An eclogite paragenesis from the Aleksod basement, Central Hoggar, south Algeria

Chemical Geology, 50 (1985) 331--347 Elsevier Science Publishers B.V., Amsterdam - - P r i n t e d in The Netherlands

331

AN ECLOGITE PARAGENESIS FROM THE ALEKSOD BASEMENT, CENTRAL HOGGAR, SOUTH ALGERIA VIOLAINE SAUTTER Laboratoire de Mindralogie, Musdum National d'Histoire Naturelle, C.N.R.S. (Centre National de la Recherche Scientifique) Laboratoire Associd L A No. 286, 75005 Paris (France) (Accepted for publication October 23, 1984)

Abstract Sautter, V., 1985. An eclogite paragenesis from the Aleksod basement, Central Hoggar, south Algeria. In: D.C. Smith, G. Franz and D. Gebauer (Guest-Editors), Chemistry and Petrology of Eclogites. Chem. Geol., 50: 331--347. Eclogite pods and metadolerite dykes occur within the central part of the Hoggar Precambrian shield, a polycyclic domain (Eburnean event: 2000 Ma; early Pan-African event: 750--600 Ma; late Pan-African event: 550 Ma). Field relations indicate that the eclogite pods are located within mylonitic zones of early Pan-African age, underlying the contact between metasedimentary and orthogneissic rocks. Dolerite dykes crosscut only the orthogneissic basement. From textural and mineralogical observations of these pods, a progressive decrease of the P--T regime during a progressive increase of shearing are obvious and correspond first to foliated hydrous eclogite appearance (pods core; 750°C; Pmin = 15 kbar) then to fotiated amphibolite formation (pods margin; 650°C; P = 4.5 kbar). Late-Eburnean dykes, showing relics of dolerite textures, are folded during the Pan-African event. This foliation corresponds to corona-forming reactions which transform primary magmatic assemblages into garnet-amphibolite parageneses. Just inside the margin a retrogression towards amphibolite-facies conditions is observed. So we must consider that the eclogitisation process occurred only inside the zone of shearing because the eclogite pods and metadolerite dykes have not had the same origin and the same evolution during the Pan-African tangential tectonic event. Eclogite pods, which crystallized at depth have been overthrusted towards supracrustal levels, whilst dolerite dykes, which cooled near the surface, have been buried towards amphibolite-facies levels.

1. I n t r o d u c t i o n The central part of the Hoggar, south Algeria, is formed by a Precambrian polymetamorphic basement. Bertrand et al. (1972) defined two different zones (Fig. 1): (1) Aleksod area: domal complexes where orthogneiss and grey banded plagioclase gneiss prevail; (2) Serkout area: mainly metasedimentary formations. The domal complexes are crosscut by

garnet amphibolite dykes bearing relict dolerite textures. In the contact zones between the orthogneissic rocks and the metasedimentary formations, basic pods with eclogite relics are to be seen. Bertrand (1974), explaining the relations between these two structures as being those an Eburnean basement (1940 + 40 Ma; Bertrand and Lasserre, 1973, phase P0) with its later cover, suggested the hypothesis of a c o m m o n origin for these different types of metabasic rocks whether occurring now in

332

shear zone in the north Serkout area on one hand, and on the other hand of those dykes which crosscut the Tala-Mellet dome (north Aleksod area; Fig. 1). 2. The eclogite pods in the north Serkout area

2.1. Field relationships In the northern Serkout area (Fig. 2) there are various zones: I -- To the NNW orthogneiss prevails, associated with pelitic gneiss, amphibolite and banded pyroxenite. The whole formation t o o k a recumbent folded shape during phase P~. In that structure, continuous amphibolite layers do not include eclogite relics. II and III -- To the SSE the rocks (essentially metasediments) are much more

'r I

Fig. 1. S k e t c h m a p o f t h e A l e k s o d a n d S e r k o u t areas from Bertrand (1974). 1 = domal gneiss complexes; 2 = m e t a s e d i m e n t a r y r o c k s ; 3 = e c l o g i t e lenses; 4 = P h a r u s i a n f o r m a t i o n (P3); 5 = i n t r u s i v e c o m p l e x (P3); 6 = e a r l y d e e p t h r u s t (P1 + P2); 7 = l a t e t h r u s t (P3). N S K = n o r t h e r n p a r t o f S e r k o u t ( S . K ) ; T . M = TalaMellet d o m e ( A l e k s o d area). S o l i d lines are late f a u l t s (P~).

pods or in dykes: t h e y might be post-Eburnean intrusives formed during a first highpressure phase (P1) during the Pan-African (s.l.) polyphased event (P1 + P2). Boullier and Bertrand (1981) considered the discontinuity existing between the basement and presumed cover as an early thrusting zone which has brought together different structural levels. Thus the relations between the eclogite-forming process and great shear thrusts pose the following question: Was the acquisition of high-pressure parageneses a prevailing feature, linked with lithostatic pressure generated by the piling of nappes, or was it localized only inside zones of shearing? In an a t t e m p t to solve this problem, a petrological study has been undertaken of basic pods regularly scattered over a large

~

t

4

,, ! 2X4,

i

~3

e.I. ~ 7

~

e.I.

o'. ~,'fl~1 "

J~tl ~//I

L

1kin i

i

Fig. 2. G e o l o g i c a l m a p o f t h e n o r t h e r n p a r t o f Serkout from J.M.L. Bertrand (unpublished data, 1 9 8 1 ) . 1 = a m p h i b o l i t i c gneiss a n d l e p t y n i t e s ; 2 = b a n d e d gneiss; 3 = p a r a g n e i s s , p y r o x e n i t e s ; 4 = m a r b l e ; 5 = i n t e r b e d d e d a m p h i b o l i t e s ; 6 = P~ m y l o n i t e ; 7 = limit o f o u t c r o p s ( s o l i d line) w i t h e r o s i o n a l c h a n n e l s ( d a s h e d line), e.l. : e c l o g i t e lenses. F o r Z o n e s I, II, III, I V see t e x t .

333

deformed than in zone I. From zone II to zone III, a gradually increasing strain is observed. Inside zone II, a kyanite S~ foliation, overprinted on Eburnean migmatites (M0), is transformed into a sillimanite-containing foliation $2. Inside zone III, heavily-deformed rocks make up a blastomylonitic band extending NNE--SSW, ~ 200 m thick. The shearing stresses ($1 + $2) have reworked the Eburnean migmatites. In this zone, a series of basic pods occur, varying in length from 0.2 to 500 m, which are quite different from the interbedded amphibolites because of the presence of eclogite relics. These basic pods, whatever their size may be, show all the same morphological and structural characteristics. The m a x i m u m length axis of the boudins coincides with the mineral lineation of the enclosing gneiss with an average orientation of N50 ° (Fig. 3). The flattening surface of the pods is parallel to the subhorizontal foliation in the band (angle of dip: S10°E). The breaking up of the basic material into the shape of boudins is therefore synchronous with the $2 shearing. The pods show two strain feetures. On one hand they are sheared in their margin conforming to the enclosing gneiss foliation $2. On the other hand, they show an oblique internal foliation, drawing S shapes, which is thus older ($1 ?). IV -- Finally, to the West a submeridian 1 m I

I

---

_

.~o

o

~

-~

~

~

Fig. 3. S c h e m a t i c s k e t c h o f e c l o g i t e l e n s e s . 1 = e c l o g i t e c o r e (oblique solid lines: S~ f o l i a t i o n ) ; 2 = garnet-amphibolite margin; 3 = schistose-amphibolite m a r g i n in c o n t a c t w i t h m y l o n i t i c g n e i s s .

fracture consists of vertical mylonites, which appeared in greenschist facies conditions (phase P3)-

2. 2. Petrography Those basic pods whose length is more than 3 m show two different concentric zones (Fig. 3). An eclogite core, more or less preserved according to the size of the boudins, is surrounded by a darker amphibolitised margin. In the innermost part of that margin, garnet amphibolites are found. The outermost part consists of foliated amphibolites.

2.2.1. Eclogite cores The most c o m m o n mineral association is the following: garnet + omphacite + quartz + zoisite ± amphibole. Accessory phases are rutile, zircon, apatite and rarely sulphides (chalcopyrite and pentlandite). Within this mineral assemblage, which is assigned to the hydrous eclogite facies (Smith, 1980), the a m o u n t of strain and the number of hydrous minerals vary. It is further subdivided into four types whose characteristics are compiled in Table I. Type 1. The eclogites are banded and show alternating coarse- and medium-grain layers with small and large amounts of garnet, respectively. Omphacite, the other major phase, is often replaced by a pale-green amphibole. Quartz either crystallizes at the grain boundaries of the minerals or is found inside omphacite as an exsolution product (cf. Smith and Cheeney, 1980). The absence of zoisite in this type is to be noted. As quartz and colourless amphibole seem to be of later age, the primary mineral assemblage was probably an anhydrous eclogite. Type 2. The eclogites are sheared. They crosscut the banded ones and preserve yet undeformed garnets mad omphacites. These eclogites are characterized by a reduction of the size of the garnets, a new crystallisation of omphacite in prisms elongated along the c-axis, and finally by the appearance of small m-type zoisite crystals (dispersion r < v accor-

334 TABLE I Summary of the main petrographic features which permit a distinction of four types of eclogite, with the mineral abbreviations used here and in other tables and figures Type

Structure

Grain size

Mineralogy with mode

1

banded eclogite

coarse < 1 cm medium = 1 mm

Gt~sOm25Qz20Amlo Gt60Om~0QzsAm~

2

sheared eclogite

fine

< 1 mm

Gt~0Om~0Qz~a-Zo~0Am ~

3

orientated concentration

coarse

> 1 cm

~-Zo~0Qz~Om~

4

banded eclogite

coarse > 1 cm medium > 1 mm

Gt~-Zo~sQz~Am~ Gt~0Zo2sQz~Am~0

Am = amphibole; Gt = garnet; Mt = magnetite; Om = omphacite; P1 = plagioclase; Px = clinopyroxene; Qz = quartz; Sym = symplectite; ~-Zo = ~-zoisite; ~-Zo = ~-zoisite. TABLE II Chronological relationships between the four eclogite types

"~

l ,%

Deformation

Mobile elements

Type

Early shear S 1

H20, SiO 2

type I without Qz and Am type 2 with a-Zo

Late fracturing

H~ O, SiO~, CaO, Fe 203, K 2O

ding t o Termier, 1898). Discontinuous elongated quartz crystals u n d e r l i n e the shearing f o l i a t i o n o f t h e r o c k , as d e f i n e d b y the o r i e n t a t i o n o f o m p h a c i t e a n d zoisite. A pale-green a m p h i b o l e crosscuts this foliation. Type 3. I t is c h a r a c t e r i z e d b y f e w centim e t r e s large c o n c e n t r a t i o n s o f o r i e n t a t e d zoisite. T h e y are f o u n d in t h e c o n t a c t z o n e s b e t w e e n t h e coarse- a n d m e d i u m - g r a i n layers o f b a n d e d eclogites ( t y p e 1) a n d t h e y c r o s s c u t t h e s h e a r e d eclogites ( t y p e 2). U n l i k e t h e f o r m e r , t h e s e zoisites are of/3 t y p e (dispersion r > v). S p r i n k l e d w i t h small inclusions o f quartz, they sometimes happen to contain also g a r n e t , o m p h a c i t e a n d / o r a m p h i b o l e associated w i t h t h e q u a r t z . T h e s e c o n c e n t r a t i o n s , u n u s u a l f r o m t h e c h e m i c a l p o i n t o f view, have t h e r e f o r e c r y s t a l l i z e d in t h e eclogite facies. Type 4. T h e eclogites are b a n d e d a n d rich in zoisite. S i t u a t e d n e a r t o t h e t y p e - 3

type 3 with ~-Zo type 1 ~ type 4 + later Am

eclogites, t h e y f o r m a r e a c t i o n z o n e , a few c e n t i m e t r e s t h i c k , a t t r i b u t e d t o a massive h y d r a t i o n o f t h e t y p e - / eclogites. T h e o r d e r in w h i c h these various t y p e s a p p e a r is i n t e r p r e t e d as t h e result o f t w o successive d e f o r m a t i o n stages, m a r k i n g a m o r e a n d m o r e i m p o r t a n t o p e n i n g u p to fluids (Table II). T h e d e d u c e d s e q u e n c e o f events is: (1) T h e f o r m a t i o n o f an a n h y d r o u s eclogite (type 1 without quartz and amphibole). (2) An early shearing e v e n t ($1) leads to t h e t r a n s f o r m a t i o n o f a n h y d r o u s eclogite i n t o h y d r o u s - e c l o g i t e b l a s t o m y l o n i t e w i t h a-zoisite ( t y p e 2). (3) A l a t e r r y t h m i c f r a c t u r i n g stage leads to ~-zoisite c o n c e n t r a t i o n ( t y p e 3). I t implies an i m p o r t a n t w a t e r , Ca a n d Si m o b i l i t y at a m i l l i m e t r e scale a n d locally t r a n s f o r m s t y p e - / eclogite into t y p e 4. (4) An even l a t e r p o s t - k i n e m a t i c a m p h i b o l e appears.

335

2.2.2. Amphibolitisation in the rim From the core to the rim of the pods, different degrees of amphibolitisation are seen (Table III). First, hydrous eclogite (type 2, superscript I) was transformed into garnet amphibolite (inner rim, superscript II), and then this was transformed into foliated amphibolite (outer rim, superscript III). This evolution has been followed step by step only from the observation of sheared eclogites (type 2) for t w o main reasons: (a) Relics of the initial paragenesis makes it possible to appreciate the static aspect of the first retrogressive stage (I -+ II). (b) The low proportion of h y d r o u s primary minerals allows us to evaluate how much the pods are open to water during the retrogression.

(a) a first stage with fine-grained reaction textures (stage IIa in Table III) forming patches at the core of the pods; during this stage the first plagioclase was formed, and amphibole may or may not form; (b) a second stage with coarse-grained reaction textures (stage IIb) during which amphibole was more and more developed. Towards the rim the appearance of the garnet amphibolites II is the result of the coalescence of these features and the clinopyroxene Ilb disappearance. During the initial stage (IIa), the locally different chemical composition exclusively controls the observed transformations. When the reaction sites are rich in quartz, the anhydrous characteristic of the retrogression is due to (Fig. 4): (a) the disappearance of zoisite; (b) a partial reaction of garnet and amphibole into a plagioclase and clinopyroxene assemblage. During the same time the omphacite breaks down into symplectites. The sum of transformations which are observed at the grain

2.2.2.1. The transformation into garnet amphibolite. The intense textural imbalance, characterizing the transformation of eclogites into garnet amphibolites without any deformation, allows the recognition of two stages (Table III; Fig. 4): T A B L E III

The d i f f e r e n t stages o f r e t r o g r e s s i o n o b s e r v e d f r o m the core to t h e rim o f a d e c a m e t r i c eclogite p o d Zone

Retrogression anhydrous

I

v Zo , \

IIa

hydrous

i Am

\

iPl

z0 \

Gt c

I

~Pl

Om~ I

Px /

Gt r

Gt c,

I I

Sym

Omr I A m Gtr I

I

IIb

Pl

I ......

Px

t

Coarse Sym Am' ~

t

t

~ /

A m Pl Gt r I

/ /

II

iQz

IIIa

iQz A m P1 Mt He I

Am

PI

Gt I

z/

i /

III

Qz A m P1

Dashed lines d e p i c t t h e c o n t i n u o u s r e a c t i o n s w h i c h o c c u r b e t w e e n t h e s e d i f f e r e n t stages; curly b r a c k e t s e m b r a c e r e a c t a n t s . r and c r e p r e s e n t the rims and t h e core o f t h e minerals, respectively.

336

HYDROUS

ANHYOROUS RETROGRESSI ON

RETROGRESSION

amphibole

Px'ilb P t , a \

z o n l n gS

a,a PxHb

" ~'~="~H20

Qz

l Or,

~Aml

',Om

~Amila

Gt~'~1A'm lla

Ptllb

'

Amll~

o

l

Arnllb" ~coa'rs e sym

Fig. 4. Retrogressive reactions: a schematic sketch of the transformation of primary eclogite assemblage (type 2; lens core) into amphibolite (lens margin) (see Table III). b o u n d a r i e s t h e n a m o u n t s to a d e h y d r a t i o n reaction: Qz + Zo I + A m I + G t I + O m Px IIa + P111a + H 2 0

(R1)

On the o t h e r h a n d , at r e a c t i o n sites witho u t q u a r t z , a green a m p h i b o l e (Am IIa) also n a m e d k e l y p h i t e , b e t w e e n garnet and o m p h a cite, crystallizes. A t the same t i m e the colourless p r i m a r y a m p h i b o l e b e c o m e s d a r k e r towards its rim. Finally, at these sites, o n l y a very small c r y p t o c r y s t a l l i n e s y m p l e c t i t e is observed a r o u n d o m p h a c i t e . This retrogression is expressed b y t h e h y d r a t i o n r e a c t i o n :

Gt I + O m I -+ A m I + H 2 0 ~ A m IIa

(R2)

To sum up, the first stage o f eclogite breakd o w n o c c u r s in a s y s t e m closed to water; H~O as a p r o d u c t o f r e a c t i o n (R1) being e n o u g h f o r t h e c o n s u m p t i o n o f r e a c t i o n (R2). T h e r e f o r e , the global t r a n s f o r m a t i o n b y reactions (R1) and (R2) is isochemical. R e a c t i o n (R2) is a c o n t i n u o u s m u l t i v a r i a n t one since garnet, a m p h i b o l e and p y r o x e n e are still p r e s e n t in the s e c o n d a r y assemblage. Write it t h e r e f o r e : Gt I + O m I + Z o ! + A m I + Q z - ~ G t IIa + Px IIa + P111a + A m IIa

(R3)

337 At more advanced stage (IIb), hydrous retrogression prevails (Table III). The transformation depends no more upon primary minerals. Indeed, it is first and foremost induced by a flux of externally derived water which is demonstrated by the amphibolitised margins of the pods. In the former anhydrous retrogression sites, the zoisite has disappeared, the secondary clinopyroxene is amphibolitised and the new plagioclase is zoned. The kelyphite (hydrous retrogression) evolves by several steps. As soon as the plagioclase appears, both garnet rim--amphibole ga and omphacite rim--amphibole IIa contacts develop on their own, respectively. When amphibole Iib has formed in sufficient amount, the secondary clinopyroxenes (omphacite IIb breakdown) are isolated from the garnets. The persistence of garnet, the massive hydration and the increasing plagioclase a m o u n t thus lead progressively to garnet amphibolite II with an equant texture towards the pod margins. 2.2.2.2. The transformation into amphibolite. The breaking down of garnet amphibolites into amphibolites was observed only in the outermost zone of the pods where the rocks have a foliated texture (Table III). The transformation is characterized by a complete pyroxene amphibolitisation and garnet breakdown in favour of a blue-green elongated amphibole l/Ia in a plagioclase groundmass. Small magnetite crystals are observed around garnet pseudomorphs. The resulting amphibole--plagioclase foliation is conformable to that of the enclosing gneiss ($2), the development of which corresponds to the polymorph inversion kyanite ($1 ?; see Section 2.1) to sillimanite. Later these schistose amphibolite margins recrystallize very locally into a greenschist assemblage (actinolitic amphibole, chlorite and epidote). 2. 2.2.3. Conclusions. The two shearing events, $1 internal and $2 external, observed in the pods are synchronous with the hydrous-

eclogite facies and amphibolite facies, respectively. The completion of the reactions during $1 is obvious in the sheared pod core (anhydrous eclogite -~ hydrous eclogite), whereas during the second deformation stage, corresponding to a gradual boudinage, hydration reactions are completed only within the outermost zone (garnet amphibolite -~ amphibolite). Thus from the core to the margin of a few decametres large lens all the steps of the retrograde continuous evolution of the $1 paragenesis can be traced. During the first stages, the static recrystallizations of eclogite into garnet amphibolite were initially induced by internally-derived water and were followed by reactions necessitating externally-derived water. The final differential shear ($2), restricted to the pod margins, allowed the completion of hydration reactions which led to the foliated amphibolites. 2.3. Whole-rock chemistry Six rocks were analysed for major elements by X-ray fluorescence spectrography with the oxidation stage of Fe determined by wet chemistry (Table IV). The different eclogite types (pod core: analyses I1, I2 and I3 are characterized by relatively high Mg and Ca contents. Type-/ eclogite mineralogical banding results from a chemical differentiation: coarse-gained horizons (analysis I2) are richer in Fe and A1 than finer ones (analysis I3). These variations could be the sign of a magmatic or an earlier metamorphic differentiation. Sheared eclogites (analysis I1) have grown rich in Si. The opening of the system during this early deformation ($1) occurs also during the later retrogression. Indeed from the core to the margin (analyses II and III) the alkali proportion as well as oxidation ratio increase, while Si leaching and an inverse Fe--Mg evolution are observed. This geochemical zonation underlines the extent of basic pods-enclosing gneiss exchanges (alkali metasomatism, oxidation). Because of that contamination, the foliated amphibolite rim (analysis III) displays

338

A r m + Spe

T A B L E IV M a j o r - e l e m e n t w h o l e - r o c k c h e m i s t r y a n d c a l c u l a t e d C.I.P.W. n o r m s f o r a series o f s a m p l e s c o l l e c t e d a c r o s s a f e w decam e t r e s large e c l o g i t e p o d II

I2

I3

II

IIIa

In

SiO 2 ( w t . % ) TiO: A1203 Fe20 ~ FeOtota 1 FeO MnO MgO CaO Na:O K20 P~O~ LOI

51.87 1.05 13.89 1.31 n.d. 11.17 0.21 8.28 11.01 1.28 -0.05 -0.45

48.74 0.62 15.32 n.d. 10.81 n.d. n.d. 9.07 13.15 1.44 0.05 0.05 0.65

49.82 0.60 12.98 n.d. 9.75 n.d. n.d. 10.02 13.56 1.75 0.02 0.07 0.08

50.37 1.35 14.68 1.69 n.d. 11.79 0.21 6.51 10.39 2.37 0.33 0.12 0.31

49.71 1.06 14.57 2.82 n.d. 11.45 0.22 6.36 11.32 2.19 0.54 0.06 0.68

48.32 0.87 15.66 2.86 n.d. 9.87 0.18 7.76 10.98 2.65 1.09 0.08 1.31

Total

99.90

99.51

99.32

99.60

100.08

100.14

5.06 0.30 12.24 35.37 22.12 12.44 . . -0.38 0.38 1.03 0.11

4.28 0.12 14.91 27.69 29.51 11.44 . -0.21 9.82 1.21 0.15

1.43 1.95 20.19 28.98 19.81 23.08 . 2.48 2.60 --0.26

1.51 3.25 10.98 28.98 23.09 27.84

-6.56 22.93 28.27 21.86 0.88 14.17 4.23 1.69 --0.17

Pyr

Gro so

B

C.I.P.W. norms:

Qz Or Ab An Di Hy Ol Mt I1 He Sp Ap

6.24 -10.91 32.52 18.33 27.97 . 1.90 2.01 --0.13

4.18 2.04 --0.13

I1 = s h e a r e d e c l o g i t e ( t y p e 2); I2 = b a n d e d e c l o g i t e ( t y p e 1), c o a r s e - g r a i n e d level; I3 = b a n d e d e c l o g i t e , m e d i u m - g r a i n e d level; II = g a r n e t a m p h i b o l i t e ( i n n e r m a r g i n ) ; n I a = p a r t l y retrograded garnet amphibolite (centre margin); III = schistose amphibolite (outer margin). LOI = H20 - 0.11 FeO wt.%. Normative minerals: Ab = albite; An = anorthite; Ap = aplite; Di = d i o p s i d e ; He = h e m a t i t e ; H y = h y p e r s t h e n e ; I1 = i l m e n i t e ; Mt = m a g n e t i t e : O1 = o l i v i n e ; O r = o r t h o c l a s e ; Qz = q u a r t z : Sp = s p h e n e . n.d. = not d e t e r m i n e d ; -- = not detected.

an olivine tholeiitic norm contrasting with the other samples which displays quartz tholeiitic norms.

2.4. Mineral chemistry 2.4. I. The primary eclogite assemblage (I) 2.4.1.1. Game t I. The different types of garnet range in composition from Alm42Pyr2s. s Gro: 7. s to Alm36Pyr38Gro2s (Table V). Using Coleman's diagram Alm--Pyr--Gro, they plot astride the band-C fields (Fig. 5). In the banded eclogites, the Mg/Fe ratio of garnet

F i g . 5.

Compositional

eclogite member

boudins plot

grains.

cores

from

from

of the

cores;

Banded

fine-grained

from end-

a n d -C e c l o g i t e

fields

eclogite

2 = porphyroclast

eclogite

coarse-grained

garnets triangular

e t al. ( 1 9 6 5 ) . S h e a r e d

= porphyroclast

small

in

(mole %). Group-B

after Coleman I

variation

depicted

(type

horizons;

1):

(type 2) : rims and

3

= garnet

4 = garnet

cores

horizons.

varies significantly according to their grain size and modal proposition: in the finegrained horizons garnets have a Mg-rich character (Alm36Pyr38) and those coming from coarse-grained horizons are more Fe-rich (Alm42Pyr28.5). In the sheared eclogites the garnet composition is more constant and shows intermediate values (Alm39 Pyr33). These mineralogical differences within the undeformed eclogite types reflects the difference in bulk chemistry (see p. 337). It is n o t clear if this is an earlier magmatic or an anhydrous eclogite synchronous metamorphic differentiation. Whichever is the case, the main effect of the shearing is to homogenize the Mg/Fe ratio of the garnets. The Ca content of the garnet does not vary significantly in the banded eclogite types, but

339

it decreases from the core to the rim in the sheared eclogite porphyroclasts. Moreover, these rims show the same composition as the small strained unzoned rounded crystals of garnet. The synkinematic zoisite appearance might explain these variations.

composition is rather constant in all eclogite types. They are highly magnesian [XMg = Mg/ (Mg+Fe) ~ 0 . 8 5 ] and the Fe 3+ content (calculated after Cawthorn and Collerson, 1974) is insignificant. Since Fetota 1 calculated as Fe 2+ yields a formula with Na < A1VI, all of the Na makes up the jadeite molecule. Its

2.4.1.2. Pyroxene I (Table VI). The pyroxene TABLE VI TABLE V Electron microprobe (EMP) analyses and calculated structural formulae of garnets from eclogites 1

2

3

4

SiO: AI:O~ TiO 2 Cr203 FeO NiO MnO MgO CaO Na20 K20

40.20 22.91 0.01 0.01 18.67 n.d. 0.22 8.86 10.41 n.d. n.d.

40.01 22.71 n.d. n.d. 19.94 n.d. 0.25 9.13 8.38 n.d. n.d.

39.59 22.67 0.04 n.d. 19.68 n.d. 0.87 7.52 10.14 n.d. n.d.

40.28 22.81 0.01 n.d. 17.49 n.d. 0.40 10.25 9.43 n.d. n.d.

Total

101.39

100.83

100.55

100.69

Number Si A1TM Al VI Ti Cr

of cations based 5.994 n.d. 4.018 -.

F e :+ Ni Mn Mg Ca Na K

.

Total

38.86 0.46 32.9 27.76

.

. .

16.002

End-member Alm Spe Pyr Gro

.

.

5.990 n.d. 4.042 0.005 .

15.989

2.489

2.176

0.112 1.696 1.644 .

0.051 2.275 1.503

15.990

16.002

.

.

r e c a l c u l a t i o n s ( m o l e %): 41.88 1.39 34.19 22.54

41.92 1.88 28.56 27.57

End members: Alm = almandine; Gro Pyr = pyrope; Spe = spessartine. n.d. = not determined;= not detected.

A m p h i b o l e I (*)

36.24 0.85 37.88 24.94 = grossular;

m

i

SiO2 ( w t . % ) A120 J TiO: Cr2 0 3 FeO NiO MnO MgO CaO Na20 K:O

54.39 7.85 0.04 n.d. 3.99 n.d. n.d. 11.49 18.31 3.78 n.d.

54.99 7.77 n.d. n.d. 3.59 n.d. n.d. 11.7 17.79 3.99 n.d.

48.11 12.00 0.32 n.d. 8.23 n.d. n.d. 16.79 9.84 2.12 0.57

47.64 13.34 0.38 n.d. 6.70 0.05 n.d. 15.63 10.12 1.95 0.82

Total

99.92

99.95

97.99

96.76

6 oxygens : 5.999 n.d. 3.999 n.d.

.

2.500 . 0.083 2.042 1.346 . . .

Omphacite I

24 oxygens:

6.000 n.d. 4.014 -.

2.323 . 0.028 1.966 1.660 .

on

EMP analyses and calculated structural formulae of omphacites and amphiboles from eclogite

Si A1TM A1VI Ti Cr F e 2÷ Ni Mn Mg Ca Na K

1.959 0.040 0.293 --0.120 . . 0.617 0.707 0.264 --

Total

4.002

XMg XNa

23 oxygens : 6.797 1.203 0.795 0.034 -0.972 .

6.772 1.228 1.007 0.041 0.015 0.797

3.536 1.490 0.580 0.103

3.312 1.547 0.537 0.147

3.998

15.510

15.532

0.84

0.85

0.78

0.80

0.27

0.29

--

--

A1VI/rv

--

-

0.66

0.89

NaM4

--

--

0.17

0.15

(Na+ K) A

--

-

0.51

0.40

. .

1.972 0.028 0.301 --0.108 . . 0.625 0.684 0.278 --

. .

* m = a m p h i b o l e in t h e m a t r i x ( t y p e 2); i = a m p h i b o l e i n c l u d e d in z o i s i t e ( t y p e 3 ). n.d. = not determined; - = not detected.

340 ratio XNa[= Na/(Na + Ca) = 0.26--0.29] places these pyroxenes in the omphacite field (0.20 < ZNa < 0.80; Rossi et al., 1983).

2.4.1.3. Amphiboles I. These belong to the subcalcic edenitic group (Leake, 1978). They are slightly substituted towards the glaucophane molecule (NaM4 = 0.15--0.20) and are highly magnesian (XMg ~ 0.80). The A1VI/ A1Iv ratio (Binns, 1967; Mottana and Edgar, 1969) enables us to distinguish between matrix amphiboles which tend to have carinthine composition (A1VI/A1 IV = 0.66) and amphibole inclusions in garnet and zoisite which belong to the barroisitic group. 2.4.1.4. Zoisites. The zoisites are slightly substituted in Fe 3÷. For 12.5 oxygens, a-zoisite contains 0.11 Fe 3÷ and ~-zoisite 0.10--0.17 Fe 3+. The lack of zoning patterns in omphacite, and the presence of zoisite, as well as the edenitic character of amphiboles and their A1VI/A1 IV ratio, all point to a rather high temperature of crystallization. 2.4.2. The retrogressive assemblages from eclogite to amphibolite 2.4.2.1. Mineralogy o f the anhydrous retrogression reaction (R1). In its first stage, IIa, the anhydrous retrogression is characterized by the appearance of a plagioclase, which presents a large composition field, and by the evolution of the omphacite into a secondary clinopyroxene progressively losing Na and A1 (pxIIa; XNa = 0.06). In fact, the plagioclase crystallizes in two extreme compositions. Zoisite breakdown gives almost pure anorthite (An9s). On the other hand, the new plagioclase which is a product of the partial amphibole and garnet breakdown in the presence of quartz, and also inside symplectites after omphacite, is rich in albite (An20-2s). At the later stage, IIb, plagioclase is zoned, and subsequently shows all possible intermediate compositions between An95 and An20. The last plagioclase is an

andesine (An3s) in equilibrium together with an Na-poor clinopyroxene (XNa = 0.06). The secondary clinopyroxene in symplecrites shows a variable composition. The welldeveloped vermicules are clearly zoned (ZNa = 0.19 in the core; XNa = 0.09 towards the rim). however, inside the double coronas isolating the primary amphibole and/or garnet from quartz (Fig. 4), the new clinopyroxene is homogeneous (XNa = 0.06). During the earliest stage of this anhydrous retrogression, the plagioclase mosaic composition indicates a limited breakdown of the primary phases (amphibole, zoisite, omphacite, grossular in garnet) and a slow diffusion of cations inside an intergranular fluid derived from the (OH) in the primary zoisite and amphibole. The secondary clinopyroxene zoning inside symplectite, and their homogeneous character in double coronas underline the important part played by quartz which favours the expulsion of the jadeite and glaucophane molecules in order to form albite. The final equilibrium (IIb), which is clear in the pod margins, indicates a more advanced breakdown of the primary and early minerals and a faster diffusion of the elements inside an intergranular fluid. The rather Narich composition of the latest formed plagioclase IIb results from the higher volume proportion of omphacite as compared to zoisite in the primary assemblage.

2.4.2.2. Mineralogy o f the hydrous retrogression. Amphibole is the most appropriate mineral to trace this continuous evolution since it is present during all the various retrogression stages from eclogite to amphibolite and locally to greenschist (Table VII; Fig. 6). Whilst from the eclogite paragenesis to stage IIb, the global chemistry remains constant, from stage IIb to the garnet amphibolites (stage II) the rocks become richer in Fe and alkalies. Moreover, the appearance of foliated amphibolite (stage III) corresponds to a high degree of oxidation. From eclogite stage I to stage IIa (constant bulk chemistry), the chemical zoning in the

341

Whole rock 11 II ilia IrP

te 40 60 58 48

ALIV I

ALl\/

2

Arrn42PYr34Gro22 Om (~d26) ; ~D /' HB EE

ITs

Jla Aim4 i Pyr33Gro26- f e PG ~ Ilia F / y / b

PI XAb

Yl . / /

HBPG

Aim39 PYr'38 Gro23 - An65

lib;

~.'o o:8

oZ

o.~//d,2

1.0 0.8 XNa(~)

06

0.4

-- PG

1.5

ul D / /

II ;

0.2

~lmso PYr17 Gro32 - An45- HB fe PG

•

1 I1/, 1/,

1

[]

Ilia;

Ill;

/ m

••

/l []

A n 6 3 - mG fe

1.0

I

BYT + ED

OLI

I +

[]

HB

® 0 0.5

~ (Na+~)A

@ 0

i 10

20

30

40

5O fe %

Fig. 6. Compositional variations in amphiboles during the retrogression. a. Edenitic substitution diagram, symbols: I (solid circles) = eclogite facies; IIa ( s o l i d stars) and IIb ( o p e n stars) = first stages of retrogression; II (triangles) = garnet-amphibolite facies; IIIa ( i n v e r t e d triangles) = garnet breakdown; III ( o p e n s q u a r e s ) = amphibolite facies. E n d - m e m b e r s ( s o l i d s q u a r e s ) : TR = tremolite, ED = edenite, PG = pargasite, TS = tschermakite. b. Plots of X ~ a in a m p h i b o l e vs. XAb in plagioclase depicting schematic topologies for the hydrous retrogression (see Table III), with garnet, p y r o x e n e and plagioclase c o m p o s i t i o n s for each stage. HB = hornblende, BYT = b y t o w n i t e , OLI = oligoclase. c. Plot of A1TM vs. fe in amphiboles where fe = 1 0 0 F e / ( F e + Mg). S y m b o l s as in Fig. 6a.

primary amphiboles is such that the edenitic and tschermakitic substitutions increase towards the rim, as manifested by a displacement of the composition towards the pargasite pole (Fig. 6a). This increase is accompanied by a decrease in XMg (XMg = 0.59) (Fig. 6c). During this evolution, the simultaneous breakdown of the glaucophane c o m p o n e n t and omphacite occurs. The next step towards stage IIb (constant bulk chemistry) is characterized by the stabilisation within the kelyphite of a pargasite IIb

saturated in AI Iv (Fig. 6a) with a plagioclase ( A n 6 j (Fig. 6b) and a garnet richer in pyrope. Thus garnet resorbs Mg released during the omphacite IIa breakdown and the amphibole recrystallization. From stage IIb to garnet amphibolite (stage II}, the rocks become richer in alkalies and Fe. This transition is defined by a decreasing amount of pargasite molecule in the amphibole II, nevertheless its fe ratio keeps on growing (Fig. 6c). Thus the amphibole II (a pargasitic hornblende rich in Fe; XMg =

342

TABLE VII EMP analyses and calculated structural formulae amphiboles from different stages of retrogression IIa

IIb

II

II I a

III

SiO 2 ( w t . % ) A120 z TiO 2 Cr20 ~ FeO NiO MnO MgO CaO Na~O K~O

41.50 16.21 0.05 n.d. 14.25 0.02 0.22 11.85 11.09 2.98 0.12

40.91 16.49 0.09 0.03 14.50 0.11 0.03 10.83 11.32 2.66 0.31

43.98 14.24 0.60 n.d. 13.98 0.07 0.09 11.02 12.00 1.39 0.52

41.24 14.47 0.52 0.15 19.66 n.d. 0.17 8.11 11.57 1.69 0.85

44.25 10.98 0.79 0.19 15.35 n.d. 0.2 11.3 12.12 1.37 1.21

Total

98.27

97.29

97.88

98.42

97.79

Number

Si A1TM A1VI Ti Cr

of

edenite + bytownite -+ hornblende + oligoclase

of cations based on 23 oxygens:

6.105 1.895 0.914 0.005

6.092 1.908 0.011 0.004

6.459 1.550 0.912 0.066 --

1.806

1.714

0.018 2.483

-1.963

0.986

ciated with plagioclase IIIa (An63) derived largely from the grossular c o m p o n e n t in the remaining garnet. At this stage of transformation, the amphibole shows both the exchange sequences between new minerals and relict ones, and the metasomatism from the enclosing gneiss. In the foliated amphibolite (stage III), the amphibole m is more actinolitic with a more important decrease of the edenitic and tschermakitic substitution. This change occurs in parallel with an Ab increase in plagioclase III, expressed by the following reaction:

6.229 1.771 0.805 0.059

6.485 1.515 0.585 0.1015

F e z+

-1.754

Ni Mn Mg Ca Na K

-0.027 2.597 1.747 0.850 0.022

0.013 0.004 2.404 1.806 0.767 --

-0.011 2.410 1.885 0.394 0.097

-0.021 1.825 1.673 0.496 0.164

-0.052 2.461 1.911 0.342 0.239

Total

15.916

15.801

15.489

15.744

15.653

XMg

0.59

0.57

0.58

0.42

0.55

n.d. = not d e t e r m i n e d ; -- = not detected.

0.55) was formed during the garnet amphibolite facies, the massive crystallization of plagioclase II (An4s) absorbing A1 in excess. This evolution, mainly controlled by the variation in bulk chemistry, is linked with the stabilization of the grossular c o m p o n e n t in almandine garnet (An67 + Na -+ An45 + Ca) (Fig. 6b). When quartz is present, the nearly constant partitioning of Na and Ca between amphibole II and plagioclase II gives evidence of an equilibrium near the hornblende-granulite-garnet-amphibolite boundary (Spear, 1981) (Fig. 6b). From the garnet amphibolite (stage II) to the foliated amphibolite (stage III), the amphiboles IIIa produced by garnet breakdown, are ferro-pargasite to magnesiohastingisite which have higher Fe (XMg = 0.42) (Fig. 6c), A1IV, alkali and Fe 3+ contents than the previous amphiboles. They are asso-

ED

BYT

HB

OLI

(Fig. 6b)

2. 4.2.3. Conclusions. The transformation eclogite-~ amphibolite is defined both by discontinuous processes and by continuous ones because of the chemical adjustment of the complex solid-solution garnet, amphibole, pyroxene and plagioclase to the new P--T--X conditions. The mineral chemical data suggest first a decrease in total pressure and then a progressively increasing PH20, foz, and finally a slight decrease in temperature. 2. 5. Physical conditions of evolution In order to locate the continuous retrograde evolution of the basic pods in P--T space, a few geothermobarometers have been chosen according to mineral pairs available in the different facies (Fig. 7).

2.5.1. Hydrous eclogite facies (I) Because of omphacite rehomogeneisation, only t h e hydrous eclogite conditions could be estimated. Fe--Mg exchange between coexisting garnet and omphacite calibrated by Ellis and Green (1979) gives the average estimate of T = 750°C (Fe 3÷ has been neglected in omphacite). The jadeite proportion in omphacite based on the Ab ~ Jd + Qz equilibrium allows only a minimum pressure estimation of eclogite recrystallization for

343

/

P kbor)

restrictions that can be made about this m e t h o d (e.g., Rollinson, 1 9 8 1 ) this geobarometer has been chosen because of the lack of other possibilities. Fe 3÷ has been neglected which leads to a minimal estimation Px of X~a_Ts. The error in P estimation brought about by this simplification is in fact negligible compared with the T uncertainty. Wood's (1977) calibration gives 6--7 kbar for an arbitrary T of 700°C.

Jd30 15 14 13 12 11 10 9 8

j v

7 6 5

/4%'~

.

4 \ \

3

•

"5"."

2

.A

\ \

1 i

500

|

600

I

I

700

800

T (°C)

Fig. 7. P - - T path inferred for the eclogite lenses and dolerite dykes. Full lines: retrograde evolution of the eclogite bodies; curves 1 and 2: from Ellis and Green (1979); curves 3 and 4: from Gasparik and Lindsley (1980); curve 5: from Wood (1977); curve 6: garnetplagioclase from Ghent et al. (1979); curve 7: garnetbiotite from Holdaway and Lee (1977) curve 8: domain of Plyusnina (1982). Dashed lines: suggested history of the metadolerite dykes: A ~ B: isobaric quick cooling; B -~ C: prograde evolution to garnet amphibolite facies.

rocks w i t h o u t plagioclase. According to Holland's (1980) and Gasparik and Lindsley's (1980) methods, which neglected aegirine contents in jadeite activity calculation, potential errors like those linked to Fe 3. estimation are avoided. This m e t h o d gives for T = 750°C minimum pressures of 14--15 kbar, corresponding to a minimal depth of 50 km.

2.5.2. Intermediate stage (IIb) Clinopyroxene--plagioclase symplectites, at last chemically unzoned (stage IIb) allow pressure estimation on the basis of the Ca-Ts + Qz-+ An equilibrium. In spite of all the

2.5.3. Foliated rim (III) and enclosing gneiss The amphibole--plagioclase equilibrium in amphibolite in the presence of epidote has been recently calibrated by Plyusnina (1982). The A1 and Ca partitioning between these two minerals gives T = 600--650°C at P = 4--5 kbar. These results are very consistent with garnet-plagioclase and garnet-biotite rim data in the enclosing sillimanite-bearing gneiss. Indeed the equilibrium 3An ~ Gt + 2Sil + Qz allows P estimation. The high Mn abundance in the garnet inhibits the application of Newton and Haselton's (1981) calibration and thus the calibrations of Ghent (1976) and Ghent et al. (1979) were chosen. The latter one gives P = 5--6 kbar for T = 600°C obtained from Holdaway and Lee's (1977) data concerning the Fe--Mg exchange between garnet and biotite.

3. Tala-Mellet dolerite dyke in the north Aleksod area

3.1. Field relationships The Tala-Mellet district consists mainly of orthogneiss rocks. Basic dykes, varying in thickness from metres to decametres, crosscut the Eburnean gneissic banding (P0). The intrusion age and/or cooling age of the basic material has been determined at ~ 1400 Ma (Bertrand et al., 1972). The whole-rock series were reworked during an event at around 1000 Ma (Bertrand, 1974), characterized by a NNE isoclinal overfolding. The leucocratic parts occur within the axial $1 foliation plane.

344

Inside the larger dykes only the border has been foliated, so within the core dolerite textures are easy recognizable (Fig. 7). 3.2. Petrography Inside the cores of the dykes, the dolerite assemblage with orthopyroxene, plagioclase and clinopyroxene is partly transformed into amphibole and garnet coronas. Towards the margin, in an intermediate zone, the recrystallization into garnet amphibolite is more advanced. Finally the dyke rims are biotite amphibolites. The relative extent of these three zones depends on the dyke size (Fig. 8). 1.5 m

X/_~-.

(a) A pale-green amphibole appears at the orthopyroxene--plagioclase contacts. (b) Garnet crystallizes at the same time as quartz forming barriers between ilmenite and plagioclase. Isolated garnet crystals inside plagioclase are also found. (c) Small double coronas of amphibole and garnet are to be seen only when orthopyroxene, ilmenite and plagioclase coexist. (d) Biotite appears as a late phase, overprinted on the above-mentioned different phases especially in the zone rich in orthopyroxene and ilmenite. These reaction textures thus display a first stage of prograde recrystallization of the primary magmatic material towards the assemblage of the garnet-amphibolite facies conditions (see the discussion in Sautter, 1983). 3.2.2. Intermediate zones The intermediate zones are characterized by a pervasive recrystallization into garnet amphibolite. The mineral shows a very fine grain size (~ 0.5 mm). The amphibole, which is the major phase, forms small green crystals. It is in equilibrium with garnet, plagioclase and biotite. Ilmenite and sphene are the predominant accessory phases.

Fig. 8. S c h e m a t i c s k e t c h o f a m e t a d o l e r i t e d y k e . 1 = d o l e r i t e core; 2 = g a r n e t a m p h i b o l i t e m a r g i n ; 3 = schistose amphibolite margin; 4 = E b u r n e a n folded (P0) l e u c o c r a t i c h o r i z o n ; 5 = E b u r n e a n l e u c o c r a t i c horizon crosscut by a metadolerite dyke.

3.2.3. Foliated margins In the foliated margins garnet has disappeared and well-developed biotite plates together with amphibole prisms delineate the obvious lineation and foliation.

3.2.1. Dyke cores The less altered central zones are constituted by a framework of cloudy zoned plagioclases, thick prismatic orthopyroxenes with brownish pigmentation, Arborescent ilmenite and apatite are the main accessory phases. These textures indicate relatively quick cooling conditions. These rocks were then transformed by the following series of reactions forming coronas and which lead to the new assemblage: garnet + amphibole + quartz + biotite.

3.2.4. Conclusions The dolerite textures, preserved in the core of these crosscutting dykes prove that the intrusion t o o k place inside a structure which was cooled after the Eburnean Orogenesis (post-Po uplift). Afterwards the history of the dykes during the early Pan-African event is characterized by a prograde metamorphic evolution at conditions that did not exceed those of the amphibolite facies. Finally, the appearance of biotite amphibolite in the dyke margins corresponds to an obvious increase in

345 partial water pressure. K/Ar dating on some amphiboles (Bertrand, 1972) gave ages of 600 Ma for the ultimate retrograde event, possibly connected with an uplift of the whole area at the end of Pan-African event (phase P3).

3.3. Mineral chemistry 3.3.1. Dyke cores: primary reactional assem blages The primary o r t h o p y r o x e n e is poor in AI and Ca. With the bronzite composition in its core, it becomes richer in Fe towards its contact with the secondary amphibole. The anorthite weight per cent in plagioclase decreases from the core to the rim in contact with amphibole and garnet neoblasts. The amphibole is magnesio-hornblende (XMg = 0.45). The garnets, of almandine type, have various compositions according to the primary reacting minerals which are involved. They are more Ca-rich inside plagioclase (Alm53Pyr,3Gro35.s), more Fe-rich atilmeniteplagioclase contacts (Alm60Pyr,gGro20), and slightly more Mg-rich in contact with amphibole. The biotites are phlogopite (XMg = 0.60) only slightly substituted towards eastonite and rich in Ti. 3.3.2. Intermediate zones: secondary garnet amphibolite Amphibole, garnet and biotite become richer in Fe. The garnet is a homogeneous almandine and coexists with a ferropargasitic hornblende (XMg = 0.40), a plagioclase (An2s) and a biotite (XMg = 0.48). 3.3.3. The biotite amphibolite rims Because of the garnet breakdown, the plagioclase has a higher An content ~An38) than the previous zone. Also Fe, A1Iv and alkalies in the amphibole increase. This amphibole is a ferroan pargasite (XMg = 0.35) and the biotite is poorer in Ti, richer in A1 and Fe (XMg = 0.38).

3.4. Physical conditions o f dyke evolution The kind of mineral assemblages appearing at different stages of the evolution of these rocks does not allow a complete study of the physical conditions. Only partitioning of Fe-Mg between garnet--biotite enables us to estimate equilibrium T from Perchuk's (1977) calibration which concerns basic rocks. The results are 600--650°C, applicable to the intermediate zone at an assumed P = 5 kbar (point C, Fig. 7) because the garnets are almandine-rich. 4. Discussion This study of metabasic rocks in pods or dykes from the Serkout and Aleksod areas underlines quite a number of dissimilarities between these areas: (a) Field relationship differences: the pods with eclogite relics (northern Serkout) are scattered along the contact between two distinct lithological formations (Fig. 2) [a metasedimentary unit {zone II) and orthogneiss (zone I)]; those orthogneiss of TalaMellet are crosscut by dolerite dykes. (b) The types of deformation assigned by phase P1 (i.e. in the pods with eclogite relics) are confined within blastomylonitic zones; the dolerite dykes outcrop only inside domains which are lightly deformed, similar to those of northern Serkout (zone I). (c) The relict mineral assemblages prior to retrogressive recrystallization under garnet amphibolite conditions are different: only the pods include obvious eclogite relics indicative of high pressure; in the dykes the primary assemblage is magmatic (low pressure). Thus diametrically opposite petrogenetic paths brought the pods and the dykes into amphibolite-facies conditions. Hence the garnet amphibolite domain is the facies of convergence. 5. Conclusions Given the structural and petrological data,

346

it is possible to conclude that the pods and dykes have n o t had the same origin, nor the same evolution. (1) The cores of the basic pods in the mylonite zone in the north Serkout area are genuine high-pressure hydrous-eclogite facies parageneses. (2) The cores of the basic dykes in the north Aleksod area are genuine low-pressure dolerite parageneses. (3) Different P--T paths lead to the petrogenetic convergence of the two different basic rock types within the garnet-amphibolite facies during the Pan-African event. (4) The host-rock units were brought together (uplift of the metasediments, burial of the orthogneiss) by a great tangential tectonic event probably related to an intracontinental orogenesis (Boullier and Bertrand, 1981). These results deny the hypothesis (Bertrand, 1974) of basement--cover relations between these different formations. (5) In this context, the eclogitisation, limited to the mylonitic soles of metasedimentary rocks thrust over orthogneiss, is not a general process linked to high pressures due to the piling of nappes. So localized tectonic overpressure should be considered responsible for eclogite appearance inside the mylonitic zones.

Acknowledgements Thanks are due to J.M.L. Bertrand for much help in the field and to Professor J. Fabri6s for helpful discussions and careful reading at various stages of preparation of this manuscript. This research has been supported by the Mus6um National d'Histoire Naturelle, Paris (microprobe facilities), the Centre de Recherches P6trographiques et G6ologiques, Nancy, and the Centre Armoricain d'Etude Structurale des Socles, Rennes (bulk-rock chemistry).

References Bertrand, J.M.L., 1974. l~volution polycyelique des Gneiss Pr4cambriens de l'Aleksod (Hoggar central, Sahara alg6rien) -- Aspects structuraux, p6trolo-

giques, g6ochimiques et g6ochronologiques. Thesis, University of Montpellier, Montpellier (published as Cent. Natl. Rech. Sci. (C,N.R.S.), Collect. C.R.Z.A. (Cent. Rech. Zones Arides), S4r. 19, 370 pp. Bertrand, J.M.L. and Caby, R., 1978. Geodynamic evolution of the Pan-African orogenic belt: a new interpretation of the Hoggar Shield (Algerian Sahara). Geol. Rundsch., 67(2): 357--388. Bertrand, J.M.L. and Lasserre, M., 1973. Age 6burn6en de la s6rie de l'Arechchoum (Hoggar central, Sahara alg6rien). C.R. Acad. Sci. Paris, S6r. D, 276: 1657--1660. Bertrand, J.M.L., Cantagrel, J.M. and Lasserre, M., 1972. Age K/Ar mesur6 sur des amphiboles dans le Pr6cambrien de l'Aleksod (Ahaggar centre-oriental, Sahara alg4rien). C.R. Acad. Sci. Paris, S6r. D, 274: 1881--1884. Binns, R.A., 1967. Barroisite-bearing eclogite from Naustdal, Sogn-og-Fjondane, Norway. J. Petrol., 8(3): 349--371. Boullier, A.M. and Bertrand, J.M.L., 1981. Tectonique tangentielle profonde et couloirs mylonitiques dans le Hoggar central polycyclique (Alg4rie). Bull. Soc. G6ol. Fr., 33(1): 17--22. Cawthorn, W.G. and Collerson, K.D., 1974. The recalculation of pyroxene end-member parameters and the estimation of ferrous and ferric iron content from electron microprobe analyses. Am. Mineral., 59: 1202--1208. Coleman, R.G., Lee, D.E., Beatty, L.N. and Brannock, W.W., 1965. Eclogites and eclogites- Their differences and similarities. Geol. Soc. Am. Bull., 76: 483--508. Ellis, D.J. and Green, D.H., 1979. An experimental study of the effect of Ca upon garnet-clinopyroxene Fe--Mg exchange equilibria. Contrib. Mineral. Petrol., 71: 13--22. Gasparik, T. and Lindsley, D.H., 1980. Phase equilibria at high pressure of pyroxenes containing monovalent and trivalent ions. In: C.T. Prewitt (Editor) Pyroxenes. Reviews in Mineralogy, Vol. 7. Mineralogical Society of America, Washington D.C., pp. 309--339. Ghent, E.D., 1976. Plagioclase--garnet--A12SiOsquartz: a potential geobarometer----geothermometer. Am. Mineral., 61: 710--714. Ghent, E.D., Robbins, D.B. and Stout, M.Z., 1979. Geothermometry, geobarometry and fluid compositions of metamorphosed calc-silicates and pelites, Mica Creek area, British Colombia. Am. Mineral., 64: 874--885. Gu4reng6, B., 1966. L'ant~cambrien de Temasint. Bureau des Recherches G6ologiques et Mini~res (B.R.G.M.), Orl6ans, Intern. Rapp. ALG.66.A2. Hensen, H.J. and Green, J.H., 1971. Experimental study of the stability of cordierite and garnet in pelitic compositions at high pressures and temperatures, III. Synthesis of experimental data and

347

geological application. Contrib. Mineral. Petrol., 38: 151--166. Holdaway, M.J., 1971. Stability of andalousite and aluminium silicate phase diagram. Am. J. Sci., 271: 97--131. Holdaway, M.J. and Lee, S.H., 1977. Fe--Mg cordierite stability in high grade pelitic rocks based o n experimental, theoretical and natural observations. Contrib. Mineral. Petrol., 63: 177-198. Holland, T.J.B., 1980. The reaction albite = jadeite + quartz determined experimentally in the range 600--1200°C. Am. Mineral., 65: 129--134. Latouche, L., 1978. I~tude p6trographique et structurale de la r6gion des Gour Oumelalen. Thesis, University of Paris VII, Paris (unpublished). Leake, B.E., 1978. Nomenclature of amphiboles. Am. Mineral., 63: 1023--1052. Mottana, A. and Edgar, A.D., 1969. The significance of amphibole compositions in the genesis of eclogites. Lithos, 3: 37--49. Newton, R.C. and Haselton, H.T., 1981. Thermodynamics of the garnet--plagioclase--A12SiO~-quartz geobarometer. In: R.C. Newton, A. Navrotsky and B.J. Wood (Editors), Thermodynamics of Minerals and Melts. Springer, New York, N.Y., pp. 131--147. Perchuk, L.L., 1977. Thermodynamic control of metamorphic processes. In: Energetics of Geological Processes. Springer, Berlin, pp. 285--352. Plyusnina, L.P., 1982. Geothermometry and geobarometry of plagioclase--hornblende bearing assemblages. Contrib. Mineral. Petrol., 80: 1 4 0 146.

Rollinson, H.R., 1981. Garnet-pyroxene thermometry and barometry in the Scourie granulites, NW Scotland. Lithos, 14(3): 225--238. Rossi, G., Smith, D.C., Ungaretti, L. and Domeneghetti, M.C., 1983. Crystal-chemistry and cation ordering in the system diopside--jadeite: A detail study by crystal structure refinement. Contrib. Mineral. Petrol., 83: 247--258. Sautter, V., 1983. Les ~clogites et les amphibolites grenat des terrains pr~cambriens de l'Aleksod: leur signification dans le cadre de l'orogen~se panafricaine. Thesis, University of Paris VI, Paris. Smith, D.C., 1980. Exceptional mineral compositions in very high pressure hydrous-eclogite-facies parageneses in the Precambrian suite of eclogite lenses in the Selje and Vartdalsfjorden Districts of the Basal Gneiss Region, SW Norway. 26th Int. Geol. Congr. Abstr. 01.3.3, p. 92. Smith, D.C. and Cheeney, R.F., 1980. Orientated needles of quartz in clinopyroxene: evidence for exsolution of SiO 2 from a non-stoichiometric supersilicic "clinopyroxene". 26th Int. G~ol. Congr., Paris, Abstr. 02.3.1, p. 145. Spear, F.S., 1981. Amphibole--plagioclase equilibria: an empirical model for the relation atbite + tremolite = edenite + quartz. Contrib. Mineral. Petrol., 77: 355--364. Stout, J.H., 1972. Phase petrology and mineral chemistry of coexisting amphiboles from Telemark, Norway. J. Petrol., 13: 99. Termier, P., 1898. Bull. Soc. G~ol. Fr., 21: 148. Wood, B.J., 1977. The activity components in clinopyroxene and garnet solid solutions and their applications to rocks. Philos. Trans. Soc. London, Ser. A, 286: 331--342.