American Mineralogist, Volume 73, pages48-56, 1988
Magnesianstaurolite in garnet-corundumrocks and eclogite from the
Donghai districtoJiangsu province,east China
Mlsaxr ENavrr
Department ofEarth Sciences,School ofScience, Nagoya University, Nagoya 464, Japan
Qr.ru. ZlNc
Department of Geology, Beijing University, Beijing, China
Ansrru,cr
Magnesian staurolite of X*, : 0.68-0.74 was found in garnet-corundum rocks and
eclogite from the Donghai district, Jiangsu province, east China. The garnet-corundum
rocks consistmainly of primary garnet,corundum, zoisite, and Na-rich phlogopite(NarO :
2.7 + 0.2 wto/o),with secondarymagresian staurolite, chlorite, Al-rich pargasite(AlrOr:
22.4 wto/omaximum) and Mg-rich allanite (MgO : 6.2 wto/omaximum). The eclogite is
composedmainly of primary garnet, clinopyroxene, corundum, kyanite, and Cr-rich zoisite (CrrO3 : 1.6 wto/omaximum) with secondarymagnesianstaurolite and calcic amphibole. Equilibrium conditions of primary minerals were estimatedat about 800-850'C and
I 1-30 kbar for the garnet-corundumrocks and 700-750t and I l-25 kbar for the eclogite.
The magnesianstaurolite occurs as pseudomorphsafter garnet, corundum, and kyanite.
Equilibrium pressureof the magnesianstaurolite is more than 1l kbar. Its equilibrium
temperatureis slightly lower than that of the primary minerals.
Cell dimensionsof the magnesianstauroliteof XM,:0.74 are a:7.873(3) A, b:
16.557(7)A, and c: 5.637(3)A. Its b axis is distinitly shorrer than that of Mg-poor
staurolite. CrrO, content of the magnesianstaurolite reaches| .2 wto/o.The Mg enrichment
of staurolite is controlled by the Mg substitution for vIAl and by the Mg + Fe substitution
in the IvFe sites.
the associated rocks in the Donghai district, Jiangsu
province, east China, magnesianstaurolite (X., : 0.68Staurolite is a common constituent mineral of pelitic 0.74) was found in garnet-corundum rocks and eclogite.
and other aluminous rocks metamorphosedunder inter- This is the most Mg-rich natural staurolite, as far as we
mediate pressureand temperature conditions. Staurolite know, among staurolites reported. The mode of occuris characteristicallylower in X". [: Mg/(Mg + Fe)] value rence, chemistry, crystallographic data and petrogenesis
than the coexisting silicate minerals: its Xr, value is usu- of the magnesianstaurolite are described in the present
ally lessthan 0.3 (e.g.,Deer et al., 1982).Iron-freestau- paper.
rolite (i.e., the pure-Mg end member) was synthesized
Gnolocrc sETTTNGAND PETRoGRAPHY
under the conditionsof i": 700-950 "C and P > I I kbar
by Schreyer(1967)and Schreyerand Seifert(1969).HellMagnesian staurolite was found in two garnet-corunman and Green (1979) experimentally produced Mg-rich dum rocks and an eclogite, from the Zhimafang and
staurolite(X-u: 0.53-0.57) from olivine tholeiitic ma- Mengzhong areas in the Donghai district, respectively.
terials under the conditions of Z: 740-760 "C and P :
Figure I showsa geologicsketch map of east China. The
24-26 kbar. These experimental works suggestthat Mg- Precambrian basementof east China is divided into two
rich staurolite could occur in Mg- and Al-rich metamor- units by the Tancheng-Lujiangfracture zone, i.e., an eastphic rocks that formed at high temperatureand pressure. ern unit (the Jiaoliao massif, age 1300-2400 Ma), and a
Recently, Mg-rich stauroliteshave been found from var- western unit (the Jilu massif, age >2400 Ma) (Tectonic
ious types of rock metamorphosedunder high pressures map compiling group, Institute of Geology, Academia
(X-" : 0.43-0.56, Ward, 1984a; 0.49, Schreyeret al., Sinica, 1974). The Donghai district is situated about 50
1984; 0.40-0.42, Grew and Sandiford, 1984; 0.52, Ni- km east of the Tancheng-Lujiang fracture zone and becollet, 1986).TheseMg-rich staurolitesalwayscoexistwith longs to the Jiaoliao massif. This district is underlain by
corundum. A silica-undersaturatedenvironment is also pelitic and mafic gneissesand serpentinized ultramafic
required for the formation of Mg-rich staurolites (e.g., rocks.
Schreyer,1967).
The garnet-corundum rocks and eclogite occur as subIn the course of our petrologic study on eclogite and rounded blocks (diameter. 5-50 cm) in ultramafic rocks
INrnooucrroN
0003-004x/88/01024048$02.00
48
49
ENAMI AND ZANG: MAGNESIAN STAUROLITE
stauroliteof magnesian
compositions
Tneue1. Bulkchemical
bearingrocks
TM-02
rs-03f
wr%
sio,
Tio,
Alro3
CrrO"
FerO.
FeO
MnO
Mgo
Nio
CaO
Na.O
K.o
l-T-l ximuertitel.-71 routt
l---l M"soroi.-Cenozoic
PrO,
H,O(+)
H,O(-)
Total
34.26
0.02
34.87
0.31
0.58
4.14
0.18
13.40
n.d.
10.90
0.15
0.05
0.33
0.32
0.12
99.63
47.0
0.58
14.1
0.79
7 76'
o.12
14.3
0.05
13.5
0.81
0.o2
0.01
Or
Ab
An
Di
0.1
6.9
34.8
25.6
Hy
13.5
ol
14.3
Mt
Cm
llm
Ap
99.0
En
Fs
Wo
En
Fs
Fo
Fa
9.5
2.7
13.4
10.5
3.0
10-g
3.4
1.6
1.2
1.1
0.0
99.1
Note.'Normativecomposition of TM-02 was calculatedon the basis of
ratio of 0.15.
Fe3+lFe2+
. Total Fe as FeO.
| [T[I[TTIr-ot" Proterozoic Poloeozoic
t Analyzed by ShiguangWang.
Proterozoic
-o^,,-^ | Io,.no"on-Eorty
Fig. 1. Geologicsketchmap of eastChina(simplifiedfrom ue (1.0) and low FerO, + FeO content (4.7 wto/o)and is
Research
Instituteof Geology,ChineseAcademyof Geological
similar to that of some diaspore bauxites (e.g., Valeton,
Sciences,
1982).TLFZ: Tancheng-Lujiang
fracturezone.
1972).This rock is alsoenrichedin MgO and CaO. These
components may have been derived from dolomite or
intruding the Precambrianpelitic gneisses.The ultramaf- dolomitic limestones, which are usually associatedwith
ic rocks in the Zhimafang area are serpentinizedphlog- bauxites. The garnet-corundum rocks are considered to
opite-bearinggarnetlherzolite. Equilibrium conditions of be metamorphosedmixtures of bauxitesand Mg-rich qarthe garnetlherzoliteare estimatedat about f: 750-850 bonate rocks, on the basis ofthe occurrenceofdiaspore
'C and P : 20-30 kbar (Zang et a1.,in prep.). The ultra- and carbonate inclusions in the corundum crystals and
mafic rocks in the Mengzhong area are completely ser- the bulk chemistry of the garnet-corundum rock.
pentinized. Bulk chemical analyses of the magnesian
Mengzhong sample
staurolite-bearingrocks are given in Table l.
The Mengzhong sample (TM-02) is massive and conZhimafang sarnples
sists of a clinopyroxene-rich $een part and garnet-rich
The Zhimafang samples (TS-03 and TS-04) are com- brown part. The sample contains garnet, clinopyroxene,
posedof a corundum-rich pink part and garnet-richwhite and subordinate amounts of corundum, kyanite, zoisite,
part. The samples contain garnet and corundum with dolomite, rutile, and apatite as primary minerals' Secsubordinateamounts of Na-rich phlogopite,zoisite, pent- ondary minerals are magnesianstaurolite, calcic amphilandite, and apatite as primary minerals. Aggregatesof bole, clinozoisite, and chlorite.
fine-graineddiaspore,margarite,dolomite, and calcite (all
Garnet occursusually as euhedral or subhedralcrystals
<0.03 mm in size)were observedas inclusions in corun- 0. l-0.5 mm in diameter. Large garnet crystals l-2 cm in
dum crystals. Secondaryminerals are magnesianstauro- diameter are sometimesobserved.The garnetcrystalsare
lite, Al-rich pargasite,chlorite, clinozoisite, Mg-rich al- usually replaced by calcic amphibole, chlorite, and clinozoisite around crystal rims and along fractures. Clilanite, and heazlewoodite(Ni.Sr).
Both garnet and corundum occur as anhedral crystals nopyroxeneoccursas rounded and subhedralcrystals0.10.1-2.0 mm in size.They are usually surroundedby ag- 0.5 mm in sizein the matrix and as fine inclusions smallgregatesof magnesianstaurolite and chlorite. Garnet is er than 0.05 mm in size in the garnet crystals.Corundum
partly replaced by Al-rich pargasite and clinozoisite is smaller than 0.03 mm in size and is rimmed by chloaround the crystal rim and along fractures. Phlogopite rite. Kyanite forms anhedral crystals smaller than 0'02
showsa subhedraland tabular form and is partly replaced mm in size and is replaced by magnesianstaurolite and
by chlorite. Magnesian staurolite constitutes acicular and/ calcic amphibole. Zoisite shows prismatic forms 0'l-0.3
or prismatic crystals and forms pseudomorphsafter gar- mm in length and occurs usually as inclusions in garnet.
net and corundum with chlorite (Fig. 2).
Some zoisite crystals contain fine-grained garnet inclugarnet-corundum
(TSThe bulk chemistry of the
rock
sions smaller than 0.03 mm in size.Magnesianstaurolite
03) is characterizedby an extremely high AlrO./SiO, val- forms anhedral crystalssmaller than 0.02 mm in size and
50
ENAMI AND ZANG: MAGNESIAN STAUROLITE
Magnesian staurolite
Fig.2. Backscattered-electron
imageof magnesian
staurolite
(sampleTS-04).Scalebar indicates200
and otherconstituents
pm. Abbreviationsfor minerals:St, magnesian
staurolite;Grt,
garnet;Crn, corundum;Chl, chlorite.
occurs as pseudomorphsafter kyanite. Magnesian staurolite crystalsare partly surrounded by calcic amphibole.
The garnet + clinopyroxene + corundum assemblage
of the Mengzhong eclogite suggestsequilibrium conditions of T:100-750 "C and P : ll-25 kbar for the
primary stage(Enami et al., 1986).Its bulk and mineral
compositions are similar to those of eclogitexenoliths in
kimberlites (Tables I and 3; also cf. Dawson, 1980).These
data suggestthat the Mengzhong eclogite was generated
from the lower crust or upper mantle.
MrNpn.q.Locy oF MAGNESTANsTAURoLITE ANL,
ASSOCIATED
MINERALS
Chemistry. HrO content of staurolite is variable and
cannot be associatedwith a fixed stoichiometry (e.g.,Juurinen, 1956;Lonker, 1983;Holdawayet al., 1986a).Li
is sometimesconcentratedin staurolite(Hietanen,1969;
Grew and Sandiford,1984;Holdaway et al., 1986b; Dutrow et a1.,1986).HrO and LirO contentscannot be estimated by err"re analysis, and so the chemical formula
of the magnesianstaurolite was calculatedon the basis of
Si + Al * Cr: 25.53 as recommendedby Holdaway et
al. (1986b).Y,O,,ZnO, CoO, CaO,Na,O, and KrO contents of the magnesianstaurolite are below the detection
limits, i.e.,lessthan 0.02wto/ofor VrO, and lessthan 0.01
wto/ofor the other elements.
The magnesianstaurolite is faintly yellow in thin section and chemically homogeneous.X., values l: N4g/
(Mg + Fe)l of the magnesianstaurolite are 0.74 + 0.01
and 0.69 + 0.01 in the Zhimafangand Mengzhongsamples, respectively.Average Si content per formula unit is
7.92 for the Zhimafang samples and 7.14 for the Mengzhong sample.Grew and Sandiford (1984) pointed out
that lower Si content is characteristicof Mg-rich staurolenvironments.Although the
ite from silica-undersaturated
present samples contain corundum, the Si contents of
these magnesianstaurolites are higher than the Si contents of Mg-rich staurolite from other corundum-bearing
samples(e.e.,7.43-7.68; Ward, 1984a).The magnesian
staurolite here occurs as pseudomorphsafter corundum
and/or kyanite, and so the relatively high Si contents are
probably due to disequilibrium with the associatedaluminous minerals. CrrO, contents of the magnesianstaurolite are up to 0.70 and 1.17wt0/ofor the Zhimafangand
Mengzhongsamples,respectively.
Figure 3 explores the variation of cation contents in
the staurolite monolayer, except for Al (FM content :
Fe * Mn + Mg + Zn -l Co + Ni + Li + Ti), for staurolites reported in the literature. Figure 3 clearly shows
that the FM content ofstaurolite increaseswith increase
FM content of Mg-rich
in I", value (: M/FM.The
staurolites(Ir, > 0.4) is higherthan 4.0, and its average
is 4.6. The FM content of Mg-poor staurolites (I., =
0.2) is mostly lower than 4.2, and its averageis 3.9. Two
Mg-poor stauroliteswith -FM > 4.6 werereportedby Griffen and Ribbe (1973)(specimens4 and l0), and the latter
has a high ZnO content of 6.86 wt0/0.Staurolites with
Y-": 0.24.4 have compositionsintermediatebetween
the above two groups (averageFM content is 4.2), except
for the unusuallyZn-enriched staurolitesvithZnO :7 .44
and 6.73 wto/oreported by Juurinen (1956) and Miyake
(1985),respectively.The compositionalvariation seenin
Figure 3 may have some uncertainities becauseof the
absenceof HrO and/or LirO data in most analyses.Thirty
complete analysesof staurolite,l including HrO and LirO,
by Holdaway et al. (1986b),however,also show that the
Chemical analyseswere performed with a lBor- electron-probe microanalyzer (nrua) JCxA-233.Accelerating
voltage, specimencurrent, and beam diameter were kept
at 15 kV, 1.2 x 10 8 A and 3 pcm,respectively.Chemical
compositions of the magnesianstaurolite and other constituent minerals are given in Tables 2 and 3, respective' Excludingoneanalysis(specimen
6-3),believedto be inacly.
curateowingto variabilityin Li content(Dutrowet al., 1986).
5l
ENAMI AND ZANG: MAGNESIAN STAUROLITE
TnaLe2. Chemicalcompositionsof magnesianstaurolites
Mengzhong
Zhimafang
TM-02
TS-03
c-1t
sio,
Tio,
Atr03
Cr,O.
FeOMnO
Mgo
Nio
CaO
Naro
KrO
Total
Si
AI
Ti
Cr
Fe2**
Mn
Mg
Ni
Ca
Na
K
X"nf
n:20I
C-27
29.8(0.3)
29.8
29.5
0.00
0.00
0.00
s5.9(0.4)
55.8
56.2
0.s3(0.08) 0.53
0.57
4.69(0.21) 4.70
4.60
0.07(0.03) 0 09
0.10
7.38(0.07) 7.37
7.35
0.16(0
05)
0.14
0 . 11
0.00
0.00
0.00
0.00
0.00
0.00
000
0.00
0.00
98.5
98.4
98.4
(si + Al +
Formulae
7.917
7.927
7.830
17.502
17.492 17.580
0.000
0.000
0 000
0.111
0 11 1
0,120
1.042
1.045
1.021
0.016
0.020
0.022
2.922
2.923
2.908
0.034
0.030
0.023
0 000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
0.000
o.74
0.74
0.74
299(0.4)
0.00
56.1(0.4)
0.54(0.09)
4.76(0.19)
0.03(0.03)
7.50(0.15)
0.16(0.03)
0.00
0.00
0.00
99.0
cr : 25.53)
7.915
17.502
0.000
0.113
1.054
0.007
2.960
0.034
0.000
0.000
0.000
0.74
n:7I
crf
28.1
28.s(0.2)
0.33
0.25(0.06)
54.4
s5.0(0.s)
0.83(0.21) 1.17
5.92(0.10) 6.09
0.00
0.00
7.36(0.11) 7.33
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
97.9
97.4
7.743
17609
0.051
0 178
1.345
0.000
2.981
0.000
0.000
0.000
0.000
0.69
7.703
17.574
0.068
0.254
1.396
0.000
2.995
0.000
0.000
0.000
0.000
0.68
Nofe.'Values in Darenthesesare the standard deviations.
. Total Fe as FeO.
t n, number of analyticalpoints; C-1 and C-2, crystals 1 and 2 for the X-ray analysis,respectively;
Cr. most Cr-richcomoosition.
+ XM,: Mg/(Mg + Fe).
FM content increaseswith increasing Ir, value in staurolite: averageFM content of nine stauroliteswith yMs>
0.2 is 4.5 and that of three stauroliteswith fM8 = 0.1 is
3.8. High FM csntent of some staurolitescan be possibly
explained by substitution of vIAl by Fe3*, which would
be seenas part of the FM components (Holdaway et al.,
1986b).Most of Mg-rich stauroliteswith fMs > 0.4, however,coexistwith zoisiteor Fe3+-poorclinozoisite(FerO, :
2.33-4.2 wto/o)and probably have little Fe3*.Staurolite
of high Yr" tends to be lower in "tAl content and vice
versa:AveragevIAl contentsare 17.3 and 11.6 for staurolites with Y-, > 0.2 and fMg < 0. 1, respectively(Holdaway et al., 1986b).Thus the high FM content of Mgrich staurolite suggeststhe preferential Mg substitution
for "IAl. Alternative explanationsfor the "'Al = Mg substitution are 2vIAl -= 3 Mg (seeWard, 1984a)or vrAl +
Mg+H.
Lonker (1983) showed the negative correlation between vIAl and (Fe + Mg) + OH contents.This negative
correlation implies that the second substitution scheme
better explains Mg enrichment in staurolite. The present
magnesianstaurolite has lower FM (4.0-4.4) and lower
Fe (l.l-1.3) concentrationsthan those of other Mg-rich
staurolites. This fact suggeststhat the Fe = Mg substitution in theI"Fe sitesalso affectsthe high Mg contentof
the magnesianstaurolite concerned.The Mg substitution
in the IvFe sites is attributed to the extremely high X.,
values of host rocks (0.84 for TS-03 and 0.77 for TM02).
X-ray data. The magnesianstaurolite was examined on
a Rigaku RAD-?BX-ray microdiffractometer with position-sensitive proportional counter (esnc,/vrocsystem),
using V-filtered CrKa radiation ()\:2.29092 A, 40 kV,
200 mA). X-ray diffraction data were obtained from the
two selectedareas of about 30 um in diameter for two
crystalsin a thin section.Averagechemical compositions
ofthe selectedareasobtained by errue analysesare shown
in Table 2.
The X-ray diffraction data are summarized in Table 4.
Peakswere indexed with respectto Schreyerand Seifert
(1969)and Borg and Smith (1969).Diffractionswith d >
6.6 A could not be observed, since 2d of the microdiffractometer rangedfrom 20" to 140" in the presentstudy.
Preliminary X-ray powder-difraction analysis of a mixture of the magnesianstaurolite and gamet crystals showed
a weak peak at d : 8.34 A, correspondingto the 020
reflection of the magnesian staurolite. Observed peaks
and their intensitiesare different betweenthe two selected
areas,becauseof the limited oscillation anglesaround 4
and X axesof the microdiffractometer. Peak positions of
the presentstudy correspondwell to those ofFe-free staurolite svnthesizedbv Schreverand Seifert (196q. An or-
52
ENAMI AND ZANG: MAGNESIAN STAUROLITE
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53
ENAMI AND ZANG: MAGNESIAN STAUROLITE
TneLe4. X-ray data of magnesianstaurolite in TS-03
Crystal2
Crystal 1
Ulo
4.53
4.43
3.549
<5
4.45
20
3.527
3.225
ZJ
3.007
95
2.618
2.542
2.387
20
100
40
3.048
3.002
2.846
2.683
15
55
15
65
2 542
55
z Jbb
Ulo
10
100
2.351
2.289
Fig. 3. Variations of FM (: Fe * Mn + Mg + Zn + Co +
Ni + Li + Ti) content of staurolites.Solid trianglesindicate aveftge FM contents.Data are from Asami and Hoshino (1980),
Asami et al. (1982),Ashworth (1975),Baltatzis(1979),CEchet
al. ( I 98 1),Enami and Zang(this paper),Foster( 1977),Fox ( I 97 l),
Gibson (1978),Green(l963), Grew and Sandiford(1984),Griffen and Ribbe (1973),Guidotti (1970),Hietanen(1969),Hiroi
(1983),Hollister (1970),Hollister and Bence(1967),Hounslow
andMoore (1967),Hudson(1980),Juurinen(1956),Kwak (1974),
Lonker(1983),Miyake (l 985),Pigage(l 976),Schreyerand Chinner (1966),Schreyeret al. (1984),Smith (1968),Tak6uchiet al.
(1972),von Knorring et al. (1979),and Ward (1984a,1984b).
40
2 257
2.099
2 069
2.054
2.003
30
30
<5
10
<5
1.776
1.765
1.706
c
<5
10
1 570
't 542
5
10
1 514
70
1.474
1.447
1.429
15
15
10
1.378
1.371
1.312
5
60
30
2.097
2.069
1.857
10
1 763
<5
1.660
1.600
10
15
1.542
1 538
JC
30
1511
1.473
1.446
5
30
1.423
J
1. 3 1 0
1.296
5
TC
3
4.52
4.42
3.555
3.526
3.227
3.052
3.007
2.853
2.684
2.620
2.545
2.392
2.370
2.355
2.292
2.260
2.102
2.070
2.O57
2.005
1.858
1.778
1.766
1.707
1.661
1.600
1.569
1.541
1.536
1.514
1.512
't.473
1.446
1.428
1.422
1.380
1.370
130
111
220
131
201
150
221
240
151
112
241
132
330
311
202
260
171
080
350
-242
401
440
172
191
223
153
243
461
282
530
192
210
173
0102
550
0120
512
1
<5
1.279
1.273
<5
<5
1.264
1.255
1.239
10
c
1.267
J
<5
1.239
1.221
10
10
thorhombic cell was assumed,and unit-cell dimensions of garnet end members are Prp54sAlm,ooGrsrrrSpsor,
calculated on the basis of 30 reflection data with 44. <
PrprrrAlm,ooGrsro
oSpso
r, and PrprorAlmrr rGrsr'Spso,
20 < 120" (excluding 0k0 reflections)are as follows: a :
for TS-03, TS-04, and TM-02, respectively(abbreviations
7.873(3)A, b : r6.5s7(7)A, and c: 5.637(3)A. rhe a after Kretz, 1983).The CrrO, content of TM-02 garnet
and c axes are similar to those of staurolites in the literreaches2.9 wto/oin the crystal core. The clinopyroxene is
ature(a : 7.850-7.891A and c: 5.635-5.667A). crifpale green in hand specimen,considering its substantial
fen et al. (1982) showedthat the b axis ofstaurolite de- CrrO. content (0.63 wto/omaximum), and is sodian augite
creasessystematicallywith decreasingmean ionic radius (NarO : 2.0 + 0.3 wto/o,
X*u:0.94) of Esseneand Fyfe
(MIR) of cations in the monolayer. The D axis of the (1967). The amphibole varies in composition from flake
magnesianstaurolite (TS-03) lies close to the regression to flake. In the TM-02, the amphibole replacinggarnet is
line of that againstthe MIR value (Fig. 4).
magnesiohornblendewith AlrO, : 12.0 + 2.1 wto/oand,
that replacing the magnesian staurolite is tschermakite
Garnetoclinopyroxene,and calcic amphibole
with maximum AlrO, of 18.8 wt0/0,according to the noThe garnet is mostly homogeneousand belongs to a menclature of Leake (1978). The amphibole in the Zhigrossular-richpyrope-almandine serieswith very low MnO
mafangsamplesis Al-rich pargasite,and its AlrO3 content
content(lessthan 0.3 wto/o).Averagecompositionsin terms is up to 22.4 wto/o.
54
ENAMI AND ZANG: MAGNESIAN STAUROLITE
Fig. 4. Relationship between mean ionic radius (MIR) of cations in the monolayer and b axis of staurolite. Regressionline
plotted was calculatedusing ten data of synthetic staurolites.Ionic radii were taken from Griffen ( I 98 1) for Zn and from Shannon
(1976)for the other elements;r: correlationcoefficient.Dataarc from C6ch et al. (1981),Enami and Zang (this study),Griffen
and Ribbe (l 973),Juurinen(1956),Miyake (1985),Schreyerand Chinner(1966),Smith (1968),Tak6uchietal. (1972),von Knorring
et aI.(1979),and Ward (1984a)for natural staurolitesand from Gritren (1981),Richardson(1966),and Schreyerand Seifert(1969)
for synthetic staurolites.
Corundum and kyanite
Corundum is a pink variety owing to its high CrrO3
content,up to 1.1wto/oin the Zhimafangsamplesand 0.9
wto/oin the Mengzhong sample. Its FerO. content is less
than 0.4 wto/0.Kyanite is also enrichedin Cr,O, (1.2 v,to/o
maximum), and its FerO. contentis 0.4 + 0.1 wto/0.
Cal'MgsorSd,for the Mengzhongsample.Fe, Mn, and
Mg components in calcite were not detected. Heazlewoodite is depleted in Fe (lessthan 0.1 wt0/o)and has a
formulaof Ni3or52.
Fonnrarronr oF MAGNESIANSTAURoLITES
Textural and chemical evidenceshowsthat the magnePhlogopiteochlorite, and margarite
sian staurolitesin theserocks are all retrogradeproducts.
Both phlogopite and chlorite have high Xr, values of The Zhimafang staurolite occurs as a pseudomorph after
about 0.96 and are colorless in thin section. Phlogopite garnet and corundum and forms an aggregatewith chlois unusuallyenrichedin Na.O (2.7 + 0.2 wtVo).NiO con- rite. The Mengzhongstaurolite occurs as a pseudomorph
tents of phlogopite and chlorite in the Zhimafang samples after kyanite. Suggestedreactionsfor the magnesianstauare up to 1.0 and Ll wt0/0,respectively.Margarite is de- rolite formations are as follows:
pletedin both Na,O + K,O (0.18wto/o)and FeO + MgO
+ 340AlrO3+ 432HrO
197(Mg,Fe).Al,Si.O,2
(0. 1 wto/o).
sarnet
Epidote group minerals
-;*;toAl,ssi,,ooo(oH)o
staurolrt€
Both zoisite and clinozoisite are colorless in thin sec+ 42(Me,Fe),,*hiJ,,O'n(oH)'u
tion. Xo.:r values [: Fe3+/(Fe3+
+ Al)] of Zhimafang and
Mengzhongzoisites are 0.030 and 0.035, respectively.
CrrO. content of the latter reaches1.6 wto/o.Clinozoisite for the Zhirnafang samples and
is depletedin Fe3*,ar.d Xr"r* valuesare about 0.10 and
8(Mg,Fe).AlrSi3O,,+ 25Al2O.'+ 2lAlrSiO5 + l2HrO
0. I 1 for the Zhimafang and Mengzhongsamples,respecgamet
kyanite
corundum
tively. Allanite in the Zhimafang sample is pale brown
- 6(Mg,Fe)oAl,ssi?
5Oo4(OH)o
and shows a weak compositional zoning consisting of
REE-rich coresand Ca-rich rims. CaO and REE contents
are 10.9 and 28.0 wto/oin the crystal core and 12.5 and for the Mengzhongsample. Grossular component in gar24.5 wto/oin the crystal rim, respectively.The MgO con- net was probably consumed for the formation of calcic
tent of allanite reaches6.2 wt0/o,and total Fe as FeO is amphibole and clinozoisite.
For the Zhimafang samples,the equilibrium temperalessthan 2.1 wIo/0,suggestingthe substitution Ca * Al +
ture of the primary garnet + corundum stagewas estiREE + Mg.
mated at about 800-850 "C by the garnet-biotite geotherOther minerals
mometer(methodsof Indaresand Martignole, 1985,and
Diasporeis enrichedin CrrO. (1.0 wt0/o),and its FerO. Hoinkes, 1986). The temperaturecalculatedusing the
contentis lessthan 0.6 wto/0.Dolomite is homogeneous, Ferry and Spear(1978) calibration for the Ca- and Mnand its averagecomposition in terms of carbonate end free system is 680 'C. The lower temperature estimates
membersis CalrrMgso,Sdo
for the Zhimafangsamplesand with the latter calibration may result from high grossular
ENAMI AND ZANG: MAGNESIAN STAUROLITE
component of garnet (35.3 molo/o).The lower pressure
limit of the primary stageis about ll-14 kbar on the
basis of the garnet * corundum assemblage(cf. Droop
and Bucher-Nurminen, 1984) and the stability field of
pyrope garnet(Schreyerand Seifert, I 969). Its upper pressurelimit is probably definedby the equilibrium pressure
ofthe host garnet-lherzoliteofabout 20-30 kbar. For the
Mengzhong sample, the equilibrium conditions of the
primary stagewere estimated at T : 700-750 "C and P
: ll-25 kbar (Enami et al., 1986).The estimatedconditions of the primary stagemay define the upper I and
P limits of magnesianstaurolite formation. The lower I
and P limits of magnesianstaurolite formation are estimated at about730-770'C and I I kbar from the stability
range of the magnesianstaurolite * chlorite assemblage
(cf. Schreyer and Seifert, 1969; Grew and Sandiford,
1984). Equilibrium conditions of the magnesianstaurolite of this study are similar to those of other Mg-rich
staurolitesin the literature.
In the presentstudy, magnesianstaurolite occursin two
characteristicrock types.TheZhimafang magnesianstaurolite occurs in highly aluminous and silica-undersaturated rocks. The Mengzhongmagnesianstaurolite, on the
other hand, occurs as a pseudomorph after kyanite in an
eclogite without normative corundum. Schreyer (1967)
suggestedthat the lack of natural magnesian staurolite
was due to the absenceof rocks having highly aluminous
bulk compositions in the upper mantle where I and P
conditions necessaryfor the formation of magnesian
staurolite may prevail (i.e., higher than I I kbar). He also
pointed out that magnesianstaurolite and quartz are lncompatible by virtue of the stable tie lines between kyanite and other magnesiansilicates.The Zhimafang samples,as well as most of Mg-rich staurolitesin the literature,
occur in metamorphosed peraluminous rocks such as
sapphirine-garnet
rock (Schreyeret al., I 984),corundumbearing phlogopite-chlorite schist (Grew and Sandiford,
(Nicollet, 1986).ThesesamI 984),and meta-anorthosite
ples suggestthat Mg-rich staurolite is formed in rocks
having restricted bulk chemistry under limited T and P
conditions. Mg-rich staurolite formation in typical mafic
rocks may require much higher pressurereaching 24-26
kbar as shown by Hellman and Green (1979). The Mengzhong sample,however, clearly showsthat Mg-rich staurolite can occur in common eclogite.Ward (1984a)reported Mg-rich staurolite in a reaction zone between
plagioclaseand orthopyroxene crystals in a metatroctolite. The Mengzhongsample and that of Wand thus suggest that Mg-rich staurolite can be formed in common
metabasitesby local reactions of aluminous and magnesian minerals.
The Mengzhongsample has bulk and mineral compositions similar to those of eclogitexenoliths in kimberlites
(Enami et al., 1986).Kyanite and other aluminousminerals are common in high-pressureeclogites(e.g., Dawson, 1980),and local breakdownof theseminerals with
decreasingtemperaturemay be expectedto form Mg-rich
staurolite.Hellman and Green(1979)suggested
that stau-
55
rolite may be widespreadin mafic rocks metamorphosed
at high pressures,as, for example, in the subductedlithosphere.We also emphasizethat careful petrographic examinations will probably reveal wide occurrence of
Mg-rich staurolite as an accessorymineral in common
metabasitesformed under high pressures,especially in
eclogites.
AcrNowr,slcMENTS
We wish to expressour sincerethanks to ProfessorK. Suwa of Nagoya
University for his critical comments on the manuscript. Thanks are also
due to Mr S. Wang of Beijing University and Mr. M. Amemiya of Rigaku
for their help in laboratory and to Mr I. Hiraiwa of Nagoya University
for preparing many thin seclions This study was supported in part by a
Grant in Aid for Scientific Researchfrom the Ministry of Education of
Japan(No. 62740481:M E ).
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