American Mineralogist, Volume 73, pages 48-56, 1988

Magnesian staurolite in garnet-corundum rocks and eclogite from theDonghai districto Jiangsu province, east China

Mlsaxr ENavrrDepartment ofEarth Sciences, School ofScience, Nagoya University, Nagoya 464, Japan

Qr.ru. ZlNcDepartment of Geology, Beijing University, Beijing, China

Ansrru,cr

Magnesian staurolite of X*, : 0.68-0.74 was found in garnet-corundum rocks andeclogite from the Donghai district, Jiangsu province, east China. The garnet-corundumrocks consist mainly of primary garnet, corundum, zoisite, and Na-rich phlogopite (NarO :2.7 + 0.2 wto/o), with secondary magresian staurolite, chlorite, Al-rich pargasite (AlrOr:22.4 wto/o maximum) and Mg-rich allanite (MgO : 6.2 wto/o maximum). The eclogite iscomposed mainly of primary garnet, clinopyroxene, corundum, kyanite, and Cr-rich zois-ite (CrrO3 : 1.6 wto/o maximum) with secondary magnesian staurolite and calcic amphi-bole. Equilibrium conditions of primary minerals were estimated at about 800-850'C andI 1-30 kbar for the garnet-corundum rocks and 700-750t and I l-25 kbar for the eclogite.The magnesian staurolite occurs as pseudomorphs after garnet, corundum, and kyanite.Equilibrium pressure of the magnesian staurolite is more than 1l kbar. Its equilibriumtemperature is slightly lower than that of the primary minerals.

Cell dimensions of the magnesian staurolite of 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-poorstaurolite. CrrO, content of the magnesian staurolite reaches | .2 wto/o. The Mg enrichmentof staurolite is controlled by the Mg substitution for vIAl and by the Mg + Fe substitutionin the IvFe sites.

INrnooucrroN

Staurolite is a common constituent mineral of peliticand other aluminous rocks metamorphosed under inter-mediate pressure and temperature conditions. Stauroliteis characteristically lower in X". [: Mg/(Mg + Fe)] valuethan the coexisting silicate minerals: its Xr, value is usu-ally less than 0.3 (e.g., Deer et al., 1982).Iron-free stau-rolite (i.e., the pure-Mg end member) was synthesizedunder the conditions of i": 700-950 "C and P > I I kbarby Schreyer (1967) and Schreyer and Seifert (1969). Hell-man and Green (1979) experimentally produced Mg-richstaurolite (X-u: 0.53-0.57) from olivine tholeiitic ma-terials under the conditions of Z: 740-760 "C and P :24-26 kbar. These experimental works suggest that Mg-rich staurolite could occur in Mg- and Al-rich metamor-phic rocks that formed at high temperature and pressure.Recently, Mg-rich staurolites have been found from var-ious types of rock metamorphosed under high pressures(X-" : 0.43-0.56, Ward, 1984a; 0.49, Schreyer et al.,1984; 0.40-0.42, Grew and Sandiford, 1984; 0.52, Ni-collet, 1986). These Mg-rich staurolites always coexist withcorundum. A silica-undersaturated environment is alsorequired for the formation of Mg-rich staurolites (e.g.,Schreyer,1967).

In the course of our petrologic study on eclogite and

0003-004x/88/01024048$02.00 48

the associated rocks in the Donghai district, Jiangsuprovince, east China, magnesian staurolite (X., : 0.68-0.74) was found in garnet-corundum rocks and eclogite.This is the most Mg-rich natural staurolite, as far as weknow, among staurolites reported. The mode of occur-rence, chemistry, crystallographic data and petrogenesisof the magnesian staurolite are described in the presentpaper.

Gnolocrc sETTTNG AND PETRoGRAPHY

Magnesian staurolite was found in two garnet-corun-dum rocks and an eclogite, from the Zhimafang andMengzhong areas in the Donghai district, respectively.Figure I shows a geologic sketch map of east China. ThePrecambrian basement of east China is divided into twounits by the Tancheng-Lujiang fracture zone, i.e., an east-ern unit (the Jiaoliao massif, age 1300-2400 Ma), and awestern unit (the Jilu massif, age >2400 Ma) (Tectonicmap compiling group, Institute of Geology, AcademiaSinica, 1974). The Donghai district is situated about 50km east of the Tancheng-Lujiang fracture zone and be-longs to the Jiaoliao massif. This district is underlain bypelitic and mafic gneisses and serpentinized ultramaficrocks.

The garnet-corundum rocks and eclogite occur as sub-rounded blocks (diameter. 5-50 cm) in ultramafic rocks

ENAMI AND ZANG: MAGNESIAN STAUROLITE 49

Fig. 1. Geologic sketch map of east China (simplified fromResearch Institute of Geology, Chinese Academy of GeologicalSciences, 1982). TLFZ: Tancheng-Lujiang fracture zone.

intruding the Precambrian pelitic gneisses. The ultramaf-ic rocks in the Zhimafang area are serpentinized phlog-opite-bearing garnet lherzolite. Equilibrium conditions ofthe garnet lherzolite are estimated at about f: 750-850'C and P : 20-30 kbar (Zang et a1., in prep.). The ultra-mafic rocks in the Mengzhong area are completely ser-pentinized. Bulk chemical analyses of the magnesianstaurolite-bearing rocks are given in Table l.

Zhimafang sarnples

The Zhimafang samples (TS-03 and TS-04) are com-posed of a corundum-rich pink part and garnet-rich whitepart. The samples contain garnet and corundum withsubordinate amounts of Na-rich phlogopite, zoisite, pent-landite, and apatite as primary minerals. Aggregates offine-grained diaspore, margarite, dolomite, and calcite (all<0.03 mm in size) were observed as inclusions in corun-dum crystals. Secondary minerals are magnesian stauro-lite, Al-rich pargasite, chlorite, clinozoisite, Mg-rich al-lanite, and heazlewoodite (Ni.Sr).

Both garnet and corundum occur as anhedral crystals0.1-2.0 mm in size. They are usually surrounded by ag-gregates of magnesian staurolite and chlorite. Garnet ispartly replaced by Al-rich pargasite and clinozoisitearound the crystal rim and along fractures. Phlogopiteshows a subhedral and tabular form and is partly replacedby chlorite. Magnesian staurolite constitutes acicular and/or prismatic crystals and forms pseudomorphs after gar-net and corundum with chlorite (Fig. 2).

The bulk chemistry of the garnet-corundum rock (TS-03) is characterized by an extremely high AlrO./SiO, val-

Tneue 1. Bulk chemical compositions of magnesian staurolite-bearing rocks

TM-02rs-03fwr%

13 .5

14.3

1 . 61 . 21 . 10.0

99.1

Note.'Normative composition of TM-02 was calculated on the basis ofFe3+lFe2+ ratio of 0.15.

. Total Fe as FeO.t Analyzed by Shiguang Wang.

ue (1.0) and low FerO, + FeO content (4.7 wto/o) and issimilar to that of some diaspore bauxites (e.g., Valeton,1972). This rock is also enriched in MgO and CaO. Thesecomponents may have been derived from dolomite ordolomitic limestones, which are usually associated withbauxites. The garnet-corundum rocks are considered tobe metamorphosed mixtures of bauxites and Mg-rich qar-

bonate rocks, on the basis ofthe occurrence ofdiasporeand carbonate inclusions in the corundum crystals andthe bulk chemistry of the garnet-corundum rock.

Mengzhong sample

The Mengzhong sample (TM-02) is massive and con-sists of a clinopyroxene-rich $een part and garnet-rich

brown part. The sample contains garnet, clinopyroxene,and subordinate amounts of corundum, kyanite, zoisite,dolomite, rutile, and apatite as primary minerals' Sec-ondary minerals are magnesian staurolite, calcic amphi-bole, clinozoisite, and chlorite.

Garnet occurs usually as euhedral or subhedral crystals0. l-0.5 mm in diameter. Large garnet crystals l-2 cm indiameter are sometimes observed. The garnet crystals areusually replaced by calcic amphibole, chlorite, and cli-nozoisite around crystal rims and along fractures. Cli-nopyroxene occurs as rounded and subhedral crystals 0.1-0.5 mm in size in the matrix and as fine inclusions small-er than 0.05 mm in size in the garnet crystals. Corundumis smaller than 0.03 mm in size and is rimmed by chlo-rite. Kyanite forms anhedral crystals smaller than 0'02mm in size and is replaced by magnesian staurolite andcalcic amphibole. Zoisite shows prismatic forms 0'l-0.3mm in length and occurs usually as inclusions in garnet.

Some zoisite crystals contain fine-grained garnet inclu-sions smaller than 0.03 mm in size. Magnesian stauroliteforms anhedral crystals smaller than 0.02 mm in size and

sio,Tio,Alro3CrrO"FerO.FeOMnOMgoNioCaONa.OK.oPrO,H,O(+)H,O(-)

Total

34.260.02

34.870.310.584.140.18

13.40n.d.

10.900 . 1 50.050.330.320.12

99.63

47.00.58

14.10.79

7 76'o . 1 2

14.30.05

13 .50.810.o20.01

Or 0.1Ab 6.9An 34.8Di 25.6

Hy

ol

MtCml lmAp

En 9.5Fs 2.7Wo 13.4En 10.5Fs 3.0Fo 10-gFa 3.4

99.0l-T-l ximuertite l.-71 routt

l---l M"soroi.-Cenozoic

| [T[I[TTI r-ot" Proterozoic - Poloeozoic

-o^,,-^ | Io,.no"on-Eorty Proterozoic

50 ENAMI AND ZANG: MAGNESIAN STAUROLITE

Fig. 2. Backscattered-electron image of magnesian stauroliteand other constituents (sample TS-04). Scale bar indicates 200pm. Abbreviations for minerals: St, magnesian staurolite; Grt,garnet; Crn, corundum; Chl, chlorite.

occurs as pseudomorphs after kyanite. Magnesian stau-rolite crystals are partly surrounded by calcic amphibole.

The garnet + clinopyroxene + corundum assemblageof the Mengzhong eclogite suggests equilibrium condi-tions of T:100-750 "C and P : l l-25 kbar for theprimary stage (Enami et al., 1986). Its bulk and mineralcompositions are similar to those of eclogite xenoliths inkimberlites (Tables I and 3; also cf. Dawson, 1980). Thesedata suggest that the Mengzhong eclogite was generatedfrom the lower crust or upper mantle.

MrNpn.q.Locy oF MAGNESTAN sTAURoLITE ANL,ASSOCIATED MINERALS

Chemical analyses were performed with a lBor- elec-tron-probe microanalyzer (nrua) JCxA-233. Acceleratingvoltage, specimen current, and beam diameter were keptat 15 kV, 1.2 x 10 8 A and 3 pcm, respectively. Chemicalcompositions of the magnesian staurolite and other con-stituent minerals are given in Tables 2 and 3, respective-ly.

Magnesian staurolite

Chemistry. HrO content of staurolite is variable andcannot be associated with a fixed stoichiometry (e.g., Ju-urinen, 1956; Lonker, 1983; Holdaway et al., 1986a). Liis sometimes concentrated in staurolite (Hietanen, 1969;Grew and Sandiford, 1984; Holdaway et al., 1 986b; Du-trow et a1., 1986). HrO and LirO contents cannot be es-timated by err"re analysis, and so the chemical formulaof the magnesian staurolite was calculated on the basis ofSi + Al * Cr: 25.53 as recommended by Holdaway etal. (1986b). Y,O,,ZnO, CoO, CaO, Na,O, and KrO con-tents of the magnesian staurolite are below the detectionlimits, i.e., less than 0.02wto/o for VrO, and less than 0.01wto/o for the other elements.

The magnesian staurolite is faintly yellow in thin sec-tion and chemically homogeneous. X., values l: N4g/(Mg + Fe)l of the magnesian staurolite are 0.74 + 0.01and 0.69 + 0.01 in the Zhimafang and Mengzhong sam-ples, respectively. Average Si content per formula unit is7.92 for the Zhimafang samples and 7.14 for the Meng-zhong sample. Grew and Sandiford (1984) pointed outthat lower Si content is characteristic of Mg-rich staurol-ite from silica-undersaturated environments. Although thepresent samples contain corundum, the Si contents ofthese magnesian staurolites are higher than the Si con-tents of Mg-rich staurolite from other corundum-bearingsamples (e.e., 7.43-7.68; Ward, 1984a). The magnesianstaurolite here occurs as pseudomorphs after corundumand/or kyanite, and so the relatively high Si contents areprobably due to disequilibrium with the associated alu-minous minerals. CrrO, contents of the magnesian stau-rolite are up to 0.70 and 1.17 wt0/o for the Zhimafang andMengzhong samples, respectively.

Figure 3 explores the variation of cation contents inthe staurolite monolayer, except for Al (FM content :

Fe * Mn + Mg + Zn -l Co + Ni + Li + Ti), for stau-rolites reported in the literature. Figure 3 clearly showsthat the FM content ofstaurolite increases with increasein I", value (: M/FM.The FM content of Mg-richstaurolites (Ir, > 0.4) is higher than 4.0, and its averageis 4.6. The FM content of Mg-poor staurolites (I., =

0.2) is mostly lower than 4.2, and its average is 3.9. TwoMg-poor staurolites with -FM > 4.6 were reported by Grif-fen and Ribbe (1973) (specimens 4 and l0), and the latterhas a high ZnO content of 6.86 wt0/0. Staurolites withY-": 0.24.4 have compositions intermediate betweenthe above two groups (average FM content is 4.2), exceptfor the unusually Zn-enriched staurolites vithZnO :7 .44and 6.73 wto/o reported by Juurinen (1956) and Miyake(1985), respectively. The compositional variation seen inFigure 3 may have some uncertainities because of theabsence of HrO and/or LirO data in most analyses. Thirtycomplete analyses of staurolite,l including HrO and LirO,by Holdaway et al. (1986b), however, also show that the

' Excluding one analysis (specimen 6-3), believed to be inac-curate owing to variability in Li content (Dutrow et al., 1986).

ENAMI AND ZANG: MAGNESIAN STAUROLITE

TnaLe 2. Chemical compositions of magnesian staurolites

5 l

Zhimafang Mengzhong

TS-03 TM-02

c-1t C-27 n : 2 0 I n : 7 I crf

sio,Tio,Atr03Cr,O.FeO-MnOMgoNioCaONaroKrO

Total

29.8(0.3)0.00

s5.9(0.4)0.s3(0.08)4.69(0.21)0.07(0.03)7.38(0.07)0.16(0 05)0.000.000 0 0

98.5

7.91717.5020.0000 . 1 1 11.0420.0162.9220.0340 0000.0000.000o.74

29.8 29.50.00 0.00

55.8 56.20.53 0.574.70 4.600 09 0.107.37 7.350.14 0 .1 10.00 0.000.00 0.000.00 0.00

98.4 98.4

28.s(0.2) 28.10.25(0.06) 0.33

s5.0(0.s) 54.40.83(0.21) 1 .175.92(0.10) 6.090.00 0.007.36(0.11) 7.330.00 0.000.00 0.000.00 0.000.00 0.00

97.9 97.4

29 9(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)0.000.000.00

99.0

SiAITiCrFe2**MnMgNiCaNaKX"nf

0.0000 . 1 1 31.0540.0072.9600.0340.0000.0000.0000.74

7.74317 6090.0510 1781.3450.0002.9810.0000.0000.0000.0000.69

7.70317.5740.0680.2541.3960.0002.9950.0000.0000.0000.0000.68

Formulae (si + Al + cr : 25.53)7 .927 7.830 7.915

17.492 17.580 17.5020.000 0 0000 1 1 1 0 , 1 2 01.045 1.0210.020 0.0222.923 2.9080.030 0.0230.000 0.0000.000 0.0000.000 0.0000.74 0.74

Nofe.'Values in Darentheses are the standard deviations.. Total Fe as FeO.t n, number of analytical points; C-1 and C-2, crystals 1 and 2 for the X-ray analysis, respectively;

Cr. most Cr-rich comoosition.+ XM, : Mg/(Mg + Fe).

FM content increases with increasing Ir, value in stau-rolite: average FM content of nine staurolites with yMs >0.2 is 4.5 and that of three staurolites with fM8 = 0.1 is3.8. High FM csntent of some staurolites can be possiblyexplained by substitution of vIAl by Fe3*, which wouldbe seen as part of the FM components (Holdaway et al.,1986b). Most of Mg-rich staurolites with fMs > 0.4, how-ever, coexist with zoisite or Fe3+-poor clinozoisite (FerO, :2.33-4.2 wto/o) and probably have little Fe3*. Stauroliteof high Yr" tends to be lower in "tAl content and viceversa: Average vIAl contents are 17.3 and 11 .6 for stau-rolites with Y-, > 0.2 and fMg < 0. 1, respectively (Hold-away et al., 1986b). Thus the high FM content of Mg-rich staurolite suggests the preferential Mg substitutionfor "IAl. Alternative explanations for the "'Al = Mg sub-stitution are 2vIAl -= 3 Mg (see Ward, 1984a) or vrAl +M g + H .

Lonker (1983) showed the negative correlation be-tween vIAl and (Fe + Mg) + OH contents. This negativecorrelation implies that the second substitution schemebetter explains Mg enrichment in staurolite. The presentmagnesian staurolite has lower FM (4.0-4.4) and lowerFe (l.l-1.3) concentrations than those of other Mg-richstaurolites. This fact suggests that the Fe = Mg substi-tution in theI"Fe sites also affects the high Mg content ofthe magnesian staurolite 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 TM-02).

X-ray data. The magnesian staurolite was examined ona Rigaku RAD-?B X-ray microdiffractometer with posi-tion-sensitive proportional counter (esnc,/vroc system),using V-filtered CrKa radiation ()\:2.29092 A, 40 kV,200 mA). X-ray diffraction data were obtained from thetwo selected areas of about 30 um in diameter for twocrystals in a thin section. Average chemical compositionsofthe selected areas obtained by errue analyses are shownin Table 2.

The X-ray diffraction data are summarized in Table 4.Peaks were indexed with respect to Schreyer and Seifert(1969) and Borg and Smith (1969). Diffractions with d >6.6 A could not be observed, since 2d of the microdif-fractometer ranged from 20" to 140" in the present study.Preliminary X-ray powder-difraction analysis of a mix-ture of the magnesian staurolite and gamet crystals showeda weak peak at d : 8.34 A, corresponding to the 020reflection of the magnesian staurolite. Observed peaksand their intensities are different between the two selectedareas, because of the limited oscillation angles around 4and X axes of the microdiffractometer. Peak positions ofthe present study correspond well to those ofFe-free stau-rolite svnthesizedbv Schrever and Seifert (196q. An or-

oo

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o x. e F *E 5 ;o c ( of .9 c?= ' i

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a

ENAMI AND ZANG: MAGNESIAN STAUROLITE

6 0 o ) 0

c E E E e b cc d c i c i c J c

, 'e o , i o r o c o N o o r S S P B S b S P S $ t t ou ? 9 e ? u ? q q r 9 o ? o o l - i l q q q q q C q q q q q C

o t d - - - d d d e c j c , l q l ^ d o i c i o o c i o c j - o o oO O N I O , ( J

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d j d d d dl8 o d - ct ci ct ct J ci ci o o o, -

- S o q E B R e - 8 8 1 3$ d p . - c i @ t r = d d l g

; < o o N o o i o o - o o o oO o o A l O O o o l , , O @ o F o o o o o 6 oF N a a O O O O I F I O O O O O O O O O O O

; cici o d d ci cilE cr d - ci d ci dd d ci ci c;

e r ' . . * o o o r S = R 8 P 8 3 3 8 : 8 8 .q q q a ? g r q q n q q l c i l q o ? g q a c q c c q q q$ o R o o o 6 o 9 . - l E o a J F o o o o r o F o o o

- e s s o e * N o r ? 8 R 8 P S 8 5 3 5 9 8 oO O O b N O - - O O O I $ r r O F O O O O 6 O @ r O O

d ct at d - d rtct i 6i cj lg O oi d ci d cici ci o o o o oo F N I O

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S o o o @ N $ @ o o o oF q J d ) O 9 0 , O O O @ I O = I f ) O O ) O O ( O

- - q o l a o l a q 6 ! q q l t l l q q q ' : \ q a q \ q C q

T d e " , i S d p d g d d l E o c i - c t o o o ; c i c j o o o

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o o o @ s o o o$ t \ O ( J F O O O @q o q o l - q q q qo o o c o o o o o o

\ f ( 9 @ N N F $ Od r @ O F O S f T r ) O O )O d ) O F O o ( o O OO O O O O F O O O

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P o r o' t o @ o

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o o o o

- e @ e e . o o @ o r N R = S g B B S g E g Sa ? C q n C q g q \ ' . C l n t : q o ? q q C q q C o ? q q3

o 3 o o o o o I o o l 3 O $ N o o o o o o F o o

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ENAMI AND ZANG: MAGNESIAN STAUROLITE 53

TneLe 4. X-ray data of magnesian staurolite in TS-03

Crystal 1 Crystal 2

UloUlo

4.534.433.549

3.0483.0022.8462.683

2 542

z Jbb

2 2572.0992 0692.0542.003

1.7761.7651.706

1 570't 542

1 514

4.45

3.5273.225

3.007

2.3512.289

2.0972.069

1.857

1 763

1.6601.600

1.5421 538

4.524.423.5553.5263.2273.0523.0072.8532.6842.6202.5452.3922.3702.3552.2922.2602.1022.0702.O572.0051.8581.7781.7661.7071.6611.6001.5691.5411.5361.5141.512't.4731.4461.4281.4221.3801.370

1 0< 540

1 5551 565

55

100

20

Z J

40

95

1 3 01 1 12 2 01 3 12 0 11 5 02 2 12 4 01 5 11 1 22 4 11 3 23 3 03 1 12 0 22 6 01 7 10 8 03 5 0

20100

6030

1 0

<5

1 01 5

JC

30

T C

530

J

5c

1 01 0

- 2 4 24 0 14 4 01 7 21 9 12 2 31 5 32 4 34 6 12 8 25 3 01 9 22 1 0 11 7 30 1 0 25 5 00 1 2 05 1 2

3030<51 0<5

c

< 51 0

51 0

70

2.6182.5422.387

Fig. 3. Variations of FM (: Fe * Mn + Mg + Zn + Co +Ni + Li + Ti) content of staurolites. Solid triangles indicate av-eftge FM contents. Data are from Asami and Hoshino (1980),Asami et al. (1982), Ashworth (1975),Baltatzis (1979), CEch etal. ( I 98 1), Enami and Zang (this paper), Foster ( 1 977), Fox ( I 97 l),Gibson (1978), Green (l963), Grew and Sandiford (1984), Grif-fen and Ribbe (1973), Guidotti (1970), Hietanen (1969), Hiroi(1983), Hollister (1970), Hollister and Bence (1967), Hounslowand Moore (1967), Hudson (1980), Juurinen (1956), Kwak (1974),Lonker (1983), Miyake (l 985), Pigage (l 976), Schreyer and Chin-ner (1966), Schreyer et al. (1984), Smith (1968), Tak6uchi et al.(1972), von Knorring et al. (1979), and Ward (1984a, 1984b).

thorhombic cell was assumed, and unit-cell dimensionscalculated on the basis of 30 reflection data with 44. <20 < 120" (excluding 0k0 reflections) are as follows: a :7.873(3) A, b : r6.5s7(7) A, and c: 5.637(3) A. rhe aand c axes are similar to those of staurolites in the liter-ature (a : 7.850-7.891 A and c: 5.635-5.667 A). crif-fen et al. (1982) showed that the b axis ofstaurolite de-creases systematically with decreasing mean ionic radius(MIR) of cations in the monolayer. The D axis of themagnesian staurolite (TS-03) lies close to the regressionline of that against the MIR value (Fig. 4).

Garneto clinopyroxene, and calcic amphibole

The garnet is mostly homogeneous and belongs to agrossular-rich pyrope-almandine series with very low MnOcontent (less than 0.3 wto/o). Average compositions in terms

of garnet end members are Prp54sAlm,ooGrsrrrSpsor,Prprr rAlm,o oGrsro oSpso r, and Prpro rAlmrr rGrsr'Spso,for TS-03, TS-04, and TM-02, respectively (abbreviationsafter Kretz, 1983). The CrrO, content of TM-02 garnetreaches 2.9 wto/o in the crystal core. The clinopyroxene ispale green in hand specimen, considering its substantialCrrO. content (0.63 wto/o maximum), and is sodian augite(NarO : 2.0 + 0.3 wto/o, X*u:0.94) of Essene and Fyfe(1967). The amphibole varies in composition from flaketo flake. In the TM-02, the amphibole replacing garnet ismagnesiohornblende with AlrO, : 12.0 + 2.1 wto/o and,that replacing the magnesian staurolite is tschermakitewith maximum AlrO, of 18.8 wt0/0, according to the no-menclature of Leake (1978). The amphibole in the Zhi-mafang samples is Al-rich pargasite, and its AlrO3 contentis up to 22.4 wto/o.

1.4741.4471.429

1 5 1 115 1 .47315 1 .4461 0

1.42353

< 51 . 3101.296

< 5< 5

1.2671 0

J

<5 1.2391.221

1.3781 .3711 .312

1.2791.273

1.2641.2551.239

54 ENAMI AND ZANG: MAGNESIAN STAUROLITE

Fig. 4. Relationship between mean ionic radius (MIR) of cations in the monolayer and b axis of staurolite. Regression lineplotted was calculated using 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: correlation coefficient. Dataarc from C6ch et al. (1981), Enami and Zang (this study), Griffenand Ribbe (l 973), Juurinen (1956), Miyake (1985), Schreyer and Chinner (1966), Smith (1968), Tak6uchi etal. (1972), von Knorringet aI. (1979), and Ward (1984a) for natural staurolites and from Gritren (1981), Richardson (1966), and Schreyer and Seifert (1969)for synthetic staurolites.

Corundum and kyanite

Corundum is a pink variety owing to its high CrrO3content, up to 1.1 wto/o in the Zhimafang samples and 0.9wto/o in the Mengzhong sample. Its FerO. content is lessthan 0.4 wto/0. Kyanite is also enriched in Cr,O, (1.2 v,to/omaximum), and its FerO. content is 0.4 + 0.1 wto/0.

Phlogopiteo chlorite, and margarite

Both phlogopite and chlorite have high Xr, values ofabout 0.96 and are colorless in thin section. Phlogopiteis unusually enriched in Na.O (2.7 + 0.2 wtVo). NiO con-tents of phlogopite and chlorite in the Zhimafang samplesare up to 1.0 and Ll wt0/0, respectively. Margarite is de-pleted in both Na,O + K,O (0.18 wto/o) and FeO + MgO(0. 1 wto/o).

Epidote group minerals

Both zoisite and clinozoisite are colorless in thin sec-tion. Xo.:r values [: Fe3+/(Fe3+ + Al)] of Zhimafang andMengzhong zoisites are 0.030 and 0.035, respectively.CrrO. content of the latter reaches 1.6 wto/o. Clinozoisiteis depleted in Fe3*, ar.d Xr"r* values are about 0.10 and0. I 1 for the Zhimafang and Mengzhong samples, respec-tively. Allanite in the Zhimafang sample is pale brownand shows a weak compositional zoning consisting ofREE-rich cores and Ca-rich rims. CaO and REE contentsare 10.9 and 28.0 wto/o in the crystal core and 12.5 and24.5 wto/o in the crystal rim, respectively. The MgO con-tent of allanite reaches 6.2 wt0/o, and total Fe as FeO isless than 2.1 wIo/0, suggesting the substitution Ca * Al +REE + Mg.

Other minerals

Diaspore is enriched in CrrO. (1.0 wt0/o), and its FerO.content is less than 0.6 wto/0. Dolomite is homogeneous,and its average composition in terms of carbonate endmembers is CalrrMgso,Sdo for the Zhimafangsamples and

Cal'MgsorSd, for the Mengzhong sample. Fe, Mn, andMg components in calcite were not detected. Heazle-woodite is depleted in Fe (less than 0.1 wt0/o) and has aformula of Ni3or52.

Fonnrarronr oF MAGNESIAN STAURoLITES

Textural and chemical evidence shows that the magne-sian staurolites in these rocks are all retrograde products.The Zhimafang staurolite occurs as a pseudomorph aftergarnet and corundum and forms an aggregate with chlo-rite. The Mengzhong staurolite occurs as a pseudomorphafter kyanite. Suggested reactions for the magnesian stau-rolite formations are as follows:

197(Mg,Fe).Al,Si.O,2 + 340AlrO3 + 432HrOsarnet

-;*;toAl,ssi,,ooo(oH)ostaurolrt€

+ 42(Me,Fe),,*hiJ,,O'n(oH)'u

for the Zhirnafang samples and

8(Mg,Fe).AlrSi3O,, + 25Al2O.' + 2lAlrSiO5 + l2HrOgamet corundum kyanite

- 6(Mg,Fe)oAl,ssi? 5Oo4(OH)o

for the Mengzhong sample. Grossular component in gar-net was probably consumed for the formation of calcicamphibole and clinozoisite.

For the Zhimafang samples, the equilibrium tempera-ture of the primary garnet + corundum stage was esti-mated at about 800-850 "C by the garnet-biotite geother-mometer (methods of Indares and Martignole, 1985, andHoinkes, 1986). The temperature calculated using theFerry and Spear (1978) calibration for the Ca- and Mn-free system is 680 'C. The lower temperature estimateswith the latter calibration may result from high grossular

ENAMI AND ZANG: MAGNESIAN STAUROLITE 55

component of garnet (35.3 molo/o). The lower pressurelimit of the primary stage is about ll-14 kbar on thebasis of the garnet * corundum assemblage (cf. Droopand Bucher-Nurminen, 1984) and the stability field ofpyrope garnet (Schreyer and Seifert, I 969). Its upper pres-sure limit is probably defined by the equilibrium pressureofthe host garnet-lherzolite ofabout 20-30 kbar. For theMengzhong sample, the equilibrium conditions of theprimary stage were estimated at T : 700-750 "C and P: ll-25 kbar (Enami et al., 1986). The estimated con-ditions of the primary stage may define the upper I andP limits of magnesian staurolite formation. The lower Iand P limits of magnesian staurolite formation are esti-mated at about730-770'C and I I kbar from the stabilityrange of the magnesian staurolite * chlorite assemblage(cf. Schreyer and Seifert, 1969; Grew and Sandiford,1984). Equilibrium conditions of the magnesian staurol-ite of this study are similar to those of other Mg-richstaurolites in the literature.

In the present study, magnesian staurolite occurs in twocharacteristic rock types. TheZhimafang magnesian stau-rolite occurs in highly aluminous and silica-undersatu-rated rocks. The Mengzhong magnesian staurolite, on theother hand, occurs as a pseudomorph after kyanite in aneclogite without normative corundum. Schreyer (1967)suggested that the lack of natural magnesian staurolitewas due to the absence of rocks having highly aluminousbulk compositions in the upper mantle where I and Pconditions necessary for the formation of magnesianstaurolite may prevail (i.e., higher than I I kbar). He alsopointed out that magnesian staurolite and quartz are ln-compatible by virtue of the stable tie lines between kya-nite and other magnesian silicates. The Zhimafang sam-ples, as well as most of Mg-rich staurolites in the literature,occur in metamorphosed peraluminous rocks such assapphirine-garnet rock (Schreyer et al., I 984), corundum-bearing phlogopite-chlorite schist (Grew and Sandiford,I 984), and meta-anorthosite (Nicollet, 1986). These sam-ples suggest that Mg-rich staurolite is formed in rockshaving restricted bulk chemistry under limited T and Pconditions. Mg-rich staurolite formation in typical maficrocks may require much higher pressure reaching 24-26kbar as shown by Hellman and Green (1979). The Meng-zhong sample, however, clearly shows that Mg-rich stau-rolite can occur in common eclogite. Ward (1984a) re-ported Mg-rich staurolite in a reaction zone betweenplagioclase and orthopyroxene crystals in a metatrocto-lite. The Mengzhong sample and that of Wand thus sug-gest that Mg-rich staurolite can be formed in commonmetabasites by local reactions of aluminous and magne-sian minerals.

The Mengzhong sample has bulk and mineral compo-sitions similar to those of eclogite xenoliths in kimberlites(Enami et al., 1986). Kyanite and other aluminous min-erals are common in high-pressure eclogites (e.g., Daw-son, 1980), and local breakdown of these minerals withdecreasing temperature may be expected to form Mg-richstaurolite. Hellman and Green (1979) suggested that stau-

rolite may be widespread in mafic rocks metamorphosedat high pressures, as, for example, in the subducted litho-sphere. We also emphasize that careful petrographic ex-aminations will probably reveal wide occurrence ofMg-rich staurolite as an accessory mineral in commonmetabasites formed under high pressures, especially ineclogites.

AcrNowr,slcMENTS

We wish to express our sincere thanks to Professor K. Suwa of NagoyaUniversity for his critical comments on the manuscript. Thanks are alsodue to Mr S. Wang of Beijing University and Mr. M. Amemiya of Rigakufor their help in laboratory and to Mr I. Hiraiwa of Nagoya Universityfor preparing many thin seclions This study was supported in part by aGrant in Aid for Scientific Research from the Ministry of Education ofJapan (No. 62740481: M E ).

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MaNuscnrpr rucEIvED Mnv 26. 1987MeNuscnrr"r AccEprED Sepreunsr 2. 1987