Petrology of a medirrm-pressure regional metamorphic terrane, Funeral Mountains, Californiat American Mineralogist, Volume 65, pages 670-689, 1980 THeooonr,C. Lasorre' Division of Geological and Planetary Sciences California Institute of Technolo gy Pasadena, Califu rnia 9l 125 Abstract The Funeral Mountains,Death Valley area,California, are ametamorphic terranecharac- terizedby kyanite-bearing assemblages in pelitic schists. Pelitic schists presewe a nearly com- plete record of the facies types for grades ranging from garnet to sillimanite, and isograds were mapped on the basisof the coexistence of garnet + chlorite, staurolite + biotite, and garnet + biotite + kyanite. Mineral inclusionsin garnet provide evidence for the stoichio- metry of the reactions chloritoid + quartz : garnet+ staurolite+ chlorite + HrO and stauro- lite + muscovite + quartz -- garnet + biotite * kyanite + HrO. Estimates of the intensivepa- rameters from the distribution of Fe and Mg between garnetand biotite and calcium between garnet and plagioclase indicate that in the high-grade region P =7200 to 9600bars and T= 6000to 700'c during metamorphism. Introduction The Funeral Mountains consist of a core of meta- morphosedsedirnentary rocks flanked by relatively unmetamorphosed Paleozoic strata. This metamor- phic core comprisesa kyanite-bearing terrane, and it has been possible to map Barrovian-type isograds that show a rangein metamorphic gradefrom below garnet zone to sillimanite zone. The metamorphic rocks range in composition, and nearly complete facies typesfor muscovite-bearing pelitic schists have beenpreserved. The geology of the Funeral Mountains and espe- cially the Chloride Cliff and Big Dune 15' Quad- ranglesis being mapped by B. W. Troxel and L. A. Wright, who kindly provided a copy of the prelimi- nary map. The geology of the southern part of the Funeral Mountains is described by McAllister (1974), and the gbologyof the Grapevine Mountains which are contiguous with the Funeral Mountains to the north is outlined by Reynolds (1974) (Fig. l). The Grapevine-Funeral Mountains chain comprises late Precambrian, Paleozoic,and Tertiary sedimentary, metasedimentary, and volcanic rocks. tContribution Number 3338. 2Present addiess: Department of Earth and Space Sciences, StateUniversity of New York at Stony Brook, Stony Brook, New York 11794. 0003-o04x/80/0708-0670$02.00 The oldestrocks are a sequence of interbedded pe- litic schist,calcite marble, micaceous quartzite, and amphibolite. Pelitic schist comprisesabout half of this sectionand is prominent in the lower and upper part. Micaceousquartzite, calcareous quartzite, and marble make up the middle part. The occurrence of amphibolite in the lower part and conglomerate in the upper part led to the correlationofthese rocks to the Pahrump Group (Troxel and Wright, 1968). The lithologies in this section are not directly correlative to the formations which compose the Pahrump Group elsewhere in the Death Valley area. The Pahrump Group is overlain by the Johnaie Formation which consistspredominantly of pelitic schist and quartzite. The Stirling Quartzite overlies the Johnnie Formation and consists of quartzite,mi- caceous quartzite, and minor carbonatelayers. Pelitic layers occur near the base of the Stirling Quartzite. In the Chloride Clif Quadrangle, the Stirling Quartz- ite is locally overlain by argillite and quartzite of the Wood Canyon Formation. This Paleozoic section is overlain by middle and upper Tertiary fluvial, la- custrine, and volcanicrocks.The generalized geology of the Chloride CliffQuadrangle, which encompasses the majority of the metamorphicrocks, is shown in Figure l. The structure of the Funeral Mountains is domi- 670
Transcript
Petrology of a medirrm-pressure regional metamorphic terrane, Funeral Mountains,Californiat
American Mineralogist, Volume 65, pages 670-689, 1980
THeooonr, C. Lasorre'
Division of Geological and Planetary SciencesCalifornia Institute of Technolo gy
Pasadena, Califu rnia 9 l 1 2 5
Abstract
The Funeral Mountains, Death Valley area, California, are ametamorphic terrane charac-terized by kyanite-bearing assemblages in pelitic schists. Pelitic schists presewe a nearly com-plete record of the facies types for grades ranging from garnet to sillimanite, and isogradswere mapped on the basis of the coexistence of garnet + chlorite, staurolite + biotite, andgarnet + biotite + kyanite. Mineral inclusions in garnet provide evidence for the stoichio-metry of the reactions chloritoid + quartz : garnet + staurolite + chlorite + HrO and stauro-lite + muscovite + quartz -- garnet + biotite * kyanite + HrO. Estimates of the intensive pa-rameters from the distribution of Fe and Mg between garnet and biotite and calcium betweengarnet and plagioclase indicate that in the high-grade region P =7200 to 9600 bars and T=6000 to 700'c during metamorphism.
IntroductionThe Funeral Mountains consist of a core of meta-
morphosed sedirnentary rocks flanked by relativelyunmetamorphosed Paleozoic strata. This metamor-phic core comprises a kyanite-bearing terrane, and ithas been possible to map Barrovian-type isogradsthat show a range in metamorphic grade from belowgarnet zone to sillimanite zone. The metamorphicrocks range in composition, and nearly completefacies types for muscovite-bearing pelitic schists havebeen preserved.
The geology of the Funeral Mountains and espe-cially the Chloride Cliff and Big Dune 15' Quad-rangles is being mapped by B. W. Troxel and L. A.Wright, who kindly provided a copy of the prelimi-nary map. The geology of the southern part of theFuneral Mountains is described by McAllister(1974), and the gbology of the Grapevine Mountainswhich are contiguous with the Funeral Mountains tothe north is outlined by Reynolds (1974) (Fig. l). TheGrapevine-Funeral Mountains chain comprises latePrecambrian, Paleozoic, and Tertiary sedimentary,metasedimentary, and volcanic rocks.
tContribution Number 3338.2Present addiess: Department of Earth and Space Sciences,
State University of New York at Stony Brook, Stony Brook, NewYork 11794.
0003-o04x/80/0708-0670$02.00
The oldest rocks are a sequence of interbedded pe-litic schist, calcite marble, micaceous quartzite, andamphibolite. Pelitic schist comprises about half ofthis section and is prominent in the lower and upperpart. Micaceous quartzite, calcareous quartzite, andmarble make up the middle part. The occurrence ofamphibolite in the lower part and conglomerate inthe upper part led to the correlation ofthese rocks tothe Pahrump Group (Troxel and Wright, 1968). Thelithologies in this section are not directly correlativeto the formations which compose the PahrumpGroup elsewhere in the Death Valley area.
The Pahrump Group is overlain by the JohnaieFormation which consists predominantly of peliticschist and quartzite. The Stirling Quartzite overliesthe Johnnie Formation and consists of quartzite, mi-caceous quartzite, and minor carbonate layers. Peliticlayers occur near the base of the Stirling Quartzite.In the Chloride Clif Quadrangle, the Stirling Quartz-ite is locally overlain by argillite and quartzite ofthe Wood Canyon Formation. This Paleozoic sectionis overlain by middle and upper Tertiary fluvial, la-custrine, and volcanic rocks. The generalized geologyof the Chloride CliffQuadrangle, which encompassesthe majority of the metamorphic rocks, is shown inFigure l.
The structure of the Funeral Mountains is domi-670
LA BOTI(A: REGIONAL METAMORPHIC TERRAN E
Fig. 1. Generalized geology of the Funeral Mountains, Death Yalley, California. The northern Funeral Mountains comprise late
Precambrian and Cambrian sedimentary and metasedimentary rocks which are folded into a northwest-trending, doubly plunging
anticline and which are cut by shallow-dipping, normal faults. The geology of the Chloride Cliff Quadrangle is modified from the
preliminary geologic map by Troxel and Wright (personal communication).
671
nated by a doubly-plunging anticline that culninatesin the high-grade metamorphic terrane near ChlorideCtiff. The high-grade metamorphic rocks in this"core" are separated from low-grade metasedimen-
tary rocks by a major fault, here called the BoundaryCanyon fault, which has a gently undulating surfaceand dips generally north. Similar gently north-dip-ping faults also displace the folded metamorphic ter-
672 I.1I BOTKA: REGIONAL METAMORPHIC TERRANE
Fig. 2. Distribution of diagnostic assemblages, metamorphic isograds, and localities of analyzed samples. Filled circles repres€ntgarnet + chlorite assemblages; lfiangles represcnt staurolite + biotite assemblages; squares represent kyanite + garnet + biotiteassemblages. Numbers indicate analyzed samples and are listed in Table l. Open circles are localities of analyzed samples that containother assemblages.
rane. Rocks as young as Oligocene are displaced bythese faults and Pliocene strata on the upper plate ofthe Boundary Canyon fault are warped (Wright andTroxel, personal communication).
Two foliation surfaces are well developed in themetamorphic rocks. The older consists of a schisto-sity that is defined by the parallel alignment of mus-covite and biotite grains and is oriented parallel to
LABOTIU: REGIONAL METAMORPHIC TERRANE
the compositional layering. A second foliation devel-oped after the growth of porphyroblasts and consistsof microfolds ("strain-slip" cleavage) whose axialplanes lie at a high angle to the bedding.
The metamorphic isograds mapped in the FuneralMountains are delineated on Figure 2. Metamorphicgrade increases from southeast to northwest, and iso-grads are cut offby the Boundary Canyon fault. Theisograds do cut across the stratigraphy, but, in gen-eral, metamorphic grade increases with stratigraphicdepth. The highest-grade rocks occur in MonarchCanyon, near the culmination of the anticline, wheresillimanite-bearing assemblages occur. Migmatitesoccur in the canyon bottom, and these high-graderocks are intruded by minor bodies of leucocratic,muscovite-bearing granitic rock.
Extensive retrograde metamorphism has affectedthe high-grade rocks in the Keene Wonder Minearea, Chloride Cliff, and along the Boundary CanyonFault, where most of the mafic minerals are alteredto chlorite. The retrograded area corresponds to amineralized area which was heavily prospected forgold in the early 1900's, and to the post-metamorphictectonic dislocation.
The age of metamorphism is not well constrained.Metamorphism occurred after the deposition of theaffected strata and before the development of theTertiary Boundary Canyon fault. Muscovite from apegmatite in Monarch Canyon yielded a K-Ar dateof 30 m.y. (Wasserburg et al., 1959), which may cor-respond to the time of mineralization and retrogrademetamorphism. Regional metamorphism may haveoccurred at the same time (late Mesozoic) as themetamorphism of the nearby Panamint Mountains(see Labotka et al.,1980\.
Methods
Analytical data were obtained by wavelength-dis-persive electron microprobe analysis on a MaterialsAnalysis Corporation Model #5-SA3 fully auto-mated microprobe. Standard operating conditionsare l5kV accelerating potential, 15 nA specimen cur-rent (brass), 15-200 sec counting time (counting sta-tistics of l%o S.D.), on-line data reduction using themethod of Bence and Albee (1968) and the correc-tion factors of Albee and Ray (1970). Reproduc-ibility in the Caltech lab is within I to 2Vo for majorelements and lOVo for minor elements (Champion elal., 1975). Probe data are presented here as points ongraphs rather than in tables. Complete analyses maybe obtained from the author upon request.
Mineral assemblages in pelitic schists
Samples were collected from several localities in
the Chloride Cliff Quadrangle in order to document
the mineral assemblages (metamorphic grade) and
the reactions responsible for the change in mineral
assemblages with increasing metamorphic grade. Si-
liceous dolomite and calciferous schist are not abun-
dant in the Funeral Mountains, and the majority of
lmm
IrmmFig. 3. Textural features ofgarnet-grade pelitic schists. (a) Relic
chloritoid (light) included in garnet (dark) in a garnet * staurolite
inclusion trail in garnet (black) in garnet + biotite + chlorite
assemblage, CP 45li (crossed nicols).
674 LABOTKA: REGIONAL METAMORPHIC TERRANE
the rocks collected are pelitic schists from the lowerStirling Quartzite, Johnnie Formation, and the pah-rump Group. On the basis of the mineral assem-blages in these pelitic schists, garnet, staurolite +biotite, and kyanite * garnet + biotite isograds weremapped.
The garnet isograd is based on the first appearanceof garnet (Fig. 2). The several mineral assemblagesobserved in the pelitic rocks south of the garnet iso-grad include:
Fig. 4. Development of secondary foliation in pelitic schist. (a)Mica foliation oriented at a high angle to inclusion trails in garnet,CP 45li (crossed nicols). (b) Iknenite (small black plates) benr intothe secondary strain-slip cleavage in a garnet + staurolite +chlorite assemblage, FML 92 (plane polarized light).
Most of these assemblages represent relatively alu-minous rock compositions, and the absence of gar-net-bearing assemblages may be due in large part toinappropriate rock compositions. These rocks are rel-atively fine-grained and the largest porphyroblasts,usually kyanite, are less than a few millirneters long.A foliation defined by a preferred orientation ofmicas is present, and this foliation is oriented approx-imately parallel to compositional layering. In manysamples the foliation is broadly folded on a smallscale, and axial planes of these microfolds lie at ahigh angle to the foliation. Chloritoid and kyaniteporphyroblasts are randomly oriented, although inone sample (FML 66) chloritoid plates appear tohave been rotated into the folded foliation.
Rocks in the garnet zone also contain a wide vari-ety of mineral assemblages, but the zone is character-izedby the coexistence of garnet and chlorite. Chlori-toid occurs in rocks just north of the garnet isograd,and the assemblages found in the lower-grade part ofthe garnet zone include:
The rocks are generally fine-grained and weakly tomoderately foliated. Two foliations are apparent.The earlier is defined by a crude parallel alignmentof fine-grained micas, oriented approximately paral-lel to bedding. A second fotation is expressed bygentle crenulation of the first, but in some cases thesecond foliation is defined by a parallel alignment ofmedium-grained muscovite, oriented at a high angleto the first. The secondary fotation is not pene-tralive, but micas occur in I to 2 mm wide zone sepa-rated by 5 mm intervals. Garnet porphyroblasts, lessthan 3 mm in diameter, contain quartz inclusiontrails which are parallel to the first foliation, but theporphyroblasts had grown prior to the developmentof the secondary foliation (CP 45Ii, Fig. 3).
Retrograde textures were identified only in chlori-
LABOTKA: REGIONAL METAMORPHIC TERRANE
toid + kyanite + chlorite assemblages, in which mm-sized magnetite grains are rimmed by hematite, al-though chloritoid is locally coated by hematite orgoethite.
Farther northwest chloritoid-bearing assemblagesare absent, and the assemblage garnet + chlorite +staurolite * muscovite * quartz + plagioclase + il-menite is dominant. Many of the garnet porphyro-blasts have inclusions of chloritoid, but chloritoid isabsent from the matrix (FML 85a, Fig. 3). The grainsize is larger than ip the lower-grade part of the gar-net zone and staurolite and garnet porphyroblastsseveral mm in diameter occur. The crenulation cleav-age is also better developed and the order of develop-ment: primary foliation, porphyroblast growth, cre-nulation, is observed.
Although staurolite is first encountered in what ishere called the garnet zone, the staurolite + biotiteisograd is defined on the basis of the first occurrenceof the association staurolite + biotite. Figure 2 illus-trates the occurrence of garnet + chlorite vs. stauro-lite + biotite assemblages and the staurolite isograd.The assemblages observed in staurolite-grade schistsinclude
These schists are very coarse-grained and stauro-lite porphyroblasts several cm long are not uncom-mon. Staurolite and kyanite tend to be aligned in thefoliation. Kinking and crenulation in the schistosityis prominent, and kyanite grains are kinked and il-menite grains are bent into the crenulation (Fig. a).Staurolite engulfs and includes garnet, and garnetporphyroblasts that occur in assemblages II and IIIare greatly embayed, and irregular in form. Thesetextures suggest that garnet is metastable in assem-blages II and III and has not been completely di-gested.
The kyanite + garnet + biotite isograd is definedby the disappearance of staurolite, which is indicatedby the stable coexistence of garnet * biotite * kya-nite (Fig. 2). The commonly observed pelitic assem-blage is garnet + biotite * kyanite * muscovite *quarlz + plagioclase * ilmenite + rutile. In MonarchCanyon sillimanite also occurs in this assemblage.Gneissic foliation characteizes the texture of the
highest-grade rocks, and migmatites which exhibitpassive flow folds and ptygmatic folds occur in Mon-
arch Canyon (Fig. 5).In thin section garnet porphyroblasts contain large
inclusions of biotite and some staurolite, and garnet
has preserved the relic composition of biotite and
staurolite. Sillimanite generally occurs as needles in
quartz, in the vicinity of kyanite grains.
The change in mineral assemblages and the preser-
vation of relic minerals as inclusions in porphyro-
blasts are indicative of prograde metamorphic reac-
tions. The variety in rock compositions in the
Funeral Mountains allows the determination of the
distribution of elements in pelitic schists and the elu-
I
DFig 5. High-grade rocks in Monarch Canyon. (a) Kyanite +
garnet + biotite schist, FML I l6b (crossed nicols). Kinkbands
may be observed in kyanite grains (lower right, arrow). (b)
Migmatite in Monarch Canyon. Melanocratic layers consist of
biotite + epidote + plagioclase + quartz, whereas the leucocratic
Iayers are generally devoid of biotite.
Kyonile
kyonile
676 LABOTKA: REGI ONAL M ETAMORPH IC TERRAN E
ALL ASSEMBLAGES INCLUDE MUSCOVITE AND QUARTZ
Fig. 6. Compositions of minerals in pelitic schist assemblages plotted in AKFM space, piojccted from muscovite + quartz. (a) Low-grade assemblages. (b) Assemblages from the lower-grade part of the garnet zone. (c) Assemblages from the higher-grade part of thegarnet zone . (d) Staurolite-zone assemblages. (e) Kyanite-zone assemblages. Dashed lines connect mmpositions of phases rhat occur asrelic inclusions.
cidation of reactions which alter the stable mineralassociation with increasing grade.
Distribution of elements in pelitic schists
AFM phases
Most of the phases in these rocks can be describedin terms of the oxides SiOr-AlrO3-MgO-FeO-HrO,and the mineral assemblages that occur in quartz *muscovite schists may be represented on the planeAl,SiO,-FeO-MgO. The use of this projection as aphase diagram is discussed by Thompson (1957). Therange in bulk composition is sufficient to allow thedetermination of the complete facies types for thissystem at some grades and of the reactions respon-
sible for the changes in topology with increasinggrade.
Analyzed mineral assemblages in pelitic schistsfrom the Funeral Mountains are illustrated in Figure6 and Table l. The relative distribution of Fe, Mg,and Al among phases within any one mineral assem-blage is consistent in all rocks exhibiting that assem-blage. For a given assemblage, tie lines do cross, butthese minor inconsistencies are attributed to minorvariations ia P, T, ar;:d er.o. For all rocks foundwithin a small area and within the same zone, unlikeassemblages do not occupy overlapping regions incomposition space, and these changes in mineral as-semblage are attributable to changes in bulk compo-sition. Four-phase assemblages occur in some of the
kyonite
s f ouro l i le
FUt
k yonite
stourol i tekelic)
I,}I B OTKA : REGI O NA L M ETA M O RP H I C TERRA N E
Tabte l. Analyzed samples, Funeral Mountains
677
sAMpLE QTZ MUSC BI0 cA CH CTD ST KY SIL PLAG ILM RUT HEM MAG 0ther Locati on Grade
I K1 2 G1 2 G9 G9 G9 G8 G7 G7 G
i l Gt ] Gi l Gl 0 G1 0 Gt J u
t 3 Go u5 G4 G3 S3 S
3 S4 G2 KI KI K
X XX XX X
X X XX
X
X XXX X
X XX X
XX X
X
X
XX XX X
XX XX XXXXXX XXXX
CP 435cC P 4 5 l ec P 4 5 1 fFML 37bFML 38cFt4L 39bFr4L 54FML 55aFML 55bFML 56FML 57FML 58FML 60FML 62FML 70aFML 72FML 75FML 94aFML IO IFML l 04aFML l04cFML 106aFML l 07aFML IO9FML I IOF M L l l 3 aFML I 16b
XXX
XX X
X X
X
XX XXX
X
XX
XX
XXX
XXXX X
XX
X XX XX XX
XX X
X XX
marg
X
XXX
XX
Af f n inera fs except quar tz were anafqzedPar = paxagon iXe, marg = MTgat iLe ; K-kgan i te , s -s tauroL i te , G-garnet ; TocaXions shown on Frg '2 '
higher-grade rocks, but the garnet in the staurolite-zone assemblage garnet * staurolite + biotite + chlo-rite appears to be altered and the garnet is believed tobe relic. Likewise staurolite in kyanite + garnet +biotite zone assemblages occurs only as inclusions ingarnet or large muscovite plates, and the staurolitealso appears to be a relic phase. This apparent con-sistency in the relative order of element partition andwith the Gibbs phase rule suggests that chemicalequilibrium was closely approached during meta-morphism and invites a closer examination of themineral chemistry and its relation to rock composi-tion and metamorphic grade.
Chlorite is the most common mineral in the lowermetamorphic grades, and its major-element chemis-try is illustrated in Figure 7. The range in total Al isrelatively small and varies from 2.7 to 3.0 cations,/formula. Chlorite that coexists with biotite tends tobe less aluminous than chlorite that coexists withrrore aluminous minerals. Chlorite with the idealcomposition of (Mg,Fe)"-,AI:'(SL-"AU" ) O,o(O H).should fall on the line Al'' : AlvI. Most analyzedchlorite corresponds to this expectation, but the sub-stitution of up to 0.2Fe'* cations/formula for Al"' is
indicated for analyses which contain an excess ofAl'' over Al'', and those analyses which shown anexcess of Al'' suggest the substitution 2A1"' + !''
P 3FmvI.
Garnet shows a larger range in composition thanchlorite, but this range is principally due to the addi-tional components MnO and CaO. All analyzed gar-nets are essentially stoichiometric (Fe,Mg,Mn,Ca)rAlrSirOr2; the amounts of TiO' and Cr'Or aregenerally less than can be detected by the probe.Garnet grains are chemically zoned (Fig. 8) and thegreatest deviations from almandine-pyrope solid so-lutions are observed in garnet-grade samples. Themaximum amount of measured grossular is 20 moleVo and up to 19 mole Vo spessartine is observed. Gar-16f 2sning patterns are of two types. In both typesmanganese decreases from core to rim, but calcium isobserved to decrease or increase from core to rim.
Neady all zoning patterns encompass composi-tional changes of less than 5 mole Vo for grossular orspessartine components. The change in grossular andspessart ine is general ly balanced by a com-plementary change in the alnandine component, andpyrope remains relatively constant. The decrease in
C HLORITE
678 LABOTKA: REGIONAL M ETAMORPH IC TERRAN E
2.O
t .8
t .6'hrIA
t . t
1.2 1.4 t .6 t .B,%rFig. 7. Al content of chlorite. The linc represents compositions
generated by the substitution rvAl + vIAl S IvSi + ttMg.Symbols indicate nature of coexisting phases. Circle : + kyanitetriangle: + garnet + chloritoid; square: + garnet + biotite;diamond: + garnet + staurolite; hexagon: + staurolite + biotit€.Units are cations/formula unit.
Mn from core to rim is most likely attributable to aRayleigh-fractionation process during garnet growth(Hollister, 1966), because garnet is essentially theonly Mn-phase present. Ilmenite may contain up to 2wt.VoMnO, but all other phases show only barely de-tectable amounts of manganese; the variations in Cacontent may be related to continuous and discontin-uous reactions involving plagioclase, and will be ex-amined in greater detail below.
Pyrope increases from about 5 mole Vo to 20 moleVo with increasing grade, and the compositional rangeencompassed by garnet zoning decreases with in-creasing grade.
'Ihe distribution of Fe, Mg, Mn, and Ca betweengarnet rirns and coexisting chlorite is shown in Fig-ure 9. Chlorite consists essentially of Fe and Mg butgarnet contains substantial amounts of Ca and Mn aswell. In the Ca-free system, the most Fe-rich chloritecoexists with garnet that contains no Mn. As theamount of Mn in garnet increases, so does theamount of Mg in chlorite. Similar relations hold inthe Mn-free system. The occurrence of apparentlycrossing tie lines can arise by the addition of Ca to
garnet as well as to the change in the intensive pa-rameters P, T, and ar,o.
The variations in biotite chemistry are illustratedin Figure 10. The range in biotite compositions isquite limited. Mgl(Mg + Fe) varies between 0.40 and0.60 and the MglFe of biotite depends greatly onmetamorphic grade and mineral assemblage (consultFig. 6). The partition of Fe and Mg between garnetand biotite has a strong dependence upon temper-ature, and will be examined more closely in order toestimate the temperature of metamorphism. The Alcontent of biotite is confined to the range 1.6 to 1.7cations,/formula, but nearly all analyses show an ex-cess of Al"t over Alrt. There is an observed excess ofup to 0.35 Alvl/formula over the amount required bythe substitution Al'' + Alrv ? Fm'I * SiI" and a sub-stitution of up to approximately 0.17 mole Vo dioc-tahedral mica is suggested.
Biotite from garnet- and staurolite-grade samplescontains a uniform level of Ti, ranging from 0.08 to0.11 cations/formula (-1.5 to 2.0 wt. Vo TiOr). Inthese assemblages ilmenite is the saturating Ti phase,but kyanite-zone biotite from rutile-bearing assem-blages contains up to 0.19 cations Tilformula (-3.25wt. Vo TiOr\.
The Na content in biotite is low and Na/(Na + K)ranges from 0.02 to 0.06 (NarO < 0.4 wt. Vo). Biotitefrom staurolite-zone assemblages tends to have moreNarO than biotite from garnet- or kyanite-zone as-semblages. Ca is below the detection limit of the mi-croprobe.
Chloritoid and staurolite consist essentially ofFeO, MgO, Al2O3, SiOr, and HrO. Mn usually occursin amounts less than 0.2 wt. 7o in staurolite and lessthan 0.4 wt. Vo in chloritoid. Zn also occurs in thesesmall amounts except in staurolite from FML 55b,which has 2.5 w.7o ZnO. Up to 0.5 wt. Vo TiO.- ispresent in staurolite. The amounts of Mn and Zn arevery small (except in garnet cores) so that the ratiosMn/Fe and Zn/Fe encompass the range 0.0 to 0.01.Nothing can be said confidently regarding the rela-tive partitioning of these elements except that garnetconcentrates Mn and staurolite concentrates Zn.
The principal variations in the chemistry of theseminerals is in the Mg/Fe ratio. Figure ll illustratesthe regular distribution of Fe and Mg among coexist-ing mafic phases. Chlorite is always the most Mg-richand biotite is always slightly more Fe-rich. Garnet isthe most Fe-rich phase and chloritoid and staurolitehave Mg/Fe values intermediate to garnet and biot-ite. The relative partition of Fe and Mg between
SPESSARTINE
LABOTKA: REGIONAL METAMORPHIC TERRANE 679
GROSSULAR
FML /O9
STAUROLITE ZONE
K YANITE ZONE
/O4c
ALMANDINE
chloritoid and staurolite is di-fficult to assess becausecoexisting chloritoid and staurolite have not been ob-served. In one sample (CP a5le) chloritoid occurs asinclusions in garnet in a garnet + staurolite * chlo-rite assemblage, and staurolite is more Fe-rich thanchloritoid, which is consistent with the observationsin Panamint Mountains assemblages (Labotka, 1980)and the suggestions of Albee (1972). Not only is theorder of Fe enrichment chlorite < biotite < chlori-toid < staurolite < garnet observed, but the distribu-tion coefficients [KD : (Mg/Fe)"/(Mg/Fe)B] showonly small variations [equal distances between twominerals represent equal Ko because log Ko : log(Mg,Fe)" - log (Mg/Fe)Bl. This relative consistencyin Fe and Mg partition among gesai5ling maficphases and constanry of total Al content in biotiteand the relatively regular Al content in chlorite sug-gest chemical equilibrium was closely approached
during metamorphism. The small variations in Ko(FelMg) are attributable to variations in the in-tensive thermodynamic parameters, or especially inthe case of garnet to the addition of Ca and Mn.
Note that in sample FML 94a, in which on a text-ural basis garnet is interpreted to be a relic in the as-semblage staurolite * chlorite + biotite, the garnetrim is more Mg-rich than staurolite. This observationis contrary to the common observation that garnet isthe more Fe-rich mineral, and this apparent reversalin Ko is consistent with the interpretation that garnetis not stable in this assemblage.
Na K Ca phases
Most assemblages contain three AFM phases inaddition to muscovite and quartz and are invariant tosmall arbitrary changes in intensive variables. Mostrocks contain Na and Ca as additional components,
'7CP435 f
Fig. 8. Garnet compositions all fall in the region mole 7o aknandine > 70Vo. I*rgth of line represents the amount of pyrope. Arrows
indicate direction of zoning from core to rim.
680 L,IIBOTKA: REGIONAL METAMORPHI C TERRANE
58, which contains the assemblage garnet * chlori-toid + chlorite. The sample has not been X-rayed,but during microprobe analysis of fine-grained whitemica, a Na-rich K-poor mica was encountered. Thecation sum of Na + K + Ca is much less than one,and the ratio Na/(Na * K * Ca) was estimated withthe assumption that only Na was lost during analysis.A systematic search for paragonite has not been per-formed, so its distribution among different assem-blages is unknown.
Margarite occurs in one low-grade assemblagewith chloritoid + chlorite (FML 72). The margariteappears as - 2 mm blades in a fine-grained matrix.
MgFe
Fig. 9. Distribution of Mn, Ca, Fe, and Mg between coexistinggarnet and chlorite.
and here the nature of the saturating phases is con-sidered. In the Funeral Mountains plagioclase, pa-ragonite, and margarite occur as the Na- and Ca-sat-urating phases.
Muscovite occurs in all assemblages in a variety oftextures ranging from fine-grained sericite to coarseplates. The composition deviates only slightly fromKAl3Si3Oro(OH)r, as shown in Figure 12. Na sub-stitutes for K in amounts up to O.l5Na/(Na + K) inlower-garnet-grade samples and approximately 0.25in muscovite coexisting with paragonite (?). The Nacontent is up to O.3Na/(Na + K) in 1[s higher-gradesamples.
The amount of phengite substitution is likewisesmall. The maximum amount of Si observed,is 3.12/formula in garnet-grade samples and about 3.06/for-mula in staurolite- and kyanite-grade samples. In thelow-grade assemblages, muscovite coexisting withbiotite contains more of the Si'v * Mg', a= AII' +Alvr substitution than muscovite from biotite-absentassemblages. All higher-grade assemblages containbiotite, which suggests that with increasing grade thephengite content of muscovite decreases.
The excess SiO, in muscovite is balanced by thesubstitution of FeO and MgO for octahedral Al.Muscovite analyses indicate 0.5 to 0.6 wt. Vo MgOand 1.0 to 2.5 wt. Vo "FeO". Most of the iron appearsto be ferric because the excess silica is nearly bal-anced by the MgO present and the total positivechange indicated is nearly always less than the theo-retical value of 22.0. Muscovite from hematite-bear-ing assemblages tends to contain more "FeO" (nearlyall of which must be FerO.) than muscovite fromhematite-free assemblages.
Paragonite has been tentatively identified in FML
o.2
1.2 1.4 r .6 r .8' ' ' tv^ , " -AI
Fig. 10. Al and Ti content of biotite. Nearly all analyses indicatean excess of octahedral Al which suggests the importance of thesubstitution vrE + 2vrAl S 3vrMg. Symbols indicate grade ofsamples. Circle: gamet zone; triangle: staurolite zone; diamond:kyanite zone. Units are cations/formula unit.
Fig. I l Distribution of Fe, Mg, Mn, and Zn between coexisting minerals. The data indicate that the order of Fe enrichment is chlorite< biotite < chloritoid < staurolite < garnet. Garnet concentrates Mn, staurolite concentrates Zn.
tor.oo.l r.o
o gornet r im. gornel coreo sfourolile
o chlorifoido b io l i tea chlorile
The distribution of Na, Ca, and K in coexisting whitemicas is illustrated in Figure 13, which shows thateven muscovite coexisting with margarite contains nocalcium.
Plagioclase occurs in most assemblages exceptchloritoid + chlorite * kyanite and garnet + chlorite+ chloritoid. Plagioclase in garnet-grade samplestends to form rounded porphyroblasts which containquartz inclusions. Plagioclase in staurolite- andkyanite-grade samples occurs in twinned grains in aquartz-plagioclase mosaic.
The compositions of plagioclase and coexistingmuscovite are shown in Figure 14. Plagioclase con-tains essentially no K and the observed composi-
tional range is A4o to Anr. lsning and grain-to-grain compositional variations are small except asnoted below. In FML 56 plagioclase An35 occurs withplagioclase Anr. The plagioclase contains multi-tudinous inclusions and only one plagioclase of com-position An, was observed. Hence it is di-fficult to de-termine whether the two compositions coexist due toimmiscibility or whether the two are related by zon-ing or reaction. No other samples were observed withthis relation.
The plagioclase in sample FML 60 is the most cal-cic feldspar observed and exhibits 2sning from a Ca-rich core (An6o) to a more sodic rim (An '). Thissample contains the most Ca-rich garnet which is
r--oa-o
-o-
a _ O o
a--o a
A O-O
a-o
a-o
a -o
-o -
a
tr-O{ O-o
O-Oa O-o
oa----------o-o
(H-a o-o
a-a-o
682 I-ABOTKA: REGI ONAL METAMORPHIC TERRAN E
Q o' '
+
.sS o..
o.2
3.OO 3.O5 3.rO 3.r5
kyonle zoneso brot i ie obsenlo + porogonitea + biolile
o "A
slouro/tle,
3.OO 3.O5 3.tO 3.15Si
Fig. 12. Paragoni te (Na) and phengi te (Si) contents ofmuscovite.
zoned (Fig. 8) toward more Ca-rich compositions,opposite to the plagioclase zoning pattern. Figure 15shows that the more Ca-rich garnets tend to coexistwith more Ca-rich plagioclase, and that becausethese are the only Ca phases present their Ca contentis controlled to some degree by the amount of Ca inthe total rock. Among these garnet-grade samplesthere is a fairly regular relation between the Na/(Na* Ca * K) ratios in muscovite and coexisting feld-spar. The more Na-rich muscovites tend to coexistwith the more Na-rich feldspars. Staurolite- andkyanite-grade samples contain plagioclase in thecompositional range An,, to An, and coexistingmuscovite is correspondingly more Na-rich (Fig. la).
Fe-Ti phases
All mineral assemblages contain Fe-Ti phases. Il-menite is the most corlmon, and it occurs in the as-semblages garnet * chlorite f biotite, garnet * chlo-rite + chloritoid, and garnet * staurolite + chlorite inthe garnet zone, and in the higher-grade assemblages
garnet + staurolite + biotite, staurolite * biotite *chlorite, kyanite + staurolite + biotite, and kyanite +garnet + biotite. Every ilnenite analysis shows moreTi than required by FeTiOr, and the presence ofminute rutile lamellae is indicated. In general MgOoccurs in amounts less than 0.1 wt. Vo and MnO lessthan 1.0 ,wt. Vo. The maximum MnO observed is 2.0vtt. Vo.
Rutile occurs in the low-grade assemblage kyanite+ chlorite + chloritoid, and the high-grade assem-blages kyanite + staurolite * biotite and kyanite +garnet + biotite.
Ferric-iron phases are observed in only the assem-blage kyanite * chlorite * chloritoid. Hematite oc-curs as small (<0.5 mm) plates and as pseudomorphsafter magnetic cubes. The remnant magnetite is es-sentially pure FerOo and the hematite rim on mag-netite is pure FerOr. The smaller matrix hematiteplates apparently contain up to 50 mole Vo FeTiO,(Fig. 16). Most of these associations contain rutile inaddition. The unusualy high TiO, content in hema-tite suggests that there are minute ilmenite lamellaepresent which may have exsolved during retrogrademetamorphism.
Mineral facies and reactions
The range in bulk composition and metamorphicgrade provide a surprisingly complete determinationof mineral facies in a Barrovian metamorphic ter-rane. The details of the facies types which involveCa, Na, Fe3* phases are at present only sketchy, butenough information exists to suggest the phase rela-tionships. Figure 17 illustrates the generalized facies
NoFig. 13. Distribution of Na, Ca, and K between coexisting white
mlcas.
LABOTKA : REGI O NA L M ETA M O RPH I C TERRA N E
muscovrle muscovite. gornel grode^ slourolile grodej kyoni le grode
Fig. 14. Distribution of Na, Ca, and K between coexisting muscovite and plagioclase.
683
p0r0gontte
No
types which describe the distribution of elementsamong coexisting facies during the metamorphism ofthe Funeral Mountains.
These AKFM rrineral facies are very similar tothose exhibited by other Barrovian terranes wherethe complete facies types have been determined,principally in the New England regional-metamor-phic terrane. The assemblages in garnet-grade rocksfrom Indian Pass are identical to those found at Mt.Grant in Vermont [Albee (1965), and Fig. l7]. Eventhe incomplete information regarding the saturatingFe'*, Ti, and Na phases is corrpatible with the Mt.
+ kyonite
Co
Grant assemblages. By analogy, the undeterminedphases may be assigned. Magnetite should occur inthe assemblages garnet + chlorite + chloritoid andgarnet + chlorite + biotite; paragonite should occurin the assemblage kyanite * cblorite + chloritoid.
The staurolite-zone assemblages shown in Figurel7 are very similar to the assemblages exhibited bythe Gassetts Schist, Vermont (Thompson et al.,1977). Fernc-iron phases do not occur in the ana-lyzed samples from the Funeral Mountains, andplagioclase rather than paragonite coexists withkyanite.
Titanium phases occur in all observed assem-
F e OU O
Fe2O3No
P/oqioc/oseFig. 15. Partition of Ca between coexisting garret and
plagioclase; the more calcic garnets coexist with more calcicplagioclases.
Fig. 16. Composition of Fe-Ti oxides. Hematite shows a wide
range in composition due to the analysis of minute ilmeniteIamellae. Dashed lines indicate probable phase relations in thelow-grade assemblage kyanite + chloritoid + chlorite.
plogioclose An50 plogioclose An5o
f s+JVg+MnTi Oz
LIIBOTKA: REGIONAL METAMORPHIC TERMNE
AKFM PELITIC SCHIST ASSEMBLAGES
FUNEflAL ilOAilTAIIVS
tourolite + Chlorire
Assembloges IncludeMuscovi le + Ouorlz
Stourolite = Go.net + gioti le + Kyonile
Mso
Fig. 17. Generalized AKFM facies series for the metamorphism of pelitic schists in a Barrovian-type terratre. Assemblages indicatedby the dashed lines are not obscrved, but are inferred to have been stable. The inversion from kvanite to sillimanite occurs after thebreakdown of staurolite.
Slou,oliie + Chlorite = Kyonite + Biotite
I.IIBOTKA: REGIONAL METAMORPHIC TERRANE
blages. Rutile occurs in aluminosilicate-bearing as-semblages, whereas ilmenite occurs in all others. Thedistribution of Fe3* and Na phases among the assem-blages is not well documented, but paragonite andmagnetite probably may occur in high-Al and low-Alassemblages respectively.
All the assemblages and the sequence of faciestypes are very similsl to those found in the regionalmetamorphic terranes in northern Vermont (Albee,1968) and in the Barrovian terrain in Scotland(Harte, 1975), except that staurolite breaks downprior to the kyanite-to-sillimanite transition. Eachfacies type is related to another by a discontinuousreaction which either represents a change in mineralcompatibilities or a termination of a phase. The reac-tions which relate the changes in facies in the systemAKFM are
(l) unknown and probably complex garnet-form-ing reactions
(2) incoming of staurolite by chloritoid * kyanite: staurolite * chlorite
(3) breakdown of chloritoid by chloritsld : garnet* chlorite * staurolite
(4) break in garnet-chlorite association by garnet+ chlorite : staurolite + biotite
(5) etimination of chlorite from most rocks bystaurolite * chlorite : kyanite * biotite
(6) breakdown of staurolite by staurolite : g&rnet* kyanite + biotite
(7) polymorphic inversion of kyanite to sillima-nite.
The stoichiometry of each discontinucius reactionmay be ascertained by the natural composition datafor mineral associations that are stable on either sideof an isograd. The present sample density does notpermit the determination of the stoichiometry ofmost reactions, but inclusions of relic mineral phasesin garnet suggest the terminal compositions of chlori-toid and staurolite.
Sample CP 45le preserves the probable terminalchloritoid as inclusions in garnet in a garnet * chlo-rite + staurolite assemblage. The compositions of thephases in this sample provide the terminal chloritoidreaction
Anorthite is derived from plagioclase and such a re-action may cause plagioclase to become more sodic(see discussion below).
The terminal composition of staurolite may also beapproxi-mated by the composition of staurolite in-clusions in garnet in the garnet * kyanite + biotiteassemblage from sample FML I10. The compositionof biotite inclusions in garnet is also used because thebiotite in the matrix has a significantly different Mg/Fe.
The reaction has been simplified by combining Caand Mn in garnet with Fe. Because muscovite ismore Na-rich than biotite, and because garnet con-tains significant Ca, any muscovils-sonsuming, gar-net-producing reaction will be accompanied by anincrease in the albite component in plagioclase. Ifboth garnet and muscovite are consumed or pro-duced (as in the crossover garnet * muscovite +chlorite : staurolite + biotite) the composition ofplagioclase will not necessarily change.
The compositions of all phases in invariant associ-ations is greatly affected by continuous and exchangereactions. The compositions of most mafic phases be-come more Mg-rich with increasing grade. Biotite isan exception, and once staurolite breaks down biotiteassociated with garnet and kyanite becomes moreFe-rich.
The compositions of coexisting muscovite andplagioclase are governed by both sasfrang€ and con-tinuous reactions. At low grades, the distribution ofsodium between muscovite and plagioclase is repre-sented by
Fig. 18. Extreme colditions of m€tamorphism based on the correspondence of observed mineral ass€mblages to experimentally
determined phase equilibria. Boundaries taken from (l) Kerrick (1968), (2) Holdaway (1971), (3) Chatterjee (1972),(4) Bird and Fawcett(1973), (5) Chauerjee and Johannes (1974), and (6) Tuttle and Bowen (1958). Staurotte stability from Ganguly (1972).
to9
5
4
3
2
Because plagioclase contains essentially no K andmuscovite contains no Ca, the equilibrium constantfor the first reaction is very large and for the secondvery small. At higher grades plagioclase coexists withkyanite, and the compositions of plagioclase andmuscovite coexisting with kyanite and qvaftz arecontrolled by the continuous reaction
affects the calcium distribution between coexistingplagioclase and garnet.
Physical conditions during metamorphism
The values of the intensive thermodynamic param-eters may be estimated by comparison of naturalmineral associations to experimental data, by calcu-lation using mineral compositions and calibratedgeothermometer-barometers, and by inference fromthe ieologic setting.
The critical observations that define the P-T fra-
jectory during metamorphism are that kyanite is thestable aluminosilicate throughout most of the teraneand that staurolite breaks down before kyanite in-verts to sillimanite.
The experinental work of Holdaway (1971), Hos-chek (1969), Ganguly (1972), and Richardson (1968)(Fig. 18) suggests that within the kyanite stabilityfield minimum conditions at the staurolite isogradare -550"C, 5 kbar; at the kyanite * biotite isograd-625"C,6 kbar; and at the kyanite isograd -700oC,
8 kbar. These are P-Z conditions where Pr,o: P,and Po, is at values within the stability field of mag-netite. Higher Po, conditions restrict apparent stauro-lite stability (Ganguly, 1972), but none of the high-grade rocks contain ferric-iron phases, so that thevalues estimated by these experiments are appropri-ate.
Quantitative estimates of the physical conditionscan be calculated from existing geothermometers.Temperatures are estimated by the relative partition-ing of Fe and Mg between garnet and biotite, fromthe oxygen isotopic catbration of Goldman and Al-bee (1977). The distribution coefficient provides tem-perature estimates for all garnet-biotite assemblages,
because K" is largely insensitive to pressure (Albee,1965). Temperatures for garnet-biotite pairs are alsocalculated with the calibrations of Ferry and Spear(1977) and of A. B. Thompson (1976) for com-parison. The results are tabulated in Table 2, andtemperatures calculated by the Goldman-Albee cali-bration correspond well to those calculated by theFerry-Spear experimental calibration, except forthree samples collected from the kyanite zone. Tem-peratures calculated with the A. B. Thompson cali-bration are in excess of the other two by 75-100.C.The high-grade pairs CP 435c, FML ll0, and FMLl16b show better correspondence in temperaturescalculated from Ferry-Spear and Thompson. Com-parison to experimental data and the correspondencebetween temperatures calculated by Goldman-Albeeand Ferry-Spear for garnet- and staurolite-gradesamples suggest that these may be realistic values.Sinilarly, temperatures calculated from Thompsonfor kyanite-sillimanite-grade sarrples seem more ap-propriate. The suggested temperature in the garnet-grade Indian Pass area is 470-500"C, at the stauroliteisograd 500-530oC, and in the high-grade MonarchCanyon area 600-700oC.
The pressure during metamorphism is difficult toestimate for most mineral assemblages. Estimatesbased on the probable amount ofoverburden are un-satisfactory because the Paleozoic section exposed to
the north (Reynolds, 1974) and to the south (McAl-hster.' 1974) amounts to 6.2 km and 7.8 km respec-tively. The thickness of the Mesozoic cover at thepresumed time of metamorphism (Cretaceous) is un-known, but if the thickness was comparable to thatestimated in the Panamints, perhaps 3 kn may beadded. A generous estimate of 12 km of cover sug-gests lithostatic pressures on the order of4 kbar, butthe mineral assemblages suggest that pressures mayhave been approximately 1.5 to 2.0 times thisamount.
In the high-grade rocks, the coexistence of garnet* plagioclase * kyanite + qvaftz allows the calcu-lation of pressure, given the temperature. Ghent(1975, 1976) suggested the use of the divariant reac-tion
3 anorthite : grossular + kyanite + quartz
as a potential geobarometer. The equilibrium con-stant is given by
o : - 3 2 7 . , 2 - 8 . 3 9 6 9 - o ' 3 4 4 8 ( P - l )T T
+ log a!ij" - 3 log aff;8
(Ghent, 1976). The activiry of anorthite in plagio-clase is 'f:!" )PA*, where yff, : 1.28 for sodic plagio-clase (Orville,1972). The activity of grossular in gar-
688 I.IIBOTKA: REGIONAL METAMORPHIC TERRANE
Table 3. Garnet-plagioclase-kyanite geobarometer and muscovite-plagioclase-kyanite geothermometer
net is given bV (fe,* -IF;:")', assuming random mixingin the three 8-fold sites; f"ii" is estimated by assuminga regular symmetric solution model for the binary so-lution grossular-almandine, which gives (Ghent,re75):
h YE?;" - (l --:€L")' w
R T
W, the regular symmetric solution parameter, de-rived from the excess free energy of mixing deter-mined by Ganguly and Kennedy (1974\, is estimatedto be +1000 callmole.
Pressures were calculated from three samples fromthe kyanite zone for each temperature calculated bythe Ko for coexisting garnet and biotite. The resultsare shown in Table 3, and show pressures from 6100to nearly 10,000 bars. The estimated temperaturerange of 600"-700'C suggests the reasonable pres-sure range of 7200-9600 bars for the high-grade ter-rane in the Funeral Mountains. The large uncer-tainty in the temperature gives a tremendousuncertainty in pressure. Despite the great uncer-tainty, the results calculated for the kyanite zonesamples are consistent with the experimental work ofGanguly (19'72) and Richardson (1968) for thebreakdown of staurolite.
The results of calculation of dr,o from the threekyanite-zone samples by the multi-variant reaction
paragonite + quartz: albite + kyanite: HrO
(see Ghent, 1975) are inconclusive. Values (Table 3)ranging from 0.250 to essentially 1.0 may be derivedfrom the estimated P-T pairs. The majority of therocks are pettic or quartzitic, whereas carbonaterocks are restricted to the high-grade regions; thefluid phase was probably rich in H,O.
The mineral assemblages in pelitic schists from the
Funeral Mountains preserve a nearly complete rec-ord of the facies series in a Barrovian metamorphicterrane. The compositions of the minerals suggestthat during metamorphism the Funeral Mountainswere buried to depths of approximately 20 km andthat dP/dT in the highest-grade area was -10 bat/
"C (-30"C/km). This gradient is steep compared togradients in the regional metamorphic terranes adja-cent to the Mesozoic batholitic belt (e.g. the Pan-amint Mountains record a gradient of - 5 bar/"C;Labotka, 1980), and the Funeral Mountains are oneof the few localities in the Cordillera that comprisean extensive kyanite-bearing terrane. This windowthrough ths higher, unmetamorphosed structural lev-els in the western Great Basin indicates that an ex-tensive regional-metamorphism terrane every bit ascomplex and petrologically interesting as terranes inother orogenic belts underlies the North AmericanCordillera.
AcknowledgmentsThis work presents some of the results from my Ph.D. thesis su-
pervised by Arden L. Albee, whose encouragement and criticism isappreciated. The study was supported by grants from the NSF(EAR75-03416A03 to A. L. Albee) and the Geological Society ofAmerica. I thank D. Rumble for his review of the manuscript.
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Manuscript received, August 8, 1979;acceptedfor publication, December 19, 1979.
