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Home , Amphibole, Epidote, Hornblende

American Mineralogist, Volume 59, pages 2240, 1974

AmphibolePairs, , Chlorite, and Plagioclasein MetamorphicRocks, Northern Sierra Nevada,California'

ANNI HtereNBN

U. S. Geological Suruey, Menlo Park, Calilornia Electron microprobe analysesby G. K. CzannNsxB

Abstract

Two kinds of - and aluminous Ca--and two to three epidote minerals coexist with chlorite and plagioclasein the meta-volcanicand meta-igneousrocks in the contact aureoles of the Cretaceous plutons in the Feather River area, northern Sierra Nevada. Small prisms and rims of actinolite occur with green outside the l-mile- wide contact aureoles of the plutons. In the zone next to the plutons, blue-greenhornblende forms rims around large grains of actinolite and also forms small prisms that are included in .Electron-microprobe analyses show that the alkali and Al content of the amphi- boles increaseswith an increasein the grade of , the ratio of NafK to tetra- hedral Al remaining close to 1:3. The ratio between two coupled substitutions, edenitic (NaAAlt' for nASi'") and tschermakitic(Al"t Al"'for Mg'r5;t"r, remains3:7 and leadsto the compositionNaooCa.Mg,"Fe2*ro(Al,Fe3*),.Sio"Al'")O*(OH), at the highest grade of meta- morphism in the Feather River area and further to NanuCarP*suR3*rnSiuAlrOrr(OH)athe ex- treme member of this actinolite-hornblendeseries. The Mg/Fe ratio changeswith the bulk com- position and with the degreeof oxidation of the host rocks. Epidote and coexist in all zonesas separategrains. In addition, many grainsof clinozoisiteinclude high index domains, lamellae, and stringersof epidote.In the highest grade zone next to the plutons, occurs with epidote,clinozoisite, and anorthitic plagioclase.Farther from the plutons, the plagioclaseis albite. There is no regular changein the distribution coefficientKn(Mg/Fe) for chlorite/amphi- bole although it is highest in the zone next to the plutons.

Introduction zone around the plutons. In the pelitic layers, the common occurrence of andalusite, cordierite, and The minerals studied occur as major constituents staurolite in the outer contact aureole and andalusite in Paleozoic(?) metavolcanic and related meta- and cordierite in the inner one indicates PZ condi- morphosed intrusive rocks around Cretaceous plu- tions of the epidote facies (Hietanen, tons in the southwestern part of the Bucks Lake 1967). Eilsewhere, assemblages such as quadrangle, northern Sierra Nevada. The general muscovite--chlorite in pelitic layers and albite- geology of this quadrangle and the and epidote-actinolite-hornblendeindicate the upper limit chemical composition of the host rocks have been of the greenschistfacies. describedin anotherreport (Hietanen, 1973);there- In the metavolcanic rocks and related intrusive fore only a short summary of the geologic setting rocks, the increase in the grade of metamorphism is and some additional features are given in the follow- indicated by the change of the color of hornblende. ing description. In the low-grade zone, the parallel to y Two metamorphic events can be recognizedin the is light green to green but becomes bluish green host rocks near the plutons. The earlier regional towards the plutons. The most interesting feature metamorphism was to the greenschistfacies and was is that two amphiboles-actinolite and aluminous modified by a later higher-temperaturecontact meta- hornblende-and two to three epidote minerals co- morphism that afiected only the rocks in a narrow exist in all zones, as shown by indices of refraction measured in immersion liquids. It was therefore 'Publication authorized by the Director, U. S. Geological highly desirable to determine the composition of Survey. these coexisting minerals. The rock samples chosen 22 AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS for this study had already been chemically analyzed Description of the Rocks (Hietanen,l9l3,Table 1). They includeintermedi- Metagabbrosample 796, O.3 mile south of the ate and basic metavolcanicrocks and related intru- Basin pluton, representsthe highest meta- sive rocks, metadiorites,and metagabbros. morphic grade. It is a coarse-graineddark rock in Samplesof meta-andesiteand metadacite (sam- which black hornblende contrasts with white an- ples 463 and464) are from successivelayers in the orthite (Ansr). Thin sectionsshow that most of the Franklin CanyonFormation, 3.2 miles east-southeast large amphibole grains are pale-greento colorless of the GraniteBasin pluton (Fig. 1). Sample465 is actinolite (yAc - 20o) mottled with blue-green from a small body of metadiorite a few meterswide amphibole (Fig. 2). All large grains are rimmed by just south of the locality of metadacitesample 464. blue-greenamphibole that is strongly pleochroic in Becauseof their proximity, thesethree rocks most blue green (y), green (F), and light green (o) and likely recrystallizedat the samePT conditionsat the has yAc = 22o. In addition, small euhedralprisms border of the greenschistand epidote amphibolite of blue-greenamphibole are included in anorthitic facies. The four other samples (Sp. 796, 546, 551, plagioclase.Most plagioclasegrains are granulated, 532) were collectedfrom 0.3 to 0.8 mile outside individual grains averaging0.01 mm in size, but the contact of the Granite Basin pluton. The rocks some are large and have mottled extinction and ir- in this contact zorre are coarsergrained and show a regular twinning lamellae. Some of these lamellae higher grade of metamorphism than those farther consistof rows of tiny small grains;others have wavy from the contact. borderssimilar to thosedescribed in an earlier report

XPLANATIOI{

Ftc. L Maior geologic units and locations of the specimens studied (796, 546, 463, 532 arrd 464) in the Feather River area. ANNA HIETANEN

included in albite. Albite is in granulated grains among the hornblende prisms and includes small Ft grains and clusters of epidote and some clinozoisite. \ Numerous scatteredanhedral to subhedralgrains of (about0.02 mm long) are includedin horn- blende; some larger grains are clusteredparallel to the foliation.Some leucoxene is includedin horn- blende and epidote minerals. Meta-andesitesample 463, 3.2 miles east-south- east of the Granite Basin pluton, is a fine-grained gray-greendense rock that is either massive or -j-.r,aar...... - weakly foliated. Major constituents are light-green to pale-green hornblende, colorless actinolite, epi- Frc. 2. Amphibole grains consisting of hornblende dote, clinozoisite, and albite. Small amounts of (shaded) and actinolite (unshaded). Black is magnetite.fn chlorite, leucoxene, magnetite, and pyrrhotite are the high grade zone (796) actinolite has rims and patches of blue green hornblende, and small prisms of hornblende ubiquitous.The minerals are clustered;aggregates are included in anorthitic plagioclase. In the low grade consisting of amphibolesand some chlorite and zone (465) large grains consist either of actinolite with epidote minerals probably were originally patchesof green hornblende,or of hornblende that includes phenocrysts.In the massiverock, green hornblende and is rimmed by actinolite. Small prisms of actinolite are occursin anhedralto subhedralgrains, 0.01 mm to included in albite. 1 mm long.Many grainshave pale-green to colorless actinolitic ends and rims; all contain inclusions of (Hietanen,1951, Fig. 2I) and are thoughtto have leucoxene.Actinolite alsooccurs as needles in albite. crystallized from zoisite at elevated temperatures Albite is in small (0.1 to 0.3 mm) grainsernbedded near the plutons. All three epidoteminerals, epidote, in a mixture of small anhedralgrains of epidote and clinozoisite, and -poor zoisite, are present. clinozoisite that is clouded by leucoxene. In some Epidote and clinozoisiteare in individual grains or in small patches,however, euhedralcrystals of epidote clusters,which are surroundedby a mass of small minerals free of inclusions are included in albite or grainsof zoisitedusted with tiny inclusions(Fig. 3). in a mixture of albite, fine-grained epidote, and Someclinozoisite grains contain patches of epidote; leucoxene.Chlorite is in clustersof small flakes next some others have twinning lamellae. All epidote to the amphiboles in the massive rock and with minerals contain inclusions of leucoxene, and are epidote minerals along the foliated shearzones. The accompaniedby somechlorite and . A few tiny opaque minerals,pyrrhotite and magnetite,are in grains of ilmenite-magnetiteoccur as an accessory. subhedralgrains, 0.01 to 0.5 mm in size. Metagabbro sample 546, O.4 mile east of the Metadiorite sample 465, which is consideredto Granite Basin pluton, is mineralogicallyand textur- be an intrusiveequivalent of the meta-andesiteand ally similar to the metagabbroof locality 796, except metadacite,is an equigranularrock in which dark- for the absenceof zoisite. a lower anorthite content green hornblendecrystals contrast with the white to (Anto) in plagioclase,and less actinolite. A few blue-greenhornblende prisms have actinolite ends. 6n Metabasaltsample 551, 0.8 mile east-northeastof L'l @ lcLt ,b the Granite Basin pluton, is a dark-greento black, t/ weakly foliated rock that consists mainly of small prisms of hornblende,opaque minerals, and albitic 464 .a plagioclase.In thin sections,hornblende prisms (0.2 ep.t-f-A'Y'/ to 1 mm long) are pleochroicin light blue-greenand OS mm 7i; light greenand havecolorless actinolite ends or, more Frc. 3. Textural relations between epidote minerals from rarely, are partly rimmed by actinolite.They are sub- the high grade zone (795) and the low grade zone (464). - cz = clinozoisite, zo = zoisite. Domains, parallel to the foliation or form radiating clusters. ep epidote, lamellae, and stringers of epidote in clinozoisite (far right) Small very light green to colorless needles of ac- are textures seen in immersed fragments. The other grains tinolite occur next to the sreen hornblendeand are are sketched from thin sections. AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS

pale green mixture of albite and epidote minerals. with the epidote in the Jurassicplutons. It is also Amphibole is in subhedralto euhedralprisms, 0.2 to possible that there is a submicroscopicintergrowth 1 mm long, that are pleochroic in light to medium of two phasesin each mineral. green;many prismshave pale greento colorlessspots Metadacite sample 464 is a light-grayish-green and ends (Fig. 2). Albite and epidoteminerals form inhomogeneousslightly foliated rock, in which dark clustersthat have outlines of former plagioclase,or patches,I to 20 mm long and consistingof chlorite, they occur as small interstitialgrains between these amphibole, clinozoisite, and epidote, are in the clusters and the hornblende prisms. Some of the matrix of albite, quartz, chlorite, and epidote min- epidoteminerals are in large (0.5 mm long) . erals. Amphibole is in small colorlessto pale-green Quartz is in small (0.01 to 0.5 mm long) grainsor prisms that have ragged ends. Chlorite, the major groupsof grains amongthe other minerals.In places, dark constituent,is colorless,has a gray to brownish clustersof very pale greenchlorite flakeswith brown- gray interferencecolor and negativeelongation, and ish gray interferencecolor occur betweenthe amphi- includes tiny grains of leucoxenealong its bole and epidote minerals. Sphene is in anhedral planes. Albite is in clusters of equant, subhedral grainsadjacent to epidoleor hornblende.Most of the grains that include epidote minerals and some small original magnetiteis altered to hematite. prisms of amphibole.Clinozoisite, the major epidote In the metadioritesample 532,1mile southof the mineral, is clustered either with chlorite or with GraniteBasin pluton, anhedralgrains (0.5 to 1 mm albite. Some clusters show the outlines of former long) of green amphibole are embeddedin a mix- pyroxeneor former plagioclase.Epidote occurseither ture consistingof albite and epidote.The amphibole as individual grains or with clinozoisite (Fig. 3). is pleochroic in light bluish green to pale green and The opaque minerals, pyrrhotite and magnetite,are includesleucoxene. Albite grainsare subhedral,0.2 in anhedralgrains 0.2 and 0.5 mm in size.Albite is to 0.5 mm long, and include numerous small sub- twinned and includes epidote minerals,chlorite, and hedral to anhedralcrystals of epidote,some of which some muscovite. are clustered.A few patches,0.5 mm long, consisting All these rocks are exceptionallypoor in potas- of tiny flakes of chlorite, occur next to amphibole sium (Hietanen,1973, Table 1); KzO contentranges grains. Magnetite is partly altered to hematite, and from 0.03 to 0.21,percent. spheneto leucoxene. The question about the possibility of equilibrium Sample 532 is from the southeasternend of an betweenthe coexistingamphiboles and epidote min- elongatebody that was emplacedalong a fault zone erals has to be consideredin the light of thermal his- (CamelPeak fault in Fig. 1). The major part of the tory of the host rocks. There were two periods of body consists of light-colored porphyritic meta- metamorphism:(1) low-grade regional metamor- trondhjemite that contains about 35 percent quartz, phism during which albite, epidote, actinolite, chlo- 25 percentalbite, 18 percentepidote and clinozoisite, rite, and quartz crystallizedfrom plagioclase,pyrox- 15 percenthornblende, and 7 percentchlorite. Small ene, "igneous" hornblende, and biotite with or grainsof ilmenite-magnetiteare envelopedby sphene. without quartz (the recrystallizationmay have not Hornblendeprisms have pleochroism7 = hght blue- been complete); (2) later contact metamorphismat - green,B = light green, d pale green. Only a few higher temperaturesgave rise to crystallization of small prisms of actinolite occur with green horn- hornblende,clinozoisite, and zoisite at temperatures blende. Most of the clinozoisite is in long prisms where epidote, actinolite, and chlorite were still among round grains of quartz that filI the fractures stable. The absenceof relict primary igneous min- in the rock. Some slenderneedles, however, are in- erals and chemical changesin bulk composition in- cluded in albite. Thus metatrondhjemitein the main dicate a thorough reconstitutionof rocks. part of this metamorphosedintrusive body has two Textural relations in the high-grade zone (Sp. amphiboles and two epidote minerals, as do the 796) suggestthat the rims of blue-greenhornblende other specimensstudied. The occurrenceof only a around actinolite may have prevented further re- green hornblende and an epidote in the metadiorite action betweenthe actinolite cores and the plagio (Sp. 532) indicates physical conditions different clasein the matrix. However, the occurrenceof blue- from those elsewhereduring the recrystallizationof green hornblende in patches in the centers of this rock; perhapsthis metadioriteis a later dike-like actinolite with its absencein parts of the rims refutes body in which epidote is late magmatic,comparable this concept and shows that a low rate of diffusion 26 ANNA HIETANEN of Al and Na was not the reasonfor preservationof (Table 1) in all the rock types exceptin metabasalt actinolitecores. The high anorthitecontent of plagio- sample551, where they are somewhathigher; r A c clase(Ane3) suggeststhat lack of availableNa was is generally21" to 26" for the hornblendeand about one of the determiningfactors, but not the only one 15ofor the actinolite. becausethe compositionof blue-greenhornblende * The indices of refraction of amphibole grains actinolite could give green hornblendewith less Na. (Table 1) can be correlated only in a general way The occurrenceof blue-greenhornblende only in the with the microprobe analyses that were done in high grade zone showsthat it crystallizedat elevated polished thin sections.The highestindices measured temperatures from bulk cornpositions similar to for actinolite are in sample 551, in which Fe/Mg those that in the low grade zone produced green ratio is higher than in the other specimens(Hiet- hornblende, albite, and actinolite. In parts of the anen, 1973). Also the hornblendein this rock has metagabbro,as in sample 546, very little actinolite higher indices of refraction than the hornblendein remainsand plagioclaseis Ant6. This sample,col- the other specimensapparently due to a high iron lected a little farther from the contact of the pluton content (see Table 2a). High aluminum content, than sample 796, is thus from a somewhatlower such as that in the blue-greenhornblende in sample temperaturezone. Although considerableamount of 796, affects indices of refraction far less than high sodium remainsin the plagioclase,a few blue-green Fe content. Specimens463 and 465 also contain hornblendeprisms in sample 546 have actinolite grains of actinolite similar to those in specimen796. ends.The actinoliteends and rims are more common Only one amphibole,the gteen hornblende,was de- and remnants of the cores more rare farther from tectedin the mineral powder from sample532. the pluton (Sp. 551). These ends and rims crys- The epidote concentrateof the powdered rocks tallized.last, probably after the maximum metamor- revealed two epidote minerals in all the samples, phic temperaturewas passed. exceptin metadioritesample 532 andin sample796, The textural relations betweenepidote and clino- which have one and three epidote minerals respec- zoisite (Fig. 3) suggestthat they coexistin equili- tively. Many grains appearedto consistof small do- brium. Clinozoisite is in anhedral or in euhedralto mains of epidote (with indices of refractioh higher subhedralcrystals next to or amongthe anhedralepi- than I.72) in clinozoisite(indices lower than 1.72). dote grains.Lamellae, stringers, and domainsof epi- The high index domainsrange from lamellaeparallel dote in clinozoisiteresemble textures due to exsolu- tion. Zoisite occursonly in the highestgrade zone; it Tegrn 1. Indices of Refraction of Amphiboles, Epidote rims grains and clusters of epidote and clinozoisite Minerals. and Chlorite and is thus later. The abundantdust-like inclusions in SqpIe 796 55r 532 465 463 464 it are probably iron oxide exsolvedwhen zoisitecrys- Rock Mete- lleta- Meta- Meta- lleta- Meta- baa.It diorlte dLortte daclte taJlizedfrom epidote. I .651 1.655 1.651 r.647 L.649 L.641 Hornblende B r.659 L.669 1.659 1,558 L.557 Optical Propertiesof Minerals Y L.677 L.679 r,671 r.671 1.670 1.671 22" 25" 24' 25' 2L" In the grade (Sp. highest zone 796) the blue- r,625 r.640 r.625 7.626 L.622 greenhornblende is stronglypleochroic; farther from Ac tlnoll te L.634 1.648 Y r.648 r.656 1,649 1.650 L.647 the pluton (Sp. 551) the colors are much lighter, r. 719 L.12r r.72L r.721 L.723 L,722 and greenhornblende in the low-gradezone has 7 = Epldote B L,7J2 1.731 7.733 L.741 7.74J L.744 L.742 r.742 1.751 1.755 1.755 light green,0 = light greento olive green,?Dd a = to 1.754 * pale green. Actinolite in all specimenshas 7 pale r.708 L,709 r.7t4 7.715 1.709 green and and a from very pale green colorless. CI lnozosite B 1.710 1.71r r.716 r.7I7 B to Y r.7r8 r.720 t.722 1,722 1,72r Many amphibole grains in samples 463, 464, and 465 have green centersand colorlessrims. The con- Zoisi te B l. 703 r.7L4 tact betweenthe centerand the rim is usuallydiffuse. I r.600 t,620 1.615 1.617 ChIori ce Many grains show intergrowthseither of fine lamel- +2V al0" soal l. small snall snall lae or of equantto elongatedomains of greenamphi- B r.59t Muscovi Ee bole in colorlessactinolite, all the green amphibole Y 1.593 t.574 1.510 1.530 7.529 showing higher indices refraction. PIaSiocIase r <1? of Each amphi- B 1.581 r. 534 bole has remarkably uniform indices of refraction 'r r.585 l. 539 1. 538 1. 538 AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS to the cleavageand perthitelike intergrowthsto small with the electron microprobe for their Ti, Mn, and equant domains of epidote in clinozoisite, all of Mg contents.Irucoxene was found to consist of 93 which still show ultrablue interferencecolor (Fig. 3, to 98 percentTiOp. at right). The contactsbetween the domainsand the host mineral seemsharp. Analytical Techniques The measurementsin the individual samples(Ta- by G. K. CznrvreNsre ble 1) show that the indices of refraction of epidote Grains of albite, amphibole,chlorite, epidote,and and clinozoisite in the samplesfrom the low grade zoisite in carbon-coated,polished thin sectionswere zone tend to be higher than those from the high analyzedwith an Ant nux-su electronmicroprobe. gradezone. In all the epidotes2V,is large.In addi- Accelerating potentials were chosen to minimize tion to clinozoisite and epidote, zoisite with gray effectsof interferenceson the important light consti- interferencecolors, parallel extinction, 2V" - 59", tuents Al, Mg, and Si (see Desboroughand Heidel, and indices of refraction lower than those of clino- l97l). The following analytical conditions were zoisite occurs in metagabbro(sample 796). The used: Ca, Fe, Na wererun at 15 kilowatts,whereas highest7 measuredis given for clinozoisitein Table Si, K, Al and Mn, Ti, Mg were run at 10 kilowatts. 1. This value is higher than would be expectedon Samplecurrent on benitoiie was 0.2 g. amperesat 15 basisof a large *2V and is due to a rangein com- kilowatts and 0.03 p amperesat 10 kilowatts, with a position. beam diameterof 2 to 3 microns. Typically, 6 to' 12 Chlorite occurs in clusters of small flakes in all points per grain were analyzedfor each set of three rocks. It is most abundantin metadacitesample 464 elements,for counting intervals of about 10 seconds and occurs only sporadically in metagabbrosample (actual count intervals were fixed by beam current 796.|t is pleochroicy = P = grseh,a = colorless termination). Raw count data were corrected for in samples796, 551, and 532, but very pale green drift, background, absorption, fluorescence, and to colorlessin the other samples.The indices of re- atomic number with the aid of the Fortran IV com- fraction of the green chlorite in sample 532 are puter programof Beeson(1,967). somewhathigher, and those in sample796 are lower Standardsfor albite, epidote, and zoisite were the than the indices of the common pale variety (Table following oxides: AlzOs, FezOe,MgO, MnOz, SiOz, I ). All chlorites have brownish gray interference TiOz, and the following three silicatesfrom Goldich, color, negative elongation, and small *2V. Ingamells,Suhr, and Anderson(1,967): albitefrom The indicesof refraction of plagioclase(Table 1) Amelia County,Virginia (Na), Or-1 (K), and Px-l. indicate that it is albite in the low grade zone and For the amphiboles, a homogeneous,hastingsitic anorthite at the highestgtade next to the pluton. amphibole[Nao.uz Ko.zs Car.zs (Alo tr Fe3*a6p Tia.37 Mgr.ozFe'*z.seMno.oo)r.tsSi6 26,{11.72O2OH )zl was em- Composition Minerals of ployed. A homogeneousbiotite [K1.seNao.67Bao.e3 Separationof minerals for wet chemical analyses (M&.snFe'*r.oa Mno.* Ab.orFe3*o.ze Cro * Tio rs)s.ss u/as not feasiblebecause of many small mineral in- Sib.?2A12.28Ozo(OH)nlwas used to analyzethe chlo- clusions,such as leucoxeneand ore minerals. Even rites, with the amphibole above as a Ca standard. a reasonablyclean fraction of blue-greenamphibole Both the amphibole and biotite were analyzed by that has beenchemically analyzed (Hietanen, 1973, wet chemical techniques by C. O. Igamells, have Table la, anlal.796h) contained a few colorless been checkedon the microprobe againstmany other amphibole grains. The major constituents-amphi- superior standards,and are routinely used as stand- boles, epidote minerals, plagioclase,and chlorite- ards becauseof their purity and homogeneity. were therefore analyzedby electron microprobe by For purposesof comparisonand further calcula- G. K. Czamanske.In each sample,several grains of tion, oxide weight percentagevalues reported in Ta- optically different types of amphibole and epidote bles 2, 3, 4, and 5 are consistentlyreported to two minerals were chosen for analyses(Tables 2a and decimal places, although in terms of expected ac- 3). Chloritewas analyzedin five samples(Table 4), curacy, no more than three significantfigures can be and plagioclasein six (Table 5). justified, e.9., SiO2will be reported as 42.97 rather Some of the accessoryminerals, such as ilmenite than 43.0. In all tables0.0 meansless than 0.005. in sample551, sphenein samples532 and 465, and Someof the spot analysesthat consistedof highly leucoxenein samples463 and 532, were checked inhomogeneouscounts were split into two parts: the 2E ANNA HIETANEN

TtB,LE?-a. Electron Microprobe Analyses,in Weight Percent,of Amphiboles from Metamorphic Rocks, Feather River Area Analyst, G. K. Czamanske, U. S. Geological Suruey

Saple 796 546 ))I Rock Metagabbro Metagabbro Metabasalt

Ana- Blue Snall Htgh A. Colorless core Low A1 Prlsm Prism Core High Al green prasm ryz< counts counfs of 15 counts parc rim for 4 for 4 core end for 16 Anal. I 2 3 4 6 7 8 9 10 II L2 13 I4

sio2 42.97 43.94 42.82 51.31 53.31 57.L6 56.43 56.65 55.75 52.LO 55.29 42.49 42.5A 41.35 412o3 16.tl L4.93 14.93 6.L7 4.55 4.O3 3.61 2.78 r.09 5.L4 ?no L3.92 11.65 L3.77 Tio2 0.19 0.15 0.25 0.13 0.09 0.06 0.10 0.08 0.04 0. 34 0.20 0.36 o.29 o.42 Fe2O3 2.63 2.45 1.88 1.55 r.80 r.44 L.7r 1.48 r.33 2.66 2.24 4.i> 4.75 4.53 FeO 9.47 8.82 6.77 5.59 6.49 5.20 6.L6 5.34 4.80 9.58 8.05 15.69 17.10 16.33 Mgo 12.76 13.70 13.96 18.10 18.90 20.47 20.34 20.58 20.64 L5.37 18.13 7.65 8.00 7.O2 MnO o.23 0.2r 0,06 0.12 0.27 0.18 0.19 0.20 0.13 0.30 o.29 0.35 0.31 o.34 CaO IL.1 2 r1.91 L2.22 L2.55 LL.76 L2.33 12.33 L2.23 L2.77 11.59 tl. 66 rt .50 11.18 tr.4) Naro r.92 l. 87 0.94 0.54 0.58 0 .80 0. 38 0.37 0.28 0.63 0.18 l?q 1.r0 1.28 Kz9 0.28 0.19 o.2I 0.03 0.02 0.00 0.00 0.00 0.00 0.02 0.00 o.34 o.37 o.28 Total 98.28 98.17 94.o4 96.o9 97.77 LoL,67LoL.zs gg.7L 98.83 97.73 98.13 97.94 97.33 96.77

tltg/Fe L.922 2.2L4 2.936 4.6L9 4.L57 5.617 4.7LL 5.50r 6.L32 2.2gO 3.zLO 0.695 0.667 0.613 Fe/Al 0.52L o.524 0.403 0.803 r.265 L.r44 1.513 r.703 3.go2 r.652 3.4L7 l.ooo L.3o2 r.o5r

S@ple 551 463 465 532 464

Rock Metab as a1 t Meta-andesite Metadlorlte Metadlorl ce Metadaclte

na- Rin of Low A1 Green Green homblende Green homblende lgh A1 SnalI Low Al Slnple Iyzed t3 counts hornblende counta prlsu count6 grain fot L4 for 28 f.ot 26 15 l6 L7 18 19 20 22 ?1 24 25 26 28 sio2 50.76 5L.64 47.30 44.r4 47.64 48.3r 48.82 49.79 48.31 47.89 49.08 47.54 50.99 56.58 56.50 Alzo 3 2,64 2.40 8.O2 6.84 7,55 7.20 6.39 5.76 7.43 7.40 6.48 6.76 4.85 0.81 0.88 TLO2 0.Il 0.07 r.48 1.34 1.14 1.11 1.33 L.29 0.86 0.83 0.88 Q.24 0.25 0.15 0.03 FerOr* 4.01 3.78 2.70 2.56 2.98 3.10 3.L6 3.00 2.9L 2.90 2.89 2.0L 2.26 L.47 r.40 FeO 14.44 13.61 9.7L 9.23 ro.74 11.20 11.40 10.80 10.48 LO.47 10.40 7.23 8.L2 5.29 5.04 Mgo 12.00 L2.L3 15.59 L6.22 14.92 14.54 14.83 t4.84 L4.35 14.50 14.7 L 18.21 L6.7L L9.32 20.47 llno 0.2r 0.19 o.25 0.23 0.31 0.40 0.43 0.42 o.44 0.37 0.44 0.16 0.20 0.19 0.18 CaO 12.06 II.77 LO.25 11.19 r0.69 10.49 10.35 10.48 LL.O2 rr.21 11.08 12.5L L2.54 13.11 13. 28 Naro 0.36 0.67 1.03 1.10 r.30 1.l0 I .02 1.13 L.23 r.2r L.zI 0.48 0.19 0.12 0.06 K2o 0.11 0.I2 0.08 0.09 0.10 0.03 0.05 0.09 0.14 0. 18 0.07 0.01 0.06 0.00 0.00

Total 96.70 96.38 96.4L 96.94 97.37 97.48 97.78 97.60 97.r7 96.96 97.24 95.15 96,r7 97.O4 97.83

Mg/Fe 1. I85 I.272 2.29O 2.506 1.981 1.853 1.855 r.959 r.953 t.976 2.OL6 3.59r 2.933 5.209 5.797 Fe/At 4.849 5.021 1.079 L.I97 L.26L r.379 r.5E3 1.663 L.25L r.253 r.424 0.949 L.486 5.786 5.064

*2O7, of total Fe expressed as Fe2o3 AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS 29

Taer.B2b. Structural Formulas for Amphiboles in Table 2a on Anhydrous Basis of 23(O)

Slre Tetrahedral M(1)--M(3) M(4) Total lons t - 8.00 I = 5.00 E = 2.00 -2+ AnaI s1 AI Al F"3+ Tr Mg F.2* Mn !e Mn Ca Na NaKCa 6. 200 1.800 0.939 0.286 0.02r 2.744 1.010 0.133 0.028 1.812 0,O27 0.5r0 0.052 6. 319 1.681 0.849 0.265 0.016 2.937 0.933 0.128 o.026 1.835 0.011 0.510 0.035 6. 346 1.654 0.954 0.210 0,028 3.083 0.725 0.r14 0 .008 1.878 o.27r 0.039 0.063 6.334 1.666 0.8r0 0.404 0.023 2.822 0.934 0.007 0.012 L.902 0.086 o.449 0.043 7 .320 0. 680 0. 358 0.166 0.014 0 .054 0.015 1,918 0.013 0.136 0.006 796 3.849 0.613 5 7.484 0.515 0.237 0.190 0.009 3.955 0.609 0.153 0.032 r.769 0.046 0.112 0.004 6 7.636 0.364 0,27L 0.145 0.006 4.O76 0.502 0 .079 0.020 r.765 0.135 0.o72

7 /.OII 0.389 0.185 0.173 0.010 4.089 0.543 0.r52 0.022 L.782 0.044 0.055 8 7.720 0. 280 0. 167 0.152 0.008 4.180 0.493 0. r15 0.o23 1.786 0.076 0.o22 9 7 .827 0.173 0.007 0.141 0.004 4,319 0.529 0.034 0. 016 L,92L 0.029 0.o47 10 7.443 0.557 0.308 0,285 0.036 1.273 1.098 0.047 0 .036 L.774 0.143 0.031 0.004 11 7.76s 0.235 0.lll o.237 0.021. 3.795 0.836 0. r09 0 .035 L.755 0.049 L2 6.360 1.640 0.816 0.490 0.04r L.707 L.946 0 .0r8 0.o44 1.844 0.094 0.281 0.065 ** 6.395 1.615 0.850 0.3L2 0.041 L.7L4 2,083 0 .059 0.o44 1. 852 0.035 0.341 0.065 13 6.472 I.528 0.559 0.543 0.033 1.813 2.O52 0. 121 0 .040 1,821 0.018 0.306 0.072 ** 6.509 1.49r 0,508 0.347 0.033 r.823 2.L89 0. 196 0 .040 r.831 o,326 0.072 0.067 55r 14 6. 303 L.697 0.777 0,520 0.048 r.595 2.060 0. 021 0 .044 r.870 0.065 0.314 0.054 ** 6.316 L.684 0.794 0.330 0.049 1.598 2.276 0.047 o.o44 r.874 0.035 o.344 0.054 15 7.563 0.437 0.027 0.450 0.0r2 2.665 L.799 o.o27 L.925 0.075 o.029 0.021 ** 7,591 0.409 0.056 0.286 0.013 2.675 1.970 0.o27 1.933 0.040 0.064 0.021 16 7.670 0.330 0.090 o.423 0.008 2.685 1.690 o.024 1.873 0.L27 0.066 0.o23 ** 7.698 0. 302 0. 120 o.269 0.008 2.695 L.852 o.o24 1.880 0.120 0.o74 0.o22 lL7 6.901 1.099 0.280 o.296 0.L62 3.390 0.872 0.313 0.031 L.602 0.054 0.238 0.015 1tt 6.986 1.014 0. 156 0.280 0.146 3,509 0.909 o.2LL 0,028 1.740 0.021 0.289 0.017 )t-' 'o 6.936 1.064 0.2J2 o.326 0.r25 3.238 r.O79 0,229 0.038 1.668 0.055 o.J02 0.019 7.023 o.977 0.256 0.339 0.121 3. r51 r. r33 o.229 0.O49 L.634 0.088 o.222 0.006

1::i- 7.O79 0.92r 0.17r 0.345 0.145 J. 2U> I. IJC 0.249 0.053 r.608 0.090 0.197 0.009 I I tc 7.203 0.797 0.185 o.327 0.140 3.200 1.148 0.159 0.052 L.624 0.165 o,L52 0.017 in 7.035 0.965 0.310 0.319 0.094 3.115 L.162 0. u4 0.054 1.719 0.113 0.235 0,026 I zt, 7 .000 1.000 0.274 0.319 0.091 3.159 r. 157 0.123 0.046 1.755 0.076 o.267 0.034 I [25 7.L32 0.868 0.24L 0.316 0.096 3 .186 1: 161 0.103 0,054 L.725 0.rr8 0,223 0.013 l'u 6.97t L.OZg 0.139 o.222 0.O27 3.980 0.633 0.254 0.020 r.726 0.137 0.002 0,239 127 7.376 o.624 0.202 0.246 0.027 3.603 0.922 0.061 0.024 1.915 0.053 0.011 0.029 128 7.934 0.066 0,068 0.155 0.0t6 4.038 0.620 0.023 1.970 0.030 0 .002 [,, 7.859 0. r4r 0.004 o.r47 0.003 4.244 0.586 0 .016 0 .005 L.979 0.016

*tlet chmical analyses from Hietanen (1973)

**The nuber of ions given on lower line are recalculated for Fero, = L2.77 of total Feo high Al and high Fe counts were combinedwith the analysis26 high Al and high Fe counts combined low Mg counts to form one analysis and the low with low Mg counts from the samegrain. Al, low Fe, and high Mg counts were calculatedas Amphiboles separate analysis. Analyses 3 and 9 were derived from analysis4 in this way. Analyses14 and 1.6arc The composition of analyzed amphiboles (Table basedon splitting counts from two grains and com- 2a) rangesfrom actinolite through actinolitic horn- bining high Al and Fe counts with low Mg counts blende to a hornblende rich in aluminum, some for 14 and low Al and Fe counts with high Mg rocks containing the two end members and some counts for 16. Analysis 28 representslow Al and others intermediate compositions.Some of the in- low Fe counts combined with high Mg counts, and termediate compositions are averagesof inhomo- 30 ANNA HIETANEN

TesLE 3. Composition of Epidote Minerals in Metamorphic Rocks, Feather fuver Areai

796 546 55t 465 532 464

Metagabbro ileta- lletabselt Hetadlorlte XetadtorlCe ktadecr.te lebbro

Zoisite Cll.nozolsite lpldot, Epldote Epldo!e Epldote Eptdote CUnozotBlt€ Eptdote

AnaI. 30 3r 32 33 34 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 5r sio2 4t.52 4L.64 40.03 39.98 39-59 38.7s 39.O7 39,23 39.10 39.70 38.28 39.28 39.26 3A.40 39.81 39,08 i9,44 39,47 39.86 38.r1 38.r0 37.57 AI2O3 33.07 33.38 32.05 30.9L 30.2t 24.86 27,oa 27.7i 27.r7 28.99 24.23 26.31 26.35 27.42 2i 26 26.96 29.61 29.32 28.13 25.23 22.6s 23.12 fe2o3i 0.23 0.11 2.34 3.58 4.87 11.58 8.27 8.72 11,55 5.76 L2,6A 9.27 9.95 8.09 8.38 8.63 4.64 5,29 6,24 LO.A7 12.85 13.14 Tlo2 0.01 0.02 0'03 0.01 0.04 0-06 0.08 0.08 0.08 0.07 0.31 0.10 0.14 0,05 o.o9 0.03 0.ll o.o4 o,o8 o.o5 0.25 0.03 Hso 0.0I 0.00 0.00 0,00 0.04 0.00 0.00 0.00 0.00 0,04 0.04 0.01 0.0r 0-00 0.00 0.00 o,o2 0.0 0.0 0.0 o,oo 0,0 xno 0.05 0.05 0.08 0.07 0.11 0.23 0.04 0.00 0.04 0.19 0.27 0.I0 0.15 0.17 0.24 0.06 0.0 o.1o o.o o.o7 o.r7 o.1o cao 23,42 23,9i 23.E9 24,12 24.24 23,0j 23.39 21.24 20,66 23.7r 22,53 23.39 23.47 23,3t 2).34 23.O8 24.1r 24.22 23.85 23.06 22.76 22.82 Ne-o 0.65 0.45 o.oo 0.00 o.oo o.oo 0.00 0.00 0.00 0.00 0.00 0.00 0,00 0.00 0.00 0,00 0,0 0.0 0.0 0.0 0.0 o.o K,0 0,02 0.05 0.04 0.01 0.00 o.02 0.07 0.07 o.o7 0.06 0.06 o.o7 o.o9 o.o4 0.06 0.07 0.03 o.o7 0.09 o.o2 o.o4 0,09

Totar 98.79 r00.62 98,46 98.68 98,20 9s.57 98.00 99.1r 9a.77 98.52 98.40 98,53 99,43 97.48 99.La 91.9L 97.96 98.51 98.55 97,41 96,52 96.81

Nuber of lons on anhydtous baste (25 oxygen elds pe! 2 fotuula unirs) si 3.100 3,086 3.031 3,037 3.0r7 3.036 3.041 3.018 3,023 3.046 3.017 3.05r 3.031 3.007 3.059 3.045 3.034 3.O29 1,O6L 3.0r5 3,059 3.020 Ar 2.r.r 2.9L7 2,a6o 2,767 2,706 2,296 2.484 2.5La 2.415 2.622 z.zsL z.4og 2.lga 2.53r 2.469 2.41b 2.6a5 2.652 2.573 2.3s3 2.r43 2.191 - re 0.013 0.006 0,r33 0.204 0.279 0.683 0.485 0.505 0.678 0-333 0.752 0.542 0.57A 0.477 0.485 0.506 0.269 0,305 0.360 0.547 0.7f7 0.795 Tl 0,0 0,001 0.002 0.0 0.002 0.004 0.005 0.005 0.005 0,004 0.019 0.006 0.008 0.003 0.005 0.001 0.006 0.002 0.005 0.003 0.015 0.002 tre 0.001 0.0 0.0 0.0 0.004 0.0 0.0 0.0 0.0 0.005 0.004 0.002 0,001 0.0 0.0 0.0 0,0 0.0 0.0 0.0 0.0 0.0 Mn 0.003 0.003 0.005 0.004 0.007 0.015 0.003 0.0 0.003 0.012 0.018 o.006 0.011 0,011 0.015 o.oo4 0.001 O.OO? O.O O.oO5 O.OI7 0.007 ca r,874 1.904 I.938 1.963 I.974 1.937 1.951 I.916 I.711 I.949 I.9o2 L.947 1.941 1.955 t.922 ] 927 1.987 r,991 r,963 1.955 1.958 1.966 Na 0.067 0.064 0.0 0.00 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0,0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 K 0.00I 0.004 0.004 0.001 0.0 0.002 0.007 0.007 0,007 0.006 0.006 0.007 0.008 0.004 o.006 o.oo7 o,oo3 0.007 0.009 o.oo2 o.oo4 0.009

Pistacite conrent ln mlecular perc€nt

Pe: o,14 0.21 4.44 6.87 9.Js 22,93 16.34 16.?r 2t.so 71.27 2s.04 $,37 tg.42 rs.s6 16.42 16.g7 9,11 ro.31 12.27 zL.s7 26.61 26.62 F"$+Al

FelA1 0.004 o.oo2 o.o47 o.o74 o.ro3 0,297 0.195 o.2oo o.274 0.r27 o.334 0,225 0.24r 0,rq9 q.r9o q-zQL g.tgg ,o,lr; p.llq u!275 0.352 o.362

rTotal Fe * Fe2O3

t Electron micraprobe analyses, in weight percent, by G. K. Czamanske, U. S. Geological Suruey.

geneous counts; they include analysis spots with made. In the wet chemical analysis of blue-green high Al and low Al counts, as well as spots with hornblendein sample796, this ratio is 0.32 (Hiet- high Fe and low Fe counts. Comparisonof the split anen, 1973, Table la, anal. 796h). This analysis analysesfrom inhomogeneousgrains with the anal- was recalculatedon anhydrousbasis of 23(O) and yses from homogeneousgrains in the same sample is shown in Table 2a under *796. The two probe showsthat the high Al and Fe countscombined with analyses(1 and 2) of high Al grainsin the same low Mg counts give composition of blue green or sample are similar to this wet chemical analysis,all green hornblende (anal. 3, t4, and 26) whereas three having about 1.9 percentNa2O and 15 to 16 the low Al and Fe counts combined with higlr Mg percentAI2OB. Analyses L2 to 16 for the amphiboles counts (anal. 9, 16, and 28) give compositionof in sample551 were alsocalculated for Fe2Os= 1.2.7 actinolite. This similarity of analysesbased on se- percent of total FeO to conform with the lower lected counts with the analyses of homogeneous p"2.1(Fez' * Fe3*) ratio in the rock analysis(anal. grains in the same samplesupports the possibility of 551, Hietanen,1973, see also Fig. 6). bimodal compositionof grainsthat give inhomogene- ous counts. It seemslikely that submicroscopicin- Structurol Formulae tergrowths of actinolite and hornblende, similar to Differencesin the compositionof analyzedamphi- those seen under the microscope although not de- bolescan be best comparedon the basisof structural tectableoptically, may give rise to the inhomogeneity formulas basedon chemistry and X-ray work of counts. (Table 2b). Accordingto the methodof calculation In all probe analyses,20 percent of total Fe was by Papikeet al. (1969), all Si and a part of Al are calculated as FezOato agree with the approximate assignedto the eight tetrahedralsites. The remaining FezOg(FeO* FezOs) ratio in the host rocks after Al, Ti and Fe3"together with Mg, Fe2*and Mn oc- an allowance to form epidote and accessorieswas cupy the five octahedralsites M(l), M(2), and AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS 31

Terre 4. Composition of Chlorite and Muscovite in this ratio is 0.55, reflectingthe high content of iron Metamorphic Rocks, Feather River Area* in the host rock.

ftlorlte Both generalized formulas are derived from 796 546 a6t 465 464 796 mainly by two coupled substitutionsthat, HetaSabbro Het.-andeslt Uetsdlorlte Hetedeclte Heta- in completion, would lead to the compositions of Bebbro s2 53 5h 55 56 57 58 59 and . The extent of these sto2 27.59 25.99 26.28 26.52 2r.46 26.98 26,O0 21.37 47.r4 coupled substitutionsdiffers; in the formula unit of At2o3 22.99 21.77 2r.o7 20,70 21,23 21.18 21.03 20.85 34.76 (NaAAlk Pt2o3 1.lr r.tl r.ll l.1l l.l1 l,1l 0.0 green hornblende,the edenitic substitution l1o2 o.0 0.06 0.02 0.05 0.04 0,07 0.0 0.04 .0.04 =,Iasit") is 0.3, and the tschermakiticsubstitution Hro 25.40 19,5r ]8.68 18.74 2r.t9 2L.45 2r.51 22.7! t.13 (Alvtfiltv = Mgor,sit") is 0.7. In the blue-green FS 9.23 L7.34 LE.26 18,58 15.16 L5.19 73,L| 72.29 0.6r l{no 0.15 0.2L 0.31 0.30 o.29 0,21 0.23 0.22 0,0 hornblendein sample 796, the correspondingvalues Ia20 0.0 0.0 0.01 0.00 0.0 0.o 0,01 0,ol o,39 are 0.5 and 1.2, and in sample551 they average Reo 0.0 '0,0 0.o 0.02 0.0 0.0 0.0 0.01 10.2s Total E6.57 86,03 t5,76 85,99 E6.49 A5.22 E3.13 83.5r 94,35 0.4 and 1.1. However, the ratio of the edenitic substitutionto the tschermakiticsubstitution remains Nuber of 16s on aDhydrous baete of 2t(0) for chlorlte sd 22(0) for Nscovite constant and close to 3;7. For this schemeof two st 5.t2i 5.367 5.472 5.515 5.561 5.489 5.440 5.4ss 6.29L coupled substitutions, the end member would be Al 2,577 2,613 2.529 2.485 2.440 2.5t2 2.560 2.545 r.709 (Al,Fes.) ^J' 2.729 2.66L 2.64t 2,544 | 2.627 2.564 2,621 2.549 3.755 hornblende Nao.oCas(Mg,Fe2. ) 3.6 r.rAlzSie F.+3 0.164 0.173 0.L74 o.rr4 0.159 o.r7o 0.175 0.173 0.0 Oze(OH)c in which edenitic substitution is 0.6 and Tr 0.0 0.009 0.006 0,003 0.007 0.006 0,011 0.0 0.004 lrc 7.413 6.006 5.796 5.E08 6.395 6.506 6,709 7.01E O.224 tschermakiticsubstitution is 1.4. Such a composition Fe l.5ll 3.001 3.179 3.23t 2.564 2.585 2.304 2.73L 0.058 is identical to the end membercomposition suggested h 0.025 0.037 0.054 0.052 0.049 0.046 0.04r 0.038 0.0 by Robinson, Ross, and Jaffe (1971') on the basis Na 0.0 0,0 0.002 0.001 0,0 0.0 0.003 0.004 0.100 K 0.0 0,0 0.0 0.004 0.0 0.0 0.0 0,003 1.743 of a more generalizedstudy. Sincethe same amount (20 percent) of total Fez*was changedto Fe!* in all x8lx8iF€+2 0.831 0,557 0.646 0.643 0.7r4 0.716 o.744 0.767 0.767 M8/Pe tot6l 4.424 l'E92 L.729 r.106 2.331 2.36L z.7os 1.o44 3.I75 * percent, Electron microprobe analyses, in weight by TesLB 5. Composition of Plagioclase in Metamorphic G. K. Czamanske, U .5. Geologicol Suntey. Rocks, Feather River Area*

M(3). A small deficiencyin thesesites in analyses 16 and 28 is considered to be due partly to an analytical error (both are composite of low Al counts) and partly to the presenceof Cr and V or sto2 46,37 50.66 69.O2 69.49 69.31 69.22 68.72 69.45 20.38 20.O2 20.O3 19.95 19,91 19,60 someother elementnot analyzed.The remainingMn Alzo: 35.54 31.45 Tlo2 o.o 0,0 0.0 0.03 0.04 0.14 0.0 0.0 possibly and some Fe occupy, together with Ca and Feo 0.0 0.0 0.16 0.09 0.0 0.ol 0.0 0.0 0.0 Na, the two M(4\ sites. The A-site is either vacant Mno o.o 0.0 0.0 0.0 0.0 o.zL 0,0 (as in actinolite) or is occupied by Na and K (in- CaO 18.50 L4.t 2 0.66 0.57 0.41 0.48 0,42 0,38 11.59 r1.58 11.64 cluding some Ca in three analyses), and the total Na2O o.61 3.23 r1.59 11.58 11,57 Kz9 0.03 0.03 0.10 0.02 0.02 o.oi o.o2 o.o2 of the octahedral occupancytogether with the excess rotal lOO.l1 99.79 r0r.91 l0l'80 r00.44 r0r'63 r00'65 101'49 valences(Al3- + Fe3** Tia*), balancesthe negative

charge of the tetrahedral sites (Al"). According to lftlber of lons on bsl6 of 32(0) this scheme, the results of calculation on an an- sl 8,514 9.236 r1.861 U.933 11.935 r1.917 11.93r 12.14 hydrous basis of 23(O) from the U.S. Geological A1 7.475 6.758 4.L29 4.O52 4.064 4.O49 4,069 3.913 0.005 0.018 0.0 0.0 Survey computer program E2lO arc shown in Table T1 0,0 0.0 0.0 0.004 Fe O.O o.O 0.023 0.014 0.0 0,001 0.0 0.0 2b. A generalizedformula for the blue-greenhigh- l{n O.O 0.0 0.0 0.0 0.0 0.031 0.0 0,0 Al amphiboleis ( Na,K ) o.uCarM&. eFe2*1.nA16. eFes*e. 3 Ca 3.639 2.8r7 0.121 0.105 0,085 0.088 0.077 0.070 Alr.?Si6.sOpp(OH)r.The green amphibolehas the Na 0.237 r.141 3.861 3.856 3.865 3.868 3.897 3.8E1 generalizedformula: (Na,K)o.aCazMgg.zFe2*r.zAlo.zK 0.006 0.007 0.021 0.004 0.005 0.007 0.004 0.004 percentaSe Tro.rFes.o.aAlSi?Os2(OH)2.In both formulas Mn, llolecular An 93.742 ?L.O6I 3.022 2.658 2.L69 2.2L6 r.941 L.175 Fe2*,and Na, substitutefor some of the Ca, so that Ab 6.LO8 28.769 96.452 97.23L 97.7L4 97.602 97.959 98.r31 the total of Ca * alkalies rangesfrom 1.9 to 2.5. Or 0.151 0.170 0.526 0.111 0.117 0.183 0.100 0.094 Most actinoliteshave M#(Fe + Mg) ratios of 0.75 * Electron microprobe analyses, in weight percent, by to 0.85; in the metabasalt(sample 551), however, G, K. Czamanske, U. S. Geologicol Suraey. ANNA HIETANEN probe analyses,the positions of points in the graphs 0.6 and the ratio (Na * K)A/Ali" = 0.3, which are only approximate. value is only a little lower than the ratio of the total Calculation of the analyses from sample 551 alkalies to the tetrahedralAl. (Table 2b) showsthat loweringof the Fe2O3con- The ratio of octahedralAl + Fe8* * Ti to tetra- tent from 2A to 12.7 percent lowers the Ali" value hedralAl (Fig. 5) is closeto 0.7 for most analyzed by 0.03 and increasesthe z4-siteoccupancy by the hornblende grains. A nearly similar ratio was sug- same amount, but that the total alkalies remain gestedby Leake (I965a, b) for the maximumpos- constant. Therefore much of the discrepancy in sible Al"i content in Ca-amphibolesin general.Only alkali/Al* ratio due to errors in estimation of Fe8. analysis point 26 in the Feather River area falls can be avoided if total alkalies instead of ,4-site significantlybelow the trend line. This analysiscom- occupancyare plotted againstthe tetrahedralAl con- bines high Al, high Fe, and low Mg counts from a tent (Fig.4). In Figure 4 all points fall closeto a spot, and therefore the possibility of analytical error trend line that connects the average for the blue- is larger than in the analysis from homogeneous green hornblende to the origin. The ratio of total areas. Analysis 26 shows an exceptionally large alkalies to the tetrahedral Al content thus remains amount of Ca and a low amount of alkalies. For constantand close to 1:3. This trend leads to an comparison, two wet chemical analyses of horn- extreme member marked by point "Hornblende" on blendein the plutonicrogks (Hietanen,1971., Table the line connectingpargasite to tschermakite.Alkati 1, anal. 474 and 522) werc plotted on Figures 4 content of the "Hornblende" is 0.66 and it has 2 and 5. The ratio of alkalies to tetrahedral Al is tetrahedralAl. The analysis26, which falls far below similar to that in the green hornblendein the meta- the trendline, has0.24 Ca ionsin the z4-site,whereas morphic rocks; the octahedralAl * Fe3* * Ti con- in most analysesCa contentis lower than2 and some tent and the extent of tschermakitic substitution. Na entersthe M(4) sites.The l-site occupancyis however, are higher. thus generallysomewhat lower than the total alkali content. In "Hornblende" the A-site occupancy is Comparisonwith the PublishedAnalyses Compton (1958) studied amphibolesfrom the pluton located Edenite contact aureole of the Bald Rock 10 to 20 miles south-southwestof Granite Basin pluton. Three of his analyseswere recalculatedon an an- hydrousbasis of 23(O) and plottedin Figures4 and 5 (Anal. points C-2, C-3, and C-4). Two of these +Ela +57 6 "..3i +51 analyses(C-2, C-3) have Ali" contentsintermediate between the green and blue-greenhornblende near Granite Basin pluton (Fig. a); the third (C-4) has a composition closer to hastingsitethan any other 02 04 06 amphibole in this region. The host rocks for these AcI Inolile Tschermokite three analysesare basaltic intrusive and extrusive Frc. 4. Plot of total alkalies against tetrahedral Al per rocks similar to those of this study. Klein (1969) formula unit on basis of 23(O) for actinolite and horn- made electronmicroprobe analysesof amphibolesin blende. Numbers I to 29 refer to microprobe analyses of Table 2. The others, added for comparison, are by wet four other specimenscollected by Compton from the chemical methods and were recalculated on the anhydrous southwestern contact aureole of the Bald Rock basis of 23(O) per formula unit. Point 796 is for the wet pluton. The given by him is similar to chemical analysis of blue-green hornblende in sample 796 that in the low gradezone near Granite Basin except (Hietanen, 1973). Analysis points 522 and, 474 are for that only epidote, but no clinozoisite,was reported. in the plutonic rocks in the study area (Hietanen, l97l); C2, C3, and C4 are for three amphi- On the basis of textural relations, Klein suggested boles from the neighboring Bidwell Bar area (Compton, that actinolite and hornblendemay have crystallized 1958); El4 and E415 are for hornblende from the Emery- in equilibrium. Klein's analyses2-6 and 2-7 for the ville area, Adirondack Mountains, New York (Engel and hornblende in metadoleriteare similar to the green Engel, 1962);963 is for hornblende in amphibolite hornblendein sample 551 in Granite Basin and and 608 for hornblende in -bearing rock, Idaho (Hietanen, 1963); Sl and 57 are for hornblende from analysis 2-5 is close to the composition of horn- Airola, Switzerland (Steiger, 1961). blendein sample465. AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS 33

Leake (1955a, b, 1968, 1971) has compiled from Emeryville, Adirondack Mountains, New York nearly 1500 analysesof calciferous and subcalci- (Engel and Engel, 1962) and two (S-1, $7) for ferous amphibolesfrom the literature and has con- hornblende in the Tremola series, Switzerland cluded that the content of Al'i is controlled by rock (Steiger,1961). Metamorphismin Emeryville was composition and pressure and that the maximum in the amphibolite facies and in medium pressures, possibleAlnr increasesas Alio increases.He has also whereasthe Tremola seriesrecrystallized at higher suggesteda nomenclaturebased on Si and Ca * pressuresas shown by the occurrenceof staurolite Na * K contentsin the half-unit cell. The analyses and kyanite in the pelitic layerssouth of St. Gotthard of the blue-greenhornblende in the Feather River (Hezner,1909; Hafner, 1958). The ratio of (Na * area fall within tschermakitichornblende, and those K)A/Alt' for the hornblendeanalysis S-1 is 1 :7, in- of the greenhornblende within magnesio-hornblende dicating a much higher contentof tschermakitemole- in his scheme.These names are not adopted here cule than is presentin the other hornblendeanalyses becauseof chemical features (Figs. 4 and 5 ) that shown in Figure 4. Frey (1969, p. 68) gives two bring the blue-greenhornblende closer to the parga- analysesof hornblende in kyanite-bearingrock at site type than to the tschermakitetype hornblende. Frodalera,south side of St. Gotthard massive.These Ernst ( 1968) usesthe namepargasite for NaCa2Mg+ and a partial reanalysisof one of them given by AlSi6Al2Oe2(OH),and the namehastingsite for the Leake (1971, p. 394) confirm the high content of correspondingferrous ferric end memberNaCa2Fez*a tschermakitein the hornblendesof this area. Fe"SioAlzOzr(OH),r. In the blue-greenhornblende Iron, ,and Titqnium Contents analyzedchemically (anal. 796), the extent of sub- stitution of Mg by Fe2. and that of octahedralAl by The iron-magnesiumratio and the con- Fe3* are both 1 :4. Thus the end member called tent of amphibolesdepend more closely on the Fe, pargasitein the graphs(Figs.4 and 5) actuallyis a Mg, and Ti contents of the host rocks than on the member that consists of 3/4 and, l/4 gradeof metamorphism. hastingsite,using the nomenclatureof 'Ernst (1968) The Mg/(Fe * Mg) ratio shows considerable and Phillips and Layton (1,964). variation but generally decreasesslightly with in- Robinson,Ross, and Jaffe (1971, Fig. 8) have creasingtetrahedral Al. All the actinolites are rich plotted A-site occupancyagainst tetrahedral Al for in magnesium. The amphibole pairs-the Al-rich a large number of hornblendeanalyses selected from hornblende and actinolite-in the metabasalt (Sp' the literature. The trend in their diagram is remark- ably similar to the trend in Figure 4 of this paper, t\ T schc?mokilr except for a much greater scatter of points. This scatter is probably due mainly to differencesin en- vironments of crystallizationbecause these analyses representa collectionof Ca-amphibolesfrom various geologic environments and diverse PT conditions. For comparison, two analysesof green hornblende from an anorthositearea in Idaho (Hietanen,1963, ;:l anal. 608, p. 43, and anal. of hornblendein sample \ 963, p. 33) are plotted in Figures4 and 5. These st hornblendescrystallized at the pressureof the triple \Oo point of kyanite, andalusite, and sillimanite 1-5 .10 kbar) and plot under (in Fig. 4) or above (in Fig. 5 ) the trend line for the hornblendesin the Feather River area, where the pressureof crystallizationwas lower (-{ kbar). The ratio of z4-siteoccupancy to tetrahedralAl for thesehornblendes is 1 :4. Similar AcI inolil. Ed!nil€ ratios ( I :4 and 1 :5 ) were calculated for two ana- Atlv gedrites (Hietanen, 1963). lyzed in this area 1959, Frc. 5. Plot of octahedral Al { Fe8* } Ti against tetra- Added for comparisonin Figure 4 are two analy- hedral Al per 23(O). Numbers refer to the same analyses ses (E-14 and E 415) for blue-greenhornblende as those in Figure 4' ANNA HIETANEN

551) are richer in iron than any other amphibole. percent TiO2, originally contained in sphene and This is due to the bulk composition of the meta- pyroxene.Metabasalt sample 551 has 2.03 percent , which has Fe/Mg = 1, whereas in all the TiOz, but most of it is in ilmenite.that was more other host rocks this ratio is lessthan l/2 (Hietanen, stablethan spheneand pyroxeneduring recrystalliza- 1973). Compton's (1958) hastingsite(C-4) with tion. only one Mg substitutingfor Fe2* is from an iron- rich layer in metabasalteast of Feather Falls. Influence ol Temperature To test the possibleinfluence of oxygen fugacity The composition of the aluminous hornblende is on the Me/(Me * Fe) ratio of hornblende, this influencedmore by the grade of metamorphismthan ratio was plotted againstFes-/(Fe2. * Fe3.) in the is the composition of actinolite. Comparison of host rocks (Fig. 6). Separatecurves were drawn by amphiboles from metagabbro (sample 796), col- visual inspectionfor amphiboleswith high, low, and lected 0.3 mile from the pluton, and those from medianAl contents.In eachof the amphibolegroups metabasalt(sample 551), 0.8 mile from the pluton, the Mg/(Mg * Fe) ratio increaseswith increasing revealsthat those closer to the pluton (sample796) degreeof oxidation of the host rocks. Bard (1,970) contain blue-greenhornblende that is richer in Alt' has found a similar relation in the Ca-amphiboles and alkalies than hornblendes from those farther from a comparablegeologic setting in Spain. from the pluton. Thin-section studies of scores of TiO2 content of the amphiboles is variable. It other specimensshow that generally near the con- does not increasewith the prograde metamorphism tacts of the plutons the hornblende is blue green, as has been found at higher gradesby other authors whereaselsewhere the amphibolesare green to pale (Binns, 1965b; Engel and Engel, 1962); rather it green. Analyzed green hornblendessuch as those in seemsto depend on the available TiOz content of metadioritesamples 465 and 532 containmuch less the host rocks. It is lowest in amphibolesfrom meta- aluminum and alkalies but have about the same gabbro 796 in which percentageof TiOz is only iron-magnesiumratio as the blue-greenhornblende 0.11 (Hietanen, 1973). It is highestin amphibolas in sample796 (l-3, Table 2b). Thus the Alt' and from the low grademeta-andesite (sample 463) and alkali contents of the hornblende increasewith in- metadiorite(sample 465) that have 0.75 and 0.54 creasinggrade of metamorphism,whereas the iron- magnesiumratio dependson the bulk composition of the host rock. fncreasein the alkali and Alt' contents of horn- blende with increasinggrade of metamorphismhas been reported by many earlier workers in different * parts of the world (e.g.,Deer,Howie, and Zussman, s 1963, pp. 304-305). Binns (1965a, b) found a Q progradeincrease in the content of alkalies,titanium, and tetrahedralaluminum in hornblendesfrom basic s hornfelsesat Tilbuster and in the Broken Hill dis- .s trict, New South Wales. However, in a similar con- q) tact zone at Moonbi, the content of Ali" in horn- \ + blende remained roughly constant. Cahill (nee Macara, 1968) describeda definiteincrease in Ail" s 12. r3.. and alkali contentsin hornblendesfrom basic horn- s felsesin southernNew South Wales.Bard (1970) has pointed out that an enrichment of alkalies and increasein Ti and Alto contentsin hornblendewith the increasingP and ? exists but that their extent varies. 7 2l+17s2+1Fe3. in hoslroc k The relation betweenthe tetrahedral Al contents of the amphibolepairs-actinolite and hornblende- FIo. 6. Plot of Mg/(Fe * Mg) in the amphiboles against FeB*/(Fd* * Fe*) in the analyzed host rocks. Numbers and the estimated temperature of recrystallization along abscissarefer to rock analysesin Hietanen (1973). in the Feather River area is shown graphicallyin AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS 35

Figure 7. The temperaturein the zone next to the contact (Fig. 1, sample 796, 546) of the plutons is estimatedat 600"C on the basisof absenceof silli- .g. .a 9.2 manite and coexistenceof andalusiteand cordierite lt r zJl in the pelitic layers.In the zoneO.6 to 1.2 miles from s 25 24 the contact (sample551 and 532), wherestaurolite \ occursiwith andalusitein the pelitic layers, tempera- t8 ture may have beenabout 500oC.For samples465, 27 22 21 463, and 464 that arc 3.2 miles from the contact, the temperatureof recrystallizationwas estimatedat o 02 04 06 otA,,r'o tz t4 16 350'C on the basisof mineral assemblagesalbite- AIIY actinolite-clinozoisite-epidote-chloriteand albite- FIc. 7. Estimated temperature of recrystallization of the chlorite-muscovite-biotitewithout garnet.Two curves amphiboles against tetrahedral Al per 23(O). Numbers connecting the actinolites on one hand and the refer to Table 2. Points (X) are for "igneous" hornblende aluminous amphiboleson the other were drawn by from Hietanen (1971). visual inspection. The points for the amphibole in sample 532 (23, 24, 25) fall in the "miscibility grains; discussed gap" betweenactinolites and hornblendesof medium grains or groups of exceptionsare from zoisiteand clinozoi- temperature.This quartz has only one amphi- below. Compositionranges with pistacite,Ca2Fe8*B bole and only one epidote mineral, suggestingthat site with little iron to as 26 molecularper- it crystallized in physical conditions that were dif- Sisor2(OH), contentsas much amounts of man- ferent from thoseof the other specimensin this zone. cent. Most analysesshow small .In the calculated It may be a later dike crystallized at igneous tem- ganese,titanium, and contentis only a little higher peraturesas shownby arows in Figure 7. The tetra- formula units, the the content hedral Al content in the "igneous" hornblendein this than the ideal three atoms, and Two analyses area (Hietanen,l97l) is plotted in Figure 7 (474, is somewhat lower than two atoms. (30 sample796 522) at temperaturesof the beginningof the melting and 31) of zoisitein metagabbro that replacescalcium. The varia- of quartz diorite and quartz in order to have some sodium is shown in Figure 8. indicate the upper limit of the possibleimmiscibility. tion of Al and total Fe as Fe3* linear trend as they The coexistenceof two amphiboles,actinolite and All analysesexcept 38 fall on a computedusing the two aluminoushornblende, has beenpreviously described should. Analysis 38 was the grain; the remainderof by Shido(1958) andby Shidoand Miyashiro (1959) highestiron counts from gives 36. Analysis38 is thus not from the Abukuma plateau, Japan, and from two the counts analysis but rather shows,together specimensof epidiorite in the Scottish Highlands. an independentanalysis of possiblevariation in According to their descriptions,the parallel inter- with analysis36, the extent individual grain. Analysis 37 growths and patches and rims of blue-greenhorn- iron content within an representsa homogeneous blende within and around actinolite are similar to from the same specimen the intergxowthsof actinolite and aluminous horn- blende in the Feather River area. Shido and Miya- shiro (1959) suggestedthat there may be a misci- bility gap betweenthe actinolite and hornblende at about 1.0 AF". The Allo content, however,is typical of the green hornblendein the Feather River area, and the miscibility gap, if. any, is a function of T \t, (Fig. 7). Also Klein (1969) suggestedthat coexist- ing amphibolesmay very likely representequilibrium pairs acrossa miscibility gap. Epidote Minerals 232t222 AI Electron-microprobeanalyses of epidote minerals FIc. 8. Plot of Fe3* against Al in the epidote minerals' (Table 3) represent relatively homogeneoussingle Numbers refer to Table 3. ANNA HIETANEN

grain; thus 36 and 37 indicatethe dominantcom- Ps 8 and Ps 28 at low temperatures'.Sterns attributed position. this to the presenceof strain in the mixed crystalsat The range of pistacite content in some individual temperaturesbelow 550"C. samplesis greater,and in others smaller,than would This can be further consideredin the light of the be expectedon the basis of the indices of refraction structure of epidote. According to Ito, Morimotq measuredon the mineral powders.Some of this dis- and Sadanag?i.(1954\, epidoteconsists of chainsof crepancy results from averagingthe counts in the (Al,Fe3.)Oo and (Al,Fe3-)Or (OH)B octahedrapar- microprobe analyses.For instance,analyses of epi- allel to b axis and bound togetherby Al,Fe3*,Ca, dote in sample465 (Table 3, anal.41 and 42) show and Si atoms.Miyashiro and Seki (1958) havesug- 18 to 19 percentiron end member (pistacite),but gestedthat the AIO and AIOH sites in the chains the highestmicroprobe counts for iron in this sample are filled mainly with Al atoms whereas the Fe8* correspond to 11 percent FeO, equivalent to 22 atoms are located betweenthe chains. In the com- molecular p€rcent pistacite. In contrast, the lowest mon epidoteCazAl:FeSigolr(OH) all the octahedral counts yield 6 percentFeO and thus about 12 mole- sites betweenthe chains would be oceupiedby Fe8. cular percent pistacite.Both clinozoisiteand epidote to give a "stable" or unstrainedstructure. were identified optically in mineral powder (Table According to the recent refinement of epidote I ). The compositionbased on the high iron countsis structure by Dollase (L971), the edge-sharingAl in agreementwith the highest indices of refraction octahedraform two types of chains: a single chain measured,but the lowest counts correspond to a of M(2) octahedraand a multiple zig-zagchain of higher content of pistacite than is indicated by the centralM(l) and peripheralM(3).These chains lowest indices of refraction. These relations are in are cross-linkedby SiOe and SizOzgroups and Ca is general agreementwith the exsolution textures seen in large cavities betweenthe chains and links. The in immersedgrains (Fig. 3). Further discrepancies single chain octahedraM(2) contain only Al atoms result from the inhomogeneityof the host rocks; the and the substitutionin the multiple M(I)-M(3) probe analyseswere made in a single polished thin chain is nonuniform, the M(3) sites containinga section, which representsa limited sampling of the larger fraction of Fe3* than the M(1) site. In the specimenfrom which mineral concentratefor immer- low-iron epidote,all Fe3*is in M(3) sites. sion work was prepared.For example,in sample532 All the analyzedepido,tes (Table 3) have con- high index domains (t = 1.754, Ps 22) were de- siderably more Al than would be necessaryto fill termined optically in some grains of bulk composi- the M(2) andM(l) sitesin chains.If it is assumed tion Ps 14 (y - 1.742). The electron-microprobe that all Fe3*is in M(3) sites, about 3/4 of these data for epidote in this sample (Table 3, anal. 43- sites are filled with Fe3* in epidotes from the low 45), however, indicate that the analyzed,glains are grade zone and only about l/3 of these sites in the homogeneousand contain 16 to 17 percentpistacite. coexistingclinozoisite contain Fe3*.In epidotesfrom Only two of the three epidote minerals in the meta- the high gradezone (Sp. 551, 546), about 1,/2 ot- gabbro sample796 were presentin the polishedthin the M(3) sites are occupiedby Fe3. atoms.These section; the third is an epidote optically similar to differencesin the extent of substitution of Fes* for epidote in samples 532 and 551. Clinozoisite in Al indicate that certain substitutionsmay be more sample796 is inhomogeneous;analyses 31 and 34, stable than someothers at a given temperature. representingdiscrete crystals, reflect the observed Myer (1966) has reportedcoexistence of ferrian limits in Ps content.Zoisite in sample796 occurs as zoisite and epidote from calc-silicaterocks in Nor- veinlets and pods and formed later than the other way. Compositionsgiven by him indicate0.11 Fe'- epidoteminerals. atoms per formula unit in ferrian zoisite (Ca2Al2.st Thus the results of electronmicroprobe work and Fe3"e.11Si3O1zOH)and 0.37 to 0.63 FeS. atoms in the indices of refraction together with exsolution coexistingepidote Caz (Al,Fe3*) 3SisO12OH. textures suggestthat two distinct phases, an iron- The stability relations between the epidote min- poor clinozoisite and iron-rich epidote, are present eralshave beenexperimentally investigated by Nitsch in all samplesexcept in sample 532. A similar rela- and Winkler (1965) and by Holdaway (1966). tionshiphas been discussedby Strens(1965) from According to Holdaway (L966),i, clinozoisite and two localities where a "miscibility gap" in the cline quafiz react to form anorthite plus grossularite'at zoisite-epidoteseries occurred between compositions about 660'C at pressureof 4 kbar. Nearly similar AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS resultswere obtained by Nitschand Winkler (1965), iron-rich epidote crystallizeswith magnesianactino- who further showed that at pressureshigher than lite at low temperaturesof the green facies. 3.5 kbar the stability field of zoisite extends to a Clinozoisite and epidote are common with horn- higher temperaturethan that of epidote. This rela- blende at intermediatetemperatures whereas zoisite tion may explain the occurrenceof zoisite near the is the only epidotemineral at very high temperatures contact of Granite Basin pluton (sample796). At and pressures.Since the major changesin composi- elevatedtemperatures near the pluton, zoisitecrystal- tion in both mineral groups are tied to the changes lized in addition to clinozoisite. in Al and Fe contents, the percentagesof Alrot and FeO for each mineral in samplesfrom the high Chlorite and Muscovite gradezone were plotted in the samegraph (Fig. 9). The compositionof chlorite (Table 4) is more Individual analysispoints for eachmineral were con- uniform than that of the amphiboles and epidote nected by a line, which shows the range of minerals.Tetrahedral Al is close to 2.5, and octa- Al2O3 and FeO contents. hedral Al is 2.55 to 2.73 in all analyzedchlorites. Mg/(Fe + Mg) ratio is 0.64 to 0.83, and the Blue-green chlorite in metagabbro sample 796 is richest in Acl inolite hornblende aluminumand magnesium. Some muscoviteoccurs with chlorite in metagab- -Zoisite bro sample796. Electron-microprobeanalysis (Table Clinozoisite 4) showsthat it is a po'tassium-aluminummica with only about 1 percent MgO and little sodium and iron. Its phengite content is about 7 molecular per- cent.

Plagioclase C,) Plagioclaseis anorthite in metagabbrosample 796 S and bytownite (Anzr) in the metagabbrosample $ 546 (Tables 1 and 5). In all the other specimens, s it is albite. The presenceof irregular twinning lamel- lae with relict granular textures in specimen 796 s ()C) suggeststhat the anorthite crystallized from zoisite q) at elevatedtemperatures. This unusual sequenceof \ crystallizationis supportedby the extreme composi- s tion of anorthitic plagioclase.The bulk composition .S. epidoie of the host rocks (Hietanen,1973) indicatesexten- s sive changesin the chemistryof all metavolcanicand meta-igneousrocks during their recrystallization.All are impoverishedin potassium,and most also in - sodium. As a result, petrochemicalnorm calculations clino yield a high anorthite content for plagioclase.An- zoisile orthitic plagioclasecrystallized in equilibrium with epidote from this compositionat temperaturesclose 4 P_. - t2 16 20 to 600'C. ActinolrlellBlue green hornblende Weighlpercenl in omphibole Distribution of Fe, Mg, and Al Among Frc. 9. Distribution of ALO' and FeO in the analyzed Homblende, Chlorite, and Epidote amphiboles and epidote minerals in the high grade zone. The occurrence of amphibole pairs and two to The range of the percentage of these oxides in minerals is three epidote minerals together raises the question shown by solid lines, and the coexisting minerals of the speciesin sample 796 are connected with broken lines. distribution of elementsamong these minerals. same of Epidote is from sample 546; optically similar epidote co- It is evidentthat the distributionchanges with the exists with the other minerals in sample 796. Line OA increase in metamorphic temperatures.Normally, shows the equal distribution' 38 ANNA HIETANEN

In the upper h'alfof Figure 9, the abscissashows sample551. Using the Fe/Al ratio of this epidote the range in Al2O3 percentagesin amphiboles,and (0.27, Sp. 551), the distributioncoefficient K1(Fe/ the ordinate showsthe rangein Al2O3percentages in Al) for epidote/clinozoisitein the highestgrade zone epidotes.For easierobservance, the actinolite-horn- is calculatedas Kp : 2.9, thus somewhatlarger than blendepairs and the zoisite-clinozoisitepa,irs in sam- the corresponding value in the low grade zone ple 796 were connectedwith broken lines. A gap (Ko = 2.6) and much larger than Kp = 1.6 in in the AlzOa contentbetween the actinolite and blue- sample551. This reversalof trend is due to a low greenhornblende at the highestgrade is wide in spite content of iron in clinozoisite(Sp. 796) that has of the overlap of the A12O3percentage in this actino- rims of zoisite dustedby iron oxide. Probably a part lite and in the gteen hornblende. In contrast, the of the iron originally present in this clinozoisitewas gap in the FeO content (lower part in Fig. 9) is exsolved upon the beginning of its transformation small; all the actinolites,however, have a lower FeO to zoisite. contentthan the hornblende.Most samplesfrom the All chlorites are rich in aluminum, but the mag- low grade zone contain actinolite * green horn- nesium/iron ratio rangesfrom 1.7 to 4.4, the chlo- blende,but in samples463, 465, and 532 only the rite in sample 796 being richest in magnesium.Dis- medium-Al hornblende was present in the polished tribution coefficients are different for amphiboles . with different Al contents, as shown in Figure 10. Only a small gap existsin the AlsOs and FeO For the hornblendesof median Al content (464h, contentsbetween the coexistingzoisite and clinozoi- 463h, 465h), Kp(MglFe) chlorite/hornblendeis '1.2, site, whereasepidote contains much lessAlzOa and generallybetween 0.7 and and there is no ob- more FeO than the coexistingclinozoisite. The gap servablechange with the temperatureof metamor- betweenthe clinozoisiteand epidotein the low grade phism (comp. 546h). However, the chlorite in the zone is distinct in samples 464 and 463, but zonenext to the pluton (in metagabbroSp. 796) is in sample 465 only an richer in magnesiumand containsless iron than any occurs. This intermediatecomposition, however, in- other chlorite.Kp(MglFe) (chlorite/hornblende)in cludesinhomogeneous counts, which, if separated, this rock is 1.9, more than twice that for the chlorite would give compositionsimilar to the clinozoisiteand and the averagemedian-Al hornblende in the low epidotein the other samples. graderocks. The Fe/Al ratio is large in the low-Al actinolites and decreasesrapidly with increasingAl content.In Summary and Conclusions actinolitichornblende this ratio is 1.6,in greenhorn- In the outer contact aureoles of the Cretaceous blende 1.1 to 1.3, and in the blue-greenhornblende plutons, Northern Sierra Nevada,where the mineral at the highestgrade it is 0.5. In the blue-greenhorn- assemblagesindicate PT conditions at the border of blendein metabasalt(sample 551), which is rich in the gteenschistand epidote amphibole facies, green Fe, the Fe/Al is of the samemagnitude as in the hornblende occurs with actinolite, chlorite, clino- green hornblendeeven if this sample comesfrom a zoisite, epidote, and albite. In the zone next to the higher grade zone. This shows that FelAl ratio plutons, where the occurrence of andalusite with generallydecreases with increasingtemperature but cordierite and prograde pseudomorphsof muscovite is alsoinfluenced by the Fe/Al ratio of the hostrock. after staurolite in interlayeredpelitic rocks indicates Comparisonof the averageFe/Al ratiosin the co- PT conditions at the higher limit of the epidote am- existing epidote and clinozoisitefrom the low-grade phibolitefacies (T - 650oC at P - 4 kbar), horn- zone (0.33 and 0.13 respectively;Sp. 463 and 464) blende is blue green, and plagioclaseis rich in the with thosefrom the high-gradezone (0.28 and 0.2, anorthite molecule.Zoisite occurs with other epidote Sp. 551 and 546) suggeststhat the differencebe- minerals. tween the Fe/Al values o'f the coexistingepidote The electron-microprobeanalyses show that the mineralsis smallerin the high-gradezone. Epidote alkali and aluminum contentsof the hornblendein- from the highestgrade zone (sample796) was not creasewith increasinggrade of metamorphism,but analyzedbecause none was found in the polished their ratio remains constant.The generalizedstruc- thin section.The indicesof refractionmeasured in tural formula of the greenhornblende is (Na,K)s.3 the mineral powder, however,suggest that epidote Ca2(Mg,Fe".)a.a (Al,FeS-,Ti ) 0.6AlSi?O2, ( OH ), and in this rock is very similar in compositionto that in that of the blue-greenhornblende is (Na,K)o uCa, AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS

(Mg,Fe)s.e(Al,Fe3*) 1.2Alr.zSio.aOzz(OH)2. The ratio though it is highest in the zone next to the plutons. of (Na + K)A/Ali" remainsclose to 0.3. The end It seemsreasonable to assumethat pressureremained member in this actinolite-hornblendeseries is the approximatelyat the samelevel during the metamor- hornblende ( Na,K) o.oCaz( Mg,Fe ) 3.6( Al,Fes.) 1.aAl2 phism and that the changesdescribed are due to the Si6Op2(OH)2,which is derivedfrom the actinolite by elevatedtemperatures near the plutons. coupled substitutions---edenitic(NaAli' for two !A References Sit") and tschermakitic(AlriAliY for Mg'rSit") at the ratio of 3:7. Beno, I. P. (1970) Composition of hornblendes formed Hercynian progressive metamorphism of the Mg/(Mg Fe) ratio in amphibolesdepends during the The * Aracene Metamorphic Belt (SW Spain). Contrib. Mineral. on the bulk compositionand the degreeof oxidation Pe trology, 28, I 17-13 4. of the host rocks. The Ti content of hornblendere- BEEsoN, M. H. (1967) A computer program for processing flects the amount of available Ti in the host rocks. electron microprobe data. U.S. Geol. Suro. Open-File The coexistenceof two Ca amphibolesimplies the Rep., lO pp. R. A. (1955a) Hornblendes from some basic horn- existence of a miscibility gap, if the amphiboles BnrNs, felses in the New England region, New South Wales. crystallizedin equilibrium. The compositionof horn- Mineral. Mag. 34, 52-65. blende in the border zone of the plutonic rocks pre (1965b) The mineralogy of metamorphosed basic vides the upper limit of the possibleimmiscibility. rocks from the Willyama Complex, Broken Hill district' Two epidote minerals-clinozoisite and epidote New South Wales. Part l. Hornblendes. Mineral, Mag. 35,306-326. with pistacitecontent from 10 to 26 percent---coexist Cenrr.r-(nee Meceu), B. I. (1968) The use of amphiboles with actinolite, hornblende,chlorite, and plagioclase to illustrate trends in contact metamorphism. IMA sth in all zones.Zoisite with anorthitic plagioclasecrys- Gen. Meet. Cambridge 1966, 189-203. tallizndat the highestgrade. Elsewhere plagioclase is Corurrow, R. R. (1958) Significance of amphibole para- Amer. albite. The miscibility gap of epidote and clinozoisite genesis in the Bidwell Bar region, California. . .Mineral. 43, 89A-907. at temperaturesbelow 600'C is suggestedby com- DBrn, W. A., R. A. Howre, lNn J. ZussueN (1963) Roclc positionsof the coexistingmineral pairs. Forming Minerals, Vol. 2. Chain Silicates' Longmans, The Fe/Al ratio in epidote decreasesslightly with Green and Co., Ltd., London, 379 PP. increasein the grade of metamorphism,whereas in DEsBoRoucH,G. A., eNo R. H. HpmBr (1971) Improved l. hornblendethe decreasein this ratio is much greater; electron microprobe analysis at low operating voltage' niagnesium and aluminum. Amer. Mineral. 56, in the high grade hornblende is one half of Silicon, Fe/Al 2129-2135. that in the low gradehornblende. There is no regular DoLLAsE, W. A. (1971) Refinement of the crystal shuctures change in Kn(Fe/Mg) for chlorite/hornblende, al- of epidote, , and hancockite. Amer' Mineral. 56, 477464. ENcu., A. E. J., ,lr.to C. G. ENcnr- (1962) Hornblendes formed during progressive metamorphism of ' KO=1.9 northwest Adirondack Mountains, New York, Geol. Soc' ( Amer. Bull. 73, 1499-1514. '-l7z:' EnNsr, W. G. (1968) Amphiboles.Yol. l,Minerals, Rocks, Springer'Verlag, Inc., New York' qr and Inorganic Materials. 125 pP. s KD= 0.5 FREv, MARTTN(1969) Die Metamorphose des Keupers vom ss Tafeljura bis zum Lukmanier-Gebiet. Beit. GeoI' Karte Schweizrl37, l-160. .s H' 465h 7 Golorcn, S. S., C. O. IlolrvrBr.rs, N. H. Suun, lNo D. .lr mineral \ .4aen 546a ANoeRsox (1967) Analyses of silicate rock and 76"ti:--- ;.2 standards.Can. l. $ci.4,747-755. s- HAFNBR, SterlN (1958) Petrographie des siidwestlichen Gotthardmassivs (zwischen St.-Gotthardpassund Nufenen- pass), Schweiz. Mineral. Petrogr. Mitt', 3E, 255-362. HrzNnn, L. (1909) Petrographische Untersuchung der kristallinen Schiefer auf der Siidseite des St' Gotthard 123456 (Tremolaserie). Neues lahrb' Mineral. Geol. Pal-RlB27, Mg/Fe in omphibote 157-218. Frc. 10. Distribution coefficient for average of MglFe HrcuNnN, ANNI (1951) Metamorphic and igneous rocks in coexisting chlorite and actinolite (a) and chlorite and of the Merrimac area, Plumas National Forest, California' hornblende (h). The numbers refer to the host rock samples. Geol. Soc. Amer. Bull. 62' 555-608. 40 ANNA HIETANEN

(1959) Kyanite-garnet gedritite near Orofino, Idaho. (1971) On aluminous and edenitic hornblendes. Amer. Mineral. 44, 539-564. Mineral. Mag. 3E, 389407. (1963) and associated rocks in the MryAsHIRo,A., eNo Y. Serr, (1958) Enlargement of the Boehls Butte quadrangle and vicinity, ldaho. U.S. Geol. composition field of epidote and and rising Suro. Prof. Pap.344-8,78 pp. temperature.Amer. I. Sci,256, 423-430. (1967) On the facies seriesin various types of met- Mvnn, G. H. (1966) New data on zoisite and epidote. amorphism. I. Geol. I75, 187-214. Amer. l. Sci. 264.364-385. ( 1971) Distribution of elements in biotite-hornblende Nrrscn, K. H., eNo H. G. F. WrNKun, (1965) Bildungs- pairs qnd in an orthopyroxene-clinopyroxene pair from bedingungen von Epidot und Orthozoisit. Beit. Mineral. zoned plutons, northern Sierra Nevada, California. Petrology,ll, 470486. Contrib. Mineral. Petrology, 30, 16l-176. PAprKE,J. f., MercolM Ross, eNo JoeN R. CLARK(1969) (1973) Geology of the Pulga and Bucks Lake quad- Crystal-chemical characterization of clino-amphiboles rangles, Butte and Plumas Counties, California. U.S. Geol. based on five new structure refinements. Minerol. Soc. Suro. Prof. Pap.73!. Amer. Spec.Pap.2, 117-136. Holoewev, M. J. (1966) Hydrothermal stability of clino- Pnrrurs, R., lNo W. LlvroN (1964) The calciferous and zoisite plus qtartz. Amer. I. Sci.2&r 643-667, alkali amphib oles. Mineral. Mag. 33, 1097-1 109. (1971) lro, T., N. Monruoro, eNo R. Seo.lNlce (1954) On the RoBrNsoN,P., M. Ross, ANDH. W. Jerrr Composi- structure of epidote. Acta Crystallogr. 7, 53-59. tion of the - series, comparisons of gedrite and hornblende, and the anthophyllite-gedrite KLEIN, CoRNerls, JR. (1969) Two-amphibole assemblages solvtrs. Amer. Mineral. 56. 1005-1041. in the system actinolite-hornblende-. lm. Mineral. 54, 212-237. Snroo, F. (1958) Plutonic and metamorphic rocks of the Nakoso and Iritono districts in the central Abukuma Lnexn, B. E. (1965a) The relationship between composition Plateau. l. Fac. Sci, Unio. Tokyo, Sec. 2, ll, Pt. 2, of calciferous amphibole and grade of metamorphism. In l3t-217. Controls of Metamorphism, W. S. Pitcher and G. W. -, lNo A. Myesrrno (1959) Hornblendes of basic Flinn, Eds, John Wiley and Sons, Inc., New York, pp. rocks. .1.Fac. Unio. Tokyo, Sec. 2,12, 299-3t8. metamorphic Sci., Pt. 1,85-102. (1965b) The relationship between tetrahedral alu- Srrrcrn, Ruoorr (1961) Die hornblende der Tremolaseries. minum and the maximum possible octahedral aluminum Teil 1. Chemismus and Dichte der Hornblenden. Schweiz. in natural calciferous and subcalciferous amphiboles. Mineral. Petrogr. Mitt. 41, 127-156. Amer. Minerar. 50, 843-851. Sttnxs, R. G. J. (1965) Stability relations of the Al-Fe (1968) A catalog of analysed calciferous and sub- epidotes. Mineral Mag. 35, 464475. calciferous amphiboles together with their nomenclature and associatedminerals. Geol. Soc. Amer. Spec. Pap. 98, Manuscript receitsed,Iune 26, 1972; accepted 2LO pp. lor publication, August 29, 1973.