Nearly Pure Iron Staurolite in the Llano Uplift and Its Petrologic Significance
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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 dynamo- thermal 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 interpreta- tion, 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 sec- tions. 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 mon- azite 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-