American Mineralogist, Volume 64, pages l-14, 1979

Zeolite facies metamorphism of basaltic rocks from the East Taiwan Ophiolite

JusN G. Lrou

Department of Geology, Sranford UniuersityS tanfo rd, C alifu rnia 94 305

Abstract

The basalt ic rocks of the fragmented East Taiwan Ophiol i te have been subjected to "ocean-f loor" zeol i te facies metamorphism. Depending on the bulk composit ion and mode of occur-rence, various mineral assemblages occur: thomsonite + analcime * chabazite; pumpellyi te *chlori te * laumontite in veins of the pi l low cores; and pumpellyi te * chlori te * K-feldspar;pumpellyi te * laumontite * thomsonite; and prehnite (about 5 weight percent FeOr) *hematite in veins of the pi l low matrices. Plagioclase phenocrysts were replaced by albite *pumpellyi te * Ca-zeoli tes, pumpellyi te * K-feldspar, analcime * chabazite * thomsonite, orby K-feldspar alone, the ol ivine phenocrysts by brown chlori te * serpentine i pumpellyi te,but the pyroxenes are well preserved. Except for local palagonit izat ion along fractures andrims, the pi l lowed glassy r ims are perfect ly fresh. The variat ions in mineral associat ion amongpil low cores, r ims, and matrices are bel ieved to have resulted from local variat ions incomposit ions of the circulat ing f luid, in the extent of recrystal l izat ion, and in temperature.The occurrence of potash feldspar suggests that the basalt ic rocks have been subjected to localmetasomatic exchange with a hydrothermal solut ion under zeol i te facies condit ions.

Pumpellyi tes characterist ical ly contain up to 25 weight percent total Fe as FeO, higher thanmost reported pumpellyi te (except julgoldite). Calculat ion of their structural formulas in-dicates that they may contain iron dominantly in the ferr ic state. Substi tut ion of Fe8+ for Al inthis phase evidently enlarges the pumpellyi te P-T stabi l i ty f ield relat ive to the zeol i te faciesassemblages under oxidizing condit ions. Pumpellyi te probably crystal l ized direct ly frompalagonit ized basalt ic glass. The basalt ic rocks of the East Taiwan Ophiol i te were hydro-thermally metamorphosed at I = 150-250'C and depths of 0.6 to 1.6 km, probably at somedistance away from the r idge.

IntroductionRecent petrologic studies of rocks dredged from

ocean floors and ridges have thrown new light on themetamorphic processes taking place near divergentlithospheric plate boundaries. The major parts of theoceanic crust have been subjected to mid-ocean ridgemetamorphism immediately after their creation at aspreading center. Varieties of this recrystall izationinclude zeolite, greenschist, and/or actinolite-calcicplagioclase and amphibolite facies metamorphism(e.9., Miyashiro et al., l97l', Bonatti et al., 1975:Coleman, 1977). l f on- land ophio l i tes are f ragmentsof oceanic crust created at mid-oceanic ridges andlater transported to convergent l i thospheric plateboundaries (e.9., Coleman, 1977), the effect of ridgemetamorphism may sti l l remain to some extent evenwhere later metamorphism is superposed on suchrocks.

0003404x/79/0102-0001$02.00 |

The l i tho logic and chemical s imi lar i t ies of the EastTaiwan Ophiolite to present-day oceanic crust andupper mantle material suggest that all l i thologies ofthis ophiolite originated in an oceanic basin (for de-tails, see Liou et al., 1977). However, the ophiolite isnot simply a fragment of normal oceanic crust of thesort formed at ridge crests, because the petrology ofthe interbedded red shales suggests that they weredeposited at significantly greater depth-below thecarbonate compensation depth (Suppe et al., 1977).Detailed field and petrographic features suggest thatthe observed stratigraphy of the ophiolite formednear the base of a deepwater fault scarp in preexistingocean crust-possibly a transform fault-that hasgiven rise to the plutonic mafic-ultramafic brecciadeposits and has been the site of later tholeiit ic ex-trusive volcanism and metamorphism. Therefore,study of mineral parageneses of basaltic rocks of this

LIOU. EAST TAIWAN OPHIOLITE

Fig I General ized geologic map of the Coastal Range ofTaiwan (modi f ied f rom Liou et a l . ,1977) The invest igated rocks ofthe East Taiwan Ophiol i le are f rom the area shown by an arrow.

ophio l i te is important to an understanding of meta-morphic and tectonic processes prevai l ing in the v i -c in i ty of a t ransform faul t in an oceanic basin.

The characteristic features of the zeolite faciesmetamorphism of ext rus ive basal t ic rocks f rom theophio l i te are the absence of penetrat ive deformat ion,the occurrence of Fe-r ich pumpel ly i te , and select iveal terat ions. The pr imary igneous st ructures and tex-tures are wel l preserved and most rocks are onlypar t ly a l tered, and recrysta l l izat ion is incomplete.Such characteristic features are consistent with thosedescr ibed by Miyashi ro et a l . (1971) and by manyother invest igators (e.g. , de Wit and Stern, 1976;Coleman, 1977) for "ocean-floor" metamorphism.

Geologic setting and field relations

The fragmented East Taiwan Ophiolite occurs asal lochthonous s labs and b locks up to one k i lometeracross in the Ple is tocene L ichi muddy m6lange, whichis exposed on the western slope of the southernCoasta l Range as a long and narrow bel t extending

from Locho to Tai tung (see Fig. l ) . The ophio l i teb locks are sporadical ly d is t r ibuted in the m6lange,but par t of the pr imary ophio l i te sequence is wel lpreserved wi th in indiv idual large s labs exposed in theKuanshan Dist r ic t . The f ie ld re lat ions and petro logyof the ophio l i te in th is d is t r ic t have been descr ibed indetaif by Liou et al. (1977): a summary of f ield rela-t ions is g iven below.

The schemat ic s t rat igraphic re lat ions are shown inFigure 2.The composi te sequence passing upward isas fo l lows: ( l ) angular polymict p lutonic breccias ofgabbro, per idot i te , and re lated rocks, wi th in ter-bedded red shale; (2) a th in (about 20 cm) layer of redshale; and (3) an extrus ive thole i i t ic sequence con-s is t ing largely of s t i l l -g lassy p i l low lavas wi th in ter-bedded th in red shales. Plagiograni t ic and d iabasicpar t ly rodingi t ized maf ic d ikes cut maf ic-u l t ramaf icbreccias. Some of these d ikes are thought to representfeeders for the extrusive basalts. The brecciated

E X T R U S I V E S E Q U E N C E

Moss ive f lows, p i l low t recc iq ondc l o s e l y p o c k e d p i l l o m i n r e g u l o rsequence; boundor ies g rodot iono l(s ix such t r ipor t i te sheets recordedby Wons, l%6) . Loyers o f redsho le occur w i th in the ex t tus ;vesequence; f ine-gro ined pumpel ly -i te -beor ins tu f f occurs be iweenp i l l o w s . . W e l l - d e v e l o p e d g l o s y

R E D S H A L E L A Y E R

Red sho le ond s i l t s tone in i r regu-ror loyers , l0 cm to more ihon 50cm th ick , o t the rop o f the p lu -

B R E C C I A T E D P L U T O N I Cs E Q U E N C E

Plu ton ic rocks , ex tens ive ly f rog-m e n t e d o n d m i x e d ; c h 6 r i c b l o c k sord s lobs o f vor ious sobbros , p lo -g iogron l tes , d ioboses , ond u l t ro -m o f i c s . B l o c k s o f s e r p e n r ; n i z e dhr rzburg i ie ond rod ing i i i zed gob-

b r o o n d d i o b o e i n f i n e l y s h e o r e d

s e , l e n i i n i t e m o h i x n e o r b 6 e .

D E C O L L E M E N T F A U L T

L I C H I M E L A N G E' f f i l ro"*- mu&rone mohix wirh^ - ^ - ^ -

, _ . 1 l I b l o c k s o f t u r b ; d ; t e s o n d s t o n e , m u d -

\ ^ ^_ - : -

LIOU: EAST TAIWAN OPHIOLITE

plutonic sequence grades upward from (a) pre-dominant ly serpent in ized harzburgi te s labs in the ser-pent in i te matr ix , to (b) angular polymict breccia ofvar ious types of gabbro, harzburgi te, d iabase, andplagiograni te, local ly wi th minor red shale cement .Pelagic red shales occur as irregular layers of l0 cmor less to more than 50 cm th ick at the top of thebrecciated p lutonic sequence and wi th in the volcanicrocks. The contact between mafic-ultramafic brecciaand over ly ing red shale layers is deposi t ional , wi ththe shale f i l l ing in around the b locks at the top of thebreccia.

The extrusive sequence progresses upward fromminor massive f lows to brecciated and dominantc lose-packed p i l lows wi th several layers of in terp i l lowred shale. Pi l low st ructures are wel l developed wi thminor in terp i l low pyroclast ic matr ix . Indiv idual p i l -lows consist of three main par ts : ( l ) a g lassy outerrim I to 20 cm thick containing scattered micro-phenocrysts of sp inel , o l iv ine, and p lagioc lase; (2) aspherul i t ic in termediate zone character ized by skele-ta l growth of spherul i tes in a g lassy matr ix ; and (3) acompact crysta l l ine core of d iabase wi th in ter-granular texture. The outer g lassy r im is dark in colorand conta ins many b lack v i t reous rounded nodulesand dul l in terst i t ia l palagoni t ized matr ices, whichcontrast wi th the l ight grey massive in ter iors of thepi l low. The chi l led margins of b lack g lass are wel ldeveloped around most p i l lows, and in some casesthese g lassy selvages are as much as 50 cm th ick. Theconspicuous d i f ferences in crysta l l in i ty , co lor , andstructure accentuate the c lass ical p i l low st ructure.Some pil lows are closely packed and contact eachother d i rect ly , whereas others may have interp i l lowmatr ices. Pumpel ly i te-bear ing green matr ices andhemat i t ic red shale are local ly abundant .

Concentr ic or radiat ing f ractures in p i l lows arecommonly fi l led by late-stage calcite, zeolites, andprehnite (Juan et al., 1964). At the outer margin theglass, which exhib i ts a round conchoidal f racture, ist raversed by a polygonal network of shr inkagecracks. No vesicular s t ructure has been found in anyglassy r ims of these p i l lowed basal ts . However, smal lvesic les in the p i l lowed cores are usual ly par t ly orwhol ly in f i l led to form amygdules. In order of abun-dance, amygdalo idal minerals inc lude chlor i te ,pumpel ly i te , natro l i te , s t i lb i te , thomsoni te, and anal-c ime. Many of these secondary minerals are a lsofound as vein lets and i r regular masses wi th in thebody of the p i l low. The lack of v is ib le vesicular f rag-ments and the minor content of f ine pyroclastic tuffsin the extrus ive sequence of the East Taiwan Ophio l -

Table I Bulk rock composi t ions and mineral contents of somebasal t ic rocks of the East Taiwan Oohiol i te

T - 9 9 C T - 2 3 r T - 1 1 2 B T - 1 1 2 C ' T - I I 2 Z(pi l1ow core) (massive basalE)

S l O z

A 1 z 0 a

T i O z

F e z O g ^

Mn0

Mgo

CaO

N a 2 O

K z o

I g n i t i o n I o s s

o l i v ine

c f inopyroxene

o P a q u e s

sphene

c h 1 o r i t e

c e l a d o n i t e

carbonate

pumpel ly i te

z e o f i t e s

s e r p e n E i n e

dev i t r i f ied

g lass

4 l . 3 6 4 9 . O 51 3 . 9 6 1 2 . 5 80 . 8 9 0 . 9 3

7 0 . 2 4 1 0 . 1 1o . 2 0 0 . 2 09 . 2 3 1 0 . 1 0

1 2 . 8 9 r 0 . 1 11 . 6 6 2 . 8 90 , 0 2 0 . 3 83 . 3 9 3 . 2 7

4 2 , L 9 4 0 . 4 2 4 4 . 8 61 3 . 0 0 8 . 9 3 L s . 2 50 . 9 3 0 . 6 7 0 . 9 9

t 4 . 3 9 1 4 . 7 6 1 2 . 6 30 . 1 6 0 . 2 1 0 . r 4

1 0 . 9 4 2 2 , 1 9 7 . 8 89 . 8 0 7 . 1 1 1 1 . 0 6r . 4 3 L . O 2 1 . 8 30 . 1 0 0 . 0 7 0 , I 26 , 4 3 4 . t 4 4 . 4 7

6015

5

t r

9 9 . 6 2

30

7

53

5

L 1

9 9 . 8 4

5 52 0

4152

I 3

9 9 . 5 2 9 9 . 2 3

105 5 4 515 20

3 4

IO

* T o t a 1 F e a s F e 2 O 3 * * t h o n s o n i t e

+ a n a l c i n e + c h a b a z i t e .

i te suggest a non-explosive deep-water (abyssal) ex-t rus ion (Moore, 1965; Jones, 1969).

Representat ive chemical composi t ions of p i l lowcore, g lassy r im, and massive f low are shown in Table| . Character is t ica l ly , the basal t ic rocks are extremelylow in KzO and TiO, content and are h igh in CaOand MgO/FeO* rat ios. They are mineralogical ly andchemical ly s imi lar to oceanic basal ts and appear to betypical abyssal thole i i te ( for deta i l , see L iou, 1974;Liou er al.. 1977\.

Petrographic descriptions ofmetamorphosed basaltic rocks

In contrast to the v i r tual ly unal tered g lassy basal tin the p i l low r ims (L iou, 1974), both p i l low cores andpi l low matr ices of the East Taiwan Ophio l i te conta inabundant secondary minerals inc luding a lb i te , pum-pellyite, K-feldspar, chlorite, smectite, celadonite,analcime, and various calcium zeolites. The presenceof such secondary minerals strongly suggests low-grade metamorphism.

With the except ion of augi te, a l l pr imary phases inthe pil low cores and massive basalts are partly re-p laced by metamorphic minerals . Many p lagioc lasephenocrysts have been albit ized and zeolit ized. Alongwi th a lb i te , laumont i te , thomsoni te, and chlor i te arethe most abundant secondary minerals . Analc ime,pumpellyite, potash feldspar, and other Ca-zeoliteswere identif ied. Olivine phenocrysts were relativelyrare; where they occurred, they are totally replaced

LIOU.' EAST TAII'YAN OPHIOLITE

Mineral

Table 2. Est imated modes for p i l low matr ices f rom theEast Taiwan Ophiol i te

cracks, well-crystall ized fine-grained and distinctlypleochroic Fe-pumpellyite occurs together with eu-hedral adularia. Textural evidence suggests that thesetwo phases grew simultaneously.

Mineral chemistryr

Albire and K-feldspar

Extensive analyses of altered plagioclases were notundertaken. The secondary p lagioc lases in the p i l lowcores, massive basalts, and pil low breccias are char-acteristically albit ic in composition (Ab 9l-97). ltshould be noted that the p lagioc lases in p i l low coresand matrix have been nearly completely albit ized,whereas plagioclases in pil low glassy rims were unal-tered and have compositions of An 70-72 (Liou,1974).

Some euhedral pr imary p lagioc lase phenocrysts inpi l low matr ices, in some pi l low cores, and in d iabasehave been tota l ly replaced by adular ia. Adular ia a lsooccurs together with pumpellyite as a major veinmineral. K-feldspars were analyzed in five samples.Their compositions are extremely uniform and highin Or content wi th very low amounts of An (0. 1-0.7mole percent) and Ab (0.7-1.8 mole percent) com-ponents and very low in minor ox ides-Fgos (

LIOU: EAST TAIWAN

tals or crystal aggregates) were selected for analysis.The resul ts are consistent ly homogeneous and thecomposi t ional var iat ions f rom one spot to another ineach sample are smal l . Ind iv idual spot analyses tendto show reciprocal variation in Al and Fe; hence,most Fe is probably Fe3+.

Calculat ion of s t ructura l formulas for pumpel ly i tefrom chemical data has been diff icult, partly becauseof unre l iab le publ ished chemical data on the f ine-grained mineral separates, and partly because of thepresumed presence of substant ia l (OH) subst i tu t ionof oxygen accompanying the in t roduct ion of t r iva lentions in to the octahedral posi t ion. Microprobe resul tsare fur ther compl icated by uncer ta int ies concerningthe oxidation state of iron. Different chemical for-mulas have been used in prev ious invest igat ions (seeAl lmann and Donnay, 1971 Passagl ia and Got tard i ,1973; Moore, l97 l ' Coombs er a l . , 1976). Assumingthe cat ion s i tes of pumpel ly i te are tota l ly f i l led andwithout excess, Coombs e/ c/. suggested the formulaWvnrrxurtyuot2lvoro+,(OH), ,, where W : Ca,Mn; X: (Mg,Fe'+,Mn)r- , (Fe3+,Al) , ; Y : Fe3+,Al ; and Z: Si ,Al . This formula was used in the present s tudy.On the basis of l6 tota l cat ions, atomic proport ionsof analyzed pumpel ly i tes were calculated. I f octahe-dra l Y s i tes consist only of Fe3+ and Al , then Fe3+occupancy in Y s i tes should equal 4 - Alv l .

For these pumpel ly i tes, Si (5.954-6.059) is consis-tent ly c lose to the ideal va lue of 6, Fe8+ in the octahe-dral Y site is characteristically high (0. 121-2.076), theoccupants of the octahedral X site are greater than 2,and Ca in the W s i te (3.834-3.996) is less than theideal 4. The XtW values near ly equal 6, suggest ingthe possib le subst i tu t ion of Fe2+ for some Ca. Thetotal absence of Al in the octahedral X site of Taiwanpumpellyite is most characteristic, as all reportedpumpel ly i te analyses conta in very h igh Al rO, exceptfor Russian Fe-rich pumpellyite (Zolotukhin et al.,1965) and ju lgold i te (Coombs et a l . , 1976). Accord-ing to Passagl ia and Got tard i (1973), the value of(Mg*Fe'z+*Mn) in the X s i te for re l iab le pump-el ly i te analyses ranges f rom 0.71 to 1.55 out of 2. Inother words, (Fe3+*Al) in the X s i te should rangebetween 1.29 and 0.45 per formula uni t . I f th is is thecase, the Fe occupying both octahedral Y and Xposi t ions in the pumpel ly i te of the East Taiwan Oph-io l i te may be dominant ly in the ferr ic s tate.

Composi t ions of pumpel ly i tes f rom Taiwan andthe composi t ional f ie lds of pumpel ly i te f rom var iousknown geologic occurrences compiled by Coombs elal. (1976) are plotted in an Al-Fe*-Mg diagram asFigure 3. It is apparent from such comparison that

OPHIOLITE

G l o u c o p h o n i c s c h i s t s ,C o l i f o r n i o

Loeche meto-son d s to ne

v o n c o u v e rI s l o n d

{ J u l g o l d i t e )

Fig. 3. Composi t ional var iat ions of analyzed pumpel ly i tes f rom

the East Taiwan Ophiol i te in Al-Fe*-Mg diagram of Coombs(1976). Composi t ional f ie lds of pumpel ly i tes are f rom Coombs er

al (1976\.

pumpel ly i tes f rom the East Taiwan Ophio l i te arehigh in tota l FeO* and low in Al rOr. They conta in upto 25 weight percent FeO* (range 9.65-25.38 weightpercent) , h igher than most repor ted pumpel ly i tes (ex-cept ju lgold i te) . Therefore, substant ia l occupanciesof Fe3+ in the Y s i te must have occurred. The h ighiron content is also reflected by the deep blue-greenpleochroic colors.

Simi lar Fe-r ich pumpel ly i te associated wi th i ron-rich prehnite and babingtonite in a metasomatizedandesine d iabase f rom Russia was descr ibed by Zolo-tukhin et a l . (1965). As shown in F igure 3, th is phaseand pumpel ly i tes f rom Taiwan are in termediatemembers of the pumpel ly i te- ju lgold i te sol id solut ionser ies. A cont inuous subst i tu t ion of Al for Fe3+ in theY octahedral site is apparent from the analyticaldata. F igure 3 a lso shows that the Mg contents inmost pumpel ly i tes f rom Taiwan are near ly constantand are h igher than in Fe-r ich pumpel ly i te f rom Rus-s ia and ju lgold i te . The Taiwanese pumpel ly i tes areFe-pumpellyite according to the classification schemeof Coombs et al. (1976) or ferropumpellyite accord-i ng t o t ha t o f Moore (1971 ) .

Zeolites

Numerous vein zeol i tes in the g lassy basal ts havebeen identif ied optically and by X-ray (e.g., Juan eta l . ,19641, Juan and Lo,197l) . These zeol i tes inc ludeanalc ime, natro l i te , thomsoni te, s t i lb i te , heulandi te,and laumont i te . Systemat ic ident i f icat ion and analy-

M g

a

a

o P i l l o w m o t r i x

. B o s o l t i c R o c k s

LIOU: EAST TAIWAN OPHIOLITE

ses of the zeolites were not undertaken. Calcium zeo-lites including natrolite, heulandite, stilbite, laumon-tite, chabazite, and thomsonite were opticallyidentified as vein and amygdaloidal minerals and asfine-grained discrete patches replacing primaryplagioclases. Vein zeolites are relatively coarse-grained, and their optical identification is relativelysimple. However, most calcium zeolites enclosed inaltered plagioclase are fine-grained aggregates, anddifferentiation among the different species is ex-tremely difficult.

Except for the analcime with 2.42 weight percentCaO replacing plagioclase in T-231, the other ana-lyzed analcimes are very uniform in composition. Theweight percent of major oxides averages about 54percent SiO2, 22.5 percent Al2O3, and 12.5 percentNarO, with other analyzed oxides present in onlyminor amounts. The analcime replacing plagioclaseinT-231contains relat ively high FqO, (

LIOU: EAST TAIWAN OPHIOLIT-E

high as 1.4 weight percent CaO, which is probablydue to the minute inc lus ion of pumpel ly i te or zeol i tes.The analyzed chlorites are higher in SiOr, in SiOr/(SiOr+Alro3) rat io , and in Alv l compared to chlo-rites from the plutonic rocks of the ophiolite (Liou eta| . ,1977). The h igh SiO, / (SiOr+Alroa) rat io may berelated to the lower metamorphic grade developed inthe extrusive rocks (zeolite facies) compared to thatof the p lutonic sequence (greenschist ) , as th is rat iohas been suggested to decrease with increasing meta-morphic grade (e.g., Ernst e/ al., 1970). The ratioFe2+/(F*++Mg) var ies considerably wi th in the ex-t rus ive chlor i tes, and these chlor i tes belong to thetalc-chlorite, penninite, and diabantite series of Hey( | 9s4).

Mineral paragenesis

Al tered basal t ic rocks in both p i l low cores andmassive f lows commonly conta in the fo l lowing min-eral associations in veins: thomsonite * celadonite;thomsoni te * analc ime * chabazi te; analc ime *chlor i te * smect i te ; and pumpel ly i te * ch lor i te .Plagioclase phenocrysts have been replaced by albite* pumpel ly i te , pumpel ly i te * adular ia, analc ime tchabazite, or by potash feldspar alone, and the oli-v ine phenocrysts by brown chlor i te * serpent ine *pumpellyite. Such replacement strongly suggests sub-stant ia l t ransport of K and Fe f rom solut ion in toplagioc lase phenocrysts and Al and Ca into o l iv ine.

For the p i l low matr ices, the mineral associat ionsare highly variable, depending on the clast-matrixratio, degree of palagonitization of basaltic clasts,extent of recrystall ization and, most importantly, in-ferred chemist ry of the hydrothermal so lut ion dur ingthe a l terat ion. The most common associat ions ob-served in veins and occasionally in amygdules are:

pumpellyite * chlorite f K-feldsparpumpel ly i te * laumont i te a thomsoni tethomsoni te * analc ime t laumont i te * K- fe ldsparprehnite * hematite * smectite * K-feldsparpumpel ly i te * laumont i te * ch lor i tepumpellyite * chlorite * calcite

Rare associations include smectite and sphene. Theprehnite * pumpellyite association was not found,whereas prehnite * hematite occur as a vein assem-blage, and smectite + opaque and potash feldsparoccur sporadically as replacements for groundmassand plagioclase respectively in small basaltic clasts.Quartz was not identif ied as a vein mineral or as oneof the replacement assemblages for primary igneousminerals. Calcite occurs as central fracture fi l l inss

and as small veinlets cross-cutting the zeolite, preh-nite, and pumpellyite veins, hence it must haveformed later than these minerals.

Microscopic examinat ion of textura l re lat ions inseveral sections has identif ied the following altera-tions and crystall ization sequence of secondary min-erals in mafic extrusive rocks of the ophiolite:

(a) palagonitization of basaltic glass and devitrif i-cat ion of g lassy mesostasis :

(b) recrystall ization of palagonitized glass topumpel ly i te , and a l terat ion of p lagioc lasephenocrysts to a lb i te * pumpel ly i te (or ca l -cium zeolites), or K-feldspar t pumpellyite,and olivine phenocrysts to chlorite * serpen-t ine A pumpel ly i te ;

(c) crysta l l izat ion of ch lor i te * pumpel ly i te t lau-mont i te a long the inner l in ings of the f issuresor amygdules, which are succeeded in turn bylaumont i te * analc ime * thomsoni te, anal -cime * chabazite * thomsonite, and finally bycalcite.

All l isted mineral associations may be depictedgraphically in the system CaAlzSirOe-Na2Al2Si2O6-HrO and in an ACF diagram, as shown in F igure 4.Three-phase assemblages such as thomsoni te * lau-mont i te + analc ime, thomsoni te * analc ime *chabazite, pumpellyite * laumontite * chlorite wereobserved as shown in the diagrams. The zeolite para-genesis of Figure 4A is consistent with the zeolitefacies metabasalts from the ocean floor described byMiyashiro et al. (1971) and metabasalts from on-landophio l i tes (e.g. , Spooner and Fyfe, 1973; Gass andSmewing, 1974); the zeolite facies assemblages havebeen considered as c lose to chemical equi l ibr ium.Phi l l ips i te and c l inopt i lo l i te , commonly repor ted indeep-sea sediments, do not appear to occur in thezeolite facies metabasalts from the East Taiwan Oph-iolite and dredged oceanic basalts. Evidently thesezeolite facies rocks have been recrystall ized at highertemperatures than those accompanying the crystall i-zation of phil l ipsite and clinopti lolite in deep-seasediments.

Most characteristic of the extrusive sequence is theoccurrence of Fe-rich pumpellyite in veins and amyg-dules and as a replacement of both o l iv ine andplagioclase. The significance of such occurrences isdiscussed in detail in the next section. On the ACFdiagram of Figure 48, compositional f ields for Fe-rich pumpellyite are plotted according to the presentchemical analyses; the compositional f ield of averagebasal t a f ter Coombs' (1963) data, and composi t ionsof the altered basaltic rocks l isted in Table I are also

LIOU: EAST TAIWAN OPHIOLITE

MINERAL ASSEMBLAGES OF THE EXTRUSIVE ROCKS

Si20s

Thomsoni te

Loumon t t t eC h o b o z i t e

S t i l b i t e Ch lo r i t e

t z

NorAl , S i rO. Notrol i te

A.

s i02

F ig 4 .Pa rageneseso f zeo l i t e f , ac i esassemb lageso f t hebasa l t i c rocks ,Eas tTa iwanOph io l i t e .D iag ramA i l l us t r a l eszeo l i t eassemb lageson an anhydrous project ion CaAlrSirOr-NarAlrs i rOr-Sior . The most common zeol i te assemblages in the metabasal ts are thomsoni te *chabazi te * analc ime and thomsoni te * laumont i te * analc ime. The dashed l ine connect ing laumont i te-alb i te is a h igher- temperaturejo in than analc ime-st i lb i te. Diagram B is an ACF diagram i l lustrat ing the zeol i te facies assemblage for basal t ic composi t ion. The dot tedcirc le refers to the basal t ic composi t ions compi led by Smith (1968) af ter Coombs'data (1963). Composi t ional f ie lds of pumpel ly i res areplot ted af ter the data of L iou et a l . (1977\. Pumpel ly i te * chlor i te + laumont i te are common in the zeol i te facies metabasal ts f rom theEast Taiwan Oohiol i te

B.Ano l c rme

shown. Apparent ly , under zeol i te fac ies condi t ions,Fe-rich pumpellyite f chlorite and Fe-rich pump-el ly i te + chlor i te * laumont i te are the most commonassemblages for basaltic compositions. The assem-blage pumpellyite * chlorite was found to replace theentire basaltic clasts of T-97C', except for init ialplagioclase microphenocrysts which have been trans-formed to potash feldspar. In addition, pumpellyite* ch lor i te * laumont i te (or other Ca-zeol i tes) com-monly occur in veins and as replacements afterplagioclase in some rocks. The observed mineral as-semblages are apparently compatible with the pro-posed relations shown in Figure 4B. Retrogressivechanges in mineral sequence from pumpellyite +chlor i te - pumpel ly i te * ch lor i te * laumont i te -laumont i te * pumpel ly i te - laumont i te * ca lc i te orprehnite * calcite observed in the vein paragenesessuggest the fluid composition is enriched in CaO/(Fe,Mg)O rat io and in COrlHrO rat io wi th t ime.Such chemical variation is consistent with metaso-matic trends for the Ordovician lavas of Australiaproposed by Smith (1968).

The phase relations i l lustrated in Figure 4 may alsoapply to the mineral association observed in the inter-flow red shales from the ophiolite (Suppe et al.,1977). Excluding the detrital minerals (plagioclase,pyroxene, quartz), interpil low shale layers character-

istically contain the zeolite facies mineral assemblagelaumont i te (or wairak i te) t pumpel ly i te + chlor i te +smect i te * hemat i te . No phi l l ips i te and c l inopt i lo t i tewere found in the red shale. The finding of wairakitein the red shale and one p i l low matr ix (H-3c) led us toconclude that wairak i te may a lso be present in thebasal t ic rocks of the ophio l i te .

Discussion

Occurrence of Fe-pumpellyite and its P-T-fO2 rela-tions with epidote

Widespread occurrences of pumpellyite in Iow-grade metamorphic terrains are increasingly beingrecognized. The composi t ion of pumpel ly i te may beused to characterize different metamorphic condi-tions. Fe-pumpellyite with total Fe as FqO3 of aboutl0 to 23 weight percent characteristically occurs inrocks of the zeolite and prehnite-pumpellyite facies,whereas Al-pumpellyite with total Fe as FqO, lessthan l0 weight percent invariably is associated withblueschist and pumpellyite-actinolite facies meta-morphism (Seki , l96 l ; Coombs eI a l . , 1976). Al -though the correlation of the Fe content of pump-ellyite with depth of burial (i.e., pressure) has beensuggested from the compilation of available chemicaldata, the quantitative effect of pressure on pumpelly-

LIOU: EAST TAIWAN OPHIOLITE

i te composi t ion and on the stabi l i ty of Fe- and Al-pumpellyite has not been investigated in relation tothe effects of bulk composition, temperature, andfluid chemistry.

The ubiqui tous h igh Fe8+ content in the pumpel ly-ites from the East Taiwan Ophiolite seems to besignificant. Evidently zeolite facies metamorphism ofthe extrusive sequence of the ophiolite took placeunder ox id iz ing condi t ions. The h igh FqO, contentsof the palagonitized basaltic glass may have beenresponsible for the Fe3+-rich nature of the pumpelly-ites. At least three questions related to the formationof pumpellyite need to be discussed: I l zeolite faciesmetamorphism took place under relatively oxidizingcondi t ions, why d id pumpel ly i te crysta l l ize instead ofepidote? What were the precursor minerals for theformat ion of pumpel ly i te? What is the min imum tem-perature required for the crystall ization of pum-pel ly i te in the ophio l i te?

As discussed in the petrographic sections, pumpel-lyite occurs along the edges of some glass shards, as atotal replacement of palagonitized glass fragments infine-grained pil low breccia, and as vein minerals inthe p i l low cores. Nei ther epidote nor prehni te werefound in pumpel ly i te-bear ing rocks. Instead, laumon-t i te and smect i te * ca lc i te , adular ia * ch lor i te , andlaumont i te * thomsoni te * analc ime are the majorcoexisting phases. Based on experimentally deter-mined stabi l i t ies of laumont i te , s t i lb i te , and analc ime(L iou , l 97 la , c ,d ; Thompson , l 97 l ) , t he pumpe l l y i t e -laumontite-bearing rocks were apparently recrys-ta l l ized under zeol i te fac ies condi t ions, presumablytemperatures less than 250"C and Pfluid less than2kbar.

For simplif ication, if we assume that Fe2+ and Fe8+are the only ions in the octahedral X s i te of thepumpellyite structure and Al in the Y site, thenpumpellyite possesses a simple composition such asCanFes+Fe2+Al iSi60r , (OH)?. This cat ion rat io isidentical to the epidote Ps 33 composition CarFes+AIrSisOrr(OH) except that epidote possesses all Fe3+,whereas idealized pumpellyite contains both Fe2+andFe3+.. The following simple reaction can be written todelineate pum pellyite-epidote relations:

2CanFe2+ Fe3+Al1Si6Or1(OHh + V2O2pumpellyite

: 4CarFes+AlrSisOrr(OH) + 5HrOepidote

This reaction possibly wil l have a positive Pfluid-?.slope and negative isobaric./Or-Z slope as shown inFigure 5.

^_too t50

too t50 200 250 300 350 400 450TEMPERATURE, OC

Fig. 5. Isobar ic log fO"-T (d iagram A) and Pf lu id-T at fO2values def ined by the hemat i te-magnet i te buf fer (d iagram B)showing the maximum stabi l i ty re lat ions of Mg- and Fe-pumpel ly i tes. Sol id l ines are exper imental ly determined stabi l i tyl imi ts for Mg-pumpel ly i te f rom Schi f fman and Liou (1977),whereas the broken line represents approximated fOr-T-Pfluidl imi ts for the react ion Fe-pumpel ly i te : epidote + H,O (see textfor explanat ion).

The ./O,-I-Pfluid stabil it ies of Fe-pumpellyitehave not been experimentally investigated. However,the low-temperature stabil ity l imit of epidote hasbeen estimated at about 220oX50oC for Ptotal l-5kbar and at 300oC, Tkbar (Seki , 1972; Tomassonand Kristmannsdottir, 1972).The appearance of epi-dote in the higher-grade part of the prehnite-pumpel-lyite facies was suggested to be a result of the reac-t ions ( l ) laumont i te * hemat i te = epidote + quartz+ Al,O3 + H,O, and (2) prehnite * hematite :

epidote +HrO (Seki , 1972). The decomposi t ion ofFe-pumpellyite according to the reaction l isted aboveshould also be considered as another l imiting reac-tion for the appearance of epidote. Evidently, asshown in Figure 5, increasing JOz may considerablydecrease the maximum temperature of Fe-pumpelly-ite stabil ity and may also increase Fe3+/(Fe3++Fe2+)in the X site and pee+/(pe3+*Al3+) in the Y site for

N

o|lo

5

a A

L E -

fo. = HM EUFFER

IIlro

F e l +

,l'i "'oII

( b ) B.

l 0 LIOU: EAST TAIWAN OPHIOLITE

pumpel ly i te , as in the case for epidote (L iou, 1973).Therefore, at a temperature of about 250'C and pres-sure of 0.5 to 2 kbar , Fe-pumpel ly i te wi th h igh Fe3+may crystall ize directly, instead of epidote, fromhighly ox id ized and palagoni t ized basal t ic g lass.

The par t i t ion of Fe8+ between pumpel ly i te andepidote under P-Z condi t ions between the maximumstabi l i ty of Mg-pumpel ly i te and the min imum stabi l -ity of Fe-epidote (curves a and b of Fig. 5) is highlydependent on temperature. With increasing temper-atures, Fe3+ may preferentially concentrate in theepidote. Therefore, epidote of the prehni te-pumpel-ly i te and b lueschist fac ies rocks conta ins a h igherpistacite component, whereas the coexisting pumpel-ly i te may be enr iched in Al (e.g. , Karmutsen metaba-sal t o f Kuniyoshi and L iou, 1976; Franciscan meta-graywacke and metabasalts, Ernst et al ., 1970).However, at temperatures lower than the low-tem-perature stabi l i ty curve of epidote (curve b of F ig. 5) ,Fe3+ wil l concentrate in the pumpellyite structure;hence zeolite facies pumpellyites may be high in Fe3+.This suggest ion is consistent wi th my chemical analy-ses of pumpel ly i tes and wi th ear l ier publ ished resul ts(e.g. , Coombs et a l . , 1976; Kuniyoshi and L iou,1976Nakaj ima et a l . , 1977). For example, pumpel ly i te inthe zeolite facies metasandstones of Hokonui, NewZealand (Boles and Coombs, 1977) conta ins about13.12 weight percent to ta l Fe as FeO* and was sug-gested to have recrystall ized at temperatures aboutI 90'C. From the analy zed pum pel lyi te com positions,the mineral assemblages, and the presence of freshunal tered basal t ic g lass in the p i l low r ims of the oph-io l i te , we conclude that the format ion temperature ofpumpel ly i te f rom palagoni te is 150' to 250'C.

Format ion of pumpellyite

The suggest ion "crysta l l izat ion of pumpel ly i te d i -rectly from palagonitized glass" needs to be docu-

Table 3. Composi t ional compar ison among f resh basal t ic g lass,palagoni t ized glass shard, and pumpel ly i te

R o c k F r e s h g l a s s * - - - - P a l a g o n i t i z e d g l a s s - - - - p u m p e l l y i E e(Light Dark Light Brownish Deepyel1ow) brom brown green green

( K - 1 r A - 1 ) ( r - 9 9 D ' ) ( r - 9 9 D ' ) ( r - 9 9 D ' ) ( r - 9 9 D ' )

S i O 2 4 9 . 4 6T i O z 0 8 7A t z o * g 1 5 , 6 4I e o 9 . 4 5M n O 0 . 1 8M c o 9 . 1 3c a o 1 2 . 4 6K z O 0 . 0 5N a 2 0 2 . 1 4Anhydrous

t o E a l 9 9 . 7 4

; C o m p o s i t i o n o f f r e s h g l a s s i s f r o m L i o u ( 1 9 7 4 )' T o t a l F e a s F e o .

mented in detail, as this feature has not been de-scribed before. Angular fragments of glass shardshave been devitrif ied and recrystall ized during pala-gonitization. Depending on t 'he extent of recrys-tall ization, differently-colored layers of palagonitewere formed and are transected by irregular pumpel-ly i te vein lets . However, most pumpel ly i tes are con-centrated in a layer adjacent to and subparallel withthe layer of l ight brown palagoni t ized g lass. The darkbrown layer appears to be isotropic, whereas the l ightbrown and brownish-green palagonites have a faintb i ref r ingence suggest ing inc ip ient crysta l l izat ion.Textural relations suggest the following parageneticsequence: ( l ) palagoni t izat ion of g lassy basal ts ( in-c ip ient recrysta l l izat ion of mineralo ids + smect i te)took p lace through the in teract ion of g lass wi th seawater; (2) pumpellyite crystall ized from palagonitizedglass at the edges of fragments or along the fracturesthrough the react ion of , palagoni te * so lut ion underzeol i te fac ies metamorphic condi t ions; and (3) crys-ta l l izat ion of laumont i te , fo l lowed by calc i te , oc-curred along open fractures.

Electron microprobe analyses (Table 3) yield anastonishing s imi lar i ty in composi t ion between pum-pel ly i te and i ts probable precursor , palagoni te. Inparticular, the brownish-green devitrif ied glasses nextto the pumpel ly i te layer possess composi t ions near lyident ica l to that of pumpel ly i te . The green palagoni teis homogeneous and extremely f ine-gra ined. How-ever, the characteristic deep green tint and the com-posi t ion shown in Table 3 suggest that the palagoni temay consist essent ia l ly of aggregates of minutepumpel ly i te crysta ls (M. Vuagnat , personal commu-n i ca t i on , 1976 ) .

From the above d iscussion, we conclude that thepumpel ly i te d id not recrysta l l ize d i rect ly f rom unal-tered basal t ic g lass under zeol i te fac ies condi t ion.Instead, when the palagoni t izat ion process haschanged the or ig inal g lass toward enr ichment inFerOr, Al rO3, and CaO and deplet ion in SiOr, NarO,and MgO, Fe-r ich pumpel ly i te may recrysta l l ize f romsuch devitrif ied glass. These pumpellyites, therefore,may contain high Fe3+ and low Alros. These proc-esses may account for the total replacement of certainglass shards by pumpel ly i te in some samples.

Occurrence oJ potash Jbldspar

Occurrences of potash fe ldspar as an auth igenicmineral in low-grade metamorphic rocks have beenextensively reported in geothermal f ields, in rocksaffected by burial metamorphism, and in hydro-thermal aureoles of ore deposits. Assemblages such

3 4 . 9 8 3 4 . 0 57 . 2 1 1 . 1 6

1 3 . 0 0 1 2 . 5 81 3 . 8 3 1 3 . 5 7

3 . 8 5 3 . 6 520.28 19 .490 . 0 6 0 . 0 40 . 0 5 0 . 0 2

a l . 2 5 8 4 . 5 5

3 4 . 2 20 . 3 7

r o . 3 2t 9 . 7 2

2 . 4 42 7 . 6 50 . 0 60 . 0 3

8 8 . 5 1

3 4 . 4 6o . 2 6

t o . l 92 2 . 5 7

2 . 5 42 I . 2 10 . 0 70 . 0 3

9 1 . 9 3

LIOU,' EAST TAIWAN OPHIOLITE l t

as quartz t a lb i te * adular ia, prehni te * adular ia *sphene, and laumont i te + adular ia * sheet s i l icatesare character is t ic . Adular ia commonly occurs in hotbr ines, as in the geothermal f ie lds of Ice land (e.9. ,Kr is tmansdot t i r , 1975). I t has been repor ted onlyonce in oceanic basalts, in metabasalt from the NorthAt fant ic Ocean (Jehl e l a l . ,1976). This may be par t lybecause oceanic basalts characterized by very lowKrO are not chemically favorable for the formationof potash fe ldspar, and par t ly because of the opt ica lsimilarity between secondary K-feldspar and albite.

Potash feldspar was identif ied in five rocks fromthe East Taiwan Ophiolite, and was analyzed by theelectron microprobe. It occurs sporadically aspseudomorphs af ter p lagioc lase microphenocrysts inpi l low cores, d iabase, and p i l low matr ices. I t a lsocrystall ized together with pumpellyite along frac-tures. The patchy nature of replacement may be duepart ly to the var iable permeabi l i ty of the rocks. Forexample, potash feldspar was not found in the freshbasal t ic g lassy r ims, whereas the basal t ic p i l low coresand p i l low matr ices conta in the character is t ic zeol i tefac ies minerals potash fe ldspar * pumpel ly i te t lau-mont i te * smect i te . The replacement of p lagioc lase(e.g., An60) by potash feldspar can be interpreted asa consequence of metasomat ic exchange involv ingthe addi t ion of K and SiO, according to the react ion:

(Nao.nCao6).4I r .6Si2.oO8 + l .6K+ + 2.4SiO,

: l .6KAls isO' * 0.4Na+ * 0.6Ca'z+

Exper imenta l s tudies by Hemley (1959) on the systemKrO-AlrO3-SiOr-HrO indicate that prec ip i ta t ion ofpotash feldspar at low temperatures (e.9., less than250'C) requi res h igh K+/H+ rat ios in the hydro-thermal solution at low fluid pressures. Therefore,a l though the host oceanic basal ts may be depletedwi th KrO, potash fe ldspar may local ly prec ip i ta tethrough interact ion of basal t ic rocks wi th K-r ich so-lut ions at re lat ive ly shal low depths. The occurrenceof potash feldspar in the basaltic rocks suggests,therefore, that these rocks have been subjected tolocal metasomat ic exchange wi th a hydrothermal so-lut ion under zeol i te fac ies condi t ions.

Selectiue alteration

Al l p i l low g lassy r ims, p i l low cores and the in ter-stit ial pyroclastic matrix must have been subjected toident ica l P-Z condi t ions dur ing zeol i te fac ies meta-morphism. Yet p lagioc lase, o l iv ine, and basal t ic g lassin the p i l low r ims remain v i r tual ly unal tered andunmetamorphosed (L iou, 1974), whereas the p i l low

cores, massive flows, and pil low matrices are highlyaltered and possess in common the mineral assem-blage a lb i te * ch lor i te * pumpel ly i te * K- fe ldspar *sphene. What caused such contrasts in mineral as-semblages and the process of alteration of primaryplagioc lase?

Similar features have been widely reported in nu-merous low-grade metamorphic sequences (see Zenand Thompson, 1974). The extent of a lb i t izat ion ofprimary plagioclase has been shown to be relatedroughly to one or more of these factors: ( l ) depth ofburial resulting in increased temperature and pres-

sure; (2) avai labi l i ty of f lu id dur ing the a l terat ion;(3)the composi t ion of the f fu id ( for a rev iew, see Kuni-yoshi and L iou, 1976). Di f ferences in P-T condi t ionsalone do not determine the extent of plagioclase alter-at ion, inasmuch as p i l low r ims, cores, and matr ices ofthe ophiolite were all subjected to the same P-7condi t ions. I t is conceivable that a more extensivealbit ization may be facil i tated by infi l tration of ahydrous f lu id phase dur ing a l terat ion. However, asshown by Kuniyoshi and L iou (1976), the composi-t ion of the f lu id ( in par t icu lar the concentrat ion of Feand Mg ions) is a crit ical factor in low-grade meta-morphic react ions. This hypothesis can a lso be sub-stantiated for the East Taiwan Ophiolite. For in-stance. there is a direct relation between albit izationof p lagioc lase and a l terat ion of pr imary o l iv ine in theextrus ive sequence. In the a lb i t ized massive f lows andpi l low cores, pr imary o l iv ines are invar iably a l teredto chlor i te and smect i te , whereas the o l iv ines in theglassy basal ts are v i r tual ly unal tered. The a l terat ionof o l iv ine to chlor i te and smect i te requi res in-troduction of AlrOr and SiOz, and release of FeO andMgO into an aqueous solut ion. Thus the in terst i t ia lf lu id probably was h ighly charged wi th Fe2+, Fe3+,and Mg'* and was able to convert the anorth i tecomponent of pr imary p lagioc lase to pumpel ly i te *chlorite. Of course, the alteration of primary magnet-ite-i lmenite opaques to magnetite-sphene inter-growths is related to the albit ization, as documentedby Kuniyoshi and L iou (1976): th is react ion may a lsohave p layed a s igni f icant ro le dur ing the metamor-phism of the ophio l i te , inasmuch as magnet i te-sphene intergrowths are abundant in both p lutonicand extrusive rocks.

Physical conditions of metamorphism

The mineral assemblages of the hydrothermal ly-altered basaltic sequence from the ophiolite are char-acteristic of the zeolite facies. Depending on the bulkcomposition, oxygen fugacity, and the mode of oc-

t2 LIOU: EAST TAIWAN OPHIOLITE

o

I Et i

.{

i t o

J

a A l a S r t o a N o A m A b S i O 2

SIo

m.s

!l

d

r500 A b+

HzoA m

Qe

A A m + W r + T h A b + P r ( r E p )1 507kmT h + A m + L m

?50"/km

(vop4i Jqu id t

2@

T E M P E R A T U R E , " CFig. 6 ' Pf lu id-7" d iagram showing the progressive mineral changes along the geothermal gradients suggested for "ocean-f foor"

metamorphism (Spooner an d Fyfe, 1973; G ass and Smewing, 197 3; de Wit and Stern, I 9761 Co leman, 1977 ) . The zeol i te assem b lages areplot ted on anhydrous diagram CaAlrSirOs-NarAlrs i ros-Sior . Also shown are exper imental ly-determined stabi l i t ies for st i lb i te (L iou.l 97 l c ) , wa i r ak i t e ( L i ou , 1970 ) , ana l c ime + qua r r z (Thompson , t 97 l ; L i ou , l 97 td ) , l aumon t i r e ( L i ou , t 97 l a ) , and p rehn i t e (L i ou , l 97 l b ) .Plagioclase composi t ions of An 50-60 are shown as black spots. Abbrevat ions: Th, thomsoni te l Ch: chabazi te, Lm: laumont i te, Wr:wairaki te, St : s t i lb i te, Hu: heulandi te, Ab: a lb i te, Am: analc ime, Na: natro l i te, ez: quartz, An: anorth i te. pr : prehni te, Zo: zois i te. Gr:grossular , and Ep: epidote.

T h , C h . A m /

T h + C h + A m{ L m )

currence, calcium zeolites and analcime together withadularia, albite, sphene, smectite-chlorite, Fe-pum-pel ly i te , and Fe-prehni te are ubiqui tous. No pre-hni te-pumpel ly i te associat ion was found in any in-vestigated rocks.

Experimental studies suggest that zeolite faciesconditions are in the range of 100-300.C and totalpressures below 3 kbar, which are consistent withthose deduced from reported burial metamorphic se-quences (for a review see Zen and Thompson, lg74),and with directly observed P-I conditions from ac-tive geothermal areas. Based on reconstructed se-quences in var ious metamorphic terra ins, Boles andCoombs (1977) est imated the min imum temperaturesof format ion for laumont i te , prehni te, and pumpel ly-i te to be about 50o, 90o and 190.C, respect ive ly .Provided these estimates are realistic, the laumontite-and pumpellyite-bearing extrusive rocks of the oph-iolite have been metamorphosed under zeolite faciescondi t ions at temperatures of about l50o to 250.C.

If ophiolite is init ially created at a spreading centerwhere geothermal gradients as high as 500o-1400.C/

km have been postulated(e.g., Cann, 1970; Spoonerand Fyfe, 1973),it must have been metamorphosed atextremely low pressures. Geothermal gradients in theorder of | 50'C/km have been suggested as beingmaintained in the oceanic crust to at least 100 kmfrom the spreading r idges (Coleman, 1977). Frominvestigations of metamorphic assemblages devel-oped in on- land ophio l i tes, Coleman and others (e.g. ,de Wit and Stern, 1976) concluded that geothermalgradients from 150'C/km to 250oClkm adequatelyexplain the observed mineral parageneses from zeo-l i te fac ies to low-rank amphibol i te assemblages in a2-3 km thick ophiolite sequence. If we take thesegeothermal gradients as having been imposed on theEast Taiwan Ophio l i te , at depths of about 0.6 to 1.6km, temperatures would have reached about 150. to250"C, enough for the recrystall ization of the basalticrocks to the observed zeolite assemblages.

The P-T conditions for the diagnostic zeolite faciesminerals such as wairakite, laumontite, sti lbite, andanalcime have been experimentally determined (forreview, see Zen and Thompson, 1974), and some of

400roo

FF''']

LTOU EAST TAIWAN OPHIOLITE l 3

system may be due to its fine-inhibi ts posi t ive opt ical ident i-

the P-T curves are summarized in Figure 6. The highgeothermal gradients of 150' -250oC/km are p lot ted,and zeolite paragenesis in the system CaAlrSirOr-NarAlrSirO.-SiOr-HrO and primary plagioclasecomposi t ion of An 50-60 are a lso shown. Immedi-ately apparent from such a plot are mineral succes-s ions encountered in the oceanic crust . They shouldchange gradually downward from thomsonite +chabazite * analcime, thomsonite i laumontite *a lb i te , and wairak i te * analc ime * thomsoni te toprehnite * albite (all assemblages a quartz * chlo-rite t sphene). Except for wairakite, these zeolite-bearing assemblages have been commonly describedfor metamorphosed basaltic rocks from dredge haulsand f rom on- land ophio l i tes. However, to my knowl-edge, wairakite has only recently been reported in thehydrothermally-metamorphosed tholeiites from thenorth Atfantic Ocean by Jehl et al. (1976). The rareoccurrence of wairakite in the ridge system, as op-posed to its common occurrence in the active geo-thermal areas, may be due partly to differences in thecomposi t ion of the hydrothermal so lut ion in thesetwo systems and partly to the diff iculty of differentia-t ing opt ica l ly between wairak i te and analc ime.

On the basis of f ield occurrences of wairakite-analc ime minerals in the a l tered volcanic rocks andact ive geothermal areas in Japan, Seki (1971) sug-gested that extensive sol id solut ions ex is t a long theanalc ime-wairak i te jo in (see a lso Surdam, 1967) atrelatively higher pressure conditions, whereas in lowpressure, active geothermal areas almost pure waira-kite is stably associated with analcime -l quartzand/or a lb i te . As shown in F igure 6, wairak i te of i ts ownbulk composi t ion is s table at temperatures rangingfrom 210'C to 320"C +10'C a long the presumedgeothermal gradients for the "ocean-floor" hydro-thermal metamorphism. For bulk composi t ions pos-sessing an excess of SiO, (e.g. , those composi t ions onthe r ight s ide of thewairak i te-a lb i te jo in) , analc ime isnot stable with respect to albite; therefore, nearlypure wairak i te + a lb i te * quar tz is the dominantassemblage within the stabil ity f ield of wairakite.Such assemblages commonly have been reported inactive geothermal areas. However, for the bulk com-positions on the left side of the wairakite-albite join,both wairakite and analcime may be stable togetherwi th a lb i te or thomsoni te (?) . Very f ine-gra inedwairakite of nearly end-member composition crystal-l izes ubiquitously together with smectite and albite inthe interaction of seawater and basalt at temperaturesof 250'-350oC and Pfluid : 500 bars (Seyfried,1977). Therefore the rarity of reported occurrences of

wairak i te in the r idgegrained nature, whichfication.

In summary, the basaltic rocks from the East Tai-wan Ophiolite were hydrothermally metamorphosedat a temperature between l50o-250'C and a depth of0.6 to 1.6 km, probably at some dis tance f rom theridge. They characteristically contain various calciumzeolites, analcime, and smectite-chlorite togetherwi th i ron-r ich pumpel ly i te , minor prehni te and adu-laria. The variation in mineral association amongpil low cores, rims, and matrices is believed to be theresult of local variations in the composition of circu-lating fluids, in the extent of recrystall ization, and intemperature. Mineral zonation observed in vein as-semblages indicates that the hydrothermal composi-t ion changed wi th t ime.

AcknowledgmentsThis research represents part of a U.S.-Republ ic of China coop-

erat ive science project to invest igate metamorphic rocks and oph-iolite in Eastern Taiwan (OlP 74/19399 and DES 74/19948) |

thank the Mining Research and Service Organizat ion and Stanford

Univers i ty for logist ical support , and W. G Ernst , John Suppe,

Ching-Ying Lan, and Y. Wang for their cooperat ion both in thef ie ld and in the laboratory. Many of the bulk rock and mineralanalyses were performed at the Univers i ty of Cal i fornia, Los Ange-

lesThis manuscr ipt has been cr i t ical ly reviewed and mater ia l ly im-

proved by W. G. Ernst , R. G. Coleman, Shingi Kuniyoshi , John

M. Ferry, and D. M Burt Preparat ion of the manuscr ipt was

supported by NSF grant GA 371'77 /L iou.

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