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Phase relations of pumpellyite-actinolite facies metabasites in the Sanbagawa metamorphic belt in central Shikoku, Japan
P]hase relations of pumpellyite-actinolite facies metabasites in. the Sanbagawa metamorphic belt in central Shikoku, Japan TAKASHi NAKAJIMA
HTHOS
Nakajima, T. 1982 10 15: Phase relations of pumpellyite--actinolite facies metabasites in the Sanbagawa metamorphic belt in central Shikoku, Japan. Lithos, Vol. 15, pp. 267-280. Oslo. ISSN 0024-4937. Sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpcUyitc-actinolite facies were examined in terms of phase equilibria in the five component system AI203-FczO3-CaOEp __ MgO-FeO. The Fe 3+ content of epidote measured as Xrc ( - Fe/(Fe + AI) of epidote) ir~ the assemblage epidote-chlorite-actinolite-pumpellyite decreases gradually towards the higher-grade, pumpellyite-free areas. The progressive change in X~ep can be detected within one metabasite bed 200 metel~ thick near the upper limit of the pumpellyite-actinolite facies. The Mg--Fe2+ substitution, as expressed by variation of Fe/(Fe + Mg) in chlorite (0.40-0.55) has little effect on the Fe3+/(Fe3+ + AI) ratios of epidote and pumpellyite in the above-mentioned assemblage. The lowest X[ p in the pumpellyite-bearing assemblage is 0.15 and hence the upper limit of the pumpellyite--actinolite facies is defined by the appearance of an Ep_ epidote--chlorite-actinolite assemblage with Xr~C.15.
Takashi Nakafima, Department of Earth Sciences, Nagoya University, Chiltusa-ku. Nagoya 464, Jepan. Present address: Geological Survey of Japan, Chugoku Office Hiroshima Godo-Chosha. 6-30. KamiHacchobori, Naka-ku, Hiroshima 730, Japan; November l~Sl.
The mapping of metamorphic zones in the Sanbagawa belt of central Shikoku has been done by Banno (1964), Ernst et al. (1970), Kurata & Banno (1974), Higashino (1975) and others. The most re,:e.nt compilation is by Banno et al. (19713), ia which the belt is divided into chlorite, garnet ($~arnet--chlorite) and biotite (biotite-gar: net-chloidte) zones. Nakajima et al. (1977) have shown that the chlorite zone can be subdivided. into a pumpellyite zone and a pumpellyite-free zone. The writer surveyed the Shirataki-Asemi area (Fig. l) in order to define the high temperature limit of the pumpellyite zone more clearly in the Sanbagawa belt. A metabasite bed about 200 meters thick and including abundant pumpellyite'bearing schi~:ts was studied in detail. This pumpeUyite-bearing bed belongs to the Main member of the Minawa Formation (Fig. 1). The purpose of this paper is (1) to examine the effect of the Mg-Fe 2+ substitution on the epidote-chk~rite-actinolite-pumpellyite equilibria (Nakajima et al. 1977), and (2) to recognize temperature variations within the pumpe!lyiteactinolite facies zone in order to refine the thermal structure of the lower-grade terrain of "t | | t". , Sanbagawa belt in central Shikoku. We will consider this pumpellyite-bearing bed to be the northern limit of the pumpellyite-a~.~inolite facies until detailed discussions are pre-
sented in a later section. The northern boundary of this bed thus separates the pumpellyite zone and the pumpelJyite-free zone. The latter corresponds to the uppermost chlorite zone and the garnet zone.
Paragenesis The size of equilibration domains in low-grade metamorphic rocks is generally considered ~o be smaller than a thin section. Different sizes of equilibration domain have been assigned by different investigators. Watanabe (1974) suggested that only phases coexisting in direct contact should be regarded as in equilibrium. Zen (1974) suggested that the equilibration domain is as large as 1 mm in diiameter. The metabasites examined in this study are completely recrystallized and seem to be relatively homogeneous in thin section, probably due to being derived mainly from reworked hyaloclastite. In this paper, the coexisting minerals within each thin section are assumed to be in equilibrium as a first approximation, unless evidence to the contrary (e.g. textural relations) is recognized. T h e S,~irataki-Asemi area covers the lowergrade region of the Shirataki and the Shiragayama areas investigated by Ernst et al. (1970)
268 Takashi Nakajima
L r r H o s 15 09,~2)
F-']
...................
ALLUVIUM
...........
IZUMI F.
............
OJO1N F.
Cretaceous)
Upper member ~]
L o w e r member
• MINAWA F.
Metagabbro
SANBAGAWA BE~
Ultramafic r o c k s ........... [l~
--
"- ....
KOBOKE F. KAWAGUCHI F.
5ANBAGAWA SOUTHERN M A R G I N A L BELT ......... ................
\
...,."
.....................
M I K A B U GREEN ROCKS CHICHIBU
BELT
FA~'.,.T
Fig. 1. Geologic sketch map of the central Shikoku area.
and Higashino (1975), respectively. Basic rocks are uncommon in the lower-grade part of the chlorite zone but are abundant in its highergrade part and in :ihe garnet and biotite zones. The constituent m;,nerals are as follows: quartz, albite, chlorite, epidote, amphiboles (actinolite, alkali amphibole, hornblende), pumpellyite, phengitic mica, g~rnet, sphene, calcite, apatite, hematite, pyrite, pyrrhotite and chalcopyrite. Mineral assemblages of the samples from the Shirataki-.Asemi .~rea are listed in Table 1. The amphiboles are of the actinolite-mhgnesioriebeckite series in tFe chlorite zone and the lowergrade part of the garnct zone. Garnet-bearing assemblages will not be discussed here because they are quite unccmmon.
Table 1 Mineral assemblages of basic rocks in the ShiratakiAsemi area.
(1) cpidote + chlorite + amphibole + hematite (2) epidote + chlorite -;- amphibole (3) epidote + chlorite + amphibole + pumpellyite (4) epidote + chlorite + amphibole + hematite + calcite (5) epidote + chlorite + amphibole + calcite (6) epidote + chlorite + amphibole + garnet (7) epidote + chlorite + amphibole + hematite + garnet All assemblages also contain: quartz + albite + sphene + phengitic mica + apatite
Mineralogy Analytical method Chemical analyses of minerals were carried out using a JEOL JXA-5A 3-channel electron-probe microanalyzer following Nakamura & Kt~,shiro (1970). Representative analyses of minerals are listed in 'Fable 2. Since tiny mineral grains and the fine chemical structure of minerals had to be examined in this study (e.g. Fig. 4), a calibration method was adopted in the analysis of epidote and chlorite to avoid possible errors arising from a slight shift of analytical point during resetting. Partial analysis of AI, Fe and Mg was done with the fixed analyzer crystals RAP, PET and RAP, respectively. The accelerating voltage and the beam current were kept at 15 kV and 0.015 I~A. When analyzing epidote, Mg was used as a monitor to check for contamination by tiny flakes or fragments of chlorite and amphibole. The calibration curves for epidote and chlorite were based on 67 and 75 full analyses respectively (Fig. 3 (a), (b)). The discrepancies between the values obtained from the calibration curves and the full analysis are below 0.5% in X ~ (= Fe3+/ (Fe3+ + AI) in epidote; total Fe = Fe 3+) and X ~ (= FEZ+/ (Fe 2+ +Mg) in chlorite; total F e = F e 2+) (Table 2). These errors are small enough to ignore in the following discussions.
Individual minerals Epidote: The grain size and the modal amount of epidote varies from rock to rock, depending on bulk rock chemistry and metamo:rphic grade. The mode of occurrence of epidote is classified into two types; one is large aggregates (some-
FS-10
~;~/r--,s-s,
FS- 4 FS- 3 FS-2
FS-4 • 5
.FS-30 .FS-25 oFS-23 -FS,-21 .FS-19A -FS-18F - FS-18B
~
FS-57
- ~----rs-6o
~
~.
--. r.4
:< --m...,....... ~ .._ ~--~
{Z-27A
SH-18 SH-14 SH-12
SII-28
SH-45
SH-53,
~"
o%---s.-7o
,----SH-7"~
~
Fig. 2. Sample locality map of the Shirataki-Asemi area. Pumpellyite-bearing rocks are showe by crosses.
""-~
w--.~
/
/
1 ,
0 L
J
2
ROCK
PUMPELLYITE- FREE o
/
PUH~LLYITE- BEARING ROCK
/
x
Chlorite Zone
.....,.~ ..~ .....~
Garnet Zone
~ '
/
Biotite Zone
km
AS 4 5 - ~ ~ m ' -
t.-
A5-61
270
Takashi Nakajima
L .60
.40
,,rs
/,"
.20
.10
/
09u)
\
X~ .50
p..j~"
.30
OS
#.¢ .40
.,,° .30
7'
.2
.,,
.6
.8
1.o IFo/IA,
~.2 I~/IFe
Fig. 3. Calibration curves used for (a) epidote and (b) chlorite. IMg, Ire and IAI represent the intensity ratios on Mg, Fe and AI, respectively; e.g. lug = (|unknownllstandard)Mg.
times up to 3-4 mm) of tiny epidote grains and the other is discrete grains isolated in the matrix. The 'discrete type epidote' often displays remarkable zoning. An interpretation of zonal structures of epidote is discussed in the following section.
Pumpellyite:
Two modes of occurrence of
pumpellyite are recognized in the study area. One of them, called pool-type, comprises monomineralic pumpeUyite pools, sometimes filling amygdules, and sometimes replacing the giasssy part of the hyaloclastite. The other, called matrix type, forms an aggregate accompanied by quartz, albite and chlorite in the matrix. Both types of
Table 2. Representative analyses of minerals. Mineral
Ep',dme
Specimen No. Assemblage,
FS-4.5 (1)
FS-37 (1)
FS-IO (3)
FS-21 (3)
SH-18 (4)
FS-10 (3)
FS-21 (3)
FS-30 (5)
FS-66 (7)
SH-18 (4)
SiOz A120~ Fe,O~* FeO* MnO MgO CaO Na,O Total
37.53 20.87 16.24
38.O1 21.81 14.08
38.26 23.61 12.46
37.38 24.93 10.18
38.46 20.15 16.10
27.66 19.04
26.14 18.53
29.55 .i8.99
27.65 19.03
30.31 17.61
O. 17 0.05 23.21
0.33 0.01 22.71
0/)7 0.02 23.65
0.09 0.04 23.55
0.35 0.13 22.70
25.30 0 26 15 30 0 24
24.44 0.30 15.34 0.31
i8.42 0.28 18.87 0.05
23.78 1.28 16.55 0.05
16.09 0.40 21.65 0.17
98.06
96.94
98.00
96.18
97.88
87 80
85.16
86.15
88.29
86.24
3.t)19 1.979 6.',~83
3.064 2.072 .854
3.030 2.2(19 .743
3.001 2.359 .615
3.(19t 1.908 .974
2A02 2.355
2.845 2.368
3.031 2296
2.~/l 2.329
3.073 2.10~i
.012 .006 2.(~10
.023 .001 1.961
.(~.i5 .002 2.(~;'6
.006 .005 2.026
.024 .016 1.955
2.220 .0!24 2.3'93 .027
2.216 .027 2.479 .036
1.580 .024 2.885 .006
2,065 .113 2.562 .003
1.365 .034 3.274 .018
12.5 .332 .335
12.5 .292 .293
12.5 .252 .253
12.5 .206 .205
12.5 .338 .339
14 .481 .485
14 .472 .476
14 .354 .3%
14 .446 .451
14 .294 .297
Si AI Fe ~' Fc:* Mn Mg Ca Na 0 X~(calc.) X~(calib.)
Chlorite
(*) total Fe as Fe 2~ or Fe~ ; (**) a sample from Omoiji-Nagasawa area (Nakajima et aI. 1977); (") preceding data recalculated to divide total Fe into Fe z* and Fe ~÷ adjusted to the structural formula; (#) calcite-bearing: XFe(ealc); X~E~and X~:~calculated from the full element analyses in tiais Table. X~(calib.): XF~. p and XF~ ch derived from the calibration me~hod.
Pumpellyite-actinolite facies membasites
LITHOS 15 0982)
271
Table 2. (continued) Mineral
l~h'nl~fl!yite
Sl~c.cimen No.
FS-10
Assemblage
(3)
(3)
(3)
HeII.OB** (3)#
SiO2 AI203 Fe203 Fee MnO MgO CaO Na20 Total
37.53 25.28
36.80 24.74
37.13 25.52
37.0 22.8
Si AI Fe 3+ Fe 2+ Mn Mg Ca Na O To~al cation
3.78 0.33 3.04 22.80 92.53 6.052 4.806 .509 .046 .731 3.900 24.5
Amphitmles
FS-lff'
1.89 2.08
92.72 6.028 4.7&3 .234 .273 .045 .727 3.882 24.5 15.972
FS.21
3.27 0.28 3.05 22.70 90.84 6.044 4.789 .449 .039 .747 3.994 24.5
FS-21"
1.56 1.87
SH-31
3.41
SH-31"
1.27 2.27
2.85 23.20 91.00
92.48
6.020 4.774 •191 .256 .039 .744 3.979
6.011 4.869
24.5 16.003
92.24
6.5 0.2 2.9 22.2 91.6 6.13 4.44
.462
5.993 4.855 .146 .314
.688 4.024 24.5
HOII.OB**"
4.7 2.2
92.0
FS-3 (I)
FS-25 (3)
56.17 2.44 20 81
54.27 1.01
10.33 1.23 6.25 97.23 7.929 •407 2.211
.90
6.06 4.39 .59 .29
.686 4.012
.72 3.94
.7 I 3.89
24.5 16.006
2-~.5
24.5 15.93
2.174 •186 1.711 23
13.73 0.18 15.69 12.47 O.33 97.68 7.904 . i 74 1.505 .022 3.408 1.946 .094 23
See footnotes at bottom of facing page. pumpeUyite often occur in the same specimen, and their chemistry is independent of the mode of occurrence. The structural formulae of pumpellyites in the study area are approximately in accordance with Ca4(AI, Fe 3+)5(Mg, Fe ~'+)Si6021-(OH)7. Amphiboles: In Fe3+-poor rocks, amphiboles are colorless to pale green actinolite; in Fea+-rich rocks, blue to bluish violet winchite (cf. Leake 1978) and magnesioriebeckite occur. Heterogeneous fine-grained amphibole, composed of actinolite, winchite and magnesiofiebeckite, is sometimes observed in the study area. Chlorite: Chlorite occurs in almost all the sampies examined. It is nearly colorle.s.~ when coexisting with pumpellyite, and is green-color¢d in hematite-beating rocks. It is chemically homogeneous within each thin section. Hematite: Ore minerals were identified under the ore microscope and microprobe. Hematite is the only Fe-oxide detected, and it occurs over the: entire area. Its habit becomes more tabular as metamo~hic grade increases. Hematite: so far examined is nearly pure Fe203, the: maximum Tie2 being 2.05 wt.%. Titanian hematite as described by ItaLy,:& Otsuk~ (1978) was not detected in the samples studied.
Calcite: Calcite usually occurs in the quartzrich layer of the relat!vely Fe3+-rich rocks which carry bluish amphibole and/or hematite. As shown in Table 1, calcite does not coexist with pumpellyite in this area.
Determination of equilibrium composition of zoned epidote It is well known that single grair,s of epidote can be remarkably heterogeneous (Tor!:=mi 1972; Raith 1976; Nakajima et al. 1977). To discuss epidote-Dumpel!y~te equilibria, at is essential to determine the domain within which epidote has equilibrated with coexisting minerals. In the Shirataki-Asemi area, three types of zonal structure are recognized in epidote; one is characterized by X Ep decreasing towards the rim ('normal zoning'), another increasing towards the rim ('rever~e zoning') and the third, with Fetich core as well as Fe-rich rim, exhibiting so to speak 'W-type zoning' (Fig. 4). T~le W-type zoning is often difficult to distinguish under the microscope from the reverse zaniag, because the i~tefference color is insensitive to composition
272 TakashiNakajima
LITHOS 15 (1982) 28 26 24 22 20 18
never coe:~ist with pumpellyite. On the other hand, normal zoning is characteristic cf the pumpellyite-bearing rocks. Fig. 6 is a pseudobinary phase diagram with excess qua~z, albite, chlorite, and actinolite, and a fixed Mg/Fe2+ ratio, schematically showing that the minimum XEep is observed when epidote coexists with pumpellyite, while the maximum X~p occurs when it coexists with hematite (Nakajima et al. 2977). The minimum XrEePcurve,which shows the decrease of X~p with increasing temperature,, represents the divariant reaction: 41 pumpeltl3dte + 2 chlorite + 47 quartz = = 71 clinozoisite + 11 actinolite + 109 water 41 Ca4AlsMgSirO21(OH)7 + 2 Mg7A]4Si4015(OH)I2 + + 47 SiO2 = 71 Ca2A13AiaOI2(OH) + (1) + 11 Ca2MgsSiaO22(OH)2 + 109 H20
L
Fig. 4. Compositional contour map of a zoned epidote. The numbers represent X~p.
for low X~. W-type zoning is a composite zonal structure which has 'normal zoning' in the core and 're~ erse zoning' at the margin. It implies that the Fe-rich rim (also of the reverse zoning) was formed a~: a later stage than that producing normal zoning. The l:.resence of two types of zoning (normal and reverse) in epidote of Sanbagawa metabasites was previously reported from the Bessi area (Horikoshi 1938) and the Kanto Mountains (Seki 1958; Toriumi 1972, 1975). Miyashiro & Seki (1958) interpreted the normal zoning as being formed during progressive metamorphism, ~,hile the reverse zoning formed during retrogres,~ive ~etamorphism. This interpretation was based on their idea that the compositional range o r epidote enlarges towards the Al-rich side as the metamorphic temperature increases. The areal distribution of zoned epidote in the Shirataki-Asemi area is shown in Fig. 5. The Wtype zoning and the reverse zoning are jointly plotted here as epidote with Fe-rich rim. Almost all the epidotes with Fe-rich rims occur on the higher-grade side of the pumpellyite-disappearante isograd. A few rocks in the pumpellyite zone have epidotes with Fe-dch rims, but they are Fe3÷..rrich rocks which do not carry pumpellyite. Therefore, the epidotes with Fe-rich rims
and shows temperature dependence of acz (= activity of the clinozoisite component of epidote solid solution Ca2AI3Si3012(OH)- Ca2AIFe3+ Si3012(OH)). This reaction is written in terms of Mg end members, namely in the AI203-Fe203CaO-MgO four component system. Even so it can well explain the natural occurrence of the minerals discussed here. The effect of Mg-Fe 2+ substitution will be discussed in detail in the following section. Epidotes in this a~semblage formed during progressive metamorphism, in equilibrium with coexisting phases, should exhibit normal zoning whereas reverse zoning in epidote would form during retrogressive metamorphism. Note particularly that the reverse zoning cannot be generated unless epidote coexists w,th pumpelly~te. The genesis of Fe-rich rims in the pumpellyite-free zone therefore cannot be explained by this model. The writer considers that the Fe-rich rim was formed by progressive metamorphism above the temperature of pumpellyite disappearance, where the epidote-pumpellyL'e equilibria is not effective. The epidote-pumpellyite equilibria has controUed the formation of normal type zoning, bu,t the Fe-rich rims of both reverse and W-type zoning were formed as a result of preferential resorption of the clinozoisite component in ep~dote during progressive metamorphism within the pumpellyite-free zone. Consequently, the compositon of Fe-rich rims of the epidote in rever:e zoning and W-type zoning should be excluded from the discussion of the phase equilibria using Fig. 6. In addition, the compositions of Fe-rich cores of normally zoned
Pumpellyite--actinolitefacies metabasites 273
[,ITHOS 15 (1982)
0
{ reverse zoning
x
n o r m a l zoning
!o o o
o
_
. . . . .
:"'~.
" " - ' ~ - ----~. . . . .
. _ _ ~
r_.~ . . . . . . . . . . . .
._....~..
O
.~...
~.....
1
/
2 ~m
Fig. 5. Areal distribution of zoned epidote. The pumpe[iyite-beafing metabasite bed occurs between the chain lines.
and W-type zoned epittotes are also excluded, as they are not considered to be in equilibrium with coexisting minerals.
Me,tamorphic zonation using epidote-pumpellyite equilibria Effect of fluid composition Recently, many investigators have suggested that the phase relations among the low-grade metamorphic minerals are strongly affected by the fluid composition, especially CO2 and 02 in basic rocks (Thompson 1971; Liou 1973; Kerrick 1974; Gla,,;sley 1974; Frost 1980). In the present study, the phase equilibria expressed in Fig. 6 are based on the assumption ~hat the chemical! system was closed except for H20 during progressive metamorphism. The minimum XEep curve was delineated by reaction (1) wlaich does not involw~ free 02. An oxidation reaction to produce epidote from pumpellyite
(e.g. Liou 1979:9) is thus not considered here. It is considered that fOe is not externally controlled (independent of paragenetic re|ations). As mentioned in the previous section, the composition of pumpeUyites in this area fits well with the ideal formula Ca4(AI, Fc3+)5(Mg, Fe2+)Si6021(OH)7, so the Fe3+O = Fe2+(OH) substitution quoted by Passaglia & Gottard". (1973) and Coombs et al. (1976) is not significant i~.~the samples studied. In the study area, calcite occurs ubiquitously but does not coexist with pumpellyite (Table 1). The paragenetic relationships are graphically represented in Fig. 7, where each phase is projected from quartz, albite and chlorite onto the CaA ~ - F e 3+ plane following Brown (1977). The paragenetic relationship expressed in Fig. 7 corresponds to that of the lowest Xco2 equil!bration of the pumpellyite-actinolite facies ('drown 1977). Ia this Xco2 range, the epidote in a,,;semblage (3), with the composition a in Fig. 7, does not coexist with calcite. Furthermore, the tact that
274
TakashiNakajima
LITHOS 15 0982)
Fe z+
+Ch + Ac
XFQ
+ Oz
O.4
Ep +Ht ~/JIIIII i ' - - ~ ~ I I I I I I I I I I I I I I I I I I I I T I '
Pp+Ht ~
"llo.,l I
oT
-~,a.J~lll~llllllllillllll|
+ Ab
Fig.6. AI-Fe3+pseudobinary phase diagramfor epidote--pumpellyiteequilibria. Hatched area representsone phase region. Abbreviations are as follows; Ht, hematite; Ep, epidote; Pp, pumpeilyite; Ch, chlorite; Ac, actinoiite; Qz, quartz; Ab, albite.
~ ~t~ ~
~ ~
~!1111"
...q
°.,
TEflPERATURE OF PURPELLYITE
sphene occurs commonly instead of ruffle + calcite + quartz in this area strongly ~uggests low Xco,. It indicates that the Jfco2 was less than 20 bars, corresponding to Xco 2---0.04, when we assume the metamorphic temperature and the total pressure to be 400°C and 7 kb (Ernst 1972). In the HzO-CO2 mixed volatile system, the equilibrium curve of reaction (1), which does not involve COz, is quite insensitive to small changes in Xco, within the low Xco 2 range. Therefore, it is concluded that X ~ of the calcite-free assemblage can be regarded as being independent of Xco" in the fluid. So wc can safely discuss the temperature dependence of XFF~ o of the calcitefree assemblage without taking Xco 2 into account. Where the Xco., is sufficiently high, the calcite-epidote join is .~table instead of the pumpellyite-actinolite join (Brown 1977), and X ~ may be influer~ced by Xco,. The calcite--epidote join enlarges the stability field of the pumpellyite-free assembla~;e, hence pumpeilyite rarely occurs when the Xco" is high in the fluid.
The sliding equilibrium of epidote and pumpellyite Ir~ the system AI203-Fe20~-CaO-MgO-FeO (~ith excess quartz and albite), XvEP in the assemblage epidote-chlorite-actinolite (assemblage (2)
ila Table 1) is a function of P,T and the Fe2+/ (Fe2+ + M g ) a n d Fea+/(Fe 3+ + Al) ratios of the ro,~ks. As shown in Fig. 6, the minimum XFEe p is realized when pumpellyite occurs with the above association (that is, assemblage (3) in Table 1), and decreases as the metamorphic temperature in~zreases for a fixed value of Xvch (Nakajima et al 1977). Fig. 8 (a) and (b) shows the frequency distributic,n of XFEp in assemblage (2) of the pumpellyitefree zone and in assemblage (3) of the pumpellyiite zone of the Shirataki-Asemi area, respectively. The epidotes shown in Fig. 8 (a) and (b) coexist with chlorite with XFe Ch_ 0.50 - 0.56. It is suggested that the oumpellyite disappearance is~grad of the study area, as defined by the abse:ace of pumpellyite in metabasite, corresponds to, the coexistence of epidote-chlorite-actinolitepumpellyRe with xE~P=0.15 and xCh=0.53 + 0.03. If we remove" the constraint that XvC~ = 0.53 +_0.03, then the minimum X Ep in assembl age (3) is a function of AFe vca as well. To examine tie effect of Mg-Fe 2+ substitution on this equili'~ria, we may vary the x~.C~and check the correlative change of the minimum XEEe p. The relationship between the mininmm XF~ and XFCe h was e:~amined for the samples from the puLmpellyiteb~aring metabasite in the Shirataki-Asemi area, ,,hich is 200 meters thick and covers a wide range of XFc~. Fig. 9 (a), (b), (c) and (d) shows the c,:mpositional ratage c,f epidote and ~Jlhesample -
PumpeUyite-actinolite facies metabasites 275
LmtOS 15 (zgsz~
A1 - 0.4Mg
Fig. L Paragenetic relations of the study area gmphficaily represented in the Ai--Ca-Fe a÷ ternary system with excess quartz, aJbite and chlorite after Brown (1977). I-I20 and CO2 are treated as mobile components. Hatched areas represent two phase re#ons. (1), (2) and (3) indicate assemblages listed in Table I. Abbreviations are as follows; Ep ss, epidote solid sDlution; Pp, pumpellyite; Hm, hematite; Cc, calcite; Cr, crossiite; MR, magnesioriebeckite; Pg, paragonite, a in the Ep ss mark~ the minimum X~p defined by assemblage (3).
- Na
#~¢#f • #
tam : a Ep ss
pe 3+
Ca
(1)
Cc
Ac
localities. On each route, specimen number increases northwards, i.e. towards higher stratigraphic levels. FS-10---25 of the Shirataki route, SH-31 and 42 of the Shimokawa route, KZ-27A and 37 of the Kozugawa route, AS-41B, 42 and 61 of the Asemi route are the samples from one metabasite bed in which pumpellyite occurs (see Fig. 2). There is a general tendency for XE~ tO decrease northwards within the metabasite bed on each route. The effect of Xre Chon Xre Epis clearly seen in Fig. 10, in which the average values of xEt, F,: are plotted against XFe. Ch The arrows on bold and dotted lines show the sequence of sample localities from south to north on the Shirataki (FS) and Asemi (AS) routes, respectively. In spite of small-scale wandering, the distinct trend of XFe, Ch decreasing from south to north is not correlated with X~.h especially if the spread of X~ep in individual samples is taken into account. The minimum X~p :appears to be almost independent of XFe.Ch Ar~other conclusion deduced from these figures, is that differences in grade of metamorphisra can be distinguished even within one me'tabasite bed of 200 meters thick. As shown in Fig. 6, IdX~/dTI increases as the temperature of equilibration increases. This may be part of the ~reason why the metamorphic grade could be dis-
,Hm
l Cr
MR
tinguished in a metabasite with such :a small thickness. : Fig. 8 (c) shows the freqaency of X~p in epidote--chlorite-actinolite-purnpellyite rocks from~ the Omoiji-Nagasawa area 'which is a typical I (a) PU~IPELLYITE-FREEZONEOF TilE SHIR~TAKI-ASEMIAREA ~" p o i n t = 76
|
15
!
20
!
!
25
3O
(b) PUP!PELLYITEZONEOF THE SHIRATAKI-ASEM[AREA )'point
= 62
i 20 (C) OMOi J]-NAGASAWA, point: =~91AREA
i 25 ~
25
i
i
30 r
35 ~
30
35
40
Fig. 8. Frequency diagram of XFE~in (a) the epidozc-'hloriteactinolite assemblage from ~he pumpcllyite-free zon~ of the Shirataki-Asemi area, (b) the epidotc-chlorite-actinolitc-pumpellyite assemblage from the pumpellyite zone of the Shirataki-Asemi area and (c) the <:t~idote-chlorite-actinol~te-pumpellyite assemblage from the Omoiji-Nagasawa area.
276 T~:~kashiNakajima
(a)
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,,
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LITHOS ~5 (1982)
.35
• is
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.2S
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EPIDOTE -;IMI~HIBOLE - CHLORITE - HEMATITE EPIDOTE -,~PHIBOLE - CHLOR!Ig EPIDOTE - ~ P H I B O L E - CHLORITE - PUMPELLYITE
Fig. 9. Change of XF~:pwith increasing temperature of metamorphism along the fear ro.tes in the Shirataki-Asemi area. Each histogram shows the frcqaency of X~Fpof the individual specimen. Numbers in parenthesis beside the specimen numbers represent ,Ch (100 x XF~) of the coexis~ting chlorite. The underlined specimen numbers indicate calcit.e-bearir~g rocks.
pumpeilyite-actinoliite facies :area (Nakajima et al. 1977). The range of AFe ,,Ch in Fig. 8 (c) is 0.450);6, a little 'wider than "'FevCh = 0.50-0.56 in (a) and (b). As the difference in XFc~has little effect on the X~ff of the above-mentioned assemblage, the difference between Fig. 8 (b) and (c) presumabrly is due to the lower temperature of equilibratior in the Omoiji-Nagasawa area than in the Shirataki-Asemi area. So we can distinguish three different grades within th~ pumpellyiteactiaolite facies of the Sanbagawa belt: the Owoiji-Nagasawa area, and the upper and lower
grade zones of file Shirataki-Asemi area. The obser:ation of this isograd s~.roagly depends on the exposure density of metabasite with appropriate bulk composition in the individual terrain• The successful mapping of progressive metamorphic zonation in some parts of the pumpe!lyite-actinolite facies zone in this study suggests that the detection of gradual temperature increases within the pumpellyite-actinolite facies zone is possible if we are blessed with good occurrence of pumpellyite-bearing metabasites.
.~s
L~OS
P~,~mpellyite-actinolitefaeriesmetabasites
15 (1982)
PUMPELLYNE-BEARING ROCK • PUHPELLYITE- FREE ~CK
277
A
o a
SIqRATAKI ROUre - - - , - - A:;Er,II ROUTE .... &....
0'25 t ..4AS-4
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IFS219A • ,/ '-FS-18F
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Fig. 10. X ~ and X~ of coexisting chlorite and epidote. Solid symbols, epidote--chlorite-actinolitepumpellyite assemblage; Open symbols, epidote--chlorite--actinolite assemblage. Arrows ind,,care the regional trend of increasing metamorphic grade.
FS-25
/
/
/
/ FS-76
/ ~AS-78B
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Discussion The high temperature limit of the pumpellyite-aainolite facies The high temperature limit of the pumpellyiteactinolite facies was previously defined by the transifion from the epidote--chlorite-actinolitepumpellyite to the epidote-chlorite-actinolite assemblages (Seki 1969; Nitsch 1971). The disappearance of pumpellyi'te is controlled by at least three parameters; (1) Xc% in the fluid as discussed in the foregoing section, (2) bulk rock chemistry as expressed in Fig. 6 and (3) metamorphic temperature. The meze disappearance of pumpellyite cannot be directly ascribed to the higher temperature of equilibration. So we should define the high temperature limit of the pumpellyite--actinolite facies using the sliding equilibrium of epidote-chlorite-actir:olite-pumpellyite (without calcite) assemblage. The clisappearance of this assemblage with increasing temperature ir~.the hig~-pressure intermediate group of metamorphism represents the transition from a four-phase assemblage to a three-phase assemblage in the five-component system. Hence a univariant equilibrium has to be defined by specifying two chemical parameters of the solid sob~ttion minerals. Fortamately, as de-
,,
0.5
scribed above, the Mg-Fe 2+ substitution of chlorite does not significantly affect the stability of this assemblage, ~s long as X ~ lies in the range 0.40-0.56 (values common ~n metabasites). Thus, let us choose X ~ as one of the chemical parameters and specify it to be in that range. Another parameter easy to specify is XrE~. This parameter is useful because the epidote-chiorite-actinolite assemblage with X Epless than the specified value indicates that the rock was formed at higher temperature than the defined univariant equilibrium. In this study, XrEP=0oT[5 was chosen as the upper limit of the pumpellyite-actinolite facies. This is the sma|lest value of X~ep in that fourphase assemblage so far reported, and thus can be used to define the high temperature limit of the pumpellyRe-actinolite facies in general. However, any value of XrEe p can be used as the zone marker in individual metamorphic terrains. We can draw several iso~ads with specified valuc~ ot X~p o,- a map to clarify the thermal structure of the terrain if the pumpellyite-bearing assemblage occurs commonly. Fig. 5 implies that the frequency of occurrence of the epidote--chlorite-actinolite-pumpellyite assemblage decreases as the metamorphic temperature increases. The pumpellyite-bearing metabasite bed studied in
FakashiNakafima
278
LrrHos 15 (1982)
]
BIOTITE ZONE ~
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:::::::::::::::::::::::::::::::::::::::::: ::::::::::::::::::::::::::::::::::::
bTg. I i. Metamorphic zone boundaries and pumpellyite disappearance isograd in central Shikoku. The disappearance isograd on the northern side i~, defined using unpublished data of the,, present writer.
detail in this paper has previously been over-. looked for this :eason Our experience in the Sanbagawa belt ~uggests that X~p = 0.'.20 or (1.25 may bt: more practical for defining the high t~'mpcratu,:e limit og the pumpellyite-ac!inolite facies. b,:cause parapellyite becomes quite rare as Xl~p of the coexisting epndote decreases below that value. In any case, the high temperature iinfit of the pumpellyite-actinolite fiacies must be defined with refeFence to the X~p of the highest grade paragene:is.
Con:ribution to the refinement of the thermal srruc?b:re of the Sanbagawa belt in central Sl:ik~ ~tcu In the S~,nbagawa metamocphic beit in centra]l Shikok u, ,~everal isograds in the strict sense have be:,r, established recently (Ranno 1977). These,. in~lt,~de Mg-Fe 2+ partition tetween garnet and chlorite, the solubility of AI in Ca-amphibole of henatite-bearing assemblages, F e 3 + / ( F e 3+ -I-- Ai) ratio of Na-amphibole coexisting with epidote, chlorite, hematite, albite and quartz, and so on. U':ing these indicators, the thermal structure of' t Sanbagawa metamorphic belt in ,central Shi-
koku has been sb-wn te he overturned on a laJs~ scale (Banno et al. 1978). However, these isograds are relevant only in the relatively highergrade part of the belt, namely, in the pumpellyite-free zone. The present stady demonstrates that progressive mineral zonafion can be found within the pumpellyite-actinolite facies zone of the Shirataki-Asemi area. Pumpellyite-bearing metabasites are observed., though spe,radically, at other localities in the Main member of the Minawa Formation. The ~umpellyite disappearance isograd therefore can be traced regionally. It is located on the low temperature side of the garnet isograd and runs !aarailel to it (Fig. 11), along the probable extensic,n of the metabasite horizon examined in detail iq the Shirataki-Asemi area. The Omoiji-Nl,gasawa area, mentioned in the foregoing sectio), includes the Sanbagawa Southern Margioal Belt and the northernmost part of the Chichbu belt (Fig. 1), corresponding to the low-grade region of the Sanbagawa metamorphic belt. Frcm Fig. 8, it is expected that the temperature decreases towards the soath, iiintc the Omoiji-Naga::,awa area. Fig. 12 is a schematic cross section (f central Shikoku in terms of metamorphic zonation; the northern part of the
LITHOS
Pumpel!yite-actinolite facies meiabasites 279
15 (1982)
Biotite Zone Garnet Zone
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. . . . . . . . . . . . . . . . .
SANBAGAWA i:
'"
SANBAGAWA BELT
i
~,..SOUTHERN .,,!= - ~ MARGINAL
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i BELT
Fig. 12. Schematic cross section of central Shikoku in terms of metamorphic zones, it is delineated combining the data of Shirataki-Asemi and Omoiji~-Nagasaw~, areas.
section follows the As~.'mi rout~.• and its extension into the Sanbagawa belt. The southern part of the figure is drawn along a N-S traverse section in the Omoiji-Nagasawa area. The overturned thermal structure is recognized clearly also in the low-grade area, where pumpe!lyite occurs stably. Acknowledgements. - Thanks are expressed to Prof. K. lshioka and Dr. K. Suwa for their continual encouragement. Prof. S. Banno kindly offered helpful advice and critical discussion greatly improving the manuscript. 1 am grateful to Prof. A. Miyashiro, Dr. C. A. Landis and Dr. K. Suwa, who all read the manuscript critically. Discussions ~vith and information presented by K. Aiba, T. Higashino, Drs. S. Maruyaraa, T. ltaya and Dr. M. Otsuki are much appreciated. Dr. S. Higashimoto and the staff of the Geological Survey of Japan gave much encouragement. I acknowledge I. Hiraiwa and S. Yogo for preparing some of the examined thin sections.
References Banno, S. 1964: Petrologic studies on Sanbagawa crystalline schists in the Bessi-lno district, central Sikoku, Japan. J. Fac. Sci. Univ. Tokyo, Sec. 11, 203-319. Banno, S. 1977: Mineral f~,cies of the Sanbagawa metamorphic belt, pp. 97--106 in Hide, K (ed.): Sanbagawa Belt, Hiroshima Univ. Press, Hiroshima. Bannc, S., Higashino, T., Ot~uki, M., Itaya, T. & Nakajima, T. 1978: Thermal structure of the Sanbagawa metamorphic belt in central Shikoku. J. Phys. Earth 26, 345-356. Brown, E. H. 1977: Phase equilibria among pumpellyite, lawsonite, epidote and associated minerals in low-grade metamorphic rocks. Contrib. Mineral. Petrol. 64, 123-136 Coombs, D S., Nakamura, Y & Vuagnat, M. 1976: Pumpellyite-acfinolite facies schists of the Taveyanne formation near Loeche, Valais, Switzerland. J. Petrol. 17, 440-4::1. Ernst, W. G. 1972: CO2-poor composition of the fluid attending Franciscan and Sanbagawa low-gr;~,de metamorphism. Geochim. Cosmochim. Acta 36, 497-504.
Ernst, W. G., Seki, Y., Onuki, H. & Gilbert, M. C. 1970: Comparative study of low-grade metamorphism in the California Coast Ranges and the outer metamorphic belt in Japan. Mem. Geol. Soc. Am. 124. Frost, B. R. 1980: Cbservations oa the boundary between zeolite facies and prehnite-pumpellyite facies. Contrib. Mineral. Petrol. 73, 365-373. Glassley, W. 1974: A model for phase equilibria in the prehnite-pumpellyite facies. Contrib. Mineral. Petrol. 43, 317332. Higashino, T. 1975: Biotite zone of Sanbagawa metamorphic terrain in the Shiragayama area, central Shikoku, Japan. J. Geol. Soc. Japan 81,653-670. Horikoshi, Y. 1938: Properties of a few constituent minerals in metamorphic rocks of the Bessi district, part 2 (epidote group). J. Geol. Soc. Japan 45, 342-351. Raya, T. & Otsuki, M. 1978: Stability and paragenesis of Fe-'l'i oxide mine-als and sphene in the basic schists of the Sanbagawa melamorphic belt in central Shikoku, Japan. J. Japan Assoc. Mln. Pet. Econ. Geol. 73, 359--379. Kerriek, D. M. 1974: Review of metamorphic mixed volatile (H20--CO2) equiliria. Am. Mineral. 59, 72%761. Kurata, H. & Banno, S. 1974: Low-grade progressive metamorphism of pelitie schists of the Sazare area, Sanbagawa metamorphic terrain in central Shikoku, Japan. J. Petrol. 15. 361-382. Leake, B. E. 1978: Nomenclature of amphiboles. Am. Mineral. 63, 1023-1052. Liou, J. G. 1973: Synthesis and stability relations of epidote Ca2AI2Fe3i3Olz(OH). J. Petrol. 14, 381-413. Lit~u, J. G. 1979: Zeolite facies metamorphism of basaltic rc.cks from the East Taiwan Ophiolite. Am. Mineral. 64, 1-14. Miyashiro, A. & Seki, Y. 1958: Enlargement of the composition field of epidote and piemontite with rising temperature. Am. J. Sci. 256, 423-430. Nakajima, T., Banno, S. & Suzuki, T. 1977: Reactions iez~.;ng to the disappearance of pumpellyite in low-grade metamorphic rocks of the Sanbagawa metamorphic belt in central Shikoku, Japan. J. Petrol. 18, 263-284. Nakamura, Y. & Kushiro, 1. 1971): Compositional relations of coexisting orthopyroxene, pigeonite and augite in a thaleiitic
280
TakashiNaka]ima
andesite from Hakone volcano. Contrib. Mineral. Petrol. 26, 265-275. Nitsch, K. H. 1971: Stabilit~itsbeziehungen yon Prei~nit- und PumpeUyit-haltigen Paragenesen. Contrib. Mineral. Petrol. 30, 240-260. Fassaslia, E. & Gottardi, G. 1973: Crystal chemistry and nomenclature of pumpellyites and julgoldites. Can. Mineral. 12, 219-223. Raith~ M. 1976: The AI-Fe(III) epidote miscibility gap in .a meLamorphic profile through the Pennic Series of the',Tauern window, Austria. Contrib. Mineral. Petrol. 57, 99--;i17. Seki, Y. 1958: Glaucophanitic regional metamorphism in the Ka~~to Mountains, central Japan. Japan. J. Geol. Grogr. 29, 233-238. Seki, Y. 1969: Facies series in low-grade metamorplaism. J. Geol. Soc. Japan 75, 255-266. ~ompson, A. B. 1971:Pco2 in low-grade metamorphism;
LrrHOS 15 (1982) zeolite, carbonate, clay mineral, prehnite relations in the system CaO-AIaO3-SiOz--CO~H20. Contrib. Min~eral. Petrol. 33, 145-161. Toriumi, M. 1972: Microprobe study of zoned epidote in the Sanbagawa rocks from the Kanto Mountains. J. Geol. Soc. Japan 78, 545-~48. Toriumi, M. 1975: Petrological study of the Sanbagawa metamorphic rocks, Kanto Mountains, Japan. Butl. Univ. Museum, Univ. Tokyo, No. 9. Watanabe, T, 1974: Metamorphic zoning of the Sanbagawa and Chichibu belts in the Koshibugawa river area, Oshika district, central Japan with special reference to pumpellyiteactinolite facies mineral assemblages. J. Geol. Soc. Japan 80, 525-53g. Zen, E-an. 1974: Prehnite- and pumpellyite-bearing mineral assemblages, west side of Apalachian metamorphic belt, Pennsylvania to Newfoundland. J. Petrol. 15, 197-242.
HTHOS
Nakajima, T. 1982 10 15: Phase relations of pumpellyite--actinolite facies metabasites in the Sanbagawa metamorphic belt in central Shikoku, Japan. Lithos, Vol. 15, pp. 267-280. Oslo. ISSN 0024-4937. Sanbagawa metabasites metamorphosed at conditions near the upper limit of the pumpcUyitc-actinolite facies were examined in terms of phase equilibria in the five component system AI203-FczO3-CaOEp __ MgO-FeO. The Fe 3+ content of epidote measured as Xrc ( - Fe/(Fe + AI) of epidote) ir~ the assemblage epidote-chlorite-actinolite-pumpellyite decreases gradually towards the higher-grade, pumpellyite-free areas. The progressive change in X~ep can be detected within one metabasite bed 200 metel~ thick near the upper limit of the pumpellyite-actinolite facies. The Mg--Fe2+ substitution, as expressed by variation of Fe/(Fe + Mg) in chlorite (0.40-0.55) has little effect on the Fe3+/(Fe3+ + AI) ratios of epidote and pumpellyite in the above-mentioned assemblage. The lowest X[ p in the pumpellyite-bearing assemblage is 0.15 and hence the upper limit of the pumpellyite--actinolite facies is defined by the appearance of an Ep_ epidote--chlorite-actinolite assemblage with Xr~C.15.
Takashi Nakafima, Department of Earth Sciences, Nagoya University, Chiltusa-ku. Nagoya 464, Jepan. Present address: Geological Survey of Japan, Chugoku Office Hiroshima Godo-Chosha. 6-30. KamiHacchobori, Naka-ku, Hiroshima 730, Japan; November l~Sl.
The mapping of metamorphic zones in the Sanbagawa belt of central Shikoku has been done by Banno (1964), Ernst et al. (1970), Kurata & Banno (1974), Higashino (1975) and others. The most re,:e.nt compilation is by Banno et al. (19713), ia which the belt is divided into chlorite, garnet ($~arnet--chlorite) and biotite (biotite-gar: net-chloidte) zones. Nakajima et al. (1977) have shown that the chlorite zone can be subdivided. into a pumpellyite zone and a pumpellyite-free zone. The writer surveyed the Shirataki-Asemi area (Fig. l) in order to define the high temperature limit of the pumpellyite zone more clearly in the Sanbagawa belt. A metabasite bed about 200 meters thick and including abundant pumpellyite'bearing schi~:ts was studied in detail. This pumpeUyite-bearing bed belongs to the Main member of the Minawa Formation (Fig. 1). The purpose of this paper is (1) to examine the effect of the Mg-Fe 2+ substitution on the epidote-chk~rite-actinolite-pumpellyite equilibria (Nakajima et al. 1977), and (2) to recognize temperature variations within the pumpe!lyiteactinolite facies zone in order to refine the thermal structure of the lower-grade terrain of "t | | t". , Sanbagawa belt in central Shikoku. We will consider this pumpellyite-bearing bed to be the northern limit of the pumpellyite-a~.~inolite facies until detailed discussions are pre-
sented in a later section. The northern boundary of this bed thus separates the pumpellyite zone and the pumpelJyite-free zone. The latter corresponds to the uppermost chlorite zone and the garnet zone.
Paragenesis The size of equilibration domains in low-grade metamorphic rocks is generally considered ~o be smaller than a thin section. Different sizes of equilibration domain have been assigned by different investigators. Watanabe (1974) suggested that only phases coexisting in direct contact should be regarded as in equilibrium. Zen (1974) suggested that the equilibration domain is as large as 1 mm in diiameter. The metabasites examined in this study are completely recrystallized and seem to be relatively homogeneous in thin section, probably due to being derived mainly from reworked hyaloclastite. In this paper, the coexisting minerals within each thin section are assumed to be in equilibrium as a first approximation, unless evidence to the contrary (e.g. textural relations) is recognized. T h e S,~irataki-Asemi area covers the lowergrade region of the Shirataki and the Shiragayama areas investigated by Ernst et al. (1970)
268 Takashi Nakajima
L r r H o s 15 09,~2)
F-']
...................
ALLUVIUM
...........
IZUMI F.
............
OJO1N F.
Cretaceous)
Upper member ~]
L o w e r member
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SANBAGAWA BE~
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--
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...,."
.....................
M I K A B U GREEN ROCKS CHICHIBU
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Fig. 1. Geologic sketch map of the central Shikoku area.
and Higashino (1975), respectively. Basic rocks are uncommon in the lower-grade part of the chlorite zone but are abundant in its highergrade part and in :ihe garnet and biotite zones. The constituent m;,nerals are as follows: quartz, albite, chlorite, epidote, amphiboles (actinolite, alkali amphibole, hornblende), pumpellyite, phengitic mica, g~rnet, sphene, calcite, apatite, hematite, pyrite, pyrrhotite and chalcopyrite. Mineral assemblages of the samples from the Shirataki-.Asemi .~rea are listed in Table 1. The amphiboles are of the actinolite-mhgnesioriebeckite series in tFe chlorite zone and the lowergrade part of the garnct zone. Garnet-bearing assemblages will not be discussed here because they are quite unccmmon.
Table 1 Mineral assemblages of basic rocks in the ShiratakiAsemi area.
(1) cpidote + chlorite + amphibole + hematite (2) epidote + chlorite -;- amphibole (3) epidote + chlorite + amphibole + pumpellyite (4) epidote + chlorite + amphibole + hematite + calcite (5) epidote + chlorite + amphibole + calcite (6) epidote + chlorite + amphibole + garnet (7) epidote + chlorite + amphibole + hematite + garnet All assemblages also contain: quartz + albite + sphene + phengitic mica + apatite
Mineralogy Analytical method Chemical analyses of minerals were carried out using a JEOL JXA-5A 3-channel electron-probe microanalyzer following Nakamura & Kt~,shiro (1970). Representative analyses of minerals are listed in 'Fable 2. Since tiny mineral grains and the fine chemical structure of minerals had to be examined in this study (e.g. Fig. 4), a calibration method was adopted in the analysis of epidote and chlorite to avoid possible errors arising from a slight shift of analytical point during resetting. Partial analysis of AI, Fe and Mg was done with the fixed analyzer crystals RAP, PET and RAP, respectively. The accelerating voltage and the beam current were kept at 15 kV and 0.015 I~A. When analyzing epidote, Mg was used as a monitor to check for contamination by tiny flakes or fragments of chlorite and amphibole. The calibration curves for epidote and chlorite were based on 67 and 75 full analyses respectively (Fig. 3 (a), (b)). The discrepancies between the values obtained from the calibration curves and the full analysis are below 0.5% in X ~ (= Fe3+/ (Fe3+ + AI) in epidote; total Fe = Fe 3+) and X ~ (= FEZ+/ (Fe 2+ +Mg) in chlorite; total F e = F e 2+) (Table 2). These errors are small enough to ignore in the following discussions.
Individual minerals Epidote: The grain size and the modal amount of epidote varies from rock to rock, depending on bulk rock chemistry and metamo:rphic grade. The mode of occurrence of epidote is classified into two types; one is large aggregates (some-
FS-10
~;~/r--,s-s,
FS- 4 FS- 3 FS-2
FS-4 • 5
.FS-30 .FS-25 oFS-23 -FS,-21 .FS-19A -FS-18F - FS-18B
~
FS-57
- ~----rs-6o
~
~.
--. r.4
:< --m...,....... ~ .._ ~--~
{Z-27A
SH-18 SH-14 SH-12
SII-28
SH-45
SH-53,
~"
o%---s.-7o
,----SH-7"~
~
Fig. 2. Sample locality map of the Shirataki-Asemi area. Pumpellyite-bearing rocks are showe by crosses.
""-~
w--.~
/
/
1 ,
0 L
J
2
ROCK
PUMPELLYITE- FREE o
/
PUH~LLYITE- BEARING ROCK
/
x
Chlorite Zone
.....,.~ ..~ .....~
Garnet Zone
~ '
/
Biotite Zone
km
AS 4 5 - ~ ~ m ' -
t.-
A5-61
270
Takashi Nakajima
L .60
.40
,,rs
/,"
.20
.10
/
09u)
\
X~ .50
p..j~"
.30
OS
#.¢ .40
.,,° .30
7'
.2
.,,
.6
.8
1.o IFo/IA,
~.2 I~/IFe
Fig. 3. Calibration curves used for (a) epidote and (b) chlorite. IMg, Ire and IAI represent the intensity ratios on Mg, Fe and AI, respectively; e.g. lug = (|unknownllstandard)Mg.
times up to 3-4 mm) of tiny epidote grains and the other is discrete grains isolated in the matrix. The 'discrete type epidote' often displays remarkable zoning. An interpretation of zonal structures of epidote is discussed in the following section.
Pumpellyite:
Two modes of occurrence of
pumpellyite are recognized in the study area. One of them, called pool-type, comprises monomineralic pumpeUyite pools, sometimes filling amygdules, and sometimes replacing the giasssy part of the hyaloclastite. The other, called matrix type, forms an aggregate accompanied by quartz, albite and chlorite in the matrix. Both types of
Table 2. Representative analyses of minerals. Mineral
Ep',dme
Specimen No. Assemblage,
FS-4.5 (1)
FS-37 (1)
FS-IO (3)
FS-21 (3)
SH-18 (4)
FS-10 (3)
FS-21 (3)
FS-30 (5)
FS-66 (7)
SH-18 (4)
SiOz A120~ Fe,O~* FeO* MnO MgO CaO Na,O Total
37.53 20.87 16.24
38.O1 21.81 14.08
38.26 23.61 12.46
37.38 24.93 10.18
38.46 20.15 16.10
27.66 19.04
26.14 18.53
29.55 .i8.99
27.65 19.03
30.31 17.61
O. 17 0.05 23.21
0.33 0.01 22.71
0/)7 0.02 23.65
0.09 0.04 23.55
0.35 0.13 22.70
25.30 0 26 15 30 0 24
24.44 0.30 15.34 0.31
i8.42 0.28 18.87 0.05
23.78 1.28 16.55 0.05
16.09 0.40 21.65 0.17
98.06
96.94
98.00
96.18
97.88
87 80
85.16
86.15
88.29
86.24
3.t)19 1.979 6.',~83
3.064 2.072 .854
3.030 2.2(19 .743
3.001 2.359 .615
3.(19t 1.908 .974
2A02 2.355
2.845 2.368
3.031 2296
2.~/l 2.329
3.073 2.10~i
.012 .006 2.(~10
.023 .001 1.961
.(~.i5 .002 2.(~;'6
.006 .005 2.026
.024 .016 1.955
2.220 .0!24 2.3'93 .027
2.216 .027 2.479 .036
1.580 .024 2.885 .006
2,065 .113 2.562 .003
1.365 .034 3.274 .018
12.5 .332 .335
12.5 .292 .293
12.5 .252 .253
12.5 .206 .205
12.5 .338 .339
14 .481 .485
14 .472 .476
14 .354 .3%
14 .446 .451
14 .294 .297
Si AI Fe ~' Fc:* Mn Mg Ca Na 0 X~(calc.) X~(calib.)
Chlorite
(*) total Fe as Fe 2~ or Fe~ ; (**) a sample from Omoiji-Nagasawa area (Nakajima et aI. 1977); (") preceding data recalculated to divide total Fe into Fe z* and Fe ~÷ adjusted to the structural formula; (#) calcite-bearing: XFe(ealc); X~E~and X~:~calculated from the full element analyses in tiais Table. X~(calib.): XF~. p and XF~ ch derived from the calibration me~hod.
Pumpellyite-actinolite facies membasites
LITHOS 15 0982)
271
Table 2. (continued) Mineral
l~h'nl~fl!yite
Sl~c.cimen No.
FS-10
Assemblage
(3)
(3)
(3)
HeII.OB** (3)#
SiO2 AI203 Fe203 Fee MnO MgO CaO Na20 Total
37.53 25.28
36.80 24.74
37.13 25.52
37.0 22.8
Si AI Fe 3+ Fe 2+ Mn Mg Ca Na O To~al cation
3.78 0.33 3.04 22.80 92.53 6.052 4.806 .509 .046 .731 3.900 24.5
Amphitmles
FS-lff'
1.89 2.08
92.72 6.028 4.7&3 .234 .273 .045 .727 3.882 24.5 15.972
FS.21
3.27 0.28 3.05 22.70 90.84 6.044 4.789 .449 .039 .747 3.994 24.5
FS-21"
1.56 1.87
SH-31
3.41
SH-31"
1.27 2.27
2.85 23.20 91.00
92.48
6.020 4.774 •191 .256 .039 .744 3.979
6.011 4.869
24.5 16.003
92.24
6.5 0.2 2.9 22.2 91.6 6.13 4.44
.462
5.993 4.855 .146 .314
.688 4.024 24.5
HOII.OB**"
4.7 2.2
92.0
FS-3 (I)
FS-25 (3)
56.17 2.44 20 81
54.27 1.01
10.33 1.23 6.25 97.23 7.929 •407 2.211
.90
6.06 4.39 .59 .29
.686 4.012
.72 3.94
.7 I 3.89
24.5 16.006
2-~.5
24.5 15.93
2.174 •186 1.711 23
13.73 0.18 15.69 12.47 O.33 97.68 7.904 . i 74 1.505 .022 3.408 1.946 .094 23
See footnotes at bottom of facing page. pumpeUyite often occur in the same specimen, and their chemistry is independent of the mode of occurrence. The structural formulae of pumpellyites in the study area are approximately in accordance with Ca4(AI, Fe 3+)5(Mg, Fe ~'+)Si6021-(OH)7. Amphiboles: In Fe3+-poor rocks, amphiboles are colorless to pale green actinolite; in Fea+-rich rocks, blue to bluish violet winchite (cf. Leake 1978) and magnesioriebeckite occur. Heterogeneous fine-grained amphibole, composed of actinolite, winchite and magnesiofiebeckite, is sometimes observed in the study area. Chlorite: Chlorite occurs in almost all the sampies examined. It is nearly colorle.s.~ when coexisting with pumpellyite, and is green-color¢d in hematite-beating rocks. It is chemically homogeneous within each thin section. Hematite: Ore minerals were identified under the ore microscope and microprobe. Hematite is the only Fe-oxide detected, and it occurs over the: entire area. Its habit becomes more tabular as metamo~hic grade increases. Hematite: so far examined is nearly pure Fe203, the: maximum Tie2 being 2.05 wt.%. Titanian hematite as described by ItaLy,:& Otsuk~ (1978) was not detected in the samples studied.
Calcite: Calcite usually occurs in the quartzrich layer of the relat!vely Fe3+-rich rocks which carry bluish amphibole and/or hematite. As shown in Table 1, calcite does not coexist with pumpellyite in this area.
Determination of equilibrium composition of zoned epidote It is well known that single grair,s of epidote can be remarkably heterogeneous (Tor!:=mi 1972; Raith 1976; Nakajima et al. 1977). To discuss epidote-Dumpel!y~te equilibria, at is essential to determine the domain within which epidote has equilibrated with coexisting minerals. In the Shirataki-Asemi area, three types of zonal structure are recognized in epidote; one is characterized by X Ep decreasing towards the rim ('normal zoning'), another increasing towards the rim ('rever~e zoning') and the third, with Fetich core as well as Fe-rich rim, exhibiting so to speak 'W-type zoning' (Fig. 4). T~le W-type zoning is often difficult to distinguish under the microscope from the reverse zaniag, because the i~tefference color is insensitive to composition
272 TakashiNakajima
LITHOS 15 (1982) 28 26 24 22 20 18
never coe:~ist with pumpellyite. On the other hand, normal zoning is characteristic cf the pumpellyite-bearing rocks. Fig. 6 is a pseudobinary phase diagram with excess qua~z, albite, chlorite, and actinolite, and a fixed Mg/Fe2+ ratio, schematically showing that the minimum XEep is observed when epidote coexists with pumpellyite, while the maximum X~p occurs when it coexists with hematite (Nakajima et al. 2977). The minimum XrEePcurve,which shows the decrease of X~p with increasing temperature,, represents the divariant reaction: 41 pumpeltl3dte + 2 chlorite + 47 quartz = = 71 clinozoisite + 11 actinolite + 109 water 41 Ca4AlsMgSirO21(OH)7 + 2 Mg7A]4Si4015(OH)I2 + + 47 SiO2 = 71 Ca2A13AiaOI2(OH) + (1) + 11 Ca2MgsSiaO22(OH)2 + 109 H20
L
Fig. 4. Compositional contour map of a zoned epidote. The numbers represent X~p.
for low X~. W-type zoning is a composite zonal structure which has 'normal zoning' in the core and 're~ erse zoning' at the margin. It implies that the Fe-rich rim (also of the reverse zoning) was formed a~: a later stage than that producing normal zoning. The l:.resence of two types of zoning (normal and reverse) in epidote of Sanbagawa metabasites was previously reported from the Bessi area (Horikoshi 1938) and the Kanto Mountains (Seki 1958; Toriumi 1972, 1975). Miyashiro & Seki (1958) interpreted the normal zoning as being formed during progressive metamorphism, ~,hile the reverse zoning formed during retrogres,~ive ~etamorphism. This interpretation was based on their idea that the compositional range o r epidote enlarges towards the Al-rich side as the metamorphic temperature increases. The areal distribution of zoned epidote in the Shirataki-Asemi area is shown in Fig. 5. The Wtype zoning and the reverse zoning are jointly plotted here as epidote with Fe-rich rim. Almost all the epidotes with Fe-rich rims occur on the higher-grade side of the pumpellyite-disappearante isograd. A few rocks in the pumpellyite zone have epidotes with Fe-dch rims, but they are Fe3÷..rrich rocks which do not carry pumpellyite. Therefore, the epidotes with Fe-rich rims
and shows temperature dependence of acz (= activity of the clinozoisite component of epidote solid solution Ca2AI3Si3012(OH)- Ca2AIFe3+ Si3012(OH)). This reaction is written in terms of Mg end members, namely in the AI203-Fe203CaO-MgO four component system. Even so it can well explain the natural occurrence of the minerals discussed here. The effect of Mg-Fe 2+ substitution will be discussed in detail in the following section. Epidotes in this a~semblage formed during progressive metamorphism, in equilibrium with coexisting phases, should exhibit normal zoning whereas reverse zoning in epidote would form during retrogressive metamorphism. Note particularly that the reverse zoning cannot be generated unless epidote coexists w,th pumpelly~te. The genesis of Fe-rich rims in the pumpellyite-free zone therefore cannot be explained by this model. The writer considers that the Fe-rich rim was formed by progressive metamorphism above the temperature of pumpellyite disappearance, where the epidote-pumpellyL'e equilibria is not effective. The epidote-pumpellyite equilibria has controUed the formation of normal type zoning, bu,t the Fe-rich rims of both reverse and W-type zoning were formed as a result of preferential resorption of the clinozoisite component in ep~dote during progressive metamorphism within the pumpellyite-free zone. Consequently, the compositon of Fe-rich rims of the epidote in rever:e zoning and W-type zoning should be excluded from the discussion of the phase equilibria using Fig. 6. In addition, the compositions of Fe-rich cores of normally zoned
Pumpellyite--actinolitefacies metabasites 273
[,ITHOS 15 (1982)
0
{ reverse zoning
x
n o r m a l zoning
!o o o
o
_
. . . . .
:"'~.
" " - ' ~ - ----~. . . . .
. _ _ ~
r_.~ . . . . . . . . . . . .
._....~..
O
.~...
~.....
1
/
2 ~m
Fig. 5. Areal distribution of zoned epidote. The pumpe[iyite-beafing metabasite bed occurs between the chain lines.
and W-type zoned epittotes are also excluded, as they are not considered to be in equilibrium with coexisting minerals.
Me,tamorphic zonation using epidote-pumpellyite equilibria Effect of fluid composition Recently, many investigators have suggested that the phase relations among the low-grade metamorphic minerals are strongly affected by the fluid composition, especially CO2 and 02 in basic rocks (Thompson 1971; Liou 1973; Kerrick 1974; Gla,,;sley 1974; Frost 1980). In the present study, the phase equilibria expressed in Fig. 6 are based on the assumption ~hat the chemical! system was closed except for H20 during progressive metamorphism. The minimum XEep curve was delineated by reaction (1) wlaich does not involw~ free 02. An oxidation reaction to produce epidote from pumpellyite
(e.g. Liou 1979:9) is thus not considered here. It is considered that fOe is not externally controlled (independent of paragenetic re|ations). As mentioned in the previous section, the composition of pumpeUyites in this area fits well with the ideal formula Ca4(AI, Fc3+)5(Mg, Fe2+)Si6021(OH)7, so the Fe3+O = Fe2+(OH) substitution quoted by Passaglia & Gottard". (1973) and Coombs et al. (1976) is not significant i~.~the samples studied. In the study area, calcite occurs ubiquitously but does not coexist with pumpellyite (Table 1). The paragenetic relationships are graphically represented in Fig. 7, where each phase is projected from quartz, albite and chlorite onto the CaA ~ - F e 3+ plane following Brown (1977). The paragenetic relationship expressed in Fig. 7 corresponds to that of the lowest Xco2 equil!bration of the pumpellyite-actinolite facies ('drown 1977). Ia this Xco2 range, the epidote in a,,;semblage (3), with the composition a in Fig. 7, does not coexist with calcite. Furthermore, the tact that
274
TakashiNakajima
LITHOS 15 0982)
Fe z+
+Ch + Ac
XFQ
+ Oz
O.4
Ep +Ht ~/JIIIII i ' - - ~ ~ I I I I I I I I I I I I I I I I I I I I T I '
Pp+Ht ~
"llo.,l I
oT
-~,a.J~lll~llllllllillllll|
+ Ab
Fig.6. AI-Fe3+pseudobinary phase diagramfor epidote--pumpellyiteequilibria. Hatched area representsone phase region. Abbreviations are as follows; Ht, hematite; Ep, epidote; Pp, pumpeilyite; Ch, chlorite; Ac, actinoiite; Qz, quartz; Ab, albite.
~ ~t~ ~
~ ~
~!1111"
...q
°.,
TEflPERATURE OF PURPELLYITE
sphene occurs commonly instead of ruffle + calcite + quartz in this area strongly ~uggests low Xco,. It indicates that the Jfco2 was less than 20 bars, corresponding to Xco 2---0.04, when we assume the metamorphic temperature and the total pressure to be 400°C and 7 kb (Ernst 1972). In the HzO-CO2 mixed volatile system, the equilibrium curve of reaction (1), which does not involve COz, is quite insensitive to small changes in Xco, within the low Xco 2 range. Therefore, it is concluded that X ~ of the calcite-free assemblage can be regarded as being independent of Xco" in the fluid. So wc can safely discuss the temperature dependence of XFF~ o of the calcitefree assemblage without taking Xco 2 into account. Where the Xco., is sufficiently high, the calcite-epidote join is .~table instead of the pumpellyite-actinolite join (Brown 1977), and X ~ may be influer~ced by Xco,. The calcite--epidote join enlarges the stability field of the pumpellyite-free assembla~;e, hence pumpeilyite rarely occurs when the Xco" is high in the fluid.
The sliding equilibrium of epidote and pumpellyite Ir~ the system AI203-Fe20~-CaO-MgO-FeO (~ith excess quartz and albite), XvEP in the assemblage epidote-chlorite-actinolite (assemblage (2)
ila Table 1) is a function of P,T and the Fe2+/ (Fe2+ + M g ) a n d Fea+/(Fe 3+ + Al) ratios of the ro,~ks. As shown in Fig. 6, the minimum XFEe p is realized when pumpellyite occurs with the above association (that is, assemblage (3) in Table 1), and decreases as the metamorphic temperature in~zreases for a fixed value of Xvch (Nakajima et al 1977). Fig. 8 (a) and (b) shows the frequency distributic,n of XFEp in assemblage (2) of the pumpellyitefree zone and in assemblage (3) of the pumpellyiite zone of the Shirataki-Asemi area, respectively. The epidotes shown in Fig. 8 (a) and (b) coexist with chlorite with XFe Ch_ 0.50 - 0.56. It is suggested that the oumpellyite disappearance is~grad of the study area, as defined by the abse:ace of pumpellyite in metabasite, corresponds to, the coexistence of epidote-chlorite-actinolitepumpellyRe with xE~P=0.15 and xCh=0.53 + 0.03. If we remove" the constraint that XvC~ = 0.53 +_0.03, then the minimum X Ep in assembl age (3) is a function of AFe vca as well. To examine tie effect of Mg-Fe 2+ substitution on this equili'~ria, we may vary the x~.C~and check the correlative change of the minimum XEEe p. The relationship between the mininmm XF~ and XFCe h was e:~amined for the samples from the puLmpellyiteb~aring metabasite in the Shirataki-Asemi area, ,,hich is 200 meters thick and covers a wide range of XFc~. Fig. 9 (a), (b), (c) and (d) shows the c,:mpositional ratage c,f epidote and ~Jlhesample -
PumpeUyite-actinolite facies metabasites 275
LmtOS 15 (zgsz~
A1 - 0.4Mg
Fig. L Paragenetic relations of the study area gmphficaily represented in the Ai--Ca-Fe a÷ ternary system with excess quartz, aJbite and chlorite after Brown (1977). I-I20 and CO2 are treated as mobile components. Hatched areas represent two phase re#ons. (1), (2) and (3) indicate assemblages listed in Table I. Abbreviations are as follows; Ep ss, epidote solid sDlution; Pp, pumpellyite; Hm, hematite; Cc, calcite; Cr, crossiite; MR, magnesioriebeckite; Pg, paragonite, a in the Ep ss mark~ the minimum X~p defined by assemblage (3).
- Na
#~¢#f • #
tam : a Ep ss
pe 3+
Ca
(1)
Cc
Ac
localities. On each route, specimen number increases northwards, i.e. towards higher stratigraphic levels. FS-10---25 of the Shirataki route, SH-31 and 42 of the Shimokawa route, KZ-27A and 37 of the Kozugawa route, AS-41B, 42 and 61 of the Asemi route are the samples from one metabasite bed in which pumpellyite occurs (see Fig. 2). There is a general tendency for XE~ tO decrease northwards within the metabasite bed on each route. The effect of Xre Chon Xre Epis clearly seen in Fig. 10, in which the average values of xEt, F,: are plotted against XFe. Ch The arrows on bold and dotted lines show the sequence of sample localities from south to north on the Shirataki (FS) and Asemi (AS) routes, respectively. In spite of small-scale wandering, the distinct trend of XFe, Ch decreasing from south to north is not correlated with X~.h especially if the spread of X~ep in individual samples is taken into account. The minimum X~p :appears to be almost independent of XFe.Ch Ar~other conclusion deduced from these figures, is that differences in grade of metamorphisra can be distinguished even within one me'tabasite bed of 200 meters thick. As shown in Fig. 6, IdX~/dTI increases as the temperature of equilibration increases. This may be part of the ~reason why the metamorphic grade could be dis-
,Hm
l Cr
MR
tinguished in a metabasite with such :a small thickness. : Fig. 8 (c) shows the freqaency of X~p in epidote--chlorite-actinolite-purnpellyite rocks from~ the Omoiji-Nagasawa area 'which is a typical I (a) PU~IPELLYITE-FREEZONEOF TilE SHIR~TAKI-ASEMIAREA ~" p o i n t = 76
|
15
!
20
!
!
25
3O
(b) PUP!PELLYITEZONEOF THE SHIRATAKI-ASEM[AREA )'point
= 62
i 20 (C) OMOi J]-NAGASAWA, point: =~91AREA
i 25 ~
25
i
i
30 r
35 ~
30
35
40
Fig. 8. Frequency diagram of XFE~in (a) the epidozc-'hloriteactinolite assemblage from ~he pumpcllyite-free zon~ of the Shirataki-Asemi area, (b) the epidotc-chlorite-actinolitc-pumpellyite assemblage from the pumpellyite zone of the Shirataki-Asemi area and (c) the <:t~idote-chlorite-actinol~te-pumpellyite assemblage from the Omoiji-Nagasawa area.
276 T~:~kashiNakajima
(a)
SHIRATAKIROUTE
Ill
F£-4 { 31.5)
J
~
:
I
•3~L
~40 . . . • JO ~3S
:IX
_L
S H-31(53.8)
---*-
vs-2s(~7.z) ~ . .~s .:~o .2s F..~-30(34.8)
.30
['h
5II-56(54.9) . .
J'L
IPr'z
.
r~
" ~,. • 3.5
ASEHI
AS-41A (53, 8)
ROUTE : M
e,rlh-,,i
,20 ~,,25 •~
.
,~0 "
i
m _
SH-7:$ __ r'R_J"l_ (43.2) r ~ - - 7 .
~z.~1
~ t
AS-42 (42.6)
~
m
AS-45(40.7)
~~,"
I-
~s-.._2_6
(55.4)
.25
i .35
m F'-I 0
_
.
AS-61(43.3) fi~ .~0 AS-?SB (39•4)
,
.~o .,2s ; ' ~ s.-9s(44.7) |
.30
(d)
AS-36A(28.5) ......
I .
.25
AS-41B(52• 6)
SH-70 143.41
.~s
~1
.J5
14t.01 ~s-64 (
;
r~
.20
t
! A m .J0 • [
KZ-62(40.71
"25 ~ ' ~ .
-• (so.t) _ ~ [] ,~/~ ~ • 20 .25 : " SH-45 (48• 6.L . . . . . " ~
. :
['~5
!
.
KZ-63(47.4)
S11-53(48.6)_.~. : r ' J L
FS-57 (44.9) . ~
.
SH-28 (49.7)
~
.~o
K,-41(5o.o)
SH-18 (30.1)
,,
SH-42
FS -:~7(37. --
! 040 :
.
~
~
.I
SH-14 (50.6)
:
FS-19A(47.2), ~| ~ FS-21(47.4)
KOZUGA~AROUTEI~j~
.J0
, ~
~
(C)
KZ-27A(45.1)
S!1-12 (41.61
|
F,q-18F (47.4)
RO.jil~m._.~ UTE -
S,qlMO~WA Sii-4 (25,7)
"
•~0~0B.~.is FS-I4 (42.2)
(b)
L
FS-4,5 (34.1) FS-X0 (48.9)
LITHOS ~5 (1982)
.35
• is
•~0
.2S
P-q;
.30
•~;s
EPIDOTE -;IMI~HIBOLE - CHLORITE - HEMATITE EPIDOTE -,~PHIBOLE - CHLOR!Ig EPIDOTE - ~ P H I B O L E - CHLORITE - PUMPELLYITE
Fig. 9. Change of XF~:pwith increasing temperature of metamorphism along the fear ro.tes in the Shirataki-Asemi area. Each histogram shows the frcqaency of X~Fpof the individual specimen. Numbers in parenthesis beside the specimen numbers represent ,Ch (100 x XF~) of the coexis~ting chlorite. The underlined specimen numbers indicate calcit.e-bearir~g rocks.
pumpeilyite-actinoliite facies :area (Nakajima et al. 1977). The range of AFe ,,Ch in Fig. 8 (c) is 0.450);6, a little 'wider than "'FevCh = 0.50-0.56 in (a) and (b). As the difference in XFc~has little effect on the X~ff of the above-mentioned assemblage, the difference between Fig. 8 (b) and (c) presumabrly is due to the lower temperature of equilibratior in the Omoiji-Nagasawa area than in the Shirataki-Asemi area. So we can distinguish three different grades within th~ pumpellyiteactiaolite facies of the Sanbagawa belt: the Owoiji-Nagasawa area, and the upper and lower
grade zones of file Shirataki-Asemi area. The obser:ation of this isograd s~.roagly depends on the exposure density of metabasite with appropriate bulk composition in the individual terrain• The successful mapping of progressive metamorphic zonation in some parts of the pumpe!lyite-actinolite facies zone in this study suggests that the detection of gradual temperature increases within the pumpellyite-actinolite facies zone is possible if we are blessed with good occurrence of pumpellyite-bearing metabasites.
.~s
L~OS
P~,~mpellyite-actinolitefaeriesmetabasites
15 (1982)
PUMPELLYNE-BEARING ROCK • PUHPELLYITE- FREE ~CK
277
A
o a
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0'25 t ..4AS-4
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AS-41B
,
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Fig. 10. X ~ and X~ of coexisting chlorite and epidote. Solid symbols, epidote--chlorite-actinolitepumpellyite assemblage; Open symbols, epidote--chlorite--actinolite assemblage. Arrows ind,,care the regional trend of increasing metamorphic grade.
FS-25
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/ FS-76
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Discussion The high temperature limit of the pumpellyite-aainolite facies The high temperature limit of the pumpellyiteactinolite facies was previously defined by the transifion from the epidote--chlorite-actinolitepumpellyite to the epidote-chlorite-actinolite assemblages (Seki 1969; Nitsch 1971). The disappearance of pumpellyi'te is controlled by at least three parameters; (1) Xc% in the fluid as discussed in the foregoing section, (2) bulk rock chemistry as expressed in Fig. 6 and (3) metamorphic temperature. The meze disappearance of pumpellyite cannot be directly ascribed to the higher temperature of equilibration. So we should define the high temperature limit of the pumpellyite--actinolite facies using the sliding equilibrium of epidote-chlorite-actir:olite-pumpellyite (without calcite) assemblage. The clisappearance of this assemblage with increasing temperature ir~.the hig~-pressure intermediate group of metamorphism represents the transition from a four-phase assemblage to a three-phase assemblage in the five-component system. Hence a univariant equilibrium has to be defined by specifying two chemical parameters of the solid sob~ttion minerals. Fortamately, as de-
,,
0.5
scribed above, the Mg-Fe 2+ substitution of chlorite does not significantly affect the stability of this assemblage, ~s long as X ~ lies in the range 0.40-0.56 (values common ~n metabasites). Thus, let us choose X ~ as one of the chemical parameters and specify it to be in that range. Another parameter easy to specify is XrE~. This parameter is useful because the epidote-chiorite-actinolite assemblage with X Epless than the specified value indicates that the rock was formed at higher temperature than the defined univariant equilibrium. In this study, XrEP=0oT[5 was chosen as the upper limit of the pumpellyite-actinolite facies. This is the sma|lest value of X~ep in that fourphase assemblage so far reported, and thus can be used to define the high temperature limit of the pumpellyRe-actinolite facies in general. However, any value of XrEe p can be used as the zone marker in individual metamorphic terrains. We can draw several iso~ads with specified valuc~ ot X~p o,- a map to clarify the thermal structure of the terrain if the pumpellyite-bearing assemblage occurs commonly. Fig. 5 implies that the frequency of occurrence of the epidote--chlorite-actinolite-pumpellyite assemblage decreases as the metamorphic temperature increases. The pumpellyite-bearing metabasite bed studied in
FakashiNakafima
278
LrrHos 15 (1982)
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bTg. I i. Metamorphic zone boundaries and pumpellyite disappearance isograd in central Shikoku. The disappearance isograd on the northern side i~, defined using unpublished data of the,, present writer.
detail in this paper has previously been over-. looked for this :eason Our experience in the Sanbagawa belt ~uggests that X~p = 0.'.20 or (1.25 may bt: more practical for defining the high t~'mpcratu,:e limit og the pumpellyite-ac!inolite facies. b,:cause parapellyite becomes quite rare as Xl~p of the coexisting epndote decreases below that value. In any case, the high temperature iinfit of the pumpellyite-actinolite fiacies must be defined with refeFence to the X~p of the highest grade paragene:is.
Con:ribution to the refinement of the thermal srruc?b:re of the Sanbagawa belt in central Sl:ik~ ~tcu In the S~,nbagawa metamocphic beit in centra]l Shikok u, ,~everal isograds in the strict sense have be:,r, established recently (Ranno 1977). These,. in~lt,~de Mg-Fe 2+ partition tetween garnet and chlorite, the solubility of AI in Ca-amphibole of henatite-bearing assemblages, F e 3 + / ( F e 3+ -I-- Ai) ratio of Na-amphibole coexisting with epidote, chlorite, hematite, albite and quartz, and so on. U':ing these indicators, the thermal structure of' t Sanbagawa metamorphic belt in ,central Shi-
koku has been sb-wn te he overturned on a laJs~ scale (Banno et al. 1978). However, these isograds are relevant only in the relatively highergrade part of the belt, namely, in the pumpellyite-free zone. The present stady demonstrates that progressive mineral zonafion can be found within the pumpellyite-actinolite facies zone of the Shirataki-Asemi area. Pumpellyite-bearing metabasites are observed., though spe,radically, at other localities in the Main member of the Minawa Formation. The ~umpellyite disappearance isograd therefore can be traced regionally. It is located on the low temperature side of the garnet isograd and runs !aarailel to it (Fig. 11), along the probable extensic,n of the metabasite horizon examined in detail iq the Shirataki-Asemi area. The Omoiji-Nl,gasawa area, mentioned in the foregoing sectio), includes the Sanbagawa Southern Margioal Belt and the northernmost part of the Chichbu belt (Fig. 1), corresponding to the low-grade region of the Sanbagawa metamorphic belt. Frcm Fig. 8, it is expected that the temperature decreases towards the soath, iiintc the Omoiji-Naga::,awa area. Fig. 12 is a schematic cross section (f central Shikoku in terms of metamorphic zonation; the northern part of the
LITHOS
Pumpel!yite-actinolite facies meiabasites 279
15 (1982)
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SANBAGAWA i:
'"
SANBAGAWA BELT
i
~,..SOUTHERN .,,!= - ~ MARGINAL
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i BELT
Fig. 12. Schematic cross section of central Shikoku in terms of metamorphic zones, it is delineated combining the data of Shirataki-Asemi and Omoiji~-Nagasaw~, areas.
section follows the As~.'mi rout~.• and its extension into the Sanbagawa belt. The southern part of the figure is drawn along a N-S traverse section in the Omoiji-Nagasawa area. The overturned thermal structure is recognized clearly also in the low-grade area, where pumpe!lyite occurs stably. Acknowledgements. - Thanks are expressed to Prof. K. lshioka and Dr. K. Suwa for their continual encouragement. Prof. S. Banno kindly offered helpful advice and critical discussion greatly improving the manuscript. 1 am grateful to Prof. A. Miyashiro, Dr. C. A. Landis and Dr. K. Suwa, who all read the manuscript critically. Discussions ~vith and information presented by K. Aiba, T. Higashino, Drs. S. Maruyaraa, T. ltaya and Dr. M. Otsuki are much appreciated. Dr. S. Higashimoto and the staff of the Geological Survey of Japan gave much encouragement. I acknowledge I. Hiraiwa and S. Yogo for preparing some of the examined thin sections.
References Banno, S. 1964: Petrologic studies on Sanbagawa crystalline schists in the Bessi-lno district, central Sikoku, Japan. J. Fac. Sci. Univ. Tokyo, Sec. 11, 203-319. Banno, S. 1977: Mineral f~,cies of the Sanbagawa metamorphic belt, pp. 97--106 in Hide, K (ed.): Sanbagawa Belt, Hiroshima Univ. Press, Hiroshima. Bannc, S., Higashino, T., Ot~uki, M., Itaya, T. & Nakajima, T. 1978: Thermal structure of the Sanbagawa metamorphic belt in central Shikoku. J. Phys. Earth 26, 345-356. Brown, E. H. 1977: Phase equilibria among pumpellyite, lawsonite, epidote and associated minerals in low-grade metamorphic rocks. Contrib. Mineral. Petrol. 64, 123-136 Coombs, D S., Nakamura, Y & Vuagnat, M. 1976: Pumpellyite-acfinolite facies schists of the Taveyanne formation near Loeche, Valais, Switzerland. J. Petrol. 17, 440-4::1. Ernst, W. G. 1972: CO2-poor composition of the fluid attending Franciscan and Sanbagawa low-gr;~,de metamorphism. Geochim. Cosmochim. Acta 36, 497-504.
Ernst, W. G., Seki, Y., Onuki, H. & Gilbert, M. C. 1970: Comparative study of low-grade metamorphism in the California Coast Ranges and the outer metamorphic belt in Japan. Mem. Geol. Soc. Am. 124. Frost, B. R. 1980: Cbservations oa the boundary between zeolite facies and prehnite-pumpellyite facies. Contrib. Mineral. Petrol. 73, 365-373. Glassley, W. 1974: A model for phase equilibria in the prehnite-pumpellyite facies. Contrib. Mineral. Petrol. 43, 317332. Higashino, T. 1975: Biotite zone of Sanbagawa metamorphic terrain in the Shiragayama area, central Shikoku, Japan. J. Geol. Soc. Japan 81,653-670. Horikoshi, Y. 1938: Properties of a few constituent minerals in metamorphic rocks of the Bessi district, part 2 (epidote group). J. Geol. Soc. Japan 45, 342-351. Raya, T. & Otsuki, M. 1978: Stability and paragenesis of Fe-'l'i oxide mine-als and sphene in the basic schists of the Sanbagawa melamorphic belt in central Shikoku, Japan. J. Japan Assoc. Mln. Pet. Econ. Geol. 73, 359--379. Kerriek, D. M. 1974: Review of metamorphic mixed volatile (H20--CO2) equiliria. Am. Mineral. 59, 72%761. Kurata, H. & Banno, S. 1974: Low-grade progressive metamorphism of pelitie schists of the Sazare area, Sanbagawa metamorphic terrain in central Shikoku, Japan. J. Petrol. 15. 361-382. Leake, B. E. 1978: Nomenclature of amphiboles. Am. Mineral. 63, 1023-1052. Liou, J. G. 1973: Synthesis and stability relations of epidote Ca2AI2Fe3i3Olz(OH). J. Petrol. 14, 381-413. Lit~u, J. G. 1979: Zeolite facies metamorphism of basaltic rc.cks from the East Taiwan Ophiolite. Am. Mineral. 64, 1-14. Miyashiro, A. & Seki, Y. 1958: Enlargement of the composition field of epidote and piemontite with rising temperature. Am. J. Sci. 256, 423-430. Nakajima, T., Banno, S. & Suzuki, T. 1977: Reactions iez~.;ng to the disappearance of pumpellyite in low-grade metamorphic rocks of the Sanbagawa metamorphic belt in central Shikoku, Japan. J. Petrol. 18, 263-284. Nakamura, Y. & Kushiro, 1. 1971): Compositional relations of coexisting orthopyroxene, pigeonite and augite in a thaleiitic
280
TakashiNaka]ima
andesite from Hakone volcano. Contrib. Mineral. Petrol. 26, 265-275. Nitsch, K. H. 1971: Stabilit~itsbeziehungen yon Prei~nit- und PumpeUyit-haltigen Paragenesen. Contrib. Mineral. Petrol. 30, 240-260. Fassaslia, E. & Gottardi, G. 1973: Crystal chemistry and nomenclature of pumpellyites and julgoldites. Can. Mineral. 12, 219-223. Raith~ M. 1976: The AI-Fe(III) epidote miscibility gap in .a meLamorphic profile through the Pennic Series of the',Tauern window, Austria. Contrib. Mineral. Petrol. 57, 99--;i17. Seki, Y. 1958: Glaucophanitic regional metamorphism in the Ka~~to Mountains, central Japan. Japan. J. Geol. Grogr. 29, 233-238. Seki, Y. 1969: Facies series in low-grade metamorplaism. J. Geol. Soc. Japan 75, 255-266. ~ompson, A. B. 1971:Pco2 in low-grade metamorphism;
LrrHOS 15 (1982) zeolite, carbonate, clay mineral, prehnite relations in the system CaO-AIaO3-SiOz--CO~H20. Contrib. Min~eral. Petrol. 33, 145-161. Toriumi, M. 1972: Microprobe study of zoned epidote in the Sanbagawa rocks from the Kanto Mountains. J. Geol. Soc. Japan 78, 545-~48. Toriumi, M. 1975: Petrological study of the Sanbagawa metamorphic rocks, Kanto Mountains, Japan. Butl. Univ. Museum, Univ. Tokyo, No. 9. Watanabe, T, 1974: Metamorphic zoning of the Sanbagawa and Chichibu belts in the Koshibugawa river area, Oshika district, central Japan with special reference to pumpellyiteactinolite facies mineral assemblages. J. Geol. Soc. Japan 80, 525-53g. Zen, E-an. 1974: Prehnite- and pumpellyite-bearing mineral assemblages, west side of Apalachian metamorphic belt, Pennsylvania to Newfoundland. J. Petrol. 15, 197-242.