THE AMERICAN MINERALOGIST, VOL 50, JULY-AUGUST, 196.5 BASIC REGIONAL METAMORPHIC ROCKS IN PART OF THE KLAMATH MOUNTAINS, NORTHERN CALIFORNIA M. J. Hor.oew,xu, Department of Geology and. Geophysics, Southern M ethodist U nirersit^t.Dallas, T exas. Arsrxlcr Regional metamorphic rocks of basic composition in parts of Cofiee Creek and Cecilville quadranglesin tectonic sequence from bottom to top are: Stuart Fork greenstone) Salmon Hornblende Schist, Grouse Ridgehornblende schist and gneiss. Probable westward thrust- ing has placed early medium-grade rocks over more recent low-grade rocks. Mineralogic differences between the three units reflect difierences in history, composi- tion, and origin. Stuart Fork plagioclase is albite, whereas that of the other units is albite to sodic andesine. The medium-grade rocks show a plagioclase composition gap in the per- isterite range. Stuart Fork epidotes cluster near the ideal composition 33 mol per cent pga+/,{lf Fe3+,but some clinozoisite occurs, and there is evidence for a clinozoisite-epidote immiscibility. Higher grade rocks contain low-iron epidote. Amphibole is actinolite, Iocally crossite to ferric glaucophane in Stuart Fork rocks, and hornblende in Salmon and Grouse Ridge rocks. Chlorite shorvs an increase in Mg content in medium-grade rocks. The mineralogic evidence suggests that the earlier, medium-grade metamorphism was of the kyanite-sillimanite type (Barrovian), and the later low-grademetamorphismwasof the high-pressure intermediate type in Miyashiro's (1961) classification of facies series. Iurnorucrron The purposeof this paper is to describe the detailedpetrology of three adjacentbasicmetamorphicunits in the Klamath Mountains;to explain, in terms of compositionand metamorphic history, why the three units are distinct; and to discuss the nature of the metamorphism. These units, describedin the regional study of Davis el ol. (ms.), and in a doctoral thesis by the writer (Univ. Calif. Berkeley, 1962) are the green- stone member of the Stuart Fork Formation, the Salmon Hornblende Schist, and the hornblende schists and gneisses of the Grouse Ridge For- mation. All three comprise what was originally named Salmon horn- blende schist by- Hershey (1901). The area of this report is that mapped by the author (Fig. 1; Davis et al., rr;'s., Pl. 1), and it includes northwestern Coffee Creek and north- eastern Cecilville quadrangles. The area, one of regionally metamor- phosed rocks intruded first by peridotitesand then by Late Jurassic (140 to 125 m.y.) trondhjemites ald qnartz diorites (Davis et al., ms.), is located 65 miles northwest of Redding, California in the central meta- morphic belt of the Klamath Mountains (Irwin, 1960,p. 10, 15). Only the basic regional metamorphic rocks will be considered in detail. The pattern of metamorphism is complex as a result of (1) a rather uniform metamorphic grade within the rocks of a given formation, (2) differences in metamorphic grade and metamorphic history between formations, 953
Transcript
THE AMERICAN MINERALOGIST, VOL 50, JULY-AUGUST, 196.5
BASIC REGIONAL METAMORPHIC ROCKS IN PART OFTHE KLAMATH MOUNTAINS, NORTHERN CALIFORNIA
M. J. Hor.oew,xu, Department of Geology and. Geophysics, SouthernM ethodist U nirersit^t. Dallas, T exas.
Arsrxlcr
Regional metamorphic rocks of basic composition in parts of Cofiee Creek and Cecilvillequadrangles in tectonic sequence from bottom to top are: Stuart Fork greenstone) SalmonHornblende Schist, Grouse Ridgehornblende schist and gneiss. Probable westward thrust-ing has placed early medium-grade rocks over more recent low-grade rocks.
Mineralogic differences between the three units reflect difierences in history, composi-tion, and origin. Stuart Fork plagioclase is albite, whereas that of the other units is albite
to sodic andesine. The medium-grade rocks show a plagioclase composition gap in the per-isterite range. Stuart Fork epidotes cluster near the ideal composition 33 mol per centpga+/,{lf Fe3+, but some clinozoisite occurs, and there is evidence for a clinozoisite-epidoteimmiscibility. Higher grade rocks contain low-iron epidote. Amphibole is actinolite, Iocally
crossite to ferric glaucophane in Stuart Fork rocks, and hornblende in Salmon and GrouseRidge rocks. Chlorite shorvs an increase in Mg content in medium-grade rocks.
The mineralogic evidence suggests that the earlier, medium-grade metamorphism wasof the kyanite-sillimanite type (Barrovian), and the later low-grademetamorphismwasofthe high-pressure intermediate type in Miyashiro's (1961) classification of facies series.
Iurnorucrron
The purpose of this paper is to describe the detailed petrology of threeadjacent basic metamorphic units in the Klamath Mountains;to explain,in terms of composition and metamorphic history, why the three unitsare distinct; and to discuss the nature of the metamorphism. Theseunits, described in the regional study of Davis el ol. (ms.), and in adoctoral thesis by the writer (Univ. Calif. Berkeley, 1962) are the green-stone member of the Stuart Fork Formation, the Salmon HornblendeSchist, and the hornblende schists and gneisses of the Grouse Ridge For-mation. All three comprise what was originally named Salmon horn-blende schist by- Hershey (1901).
The area of this report is that mapped by the author (Fig. 1; Daviset al., rr;'s., Pl. 1), and it includes northwestern Coffee Creek and north-eastern Cecilville quadrangles. The area, one of regionally metamor-phosed rocks intruded first by peridotites and then by Late Jurassic (140to 125 m.y.) trondhjemites ald qnartz diorites (Davis et al., ms.), islocated 65 miles northwest of Redding, California in the central meta-morphic belt of the Klamath Mountains (Irwin, 1960, p. 10, 15). Onlythe basic regional metamorphic rocks will be considered in detail. Thepattern of metamorphism is complex as a result of (1) a rather uniformmetamorphic grade within the rocks of a given formation, (2) differencesin metamorphic grade and metamorphic history between formations,
953
954 M. J. EOLDAWAY
Frc. 1. Generalized geologic map of the area discussed in this report. The location of the
report area and the California Klamath Mountains is shor,vn on the index map of northern
California at Iower left.
and (3) retrogressive metamorphism in some units. Accordingly, there isno simple series of progressive metamorphic zones.
On the largest scale, the metamorphic units comprise an antiform;the compiete structural sequence from bottom to top is: Stuart Forkmetacherts followed by Stuart Fork greenstone, Salmon HornblendeSchist, Grouse Ridge hornblende schist, and finally Grouse Ridge mica-ceous and calcareous metasediments. During Pennsylvanian (?) time(Lanphere and Irwin, 1965), the structurally highest Grouse RidgeFormation was metamorphosed largely to the staurolitel zone, while the
I The metamorphic facies and subfacies classification used in this report is essentialiy
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METAMORPHIC ROCKS OF I:HE KLA],IATH MOUNTAINS 955
underlying Salmon formation was metamorphosed to the garnet zoneand in part the staurolite zone. Metamorphism largely in the chloritezone. affected all units. It is possible that the Salmon and Grouse Ridgeformations were metamorphosed to medium grade, and later thrust overthe Stuart Fork Formation. Low-grade progressive metamorphism ofthe Stuart Fork rocks and retrogressive metamorphism of the allochthon-ous (?) units pre-dated and, in part, post-dated thrusting. G. A. Davis(writt, comm., 1963) believes a thrust-fault relationship between theGrouse Ridge and Salmon formations may exist, although evidence forsuch thrust-faulting is not conclusive. A tectonic relationship betweenthe two formations could explain apparent higher grade of metamor-phism exhibited in several areas by the structurally highest Grouse Ridgerocks.
Besrc MBraMoRpHrc UNrrs
The sequence: Stuart Fork greenstone, Salmon hornblende schist,Grouse Ridge hornblende schist, comprises a continuous series of basicmetamorphic rocks which at first sight appear indivisible. However, eachunit is distinct, and differences between the units have considerable bear-ing on the geologic history of the area. The Stuart Fork greenstones arecommonly fine-grained, unlineated rocks, which only show a foliationwhen near plutons. fn abrupt contact with these rocks are the Salmonhornblende schists, which are coarser and show preferred orientation ofhornblende prisms producing both planar schistosity and lineation. Theserocks are in many places separated from the overlying Grouse Ridgerocks by a thin zone of Salmon Hornblende Schist (a shear zone?) con-taining numerous thin feldspathic lenses, many of which have becomerootless isoclinal folds. Finally, the Grouse Ridge hornblende schists maybe distinguished by coarse grain size, local gneissic banding, poorly de-veloped planar schistosity and lineation, and presence of micaceous orcalcareous layers and periodotite-serpentine intrusions. The mineralcontent and variability of rocks of the three units are summarized inTable 1. A brief petrographic description of each unit is given below.
Stwart Forh greenstone. Most of the Stuart Fork greenstones exhibit littleor no preferred orientation of minerals, but some show primary igneous
that of Turner in Turner and Verhoogen (1960) and Fyfe et ol. (1958) modified from Eskola(1939). For brevity subfacies are referred to by their zone names as shown below. Thus theuse of a zone name does not necessarily imply the presence of that mineral.
Chloritez. Greenschistfac., quartz-albite-muscovite-chloritesubf.Biotite z. Greenschist lac., quaftz-albite-epidote-biotite subf.Garnet z. Greenschist fac., quartz-albite-epidote-almandinesubf.Staurolite z. Amphibolite fac.. staurolite-almandine subf.
956 M. J. IIOLDAWAY
textures (Davis et al., ms.). The principal mineral assemblage is actino-lite-epidote (and,/or clinozoisite) -chlorite-albite-quartz-leucoxene. Cal-cite, hematite and ore are possible minor constituents. Crossite or ferricglar.rcophane occrrrs with epidote in a few specimens from northeasternCecilvi l ie quadrangle on the contact between greenstones and sil iceousrocks. The blue amphibole commonly fi l ls veinlike areas in greenstonesbut locally occurs as the dominant mineral in schists. In blue amphibolerocks actinolite is absent or is largell ' replaced b-v chlorite and green
biotite.
Salmon, Hornblende Schist. The mineral assemblage of the Salmon schistsis hornblende-albite (locally oligoclase)-epidote-chlorite-quartz-sphene.The rocks are reasonably restricted in composition in that clinopyroxeneand calcite are rare and garnet is absent. The occurrence of clear albitewith no indication of more calcic plagioclase suggests that most Salmonrocks 'nvere metamorphosed only to the garnet zone, but metamorphicgrade reached the staurolite zone in the few oligoclase-bearing rocks inthe eastern part of the area.
Most Salmon rocks contain well defined veins of actinolite, albite, andqtartz, but some rocks, near the Stuart Fork greenstones in Cecilvi l lequadrangle, contain calcite or chlorite which formed retrogressively dur-ing deformation after the main metamorphism, possibly during thrust
'I'aer-e 1. Avnltcns mgo RnNcBs or MtNrn.q.l Colrposttl<tn lon B,lsrc Rocrs ol rxr'Innrr Fonuarroxs. Voluur Pnn CrNr Basao oN Vrsuar Esrrulms
6. Compositional range in the Grouse Ridge hornblende schists.
METAMORPIIIC ROCKS OF THD KLAMATH MOUNTAINS 957
faulting. In some rocks of the formation ruti le is rimmed b1- an opaquemineral (i lmenite?) and in others the opaque mineral is rimmed bysphene. The sphene rims may not be retrogressive as was the case for theBessi district (Banno and Kanehira, 1961), because the occurrence ofsphene rims apparently does not correlate with known retrogressive min-erals. Retrogressive effects in Salmon rocks can best be ascribed to condi-tions of the chlorite zone andfor biotite zone.
Growse Ri,dge hornblende schist. The coarser Grouse Ridge rocks aresimilar in mineral content to the Salmon schists except for a greater de-gree of variabil ity. Most rocks contain hornblende-albite, oligoclase, orandesine-epidote-chlorite-quartz-sphene-ore, although any mineral otherthan hornblende may be absent. Small amounts of l ight brown biotite,calcite, diopside, or almandine produce greater varietl, than is seen inSalmon schists. Of the minerals present onlv diopside and almandine aremutuallv exclusive, their place being taken by hornblende. However,in southwestern Coffee Creek quadrangle, Davis (writt. comm., 1965)reports coexistence of much garnet and diopside in two Grouse Ridgerocks. The garnet in one such specimen was identif ied as calcium-bearing almandine on the basis of refractive index, unit cell size and al-teration to chiorite.
Retrogressive metamorphism is more extensive in Grouse Ridge rocksthan in the other units. In most of them hornblende has been partlv re-placed along fractures and grain boundaries by pale actinolite. Plagio-clase commonly contains saussurit ic patches or coarser clinozoisite inclu-sions, suggesting alteration from a more calcic composition. Garnet,where present, is partly altered to chlorite and light brown biotite. Tracesof biotite in other basic rocks are retrogressive, resuiting from release ofKrO as hornbiende (.36 to .737aK2O, Table 3) is replaced by actinolite.Chlorite replaces some of the hornblende in a few rocks, but muchchlorite appears to be the result of progressive metamorphism. Theretrogressive metamorphism was in the chlorite and probably biotitezones (a few Grouse Ridge pelit ic rocks contain fine-grained biotite n'hichmay be retrogressive).
Mrrqpnarocv lul Cnn,urstnv
Optical and chemical data have been obtained for a number of min-erals in the basic metamorphic rocks with a view to showing differencesand similarities within the three formations and between these rocks andother metamorphic terranes. The data from which the diagrams in thissection were prepared are presented in table form in the appendix.
G R O U S E
M. J, HOLDAWAV
4 0
P L A G I O C L A S E O / O A N
Frc. 2. Frequency diagram of plagioclase composition in basic metawrorphic rocks
described. Each square represents a single measurement. Between one and three measure-
ments were made for most specimens with suitable plagioclase.
Plagioclase. Plagioclase compositions for rocks in the three formationswere determined by the Federow method using the curves of Slemmons(1962) and are considered to be accurate to two or three mol per cent(Fig. 2). Plagioclase of the Stuart Fork greenstones is entirely albite Anoto An2, which in every case coexists with epidote or clinozoisite. The otherformations contain oligoclase and sodic andesine as well as albite.
The average composition of the Grouse Ridge plagioclase is more calcicthan that of the Salmon rocks. For Grouse Ridge rocks, 32/6 oI the meas-urements yielded plagioclase more calcic than An26, while only t8/e oIthe measurements in Salmon rocks were above An26. As noted above, theGrouse Ridge plagioclase also shows greater evidence of retrogressionfrom calcic compositions than the Salmon plagioclase. Because plagio-clase in both formations coexists with epidote, the above evidence sug-gests that progressive metamorphism of the Grouse Ridge rocks reacheda higher grade on the average (largely staurolite zone) than the Saimonrocks (largelv garnet zone). An alternative explanation is discussed inthe section on epidote minerals.
The compositional gap in plagioclase commonly seen in regional meta-
3 0
S A L M O N
S T U A R T F O R K
METAMORPHIC ROCKS OF THE KLAMATH MOANTAINS
morphic rocks (e.9. deWaard, 1959) is exemplified by the frequency dia-grams for plagiociase composition in Salmon and Grouse Ridge rocks.OnIy one determination gave a value between Anrz and An22. Neverthe-less, the compositional gap is much less than would be expected from thecompositions of the exsolved phases in peristerites, An6 to An3 and An2sto An36 (Laves, 1954). There are two possible explanations for this. (1)The plagioclase crystallization may have been at a high enough tempera-ture that the composition gap was significantly narrower than that for
Fro. 3. Possible modifications of Ramberg's (1949) plagioclase-epidote equilibriumcurve for constant Psre and P61 as influenced by peristerite unmixing. The upper curvewould exist if the plagioclase-epidote equilibrium is always greater than the top of theperisterite solvus, rvhile the lower curve would exist if unmixing occurred at the tempera-ture of plagioclase-epidote crystallization.
unmixed peristerites (Fig. 3). The sodic plagioclases might now be peris-terites in themselves as a result of slow cooling after initial crystalliza-tion. (2) Crystailization might have taken piace at such temperaturesthat plagioclase of all compositions lay above the solvus. The scarcity ofcompositions between An12 and An22 would then result from a flatteningof the epidote-plagioclase equilibrium curve as it passed over the solvus(Fig. 3). The existence of the peristerite immiscibil i tv at low temperaturesis due to positive heat of solution of Ane and An25 which presumabl.vexists in the more ordered plagioclases. This means that at low enoughtemperatures free energ_v of solution becomes positive according to therelation.
4 02 0
A G " : 4 1 1 " - T A S "
M, T. HOLDAWAY
despite the fact that entropy of solution (AS") is alwa1's a positive quan-tity. This positive heat of solution would sti l l exist in relatively orderedfeldspars above the solvus. Its effect wouid be to reduce the free energr- ofsoiution of albite in anorthite for the peristerite range. This reduction inturn would tend to flatten the epidote-plagioclase equil ibrium curve butnot make it strictly horizontal. In the peristerite range, a small tempera-ture change would produce a large composition change, and sodic oligo-clase wouid be less common but not altogether absent. This explains theexistence of rare metamorphic plagioclases within the gap (see also de-Waard, 1959) which must otherwise be explained b1'metastabie cr1,s1u1-Iization. According to this explanation manv of the plagioclases mightnow be peristerites due to unmixing during cooiing.
Epidote mi.nerals. Ciinozoisite and epidote have been identified in thebasic metamorphic rocks. The Stuart Fork greenstones contain stronglypleochroic epidote and iarge areas of colorless clinozoisite in minutegrains with anomalous blue interference colors, identified on the basis ofr-ray and refractive index. Salmon schists contain moderately pleochroicepidote, and Grouse Ridge rocks contain nearlv colorless epidote. BothSalmon and Grouse Ridge epidotes are weakll ' zoned and have morealuminous rims.
Approximate compositions of epidote minerals from the three forma-tions, based on 2V or refractive index measurements, are presented inFigure 4. The approximate nature of these determinations is empha-sized; any- composition ma.v be off by as much as 5 mo1 per cent. TheStuart Fork epidote minerals are in the ranges Ps2s1 to Ps3s and Psz toPs16, Salmon epidotes are Ps1 to Ps3a, and Grouse Ridge epidotes arePs11 to Ps2a. Most samples show a range of composition, the mean ofwhich is plotted in Figure 4.
In a general way, there is a decrease in iron content of the iron epidotewith increasing grade of metamorphism. The effect of increasing grade toproduce more aluminous epidotes is probablv not the result of dehydra-tion of the more iron-rich epidotes at lower temperatures. Indeed, workin progress by the author shows that the breakdown temperature of epi-dote and quartz to anorthite and grossularite is dependent on iron con-tent, but the more iron-rich epidotes persist to higher temperatures.Earlier work by Winkler and Nitsch (1963) and by S. Merrin (1962, un-pubt. Ph.D. Thesis, Pennsylvania State Univ.) indicated that iron con-tent of the s1'stem has no effect on this reaction. The temperature depen-
1 Ps:s indicates 29 mol per cent of the hypothetical molecule CazFe:SisO12(OH), 71
mol per cent Ca:AIsSLOlr(OH).
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METAMORPIIIC ROCKS OF THE KLAMATII MOUNTAINS 961
Frc. 4. Epidote composition in basic metamorphic rocks described, as estimated from2V ('frdger, 1959) or from B (Deer et al.,1962). The progression from left to right withinrocks of a given unit is toward more aluminous compositions. A vertical line on the symbolindicates coexistence of epidote and clinozoisite.
dence on iron content of epidotes might weli be the same when the reac-tion involves hornblende instead of grossularite.
At least three factors may be important in explaining the variation iniron content of the epidotes.
(1) In Stuart l'ork rocks, which crystallized at Iowest temperatures, the tendency for theiron epidote to have a composition near Ps33 is in agreement with the observations ofMiyashiro and Seki (1958a) on low-grade rocks. There is also a composition gap betweenPsro and Pszg. In two rocks clinozoisite and epidote crystallized together in apparent equi-librium.The scarcity of clinozoisite and two-epidoteassemblages in these rocks is a reflec-
tion of the generally high ratio of Fe3+/Al*Fe3+. Strens (1963) has also observed such a gap(Psr3 to Pszr) in very low grade rocks and explains it as a miscibility-gap between clinozoi-site and epidote. The Salmon and Grouse Ridge epidotes, which crystallized at higher tem-peratures, filI in the gap, and presumably formed at temperatures above the solvus. (2)
With increasing temperature one might expect a progressive reduction of the iron, unless
the system was closed to oxygen allowing oxygen pressure to build up. (3) Differences ininitial content of FezOs are likely to be important, especially in Grouse Ridge rocks whichshow low Fe:Os in the amphiboles as well (Table 3). Possibly, Iower content of Fe2OB, pro-
ducing more aluminous epidotes in the Grouse Ridge rocks, favored reaction of albite and
epidote to oligoclase at slightly lower temperatures (see above), hence producing more
962 M. J. HOLDAWAY
calcic plagioclase under the same conditions of metamorphism as those which afiected theSalmon rocks.
In summary, the differences in epidote composition between StuartFork and Salmon basic rocks may be explained by an immiscibil i ty atlow temperatures and a greater variabil ity in init ial FezOa content ofStuart Fork rocks. Differences between Salmon and Grouse Ridge epi-dotes reflect either init iai differences in FezOr content or conceivably pro-gressive reduction of iron at higher temperatures if such higher tempera-tures existed.
The occurrence of iron-poor epidote minerals in the basic rocks of theKlamath Mountains has bearing on the use of clinozoisite as an indicatorof metamorphic grade. Hamilton (1963) has mapped a clinozoisite iso-grad between the biotite and garnet isograds in basic contact-relatedmetamorphic rocks near the Idaho bathoiith. It is the contention of thepresent writer that there is no reason for the instabil it,v of clinozoisite inchlorite-zone rocks of any metamorphic terrane. Clinozoisite is certainlypresent in chlorite-zone rocks of the Klamath Mountains. Two factorsmay have played a role in the observation of a clinozoisite isograd in theIdaho area.
(1) T'he clinozoisite ivhich did appear (identified by its Iack of color) may have been iron-poor epidote like that of the Grouse Ridge Formation, in u'hich case it u'ould not have ap-peared until temperatures were high enough to insure stable compositions in the miscibilitygap. In this case two epidote minerals might have appeared at lower grades, the one strongly
colored and Fe-rich, and the other true clinozoisite, which could have gone unobserved (2)
Rock compositions in the lower grades could have been rich enough in iron so that oniy epi-
dote rrithout true clinozoisite appeared.
The present study suggests that one can use iron-poor epidote (Psrr, toPszr,) as a zone indicator dependent on an epidote immiscibility at lowtemperature. In the Klamath Mountains iron-poor epidote is apparentlynot stable in the chlorite zone but is stable throughout the garnet andstaurolite zones. Hence its f i.rst appearance rvould occur between thebiotite and garnet isograds.
The more aluminous rims seen on epidotes of many Grouse Ridge andSalmon rocks pose a problem. Banno and Kanehira (1961) show thatiron-rich rims in epidote of basic schists result from retrogressive meta-morphism concurrently with chloritization and armoring of rutile bysphene. In the Klamath Mountain rocks the effect on epidote is oppositeeven in retrogressed rocks, suggesting that either the epidote rims are theresult of progressive metamorphism, or preferably that they are retro-gressive, resulting from a mechanism not operative in the Japaneserocks (e.g., introduction of solutions poor in FezOs).
METAMORPHIC ROCKS OF THE KLAMATH TIOUNTAINS 963
Amphibole. Actinolite, hornblende and crossite have been studied opti-cally and chemically. Measurements of B and 2Y have been made to aidin showing differences between amphiboles of the three units and to re-late optical properties to composition.
1. Calciferous Amphibole. All the calciferous amphiboles are similarin appearance, but those of the Stuart Fork greenstone are fibrous tovarying degrees and locall.v occur as glomeroblastic replacement offormer augite phenocrysts. The optical properties of amphiboles in the
three units are summarized in Table 2. Grouse Ridge and Stuart Forkamphiboles have similar pleochroism while Salmon amphiboles aredarker. A 2V-P plot (Fig. 5) of the type used by Shido and Miyashiro(1959, p. 86) for calciferous amphiboles of basic regional metamorphicrocks indicates that the amphiboles of the three units are optically dis-tinct from each other and that Stuart Fork amphiboles are actinolitewhile those of the Grouse Ridge and Salmon rocks are hornblende.
Chemical analyses (Table 3) were made for the nine calcilerousamphiboles numbered in Fig. 5. The amphiboles were separated fromcoexisting minerals by fine grinding and repeated separations with theisodynamic separator and heavy liquids. Impurities, chiefly in the formof minute epidote inclusions, were estimated to be less than one per centin all but three of the samples; 46578 contained about one per cent ofimpuritv, C-57 contained about two per cent of epidote, and C-165 con-tained about four per cent of clinozoisite. The latter two samples, fromthe Stuart Fork greenstone, were among the best materials available andeven grinding to less than 325 mesh per inch did not effect the removal ofall the inclusions. The analyses of these two rocks were corrected for theimpurity using appropriate epidote analyses. The changes are small, ex-cept for AlzOs in sample C-165, which is reduced by t. l/6. Structuralformulae are given for both the corrected and uncorrected analyses.
Structural formulae, calculated on the basis of 24 (O, OH) are in rea-sonable agreement with those of other hornblende and actinolite anlayses.
I2v*
964 M. J. HOLDAWAY
The maximum tetrahedral aluminum in the corrected Stuart Fork actino-lites is 0.66, which is high for actinolite according to Deer et aI. (1963);
however, the most aluminous actinolite cited b1' Compton (1958)
contains 0.53 Alrv, and the least aluminous metamorphic true hornblendecontains 0.84 AUv. Furthermore the actinolites fall in or very near the
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Flc. 5. B-2V diagram for caiciferous amphiboles in basic metamorphic rocks described.
Circles-Stuart I'ork actinolite, dots-Grouse l{idge hornblende, r's-Salmon hornblende.The heavy line approximately divides the diagram into fields of actinolite and hornblende
for basic regional metamorphic rocks of the cental Abukuma Plateau (Shiclo and Miyashiro,
19s9).
actinolite field in a Mg-[Fv diagram (Fig.6; Shido and Miyashiro,1959). All the other calciferous amphiboies fall in the hornblende fieid.Non-garnetiferous Grouse Ridge rocks contain hornblende with 1.14 to1.54 Alrv, Salmon hornblendes contain I.32 to 1.60, and hornblende froma garnet-bearing Grouse Ridge schist contains 1.79. Thus the miscibil i tvgap between low-grade actinolite and hornblende suggested b,v Shido andMiyashiro is not disproven bv available anall'5ss of Klamath Mountainamphiboles.
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A C T i N O L I T E
A t r Y - +
Fro. 6. Mg-AIrv diagram for calciferous amphiboles of basic metamorphic rocks. Thecomposition fields ol actinolite and common hornblende are after Shido and Miyashiro(1959). Symbols as in Fig. 5. The positions of symbols are recalculated on the basis of0:23 to be consistent with the composition fields of Miyashiro's diagram.
Calcium content is normall all samples contain between 1.61 and 1.95Ca. The sum of atoms in the Y sites varies from 4.96 to 5.16. The contentof the vacant site (Na+K*Ca-2) is less than 0.09 in the actinolitesand 0.26 to 0.43 in the hornblendes. The unbalanced tetrahedral charge(Alrv less octahedral trivalent ions), which should be balanced by Na*Kin the vacant site, is in each case within 0.25 of the content of the vacantsite. This indicates only approximate agreement with the accepted solidsolution substitutions Al(Al, Fe3+) for Si(Mg, Fe2+) and Al(Na, K) for Si.
Several samples contain siightly more than the requisite two OH-, butonly one (46578 with 2.39) is considered to be unusually high. This maybe due to an error in analysis or the replacement of some oxygen in thelattice by OH- (Nicholls and Zussman, 1955).
Due to the large number of variables, no attempt will be made to cor-relate optical and chemical variation in detail. However, the grouping ofopticai data for amphiboles of each formation (Fig. 5) should be ex-plained. The Stuart Fork actinolites presumably owe their low 2V andrefractive indices to Iow content of AIrOs compared with that of the horn-blendes. Reduction in alkalis might contribute to the lowering of 2V inmagnesian actinolites (Winchell and Winchell, 1951, p. 434-435). In partthe low FeO contributes to low refractive indices. The optical differencesbetween Grouse Ridge and Salmon hornblendes cannot be explained byvariation in A12O3, FeO, MgO, or alkalis because hornblendes of each
t 8t 6t 4r o
METAMORPHIC ROCKS OF THE KLAMATH MOUNTAINS 967
unit include approximately the same ranges of each of these. The singlechemical characteristic which distinguishes Grouse Ridge from Salmonhornblendes is content of ferric iron; Grouse Ridge hornblendes containbetween 0.24 and 0.45 Fe3+, whereas Salmon hornblendes contain 0.47to 0.63. Apparently the higher ferric iron increases refractive index andreduces 2V. The differences in other elements, which are not characteris-tic of a given formation, then superimpose their effects on the positionsfor the minerals on the diagram. The suggested control by ferric iron ofthe differences between properties of Grouse Ridge and Salmon horn-blendes is in gocd agreement with the darker color of hornblende andepidote in the Salmon Hornblende Schist.
Most of the analyzed hornblendes coexist with sodic plagioclase andepidote with or without chlorite; but two specimens are unusual withrespect to coexisting minerals (Table 3). Hornblende from specimen C-69is in apparent equilibrium with coarse-grained muscovite. The presenceof muscovite in the rock has little effect on the potassium content of thehornblende, although K is higher in this hornblende than in any otheranalyzed. Specimen 46578 is a garnet amphibolite whose hornblendecontains more FeO and AIrOe than any other specimen analyzed. Thisimplies that both elements act together as controls on the formation ofalmandine-rich garnet in amphibolites. Possibl-v FeO is the more impor-tant, because magnesian compositions would favor a magnesian chloriteat this grade. Apparently Grouse Ridge hornblendes with the highestrefractive index and 2Y are those which occur with garnet (Fig. 5).
Shido and Miyashiro (1959) have differentiated basic metamorphicrocks of the central Abukuma plateau in terms of three zones of pro-gressive metamorphism: A, characterized by actinolite; B, characterizedby bluish green common hornblende; and C, characterized by green tobrown common hornblende. The actinolites of zone A differ from thehornblendes of zone B in having more Mg and less AI, while the horn-blendes of zone B commonly differ from those of zone C in having lessalkalis and less titanium. The Salmon and Grouse Ridge hornblendes areoptically and chemically similar to those oI zone B, which cr1-stallized atapproximately the same grade. However, the Klamath Mountain horn-blendes differ from other hornblendes in rocks of the same grade in thatthey contain slightly more alkalis. The Klamath specimens average 0.56NalK, Dalradian hornblendes average 0.51, and central Abukumahornblendes average 0.46 (Shido and Miyashiro, 1959).
In summary calciferous amphiboles of the Stuart Fork Formation areestablished as actinolites, and those of the Salmon and Grouse Ridgeformations are hornblende. Grouse Ridge and Salmon hornblendes differin that the Grouse Ridse hornblendes contain less ferric iorn. Grouse
968 M. I. HOLDAI|/AY
Ridge hornblendes richest in ferrous iron and aluminum commoniy occurwith garnet. The hornblendes are similar to those of comparable gradefrom other metamorphic terranes, except for slight11. higher alkali con-tent.
2. BIue Amphibole. In northeastern Cecilvi l le quadrangle, blue amphi-bole occurs locallf in the Stualt Fork greenstones at the contact withmetacherts of the same formation. One blue schist (specimen 121) is com-posed largely of highly fibrous blue amphibole showing variable opticalproperties. Only approximate and incomplete optical properties wereobtainable due to the fibrous and variable nature of the amphibole. Mostabundant is f ibrous material with anomalous brown to blue interferencecolors and other optical properties as given in Table 4. Less common isnon-fibrous crossite with normal interference colors. There appears to be
T.qrln 4. Oprrcer, Pnopnnrms ol BruB Aulnmor,r, Slrcrutm 121.INorces enn *.005, 2Vo rs Esrrrremo
complete variation between the types: intergrowths of anomalous brownand anomalous blue, and of anomalous blue and normal amphibole arecommon in the rock. The anomalous brown mineral, ferric glaucophane,has the pleochroic scheme : X : l ight yellow, Y : violet-blve, Z :greenishblue. The anomalous biue and normal mineral, both crossite, haveX : Iight vellow, Y : greenish blue, Z : violet-blue. The extinction anglein all the blue amphibole is 3o * 3o. A trace of green amphibole in the spec-imen (a: I.640,2\c-- 15", moderate birefringence) is either actinolite orabnormal glaucophane (Winchell and Wincheil, 1951, p. aa3). Iron-richchlorite and epidote also coexist with the crossite-ferric glaucophane in-tergrowth.
The blue amphibole (specimen 121) was chemically analyzed (Table3). Impurit ies of epidote and green amphibole comprised less than oneper cent of the analyzed sample. The analysis represents the averagecomposition of the blue amphibole consisting of both ferric glaucophaneand crossite. Minor abnormalit ies in the composition mav be related to
METAMORPHIC ROCKS OF THE KLAMATH MOUNTAINS 969
the fi.brous nature of the mineral. Both the X and Y positions are incom-pletely filied; there are insufficient trivalent ions and Na, whereas Ca ishigh. The anal1'sis actualiy represents a solid solution ol 8316 crossite,l7o/s actinolite. The content of water is high and specific gravity is low.Most of the individual chemical abnormalit ies mal, be seen in othercrossites and ferric glaucophanes, but the overall deficiencf in the X andY sites is unusual (Deer et al., t963).
Unlike the blue amphibole from collected samples in the area of thisstudl-, glaucophane or crossite where seen elsewhere in the KlamathMountains coexists with lawsonite in some samples from each locality.Glaucophane schist has been found in southern Cecilville quadrangie b1'G. A. Davis (writt. comm., 1964), and in southwestern Helena quad-rangle by D. P. Cox (1956, unpubl. Ph.D. Thesis, Stanford Univ.). Inthese two areas it is believed to represent local high-pressure conditionsin the glaucophane schist facies, while in northeastern Cecilvi l le quad-rangle blue amphibole and epidote without lawsonite or jadeite implyIower pressure conditions equivaient to the glaucophane-schist facies,crossite subfacies of Misch (1959) or the epidote-glaucophane subfaciesof Miyashiro and Seki (1958b). According to Turner and Verhoogen(1960, p. 543) such rocks represent conditions transitional to the green-schist facies.
Chlorite. Chlorite occurs in many- basic rocks from all three units, and itsoptical properties vary as does its genesis. Progressive metamorphicchlorite is present in most Stuart Fork greenstones. In Salmon andGrouse Ridge basic rocks the mineral is prograde, retrograde (replacinghornblende or garnet), or absent.
The chlorites of the actinolite- and hornbiende-bearing rocks fall intothree groups according to optical properties: (1) l ight green chlorite withanomalous brown interference colors, small positive 2V, and B:1.612 to1.625; (2) green chlorite, with anomalous brown interference colors andsmall positive 2V or anomalous blue colors and small negative 2Y, and
0:1.627 to 1.628; (3) green chlorite with anomalous blue interferencecolors, small negative 2V, and B:1.630 to 1.636. The transition fromoptically positive to optically negative (at 1.627 to 1.628), which occursas iron content of the chlorite increases, was also observed by Wiseman(1934, p. 361) in Dalradian basic metamorphic rocks. He found the tran-sit ion occurred at B: 1.631. According to the classification scheme of Hey(1954) the present chlorites are aluminous iron pycnochlorite and aiumi-nous magnesian brunsvigite; their major compositional variation is iniron content rather than Si/Al ratio. For the purpose of this discussionchlorites with P> 1.628 wil l be termed Fe-chlorites, while those with
970 M. J. HOLDAWAY
P<1.628 wil l be termed l{g-chlorites. No chlorite with B) 1.636 coexistswith actinolite or hornblende in Klamath Mountain rocks as Wisemanobserved in Dalradian rocks. A dark green magnesian daphnite (B:1.655) coexists with crossite, but probably not with actinolite inStuart Fork specimen l2l.
Figure 7 shows plots of chlorite B against amphibole B for the threemetamorphic units. The Stuart Fork chlorites are both Mg- and Fe-chlorites. Their indices show a reasonable straight l ine relationship withthe amphibole indices, suggesting that FeO/FeO*MgO varies together
t 6 5 1 6 6 1 6 7
A M P H I B O L E B
Fro. 7. Diagram showing refractive indices of coexisting chlorite and amphibole in thebasic regional-metamorphic rocks described. Symbols as in Fig. 5. A large circle surround-ing the symbol indicates that the chlorite is retrogressive.
in the chlorite and actinolite. The position of the line is about the same asthat of Wiseman's (1934) curve. Progressive metamorphic chlorites ofthe Salmon and Grouse Ridge basic schists when present are consistentll,Mg-chlorites as would be expected under garnet- and staurolite-zonemetamorphism (Turner and Verhoogen, 1960). The iack of coincidenceof the Grouse Ridge and Salmon curves is principally a function of theamphiboie refractive indices, which in these units are dependent on sev-eral factors besides FeO/FeO*MgO. The linear reiations seen for thechlorite z's. amphibole indices are based on scanty data and may be inpart fortuitous.
1 6 3
tdF
E.
i , r zo
METAMORPHIC ROCKS OF TIIE KLAMATH MOUNTAINS 97r
Retrogressive metamorphism of the medium-grade rocks producedIargel-v Fe-chlorites whose indices do not fall in l ine with the progradechlorite indices. During retrogression of some of the Salmon rocks it ispossible that the amphibole indices changed also, tending to producemineral compositions more l ike those of Stuart Fork greenstones. Theconditions of retrogressive metamorphism of the Salmon and GrouseRidge rocks may well have been those extant during the Stuart Forkprogressive metamorphism.
In summary, Stuart Fork chlorites show a wide range of iron content,progressive metamorphic chlorites of the higher-grade rocks are re-stricted to the more magnesian compositions, and retrogressive chloritesof the higher-grade rocks are Fe-rich, l ike many of the Stuart Forkchlorites. Prograde chlorites of any given formation show systematiccompositional relationships with coexisting amphiboles.
Colrpenrsorq ol LrrHoLoGrES
The following points selve to compare the l ithologies of the three basicunits:
(11 Plagioclase coexisting with epidote varies from albite in Stuart Fork rocks to albite andoligoclase in Saimon rocks, and becomes a little more calcic on the average in Grouse Ridgerocks. (2) Epidote shows a lorv-temperature composition gap in Stuart Fork rocks, butshorvs no gap in the higher grade rocks. l-he most aluminous epidotes occur in Grouse Ridgerocks. (3) Amphibole is actinolite (some biue amphibole) in Stuart Fork greenstones, horn-blende in Salmon rocks, and hornblende lr'ith low ferric iron in Grouse Ridge rocks (4)
Progressive metamorphic chlorites may be iron rich in the Stuart Fork Formation, but mustbe magnesian in the other units. (5) Rock composition is most homogeneous in the SaimonHornblende Schist, least homogeneous, with metasedimentary interbeds, in the GrouseRidge Formation (6) Some primary volcanic textures are preserved in the fine-grainedStuart Fork greenstones, but such textures are obliterated by more complete metamorph-ism and deformation in the overlying units. Grouse Ridge basic rocks are more coarse-grained and shon less preferred orientation than Salmon schists.
Differences betlveen the three units may be explained in terms of thecomposition and history of the rocks. Contrast in metamorphic grade ex-plains the textural and mineralogical differences between the Stuart Forkand Salmon basic rocks, both of which were probably derived from vol-canics. A small change in grade might also explain the apparent differ-ence between Salmon and Grouse Ridge plagioclase. However, most ofthe contrast between Salmon and Grouse Ridge basic rocks is best ex-plained by differences in initial bulk composition and pre-metamorphicorigin. The composition and homogeneity of the Salmon rocks attest totheir origin as basic igneous rocks, probably volcanic. The Grouse Ridgerocks, with less ferric iron, probably came from a more reducing pre-metamorphic environment (alternatively iron was reduced to a greater
972 M. J. IIOLDAWAY
extent during metamorphism at a slightly higher grade). Presence of cal-cite in some rocks and almandine in others suggests respectively cal-careous and possibly pelitic contamination of original basic igneous ma-terial. Occurrence of metasedimentary interbeds indicates that thedepositional environment was at least partly sedimentary. The mostplausible origin of Grouse Ridge basic rocks is as water-laid or reworkedtufi or volcanic detritus contaminated by varying amounts of carbonateor shale. A sedimentary origin might also explain the coarser metamor-phic texture of the Grouse Ridge basic rocks, because transmission offluids and recrytallization probably proceed differently in a sedimentthan in a volcanic rock. Alternatively, a higher metamorphic grade in theGrouse Ridge rocks may favor a coarser, less foliated texture.
In conclusion, basic rocks of the three Klamath Mountain meta-morphic units are, in fact, distinct from each other in the area studied,thus justifying their status as parts of three different formations. Differ-ences between the units were produced by differences in metamorphicgrade, bulk composition, and pre-metamorphic history.
Acn ol Morauonpurslr
Petrologic evidence has established the existence of a medium-grademetamorphic event and a low-grade metamorphic event in the KlamathMountains. Recent work may indicate that these metamorphic eventsare significantly separated in time.
Lanphere and Irwin (1965; personal communication from Lanphere)have dated hornblende from Salmon (or Grouse Ridge) hornblende schistexposed at the western edge of the metamorphic belt in Weaverville andHelena quadrangles. Potassium-argon ages of hornblende unafiected byLate Jurassic events are 27O, 272, and 286 m.y. suggesting that medium-grade metamorphism was probably Pennsylvanian in age.1 Ages as youngas 135 m.y. on "Abrams" metasediments considered to be equivalent toGrouse Ridge (see also Davis and Lipman,1962) may have resulted fromLate Jurassic retrogressive metamorphism or contact metamorphism as-sociated with Jurassic plutonics.
West and north of the area of this report rocks as young as Late
Jurassic age have been metamorphosed to low grade (Irwin, 1960). Davis(1964) has correlated the rocks of the Stuart Fork Formation withslightly less metamorphosed rocks of the western belt of the KlamathMountains which yield fossils of Pennsylvanian to Triassic (?) age. Theoldest dated pluton of the Klamath Mountains, the Russian Peak, batho-
I Additional confirmation of the existence of Paleozoic metamorphism in the SierraNevada-Klamath Mountain system is given by a 307+30 m.y. age on tremolite in ser-pentinite in the Sierra Nevada foothills near Visalia, California (Putman and Alfors, 1965).
METAMORPIIIC ROCKS OF TIIE KLAMATH MOUNTAINS 973
lith, is unaffected by metamorphism and has a potassium-argon age of140 m.y. on biotite (Davis et al., ms.). Thus the age of low-grade meta-morphism of the Stuart Fork Formation and probably of retrogressivemetamorphism of the Salmon and Grouse Ridge formations may beJurassic.
NarunB ol Kralraru MouNrarN RBcrowar Ml'ralronpnrslr
In this discussion, evidence from all regional metamorphic rocks willbe used, including some from outside the report area.
The available petrologic data are consistent with the preferred struc-tural interpretation for the area. During late Paleozoic time, the Salmonand Grouse Ridge formations were deformed by shearing and meta-morphosed to the garnet and staurolite zones. Later, during Jurassic (?)time the Stuart Fork rocks, which were at that time widely separatedfrom the middle-grade rocks, underwent chlorite-zone and local glauco-phane-schist metamorphism. Toward the close of metamorphism theSalmon-Grouse Ridge sequence was thrust over the Stuart Fork Forma-tion and retrogressively metamorphosed during the closing stages ofregional metamorphism. To this must be added the possibility of anotherthrust separating the Grouse Ridge Formation from the underlyingunits. (G. A. Davis, writt. comm., 1963). The root zone of the thrustor thrusts is not definitely known. Stuart Fork rocks continue westwardwith reduction in the degree of metamorphic recrystallization, whereasthe eastern boundary of the central metamorphic belt is more abruptand is probably a high-angle fault or thrust fault which brings slightlymetamorphosed Ordovician and Silurian rocks into view on the east.It appears l ikely, then, that the root zone is under the lower Paleozoicrocks to the east and that the sense of the thrust or thrusts is east overwest .
There is incomplete evidence to the effect that metamorphic grade in-creased from west to east in both the Paleozoic and Jurassic (?) meta-morphic events and that isograds had trends between N. 15o E. andnorth. Consider first the autochthonous (?) rocks, the Stuart Fork andwestern Paleozoic and Triassic rocks, which were affected by the low-grade event. West of the central metamorphic belt these rocks are incipi-ently metamorphosed probably to the chlorite zone (D. P. Cox, 1956, un-publ. Ph.D. Thesis, Stanford Univ.). Proceeding eastward, one encount-ers an increase in degree of recrystallization and finally local developmentof glaucophane schists, all of which are located near a 34-mile line trend-ing N 15' E. between the northeast corner of Cecilvi l le quadrangle andthe southern edge of Helena quadrangle to the south. East of this line,most Stuart Fork rocks are completely recrystallized in the chlorite zone,
M. J. HOLDAWAY
but pre-metamorphic textures are locally preserved. The metamorphicgrade of Stuart Fork rocks in the area of this study never rises above thechlorite zone, and all Stuart Fork rocks not affected by contact meta-morphism are not more than four miles east of the zone of local glaucophanitic metamorphism. Ilowever, south of the eastern part of the area,in the Minersvil le quadrangle Stuart Fork rocks were metamorphosed tothe biotite zone (Lipman ia Davis et al. ms.). These rocks might havesuffered from a local increase in regional metamorphism associated with acluster of plutons, but Lipman feels that, since the biotite-bearing sil i-ceous rocks are several miles from plutons, they have a purely regionalmetamorphic origin. These Stuart Fork rocks are nine miles from the l ineof local glaucophanitic metamorphism and could well be in a regionalbiotite zone. Thus, there is evidence, albeit somewhat meager, thatStuart Fork rocks show progressive increase in grade through the follow-ing steps as one proceeds S. 75" E.: incipient metamorphism-+(glauco-phane schist)---+chlorite zone---+biotite zone.
The evidence for increasing grade toward the east is even less certain inthe allochthonous (?) units metamorphosed during the Paleozoic era. Allthe regionally metamorphosed Salmon rocks in the area (and probablymost such rocks in the central metamorphic belt) are albite-bearing savea few at the eastern edge which contain oiigoclase. Hence their grade isprimarily in the garnet zorte, increasing locally to the staurolite zone atthe eastern edge. Western exposures of the Grouse Ridge Formation inthe area of this report (Fig. 1) are rocks of the garnet zone, whereas manyof the eastern Grouse Ridge rocks, east of the Salmon outcrop area, havebeen metamorphosed to the staurolite zone. The suggestion of eastwardincrease in grade is weakened by the recent recognition of staurolite-zonewestern Grouse Ridge rocks south of Cecilvi l le (G. A Davis, writt.comm., 1964). These conclusions are tentative and it remains to be seenwhether future work wil l substantiate, modify or refute them.
Each metamorphic episode in the Klamath Mountains may be as-signed to a facies series (Miyashiro, 1961). The facies series classificationis based principally on differences in rock pressure for various meta-morphic terranes. By comparison with several standard facies series, onecan relate the metamorphism for a given time and area" to one of the fivefacies series: (from high to low pressure) jadeite-glaucophane type, highpressure intermediate type, kyanite-sil l imanite type (normal Barrovian),low-pressure intermediate type (Buchan), and andalusite-sil l imanite type(central Abukuma).
Evidence bearing on a facies series for the Salmon-Grouse Ridge se-quence is as follows:
(1) Hornblendes have a slightly higher average alkali content than those of Dalradian epi-
METAMORPHIC ROCKS OF TIIE KLAMATH MOUNTAINS 975
diorites or central Abukuma basic schists. Shido and Miyashiro (1959) interpret an increase
of alkalis in amphiboles which crystallized at a. given grade as related to increased load
pressure, because alkalis r,vould otherwise cause the formation of more feldspar with lower
density than the amphibole. Other factors which increase alkali content in amphiboles
(e.g. increased metamorphic grade or greater alkalis in the rock) can probably be ruled out
for the Kalamath Mountain rocks when comparing them with the Dalradian or central
Abukuma rocks (2) No kyanite or staurolite appear in the pelitic rocks, but neither do
sillimanite, andalusite, or cordierite. The rocks u'ere simply not aluminous enough for these
minerals to form. (Cordierite and andalusite do occur in Stuart Fork contact metamorphic
rocks whose genesis followed the lorver greenschist retrogressive metamorphism). (3)
Almandine rich garnets occur in both basic and pelitic Grouse Ridge rocks. Their absence
in Salmon rocks is dependent on composition, not grade. The characteristics of the Salmon-
Grouse Ridge terrane are similar to those of the kyanite-sillimanite facies series rvith
pressures perhaps on the high side as indicated by the alkali content of the hornblendes.
Evidence bearing on the facies series of the Stuart Fork rocks affectedby Jurassic (?) metamorphism is as follows.
(1) Crossite, glucophane, and lar,r'sonite are locally developecl. (2) Most of the rocks are
restricted to the chlorite zone and possibly the biotite zone
The scarcity of glaucophanitic rocks and the abundance of greenschistsindicate that the facies series here was high-pressure intermediate ratherthan glaucophane-jadeite series. There is a good possibil i ty that theglaucophane schists resulted from localized tectonic over-pressure orincrease in fluid pressure, and that their occurrence along a N. 15o E. l ineis because this zone underwent metamorphism at conditions closest tothose of the glaucophaneschist facies.
AcTNowTBoGMENTS
The author wishes to express appreciation for the financial support of
the University of California for field work and the Geological Society ofAmerica for chemical analyses. F. J. Turner crit ically read the Ph.D.dissertation on which much of this report is based. G. A. Davis, P. W.Lipman, and W. D. Romey reviewed the manuscript and offered many
helpful suggestions.
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