Geology and petrology of som'e polymetamorphosed amphibolites and associated rocks in northeastern Taiwan
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.
schist, marble, gneiss, amphibolite, 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 + plagioclase lower Cenozoic formations. Some dikes I
(An4o to An52), + epidote (Ps4 to Ps 15 ) + may be as young as Miocene. The whole
c sphene 2 rutile ? quartz was intruded complex--basement, dike, 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 Pliocene-Pleistocene 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, actinolite + 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 amphiboles 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 Cretaceous 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
amphibole, 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) mafic 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 Greenschist 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 Philippine Sea plate 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 terrane 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 slate 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 schists, and (5) Tertiary cover rocks
(slates + 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. 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 Ichimura, 1944; Fuh, 1963), mineral' basement rocks is eypased extensively . 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 (gneisses 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 -. serpentinites. This assemblage of foliated. Most of the schistosity 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 616 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 greenschists 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 mica, 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 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 617 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 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 618 feldspathized. Many irregular and dike are 87 t 5 m.y. (Yen and Rosenblurn, lenticular pegmatite 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 Eocene-Oligocene 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 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 619 gouges within brecciated amphibolites, to those described above. However, presence of retrograde talc-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 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 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 6.2 0 - 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 ' granite; 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. 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 Figure 2. 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 , 622 .I C Figure 2. (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 623 the included amphibolites contain both amphibolite and serpentinite 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 diabase. Some of exist over a disl-ance of 0.5 m; (2) both these dike's'have been.-. described previously 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 624 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. . 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 627 ? Figure 4. 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 628 .. C D Figure 4. (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 629 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 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 - 631 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’ , 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 632 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. 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 n I 0.2r ..a I .c * E --'_'. 17 Figure 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 636 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 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 638 . ., TABLE 1. BULK ROCK COMPOSITIONS AND FlINERAL ASSEMBLAGES OF AMPHIBOLITES FROM NORTHEASTERN TAIWAN 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 . 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 639 TABLE 1. (continued) 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 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 interference coldrs and have been clinozoisite + quartz +'plagioclase, 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 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 642 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 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 - 614 TABLE 2. BULK-ROCK COMPOSITIONS AND MINERAL A~SEMBLAGES OF THE GREENSCHIST FACIES ROCKS FROM NORTHEASTERN TAIWAN - ~ ~~ ~~ 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. 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 615 TABLE 2. (continued) 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 / - ___- 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 646 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 < 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 647 TABLE 3. BULK ROCK COMPOSITIONS AND EINERAL ASSEMBLAGES OF THE GRANITIC ROCKS FROM NORTHEASTERN TAIWAN 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. 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 648 - 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 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 650 (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 Plagioclases 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 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 651 TABLE 4. COMPOSITIONAL RANGES GIOCLASES IN AMPHIBOLITES 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. 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 652 TABLE 4. (contime$ 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 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 653 TXBLE 5. CHEMICAL COMPOSITION OF PLAGIOCLASES IN THE GREENSCHISTS FROM NORTHEASTERN TAIWAN - T-41A* T- 12C* T-334A* T-334C" T-336* 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. 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 654 TABLE 5. (continued) - 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 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 6 55, TABLE 5. (continued) ~~ 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 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 656 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. 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 657 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 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 658 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, 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 659 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 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 668 TABLE 6. CHEMICAL ANALYSES OF AMPHIBOLES IN .4MPHIBOLITES FROM NORTHEASTERN TAIWAN c 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. 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 _-661 TABLE 6. (continued) 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 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 662 TABLE 6. (continued) . 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 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 663 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. 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 664 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 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 1.20 I r\ I I I I I \\ 0 \ \ \ .* \ U \ A \ \ +- 1.00 -0 \ S. 0 3 '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. 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 667 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 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 668 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 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 669 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 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 67b .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 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 6.7 1 TABLE 8. CHEMICAL ANALYSIS OF EPIDOTES IN AMPHIBOLITES FROM NORTHEASTERN TAIWAN 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 . 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 672 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 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 673 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_--- 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 674 TABLE 9. CHEMICAL COMPOSITION OF EPIDOTES IN THE GREENSCHISTS FROM NORTHEASTERN TAIWAN 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. 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 6% TABLE 9. (continukd) 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 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 676 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 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 677 I , 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 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 678 ~~ 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 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 679 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'. 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 680 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 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 681 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$h Fef3/(Fe+' + Al) ratio, and they metamorphism; none of it appears to be are relatively uniform in composition; a stable associate of hornbwnde or Ps vdlues range from 20 to 26, similar calcic plagioclase, although it does Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/92/5_Part_II/609/3429806/i0016-7606-92-5-609.pdf by guest on 01 October 2021 682 \ \ ?\ \ \ \ x\ \ \ \ \ \X* x\\ x\x '\+ h \ \ \ \ \ \ \ 'A 'A \ \ \ \ 2.0 2.2 2.4 2.6 218 3.0 A1"'atoms per formula unit Figure 12. Proportions of octahedrally coordinated ferric iron and aluminum in epidotes ofgreenschist,diaba$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 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 684 TABLE 10. CHEMICAL COMPOSITION OF CHLORITES IN AMPHIBOLITES FROM NORTHEASTERN TAIWAN 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. > 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 685 TABLE 10. (continued) 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 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 68G TABLE 11. CHEMICAL COMPOSITION OF CHLORITES IN THE GREENSCHISTS FROM NORTHEASTERN TAIWAN 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. 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 TABLE 11. (continued) , 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 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 688 .-t 5 8.0 / / / / / -U talc-chlorite,/ . . t/ / I (a) i / .b10 + b a ~6.0 ’ 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.‘ 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 El 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) 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 690 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 micas 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 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 691 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. 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 692 TABLE 12. CHEMICAL COMPOSITION OF WHITE MICAS IN AMPHIBOLTTES FROM NORTHEASTERN TAIWAN 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 . 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 6 9 3.. 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 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 694 TABLE 13. CHEMICAL COMPOSITION OF WHITEMICAS 'IN THE G~ENSCHISTSAND GRANITIC ROC? FROM NORTHEAST TAIWAN 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 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 696 ~ 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, 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 ' 697 TABLE 14. 'CHEMICAL COMPOSITION OF BIOTITES IN AMP'HIBOLITES AND GREENSCHISTS FROM NORTHEASTERN TAIWAN 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. 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 698 TABLE 14. (continued) 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 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 699 TABLE 14. (continued) c 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 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 700 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 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 703 TABLE 15. CHEMICAL COMPOSITION OF GARNET AND PYROXENE IN THE AMPHIBOLITE AND GRANITIC ROCKS FROM NORTHEASTERN TAIWAN 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. 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 704 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, 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 705 TABLE 16. CHEMICAL COMPOSITION OF SPHENES IN THE AMPHIBOLITES, GREENSCHISTS AND GRANITIC ROCKS FROM NORTHEASTERN TAIWAN 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 . 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 - as well as moderate amounts of U203 magnetite,, And thrge rutiles; chemical ' 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; 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 '707 TAFU 17. CHEMICAL COMPOSITION OF LAUMOVITE, ILMENITE, MAGNETITE, AND RUTILE IN THE AMPHIBOLITE, GREENSCHIST, kND GRANITIC ROCKS FROM NORTHEASTERN TAIWAN '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 . 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 708 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. 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 709 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 \ 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 1000 .<\ 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. 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 711 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. 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 712 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. 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 713 1.6 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. 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 714 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. 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 715 - 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. 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 716 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 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 '717 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. 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 718 0.4 - I -I I I I I 0.2 ‘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. 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 719 1.5 / 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’?. 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 720 = 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 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 721 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. 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 722 I .5 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. 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 723 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. 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 724 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 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 725 _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 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 726 .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. 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 727 + 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 olivine -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 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 728 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) 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 729 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. 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 730 .I 1c I I I I I I 300 400 500 600 700 800 Temperature & OC Figure 26. 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 b and an inferred temperature less than clinozoisite is consistent with this 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 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 732 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 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 733 ? 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 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 734 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 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 735 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 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 -736 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 + 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 737 ~~~ 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 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 738 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 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 733 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 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 740 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, Geochemistry of John Suppe, E. D. Ghent, and metamorphic rocks from eastern Taiwan: ' A. Miyashiro. 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