P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria

Lithos, 19 (1986) 219-234 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

219

P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria F.S. SPEAR 1 and G. FRANZ 2 Department o f Geology, Rensselaer Polytechnic Institute, Troy, N Y 12180 (U.S.A.) 2Faehgebiet Petrologie, Technische Universitiit Berlin, Strasse des 17. Juni 135, D-I O00 Berlin 12 (Federal Republic o f Germany)

LITHOS

Spear, F.S. and Franz, G., 1986. P-T evolution of metasediments from the Eclogite Zone, south-central Tauern Window, Austria. In: W.L. Griffin (Editor), Second International Eclogite Conference. Lithos, 19; 219-234. Petrologic data on the paragenesis of (I) kyanite-zoisite marbles and (II) garnet-chloritoid quartz-mica schists are presented with the goal of providing constraints on the pressure-temperature evolution of the Eclogite Zone, Tauern Window, Austria. The peak metamorphic assemblages in the two rock types are: (I) kyanite + zoisite + dolomite + quartz; zoisite + muscovite + dolomite + calcite + quartz; and (II) garnet + chloritoid + kyanite + muscovite + quartz + epidote -+ dolomite -+ Zn-staurolite. The estimated peak metamorphic conditions are 19 -+ 2 kbar, 590 -+ 20°C. Secondary alteration of the kyanite-zoisite marbles was accomplished in two stages. The early stage resuited in the production of margarite, paragonite, secondary muscovite and chlorite and the later stage resuited in the formation of sudoite (a di/trioctahedral Mg-A1layer silicate) and kaolinite. The early alteration is bracketed at conditions between 3 and 10 kbar, 450-550°C and the later alteration between 200 and 350°C, P <~ 3 kbar. The P-T path is characterized by maximum burial to approximately 19 kbar (60-70km) (at-~590°C), followed by nearly isothermal decompression to approximately 10 kbar (30 km), and then more gradual decompression with cooling to approximately 3 kbar (10 km). Alteration was apparently accomplished by the influx of H20-rich fluids, with the composition of the fluid locally buffered by the mineral assemblage. (Received November 20, 1985 ; accepted after revision March 11, 1986)

1. I n t r o d u c t i o n The Eclogite Z o n e (EZ) in the south-central Tauern Window, Austria, consists o f a lithologically diverse sequence o f calc-mica schists, mica schists, metaquartzites, siliceous dolomites, and diverse types o f eclogites including amphibole-, epidote- and mica-bearing varieties. The stratigraphy within the EZ is c o n t i n u o u s on a scale o f tens o f meters to kilometers (Raith et al., 1980). The i n t e r p r e t a t i o n o f the tectonic e v o l u t i o n o f the EZ is that it was subducted as a c o h e r e n t unit during an early phase o f the Alpine orogeny, and was then thrust up into a shallower structural level (see, e.g., England and Holland, 0024-4937/86/$03.50

© 1986 Elsevier Science Publishers B.V.

1979; Holland, 1979; Franz and Spear, 1983; and references therein for a discussion o f the general setting o f the EZ and vicinity). The p e t r o l o g y o f several different lithologies from the EZ has been described in the literature, with the ~mphasis on garpet-omphacite eclogites b y Abraham et al. (1974), Miller (1974, 1977) and Raith et al. (1977), on vein assemblages in eclogites by Holland (1979), and on siliceous dolomites by Franz and Spear (1983). All authors conclude that the peak m e t a m o r p h i c temperatures experienced by the EZ were on the order o f 500 ° to 600°C. Pressure estimates vary between a p p r o x i m a t e l y 10 and 20 kbar, and there is

220

2, Sample description and petrography

a controversy about the composition of the fluid phase. Holland (1979) and Franz and Spear (1983) calculated a high H~O/CO~ ratio in metabasaltic as well as in carbonate-rich assemblages, whereas Luckscheiter and Morteani (1980) observed high-density, CO:-rich, fluid inclusions in quartz veins from the EZ. Miller (1977) also states that PH~O was less than Ptotal during the eclogite metamorphism. The purpose of this paper is to present new petrologic data on the paragenesis of different types of metasediments at eclogite-facies conditions with the goal of: (1) providing constraints on the P- T evolution of the EZ; and (2) describing the phase relations in different systems at high Ptotat and temperature. High-pressure phase relations in metacarbonates and quartz-mica schists are incompletely understood and the rocks from the EZ provide new data on metamorphic facies at high pressure. These rocks also display a series of complex post-peak metamorphic reactions. Of particular interest is the retrograde formation of sudoite, a di/trioctahedral chlorite together with kaolinite.

!

The samples studied in this report come from outcrops in two areas in the EZ, Frosnitz Valley-Knappenhaus-Dabemitzach and from the vicinity of Lake Raneburg (see sample map, Fig. 1). The samples are classified into two groups: (l) Kyanite-zoisite bearing marbles. The minerals are characterized by a high Mg/Fe ratio, and the rocks are rich in carbonate minerals. (II) Quartz-mica schists. The minerals are characterized by a low Mg/Fe ratio and the rocks are carbonate-free or poor. Each group displays textural evidence that can be used to constrain the prograde, peak metamorphic and retrograde (post-peak metamorphic) portions of the pressure-temperature (P 7) path. Phases believed to have been part of the peak metamorphic mineral assemblage will be labeled as primary (P) whereas phases that were clearly formed during retrograde processes will be labeled as secondary (S). The textural criteria for distinguishing primary, or a

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q*.Klrzite staurolite-quorzlte

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zo~ite-dolomitemarble

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Fig. 1. A. Simplified tectonic map of the western Tauern Window, Austria, showing the location of the Eclogite Zone (in black). Box shows location of (B). B. Geologic sketch map of the Eclogite Zone and vicinity showing sample localities.

221 "peak metamorphic mineral" are one or more of the following: (1) large porphyroblastic habit, (2) not obviously produced in an alteration reaction, (3) its presence as an abundant and therefore always present phase (e.g., dolomite and quartz), or (4) systematic element partitioning with other "peak" phases. Secondary minerals are distinguished by their occurrence as alteration products of primary minerals.

as evidenced by the variable proportions of carbonate vs. silicate minerals. The aluminous kyanite-bearing marbles most likely resulted from kaolinite (_+ illite/ montmorillonite/quartz) deposition in a carbonate platform environment. There are two principal peak (primary) metamorphic assemblages in the kyanite marbles: Kyanite (Ky) + zoisite (Zoi) + dolomite (Dol)

2.1. Kyanite-zoisite marbles In outcrop and hand sample, the kyanite marbles occur as layers 5 - 1 0 cm thick and are continuous for tens of meters. Generally they occur within a series of dolomitic marbles, tremolite marbles, calc-mica schists and quartzites. The overall appearance of these units is one of a continuous stratigraphic succession within a predominantly dolomitic sequence. Lithologically distinct layers differ in their SiO2, Al~O3, K~O, Na~O and MgO contents

+ quartz (Qz) -+ phengitic muscovite (Ph) and Zoi + Dol + Ph + Qz + calcite (Cc) Textural criteria do not allow us to distinguish if there is primary chlorite (Chl) + Qz. There is definitely abundant late Chl + Cc + Qz, similar in appearance to that described by Franz and Spear (1982, fig. 3D). There are also isolated flakes of

Fig. 2. Photomicrograph of kyanite marble (sample 82-11 ; plane polarized light, real length of picture is 1.6 mm). Primary kyanite (Ky), dolomite (Do) and muscovite (Phi) are altered to margarite (Ma), secondary muscovite (PhlI), paragonite (Pa) and chlorite (Chl).

222

Fig. 3. Photomicrograph of zoisite marble (sample 79-80; plane polarized light, real length of picture is 1.6 mm), showing the decomposition of zoisite (Zoi), dolomite (Do) and quartz (Qz) into paragonite (Pa), secondary muscovite (Phil) and chlorite (Chl). Also note the production of kaolinite (Kao) along cracks within zoisite and in the reaction zone between zoisite and dolomite. chlorite in a carbonate matrix, which could be primary chlorite, but no unequivocally primary Chl 4- Qz. Secondary mineral assemblages in the kyanite marbles are characterized by hydration reactions along grain boundaries and fractures, as shown in Figs. 2- 4. Details of the alteration sequence will be discussed in Section 4 on phase relations but the general features include: (1) the production of chlorite and margarite along kyanite-dolomite grain boundaries; (2) the formation of chlorite + calcite at the expense of zoisite + dolomite + quartz (Fig, 3); (3) the production of margarite, chlorite and paragonite at the expense ofkyanite + dolomite + muscovite (Fig. 2); (4) the production of secondary muscovite, paragonite and chlorite as a replacement of zoisite and dolomite (Fig. 3);

(5) the formation of sudoite along cracks and cleavage planes within kyanite and mixtures of sudoite and kaolinite (+ quartz) in places where kyanite and dolomite have been replaced by margarite and chlorite (Fig. 4); (6) the production of kaolinite along cracks within zoisite and between zoisite and its decomposition products, paragonite, muscovite and chlorite (Fig. 3).

2.2. Quartz-mica schists The quartz-mica schists also show a clear stratigraphic sequence. Different layers range in composition from almost pure white quartzite to mica-bearing quartzite, garnet-quartzite, or chloritoid-mica quartzite with transitions into mica schists or carbonatemica schists that are both gradational and sharp. The thickness of the quartzite layers ranges from centimeters to several decimeters.

223

Fig. 4. P h o t o m i c r o g r a p h of k y a n i t e marble (sample 82-19; plane polarized light, real length of picture is 1.6 mm), showing the d e c o m p o s i t i o n of k y a n i t e (Ky), d o l o m i t e (Do and quartz (Qz) into margarite (Ma) and sudoite (Sud) or a m i x t u r e of sudoite and kaolinite (Kao). Cracks in k y a n i t e are filled with sudoite.

TABLV l Microprobe analyses of chlorite, sudoite and kaolinite Mineral Chl No. 79-80

Kao 79-80

Chl 79-80

Kao 79-80

Sud Sud Sud/ Sud/ Sud/ Sud/ Sud/ Sud/ Kao Chl Chl 82-19A 82-19A Kao Kao Kao Kao Kao Kao 82-19C 82-10C 82-11C 82-19C 82-19C 82-19C 82-19C 82-19C 82-19C

45.81 36.70 0.18 0.25 0.29

32.92 35.72 0.31 14.12 -

37.44 36.27 0.25 11.65 0.07

39.73 35.44 0.21 9.tl 0.09

40.62 36.48 0.16 7.00 0.07

40.55 36.49 0.18 5.51 0.10

44.37 37.98 2.85

44.96 37.51

45.17 37.84

0.84 0.01

83.07 *2 84.93 *= 85.67

84.58

84.33

82.83

85.20

SiO 2 AI=O3 FeOto t MgO CaO

28.96 24.17 1.28 31.08 0.07

44.52 37.97

0.60

29.49 23.68 2.11 30.47 0.15

~,1

85.56

83.09

85.90

83.22

Si A1TM A1VI l'c Mg Ca

5.47 2.53 2.86 0.20 8.76 0.01

7.95 0.05 7.94 -

8.15

0.12

5.57 2.43 2.85 0.33 8.58 0.03

E oct

11.83

8.06

11.79

34.10 36.26 0.15 14.42 -

0.38 0.01

28.72 24.23 1.62 31.08 0.03

28.33 24.17 1.51 32.09 ---

83.32

83.40

85.67

86.10

5.43 2.57 2.84 0.26 8.77 0.01

5.34 2.66 2.71 0.24 9.02

11.87

11.97

6.20 1.80 5.98 0.02 3.91

6.69 1.31 6.33 0.04 3.10 0.01

7.13 0.87 6.63 0.03 2.44 0.02

7.27 0.73 6.97 0.03 1.87 0.01

7.37 0.63 7.18 0.03 1.49 0.02

7.76 0.24 7.59

7.99 0.01 7.85

8.02

7.70 0.03 0.06 0.06

6.13 1.87 5.98 0.05 3.92 -

0.74

0.22

0.10

7.85

9.95

9.91

9.49

9.12

8.88

8.72

8.33

8.08

8.02

1 ormulas calculated on the basis of 28 oxygens. * ~TiO 2, MnO. K~O and Na20 in all cases below detection limit. ,2 1 ~ 0.10 w t / L

7.91

224 Primary mineral assemblages in the quartz-mica schists include quartz + phengite + garnet + chloritoid +_ kyanite -+ staurolite _+ zoisite/epidote +- sulfides _+ rutile _+ titanJte -+ apatite +- zircon +- tourmaline. Chlorite is also present as a secondary (retrograde) mineral, generally as an alteration of garnet. Phengite occurs both as a primary and secondary mineral.

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3. Mineral chemistry

pheng,te]l S

Mineral compositions have been determined by electron microprobe analysis; representative analyses o f selected minerals from kyanite-zoisite marbles and mica schists are presented in Tables 1 - 3 and are discussed below.

3.1. Kyanite-zoisite marbles

10 F 2+ etot 2+ Feto t 4- Mg

i~

20 NO

Nc2KeCc

Fig. 5. Compositions of primary (phengite I) and secondary (phengite II) muscovites from kyanite-zoisite marbles. Divalent cations (Mg + Fe + Mn) per octahedral layer in phengite plotted against XFe = Fetot/(Fetotal + Mg) and "paragonite content" XNa = Na/(Na + K + Ca). Lines connect analyses on the same crystal (core-margin) or adjacent crystals (o = sample 82-10; u = 82-11; a = 82-16;• = 79-80).

The composition of most o f the primary phases in the kyanite-zoisite marbles are quite uniform. Kyanite, zoisite, dolomite and calcite are all very close to their stoichiometric end-member formulae. White micas (primary and secondary muscovite, paragonite and margarite) display variable compositions as discussed below. Chlorite is a clinochlor and is restricted in composition with low Fe contents [Fe/(Fe + Mg) = 0.02-0.03] and 5.3 5.5 Si per formula unit (based on 28 oxygens; :see Table 1).

The composition of the late white micas are also shown in the triangular diagram muscovite-paragonite-margarite (Fig. 6). Margarite has up to 4 0 - 5 0 mole% paragonite component, paragonite 1 5 - 2 0 mole% margarite component and up to 25 mole% muscovite component. Ca component in muscovite is negligible, and K component in margarite is near 10% maxinmm.

3.1.1. White micas. Four different types of white micas are present in the kyanite-zoisite marbles: primary muscovite (Phi), secondary muscovite (PhII), paragonite and margarite. Representative compositions are given in Table 2. Secondary muscovite is observed as either small crystals (together with other secondary micas and chlorite) or as rims on primary muscovite porphyroblasts. The compositions of primary and secondary muscovites are plotted in Fig. 5. Primary muscovites have phengite contents of 3 0 - 4 0 % and paragonite contents of 5 - 1 0 % (see Fig. 5). These values are similar to phengite and paragonite contents of other muscovites from blueschist- and eclogite-facies rocks as reported in the literature (see, e.g., Ahn et al., 1985, fig. 1). Secondary muscovites have lower phengite contents ( 1 0 - 2 0 % ) and higher paragonite contents ( 1 2 - 1 8 % ) and Fe/(Fe + Mg) than do the primary muscovites (compare analyses 5 and 6, sample 82-11 and analyses 7, sample 82-16 in Table 2).

3.1.2. Sudoite. The occurrence of sudoite in these rocks is of special interest, because sudoite has only been reported from a few other localities in metamorphic rocks (for a review, see Fransolet and Schreyer, 1984). Representative microprobe analyses of homogeneous grains of sudoite (sample 82-19A) are given in Table 1 and plotted in Fig. 7. The mineral contains approximately 23 wt.% S i Q , 36.5 wt.% A1203 and 14 wt.% MgO, with a total near 85 wt.%. All other elements are below 0.5 wt.%, including F, which is 0.1 wt.%. It has a low birefringence and is similar in appearance to chlorite, though sudoite is generally finer grained. The amount of material was too small to separate for X-ray analyses but the only phase approaching this composition is the sudoite reported by Fransolet and Bourguignon (1978), as shown in Fig. 7A (point FB). The only other mineral similar in composition to sudoite is vermiculite, but it can be distinguished from sudoite by its higher H20 contents (~20 wt.%). Because o f the absence of X-

225

TABLE 2 Microprobe analyses of micas from carbonate rocks, EZ Mineral No.

Pa .1 82-10

Pa*: 82-12

Ma*: 82-12

Ma .3 82-10

Ma.4 82-12

Ph .5 82-11

Ph .6 82-11

Ph .7 82-16

Ph .7 82-16

Ph *a 79-80

SiO 2 TiO 2 AI~O3 FeOto t MgO MnO CaO Na20 K20

46.19 0.02 39.64 0.13 0.02 0.58 6.50 1.72

43.77 41.00 0.04 1.92 6.28 1.29

38.04

34.10

34.40

44.80

6.93 3.53 0.70

46.75 0.08 0.30 0.01 8.94 2.64 0.55

46.81 0.20 0.04 9.96 1.80 0.27

51.23 0.15 29.02 0.18 3.85 0.03 0.05 0.50 10.63

47.78 0.06 35.08 0.13 1.44 0.16 1.28 9.68

50.89 0.10 29.02 0.53 3.82 0.03 0.06 0.38 10.74

46.73 0.02 36.41 0.33 0.69 0.07 0.04 1.07 10.18

47.59 0.09 35.28 0.34 0.96 0.15 1.24 8.99

"2

94.80

94.30

94.32

93.37

93.48

95.64

95.61

95.57

95.54

94.64

Si A1TM

5.95 2.05

5.70 2.30

5.01 2.99

4.58 3.42

4.60 3.40

6.74 1.26

6.23 1.73

6.72 1.28

6.16 1.84

6.29 1.71

AIVI Fe Mg Mn Ti

3.97 -0.03 -

3.99

3.96 0.06 -

3.98 0.01 0.06 -

3.98 0.04 0.01

3.24 0.02 0.75 0.02

3.71 0.01 0.28

3.82 0.04 0.14 0.01

0.01

3.23 0.06 0.75 0.01

3.78 0.04 0.19 0.01

~,2

4.00

4.00

4.02

4.05

4.03

4.03

4.01

4.05

4.01

4.02

K Ca Na

0.28 0.08 1.62

0.22 0.27 1.58

0.12 0.98 0.90

0.09 1.29 0.69

0.05 1.43 0.47

1.78 0.01 0.13

1.62 0.02 0.33

1.81 0.01 0.10

1.71 0.01 0.28

1.52 0.02 0.32

>2

1.98

2.07

2.00

2.07

1.95

1.92

1.97

1.92

2.00

1.86

0.32

0.01 -

-

* ~Coexisting with phengite, * 2 coexisting Ma/Pa; * 3coexisting with chlorite and paragonite; * 4 coexisting with phengite; * s core; *6rim, coexisting with margarite; *Tprimary phengite and secondary phengite coexisting; *S secondary phengite after zoisite. ray d a t a on o u r sample, the a s s i g n m e n t o f the n a m e

• 79-80

o 82-,0 o

sudoite m u s t be tentative.

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82-,

o 82-~2 o a2-16

N o t all o f the crystals with the optical a p p e a r a n c e o f s u d o i t e are chemically h o m o g e n e o u s . A r e a c t i o n rim b e t w e e n k y a n i t e and d o l o m i t e (sample 82-19; see Fig. 4), and also several analyses from samples 82-19C and 82-12, s h o w a wide range o f c o m p o s i t i o n s bet w e e n 0 and 14 wt.% MgO (Table 1, Fig. 7). These c o m p o s i t i o n s can be d e s c r i b e d as m i x t u r e s o f sudoite and kaolinite because a true solid s o l u t i o n b e t w e e n d i / t r i o c t a h e d r a l and d i o c t a h e d r a l chlorite should lie on the line s u d o i t e - d o n b a s s i t e . It is i n t e r e s t i n g to n o t e t h a t the actual c o m p o s i t i o n o f t r i o c t a h e d r a l chlorite

IV~

~

.

.

"

'Ca

Fig. 6. C o m p o s i t i o n o f secondary w h i t e micas f r o m k y a n i t e zoisite marbles in terms o f muscovite-paragonite-margarite content [Na = Na/(Na + K + Ca); K = K / ( N a + K + C a ) ; C a = Ca/(Na + K + Ca)] Lines connect analysis points on adjacent

crystals.

f r o m these rocks is e x a c t l y collinear w i t h sudoite and d o n b a s s i t e , indicating that the c l i n o c h l o r c o m p o s i t i o n w i t h Si ~ 5 . 5 per f o r m u l a u n i t is f a v o u r e d b y crystal c h e m i s t r y . These i n t e r g r o w t h s o f kaolinite and s u d o i t e m u s t be very fine grained because backs c a t t e r e l e c t r o n imaging at a resolution o f 1 u m

226 failed to reveal any mixed phase character of these grains.

~rophylite

3.1.3. Kaolinite. Pure kaolinite is found as alterations within zoisite (sample 7 9 - 8 0 , Fig. 3) with a composition close to stoichiometric kaolinite (Table 1; Fig. 7B), often together with chlorite, secondary muscovite and paragonite. X-ray work with a Gandolfi camera on material taken from a polished section showed a strong 7 A line and a weak 14 A line, supporting this assignment.

3. 2. Quartzites and mica schists Representative compositions of primary garnet, chloritoid and staurolite from quartz-mica schists are given in Table 3. Garnet is largely a solid solution among almandine, pyrope and grossular with only minor spessartine component. Zoned crystals show an increase in pyrope and grossular and a decrease in almandine and spessartine content from core to rim. Chloritoid is largely an Fe-Mg solution with Fe/(Fe + Mg) ranging fiom approximately 0.7 to 0.85. No highly magnesian chloritoids, such as those reported by Chopin and Schreyer (1983) were found, probably owing to the restricted range of bulk Fe/(Fe + Mg) found in the quartz-mica schists. Staurolite is found in a few samples of quartz-mica schist from the'. Frosnitz Valley area (samples 79-70 and 81-49; see Fig. 1). These staurolites are worthy of note because of the high ZnO contents ( 6 - 8 wt.%; see Table 3). A comparison of the compositions of these staurolites with others reported in the literature is given in Fig. 8. As can be seen, the staurolites from the Frosnitz Valley have some of the highest ZnO contents of any reported in the literature. No Zn-free staurolites were observed and it is likely that the stability of Zn-free staurolite is exceeded at the P-T conditions of the EZ, and that these staurolites are stabilized by their zinc contents.

4. Phase equilibria

4.1. Kyanite-zoisite marbles 4.1.1. Primary assemblages. The phase relations of the peak metamorphic (primary) mineral assemblages can be represented by the chemical system S i O : A I ~ O : CaO-MgO-Na30-K~O-H~O-CQ. Other elements are

M90'

;~./203

MgO'

~AI203

Fig. 7. A. Idealized compositions of sheet silicates in the system SiO:MgO-AI~O3(+ H~O), showing directions of phengite (MgSi = 2A1) and di/trioctahedral (3Mg = 2AI) substitutions. BB = sudoite composition of Brindley and Brown (1980); FB = sudoite composition of Fransolet and Bourguignon (1978). B. Measured compositions of chlorites, sudoites, sudoite/ kaolinite mixtures and pure kaolinites from kyanite-zoisite marbles. present in minor amounts only, and will not seriously distort the phase relations (see Tables 1 and 2). Quartz is ubiquitous, so projection can be made into the tetrahedron AI~O:MgO-CaO-Na20 or AI~O3-MgOCaO-K~O. The presence o f K~O or Na~O stabilizes a white mica, so K~O and Na20 can be listed as a single component R~O with the understanding that one or two white micas may be present. Projection from H20 and C02 presupposes that the chemical potentials of these components (the H20/CO~ ratio) are fixed externally or that all assemblages exist at equal

227

TABLE 3 Representative microprobe analyses of garnet, chloritoid, staurolite from quartzites, EZ Mineral Gt No. 79-70 SiO 2 TiO 2 AI=O3 FeOto t MgO MnO CaO

36.88

Gt .1 81-46

Gt *= 81-49

Gt .3 81-67

38.37 . 21.99 37.32 4.49 0.09 0.75

38.12 . 21.14 35.06 1.10 1.28 6.18

Gt .4 81-67

Ctd 81-46

Ctd 81-49

Ctd 81-67

Sta 81-49

Sta 79-70

Sta 79-70

38.76

23.77

22.50 32.14 3.05 0.31 6.94

40.09 24.38 3.10 0.03 -

24.38 0.01 41.88 21.86 5.15 0.06 -

24.34 41.36 26.64 2.55 0.15 -

29.72 0.44 55.67 9.00 1.10 5.95

29.36 0.30 54.76 7.40 1.09 7.40

28.27 0.47 52.71 7.07 0.88 7.92

101.88

100.31

97.32

Si A1 Ti Fe Mg

7.944 17.540 0.088 2.012 0.438

7.960 17.497 0.062 1.677 0.440

7.954 17.475 0.100 1.664 0.367

Zn

1.175

1.593

1.644

21.51 38.39 3.84 0.06 0.51

36.84 . 21.57 39.27 3.01 0.01 0.94

2

101.19

101.64

103.1i

102.88

103.70

91.37

93.34

95.04

Si A1

2.946 2.025

2.944 2.032

2.982 2.014

3.004 1.963

2.974 2.034

1.988 3.952

1.966 3.980

1.973 3.952

Fe Mg Mn

2.565 0.457 0.004

2.624 0.359 0.001

2.426 0.520 0.006

2.311 0.129 0.085

2.062 0.349 0.020

1.706 0.387 0.002

1.474 0.619 0.004

1.806 0.308 0.010

Ca

0.044

0.080

0.062

0.522

0.570

-

-

Aim Pyr Gro Spe

83.4 14.8 1.4 0.1

85.6 11.7 2.6 -

80.4 17.2 2.0 0.1

75.8 4.2 17.1 2.8

68.7 11.6 19.0 0.6

.

.

ZnO

"1 Coexisting with ctd 81-46; ,2 coexisting with ctd 81-49; *a core; *4rim, coexisting with Ctd 81-67.

Z

t i g h t l y c o n s t r a i n e d . This p o i n t will be addressed later, b u t for t h e m o m e n t it will be assumed t h a t it is valid t o p r o j e c t t h r o u g h these c o m p o n e n t s . T h e phase relations for the peak m e t a m o r p h i c assemblages are s h o w n in Fig. 9. T h e y f o r m d i s t i n c t four phase v o l u m e s in this p r o j e c t i o n : Z o i + Cc + Dol + m i c a (+ Qz + H~O + CO2) and Ky + Z o i + Dol + m i c a (+ Qz + H~O + C Q )

Fg"

Mg

Fig. 8. Zn-staurolite analyses from quartzites, EZ (dots) compared with Zn-staurolite from Griffen and Ribbe (1973) and Ashworth (1975) (open circles). values o f these c o m p o n e n t s . T h e r e is c o n s i d e r a b l e evid e n c e t h a t d u r i n g p e a k m e t a m o r p h i c c o n d i t i o n s the fluid p h a s e was H 2 0 rich in these and o t h e r lithologies w i t h i n t h e EZ, and t h a t the H20/CO~ ratio was

These p h a s e v o l u m e s o c c u p y a large p o r t i o n o f the c o m p o s i t i o n space w i t h n o a p p a r e n t crossing tie-lines, s u b s t a n t i a t i n g the h y p o t h e s i s t h a t these r e p r e s e n t e q u i l i b r i u m peak m e t a m o r p h i c assemblages. Several p o i n t s s h o u l d be m a d e a b o u t the peak m e t a m o r p h i c assemblages: ( 1 ) D o l o m i t e and q u a r t z is a p p a r e n t l y a stable association, in a g r e e m e n t to the findings o f F r a n z and S p e a r ( 1 9 8 3 ) , w h e r e diopside + Dol + Qz was the stable early association. (2) T h e associations Z o i + Dol and Ky + Dol are stable.

228

Ai203

and

Ky %

K20

~\

Ky + Zoi + Dol + mica (+ Qz)

+ H20/CO2

X

oi \

Na20 ~

i Tr,

\/ ~D°l(D~°) Cc CaO

Fig. 9. Phase relations of primary mineral assemblages in kyanite-zoisite marbles in the system A1203-CaO-MgO-(Na20, K20). Projection is from quartz and a fixed H20/CO r Abbreviations: Ky = kyanite; Pa = paragonite; Phi = phengite I (primary muscovite); Zoi = zoisite; Dol = dolomite, Cc = calcite; Tre = trernolite; Dio = diopside.

(3) The upper pressure and temperature stability limit o f phengitic muscovite has not been exceeded. (4) Kyanite + zoisite + quartz is stable in preference to assemblages that contain margarite or lawson° ite + quartz (i.e. the stabilities o f margarite + quartz and lawsonite + quartz have been exceeded). (5) The upper stability o f paragonite is not exceeded. This will be discussed below with reference to Holland's (1979) observations that paragonite + Ky + jadeite + quartz assemblages are stable in veins in the eclogites. (6) The associations Chl + Cc, Chl + Zoi and Ky + Cc are not observed. Franz and Spear (1983) reported a third early mineral assemblage in metacarbonate rocks from the EZ: Zoi + diopside (Dio) + tremolite (Tre) + Cc + Dol + Qz This assemblage is invariant on a T-Xco 2 section and must have crystallized at a different XCO, from the other two primary assemblages: Zoi + Cc + Dol + mica (+ Qz)

4.1.2. Secondary assemblages. Textural criteria combined with data on the chemistry and stability o f the minerals indicate two distinct phases o f alteration. (1) Kyanite + dolomite + muscovite 1 and zoisite + dolomite + quartz are replaced by chlorite + margarite + paragonite + muscovite II and by chlorite + muscovite II + paragonite + calcite, respectively. (2) Sudoite and sudoite/kaolinite mixtures are produced along cracks in kyanite and in reaction zones between kyanite and dolomite. Kaolinite is produced as a decomposition product along cracks in zoisite and also within pseudomorphs of white mica, chlorite and calcite after zoisite. A phase diagram for the early stages of alteration listed in (1) above is presented in Fig. 10. This diagram shows how the peak metamorphic associations kyanite + dolomite and zoisite + dolomite have been replaced by chlorite + zoisite, chlorite + margarite and chlorite + calcite assemblages (compare with Fig. 9). The plotting positions of sudoite and kaolinite are also shown, but tie-lines have not been drawn be-

AI203 Ph~/~,~%, \ ~

+ H20/C02

" Ch

\ \',,111 "-,~ce OaO Fig. i0. Secondary mineral assemblages from kyanite-zoisite marbles projected from quartz and H20/CO 2. Additional abbreviations to those explained in the caption to Fig. 9 include: Ch = chlorite; Ma = maxgarite; Sud = sudoite; Phll= phengite 1I (secondary muscovite); Kao = kaolinite.

229

cause it is not clear with which phases they stably coexist. From thin-section observations it appears that sudoite most likely coexists with chlorite + margarite + kyanite + white mica. Several reactions can be listed that describe the transition between the peak metamorphic assemblages (Fig. 9) and the secondary mineral assemblages (Fig. 10): kyanite + dolomite + H20 chlorite + margarite + C0~ and zoisite + dolomite + quartz + H20 chlorite + calcite + CO~ The production of secondary paragonite and muscovite is somewhat more problematical. In some reaction zones, the amount of secondary white mica produced cannot be balanced by primary K- or Na-bearing phases. In these cases, Na and K must come from the fluid phase via reactions such as: kyanite + nmscovite I + dolomite + NaCI + H20

4. 2. Quartz-mica schists 4.2.1. Primary assemblages. The principal peak metamorphic mineral assemblage in the quartz-mica schists is quartz + muscovite -+ paragonite + garnet + chloritoid + kyanite -+ dolomite -+ epidote; these phase relations are depicted in an AFM diagram in Fig. 1 1. Note that staurolite is not plotted in Fig. 11 because it is presumably stabilized by its Zn content. It is not clear whether primary chlorite + quartz is stable. The facies type depicted in Fig. 11 is slightly different from those presented by Chopin and Schreyer (1983, fig. 2) in their summary of phase relations in petites from the eclogite facies. Specifically, choritold, which is shown by Chopin and Schreyer to extend to the Fe-side, is restricted in the Tauern eclogites to intermediate Fe-Mg contents. Untortunately, the bulk compositions of the quartz-mica schists from the EZ are too restricted to provide additional constraints on the AFM topology. However, Miller (1977) reports tire association talc + kyanite ± Mgchloritoid from the EZ, so it is possible that the AFM facies type resembles that of fig. 2D of Chopin and Schreyer (1983).

margarite + chlorite + muscovite II + paragonite + CaCI~ + C02 or zoisite + dolomite + NaC1/KC1 + H=O

4.2.2. Secondary assemblages. The only retrograde assemblages fomled in the quartz-mica schists are the production of secondary chlorite after garnet and biotite after garnet and phengite. The secondary

chlorite + paragonite + muscovite II + CaCI= + CO=

Kyanite A A

The kaolinite alteration of zoisite and kaolinite/sudoire alteration of kyanite must, in a similar way, involve liberation o f Ca into fluid:

//

zoisite + HC1 + H~O e- kaolinite + CaCI~ and

/,,Chloritoid

kyanite + dolomite + HC1 + H~O sudoite + kaolinite + CaCI~ + CO2 Two features o f these reactions should be noted: (1) they are all hydration reactions and most of them also liberate CO~; and (2) there is evidence that Na and K have been introduced into and Ca removed from the rocks during 'alteration. Both of these points suggest that the alteration was accomplished by infiltration of an aqueous and possibly chloride-bearing fluid during retrogradation. Possible sources of this fluid will be discussed later.

j

Garne;

Chlorite

M Biotite Fig. 11. AFM diagram (projected from quartz, muscovite and H20) showing plotting positions of primary mineral assemblages in quartz-mica schists (solid lines). Secondary biotitechlorite is shown with dashed lines.

230

TABLE 4 Reactions for the petrogenetic grid of Fig. 12

/' / / / 'i?

Fig. 12. P-T diagram for the evolution of metasediments from the Eclogite Zone, The interred P-T path is shown by large dashed lines and arrows; crosshatched areas are estimated P-T conditions for peak metamorphic conditions (P), the early alteration sequence (A1) and the late alteration sequence (A2). Reactions are listed in Table 4. c h l o r i t e differs f r o m t h e late c h l o r i t e in the k y a n i t e zoisite m a r b l e s in t h a t it h a s a m u c h h i g h e r Fe/Mg. T h e p l o t t i n g p o s i t i o n o f these s e c o n d a r y minerals are also s h o w n o n t h e AFM d i a g r a m o f Fig. 11.

5. Conditions of metamorphism The pressure-temperature conditions of metamorp h i s m o f t h e E Z have b e e n c o n s t r a i n e d b y a p p l i c a t i o n o f p h a s e e q u i l i b r i a a n d m i n e r a l stability criteria to the p r i m a r y a n d s e c o n d a r y m i n e r a l assemblages. A P-T d i a g r a m s u m m a r i z i n g the r e l e v a n t e q u i l i b r i u m curves is p r e s e n t e d in Fig. 12; r e l e v a n t r e a c t i o n s are summ a r i z e d in T a b l e 4.

5.1. Peak metamorphic conditions E s t i m a t e s o f peak m e t a m o r p h i c c o n d i t i o n s for t h e T a u e r n eclogites have b e e n given b y Miller ( 1 9 7 7 ) , H o l l a n d ( 1 9 7 9 ) , R a i t h et al. ( 1 9 8 0 ) , and F r a n z a n d Spear (1983). Holland (1977) estimated peak metam o r p h i c c o n d i t i o n s o f 19.5 ± 2.5 k b a r , 6 2 0 +- 30°C b a s e d o n t h e o c c u r r e n c e o f the vein assemblage k y a n i t e + p a r a g o n i t e + o m p h a c i t e + q u a r t z . This is in g o o d a g r e e m e n l w i t h t h e e s t i m a t e o f F r a n z a n d Spear ( 1 9 8 3 ) o f 18 125 k b a r , 6 0 0 -+ 3 0 ° C b a s e d o n equilibria in m e t a c a r b o n a t e assemblages. D a t a f r o m the p r e s e n t s t u d y p e r m i t the f o l l o w i n g limits to be placed o n t h e p e a k P-T c o n d i t i o n s . R a o and J o h a n n e s ( 1 9 7 9 ) have d e t e r m i n e d e x p e r i m e n t a l l y t h a t the assemblage k y a n i t e + c h l o r i t o i d + a l m a n d i n e + q u a r t z + H 2 0 (in the p u r e - F e s y s t e m ) is l i m i t e d b y

No.

Assemblage/reaction

Reference*

a b

Dio-Tre-Do-Cc-Qz Tre-Ta-Do-Cc-Qz

(1) (1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

Jad + Qz ~ Ab Par ~ Jad + Kya + Qz + H20 muscovite - granite melting Ab + Qz + H20 ~ melt Par + Qz ~ Ab + AISi + 1t20 Mar + Qz ~ An + AISi + H20 An + H20 ,~ Zoi + Kya + Qz Mar + Qz ;- Zoi+ Kya + tt~O Law ~- Zoi+ Mar + Qz + H20 Law ~ Zoi + Kya + Qz + Ict20 Kao + Q z ~ Py + 1420 P y ~ Kya+ Qz+ tt20 Kao + C h l ~ Sud + Qz + 1420 Sud + Q z ~ Car Sud + Qz ~ Py + Chl + ft20 Sud ,~ Chl + And + Qz + H20 Car ~ Chl + Kya + Qz + H20 Chl + Qz =- Tal + Car + tt20 Car - Chl + Kya + Tal + tt20 Chl + Qz -~ Tal + Kya + 1t20 Ctd + Kya ~ Sta + Qz + 1420 Ctd + Qz ~- Sta + Atm + ft20 Sta + Qz ~" Ahn + Kya + H20 Ctd + Kya ,~ Sta + Aim + tt20 Ctd + Qz ~ Alm+ Kya + H20

(2) (3) (4) (5) (5) (6) (6) (6) (6) (6) (6) (6) (7) (7) (7) (7) (8) (8) (8) (8) (9) (9) (9) (9) (9)

*References: 1 = Franz and Spear (1983); 2 = Holland (1980); 3 = llolland (1979); 4 = lluang and Wyllie (1973); 5 = Chatterjee (1974); 6 = Chatterjee et al. (1984); 7 = Fransolet and Schreyer (1984); 8 = Chopin and Schreyer (1983); 9 = Rao and Johannes (1979). an i n v a r i a n t p o i n t at a p p r o x i m a t e l y 16 k b a r , 590°C. The n a t u r a l assemblages f r o m the quartzmica schists c o n t a i n some Mg, w i t h Mg p r e f e r r e d b y c h l o r i t o i d (XFe,Gar/XFe,Ctd = 1.15), resulting in an increase in t h e e s t i m a t e d t e m p e r a t u r e o f this e q u i l i b r i u m o f p r o b a b l y a few degrees. This assemblage t h u s provides a low P / u p p e r T limit o f a p p r o x i m a t e l y 5 9 0 ° C at 16 kbar. T h e a b s e n c e o f a n y signs o f m e l t i n g in the diverse b u l k - c h e m i c a l composit i o n s also places an u p p e r t e m p e r a t u r e limit o f 6 1 0 6 3 0 ° C a t P H 2 0 = 1 5 - 2 0 k b a r (curve 3). A minimum temperature estimate of 550--560°C at PH20 o f 15 20 k b a r is given b y the a b s e n c e o f c a r p h o l i t e + q u a r t z associations (curves 17 and 19). A n o t h e r l o w e r - T e s t i m a t e is i n d i c a t e d b y the ab-

231 sence of lawsonite and the presence of kyanite + zoisite + quartz, which, based on the calibration of Newton and Kennedy (1963) provides estimates of 525 590°C at PH~O of 15 and 20 kbar, respectively (curve 10). Lower-pressure estimates of 14-16 kbar at 5 0 0 600°C are also given by the absence of albite and the presence ofjadeitic pyroxene (curve 1). An upper PH20 limit is given by the intersection of the chloritoid-almandine-kyanite-quartz equilibrium (curve 25) with the lawsonite-zoisite-kyanite-quartz equilibrium at PH~O < 20 kbar, which is consistent with Holland's (1979) breakdown curve for paragonite omphacite jadeites0 + kyanite + H~O (curve 2). The absence of primary chlorite-quartz associations (chlorite could not unequivocally be identified as a primary phase) also limits the lower P-T peak metamorphic conditions (curves 18 and 20). Two other curves are shown in Fig. 12 that constrain the peak P-T estimates. Miller (1977) reports kyanite + talc associations from the EZ and the stability of this association is given by curve 20. Curve a is the univariant P-T trace of the isothermal invariant point A1 (tremolite-diopside-quartz-calcite-dolomite) of Franz and Spear (1983, fig. 4), along which the metacarbonates of the EZ are believed to have crystallized. These constraints limit the peak metamorphic conditions to the high-pressure crosshatched labeled P in Fig. 12. Pressures range from 16 to 21 kbar and temperatures range from 560 ° to 620°C, with preferred values at approximately 19 kbar, 590 ° C.

al., 1977) with natural compositions gives temperatures greater than 500°C. Comparison of the natural muscovite-paragonite pairs with the solvus of Chatterjee and Froese (1975), also indicates minimum temperatures of approximately 500 ° C. Franz and Spear (1983) report the formation of secondary talc in metacarbonate rocks from the EZ. These talc-bearing assemblages are believed to have formed at an isothermal (or isobaric) invariant point, which traces out a univariant P-T loop as depicted by curves b in Fig. 12. If the talc alteration occurred at the same time as the alteration of the kyanitezoisite marbles, then the first alteration sequence is constrained to the crosshatched area labeled A1 in Fig. 12. The second phase of 'alteration produced sudoite -+ kaolinite + quartz. Stability fields for sudoite + quartz and kaolinite + quartz are shown in Fig. 12 as curves 13, 14, 15 and 16. The maxinmmP-Tconditions where kaolinite-sudoite mixtures could have formed are approximately 340°C and 7 kbar and the minimum T is approximately 210°C. Late-stage, low-density, H~O-rich, fluid inclusions in dolomite with homogenization temperatures between 280 ° and 310°C (S. Thomas, unpublished data, 1985)provide an additional temperature constraint and an upper pressure limit of 2.5-3 kbar. We theretore conclude that the second phase of alteration took place at 250 ° < T < 310°C and P < 3 kbar, as shown by the crosshatched region, labeled A2 in Fig. 13. 6. Discussion: P-T and tectonic evolution of the

eclogite zone 5.2. P-T conditions o f alteration The first episode of alteration produced the minerals margarite (+ quartz), paragonite, secondary muscovite and chlorite in the kyanite marbles and chlorite + biotite in the quartz-mica schists. Margarite + quartz stability is depicted in Fig. 12 (curves 6 and 8) and restricts the alteration to have occurred at PH20 < 10 kbar and T < 550°C. The presence of paragonite (+ quartz) as an alteration mineral restricts the temperature to the low-T side of the paragonite + quartz stability curve (curve 5; T < 550 ° at 5 kbar, T < 680 ° at 10 kbar). The composition of coexisting secondary white n~icas (Fig. 6) also helps constrain the temperature of alteration. Comparison of the experimentally determined paragonite-margarite solvus (Franz et

Figure 12 depicts a postulated P-T path for the rocks of the EZ. The prograde portion of the P-T path is not well constrained, but some aspects of the paragenesis can be inferred from textural observations of pseudomorphs and mineral inclusions within these porphyroblasts. In the kyanite-zoisite marbles, the large kyanite crystals are almost always associated with quartz. In only a few places, isolated kyanite crystals completely surrounded by dolomite have been observed. The ratio of quartz to kyanite is approximately 3 : 1 to 5 : 1. The most likely reaction sequence of aluminum silicates in these rocks is kaolinite pyrophyllite ~ kyanite. Production of kyanite from original kaolinite would generate quartz + kyanite in a volume ratio of approximately 1 : 2, therefore

232 the original segregations could have been kaolinite + quartz. Inclusions are,. found within both kyanite and zoisite. Kyanite contains inclusions of dolomite, mtile and quartz; zoisite contains inclusions of kyanite as well as fluid inclusions. Large dolomite blasts contain inclusions of white mica and fluid inclusions. In some samples (e.g., 82-13) kyanite and zoisite contain inclusions that are mixtures of mica (mostly margarite) and chlorite. There is extensive late-stage margarite in these rocks, but these inclusions have habits that are very suggeslive of pseudomorphs after prismatic crystals. A possible precursor to these pseudomorphs is lawsonite which has the appropriate crystal habit. During the progressive metamorphism lawsonite would decompose to zoisite + kyanite and late-stage alteration would produce margarite. In the quartz-mica schists, pseudomorphs within garnet have a rectangular shape. They consist of epidote + quartz + paragonite + phengite + opaque minerals ± chloritoid -+ rutile. Both, shape and mineral assemblages, are consistent with the interpretation as lawsonite pseudomorphs. The reactions: lawsonite + albite ~ zoisite + paragonite + quartz and lawsonite + jadeite ~ zoisite + paragonite + quartz have been proposed by Franz and Althaus (1977) and experimentally verified by Heinrich and Althaus (1979). The presence of phengite and accessory minerals indicates that the transformation of lawsonite was even more complex, involving a K phase and probably Mg/Fe minerals. Also, the occurrence within garnet shows that the alteration of lawsonite (+ other phases) must have taken place after the formation of almandine-rich garnet. Therefore lawsonite grew before garnet, but its breakdown must have taken place within the garnet stability field. Collectively, these observations constrain the prograde P-T path only to have crossed the kaolinite + quartz, and lawsonite stability curves (curves 11, 9, and 10, respectively). The prograde path shown in Fig. 12 is completely conjectural, but has been placed as shown to conform to a more-or-less typical subduction path. The P-T path is drawn to intersect the peak metamorphic conditions, indicated by the crosshatched region labeled P, which presumably represent the

maximum extent of subduction of the EZ to a depth of approximately 60 70 km. The uplift path is drawn to intersect the P-T conditions of the two stages of alteration, A1 and A2. The present tectonic position of the EZ, which is sandwiched between the Upper and Lower Schieferhiille units, indicates that the EZ must have been emplaced tectonically between these two units during some part of its P-T evolution. The maximum pressures achieved by the Lower and Upper Schieferhtille units are 10 and 7 kbar, respectively (Selverstone et al., 1984; Selverstone, 1985; Selverstone and Spear, 1986). It is therefore possible that the EZ could have been emplaced within these two units at a maximum depth of 2 0 - 3 5 kin, although it is not possible to rule out shallower depths of emplacement. According to England and Holland (1979) buoyancy forces could have been responsible for the uplift of the EZ. Calculated uplift rates are on the order of 40 mm yr -~, which would require approximately 1 Ma for the uplift of the EZ from 70 to 30 kin. The first episode of alteration of the EZ rocks (A1) occurred at depths of 15-30 kin, which is similar to the maximum depths experienced by the Upper and Lower Schieferhiille units. It is therefore possible that the fluids responsible for this alteration were generated by dehydration reactions that were proceeding in the Lower and/or Upper Schieferhiille. The alteration reactions are all hydration reactions and some of the reactions require an addition of alkalis and removal of Ca, both of which suggest that the alteration agent was an infiltrating H~O-rich fluid. The amount of fluid infiltration could not have been excessive, however, because Franz and Spear (1983) have shown that the talc-producing reactions buffered the fluid to increasingly CO~-rich compositions. In other words, the H20 to produce the first alterations was likely supplied by infiltrating aqueous fluids, but the buffer capacity of the rocks was not exceeded so that the composition of the fluid in equilibrium with the rocks was buffered by the mineral assemblage to relatively CQ-rich compositions. This C O s i c h fluid may have been trapped as fluid inclusions in quartz from eclogites, representing the early generation of fluid inclusions described by Luckscheiter and Morteani (1980). The late-stage alteration most likely occurred at shallow depths (less than 10 km). It is not probable that dehydration reactions in the Upper and Lower Schieferhiille were still proceeding at this shallow

233

level, so a d i f f e r e n t

source is required. T w o pos-

sibilities are: (1) t h e e x s o l u t i o n o f fluid f r o m prim a r y m i n e r a l s d u r i n g d e c o m p r e s s i o n a n d cooling, a f t e r a m e c h a n i s m similar t o t h a t discussed b y S p e a r a n d S e l v e r s t o n e ( 1 9 8 3 ) ; a n d (2) d e e p c i r c u l a t i n g g r o u n d w a t e r s . This fluid m a y also be r e p r e s e n t e d in t h e late-stage, H~O-rich fluid i n c l u s i o n s r e p o r t e d b y L u c k s c h e i t e r and M o r t e a n i ( 1 9 8 0 ) . Acknowledgements T h i s w o r k was s u p p o r t e d b y grants No. F r 5 5 7 / 3-1 ( D F G to G.F.), N S F g r a n t s E A R - 8 3 0 6 3 7 8 and E A R - 8 5 1 4 6 5 9 ( t o F.S.S.) a n d the MIT-TUB exchange p r o g r a m . We t h a n k S. T h o m a s a n d J. Selvers t o n e for t h e i r help d u r i n g field w o r k a n d for m a n y h e l p f u l discussions, a n d K. B u c h e r - N u r m i n e n for a very careful review.

References Abraham, K., H6rmann, P.K. and Raith, M., 1974. Progressive metamorphism of basic rocks from the Southern Hohe Tauern area, Tyrol (Austria). Neues Jahrb. Mineral. Abh., 122: 1-35. Ahn, J.H., Peacor, D.R. and Essene, E.J., 1985. Coexisting paragonite-phengite in blueschist eclogite: a TEM study. Am. Mineral., 70:1193 1204. Ashwoth, J.R., 1975. Staurolite at anomalously high grade. Contrib. Mineral. Petrol., 53." 2 8 1 - 2 9 1 . Brindley, G.W. and Brown, G. (Editors), 1980. Crystal structures of clay minerals and their X-ray identification. Mineral. Soc., London, Monogr. No. 5. Chatterjee, N.D., 1974. Crystal-liquid-vapour equilibria involving paragonite in the system NaA1Si30:A120: SiO:H~O. Ind. J. Earth Sci., 1: 3-11. Chatterjee, N.D. and Froese, E., 1975. A thermodynamic study of the pseudo-binary join muscovite-paragonite in the system KAISi3Os-NaA1Si3Os-A1203-SiO~-H:O. Am. Mineral., 60: 9 8 5 - 9 9 3 . Chatterjee, N.D., Johannes, W. and Leistner, H., 1984. The system CaO-A120:SiO:H20: new phase equilibria data, some calculated phase relations, and their petrological applications. Contrib. Mineral. Petrol., 88: 1-13. Chopin, C. and Schreyer, W., 1983. Magnesiocarpholite and magnesiochloritoid: two index minerals of pelitic blueschists and their preliminary phase relations in the model system MgO-A120:SiO:H20. Am. J. Sci., Orville Vol., 283-A: 7 2 - 9 6 . England, P.C. and Holland, R.J.B., 1979. Archimedes and the Tauern eclogites: The role of buoyancy in the preservation of exotic eclogite blocks. Earth Planet. Sci. Lett., 44: 287 294. Fransolet, A.-M. and Bourguignon, P., 1978. Di/trioctahedral chlorite in quartz veins from the Ardennes, Belgium. Can. Mineral., 16: 365-373.

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