Amphibole Pairs, Epidote Minerals, Chlorite, and ... · minerals coexist with chlorite and...

American Mineralogist, Volume 59, pages 2240, 1974

Amphibole Pairs, Epidote Minerals, Chlorite, and Plagioclase inMetamorphic Rocks, Northern Sierra Nevada, California'

ANNI HtereNBN

U. S. Geological Suruey, Menlo Park, CalilorniaElectron microprobe analyses by G. K. CzannNsxB

Abstract

Two kinds of amphiboles-actinolite and aluminous Ca-amphibole-and two to three epidote

minerals coexist with chlorite and plagioclase in the meta-volcanic and meta-igneous rocks in

the contact aureoles of the Cretaceous plutons in the Feather River area, northern SierraNevada. Small prisms and rims of actinolite occur with green hornblende outside the l-mile-

wide contact aureoles of the plutons. In the zone next to the plutons, blue-green hornblende

forms rims around large grains of actinolite and also forms small prisms that are included inplagioclase. Electron-microprobe analyses show that the alkali and Al content of the amphi-

boles increases with an increase in the grade of metamorphism, the ratio of NafK to tetra-

hedral Al remaining close to 1:3. The ratio between two coupled substitutions, edenitic(NaAAlt ' for nASi'") and tschermakit ic (Al"t Al" ' for Mg'r5;t"r, remains 3:7 and leads to the

composit ion NaooCa.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)a the ex-

treme member of this actinolite-hornblende series. The Mg/Fe ratio changes with the bulk com-position and with the degree of oxidation of the host rocks. Epidote and clinozoisite coexist in

all zones as separate grains. In addition, many grains of clinozoisite include high index domains,lamellae, and stringers of epidote. In the highest grade zone next to the plutons, zoisite occurswith epidote, clinozoisite, and anorthitic plagioclase. Farther from the plutons, the plagioclase isalbite. There is no regular change in the distribution coefficient Kn(Mg/Fe) for chlorite/amphi-bole although it is highest in the zone next to the plutons.

Introduction

The minerals studied occur as major constituentsin Paleozoic(?) metavolcanic and related meta-morphosed intrusive rocks around Cretaceous plu-tons in the southwestern part of the Bucks Lakequadrangle, northern Sierra Nevada. The generalgeology of this quadrangle and the petrography andchemical composition of the host rocks have beendescribed in another report (Hietanen, 1973);there-fore only a short summary of the geologic settingand some additional features are given in the follow-ing description.

Two metamorphic events can be recognized in thehost rocks near the plutons. The earlier regionalmetamorphism was to the greenschist facies and wasmodified by a later higher-temperature contact meta-morphism that afiected only the rocks in a narrow

'Publication authorized by the Director, U. S. GeologicalSurvey.

zone around the plutons. In the pelitic layers, the

common occurrence of andalusite, cordierite, and

staurolite in the outer contact aureole and andalusite

and cordierite in the inner one indicates PZ condi-tions of the epidote amphibolite facies (Hietanen,

1967). Eilsewhere, mineral assemblages such asmuscovite-biotite-chlorite in pelitic layers and albite-epidote-actinolite-hornblende indicate the upper limit

of the greenschist facies.In the metavolcanic rocks and related intrusive

rocks, the increase in the grade of metamorphism is

indicated by the change of the color of hornblende.In the low-grade zone, the pleochroism parallel to yis light green to green but becomes bluish green

towards the plutons. The most interesting featureis that two amphiboles-actinolite and aluminous

hornblende-and two to three epidote minerals co-

exist in all zones, as shown by indices of refractionmeasured in immersion liquids. It was thereforehighly desirable to determine the composition ofthese 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- Metagabbro sample 796, O.3 mile south of theate and basic metavolcanic rocks and related intru- Granite Basin pluton, represents the highest meta-sive rocks, metadiorites, and metagabbros. morphic grade. It is a coarse-grained dark rock in

Samples of meta-andesite and metadacite (sam- which black hornblende contrasts with white an-ples 463 and464) are from successive layers in the orthite (Ansr). Thin sections show that most of theFranklin Canyon Formation, 3.2 miles east-southeast large amphibole grains are pale-green to colorlessof the Granite Basin pluton (Fig. 1). Sample 465 is actinolite (yAc - 20o) mottled with blue-greenfrom a small body of metadiorite a few meters wide amphibole (Fig. 2). All large grains are rimmed byjust south of the locality of metadacite sample 464. blue-green amphibole that is strongly pleochroic inBecause of their proximity, these three rocks most blue green (y), green (F), and light green (o) andlikely recrystallized at the same PT conditions at the has yAc = 22o. In addition, small euhedral prismsborder of the greenschist and epidote amphibolite of blue-green amphibole are included in anorthiticfacies. The four other samples (Sp. 796, 546, 551, plagioclase. Most plagioclase grains are granulated,532) were collected from 0.3 to 0.8 mile outside individual grains averaging 0.01 mm in size, butthe contact of the Granite Basin pluton. The rocks some are large and have mottled extinction and ir-in this contact zorre are coarser grained and show a regular twinning lamellae. Some of these lamellaehigher grade of metamorphism than those farther consist of rows of tiny small grains; others have wavyfrom the contact. borders similar to those described in an earlier report

XPLANATIOI{

Ftc. L Maior geologic units and locations of the specimens studied (796, 546, 463, 532 arrd464) in the Feather River area.

ANNA HIETANEN

-j-.r,aar.........-

Frc. 2. Amphibole grains consisting of hornblende(shaded) and actinolite (unshaded). Black is magnetite. fnthe high grade zone (796) actinolite has rims and patchesof blue green hornblende, and small prisms of hornblendeare included in anorthitic plagioclase. In the low gradezone (465) large grains consist either of actinolite withpatches of green hornblende, or of hornblende that includesand is rimmed by actinolite. Small prisms of actinolite areincluded in albite.

(Hietanen, 1951, Fig. 2I) and are thought to havecrystallized from zoisite at elevated temperaturesnear the plutons. All three epidote minerals, epidote,clinozoisite, and iron-poor zoisite, are present.Epidote and clinozoisite are in individual grains or inclusters, which are surrounded by a mass of smallgrains of zoisite dusted with tiny inclusions (Fig. 3).Some clinozoisite grains contain patches of epidote;some others have twinning lamellae. All epidoteminerals contain inclusions of leucoxene, and areaccompanied by some chlorite and quartz. A few tinygrains of ilmenite-magnetite occur as an accessory.

Metagabbro sample 546, O.4 mile east of theGranite Basin pluton, is mineralogically and textur-ally similar to the metagabbro of locality 796, exceptfor the absence of zoisite. a lower anorthite content(Anto) in plagioclase, and less actinolite. A fewblue-green hornblende prisms have actinolite ends.

Metabasalt sample 551, 0.8 mile east-northeast ofthe Granite Basin pluton, is a dark-green to black,weakly foliated rock that consists mainly of smallprisms of hornblende, opaque minerals, and albiticplagioclase. In thin sections, hornblende prisms (0.2to 1 mm long) are pleochroic in light blue-green andlight green and have colorless actinolite ends or, morerarely, are partly rimmed by actinolite. They are sub-parallel to the foliation or form radiating clusters.Small very light green to colorless needles of ac-tinolite occur next to the sreen hornblende and are

included in albite. Albite is in granulated grainsamong the hornblende prisms and includes smallgrains and clusters of epidote and some clinozoisite.Numerous scattered anhedral to subhedral grains ofilmenite (about 0.02 mm long) are included in horn-blende; some larger ore grains are clustered parallelto the foliation. Some leucoxene is included in horn-blende and epidote minerals.

Meta-andesite sample 463, 3.2 miles east-south-east of the Granite Basin pluton, is a fine-grainedgray-green dense rock that is either massive orweakly foliated. Major constituents are light-greento pale-green hornblende, colorless actinolite, epi-dote, clinozoisite, and albite. Small amounts ofchlorite, leucoxene, magnetite, and pyrrhotite areubiquitous. The minerals are clustered; aggregatesconsisting of amphiboles and some chlorite andepidote minerals probably were originally pyroxenephenocrysts. In the massive rock, green hornblendeoccurs in anhedral to subhedral grains, 0.01 mm to1 mm long. Many grains have pale-green to colorlessactinolitic ends and rims; all contain inclusions ofleucoxene. Actinolite also occurs as needles in albite.Albite is in small (0.1 to 0.3 mm) grains ernbeddedin a mixture of small anhedral grains of epidote andclinozoisite that is clouded by leucoxene. In somesmall patches, however, euhedral crystals of epidoteminerals free of inclusions are included in albite orin a mixture of albite, fine-grained epidote, andleucoxene. Chlorite is in clusters of small flakes nextto the amphiboles in the massive rock and withepidote minerals along the foliated shear zones. Theopaque minerals, pyrrhotite and magnetite, are insubhedral grains, 0.01 to 0.5 mm in size.

Metadiorite sample 465, which is considered tobe an intrusive equivalent of the meta-andesite andmetadacite, is an equigranular rock in which dark-green hornblende crystals contrast with the white to

Ft\

OS mm

6nL ' l

l c L tt /

464

7i;

@,b.a

ep.t-f-A'Y ' /

Frc. 3. Textural relations between epidote minerals fromthe high grade zone (795) and the low grade zone (464).

ep - epidote, cz = clinozoisite, zo = zoisite. Domains,lamellae, and stringers of epidote in clinozoisite (far right)are textures seen in immersed fragments. The other grains

are sketched from thin sections.


pale green mixture of albite and epidote minerals.Amphibole is in subhedral to euhedral prisms, 0.2 to1 mm long, that are pleochroic in light to mediumgreen; many prisms have pale green to colorless spotsand ends (Fig. 2). Albite and epidote minerals formclusters that have outlines of former plagioclase, orthey occur as small interstitial grains between theseclusters and the hornblende prisms. Some of theepidote minerals are in large (0.5 mm long) crystals.Quartz is in small (0.01 to 0.5 mm long) grains orgroups of grains among the other minerals. In places,clusters of very pale green chlorite flakes with brown-ish gray interference color occur between the amphi-bole and epidote minerals. Sphene is in anhedralgrains adjacent to epidole or hornblende. Most of theoriginal magnetite is altered to hematite.

In the metadiorite sample 532,1mile south of theGranite Basin pluton, anhedral grains (0.5 to 1 mmlong) of green amphibole are embedded in a mix-ture consisting of albite and epidote. The amphiboleis pleochroic in light bluish green to pale green andincludes leucoxene. Albite grains are subhedral, 0.2to 0.5 mm long, and include numerous small sub-hedral to anhedral crystals of epidote, some of whichare clustered. A few patches, 0.5 mm long, consistingof tiny flakes of chlorite, occur next to amphibolegrains. Magnetite is partly altered to hematite, andsphene to leucoxene.

Sample 532 is from the southeastern end of anelongate body that was emplaced along a fault zone(Camel Peak fault in Fig. 1). The major part of thebody consists of light-colored porphyritic meta-trondhjemite that contains about 35 percent quartz,25 percent albite, 18 percent epidote and clinozoisite,15 percent hornblende, and 7 percent chlorite. Smallgrains of ilmenite-magnetite are enveloped by sphene.Hornblende prisms have pleochroism 7 = hght blue-green, B = light green, d - pale green. Only a fewsmall prisms of actinolite occur with green horn-blende. Most of the clinozoisite is in long prismsamong round grains of quartz that filI the fracturesin the rock. Some slender needles, however, are in-cluded in albite. Thus metatrondhjemite in the mainpart of this metamorphosed intrusive body has twoamphiboles and two epidote minerals, as do theother specimens studied. The occurrence of only agreen hornblende and an epidote in the metadiorite(Sp. 532) indicates physical conditions differentfrom those elsewhere during the recrystallization ofthis rock; perhaps this metadiorite is a later dike-likebody in which epidote is late magmatic, comparable

with the epidote in the Jurassic plutons. It is alsopossible that there is a submicroscopic intergrowthof two phases in each mineral.

Metadacite sample 464 is a light-grayish-greeninhomogeneous slightly foliated rock, in which darkpatches, I to 20 mm long and consisting of chlorite,amphibole, clinozoisite, and epidote, are in thematrix of albite, quartz, chlorite, and epidote min-erals. Amphibole is in small colorless to pale-greenprisms that have ragged ends. Chlorite, the majordark constituent, is colorless, has a gray to brownishgray interference color and negative elongation, andincludes tiny grains of leucoxene along its cleavageplanes. Albite is in clusters of equant, subhedralgrains that include epidote minerals and some smallprisms of amphibole. Clinozoisite, the major epidotemineral, is clustered either with chlorite or withalbite. Some clusters show the outlines of formerpyroxene or former plagioclase. Epidote occurs eitheras individual grains or with clinozoisite (Fig. 3).The opaque minerals, pyrrhotite and magnetite, arein anhedral grains 0.2 and 0.5 mm in size. Albite istwinned and includes epidote minerals, chlorite, andsome muscovite.

All these rocks are exceptionally poor in potas-sium (Hietanen, 1973, Table 1); KzO content rangesfrom 0.03 to 0.21, percent.

The question about the possibility of equilibriumbetween the coexisting amphiboles and epidote min-erals has to be considered in the light of thermal his-tory of the host rocks. There were two periods ofmetamorphism: (1) low-grade regional metamor-phism during which albite, epidote, actinolite, chlo-rite, and quartz crystallized from plagioclase, pyrox-ene, "igneous" hornblende, and biotite with orwithout quartz (the recrystallization may have notbeen complete); (2) later contact metamorphism athigher temperatures gave rise to crystallization ofhornblende, clinozoisite, and zoisite at temperatureswhere epidote, actinolite, and chlorite were stillstable. The absence of relict primary igneous min-erals and chemical changes in bulk composition in-dicate a thorough reconstitution of rocks.

Textural relations in the high-grade zone (Sp.796) suggest that the rims of blue-green hornblendearound actinolite may have prevented further re-action between the actinolite cores and the plagioclase in the matrix. However, the occurrence of blue-green hornblende in patches in the centers ofactinolite with its absence in parts of the rims refutesthis concept and shows that a low rate of diffusion

26 ANNA HIETANEN

of Al and Na was not the reason for preservation ofactinolite cores. The high anorthite content of plagio-clase (Ane3) suggests that lack of available Na wasone of the determining factors, but not the only onebecause the composition of blue-green hornblende *actinolite could give green hornblende with less Na.The occurrence of blue-green hornblende only in thehigh grade zone shows that it crystallized at elevatedtemperatures from bulk cornpositions similar tothose that in the low grade zone produced greenhornblende, albite, and actinolite. In parts of themetagabbro, as in sample 546, very little actinoliteremains and plagioclase is Ant6. This sample, col-lected a little farther from the contact of the plutonthan sample 796, is thus from a somewhat lowertemperature zone. Although considerable amount ofsodium remains in the plagioclase, a few blue-greenhornblende prisms in sample 546 have actinoliteends. The actinolite ends and rims are more commonand remnants of the cores more rare farther fromthe pluton (Sp. 551). These ends and rims crys-tallized. last, probably after the maximum metamor-phic temperature was passed.

The textural relations between epidote and clino-zoisite (Fig. 3) suggest that they coexist in equili-brium. Clinozoisite is in anhedral or in euhedral tosubhedral crystals next to or among the anhedral epi-dote grains. Lamellae, stringers, and domains of epi-dote in clinozoisite resemble textures due to exsolu-tion. Zoisite occurs only in the highest grade zone; itrims grains and clusters of epidote and clinozoisiteand is thus later. The abundant dust-like inclusions init are probably iron oxide exsolved when zoisite crys-taJlized from epidote.

Optical Properties of Minerals

In the highest grade zone (Sp. 796) the blue-green hornblende is strongly pleochroic; farther fromthe pluton (Sp. 551) the colors are much lighter,and green hornblende in the low-grade zone has 7 =light green,0 = light green to olive green, ?Dd a =pale green. Actinolite in all specimens has 7 * palegreen and B and a from very pale green to colorless.Many amphibole grains in samples 463, 464, and465 have green centers and colorless rims. The con-tact between the center and the rim is usually diffuse.Many grains show intergrowths either of fine lamel-lae or of equant to elongate domains of green amphi-bole in colorless actinolite, all the green amphiboleshowing higher indices of refraction. Each amphi-bole has remarkably uniform indices of refraction

(Table 1) in all the rock types except in metabasaltsample 551, where they are somewhat higher; r A cis generally 21" to 26" for the hornblende and about15o for the actinolite.

The indices of refraction of amphibole grains(Table 1) can be correlated only in a general waywith the microprobe analyses that were done inpolished thin sections. The highest indices measuredfor actinolite are in sample 551, in which Fe/Mgratio is higher than in the other specimens (Hiet-anen, 1973). Also the hornblende in this rock hashigher indices of refraction than the hornblende inthe other specimens apparently due to a high ironcontent (see Table 2a). High aluminum content,such as that in the blue-green hornblende in sample796, affects indices of refraction far less than highFe content. Specimens 463 and 465 also containgrains of actinolite similar to those in specimen 796.Only one amphibole, the gteen hornblende, was de-tected in the mineral powder from sample 532.

The epidote concentrate of the powdered rocksrevealed two epidote minerals in all the samples,except in metadiorite sample 532 and in sample 796,which have one and three epidote minerals respec-tively. Many grains appeared to consist of small do-mains of epidote (with indices of refractioh higherthan I.72) in clinozoisite (indices lower than 1.72).The high index domains range from lamellae parallel

Tegrn 1. Indices of Refraction of Amphiboles, EpidoteMinerals. and Chlorite

SqpIeRock

55r 532 465 463 464lleta- Meta- Meta- l leta- Meta-baa. I t d io r l te dLor t te andes i te dac l te

796Mete-

gabbro

Hornblende

Ac t lnol l te

Ep ldote

C I l n o z o s i t e

Z o i s i t e

C h I o r i c e

Muscovi Ee

P I a S i o c I a s e

r . 6 0 0a l0"

r . 5 9 t1 . 5 9 3

t . 5 7 4 1 . 5 1 01 . 5 8 1 r . 5 3 4r . 5 8 5 l . 5 3 9

BY

Y

B

BY

B

I+ 2 V

BY

B'r

I . 6 5 1 1 . 6 5 5 1 . 6 5 1r . 6 5 9 L . 6 6 9 1 . 6 5 9L . 6 7 7 L . 6 7 9 r , 6 7 1

22" 25" 24'

r , 6 2 5 r . 6 4 0L . 6 3 4 1 . 6 4 8r . 6 4 8 r . 6 5 6

r . 7 1 9 L . 1 2 r r . 7 2 LL , 7 J 2 1 . 7 3 1 7 . 7 3 3L . 7 4 4 L . 7 4 2 r . 7 4 2

t o 1 . 7 5 4

r . 7 0 8 L , 7 0 91 . 7 1 0 1 . 7 1 rr . 7 r 8 r . 7 2 0

l . 7 0 3r . 7 L 4

r . 6 4 7 L . 6 4 9 L . 6 4 11 , 5 5 8 L . 5 5 7r . 6 7 1 1 . 6 7 0 1 . 6 7 1

25' 2L"

r . 6 2 5 7 . 6 2 6 L . 6 2 2

1 , 6 4 9 1 . 6 5 0 L . 6 4 7

r . 7 2 1 L . 7 2 3 L , 7 2 2L . 7 4 1 7 . 7 4 J1 . 7 5 1 1 . 7 5 5 1 . 7 5 5

r . 7 t 4 7 . 7 1 5 1 . 7 0 9r . 7 1 6 r . 7 I 7t . 7 2 2 1 , 7 2 2 1 , 7 2 r

1 . 6 1 5 1 . 6 1 7small snall snall

1 . 5 3 0 7 . 5 2 9r < 1 ?

1 . 5 3 8 1 . 5 3 8

t , 6 2 0soal l.


to the cleavage and perthitelike intergrowths to smallequant domains of epidote in clinozoisite, all ofwhich still show ultrablue interference color (Fig. 3,at right). The contacts between the domains and thehost mineral seem sharp.

The measurements in the individual samples (Ta-ble 1) show that the indices of refraction of epidoteand clinozoisite in the samples from the low gradezone tend to be higher than those from the highgrade zone. In all the epidotes 2V,is large. In addi-tion to clinozoisite and epidote, zoisite with grayinterference colors, parallel extinction, 2V" - 59",and indices of refraction lower than those of clino-zoisite occurs in metagabbro (sample 796). Thehighest 7 measured is given for clinozoisite in Table1. This value is higher than would be expected onbasis of a large *2V and is due to a range in com-position.

Chlorite occurs in clusters of small flakes in allrocks. It is most abundant in metadacite sample 464and occurs only sporadically in metagabbro sample796.|t is pleochroic y = P = grseh, a = colorlessin samples 796, 551, and 532, but very pale greento colorless in the other samples. The indices of re-fraction of the green chlorite in sample 532 aresomewhat higher, and those in sample 796 are lowerthan the indices of the common pale variety (TableI ). All chlorites have brownish gray interferencecolor, negative elongation, and small *2V.

The indices of refraction of plagioclase (Table 1)indicate that it is albite in the low grade zone andanorthite at the highest gtade next to the pluton.

Composition of Minerals

Separation of minerals for wet chemical analysesu/as not feasible because of many small mineral in-clusions, such as leucoxene and ore minerals. Evena reasonably clean fraction of blue-green amphibolethat has been chemically analyzed (Hietanen, 1973,Table la, anlal. 796h) contained a few colorlessamphibole grains. The major constituents-amphi-boles, epidote minerals, plagioclase, and chlorite-were therefore analyzed by electron microprobe byG. K. Czamanske. In each sample, several grains ofoptically different types of amphibole and epidoteminerals were chosen for analyses (Tables 2a and3). Chlorite was analyzed in five samples (Table 4),and plagioclase in six (Table 5).

Some of the accessory minerals, such as ilmenitein sample 551, sphene in samples 532 and 465, andleucoxene in samples 463 and 532, were checked

with the electron microprobe for their Ti, Mn, andMg contents. Irucoxene was found to consist of 93to 98 percent TiOp.

Analytical Techniquesby G. K. CznrvreNsre

Grains of albite, amphibole, chlorite, epidote, andzoisite in carbon-coated, polished thin sections wereanalyzed with an Ant nux-su electron microprobe.Accelerating potentials were chosen to minimizeeffects of interferences on the important light consti-tuents Al, Mg, and Si (see Desborough and Heidel,l97l). The following analytical conditions wereused: Ca, Fe, Na were run at 15 kilowatts, whereasSi, K, Al and Mn, Ti, Mg were run at 10 kilowatts.Sample current on benitoiie was 0.2 g. amperes at 15kilowatts and 0.03 p amperes at 10 kilowatts, with abeam diameter of 2 to 3 microns. Typically, 6 to' 12points per grain were analyzed for each set of threeelements, for counting intervals of about 10 seconds(actual count intervals were fixed by beam currenttermination). Raw count data were corrected fordrift, background, absorption, fluorescence, andatomic number with the aid of the Fortran IV com-puter program of Beeson (1,967).

Standards for albite, epidote, and zoisite were thefollowing oxides: AlzOs, FezOe, MgO, MnOz, SiOz,TiOz, and the following three silicates from Goldich,Ingamells, Suhr, and Anderson (1,967): albite fromAmelia County, Virginia (Na), Or-1 (K), and Px-l.For the amphiboles, a homogeneous, hastingsiticamphibole [Nao.uz Ko.zs Car.zs (Alo tr Fe3*a 6p Tia.37Mgr.ozFe'*z.seMno.oo) r.tsSi6 26,{11.72O2OH )zl was em-ployed. A homogeneous biotite [K1.seNao.67Bao.e3(M&.sn Fe'*r.oa Mno.* Ab.or Fe3*o.ze Cro * Tio rs)s.ssSib.?2A12.28Ozo(OH)nl was used to analyze the chlo-rites, with the amphibole above as a Ca standard.Both the amphibole and biotite were analyzed bywet chemical techniques by C. O. Igamells, havebeen checked on the microprobe against many othersuperior standards, and are routinely used as stand-ards because of their purity and homogeneity.

For purposes of comparison and further calcula-tion, oxide weight percentage values reported in Ta-bles 2, 3, 4, and 5 are consistently reported to twodecimal places, although in terms of expected ac-curacy, no more than three significant figures can bejustified, e.9., SiO2 will be reported as 42.97 ratherthan 43.0. In all tables 0.0 means less than 0.005.

Some of the spot analyses that consisted of highlyinhomogeneous counts 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 AreaAnalyst, G. K. Czamanske, U. S. Geological Suruey

S a p l e

R o c k

7 9 6

Me tagabbro

546

Me tagabbro

) ) I

M e t a b a s a l t

Ana-

ryz<

pa rc

Anal .

Bluegreen

rim

I

Snallp rasm

2

Htgh A.

counts

f o r 4

3

Color less core

4 6 7 8

Low A1

counfs

for 4

9

Pr lsm

core end

10 I I

Prism

L2

Core

of 15

13

High Al

counts

fo r 16

I 4

sio2

412o3

Tio2

Fe2O3

FeO

Mgo

MnO

CaO

Naro

Kz9

42 .97

16 . t l

0 . 1 9

2 . 6 3

9 . 4 7

1 2 . 7 6

o . 2 3

IL.1 2

r . 9 2

0 . 2 8

4 3 . 9 4 4 2 . 8 2

L 4 . 9 3 1 4 . 9 3

0 . 1 5 0 . 2 5

2 . 4 5 1 . 8 8

8 . 8 2 6 . 7 7

1 3 . 7 0 1 3 . 9 6

0 . 2 r 0 , 0 6

r 1 . 9 1 L 2 . 2 2

l . 8 7 0 . 9 4

0 . 1 9 o . 2 I

5 1 . 3 1 5 3 . 3 1

6 . L 7 4 . 5 5

0 . 1 3 0 . 0 9

1 . 5 5 r . 8 0

5 . 5 9 6 . 4 9

1 8 . 1 0 1 8 . 9 0

0 . 1 2 0 . 2 7

L2 .55 LL .76

0 . 5 4 0 . 5 8

0 . 0 3 0 . 0 2

57 .L6 56 .43 56 .65 55 .75

4 . O 3 3 . 6 1 2 . 7 8 r . 0 9

0 . 0 6 0 . 1 0 0 . 0 8 0 . 0 4

r . 4 4 L . 7 r 1 . 4 8 r . 3 3

5 . 2 0 6 . L 6 5 . 3 4 4 . 8 0

20.47 20.34 20.58 20.64

0 . 1 8 0 . 1 9 0 . 2 0 0 . 1 3

L 2 . 3 3 1 2 . 3 3 L 2 . 2 3 L 2 . 7 7

0 . 8 0 0 . 3 8 0 . 3 7 0 . 2 8

0 . 0 0 0 . 0 0 0 . 0 0 0 . 0 0

52.LO

5 . L 4

0 . 3 4

2 . 6 6

9 . 5 8

L5 .37

0 . 3 0

1 1 . 5 9

0 . 6 3

0 . 0 2

55 .29? n o

0 . 2 0

2 . 2 4

8 . 0 5

1 8 . 1 3

o . 2 9

t l . 6 6

0 . 1 8

0 . 0 0

4 2 . 4 9

L3 .92

0 . 3 6

4 . i >

1 5 . 6 9

7 . 6 5

0 . 3 5

r t .50l ? q

o . 3 4

4 2 . 5 A

1 1 . 6 5

o . 2 9

4 . 7 5

1 7 . 1 0

8 . 0 0

0 . 3 1

1 1 . 1 8

1 . r 0

o . 3 7

4 1 . 3 5

L3 .77

o . 4 2

4 . 5 3

1 6 . 3 3

7 . O 2

o . 3 4

t r . 4 )

1 . 2 8

o . 2 8T o t a l 9 8 . 2 8 9 8 . 1 7 9 4 . o 4 9 6 . o 9 9 7 . 7 7 L o L , 6 7 L o L . z s g g . 7 L 9 8 . 8 3 9 7 . 7 3 9 8 . 1 3 9 7 . 9 4 9 7 . 3 3 9 6 . 7 7

t l t g / F e L . 9 2 2 2 . 2 L 4 2 . 9 3 6 4 . 6 L 9 4 . L 5 7 5 . 6 1 7 4 . 7 L L 5 . 5 0 r 6 . L 3 2 2 . 2 g O 3 . z L O 0 . 6 9 5 0 . 6 6 7 0 . 6 1 3F e / A l 0 . 5 2 L o . 5 2 4 0 . 4 0 3 0 . 8 0 3 r . 2 6 5 L . r 4 4 1 . 5 1 3 r . 7 0 3 3 . g o 2 r . 6 5 2 3 . 4 L 7 l . o o o L . 3 o 2 r . o 5 r

S @ p l e

R o c k

5 5 1

Me tab as a1 t

463

Meta-andes i te

465

Metad lo r l te

532

Metadlor l ce

464

M e t a d a c l t e

na-

Iyzed

Rin of

t 3

15

Low A1

counts

fo t L4

l6

Green

hornblende

L7 18

Green homblende

19 20 22

Green homblende

? 1 24 2 5

lgh A1

counta

f o r 28

26

SnalI

prlsu

Low Al

count6

f .o t 26

2 8

Slnp le

gra in

sio2Alzo 3TLO2

FerOr*

FeO

Mgo

llno

CaO

Naro

5 0 . 7 6 5 L . 6 4

2 , 6 4 2 . 4 0

0 . I l 0 . 0 7

4 . 0 1 3 . 7 8

14 .44 13 .61

1 2 . 0 0 L 2 . L 3

0 . 2 r 0 . 1 9

12 .06 I I . 77

0 . 3 6 0 . 6 7

0 . 1 1 0 . I 2

47 .30 44 . r4

8 . O 2 6 . 8 4

r . 4 8 1 . 3 4

2 . 7 0 2 . 5 6

9 . 7 L 9 . 2 3

15 .59 L6 .22

o . 2 5 0 . 2 3

LO.25 11 .19

1 . 0 3 1 . 1 0

0 . 0 8 0 . 0 9

4 7 . 6 4 4 8 . 3 r

7 , 5 5 7 . 2 0

1 . 1 4 1 . 1 1

2 . 9 8 3 . 1 0

ro .7 4 11 .20

14 .92 14 .54

0 . 3 1 0 . 4 0

r 0 . 6 9 1 0 . 4 9

r . 3 0 1 . l 0

0 . 1 0 0 . 0 3

48 .82 49 .79

6 . 3 9 5 . 7 6

1 . 3 3 L . 2 9

3 . L 6 3 . 0 0

1 1 . 4 0 1 0 . 8 0

14 .83 t 4 .84

0 . 4 3 0 . 4 2

10 .35 10 .48

I . 0 2 1 . 1 3

0 . 0 5 0 . 0 9

4 7 . 5 4 5 0 . 9 9

6 . 7 6 4 . 8 5

Q . 2 4 0 . 2 5

2 . 0 L 2 . 2 6

7 . 2 3 8 . L 2

1 8 . 2 1 L 6 . 7 L

0 . 1 6 0 . 2 0

12.5L L2.54

0 . 4 8 0 . 1 9

0 . 0 1 0 . 0 6

5 6 . 5 8 5 6 . 5 0

0 . 8 1 0 . 8 8

0 . 1 5 0 . 0 3

L . 4 7 r . 4 0

5 . 2 9 5 . 0 4

L9 .32 20 .47

0 . 1 9 0 . 1 8

13 . 11 13 . 28

0 . 1 2 0 . 0 6

0 . 0 0 0 . 0 0K2o

4 8 . 3 1 4 7 . 8 9 4 9 . 0 8

7 . 4 3 7 . 4 0 6 . 4 8

0 . 8 6 0 . 8 3 0 . 8 8

2 . 9 L 2 . 9 0 2 . 8 9

10 .48 LO .47 10 .40

L 4 . 3 5 1 4 . 5 0 1 4 . 7 L

o . 4 4 0 . 3 7 0 . 4 4

LL .O2 r r . 21 11 .08

L . 2 3 r . 2 r L . z I

0 . 1 4 0 . 1 8 0 . 0 7

T o t a l 9 6 . 7 0 9 6 . 3 8 9 6 . 4 L 9 6 . 9 4

M g / F e 1 . I 8 5 I . 2 7 2 2 . 2 9 O 2 . 5 0 6

F e / A t 4 . 8 4 9 5 . 0 2 1 1 . 0 7 9 L . I 9 7

9 7 . 3 7 9 7 . 4 8 9 7 . 7 8 9 7 . 6 0

1 . 9 8 1 1 . 8 5 3 1 . 8 5 5 r . 9 5 9

L . 2 6 L r . 3 7 9 r . 5 E 3 1 . 6 6 3

9 7 . r 7 9 6 . 9 6 9 7 . 2 4

r . 9 5 3 t . 9 7 6 2 . O L 6

L .25L r . 253 r . 424

9 5 . 1 5 9 6 , r 7 9 7 . O 4 9 7 . 8 3

3 . 5 9 r 2 . 9 3 3 5 . 2 0 9 5 . 7 9 7

0 . 9 4 9 L . 4 8 6 5 . 7 8 6 5 . 0 6 4

*2O7, o f to ta l Fe expressed as Fe2o3


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

29

S l reTotal lons

Tetrahedralt - 8 . 0 0

M(1) - -M(3)I = 5 . 0 0

M ( 4 )E = 2 . 0 0

AnaI s1 AI Al F"3+ Tr Mg F.2* Mn - 2 +! e Mn C a N a N a K C a

796

l L 7

1 t t) ' ot - '1::I

i -

I t c

inI zt,I[ 2 5

l 'u1 2 71 2 8

[ , ,

5

6

7

8

9

10

11

L 2 6 . 3 6 0* * 6 . 3 9 5

1 3 6 . 4 7 2* * 6 . 5 0 9

14 6 . 303* * 6 . 3 1 6

15 7 .563* * 7 , 5 9 1

1 6 7 . 6 7 0* * 7 . 6 9 8

6 . 901

6 . 9 8 6

6 .936

7 .023

7 .O79

7 .203

7 . 0 3 5

7 .000

7 .L32

6 . 9 7 t

7 . 3 7 6

7 . 934

7 . 8 5 9

1 .800 0 .939

1 . 6 8 1 0 . 8 4 9

1 . 6 5 4 0 . 9 5 4

1 . 6 6 6 0 . 8 r 0

0 . 680 0 . 358

0 . 5 1 5 0 . 2 3 7

0 . 3 6 4 0 , 2 7 L

0 . 3 8 9 0 . 1 8 5

0 . 280 0 . 167

0 . 1 7 3 0 . 0 0 7

0 . 5 5 7 0 . 3 0 8

0 . 2 3 5 0 . l l l

1 . 6 4 0 0 . 8 1 61 . 6 1 5 0 . 8 5 0

I . 5 2 8 0 . 5 5 91 . 4 9 r 0 , 5 0 8

L . 6 9 7 0 . 7 7 7L . 6 8 4 0 . 7 9 4

0 .437 0 .0270 .409 0 .056

0 . 3 3 0 0 . 0 9 00 . 302 0 . 120

1 . 0 9 9 0 . 2 8 0

1 .014 0 . 156

1 .064 0 .2J2

o . 9 7 7 0 . 2 5 6

0 . 9 2 r 0 . 1 7 r

0 . 7 9 7 0 . 1 8 5

0 . 9 6 5 0 . 3 1 0

1 . 0 0 0 0 . 2 7 4

0 . 8 6 8 0 . 2 4 L

L . O Z g 0 . 1 3 9

o .624 0 .202

0 . 0 6 6 0 , 0 6 8

0 . r 4 r 0 .004

0 . 2 8 6 0 . 0 2 r

0 . 2 6 5 0 . 0 1 6

0 . 2 1 0 0 , 0 2 8

0 . 4 0 4 0 . 0 2 3

0 . 1 6 6 0 . 0 1 4

0 . 1 9 0 0 . 0 0 9

0 . 1 4 5 0 . 0 0 6

0 . 1 7 3 0 . 0 1 0

0 . 1 5 2 0 . 0 0 8

0 . 1 4 1 0 . 0 0 4

0 , 2 8 5 0 . 0 3 6

o . 2 3 7 0 . 0 2 1 .

0 . 4 9 0 0 . 0 4 r0 . 3 L 2 0 . 0 4 1

0 . 5 4 3 0 . 0 3 30 . 3 4 7 0 . 0 3 3

0 , 5 2 0 0 . 0 4 80 . 3 3 0 0 . 0 4 9

0 . 4 5 0 0 . 0 r 20 .286 0 .013

o .423 0 .008o . 2 6 9 0 . 0 0 8

o .296 0 .L62

0 . 2 8 0 0 . 1 4 6

o .326 0 . r 25

0 . 3 3 9 0 . 1 2 1

0 . 3 4 5 0 . 1 4 5

o . 3 2 7 0 . 1 4 0

0 . 3 1 9 0 . 0 9 4

0 . 3 1 9 0 . 0 9 1

0 . 3 1 6 0 . 0 9 6

o . 2 2 2 0 . O 2 7

0 . 2 4 6 0 . 0 2 7

0 . 1 5 5 0 . 0 t 6

o . r47 0 .003

2 . 7 4 4 1 . 0 1 0

2 . 9 3 7 0 . 9 3 3

3 . 0 8 3 0 . 7 2 5

2 .822 0 .934

3 . 8 4 9 0 . 6 1 3

3 . 9 5 5 0 . 6 0 9

4 . O 7 6 0 . 5 0 2

4 . 0 8 9 0 . 5 4 3

4 . 1 8 0 0 . 4 9 3

4 ,319 0 .529

1 . 2 7 3 1 . 0 9 8

3 . 7 9 5 0 . 8 3 6

L .707 L .946L . 7 L 4 2 , 0 8 3

1 . 8 1 3 2 . O 5 2r .823 2 .L89

r . 595 2 .0601 . 5 9 8 2 . 2 7 6

2 . 6 6 5 L . 7 9 92 . 6 7 5 1 . 9 7 0

2 .685 1 .6902 .695 L .852

3 . 3 9 0 0 . 8 7 2

3 , 5 0 9 0 . 9 0 9

3 .238 r .O79

3 . r 51 r . r 33

J . 2 U > I . I J C

3 . 200 1 . 148

3 .115 L .162

3 . 1 5 9 r . 1 5 7

3 . 186 1 : 161

3 . 9 8 0 0 . 6 3 3

3 . 6 0 3 0 . 9 2 2

4 . 0 3 8 0 . 6 2 0

4 . 2 4 4 0 . 5 8 6

0 . 1 3 3

0 . 1 2 8

0 . r 1 4

0 . 0 0 7

0 . 054

0 . 1 5 3

0 . 079

0 . r 5 2

0 . r 1 5

0 . 0 3 4

0 .047

0 . r 09

0 . 0 r80 . 059

0 . 1 2 10 . 196

0 . 0210 .047

o .o27

1 . 8 1 2 0 , O 2 7

1 . 8 3 5 0 . 0 1 1

1 . 8 7 8

L .902 0 .086

1 , 9 1 8 0 . 0 1 3

r . 7 6 9 0 . 0 4 6

r . 7 6 5 0 . 1 3 5

L . 7 8 2 0 . 0 4 4

1 . 7 8 6 0 . 0 7 6

L ,92L 0 .029

L . 7 7 4 0 . 1 4 3

L . 7 5 5 0 . 0 4 9

1 . 8 4 4 0 . 0 9 41 . 852 0 .035

1 , 8 2 1 0 . 0 1 8r . 8 3 1

r . 8 7 0 0 . 0 6 5r . 8 7 4 0 . 0 3 5

L . 9 2 5 0 . 0 7 51 . 9 3 3 0 . 0 4 0

1 . 8 7 3 0 . L 2 71 . 8 8 0 0 . 1 2 0

L . 6 0 2 0 . 0 5 4

1 . 7 4 0 0 . 0 2 1

1 . 6 6 8 0 . 0 5 5

L .634 0 .088

r . 6 0 8 0 . 0 9 0

L . 6 2 4 0 . 1 6 5

1 . 7 1 9 0 . 1 1 3

1 . 7 5 5 0 . 0 7 6

L . 7 2 5 0 . r r 8

r . 7 2 6

1 . 9 1 5

1 . 9 7 0 0 . 0 3 0

L . 9 7 9 0 . 0 1 6

0 . 5 r 0 0 . 0 5 2

0 . 5 1 0 0 . 0 3 5

o . 2 7 r 0 . 0 3 9 0 . 0 6 3

o . 4 4 9 0 . 0 4 3

0 . 1 3 6 0 . 0 0 6

0 . 1 1 2 0 . 0 0 4

0 . o 7 2

0 . 0 5 5

0 .o22

0 .o47

0 . 0 3 1 0 . 0 0 4

0 . 2 8 1 0 . 0 6 50 . 3 4 1 0 . 0 6 5

0 . 3 0 6 0 . 0 7 2o , 3 2 6 0 . 0 7 2 0 . 0 6 7

0 . 3 1 4 0 . 0 5 4o . 3 4 4 0 . 0 5 4

o .029 0 .0210 . 0 6 4 0 . 0 2 1

0 . 0 6 6 0 . o 2 30 . o 7 4 0 . o 2 2

0 . 2 3 8 0 . 0 1 5

0 . 2 8 9 0 . 0 1 7

o . J 0 2 0 . 0 1 9

o . 2 2 2 0 . 0 0 6

0 . 1 9 7 0 . 0 0 9

o , L 5 2 0 . 0 1 7

0 . 2 3 5 0 , 0 2 6

o . 2 6 7 0 . 0 3 4

0 , 2 2 3 0 . 0 1 3

0 . 1 3 7 0 . 0 0 2 0 , 2 3 9

0 .053 0 .011 0 .029

0 . 002

6. 200

6 . 319

6 . 346

6 . 3 3 4

7 .320

7 . 4 8 4

7 .636

/ . O I I

7 . 720

7 .827

7 . 4 4 3

7 . 7 6 s

0 . 0 2 8

o .026

0 .008

0 . 0 1 2

0 .015

0 . 0 3 2

0 . 0 2 0

0 .022

0 .o23

0 . 016

0 . 036

0 . 035

0 . o 4 40 .o44

0 . 0400 . 040

0 . 044o . o 4 4

0 .o27

0 .005

55r

o .024o .o24

0 . 3 1 3 0 . 0 3 1

o .2LL 0 ,028

0 ,229 0 .038

o . 2 2 9 0 . O 4 9

0 .249 0 .053

0 . 1 5 9 0 . 0 5 2

0 . u 4 0 . 0 5 4

0 . 1 2 3 0 . 0 4 6

0 . 1 0 3 0 , 0 5 4

0 . 2 5 4 0 . 0 2 0

0 .061 0 .024

0 . 0 2 3

0 . 016

* t l e t c h m i c a l a n a l y s e s f r o m H i e t a n e n ( 1 9 7 3 )

* * T h e n u b e r o f i o n s g i v e n o n l o w e r l i n e a r e r e c a l c u l a t e d f o r F e r o , = L 2 . 7 7 o f t o t a l F e o

high Al and high Fe counts were combined with thelow Mg counts to form one analysis and the lowAl, low Fe, and high Mg counts were calculated asseparate analysis. Analyses 3 and 9 were derivedfrom analysis 4 in this way. Analyses 14 and 1.6 arcbased on splitting counts from two grains and com-bining high Al and Fe counts with low Mg countsfor 14 and low Al and Fe counts with high Mgcounts for 16. Analysis 28 represents low Al andlow Fe counts combined with high Mg counts, and

analysis 26 high Al and high Fe counts combinedwith low Mg counts from the same grain.

Amphiboles

The composition of analyzed amphiboles (Table2a) ranges from actinolite through actinolitic horn-blende to a hornblende rich in aluminum, somerocks containing the two end members and someothers intermediate compositions. Some of the in-termediate compositions are averages of inhomo-

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

AnaI .

796

Metagabbro

546

ileta-lebbro

55t

lletabselt

4 6 5

Hetadlor l te

532

XetadtorlCe

464

ktadecr.te

Zoisi te

30 3r

Cl l .nozo ls i te

32 33 34

lpldot, Ep ldo te

36 37 38

Ep ldo !e

39 40

Ep ldo te

4 1 4 2

Eptdote

4 3 4 4 4 5

CUnozotBlt€

46 47

Eptdote

48 49 50 5r

30 ANNA HIETANEN

s i o 2 4 t . 5 2 4 L . 6 4 4 0 . 0 3 3 9 . 9 8 3 9 - 5 9 3 8 . 7 s

A I 2 O 3 3 3 . 0 7 3 3 . 3 8 3 2 . 0 5 3 0 . 9 L 3 0 . 2 t 2 4 . 8 6

f e 2 o 3 i 0 . 2 3 0 . 1 1 2 . 3 4 3 . 5 8 4 . 8 7 1 1 . 5 8

T l o 2 0 . 0 1 0 . 0 2 0 ' 0 3 0 . 0 1 0 . 0 4 0 - 0 6

H s o 0 . 0 I 0 . 0 0 0 . 0 0 0 , 0 0 0 . 0 4 0 . 0 0

x n o 0 . 0 5 0 . 0 5 0 . 0 8 0 . 0 7 0 . 1 1 0 . 2 3cao 23,42 23,9i 23.E9 24,12 24.24 23,0j

N e - o 0 . 6 5 0 . 4 5 o . o o 0 . 0 0 o . o o o . o o

K , 0 0 , 0 2 0 . 0 5 0 . 0 4 0 . 0 1 0 . 0 0 o . 0 2 0 . 0 7 0 . 0 7 o . o 7 0 . 0 6 0 . 0 6 o . o 7 o . o 9 o . o 4 0 . 0 6 0 . 0 7 0 . 0 3 o . o 7 0 . 0 9 o . o 2 o . o 4 0 , 0 9

T o t a r 9 8 . 7 9 r 0 0 . 6 2 9 8 , 4 6 9 8 . 6 8 9 8 , 2 0 9 s . 5 7 9 8 . 0 0 9 9 . 1 r 9 a . 7 7 9 8 . 5 2 9 8 . 4 0 9 8 , 5 3 9 9 , 4 3 9 7 . 4 8 9 9 . L a 9 1 . 9 L 9 7 . 9 6 9 8 . 5 1 9 8 . 5 5 9 7 , 4 1 9 6 , 5 2 9 6 . 8 1

Nuber of lons on anhydtous baste (25 oxygen elds pe! 2 fotuula unirs)

s i 3 . 1 0 0 3 , 0 8 6 3 . 0 3 1 3 , 0 3 7 3 . 0 r 7 3 . 0 3 6 3 . 0 4 1 3 . 0 1 8 3 , 0 2 3 3 . 0 4 6 3 . 0 1 7 3 . 0 5 r 3 . 0 3 1 3 . 0 0 7 3 . 0 5 9 3 . 0 4 5 3 . 0 3 4 3 . O 2 9 1 , O 6 L 3 . 0 r 5 3 , 0 5 9 3 . 0 2 0A r 2 . r . r 2 . 9 L 7 2 , a 6 o 2 , 7 6 7 2 , 7 0 6 2 , 2 9 6 2 . 4 8 4 2 . 5 L a 2 . 4 1 5 2 . 6 2 2 z . z s L z . 4 o g 2 . l g a 2 . 5 3 r 2 . 4 6 9 2 . 4 1 b 2 . 6 a 5 2 . 6 5 2 2 . 5 7 3 2 . 3 s 3 2 . r 4 3 2 . 1 9 1

r e - 0 . 0 1 3 0 . 0 0 6 0 , r 3 3 0 . 2 0 4 0 . 2 7 9 0 . 6 8 3 0 . 4 8 5 0 . 5 0 5 0 . 6 7 8 0 - 3 3 3 0 . 7 5 2 0 . 5 4 2 0 . 5 7 A 0 . 4 7 7 0 . 4 8 5 0 . 5 0 6 0 . 2 6 9 0 , 3 0 5 0 . 3 6 0 0 . 5 4 7 0 . 7 f 7 0 . 7 9 5

T l 0 , 0 0 , 0 0 1 0 . 0 0 2 0 . 0 0 . 0 0 2 0 . 0 0 4 0 . 0 0 5 0 . 0 0 5 0 . 0 0 5 0 , 0 0 4 0 . 0 1 9 0 . 0 0 6 0 . 0 0 8 0 . 0 0 3 0 . 0 0 5 0 . 0 0 1 0 . 0 0 6 0 . 0 0 2 0 . 0 0 5 0 . 0 0 3 0 . 0 1 5 0 . 0 0 2

t r e 0 . 0 0 1 0 . 0 0 . 0 0 . 0 0 . 0 0 4 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0 0 5 0 . 0 0 4 0 . 0 0 2 0 , 0 0 1 0 . 0 0 . 0 0 . 0 0 , 0 0 . 0 0 . 0 0 . 0 0 . 0 0 . 0

M n 0 . 0 0 3 0 . 0 0 3 0 . 0 0 5 0 . 0 0 4 0 . 0 0 7 0 . 0 1 5 0 . 0 0 3 0 . 0 0 . 0 0 3 0 . 0 1 2 0 . 0 1 8 o . 0 0 6 0 . 0 1 1 0 , 0 1 1 0 . 0 1 5 o . o o 4 0 . 0 0 1 O . O O ? O . O O . o O 5 O . O I 7 0 . 0 0 7

c a r , 8 7 4 1 . 9 0 4 I . 9 3 8 1 . 9 6 3 I . 9 7 4 1 . 9 3 7 1 . 9 5 1 I . 9 1 6 I . 7 1 1 I . 9 4 9 I . 9 o 2 L . 9 4 7 1 . 9 4 1 1 . 9 5 5 t . 9 2 2 ] 9 2 7 1 . 9 8 7 r , 9 9 1 r , 9 6 3 1 . 9 5 5 1 . 9 5 8 1 . 9 6 6

N a 0 . 0 6 7 0 . 0 6 4 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 . 0 0 . 0 0 . 0

K 0 . 0 0 I 0 . 0 0 4 0 . 0 0 4 0 . 0 0 1 0 . 0 0 . 0 0 2 0 . 0 0 7 0 . 0 0 7 0 , 0 0 7 0 . 0 0 6 0 . 0 0 6 0 . 0 0 7 0 . 0 0 8 0 . 0 0 4 o . 0 0 6 o . o o 7 o , o o 3 0 . 0 0 7 0 . 0 0 9 o . o o 2 o . o o 4 0 . 0 0 9

Pistaci te conrent ln mlecular perc€nt

P e : o , 1 4 0 . 2 1 4 . 4 4 6 . 8 7 9 . J s 2 2 , 9 3 1 6 . 3 4 1 6 . ? r 2 t . s o 7 1 . 2 7 2 s . 0 4 $ , 3 7 t g . 4 2 r s . s 6 1 6 . 4 2 1 6 . g 7 9 , 1 1 r o . 3 1 1 2 . 2 7 z L . s 7 2 6 . 6 1 2 6 . 6 2F"$+Al

F e l A 1 0 . 0 0 4 o . o o 2 o . o 4 7 o . o 7 4 o . r o 3 0 , 2 9 7 0 . 1 9 5 o . 2 o o o . 2 7 4 0 . r 2 7 o . 3 3 4 0 , 2 2 5 0 . 2 4 r 0 , r q 9 q . r 9 o q - z Q L g . t g g , o , l r ; p . l l q u ! 2 7 5 0 . 3 5 2 o . 3 6 2

rTo ta l Fe * Fe2O3

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

3 9 . O 7 3 9 , 2 3 3 9 . 1 0 3 9 . 7 0 3 8 . 2 8 3 9 . 2 8 3 9 . 2 6 3 A . 4 0 3 9 . 8 1 3 9 , 0 8 i 9 , 4 4 3 9 , 4 7 3 9 . 8 6 3 8 . r 1 3 8 . r 0 3 7 . 5 7

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

8 . 2 7 8 . 7 2 1 1 , 5 5 5 . 7 6 L 2 , 6 A 9 . 2 7 9 . 9 5 8 . 0 9 8 . 3 8 8 . 6 3 4 . 6 4 5 , 2 9 6 , 2 4 L O . A 7 1 2 . 8 5 1 3 . 1 4

0 . 0 8 0 . 0 8 0 . 0 8 0 . 0 7 0 . 3 1 0 . 1 0 0 . 1 4 0 , 0 5 o . o 9 0 . 0 3 0 . l l o . o 4 o , o 8 o . o 5 0 . 2 5 0 . 0 3

0 . 0 0 0 . 0 0 0 . 0 0 0 , 0 4 0 . 0 4 0 . 0 1 0 . 0 r 0 - 0 0 0 . 0 0 0 . 0 0 o , o 2 0 . 0 0 . 0 0 . 0 o , o o 0 , 0

0 . 0 4 0 . 0 0 0 . 0 4 0 . 1 9 0 . 2 7 0 . I 0 0 . 1 5 0 . 1 7 0 . 2 4 0 . 0 6 0 . 0 o . 1 o o . o o . o 7 o . r 7 o . 1 o

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 22.820 . 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 . 0 0 . 0 o . o

geneous counts; they include analysis spots withhigh Al and low Al counts, as well as spots withhigh Fe and low Fe counts. Comparison of the splitanalyses from inhomogeneous grains with the anal-yses from homogeneous grains in the same sampleshows that the high Al and Fe counts combined withlow Mg counts give composition of blue green orgreen hornblende (anal. 3, t4, and 26) whereasthe low Al and Fe counts combined with higlr Mgcounts (anal. 9, 16, and 28) give composition ofactinolite. This similarity of analyses based on se-lected counts with the analyses of homogeneousgrains in the same sample supports the possibility ofbimodal composition of grains that give inhomogene-ous counts. It seems likely that submicroscopic in-tergrowths of actinolite and hornblende, similar tothose seen under the microscope although not de-tectable optically, may give rise to the inhomogeneityof counts.

In all probe analyses, 20 percent of total Fe wascalculated as FezOa to agree with the approximateFezOg(FeO * FezOs) ratio in the host rocks afteran allowance to form epidote and accessories was

made. In the wet chemical analysis of blue-greenhornblende in sample 796, this ratio is 0.32 (Hiet-anen, 1973, Table la, anal. 796h). This analysiswas recalculated on anhydrous basis of 23(O) andis shown in Table 2a under *796. The two probeanalyses (1 and 2) of high Al grains in the samesample are similar to this wet chemical analysis, allthree having about 1.9 percent Na2O and 15 to 16percent AI2OB. Analyses L2 to 16 for the amphibolesin sample 551 were also calculated for Fe2Os = 1.2.7percent of total FeO to conform with the lowerp"2. 1(Fez' * Fe3*) ratio in the rock analysis (anal.551, Hietanen, 1973, see also Fig. 6).

Structurol Formulae

Differences in the composition of analyzed amphi-boles can be best compared on the basis of structuralformulas based on crystal chemistry and X-ray work(Table 2b). According to the method of calculationby Papike et al. (1969), all Si and a part of Al areassigned to the eight tetrahedral sites. The remainingAl, Ti and Fe3" together with Mg, Fe2* and Mn oc-cupy the five octahedral sites M(l), M(2), and

AMPHIBOLE, EPIDOTE, CHLORITE, AND PLAGIOCLASE IN METAMORPHIC ROCKS 31

Terre 4. Composition of Chlorite and Muscovite inMetamorphic Rocks, Feather River Area*

this ratio is 0.55, reflecting the high content of ironin the host rock.

Both generalized formulas are derived fromtremolite mainly by two coupled substitutions that,in completion, would lead to the compositions ofedenite and tschermakite. The extent of thesecoupled substitutions differs; in the formula unit ofgreen hornblende, the edenitic substitution (NaAAlk=,Iasit") is 0.3, and the tschermakitic substitution(Alvtfiltv = Mgor,sit") is 0.7. In the blue-greenhornblende in sample 796, the corresponding valuesare 0.5 and 1.2, and in sample 551 they average0.4 and 1.1. However, the ratio of the edeniticsubstitution to the tschermakitic substitution remainsconstant and close to 3;7. For this scheme of twocoupled substitutions, the end member would behornblende Nao.oCas (Mg,Fe2. ) 3.6 (Al,Fes.) r.rAlzSieOze(OH)c in which edenitic substitution is 0.6 andtschermakitic substitution is 1.4. Such a compositionis identical to the end member composition suggestedby Robinson, Ross, and Jaffe (1971') on the basisof a more generalized study. Since the same amount(20 percent) of total Fez* was changed to Fe!* in all

TesLB 5. Composition of Plagioclase in MetamorphicRocks, Feather River Area*

sto2Alzo:Tlo2

46,37

35.54

o . o

0 . 0

o . o

18.50

o . 6 1

0 . 0 3

5 0 . 6 6

3 1 . 4 5

0 , 0

0 . 0

0 . 0

L4.t 2

3 . 2 3

0 . 0 3

69.O2 69.49 69 .31

20.38 20.O2 20.O3

0 . 0 0 . 0 3 0 . 0 4

0 . 1 6 0 . 0 9 0 . 0

0 . 0 0 . 0 0 . 0

0 . 6 6 0 . 5 7 0 . 4 1

r 1 . 5 9 1 1 . 5 8 1 1 , 5 7

0.10 0 .02 0 .02

69.22 68.72 69.45

19.95 19 ,91 19 ,60

0 . 1 4 0 . 0 0 . 0

0 . o l 0 . 0 0 . 0

o . z L 0 , 0 0 . 0

0 . 4 8 0 , 4 2 0 , 3 8

11.59 r1 .58 11 .64

o.oi o.o2 o.o2

Feo

Mno

CaO

Na2O

Kz9ro ta l lOO. l1 99 .79 r0 r .91 l0 l '80 r00 .44 r0 r '63 r00 '65 101 '49

f t lo r l te

796 546

HetaSabbro

s 2 5 3

a6t

Het.-andeslt

5h 55

465

Uetsdlor l te

56 57

464

Hetedeclte

58 59

7 9 6

Heta-

Bebbro

sto2At2o3Pt2o3l1o2Hro

FS

l{no

Ia20

27.59 25.99

22.99 21.77

1 . l r r . t l

o . 0 0 . 0 6

2 5 . 4 0 1 9 , 5 r

9 . 2 3 L 7 . 3 4

0 . 1 5 0 . 2 L

0 . 0 0 . 0

0 . 0 ' 0 , 0

26.28 26.52

2r.o7 20,70

. 0 . 0 4 0 . 0 2

] 8 . 6 8 1 8 . 7 4

L E . 2 6 1 8 , 5 8

0 . 3 1 0 . 3 0

0 . 0 1 0 . 0 0

0 . o 0 . 0 2

2r.46 26.98

2 1 , 2 3 2 1 . 1 8

r . l l l . 1 l

0 . 0 5 0 . 0 4

2r. t9 2L.45

1 5 . 1 6 L 5 . 1 9

o . 2 9 0 , 2 1

0 . 0 0 . o

0 . 0 0 . 0

26,O0 21.37 47.r4

2 1 . 0 3 2 0 . 8 5 3 4 . 7 6

l . l 1 l , 1 l 0 . 0

0 , 0 7 0 . 0 0 . 0 4

2 r . 5 1 2 2 . 7 ! t . 1 3

73,L| 72.29 0.6r

0 . 2 3 0 . 2 2 0 , 0

0 , 0 1 0 , o l o , 3 9

0 . 0 0 . 0 1 1 0 . 2 sReoT o t a l E 6 . 5 7 8 6 , 0 3 t 5 , 7 6 8 5 , 9 9 E 6 . 4 9 A 5 . 2 2 E 3 . 1 3 8 3 . 5 r 9 4 , 3 5

Nuber of 16s on aDhydrous baete of 2t(0) for chlor l te sd 22(0) forNscovite

s t 5 . t 2 i 5 . 3 6 7 5 . 4 7 2 5 . 5 1 5 5 . 5 6 1 5 . 4 8 9

A l 2 , 5 7 7 2 , 6 1 3 2 . 5 2 9 2 . 4 8 5 2 . 4 4 0 2 . 5 t 2

^ J ' 2 . 7 2 9 2 . 6 6 L 2 . 6 4 t 2 , 5 4 4 | 2 . 6 2 7 2 . 5 6 4

F . + 3 0 . 1 6 4 0 . 1 7 3 0 . L 7 4 o . r r 4 0 . 1 5 9 o . r 7 o

Tr 0.0 0.009 0.006 0,003 0.007 0.006

lrc 7.413 6.006 5.796 5.E08 6.395 6.506

F e l . 5 l l 3 . 0 0 1 3 . 1 7 9 3 . 2 3 t 2 . 5 6 4 2 . 5 8 5

h 0 . 0 2 5 0 . 0 3 7 0 . 0 5 4 0 . 0 5 2 0 . 0 4 9 0 . 0 4 6

N a 0 . 0 0 , 0 0 . 0 0 2 0 . 0 0 1 0 , 0 0 . 0

K 0 . 0 0 , 0 0 . 0 0 . 0 0 4 0 . 0 0 . 0

x8lx8iF€+2 0.831 0,557 0.646 0.643 0.7r4 0.716

M8/Pe tot6l 4.424 l 'E92 L.729 r.106 2.331 2.36L

* Electron microprobe analyses, in weight percent, byG. K. Czamanske, U .5. Geologicol Suntey.

M(3). A small deficiency in these sites in analyses16 and 28 is considered to be due partly to ananalytical error (both are composite of low Alcounts) and partly to the presence of Cr and V orsome other element not analyzed. The remaining Mnand some Fe occupy, together with Ca and possiblyNa, the two M(4\ sites. The A-site is either vacant(as in actinolite) or is occupied by Na and K (in-cluding some Ca in three analyses), and the totaloccupancy together with the excess of the octahedralvalences (Al3- + Fe3* * Tia*), balances the negativecharge of the tetrahedral sites (Al"). According tothis scheme, the results of calculation on an an-hydrous basis of 23(O) from the U.S. GeologicalSurvey computer program E2lO arc shown in Table2b. A generalized formula for the blue-green high-Al amphibole is ( Na,K ) o. uCarM&. eFe2*1.nA16. eFes*e. 3Alr.?Si6.sOpp(OH)r. The green amphibole has thegeneralized formula: (Na,K)o.aCazMgg.zFe2*r.zAlo.zTro.rFes.o.aAlSi?Os2(OH)2. In both formulas Mn,Fe2*, and Na, substitute for some of the Ca, so thatthe total of Ca * alkalies ranges from 1.9 to 2.5.Most actinolites have M#(Fe + Mg) ratios of 0.75to 0.85; in the metabasalt (sample 551), however,

l f t lber of lons on bsl6 of 32(0)

s l 8 ,514 9 .236 r1 .861 U.933

A1 7 .475 6 .758 4 .L29 4 .O52

T 1 0 , 0 0 . 0 0 . 0 0 . 0 0 4

F e O . O o . O 0 . 0 2 3 0 . 0 1 4

l {n O.O 0 .0 0 .0 0 .0

Ca 3 .639 2 .8r7 0 .121 0 .105

Na 0 .237 r .141 3 .861 3 .856

K 0 .006 0 .007 0 .021 0 .004

llolecular percentaSe

An 93.742 ?L.O6I 3.022 2.658

Ab 6.LO8 28.769 96.452 97.23L

Or 0 .151 0 .170 0 .526 0 .111

* Electron microprobe analyses, in weight percent, by

G, K. Czamanske, U. S. Geologicol Suraey.

5 . 4 4 0 5 . 4 s s 6 . 2 9 L

2 . 5 6 0 2 . 5 4 5 r . 7 0 9

2 , 6 2 1 2 . 5 4 9 3 . 7 5 5

0 . 1 7 5 0 . 1 7 3 0 . 0

0 , 0 1 1 0 . 0 0 . 0 0 4

6,709 7.01E O.224

2 . 3 0 4 2 . 7 3 L 0 . 0 5 8

0 . 0 4 r 0 . 0 3 8 0 . 0

0.003 0.004 0.100

0 . 0 0 , 0 0 3 1 . 7 4 3

o . 7 4 4 0 . 7 6 7 0 . 7 6 7

z . 7 o s 1 . o 4 4 3 . I 7 5

11.935 r1 .917 11 .93r 12 .14

4.064 4 .O49 4 ,069 3 .913

0.005 0 .018 0 .0 0 .0

0 . 0 0 , 0 0 1 0 . 0 0 . 0

0 . 0 0 . 0 3 1 0 . 0 0 , 0

0 ,085 0 .088 0 .077 0 .070

3.865 3 .868 3 .897 3 .8E1

0.005 0 .007 0 .004 0 .004

2.L69 2 .2L6 r .941 L .175

97.7L4 97 .602 97 .959 98 . r31

0.117 0 .183 0 .100 0 .094

probe analyses, the positions of points in the graphsare only approximate.

Calculation of the analyses from sample 551(Table 2b) shows that lowering of the Fe2O3 con-tent from 2A to 12.7 percent lowers the Ali" valueby 0.03 and increases the z4-site occupancy by thesame amount, but that the total alkalies remainconstant. Therefore much of the discrepancy inalkali/Al* ratio due to errors in estimation of Fe8.can be avoided if total alkalies instead of ,4-siteoccupancy are plotted against the tetrahedral Al con-tent (Fig.4). In Figure 4 all points fall close to atrend line that connects the average for the blue-green hornblende to the origin. The ratio of totalalkalies to the tetrahedral Al content thus remainsconstant and close to 1:3. This trend leads to anextreme member marked by point "Hornblende" onthe line connecting pargasite to tschermakite. Alkaticontent of the "Hornblende" is 0.66 and it has 2tetrahedral Al. The analysis 26, which falls far belowthe trend line, has 0.24 Ca ions in the z4-site, whereasin most analyses Ca content is lower than2 and someNa enters the M(4) sites. The l-site occupancy isthus generally somewhat lower than the total alkalicontent. In "Hornblende" the A-site occupancy is

+ E l a + 5 7

+ 5 1

0.6 and the ratio (Na * K)A/Ali" = 0.3, whichvalue is only a little lower than the ratio of the totalalkalies to the tetrahedral Al.

The ratio of octahedral Al + Fe8* * Ti to tetra-hedral Al (Fig. 5) is close to 0.7 for most analyzedhornblende grains. A nearly similar ratio was sug-gested by Leake (I965a, b) for the maximum pos-sible Al"i content in Ca-amphiboles in general. Onlyanalysis point 26 in the Feather River area fallssignificantly below the trend line. This analysis com-bines high Al, high Fe, and low Mg counts from aspot, and therefore the possibility of analytical erroris larger than in the analysis from homogeneousareas. Analysis 26 shows an exceptionally largeamount of Ca and a low amount of alkalies. Forcomparison, two wet chemical analyses of horn-blende in the plutonic rogks (Hietanen, 1971., Table1, anal. 474 and 522) werc plotted on Figures 4and 5. The ratio of alkalies to tetrahedral Al issimilar to that in the green hornblende in the meta-morphic rocks; the octahedral Al * Fe3* * Ti con-tent and the extent of tschermakitic substitution.however, are higher.

Comparison with the Published Analyses

Compton (1958) studied amphiboles from thecontact aureole of the Bald Rock pluton located 10to 20 miles south-southwest of Granite Basin pluton.Three of his analyses were recalculated on an an-hydrous basis of 23(O) and plotted in Figures 4 and5 (Anal. points C-2, C-3, and C-4). Two of theseanalyses (C-2, C-3) have Ali" contents intermediatebetween the green and blue-green hornblende nearGranite Basin pluton (Fig. a); the third (C-4) hasa composition closer to hastingsite than any otheramphibole in this region. The host rocks for thesethree analyses are basaltic intrusive and extrusiverocks similar to those of this study. Klein (1969)made electron microprobe analyses of amphiboles infour other specimens collected by Compton from thesouthwestern contact aureole of the Bald Rockpluton. The mineralogy given by him is similar tothat in the low grade zone near Granite Basin exceptthat only epidote, but no clinozoisite, was reported.On the basis of textural relations, Klein suggestedthat actinolite and hornblende may have crystallizedin equilibrium. Klein's analyses 2-6 and 2-7 for thehornblende in metadolerite are similar to the greenhornblende in sample 551 in Granite Basin andanalysis 2-5 is close to the composition of horn-blende in sample 465.

Edenite

ANNA HIETANEN

Tschermokite0 2 0 4 0 6

Ac I Ino l i le

Frc. 4. Plot of total alkalies against tetrahedral Al performula unit on basis of 23(O) for actinolite and horn-blende. Numbers I to 29 refer to microprobe analyses ofTable 2. The others, added for comparison, are by wetchemical methods and were recalculated on the anhydrousbasis of 23(O) per formula unit. Point 796 is for the wetchemical analysis of blue-green hornblende in sample 796(Hietanen, 1973). Analysis points 522 and, 474 are forhornblendes in the plutonic rocks in the study area(Hietanen, l97l); C2, C3, and C4 are for three amphi-boles from the neighboring Bidwell Bar area (Compton,1958); El4 and E415 are for hornblende from the Emery-ville area, Adirondack Mountains, New York (Engel andEngel, 1962);963 is for hornblende in garnet amphiboliteand 608 for hornblende in kyanite-bearing rock, Idaho(Hietanen, 1963); Sl and 57 are for hornblende fromAirola, Switzerland (Steiger, 1961).

6 "..3i


Leake (1955a, b, 1968, 1971) has compilednearly 1500 analyses of calciferous and subcalci-ferous amphiboles from the literature and has con-cluded that the content of Al'i is controlled by rockcomposition and pressure and that the maximumpossible Alnr increases as Alio increases. He has alsosuggested a nomenclature based on Si and Ca *Na * K contents in the half-unit cell. The analysesof the blue-green hornblende in the Feather Riverarea fall within tschermakitic hornblende, and thoseof the green hornblende within magnesio-hornblendein his scheme. These names are not adopted herebecause of chemical features (Figs. 4 and 5 ) thatbring the blue-green hornblende closer to the parga-site type than to the tschermakite type hornblende.Ernst ( 1968) uses the name pargasite for NaCa2Mg+AlSi6Al2Oe2(OH), and the name hastingsite for thecorresponding ferrous ferric end member NaCa2Fez*aFe"SioAlzOzr(OH),r. In the blue-green hornblendeanalyzed chemically (anal. 796), the extent of sub-stitution of Mg by Fe2. and that of octahedral Al byFe3* are both 1 :4. Thus the end member calledpargasite in the graphs (Figs.4 and 5) actually is amember that consists of 3/4 pargasite and, l/4hastingsite, using the nomenclature of 'Ernst (1968)and Phillips and Layton (1,964).

Robinson, Ross, and Jaffe (1971, Fig. 8) haveplotted A-site occupancy against tetrahedral Al fora large number of hornblende analyses selected fromthe literature. The trend in their diagram is remark-ably similar to the trend in Figure 4 of this paper,except for a much greater scatter of points. Thisscatter is probably due mainly to differences in en-vironments of crystallization because these analysesrepresent a collection of Ca-amphiboles from variousgeologic environments and diverse PT conditions.For comparison, two analyses of green hornblendefrom an anorthosite area in Idaho (Hietanen, 1963,anal. 608, p. 43, and anal. of hornblende in sample963, p. 33) are plotted in Figures 4 and 5. Thesehornblendes crystallized at the pressure of the triplepoint of kyanite, andalusite, and sillimanite 1-5kbar) and plot under (in Fig. 4) or above (in Fig.5 ) the trend line for the hornblendes in the FeatherRiver area, where the pressure of crystallization waslower (-{ kbar). The ratio of z4-site occupancy totetrahedral Al for these hornblendes is 1 :4. Similarratios ( I :4 and 1 :5 ) were calculated for two ana-lyzed gedrites in this area (Hietanen, 1959, 1963).

Added for comparison in Figure 4 are two analy-ses (E-14 and E 415) for blue-green hornblende

from Emeryville, Adirondack Mountains, New York(Engel and Engel, 1962) and two (S-1, $7) forhornblende in the Tremola series, Switzerland(Steiger, 1961). Metamorphism in Emeryville wasin the amphibolite facies and in medium pressures,whereas the Tremola series recrystallized at higherpressures as shown by the occurrence of stauroliteand kyanite in the pelitic layers south of St. Gotthard(Hezner, 1909; Hafner, 1958). The ratio of (Na *K)A/Alt' for the hornblende analysis S-1 is 1 :7, in-dicating a much higher content of tschermakite mole-cule than is present in the other hornblende analysesshown in Figure 4. Frey (1969, p. 68) gives twoanalyses of hornblende in kyanite-bearing rock atFrodalera, south side of St. Gotthard massive. Theseand a partial reanalysis of one of them given byLeake (1971, p. 394) confirm the high content oftschermakite in the hornblendes of this area.

Iron, Magnesium, and Titqnium Contents

The iron-magnesium ratio and the titanium con-tent of amphiboles depend more closely on the Fe,Mg, and Ti contents of the host rocks than on thegrade of metamorphism.

The Mg/(Fe * Mg) ratio shows considerablevariation but generally decreases slightly with in-creasing tetrahedral Al. All the actinolites are richin magnesium. The amphibole pairs-the Al-richhornblende and actinolite-in the metabasalt (Sp'

t \T s c h c ? m o k i l r

; : l

\st\ O o

Ac I ino l i l .

Frc. 5. Plot of octahedral Al { Fe8* } Ti against tetra-

hedral Al per 23(O). Numbers refer to the same analyses

as those in Figure 4'

E d ! n i l €A t l v

. 1 0

ANNA HIETANEN

551) are richer in iron than any other amphibole.This is due to the bulk composition of the meta-basalt, which has Fe/Mg = 1, whereas in all theother host rocks this ratio is less than l/2 (Hietanen,1973). Compton's (1958) hastingsite (C-4) withonly one Mg substituting for Fe2* is from an iron-rich layer in metabasalt east of Feather Falls.

To test the possible influence of oxygen fugacityon the Me/(Me * Fe) ratio of hornblende, thisratio was plotted against Fes-/(Fe2. * Fe3.) in thehost rocks (Fig. 6). Separate curves were drawn byvisual inspection for amphiboles with high, low, andmedian Al contents. In each of the amphibole groupsthe Mg/(Mg * Fe) ratio increases with increasingdegree of oxidation of the host rocks. Bard (1,970)has found a similar relation in the Ca-amphibolesfrom a comparable geologic setting in Spain.

TiO2 content of the amphiboles is variable. Itdoes not increase with the prograde metamorphismas has been found at higher grades by other authors(Binns, 1965b; Engel and Engel, 1962); rather itseems to depend on the available TiOz content ofthe host rocks. It is lowest in amphiboles from meta-gabbro 796 in which percentage of TiOz is only0.11 (Hietanen, 1973). It is highest in amphibolasfrom the low grade meta-andesite (sample 463) andmetadiorite (sample 465) that have 0.75 and 0.54

7 2l+17 s2+1Fe3. in hosl roc kFIo. 6. Plot of Mg/(Fe * Mg) in the amphiboles against

FeB*/(Fd* * Fe*) in the analyzed host rocks. Numbersalong abscissa refer to rock analyses in Hietanen (1973).

percent TiO2, originally contained in sphene andpyroxene. Metabasalt sample 551 has 2.03 percentTiOz, but most of it is in ilmenite. that was morestable than sphene and pyroxene during recrystalliza-tion.

Influence ol Temperature

The composition of the aluminous hornblende isinfluenced more by the grade of metamorphism thanis the composition of actinolite. Comparison ofamphiboles from metagabbro (sample 796), col-lected 0.3 mile from the pluton, and those frommetabasalt (sample 551), 0.8 mile from the pluton,reveals that those closer to the pluton (sample 796)contain blue-green hornblende that is richer in Alt'and alkalies than hornblendes from those fartherfrom the pluton. Thin-section studies of scores ofother specimens show that generally near the con-tacts of the plutons the hornblende is blue green,whereas elsewhere the amphiboles are green to palegreen. Analyzed green hornblendes such as those inmetadiorite samples 465 and 532 contain much lessaluminum and alkalies but have about the sameiron-magnesium ratio as the blue-green hornblendein sample 796 (l-3, Table 2b). Thus the Alt' andalkali contents of the hornblende increase with in-creasing grade of metamorphism, whereas the iron-magnesium ratio depends on the bulk compositionof the host rock.

fncrease in the alkali and Alt' contents of horn-blende with increasing grade of metamorphism hasbeen reported by many earlier workers in differentparts of the world (e.g.,Deer, Howie, and Zussman,1963, pp. 304-305). Binns (1965a, b) found aprograde increase in the content of alkalies, titanium,and tetrahedral aluminum in hornblendes from basichornfelses at Tilbuster and in the Broken Hill dis-trict, New South Wales. However, in a similar con-tact zone at Moonbi, the content of Ali" in horn-blende remained roughly constant. Cahill (neeMacara, 1968) described a definite increase in Ail"and alkali contents in hornblendes from basic horn-felses in southern New South Wales. Bard (1970)has pointed out that an enrichment of alkalies andincrease in Ti and Alto contents in hornblende withthe increasing P and ? exists but that their extentvaries.

The relation between the tetrahedral Al contentsof the amphibole pairs-actinolite and hornblende-and the estimated temperature of recrystallizationin the Feather River area is shown graphically in

*sQ

ss.sq)

\+

ss 1 2 .

r3 . .


Figure 7. The temperature in the zone next to thecontact (Fig. 1, sample 796, 546) of the plutons isestimated at 600"C on the basis of absence of silli-manite and coexistence of andalusite and cordieritein the pelitic layers. In the zone O.6 to 1.2 miles fromthe contact (sample 551 and 532), where stauroliteoccursi with andalusite in the pelitic layers, tempera-ture may have been about 500oC. For samples 465,463, and 464 that arc 3.2 miles from the contact,the temperature of recrystallization was estimated at350'C on the basis of mineral assemblages albite-actinolite-clinozoisite-epidote-chlorite and albite-chlorite-muscovite-biotite without garnet. Two curvesconnecting the actinolites on one hand and thealuminous amphiboles on the other were drawn byvisual inspection. The points for the amphibole insample 532 (23, 24, 25) fall in the "miscibilitygap" between actinolites and hornblendes of mediumtemperature. This quartz diorite has only one amphi-bole and only one epidote mineral, suggesting thatit crystallized in physical conditions that were dif-ferent from those of the other specimens in this zone.It may be a later dike crystallized at igneous tem-peratures as shown by arows in Figure 7. The tetra-hedral Al content in the "igneous" hornblende in thisarea (Hietanen, l97l) is plotted in Figure 7 (474,522) at temperatures of the beginning of the meltingof quartz diorite and quartz monzonite in order toindicate the upper limit of the possible immiscibility.

The coexistence of two amphiboles, actinolite andaluminous hornblende, has been previously describedby Shido (1958) and by Shido and Miyashiro (1959)from the Abukuma plateau, Japan, and from twospecimens of epidiorite in the Scottish Highlands.According to their descriptions, the parallel inter-growths and patches and rims of blue-green horn-blende within and around actinolite are similar tothe intergxowths of actinolite and aluminous horn-blende in the Feather River area. Shido and Miya-shiro (1959) suggested that there may be a misci-bility gap between the actinolite and hornblende atabout 1.0 AF". The Allo content, however, is typicalof the green hornblende in the Feather River area,and the miscibility gap, if. any, is a function of T(Fig. 7). Also Klein (1969) suggested that coexist-ing amphiboles may very likely represent equilibriumpairs across a miscibility gap.

Epidote Minerals

Electron-microprobe analyses of epidote minerals(Table 3) represent relatively homogeneous single

. g . . a 9 . 2l t

r zJl25 24

t 8

27 22 21

o 0 2 0 4 0 6 o t A , , r ' o t z t 4 1 6

FIc. 7. Estimated temperature of recrystallization of theamphiboles against tetrahedral Al per 23(O). Numbersrefer to Table 2. Points (X) are for "igneous" hornblendefrom Hietanen (1971).

grains or groups of grains; exceptions are discussedbelow. Composition ranges from zoisite and clinozoi-site with little iron to epidotes with pistacite, Ca2Fe8*BSisor2(OH), contents as much as 26 molecular per-cent. Most analyses show small amounts of man-ganese, titanium, and potassium. In the calculatedformula units, the silicon content is only a little higherthan the ideal three atoms, and the calcium contentis somewhat lower than two atoms. Two analyses(30 and 31) of zoisite in metagabbro sample 796have some sodium that replaces calcium. The varia-tion of Al and total Fe as Fe3* is shown in Figure 8.All analyses except 38 fall on a linear trend as theyshould. Analysis 38 was computed using the twohighest iron counts from the grain; the remainder ofthe counts gives analysis 36. Analysis 38 is thus notan independent analysis but rather shows, togetherwith analysis 36, the extent of possible variation iniron content within an individual grain. Analysis 37from the same specimen represents a homogeneous

t,\

A I

FIc. 8. Plot of Fe3* against Al in the epidote minerals'

Numbers refer to Table 3.

s\

A I I Y

2 3 2 t 2 2 2

ANNA HIETANEN

grain; thus 36 and 37 indicate the dominant com-position.

The range of pistacite content in some individualsamples is greater, and in others smaller, than wouldbe expected on the basis of the indices of refractionmeasured on the mineral powders. Some of this dis-crepancy results from averaging the counts in themicroprobe analyses. For instance, analyses of epi-dote in sample 465 (Table 3, anal. 41 and 42) show18 to 19 percent iron end member (pistacite), butthe highest microprobe counts for iron in this samplecorrespond to 11 percent FeO, equivalent to 22molecular p€rcent pistacite. In contrast, the lowestcounts yield 6 percent FeO and thus about 12 mole-cular percent pistacite. Both clinozoisite and epidotewere identified optically in mineral powder (TableI ). The composition based on the high iron counts isin agreement with the highest indices of refractionmeasured, but the lowest counts correspond to ahigher content of pistacite than is indicated by thelowest indices of refraction. These relations are ingeneral agreement with the exsolution textures seenin immersed grains (Fig. 3). Further discrepanciesresult from the inhomogeneity of the host rocks; theprobe analyses were made in a single polished thinsection, which represents a limited sampling of thespecimen from which mineral concentrate for immer-sion work was prepared. For example, in sample 532high index domains (t = 1.754, Ps 22) were de-termined optically in some grains of bulk composi-tion Ps 14 (y - 1.742). The electron-microprobedata for epidote in this sample (Table 3, anal. 43-45), however, indicate that the analyzed, glains arehomogeneous and contain 16 to 17 percent pistacite.Only two of the three epidote minerals in the meta-gabbro sample 796 were present in the polished thinsection; the third is an epidote optically similar toepidote in samples 532 and 551. Clinozoisite insample 796 is inhomogeneous; analyses 31 and 34,representing discrete crystals, reflect the observedlimits in Ps content. Zoisite in sample 796 occurs asveinlets and pods and formed later than the otherepidote minerals.

Thus the results of electron microprobe work andthe indices of refraction together with exsolutiontextures suggest that two distinct phases, an iron-poor clinozoisite and iron-rich epidote, are presentin all samples except in sample 532. A similar rela-tionship has been discussed by Strens (1965) fromtwo localities where a "miscibility gap" in the clinezoisite-epidote series occurred between compositions

Ps 8 and Ps 28 at low temperatures'. Sterns attributedthis to the presence of strain in the mixed crystals attemperatures below 550"C.

This can be further considered in the light of thestructure of epidote. According to Ito, Morimotqand Sadanag?i. (1954\, epidote consists of chains of(Al,Fe3.) Oo and (Al,Fe3-) Or (OH)B octahedra par-allel to b axis and bound together by Al,Fe3*, Ca,and Si atoms. Miyashiro and Seki (1958) have sug-gested that the AIO and AIOH sites in the chainsare filled mainly with Al atoms whereas the Fe8*atoms are located between the chains. In the com-mon epidote CazAl:FeSigolr(OH) all the octahedralsites between the chains would be oceupied by Fe8.to give a "stable" or unstrained structure.

According to the recent refinement of epidotestructure by Dollase (L971), the edge-sharing Aloctahedra form two types of chains: a single chainof M(2) octahedra and a multiple zig-zag chain ofcentral M(l) and peripheral M(3).These chainsare cross-linked by SiOe and SizOz groups and Ca isin large cavities between the chains and links. Thesingle chain octahedra M(2) contain only Al atomsand the substitution in the multiple M(I)-M(3)chain is nonuniform, the M(3) sites containing alarger fraction of Fe3* than the M(1) site. In thelow-iron epidote, all Fe3* is in M(3) sites.

All the analyzed epido,tes (Table 3) have con-siderably more Al than would be necessary to fillthe M(2) and M(l) sites in chains. If it is assumedthat all Fe3* is in M(3) sites, about 3/4 of thesesites are filled with Fe3* in epidotes from the lowgrade zone and only about l/3 of these sites in thecoexisting clinozoisite contain Fe3*. In epidotes fromthe high grade zone (Sp. 551, 546), about 1,/2 ot-the M(3) sites are occupied by Fe3. atoms. Thesedifferences in the extent of substitution of Fes* forAl indicate that certain substitutions may be morestable than some others at a given temperature.

Myer (1966) has reported coexistence of ferrianzoisite and epidote from calc-silicate rocks in Nor-way. Compositions given by him indicate 0.11 Fe'-atoms per formula unit in ferrian zoisite (Ca2Al2.stFe3"e.11Si3O1zOH) and 0.37 to 0.63 FeS. atoms incoexisting epidote Caz (Al,Fe3*) 3SisO12OH.

The stability relations between the epidote min-erals have been experimentally investigated by Nitschand Winkler (1965) and by Holdaway (1966).According to Holdaway (L966),i, clinozoisite andquafiz react to form anorthite plus grossularite 'at

about 660'C at pressure of 4 kbar. Nearly similar


results were obtained by Nitsch and Winkler (1965),who further showed that at pressures higher than3.5 kbar the stability field of zoisite extends to ahigher temperature than that of epidote. This rela-tion may explain the occurrence of zoisite near thecontact of Granite Basin pluton (sample 796). Atelevated temperatures near the pluton, zoisite crystal-lized in addition to clinozoisite.

Chlorite and Muscovite

The composition of chlorite (Table 4) is moreuniform than that of the amphiboles and epidoteminerals. Tetrahedral Al is close to 2.5, and octa-hedral Al is 2.55 to 2.73 in all analyzed chlorites.Mg/(Fe + Mg) ratio is 0.64 to 0.83, and thechlorite in metagabbro sample 796 is richest inaluminum and magnesium.

Some muscovite occurs with chlorite in metagab-bro sample 796. Electron-microprobe analysis (Table4) shows that it is a po'tassium-aluminum mica withonly about 1 percent MgO and little sodium andiron. Its phengite content is about 7 molecular per-cent.

Plagioclase

Plagioclase is anorthite in metagabbro sample 796and bytownite (Anzr) in the metagabbro sample546 (Tables 1 and 5). In all the other specimens,it is albite. The presence of irregular twinning lamel-lae with relict granular textures in specimen 796suggests that the anorthite crystallized from zoisiteat elevated temperatures. This unusual sequence ofcrystallization is supported by the extreme composi-tion of anorthitic plagioclase. The bulk compositionof the host rocks (Hietanen, 1973) indicates exten-sive changes in the chemistry of all metavolcanic andmeta-igneous rocks during their recrystallization. Allare impoverished in potassium, and most also insodium. As a result, petrochemical norm calculationsyield a high anorthite content for plagioclase. An-orthitic plagioclase crystallized in equilibrium withepidote from this composition at temperatures closeto 600'C.

Distribution of Fe, Mg, and Al AmongHomblende, Chlorite, and Epidote

The occurrence of amphibole pairs and two tothree epidote minerals together raises the questionof distribution of elements among these minerals.It is evident that the distribution changes with theincrease in metamorphic temperatures. Normally,

iron-rich epidote crystallizes with magnesian actino-lite at low temperatures of the green schist facies.Clinozoisite and epidote are common with horn-blende at intermediate temperatures whereas zoisiteis the only epidote mineral at very high temperaturesand pressures. Since the major changes in composi-tion in both mineral groups are tied to the changesin Al and Fe contents, the percentages of Alrotand FeO for each mineral in samples from the highgrade zone were plotted in the same graph (Fig. 9).Individual analysis points for each mineral were con-nected by a solid line, which shows the range ofAl2O3 and FeO contents.

4 P_. t2 16 20Act ino l r le l lB lue - green hornblende

We igh l pe rcen l i n omph ibo leFrc. 9. Distribution of ALO' and FeO in the analyzed

amphiboles and epidote minerals in the high grade zone.The range of the percentage of these oxides in minerals is

shown by solid lines, and the coexisting minerals of the

same species in sample 796 are connected with broken lines.

Epidote is from sample 546; optically similar epidote co-

exists with the other minerals in sample 796. Line OA

shows the equal distribution'

C,)

S$ssC)()q)

\

s.S.s

A c l ino l i te

-Zo i s i t e

Cl inoz o is i te

e p i d o i e

c l i no -zo i s i l e

B lue -g reenhornb lende

3 8 ANNA HIETANEN

In the upper h'alf of Figure 9, the abscissa showsthe range in Al2O3 percentages in amphiboles, andthe ordinate shows the range in Al2O3 percentages inepidotes. For easier observance, the actinolite-horn-blende pairs and the zoisite-clinozoisite pa,irs in sam-ple 796 were connected with broken lines. A gapin the AlzOa content between the actinolite and blue-green hornblende at the highest grade is wide in spiteof the overlap of the A12O3 percentage in this actino-lite and in the gteen hornblende. In contrast, thegap in the FeO content (lower part in Fig. 9) issmall; all the actinolites, however, have a lower FeOcontent than the hornblende. Most samples from thelow grade zone contain actinolite * green horn-blende, but in samples 463, 465, and 532 only themedium-Al hornblende was present in the polishedthin section.

Only a small gap exists in the AlsOs and FeOcontents between the coexisting zoisite and clinozoi-site, whereas epidote contains much less AlzOa andmore FeO than the coexisting clinozoisite. The gapbetween the clinozoisite and epidote in the low gradezone is distinct in samples 464 and 463, butin sample 465 only an intermediate compositionoccurs. This intermediate composition, however, in-cludes inhomogeneous counts, which, if separated,would give composition similar to the clinozoisite andepidote in the other samples.

The Fe/Al ratio is large in the low-Al actinolitesand decreases rapidly with increasing Al content. Inactinolitic hornblende this ratio is 1.6, in green horn-blende 1.1 to 1.3, and in the blue-green hornblendeat the highest grade it is 0.5. In the blue-green horn-blende in metabasalt (sample 551), which is rich inFe, the Fe/Al is of the same magnitude as in thegreen hornblende even if this sample comes from ahigher grade zone. This shows that FelAl ratiogenerally decreases with increasing temperature butis also influenced by the Fe/Al ratio of the host rock.

Comparison of the averageFe/ Al ratios in the co-existing epidote and clinozoisite from the low-gradezone (0.33 and 0.13 respectively; Sp. 463 and 464)with those from the high-grade zone (0.28 and 0.2,Sp. 551 and 546) suggests that the difference be-tween the Fe/Al values o'f the coexisting epidoteminerals is smaller in the high-grade zone. Epidotefrom the highest grade zone (sample 796) was notanalyzed because none was found in the polishedthin section. The indices of refraction measured inthe mineral powder, however, suggest that epidotein this rock is very similar in composition to that in

sample 551. Using the Fe/Al ratio of this epidote(0.27, Sp. 551), the distribution coefficient K1(Fe/Al) for epidote/clinozoisite in the highest grade zoneis calculated as Kp : 2.9, thus somewhat larger thanthe corresponding value in the low grade zone(Ko = 2.6) and much larger than Kp = 1.6 insample 551. This reversal of trend is due to a lowcontent of iron in clinozoisite (Sp. 796) that hasrims of zoisite dusted by iron oxide. Probably a partof the iron originally present in this clinozoisite wasexsolved upon the beginning of its transformationto zoisite.

All chlorites are rich in aluminum, but the mag-nesium/iron ratio ranges from 1.7 to 4.4, the chlo-rite in sample 796 being richest in magnesium. Dis-tribution coefficients are different for amphiboleswith different Al contents, as shown in Figure 10.For the hornblendes of median Al content (464h,463h, 465h), Kp(MglFe) chlorite/hornblende isgenerally between 0.7 and '1.2, and there is no ob-servable change with the temperature of metamor-phism (comp. 546h). However, the chlorite in thezone next to the pluton (in metagabbro Sp. 796) isricher in magnesium and contains less iron than anyother chlorite. Kp(MglFe) (chlorite/hornblende) inthis rock is 1.9, more than twice that for the chloriteand the average median-Al hornblende in the lowgrade rocks.

Summary and Conclusions

In the outer contact aureoles of the Cretaceousplutons, Northern Sierra Nevada, where the mineralassemblages indicate PT conditions at the border ofthe gteenschist and epidote amphibole facies, greenhornblende occurs with actinolite, chlorite, clino-zoisite, epidote, and albite. In the zone next to theplutons, where the occurrence of andalusite withcordierite and prograde pseudomorphs of muscoviteafter staurolite in interlayered pelitic rocks indicatesPT conditions at the higher limit of the epidote am-phibolite facies (T - 650oC at P - 4 kbar), horn-blende is blue green, and plagioclase is rich in theanorthite molecule. Zoisite occurs with other epidoteminerals.

The electron-microprobe analyses show that thealkali and aluminum contents of the hornblende in-crease with increasing grade of metamorphism, buttheir ratio remains constant. The generalized struc-tural formula of the green hornblende is (Na,K)s.3Ca2(Mg,Fe".) a.a (Al,FeS-,Ti ) 0.6AlSi?O2, ( OH ), andthat of the blue-green hornblende is (Na,K)o uCa,


(Mg,Fe) s.e(Al,Fe3*) 1.2Alr.zSio.aOzz(OH)2. The ratioof (Na + K)A/Ali" remains close to 0.3. The endmember in this actinolite-hornblende series is thehornblende ( Na,K ) o.oCaz ( Mg,Fe ) 3.6 ( Al,Fes. ) 1. aAl2Si6Op2(OH)2, which is derived from the actinolite bytwo coupled substitutions---edenitic (NaAli' for !ASit") and tschermakitic (AlriAliY for Mg'rSit") at theratio of 3:7.

The Mg/(Mg * Fe) ratio in amphiboles dependson the bulk composition and the degree of oxidationof the host rocks. The Ti content of hornblende re-flects the amount of available Ti in the host rocks.

The coexistence of two Ca amphiboles implies theexistence of a miscibility gap, if the amphibolescrystallized in equilibrium. The composition of horn-blende in the border zone of the plutonic rocks prevides the upper limit of the possible immiscibility.

Two epidote minerals-clinozoisite and epidotewith pistacite content from 10 to 26 percent---coexistwith actinolite, hornblende, chlorite, and plagioclasein all zones. Zoisite with anorthitic plagioclase crys-talliznd at the highest grade. Elsewhere plagioclase isalbite. The miscibility gap of epidote and clinozoisiteat temperatures below 600'C is suggested by com-positions of the coexisting mineral pairs.

The Fe/Al ratio in epidote decreases slightly withincrease in the grade of metamorphism, whereas inhornblende the decrease in this ratio is much greater;Fe/Al in the high grade hornblende is one half ofthat in the low grade hornblende. There is no regularchange in Kn(Fe/Mg) for chlorite/hornblende, al-

KO= 1.9

( '-l7z:'

KD = 0.5

.s.lr\

s-

1 2 3 4 5 6Mg/Fe in omphibote

Frc. 10. Distribution coefficient for average of MglFein coexisting chlorite and actinolite (a) and chlorite andhornblende (h). The numbers refer to the host rock samples.

though it is highest in the zone next to the plutons.It seems reasonable to assume that pressure remainedapproximately at the same level during the metamor-phism and that the changes described are due to theelevated temperatures near the plutons.

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qr

sss

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76"ti:--- ;.2

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Amphibole Pairs, Epidote Minerals, Chlorite, and ... · minerals coexist with chlorite and...

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