosher, and N. Walker The stability relations ofandalusite: Therrnochemicaldata and phasedia- Geology Foundation also helped to defray the costsof publication. gram for the aluminum silicates.American Mineralogist, 78, 298-315. - (l 993b) Geothermobarometryin pelitic schists:A rapidly evolving RrrnnrNcns crrED field. American Mineralogist, 78, 681-693. Holdaway, M.J., Mukhopadhyay, 8., Dyar, M.D., Dutrow, B.L., and Albee, A.L., and Ray, L. (1970) Conection factors probe for electron Rumble, D., III (1991) A new perspectiveon staurolite crystal chem- microanalysisof silicates,oxides, carbonates, phosphates, and sulfates. istry: Use of stoichiometric and chemical end-members for a mole- Analytical Chemistry, 42, t4O8-1414. fraction model. American Mineralogist, 76, l9lD-1919. Bardossy,G., Mesko, L., Panto,G., and Sajgo, (1970) C. k m6tamorphis- Irtargo, C.M.R., and lamb, W.M. (1991)Granite emplacementpressures me de contact de la bauxite de B6darieuxet quelques g6n6raux aspects and metamorphism in the Llano Uplift of central Texas. Geological du m6tamorphisme des bauxites (Contact metamorphism of the B6- Society of America Abstracts with Programq23,446. darieux bauxite and some general aspectsof bauxite metamorphism) - (1992) Comparative thermometry on pelitic rocks and marbles of (in French). Bulletin de Soci6teG6ologique de France, 12, 856-869. the Llano Uplift, Texas.Geological Societyof America Abstractswith Bebout,G.E., and Carlson,W.D. (1986) Fluid evolution and transport Programs,24, 219. during metamorphism: Evidence from the Llano Uplift, Texas. Con- -(1993) P-I-Xconditions of calc-silicateformation: Evidencefrom tributions to Mineralogy and Petrology, 92,518-529 fluid inclusionsand phaseequilibria, Llano Uplift, central Texas,USA. Berman, R.G. (1990) Mixing propertiesof Ca-Mg-Fe-Mn garnets. Amer- Journal of Metamorphic Geology, I l, 89- 100 ican Mineralogist, 75, 328-344. Nelis, M.K., Mosher, S., and Carlson,W.D. (1989) Grenville-ageorogeny Bohlen,S.R., Montana, A, and Kerick, D.M. (1991) Precisedetermi- in the Llano Uplift of central Texas:Deformation and metamorphism nations ofthe equilibria kyanite : sillimanite : and kyanite andalusite of the Rough Ridge Formation. GeologicalSociety ofAmerica Bulletin, and a revised triple point polymorphs. for Al,SiO, American Miner- l0l,876-883. alogist,76, 677-680. Richardson, S.W. (1968) Staurolite stability in a part ofthe system Fe- Carlson,W D. (1992) Polymetamorphism in the Llano Uplift. Geological Al-Si-O-H. Journal of Petrology,9, 467-488. Society of America Abstracts with Programs, South-Central Section, Schreyer,W., and Chinner, G.A (l 966) Staurolite-quartzitebands in ky- 24,6. anite quartzite at Big Rock, Rio Arriba County, New Mexico. Contri- Carlson,W.D, and C.D. (1991) Johnson, Coronal reactiontexrures ln butions to Mineralogy and Petrology, 12,223-244. garnet amphibolites of the Llano Uplift. American Mineralogist, 76, Schwarze,E.T. (1990) Polymetamorphism in the Llano Uplift: Evidence 756-772. from geothermobarometryand compositional zoning in g rnet,275 p. Carlson, W.D., and Nells, M K. (1986) An occurrenceof staurolite in the Master's thesis,University of Texas, Austin, Texas. Llano Uplift, central Texas.American Mineralogist, 71,682-685. Trommsdorfl V., and Evans,B.W. (1972) Progressivemetamorphism of Carlson, W.D., and Schwarze,E.T. (1993) Petrologicand tectonic signif- antigorite schistin the Bergelltonalite aureole(Italy). American Journal icance of geographicvariations in garnet growth zoning in the Llano of Science.272. 423-437. Uplift. Geological Society of America Abstracts with Progams, 25, Wilkerson, A., Carlson, W.D., and Smith, D. (1988) High-pressuremeta- l0l-102. morphism during the Llano orogenyinfered from Proterozoiceclogite Day, H.W, Chemosky, (1985) J.V., and Kumin, H.J. Equilibria in the remnants. Geology, 16, 39 l-39 4. system MgO-SiO,-H,O: A thermodynamic analysis. American Min- Znn, E-an (1981) Metamorphic mineral assemblagesof slightly calcic pe- eralogist,70, 237-248. litic rocks in and around the Taconic allochthon, southwestern Massa- Ferry, J M., and Spear,F.S. (1978) Experimental par- calibration of the chusettsand adjacentConnecticut and New York U S. GeologicalSur- titioning of Fe and Mg garnet between biotite and Contributions to vey ProfessionalPaper, I I 13, 128p. Mineralogyand Petrology,66, ll3-117. Garrison, J.R , Jr. (198l) Coal Creek serpenrinite,Llano Uplift, Texas:A fragment of an incomplete Precambrian ophiolite. Geology, 9, 225- MeNuscnrrr RECETvEDMav 17, 1993 230. M,c.NUscRrPrAccEF.rED Sempuser 25. 1993