Coexisting Paragonite and Quartz in Sillimanitic Rocks from New Mexico

American Mineralogist, Volume 69, pages 79-87, 194

Coexistingparagonite and quartz in sillimanitic rocks from New Mexico

Jerpnpv A. GneMsr-rxc Department of Geology University of New Mexico Albuquerque, New Mexico 87131

Abstract

Paragonitehas been identified by electron microprobe and X-ray diffraction analysis in four specimensof metamorphic rock from northern New Mexico. The sodic mica coexists with quartz in three of these rocks. One comesfrom an area where kyanite, andalusiteand sillimanite coexist. The other two come from widely separatedareas where sillimanite is the only polymorph of AIzSiOsto be found, and sillimanite coexists with paragonite and quartz in each. The paragonite-quartz-sillimaniteassemblages appear to be stable. These are the first reported occurrencesofthis assemblage,and its presencesupports the Al2SiO5 triple point of Holdaway (1971). Muscovite occurs in two of the paragonite-sillimanite quartzites. Because these two samplescrystallized at similar temperatures,and define a solvus, IV(K+Na) ratios should be similar for the two muscovitesand for the two paragonites.However, large differences in K/(K+Na) of muscovite exist between the two samplesand smaller differencesin that ratio occur between the two paragonites.These variations may be related to differencesin the celadonitecontent of muscovite in the two rocks. Consistentwith tentative suggestions of previous workers, data presented here suggest that, as Fe and Mg are added to muscovite-paragonitepairs, IV(K+Na) increasesin muscovite and may decreaseslightly in paragonite. This relationship has serious implications for muscovite-paragonitesolvus geothermometry since most natural muscovitesand paragoniteshave small amounts of Fe and Mg. Generally, muscovite-paragonitesolvus geothermometryseems to yield inconsis- tent and unreasonablyhigh temperatures.

Introduction Holdaway, 1978; Grambling, 1981, 1982; Hodges and No assemblageswith coexisting paragoniteand quartz Spear, 1982;Kieffer, 1982).A restricted stability field for are known from sillimanite-grade metamorphic rocks paragonite.4uartz-sillimanite should exist in nature if (Chatterjee, 1972), but paragonite-quartz assemblages Holdaway's phase diagram for AlzSiOs is correct, be- are relatively common in rocks containing kyanite or cause Holdaway's sillimanite field overlaps Chatterjee's andalusite(Zen and Albee, 1964;Guidotti, 1968;Thomp- paragonite + quartz field between 3 and 5.5 kbar, 500- son et al., 1977;Rumble,1978; Hoffer, 1978;Labotka, 600"C. The conspicuous absence of paragonitenuartz- 1980;Mohr and Newton, 1983).Chatterjee (1972) has sillimanite assemblagesfrom the published literature is interpreted this to mean that the assemblageparagonite + problematic for supportersof Holdaway's (1971)Al2SiO5 quartz breaks down to albite + AlzSiO5at a temperature invariant point. below the Al-silicate invariant point. According to experi- If the assemblageparagonite-quartz-sillimanite exists mental studies of paragonite + quartz breakdown, this in nature, the Precambrian terrane of north-central New would require the Al2SiO5invariant point to occur at a Mexico should be an ideal place to find it. Rocks which temperature above 550"C (Greenwood, 1976). Such a contain coexisting kyanite, andalusite and sillimanite are temperature would support the experimental determina- widespread, petrologic evidence suggeststhat the coex- tion of the kyanite-andalusite-sillimanite equilibrium by isting Al-silicates crystallized close to equilibrium, and Richardson et al. (1969'1. common mineral assemblagesindicate metamorphiccon- Recent field and thermodynamic studies have suggest- ditions of 500-550'C, 3.7-4.7 kbar (Holdaway, 1978; ed that the lower-temperature Al2SiO5 invariant point Grambling,1981, 1982). proposedby Holdaway (1971)may be more applicable to The present study documents the existence of four natural systemsthan that of Richardsonet a/. (Navrotsky samples which contain paragonite from this area. All et al., 1973;Anderson et al., 1977;Carmichael, 1978; samplesoccur in sillimanite-graderocks, three samples m03{n4x84/0102-0079$02.00 79 EO GRAMBLING: PARAGONITE IN SILLIMANITIC ROCKS

tosity and local transposition. F2 folds are upright and tight to isoclinal with easterly strike and steeply south- dipping axial planes. F3 folds are localized, gentle, open, upright structureswith northerly trend, best developedin the Picurisand TruchasRanges. Peak metamorphic conditions were attained after F2 folding as documented by porphyroblasts which cut 52 cleavage and by isograds which cut axial planes of F2 folds (Grambling,l98l). Metamorphicgrade is character- ized by the widespreadcoexistence ofkyanite, andalusite and sillimanite. The three polymorphs occur in the Picuris Range together with either chloritoid or staurolite, and 77-A4c Holdaway (1978)inferred conditionsof 530'C, 3.7 kbar Irudros for their coexistence.Grambling (1981) recognized kyanite, andalusite and sillimanite coexisting in part of the Truchas Range and calculated P and T as 535"C, 4 kbar based on the assemblageschloritoid-staurolite-Al2sio5, cordierite-biotite-chlorite-muscovite-Al2SiO5, and on garnet-biotite geothermometry. The three Al-silicates coexist only in the central part of the Truchas Range. Elsewhererocks contain a systematicdistribution of one or two polymorphs and can be divided into a kyanite zone, a sillimanite zone and a zone where kyanite and andalusitecoexist (Fig. 2). Three coexistingAl-silicates

Fig. l. Geologicmap of the southernSangre de Cristo Mountains,New Mexico,showing sample localities. Geology takenfrom Montgomery (1953), Miller et al. (1963\,Gresens and Stensrud(1974), Grambling (1981 and unpublished data), Robert- sonand Moench(1979), Grambling and Codding(1982), Hol- combeand Callender(1982), and Grambling,Williams and Codding(1983). contain quartz, and two samples contain coexisting paragonite-quartz-sillimanite.

Regional geology 77-244c The Picuris, Truchas and Rio Mora Rangesform three fault-bounded uplifts in the southern Sangre de Cristo Mountains,New Mexico (Fig. l). Eachuplift consistsof a core of Precambrian igneous and metamorphic rocks overlain locally by a thin veneer of unmetamorphosed Paleozoic cover. Precambrian metamorphic lithologies can be divided into two stratigraphic groups, the Vadito and Ortega(Montgomery, 1953; Miller et al.,1963;P. E. Sllars*b Long, 1976; Grambling and Codding, 1982).The older ek!,utite) Vadito Group includes amphibolite, metarhyolite, meta- arkoseand schist.The youngerOrtega Group consistsof K)lm massive,crossbedded orthoquartzite (the Ortega Quartz- ite) overlain by a sequenceof pelitic schist, schistose quartzite and graphitic schist. Fig.2. Map showingthe distributionof Al2SiO5minerals in These stratigraphic units have been deformed by two the southernSangre de Cristo Mountains. Data from the Truchas episodes of folding, with local overprinting by third- Rangeand Rio Mora area:re from Grambling(1981, 1982 and generation (Nielsen, folds 1972;Grambling and Codding, unpublisheddata). Data from the Picuris Rangeare only approxi- 1982;Holcombe and Callender,l9E2). Fr folds are isocli- mate, taken from Montgomery (1953), Holdaway (1978), nal, recumbent structures with a strong axial-planeschis- McCarty (1983)and Grambling (unpublisheddata). GRAMBLING: PARAGONITE IN SILLIMANITIC ROCKS EI

Mineralogy Of the 56 mica samples analyzed by electron microprobe, only four containedparagonite. Localities ofthese four samplesare shown in Figures I andZ. All paragonite- bearing sampleswere collected from the lowest 250 m of the 1000m-thick Ortega Quartzite. Pure paragonite separates were obtained from two specimens,77 -163 and 77-244c. X-ray diffraction analysis has confirmed that the sodic mica is paragonite. Sample 77-163 contains the lM polymorph with de61of 9.69A, whereas sample 77-244ccontains the more common 2M1 polymorphwith dss2of 9.684. White mica in sample8l- 42 was analyzedbut yielded a difuse X-ray pattern. The diffusepattern probably representsthe fine intergrowth of muscovite and paragonitethat can be seenin scanningX- ray photographsof this sample (Fie. 3). Fig. 3. X-ray photographshowing the distributionof K in Microprobe analysesof paragonite(Table l) show that intergrownmuscovite and paragonite, sample 81-42. Scale bar is its compositionvaries considerably.One specimen(78- l0 pm long. 86b) has nearly pure NaAlrSirOro(OH)2, with traces of FeO and F as the only impurities. Paragoniteis less pure in other samples,with the most abundant impurity being also appear in Rio Mora. Kyanite and sillimanite are K which replacesup to 14moleVo of the Na. Calciumis an -244c widespread,but crystalsofandalusite occur only alonga important substituentin specimen77 where it occu- single bed (the Vadito-Ortega contact) where they are pies 1l moleVoof the alkali site. This sample has excess enrichedin Fe2O3and Mn2O3which have stabilizedthe andalusiteto abnormally high pressure and temperature. whitemicas from paragonitic P-? conditionsat Rio Mora were near 550'C, 4.7 kbar Table l. Microprobeanalyses of rocks.Note that paragonite in 78-86boccurs only as inclusions in during metamorphism(Grambling, 1982). garnetand does not coexistwith muscoviteor quartz. The age of metamorphismacross the region is not well- documented.However, similar rocks in nearby areas 77- 77- 77- 78- 78- 81- 6r- 163 2A4c 244c 8qb 85b 42 42 have yielded apparent metamorphic agesnear 1425m.y. PPUP'uP u (L. E. Long, 1972;Gresens, 1975). Fe0 .62 2.99 r.rr 1.64 .7t 1.40 ugo .06 .04 .91 .04 .03 .20 Methods h0 .00 .01 .01 .00 .02 ,00 .00 Tt0, .08 .05 .2E .O2 .22 ,08 .28 Al-0^ 39.75 35.19 40,43 35.88 40.47 37.29 This study is based on 56 specimens, four from the 45.20 44.53 44.77 47.73 46,3t 45.20 45.12 Picuris Range and the rest divided evenly between the K.o .96 t, r8 9.42 .06 7.97 1,41 1.2E Truchas and Rio Mora uplifts. Each Ng.o 5.88 6.30 t.52 7.64 2.t0 5,84 specimen contains ced .06 1.s6 .00 .o2 ,00 .00 .00 white mica. Micas were analyzed on an automated enr-- Ba0 n. d. .03 .r0 .06 .28 eux microprobeat the University of New Mexico in a F n. d. .04 .24 .30 .49 .t2 .20 n. d. -.02 -.10 _.05 -,08 systematic search for paragonite. Operating conditions TotaI 94.31 95.25 95.55 97.25 95.98 93.89 94.42 were l5 kV, 0.005-0.05pA samplecurrent on brass,l0- catlona beaed on 11 oxygen6 30 secondscounting time and a beam diameter of l-3 rr.m. Fe .03 .L7 .06 .09 .o4 ,0E ug ,01 .00 .09 .00 .04 .00 ,02 Qualitative X-ray elemental photographs were taken at Un .00 ,00 .00 .00 .00 .00 .00 the sameconditions on the JEoL Superprobe733 in the T1 ,01 .00 .01 .00 .ol .00 .01 A1 3.03 3.L4 2.79 2.99 2.86 3.10 Department of Geology. Data were reduced using a sl 2.97 2.A5 3.01 2.99 3.04 2.94 3.00

Bence-Albee (1968)correction scheme,with natural and .08 .10 .81 .00 .61 .12 .62 Ns .86 .74 .2t .93 .74 .32 synthetic silicate and oxide minerals as standards.Error Ca .00 . rr .00 .00 .00 .00 .00 estimatesare -r3 percent of the amounts presentfor most Ba n.d. .00 .01 .00 .00 .00 .0r -r10 F1 .01 .05 .06 .02 .04 elements, relativepercent for BaO and t50 relative .F+OH' percent for F. The large errors on F analysesreflect the K ,11 .79 t.00 .14 .66 fact that F occurs near the limits of detection in most (+Ne samples. Micas were analyzed in three samples using X-ray n.d. = not detemlned - paragonlte diffraction techniques. Mica grains were hand-picked P from the samples,crushed, mounted on glass fibers and lcalculated dssuntng (F + on) - 2 2lncluelon analyzedin a Debye-Scherrer X-ray camera. ln garnet 82 GRAMBLING: PARAGONITE IN SILLIMANITIC ROCKS

Table 2. Mineral assemblagesand selectedcompositions (from microprobe analyses)in paragonite-bearingrocks

crd 2 ""r 77-163 x x .97 .95 .90 .88 .95

77-244c x x x x x x .08 x

78-86br x .95 x

8L-42 x x x .94 .95 x

in garnet -ret!ogradeilncluslon

Nuobers represent Fe/(Fe + I'tg) for sllicates, or PeTio-/(FeTtO- + Fe^O^) for oxldes J J ZJ

AI, a correspondingdeficiency in Si and full occupancyof Petrology the alkali site, suggestingthat Ca enters the structure through substitution toward margarite, CaAla Two paragonite-bearingsamples (77 -163, 78-86b) come Si2Org(OH)2.Other sampleshave negligibleamounts of from rare garnet-rich veins that are exposed in cliffs of calcium. Minor amounts of Fe and Mg occur in several Ortega Quartzite, I km northwest of the summit of East samples,presumably occupying octahedralsites. They PecosBaldy in the south-centralTruchas Range (Fie. l). may enter the structure through the celadonite substitu- The veins cut across well-developedfestoon crossbed- tion (AlvI + AlIv = Ge, Mg)vt + Sirv). Nearly all the ding in the quartzite. Veins contain garnet, chloritoid, paragoniteshave slight deficienciesin alkalis (c/. Henley, staurolite, quartz, white mica, apatite, tourmaline, large 1970;Hock, 1974;Baltatzis and Wood,1977i Rumble, (0.1 mm), frosted, pitted and rounded zircon crystals, 1978)which may suggestthe presenceof hydronium rutile and traces of retrograde chlorite. Veins consist of (HrO*) in their alkali sites (Brown and Norrish, 1952). 90-95 modal percent euhedral garnet at their centers, However, alternative interpretations such as substitution grading into sieve-textured intergrowths of garnet, stau- of (Fe,Mg)vt into normally unfilled octahedral sites, rolite, chloritoid and quartz near their margins, then coupled with two vacancies on alkali sites, are also graduallygrading into garnet-freeOrtega Quartzite. Veins consistentwith analytical data. Additional substituents are up to 15 cm thick, irregular in shape, commonly include trace amounts of fluorine and barium. No parago- anastomosingwithin the quartzite, and generally have a nite had detectableamounts of chlorine. near-vertical orientation. Dozens of these veins occur at Muscovite shows similarly large variations from its this locality. The origin of theseveins is not well under- end-membercomposition of KAlSirOro(OH)2.The most stood. A tentative interpretation is that they representthe abundantsubstituent is Na for K with up to 34 moleVoof metamorphosedequivalents of detrital clays and heavy the potassiumreplaced by sodium (Table l). Up to 3.9 minerals strapped in vertical fissures during or slightly wt.Vo (FeO+MgO) replaces Al2O3, presumably through after accumulation of the Ortega Quartzite. Such an substitution toward celadonite but possibly involving interpretation is consistentwith preliminary U-Pb studies someFe2O3 as well. The most celadoniticmuscovite (77- of the zircons. The zircons yield an array of data points 244c)coexists with hematite, suggestingthe possibility of which indicate an age well in excessof 1700m.y., the Fe3+in the muscovitestructure. Like paragonite,musco- approximate depositional age of the quartzite (S. Bow- vite shows slight deficienciesin alkalis relative to its ideal ring, pers. comm., 1983). chemistry. Trace amountsof Ti, Ba and F can be found in Regardless of their origin, the veins seem to have several samples,but none contain detectable concentra- equilibrated at peak metamorphic conditions. Their min- tions of chlorine. eral assemblagesare compatible with those of the sur- Other minerals found in these rocks are summarizedin rounding metamorphic rocks, and rare biotite-bearing Table 2. Garnet forms solid solution between almandine veins give garnet-biotite temperature of 510'C using the and pyrope, with negligibleamounts of Mn or Ca. Chlori- geothermometerof Ferry and Spear(1978; cf. Grambling, toid, staurolite and chlorite consist of nearly pure Fe-Mg l9E3). solutions, with less than 0.3 wt.% ZnO in staurolite of Paragonite does not seem to be in equilibrium with specimen8l-42 the only significantimpurity. Gahnitehas quartz in one of these samples (78-86b).Paragonite oc- compositionFee6Mg.16Mn s2ZnssAl2Oa in the one sam- curs only as inclusions in garnet and never touches ple in which it occurs. Sillimanite, quartz and rutile have quartz. The matrix of the sample contains muscovite, compositionsAl2SiO5, SiO2 and TiO2, respectively. quartz and other mineralslisted above, but no paragonite. GRAMBLING: PARAGONITE IN SILLIMANITIC ROCKS 83

tions suggestthat the rock contains a stableassemblage of paragonite,quartz and sillimanite. Specimen 77-244c was collected 4 km north of the regional kyanite-sillimanite isograd (Fig. 2). Its metamorphic temperature must have been somewhat higher than that of the Al2SiO5triple point. Quartzites near Middle Truchas Peakcontain coexisting chloritoid and sillimanite (Grambling, 1981, 1983), restricting temperatureto a maximum of 560"C according to experimental work of Ganguly (1977). A reasonable estimate of metamorphic conditions would be 530-550"C,4-4.5 kbar based on these constraints and the Al2SiO5 triple point of Holdaway (1971). The fourth paragonite-bearingsample (81-42) comes from sillimanite-zonerocks in the Rio Mora region, 3 km west of the villageof Gascon,New Mexico. This sample Fig. 4. Photomicrograph of coexisting paragonite, was collected from a I cm thick aluminous bed in Ortega quartz silfimanite and in specimen77-244c. Photograph is I mm from outcropsat 3110m elevation(Figs. 1, 2). long. Quartzite Its assemblageconsists of sillimanite-garnet-staurolite- paragonite-muscovite-quartz with minor ilmenite and rutile. Mineral compositionsare listed in Table 2. Silli- manite includes coarse and fibrolitic crystals up to 3 mm However, the other garnet-vein sample has paragonite in length,typically alignedparallel to 51. Muscovite and in direct contact with quartz. No muscovite occurs in 77- paragoniteform plates up to 4 mm across, with the two 163,but otherwisethe sampleappears similar to 78-86b. micas showing epitaxial intergrowths parallel to (001) Minerals coexistingwith paragoniteand quartz are chlori- (Figure 3). Most micas are oriented parallel to 51. Parago- toid, staurolite, garnet, zircon, rutile and trace amounts nite and sillimanite commonly occur in direct contact, of retrograde chlorite which occur concentrated along and quartz is ubiquitous. Again, textural observations garnet grain boundaries. The complete mineral assem- indicate stable coexistence of paragonite, quartz and blageis listed in Table2. Paragoniteforms crystals up to 3 sillimanite. mm in length,in placesintergrown with chloritoid. Crys- The kyanite-sillimanite isograd forms a horizontal, tals are subhedralto euhedral, are never associatedwith nearly planar surface at Rio Mora. Sample 8l-42 was chlorite, and do not appe.r to be retrograde in origin. collected from sillimanite-rich outcrops 300 m below the Although sample77-163 has no Al-silicates, it comesfrom isograd, along the face of an 850 m-high cliff that bounds the central part of the Truchas Range where kyanite, the easternedge of the Rio Mora region. Grambline (1982) andalusiteand sillimanitecoexist (Fie. 2). Rocks with all has estimatedmetamorphic conditions as 4.7 kbar, 550'C three polymorphs of Al2SiO5were collected 75 m north at the isograd. Such a temperatureis consistent with the and 100m south of this station. Evidence suggeststhat generalabsence of chloritoid from sillimanitic quartzite at the rock has a stable associationof paragonite + quartz Rio Mora (cf. Ganeuly,1977). and crystallized at conditions near the Al2SiO5invariant Although the three samplesof paragonite-quartz come point. from widely separated areas, none has any textural A third paragonite-bearingsample (77-244c)was col- features suggestive of disequilibrium. All have inter- lected from the eastern summit of Middle Truchas Peak growths of paragonite and qtartz, and sillimanite forms (Fig. 1). The sample representsa slightly schistose, an integral part of the mineral assemblagein two of them. sillimanitic bed in Ortega Quartzite and contains musco- The geometry of mapped Al2SiO5isograds across the vite, paragonite,quartz, sillimanite,hematite, rutile and region (Grambling, 1981, 1982)requires that sillimanite scatteredgrains of bluish, Zn-ich spinel (Table 2). Silli- was a stable phase in all paragonitelocalities. The domi- maniteneedles and hematiteplates define the planeof 51 nant alignment of paragoniteand sillimanite parallel to 51 foliation. Nearly all micas are oriented parallel to 51. The in two samples(77-244c, 8l-42) provides textural evi- two micas occur as separatecrystals up to 1.5 mm in dencethat the paragonitewas present during F1 deforma- Iength, never as intergrowths, yet the two micas are in tion, so the paragonite cannot be a retrograde phase. direct contact in severalplaces and invariably lie within a Paragoniteshows random orientation in one sample (77- few millimeters of each other. Both micas occur in direct 163)and rare grains cut across52 in other samples(77- contact with sillimanite and quartz, and intergrowths of 244c, 8l-42), suggestingthat paragonite partially recrys- paragonite, sillimanite and quartz are common (Fig. a). tallized after F2 folding. Therefore the paragonitedid not No retrogrademinerals can be found in the specimenand behave as a non-reactive, relict phase. Several of the the rock seems totally unweathered. Textural observa- samples (77-163, 77-2tAc) have paragonite in crystals 84 GRAMBLING: PARAGONITE IN SILLIMANITIC ROCKS

77-244c,all calculated temperatures are too high to be consistentwith paragonite + quartz stability (Chatterjee, 1972),especially since K does not enlargethe stability of paragonite+ quartz by more than afew degrees(Chatter- Qp o ^do ovb jee and Froese, 1975).All but the 550'C temperatureare o also inconsistentwith other observed mineral assemblages. Fig. 5. Distributionof K, Na, and (Mg+Fe)in muscovite Previous studiesofmuscovite-paragonite pairs in natu- and paragonite.Data come from all analyzedmuscovite- ral samples have met with similar difficulties (Guidotti paragoniterocks which represent a temperatureinterval ofabout and Sassi,1976;Baltatzis and Wood,1977i Katagasand 520-550'C.Tie linesjoin the two muscovite-paragonitepairs, Baltatzis, 1980).Temperatures calculated from the mus- andthe dashed line connectsthe two muscovitesof thesepairs. covite limb of the solvus tend to disagreewith tempera- This dashedline approximatelyseparates paragonite-free rocks tures calculated from the paragonite limb, and both from rockswith muscovite+ paragonite.Its slopesuggests that with values calculated from the solvusbetween muscovite and paragonitemay broadenas temperatures may disagree (Mg+Fe)are added to muscovite. other phaseequilibria. Possible reasons for thesediscor- dant temperaturesinclude deviations of natural samples from experimentalmineral compositions, potential effects totally separatefrom muscovite, suggestingthat the pa- of hydronium(H:O*), poorly understoodeffects of vari- ragonite did not form by exsolution during cooling. No ablepressure on the solvus(c/. Guidotti andd Sassi,1976 paragonite-bearingrock contains plagioclase,an expect- with Chatterjeeand Froese, 1975),possibly-erroneous ed breakdown product ofparagonite + quartz. It appears experimental data and thermodynamic extrapolations that the assemblageparagonite-quuutz was stable under from those data (Essene, 1982),and partial re-equilibra- prevailing sillimanite-grade metamorphic conditions in tion of muscovite-paragonitepairs during cooling. thesethree localities. The scatter of calculated solvus temperatures in the New Mexico rocks may be related to the considerable Solvus relationships content of impurities in the analyzedmicas, especiallyFe Sodic muscovite and potassic paragonitecoexist in and Mg in muscovite. Figure 5, a phase diagram separat- samples77-244c and 8l-42. Low-potassiumparagonite ing the effects of K, Na and (Fe+Mg), illustratesthis (samples77-163,78-86b [as inclusions in garnet])does not suggestion.The diagram includes data from all 56 ana- coexistwith muscovite,and low-sodiummuscovite (ma- lyzed specimens.It assumesthat all micascrystallized at trix, sample 78-86b)does not coexist with paragonite, similar temperatures,thus neglectingthe 530-560'C tem- consistentwith the interpretationthat mica compositions perature range encompassedby the samples. It further preserveevidence of equilibrium.Compositions of coex- assumesthat Fe3+ is not a major substituentand that isting micas might be useful for geothermometrybecause other substituentssuch as Ca or Ti have negligible effect coexistence of two white micas defines a solvus in the on the solvus.With only two exceptionsthe white micas systemKAl3Si3O rg(OH)rNaAlrSirO ro(OH)2. separateinto three distinct fields: muscovite only, musco- Three diferent experimental and theoretical calibra- vite + paragonite, and paragonite only. Although both tions of the muscovite-paragonitesolvus have been pub- muscovite-paragoniterocks crystallized at similar P-T lished recently (Eugster et al., 1972;Thompson, 1974; conditions,tie lines between muscovite and paragonite Chatterjeeand Froese, 1975).Samples reported in this seemto show considerabledependence on (Fe+Mg) in study havecompositions which do not plot on the solvus muscovite.The ratio IV(K+Na) of muscoviteincreases of Chatterjeeand Froese(1975) for any reasonablepres- sharply and trU(K+Na) of paragonitemay decreaseslight- sures or temperatures. Samples do fall on the solvus of ly as (Fe+Mg) increases.The scantydata base is support- Eugster et al. (1972) or Thompson O970. However, ed by the generalabsence ofmuscovite analysesfrom the temperaturescalculated from solvus relationships seem region to the right ofthe dashedline in Figure 5. The data inaccurate. Temperature calculated from the muscovite suggestthat addition of a celadonite component to mus- limb in sample 77-244c(550'C) differs considerably from covite may effectively "broaden" the solvus between temperature calculated from the paragonite limb of the muscoviteand paragonite.Such a conclusionis consis- solvusin the samesample (620'C), using the Eugstere/ tent with observations of Hock (1974)and Katagas and al. (1972)solvus. The other muscovite-paragonitesample Baltatzis (1980)in other areas. The same phenomenon (81-42) yields concordant but significantly higher tem- could also explain unusual observationsmade by Mohr peraturesof 675-680"C.Extrapolation of Thompson's andNewton (1983),who observedan abruptnarrowing of (1974)solvus yields similar results. These temperatures the muscovite-paragonite solvus across their kyanite were calculated at 2.07 kbar, so pressure corrections isograd in North Carolina. The narrowing of the solvus suggestedby Thompson (1974) or Guidotti and Sassi correspondsto an abrupt decreasein (Fe+Mg) in musco- (1976)would raise all temperatures by 30-50'C. Except vite, the samerelationship observed in the presentstudy. for the 550'C temperature calculated from muscovite in Considering the meager data base presented here, it GRAMBLING: PARAGONITE IN SILLIMANITIC ROCKS E5

remainspossible that somesubstituent besides (Fe+Mg) biotite geothermometry and by silicate phase relations causesthe observedbroadening of the muscovite-parago- (Holdaway, 1978; Grambling, 1981, 1982, 1983and in nite solvus. However, Thompson et al. (1977)suggest prep.). The garnet-biotite geothermometerof Ferry and that Ca has a negligible effect on tr?(K+Na) in musco- Spear(1978) yields temperatures of500t25'C from Pecos vite-paragonite pairs, and few other substitutions occur Baldy, slightly south of paragonite localities 77-163 and in the white micas from New Mexico. Further analytical 78-86b;510+25"C from garnet veins at locality 77-163; work is underwayto test theserelationships. and 550t25"C from Rio Mora. A small temperature In any case it appears that the anomalously high difference between the southern Truchas Range and Rio temperaturescalculated from the muscovite-paragonite Mora is consistent with ditrerences in mineral assem- solvusare not causedby (Fe+Mg). Extrapolationof data blages between the areas. Even if the garnet-biotite in Figure 5 to a celadonite-freecomposition suggeststhat geothermometer is incorrectly calibrated, the tempera- Fe, Mg-free muscovite coexisting with sodic mica should ture differences should be real. Therefore, it seemscer- have about 42 moleVoNa, which would give a solvus tain that the paragonite + quartz rocks in the southern temperatureofabout 725'C(Eugster et al.,1972),consid- Truchas Rangecrystallized at least 50"C below the maxi- erably too high for these rocks. The reason for the mum stability of that assemblage,i.e., below 550"C. anomalously high temperature is unknown. Results of Becausesurrounding rocks have sillimanite, the Al2SiO5 this and other studies(Baltatzis and Wood, 1977;Katagas triple point must also lie below 550"C. and Baltatzis, 1980;Essene, 1982)suggest that musco- Al[ mineral assemblagesthat can be considered diag- vite-paragonite solvus geothermometry should not be nostic of the Al2SiO5invariant point of Holdaway (1971) used until problems of calibration and mineral composi- have now been found. Holdaway (1978) predicted that tion are resolved. paragonite-quartz-sillimanite assemblages might be found in the Precambrianrocks of northern New Mexico. Conclusions and this report substantiateshis prediction.Other assem- Paragoniteand quartz coexist in apparent equilibrium blagesdiagnostic of the Holdaway (1971)invariant point in Precambrian metamorphic rocks of northern New include chloritoid-sillimanite-quartz and kyanite-cordi- Mexico. They occur in an area containing kyanite-anda- erite, and both occur in the area (Holdaway, 1978; lusite-sillimanite and in two areaswhere sillimanite is the Grambling,l98l). only polymorph of Al2SiO5present. These are the first It is likely that paragonite-quartz assemblageswill be reported occurrencesofassemblages with paragoniteand found in other areas where kyanite, andalusite and silli- quartz in a sillimanite-grade area, although Holdaway manite coexist. The apparent scarcity of rocks with (1978)documented the occurrence of muscovite-parago- paragonite-quartz-sillimanite is in part an artifact of nite-quartz in kyanite-andalusite rocks from the Picuris analyticaltechniques. It is expensiveand time-consuming Range. to perform microprobe and X-ray analyseson micaceous Becauseno samplescontain the univariant assemblage rocks, yet paragoniteand muscovite have virtually identi- paragonite-quartz-sillimanite-albite, it is difficult to de- cal optical and physical properties so are difficult to termine an accurate temperature of metamorphism from distinguish otherwise. The scarcity of this assemblagein the paragoniteitself. However, coexistenceof paragonite schists may also result in part from a combination of + quartz requires temperature less than 6fi)"C at P less Xu2o 1l and Ca-bearingbulk compositions. than 5 kbar (Chatte{ee, 1972).This maximum stability temperatureis not significantly affectedby substitution of Acknowledgments K for Na, and solid solution of paragonitetoward marga- I thank GeorgeConrad for maintainingthe electronmicro- rite or celadonite should reduce the thermal stability of probeat the Universityof New Mexicoin excellentcondition, paragonite quartz (Chatterjee + and Froese, 1975). and Mike Williamsfor obtainingX-ray analysesof paragonite. Chattedee and Froese predicted that significant Fe3+ Researchwas supportedby the NationalScience Foundation might stabilize paragonite + quartz to temperatures undergrant EAR-81 15530 and by theDepartment ofGeology and above 600"C. Hematite coexists with paragonite in one Instituteof Meteoritics,University of New Mexico. specimen (77-244c), but the total iron content of that paragoniteis only 0.6 wt.Vo. Other paragonitesoccur in References more reduced assemblagesand contain similar amounts of iron, so it is not clear that the Fe in paragonite is Anderson,P. A. M., Newton,R. C. andKleppa, O. J. (1977)The oxidized. Iron contents near 0.6 wt.Vo (as FeO) are enthalpychange of the andalusite-sillimanitereaction and the Al2SiO5diagram. American of Science,277,585-593. common in natural paragonites(Henley, 1970;Baltatzis Journal Baltatzis,E. and Wood,B. I. (1977)The occurrenceof parago- and Wood, 1977;Hoffer, 1978;Katagas and Baltatzis, nite in chloritoidschists from Stonehaven,Scotland. Mineral- 1980),and such negligibleconcentrations should not have ogicalMagazine, 41, 2ll-216. a significanteffect on experimentally determined stability Bence,A. E. and Albee,A. L. (1968)Empirical correction curves,assuming Henry's Law behavior. factorsfor the electronmicroanalysis of silicatesand oxides. Temperatures below 600'C are supported by garnet- Journalof Geology,76,382403. 86 GRAMBLING: PARAGONITE IN SILLIMANITIC ROCKS

Brown, G. and Norrish, K. (1952)Hydrous micas. Mineralogical north Norway. Mineralogical Magazine, 37, 693-1M. Magazine, 29,629-632. Hdck, V. (1974)Coexisting phengite, paragonite and margaritein Carmichael, D M. (1978) Metamorphic bathozonesand batho- metasedimentsof the Mittlere Hohe Tauern, Austria. Contri- grads: a measureof the depth of post-metamorphicuplift and butions to Mineralogy and Petrology, 43,261-273. erosion on the regional scale. American Journal of Science, Hodges,K. V. and Spear,F. S. (1982)Geothermometry, geobar- 27E,769-:t97. ometry and the AlzSiOs triple point at Mt. Moosilauke, New Chatterjee, N. D. (1972)The upper stability limit of the assem- Hampshire.American Mineralogist, 67, I I l8-l 134. blage paragonite+ quartz and its natural occurrences.Contri- Hoffer, E. (197E)On the "late" formation of paragoniteand its butionsto Mineralogyand Petrology,34,288-303. breakdown in pelitic rocks of the southern Damara orogen Chatterjee,N. D. and Froese, E. (1975)A thermodynamic study (Namibia). Contributions to Mineralogy and Petrology, 67, of the pseudobinaryjoin muscovite-paragonitein the system 209-2t9. KAlSi3OrNaAlSi3OrAl2O3-SiOrH2O. American Mineral- Holcombe,R. J. and Callender,J. F. (1982)Structural analysis ogist, 60, 985-993. and stratigraphicproblems of Precambrianrocks of the Picuris Essene,E. J. (1982)Geologic thermometry and barometry. In J. Range,New Mexico. Geological Society of America Bulletin, M. Ferry, Ed., Characterization of Metamorphism through 93, r38-r49. Mineral Equilibria, Reviews in Mineralogy, 10, p. 153-206. Holdaway, M. J. (1971)Stability of andalusiteand the aluminum Mineralogical Society of America, Washington, D.C. silicate phasediagram. American Journal of Science,271, 97- Eugster,H. P., Albee,A. L., Bence,A. E., Thompson,J. B. Jr. 131. and Waldbaum, D. R. (1972)The two-phaseregion and excess Holdaway, M. J. (1978)Significance of chloritoid-bearing rocks mlxing properties of paragonite-muscovitecrystalline solu- in the Picuris Range, New Mexico. Geological Society of tions. Journal of Petrology, 13, 147-179. America Bulletin. 89. lN4-1414. Ferry, J. M. and Spear, F. S. (1978)Experimental calibration of Katagas, C. and Baltatzis, E. (1980) Coexisting celadonitic the partitioning of Fe and Mg between biotite and garnet. muscovite and paragonitein chlorite zone metapelites.Neues Contributions to Mineralogy and Petrology, 66, ll3-117. Jahrbuchfiir Mineralogie, Monatshafte, 215-222. Ganguly, I . (1977)Compositional variables and chemicalequilib- Kietrer, S. W. (1982)Thermodynamics and lattice vibrations of rium in metamorphism. tn S. K. Saxenaand S. Bhattacharji, minerals: 5. Applications to phaseequilibria, isotopic fraction- Eds., EnergeticsofGeological Processes, p. 251-284.Spring- ation, and high-pressurethermodynamic properties. Reviews er-Verlag, New York. of Geophysicsand SpacePhysics, 20, 827-849. Grambling, J. A. (1981) Kyanite, andalusite,sillimanite and Labotka, T. C. (1980)Petrology of a medium-pressureregional related mineral assemblagesin the Truchas Peaksregion, New metamorphic terrane, Funeral Mountains, California. Ameri- Mexico. American Mineralogist, 66, 702J22. can Mineralogist, 65, 670-689. Grambling, J. A. (1982) Mn andalusite in kyanite-sillimanite Long, L. E. (1972)Rb-Sr chronology of Precambrianschist and rocks from Rio Mora, New Mexico (abstract). Geological pegmatite, La Madera quadrangle, northern New Mexico. Society of America Abstracts with Programs, 14, 500. Geological Society of America Bulletin, 83,3425-3432. Grambling, J. A. (1983)Reversals in Fe-Mg partitioning between Long, P. E. (1976) Precambrian granitic rocks of the Dixon- chloritoid and staurolite. American Mineralogist, 68, 373-388. Penasco area, northern New Mexico: a study in contrasts, Grambling, J. A. and Codding, D. B. (1982) Sttatigraphic and Ph.D. thesis,Stanford University, Stanford. structural relationship of multiply-deformed Precambrian McCarty, R. (1983)Structural geology and petrographyof part of metamorphic rocks in the Rio Mora area, New Mexico. the Vadito Group, Picuris Mountains, New Mexico. M.S. Geological Society of America Bulletin, 93, 127-137. thesis, University of New Mexico, Albuquerque. Crambling,J. A., Williams,M. L. and Codding,D. B. (1983)Mn Miller, J. P., Montgomery, A. and Sutherland,P. K. (1963) and Cr-rich marker horizons in multiply-deformed Proterozoic Geology of part of the southern Sangrede Cristo Mountains, metamorphicrocks, northern New Mexico. (abstr.) Geological New Mexico. New Mexico Bureau of Mines and Mineral Society of America Abstracts with Programs, 15, 424. Resources,Memoir ll. Greenwood, H. J. (1976)Metamorphism at moderate tempera- Mohr, D. W. and Newton, R. C. (1983) Kyanite-staurolite turesand pressures.In D. K. Baileyand R. Macdonald,Eds., metamorphismin sulfidic schistsof the AnakeestaFormation, The Evolution of the Crystalline Rocks, p. 187-259.Academic Great Smoky Mountains, North Carolina. American Journal of Press,New York. Science,283, 97-134. Gresens,R. L. (1975)Geochronology of Precambrianmetamor- Montgomery, A. (1953) Precambrian geology of the Picuris phic rocks, north-central New Mexico. Geological Society of Range, north-central New Mexico. New Mexico Bureau of American Bulletin. 87. 1444-1M8. Mines and Mineral Resources,Bulletin 30. Gresens,R. L. and Stensrud,H. L. (1974)Recognition of more Navrotsky,A., Newton, R. C. and Kleppa,O. J. (1973)Silliman- metarhyolite occurrences in northern New Mexico and a ite disordering enthalpy by calorimetry. Geochimica et Cos- possible Precambrian stratigraphy. Mountain Geologist, ll, mochimica Acta, 37, 2497-2508. 109-124. Nielsen, K. C. (nD Structural evolution of the Picuris Moun- Guidotti, C. V. (1968) On the relative scarcity of paragonite. tains, New Mexico. M. S. thesis,University of North Caroli- American Mineralogist, 53, 963-974. na, Chapel Hill. Guidotti, C. V. and Sassi,F. P. (1976)Muscovite as a petroge- Richardson.S. W., Gilbert, M. C., and Bell, P. M. (1969) netic indicator mineral in pelitic schists. Neues Jahrbuch fiir Experimental determination of kyanite-andalusiteand andalu- Mineralogie, Abhandlungen, 127, 97-142. site-sillimanite equilibria: the aluminum silicate triple point. Henley, K. J. (1970) Application of the muscovite-paragonite AmericanJoumal of Science.276,259-272. geothermometerto a staurolite-gradeschist from Sulitjema, Robertson,J. M. and Moench, R. H. (1979)The Pecosgreen- GRAMBLING: PARAGONITE IN SILLIMANITIC ROCKS 87

stone belt: Proteozoic volcano-sedimentary sequencein the Thompson,A. B., Lyttle, P. T. and Thompson,I. B. Jr. (1917) southem Sangre de Cristo Mountains, New Mexico. New Mineral reactionsand A-Na-K and A-F-M facies types in the Mexico Geological Society Guidebook, 30, 165-174. Gassetsschist, Vermont. American Journal of Science, 277, Rumble, D. III (1978)Mineralogy, petrology, and oxygen isoto- lt24-lt5t. pic geochemistryof the Clough Formation, Black Mountain, Zen, E-an and Albee, A. L. (1974) Coexistent muscovite and western New Hampshire, U.S.A. Journal of Petrology, 19, paragonitein pelitic schists. American Mineralogist, 49,904- 317-340. 925. Thompson, A. B. (1974) Calculation of muscovite-paragonite- alkali feldspar phase relations. Contributions to Mineralogy Manuscript received, March 2, l9E3; and Petrology, 44, 173-194. acceptedfor publication, August 23, 19E3.