P?T conditions for the eclogitic re-equilibration of the metaophiolites from Val d'Ala di Lanzo...

J . tnmimorphic Gc~ol. 1986. 4, 161-178

P-T conditions for the eclogitic re-equilibration of the metaophiolites from Val d’Ala di Lanzo (internal Piemontese zone, Western Alps)

R. SANDRONE, L. LEARDI & P. ROSSETTI, Dipartimento di Georisorse e Territorio, Politecnico di Torino, corso Duca degli A bruzzi 24, 10129 Torino, Italy R. COMPAGNONJ. Dipartimento di Scienze della Terra, Universita’ degli Studi di Torino, via Valperga Caluso 37, 10125 Torino, Italy

Abstract. A study has been made of the high- pressure early-Alpine re-equilibration in the eclogites and metasedimentary cover of the Val d’Ala di Lanzo ophiolite. All of the main high- pressure minerals have been analysed and their compositions used to determine re-equilibration temperatures. The minimum conditions proposed ( P = I .3 GPa. T = 450460°C) are also indicated by the presence of a jadeite+quartz- bearing metagranite.

The temperaturesare compared with those reported for similar eclogites from the Voltri Group, the Aosta Valley and the Valais. Com- parison of recalculated temperatures shows that the temperature (and probably the pressure) of the eclogitic re-equilibration increased in the Aosta Valley and the Valais, in keeping with what has been observed i n the internal Penninic basement of the Gran Paradiso and Monte Rosa crystalline massifs.

Key-words: geothermobarometry ; high-pressure metamorphism; metaophiolite; Western Alps

INTRODUCTION

Determination of the peak conditions of the early-Alpine high-pressure metamorphic event in the internal Piemontese Zone has been the subject of much work for more than a decade: for a review of the literature, see Bocquet (1974) and Ernst (1981). the relevant abstracts of the First International Eclogite Conference, Clermont-Ferrand (Terra Cognita. 2 , 305-309. 1982) and Messiga, Piccardo & Ernst (1983).

The temperature ( T ) for the climax of the event, at a constant pressure (P) of 1.0 GPa (10 kbar), as suggested by Ernst (1976). has been estimated using the garnet4inopyroxene Fe-

Mg exchange geothermometer of Ellis & Green (1979) as 527°C for eclogitesof the Zermatt-Saas zone (Ernst. 1981). and 431°C and 508°C for rneta-FeTi-gabbro and meta-Mg-gabbro, respectively, from the Voltri Group (for further details see Messiga eta/. , 1983).

Chinner & Dixon (1973). however, have suggested a higher minimum pressure and more recent work is also pointing in this direction. Oberhaensli (1980) used both the Rgheim & Green (1974) and the Ellis & Green (1979) calibrations of the garnet-clinopyroxene Fe-Mg exchange geothermometer, the content of the jsdeite end-member in omphacite (Kushiro, 1969), and Krogh & Riheim’s (1978) garnet- phengite geothermometer to estimate a P-T range from 1.0 GPa and 400°C to 1.8 GPa and 765°C for the early-Alpine metamorphic re- equilibration of the pillow lavas from the Zermatt-Saas zone (cf. Bearth, 1967, 1973, 1974).

Pognante (1981. 1982) has suggested minimum conditions of 1.2-1.3 GPa and 400450°C for the coronitic eclogite re-equilibration of FeTi-gabbros from the Orsiera-Rocciavre Klippe (lower Val di Susa) based upon X-ray recognition of the jadeite+quartz paragenesis, and temperatures calculated using the Ellis & Green (1979) calibration of the garnet-clinopyroxene Fe-Mg exchange geothermorneter. Sandrone & Compagnoni (1983) have suggested conditions of 1.5 GPa and about 500°C for the Lanzo Massif, using the same geothermometer on assemblages in metagabbros and metabasalts and in view of the widespread jadeite+quartz paragenesis (see also Compagnoni. Radicati di Brozolo & Sandrone, 1984).

This paper presents the results of a study of the high pressure parageneses from metamorphic FeTi-gabbros (= coarse-grained eclogites),

161

162 R. Sandrone et at.

FeTi-basalts (= fine-grained eclogites), albite- granite (= jadeite + quartz-bearing metagranite), and micaschists from the ophiolite cover outcropping in the Val d'Ala di Lanzo. Work on these rocks has allowed a more accu- rate assessment to be made of the high-pressure metamorphic peak in this portion of the internal Piemontese zone. The data obtained are compared and discussed in the light of similar high- pressure re-equilibration in the Piemontese zone from the Voltri Group in Liguria to the Zermatt- Saas zone in Switzerland.

GEOLOGICAL FRAMEWORK

The middle upper Val d'Ala (Fig. 1) mainly consists of a metamorphic ophiolite (Leardi, Rossetti & Compagnoni, 1986) which belongs to the internal Piemontese zone of the Western Alps (Bearth, 1967, 1973, 1974; Dal Piaz, 1974; for a review see also Compagnoni. Piccardo & Sandrone, 1983); all the members of an oceanic- type sequence occur, though not always in their original setting.

Ultramafics The ultramafic rocks consist of kilometre-scale masses of antigorite serpentinite several hundred metres thick. In addition to folded veins which contain chlorite, diopside, Ti-clinohumite and metamorphic olivine, they often contain decimetre-scale boudins or lenses of garnet, diopside, epidote and idocrase rodingites. The latter often include pegmatoid-grained clinopyroxene relics indicative of the original gabbroic nature of the parent rock. The antigorite serpentinite masses are usually surrounded by foliated serpentinites or serpentinite schists.

lntrusives These rocks are quantitatively subordinate to both the ultramafics and the volcanics. They consist of predominant metamorphosed Mg- gabbro with minor FeTi-gabbro. The Mg- gabbro, transformed into sheared flaser gabbro, is usually intercalated between serpentinite and metabasalt and occurs as bodies up to several kilometres in length and up to l00m thick. The FeTi-gabbro, converted to coarse-grained eclogite, occurs as lenses in serpentinite and boudinaged layers within the meta-Mg-gabbros. In one locality only (Fig. l ) , a jadeite+quartz- bearing metagranite derived from an albite- granite protolith (Leardi et al . , 1986) is present and is intimately associated with the coarse- grained eclogite.

Volcanics Metabasaltic rocks, derived from a MORB-type protolith (Leardi et al . , 1986). occur as homogeneous masses, up to 400m thick, of actinolite-epidote-chlorite-albite greenstone (= 'prasinite' in the Alpine literature). in which all the primary magmatic features are completely obliterated by Alpine deformation and recrystallization. Their structural characters vary considerably from one place to another. Several different facies have been recognized: prasinites S.S. ; prasinites with large albite poikiloblasts, locally containing boudinaged omphacite-bearing veins; amphibolitic prasinites; banded prasinites with epidote-rich and actinolite-rich layers; banded prasinites with glaucophane-rich and Fe-epidote-rich layers; glaucophanic prasinites; fuchsite-bearing foliated prasinites and chlorite-albite-green- stones (= 'ovardite' in the Alpine literature). FeTi-basalts, transformed into fine-grained eclogite, usually occur as discontinuous layers within glaucophane-bearing prasinite.

Sedimentary cover The cover of the ophiolite is scanty and very rarely retains its original stratigraphical position. It usually occurs as synforms within folded meta-ophiolite, especially the metabasalts, and consists of micaceous quartzite and minor micaschist (very probably derived from radiolarian chert). overlain by calcschist. locally grading to impure marble.

PETROGRAPHY

All of the lithologies examined in t h e Val d'Ala display an Alpine evolution of the type reported for the other ophiolites of the inner Piemontese domain (cf. Compagnoni ct a/. , 1983. and references therein); only the serpentinites. meta-Mg- gabbros and coarse-grained eclogites display mineralogical and textural relics of their protoliths. The derivation of the massive serpentinites from tectonized lherzolite is indicated by clinopyroxene relics with a tectonite structure, still recognizable thanks to the essentially pseudo- morphic nature of serpentinization. Both the meta-Mg-gabbros and the coarse-grained eclogites locally preserve relics of the original magmatic clinopyroxene.

Two main metamorphic events can be recognized: the early-Alpine event, characterized by high pressure parageneses; and the Lepontine event, characterized by typical associations of the greenschist facies. The signs of this evolution are not equally evident in all the lithologies. The

m

2

......... ........ ......... ........ ......... ......... ......... I ......... F13mrl

......

164 R. Sandrone et al.

FeTi-gabbros and basalts, albite-granite and micaschists. for example, offer a better record of the early-Alpine event, whereas the rnetabasalts usually have been more thoroughly re-equili- brated into greenschists and display their early- Alpine minerals only as relics. Since the purpose of this paper is to describe the P-Tconditions of the high-pressure early-Alpine event, our atten- tion will be confined to the lithologies that best illustrate assemblages associated with that event.

The typical high-pressure assemblage in the eclogites consists of garnet and omphacitic/ chloromelanitic clinopyroxene in roughly equal proportions, and smaller amounts of rutile, with apatite, quartz, zoisite and allanite as accessories. Growth of the minerals of the eclogite paragenesis took place in two stages, separated by an episode of deformation. Subsequent par- tial re-equilibration, still under high-pressure conditions, resulted in the development of glaucophane * white micas. During the Lepontine event, there was a limited growth of albite, chlorite, blue-green amphibole, actinolite, clinozoisite/epidote, white micas and green biotite.

The metamorphosed albite-granite consists of a jadeite+quartz assemblage and accessory glaucophane, rutile, zircon, apatite and allanite. The jadeite is zoned and displays strong un- dulatory extinction. Its composition in the rock varies gradually towards the contact with the host eclogite, as shown by the change of Na- pyroxene from colourless to pale green. Jadeite crystals are usually surrounded by an alteration rim of albite and aegirine. The greenschist facies retrogression, especially well developed in the core of quartz-jadeite veins, produced widespread alterations of jadeite to albite and minor white mica k blue-green amphibole.

In the micaschists of the cover, the high- pressure event produced a garnet-paragonite- phengite I - zoisite-chloritoid-rutile assemblage, whereas the greenschist facies event developed a phengite I1 - chlorite-biotite-epidote-sphene assemblage, Tourmaline, apatite and opaques are common accessories. An interesting feature of the calcschist assemblages is the local appear- ance of rhornbic sections, up to about 2 rnm across, of clinozoisite+white mica pseudo- rnorphsafterlawsonite(cf. Fry& Fyfe, 1971).

MINERAL CHEMISTRY More than 200 analyses of the high pressure minerals (garnet, Na-clinopyroxene. Na-amphibole. white micas) have been made on three

samples of metamorphic FeTi-gabbro, two samples of albite-granite, one sample of FeTi- basalt and one sample of micaschist from the cover. The analyses (unless otherwise stated) were made on the rims of contacting mineral phases and in portions of fine-grained rocks with metamorphic-textural re-equilibration.

Analytical methods All phases were analysed in a wavelength- dispersive ARL SEMQ instrument. Operating conditions were 15 kW accelerating voltage, 0.020 FA sample current on brass, and beam spot size 2-10 pm, depending on the mineral grain size; except for spot selection and focus, the instrument operation was computer con- trolled with on-line data reduction, using the MAGIC IV program (Colby, 1968). Natural minerals were used as standards. I n Tables 1 to 6, data are reported to two decimal places; the accuracy, however, is probably almost one order of magnitude lower for the major oxides.

The Fe'+/Fe3+ proportions were estimated using the method of Papike. Cameron & Baldwin (1974) and Cawthorn & Collerson (1974) for clinopyroxenes, Ryburn, RAheim & Green (1975) for garnets, Papike er al. (1974) for amphiboles, and Laird & Albee (1981) for white micas. End-member assignments were made from the formula proportions with the exception of garnet, for which the method suggested by Rickwood (1968) was followed.

Na-clinopyroxenes Fourteen rim analyses of Na-clinopyroxenes from three meta-FeTi-gabbros (analyses I , 2-5, 6-9) and one meta-FeTi-basalt (analyses 10-14) are listed in Table 1. The cation sum is close to the theoretical value 4. Because the Si content is quite homogeneous and close to 2.00. almost all the Al content appears to be octahedrally coor- dinated. The low values of the AIV'Ma ratio (average 0.49, range 0.26-0.76) suggest that most Na enters the acmite molecule. The Fe,,,, afomic content is significantly high, though highly variable (average 0.35, range 0.22-0.55). TiOz, MnO, Cr203 and K 2 0 are negligible. The Fe*+/Mg ratio ranges from 0.13 to 0.31 (average 0.22) in the meta-FeTi-gabbro Na-clinopyroxenes. Values are significantly higher in meta- FeTi-basalts (0.39-0.70. average 0.41).

The scatter for any given specimen probably depends on the textural position of the mineral analysed, which is sensitive to the chemical composition of the associated phases (e.g. Cimmino

P-T conditions, Val d’Aia di Lanzo 165 Table I. Rcprcsentativc electron microprobe analyses and atomic proportions (basis 6 oxygens) for Na-clinopyroxenes from eclogitic rocks. Analyses 1-9: meta-FeTi-gabbros; 10-14: meta-FeTi-basalt

~~

Analysis I 2 3 4 5 6 7 8 9 10 I 1 12 13 14

SiOz TiOz

FeO, MnO

CaO N a 2 0

CrlOl TotA

Si Al Al Fe’ +

Fe’+

Ca Na Total

AlzOl

MgO

KzO

Mg

55.05 55.72 55.76

5.93 8.76 8.26

0.06 n.d. n.d.

0.05 0.05 0.M

10.79 7.18 7.84

7.55 7.54 7.80 13.07 12.73 12.23 6.98 7.38 7.41

0.02 0.02 -

- _ -

55.75 55.69 54.80 0.07 o.09 0.11 9.05 8.05 3 . 7 ~

0.08 n.d. 0.04 7.37 7.05 8.68

12.51 12.03 15.10 7.49 7.66 5.82

7.29 8.74 12.21

- - - 0.02 - -

9Y.4X Y9.3X 99.74 99.61 99.33 100.54

2.00 2.00 2.00 2.00 2.00 1.99 0.01 - _ _ - _

0.26 0.37 0.36 0.38 0.34 0.15 0.09 0.08 0.08 0.08 0.09 0.12 0.24 0.14 0.15 0.14 0.18 0.26 0.41 0.40 0.42 0.40 0.38 0.47 0.51 0.52 0.47 0.48 0.47 0.59 0.49 0.49 0.52 0.52 034 0.41 4.on 4.00 4.00 4.00 4.00 4.00

54.88

3.04

0. I0

17.12 4.69

n.ob

9.50

10.89

0.01 n.ni

l00.30

I .99 0.01 0. 12 0.08

0.67

0.21 0.59

0.33 4.00

54.07 54.75 54.52 54.48 55.10 0.03 0.02 0.08 0.06 - 4.51 3.61 5.13 5.38 5.12

12.39 10.74 13.53 13.54 13.82 0.12 0.10 - n.d. 0.03 6.35 9.65 6.59 6.48 6.34

14.14 16.32 12.76 12.49 12.236

- n.ni - - - 7.68 . 4.63 7.02 7.12 7.19

0.05 - - 0.02 99.29 99.88 99.63 99.55 100.48

-

1.99 2.00 2.00 2.00 2.01

0.19 0.16 0.22 0.23 0.22 0.10 0.16 0.14 0.14 0.14 0.28 0.17 0.28 0.28 0.28 0.42 0.52 0.36 0.3s 0.34 0.56 0.M 0.50 0.49 0.50

- 0.01 - - -

0.45 0.33 0.50 0.5 I 0.51 4.00 4.00 4.00 4.00 4.00

54.38 53.11

5.35 2.74 14.53 16.27 n.d. 0.15 5.86 6.35

7.58 5.26

0.10 -

11.88 15.27

0.01 0.01 0.04 -

99.73 99.16

1.99 2.00 0.01 - 0.23 0.12 0.13 0.26 0.31 0.26

0.47 0.62 0.32 0.36

0.54 0.38 4.00 4.00

& Messiga, 1983). Marked, systematic differ- ences were also noted between one specimen and another, which pointed to a close relation between the chemical composition of the mineral phases and that of the system as a whole.

Calculation of the end-members (Cawthorn & Collerson. 1974) reveals a jadeite and acmite molecule content of approximately 13-39% and 13-35%, respectively. The jadeite-acmite- augite diagram (Fig. 2) of Essene & Fyfe (1967) indicates chloromelanite compositions for most Na-pyroxenes from meta-FeTi-gabbro and rneta-FeTi-basalt, and also shows their distribu- tion into the fields of omphacites, aegirine- augites and sodic augites.

Five jadeite analyses from the jadeite+quartz- bearing metagranite are also shown in Table 2. Despite the number of analyses from areas in which the jadeites are best preserved, the stoichiornetric quality of the results is always low. A SEM study was made with an EDS EDAX microanalytical system installed on a Philips 501 SEM (beam diameter=().] p m ; beam acceleration = 30 kV), to determine the cause of the low stoichiometry of jadeite; in particular, the possible presence of submicroscopic intergrown phases was investigated. The check merely revealed an inhornogeneity of the main cations (especially Fe) of the same order as that noted with the microprobe.

Similar stoichiornetric problems have been encountered in other Alpine jadeites from the

Table 2. Representative electron microprobe analyses and atomic proportions (basis 6 oxygens) for jadeites from metamorphic albite-granite

SiOz Ti02 A1203 FeO, MnO MgO CaO Na20

CrZO, Total

Si Al Fe” Fe’’

Ca Na Total

KZO

Me.

57.94 60.17 59.64 58.56 59.13

16.58 21.45 16.44 17.16 17.19 0.14 0.01 0.02 0.04 0.01

5.11 1.44 4.04 4.08 4.02 n.d. 0.03 n.d. 0.01 0.03 1.67 0.17 0.74 0.89 0.77 1.80 0.37 1.54 1.76 1.32

13.48 14.01 13.70 16.23 14.55 0.03 - - 0.01 0.01

96.75 97.68 06.12 98.74 97.03

2.05 2.08 2.13 2.05 2.08 0.69 0.87 0.69 0.71 0.71 - 0.04 0.12 - -

0 . 1 5 - - 0.12 0.12 o . 0 ~ o.01 0.04 0.05 0.04 0.07 mi 0.06 0.07 0.0s 0.93 0.94 0.95 i . i n 0.99

0.03 - - - -

3.98 3.95 3.99 4.10 3.99

rneta-gabbros of the Lanzo Massif (Compagnoni & Sandrone, unpublished data) and the Orsiera- Rocciavre Klippe (U. Pognante, personal corn- munication). In keeping with what had been observed in the thin sections, the analyses showed that the jadeite becomes richer in acmite as the


J d 5 0 A c m Fig. 2. Ternary diagram Jadeite (JdtAcmite (AcmtAugite (Aug) after Essene & Fyfe (1%7) for Na- clinopyroxenes from the Val d'Ala rocks; a. b, c: meta-FeTi-gabbros; d: meta-FeTi-basalt; open circles: jadeite+quartz-bearing metagranite. Analytical data are compared with the compositional fields of clinopyroxenes from the Voltri Group (I: meta-Mg-gabbros; 2: meta-FeTi-gabbros) and Monviso, Cottian Alps (3: meta-FeTi-gabbros). Data from Lombard0 er al. (1978). Messiga & Piccardo (1980) and Messiga er d. (1983).

meta-FeTi-gabbro contact is approached. A maximum jadeite molecule content of 93% can be observed in the analyses which display the highest stoichiometry. The jadeites in the central part of the veins are very likely to be even purer, both because they are further away from the contact and also because they appear to have completely retrogressed to albite. I t is well known that destabilization of Na-clinopyroxene is easier the higher its end-member jadeite content.

The secondary Na-clinopyroxene showed compositions on the boundary between aegirine and aegirine-augite on the diagram of Essene & Fyfe ( 1967).

Garnets Fourteen garnets from three meta-FeTi- gabbros and one meta-FeTi-basalt were

analysed at the same time as the associated clinopyroxenes of Table 1. The results are shown in Table 3 and Fig. 3. Fe3+ is certainly present, since formation of the andradite molecule will significantly reduce the number of cations not allocated to end-members. The garnets consist predominantly of almandine (58-73%) with subordinate pyrope (Z-l6%). andradite (0-14%) and grossular (&24%). The latter displays the most significant variation even within the same specimen. The mean Mn content is around 1% (04%). Ti02 and Cr203 are absent or negligible. As can be seen in Fig. 3a. the garnet composition in a given specimen of meta-FeTi- gabbro varies slightly more than that in the corresponding metabasalt and also has a higher pyrope content.

The compositions of garnets from a micaschist are listed in Table 4 and plotted in Fig. 3b. They are taken from six analyses of a garnet rim in

P-T conditions, Val d’Ala di Lanzo 167

Table 3. Representative electron microprobe analyses and atomic proportions (basis 24 oxygens) for garnets from eclogitic rocks. Analyses 1-9: meta-FeTi-gabbros; 10-14: meta-FeTi-basalt

Analysis 1 2 3 4 5 6 7 u 9 i n I I 12 13 14

S i 0 2 TiOz

FeO, MnO

CaO Cr201

Total

Si Al A1 Ti Fe” Fe’+ Mn Mg Ca Cr

Total

A1201

MgO

37.77 37.04 36.74 0.10 0.04 0.03

18.47 19.27 18.08

3.30 4.46 3.55

- - 0.04

I ( w ) . ~ o 99.73 1w.42

6.02 5.93 5.89 - 0.07 0.11

0.01 -

3.90 4.13 4.15 0.45 0.50 0.78 0.14 - 0.79 1.07 0.85 1.22 0.74 0.90

32.63 34.63 36.72 1.01 n.d. n.d.

7.12 4.29 5.26

3.47 3.56 3.31 -

-

- 0.01

16.00 16.00 16.00

-

36.73 36.76 37.92 37.64 37.62 37.15 37.32 36.98 36.60 36.89 36.85 n.05 0.21 0.13 0.11 0.09 - 0.08 - 0.12 - -

17.44 18.31 18.01 17.96 17.88 17.09 17.94 1 8 . ~ 18.54 18.82 18.18 36.1s 35.21 30.95 32.02 30.66 35.77 34.76 35.16 34.51 35.42 34.70

1.30 n.d. 0.83 0.76 1.03 0.43 0.71 n.d. 0.24 n.d. 0.56 3.2s 3.07 2.76 3.17 2.89 2.77 1.62 1.59 1.04 1.31 0.80

0.13 0.02 - 0.01 0.01 - 0.02 - 0.01 - 99.59 99.25 100.48 io0.01 99.79 99.73 99.75 99.27 99.26 1no.m 99.50

0.03 0.04 - 3.31 3.46 3.38 3.39 3.38 3.27 3.43 3.48 3.55 3.57 3.50

4.20 4.21 3.62 3.75 3.56 4.19 4.27 4.36 4.23 4.37 4.27 0.71 0.57 0.50 0.53 0.54 0.66 0.45 0.43 0.48 0.43 0.46 0. I8 - n . i i 0.10 0.14 0.06 o.10 - 0.03 - 0.08 0.79 0.74 0.66 0.76 11.69 0.67 0.39 0.39 0.25 0.32 0.20 0.80 1.01 1.67 1.43 1.65 1.12 1.30 1.29 1.46 1.30 1.47 0.02 -

16.01 16.00 ih.m 16.00 i6.00 I ~ . ( M I 1 6 . w 16.00 16.00 16.00 16.00

4.59 5.83 9.80 8.32 9.59 6.43 7.38 7.40 8.32 7.52 8.41 -

5.97 5.96 6.04 6.02 6.03 6.02 6.06 6.04 5.98 5.97 6.02 0.02 0.03 -

0.01 0.02 0.02 n.01 0.01 - 0.01 - 0.01 -

- - - - -

-

- - - - - - _ - -

Table 4. Representative electron microprobe analyses and atomic proportions (basis 24 oxygens) for garnets from a micaschist. Analyses 1-6: rims; 7-10: cores

Analysis I 2 3 4 5

SiOl TiOz

FeO, M n O MgO CaO CrZOJ Total

Si A1 Ti Fe” Fe.” Mn

Ca Cr Total

All01

Mg

37.27 -

18.10 32.02

1.53 I .25 9.71 0.07

i(n).ns

36.97 37.30 36.87 36.98 0.03 0.07 0.10 0.1 I

33.71 33.03 31.48 33.08

1.07 1.04 0.96 1 . ~ 8.17 8.90 8.93 8.62 - 0.07 0.02 0.01

17.90 18.32 19.25 18.15

1.78 1.57 1.81 1.64

99.63 100.30 99.42 99.65

6

37.32

19. I3 31.73

I .86 0.95 8.78

99.84

-

0.06

o.01

7

36.91 -

0.03 18.40 33.76

2.61 0.88 7.37

99.96

-

8 9 in

37.03 36.68 36.89

18.14 18.33 18.28

3.35 2.51 2.70

7.90 7.16 7.34

0.09

32.69 34.06 33.91

0.7s 0.84 0.93

0.01 - 0.02

- -

99.87 99.58 100.16

6.02 6.03 6.02 5.99 6.02 6.04 6.01 6.03

0.01 0.01 0.01 0.m - - 3.83 4.10 4.03 3.97 4.04 4.04 4.16 4.01 0.50 0.50 0.43 0.31 0.46 0.26 0.44 0.45 0.21 0.25 0.22 0.25 0.23 0.25 0.36 0.46 0.30 0.26 0.25 0.23 0.26 0.23 0.21 0.18

0.01 - o.ni - - - - -

3.45 3.44 3.49 3.69 3.48 3.65 3.53 3.49 - -

1.68 1.43 1.54 1.55 1.50 1.52 1.29 1.38

16.00 16.01 16.00 16.00 16.00 16.00 16.00 16.00

6.00 6.00 3.53 3.50 - 0.01

0.47 0.48 0.35 0.37 0.20 0.23

4.19 4.13

1.26 1.28

16.00 16.00


a

C

b

Fig. 3. Ternary diagram (Ca+Mn)-Mg-Fe'+ for garnets from meta-FeTi-gabbros (fields a. b and c of Fig. 3a). a meta-FeTi-basalt (field d of Fig. 3s) and a micaschist (Fig. 3b) from Val d'Ala. In Fig. k. the analysed garnets (full line fields) are compared with compositional fields of meta-FeTi-gabbros garnets from Monviso and Voltri Group (dotted line), and meta-Mg-gabbros garnets from Voltri Group (dashed line). Data from Ernst (1976), Lornbardo er a/. (1978). Messiga & Piccardo (1980) and Messiga er 01. (1983).

P-T conditions, Val d'Ala di Lanzo 169 contact with phengite and four from a garnet core. Their two main features are a constant Mg value and an Fe2+/(Ca+Mn) ratio ranging from 2.03 to 3.51. Their Mn content is usually higher than that of the eclogites. The core analyses display a higher Mn content, lower Ca values and a slightly higher Fe*+/Mg ratio than those for the rim.

White micas Twelve analyses of white micas from a micaschist specimen are reported in Table 5. The sum of the oxides analysed is about 96% by weight. Ascan beseen in Figs4and5, thesedioctahedral micas include paragonites and phengites.

Phengite I and phengite I1 can be distin- guished on account of their marked difference in celadonite content, which ranges from 45% to 55% and from 17% to 35%. respectively (Fig. 4a). Phengite I also has less Na (Fig. 4b) and AI"' (Fig. 5a) and more Fe (0.13-0.17, average 0.15, as opposed to 0.07-0.12, average 0.10 for phengite 11) and Mg (0.26-0.38, average 0.31, as opposed to 0.09-0.12, average 0.11). The average Mg/(Mg+Fe) ratio is about 0.7 for phengite I and 0.5 for phengite XI. The K content is about the same in both phengite groups: range 0.604.75 atoms per formula unit, average slightly higher in phengite I (Fig. 4b).

Paragonites have Na values of about 0.8 per formula unit and very low K values (0.05-0.07).

Their Fe+Mg contents are also extremely low (less than 0.05) and their AI"' and AllV values are thus close to the stoichiometric values. These correspond to a very high paragonite content (over 90%) and only 5-9% celadonite (Fig. 4a). Ti, Mn, Ca and Cr are always very low. Ti is almost always absent from paragonitic micas, whereas their Ca content, though very low (about 0.01) is much higher than that of phengites, as was to be expected for the higher solid solubility of the margaritic molecule (cf. Guidotti, 1984).

Sodic amphiboles The analyses of 10 sodic amphiboles from one meta-FeTi-gabbro (analyses 1-3). one meta- FeTi-basalt (analyses 4-7). and one metamorphic albite-granite (analyses 8-10) are listed in Table 6. All these amphiboles are classified as glaucophanes (Leake, 1978; Hawthorne, 1981). The cation sums range between 15.12 and 15.43 (average 15.27) and the sums of the octahedral cations (Mn, Fe2+. Fe3+ , Mg, Cr, Ti, AIV') sys- tematically exceed the theoretical value of 5.00 (average 5.15, range 5.00-5.31). Only minor amounts of Al (average 0.03, range 0.01-0.07) and Na (average 0.25. range 0.10-0.43) respectively substitute for Si in the T-sites and occupy the A-sites. Contents of K and Mn appear to be negligible, and Cr is virtually absent. The Fe-'+/ Fe2+ ratio shows a wider range in those

Table 5. Representative electron microprobe analyses and atomic proportions (basis 1 I oxygens) for white micas from a micaschist. Analyses 1-5: phengite I: 7-9: phengite 11: 10-12: paragonite

Analysis 1 2 3 4 5 6 7 8 9 10 I I I2

SiO: TiO:

FeO, M n O

CaO NazO

CrrO,

Total

Si Al Al Ti Fe

K Na Ca

Total

A1201

MgO

KZO

Mg

~~~~~~ - ~~ ~~~~

53.66 54.03 54.24 54.11 53.33 53.92 49.48 49.72 49.44 48.53 48.36 4 8 . ~ 0 0.25 - 0.12 0.21 0.20 0.25 11.27 0.13 0.24 0.06 0.02 0.08

2.7s 2.51 2.60 3.12 2.81 2.44 2.03 2.27 1.55 0.36 0.41 0.34

3.75 3.1s 3 .1s 3.63 3.74 2.93 1.1s 1.21 1.18 0.15 0.23 0.16 - 0.03 0.02 - - - - 0.05 0.01 0.13 0.14 0.18 0.13 0.17 0.09 0.24 0.20 0.15 0.49 0.62 0.93 6.88 6.46 6.72 7.64 8.78 8.11 7.67 8.92 8.07 8.08 8.05 7.75 0.74 0.91 0.67 - _ 0.05 - - 0.03 0.07 0.02 0.02 o.ni n.ni 0.02

27.24 27.98 28.38 27.21 26.10 29.02 33.94 33.91 34.67 39.45 39.36 39.23

_ _ 0.02 0.02 0.01 0.01 o.ns o.oi 0.03 - - 0.0 I

95.92 96.65 96.78 96.21 95.31 96.82 95.56 95.99 95.83 96.31 95.90 95.91

3.48 3.49 3.49 3.50 3.51 3.46 3.23 3.23 3.21 3.06 3 . ~ 3 . ~ 0.52 0.51 0.51 0.50 0.49 0.54 0.77 0.77 0.79 0.94 0.94 0.94

0.01 - 0.01 0.01 0.01 0.01 0.01 0.01 o . o i - - - 0.15 0.14 0.14 0.17 0.16 0.13 0.11 0.12 0.08 0.02 0.02 0.02 0.36 0.30 0.30 0.35 0.37 0.28 0.11 0.12 0.11 0.01 0.02 0.02 0.63 0.72 0.66 0.63 0.75 0.66 0.67 0.67 0.64 0.06 0.07 0.05 0.02 0.02 o.oi 0.02 0.03 0.02 0.06 0.08 0.12 0.84 0.79 0.82

0.01 0.01 0.01

1.60 1.63 1.64 1.58 1.54 1.66 1.84 1.83 1.86 1.98 1.99 1.98

- - - - - - - - -

6.77 6.81 6.76 6.76 6.86 6.76 6.80 6.83 6.82 6.92 6.90 6.90

170

7 0 -

-

5 0

-

30 -

-

10 -

R. Sandrone et al.

% c e l a d .

a

-5, a

**

a . a

*.,

.9

. 7

.6

b

% I I I I I I I I I I

10 30 5 0 7 0 90

% p a r a g . Fig. 4. Celadonite vs. paragonite contents (a) and potassium vs. sodium contents per formula unit (b) of white micas from a Val d'Ala micaschist (triangles: phengite I: points: phengite 11; open circles: paragonites). The three groups of white micas appear to be clearly different in celadonite-content (a). as well as in sodium content (b).

Table 6. Representative microprobe analyses and atomic proportions (basis 23 oxygens) for Na-amphiboles from eclogitic rocks (analyses 1-3: meta-FeTi-gabbros; 4-7: meta-FeTi-basalt) and metamorphic albite-granite (analyses RIO)

Analysis 1 2 3 4 5 6 7 8 9 10

SiOz TiO? A1201 FeO, MnO

CaO Na2O

Cr203 Total

Si Al Al Ti Fez+ Fe3+ Mn Mg Ca Na K Total

MgO

KZ0

58.54 57.99 57.93 55.90 56.46 56. I6 56.67 57.74 58.03 S8.22

10.00 9.13 8.60 7.63 7.93 7.19 7.73 9.15 8.70 8.94 9.93 10.60 10.73 15.93 16.80 15.98 15.80 13.06 11.69 11.71

0.06 0.07 0.03 0.07 - - - 0.09 0.07 -

0.05 0.09 - - - - - 0.03 - - 12.22 12.45 12.02 10.30 9.16 10.55 9.90 9.99 11.00 10.94 0.35 0.61 0.74 0.54 0.82 mi 0.37 0.20 0.22 0.27

0.09 0.02 0.08 0.03 0.04 0.03 0.07 0.12 0.10 0.13 6.93 7.10 8.38 7.42 6.59 7.35 7.30 7.90 8.22 8.07

98.12 98.02 98.60 97.82 97.80 97.88 97.84 98.25 98.09 98.28

7.98 7.96 7.97 7.93 0.02 0.04 0.03 0.07 1.59 1.44 1.36 1.20 0.01 0.01 - 0.01 1 . 1 1 1.16 1.17 1.80 0.03 0.06 0.06 0.09 - 0.01 0.01 - 2.48 2.55 2.46 2.18 0.05 0.W 0.1 I 0.08 1.83 1.89 2.23 2.04 0.02 - 0.01 -

7.99 7.98 0.01 0.02 1.31 1.18

1.83 1.90 - -

0.16 - - - 1.93 2.23 0.12 0.09 1.81 2.02 0.01 -

7.99 7.96 0.01 0.04 1.27 1.44 - 0.01 1.68 1.13 0.18 0.38 - _ 2.08 2.05 0.06 0.03 2.00 2.11 0.01 0.02

7.98 7.98 0.02 0.02 1.39 1.43 0.01 - 1.01 1.01 0.34 0.33 - - 2.25 2.24 0.03 0.04 2.19 2.14 0.02 0.02

15.12 15.21 15.41 15.40 15.17 15.42 15.28 15.17 15.24 15.21

P-T conditions, Val d’Ala di Lanzo 171

F e

0 Y + a I + C

- > -

I I I I I I I I I

3 .O 3.5 Fig. 5. Ternary diagram AIV’-Fe’+-Mg (a) and octahedral occupancy (AIV’+ Mg+Fe”) vs. Si content (b) of white micas from an ophiolitic cover micaschist of Val d’Ala (symbols as in Fig. 4). The dioctahedral nature of white micas is evident from both diagram a and b: while diagram b shows the increase of the celadonite-substitution from paragonite to phengite I’ through phengite 11.

glaucophanes from meta-FeTi-basalt (range 0.00-0.11) than those from meta-FeTi-gabbro (range 0.02-0.05), whereas in glaucophanes from the jadeite +quartz-bearing metagranite i t is close to 0.33. Meta-FeTi-gabbro glaucophanes also exhibit higher Mg-contents (average 2.50 instead of 2.10 and 2.18 atoms per formula unit) and Mg/(Mg+FeZ+) ratio (average 0.69 as opposed to 0.54 from meta-FeTi-basalt and metamorphic albite-granite, respectively).

METAMORPHIC CONDITIONS

The metamorphic conditions during the early- Alpine event in the ophiolite of Val d’Ala and its

cover can be estimated by using the petrogenetic grid shown in Fig. 6. This has been constructed from both experimental curves and curves derived from geothermometric calculations.

The albite = jadeite+quartz reaction has been calibrated by many investigators. Except for the data of Newton & Smith (1967) and Newton & Kennedy (1968). all data were obtained at T s 600°C. Curve 1 in Fig. 6 is extrapolated from the most reliable P value at 600°C (= 1.63 GPa; cf. Johannes. Bell, Boettcher, Chipman, Hays, Mao, Newton & Seifert. 1971; Hays & Bell, 1973; Holland, 1980). assumingadPldTslopeof 1.87 MPa/”C, calculated from the thermodynamic data of Robie, Hemingway & Fisher


(1978) for low albite and quartz and Holland (1980) for jadeite. This curve can be regarded as a good approximation for the conditions under which the albite-granite was metamorphosed, bearing in mind the fact that the jadeite composition is close to that of the pure end-member. It also provides a lower pressure limit for the eclogite event.

Maresch (1977) has proposed a temperature of about 550°C as the upper stability limit for glaucophane. This value (line 2, Fig. 6) can be assumed as the upper T limit for the high pressure re-equilibration.

Lawsonite-out/zoisite-in reactions have been calibrated by Newton & Kennedy (1963), Nitsch (1972, 1974) and Chattejee. Johannes & Leistner (1984). The lawsonite = zoisite+

1.5

1.0

kyanite+quartz+HzO equilibrium (line 3. Fig. 6) has been studied by Newton & Kennedy (1963) under high P conditions (1.2-3.3 GPa) and by Nitsch (1972) under lower P conditions (0.4-0.7 GPa). Nitsch (1974) has since calibrated the lawsonite = zoisite+margarite+ quartz+H20 reaction between 0.4 and 1.0 GPa (line 4). These curves assume P H ~ o = PI,,,. Fry (1972) has shown on theoretical grounds that lawsonite breaks down at a lower temperature if PH,o< Pt,,c. On the basis of new experimental data in the P range of 1.0-2.0 GPa Chatterjee el al. (1984) suggest lower T for the above reactions (lines Sa and 5b of Fig. 6, respectively).

The metamorphic temperatures can also be estimated from the compositions of white micas and the partitioning of Fe2+ and Mg between

400 5 0 0 6 0 0 Fig. 6. Petrological grid showing the minimum P-T conditions (shaded area) inferred for the early-Alpine eclogitic re-equilibration in the Val d'Ala rocks. The following equilibrium lines (full lines: experimentally determined curves: dashed lines: extrapolated curves; dotted-dashed lines: geothermometric determinations) are reported: albite =jadeite+quartz ( I ) ; upper stability limit for glaucophane (2); lawsonite =zoisite+kyanite+quartz+H20 (3 and Sa); lawsonite = zoisite+margarite+quartz+HIO (4 and 5b): isotherms calculated with the muscovite-paragonite solvus curve (6a. 6b. 6c): garnetklinopyroxene average equilibrium curves (7 and 8); garneuphengite average equilibrium curve ( 9 ) . See text for further details and references.

P-T conditions, Val d'Ala di Lanzo 173 coexisting garnet4inopyroxene pairs in the eclogites and garnet-phengite pairs in the metasedimentary cover.

Eugster. Albee, Bence, Thompson & Waldbaum (1972) have outlined the muscovite- paragonite solvus curve between 300°C and 600°C at 0.207 GPa with Pl,,,= PH,O. Use of this calibration for the Val d'Ala rocks is rendered uncertain, both because the effect of the celadonite molecule on the solvus curve is unknown and because t h e experimental P value is at least 1 GPa lower than that expected for these rocks (see also discussion by Maresch & Abraham, 1981). In view of the work of Chatterjee & Froese (1975) and Blencoe (1977). Maresch & Abraham (1981) suggest that the temperature obtained be taken as a minimum value. The K/(K+Na+Ca) ratio of >0.95 for phengite I suggests inexplicably lower temperature values (see Fig. 9 of Eugster eral.), elsewhere observed for high pressure re-equilibrations (cf. Laird & Albee, 1981). By contrast, phengite 11, which was probably generated during decompression, displays a chemistry closer to that of muscovite, and has a K/(K+Na+Ca) ratio of 0.85-0.92, which corresponds to temperatures of 30& 450°C. These higher values can be regarded as minima for the high pressure event (line 6a of Fig. 6). since all the f-T trajectories proposed for the post-eclogite metamorphic evolution in the Western Alps suggest that the considerable release of pressure was accompanied by a slight fall in temperature (e.g. Ernst & Dal Piaz, 1978; Ernst, 1979, 1981; Williams & Compagnoni, 1983). Lastly, paragonites, with their 0 .06408 molar muscovite contents, point to temperatures in the range 480-550°C (lines 6b and 6c of Fig. 6).

The garnet-clinopyroxene geothermometer has been calibrated by many investigators. Only the calibrations of Riheim & Green (1974) and Ellis & Green (1979). however, were obtained for eclogiticcompositions. Thecalibration conditions used by Riheim 8i Green were2.04.0GPa and 600-1500"C. Linear extrapolation of their equation to the Val d'Ala eclogites, therefore, is clearly risky owing to these higher P and Tcalibration values. The Ellis & Green geothermometer has the advantage that Ca activity is considered. However, both the lowest values of P (2.4 GPa) and T (750°C) used in calibration are even higher, so that extrapolation is even more risky.

Temperatures were calculated by both methods for fourteen pairs of fine-grained garnets and clinopyroxenes, whose contiguity suggested the attainment of equilibrium. The

near-rim representative analyses given in Tables 1 and 3 were used. The mineral pairs yield KD (=(Fe/Mg)garnet/(FelMg)clinopyroxene) values in the range 20-40 (see Fig. 7). At a confining pressure of 1.3 GPa, as suggested by line 1 of Fig. 6, the temperatures given by the two calibrations are in good agreement: there is a difference of 15°C (449°C and 464°C) in the averages. The difference in temperature determined from the same pairs is 440°C. except for No. 6 and No. 8 (Tables 1 and3) which have high XF&cl (0.28) and a difference in calculated Tof more than 60°C. The average values given by the calibrations of Ellis & Green and Riiheim & Green are shown as curves 7 and 8, respectively, on Fig. 6. The relationships between KD values and Ca contents in garnet are shown on the lnKD vs. X&et diagram of Fig. 8. This has been redrawn employing the Ellis & Green equation at 1.3 GPa. Representative points cluster around the 450°C isotherm and a mean nominal temperature of 464°C (standard devia- tion rf: 37°C and range 428-541°C) is obtained for the eclogitic recrystallization of the Val d'Ala FeTi-metabasics with 1.3 GPa as the confining pressure.

The garnet-phengite geotherrnometer has been calibrated by Krogh & RIheim (1978) and Green & Hellman (1982). Krogh & Riheim have calibrated their equation in the ternpera- ture range 700--1000°C at 3.0 GPa on basaltic composition and have also tested it for garnet/ phengite pairs from schists enveloping phengite- bearing eclogites. Green & Hellman have studied the Fe/Mg exchange reaction of coexisting garnet and phengite, in the presence of quartz and water, at 2.0-3.5 GPa and 800- 1OOO"C; this shows the reaction to be dependent on the Ca-content of the garnet and on the mg (= 100 Mg/(Mg+Fe)) of the bulk composition. The use of these two thermometers for the cover of the Val d'Ala ophiolites is thus subject to similar extrapolation difficulties as the garnet- clinopyroxene geothermometers.

Temperatures have been calculated by these two methods for six pairs of contiguous garnet and phengite grains from a micaschist. The first six analyses in Tables 4 and 5 were used. The mineral pairs yield KD (=(Fe/Mg)garnet/(Fe/ Mg)phengite) values ranging from 34.6-37.9 and the representative points are shown on the (Fe/Mg),,, vs. (Fe/Mg)pheng diagram of Fig. 7. For a confining P of 1.3 GPa, the equation of Green & Hellman for a low Ca-system with mg=67, which closely corresponds to the chemical characters of the Val d'Ala micaschist analysed, gives temperatures ranging from


Fig. 7. Plot of analysed garnedomphacite pairs (dots) and garnet/phengite pairs (triangles) on a (Fe/Mg)garnet vs. (Fe/Mg)ciinopyroxene diagram for the eclogites and (Fe/Mg)garnet vs. (Fe/Mg)phengite diagram for a micaschist.

0.1 0.2 0.3 0.4 Fig. 8. Plot of representative analysed garnet/omphacite pairs from the Val d'Ala eclogites on a InK,, (garnetlclinopyroxene) vs. k$Lc, diagram (redrawn employing the equation of Ellis & Green (1979) at a constant 1.3 GPa pressure value).

454°C to 462°C (average 459°C). These are close to the 464°C and 449°C average temperatures determined for garnetlclinopyroxene pairs from the eclogites. By contrast, the equation of Krogh & Rhheim (1978) yields temperatures c. 80°C lower. These values are certainly less reliable than those obtained with the Green & Hellman

equation, since the calibration of a system with a very different chemistry is involved. The average equilibrium curve obtained by the Green & Hellman calibration is numbered 9 in Fig. 6.

Summing up, therefore, curve 1 in Fig. 6 and the best fit for the temperatures offered by the

P-T conditions, Val d'Ala di Lanzo 175

various geothermometers point to 1.3 GPa and 450460°C as the minimum conditions for the eclogitic climax in the Val d'Ala.

DISCUSSION AND CONCLUSIONS

The minimum pressure conditions suggested here for the eclogitic climax are based on one of the reactions that has been most widely studied from both the experimental and the theoretical standpoint and which is thought to have a good degree of reliability. The estimate of temperature is thought to be equally reliable. Similar values are obtained from two garnet- clinopyroxene thermometers in the eclogites and by a garnet-phengite thermometer in the cover metasediments. Further corroboration is obtained from the muscovite-paragonite solvus curve of Eugster et al. (1972), and the glaucophane (Maresch, 1977) and chloritoid (Rao & Johannes, 1979) stability fields.

There is, however, a conflict with the lawsonite stability data (curves 3, 4 and 5 , Fig. 6), which show this mineral as a stable phase under the inferred P-Tconditions. In Val d'Ala, on the other hand, zoisite is the stable phase together with the high pressure minerals. Evidence of lawsonite is only present as pseudomorphs con- sisting of zoisite+white mica in some of the cover calcschists. The most plausible explanation is that the PH,O was not the same in the different lithologies. Since the lawsonite-out/ zoisite-in curves shift towards lower temperatures when PHIO<Ptl,, (Fry, 1972), it may reasonably be inferred that zoisite formed in the more anhydrous lithologies (especially meta- basites) and that lawsonite developed, at the same time, where f H , O - f r , , , . Destabilization of lawsonite then occurred during the subsequent decompression stage.

Our P-T values agree with those reported by Pognante (1981,. 1982) for the coronitic associations of the meta-FeTi-gabbros of the nearby Orsiera-Rocciavre ophiolite-klippe. Messiga et al. (1983) have used the Ellis & Green geothermometer on meta-Mg-gabbros to propose a mean temperature of 508°C for the eclogite event in the Voltri Group. Their data also include the temperature estimate from an 'inter- mediate' metagabbro in which estimates of Tare 4 2 2 4 0 ° C (average 431°C). Previously, Ernst & Dal Piaz (1978), using the RBheim & Green geothermometer, had suggested a mean temperature of 470°C (for P = 1 .O 2 0.2 GPa) for the eclogitic rocks of the Breuil-St Jacques area (Aosta Valley). The same analyses were then used by Ernst (1981) to calculate a mean tem-

perature of 527°C at the same P with the Ellis & Green thermometer. The study of Oberhaensli (1980) of eclogitic pillow-basalts from Pfulve and Rimpfischhorn (Zermatt-Saas Zone, Valais) indicates a mean P of 1.4 GPa and a mean T of 600°C (1980). The range of values which he obtained (1.0-1.6 GPa and 4W700"C) are regarded as representative of the f-T path during early-Alpine re-equilibration. Similar conditions (1 .O-I .5 GPa and 500-700°C) have been suggested by Chinner & Dixon (1973) from paragenetic evidence for the well-known Allalin gabbro (Bearth, 1967).

To compare the temperature data from the different authors in an internally consistent way, KD, X:&.,,,, (and hence r ) were recalculated from the above literature data using the crystal- chemical methods employed in this paper. Re- calculation of the Ernst (1976) KD values for the Voltri Group gives a range of 22.3-53.7 (average 36.1). This is substantially in agreement with the Val d'Ala range: 20.6-39.9 (average 27.6). The Breuil-St Jacques area and Zermatt-Saas Zone, on the other hand, gives much lower values: range 5.8-14.5 (average 11.5). The nominal temperature obtained with the different methods are compared in Table 7. In agreement with the corresponding mean K D , they show that the re-equilibration temperature rises in the direction of the Central Alps. The Zermatt-Saas Zone eclogites, in fact, give values about one hundred degrees higher than those of other areas.

Taken as a whole, the data for the Piemontese ophiolites show that nominal temperature values calculated using the RBheim & Green geothermometer are more uniform for samples from the same area than for the several areas examined. They are, in fact, usually lower than those calculated using the Ellis & Green geothermometer. Irrespective of the method used, however, there is a distinct increase in temperature from Liguria to the Valais. The difference of 124°C between the means obtained from meta-FeTi-gabbros by Ernst (1976) (384°C) and from meta-Mg-gabbros by Messiga et al. (1983) (508°C) is too high to be reasonably accepted. It is thus more logical to accept the nominal temperatures offered by the RAheim & Green method, since this gives 436°C and 403°C respectively for the different, but intimately associated, Mg- and FeTi-gabbros of the Voltri group, i.e. an acceptable difference of only 33°C. The conclusion to be drawn is that the RBheim & Green method provides values that are closer to reality, at least for this P-Trange.

An alternative explanation, however, may be

Tab

le 7

. K

,, a

nd n

omin

al t

empe

ratu

res

for t

he e

clog

itic

re-e

quili

hrat

ion

of t

he W

este

rn A

lps

met

abas

ics.

Ana

lytic

al d

ata

from

the

lite

ratu

re a

re

reca

lcul

ated

acc

ordi

ng t

o t

he p

roce

dure

fol

low

ed i

n th

is p

aper

Are

a Pr

otol

ith

~~

Fe T

i -ga

bbro

s

'Inte

rmed

iate

' ga

bbro

Fe T

i - ga

bbro

s

Vol

tri G

roup

M

g - ga

bbro

s

Bre

uil - St

Jac

ques

(Z

erm

att)

Zer

mat

t Fe

Ti - ba

salts

V

al d

'Ala

Fe

Ti - ga

bbro

s Fe

Ti - ba

salt

Tem

pera

ture

s ("

C) c

alcu

late

d ac

cord

ing

to t

he g

eorh

erm

omet

ers o

f

K,,

R

Hhe

im &

Gre

en

Elli

s &

Gre

en

Ref

eren

ces

Ave

rage

R

ange

Ern

st (

1976

; 19

81)

36. I

22

.2-5

3.7

Mes

siga

ct a

/. (

1983

) 26

.4

22.6

29.3

M

essi

gn e

t a/.

( 19

83)

30.5

28

.4-3

2.5

Ern

st &

Dal

Pia

z (1

978)

I I

.5

5.8-

14.5

E

rnst

(19

81)

Obe

rhan

sli

(198

0)

15.1

5

.63

8.2

Thi

s pa

per

27.6

20

.639

.9

Ave

rage

R

ange

403

356-

458

4.3b

42

24

55

41

8 4

1M

26

581

520-

698

557

391-

706

449

40

14

83

Ave

rage

R

ange

384

338-

449

508

483-

544

431

422-

440

625

561-

771

59Y

38S

747

464

428-

541

I .(I I .3

P-T conditions, Val d'Ala di Lanzo 177

derived from the experimental demonstration (Green & Ringwood, 1967) that basaltic rocks with a higher Fe/Mg ratio (i.e. FeTi-basalts in the present case) develop eclogitic parageneses at lower pressures than normal basalts. It could, therefore, be argued that the meta-FeTi- gabbroshasalts record an earlier stage (i.e. lower P-T) of the subduction process than meta-Mg-gabbros.

Whichever calibration is used, however, there is clearly a rise in the re-equilibration temperature from the Aosta Valley to the Valais, probably also accompanied by an increase in pressure (Oberhaensli, 1980). Similar conclusions emerge from the high-pressure parageneses in the internal Penninic basement, i.e. the Gran Paradiso and Monte Rosa crystalline massifs (cf. Chopin. 1984, with references therein).

ACKNOWLEDGEMENTS

Constructive reviews of the draft manuscript were obtained from J. Demons. University of Nancy, and A. Mottana, University of Rome. Mineral analyses were carried out by means of the ARL SEMQ instrument installed and main- tained by C.N.R. (National Research Council) at the Department of Earth Sciences of the Uni- versity of Milan. I. Memmi provided assistance with the SEM analyses performed on jadeite at the Department of Earth Sciencesof the Univer- sity of Siena. This study was carried out with the financial support of C.N.R. (Centro di Studio per i Problemi Minerari and Centro di Studio sui problemi dell'orogeno delle Alpi Occidentali, Turin) and M. P. I. (grant 40% to R. Sandrone). The authors are grateful to these persons and in- stitutions for their help.

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