Geology and petrology of som'e polymetamorphosed and associated rocks in northeastern

J. G. LlOU Department of Geology, Stanford University, Stanford, California 94305 W. G. ERNST Department of Exth and Space Sciences, University of California, Los Angeles, ?alifornia 90024 DIANE E. MOORE U. S. GeologicalSurvey, Menlo Park, California 94025

Geological Society of America Bulletin, Part II, v. 92, p. 609:- 748, 26 figs'., 17 tables, May, 1981, Doc. no. M10501

the expense of hornblende, and (4) ABSTRACT production of biotite + muscovite +

The pre-Tertiary metamorphic jomplex K-eldspar in the part1y;mctasomatized

of northeastern Taiwan consists of arnphibolites, The metamorphic-igneous.

, marble, , , complex was later intruded by thin diabasi'c aikes; the older of these dikes (meta)granodiorite, and minor, serpentinized pe.ridotite. Fault-bounded are believed to be,feedeis for basaltic

and foliated_amphibolite with the flows and pyroclastics in the overlying

assemblage green hornblende + lower formations. Some dikes I

(An4o to An52), + (Ps4 to Ps 15 ) + may be as young as . The whole

c sphene 2 rutile ? quartz was intruded complex--basement, , and cover '

by granitic rocks about 87 m.y. ago rocks--was metamorphosed under+g;eenschist -c or earlier. The most apparent thermal ,facies conditions during

effects include (1) transformation of - collisioii between

green hornblende to brown hornblende, the China and the Philippine Sea plates.

(2) transf&ation of epidote to Amphibolite has been partly .? symplectic intergrowths of - recrystallized to the assemblage

r -i clinozoisite + plagioclase + quartz, + chloritz + epidote

(3) crystallization of clinopyroxene at ("15 to. Ps 27 ) + quartz + sphene. 609

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Both nonfoliated dike rocks and foliated fractionations characteristic of lower

basaltic flows in the Cenozoic sequence grades; (2) the Mg/Fe ratio of chlorite

carry the asssemblage albite +' is 2.0 in amphibolite, 1.2 to 1.5 in

a'ctinolite + chlorite + epidote altered amphibolite, and 0.9 to 1.0 in

(PS~~to Ps~~) + biotite + quartz + grecnschlst ; and (3) the amphibolite

sphene. Zeolite facies rccrystalliza- facies hornblendes contain high

tion occurred locally along fractures in pargasite-tschermakite proportions

some amphibolite and produced- (A1203 = 8 to 13 wt %; Ti0 = less than 2 laumontite + cpidote (Ps ) + chlorite. 0.6 wt %; Pig& + Fe ratio = 0.54 to 32 9 .< Bulk-rock X-ray fluorescence (XW) 0.65), the thermally recrystallized

compositions of 14 amphibolites fall brown hornblendes .contain

within the range of low-K and low-Ti T.9 %), substantially more Ti02 (1.0 to wt

tholeiite, whereas 7 diabasic dike rocks slightly c.lower Si and hence higher

and 2 basaltic rocks of the Cenozoic ill1" and elevated total alyninum * r sequencrare alkali basalts with contents compared to the green and

relatively higher 'TiO2 (0.7 to 2.7 wt %), blue-green hornblendes, whereas the '_ Na 0 (1.6 to 4.0 wt %), and K20 2 gr'eenschist facics are .I (0.6 to 0.8 wt 2). Thermally Al 0 actinolitic, with 23less than recrystallized amphibolites enclosed in 5.9 wt %. Fractionation of elements-

the Upper granitic intrusion between coexisting phases in the

are enriched in Si02, A1203, and K20 amphibolites and associated rocks

/ and depleted in total Fe, MgO, and CaO. are systematic and, in general, suggest

& I. Microprobe analyses of coexisting phases a close approach to chemical equilibrium;

show.:that (1) zoning of. epidote, in most cases, the exchange reactions

, and plagioclase reflects appear to be of the ion-for-ion type,

changes in pressure-temperature exce3t where several structurally

conditions--with more pronounced distinct crystallographic sites are

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involved in the partitioning. intermediate-pressure recrystallization

Comparisons with experimentally event prior to or during Cretaceous

determined phase equilibria and element time. Cretaceous calc-alkalic plutonism

distributions among coexisting minerals and thermal metamorphism attended

yield the following estimates of marked convergent plate motion.. Only

pressure-temperature conditions for the minor (slightly alkalic) magmatic

various recognized stages of metamorphic activity was associated with a I r." r." recrystallization: (1) amphibolite A ' hypothesized early Cenozoic rifting

facies metamorphism at pressures of about of the Asiatic continental margin. .I;

5 kb anditemperatures approaching facies recrystallizati'on,

650 'c; (2) accompanying the granitic which apparently took plare diiring I intrusion; 'potassium metasomatism and Pliocene-Pleistocene time, is inferred

thermal metamorphism at about 700 OC, to reflect the collisl'on of the Chinese

judging from the incipient breakdown of shelf + slope + rise with the western iJ epidote-clinozoisite in the amphibolite edge of the and the formation of clinopyroxenc at the (=..,Luzonarc) . expense of hornblende; and (3) INTRODUCTION Pliocene-Pleistocene greenschist facies

metamor'phism at 350 to 475 'Cand P total The pre-Tertiary basement of Taiwan

of no more than about 5 kb. is well exposed in the Central Range. -3 Cretaceous or earlier sea-floor . , Metamorphis? of this involves

spreading apparently generated the several differeqt stages that have not

Suao-Nanao basaltic + ultramafic been ,previously documented. The effects

prbtolith; these,rocks, overlain by of multistage recrystallization and

younger sediments, were transported to deformation are present in a variety of

and sequestered at the Asiatic continentap metamorphic rocks of the pre-Tertiary

margin, accompanied by an complex but are especiall'y well

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displayed in the amphibolites. At the greenschist facies conditions during

northern tip of,the metamorphic basemeot Pliocene-Pleistocene time (Suppe. and

complex in the Suao-Nanao area (Fig. l), ' others, 1981). Although the

strongly foliated amphibolites are bounde amphibolites have been much modified

by high-angle thrust faults; these by'later deformation and recrystallization,

masses occur as lcnticular bodies with. the effects of.the &rly events are

dimensions of 5 to 10 km by 1 to 2 km. preserved in most amphibolites in the The amphibol ites have been intruded by Suao-Nanao area. -- . ,. Cretaceous granitic rocks; distinct These amphibolites are perhaps ,.. thermal ef,fec ts , including migma t ization the oldest rocks in Taiwan. Mineral

of amphibolite, are displayed adjacent to assemblages identified in them may be

,the intrusions. The metamorphic-igneous correlated with various metamorphic ., complex was later subjected to further and tectonic events, including

deformation and retrograde *? (1) preintrusive amphjbolite facies ,. J - recrystallization. After an erosional recrystallization, (2) synintrusive

interval, .the complex, along with other migmatization and thermal metamorphism,

pre-Tertiary units, was.covered by and (3) -postintrusive greenschist facies

an unconformable series of Tertiary, and zeolite facies recrystnlJization. 1 chiefly sedimentary, rocks, now Therefore, investigation of deformation

(Suppe and -others, 1976). The and petrology of t~hepolymehimorphic

pre-'l'ertiary basement Lcomplex is cut . amphibolites and associated granitic

by thin diabasic dikes, which are plutons and- the much younger diabasic

believed to represent feeders for the dikes should shed light on the

basaltic flows and pyroclastic rocks petrotectonic evolution of the Central a' intercalated in the Tertiary formations. Range of Taiwan. Except for

/I ' The pre-Tertiary basement, dike, and reconnaissance field surveys and

covcr rocks wcrc thcn mctamorphoscd undcr petrographic studies (Yen, 1954a, 1954b;

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Figure 1. General geologic map of Suao-Nanao area of Taiwan showing

sample localities and five iiidjor litliologic units: (1) amphibolites,

(2) granitic intrusions (orthogneisses), (3) quartzofeldspathic

paragneisses, (4) marbles and , and (5) Tertiary cover rocks

( + greenstones). This map is based on our field study as well . as earlier works by Chen (1977) and Suppe and others (1976)

Figure 1 appears on the following frame.

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E X PLANAT ION 309 SAMPLE LOCALITY

_-**-- LITHOLOGIC CONTACT .--FAULT '\T TRAMWAY

SLATE 0SCHIST t MARBLE QUARTZ~FELDSPATHIC El PARAGNEISS METAGRANITIC ROCKS AL!P!

Figure 1.

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parageneses and crystal chemistries of along the eastern siope of the Central

.I the amphibolites have not' been delineated -Range; it has been-subjected to

previously. Such stud-ies have been multistage deformation and metamorphism, ., .* undertaken as part of our new as evidenced by occurrence of

investigation of the regional metamorphisr incompatible mineral associations

and deformation of the Central Range, and superpositions of foliatioi.

, and the results are described in this Amphibolite is extensively exposed in

report. the Suao-Nanao district, shown in

Figure 1. The five major subparallel GEOLOGIC SETTING AND.FIELD RELATIONS lithologic units' trpd approximately

The geology of the Suao-Nanao district, east-west. Mapped lithologies include

as shown in Figure 1, has been studied well-foliated 'amphibolites, metamorphosed

i and discussed in recent years by granitic plutons ( and

numerous workers, especially Yen (1954a,. migmatites), quartzofeldspathic gneisses,

1954b1, Fuh (1962), Ho (1975), and thick sequences of interbedded marble

Suppe and others (1976). The pre-Tertiary and pelitic schist, and slaty Tertiary

basement compltx is overlain unconformably cover rocks. Locally abundant,

by lower Tertiaky strata (Suppe and thin diabasic and lamprophyric' dikes

others, 1976). The basement rocks, and pegmatitic pods crosscut the major

grouped together under the general pre-Tertiary rocks: Metamorphosed -3- Ic stratigraphic term "Tanano Schist" ultramafic rocks and rodi'pgites occur

(Yen, 1960),. consist of a variety of sporadically in the area but are too

schists and marbles, together with small to be designated on the map. 5- ! a minor amount of gneisses, amphibolites, Except for the young mafic dikes,

migmatites, metabasalts, and other rocks in this area are well -. . This assemblage of foliated. Most of the schistosity

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strikes nearly east-west and dips 200 m thick, and some finely banded

predominantly to the south. Intensive biotite + epidote + chlorite t multistage deformation and are laminated with marbXes

r-. polymetamorphism have locally obscured and/or with graphite-bearing pelitic

details of the pre-Tertiary sequence. schists. Marbles show variable color -_ C. At least twd foliations are present in and grain size and range from massive b the basement rocks, with the earlier to well foliated. They consist mainly

schistosity strongly folded; fold axes of equigranular dalcite, with quartz,

andathe younger foliation are subparallel white , chloiite, phlogopite, sphene,

to the slaty cl.eavage in the Tertiary and graphite as minor accessories.

cover rocks. Lenses of dolomitic marble are sparsGly

The general geologic features of the intercalated.

\ five major units are described below. The pelitic schists are medium to r fine grained and consist of di'ffering Schist and Marble Unit amounts of quartz, albite, chlorite,

Graphite-bearing pelitic schists white mica, green biotite,..sphene,

with subordinate!,hinount s of graphite, and pyrite: calcite is locally

chlorit6-rich greenschist intercalated present. The chlorite-rich schists

with gray to white marbles are are finely laminated and contain variable

t. a. extensively exposed in this and other proportions of quartz, albite, chlorite,

areas of the Central Range.' This unit' green to olive-brown biotite, epidote,

is the most abundant lithology in the calcite, sphene, and opaques in .

basement complex of the Central -.Range different compositional layers. (details will be given elsewhere). The Actinolite is\present only in those \ thickness of these lithologies varies rocks lacking calcite-evidently the

from area to area; for example, marble occurrence of actinolite in the I/ beds range from a few millimetres to greenschist facies metabasites is

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controlled by a low fugacity of C02, as textures, and presence of abundant

dohmented elsewhere (for example, quartz, biotite, and untwinned or

Hdrte and Graham, 1975). simply twinned albitic plagioclase; -e The mineral assemblages of various These rocks also contain muscovite, schists and marbles indicate that these chlorite, rare epidote,,sphene, and

rocks were,thoroughly recrystallized garnet. Both bio,tite and garnet

under pressure-temperature conditions have been partially or completely

of the biotite zone of the greenschist replaced by chlorite,

* .. facies. Me tagranitic .Rocks Quartzofeldspathic Paragneiss The granitic orthogneiss occurs

The gneissic rocks exposed in the as a narrow lenticular body extending

southern part of the area have been in an east-west direction for more

investigated by Yen (1954a, 1954b) and than 6 km, with an average width of

Fuh (1yW. Both metasandstone and about 2 km. The body' is hounded by

graphitic metashale have been suggested amphibolite to the north and

as protoliths for the paragneisses. metasedimentary gneisses to the south.

Contacts between orthogneisses on the The contacts with both country rocks

north and paragneisses on the south are are irregular and show characteristic

irregular and gradational. intrusive features, including contact

Quartzofeldspathic paragneiss is metamorphism of the country rocks, . ,-. differentiated from metagranitic rocks, lit-par-lit injection, migmatization,

as described below, on the basis of and the presence of abundant "xenoliths 1. field relations and lithologic characters. of country rock. Some of the inc1;ded

In brief, metasedimentary gpeisses.are amphibolite and paragneiss fragments

characterized by coarse grain size, are as much as several metres long,

gneissic structure, relict clastic and they are extensively alt;?red and

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feldspathized. Many irregular and dike are 87 t 5 m.y. (Yen and Rosenblurn,

lenticular dikes 3 to 50 m thicl 1964; C. Y. Shih, 1972, personal

occur subparallel to the 'foliation of the commup.), whereas K-Ar dating of

metagranites and adjacent paragneisses recrystallized biotite from the

(C. Y. Lan,. 1978, personal commun.). orthogneisses yields an apparent age

The metagranitic orthogneisses are of 39 2 0.8 m.y. (Juan and others,

light-gray, rather homogeneous rocks 1972). Evidently, the minimum age of

with distinct relict granitic textures. last equilibration of minerals in the

Foliation defined by the concentration granitic orthogneiss is Late Cretaceous,

of mica flakes is nearly concordant with The intrusion could have occurred much

that of pelitic schist and mfrble. earlier. The - date is

Chemical compositions and modes (see,for prisumably a Mesozoic value overprinted

example, Fuh, 1962; Table 3 and Fig. 4 by the Pliocene-Pleistocene metamorphism,

here) indicate that they are peraluminous inasmuch as the Tertiary cover sequence

granodiorites consisting of abundant shows no evidence of disturbance during

sodic plagioclase, quaytz, biotite, and early Cenozoic time.

muscovite, with minor chlorite, garnet, Amphibolites .. epidote-clinozoisite, sphene, and iron

oxides, The ubiquitous presence of Two lenticular bodies of amphibolite

albite, chlorite, biotite, and epidote trend nearly east-west for more than

suggests that pervasive greenschist 5 km in the Suao-Nanao area. The

I facies metamorphism took place after northern body is bounded by two ,

intrusion of the granitic msses. high-angle faults and is in sharp contact

Radiometric dating of the granitic and with the marble and graphite-bearing

country rocks has scarcely begun. Bdth pelitic schists. Features such as

K-Ar and Rb-Sr ages of muscovite and truncation of foliations at the contacts,

a mineral isochron from 3 pegmatite well-developed slickenside and fault

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gouges within brecciated amphibolites, to those described above. However,

presence of retrograde -chlorite the southern part of the amphibolite

schists, and sericitized rocks with .body adjacent to the orthogneiss .) copper mineralization are abundant contact exhibits characteristic

along the fault zones. The amphibolite intrusive features. For example,

is well foliated, with a dark-green to amphibolite is interleaved with injpcted

greenish black color. Some granitic layers and quartzose bands of

coarse-grained amphibolites show - various thicknesses, Quartz veins

distinct layers of plagioclasg a& and granitic pegmatitic dikes are

clinozoisite, whereas others contain common. Evidently, strongly foliated .... alternating bands of coarse- and and tightly folded amphibolitic units

fine-grained varieties. These features were intruded by granitic magma.

suggest that the protolith-may have I Migmatization and high-temperature

been basaltic flows and tefs. In the alteration of amphibolites must have

northern body, pegmatite dikes:and occurred during the intrusion. Some +.* . .. L granitic layers were not found, and of these thermal effects have been

quartz veins are relatively rare, The described by Fuh (1963). AS shown

northern body is spatially separated in Figure 2, C, foliation of the relict

from the orthogneiss, and this amphibolite can be traced locally

amphibolite body apparently is free from one block to adjacent layers; in

from the thermal effects of granitic other places, individual rotated

intrusion. ?-'Photographs of some . amphibolite fragments of diverse sizes ,..-

occurrences are given in Figure"2; * occur as xenoliths in the granitic

The megascopic features of the s6uthern rocks (Fig. 2, D). Leucocratic rocks

amphibolite body and the bounding wit.h abundant quartz + epidote +

northern'high-angle fault contact with plagioclase were developed around the

marble and pelitic schist are similar rim of the amphibolite blocks. Most of

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- Figure 2. Sorne field views of amphibolites and their contact relations

with country rocks, northeastern Taiwan. A. Well-foliated coarse-grained

amphibolite with distinct segregation layers of plagioclase -+ clinozoisite

(T-8A). B. Fault contact between foliated amphibolite to right and

chloritle schist to left (T-333). C. Folded amphibolite intruded by '

; note that foliation of amphibolite ca2/be traced from one block

to adjacent layers. Leucocratic rock contains abundant quartz, clinozoisite, 0 and plagioclase (T-12). D. Amphibolite blocks as xenoliths in migmatized

amphibollte. TLin layers of lcplhocratic rock occur at margins of xenoliths;

other blocks with abundant biotite-rich rims are. strongly weathered at

this stream exposure (T-12).

Figure 2 appears on the following frames.

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Figure 2.

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C

Figure 2. (Continued)

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the included amphibolites contain both amphibolite and have

biotite and muscovite; accordingly, identical foliations; and (3) consistent

K metasomatism may have been significant mineral assemblages among amphibolite,

during the interaction of granitic melt rodingite, and serpentinite are observed.

and amphibolite. Some of the amphibolites may have be-

Depending on the effects of granitic derived from gabbros that originally

intrusion, the amphibolitic rocks in were associated with harzburgites;

this area may be divided, therefore, into During serphtinization of the dep'leted

three groups:. (1) amphibolites showing ultramafics, the gabbros at the contact

no apparent thermal effects, (2) thermall: evidently were rodingitized into

recrystallized amphibolites-occurring Ca-A1 - rich rocks as described f&m

.adjacent to the granitic orthogneisses; other areas by Coleman (1966, 1967) and

and (3) migmatized and contaminated Dal Piaz (1969). During regional

amphibolites, includiw- xenoliths metamorphism, the mafic igneous

displaying the results of strong rock - rodingite - serpentinite sequence

K metasomatism. was recrysta$l$z-ed .to amphibolite,

The amphibolites are locally metarodingite (diopside-grossular-

associated with serpentinite pods about vesuvianite), and metaserpentinite

20 by 200 m in areal dimensions(Fig. 3). (antigorite + edenitic amphibole),

Thin rodingite zones, -1 to 1.5 m thick, respectively.

occur at contacts between amphibolite Diabasic Dike Rocks and serpentinite. Evidence suggests

that the mafic and ultramafic rocks were The pre-Tertiary metamorphic complex

incontact prior to regional amphibolite in the Suao-Nanao area is transected

facies metamorphism: (1) gradattonal by numerous dikes, including pegmatite,

transitions from rodingite to amphibolite lamprophyre, and . Some of exist over a disl-ance of 0.5 m; (2) both these dike's'have been.-. described previously

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Figure 3. Sea-cliff section of ,schist-amphibolite contact 1 to 2 km south-southeast of

Tung-ao (see Fig. l), provided by John Suppe, showing details of later crosscutting

diabasic dikes,. F = fault: screened pattern dipping to north = dikes; clear pattern with

folded layers = quartz + mica and quartz schists. *

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 (for example, Tuh, 1962;.C.r Y. Lan, 1978, in the thicker dikes; and (3) they

personal commun.). Diabasic dikes were contain the greenschist facies assemblage

selected for detailed study in the actinolite + chlorite + albite +

investigation described here in order to epidote + biotite quartz ? calc!.te +

compare the mineral parageneses and sphene'.

crystal chemistries of amphibolite and Slaty Tertiary Cover Rocks metadiabase. At least some of the

diabasic dikes appear to represent the In contrast to the thoroughly and

.feeders for the basaltic rocks of the multiply recrystallized and foliated

Tertiary cover formation and have thus rocks of the basement complex, the

9 been subjected only to the late-stage ' post-Mesozoic supeyjacent series *\ \ metamorphism. This accounts for the consists of sedimentary and minor

fact that their metamorphic mineral volcanic rocks that are overprinted by

assemblages 3re identical to those one or more phases of greenschist

developed terminally in the facies deformation and metamorphism.

polymetamorphosed amphibol3tes. Rock types include pelitic slates', Diabasic dikes of a few centimetres fine-grained phyllites, quartz-rich. \ to more thrtn 10 m in thickness occur metasandstones, and a variety of

within the amphibolites, metagranites, interbedded metavolcanic rocks,

and pelitic schists. Their attitudes including amygdaloidal metabasaltic

vary from place to place; some are flows, massive -greenstones, tuffaceous ,. ., paralleL to the foliation, whereas others metasedimentary rocks, and coarse-grained

crosscut schistosity of the country meta-agglomerates. The relatively

rocks (Fig. 4). The characteristic massive metagabbroic and metadiabasic

features of these dike rocks ye (1) they rocks contain relics of primary augite,

are massive and lack visibl4 foliation; now largely replaced by uralitic

(2) chilled margins are locauy developed hornblende, saussuritized calcic

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 Figure 4. Some field views of diabasic dikes intruding Flesozoic

basement complex, northeastern Taiwan. A. Subparallel dikes intruding

migmatite and granitic rocks at locality T-12. One dike, about 1.2 m

thick, is pointed out by John Suppe, and other dike, about 20 cm

thick, lies beneath0 hammer. B. Close-up view of thin dike of photo A

showing amphibolite xenoliths and massive migmatite. Note that dike is

' irregular in thickness. C. Nassive 'diabasic dike about 1.5 m thick .. transecting pelitic schist and quartz vein at locality T-334. D. Thin

'diabasic dike forming boudinaged lens lying within main folia.tion of

marble. Note that jointing in dike and marble is continuous.

Figure 4 appears on the following frames. .

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?

Figure 4.

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.. C

D

Figure 4. (Continued)

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plagioclase, and the greenschist facies hinge zone near the south end of the

assemblage described above. These outcrop (Fig. 3). The diabasic dikes -. recrystallized sedimentary and volcanic dip gently to the north, crosscut the

rocks exhibit the effects of a distinct, outcrop-scale folds of the schists and I single-stage penetrative deformation amphibolites, and are themselves

of Pliocene-Pleistocene age. unfoliated (Fig. 4). Therefore, the

dikes postdate the main penetrative Contact Relations of the Mafic Dikes deformation displayed in the Tung-ao

Contact relations between the area.

amphibolites and diabasic dikes are Regional considerations suggest

well displayed along the beach cliffs that at least some of the diabasic

south of Tung-ao (see Fig. 3), where dikesarcearly Tertiary in age. The

various interlayered south-dipping Tanano schists arc unconformably overlain

quartz-mica schists, amphibolites, bxl Paleogenea slates about 4 ,km north of

and ultramafic rocks are exposed. The Tung-ao (Fig. 1). The unconformity is

0 schistose layering is accentuated by now overturned and dips 70 to 80' to

quartz segregations and is folded the south (Suppe and others, 1976).

into nearly isoclinal, north-vergent, The lower part of the slaty Paleogene

outcrop-scale folds with east-plunging sequence contains basaltic tuffs,

axes lying within the planc of the flows, dikes, and sills that have

well-developed axial-plane foliation. undergone a metamorphism similar to that

This strong metamorphic folding, which of the dikes within the basement rocks.

is displayed by both schists and Some of the dikes may be as young as

amphibolites, does not appear to be Miocene, because scattered extrusive and

the 'first major deformation, because intrusive basalts are present within

refolded folds of diverse orientations the Tertiary section of the Hsueishan are widely exposed in the major synformal Range and western foothills (Ho, 1975;

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 Lo and Goles, 1976; Juan and others, 1979) recrystallization ,fractures. These - aloqg The north-dipping dikes near Tung-ao (Fig. four stages of metamorphism are developed .. 3) are approximately perpendicular to the to differing extents in various amphibolite

overturned sputh-dipping unconformity. samples; corresponding mineral assemblages

. Therefore, the dikes may have been vertica are identified in granitic and metadiabasic

feeders for the Paleogene extrusive basalt rocks.

,above the unconformity near Suao, although Except for xenoliths in the granitic

this geometry requires that the basement ortho gne isses , the "primary metamorphic

kocks containing the dikes have been phase assemblage is well preserved in most

overturned rigidly without any Tertiary amphibolites from the Suao-Nanao area.

distortion. Mineral associations produced by the later

three stages are locally developed and are MINERAL PARAGENESIS OF AMPHIBOLITES relatively minor. The thermal effects are AND METADIABASIC ROCKS best shorjn by amphibolites adjacent to the

From field relations described above, granitic rocks. The greenschist recrystal-

the amphibolitic rocks from the Suao-Nanao lization is better developed along secondary

area evidently have been subjected to at foliationsor near the margins of the j least three--and probably four--metamorphic omphibolitebodiet; where brecciation has

events after the initial amphibolite enhanced the cryshallization of retrograde

'recrystallization: (1) thermal metamorphism assemblages. Zeolite alterations occur

due to the intrusion of Cretaceous sporadically as veins ,crosscutting the

granitic rocks; (2) and (3) greenschist various lithologic units. Although the

facies metamorphism during pre-Cenozoic "primary" amphibolite stage and the

back reaction(?) and Pliocene-Pleistocene . subsequent thermal upgrading have been

time (most unambiguously developed,howcver, briefly described before (Yen, 1954a, 1954b;

in the crosscutting Tertiary metadiabasic Fuh, 1962), the greenschist overprinting

dike rocks); and (4) zeolite facies . and formation of late laumontite + epidote

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veins have not been identified in the clear blue-green color parallel to 2, and

amphibolites prior to our work. Many a greenish yellow color parallel to X.

amphibolites do show relics of all four Most hornblende crystals range in length

stages in representative hand specimens, from 1 to 10 mm and in thickness from 0.05

however. Paragenetic relations are to 0.4 mm; they define a pronounced

illustrated in Figure 5 and summarized lineation. Short prismatic hornblendes

briefly :below. occur in a few specimens.

Plagioclase is chiefly andesine, Amphibolite Stage although both oligoclase and labradorite

The amphibolite stage is characterized , occur. Twinning is commonly not

by pronounced foliation and gneissic visible. Plagioclase cr$stalls are

banding. The amphibolite assemblage typictlly elongated parallel to the

consists of highly elongate, parallel lineation of the rock. Some grains

crystals of blue-green hornblende, are intensely sericitizedand altered

clinozoisite-epidote, and plagioclase to aggregates of clinozoisite and

with subordinate amounts of quartz, white mica.

sphene, rutile, and ilmenite. The Epidote-clinozoisite occurs as

well-recrystallized rocks are nematoblastic granular or crudely tabular forms

in texture, and are medium to coarse associated with hornblende and > gqained. Photomicrographs are shown in plagioclase, or as slender idioblastic

- .-z- Figurks 6 and 7; estimated modes;”as well crystals elongated parallel to the

as bulk chemical compositions, are listed ,schistosity. Some clinozoisite crystals

in Table 1. have good cleavage parallel to (001)

Hornblende ranges from 50% to 80% and are elongated along the c axis, in

I.,.. . by volume in most”amphibo1ites. The many cases being 3 mm or more in‘length 4 most striking characteristics are its (Fig. 6, A). They exhibit parallel

uniformly long slender crystal habit, extinction and abnormal blue’ ,

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Figure 5. Schematic mineral parageneses for polymetamorphic

amphibolites and for Tertiary diabasic dike rocks from

northeastern Taiwan. Mineral assemblages for (1) amphibolite c facies, (2) thermal-metamorphic, and (3) greenschist facies stages

are shown, 'Dhshe'd lines indicate presence of only minor amounts or

sporadic occurrences of a mineral. Greenschist fncies metamorphism

affected all rocks of Suao-Nanao area in Pliocene-Pleis.tocene time,

but earlier retrograde event may have occurred in amphibolite-granitic

gneiss basement complex prior to deposition of Cenozoic cover sequence.

Figure 5 appears on the following frame.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 nnn \ Stage Mesozoic ? Cenozoic. Amp hi b ol ite Facies Granitic ’- - Greenschist Facies Mineral \ Metamorphism I Intrusion 1 Metamorphism AMPHIBOLITES plagioclase An 40-52 An 16-57 An 00-07 quartz Ca-amphibole gre$n hb brown hb act inoli te chlorite biotite -----==- garnet white mica clinopyroxene m- epidote-cz PS 04-15 PS 05-14 PS 15-27 calcite rutile sphene ilmenite

METADIABASE! plagioclase An 01-11 quartz Ca-am phi bole actinolite I I chlorite I biotite - white mica c. epidote-cz .: ca Icite sphene magnetite ; pyrite

Figure 5. >

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 Figure 6. Photomicrographs of sqme polymetamorphosed amphibolites

from northeastern Taiwan. Except for E, these amphibolites wgre not

affected by granitic intrusion. A th;ough E, plane- polarizer;,F,

crossed polarizers. A. Well-foliated amphiboli?; with abundant

hornblende, saussuritized plagiGclase, and one long, slender

clinozoisite crystal (Cz) with good (001) clcibvage (T-5).

B. Well-foliated amphibiolite containing abundant hornblende (Hb) ,

epidote (Ep) , and saussuritized plagioclase. Formation of

saussurite is believed to have been due to later greenschist facies c. recrystallization (T-7). C. Folded amphibolite with abundant

hornblende (Hb), epidote (Ep), apd plagioclase (Pl) (T-40A).

D. Folded amphibolite with abundant lenticular sphene together

with hornblende (Hb), pl~gioclase (Pl), epidote (Ep), and quartz

(Qz) (T-395E). E. Thermally recrystallized amphibolite showing

granoblastic texture; this specimen contains brown hornblende (Hb), - clinopyroxene, epidote (Ep), and plagioclase (Pl) (T-8R). F. Folded

amphibolite showing develdpment of greenschist facies miner-als

c actinolite + chlorite 2 epidote (Ep) along hinges of fold axis (T-40A).

, Figure 6 appears on the following frank.

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.c *

E --'_'. 17 Figure

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Figure 7. Photomicrographs of thermally metamorphosed amphibolite

(A through E) and migmatized amphibolite (F) from northeastern Taiwan

(all with plane polarizer). A, R. Thermally metamorphosed amphibolite

with abundant symplectite intergrowth of clinozoizite + quartz +

plagioclase, zoned hornblende, and saussuritized plagioclase. Vein

material in B contains epidote, quartz, albite, and chlorite (T-8A).

C: Symplectic clinozoisite + rutile + quartz (Qz) in

clinopyroxene-bearing amphibolite (T78B). D, E. \Jell-foliated

amphibolite with abundant hornblende (light green and very elongate),

symplectite clinozoisite (Ep), plagioclase (Pl), quartz (Qz), and I - sphene. Vein in D contains laumontite, chlorite, and epidote

(TPY 6617). F. Migmatized amphibolite showing replacement of

hGrnblende (Hb) by biotite (Bi) and crystallization of potassium

feldspar (Ks). Saussuritized plagioclase (Pl) also present (T-12H).

Figure 7 appears on the following frame.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 ' UJUJZ'O ' 3

Y

UJWZ'O I W

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T-6* T-7h T-8A* T-33C T-38A T-40A T-l2A* T-12R*

S i02 48.69 47.91 48.98 49.13 49.43 49.50, 58.10 46.96 T i02 0.97 1.27 1.20 1.04 0.92 0.99 1.91 1.70 13.48 11.04 14.33 12.06 12.62 12.41' 14.54 13.28 9.63 10.85 10.33 9.80 9.46 9.23 8.34 13.07 0.156 ' 0.173 0.132 0.148 0.139 0.157 0.136 0.259 MgO 11.21 12.55 9.11 11.25 11.13 11.20 4.78 io.11 CaO 10.39 12.53 10.68 12.07 12.12 12.36 5.52 8.91 K20 0.40 0.05 0.99 0.07 0.12 0.10 2.60 1.94 Na20 2.75 1.85 2.42 2.25 2.38 2.14 2.06 0.88 L.O.I. 1.59 1.19 0.92 1.51. 1.10 1.31 1.35 2.28.' p2°5 0.14 0.20 0.21 0.16 0.16 0.15 0.23 0.23 S 0.04 0 43 0.02 0.04 0.04 - 0.04 0.09 0.03 Anhydrous 99.45 99:m64 99.32 99.53 . 99.62 99.59 99.66 99.75 Total

- - __ - - __ - r Hornblende 55 65 65 50 55 55 10 50 Plagioclase 25 3 30 25.- 20 35 5 Quartz 2 10 3 25 15 Ep idote 10 15 15 10 15 4 5 Chlorite 2 3 1.1 2 V 5 Biotite 20: . Sphene 7 3 4 5 5 5 3 Opaque 1 2 tr tr tr 2 1 2 White mica tr 1 tr 10 Carbonate tr Symplec t ite 10 20 3 tr 5 Laurnon t ite tr

PJOte: XRF analyses by G. Sturmner, University"of California, Los Angeles. qhermally recrystallized amphibolites. ?Total Fe as Fe2O3 .

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T-12" T-12" T-327E T-328C --T G- 1 G-3 \

S i02 49.62 63.27 48.20 48.30 46.54 48.36 Ti02 1.45 0.74 1.10 0.99 1.34 1.41 A1203 11.59 15.70 12.14 12.54 12.06 12.42 Fe203t 12.01 5.91 10.45 10.11 11.85 12.08 . MnO 0.217 0.081 0.174 0.165 Oi202 0.182 ' MgO 9.85 3.52 12.13 11.71 12.62 10.07 CaO 11.62 3.27 11.09 11-01 10.45 11.72 K20 0.71 2.48 0.12 0.11 0.21 0.20 Na20 1.01 2.48 2.27 2.42 2.03 1.88 L.O.I. 1.16 1.93 1.91 2.04 2.46 1.43 p2°5 0.22 0.06 S 0.06 0.21 Anhydrous 99.52 99.65 99.58 99.40 99.76 99.75 Total

Hornblende 80 20 55. 60 65 55 Plagioclase 5 20 15 18 20 Quartz 5 20 - tr tr tr Epidote 5 40 15 10 12 10 Chlorite 5 5 tr 7 Biotite Sphene 2 5 5 5 5 5

Opaque ' 1 tr tr tr tr tr \Jhite mica 2 10 tr 3 Carbonate tr tr S ymplec t i t e 5 5 Laumont i te

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identified as zoisite (for example, Fuh, (3) crystallization of clinopyroxene

1962). However, Fe 0 contents greater of 23 at the expense hornblende, and than 2% by weight suggest that they (4) formation of biotite + muscovite +

are clinozoisite. Although this phase K-feldspar in the metasomatized

is ubiquitous, it is a relatively amphibolites.

minor constituent in the amphibolite and Hornblendes in the thermally altered

is typically zoned with a more pistacitic amphibolites have greenish cores / core. characterized by lower refractive

indices compared to the dark-brown Thermal Mhtamorphic Stage rims. Some hornblendes are altered

During the intrusion of about to biotite along grain margins or

87-m.y.-old granitic rocks, some cleavage traces; the amount of biotite

amphibolites were- prograded and locally replacement increases toward the margins 0 K metasomatized; they were transformed of the xenoliths and toward the

to biotite + muscovite + garnet + introsive contacts. Where hornblende

K-feldspar - rich rocks, but minor is biotitized, quartz is abundant.

relict amphibolitic assemblages have Plagioclase becomes more sodic in the

been preserved locally. Well-foliated metasomatically altered amphibolites,

amphibolite fragments of'various with compositions ranging down to

sizes occur as xenoliths iq the oligoclase; K-feldspar occurs locally. 4,) granitic rocks. -The most apparent Transformation of coarse-grained

thermal effects on the preexisting epidote to a symp1ectic.intergrowth

phase assemblages include (1) transforma- of clinozoisite + quartz +I plagioclase

tion of green hornblende to brown is a characteristic feature of many

hornblende, (2) con.version of epidote upgraded amphibolites. Several

to a symplectic intergrowth of examples are shown in Figure 7. ,

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 The fine-grained vermicular clinozoisites in diameter; some crystals exhibit fine

are irregular. in fo& and possess abnormal exsolution lamellae. The other associated

blue interference colors; they are minerals include fine-grained rutile.

compositionally. heterogeneous, with Except for a few veinlets of albite, ~

varying amounts of Fe and Al. no chlorite or other greenschist

Clinopyroxene-bearing, thermally minerals were found in this sample. .

metamorphosed amphibolite was found The growth of granoblastic clinopyroxene

only in one sample, T-8R; this specimen, and brown hornblende, the absence of

which was collected about 100 m away greenschist and K-bearing phases, and

from the intrusive contact (see Fig. 1) the presence of symplectic clinozoisite +

is closer to the granitic body than quartz + plagioclase all indicate

other thermoamphibQQte 'samples. that the amphibolite was isochemically ,o . Differing from other altered amphibolites, recrystallized under pressure-temperature

this rock contains about 20% by volume conditions of the upper amphibolite

clinopyroxene but neither muscovite faciesduringthe intrusion of granitic

nor biotite. The original amphibolitic magma.

foliatibn is well preserved and defined The amphibolitic xenoliths within

by alternating layers of brown hornblende the orthogneiss typically are rimmed

(30%) and calcic plagioclase (30%)+ with either codrse-grained biotite or

quartz (15%) + epidote (5%). a leucocratic variety of the granitic

Granoblastic texture iz locally well rock. The biotite aggregates vary in

dev'eloped, reflecting growth of thickness from a few millimetres to

clinopyroxene and stubby hornblende more than 5 cm; in some cases,

crystals. The crystallization of coarse-grained biotite aggregates

clinopyroxene at the expense of totally replace-small- amphibolite

hornblende is apparent, and the fragments; suggesting completion of

clinopyroxene rangek from 0.02 to 0.35 mm reaction between the host xenolith and

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the invading granitic magma. The earlier amphibolitic assemblages;

lcucocratic rocks are coarse grained these new minerals are best developed

and contain quartz, plagioclase, along.fracture zones and along the

clinozoisite, muscovite, and sphene. hinges of later folds. Actinolitic

_I amphibole is very weakly pleochroic, Greenschist Facies Stage . from pale green to colorless, and

30th the amphibolites and granitic occurs 5s rims on the pre-existing

orthogneisses have been subjected toI .. hornblende or along its cleavage traces.

greenschist facies recrystallizatiorp Near fault zones or in the margins

in Pliocene-Pleistocene and possibly of the bodies, some amphibolites-have

also latest Cretaceous time. The most been replaced by massive aatinolite + I recent transformation evidently is chlorite aggregates., Elsewhere,

coeval with greenschist facies chlorite mainly occurs as platy

metamorphism of the young, undeformed crystals or as radial aggregates in

diabasic dikes that cut the slaty the interstices of the high-rank

Tertiary cover rocks; Because of rare precursor assemblage. Fine-grained L-i occurrences of greenschist assemblages epidote is present as aggregates or

in the amphibolites, the various possible as rips around coarse-grained , - stages .of Y'reenschist facies clinozoisite and characteristically recrystallization are impossible to has higher birefringence. Albite

distinguish; therefore, they are generally occurs as an irregular

simply 'combined in this report as one replacement of coarse-grained plagioclase metamorphic eP isode. or as fine-grained intergrowths with Low-grade, fine-grained phases such clinozoisite. Some albite

as albite, actinolite, chlo'rite, concentrations travefse the rocks as

epidote, and sphene occur in small .minute veins. Saussuritization of

amounts compared to the persisting earlier, more calcic plagioclase to

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 fibrous clinozoisite + albite is the fine-grained aggregates of actinolite,

most characteristic feature of the albitic plagioclase, quartz, epidote,

greenschist facies recrystallization, chlorite, green or tan biotite, and

examples of which arc! shown in Figure 6, sphene. A and B. Retrogressive recrystallization Zeolite Facies Recrystallization of migmatized amphibolite is indicated

by the occurrence of albi'te and epidote The assemblage laumontite +

and chloritization of biotite and quartz + Fe-rich epidote occurs along

hornblende. Actinolitic amphibole is fractures in a few of the amphibolite

rare. Rimming of sphene around ilmenite specimens; greenschist assemblages of

is ubiquitous in amphibolites and their the metadiabasic dike $ocks were

metasomatic products, and in the replaced locally by carbonate. The

granitic orthogneisses. presence of,laumontite and iron-rich

The Tertiary diabasic dikes exhibit epidote suggests that the amphibolite

the clearest record of greenschist ' and associated rocks in this area were

facies metamorphism in this area, subjected to a final stage of incipient,

inasmuch as they lack the pre-Tertiary zeolite facies metamorphism, except

amphibolitic phase assemblages. They where CO 2 activity was high. are thoroughly recrystallized; in BULK-ROCK CHEMISTRY

general, they have not been i

penetratively deformed, and therefore Compositions of 14 amphibolites,

schistosity is not well developed, 12greenschists, and 3 granitic rocks

except locally in some fault zones. were obtained, employing XRF techniques,

Primary d iabas ic text ures/&d re1 ic t by G. Stummer (University of California,

igneous minerals are locally preserved, Los Angeles; for analytical details,

Modes of these diabasic rocks are see Cortesogno and others, 1977). The

listed in Table 2. They contain results are listed in Tables 1, 2, and

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- ~ ~~ ~~ S i02 51.20 58.04 54.66 50.60 47.99 54.07 Ti02 ' 1.56 1.35 1.04 0.71 1.28 0.89 A1203 12.69 13.21 13.33 13.93 12.31 13.74 F e 2O3-bk 13.21 9.49 11.42 9.37 10.73 11.14 MnO 0.188 0.198 0.189 0.151 0.169 0.182 MgO 5.95 4.50 5.21 9.59 11.40 5.81 CaO 8.15 5.30 6.93 9.12 10.68 7.23 K20 0.83 0.60 0.61 ' 1.59 0.28 0.79 Na20 2.49 4.05 2.73 1.67 ' 1.59 2.37 p2°5 0.19 0.39 S 0.07 0.14 L.O.I. 3.15 2.31 3.39 3.12 3.14 3.45

Total 99.68 99.58 99.51 99.85 99.57 .-99.67

Amph ibo le 35 15 10 45 50 20. Albite 35 50 30 10 5 15 Quartz I 5 4 10 2 5 10 Epidote 10 15 15 14 20 20 Chlorite 9 2 15 5 7 10 White mica 10 Biotite 5 4 10 10 2 . 10 Sphene 5 6 10 5 8 3 Opaque s 1 4 tr tr tr tr Carbonates 5 4 3 2

Note: XRF analyses by G. Sturmner, University of California, Los Angeles. "Djabasic dike rocks. +Chlorite schists interbedded wirh marble and pelitic schist. 5 Tertiary met a-bbsa 1tic racks . ""Total Fe as Fe20;3.

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T-33 lBt T-3B" T-303Bt. T-300Bt T-1B ' T-324C' S i02 48.80 48.85 42.82 ~28.87 48.41 50.14 T i02 1.50 1.61 2.11 4.87 2.30 2.69 A1203 11.63 12.78 13.16 12.23 12.85 11.31 F e 203 -.w 11.99 11.10 13.52 17.67 13.10 11.33 MnO 0.187 0.179 0.142 0.408 0.18 0.28 MgO 10.98 8.20 9.14 22.80 9.48 11.22 CaO 9.68 9.17 7.69 5.28 5.60 5.17 K20 0.07 0.05 1.22 0.002 0.26 0.25 Na20 2.10 2.51 1.01 n.d. 3.59 2.38 2O5 -0.27 '3 s, 0.08 L.O.I. 2.54 5.01 8.85 8.34 3.65 4.91 Total 99.48 99.81 99.66 99.47 99.42 99.68 .. Amphibole 60 .I 12 27 25 Atbite 5 45 10 5 55 20 Quartz 10 . 30 15 5 Epidote 15 4- 25 1 tr Chlorite 5 15 ' 25 45 10 35 White mica 15 Biotite 1 5 Sphene 5 7 5 6 10 Opaques tr 2 5 5 ' tr tr Carbonates 15 15 / - ___-

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F -3, respectively, and plotted on AFM and intrusions (for example, T-6,- T-8A), and / ACF diagrams in Figure 8 for comparison. those included as xenoliths in the <

Although most of the iron probably is orthogneisses (for example, T-l2A, T-12B,

present as FeO, oxidation states were T-l2G)--are characteristically higher 7-

not determined and iron is reported in K20, A1203, and Si02 and lower in

as Fe 0 in the tables; oxide totals MgO and CaO. This increment of K 23 2 0 include ignition losses. and A1203 is reflected in the

Most Suao-Nanao amphibolites containing crystallization of muscovite and

.green hornblende have uniform bulk biotite in these rocks. Compositions of

compositions: SiO averages about 2 migmatized amphibolites vary considerably 48: wt %, Ti0 ranges from 0.4 to 1.4 2 even within a short distance. For

wt %, and K 0 averages about 0.12 wt % ~ 2 example, samples T-12G-1 and T-12G-3 are (the range is 0.05 to 0.21 wt %). about 5 cm apart; the former has a bulk

Compared with Fresh igneous rock series, chemistry similar to the original

the proportions of total alkalis versus amphibolite, whereas the latter is

silica indicate that the amphibolites much higher in SiO K20, and N 0 2’ 23 lie chiefly in the tholeiitic basalt and lower in FeO, MgO, and CaO and

field as defined by Kuno (1966). As approaches a composition similar to

shown in Figure 8, the overall granitic rocks T-12E and T-342 listed

compositions of amphibolites fall in Table 3.

within

Coombs (1963) and are quite comparable greenschist facies dike rocks exhibit

with present-day oceanic tholeiites broader chemical variations compared

(Shido and others, 1971). to the normal amphibolites, as shown

On the other hand, amphibolites in Table 2 and Figure 8. The young meta-

that carry brown hornblende--both diabasic dike rocks and their genetically

those very close to the metagranitic related Tertiary metabasaltic flows <

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T-12EI T-340A T-342

S iO2 60.46 71.58 57.09 Ti02 0.75 0.04 1.48 A1203 14.89 16.79 16U3 Fe2O3* 6.69 1.00 .- 7.19 MnO 0.11 0.07 0.13 MgO 4.50 0.32 5.15 CaO 2.76 0.66 5.32 K20 4.55 3.51 2.56 Na20 2.61 4.39 2.22 Loss on ignition 2.16 1.32 2.11 Total 99.48 99.68 99.48

Amph ibo le 5 P 1agi oc lase 40 20 25 K- f eld spar 25 Quartz 30 25 20 10 Epidote * q--A Chlorite 2 5 Biotite 15 10 Muscovite 10 20 15 Garnet tr 5 3 Symplec t ite 2 Calcite tr Sphene 3 Opaque s 5 tr 2

?l’otal Fe as Fe203.

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- Figure 8. Compositions of analyzed amphibolites, greenschists, metamorphosed dikes, and

granitic arid metabasaltic rocks from northeastern Taiwan pl-otted in classical AFN.,,

and' ACF (mole Z) diagrams. For ACF plot, 10 wt Z of total Fe was assumed to be Fe203, whereag

for AFX diagram, all iron was calculated as ferrous. Compositional fields for oceanic

tholeiites from Shido and others (1971) and basalts from Coombs (1963) arc also shown for

comparison.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 are enriched in K 0, Fe 0 Ti02, and analyses, amphibolites apparently were 2 2 3' SiO and depleted in MgO and CaO relative 2 der.ived mainly from mafic intrusive to the amphibolites, and they have chemic racks, whereas chlorite schists

affinities with alkalic basalts. represent hydrated or altered extrusive . Compared with fresh igneous rock series, rocks., and paragneisses are chiefly

the Tertiary dike rocks lie very close metasandstones (Chu and Shieh, 1979).

to the boundary between the fields of

MINERAL CHEMISTRY , high-alumina and tholeiitic basalts, Analytical Methods after Kuno (1966). The high K 0 and 2 ,.r.: Ti02, and low CaO contents account Chemical compositions of minerals

for the fact that these greenschist were analyzed with an AIU-EMX electron. .

facies dike rocks contain aluminous microprobe at Stanford University.

biotite in addition to actinolite + , Analytical conditions were as follows: ,

chlorite + epidote + albite + sphene. accelerating voltage 15 kv;' specimen '

On the other hand, pre-Tertiary current 0.20 PA on standard benitoite;

greenschists (for example, T-331B) spot size about 1 to 3 u; and counting

interbedded with marbles and pelitic time 10 s. Standards employed were

.schists are of tholeiitic affinity, with well-determined minerals such as

low K20 and Ti0 The chlorite schists clinopyroxene, kaersut ite, rhodonite, ~ 2' .. (for example, T-303R and T-300B) are and bytownite, which were previously . >. tuffaceous rocks with broad ranges in used for lunar sample analysis at the composition.. ThesQresults are consistent Johnson Space Center, Houston during,. with those of Chen (1977), who published the. period 1970 - 1972. Secondary

many analyses of greenschists and standards were also employed to check

amphibolites in the basement complex of routine analyses. Corrections for atomic

the Central Range of Taiwan. number, absorption,and fluorescence

On the basis of oxygen isotope were made with use pf the MAGIC program

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(Colby, 1971). whit'e mica. Cation proportions

Fo? each studied rock sample, minerals were calculated on an anhydrous basis,

crystallized from various stages of using the program of Jackson and

metamorphism were identified and others (1967).

selected for analysis. Each crystal was Plagioclase subjected to at least three spot

analyses, depending on the size of from nine amphiboldtes,

crystal. Where significant zoning one greenschist, two granitic rocks,

was found, such as in amphibole and six Tertiary dike rocks, and three

epidote, and analyticil data are presented metabasalts were analyzed; the results

for core and rim compositions. Where are shown in Tables 4 and 5 and plotted

variation in composition for each phase in Figure 9. Compositions of plagioclase

is significant, ranges are shown in the vary considerably, depending on the

tables. (Note:each analysis presented rock type and grade of metamorphism.

in Tables 4 through 17 represents a single For most samples, only the compositional

point, and the range indicated is the ranges are listed in the tables, and

maximum variation detected among for those rocks with relatively uniform

10 to 15 spot analyses in a rock.) plagioclase composition, the average

In general, the analyses are believed composition is listed.

to be accurate to +2% of the amount PZagiscZase in Amphibozites. As ,. present for major elements and 25%. evident from examination-of Table 4,

for minor elements. Present technqfiues do /plagioclase in the amphibolites

not allow for routine. distinction of qhows a wide range of compositions,

Fe+2 aKd Fe+3; therefore, total iron refleeting polyrPycystallizat ion of

was considered as Fe+2 in amphibole, the host rocks under various conditions.

chlorite, biotite, garnet, and sphene, Correlation of plagioclase compositibix

and Fe+3 in plagioclase, epidote, and with specific metamorphic stages is

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T-8A T-8B* T-38A T-40A T- 12A*

S i02 54.68-57.03 53.34-55.02 57.51-60.17 57.51-58.93 56.27- 66.68 A1203 26.95-25.46 29.43-28.66 25.76-23.93 27.54-26.28 27.20- 21.90 Fez035 0.11- 0.16 0.11- 0.10 0.02- 0.06 0.06- 0.12 0.01 -- CaO 10.03- 8.26 11.80-10.72 8.28- 4.92 8.55- 7.28 9.29- 3.20

K20 0.10- 0.11 0.16- 0.16 0.23- 0.10 -7 0.09- 0.10 0.14- 0.11 Nqo 5.88- 6.90 4.76- 5.36 6..72- 8.49 6.67- 7.10 5.61-. 9.13 Anhydrous Total 97.74-97.91 99.60-100.02 98.53-97.66 100.42-99.81 98.53-101.02 1.: Si 2.519-i. 609 2.422-2.478 2.612-2.713 2.5642.631 2.556-2.892 AlIV 1.463-1.373 1.575-1.522 1.379-1.269 1.436-1/369 1.444-1.108 A 1vI 0,011 0.011-0.014 0.012-0.012 Fe+3 0.004-0.006 0.004-0.003 0.001-0.002 0.002-0.004 Ca 0.495-0.405 0.574-0.517 0.403-0.239 O.tO8-0.348 0.452-0.149 Na 0.525-0.612 0.419-0.468 0.592-0.747 0.577-0.615 0.494-0.768 K 0.006-0.006 0.009-0.009 0.013-0.006 0.005-0.006 0.008-0.006

~~ ~~ ~ ~~~ An 48.2-39.6 57.3-52.0 40.0-24.1 41.2-35.9 47.4-16.1 Ab 51.2-59.8 41.8-47.1 58.7-75.3 58.3-63.5 5 1.8-83.2 Or 0.6- 0.4 0.9- 0.9 1.3- 0.6 0.5- 0.6 0.8- 0.7

*Thermally recrystallized amphibolite. ?Vein albite. §Total Fe as Fc2o3.

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T-12G-1" T-12H* T-335B T-337A v.a.+ S iO2 53.68-57.96 58.18-61.52 57.99-55.34 .68.02; 67.83 A1203 29.07-27.75 25.85-23.09 25.86-27.51 19.19 19.51 Fez03 5 0.13- 0.03 0.21- 0.04 0.42- 0.51 0.16 0.07 CaO 11.63- 7.33 7.48- 4.52 7.96-10.51 0.78 0.04 K20 0.10- 0.12 0.10- 0.13 0.14- 0.17 iO.17 1L.48 Na20 4.82- 6.72 7.30- 8.63 7.05- 5.33 11.78 0.13 Anh y d rou a Total 99.42-99.91 99.12-97.93 99.40-99.37 100.09 99.06

2.439-2.584 2.623-2.778 2.612-2.509 2.981 2.990 1.55.7-1.416 1.374-1.222 1.373-1.470 0.992 1.010 0.042 0.006 0.004 0.004-0.001 0.007-0.001 0.014-0.017 0.005 0.002 0.566-0.350 0.361-0.219 0.384-0.511 0.037 0.019 0.425-0.58 1 0.638-0.755 0.616-0.469 1.001 %.981 0.006-0.007 0.006-0.008 0.008-0.010 0.009 0.007

~~ ~~ An 56.8-37.3 35.9-22.3 38.1-5 1.6 3.5 1.9 Ab 42.6-61.9 63.5-76.9 61.1-47.4 95.6 97.4 Or 0.6- 0.8 0.6- 0.8 0.8- 1.0 0.9 0.7

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S iO2 67.21- 67.68 66.16- 66.10 66.50- 65.02 68.87 64.38 A1203 20.47- 20.72 21.04- 21.28 21.25- 22.35 18.77 22.37 Fe2035 0.29- 0.17 0.44- 0.35 0.18- 0.14 0.08 0.38 C a0 0.94- 0.85 2.33- 1.71 1.68- 1.73 0.13 2.54 Na20 11.19- 11.14 10.48- , 10.84 10.90- 9.78 12.07 10.84 K20 0.15- 0.17 0.18- 0.16 0.13- 0.14 0.15 0.11 Anhy d rou s Total 100.25-100.72 100.62-100.44 100.68- 99.13 100.06 100.61

Si 2.923- 2.943 2.896- 2.895 2.904- 2.872 3.012 2.802 AlIV 1.049- 1.057 1.085- 1.099 1.094- 1.128 , 0.987 1.190 AlVI 0.004 0.036 Fe+3 0.010- 0.006 0.015- 0.012 0.006- 0.005 0.003 0.012 Ca 0.045- 0.040 0.109- 0.080 0.079- 0.053 0.006 0.118 Na 0.944- 0.939 0.889- 0.921 0.923- 0.838 r.023 0.915 K 0.008- 0.009 0.010- 0.009 0.007- 0.008 0.008 0.006

~~~~~ ~~ ~~ An 4.5- 4.0 10.8- 7.9 7.8- 5.9 0.5 11.4 Ab 94.7 -95.0 88.2-91.2 91.5-93.2 98.6 88.0 Or 0.8- 1.0 1.0- 0.9 0.7- 0.9 0.8 0.6

9: Dike rocks. ? Greenschist interbedded with marble and pelitic schists. Tertiary metabasaltic rocks. "X-AGranitic rocks. ?iK-feldspar. §§Total Fe as Fe203.

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- T-331B t T-3BJr T-1B 5 T-324C 5

SiO2 ' 67.67- 67.54 53.05- 63.92 67.05-66.45 68.39

A1203 19.54- 20.22 28.57- 22.76 ~ 20.25-19.93 19.60 F903 55 0.13- 0.13 0.59- 0.28 0.34- 0.21 0.17 CaO 0.41- .1.12 11.99- 3.70 0.81- 0.39 0.25 Na20 11.37 10.77 . 4.75- 9.42 10.73- 9180 11.21 K20 0.06- 0.08 0.16- 0.13 0.20- 1.62 0.11 Anhydrous Total 99 .I9 99.87 99.10-100.21 99.37-98 a.40 99.72

Si 2.982- 2.958 2.428- 2.816 2.952- 2.965 2.293 AlIV 1.015- 1.042 1.541- 1.182 1.048- 1.036 1.007 Al VI 0.002 0.003- 0.012 0.003 Fe+3 0.004- 0.004 0.020- 0.009 0.011- 0.007 0.006 Ca 0.019- 0.053 0.058- 0.175 0.038- 0.019 0.012 Na 0.972- 0.915 0.421- 0.805 0.916- 0.848 0.951 K 0.002- 0.005 0.009- 0.007 0.011- 0.092 0.006

An 2.0 - 5.4 . 57.8-17.7 3.9- 2.0 1.2 Ab 97.71-94.1 41.4-81.6 94.9-88.4 '98.2 Or 0.3-. 0.5 0.8- 0.7 1.2- 9.6 0.6

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~~ S iO2 67.83 64.71 59.39-65.72 56.01-57.70 A1203 19.51 18.91 25.57-21.44 27.37-26.62 Fez035 § 0.07 0.15 0.02- 0.06 0.06- 0.04 C a0 0.04 0.04 7.48- 2.31 10.58- 8.07 Na20 11.48 2.12 7.06- 9.91 5.79- 7.27 K20 0.13 12.91 0.27- 0.11 0.16- 0.14 * Anhydrous .

Total ’ 99.06 98.84 99.79-99.54 99.95-99.85

--,t Si 2.990 2.986 2.654-2.896 2.524-2.589 AlIV 1.010 1.024 1.346-1.104 1.454-1.408 N VI 0.004 0.005 0.001-0.009 Fe+3 0.002 0.050 0.001-0.002 0.002-0.002 Ca 0.019 0.019 0.358-0.109 0.511-0.388 Na 0.981 0.190 0.612-0.847 0.505-0.633 K 0.007 0.760 0.016-0.006 0.009-0.008

~ ~ ~ ~ An 1.9 2.0 36.3-1 1.3 49.9-37.7 Ab 97.4 19.6 62.1-88.1 49.3-61.5 Or 0.7 78.4 1.6- 0.6 0.8- 0.8

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greenschists, metagranites and Tertiary metamafic B flows and dikes t- $+

(X*-* >(OX "X x x-x *x-x

I I I I I I I I I *-' amphibolifes and metasomatized amphibolites I 0 8--.

4 I 0-0 0 i -- I I I I I I I I 0 10 20 30 40 50 60 70 . 80 90 100 Mole percent An

Figure 9. Compositional ranges of plagioclases from amphibolites (circles), thermally

metamorphosed amphibolites ("thermoamphibolites") (solid dots), metadiabasic dike rocks

(X), Tertiary metabasalts (+), greenschists (*), and metagranites (triangles). Ranges

shown represent at least six analyses for three to four plagioclase grains in each sample.

For relatively uniform plagioclase, average composition is shown, For simplification,

minor amount of ofthoclase component was not plotted.

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difficult because of the obscuring amphibolite plagioclase. Such

overprint during subsequent plagioclasehas both a larger compositional

recrystallization. Some coarse, clear range and a higher average Ab component.

plagioclase grains in the amphibolites Many of these altered amphibolites,

distant from the granitic intrusion as shown in Figure 7, contain

An An are andesine ranging from 40 to 52' symplectic intergrowths of clinozoisite + Such plagioclase is free from inclusions quartz + plagioclase. The intergrown

and saussuritization. This compositional plagioclase crystals are difficult P '?> range may be representative of the to analyze, but microprobe data

plagioclase produced during the initial indicate that they are roughly

stagebof amphibolite facies metamorphism. oligoclase in composition.

Subsequent recrystallizations during It should be pointed out that both

the granitic intrusion and greenschist amphibolites and thermally recrystallized

facies overprinting have modified most amFhibolites have been subjected to later

of the amphibolite plagioclase. Because 'stageseof greenschist facies metamorphism.

of the general increase in Ab component Therefore, whether the compositionsj

relative to the original plagioclase, analyzed from the thermally metamorphosed

the later, low-grade metamorphism 'might amphibolite represent the effect of

be regarded as having provided the metasomatic depletion of CaO during

more substantial compositional change. the granitic intrusion (for example,

However, except for the Table 1, T-12A, T-12G-1, T-12H), or

clinopyroxene-bearing amphibolite, T-8B. the effect of low-temperature

most plagioclase in thermally recrystallization during greenschist I i? recrystallized amphibolites (for facies metamorphism--or both--is

example, Td12A, T-12G-1, and T-12?1) difficult to determine. The

also tends to more sodic compositions amphibolite, T-8B, contains abundant

(as low as Anl6) compared to normal granoblastic clinopyroxene after

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hornblende due to isochemical thermal' have been subjected to greenschist

metamorphism; upgrading presumably facies metamorphism, Although

caused the plagioclase to increase in, overprinted, they preserved primary

An content from andesine to 1-abradorite' igneous textures and are nonfoliated.

(An5* to An57). Most of the plagiqclase All rocks analyzed contain albite +

in this rock is free from subsequent actinolite + epidote + chlorite ?

saussuritization. quartz ? sphene/+ biotite.

As mentioned in the previous sections, Fine-grained xenoblastic ,crystals

the effects of greenschist facies of albitic plagioclase konstitute

recrystallization are best shown in the about 10% to 50% of every investigated 1 amphibolites away from the granitic . metadiabase and metabasalt. Six samples intrusion. Fine-grained albite less were analyzed for their plagioclase

than Ano7 (Table 4) occurs as an compositions (Table 5). Except for

irregular replacement of amphibolitic plagioclase in T-3B, most other

plagioclase, as fibrous mixtures with plagioclases are uniformly sodic and

clinozoisite in saussurite (for range in An content from An to An 2, 02 11, example, see Fig. 7), or as thin, consistent with greenschist facies

minute albite veins transecting the albite compositions reported in the

host amphibolite. Such albites are literature (for example, Kuniyoshi and

characteriskcally low in An'and Or Liou, 1976). Equilibrium pairs of

contents. plagioclase, defining a peristerite \ PZagiocZase in the Metamorphosed gap, were not found. The metadiabasic

Tertiary Dikes and Metabasaltic Rocks. dike rock T-3R occurs as boudins in the

Some of the diabasic dikes probably pre-Tertiary marble and is not

recresent feeders for the-basaltic thoroughly recrystallized. Relict

rocks of the Tertiary cover formation; porphyritic texture is well preserved

both the dikes and the mafic flows and the rock is not schistose. Patchy,

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fibrous, and fragmented Ca amphiboles greenschists and chlorite schists

appear to pseudomorph clinopyroxene, interbedded with marble and pelitic

whereas plagioclase phenocrysts are schists from this area, no calcite

warsegrained, zoned, and twinned. was found in this rock. The composition

Plagioclase. with 2V about 70°(- ) may of the albite is Ano2 to Anl6.

have retained the original igneous On the other hand, except for some

composition of An Compositional saussuritized grains that replaced 58‘ were readjustment during greenschist facies by albitic plagioclase + clinozoisite, - metamorphism was incomplete, and zoned plagioclases in the quartz

a range of plagioclase composition diorites (T-12E and T-342) from this

of Anl7 to An58 exists (Table 5). area preserved their primary core

However, most analyzed plagioclase grains compositions of An 36 and An50’ in this rock have compositions between respectively, a’s shown in Table 5.

Anl7 and An 25‘ Amphiboles PZagioclase in the Pre-Tertiary

Greenschists and Metagranitic Rocks. Analyses of 28 calcic amphiboles

Greenschists including chlorite schist froq twelve amphibolites, one granitic

are ubiquitous in the pre-Tertiary rockj one greenschist, seven Tertiary

basement complex. For comparison, dike rocks, and two metabasalts are

plagioclases from one representative presented in Tables 6 and 7. Only

greenschist and from two metagranitic eight elements were analyzed for each

rocks from the investigated area amphibole, but qualitative scanning t suggests that MnO and Cr 0 contents were analyzed (Table 5). The greenschist, 23 T-331B, is a strongly foliated of these amphiboles are negligible.

fine-grained rock that contains albite, Slightly low anhydrous totals for the d actinolite, epidote, chlorite, and analyzed Ca amphiboles, averaging

sphene (Table 2). Unlike most other 97.32 wt %, may reflect the fact that

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TPY 66 17 T-40A T-38A T-8A* T-8B* Green Hb Actinolite Green Hb Green Hb Brown Hb Brown Hb

S iO2 46.99-46.47 55.55-54.07 48.31-48.70 47.59-48.35 a 46.89-45.32 45.71 Ti02 0.56- 0.10 0.04- 0.05 0.28- 0.37 0.45- 0.30 1.16- 1.47 A1203 10.73- 9.47 2.18- 3.94 11.37 13.29 11.03- 9.62 9.89-11.45 Fed 15.05-14.49 11.31-11.65 11.00-10.09 11.84-11.45 13.36-13.68 A::;14 .,64 MgO 11.15-14.54 15.45-15.69 13.37-12.21 12.96-13.50 11.86-11.42 10.87 CaO 11.92-11.47 12.58-12.47 11.80-11.33 11.82-11.89 11.42-10.92 * 12.12 K20 0.28- 0.12 0.06- dhO 0.10- 0.09 0.15- 0.14 0.75- 0.96 0,88 Na20 1.15- 0.51 0.21- 0'.42 1.51- 1.47 1.32- 1.32 1.20- 1.30 1.08 . Anhydrous Total 97.84-97.17 97.38-98.40 97.74-97.55 97.16-96.58 96.54-96.51 97.38

Si 6.877-6.780 7.922-7.666 6.926-6.918 6.901-7.040 6.925-6.719 6.759 AlIV 1.123-1.220 0.078-0.334 1.074-1.072 1.099-0.960 1.075-1.281 1.241 AlVI 0.731-0.411 0.289-0.326 0.847-1.153 0.786-0.691 0.647-0.720 0.689 Ti 0.062-0.011 0.004-0.006 0.030-0.040 0.049-0.033 0.129-0.164 0.1:1 Fe+2 1.845-1.771 1.351-1l.383 1.319-1.198 1.436-1.395 1.650-1.696 1.81G Mg 2.435-3.166 3.288-3.319 2.856-2.585 2.800-2.929 2.611-2.523 2.395 Ca 1.872-1@52 1.925-1.897 1.812-1.724 1.837-1.855 1.807-1.734 1.920 K * 0.053-0.022 .0.010-0.019 0.019-0.017 0.028-0.026 0.142-0.182 0.165 Na 0.328-0.144 /' 0.058-0.116 0.420-0.405 0.371-0.373 0.344--0.374 0.309

b Pate: Atomic ratios were calculated on the basis of 23 oxygens. "Thermally recrystallized amphibolites. -t Total Fe as FeO.

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T-12G-3$~ T-335B T-337A Green Hb Act inol ite _Green Hb Ac tinol ite rim core

S iO2 47,06-46.25 55.96-55.37 47.47-45.86 52.24-53.03 44.98-47.39 Ti02 0.95- 0.36 0.08 -- 0.30- 0.41 0.08- 0.05 0.64- 0.23 A12Q3 10.09-10.11 0.95- 2.79 10.35-13.08 5.86- 5.48 13.33- 9.92 FeOt 18.03-14.60 14.60-11.12 12.36-12.25 1I .65-12.06 13.79-14.28 MgO 9.30-13.75 13.66-16.17 12.59-11.58 14.07-14.08 10.97-12.96 CaO 11.66-12.61 11.91-12.84 11.58-11.13 12.01-11.94 11.39-12.08 K20 0.64- 0.51 0.06- 0.09 0.20- 0.26 0.05- 0.15 0.09- 0.10 Na20 . 0.98- 0.65 0.09- 0.22 1.32- 1.94 0.03- 0.08 - 1.99- 1.06 Anhydrous I'Y Total 98.69-98.87 97.31-98.61 96.16-96.47 95.99-96.87 97.18-98.01

Si 6.931-6.406 g. 088-7.807 6.972-6.721 7.576-7.630 6.608-6.901 AlIV ' 1.069-1.594 -- 0.193 1.028-1.279 0.424-0.370 1.392-1.099 AlVI 0,682-0.790 . 0.162-2.271 0.763-0.981 0.578-0.560 0.917-0.604 Ti 0.105-0.037 0.029 -- 0.034-0.045 0.009-0.005 0.071-0.025 Fe+2 2.220-1.691 1.765-1.313 1.518-1.501 1.412-1.451 1.694-1.739 Mg -2.041-2.838 2.911-3.404 2.756-2.529 3.041-3.019 2.402-2.812 Ca 1.840-1.872 1.844-1.942 1.822-1.748 1.866-1.840 1.793-1.885 K 0.12070.090 -- -- 0.037-0.049 0.009-0.028 0.018-0.019 Na 0.28070.183 0.026 -0.059 0.376-0.55 1 0.009-0.023 0.567-0.299

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. T-12B* T-12A* T-lZH* T-12G-2* Brown Hb Actinolite Brown Hb Actinolite Brown Hb Brown Hb rim core

S i02 45.95-47.04 54.80-54.64 41.59-43.53 53'.-03 46.04-44.97 44.25-47.85 Ti02 1.07- 1.32 -- 0.02 0.61- 1.16 -- 1.17- 1.51 1.86- 0.74 A1203 12.15-10.97 14.42-11.56 ?.64 10.79-1 0.94 11.60- 8.31 FeO t 15.20-15.90 18.31-19.0 18.39-18.07 f6.18 15.51-15.46 15.42-13.66 MgQ 10.24-11.32 11.82-11.42 6.95- ,7.94 12.61 10.60 -1 0.39 9.86-12.37 CaO 11.27-10.50 12.57-12.71 11.46-11.42 12.09 11.62-11.64 11.51-12.00 K20 I 0.76- 0.63 0.09- 0.09 ,1.36- 1.19 0.14 0.87- 1.03 '1.01- 0.62 Na20 1.26- 1.08 0.04- 0.06 1.25- 1.16 0.23 1.44- 1.40 1.13- 0.82 :':: '> Anhydrous 1 Total 97.80-98.76 98.44-98.99 96.03-96.03 96.93, 98.04-97.35 96.64-96.37 ..

6.738-6.834 7.938-7.965 6.390-6.661 7.803 6.783-6.695 6.632-6.653 1.262-1.166 0.062-0.035 1.610-1.339 0.197 1.217-1.305 1.368-1.347 0.841-0.712 0.077-0.142 1.002-0.746 0.261 0.657-0.615 0 .' 68 2 -0.0 15 0.118-0.144 -- 0.002- 0.071-0.133 0.132-0.171 0.210-0.077 1.867-1.932 2.238-2.318 2.362-2.312 1.991 1.911-1.925 1.933-1.588 ' 2.241-2.451 2.575-2.480 1.592-1.810 2.765 2.327-2.305 2.202-2.563 1.773-1.634 1.968-1.985 1.887-1.872 1.906 1.834-1.857 1.848-1.788 0.358-0.304 0.011-0.018 0.266-0.232 0.026 0.163-0.195 0.193-0.110 0.143-0.117 0.018-0.018 0.373-0.344 0.065 0.205-0.204 0.328-0.221

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TABLE 7. CHEMICAL COMPOSITION OF AMPHIBOLES IN THE GREENSCHISTS ’ FROM NORTHEASTERN TAIWAN

T-12C” T-41AJC T-334A* T-334C-k T-335E* T-336*

S iO2 43.57-52.48 52.24 48.01 -51.45 55.06-51.91 53.40 52.78-52.74 Ti02 3.69- 0.04 0.02 0.80- 0.08 A1204 10.13- 2.50 2.86 5.47- 3.98 2.55- 4.57 3.30 3.47- 3.03 FeOt 15.76-15.03 17.17 19.82-16.29 10.79-12.53 12.93 15.98-16.10 MgO . 12.54-13.16 12.63 12.07-14.08 16.49-1 5.64 14.64 13.04-12.96 CaO . 10.20-1 2.64 12.48 10.01-12.15 2.73-12.29 12.55 12.60-12.16 K20 0.20- 0.13 0.15 0.29- 0.17 0.10- 0.11 0.21 0.13- 0.18 Na20 2.26- 0.19 0.27 0.69- 0.23 0.22- 0.22 0.39 0.22- 0.30 Anhydrous Total 98.34-96.17 97.82 97.17-98.35 97.93-97.26 97.50 98.22-97.46

Si 6.435-7.766 7.679 7.215-7.495 7.812-7.502 7.314 7.669-7.724 A1Iv 1.565-0.234 0.321 0.785-0.505 0.188-0.498 0.286 0.331-0.276 AlVI 0.199-0.202 0.174 0.184-0.179 0.238-0.281 0.275 0.263-0.247 Ti 0.410-0.004 0.002 0.090 Fe+2 1.947-1 -860 2.111 2.491-1.982 1.280-1.515 1.562 1.942-1.972 Mg 2.760-2.902 2.766 2:704-3+Q57 3.485-3.369 3.152 2.824-2.829 Ca 1.614-2.004 1.965 1.612-1.897 1.934-1.903 1.942 1.962-1.908 K 0.037-0.025 0.028 0.056-0.032 0.016-0.020 0.039 0.024-0.034 Na 0.648-0.055 0.078 0.201-0.065 0.061-0.062 0.109 0.062-0.085

JJote: Atomic ratios werf: calculated on the basis of 23 oxygens. -k Diabasic dike rocks. t Greenschists interbedded with marble and pelitic shist. 3 Tertiary metabasaltic rocks. %--\Granitic rocks. tflotal Fe as FeO.

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TABLE 7. (continued)

T-3B* T-33 1Bf T-IB§ T-324Cs T-342**

,SiO2 53.26-52.41 47.14-53.37 44.43-54.64 44.30-+!.a3 T i02 0.04- 0.03 0.22- 0.04 0.64- 0.03 1.46- q6 A1203 2.93- 7.76 8.73-- 2.95 2.48- 5.04 8.70- 1.99 11.45-14.26 Fe0f-k 14.22-13.96 14.46-13.47 14.55-14.54 14.49-11.53 17.59-16.99 MgO 14.02-10.9a 13.27-15.37 13.79-14.66 15.83-15.55 8.53- 8.06 CaO 12.47-11.20 11.52-12.52 12.60-12.74 12.75-13 .oa 11.21-11.55 K20 0.10- 0.13 0,31- 0.29 0.13- 0.10 0.10- 0.09 1.11- 0.84 Na20 0.30- 1.83 1.44- 0.65 0.23- 0.22 0.14- 0.17 1.16- 1.25. Anhydrous Total 97.28-98.31 97.07 -98.64 96.60-97.42

~~ 7.741-7.51 1 6.957-7.65i 7.761 -7.338 6.602-7. a59 \ 6.689-6.543 0.259-0.489 1.043-0.348 0.239-0.662 1.398-0.141 1.311-1.457 0.243-0. a22 0.475-0.151 0.191-0.20a 0.126-0.197 0.727-1.053 0.004-0.003 0.024-0.004 0.072-0.003 0.166-0.063 1.72811.673 1.785-1.615 1.787-1.781 1.aoi-i .3a7 2.221-2.121 3.036-2.345 2.919-3.285 3.019-3.199 3 .'506-3.334 1.920-1.793 1.942-1.720 1 .a22-1.923 1.983-1.999 2.030-2.016 , I. 814-1.848 0.01 9-0.012 0.058-0.053 0.025-0.019 0.01 9-0.01 7 0.214-0.160 o.oa4-0.5oa 0.412-0.178 0.065-0.062 0.040-0.047 0.339-0.362

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 a substantial proportion of the iron is 0.6 wt %) and high in alumina (8to 15 wt%).

Fe+3, rather than exclusively divalent as On the basi? cf 23 oxygens--assumin? one-

shown in the tables, Of course, the H 0 per formula unit--the Si content ranges 2 addition of the H 0 content typical of Alv' 2 from 6.87 to 7.04, from 0.73 to 0.98, amphibole, about 2.0 wt %, would bring Mg+2 from 2.44 to 3.04, and Fe+2 from 1.32

these analyses up to nearly 100 %. to 1.85. According to the method of Misch

Ca AmphiboZes in kmphibolites. Three and Rice (1975) for calculation of

distinct varieties of Ca amphibole were end-member proportions, the average green

identified in the amphibolites--green (and hornblende consists of 10 mol %

blue-green) hornblende, brown hornblende, richterite-ferrorichterite, 22%

\ and actinolite. They aqe interpreted to pargasite-ferrohastingsite', 30%

rcprescnt arnphibolcs that crystallized tschermakite-ferrotschermakite, 35%

during the metamorphic stages of tremolite-actinolite, and 3%

amphibolite facies, granitic intrusion anthophyllite-cummingtonite.

(upper amphibolite facies), and Locag'thermal effects due to the

greenschist facies, respectively. granitic intrusion are manifestled by

The blue-green hornblende, whichdefine: changes in pleochroic color of

a distinct lineation for therrocks, occurs Ca amphibole from bluish green or

as long slender crystals in well-foliated greenish brown to brown, and the

amphibolites away from the granitic crystal habit from a long, slender form

intrLsion (see Fig. 6). Individua1,grains to relatively short, stubby, prismatic c of 'T-these amphiboles are compgkitionally grains. Such conversions are incomplete

homogexous; however, sight compositional . in some specimens, and many granoblastic

variations were found among grains and hornblende crystals in the \ between samples. As shown in Table 6 and contact-metamorphosed and migmatized

Figure 10, the green hornblendeis lbw in amphibolite exhibit a faint color

Si02 (45 to 49 wt %) and Ti0 (less than zoning from a green core to a 2

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'uc 0.80 X

't6 0.60 n co 040 0 -k U

0.00

6.40 6.60 6.80 7.00 7.20 Z40. 7.60 I 7.80 8.00 Si atoms per formula unit

Figure 10. Proportions of octahedrally coordinated aluminum and silicon atoms per formula

unit for calcic amphiboles from amphibolites, migmatized amphibolites, greenschists,

metadiabasic dikes, and metabasaltic and granStic rock6 from northeastern Taiwan. Symbols

as in Figures 8 and 9. Actinolikes from amphibolites shown as inverted solid triangles.

Principalcalcic amphibole substitutions are shown. As pointed.out by Laird (1980),

glaucophane substitution, not illustrated in this plot, can be significant in calcic

amphibol.cs in mafic schists.

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light-brown rim. Such a color change have slightly lower Si contents and

in part may be correlated with an therefore higher Al'" and total aluminum increase in Ti0 content in brown 2 contents than the green and blue-green

hornblende, as shown in Table 6; hornblendes. Systematic differences '

brown hornblendes contain substant idly in Fe and Mg are not apparent, The

more titania, 1.0 to 1.9 wt %, than the average calculated brown hornblende

green hornblendes. This relati-onship end-member composition after Misch and

is well illustrated by comparing Rice (1975) is 14. mol % /

analyzed hornblende grains in T-12&2 richterite-ferrorichterite, 24%

and T-12G-3; these two samples are pargasite-ferrohastingsite, 33%

5 cm apart, and the former is about tschermakite-ferrotschermakite, 26% 'I 4 cm from the metasomatized amphibolite tremolite-actinolite, and 3%

T-12G-1. Approaching the intrusive anthophyllite-cummingtonite. Heating \ contact, for sample T-12G-3, Ti02 and granitic metasomatism apparently

increases from 0.36 wt % in green have not significantly modified the

hornbleqde cores to 0.96 wt % in the major components of hornblende, as c greenish brown rims of the same grain; shown in Figure 10. However, the

for T-12G-2, the corresponding green recrystallized brown hornblendes

core and brown rim values are 0.74- and contain slightly higher richterite,

1.86 pt % Ti02. This increase in tschermakite, and probably pargasite

Ti content of hQrnble,nde with increasing components and lower tremolite-actinolite-

metamorphic grade has been noted by components. This ,relationship is

Shido (1958), Engel and Engel (1962), consistent with previous studies

Leake (1968), Ernst (1972), Graham (1974) (for example, Banno, 1964, Cooper,

and Laird (1980), among others. 1972, Yagi and others, 1975). Moreover,

As is evident from Figure 10, the the brown hornb'lendes carry substantially - thermally recrystallized brown hornblendes more potasium oxide (K20 = 0.62 to

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1.36 wt %) than the green and blus-green and thermally metamorphosed and

hornblendes (K 0 = 0.09 to 0.64 wt %). migmatized amphibolites. 2 Actinolite occurs as rims around ActinoZite in Tertiamj Metadikes and

hornblende and along cleavage traces, Other Greenschist Facies Rocks. Compared

or as fine-grained aggregates in to the amphibolites, the Tertiary

fracture zones of some amphibolites; metamorphosed diabasic and metabasaltic

these late-stage amphiboles rocks are relatively heterogeneous in

recrystallized during the greenschist chemical and mineral compositions. Some

facies metamorphism(s). mafic units are richer in Ti02 and K20,

A few actinolitic amphiboles were ‘and thereforeboth muscovite and biotite

analyzed (Table 7; Figure 10). The . are present in addition to actinolite,

. actinolites are distinctly different epidote, and chlorite; one rock contains

from the hornblendes described above primary brown hornblende. All the

in that they are characteristically recrystall ized diabasic dikes and

high in Si and low in NIV, AlV1, and related greenstones were recrystallized

Ti. Al2O3,6ontent ranges from 0.8 during Pliocene-Pleistocene greenschist

to 5.9 wt %, ‘and Ti0 content is 1.eSs facies metamorphism. The rocks are 2 than 0.1 wt %. The calculated average massive and fine graincd,,with local 9 end-member composition consists of preservation of relict diabasic textures,

92 mol 2 tremolite-actinolite, 5% igneods amphibole, and plagioclase.

tschermakite-ferrotschermakite, and As shown in Table 7, all analyzed

3% glaucophane-ferroglaucophane amphiboles=except the relict brown

component; less than 1 mol % of hornblende core iq T-l2C--are.

anthophyllite and richterite components Al-actinolites, containing neglig3le

are present in these actinolites. Ti02 and very low A1 0 Na 0, and 2 3’ 2 Similar compositions were f_ound for K20. The Al 0 content ranges from 23 all actinolites in both awibolites 2.5 to 8.7 wt 2; most of the.actinolites

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5' i 'carry less than 4 wt %. TheP alculated analyses of highly aluminous and sodig average end-member composition consists igneous'relics include that of sample

of 90 mol % tremolite-actinolite, 7% T-331B and possibly T-324C. 1 C I tschermakite and pargasitc, and 3% Primary Nornb Zende in Metagranitic

glaucophane-ferroglawophane components. Rocks. Minor primary hornblende occurs

As shown in Figure 10 and Tables 6 and7, . in some granitic rocks (for example,

compared to actinolites in the T-342) where it has been replaced by

amphibol ites , the analyzed act inolites a combination of various greenschist

froin the Tertiary rocks are relatively facies minerals, including biotite,

heterogeneous in compositon, in actinolite, clinozoisitc, chlorite, and d general being higher in Al'", K20, and sph'ene. Some hornblende crystals show

Na 0 and lower in Si content. The a .2 zoning from greenish core to a contrast in amphibole composition brownish rim or vice verdk. .The range

appears to be due to diffprences in of analyses listed in Table 7 indicates ,' bulk-rock chemistry, refkecting the that the igneous hornblendes in \ more a,&minous and alkalic original metagranite contain more A1203, Ti02,

nature of the Tertiary dikes and K.0, 2 and Na 20 than most brown hornblendes basaltic flows. in the metasomatized and thermally

The fine-grained brown hornblende metamorphosed amphibolites, but -1 3.7 % core in T-12C contains wt Ti02 the data are too few to allow detailed and 2.26 wt % Na20, higher than any conclusions.

other analyzed amphiboles from this Epidote-Clinozoisite Minera$s area. The textural rel-ationsand

composition suggest that this hornblende An epidote-clinozoisite phase is

is primary and was crystallized from an ubiquitous in recrystallized mafic

alkaline basaltic magma (see Table 2 for rocks of the Suao-Nanao area. e bulk-rock composition). Similar Compositions were obqined for this

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.I

mineral in five amphibolites, seven was higher than that defined by the

thermally metamorphosed and migmatitic hematite-magnetite (HM) buffer curve ~

amphibolites, ten greenschists, and (Keskinen and Liou, 1979). Therefore, \ two metagranitic rocks; results are the only significant chemical variable

li,sted in Table 8 and 9. For each sample, of the epidote-clinozoisite minerals

at least five grains were analyzed and the crystallized in the various stages of

compositional range determined; metamorphism and in the different rock

maximum variations in core and rim types involves substitution of octahedral

compositions are listed in the tables, Al"' and Fe+3, as shown in Figure'll.

Equilibrium pairs were not recognized. Epidote-Clinozoisite in Amphibolites.

Qiialitative scanning for all elements On the basis of chemistry, optical

suggests that the analyzed properties, and texture, at least seven

epidote-clinozoisites consist different modes of occurrence of

essentially of Si, Ti, N, 'Fe, Mn, Ca, epido te-cl inozois fte minerals were

(0 and H), with negligible amounts ' identified in the amphibolites: (1) some

(less than 0.05 wt %) of Na, K, and Cr. optically homogeneous clinozoisite

Ti0 is minor and does not appear to 2 crystals with abnormal blue interference vary systematically. The yery low colors are markedly elongatnd normal / MnO (0.03 to 0.33 wt %) in all to the 001 cleavage (that is, parallel b analyzed epidote-clinozoisites is to the c axis), typically being 3 nm

believed to be related to the paucity or more in length (Fig. 6, A); (2)

of manganese in these rocks (see Tables abundant slendfr, deformed, zoned

1 and 2) and to low fo conditions epidotc grains.have blue interference 2 8 during recrystallization. The colors in the cores and more

piemontite component in the epidote birefringent rims (Fig. 6, C); (3)

should be high only if the prevailing many short, stubby, zoned epidote

oxidation state during the crystallization crystals with highly birefringent

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T-8B* T -3 8A T-8A* core rim core rim granular -core rim

~ ~ ~~~~~~~ ~

S iO2 " 39.37-38.96 38.54-38.17 38.59 38.56-38.54 T i02 0.14- 0.06 0.09- 0.07 0.07 0.02- 0.00 A1203,- 30.03-29.32 28.85-26.68 23.77 28.50-27 -58 Fez03 1 5.44- 6.48 5.70- 9.15 11.58 5.78- 6.85 MnO 0.08- 0.08 0.05- 0.08 0.14 0.12- 0.03 CaO 23.93-23.83 23.40-22.85 23.07 23.38-25.53 Anhydrous Total 98.99-98.74 96.72-97.00 97.22 96.34-96.53

Si 6.006-5.984 6.03P-6.022 6.133 6.055-6.067 AlIV 0.016 A 1vI 5.399-5.292 5.321-4.961 4.452 5.274-5.117

Ti 1 0.016-0.007 0.01 1-0.007 0.007 0.002 Fe+3 0.625-0.749 0.671 1.086 1.395 0.683-0.812 Mn 0.010-0.010 0.007-0.011 0.019 0.016-0.004 Ca 3.911-3.922 3.923-3.862 3.926 3.934-3.969

Ps 10.4-12.4 8.9-17.9 23.2 11.4-13.7

IJote: Atomic ratios were calculated on the basis of 25 oxygens. *T henna 11y re crys t a 11 i zed amph ib o 1i t e . 'fTotal Fe as Fez03 .

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TABLE 8. lcontinuec')

T-40A TPY-66 17 T-l2A* T-l28B* core rim granular core rim vein core tim core rim

S iO2 38.47-38.40 38.08 38.66-37.93 37.65 38.71-38.93 38.9'3-38.75 Ti02 0.17- 0.19 -- -- 0.07 0.04 0.22- 0.17 -7 -- A1203 29.77-28.61 24.41 33.09-28.93 22.05 28.29-29.21 30.19-30.93 Fe2O3:' 4.95- 6.60 12.52 2.52- 7.45 15.74 6.58- 5.62 5.54- 4.56 MnO 0.09- 0.13 0.06 0.14- 0.16 0.15 0.31- 0.28 0.19- 0.16 CaO 23.43-23.31 23.11 24,37-23.86 23.25 23.65-23.28 23.54-24.53 Anhydrous Total 96.80-97.22 98.68 98.79-98.38 98.88 14.2 - 12.0 11.5 - 9.5

Si 5.989-5.993 5.988' 5.861-5.884 5.989 6.019-6.035 5.976-5.916 AlIV 0.011-0.007 0.139-0.116 0.011 0.024-0.084 AlVI 5.451-5.255 4.609 5.774-5.167 4.124 5.184-5.337 5.437-5.482 Ti 0.020-0.022 0.008 0.005 0.026-0.020 Fe+3 0.580-0.775 1.473 0.288-0.870 1.885 0.770-0.657 0.640-0.524 Mn 0.012-0.017 0.009 0.018-0.021 0.020 0.041-0.037 0.025-0.021 Ca 3.908-3.898 3.891 3.959-3.966 3.963 3.940-3.866 3.871-4.013

PS 9.6-12 .a 24.6 5.1-14.9 32.3 4.12-12.0 11,5-9.5

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T-1 2G- 1" T-l2G-2* T-12G-39~ T-335B T-337A

core rim core rim core rim core rim , granular

S iO2 38.87-39.43 39.00-39.21 38.24-38.56 39.09-39.01 38.15 37.74 T i02 0.09 -- 0.03- 0.07 -- 0.07 *I203 28.83-29.86 30.24-31.60 28.85-32.09 28.98-27.82 23.. 58 28.71 Fez03 -i- 6.53- 5.58 4.47- 2.70 6.21- 2.78 5.74- 6.70 13.84 5.59 MnO 0.09- 0.15 0.07- 0.08 0.12- 0.15 0.23- 0.14 0.08 . 0.08 CaO 23.77-23.59 24.49-24.64 23.88-24.22 23.07-23.17 23.12 23.79 Anhydrous Total 98.18-9d. 61 98.28-98.23 98.41-97.70 97.14-96.90 98.7.7 95.98

~~ ~~~~ ~ ~~ Si 6.007-6.035 5.989-5.984 5.968-5.912 6.075-6.020 5.977 5.933 AlIV 0.011-0.016 0.032-0.088 0.003, -7 0.067 AlVI 5.251-5.386 5.462-5.669 5.275-5.710 5.310-5.194 4.360 5.338 Ti 0.011 0.004-0.008 -- 0.008 Fe+3 0.759-0.643 0.517-0.310 0.729-0.321 0.671-0.799 1.643 0.661 Mn 0.012-0.019 0.009-0.010 0.016-0.020 0.029-0.019 0.010 0.0 1ope Ca 3.936-3.868 4.029-4.029 3.993-3.978 3.842-3.932 3.891 4.007

PS 12.6-10.7 9.0-5.0 '13.0-7.6 , 11.3-13.3 27.4 11

-___-l_---

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T-12C* T-41A* T-3 34A* T-334C-k T-335E*

~~~ S i02 38.54-37.42 37.89-37.70 37.65-36.93 38.46-38.79 38.75-38.68 T i02 0.14- 0.04 0.03- 0.14 0.19- 0;02 0.08- 0.01 A1203 26.36-24.13 23.63-21.22 26.73-24..28 30.37-29.56 29.15-29.04 Fe203-tt 9.21-12.21 12.97-16.41 10.48-13.03 6.50- 6.98 5.72- 6.28 MnO 0.11- 0.16 0.16- 0.17 0.18- 0.18 0.06- 0.04 0.03- 0.04 CaO 23.89-23.36 23.31-23.05 23.58-23.19 23.61-23.56 24.10-24.04 Anhydrous Total 98.24-97.31

Si 6.023-5.977 6.020-6.023 5.880-5.899 5.884-5.950 6.005-5.985 AlIV 0.023 0.120-0.101 0.116-0.050 0.015 AlVI 4.855-4.520 4.425-3.995 4.799-4.469 5.360-5.295 5.324-3.280 Ti 0.017-0.005 0.004-0.017 0.022-0.002 0,009-0.001 Fe+3 1.983-1.468 1.55171.973, 1.232-1.566 0.748-0.802 0.667-0.7.31 Mn 0.015-0.022 0.022-0.023 0.024-0.024 0.008-0.005 0.004-0.005 Ca 4.000-3.998 3.968-3.945 3.945-3.969 3.870-3.872 4 .d01-3.985 r... - Ps 18 - 24 26 - 23 21 - 26 12 * 13 11 - 12

!?ate: Atomic ratios were calculated on the basis of 25 oxygens. * Diabasic dike rocks. -b Greenschists interbedded with marble and pelitic schist. 9 Tertiary metabasaltic rocks.: +kGranitic rocks. ttTotal Fe as Fe203.

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T-336* T-3 37 A* T-3B* T-33 1B?

S i02 38.51-37.95 37.94 38.60-37.89 38.12-37.62

T i02 0.12 0.06 0.06- 0.04 ~ 0.04- 0.06 A1203 26.60-23.68 23.05 29.27-27.28 26.88-24.21 Fe203:t 8.49-13.53 13.50 6.04- 8.37 10.07-12.99 MnO 0.12- 0.09 0.10 0.14- Ot09 0.15- 0.07 CaO 23.93-23.32 23.61 23.80-25.58 23.65-23.35 Anhydrous Total 97.65-98.69 98.24 97.90-97.25 98.71-98.30

Si 6.041-5.994 6.028 5.976-5.963 5.945-5.957 A1Iv 0.006 0.024-0.037 0.055-0.043 AlVI 4.9'18-4.402 4.317 5.317-5.023' 5.849-4.476 Ti 0.014 0.007 0.005-0.007 0.005-0.007 Fe+3 1.002-1.608 1.614 0.704-0.991 1.182-1.548 Mn 0.016-0.012 0.013 0.018-0.012 0.020-0.009 Ca 4.022-3.947 4.019 3.948-3.976 8.952-3.962 , Ps 17 - 27 27 12 - 16 20 - 26

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T-1B 5 T-12E** T-342**

4

S iO2 38.52-38 .$1 37.82-37.69 38.56-38.97 T i02 0.08 0.02- 0.15 *l203 27.88-27.99 29.36-27 .d5 31.48-29.10 Fe203f-f 7.85- 7.29 6.44~9.93 2.43- 5.76 MnO 0 a08- 0.08 0.33- 0.04 0.16- 0.25 CaO 23.54-23.52 23.58-23.75 23.77-23.17 Anhydrous

Total ' 97.95-98.09' 97.52-98.76 96.43-97.41

Si 5.996-6.009 5.896-5.873 5.982-6.045 . AlIV 0.004- 0.104-0.127 0.018 A 1vI 5.11 1-5.161 5.307-4.997 5.738-5.322 Ti 0.009 0.003 -0.0 1.8 Fe+3 0.920-0.858 0.755-1.165 0.283-0.673 Mn 0.01 1-0.01 1, 0.044-0.006 0.021 -0.033

Ca I 3.926-3.942 3.939-3.965 3.951-3.851

Ps 15 - 16 13 - 19 5 - 11

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, 1.0 t m- c

3 .' 0 *\ - 0.8 \\ \ Greenscbisf fucies

\ .\\< c\ z \ + 0.6 \ t Q \ Amphibolites A\\O RmphiBoMe hcie: a 0.4 0 rim @A. 8 core aw E e granular a 0 & t a vein \ \ 00 \ Metasornotized arrrphilfolites \ AA A rim \a I \3 \ \ A core \ \

I \ I' ' I I

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~~

cores occur in the thermally recrystallize to the minimum Fe+3 content of the cores

and metasomatized amphibolites of some second-type epidotes in.

(Fig. 6, B, E); (4) symplectectic amphibolites apparently unaffected

iniergrowths of clinozoisite together by granitic intrusion. Therefore,

with quartz + sodic plagioclase , the range Ps to Ps may represent 04 05 pseudomorph earlier epidote crystals the composition of clinozoisite

(Fi&. 7); (5) xenomorphic and fine-grained initially crystB33Lized from the I epidote aggregates haye formed marginal mafic rocks during amphibolite

to coarse-grained epidote-clinozoisitc facies metamorphism. It should be

4- crystals or have formed along'hinges noted in Table 8 that the core

of the second foliation (Fig. 6, D, F); composition of the semnd type of

(6) fibrous, fine-grained clinozoisite epidote-clinozoisite mineral in the

occurs in saussuritized plagioclase; amphibolites away from the granitic

and (7) fine-grained, granular epidote intrusion varies from Ps to Psl0. 05 occurs with laumontite as a vein In zoned grains, chemical variation is

mineral. dependent on the level of section

These contrasting occurrences of of the crystals; therefore, those

epido te-cl inozoisi te differ with minimum Fc+~contents were

significantly in composition and appear selected as the most representative 1

to correspond to the four different core compositions $01 clinozqisit c

stages of metamorphic crystallization crystallized during the amphibolite

described in the previous section. fac ies metamorphism. Subsequent

Elongate clinozoisite crystals of the deformation and recrystallization

first type are homogeneous in composition' of the amphibolites contemporaneous

with Fe+3 ranging from less than with granitic intrusion and in various

0.12 to 0.15 atoms per formula unit episodes of greenschist facies

(Pso4 to Psd5). This range is similar -metamorphism have signif ic'antly

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modified the compositions of the original from the contact metamorphosed and

clinozoisites, in general by enriching migmatized amphibolites are more

their rims in the pistacite component aluminous than corresponding phases +3 (PsI2 to PsI7). The increase in Fe in the other rocks (Fig. 11). Evidently,

content of the epidote-clinozoisite rising temperature caused a decrease

solid solution with decreasing in the Fe+3 content in the

metamorphic grade has been documented epido te-clinozo isite solid solution,

before (for example, Miyashiro and Seki, and some crystals were evendreplaced

1958; Ernst, 1972; Hsrmann and Raith, by a symplectic intergrowth of

.#

, 1973) and appears to be a generally clinozo-isite .(type four) + plagioclase +

I applicable phenomenon for an quartz. Optical and qualitative

isochemical metamorphic rock series. microprobe analyses indicate that the

'1 The third type, stubby epidote symplectic clinozoisites are'extremely

in thermallz metamorphosed amphibolite, low in Fe+3 content (less than 0.08

shows different zoning relations Fe+3 atoms per formula unit) but it

compared to the epidote-clinozoisite is difficult to obtain reliable +3 minerals described above. The Fe 1 chemical data for such intergrowths.

atomic concentration per formula unit The xenomorph'ic and fine-grained

ranges from 0.41 to 0.27 (PsI4 to Pso9) epidote crystals (type five) were

in the corcs and 0.34 to 0..14 (Ps to analyzed from two amphibolite 11

' Ps ) in the rims. Considering al.1 samples (T-38A and T-40A); petrographic 05 the rocks affected by granitic evidence suggests that they

ihtrusion, significant overlap between crystallized during the later greenschist

the epidote core and rim compositions facies mbtdmorphism. .. Although

is evident, but for individual samples minor zoning exists in each grain, with

the core invariably contains higher, consistent !enrichment in Fe+3 toward

Ps than the rim, Moreover, epidotes the margin, because of the small grain'.

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size, rim and core compositions were not The fine-grained epidote associated

differentiated. As shown in Table 8 with laumontite and chlorite in veins

and Figure 11, the greenschist transecting amphibolite was identified

facies epidotes in the amphibolite only in sample TPY-6617. This

exhibit a wide compositional range from low-temperature epidote is extremely

Ps to Ps arid they are , pistacitic (Ps32), as shown in Table 15 27.7 +3 characteristically higher in Fe 8 and Figure 11.

content than the epidote-clinozoisite Epidote in Tertiary Dikes and Other

phase that formed earlier in the same Greenschist Facies Rocks. Epidote

rock. The wide range in composition of is widespread in all metamorphosed

the greenschist facics epidotcs may be Tertiary mafic rocks (dike and bas'altic

related to contrasting nucleation cover rocks) investigated in this

sites. Thos@=$&t pseudomorphed study. It occurs as fine-grained,

earlier epidote-clinozoisites contain granular xenoblastic crystals after

lower Fe+3 than those that plagioclase and clinopyroxene. The

crystallized together with chlorite at epidote in many diabasic dikes is

the hinges of second-generation folds. distinctly pleochroic, with higher

Many saussuritized plagioclase crystals birefringence than that in the

contain fibrous cl inozoisite (type amphibolite. Zoning is not significant

six above), which appears to have except in some large crystals in

crystallized together with albitic which the rims are characteristically

plagioclase during the greenschist enriched in Fc+~. Microprobe analysis

facies stage; this clinozoisite of epidote graiils yields approximately +3 contains very little Fe+3 according to stoichiometric pistacite, Ca3(A1,Fe )

qualitative spectral scanning A1 Si 0 with the principal 2 3 12'NH), (similar relations have been described variation occu4ring in Fe+3/(Fef3 -k Al).

by Eriaiiii and Banno, 1977). Table 9 lists the compositional ranges

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of all analyzed epidote minerals, and to the epidotes in most of the

Figure 12 illustrates their variations. recrystallized Tertiary dike rocks.

Ap?arently, except for a few rocks On the other hand, metagranitic

,(for example, T-334C and T-335E), the rocks (for example, T-12E and T-342)

analyzed epifiotes contain higher contain nassive coarse-grained

proportions of the pistacite component epidote-cl-jriozoisite pseudomorphous

than the corresponding phase in the after plagioclase and hornblende. The

arnphibolites. Values for the analyzed epidote-clinozoisites are

Fc&~/(F~+~+ N) ratio are as high as uminous, with 0 contents ranging a1 'Fe23 0.24 to 0.27. The epidote of the from 2.43 to 9.93 wt 2. Such low

two greenschist facies dike rocks Ps proportions in the epidote mentioned above is slightly. more minerals may be related to low Fe aluminous and homogeneous. The contents of the granitic rocks and

epidotes are finer in grain size than possible low f cpnditions during O2 the others, and the analysis listed the greenschist facies recrystallization

in the tabl'e may be mainly a reflection of the orthogneisses.

of their core compositions. Chlorite The pre-Tertiary greenschist (one

representative sample, T-331B) contains ' Chlorite is ubiquitous in both

abundant granular, medium-grained amphibolites and greenschists

idioblastic epidote granules, which, investigated in the present study.

along with chlorite and actinolite, Petrographic evidence indicates that

dafine a strong foliation in the rock. chlorite in the amphibolite formed

The epidotes characteristically have a during the later greenschist faci'es

h$h Fef3/(Fe+' + Al) ratio, and they metamorphism; none of it appears to be

are relatively uniform in composition; a stable associate of hornbwnde or

Ps vdlues range from 20 to 26, similar calcic plagioclase, although it does

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\ \ ?\ \ \ \

x\ \ \ \ \ \X* x\\ x\x '\+ h

\ \ \ \ \ \ \ 'A 'A \ \ \ \

2.0 2.2 2.4 2.6 218 3.0 A1"'atoms per formula unit

Figure 12. Proportions of octahedrally coordinated ferric iron and aluminum in epidotes

ofgreenschist,diaba$c dike, and basaltic and granitic rocks; kompare with Figure 11.

Symbols as in Figures 8 and 9.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 seem to be systematically more magnesian of H 0 as 12.0 wt % (Ernst, 1977). 2 than greenschist chlorite. Cation proportions appear to be

Chemical compositions of chlorites reasonable. , from seven amphibolites, including The variations in major elements

the chlorite in a laumontite-bearing , Si, N, Fe, and Mg for the analyzed

vein, and twelve greenschist facies rocks chlorites are shown in Figure 13.

(seven Tertiary metadiabasic dikes, two Except for the analyzed chlorite

metabasaltic cover rocks, one from the laumontite vein, all chlorites

pre-Tertiary greenschist, and two , are remarkably uniform in, composition, .

metagranitic rocks) were obtained; irrespective of their occurrence. On

analytical results art listbd in the basis of one formula unit, the Si

.Tables 10 and 11. Total Fe is expressed content.,ranges from 5.3 to 5.6, and

as FeO. Structural formulas 6alculated NV1 ranges from 2.4 to 2.8. The

on the basis of 28 oxygens are also Fe+2/(Fe+2 + FIg) ratio varies from

presented in the tables. ChlBites 0.20 to 0.45, apparently chiefly as

were analyzed for at least six elements a function of host-rock composition.

(Si, N, Fe, Fig, Mn, and Ca); in,some Pressure-temperature conditions must

samples, Ti, K, and Na were alsq also exert some control, because high-grade

determined. As shown in the tables, chlorites are consistently more mapesian

all analyzed chlorites ape very , than greenschist facies chlo'rites. The TV- low in Ca, Ti, and alkalis; Fin0 amount of Alvl exceeds that of Al

contents range from 0.2 to 0.9 wt 2, in most samples, but the differences

with most'chlorites carrying less than qre small, suggesting a simple

0.4 wt %. Anhydrous totals for these clinoehlore-type coupled substitution

chlorites are slightly lower than the of NI" + ~1"for si + Mg. AS is

ideal value of 88 wt %, partly- apparent frorh the figure, the analyzed refleccs ing a possible underestimation chlorites are Uustered. in compositional

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T-40A T-38A T-8A TPY-66 17 vc in I S i02 26.49-26.,17 26.66-26.61 26.50-26.25 . 28.17-27.99 31.251’ A 203 22.26-22.05 21.78-21.75 20.79-20.44 22.99-22J 1 21.82 Few 16.02-17.43 16.89-18.13 23.23-22.98 13.75-15.52 18.62 MnO 0.’18- 0.19 0.14- 0.20 0.21- 0.21 0.35- 0.39 0.69 MgO . 20.69-19.82 21.10-20.17 16.54-16.57 23.46-22.28 18.07 CaO , 0.05- 0.07 0.08- 0.10 0.07- 0.08 0.09- 0.05 0.18 Anhydrous Total 85.69-85.73 86.66-86.95 87.43-86.53 88.81-88.33 90.63

Si 5.423-5.400 5.425-5.432 5.530-5.535 5.483-5.533 6.061 AlIv 2.577-2.600 2.575-2.568 2.470-2.465 2.517-2.467 1.939 AlVI 2.795-2.764 2.650-2.666 2.644-2.615 2.758-2.686 3 -050 Fe+2 2.743-3.008 2.874-3.094 4.072-4.051 2.239-2.566 3.021 Mn 0.031-0.034 0.024-0.034 0.038-0.038 0.057-0.065 0.113 Mg 6.311-6.097 6.398-6.135 5.144-5.207 6.8.06-6.564 5.223 Ca 0.0 11-0.0 15 0.017-0.022 0.01 5-0.01 8 O.O~~-O.’o 11 0.037 Fe/(Mg+Fe) 0.303-0.330 0.310-0.335 0.442-0.438 0.248-0.281 0.366

Note: Atomic ratios were calculated,on the basis of 28 oxygens. *Total Fe as FeO. >

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T-12B IS-12G-2 T-335B

S iO2 26.52- 6.14 26.40-26.85 26.86-26.63 A1203 21.92- $ 2.19 21.41-21.43 20.08-20.32 FeO-2 23.30-23.44 21.67-21.22 '19.82-20.61 MnO 0.91- 0.91 0.23- 0.22 0.27- 0.30 Me0 16.57-17.31 17.68-17.80 17.53-17.24 CaO 0.11- 0.iO 0.07- 0.12 0.08- 0.04 Anhydrous Total 89.33-90.08 27.45-87.25 84.63-85.14

5.426-5.312 5.460-5.524 5.943-5.643 2.574-2.688 2.540-2.476 2.057 -2.357 2.712-2.627 2.680-2.722 2.819-2.538 3.986-3.983 3.748-3.633 3.414-3.653 0.157-0.156 0.040-0.038 0.005-0.005 5.052-5.242 5.449-5.458 5.379-5.444 0.025-0.022 0.015-0.026 0.017-0.008 0.440-0.432 0.408-0.400 0.388-0.402

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T-12C* .T-41A* T-334AQ T-335Ek T-336* T-337A*

S iO2 26.33-29.61 26.44-27.64 25.33-25.02 25.96-26.05 26.15-26.65 30.20-25.61 T i02 0.03- 0.29 0.02 A1203, 19.31-17.71 20.01-18.31 19.53-20.16 21.95-21.56 20:91-20.10 19.63-19.60 FeO f'i. 25.62-23.68 ' 26.61-25.87 25.79-27.05 18.66-20.70 25.23724.48 . 21.63-24.38 MnO 0.63- 0.58 0.26- 0.35 MgO 14.19-1 3.07 14.15-15.50 16.62-16.92 . 19.89-17.76 14.93-15.06 15.45-16.77 CaO 0.08- 0.10 0.06- 0.03 0.07- 0.05 0.02- 0.05 0.05- 0.07 0.09- 0.11 K20 0.72- 2.57 0.12- 0.10 0.04- 0.05 0.06- 0.24 Na20 0.03- 0.04 0.12- 0.19 0.01- 0.04 0.02- 0.05 Anhydrous Total 86.31-87.07 87.90-87.92 87.58-89.71 86.53-86.21 87.34-86.64 87.26-86.82

Si 5.671-6.279 5.600-5.823 5.381-5.230 5.343-5.438 5.516-5.652 . 6.206-5.462 AlIV , 2.329-1.721 2.400-2.177 2.619-2.770 2.657-?. 562 2.484-2.348 1.794-2.538 AlVI 2.579-2.706 2.596-2.371 2.272-2.197 2.668-2.743 2.715-2.676 2.962-2.390 Ti 0.005-0.047 0.003 Fe+2 4.615-4.199 4.714-4.558 4.582-4.729 3.212-3.614 4.'45 1-4.342 3.7 17-4.348 Mn 0.113-0.104 0.005r0.006 Mg 4.554-4.131 4.466 -4.86 6 5.263-5.271 6.102-5.526 4.694-4.760 4.732-5.330 Ca 0.018-0.023 0.013-0.006 0.016-0.01 1 0.004-0.01 1 0.011-0.016 . K 0.197-0.348 0.033-0.027 0.011-0.013 0.016-0.065 Na 0.013-0.016 0.049-0.118 0.004-0.016. 0.008-0.021 Fe/ (Mg+Fe) 0.503-0.504 0.514-0.484 0.465 -0.47 3 0.345-0.395 -0.487-0.477 0.44'0-0.449

Iiote: '>Atomic ratios were calculated on the basis of 28 oxygens.

Q Diabasic dike rocks. f Greenschists interbedded with marble and pelitic schist. 5 Tertiary meta-basaltic rocks. *;Granitic rocks. 'fTTotal Fe as FeO.

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25.52-25.94 26.52-26.76 26.14-26.33 26.91-26.61 24.77-25.36 25.23 0.04- 0.04 ’ 0.06 0.04- 0.06 21.49-21.61 20.94-20.62 21.16-21.17 20.86-20.97 22.71-21.36 21.81 25.02-24.49 22.14-22.63 24.04-23.82 20.68-20.71 25.33-24.03 25.89 0.33- 0.35 0.30- 0.31 0.40 15.45-15.33 17.04-17.06 15.97-15.98 17.66-17.90 14.50-15.61 13.00 0.12- 0.11 0.15- 0.18 0.06- 0.06 0.07- 0.02 0.04- 0.04 0;05 0.01 0.13- 0.10 0.06- 0.11 0.03- 0.04 0.04- 0.02

87.93-87.84 86.86-87.30 87.68-87.67 86.39-86.34 87.47-86.58 86.37

Si 5.357-5.427 5.533-5.568 5.466-5.496 5.598-5.543 5.235-5.386 5.430 AlI” . 2.643-2.573 2.467-2.433 2.534-2.504 2.402-2.457 2.765-2.714 2.570

AlVI 2.675-2.757 2.682-2.624 2.681-2.705 2.713-2.691 2.894-2.633 I 2.965 Ti 0.006-0.006 0.009 Fe+2 4.392-4.285 3.863-3.938 4.204-4.157 3.598-3.608 4.476-4.268 4.660 Mn ’ 0.059-0.062 0.053-0’.055 0.008 % 4.833-4.779 5.299-5.291 4.976-4.970 5.476-5.558 4.566-4.951 4.169 Ca 0.025-0.025 0.034-0.040 0.013-0.013 0.016-0.005 0.006-0.006 K 0.003 0.035-0.026 0.014-0.027 Na 0.476-0.473 0,422-0.427 0.458-0.456 0.012-0.016 0.016-0.008 Fe/(Mg+Fe) 0.397-0.394 0.495-0.486 0.528

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’ El t,0 ‘0

-Fe/(Mg + Fe) ’

Figure 13a. Compositional variations -of chlorites from amphibolites, greenschist,

and metamorphosed diabasic, kdnltic, and granitic rocks, Symbols as in Figures

8 and d. Open sq.uare = chlorite from vein in amphibolite. Classification of

chlorites aftcr Iley (1954), assuming all iron present is d‘ivalent. Proportiong of *, fourfold and sixfold coordinated aluminum; dashed line indicates equal amounts

of aluminum in both types of struc~ur$lsite and represents serpentine-clinochlore- 1, corundophilite - type substitutrion.

Figure 13b appears on the following frame.‘

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I

/ -. 20 I I. I I 1 1.6 ',I 8 20 -22 24 2.6 &28 AI'" atoms per' formula unit

Figure 13b. Compositional variation< of chlorites from amphiboli tes, greenschist,

and metamorphosed dinbasic, basaltic, and grnnitic rocks. Symbols as in Figut-es

8 and 9, Open square = chlorite from.vein in amphibolite. Classificac.Zbn of

chlorites after Hey (1954), assuming all iron present is divalent. Pdoportions,of

fourfold 'and sixfbld coordinated alumidrim; dashed line indicates equdl amounts

of aluminum in both types of structural site and repre'sents serpentine-clinochlore-

corundophilit6 - type substitution.

Figure 13. ,(Continued)

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space and are classified as pycnochlorite that are distant from the intrusive

and ripidolite. Presumably, this metagranitic rocks. However, both

compositional uniformity reflects ~ the grain size and the amount of

crystallization under greenschist white mica increase with increasing

facies conditions. effect of thermal metamorphism.

The chlorite in the laumontite-bearing For instance;: some metasomatiz'ed ..,-in vein of amphibolite sample TPY-6617 amphibolite xenoliths within the

occurs as spherulitic aggregates with metagranitic orthogneisscs,

a ring diameter, less than 0.05 mm. as much as r0 vol X of

The chlorites are pale green and of coarse-grained white mica occurs.

very low birefringence; as shhwn in Apparently the proportions of

7 Table 10, this phase is characterize: in the amphibolites reflect the

by extremely high Si (hence lower introduction of K 0 during granitic 2 IV ) and higher Alvl compared to . intrusion. Nuscovite is Abiquitous Al - the groundmass (greenschistic) in the granitic rocks; some crystals

chlorite in the same sample. arc coarse euliedra and appear to

Presumably; the chlorite 3- laumontite represent a primary. igneous phase.

vein formed at much lower temperatures- Other fine-grained interstitial

than attended the greenschist facies white mica flakes associated with

metamorphism; this wQi?ld appear to sphene granules seem to be

account for its enrichment in Si products of the later greenschist '

(Ernst, 1972). facies metamorphism. Piost of the

Tertiary metadiabasic dikes and White Mica related\metabasaltic rocks 'are

' Trace ampunts of fine-grained devoid of'white mica. Only one dike

3, white mica occur in a few amphibolites rock (T-334C) contains abundant

(for example, T-335B and TPY-6617) fine-grained xenoblastic white mica

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flakes aligned subparallel to the , conclusions. Cipriani and others

foliation. The occurrence of white (1971) havk suggested that the chemistry

mica in this grecnschist facies metadike of white mica is controlled lllainly - is apparently related towhe high K20 by temperature and pressure rather than

md A1203 content of this rock (Table 2). by bulk-rock composition. Howevef,

\bite micas were analyzed from eight this effect was not clearly demonstrated

amphibolites; one diabasic dike, gd in the study described here. In the

two granitic rocks; the results are absence of, low-variance phase .

listed in Tables 12 and 13; All assenblages, white mica compositions

analyzed white micas are low in Ti0 should reflect the chemistry of the 2' NnO and CaO. Nost white micas in the parent lithology (bee also Ernst,

amphibolites contain less,Fhan' 0.50 1963). The analyzed amph+bolites consist

wt 2 Na 0, except for two with 1.4 to ' 2 not of a single paragenesis but of 1.8 wt 2. Fe 0 ranges from 0.4 to 4.0 intergradational mineral- assemblages 23 wt Z and MgO from 0.2 to 3 wt %,. that crystallized successively'in

The N203 content ahd Al1"/Si ratios various metamorphic stages, as

of the-analyzed white micas arc compatible described above. The white micas may

with those val,uesyof white micas from ham recrystallized in different events

rocks subjected to greenschist and without total re-equilibration. This

amphibolite facies conditions (see, may account for some variation in

for example, Deer and others, 1964). white mica composition 'even within a

compositions of white micas in the single rock9 specimen. Similar

other analyzed rocks are similar to conclusions have been deduced by

those in the thermally recrystallized John Suppe (1980, personal commun.), .

amphibolites. on the basis of radiometric data for

There are too few data on white micb white micas from Californian blueschist

compositions to draw detailed facies rocks.

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TPY-66 17 T-8kk T-12A* T-12B*

~~~~~ ~ 50.07-48.10 47.47-47.79 47.86-47.71 49.66-51.44 . -- 0.09 0.01- 0.13 0.13- 0.09 -- 0.02 34.94-37.13 31.32-31.63 31.21-30.47 3.3.44-29.63 0.48- 0.48 2.38- 1.77 3.87: 4.03 1.38- 2.01 0.19- 0.86 0.92- 1.89 2.24- 2.55 0.75- 2.23 0.2 2-’ 0.0 1 0.10- 0.08 0.05- ‘0.05 -- 0.03 8.44- 8.77 10.16-10.96 9.76- 9.58 8.89- 9.88 1.39- 1.26 0.41- 9.33‘ 0.14- 0.14 1.81- 1.71 , 95.74-96.70 92.77-94.58 95.24-94.62 95.93-96.00

.6.491-6.201 6.471-6.414 6.37 2-6.400 6.918-6.742 1.509-1.799 1.529-1.586 1.628-1.600 1.072-1.258 3.822-3.844 3.504 3.418 3.271-3.216 3.576-3.317 0.009 0.001-0.013 0.013-0.019 -- 0.002 0.047 -0.04-6 0.244-0.179 0.388 -0.407 0.122-0.198 0.037-0.165 0.187-0.378 0.455-0.509 0.132-0.435 0.030-0.002 0.015-0.011 0.007-0.007 -- 0.004 1.390-1.442 1.768-1.877 1.658-1.639 1.338-1.645 0.349-0.314 0.108-0.085 0.01 8-0.037 0.414-0.181

?Iota: Atomic ratios were ‘calculated on the basis of 22 oxygens .I :kT herma 11 y re c r y s t a 11 i z ed amph i b o 1i t e s . tTotal Fe as Fez03 .

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TABLE 12. (continued) 0

T-12G-l* ' T-12G-2" T-12G-3* T-335B

S iO2 47.77 -48.35 46.66-46.01 47.40-52.04 48.71 Ti02 0.87- 0.03 -- -- 0.01- 0.01 0.53 A1203 32.25-32.58 32.90-34.75 31.18-28.33 28.51 Fez03 t 2.47- 2.31 1.48- 1.14 1.62- 1.02 3.04 MgO 1.60- 2.06 1.43- 0.73 2.13- 1.95 3.18 ' CaO -- 0.08 0.07 -- 0.67- 0.02 0.28 K20 10.62-10.77 10.95-10.99 10.64- 8.66 10.91 Na20 0.42- 0.17 0.23- 0.22 0.17- 1.06 0.21 1 Anhydrous Total 96.00-96.35 93.72-q.84 93.81-93.68 95.36

~~ ~ ~~ ~~ 6.322-6.364' 6.314-6.207 6.412-6.970 6.524 1'. 678-1.636 1.686-1.793 1.588-1.030 1.476 3.353-3.419 3.562-3.734 3.385-3.401 3.026 Ti 0.087-0.003 -- -- 0.012-0.012 0.053 Fe+3 0.246-0.223 0.151-0.116 0.165-0.102 0.306 Mg 0.31 6-0.404 0.151-0.147 0.429-0.386 0.635 Ca -- 0.011 0.011 C .097-0.003 0.040 K 1.792-1.808 1.889-1.892 1.837-1 -465 1.864 Na 0.108-0.043 0.060-0.057 0.044 -0.273 0.055

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Metadiabasic rock Granitic rocks r T-334C T-12E . T-342

48.14 46.35 46.6.7 0.06 0.94 0.20 30.53 33.89 ' 33.60 2.71 1.81 2.28 2.84 1'. 17 1.33 0.04 0.02 10.98 10.56 10.55 0.23 0.47 0.65

95.52 95.21 95.02

6.422 6.223 6.224 1.378 ' 1.777 1.776 3.223 3.588 3.506 0.006 0.095 0.020 0.273 0.182 0.229 0 ,,5 64 0.234 0.264 0.006 0.003 1.869 1.809 1.795 0,059 0.123 0.168

Note: Atomic ratios were calculated on the bas,is of 22 oxygens. *Total Fe as Fe2q .

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 The biotite-rich metasomatized Biotite amphibolite grades into biotitic

Similar to the white mica, biotite in (10 to 15 vol %) garnet- and

the amphibolites is confined to those hornblende-bearing granitic rocks.

rocks that have been metasomatized Biotites are coarse-grained and

appreciably during granitic intrusion. idioblastic in form, and some are in

) Coarse-grained bibtite books are nearly contact with granoblastic garnet, However,

ubiquitous at contacts between the paragenetic relation between garnet ,

xenolithic amphibolite and the and biotite in the orthogneiss is

surrounding granitic orthogneiss. ambiguous. In some samples (for

TJithin the amphibolites, as much as example, T-l2E), both garnet and

5 vol % medium-grained biotite has ,biotite appear to be magmaticAhases,

\ replaced hornblende and plagioclase. whereas in sample T-342, biotite forms

In s-ome amphibolites, larger amounts as a replacement product after hornblende

6f biotite, together with closely and‘ garnet. Both garnet and- biotite

associhted porphyroblastic’garnet, in the orthogneiss exhibit later

may have crystallized simultaneously alteration to chlorite.

during the thermal metamorphism. These In the recrystallized dinbasic dike

two minerals have been partly replaced rocks, yellowish brown to light-green

by chlorite alonE cleavages and fractures biotite is ubiquifous and amounts to

during the. subsequent greenschist bcies 2 to 10 vol %. Prebiously, these

e recrystallization. In some *amphibolites, biotites were considered to be primary,

however, pale-brown to nearly colorless , and the dike rocks were commonly biotite rims coarse-grained biotite ; referred to as lamprophyres -(for

such late-formed biotite may have grown ’example, Yen, 1954a, 1954b; Ho, 1975).

during the gieenschist facies However, petrographic and cor2pdsitionali

metamorphism. data collected in this study do not

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~

support this conclusion, .The biotites uniform in compgsition, wtth high total

occur' as fine-grdped xenoblastic iron and MgO and moderate amounts of

crystals after relict clinopyroxene The Mg/Fe ratio ranges from A1203. and- plagioclase (these primary phases *0.7 to 1.2 and Alvl from 0.57 tco 0..68; t have well-preserved crystal forms, such variations are apparently related

although the clinopyroxene has been to bulk-rock composition; Compared to , B B totally'replaced). The biotity are amphibolitic biotites, three analyzed ,. intergrown with typical greenschist c biotit/es from orthogneisses contain facies minerals such as chlorite, higher Ti02 (1.4 to 2.1 wt %) and

4 actinolite, epidote, and granular Alvl (0.78 to 0.72) and have similar.

-sphene. Althdugh most metadiabasic Mg/Fe ratios (0.8 to 0.9). 2 ' rocks dire massive and preserve primary, The greenschist facies biotites in I textures, some biotite, together with both amphibolites and metadiabasic rocks-

' VI chlorite ,and aETinolite, defines a contain less Ti02 and Al (Fig. 14), d very weak foliation; such parallel more Si02, and a*higher Mg/Fe ratio

orientation suggests that'these phases than those in the amphibolite and s crystallized during moderate deformation orthogneiss. This is apparent when the

in the greenschist facies stage. compositions of the earlier-and later 1 Compositions of biotites were formed biotite in the same amphkbolite

analyzed for four amphibolites, are compared (for example, T-12A, ._. seven diabasic and basaltic ,rocks, and T-12G-1, Znd T-12H). The biotites in two metagranitic rocks; ,results are + the metadiabasic rocks are very listed in Table 14 and shown graphically heterogeneous in aggregate composition, 4 in Figure 14. All analyzed bio-tites as shown inqigure 14, alLhough only

are low in Na 0 and CaO; MnO was not 2 a small variation occurs in a single analyzed. Biotites in the thermally specimen (see Table 14). The Mg/Fe ". ' recrystallized amphibolites are ratio rapges from 0.9 to 1.6,

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Amph iboli t e ,J ,J

1 T-8A* T-12A* T-12G-1* T-12H*

SiO2 , 36.96-37.18 36.10-35.67 37.97-36.28 36.86-37.24 T i02 2.04- 2.47 1.34- 2.10 1.06- 2.31 2.37' 1.05 A1203 16 75-16.54 17.47-16.80 17.39-17.36 16.08-16.67 - FeO-i--? 16.60-16.88. 21.14-21.36 15.88-20.07 18.00-17.32 MgO 11.15-11.13 9.17- 8.88 ' 12.69- 9.54 10.98-11.85 CaO 0.19- 0.14 0.38- 0.04 0.04- 0.02 0.04- 0.08 K20 9.87- 9.39 8.89: 9.72 9.73- 9.51 Na20 * 0.08- 0.07 0.09- 0.06 0.08- 0.05 0.05-* - 'g0. *I3 6 Anhydrous - Tdtal 93.65-93.76 94.58-94.62 94.B-95.15 93.99-93.30

Sl 5.569-5.672 5.569-5.542 5.633-5.548 5.6574.706 AlIV 2.431-2.328 + 2.431-2.458 2.367 -2.45 2 2.343-2.294 AlVI ' 0.593-0.647 ?0.746-0.619 0.674-0.678- 0.567-0.717 Ti 0.235-0.283 0.155-0.246 0.130-0.266 0.273-0.121 Fe+2 2.125-2.153 2.727-2.775 1.970-2.566 2.3 10-2.220 Mg 2.544-2.531 2.108-2.056 1.805-2.174 2.511-2.706 - Ca 0.031-0.023 0.063-0.006 0.006-0.004 0.006-0.013 K 1.928-1.821 1.750-1.927 1.841-1.856 1.881-1.766 Na '0.024-0.020 0.028-0.019 0.023-0.015 0.015-0.018 MgIFe , 1-19-41.18 0.77 -0.74 1.42 -0.85 1.09'-1.22

Note:Atomic ratios were calculated on the basis of 22 oxygens. *Thermally recrystalrized amphibolites. For T-12A, T-12G-1, and T-12H, biotite may have recrystallized in the greenschist facies condition. +Tertiary metarbasaltic rocks. SDiabasic dike rocks. **Greenschists interbedded with marble and pelitic schist. ++Total Fe as FeO.

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Greenschis t

S i02, 37.10-37.36 37.. 22-36.99 37.76-35.30 36.30-36.28 38.31-38.25 Ti02 0.70- 0.55 1.22- 0.72 0.96- 0.55 0.82- 0.93 0.61- 0.58

. A1203 17.35-17.42 16.62-16.43 15.92-17.38 1,7.02-17,35 a 18.83-17.72 FeO i? 19.99-19.12 20.72-19.62 20.15-21.88 20.28-20.62 15.86-15.66 MgO 11.35-11.75 10.29- 9.98 lb.2 1-1 1.24 11.83-1 1.6.4 12.27-12.39 C a0 0.21- 0.23 0.17- 0.21 0.94- 0.12 0.01- 0.01 0.06- 0.11 K20 8.88- 8.72 9.29- 9.33 9.Y5- 8.62 9.15- 9.26 9.11- 9.14 Na20 0.14- 0.15 0.07- 0.09 0405- 0.05 0.01 0.09- 0..07 Anh y dtous Total 94.70-95.29 95.59-93.37 94.42-95.14 95.42-96.10 95.13-93.92

~~__~~~~~ ~~ Si 5.639-5.637 5.667-5.750 5.805-5.435 5.532- 5.499 5.682- 5.753 AIIV 2.361-2.363 2.333-2.250 ’ 2.195-1.565 2.468- 2.501 2.318- 2.247 AlVI 0.748-0.736 0.650-0.758 0.690-0.590 0.588- 0.599 0.973- 0.894 Ti 0.080-0.063 0.140-0.084 0.111-0.063 0.094- 0.106 0.068- 0.066 Fe+2 2.414-2.41z 2,639-2.548 : 02.591-2.817 2.584- 2.614 1.967- 1.970 2.57 1-2.642 2.335-2.310 2.339-2.579 2.687- 2.630 2.712- 2,778 O.x)34-0.037 0.027-0.035 0.006-0.019 0.002- 0.002 0.010- 0.018 1.722- 1.681 1.804-1.848 1.794-1.693 1.779- 1.791 1.724- 1.754 0.024-0.044 0.020-0.028 0.015-0 .:015 0.003 0.026- 0.020 1.07 -1.10. 0.88 -0.94 I. 25 -0.92 1.04 - 1.01 1.38 - ).41

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Greensch ist Granitic Rocks I

$ -, T-336 4 T-331Bwx T-324Cf T-12E T-342

38.93-38.37 37.10 36.99-37.75 35.37 36.38-36.52 0.93- 0.84 1.22 0.49- 0.51 2.12 1.43- 1.71 17.36-16.97 17.00 17.04-16.05 18.73 18.23-17.58 18.64 -'I a. 98 19.12 16.97-16.31 19.05 20.03-20.57 10.60-10.58 11.62 15';49-13.24 9.64 9.42- 9.09 0.09- '0 -04 0.32 0.05- 0.09 0.01 8.86- 8.92 8.30 8.13- 9.06 9.60 ;:;;; ;$; 0.12- 0.06 0.08 0 0.14 0.09 0.15- 0.11

95.51-94.74 94.76 95 3.15 93,.r 61 95.35-95.03

3- 5.818-5.804 5.625 5.518-5.766 5.421 5.542-5.594 #2.182-2.196 2.375 2.482-2.234 2.579 2.458-2.406 0.876-0.830 0.662 0.514-0.656 0.804 0. a 15 -0.7 70 0.105-0.096 0.139 0.055-0.059 0.244 0.164-0.197 2.330-2.401 2.424 2.1'1 7 -2.084 2.441 2.552-2.635 -2.361-2.386 2.626 3.444-3.014 2.202 2.138-2.076 '0.014-0.007,'' 0.052 0.008-0.015 0.002 0.060-0.006 1.689-1L2.l *I.605 1.547-1.765 1.877 1.816-1.841 0.035-0.018 ' 0.024 0.035-0.042 0.028 0.044-0.033 1.01 -0.99 1.08 1.63- 1.45 0.90 0.84- 0.79 / r

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X 0 . x--x. 'X + XX "L X + ', * ;* 0.4 I Or8 -1.0 1.2 '\ 44' y, 1.6

Figure 14: Compositional variations of biotites from migmatized amphibolites,

greenschists, and metamorphosed diabasic, basaltic, and granitic rocks. Symbols 0 as in Figures 8 and 9. (a) Ti02 (wt X) versus Fe/Mg.

Figure 14(b),appears on the following frame. -

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 .701 c 7 . 1.0 I I I I 1 .x

.-t c 0.9 X 9 3

x A m

0 X 0 ++ b 0 a 0.7 - X e * 0 X *

L

.. , >- X xX @ a e , * 0.5

0.8 1.0 - 1.2 1.4 1.6 ' Mg/Fe

\ Figure 14. 'Compositional variations of biotites from mipatized amphibolites,

.1 greeschists, and metamorphosed diabasic, basaltic, and granitic rocks. Symbols

as in Figures 8 and 9. (b) Al"' per formula unit versus .Fe/Mg.

Figure 14. (Continued)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 Alvl from 0.5 to 0.97, and A1203 from facieq alteration, and most garnets and

16 to 19 wt %. Such a large chemical some biotites were-&placed by chlorite. 4 .> variation appears to he due to significant Garnets from one metasomatized

differences in bulk-rock composition. . amphibolite and two granitic rocks

The low Ti, high AIV1, and Si are were 'analyzed; the. results are listed

characteristic features of greenschist in Table 15. Because of the fine grain

facies biotites from metabasaltic size and extensive.alteration to chlorite, ' rocks (Cooper, 1972) and are distinctly rim and core compositions of the relict

different from titanobiotites from garnets yere not differentiated. All

lamprophyres (Yagi and others, 197'5; analyzed garnets contain negligible

Rock, 1977; Cooper, 1979). amounts of Na 220, K 0, and Ti02, as

indicated by microprobe reconna'issance, Garnets ,. ' and their concentrations- hme not been

Garnet was not found in amphibolites listed in the table. The analyses are

from this area except for those uniform in composition, with val6es of

met asomat ized amphiholi tes included silica and alumina ranging. from, 3f.4 to in the granitic prthogneiss; in contrast, 38.5 wt % and 21.7 to 23.0 wt. %, - * Y minor amounts of garnet are widespread respectively, Except for garnet .i~.

'I in the metagranitic rocks and pGgmatites T-342 which coexists with hornblende, i (C. Y. Lan, 1978, personal commun.). minor variations among the other

Garnets are xenoblastic and medium to three garnets involve spessartine and '

fine grained, and they have been altered grossular components; they contain / to chlorite along fractures. Textuial high almandine (65 to 70 mol %)'and

relations and occurrences suggest that ' low pyrope (about 16 mol X) proportiqns. . .. a\* garnet and biotite (or hofnblende in Garnet T-342 may not be stable with

T-342) crystallized directly from magma. biotite and is significantly highpr

They were later subjected to greenschist in grossular and lower in almandine

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Garnets Pyroxene T- 12G- l* T-12ET T-342 t T-8B*

S i02 . 37.42-38.06 38.40 38.49 52.78-52 .+26 Ti02 0.04- 0.16 A1203 21.69-21.94 22.98 21.82 0.84- 1.70 FeO § 29,87-29.65 28.55 23.45 . 9.09- 9.61 MnO 4.73- 5.45 6.77 6.12 0.34- 0.35 MgO 3.98- 1,39 4.13 1.48 12 -36-1 1-95’ CaO 2.77- 2.44 2.06 10.07 23.68-23.42 Na2Q 0.23- 0.30 Anhydrous

total , 100.46-98.93 102.87 101.42 ~ 99.36-92.73

Si 2.972-3.065 2.965 3.012 1.982-1.969 AlIV 0.028 0.035 0.008-0.03 1 AlVI 2.003-2.083 2,957 2.013 0.029-0.045 Ti 0.001-0.005 Fe+2 1.984-1.997 1.844 1.535 0.287-0.303 Mn 0.318-0.372 0.443 0.406 0:Oll-0.011

Mg 0.471-0.167 0.475 ‘I, 0.173 0.695-0.671 Ca 0.236-0.211 0.170 0.844 0.958-0.945 Na , 0.017-0.022 Pyrope 16 - 16 15 6 Spe s sart i ne 11 14 16 -* 14 Gros su lay 8- 7 6 28 Almandine 65 - 95 63 52 Enstat i te 36 - ‘35‘ Ferrosilite 15 - 16 Wok1 aston ite 49 - 49

??he rma 11 y rec rys t a 11 i z ed amph ibo1 i t e . ?Granitic rocks. §Total Fe as FeO.

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A ..- and ’pyrope. The few reconnaissance facies metabasaltic rocks (for exdm$e,

data listed in Table 15 suggest that’ see Kretz, 1963). I garnet assoqiated with biotite is Sphene * higher in almandine and lower in

grossular than that coexisting with Sphene is ubiquitous inqhe

hornblende. More data are needed to investigaxed rocks from this areA. It

support this suggestion, however. wcurs as a coar5e-grainedteuhedral -7 primary phase in granitic :rocks, as Clinopyroxene medium- to fine-grained idioblastic-

As mentioned in the previous section, lenses parallel to the foliation in

clinopyroxene was found only in one amphibolite, and as fine-grained dus-ty

thermally metamorphosed amphibo 1ite aggregates around relict ilmenite grains,

(T-8Bj, where it occurs abundantly- as in both metsdsabasic dikes and some

granoblastic crystals in the ‘rock. metagranitic rocks. Some of the sphenes

Eight spot analyses for three selected in the greenschist facies rocks are

clinopyroxene grains yield .the exceedingly fine-grained aggregates. . c ...” compositional range listed in Table 15. Sphenes representative of the f The clinopyroxene is extremely various metamorphic stages were

homogeneous. It is low in Ti02 (less . analyzed from three amphibolites,

than 0:20 wt %), Na20 (0.2 to 0.3 wt %), five gfeenschists, and two granitic

MnO (0.35%), and A1203 (0.84 to 1.70 rocks; the results are listed in z wt %). In terms of end’member Table 16. The analyses are remarkably

! composition, the analyzed clinopyroxene homogeneous in cornpodition, regardless

is classified as salite with 49 mol % of rock type, mode of occurrence,

Wo, 16% Fs, and 35% En. This is a crystal form, and crystal*&ze. They

characteristic composition €or contain nearly constant proportions of’

clinopyroxene from upper amphibolite the major oxides’ Si02, Ti02, and CaO,

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Amphibolite . Greenschis t s Gtanit ic Rocks T-40A T-12H T-12B T-334A* T-335E* T-3B* T-331B-i- T-lBS T-12E T-342

S iO2 30.02 29m 29.89 30.48 '30.26 31.65 30.51 30.57 30.65 30.56 Ti02 37.29 38.52 37.75 34.98 39.00 36.47 37.76 34.74 38.12 34.93 All203 1.47 1.21 . .o.ai 2.37 1.28 2.17 .i.o9 2.81 1.40 2.08 Fe O3** 0.32 0.84 1.01 0.81 0.64 1.98 0.59 1.88 0.23 0.39 Mg 6 0.01 0.03 0.23 0.21 0.02 CaO" 28.26 28.30 29.06 28.29 29.05 27.77 28.45 28.16 26.94 28.23 K20 0.06. 0.03 Na20 0.01 , 0.01 0.01 Anhydrous Total . 97.43 98.13 98.56 96.96 100.25 100.27 98.40 98.36 97.35 96.26

* Diabasic dike rocks. 1- Greenschist interbedded 'with marble and pelitic schist. 5 Tertiary metabasaltic rock. *kTotal Fe. as Fe203 .

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and Fe 0 Si02 ranges from 29.9 I data are listed in Table 17. The analyzed 2 3' *- to 31.6 wt %, Ti02 34.7% to 39.0%, ilmenite grains have a'very uniform I CaO 26.9% to 29.6%, A1203 0.8% to 2.4%. composition and are clos; #to ideal -I I and Fe203 0.2% to 2.0%. hlotHer stoichiometry. 'The analized magnetite

analyzed oxides are present in I bs a very l6w oxide total, with 81 4 insignificant amounts. Low-titania wt % total FeO (actually most is present

Sphenes contain appreciable amounts as ferric iron), 0.21% Ti02, and 0.71%

0 and Fe203; apparently, minor A1203. This magnetite may contain of A1.2 3 +3 substitutions amongTi, and Fe significant amounts of Cr 0 (on the Al, 23 occur. However, these chemical variations order of 8%), which was opt' analyzed _'*- do not appear to be systematic with in this.siqdy. The rutile may have been

regard to formation in different replaced by mlnor amounts of

metamorphic stages, cryptocrystalline sphene during the . Lr greenschist facies metamorphism; Laumontite and Opaques .. therefore, the analyses contain miqr

Laumontite, together with Fe-rich but significant amounts of SiO FeO, ! 2' chlorite and epidote, occurs in some and CaO.

veins transecting host amphibolites. ELEMENT PARTITIONING The result of a single analysis is Principles listed in Table 17. This laumontite

closely approaches the ideal composition The theory of element fractionation

or CaAl Si 0 -4H 0, with mino: amounts 2 412 2 between coexisting natural rock-forming 0, of Fe203, K 2 and Na20. minerals, first discussed systematically For completeness, several opaque by Lmbetg and deVore (1951), has been

phases were selected for analysis. elaborated on by numerous authors (for

These include three ilmenites, one example, Kretz, 1959, 1960, 1961, 1963;

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'Laumont ite I lmen i te Magnetite Rutife T'PY-6617* T-12Ht T-12B1 T-3425 T-12E5 T-335B" T-12E5 I S iO2 - 51.75 0.10 .O .04 0.07 0.12 0.68 0.76 T i02 54.22 54.35 55.07 0.21 96.95 98.26 *l2O3 21.51 . 0.15 0.11 0.63 0.73 0.24 0.19 FeO" .- 44.19 45.17 44.78 80.80 0.49 0.24 Fe203i-t 0. lo MnO 0.12 0.07 MgO 0.31 . 0.18 0.14 0.06 0.03 0.34 CaO 10.64 0.17 0.15 0.10 0.03 0.98 1.28 K20 0.83 0.08 0.05 !a20 0.01 0.01 Anhydrous .Tot a 1 84.85 9.22 100.05 100.79 82.07 99.44 101.07 . ,- . * ,App$ibolite. t Thermally recrystal1:zed amphibolite. .§ Gran'itic rocks. "Total Fe as FeO. ttTotal Fe asFe2O3 .

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Mueller, 1960, 1961, 1962; Albee, 1965; however, regulqr behavior of the

Ernst, 1964, 1970; Ernst and others, participating solid solutions is

1970; Saxena, 1966, 1968a, 1968b, 1968~). reflected in exchange diagrams such

A recent summary and exposition of the as presented here!:by a close approach

principles has .&en provided by to linear trends. Departyes from these

Saxena (1973). Stated 'simply, the trends reflect (1) failure to achieve

partitioning of elements between chemical equilibrium; (2) superposition I competing minerals with,contrasting of phase relations from several

structural sites is a function of different pressure-temperature environ-

tcmFerature and, to a much lesser ments on the same plots; (3) influence a ' extent, pressure. or. an,iGn-'for-ion of other element variations ignored

exchange reaction of the sort in the diagrams; or (4) inaccurate

I: F%b + Mggi = MgHb + FeBi, an chemical analyses. In general, all

equilibrium constant, KD, may be of these effects are additive and

defined on the basis of the atomic result in a dispersion of the data

proportions of the exchangeable cations which obscures regularities in the

in the analyzed minerals as actual partitioning. ..

The distributions Df--Naversus K,

~ Thjs equation is devoid of expon5nts Na versus Ca, Fe versus Mg, and Fe

that would be requij@ by the versus Al will now be examined '

stoichiometry if groups of sites and for various co-existing pairs of the

atoms were actually involved rather following phas_es: plagioclase, calcic

than the single pairs of exchangeable amphiboles, epidote, biotite, white

cations as above (see Mueller, 1969; mica, and chlorite. In general, more

Grover and Orville, 1969; Thompson, pronounced fractionation (that is,KD

1969; Spear, 1980). Regardless of the departs more markedly from unity) is

details bf the element partitioning, favored by lower temperatures.

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chemical range is quite limited. Alkali Na/K Partitioning equilibration between these phases seems

The distributions of Na and K between plausible, but the contrasting

coexisting plagioclase'and calcic parageneses do not exhibit obvious amphiboles are shown in Figure 15. An differences.. in partitioning. Plagioclase apparent close approach to chemical concentrates sodium intensely over biotite

equilibrium is evident, Hprnblende + (KD = 7,600), a reflection of the large I plagioclase pairs from amphibolites interlayer cation site in micas which

show sodium moderately concentrated accommodates K preferentially ovet

in plagioclase relative to hornblende smaller monovalent cations.

(KD = 4.6). In contrast, the partitioning W/K partitioning for white mica + 17. of Na/K is much more pronounced for * amphibole pairs is shown in Figure

feldspars that equilibrated with A close approach to equilibrium is not

actinolite rims in greenschists and apparent for any of the rocks

other low-grade metamorphic rocks (amphibolites, a metagraRite, and a

(K< 45). Surprisingly, the thermally metadiabase dike). However, amphibole

metamorphosed amphibolites and . strongly concentrates sodium over

orthogneiss display similarly strong . potassium compared to the white mica (KD = 0.019), of fractionations--perhaps fortuitously another manifestation the optimum size of the mica interlayer due to the fact that K metasomatism

of the parental rocks may have caused alkali position for potassium. An appar,ent equilibrium partitioning enrichment of 'the hornblendes in K of Na and K between coexisting white relative to plagioclases. mica + biotite is shown in Figure 18. Fractionation of Na and K between Most of the samples are from metasomatically plagioclase and biotite pairs is altered amphibolites and metagranites, illustrated in Figure 16, Mineral but one sample represents an analyzed compositions are systematic, but the \

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.<\ G A'

'6100 0

zU W .

1.0 .-.... 10 100 (Na/K) amp h i bole

.\-__. Figure 15. Fractionation of sodium versus potassium between plagioclase and calcic

amphibole pairs. Symbols as in Figures 8 and 9.

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IOOC I

0.001 8.01 ~ 0.1 biotite

Figure 16. Fractionation of sodium versus potassium between plagioclase and biotite pairs.

Symbols 3s in Figures 8 and 9.

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1.o I I

0

-- /- 0.1 A / / / /

/ / /

/0.01

0.1 1.O 10 (Na/K) amphibole

Figure 17. Fractionation of sodium versus potassium between white mica and' calcic amphibole >- pairs. Symbols as in Figures 8 and 9.

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0 / Q.1 00 S 0 3 @0 n ’/

0’- x 0 0 0’@ 0

0.01

I 1 0*001 0.01 0.1

Figure 18. Fractionation of sodium versus potassium between white mica and biotite pairs.

Symbols- as in, Figares 8 and 9.

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pair from a'metadiabase. A marked and orthogneiss exhibit a m2derately

contrast in fractionation is not evident close approach to systematic behavior,

for assemblages from different rock types with sodium moderately concentrafed

probably reflecting the fact that all in fhe plagioclase relative to the

I. pairs were generated or annealed more potassic hornblendes (K = 6..6). D during the late-stage greenschist Wide scatter is evident for coexisting ,.

facies recrystallization. All show a * pairs from greenschists and related

' moderate elevation .in sodium/potassium lower grade metabasaltic rocks,

ratio2 of white mieas compared to the indicating nonattainment of chemical

associated biotite (KD = 3.4). equilibrium or complex behavior due

to other ions not considered in this Na/Ca Partitioning treatment. A tendency is evident for I Distributions of Na and Ca between Na/Ca ratios of plagioclase to be much

coexisting plagioclase and calcic greater than corresponding values for

amphiboles are shown in Figure 19. actinolite (KD-r 450), .as would be

Because of the involvement,of at least appropriate for lower temperature

two contrasting structural sites in assemblages; however, broad dispersion

amphiboles, Na/Ca fractionation is of the data points does not encourage

expected to be complex. The relationship confidence in the idicated numerical

shown in Figure 19 to some extent value,,o&-KD. What can be concluded

reflects the ratio of Na/Ca in the from the diagram is that amphibole

M(4) site of the analyzed amphiboles accommodates Na less readily than

as a function of the mole fraction of doeas plagioclase--presumably because

albite component in the plagioclase. in actinolite the M(4) site is occupied

Details of such partitioning have' by Ca and,the A structural site is

been discussed by Spear (1980). too large to carry appreciable amounts

Mineralogic pairs from the amphibolites of sodium.

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1oc

+ o? ,

, .’ x /- XI’. / X/ / / / X //

4 .O 0.01 0.1 1.o (Na/Ca)amphibole

Figure 19. Fractionation of sodium versus calcium between plagioclase and calcic amphibole

pairs. Symbols as in Figures 8 and 9.

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between epidote rims sand white mica Fe/Altotal Partitioning i is presented in Figure 21. As'with the

The fractionation of iron (presumed previous diagram, dispersion of the data

to be chiefly ferric) and &tal A1 apparent. Most analyzed pairs .. is are between coexisting plagioclase + epidote from amphibolitGs and orthogneisses, pairs is illustrated in Figure 20. but a single epidote +white mica pair . Chemical relationships are reasonably from a metadiabase exhibits a comparable

systematic when it is*appreciated. that eiement distribution. Because the white

the scatter appareht in the figure is micas are phengitic, the exchange

<,C probably the result of high percentage reaction involves mainly octahedral

' .w errors associated with the very low . sites in both minerals. Althouih the

absolute concentrations of iron in fractionation is much less pronounced

the plagioclase. It is not certain because of the similarity,-of these

that equilibrium has been established crystallographic positions, epidote

or that a contrast in partitioning rims concentrate iron relative to the

typifies the different paegeneses. layer silicate (KD = 2.6).

Nevertheless, it is evident that * . Fe/Mg Partitioning epidote rims s.trongly concentrate Fe

relative to Al compared to associated Iron (principally ferrous) versus

plagioclase (K = 0.0018). This 'magnesium distribution between D phenomenon undoubtedly reflects the coexisting chlorite + hornblende pairs

large size of the octahedral pi(3) is illustrated in Figure 22. Most .

site in epidote which accommodates samples plotted represent retrograde

ferric iron versus the exclusively phases from amphibolites; a single

small .tetrahedral sites available in greenschist pair displays siklar

plagioclase. element behavior. The distrib'ution is total Fractionation of Fe versus Al systematic, but fractionation is

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I I

0.01

' /- / .- .- .a004

0.01 0.1 1.o otal (Fe/Alt Iepidota rims

Figure 20.. Fractionation of (ferric) iron versus tatal aluminum betweea plagioclase and

epidote rim mineral pairs. Symbols as in Figures 8 and 9.

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‘A /’ 0.2 /”‘

x 0 ,i: / / A ;‘ 8 0 e* /’ / 01 / /a / / / / / / ‘0.07 / / 0.05 0.04

0.02 0.03 0.05 0.07 0.1 tOtd) (Fe/AI white mica

Figure 21. Fractionation of (ferric) iron versus total aluminum between epidote rims and

white mica pairs. Symbols as in Figures S and 9.

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/ 1.o. / -- -/ -

01 * 0.2 0.3 0.4 0.5 0.7 1.0 1.5

Figure 22. Fractionation of (ferrous) iron versus magnesium between chlorite and -* hornblende palrs. Symbols as in Figures 8 and’?.

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= nonexistent (KD 1.06). Although a closc fractionation. I' I' approach to chemical dquilibrium seems Fe/Mg partitioning between coexisting

to be indicated, it is possible that chlorite and biotite is shown in

chlorite has simply replaced pre-existing Figure 24. Again, a close approach

hornblende,,. inheriting the Fe/Mg ratio to equilibrium is indicated by the of its mineralogic precursor in the systematic fractionation behavior,

process. regardless of paragenesis. Biotite

The partitioning of Fe and Mg between concentrates iron relative to magnesium

chlorite and actinoliteis presented in compared to chlorite; the fractionation

Figure 23. Again, relationships are is slight (KD = 0.89), however, because

quite systematic, with no evident of the similarity of sixfold coordinated

variation of K D as a function of host-rosk structural sites in both layer mineralogy (amphibolite, greenschist, silicates.

Tertiary metabasalt, or metadiabase). f< Summary of Element-Partitioning Data Here, however, chlorite displays a slight

preference €or iron over magnesium Ftactionations of elements between

relative to the associated actinolite doexisting phases in Suao-Nanao

(KD = 1.41). Because of adherence to amphibolites, orthogneisses, greenschists,

the linear, ion-for-ion exchange reaction and Tertiary metamafic rocks are

and because (in contrast to pre-existing systematic and, in general, suggest

hornblende) the actinolite is a new, a close approach to chemical equilibrium.

low-grade metamorphic phase that grew In many cases, especially for competing

during the chloritization proc&ss, phases that contain similar

equilibrium seems to have beeh attained. crystallographic environments, the

The similarity.of octahedral structural exchange reaction appears to be of the

positions €or these divalent cations is ion-for-ion type. Some early amphibolitic

probaby responsible for the weak and metagranitic fractio%ations are

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I I I I I I

‘. ’ / / /, 1.o

-0 /- i 0.5 / 0- 3 0.4 \ 2 0.3 *\ -f 0.2

- ._ 0.1 0.2 0.3 0.4 0.5 0.7 1.0 1.5 *

I

Figure 23. Fractionation of (ferrous) iron versus magnesium between chlorite and

actinolite pairs. Symbols as in Figures 8 and 9.

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1.O

0.7 / '-+ / / / 0.5 0.4 0.3

0.2

01' I I I I 1. I

Figure 24. Fractionation of (ferrous) iron versus magnesium between chlorite and biotite I pairs. Symbolds as in Figures 8 and 9.

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preserved--especially those involving to brown hornblende k clinopyroxene +

relict(?) hornblende--but in-most cases plagioclase + sphene -f rutile

-. the higher grade, less pronounced compatibilities, whereas others were

partitioning has been overprinted by variably metasomatized and migmatized

the younger greenschist facies to biotite-bearing assemblages.

recrystallization involving more marked Symplectic breakdown of clinozoisite

element fractionations. to plagioclase + quartz also occurred.

(3) Greenschist facies metamorphism PETROLOGIC DISCUSS ION of Pliocene-Pleistocene, and possibly

From the preceding roc& and mineral also latest Cretaceous, age partly

analyses, combined with the observed converted pre-existing amphiboaite

petrographic relationships, several assemblages to albite (Anoo to Ano7) + I conclusions have been drawn regarding epidote (PsI5 to Ps ) + chlorite + , 27 the polymetamorphism of the Suao-Nanao actinolite + granular sphene along

amphibolites and related rocks. fracture zones and at the hinges of

(1) Low-potassium low-Ti tholeiitic later folds, Diabasic dike rocks

gabbros, basalts, and tuffs have been were transformed to massive aggregates

thoroughly deformed and rec-rysfallized of actinolite + quartz + albite +

to strongly foliated amphibolites during chlorite + epidote (PS~~to Ps 26 ) +

Mesozoic medium-grade metamorphism. biotite + granular sphene. These G.s b Typical assemblages consist of green petrologic conclusions are discussed’

hornblende t plagioclase (An 40 to An52) + below in more detail in terms of the clinozoisite + sphene t quartz ? ilmenite; physical.conditions of metamorphism ‘*I,’ no garnet, biotite, or chlorite were of these various stages, the-metamorphic

formed in this stage. (2) During granitic reactions themselves, and comparisons 9 intrusion about 87 m.y. ago, some with other experimental and petrographic

amphibolites were isochemically prograded results.

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where epidote is associated with plagioclase Hornblende-Plagioclase-Clinozoisite and hornblende in rocks of the epidote Assemblages in Amphibolite amphibolite facies, the plagioclase is

Phase relations between greenschist either albite or peristerite. Therefore,

and amphibolite assemblagzs for basaltic in metabasaltic systems, reactions such

bulk-rock compositions have been as (1) albite + chlorite + clinozoisite

extensively investigaFFd both in the -(epidote) + quartz = hornblende + 'T. ,_ ? field and in the laboratory (Cooper, 1972; plagioclase + H 0, and (2) albite + 2 Liou and others, 1974; Kuniyoshi and Liou, actinolite = hornblende + quartz t 0 H2 1976; Best, 1978; Spear, 1980, 1981). have been suggested to explain the

Mineral assemblages such as albite + transformation of epidote amphibolites to

epidote.4 chlorite + actinolite + sphene amphibolite facies assemblages (for example,

(= greenschist facies), albite + epidote Cooper, 1972; Liou and others, 1974).

d;, . (or pistacite) + chlorite + hornblende The amphibolites from the Suao-Nanao

(= epidote amphibolite facies), district invariably contain

hornblende + plagioclase ? garnet hornblende-plagioclase-clinozoisite,

(= amphibolite facies), and plagioclase + with minor sphene and ilemenite. The

actinolite + chlorite (= Ca plagioclase - question is why these mafit amphibolites

actinolite hornfels facies) have been contain clinozoisite instead of garnet

assigned to certain pressure-temperature if they were metamorphosed under

fields (for example, see Turner, 1981; pressure and temperature conditions

Ernst, 1973). Metamorphic reactions of the amphibolite facies.

related to these transformations have also Schematic pressure-temperature

been suggested. For most amphibolites, relations for amphibole, epidote,

'hornblende and plagioclase occur a chlorite, pligioclase, aid almandine

together with garnet rather than with an garnet in the system Ca0-Na20-Al23 0 -

epidote-clinozoisite. On the other hand, (FeO + Mg0)-Si02-H20 are shown in r

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_I

Figure 25. This diagram was constructed plagioclase-bearing assemblages, ,

employing the assumptions that schematically shown as dashed lines (1) both SiO and H 0 are present in 2 2 ... in the figure. Also listed on the excess; (2) plagioclase and garnet are diagram are the mineral assemblages

qf anorthite and alpqnd2ne (+ pyrope) I in each pressure-temperature field for

end-member compositions; (3) chlorite two bulk compositions, X and Y, that lie

has a fixed composition of on the Ca-rich and Ca-poor sides,

(Mg,Fe)5A12Si301,(OH)8 (clinochlore) ; respectively, of the join

(4) all iron in amphibole and chlorite amphibole-anorthitc. The topological is ferrous, whereas epidote contains arrangement-* - of the reactions around exclusively Fe+3;' and (5) only one the invariant point holds as long as

calcic amphibole, with a composition the compositional configuration among

between actinolite Ca2 (Fe,Mg) 5Si8022 (OH) the ftve phases remains as illustrated

4 and hornblende (Ca2(Mg,Fe),A12Si7022(OH)2, in the insert diagram of Fig- 25.

is stable in this pressure-temperature Because of the compositional variability

range (although immiscibility of of phases in nature, because these

tremolite and hornblende has been reactions depend markedly on fo , and 2 2+ suggested--for example, Misch and Rice, because variations of Fe /Mg in 1975). The pressure-temperature slopes chlorite, garnet, and'iamphibole are

for the five univariant curves radiating complex; with iron and magnesium

from an invariant point were estimated behaving as independent components,

on the basis of avaslable experimental the relationship shown schematically

data for dehydration reactions and oh-. in Figure 25 could be significantly

natural mineral parageneses. modified. Nevertheless, this diagram

-SS The introduction of NaAlSi 0 38 not only explains the disposition of component into the plagioclase expands mineral assemblages for basaltic

the stability fields fdr compositions recrystallized under various

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.a .f P

Figure 25. 'Schemati'c,phase relations (solid lines) for assemblages involving anorthite

(An, CaAl Si 0'), epidote (Ep, Xa2( M,Fe I3Si3Ol2(0H)), garnet (Ga, (Fe,Mg)3A12Si3012), 2 28 chlorite.(Ch, (Mg,Fe) Al Si 0 (OH)8), and amphibole 5. 2 3 10 (Am, Ca;(Fe;Mg) Si 0 (OH)2-Ca2(Fe,Mg) 4 A12 Si722 0 (OH)2) in 'plresence "of excEss 'quartz and aqueous 5 8 22 fluid in system CaO*Na20*(M2O3 + Fe.. 23 0 )*(FeO + Elg0)*SiO2'*H20. Effect produced % introduction of albite- component into plagioclase is Schematically.shown as dotted lines. Mineral assemblages for bulk co.mpositions X and Y are also indicated.

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+ metamorphic conditions, it also accounts Physical Conditions for Amphibolite for differences in mineral assemblages ' Facie's Metamorphism qt the same metamorphic grade.

SpeEif iCjilly , the greenschis t assemblage The stability of hornblende and

is skable-q-low temperatures, and the its associated phases for an

-P1 +-.ChL (or Ep) 33actinolite association tholeiite composition has been

occurs as a transition zone between determined recently by Spear (1981); .. greenschist 'and amphibolite assemblages results indicate that the typical SJ at low pressures, whereas the epidote amphibolite assemblage hornblende +

amphibolite facies ik restricted to plagioclase can exist over a wide

relatively high pressures. For range of pressure, tempergture, and f amphibolite'facies conditions, depending conditions. The upper limitjis fo2 on the bulk composition for basaltic marked by the appearance of clinopyroxene

rocks,,clinozoisite + plagioclase + at795 0 C, 2 kb Pfluid, and about

hornblende, plagioclase + hornblende, 800 OC, 4 kb, with fo' defined by the 2 and plagioclase + hornblende + garnet QFM buffer. The lower thermal stgbility

are the principal assemblages. Apparently 'limit determined by Liou and others

th'e clinozoisite + plagioclase + (1974) is defined by the growth of,

hornblende asgociation, which commonly chlorite at the expense of hornblende +

occurs in amphibolites from-the plagioclase at 550 '8, 2 kb Pfluid,

Suao-Nanao region, is a rather atypical and QFM buffer; the amphi6olitic

amphibolite assemblage but f's favored assemblage is entirely replaced by

in metabasaltic rocks with slightly greenschist mineral compatibilities

higher Ca contents than normal at 475 OC. Within this wide temperature

tholeiites. range of 550 to 790 OC, Spear (1981)

further subdivided the amphibolite

assemblage depending on the occurrence

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or #onoccurrence of sphene. The upper -temperature-pressure relations are

_th'ermal stability limit of sphene-bearing dependent on the compositional variability

amphibolite for the tholeiitic bulk of epidote, garnet, and plagioclase. This

0 composition was located at 575 C, 2 kb, reaction separaty the epidote amphibolite fj and about 730 OC, 5 kb Pfluidfior the assemblage at hT& pressure and low

oxidation state provided by the OF'M buffer temperature from the amphibolite I'

Sphene-bearing amphibolite is favored assemhiage garnet + hornblende + plagioclase

by low temperature, high pressure, at lower pressure and higher temperature.

and high fo , whereas ilmenite or Where fo diminishes from values 2 2 hematite is more common in higher appropriate to the HM buffer down to

grade amphibolite assemblages. These those of QFM buffer, this reaction

experimentally determined stabilities, would be significantly displaced

as well as ";he ch2racteristic mineral toward .lowe; temperature reflecting

, ,. I assemblage for the olivine tholeiite introduction of more pyralspite component

composition, are "~'inFigure 26. into the garnet. Indeed,'Liou and

Also shown in Figure 26 are others (1974) experimentally concluded

experimentally determined that albite + epidote becomes unstable .

pressure-temperature stabilities of at T = 475 OC, 2 kb, and QFM buffer for

zoisite + quartz (Newton, 1966), basaltic bulk compositions. ,epidote (Ps ) + quartz at the HM buffer 33 The attending pressure for (Liou, 1973), and albite + epidote .-. amphibolite facies metamorphism in

(Ps >" + quartz at the HM buffer (Best, the Suao-Nanao area is difficult 33 I

1978) [see also epidote (Ps ) + quartz , 25 to determine. However, on the basis at the QF'M buffer (Liou, 1973)]. The of pressure of aluminous green

reaction'albite + epidote (Ps ) + hoinblende intermediate in 33 quartz = plagioclase (An ?) + grandite + composition among barroisite, 30 H 0 was suggested to be trivariant because 2 pargasite, and.actinolite (Ernst, 1979)

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Figure 26. Pfluid$ diagram showing stability relations of mineral

assembla' es for various metamorphic facies ip system of basaltic composition.

Experimentally7 determined stabilities are shown for pumpellyite (Pm)

(Schiffman and Liou, 1980), prehnite (Pr) (Liou, 1971), zoisite (Zo) + quartz

(Qz) .(Newton, 19661, albite (Ab) + epidote (Ps~~)(Ep) + quartz (Best, 1978),

epidote .(Ps ) + quartz (Liou, 1973), chlorite-decreasing and chlorite-out 33 (Liou and others, 1974), sphene-out and clinopyroxene-in for basaltic I

compositions (Spear, 1981). Basaltic assemblages f quartz for various

-T conditions are also shown. Act = actinolite; Ch = chlorite; 'fluid L Sp = sphene; Cz = clinozoisite; Gr = grossular; An = anorthite; P1 = plagioclase;

Hb = hornblende; HM = hematite-magnetite; I1 = ilmenite; Cpx = clinopyroxene.

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.I 1c I I I I I I

300 400 500 600 700 800 Temperature & OC

Figure 26.

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that of the invading granitic magma suggestion, although the bulk-rock

(for example,see Luth 2nd others, 1964), compositipn plays a significant role

lithostatic ("fluid) -pressures may be conceqning the absence or presence of

estimated as'approcaching 5 kb for a clinozoisite in the amphibolite, as

temperature range of, say, 550 eo 700 OC. discussed in the previous section.

Suunferred conditions would be The rare occurrence of quartz and the

consistent with the hypothesis that the lack of hematite suggest that the

amphibolite in this area is the oldest metamorphism took place at oxygen

(and probably the most deeply bu;ied) fugaclties lower than those defined

bagement rock in Taiwan. by the HM buffer (see Spear, 1981, for

Amphibolites from the study area are the role of fo regarding modal quartz 2 characterized by hornblende + in amphibolite).

plagioclase + clinozoisite + sphene Thermal Metamorphism and Metasomatism

ilmenite; quartz is less common, and I

hematite and garnet were not found. The apparent thermal and chemical

The ubiquitous occurrence of sphene in effects of granitic intrusion on the

the amphibolite assemblage indicates amphibolite assemblages, as discussed

that pressurg-temperature conditions of in previous sections, include (1)

recrystallizat ion were bounded transformation of green hornblende to

'by the sphene-out reaction at high brown hornblende, (2) crystallization .

temperature and the chlorite-out of clinopyroxene at the expense-of

reaction at low temperature. In other hornblende, (3) conversion of

words, the amphibolite facies metamorphism clinozoisite to symplectic intergrowths

probably took place between about of plagioclase + quartz, and (4)

550 and 700 OC and at values of P formation of biotite and whLte mica + fluid ., f on the order of 5 kb. The presence of orthoclase in the metasomatized

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amphibolites. governed by reactions of the sbrt

The appearance of clinopyroxepe, as -hornblendel = hornblende + ..'. 2 . \ shown in Figure 26 sets the maximum plagioclase k clinopyroxene + H 2 0, ,- temperature for the thermal metamorphic where product hornblende2 is more

event at about 800 OC assuming pargasitic and Ti-rich than reactant

- = 5 kb and fo defined hornblende The clinopyroxene-bearing 'fluid 'total 9 1' L by the QFM buffer (Spear, 1981). At amphibolite T-8B contains higher modal

f higher than QFM, clinopyroxene percentages of calcic plagioclase. O2 appears in metabasaltic bdlk compositions (30%) and quartz (15%), has a trace

at lower temperatures (for example, of rutile, and lacks sphene; plagioclase

710 OC, 1 kb, and HM buf'fer). It is slightly more cilcic (Ans2 to An57)

should be emphasized that the than those in the original amphibolites.

experimental studies were carried out Therefore,a reaction similar to that

- therefore, Figure 26 proposed by Spear, such as hornblendel at 'H,O - 'tot.4 + shows'-the maximum thermal stabilities for plagioclasel + sphene = hornblende2 +

hydrous minerals, For instance, at plagioclase clinopyroxene quartz 2 + + + PH20 - 0.5 Ptotal, the appearance of rutile + H20 may have occurred during

clinopyroxene in the amphibol i t e apparently isochemical thermal

assemblage was calculated to occur metamorphism. The observed compositional

at about 700 OC for a fluid pressure of variation from green to brown hornblende

about 2.5 kb (QFM buffer)& Therefore, reflecting increase of Al, Na, and Ti

it is reasonable to suggest that the and decrease of Si in the amphibole

maximum temperature for thermal are consistent with recent experimental

metamorphism of some amphibolites in the studies (Spear, 1981).

Suao-Nanao area may have approached Other metamafic units, particularly

700 'C. Spear (1981) saggested that the those amphibolitic inclusions in the

appearance of clinopyroxene is orthogneiss, were subjected to

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? K metasomatism during the Cretaceous others, 1979), tfiese values tend to

L: granitic intrusion, The physical be slightly lower tha%,temperature

conditions for metasomatism should estimates for the same exchange reaction

have involved temperatures at least as as calibrated by Thompson (1976) high as those attending the isochemical and by Ferry and’Spear (1978). It

thermal metamorphism of the wallk-ock should be noted that both garnet

amphibolites discussed above. Many and biotite in these rocks have been

,”me tasoma t ic amphiboli tes contain b io t i t e , partly chloritized, and some Ti-poog

’ muscovite, and a trace of K-feldspar, I biotite may have ‘been chemically

together with hornblende, sodic modified during the subsequent

plagioclase, and very minor garnet. greenschist facies metamorphism,

Assuming garnet and biotite formed at Acccordingly, using different biotite

equilibriumk during this stage, and compositions from the same rock

using the equation of Goldman and Albee (for example, T-12G-1), the Fe-Mg

. . (1977, p. 760) for the biotite-garnet distribution of garnettbio ti te yields

geothermometer, the garnet-biotite a temperature of recrystpllization of

pair in the orthogneiss sample T-12E 410,OC. For another orthogneiss

yields an apparent temperature of (T-342), the calculated apparent

0 crystallization of 711 ,C and the temperature is 450 0C. These much

metasomatized amphibolite T-12G-1 lower temperatures ref lectv kither

0 a temperature of 675 C. These greenschist facies metamorphism-for

calculated temperatures based on the these rocks or nonequilibration between

element partitioning betwe& biotite and garnet and Ti-poor biotite.

garnet are consistent with those One of the characteristic features

derived from experimental phase of the metasomatized amphibolites is

equilibria. As pointed out by the transformation of hornblende to

several authors, (for example, ‘Ghent and biotite. Depending on the extent of

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the alteration, some hornblendes have consumed during this transformation

been partially replaced by biotite (see point 1); and (4) the metasomatized

along fractures and grain margins, amphiboutes are higher in K20, Na20,

whereas others evidently were totally A1203, and SiO2 and lower in MgO, FeO,

biotitized; the biotitefhornblende CaO, and Ti02 than the original

ratio increases toward the margins of amphibolites. These textural,

the amphibolite inclusions in the minefalogical, and chemical variations I orthogneiss, and ultimately a thin Suggest that the amphibolite may have

biotite-rich zone is seen to have exchanged material with invading granitic

developed as the end product of such magma according to the following

a chemical alteration. Accompanying reaction: hornblende + plagioclase + this transformation are the follow'ing (An4o to An ) + sphene + K+ + Na + 52 features: (1) sphene becomes less Al 0 + SiO + H20 (granitic melt) = 23 2 common, quartz becomes increasingly biotite + muscovite + plagioclase

abundant, and traces of white mica, (Anl5 to An 30 ) + quartz + K-feldspar + garnet, and K-feldspar appear in FkO_-+ MgO + CaO + Ti02.. The FeO, MgO,

the biotite-rich amphibolite; (2) Tib2, and CaO released from the reaction

plagioclase becomes more sodic and may have been incorporated in the

changes composition from An 40 to An52 liquid phase o.r-mayhave promoted the in original amphibolite to Anl6 to An30 crystallization of additional mafic in the biotite-bearing amphibolite material (amphibold?). Depending

(moreover, some plagioclases were on the extent of transformation,

replaced by clinozoisite together with the disposal of such removed constituents

quartz); (3) the Ti02 content of may have significantly affected the

biotite is about 1 to 2 wt % higher composition of granitic melt. In support

than that in both brown and green of this hypothesis., granitic rocks

hornblendes, and sphene may be adjacent to the amphibolite body vary

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extensively in mineral content and in incipient instability of clinozoisite

bulk chemistry, as shown in Table 3. during the granitic intrusion.

In a single specimen, symplectite Clinozoisite Symplectic Intergrowths occurs uniformly in the rock and is

Transformation of coarse-grained i in sharp contact with adjacent hornblende clinozoisite to a symplectic intergrowth grains; intergrowths clearly parallel

of plagioclase + quartb + clinozoisite the main foliation and pseudomorph

is another characteristic feature of the original clinozoisite. The

many thermally metamorphosed amphibolites. mineralogic texture and undisturbed

As shown in Figure 7, the fine-grained fabric suggest that formation of I vermicular plagioclase + quartz is symplectite occurred essentially

included within the host clinozoisite isochemically at nearly constant volume.

as symplectite. In amphibolites Similar intergrowths of epidote +

adjacent to the contact with the quartz (+ actinolite) have been

orthogneiss, nearly all clinozoisite described by Rivalenti and Rossi

grains show such textures, and the (1972) in amphibolite inclusions in

I vermicular inclusions .of quartz and Precambrian gneiss from southwest

plagioclase become coarser in grain .- Greenland. . According to them,

size and denser in concentration: in quartz-epidote and quartz-actinolite

contrast, for amphibb3ites some distance symplectites always occur along grain

away from the contact, symplectic texture boundaries between plagioclase and

is sparse and has developed in some hornblende, reflecting a reaction such

amphibolites where green hornblende as plagioclase + hornblende = epidote +

still prevails. In metasomatized quartz + actinolite. This rea'ction

amphibolites, such intergrowths and occurs as "a consequence of the a clinozoisites were not found. Apparently, different environmental conditions

the formation of symplectite is due to and Pco higher in the migmatitic 2

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environment), which causes an instability th'erefore, breakdown of clinozoisite

between calcic plagioclase and to cakic plagioclase (+ quartz)

hornblende" (Rivalenti and Rossi, 1972, occurred. The reaction may not

p. 55). have run to' completidp partly because

However, the spatial relation of for small intrusions, high temperature

symplectic texture with plagioclase and may not have prevailed long enough

hornblende was not found in the and possibly because of the metastable

investigated amphibolites of the persistence of clinozoisite at

Suao-Nanao area. In some samples 10 to 20 OC higher than its stablility

(for example, Fig. 7, E) , symplectite limit (this phenomenon is commonly

6 is not concentrate5along contacts observed in laboratory studies;

between plagioclase and hornblende. f_or example, Liou, 1973).

Typically, many such intergrowths Greenschist Facies Metamorphism occur within pre-existing clinozoisite

crystals where plagioclase is not Only rare occurrences of the

1- ;- spatially associated. Apparently, greenschist assemblage albite +

the formation of symplectite in the actinolite + chloritq + epidote +

clinozoisite is due to clinozoisite quartz + granular sphene were found

instability during the thermal in the amphibolites. These associations

metamorphism. As shown in Figure 26, are best developed along fracture

clinozoisite is stable up to about zones and along hinges of later folds.

0 , 650 C at PH = = 5 kb and the However, the Tertiary rocks of basaltic 'total 2 QFM buffer. Attending intrusion of small composition, including dike rocks and

granitic bodies, some amphibolites may lavas within the cover series, are

have been subjected to recrystallization thoroughly recrystallized to fine-grained

at temperatures up to 670 to 700 0C, as aggregates of actinolite + albite +

discussed in the previous section; epidote + quartz + chlorite 2 biotite +

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~~~

sphene. Experimentally determined chlorite assemblage may appear at

pressure and temperature conditions for 310 OC and 5 kb Pfluid(Schiffman and

the greenschist.assemblagesfor Liou, 1980). On the basis of the

metabasaltic compositions are i%dicated experimentally determined phase

in Figure 26. relations for basaltic compositions

..-Hydrothermalmineralization accompanied shown in Figure 26, together with

or followed greenschist facies the effects of compositional variability

metamorphism in the Suao-Nanao area. ratio for the and lower Pfluid lP total Iron sulfides (chalcopyrite, pyrite, natural assemblages, it seems

sphalerite, and pyrrhotite) were reasonable to conclude that the

produced along fault and fracture amphibolite, the orthogneiss, and the

zones at the contact between amphibolite basaltic rocks from the Suao-Nanao

and graphite-bearing pelitic schist area were subjected to 'greenschist

(locality T-7). On the basis of the facies recrystallization at 350 to

compositions of sphalerite, these late 475 OC and P-total of qo more than

hydrothermal veins were estimated by about 5 kb.

0 Huang (1979) to have formed at 310 C The nearly ubiquitous occurrence and 5 kb Plead.,. The crystallization of biotite in the metamorphosed of sulfide ores along fractures may have diabasic dike rocks and Tertiary

occurred at a very low P metabasaltic rocks of the cover series f1 uid" 1i thos t a t ic ratio (1/3), whereas the greenschist is characteristic of the upper

facies metamorphism probably to'ok place greenschist facies. The moderate

ratios. at higher PfluidlP lithostatic K20 content of these rocks

Therefore, the estimated 310 OC may be (0.3 to 1.6 wt %) could account for

about 50 OC' too low for greenschist its presence, inasmuch as biotite

facies metamorphism inasmuch as the appears at lower grades in metabasalts

(unobserved) pumpellyite i- actinolite -I- than in the dominant Suao-Nanao pelitic

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and quartzoLrldspathic schists (Cooper, The lithologic precursor is unknown,

1972). due to the total obliteration of original

textural features accompanying the TECTONIC IMPLICATIONS amphibol i t e fac ies recrystall izat ion

Fewdefiniteconclusions may be reached and penetrative deformation. Judging

regarding the large-scale structural from the tholeiitic bulk chemistry of

significance of the Suao-Nanao . the amphibolites (Fig. 8) and from the

amphibolites and related rocks in the association with ultra6zfic lenses,

tectonic development of northeastern the protolith was oceanic in its

1 Taiwan. Several relationships seem affinities. Marble, quartz schist

reasonably clear, however. Insofar (metachert?), and pelitic schist may

as appropriate, this scenario now to represent chemical and fine clastic

be presented applies simply to the sediments deposited on this oceanic -._ basement of the Central Range; inferred crust, The original basalt +

plate motions are only indirectly gabbro + minor peridotite and

related to the now-active region located associated metasedimentary units were

in easternmost Taiwan, which is subjected to amphibolite facies

responding to complex interactions metamorphism at temperatur$s approaching

among the Philippine, Pacific, and . 650 OC. An Al-rich green hornblende

Asiatic lithospheric plates (Chai, 1972; characterized this stage of amphibolite

Bowen and others, 1978; Hamilton, 1979; generation.

Suppe and others, 1981). Later calc-alkaline plutonism

The amphibolites represent a caused thermal upgrading, converting

remobilized basement terrane and, the lower-rank amphibolites to brown

along with the associated metasedimentary hornblende 2 clinopyroxene - bearing

strata, constitute the highest grade garnet amphibolites about 87 m.y. ago.

and perhaps .the oldest rocks in Taiwan. K metasomatism induced by granitic

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intrusion resulted in the widespread convergent plate motion. Uplift and

production of biotite in the metamafic cooling of the basement, early Tertiary

rocks. Temperatures evidently reached rifting of the Asiatic continental

about 700 OC, judging from the incipient margin, and formati6n of the South

breakdown of 6pPdote-clinozoisite in China Sea we&-followed by partial

the amphibolites and the rare production consumption and impaction of the Chinese

of clinopyroxene. Clearly,- the lithologic margin (including the Suao-Nanao complex) 7 assemblage and the thermal regime with the western edge of the Luzon

recorded by the rocks testify to& . arc (Philippine Sea plate) in

island-arc environment during Late Pliocene-Pleistocene time. This

Cretaceous time. The subsequent history movement has continued south of Taiwan

involved latest Cretaceous - earliest to the present.

Tertiary erosion, exclusively clastic ACKNOWLEDGMENTS deposition, and minor mafic vol'canism

of high-K basalts, followed by This paper presents research

Pliocene-Pleistocene metamorphism. accomplished during the\tenure of a

Thus, we may speculate that early(?)' - United States-Republic of Cpina

Mesozoic sea-floor spreading generated scientific 'cooperative project, supported

the Suao-Nanao basaltic + ultramafic by National Science Foundation Grant

protolith; this unit, overlain by deep FAR 77-23533. Facilities at Stanford

marine gtrata, was transported to and University and the University of

sequestered at the Asiatic continental California, Los Angeles provided bases

margin, accompanied by an for the laboratory and analytical studies.

intermediate-pressure recrystallization The Mining Research,and Service '

event prior to or during Cretaceous time, Organization of Taiwan and the National

Late Cretaceous calc-alkaline plutonism Taiwan University supported the field

and thermal metamorphism attended marked investigations. Liou received funding

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during 1978-1979 from the JohnSimon Best, N. F., 1978, Stability of the \ ' Guggenheim Memorial Foundation, and assemblage epidote-albite-quartz: , partial support, from National Science Progress in Experimental Petrology,

Foundation &ant EAR 79-09138 for 1975-78, p. 156-159.

preparation of the manuscript and Bowen, C. , Lu, R. S., Lee, C. S., 'and * .. for microprobe charges at Stanford. Schouten, H., 1978, Plate convergence

We thank the institutions named above and accretion in Taiwan-Luion region:

. for support and our Chinese colleagues American Association of Petroleum

C. S. Ho, T. P. Yen, C. Y. Lan, T. T.- Geologists Bulletin, v. 62, p, 1645-1672. Feng, Y. Wang, C.. W. Lee, and C. Y. Meng Chai, B.H.T., 1972, Structural and

for informative discussions and tectonic evolution of Taiwan: American

encouragement. The manuscript was Journal of Science, v. 272, p.389-422.

reviewed and materially improved by Chen, J. C., 1977, of

John Suppe, E. D. Ghent, and metamorphic rocks from eastern Taiwan: '

A. Miyashiro. Acta Geologica Taiwanica, no. 19,

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MANUSCRIPT ~ECEIVED BY THE SOCIETY

APRIL 21, 1980

REVISED MANUSCRIPT RECEIVED OCTOBER 6, 198

MANUSCRIPT ACCEPTED OCTOBER 31; 1980

Printed in U.S.A.

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