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Kyanite, margarite and paragonite in pseudomorphs in amphibolitized eclogites from the Betic Cordilleras, Spain
Chemical Geology, 50 (1985) 129--141 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
129
KYANITE, MARGARITE AND PARAGONITE IN PSEUDOMORPHS IN AMPHIBOLITIZED ECLOGITES FROM THE BETIC CORDILLERAS, SPAIN M A R I A T E R E S A G O M E Z - P U G N A I R E I, D A R I O V I S O N A ~ a n d G E R H A R D F R A N Z 3 1Departamento de Petrologia y de Investigaciones Geol6gicas, C.S.I.C. (Consejo Superior de Investigaciones Cientifieas), Facultad de Ciencias, Granada (Spain) 2Istituto di Mineralogia e Petrologia, Universita di Padova, Padova (Italy) 3Institut fiir Angewandte Geophysik, Petrologic und LagerstEttenforschung, Teehnische Universitiit, 1000 Berlin 12 (Federal Republic o f Germany) (Accepted for publication October 23, 1984)
Abstract Gomez-Pugnaire, M.T., Visona, D. and Franz, G., 1985. Kyanite, margarite and paragonite in pseudomorphs in amphibolitized eclogites from the Betic Cordilleras, Spain. In: D.C. Smith, G. Franz and D. Gebauer (Guest-Editors), Chemistry and Petrology of Eclogites. Chem. Geol., 50: 129--141. Pseudomorphs in amphibolitized eclogites of the Nevado--Fil~bride Complex, Betic Cordilleras, Spain, are described. Three different textural and mineralogical types can be distinguished. All contain a combination of the minerals kyanite, margarite, paragonite, epidote and quartz. Electron microprobe data indicate that the solid solution between paragonite and margarite is extensive; intermediate members Ma3sPa6s to MasoPas0 were found. The mineral assemblages and textures are explained by the successive crystallizations: from lawsonite to kyanite + epidote + quartz; kyanite + epidote to margarite + quartz; margarite + quartz + Na ÷ to paragonite + epidote; lawsonite + albite or jadeite to paragonite + epidote + quartz; and kyanite + (Na ÷ + Ca :+) to paragonite + margarite. Possible P--T paths are discussed.
1. I n t r o d u c t i o n In t h e P e r m o - T r i a s s i c series o f t h e m e t a morphic Nevado--Fil~ibride Complex, Betic C o r d i l l e r a s ( S p a i n ) , b o d i e s o f m e t a b a s i t e s are c o m m o n . M o s t are d i s c o r d a n t , b u t s t r a t i f o r m u n i t s d o o c c u r . B o t h t y p e s a r e seen s c a t t e r e d t h r o u g h t h e A l p i n e - c o v e r m e t a p e l i t e series ( N i j h u i s , 1 9 6 4 ; Puga, 1 9 7 1 ; G o m e z - P u g n a i r e , 1 9 8 1 ) . Fig. 1 s h o w s a s k e t c h o f t h e t e c t o n i c units of the Betic Cordilleras and the sample l o c a l i t i e s ( S i e r r a de Baza). M o s t o f t h e s e m e t a b a s i t e s have a l k a l i n e - - b a s a l t i c a f f i n i t i e s ( G o mez-Pugnaire, 1981) and are now a m p h i b o 0009-2541/85/$03.30
lites. In s o m e cases relics o f e c l o g i t e a s s e m blages o f an e a r l y A l p i n e m e t a m o r p h i c e v e n t are p r e s e r v e d . The association kyanite + epidote + quartz a n d t h e o c c u r r e n c e o f large f l a k e s o f p a r a g o n i t e in t h e s e r o c k s w a s d e s c r i b e d b y P u g a (1971) and Gomez-Pugnaire (1981). However, m a r g a r i t e has n o t p r e v i o u s l y b e e n described from the metabasites or metapelites of the Betic Cordilleras. Aggregates of calcite + e p i d o t e + p a r a g o n i t e + a l b i t e have b e e n i n t e r p r e t e d as p s e u d o m o r p h s o f l a w s o n i t e p o r p h y r o b l a s t s b y Puga ( 1 9 7 7 ) . In t h i s p a p e r , aggregates o f paragonite + margarite + e p i d o t e
© 1985 Elsevier Science Publishers B.V.
130
:ii
'3"
,~=,,,,r'TTTT'Nevado~ Filabride i!~ ! !! ! Complex [ ~ Alpujarride and
Malaguide Complexes
~ ~
External J Zones Volcanic rocks
Post-orogenic materials
Fig. 1. Sketch of tectonic units of the central and eastern parts of the Betic Cordilleras, southern Spain (from Julivert et al., 1974). + k y a n i t e , in some cases w i t h c o r o n i t i c textures, are described, and i n t e r p r e t e d as breakd o w n p r o d u c t s o f lawsonite. Minerals reactions are d e d u c e d f r o m t e x t u r e s and, c o m bined w i t h available e x p e r i m e n t a l d a t a , possible P - - T p a t h s for the m e t a b a s i t e s are discussed. 2. P e t r o g r a p h y o f the p s e u d o m o r p h s replacement textures
and
garnet + q u a r t z + rutile + p y r i t e + apatite. T h e o r d e r o f listing o f these minerals c o r r e s p o n d s to decreasing a b u n d a n c e in t h e rocks. The r o c k s t y p i c a l l y show the b r e a k d o w n o f o m p h a c i t i c p y r o x e n e into s y m p l e c t i t i c aggregates o f a m p h i b o l e + albite, r e p l a c e m e n t o f g l a u c o p h a n e b y sodic--calcic and sub-calcic a m p h i b o l e s , and t h e characteristic p s e u d o m o r p h s as described below.
2.1. Pseudomorphs The m e t a b a s i t e s w h i c h c o n t a i n t h e pseud o m o r p h s are intensely a m p h i b o l i t i z e d eclogites. T h e y are massive r o c k s w i t h o u t a foliat i o n and w i t h a large range o f variation in grain size. Generally an original igneous fabric is preserved. T h e t y p i c a l assemblage is: a m p h i b o l e + e p i d o t e + plagioclase + paragonite + margarite + k y a n i t e + o m p h a c i t e +
T h e c o m p o s i t i o n and t h e t e x t u r e s o f t h e p s e u d o m o r p h s are variable. T h r e e t y p e s can be distinguished: T y p e A consists o f large crystals o f k y a n i t e (length 1--2 m m ) , c o r r o d e d b y decussate w h i t e m i c a [Plate I, ( A ) ] . In s o m e cases o n l y a few relics o f k y a n i t e are preserv-
PLATE I A. Type-A pseudomorphs: kyanite crystals (Ky) corroded by white mica (crossed nicols), sample No. C 668; real length of the picture is 4.5 ram. B 1 . Type-A pseudomorphs, detail: few relics of kyanite (Ky), surrounded by margarite (M) and paragonite (Pa). Margarite always occurs as rims around kyanite, and paragonite as rims around margarite (crossed nicols), sample No. C 668; real length of the picture is 1.2 mm. B2. X-ray fluorescence scanning pictures for Ca for the upper left part of B 1; same scale. C. Type-B pseudomorphs with white mica (M, margarite and paragonite), kyanite (Ky), and a coronitic rim of epidote (E), (crossed nicols), sample No. SF 81 ; real length of the picture is 4.7 mm. D. Type-B pseudomorphs, sample No. SF 81 ; real length of the picture is 4.7 mm. E. Type-C pseudomorphs: paragonite + quartz (low relief) and epidote (high relief), crossed nicols), sample No. C 858;real length of the picture is 4.2 mm.
132 ed [Plate I, (B1) and (B2)]. Large kyanites are optically continuous, small crystals may be either optically continuous or discontinuous. Rarely small amounts of carbonate occur in the centre of the pseudomorphs: in one case additional chlorite was observed. Type B shows a coronitic texture (width of the corona 2--2.5 mm). Their shape is typically rhombohedral and prismatic, mostly euhedral to subhedral. They have a core of white mica with small irregularly--distributed and differently-orientated crystals o f kyanite, and a corona of epidote [Plate I, (C) and (D)]. In both cases A and B, paragonite and margarite are both present in the micaceous aggregates. Type C. Quartz and epidote are c o m m o n l y associated with large flakes of decussate paragonite. The replaced mineral was euhedral. The rectangular habit is evident in Plate I, (E). They reach a length of 2--4 mm, and occur frequently in the metabasites. Exceptionally small relics of kyanite rimmed by margarite are included in the large flakes of paragonite.
2.2. Paragonite and margarite Both minerals occur and coexist exclusively in the pseudomorphs. The paragonite crystals are generally larger than the margarite crystals which often occur as inclusions isoorientated in the paragonite flakes. In the pseudomorphs A and B, relics of kyanite are always surrounded by a rim of margarite. A second rim of paragonite around margarite is also evident in Plate I, (B1) and (B2). Margarite and paragonite are easily distinguished microscopically by the higher refractive index and lower birefringence of margarite.
2.3. Amphibole Green and blue-green amphiboles are the main constituents of the matrix of the metabasites. They occur as crystals with variable dimensions replacing glaucophane, garnet and omphacite. In the last case, amphibole coexists
with albite in symplectitic intergrowths (Gomez-Pugnaire et al., 1979).
2.4. Epidote minerals Clinozoisite and pistacite are the d o m i n a n t epidote-group minerals in these rocks, but zoisite occurs also. The three minerals have been identified microscopically, but within the chemically analyzed pseudomorphs only clinozoisite and pistacite have been detected. Epidote group minerals are abundant only in association with micas; in fact, within the matrix t h e y are generally scarce, and this scarcity is especially evident in eclogites which are only slightly altered into amphibolites.
2.5. Plagioclase Plagioclase is a very abundant mineral in the matrix of some metabasites, especially in the amphibolitized eclogites. It occurs as small, rounded crystals of albite with abundant inclusions o f amphiboles and opaque minerals, or as part of the above-mentioned symplectitic intergrowths. Sometimes the crystals show a rim of sodic oligoclase.
2.6. Garnet and omphacite Both minerals occur together with glaucophane as relics of the eclogite mineral paragenesis which crystallized in the older metamorphic event. Omphacite has been replaced by amphibole or a symplectitic intergrowth; garnet has been replaced by amphibole.
2.7. Accessory minerals Rutile is the characteristic accessory mineral in these rocks. It occurs randomly t h r o u g h o u t amphibole- and epidote-rich areas, and is c o m m o n l y associated with quartz replacing crystals of titano-magnetite (see Gomez-Pugnaire et al., 1979, fig. 5). It was not observed within the pseudomorphs. Minor amounts of euhedral pyrite, in some cases completely replaced by limonite, and apatite crystals were found in all rock types.
39.9 59,6 0.5
50.7 49.0 0.3
2.015
1.980 34.5 64.9 0.6
1.980
0.683 1.286 0.011
4.093
5.118 2.882 4.029 0.064
94.90
39.27 44.99 0.59 4.89 5.09 0.07
6.5 91.4 2.1
1.920
0.125 0.755 0~040
4.008
6.017 1.983 3.922 0.086
95.43
46.88 39.03 0.80 0.91 7.05 0.25
4.1 91.8 4.1
1.902
0.078 1.746 0.078
4.027
6.032 1.968 3.935 0.092
95.37
47.21 39.20 0.86 0.57 7.05 0.48
5
6.2 90.9 2.9
2,074
0,129 1.880 0.065
3.991
5.894 2.106 3.923 0.068
96.44
46.48 40.34 0.65 0.95 7.65 0.37
6
2.5 94.8 2.7
1,892
0.048 0.793 0.051
4.064
5.912 2.088 4.018 0.046
95.17
46.28 40.55 0.43 0.35 7.24 0.31
* T o t a l iron as FeO. S t r u c t u r a l f o r m u l a e b a s e d u p o n t w e n t y - t w o o x y g e n s .
Matgatite Paragonite Muscovite
1.022 0.988 0.005
4.179
4.501 3.499 4.103 0.076
94.91
34.15 48.93 0.69 7.24 3.87 0.03
0.791 1.180 0.009
4.064
VI
Ca Na K
5.109 2.891 3.991 0.073
94.92
Z
Si AI I V A 1VI Fe
39.14 44,73 0.67 5.66 4.66 0.06
AI~O 3 FeO* CaO Na20 K~O
SiO~
4
7
3
1
2
C 668
SF81
2.0 96.2 1.8
1.966
0.040 1.891 0.035
4.087
5.903 2.097 4.014 0.063
95.24
46.07 40.46 0.59 0.29 7.61 0.22
8
5.1 89.5 5.4
1.886
0.097 1.688 0.101
4.044
5.911 2.089 4.014 0.030
94.14
45.74 40.07 0.28 0.70 6.74 0.61
9
2.3 94.5 3.2
2.032
0.047 1.920 0.065
3.984
6.015 1.985 3.929 0.060
95.20
45.63 40.79 0.55 0.33 7.51 0.39
10
37.3 61.0 1.7
2.038
0.760 1.244 0.034
4,046
5.112 2.888 3.996 0.050
95.18
39.27 44.86 0.46 5.45 4.93 0.21
11
62.5 37.3 0.2
1.980
1.236 0.739 0.005
4.195
4.262 3.738 4.127 0.068
93.51
31.74 49.70 0.61 8.59 2.84 0.03
12
M i c r o p r o b e analyses, c a l c u l a t e d f o r m u l a e , a n d e n d m e m b e r s of w h i t e m i c a s f r o m p s e u d o m o r p h s of t y p e s A and B
TABLE I
81.9 18.1 --
1.967
1.611 0.356 --
4.115
4.133 3.867 4.057 0.058
94.14
30.85 50.18 0.52 11.22 1.37 --
13
72.4 27.6 --
2.043
1.479 0.564 --
4.146
4.162 3.838 4.084 0.062
94.90
31.35 50.63 0.56 10.40 1.96 --
14
70.7 29.2 0.1
2.015
1.425 0.533 0.002
4.134
4.247 3.753 4.043 0.091
94.86
31,96 49.78 0.82 10.01 2.28 0.01
15
C884
11.3 84.9 3.8
1.926
0.219 1.634 0.073
4,012
5.885 2.115 3,947 0.065
93.78
45.20 39.50 0.60 1.57 6.47 0.44
16
134
solution with margarite. These values agree very well with the data on paragonite given b y Deer et al. (1962), Ackermand and Morteani (1973), and H6ck (1974). The content of
3. Analytical data
3.1. Margarite and paragonite The chemical compositions o f margarite and paragonite from five typical rock samples of metabasites were analyzed with an electron microprobe. Tables I and II show the oxide analyses, the calculated cation formulae, and the proportion of the end-member micas (paragonite--margarite--muscovite) of twentyfour white-mica grains. The analyses are tabulated b y the pseudomorph t y p e (Table I: types A and B; Table II: type C). The compositions of the white micas in terms o f the end-members muscovite--parago nite--margarite are represented in Fig. 2. 2--13 mole% muscovite are in solid solution with paragonite (average 8 mole%), b u t only 0.2--0.3 mole% muscovite is in solid
Mus
Mus
50
50
40
~0
30
30
20
20
10
P|
10
10
20
30
40
50
60
70
M i c r o p r o b e analyses o f p a r a g o n i t e f r o m p s e u d o m o r p h s o f t y p e C GW 4 5
C 858
1
2
3
4
5
6
7
8
45.52 40.23 0.71 0.28 7.48 0.95
45.73 40.81 0.55 0.35 6.96 0.82
45.50 40.44 0.79 0.26 7.16 0.43
45.35 40.75 0.79 0.19 7.20 0.71
45.34 40.68 .0.74 0.41 7.41 0.90
45.28 40.61 0.69 0.17 7.20 1.24
46.77 39.02 0.97 0.20 6.86 1.48
45.82 40.48 0.48 0.76 7.33 0.44
95.17
95.22
94.48
94.99
95.48
95.19
95.30
95.31
Si A1TM Al VI Fe
5.858 2.142 3.961 0.076
5.859 2.141 4.022 0.058
5.866 2.134 4.011 0.085
5.833 2.167 4.011 0.085
5.819 2.181 3.973 0.079
5.829 2.171 3.991 0.074
6.013 1.987 3.924 0.104
5.867 2.133 3.976 0.051
VI
4.037
4.080
4.096
4.096
4.052
4.065
4.028
4.027
Ca Na K
0.039 1.867 0.161
0.048 1.729 0.134
0.035 1.789 0.071
0.026 1.796 0.116
0.056 1.843 0.146
0.023 1.798 0.204
0.027 1.709 0.241
0.104 1.820 0.072
E
2.067
1.911
1.895
1.938
2.045
2.025
1.977
1.966
Margarite Paragonite Muscovite
1.9 90.3 7.8
2.5 90.5 7.0
1.9 94.4 3.7
90
Fig. 2. C o m p o s i t i o n o f the w h i t e mica (see Tables I and II) in the diagram p a r a g o n i t e - - m a r g a r i t e - - m u s c o vite. F u l l c i r c l e s = sample SF 8 1 , t y p e B; o p e n c i r c l e s = sample C 8 5 8 , t y p e C; s q u a r e s = sample GW 4 5 , t y p e C; c r os s e s = sample C 6 6 8 , t y p e A ; a s t e r i s k s = sample C 8 8 4 , t y p e B.
T A B L E II
SiO 2 Al~O 3 FeO* CaO Na20 K20
80
1.3 92.7 6.0
2.7 90.1 7.2
1.1 88.8 10.1
1.4 86.4 12.2
5.2 91.2 3.6
* T o t a l iron as FeO. S t r u c t u r a l f o r m u l a e calculated o n a t w e n t y - t w o o x y g e n basis.
135
the muscovite component in margarite is considerably lower in the micas from the Betic Cordilleras (this study) than in micas from the Eastern Alps, Italy (Ackermand and Morteani, 1973; HSck, 1974). The microprobe analyses of margarite and paragonite show a larger range of miscibility of these two minerals than that indicated by data on the natural micas from the Eastern Alps (Ackermand and Morteani, 1973) and from the Swiss Alps (Frey et al., 1982). Three analyses of the white mica (sample SF 1, Nos. 1 and 3; sample C 668, No. 5) contain 35--40
mole% margarite component, which is ~ 2 0 mole% more than that previously reported in solution with paragonite, whilst one analysis (sample SF 81, No. 2) has 50 mole% margarite. This suggests a very small miscibility gap in the solid-solution series paragonite--margarite (see Fig. 2). It is difficult to compare the analytical results of these micas with the experimental data given by Franz et al. (1977) because of the unknown influence of the muscovite component on the solid-solution behaviour. If (1) this influence can be neglected, and if (2)
TABLE III
Microprobe analyses of amphiboles and garnets Sub-calcic amphiboles
Sodic--calcic amphiboles
Garnets
C 884
SF 1
SF 1
GW 45
C 884
5
1
2
1
2
4
SiO 2 TiO 2 A1203 FeO* MnO MgO CaO Na~O K20
42.34 0.15 16.47 16.50 0.48 8.22 8.72 4.73 0.46
43.46 0.44 16.58 16.38 0.51 7.61 8.22 5.20 0.34
41.72 0.21 19.09 15.61 0.65 6.33 9.38 3.77 0.78
44.48 0.28 15.53 15.66 0.16 8.82 8.15 5.20 1.02
45.58 0.21 15.73 16.19 0.05 8.59 7.37 5.62 1.06
37.42 -21.84 26.17 3.80 3.21 8.31 ---
38.34 -21.42 26.58 0.69 5.43 7.35 ---
:~
98.07
98.74
97.54
99.30
100.40
100.75
99.81
1
Si A1TM A1VI Ti Fe Mn Mg Ca Na
6.295 1.705 1.182 0.016 2.058 0.060 1.819 1.390 0.475
6.389 1.611 1.263 0.049 2.014 0.063 1.667 1.295 0.648
6.200 1.800 1.545 0.024 1.940 0.082 1.402 1.494 0.506
6.489 1.511 1.161 0.031 1.911 0.020 1.918 1.274 0.686
6.565 1.435 1.235 0.022 1.950 0.006 1.843 1.137 0.807
5.899 -4.056 -3.450 0.507 0.754 1.404 --
6.001 -3.952 -3.479 0.091 1.267 1.233 --
Na K
0.888 0.087
0.834 0.064
0.580 0.148
0.785 0.190
0.763 0.195
---
---
56.4 8.3 12.3 23.0
57.3 1.5 20.9 20.3
Almandine Spessartine Pyrope Grossu|ar
*Total iron as FeO. Structural formulae based upon twenty-three (amphiboles) and twenty-four (garnets) oxygens.
136 eclogites o f t h e Sierra de Baza ( G o m e z - P u g naire, 1 9 8 1 ) .
t h e r e are no fine-grained i n t e r g r o w t h s o f paragonite--margarite below the resolution of the microprobe beam, the intermediate comp o s i t i o n s i n d i c a t e a r a t h e r high t e m p e r a t u r e o f f o r m a t i o n o f t h e n a t u r a l micas. A t t e m p e r atures o f 6 0 0 ° C t h e e x p e r i m e n t a l d a t a d o n o t allow t h e u n e q u i v o c a l d i s t i n c t i o n o f w h e t h e r t h e r e still exists a m i s c i b i l i t y gap o r n o t .
3.3. A m p h i b o l e s R e p r e s e n t a t i v e analyses o f a m p h i b o l e s are given in T a b l e III. T h e y are r e p r e s e n t a t i v e o f t h e m o s t a b u n d a n t a m p h i b o l e s in t h e s e r o c k s . O n l y t w o relics o f b l u e - a m p h i b o l e , o p t i c a l l y i d e n t i f i e d as g l a u c o p h a n e , have b e e n f o u n d , b u t t h e grains w e r e t o o small t o analyse. T h e t e r m s sub-calcic and sodic--calcic a m p h i b o l e s are used h e r e following L e a k e ( 1 9 7 8 ) . S a m p l e SF 1 has b o t h sub-calcic a m p h i b o l e s w i t h a c o m p o s i t i o n closely related to pargasite, and sodic--calcic a m p h i b o l e (katop h o r i t e ) . T h e a m p h i b o l e s o f samples C 884 and GW 45 do n o t show significant v a r i a t i o n s in c o m p o s i t i o n and o n l y o n e analysis f r o m each is listed.
3.2. Garnet R e p r e s e n t a t i v e m i c r o p r o b e analyses o f g a r n e t c o e x i s t i n g w i t h t h e m i n e r a l s t h a t are n o w seen in t h e p s e u d o m o r p h s are s h o w n in T a b l e III. T h e s e g a r n e t s p r o b a b l y also c o e x i s t ed w i t h t h e m i n e r a l t h a t has beer~ p s e u d o m o r p h o s e d . T h e y are a l m a n d i n e - r i c h garnets with a homogeneous composition from core to rim. G r o s s u l a r and p y r o p e c o m p o n e n t s (20 m o l e % e a c h ) are similar, and o n l y o n e grain s h o w s a significant a m o u n t o f spessartine c o m p o n e n t w i t h c o n c o m i t a n t d e p l e t i o n o f p y r o p e c o n t e n t . This c o m p o s i t i o n is v e r y similar to t h a t o f garnets f r o m t h e c o r o n i t i c
3.4. Epidote-group minerals T h e c o m p o s i t i o n o f six crystals is given in T a b l e IV. T h e pistacite c o n t e n t varies f r o m Pist34Zoi66 to Pists0Zoi:0. T h e r e are no t e x -
TABLE IV Microprobe analyses of epidote from pseudomorphs of type B SF 81 1
2
3
4
5
6
SiO 2 A1203 Fe203* CaO
36.37 24.40 13.31 23.96
36.90 29.48 7.68 24.03
37.99 26.98 9.15 23.94
38.20 27.78 7.98 24.10
38.47 27.88 7.61 24.04
37.98 27.54 8.51 23.89
~:
98.04
98.09
98.06
98.06
98.00
97.92
Si A1 Fe Ca
Pistacite Zoisite
2.903 2.296 0.800 2.049
2.877 2.709 0.451 2.007
2.974 2.490 0.539 2.008
2.978 2.553 0.468 2.013
2.994 2.559 0.446 2.005
2.968 2.539 0.501 2.000
8.048
8.044
8.011
8.012
8.004
8.008
80 20
44 56
54 46
47 53
45 55
50 50
*All iron considered as Fe20 ~. Structural formulae based upon 12.5 oxygens.
137
tural differences between the epidotes of different composition. Individual grains are chemically h o m o g e n e o u s . The chemical differences may be due to successive crystallization, or to local buffering of the o x i d a t i o n ratio by the mineral assemblage.
Type-A pseudomorphs consist of kyanite, replaced by white mica. Similar textures o f the transformation of kyanite were found in the surrounding metapelites o f the Sierra de Baza (Gomez-Pugnaire, 1979). This suggests ionic reactions like: 2ky+Ca 2÷+2H20~
4. Mineral reactions
lma+2H
÷
(1)
3 ky + 2Na + + 3SIO2 + 3H20 ¢- 2 pa + 2H ÷ (2)
In the following paragraphs we give an interpretation of h o w the pseudomorphs and their mineral assemblages could have been formed.
where ky = kyanite, ma = margarite and pa = paragonite, The reactions are balanced to constant A1 with a mica formula XY2Z4010(OH)2,
TABLE V
Petrographical characteristics o f p s e u d o m o r p h s o f t y p e s A , B a n d C, and possible mineral reactions End-members
Textures
Possible reactions and reaction s e q u e n c e
large k y core, o p t i c a l l y c o n t i n u o u s , decussate w h i t e m i c a , m a replacing k y , p a replacing
2 k y + Ca ~+ + 2 H ~ O ¢- 1 m a + 2 H ÷
[ s a m p l e s in P l a t e I]
Type-A pseudomorph: k y , m a , p a , + chlorite + carbonate [GW R , C 668, P l a t e I, ( A ) and (B)]
ma
(1)
3 ky + 2Na + + 3SiO 2 + 3H20 = 2 pa + 2H +
(2)
1 m a + S i 4 + + N a +~- 1 p a + A 1 3 + + C a
(6)
2+
3 m a + 4 N a + + 2 H + + 6SiO~ ~ 4 p a + 3 C a 2+
(6a)
Type-B pseudomorph: epi, ky, ma, pa [C 884, P l a t e I, ( D ) and S F 8 1 - 1 , P l a t e I, (C)]
s m a l l k y in w h i t e m i c a , m a b e t w e e n pa and k y ,
corona o f epi
(a) 41aw~
2zoi+
1 ky+
51aw~- 2zoi+
(3)
1 qz+7H~O
lma+2qz+8H20
(4)
lky+
1 qz+7H20
(3)
4ma+3qz
(5)
or (b), first: 41aw~- 2zoi+
then: 2zoi+bky+3H20~
1 m a + S i 4+ + N a + ~ l p a + 3ma+4Na
A l 3+ + C a 2+
+ + 2 H + + 6 S i O 2¢- 4 p a +
3 C a 2+
(6) (6a)
Type-C pseudomorph: epi, pa, qz [C 858, G W (E)]
45,
P l a t e I,
aggregates o f decussate p a + epi + q z
4law + lab~
2 zoi+2qz+
1pa+6H20
(7)
2zoi+lqz+
1pa+6H20
(8)
or • 4law+
ljd¢
or 4an+
For abbreviations see the t e x t .
1 ab+ 2H20~- 2zoi+
2 qz+
lpa
(9)
138 Although these mineral transformations are written as ionic exchange reactions, t h e y do n o t require a large-scale metasomatism. During breakdown of omphacitic pyroxene into amphibole and albite near to the kyanite crystals, Ca as well as Na and Si would be ionic species in a fluid phase. By this process the Ca2+/H ÷ and Na+/H ÷ ratios of the fluid are significantly changed, which is of great influence upon the stability of aluminium silicate. The textures of these pseudomorphs [see Plate I, (B1) and (B2)] indicate that first margarite and then paragonite replace kyanite, or that paragonite replaces margarite [see reactions (6) and (6a) below, and Table V). Type-B pseudomorphs show a more complicated mineralogy and texture. Their shape indicates that t h e y were formed after lawsonire. However, no relics o f lawsonite have been found. There are two different extreme possibilities to explain their formation from lawsonite [which are called as (a) and (b) in the following section] : (a) In the first stage lawsonite was altered (more or less simultaneously) according to the following reactions (3)--(5) (Newton and Kennedy, 1963; Nitsch, 1973; Chatterjee, 1976; Holland, 1979) at P--T conditions where these three reactions meet in an invariant point: 4 law ~ 2 zoi + 1 k y + 1 qz + 7H20
(3)
51aw-~2zoi+lma+2qz+8H20
(4)
2 zoi + 5 ky + 3H20 ~ 4 ma + 3 qz
(5)
where law = lawsonite, zoi = zoisite and qz = quartz. The pseudomorphs would then consist of margarite, quartz, kyanite and zoisite. It may seem improbable that the P--T conditions were exactly at the invariant point, but it must be remembered, that this "invariant" equilibrium becomes at least bivariant due the Fe 3+ content of epidote. In the later stage, margarite was altered into paragonite by the exchange reactions: Ima+Na
÷+Si 4÷~- ipa+Ca
2÷+Al 3+
(6)
3 m a + 4 N a ÷ + 2 H ÷ + 6 S i O 2 ~ 4 p a + 3 C a 2÷ (6a) where reaction (6) is balanced with respect to 1 "mica u n i t " , and reaction (6a) is balanced with respect to constant A1. These reactions can explain the lack of quartz in the pseudomorphs, which should be present according to reactions (3)--(5), and also the rim of epidote around the pseudomorphs. The process is similar to the ionic reaction formulated above for pseudomorphs of type A. Ca 2÷ and A13÷, released by this process, could produce the monomineralic epidote corona of the pseudomorphs. Such monomineralic zones are typical of metasomatism and point to the high mobility of many components (Table V). (b) Lawsonite was transformed into zoisite + kyanite + quartz only (not margarite) by crossing the boundary of reaction (3). In a later stage margarite was produced by a retrograde hydration reaction (5), and in a yet later stage [similar to possibility (a)], margarite was transformed into paragonite by high activity of Na ÷ (Table V). Type-C pseudomorphs are easily explained by breakdown of the association lawsonite + albite, or lawsonite + jadeite, according to reaction (7) or reaction (8) (Franz and Althaus, 1977; Heinrich and Althaus, 1980; Heinrich, 1981): 41aw+lab~-
lpa+2zoi+2qz+6H20
(7)
41aw+ljd~
lpa+2zoi+lqz+6H20
(8)
where ab = albite and jd = jadeite (see Fig. 3). It is tacitly assumed that albite or jadeite were transported from the matrix to the lawsonite as components of the fluid phase in order to maintain the euhedral shape of the pseudomorphs. The shape of the pseudomorphs C [see Plate I, (E)] leaves open the interpretation that the pseudomorphosed mineral was not lawsonite, but plagioclase. In this case an anorthite-rich plagioclase may have been transformed directly into the assemblage
139 20
eclogite paragenesis, otherwise these pseudomorphs never passed through a stage of eclogite equilibrium without plagioclase.
PH2o (k bar) 18
5. Metamorphic conditions
16
t2
tO
I
I
I
|
I
300
400
5.00
600
700
r ( "C )
Fig. 3, E x p e r i m e n t a l l y - d e t e r m i n e d o r calculated e q u i l i b r i u m curves and possible e x t r e m e P - - T p a t h s (a) and (b) for the d e v e l o p m e n t o f t h e rocks. E n circled n u m b e r s refer to r e a c t i o n n u m b e r s in t h e t e x t [DH = D e l a n e y and Helgeson ( 1 9 7 8 ) ; C H = C h a t t e r jee ( 1 9 7 1 , 1972, 1976); F A = F r a n z and A l t h a u s ( 1 9 7 7 ) ; H A = Heinrich and A l t h a u s ( 1 9 8 0 ) ; N = N i t s c h ( 1 9 7 3 ) ; N K = N e w t o n and K e n n e d y ( 1 9 6 3 ) ; S N = S t o r r e and N i t s c h ( 1 9 7 4 ) ; H = Holland ( 1 9 7 9 ) ; V = Velde ( 1 9 7 1 ) ] . F o r a b b r e v i a t i o n s and r e a c t i o n n u m b e r s see t h e t e x t ( e x c e p t co = c o r u n d u m ) .
paragonite + zoisite + quartz: 4an+lab+2H20,~lpa+2zoi+2qz
(9)
where an = anorthite. Note that this is a hydration reaction, whereas the above-mentioned reactions are dehydration reactions (Table V). This last reaction also necessitates that the assemblage paragonite + zoisite + quartz was the
Since the stability fields of the abovementioned mineral associations have been determined experimentally b y various authors, it is seductive to try to reconstruct the metamorphic P --T history of the rocks. The relevant equilibrium curves are shown in Fig. 3, together with possible P--T paths. We start from the assumption that the rocks were first metamorphosed at low temperatures/high pressures. This hypothesis is supported b y the occurrence of relics of glaucophane and omphacite. The possibility that the type-C pseudomorphs were formed directly from plagioclase can therefore safely be excluded, because this would require a P--T path from low to intermediate temperatures/low pressures to P --T conditions near to the b o u n d a r y of the paragonite + zoisite + quartz stability field [No. (9) in Fig. 3]. The pseudomorphs therefore are more likely to have formed after lawsonite, and the rocks must have passed through the lawsonite (and lawsonite + albite or lawsonite + jadeite) stability field, which is somewhere left of reactions (3), (4), (7) and (8) in Fig. 3. The invariant point, where zoisite + kyanite + margarite + lawsonite + quartz + water coexist, was calculated by Chatterjee (1976) at 8.5 kbar and 480°C. Experimental data on the reactions (3) and (4) b y Newton and Kennedy (1963) and Nitsch (1973) indicate slightly lower temperatures for this point. If the calculated values of Holland (1979) are accepted for the reaction (5) the invariant point lies at 11 kbar, 500°C. Therefore the formation of zoisite, kyanite, margarite and quartz (type-B pseudomorphs) must have happened at pressures above at least 8 kbar and temperatures of 400--500°C. Now two extreme possibilities of P--T paths are discussed. Path (a) is consistent with reaction sequence (a) for type-B pseudo-
140 m o r p hs (see Table V): during t he uplift of t h e rocks, t e m p e r a t u r e s rose and t he P - - T path followed reaction (5), where zoisite and kyanite are replaced by margarite and quartz. Nevertheless, the position o f this curve is uncertain, because the calculated data refer to pure margarite and the natural margarite o f t h e p s e u d o m o r p h s contains up to 50 mole% paragonite. Storre and Nitsch (1974) used a natural margarite--paragonite solid solution for their experiments and f o u n d a much steeper slope (see Fig. 3) than the calculated one. T h e paragonite c o n t e n t o f t he margarite is t h e r e f o r e o f critical influence on the considered P - - T path. This is i m p o r t a n t for the interp re t a t io n o f the results presented here, because it opens the possibility t hat the rocks experienced further compression and heating, following path (b), consistent with reaction sequence (b) in Table V. Of course, any intermediate path b e t w e e n (a) and (b) is possible, too. T h e t r a n s f o r m a t i o n of margarite into paragonite [reactions (6) and (6a)] in pseudomorphs A and B is the last event which can be traced in th e rocks. It must have t aken place at still high pressures, as indicated b y the experimental data o f Franz and Althaus (1977) on the stability field of paragonite + zoisite + quartz. Due to th e high a m o u n t o f Fe 3÷ in t he epidote, th e experimentally-determinated pressures are n o t directly applicable to t he natural systems, but t h e y were pr oba bl y on the order o f PH:O o f 6--8 kbar [see curve (9) in Fig. 3]. The e x t r e m e l y high miscibility bet w e e n paragonite and margarite indicates high temperatures in the order o f 600°C. This m a x i m u m t e m p e r a t u r e is consistent with the data given b y Gomez-Pugnaire (1979) for the surrounding metapelites. 6. Conclusions (1) Microprobe analyses of white mica in some amphibolitized eclogites indicate an extensive solid solution be t w een paragonite and margarite near to the closure of the miscibilit y gap.
(2) These micas occur t oget her with kyanite, epidote and quartz in p s e u d o m o r p h s which can be interpreted as p s e u d o m o r p h s after lawsonite and after kyanite. (3) Small-scale metasomatism produces the corona t e x t u r e of some of the p s e u d o m o r p h s and points to the i m port ance of mobile components (especially Na, Ca, but also Si and A1) in high-pressure rocks. (4) With the presently available data it is impossible to use the system Na20--CaO-A1203--SiO2--H20 to derive exact P --T paths, but some constraints can be made. Acknowledgements This paper has been made possible by t he financial support of Programs o f Collaboration between the C.S.I.C. (Spain) and the C.N.R. (Italy). The authors thank Professors F.P. Sassi and M. Mufioz as Italian and Spanish directors of these Programs. T he electron microprobe analyses have been made at Istituto di Mineralogia of Padua (Italy). T h e y thank also Drs. G.M. Molin and G. Da Roit b y their collaboration. References Ackermand, D. and Morteani, G., 1973. Occurrence and breakdown of paragonite and breakdown of paragonite and margarite in Greiner Schiefer Series (Zillertal Alps, Tyrol). Contrib. Mineral. Petrol., 40: 293--304. Chatterjee, N.D., 1971. Preliminary results on the synthesis and upper stability of margarite. Naturwissenschaften, 58: 147. Chatterjee, N.D., 1972. The upper stability limit of the assemblage paragonite + quartz and its natural occurrence. Contrib. Mineral. Petrol., 34: 288-303. Chatterjee, N.D., 1976. Margarite stability and compatibility relations in the system CaO--A1203SiO2--H20 as a pressure--temperature indicator. Am. Mineral., 61 : 699--709. Deer, W.A., Howie, R.A. and Zussman, J., 1962. An Introduction to The Rock-forming Minerals. Longman, London, 528 pp. Delaney, J.M. and Helgeson, H.C., 1978. Calculation of the thermodynamic consequence of dehydration in subducting oceanic crust to 100 kb and 600°C. Am. J. Sci., 278: 638--686.
141
Franz, G. and Althaus, E., 1977. The stability relations of the paragenesis paragonite--zoisite-quartz. Neues Jahrb. Mineral. Abh., 130: 159-167. Franz, G., Hinrichsen, T. and Wannemacher, E., 1977. Determination of the miscibility gap on the solid solution series paragonite--margarite by means of infrared spectroscopy. Contrib. Mineral. Petrol., 59: 307--316. Frey, M., Bucher, K., Frank, E. and Schwander, H., 1982. Margarite in the Central Alps. Schweiz. Mineral. Petrogr. Mitt., 62: 21--45. Gomez-Pugnaire, M.T., 1979. Some considerations on the highest temperature reached in the outcropping rocks of the Nevado--Fil~bride Complex in the Sierra de Baza area during the Alpine metamorphism. Neues Jahrb. Mineral. Abh., 135: 75--87. Gomez-Pugnaire, M.T., 1981. Evoluci6n del metamorfismo alpino en el Complejo Nevado-Fil~ibride de la Sierra de Baza (Cordilleras B6ticas, Espafia). Tecniterrae, No. 4 1 , 1 3 0 pp. Gomez-Pugnaire, M.T., Mottana, A., Bocchio, R., Liborio, G. and Abraham, K., 1979. Coronitic eclogites in the Sierra de Baza (Betic Cordilleras, Spain). Neues Jahrb. Mineral. Abh., 136: 42--62. Heinrich, W., 1981. Paragonit in Hochdruckgesteinen Experimentelle Untersuchungen zu B i l d u n g s und Abbaureaktionen. Thesis, University of Karlsruhe, Karlsruhe, 111 pp. Heinrich, W. and Althaus, E., 1980. Die obere Stabilit//tsgrenze yon Lawsonit plus Albit bzw. Jadeit, Fortschr. Mineral. Beiheft, 58: 49--50. H6ck, V., 1974. Coexisting phengite, paragonite and -
-
margarite in metasediments of Mittlere Hohe Tauern, Austria. Contrib. Mineral. Petrol., 43: 261--273. Holland, T.J.B., 1979. High water activities in the generation of the high pressure kyanite eclogites in the Tauern Window, Austria. J. Geol., 87 : 1--27. Julivert, M., Fontbot~, J.M., Riveiro, A. and Conde, L., 1974. Mapa TectSnico de la Penfnsula Ib~rica y Baleares. Inst. Geol. Mineral., Madrid (scale 1 : 1,000,000). Leake, B.E., 1978. Nomenclature of amphiboles. Mineral. Petrogr. Acta, 22: 195--224. Newton, R.C. and Kennedy, G.C., 1963. Some equilibrium reactions in the join CaA12Si20,--H~O. J. Geophys. Res., 68: 2967--2983. Nijhuis, H.J., 1964. Plurifacial Alpine metamorphism in the south-eastern Sierra de los Filabres, South of Lubrfn, SE Spain. Thesis, University of Amsterdam, Amsterdam, 151 pp. Nitsch, K.H., 1973. Neue Erkentnisse zur Stabilit~t yon Lawsonit. Fortschr. Mineral., 50: 34--35. Puga, E., 1971. Investigaciones geoldgicas en Sierra Nevada Occidental. Thesis, University of Granada, Granada, 269 pp. Puga, E., 1977. Sur l'existence dans le Complexe de la Sierra Nevada (Cordill~re b~tique, Espagne) d'~clogites et sur leur origine probable ~ partir d ' u n e crofite oc~anique m4sozoique. C.R. Acad. Sci. S~r. D, 285: 1379--1382. Storre, B. and Nitsch, K.H., 1974. Zur Stabilit~/t yon Margarit in System CaO--A1203--SiO2--H20. Contrib. Mineral. Petrol., 43: 1--24. Velde, B., 1971. The stability and natural occurrence of margarite. Mineral. Mag., 38: 317--323.
129
KYANITE, MARGARITE AND PARAGONITE IN PSEUDOMORPHS IN AMPHIBOLITIZED ECLOGITES FROM THE BETIC CORDILLERAS, SPAIN M A R I A T E R E S A G O M E Z - P U G N A I R E I, D A R I O V I S O N A ~ a n d G E R H A R D F R A N Z 3 1Departamento de Petrologia y de Investigaciones Geol6gicas, C.S.I.C. (Consejo Superior de Investigaciones Cientifieas), Facultad de Ciencias, Granada (Spain) 2Istituto di Mineralogia e Petrologia, Universita di Padova, Padova (Italy) 3Institut fiir Angewandte Geophysik, Petrologic und LagerstEttenforschung, Teehnische Universitiit, 1000 Berlin 12 (Federal Republic o f Germany) (Accepted for publication October 23, 1984)
Abstract Gomez-Pugnaire, M.T., Visona, D. and Franz, G., 1985. Kyanite, margarite and paragonite in pseudomorphs in amphibolitized eclogites from the Betic Cordilleras, Spain. In: D.C. Smith, G. Franz and D. Gebauer (Guest-Editors), Chemistry and Petrology of Eclogites. Chem. Geol., 50: 129--141. Pseudomorphs in amphibolitized eclogites of the Nevado--Fil~bride Complex, Betic Cordilleras, Spain, are described. Three different textural and mineralogical types can be distinguished. All contain a combination of the minerals kyanite, margarite, paragonite, epidote and quartz. Electron microprobe data indicate that the solid solution between paragonite and margarite is extensive; intermediate members Ma3sPa6s to MasoPas0 were found. The mineral assemblages and textures are explained by the successive crystallizations: from lawsonite to kyanite + epidote + quartz; kyanite + epidote to margarite + quartz; margarite + quartz + Na ÷ to paragonite + epidote; lawsonite + albite or jadeite to paragonite + epidote + quartz; and kyanite + (Na ÷ + Ca :+) to paragonite + margarite. Possible P--T paths are discussed.
1. I n t r o d u c t i o n In t h e P e r m o - T r i a s s i c series o f t h e m e t a morphic Nevado--Fil~ibride Complex, Betic C o r d i l l e r a s ( S p a i n ) , b o d i e s o f m e t a b a s i t e s are c o m m o n . M o s t are d i s c o r d a n t , b u t s t r a t i f o r m u n i t s d o o c c u r . B o t h t y p e s a r e seen s c a t t e r e d t h r o u g h t h e A l p i n e - c o v e r m e t a p e l i t e series ( N i j h u i s , 1 9 6 4 ; Puga, 1 9 7 1 ; G o m e z - P u g n a i r e , 1 9 8 1 ) . Fig. 1 s h o w s a s k e t c h o f t h e t e c t o n i c units of the Betic Cordilleras and the sample l o c a l i t i e s ( S i e r r a de Baza). M o s t o f t h e s e m e t a b a s i t e s have a l k a l i n e - - b a s a l t i c a f f i n i t i e s ( G o mez-Pugnaire, 1981) and are now a m p h i b o 0009-2541/85/$03.30
lites. In s o m e cases relics o f e c l o g i t e a s s e m blages o f an e a r l y A l p i n e m e t a m o r p h i c e v e n t are p r e s e r v e d . The association kyanite + epidote + quartz a n d t h e o c c u r r e n c e o f large f l a k e s o f p a r a g o n i t e in t h e s e r o c k s w a s d e s c r i b e d b y P u g a (1971) and Gomez-Pugnaire (1981). However, m a r g a r i t e has n o t p r e v i o u s l y b e e n described from the metabasites or metapelites of the Betic Cordilleras. Aggregates of calcite + e p i d o t e + p a r a g o n i t e + a l b i t e have b e e n i n t e r p r e t e d as p s e u d o m o r p h s o f l a w s o n i t e p o r p h y r o b l a s t s b y Puga ( 1 9 7 7 ) . In t h i s p a p e r , aggregates o f paragonite + margarite + e p i d o t e
© 1985 Elsevier Science Publishers B.V.
130
:ii
'3"
,~=,,,,r'TTTT'Nevado~ Filabride i!~ ! !! ! Complex [ ~ Alpujarride and
Malaguide Complexes
~ ~
External J Zones Volcanic rocks
Post-orogenic materials
Fig. 1. Sketch of tectonic units of the central and eastern parts of the Betic Cordilleras, southern Spain (from Julivert et al., 1974). + k y a n i t e , in some cases w i t h c o r o n i t i c textures, are described, and i n t e r p r e t e d as breakd o w n p r o d u c t s o f lawsonite. Minerals reactions are d e d u c e d f r o m t e x t u r e s and, c o m bined w i t h available e x p e r i m e n t a l d a t a , possible P - - T p a t h s for the m e t a b a s i t e s are discussed. 2. P e t r o g r a p h y o f the p s e u d o m o r p h s replacement textures
and
garnet + q u a r t z + rutile + p y r i t e + apatite. T h e o r d e r o f listing o f these minerals c o r r e s p o n d s to decreasing a b u n d a n c e in t h e rocks. The r o c k s t y p i c a l l y show the b r e a k d o w n o f o m p h a c i t i c p y r o x e n e into s y m p l e c t i t i c aggregates o f a m p h i b o l e + albite, r e p l a c e m e n t o f g l a u c o p h a n e b y sodic--calcic and sub-calcic a m p h i b o l e s , and t h e characteristic p s e u d o m o r p h s as described below.
2.1. Pseudomorphs The m e t a b a s i t e s w h i c h c o n t a i n t h e pseud o m o r p h s are intensely a m p h i b o l i t i z e d eclogites. T h e y are massive r o c k s w i t h o u t a foliat i o n and w i t h a large range o f variation in grain size. Generally an original igneous fabric is preserved. T h e t y p i c a l assemblage is: a m p h i b o l e + e p i d o t e + plagioclase + paragonite + margarite + k y a n i t e + o m p h a c i t e +
T h e c o m p o s i t i o n and t h e t e x t u r e s o f t h e p s e u d o m o r p h s are variable. T h r e e t y p e s can be distinguished: T y p e A consists o f large crystals o f k y a n i t e (length 1--2 m m ) , c o r r o d e d b y decussate w h i t e m i c a [Plate I, ( A ) ] . In s o m e cases o n l y a few relics o f k y a n i t e are preserv-
PLATE I A. Type-A pseudomorphs: kyanite crystals (Ky) corroded by white mica (crossed nicols), sample No. C 668; real length of the picture is 4.5 ram. B 1 . Type-A pseudomorphs, detail: few relics of kyanite (Ky), surrounded by margarite (M) and paragonite (Pa). Margarite always occurs as rims around kyanite, and paragonite as rims around margarite (crossed nicols), sample No. C 668; real length of the picture is 1.2 mm. B2. X-ray fluorescence scanning pictures for Ca for the upper left part of B 1; same scale. C. Type-B pseudomorphs with white mica (M, margarite and paragonite), kyanite (Ky), and a coronitic rim of epidote (E), (crossed nicols), sample No. SF 81 ; real length of the picture is 4.7 mm. D. Type-B pseudomorphs, sample No. SF 81 ; real length of the picture is 4.7 mm. E. Type-C pseudomorphs: paragonite + quartz (low relief) and epidote (high relief), crossed nicols), sample No. C 858;real length of the picture is 4.2 mm.
132 ed [Plate I, (B1) and (B2)]. Large kyanites are optically continuous, small crystals may be either optically continuous or discontinuous. Rarely small amounts of carbonate occur in the centre of the pseudomorphs: in one case additional chlorite was observed. Type B shows a coronitic texture (width of the corona 2--2.5 mm). Their shape is typically rhombohedral and prismatic, mostly euhedral to subhedral. They have a core of white mica with small irregularly--distributed and differently-orientated crystals o f kyanite, and a corona of epidote [Plate I, (C) and (D)]. In both cases A and B, paragonite and margarite are both present in the micaceous aggregates. Type C. Quartz and epidote are c o m m o n l y associated with large flakes of decussate paragonite. The replaced mineral was euhedral. The rectangular habit is evident in Plate I, (E). They reach a length of 2--4 mm, and occur frequently in the metabasites. Exceptionally small relics of kyanite rimmed by margarite are included in the large flakes of paragonite.
2.2. Paragonite and margarite Both minerals occur and coexist exclusively in the pseudomorphs. The paragonite crystals are generally larger than the margarite crystals which often occur as inclusions isoorientated in the paragonite flakes. In the pseudomorphs A and B, relics of kyanite are always surrounded by a rim of margarite. A second rim of paragonite around margarite is also evident in Plate I, (B1) and (B2). Margarite and paragonite are easily distinguished microscopically by the higher refractive index and lower birefringence of margarite.
2.3. Amphibole Green and blue-green amphiboles are the main constituents of the matrix of the metabasites. They occur as crystals with variable dimensions replacing glaucophane, garnet and omphacite. In the last case, amphibole coexists
with albite in symplectitic intergrowths (Gomez-Pugnaire et al., 1979).
2.4. Epidote minerals Clinozoisite and pistacite are the d o m i n a n t epidote-group minerals in these rocks, but zoisite occurs also. The three minerals have been identified microscopically, but within the chemically analyzed pseudomorphs only clinozoisite and pistacite have been detected. Epidote group minerals are abundant only in association with micas; in fact, within the matrix t h e y are generally scarce, and this scarcity is especially evident in eclogites which are only slightly altered into amphibolites.
2.5. Plagioclase Plagioclase is a very abundant mineral in the matrix of some metabasites, especially in the amphibolitized eclogites. It occurs as small, rounded crystals of albite with abundant inclusions o f amphiboles and opaque minerals, or as part of the above-mentioned symplectitic intergrowths. Sometimes the crystals show a rim of sodic oligoclase.
2.6. Garnet and omphacite Both minerals occur together with glaucophane as relics of the eclogite mineral paragenesis which crystallized in the older metamorphic event. Omphacite has been replaced by amphibole or a symplectitic intergrowth; garnet has been replaced by amphibole.
2.7. Accessory minerals Rutile is the characteristic accessory mineral in these rocks. It occurs randomly t h r o u g h o u t amphibole- and epidote-rich areas, and is c o m m o n l y associated with quartz replacing crystals of titano-magnetite (see Gomez-Pugnaire et al., 1979, fig. 5). It was not observed within the pseudomorphs. Minor amounts of euhedral pyrite, in some cases completely replaced by limonite, and apatite crystals were found in all rock types.
39.9 59,6 0.5
50.7 49.0 0.3
2.015
1.980 34.5 64.9 0.6
1.980
0.683 1.286 0.011
4.093
5.118 2.882 4.029 0.064
94.90
39.27 44.99 0.59 4.89 5.09 0.07
6.5 91.4 2.1
1.920
0.125 0.755 0~040
4.008
6.017 1.983 3.922 0.086
95.43
46.88 39.03 0.80 0.91 7.05 0.25
4.1 91.8 4.1
1.902
0.078 1.746 0.078
4.027
6.032 1.968 3.935 0.092
95.37
47.21 39.20 0.86 0.57 7.05 0.48
5
6.2 90.9 2.9
2,074
0,129 1.880 0.065
3.991
5.894 2.106 3.923 0.068
96.44
46.48 40.34 0.65 0.95 7.65 0.37
6
2.5 94.8 2.7
1,892
0.048 0.793 0.051
4.064
5.912 2.088 4.018 0.046
95.17
46.28 40.55 0.43 0.35 7.24 0.31
* T o t a l iron as FeO. S t r u c t u r a l f o r m u l a e b a s e d u p o n t w e n t y - t w o o x y g e n s .
Matgatite Paragonite Muscovite
1.022 0.988 0.005
4.179
4.501 3.499 4.103 0.076
94.91
34.15 48.93 0.69 7.24 3.87 0.03
0.791 1.180 0.009
4.064
VI
Ca Na K
5.109 2.891 3.991 0.073
94.92
Z
Si AI I V A 1VI Fe
39.14 44,73 0.67 5.66 4.66 0.06
AI~O 3 FeO* CaO Na20 K~O
SiO~
4
7
3
1
2
C 668
SF81
2.0 96.2 1.8
1.966
0.040 1.891 0.035
4.087
5.903 2.097 4.014 0.063
95.24
46.07 40.46 0.59 0.29 7.61 0.22
8
5.1 89.5 5.4
1.886
0.097 1.688 0.101
4.044
5.911 2.089 4.014 0.030
94.14
45.74 40.07 0.28 0.70 6.74 0.61
9
2.3 94.5 3.2
2.032
0.047 1.920 0.065
3.984
6.015 1.985 3.929 0.060
95.20
45.63 40.79 0.55 0.33 7.51 0.39
10
37.3 61.0 1.7
2.038
0.760 1.244 0.034
4,046
5.112 2.888 3.996 0.050
95.18
39.27 44.86 0.46 5.45 4.93 0.21
11
62.5 37.3 0.2
1.980
1.236 0.739 0.005
4.195
4.262 3.738 4.127 0.068
93.51
31.74 49.70 0.61 8.59 2.84 0.03
12
M i c r o p r o b e analyses, c a l c u l a t e d f o r m u l a e , a n d e n d m e m b e r s of w h i t e m i c a s f r o m p s e u d o m o r p h s of t y p e s A and B
TABLE I
81.9 18.1 --
1.967
1.611 0.356 --
4.115
4.133 3.867 4.057 0.058
94.14
30.85 50.18 0.52 11.22 1.37 --
13
72.4 27.6 --
2.043
1.479 0.564 --
4.146
4.162 3.838 4.084 0.062
94.90
31.35 50.63 0.56 10.40 1.96 --
14
70.7 29.2 0.1
2.015
1.425 0.533 0.002
4.134
4.247 3.753 4.043 0.091
94.86
31,96 49.78 0.82 10.01 2.28 0.01
15
C884
11.3 84.9 3.8
1.926
0.219 1.634 0.073
4,012
5.885 2.115 3,947 0.065
93.78
45.20 39.50 0.60 1.57 6.47 0.44
16
134
solution with margarite. These values agree very well with the data on paragonite given b y Deer et al. (1962), Ackermand and Morteani (1973), and H6ck (1974). The content of
3. Analytical data
3.1. Margarite and paragonite The chemical compositions o f margarite and paragonite from five typical rock samples of metabasites were analyzed with an electron microprobe. Tables I and II show the oxide analyses, the calculated cation formulae, and the proportion of the end-member micas (paragonite--margarite--muscovite) of twentyfour white-mica grains. The analyses are tabulated b y the pseudomorph t y p e (Table I: types A and B; Table II: type C). The compositions of the white micas in terms o f the end-members muscovite--parago nite--margarite are represented in Fig. 2. 2--13 mole% muscovite are in solid solution with paragonite (average 8 mole%), b u t only 0.2--0.3 mole% muscovite is in solid
Mus
Mus
50
50
40
~0
30
30
20
20
10
P|
10
10
20
30
40
50
60
70
M i c r o p r o b e analyses o f p a r a g o n i t e f r o m p s e u d o m o r p h s o f t y p e C GW 4 5
C 858
1
2
3
4
5
6
7
8
45.52 40.23 0.71 0.28 7.48 0.95
45.73 40.81 0.55 0.35 6.96 0.82
45.50 40.44 0.79 0.26 7.16 0.43
45.35 40.75 0.79 0.19 7.20 0.71
45.34 40.68 .0.74 0.41 7.41 0.90
45.28 40.61 0.69 0.17 7.20 1.24
46.77 39.02 0.97 0.20 6.86 1.48
45.82 40.48 0.48 0.76 7.33 0.44
95.17
95.22
94.48
94.99
95.48
95.19
95.30
95.31
Si A1TM Al VI Fe
5.858 2.142 3.961 0.076
5.859 2.141 4.022 0.058
5.866 2.134 4.011 0.085
5.833 2.167 4.011 0.085
5.819 2.181 3.973 0.079
5.829 2.171 3.991 0.074
6.013 1.987 3.924 0.104
5.867 2.133 3.976 0.051
VI
4.037
4.080
4.096
4.096
4.052
4.065
4.028
4.027
Ca Na K
0.039 1.867 0.161
0.048 1.729 0.134
0.035 1.789 0.071
0.026 1.796 0.116
0.056 1.843 0.146
0.023 1.798 0.204
0.027 1.709 0.241
0.104 1.820 0.072
E
2.067
1.911
1.895
1.938
2.045
2.025
1.977
1.966
Margarite Paragonite Muscovite
1.9 90.3 7.8
2.5 90.5 7.0
1.9 94.4 3.7
90
Fig. 2. C o m p o s i t i o n o f the w h i t e mica (see Tables I and II) in the diagram p a r a g o n i t e - - m a r g a r i t e - - m u s c o vite. F u l l c i r c l e s = sample SF 8 1 , t y p e B; o p e n c i r c l e s = sample C 8 5 8 , t y p e C; s q u a r e s = sample GW 4 5 , t y p e C; c r os s e s = sample C 6 6 8 , t y p e A ; a s t e r i s k s = sample C 8 8 4 , t y p e B.
T A B L E II
SiO 2 Al~O 3 FeO* CaO Na20 K20
80
1.3 92.7 6.0
2.7 90.1 7.2
1.1 88.8 10.1
1.4 86.4 12.2
5.2 91.2 3.6
* T o t a l iron as FeO. S t r u c t u r a l f o r m u l a e calculated o n a t w e n t y - t w o o x y g e n basis.
135
the muscovite component in margarite is considerably lower in the micas from the Betic Cordilleras (this study) than in micas from the Eastern Alps, Italy (Ackermand and Morteani, 1973; HSck, 1974). The microprobe analyses of margarite and paragonite show a larger range of miscibility of these two minerals than that indicated by data on the natural micas from the Eastern Alps (Ackermand and Morteani, 1973) and from the Swiss Alps (Frey et al., 1982). Three analyses of the white mica (sample SF 1, Nos. 1 and 3; sample C 668, No. 5) contain 35--40
mole% margarite component, which is ~ 2 0 mole% more than that previously reported in solution with paragonite, whilst one analysis (sample SF 81, No. 2) has 50 mole% margarite. This suggests a very small miscibility gap in the solid-solution series paragonite--margarite (see Fig. 2). It is difficult to compare the analytical results of these micas with the experimental data given by Franz et al. (1977) because of the unknown influence of the muscovite component on the solid-solution behaviour. If (1) this influence can be neglected, and if (2)
TABLE III
Microprobe analyses of amphiboles and garnets Sub-calcic amphiboles
Sodic--calcic amphiboles
Garnets
C 884
SF 1
SF 1
GW 45
C 884
5
1
2
1
2
4
SiO 2 TiO 2 A1203 FeO* MnO MgO CaO Na~O K20
42.34 0.15 16.47 16.50 0.48 8.22 8.72 4.73 0.46
43.46 0.44 16.58 16.38 0.51 7.61 8.22 5.20 0.34
41.72 0.21 19.09 15.61 0.65 6.33 9.38 3.77 0.78
44.48 0.28 15.53 15.66 0.16 8.82 8.15 5.20 1.02
45.58 0.21 15.73 16.19 0.05 8.59 7.37 5.62 1.06
37.42 -21.84 26.17 3.80 3.21 8.31 ---
38.34 -21.42 26.58 0.69 5.43 7.35 ---
:~
98.07
98.74
97.54
99.30
100.40
100.75
99.81
1
Si A1TM A1VI Ti Fe Mn Mg Ca Na
6.295 1.705 1.182 0.016 2.058 0.060 1.819 1.390 0.475
6.389 1.611 1.263 0.049 2.014 0.063 1.667 1.295 0.648
6.200 1.800 1.545 0.024 1.940 0.082 1.402 1.494 0.506
6.489 1.511 1.161 0.031 1.911 0.020 1.918 1.274 0.686
6.565 1.435 1.235 0.022 1.950 0.006 1.843 1.137 0.807
5.899 -4.056 -3.450 0.507 0.754 1.404 --
6.001 -3.952 -3.479 0.091 1.267 1.233 --
Na K
0.888 0.087
0.834 0.064
0.580 0.148
0.785 0.190
0.763 0.195
---
---
56.4 8.3 12.3 23.0
57.3 1.5 20.9 20.3
Almandine Spessartine Pyrope Grossu|ar
*Total iron as FeO. Structural formulae based upon twenty-three (amphiboles) and twenty-four (garnets) oxygens.
136 eclogites o f t h e Sierra de Baza ( G o m e z - P u g naire, 1 9 8 1 ) .
t h e r e are no fine-grained i n t e r g r o w t h s o f paragonite--margarite below the resolution of the microprobe beam, the intermediate comp o s i t i o n s i n d i c a t e a r a t h e r high t e m p e r a t u r e o f f o r m a t i o n o f t h e n a t u r a l micas. A t t e m p e r atures o f 6 0 0 ° C t h e e x p e r i m e n t a l d a t a d o n o t allow t h e u n e q u i v o c a l d i s t i n c t i o n o f w h e t h e r t h e r e still exists a m i s c i b i l i t y gap o r n o t .
3.3. A m p h i b o l e s R e p r e s e n t a t i v e analyses o f a m p h i b o l e s are given in T a b l e III. T h e y are r e p r e s e n t a t i v e o f t h e m o s t a b u n d a n t a m p h i b o l e s in t h e s e r o c k s . O n l y t w o relics o f b l u e - a m p h i b o l e , o p t i c a l l y i d e n t i f i e d as g l a u c o p h a n e , have b e e n f o u n d , b u t t h e grains w e r e t o o small t o analyse. T h e t e r m s sub-calcic and sodic--calcic a m p h i b o l e s are used h e r e following L e a k e ( 1 9 7 8 ) . S a m p l e SF 1 has b o t h sub-calcic a m p h i b o l e s w i t h a c o m p o s i t i o n closely related to pargasite, and sodic--calcic a m p h i b o l e (katop h o r i t e ) . T h e a m p h i b o l e s o f samples C 884 and GW 45 do n o t show significant v a r i a t i o n s in c o m p o s i t i o n and o n l y o n e analysis f r o m each is listed.
3.2. Garnet R e p r e s e n t a t i v e m i c r o p r o b e analyses o f g a r n e t c o e x i s t i n g w i t h t h e m i n e r a l s t h a t are n o w seen in t h e p s e u d o m o r p h s are s h o w n in T a b l e III. T h e s e g a r n e t s p r o b a b l y also c o e x i s t ed w i t h t h e m i n e r a l t h a t has beer~ p s e u d o m o r p h o s e d . T h e y are a l m a n d i n e - r i c h garnets with a homogeneous composition from core to rim. G r o s s u l a r and p y r o p e c o m p o n e n t s (20 m o l e % e a c h ) are similar, and o n l y o n e grain s h o w s a significant a m o u n t o f spessartine c o m p o n e n t w i t h c o n c o m i t a n t d e p l e t i o n o f p y r o p e c o n t e n t . This c o m p o s i t i o n is v e r y similar to t h a t o f garnets f r o m t h e c o r o n i t i c
3.4. Epidote-group minerals T h e c o m p o s i t i o n o f six crystals is given in T a b l e IV. T h e pistacite c o n t e n t varies f r o m Pist34Zoi66 to Pists0Zoi:0. T h e r e are no t e x -
TABLE IV Microprobe analyses of epidote from pseudomorphs of type B SF 81 1
2
3
4
5
6
SiO 2 A1203 Fe203* CaO
36.37 24.40 13.31 23.96
36.90 29.48 7.68 24.03
37.99 26.98 9.15 23.94
38.20 27.78 7.98 24.10
38.47 27.88 7.61 24.04
37.98 27.54 8.51 23.89
~:
98.04
98.09
98.06
98.06
98.00
97.92
Si A1 Fe Ca
Pistacite Zoisite
2.903 2.296 0.800 2.049
2.877 2.709 0.451 2.007
2.974 2.490 0.539 2.008
2.978 2.553 0.468 2.013
2.994 2.559 0.446 2.005
2.968 2.539 0.501 2.000
8.048
8.044
8.011
8.012
8.004
8.008
80 20
44 56
54 46
47 53
45 55
50 50
*All iron considered as Fe20 ~. Structural formulae based upon 12.5 oxygens.
137
tural differences between the epidotes of different composition. Individual grains are chemically h o m o g e n e o u s . The chemical differences may be due to successive crystallization, or to local buffering of the o x i d a t i o n ratio by the mineral assemblage.
Type-A pseudomorphs consist of kyanite, replaced by white mica. Similar textures o f the transformation of kyanite were found in the surrounding metapelites o f the Sierra de Baza (Gomez-Pugnaire, 1979). This suggests ionic reactions like: 2ky+Ca 2÷+2H20~
4. Mineral reactions
lma+2H
÷
(1)
3 ky + 2Na + + 3SIO2 + 3H20 ¢- 2 pa + 2H ÷ (2)
In the following paragraphs we give an interpretation of h o w the pseudomorphs and their mineral assemblages could have been formed.
where ky = kyanite, ma = margarite and pa = paragonite, The reactions are balanced to constant A1 with a mica formula XY2Z4010(OH)2,
TABLE V
Petrographical characteristics o f p s e u d o m o r p h s o f t y p e s A , B a n d C, and possible mineral reactions End-members
Textures
Possible reactions and reaction s e q u e n c e
large k y core, o p t i c a l l y c o n t i n u o u s , decussate w h i t e m i c a , m a replacing k y , p a replacing
2 k y + Ca ~+ + 2 H ~ O ¢- 1 m a + 2 H ÷
[ s a m p l e s in P l a t e I]
Type-A pseudomorph: k y , m a , p a , + chlorite + carbonate [GW R , C 668, P l a t e I, ( A ) and (B)]
ma
(1)
3 ky + 2Na + + 3SiO 2 + 3H20 = 2 pa + 2H +
(2)
1 m a + S i 4 + + N a +~- 1 p a + A 1 3 + + C a
(6)
2+
3 m a + 4 N a + + 2 H + + 6SiO~ ~ 4 p a + 3 C a 2+
(6a)
Type-B pseudomorph: epi, ky, ma, pa [C 884, P l a t e I, ( D ) and S F 8 1 - 1 , P l a t e I, (C)]
s m a l l k y in w h i t e m i c a , m a b e t w e e n pa and k y ,
corona o f epi
(a) 41aw~
2zoi+
1 ky+
51aw~- 2zoi+
(3)
1 qz+7H~O
lma+2qz+8H20
(4)
lky+
1 qz+7H20
(3)
4ma+3qz
(5)
or (b), first: 41aw~- 2zoi+
then: 2zoi+bky+3H20~
1 m a + S i 4+ + N a + ~ l p a + 3ma+4Na
A l 3+ + C a 2+
+ + 2 H + + 6 S i O 2¢- 4 p a +
3 C a 2+
(6) (6a)
Type-C pseudomorph: epi, pa, qz [C 858, G W (E)]
45,
P l a t e I,
aggregates o f decussate p a + epi + q z
4law + lab~
2 zoi+2qz+
1pa+6H20
(7)
2zoi+lqz+
1pa+6H20
(8)
or • 4law+
ljd¢
or 4an+
For abbreviations see the t e x t .
1 ab+ 2H20~- 2zoi+
2 qz+
lpa
(9)
138 Although these mineral transformations are written as ionic exchange reactions, t h e y do n o t require a large-scale metasomatism. During breakdown of omphacitic pyroxene into amphibole and albite near to the kyanite crystals, Ca as well as Na and Si would be ionic species in a fluid phase. By this process the Ca2+/H ÷ and Na+/H ÷ ratios of the fluid are significantly changed, which is of great influence upon the stability of aluminium silicate. The textures of these pseudomorphs [see Plate I, (B1) and (B2)] indicate that first margarite and then paragonite replace kyanite, or that paragonite replaces margarite [see reactions (6) and (6a) below, and Table V). Type-B pseudomorphs show a more complicated mineralogy and texture. Their shape indicates that t h e y were formed after lawsonire. However, no relics o f lawsonite have been found. There are two different extreme possibilities to explain their formation from lawsonite [which are called as (a) and (b) in the following section] : (a) In the first stage lawsonite was altered (more or less simultaneously) according to the following reactions (3)--(5) (Newton and Kennedy, 1963; Nitsch, 1973; Chatterjee, 1976; Holland, 1979) at P--T conditions where these three reactions meet in an invariant point: 4 law ~ 2 zoi + 1 k y + 1 qz + 7H20
(3)
51aw-~2zoi+lma+2qz+8H20
(4)
2 zoi + 5 ky + 3H20 ~ 4 ma + 3 qz
(5)
where law = lawsonite, zoi = zoisite and qz = quartz. The pseudomorphs would then consist of margarite, quartz, kyanite and zoisite. It may seem improbable that the P--T conditions were exactly at the invariant point, but it must be remembered, that this "invariant" equilibrium becomes at least bivariant due the Fe 3+ content of epidote. In the later stage, margarite was altered into paragonite by the exchange reactions: Ima+Na
÷+Si 4÷~- ipa+Ca
2÷+Al 3+
(6)
3 m a + 4 N a ÷ + 2 H ÷ + 6 S i O 2 ~ 4 p a + 3 C a 2÷ (6a) where reaction (6) is balanced with respect to 1 "mica u n i t " , and reaction (6a) is balanced with respect to constant A1. These reactions can explain the lack of quartz in the pseudomorphs, which should be present according to reactions (3)--(5), and also the rim of epidote around the pseudomorphs. The process is similar to the ionic reaction formulated above for pseudomorphs of type A. Ca 2÷ and A13÷, released by this process, could produce the monomineralic epidote corona of the pseudomorphs. Such monomineralic zones are typical of metasomatism and point to the high mobility of many components (Table V). (b) Lawsonite was transformed into zoisite + kyanite + quartz only (not margarite) by crossing the boundary of reaction (3). In a later stage margarite was produced by a retrograde hydration reaction (5), and in a yet later stage [similar to possibility (a)], margarite was transformed into paragonite by high activity of Na ÷ (Table V). Type-C pseudomorphs are easily explained by breakdown of the association lawsonite + albite, or lawsonite + jadeite, according to reaction (7) or reaction (8) (Franz and Althaus, 1977; Heinrich and Althaus, 1980; Heinrich, 1981): 41aw+lab~-
lpa+2zoi+2qz+6H20
(7)
41aw+ljd~
lpa+2zoi+lqz+6H20
(8)
where ab = albite and jd = jadeite (see Fig. 3). It is tacitly assumed that albite or jadeite were transported from the matrix to the lawsonite as components of the fluid phase in order to maintain the euhedral shape of the pseudomorphs. The shape of the pseudomorphs C [see Plate I, (E)] leaves open the interpretation that the pseudomorphosed mineral was not lawsonite, but plagioclase. In this case an anorthite-rich plagioclase may have been transformed directly into the assemblage
139 20
eclogite paragenesis, otherwise these pseudomorphs never passed through a stage of eclogite equilibrium without plagioclase.
PH2o (k bar) 18
5. Metamorphic conditions
16
t2
tO
I
I
I
|
I
300
400
5.00
600
700
r ( "C )
Fig. 3, E x p e r i m e n t a l l y - d e t e r m i n e d o r calculated e q u i l i b r i u m curves and possible e x t r e m e P - - T p a t h s (a) and (b) for the d e v e l o p m e n t o f t h e rocks. E n circled n u m b e r s refer to r e a c t i o n n u m b e r s in t h e t e x t [DH = D e l a n e y and Helgeson ( 1 9 7 8 ) ; C H = C h a t t e r jee ( 1 9 7 1 , 1972, 1976); F A = F r a n z and A l t h a u s ( 1 9 7 7 ) ; H A = Heinrich and A l t h a u s ( 1 9 8 0 ) ; N = N i t s c h ( 1 9 7 3 ) ; N K = N e w t o n and K e n n e d y ( 1 9 6 3 ) ; S N = S t o r r e and N i t s c h ( 1 9 7 4 ) ; H = Holland ( 1 9 7 9 ) ; V = Velde ( 1 9 7 1 ) ] . F o r a b b r e v i a t i o n s and r e a c t i o n n u m b e r s see t h e t e x t ( e x c e p t co = c o r u n d u m ) .
paragonite + zoisite + quartz: 4an+lab+2H20,~lpa+2zoi+2qz
(9)
where an = anorthite. Note that this is a hydration reaction, whereas the above-mentioned reactions are dehydration reactions (Table V). This last reaction also necessitates that the assemblage paragonite + zoisite + quartz was the
Since the stability fields of the abovementioned mineral associations have been determined experimentally b y various authors, it is seductive to try to reconstruct the metamorphic P --T history of the rocks. The relevant equilibrium curves are shown in Fig. 3, together with possible P--T paths. We start from the assumption that the rocks were first metamorphosed at low temperatures/high pressures. This hypothesis is supported b y the occurrence of relics of glaucophane and omphacite. The possibility that the type-C pseudomorphs were formed directly from plagioclase can therefore safely be excluded, because this would require a P--T path from low to intermediate temperatures/low pressures to P --T conditions near to the b o u n d a r y of the paragonite + zoisite + quartz stability field [No. (9) in Fig. 3]. The pseudomorphs therefore are more likely to have formed after lawsonite, and the rocks must have passed through the lawsonite (and lawsonite + albite or lawsonite + jadeite) stability field, which is somewhere left of reactions (3), (4), (7) and (8) in Fig. 3. The invariant point, where zoisite + kyanite + margarite + lawsonite + quartz + water coexist, was calculated by Chatterjee (1976) at 8.5 kbar and 480°C. Experimental data on the reactions (3) and (4) b y Newton and Kennedy (1963) and Nitsch (1973) indicate slightly lower temperatures for this point. If the calculated values of Holland (1979) are accepted for the reaction (5) the invariant point lies at 11 kbar, 500°C. Therefore the formation of zoisite, kyanite, margarite and quartz (type-B pseudomorphs) must have happened at pressures above at least 8 kbar and temperatures of 400--500°C. Now two extreme possibilities of P--T paths are discussed. Path (a) is consistent with reaction sequence (a) for type-B pseudo-
140 m o r p hs (see Table V): during t he uplift of t h e rocks, t e m p e r a t u r e s rose and t he P - - T path followed reaction (5), where zoisite and kyanite are replaced by margarite and quartz. Nevertheless, the position o f this curve is uncertain, because the calculated data refer to pure margarite and the natural margarite o f t h e p s e u d o m o r p h s contains up to 50 mole% paragonite. Storre and Nitsch (1974) used a natural margarite--paragonite solid solution for their experiments and f o u n d a much steeper slope (see Fig. 3) than the calculated one. T h e paragonite c o n t e n t o f t he margarite is t h e r e f o r e o f critical influence on the considered P - - T path. This is i m p o r t a n t for the interp re t a t io n o f the results presented here, because it opens the possibility t hat the rocks experienced further compression and heating, following path (b), consistent with reaction sequence (b) in Table V. Of course, any intermediate path b e t w e e n (a) and (b) is possible, too. T h e t r a n s f o r m a t i o n of margarite into paragonite [reactions (6) and (6a)] in pseudomorphs A and B is the last event which can be traced in th e rocks. It must have t aken place at still high pressures, as indicated b y the experimental data o f Franz and Althaus (1977) on the stability field of paragonite + zoisite + quartz. Due to th e high a m o u n t o f Fe 3÷ in t he epidote, th e experimentally-determinated pressures are n o t directly applicable to t he natural systems, but t h e y were pr oba bl y on the order o f PH:O o f 6--8 kbar [see curve (9) in Fig. 3]. The e x t r e m e l y high miscibility bet w e e n paragonite and margarite indicates high temperatures in the order o f 600°C. This m a x i m u m t e m p e r a t u r e is consistent with the data given b y Gomez-Pugnaire (1979) for the surrounding metapelites. 6. Conclusions (1) Microprobe analyses of white mica in some amphibolitized eclogites indicate an extensive solid solution be t w een paragonite and margarite near to the closure of the miscibilit y gap.
(2) These micas occur t oget her with kyanite, epidote and quartz in p s e u d o m o r p h s which can be interpreted as p s e u d o m o r p h s after lawsonite and after kyanite. (3) Small-scale metasomatism produces the corona t e x t u r e of some of the p s e u d o m o r p h s and points to the i m port ance of mobile components (especially Na, Ca, but also Si and A1) in high-pressure rocks. (4) With the presently available data it is impossible to use the system Na20--CaO-A1203--SiO2--H20 to derive exact P --T paths, but some constraints can be made. Acknowledgements This paper has been made possible by t he financial support of Programs o f Collaboration between the C.S.I.C. (Spain) and the C.N.R. (Italy). The authors thank Professors F.P. Sassi and M. Mufioz as Italian and Spanish directors of these Programs. T he electron microprobe analyses have been made at Istituto di Mineralogia of Padua (Italy). T h e y thank also Drs. G.M. Molin and G. Da Roit b y their collaboration. References Ackermand, D. and Morteani, G., 1973. Occurrence and breakdown of paragonite and breakdown of paragonite and margarite in Greiner Schiefer Series (Zillertal Alps, Tyrol). Contrib. Mineral. Petrol., 40: 293--304. Chatterjee, N.D., 1971. Preliminary results on the synthesis and upper stability of margarite. Naturwissenschaften, 58: 147. Chatterjee, N.D., 1972. The upper stability limit of the assemblage paragonite + quartz and its natural occurrence. Contrib. Mineral. Petrol., 34: 288-303. Chatterjee, N.D., 1976. Margarite stability and compatibility relations in the system CaO--A1203SiO2--H20 as a pressure--temperature indicator. Am. Mineral., 61 : 699--709. Deer, W.A., Howie, R.A. and Zussman, J., 1962. An Introduction to The Rock-forming Minerals. Longman, London, 528 pp. Delaney, J.M. and Helgeson, H.C., 1978. Calculation of the thermodynamic consequence of dehydration in subducting oceanic crust to 100 kb and 600°C. Am. J. Sci., 278: 638--686.
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Franz, G. and Althaus, E., 1977. The stability relations of the paragenesis paragonite--zoisite-quartz. Neues Jahrb. Mineral. Abh., 130: 159-167. Franz, G., Hinrichsen, T. and Wannemacher, E., 1977. Determination of the miscibility gap on the solid solution series paragonite--margarite by means of infrared spectroscopy. Contrib. Mineral. Petrol., 59: 307--316. Frey, M., Bucher, K., Frank, E. and Schwander, H., 1982. Margarite in the Central Alps. Schweiz. Mineral. Petrogr. Mitt., 62: 21--45. Gomez-Pugnaire, M.T., 1979. Some considerations on the highest temperature reached in the outcropping rocks of the Nevado--Fil~bride Complex in the Sierra de Baza area during the Alpine metamorphism. Neues Jahrb. Mineral. Abh., 135: 75--87. Gomez-Pugnaire, M.T., 1981. Evoluci6n del metamorfismo alpino en el Complejo Nevado-Fil~ibride de la Sierra de Baza (Cordilleras B6ticas, Espafia). Tecniterrae, No. 4 1 , 1 3 0 pp. Gomez-Pugnaire, M.T., Mottana, A., Bocchio, R., Liborio, G. and Abraham, K., 1979. Coronitic eclogites in the Sierra de Baza (Betic Cordilleras, Spain). Neues Jahrb. Mineral. Abh., 136: 42--62. Heinrich, W., 1981. Paragonit in Hochdruckgesteinen Experimentelle Untersuchungen zu B i l d u n g s und Abbaureaktionen. Thesis, University of Karlsruhe, Karlsruhe, 111 pp. Heinrich, W. and Althaus, E., 1980. Die obere Stabilit//tsgrenze yon Lawsonit plus Albit bzw. Jadeit, Fortschr. Mineral. Beiheft, 58: 49--50. H6ck, V., 1974. Coexisting phengite, paragonite and -
-
margarite in metasediments of Mittlere Hohe Tauern, Austria. Contrib. Mineral. Petrol., 43: 261--273. Holland, T.J.B., 1979. High water activities in the generation of the high pressure kyanite eclogites in the Tauern Window, Austria. J. Geol., 87 : 1--27. Julivert, M., Fontbot~, J.M., Riveiro, A. and Conde, L., 1974. Mapa TectSnico de la Penfnsula Ib~rica y Baleares. Inst. Geol. Mineral., Madrid (scale 1 : 1,000,000). Leake, B.E., 1978. Nomenclature of amphiboles. Mineral. Petrogr. Acta, 22: 195--224. Newton, R.C. and Kennedy, G.C., 1963. Some equilibrium reactions in the join CaA12Si20,--H~O. J. Geophys. Res., 68: 2967--2983. Nijhuis, H.J., 1964. Plurifacial Alpine metamorphism in the south-eastern Sierra de los Filabres, South of Lubrfn, SE Spain. Thesis, University of Amsterdam, Amsterdam, 151 pp. Nitsch, K.H., 1973. Neue Erkentnisse zur Stabilit~t yon Lawsonit. Fortschr. Mineral., 50: 34--35. Puga, E., 1971. Investigaciones geoldgicas en Sierra Nevada Occidental. Thesis, University of Granada, Granada, 269 pp. Puga, E., 1977. Sur l'existence dans le Complexe de la Sierra Nevada (Cordill~re b~tique, Espagne) d'~clogites et sur leur origine probable ~ partir d ' u n e crofite oc~anique m4sozoique. C.R. Acad. Sci. S~r. D, 285: 1379--1382. Storre, B. and Nitsch, K.H., 1974. Zur Stabilit~/t yon Margarit in System CaO--A1203--SiO2--H20. Contrib. Mineral. Petrol., 43: 1--24. Velde, B., 1971. The stability and natural occurrence of margarite. Mineral. Mag., 38: 317--323.