Eclogite-plagiopyrigarnite relations in the catazonal complexes of northwest Spain

Chemical Geology, 50 (1985) 163--171 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

163

ECLOGITE-PLAGIOPYRIGARNITE RELATIONS IN THE CATAZONAL COMPLEXES OF NORTHWEST SPAIN R.P. K U I J P E R ~, D.E. V O G E L 2 and E. D E N T E X 3 ~Holland Sea Search N.V., 2509 GB The Hague (The Netherlands) 2Laboratorium voor Physicochemische Geologie, Katholieke Universiteit, 3030 Leuven (Belgium) 3Instituut voor Aardwetenschappen, Rijksuniversiteit Utrecht, 3508 TA Utrecht (The Netherlands)

(Accepted for publication October 23, 1984)

Abstract Kuijper, R.P., Vogel, D.E. and Den Tex, E., 1985. Eclogite--plagiopyrigarnite relations in the catazonal complexes of northwest Spain. In: D.C. Smith, G. Franz and D. Gebauer (Guest-Editors), Chemistry and Petrology of Eclogites. Chem. Geol., 50:163--171. Eclogites and plagiopyrigarnites in the catazonal complexes have identical major- and trace-element bulk compositions and very similar lattice fabrics of their constituent minerals. On the other hand, the chemical composition of and the element partitioning between the respective clinopyroxenes and garnets are significantly different. The dimensional fabric of the respective clinopyroxenes is also different, indicating dynamic crystallization for the eclogites followed by static recrystallization during their transformation to plagiopyrigarnites. The eclogites are of B-type facies and the plagiopyrigarnites are high-pressure granulites of the clinopyroxene + almandine ± hornblende subfacies. Oriented omphacite of the eclogites is gradually transformed into mosaic-equigranular sodic augite and plagioclase via symplectitic intergrowths in transitional rock types, indicating an increase in temperature possibly accompanied by a slight drop in pressure. The inferred P--T--time path is supported by the temperature and pressure of coeval recrystallization established in closely associated peridotite bodies.

1. G e o l o g i c a l s e t t i n g

Several m e s o - and c a t a z o n a l c o m p l e x e s w i t h d e p o s i t i o n a l and igneous p r o t o l i t h ages o f the r o c k s ranging f r o m 2.5 G a to 470 Ma o c c u r in t h e p r e d o m i n a n t l y Variscan o r o g e n i c b e l t ( H e s p e r i a n Massif) o f t h e n o r t h w e s t e r n I b e r i a n p e n i n s u l a ( A r p s et al., 1 9 7 7 ; D e n T e x , 1981). Mafic and u l t r a m a f i c r o c k s f o r m up to 75% in s u r f a c e area o f t h e c a t a z o n a l C a b o Ortegal C o m p l e x at t h e n o r t h w e s t e x t r e m i t y o f the H e s p e r i a n Massif, w h e r e eclogite- and high-pressure granulite-facies m e t a m o r p h i s m

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has a f f e c t e d t h e r o c k s b e t w e e n a p p r o x i m a t e ly 1 G a a n d 350 Ma B.P. (van Calsteren et al., 1 9 7 9 ; K u i j p e r , 1979). In t h e c a t a z o n a l Sobrad o m T e i j e i r o C o m p l e x high-pressure granulitefacies r o c k s o f m a f i c c o m p o s i t i o n c o n t a i n vestiges o f eclogite-facies p a r a g e n e s e s ( K u i j p e r , 1 9 7 9 ) (Fig. 1). T h e r o c k s in t h e s e c o m p l e x e s have b e e n s u b j e c t to p o l y p h a s e d e f o r m a t i o n and m e t a m o r p h i s m , resulting in i n t e r f e r i n g fold p a t t e r n s , b o u d i n a g e and ( b l a s t o ) m y l o n i t e s - - t h e latter c o n c e n t r a t e d along t h e p e r i p h e r y o f t h e c o m p l e x e s - - and in progressive h y d r a t i o n a n d r e t r o g r a d a t a t i o n o f t h e m i n e r a l parageneses.

© 1985 Elsevier Science Publishers B.V.

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2. Pertinent rock associations In the two catazonal complexes referred to above, high-temperature peridotite bodies (Vogel, 1967; Maaskant, 1970; van Calsteren et al., 1979), containing veins of garnet- and spinel-pyroxenite and of pargasite- or phlogopite-rich rock, occur in close association with more or less overprinted eclogites (B-type of Coleman et al., 1965), (plagio)pyrigarnites (as defined b y Vogel, 1967), felsic kyanite-garnet granulites, more or less banded and migmatic k yanite--garnet--biotite--muscovite paragneisses, partially coronitic metagabbros, partially garnet-bearing amphibolites and some granitic orthogneisses (Vogel, 1967; Hubregtse, 1973; Kuijper, 1979). The eclogites form layers, lenses and boudins in the banded and migmatic paragneisses, the (plagio)pyrigarnites constitute more or less massive bodies adjacent to kyanite--garnetbearing gneisses and to high-temperature peridotite outcrops, while in the Sobrado-Teijeiro area eclogites and (plagio)pyrigarnites occur irregularly dispersed, forming a heterogeneous unit of mafic rocks (Kuijper, 1979). Amphibolite- and greenschist-facies parageneses have been overprinted on them, especially so in the vicinity of major zones of tectonic movement. 3. Eclogite and plagiopyrigarnite: similarities and differences

3.1. Bulk-chemical compositions The major- and trace-element composition of the eclogites and plagiopyrigarnites from the Cabo Ortegal Complex is very similar. Both are predominantly quartz-normative tholeiitic basalts close to the pigeonite-hypersthene trend line in the AFM triangle (Fig. 2), the plagiopyrigarnites having somewhat higher FeO/MgO ratios than the eclogites. Chondrite-normalized La/Sm ratios are also nearly identical, being situated slightly above the unit line, close to the average of continental basalts (Fig. 3). A similar behaviour is shown by their K/U and K / R b vs. K

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ratios, all lying in the areas where continental tholeiites are expected. On the other hand unpublished U--Pb, Sm--Nd and new rareearth element (REE) data on the eclogites led Peucat et al. (1983) to consider their protoliths as m i d ~ c e a n ridge basalts (MORB). Anyway, the bulk-chemical compositions led van Calsteren (1978) to the conclusion that b o t h rock types have had a c o m m o n origin as slightly K-, U- and LREE-enriched quartz- or olivinenormative tholeiites. The close spatial association of eclogites and plagiopyrigarnites in the Sobrado--Teijeiro Complex supports this conclusion (Kuijper, 1979). It should be realized, moreover, that the 5'SO-values of the eclogites, varying b e t w e e n +1.5 and +5.8%0 SMOW, are much lower than those of MORB but in the range of Tertiary basalts and gabbros in Scotland (U.K.) which may have interacted with light meteoric waters at high temperatures (Vogel and Garlick, 1970). Moreover, according to Forester and Javoy (1979) the 51SO-values of closely associated eclogites, plagiopyrigarnites, amphibolites and gneisses from Cabo Ortegal (between +1.5 and +11.6°/00 SMOW) differ significantly, suggesting that the rocks have behaved as closed systems for oxygen isotope exchange during metamorphism.

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from the Sobrado--Tejeiro Complex. The lower jadeite contents of clinopyroxenes in symplectitic eclogites and plagiopyrigarnites are, at least partly, due to the presence of "albite" in these rocks. The symplectites consist of fine wormlike intergrowths of sodic plagioclase (~ An33) and a sodic augite containing 18--23 mole% jadeite in the Cabo Ortegal Complex. They develop from the grain boundaries of adjoining omphacite crystals and may completely replace them (Fig. 5). The stable garnets [PY29-s1, (Alm + Spess)29--4~, (Gross + Andr)20-2b] of the Cabo Ortegal and Sobrado--Tejeiro eclogites and coronitic metagabbros have compositions within the range of group-B eclogite-garnets according to Coleman et al. (1965). They generally contain rutile-dust inclusions in more or less regular patterns and exhibit kelyphitic plagioclase---amphibole corona's. The plagiopyrigarnite-garnets which never contain rutile-dust inclusions, have the composition PY22--37, (Alm + Spess)3s--s3, (Gross + Andr)22--29 (Vogel, 1967; Kuijper, 1979).

Fig. 3. Diagram of the chondrite-normalized La and Sm concentrations with the continental basalt average (e) as reference. Granulites = plagiopyrigarnites (from van Calsteren, 1978, fig. 4).

3.2. Mineral compositions and element partitioning The stable clinopyroxenes of the Cabo Ortegal eclogites are omphacites containing 30--33 mole% jadeite or omphacitic chloromelanites [according to TrSger's (1962) classification] as determined by wet-chemical and electron microprobe analysis, whereas the pyroxenes of the plagiopyrigarnites and the symplectitic pyroxenes of the eclogites are sodic augites with a higher acmite/jadeite ratio and only 10--23% of the jadeite mole in solid solution (Vogel, 1967). Kuijper (1979) found jadeite contents between 4.6 and 17.4 mole% in clinopyroxenes of granofelsic plagiopyrigarnites and symplectitic eclogites

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Fig. 5. Outward coarsening clinopyroxene--plagioclase symplectite growing from an (invisible) omphacite grain beyond the lower right-hand corner into another one (upper left-hand), which has previously exsolved plagioclase platelets and rods (light grey).

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between coexisting clinopyroxenes and garnet differ significantly in eclogites and plagiopyrigarnites from the Mellid Complex (Hubregtse, 1973) and range from 10 to 8 in the Cabo Ortegal eclogites, and from 7 to 5 in the Mellid plagiopyrigarnites and coronitic metagabbros (Kuijper, 1979). 3.3. Fabric

Omphacites, kyanites and a-zoisites in the eclogites generally exhibit preferred lattice and dimensional orientations parallel to an inferred early fold axis (B) and axial plane (S) (Engels, 1972). Omphacite has a pianolinear to linear fabric with occasionally almost perfect orientation of [001] perpendicular to S, and [010] parallel to S (Fig. 6, upper part). This fabric is the dominant t y p e in the clinopyroxenes of plagiopyrigarnites

from Cabo Ortegal, where planolinear fabrics are rare. But -- in contrast to the adjacent eclogites -- the former rocks exhibit granofelsic textures devoid of any preferred dimensional orientation (Engels, 1972) (Fig. 6). Identical omphacite fabrics have been reported b y Helmstaedt et al. (1972) from eclogite-xenoliths in kimberlite-bearing breccias from the eastern Colorado Plateau (U.S.A.). It is possible that the difference in emphasis b e t w e e n the eclogite and plagiopyrigarnite fabrics is due to a change in the deformation mechanism, i.e. to the operation of different slip systems in the omphacite lattice, related to the different P - - T regimes of the eclogite- and the high-pressure granulite-facies, or to the t y p e of strain. It cannot be excluded, however, that the planolinear fabric represents a flattening regime obtaining in the limbs, and the linear fabric a constrictional regime in the hinges of early folds (Helmstaedt et al., 1972). The cpx--plag symplectites, mentioned in section 3.2, show various microstructures, resembling b o t h dis-

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continuous and continuous precipitation, as evidenced b y regular vs. irregular rods and platelets of the two intergrown minerals (van Roermund, 1982; Boland and van Roermund, 1983). In eclogites from the Basal Gneiss Complex, western Norway, continuous precipitation is restricted to high-temperature (800--850°C) eclogite occurrences, whereas discontinuous precipitation is present throughout the whole Western Gneiss Region (temperatures ranging from 850 ° to 550°C). Where b o t h precipitation mechanisms have operated in the same sample, continuous precipitation preceded the discontinuous one (Hernes, 1954; Mysen and Griffin, 1973). In one transitional rock sample from the

Sobrado--Tejeiro Complex we have observed the following sequence o f microstructures (Fig. 5): (I) regular rods and platelets o f plagioclase in clinopyroxene; (2) finegrained cpx--plag symplectite advancing from a neighbouring omphacite grain and corroding type-/ symplectites; and (3) frontal lobes of coarser-grained symplectite. The coarser-grained frontal lobes may have been formed in two ways: (a) driven b y chemical free-energy differences due to an increase in temperature, or (b) b y surface free-energy differences as a result of decompression. In order to establish the correct pressure--temperature--time relationships between the three microstructures, referred to above, we shall need to investigate

169 I

more transitional rock samples and to analyse the detailed chemistry of the intergrown minerals concerned (H.L.M. van Roermund, pers. c o m m u n . , 1982). 3.4. Mineral parageneses and metamorphic facies

The eclogites have co-stable parageneses consisting of omphacite + garnet + rutile + quartz + a-zoisite + kyanite -+ carinthine + zircon + apatite. Sodic plagioclase, if present, is always secondary. Thus the stable paragenesis is t h a t of the eclogite facies as defined by Eskola (1921). The plagiopyrigarnites, on the other hand, are composed of co-stable sodic augite + garnet + sodic plagioclase + quartz + rutile _+ ~-zoisite +- brown-green hornblende. They never contain stable kyanite and qualify as t y p o m o r p h i c parageneses of the high-pressure granulite facies (clinopyroxene + almandine + hornblende subfacies). The symplectitic eclogites also have mosaic-textured parageneses transitional between the eclogite- and highpressure granulite-facies. They range from garnet + clinopyroxene to symplectitic clinopyroxene + blastic plagioclase + kyanite -+ pargasite/tschermakitic hornblende + rutile + quartz +- a-zoisite + ~-zoisite. All parageneses and mineral compositions can be found in Vogel (1967), Hubregtse (1973) and Kuijper (1979). 4. P - T path of the eclogite--plagiopyrigarnite transition

On the basis of jadeite contents (30--33 mole%) of omphacite and Kd-values (10--8) for (FeO/MgO)ga/(FeO/MgO)cpx the P--T field of eclogite equilibration at Cabo Ortegal can be fairly well constrained (Fig. 7). Kuijper (1979) arrived at constraints between 10 and 11 kbar and 580--640°C. U s i n g t h e revised isopleths for jadeite mole fractions in omphacite in the system diopside--jadeite--quartz (Gasparik and Lindley, 1980) we conclude, however, that 13--15 kbar and 590--670°C

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are more realistic constraints for P and T. In view o f their association with "albite", the jadeite contents (10--23 mole%) of clinopyroxenes from plagiopyrigarnite and of completely symplectitized omphacite (18--23 mole%) in transitional rock types cannot be used with confidence to constrain the pressure conditions of equilibration. The fabric evidence of the symplectites indicates a rise rather than a fall of temperature in going from eclogite to plagiopyrigarnite (cf. Section 3.3). This is supported by the temperature and pressure values of re-equilibration during crustal emplacement (800--900°C at 10--15 kbar), obtained by Maaskant (1970) on the contiguous peridotite/pyroxenite bodies. The inferred pressure range (10--15 kbar) of re-equilibration of these diapiric bodies may have been overestimated, since it

170

was based o n O ' H a r a ' s ( 1 9 6 7 ) /~-parameter o f the A1203 c o n t e n t o f c l i n o p y r o x e n e , b u t even so the circumstantial evidence indicates a t e m p e r a t u r e rise r a t h e r t h a n a pressure d r o p in the i m m e d i a t e vicinity o f t h e t r a n s f o r m i n g rocks, and w i t h i n t h e general t i m e span o f the process. T h e c o n c l u s i o n is t h a t heating, whether or not accompanied by decompression ( d o u b l e arrow bar in Fig. 7), is t h e m o s t p r o b a b l e m e c h a n i s m t o have caused t h e t r a n s i t i o n f r o m eclogite t o plagiopyrigarnite in the c a t a z o n a l c o m p l e x e s o f n o r t h w e s t Spain. This can be c h e c k e d w h e r e Kd-values for (FeO/MgO)ga/(FeO/MgO)cpx of totally s y m p l e c t i t i z e d eclogites o r p l a g i o p y r o g a r n i t e s are available, as is the case f o r the Mellid Complex, where Hubregtse (1973) obtained Kd-values b e t w e e n 7 and 5, c o n f i r m i n g t h e rise in t e m p e r a t u r e t o ~ 8 5 0 ° C , p r o b a b l y a c c o m p a n i e d b y a d r o p in pressure. 5. Conclusions S i m u l t a n e o u s eclogite- and high-pressure granulite-facies m e t a m o r p h i s m in t h e catazonal c o m p l e x e s o f n o r t h w e s t Spain is ruled o u t by: (1) t h e virtually identical b u l k - c h e m ical c o m p o s i t i o n o f eclogites and plagiopyrigarnites f o r m a j o r and t r a c e elements, (2) the fabric e v o l u t i o n f r o m d i m e n s i o n a l l y pianolinear eclogite via s y m p l e c t i t i z e d eclogite t o granofelsic plagiopyrigarnite, (3) the P - - T p a t h o f the eclogite--plagiopyrigarnite transit i o n implying a rise in t e m p e r a t u r e o f r o u g h l y 200°C and perhaps a small d r o p in pressure as well. M o r e o v e r , a p r e l i m i n a r y s t u d y o f t h e U/ Pb w h o l e - r o c k systematics appears to s u p p o r t a n age f o r the eclogite-facies m e t a m o r p h i s m in the interval b e t w e e n 1.0 and 0.5 Ga (Kuijper, 1979; Kuijper et al., 1 9 8 2 ) , b u t t h e i r Sr-isotope systems w e r e a p p a r e n t l y n o t reh o m o g e n i z e d , as testified b y t h e i r failure t o p r o d u c e a R b / S r i s o c h r o n , o n t h e same scale as t h o s e o f the h o r n b l e n d e - b e a r i n g plagiopyrigarnites, w h i c h w e r e shown t o have closed at 362 +- 66 Ma b y van Calsteren et al. ( 1 9 7 9 ) . T h u s we believe to have a d d u c e d m o r e evid e n c e f o r a t h e r m a l e v e n t in t h e Early Palaeo-

zoic, possibly a c c o m p a n i e d b y s o m e d e c o m pression, to have caused the m o r e or less complete c o n v e r s i o n o f earlier eclogites into plagiopyrigarnites p r i o r to t h e i r h y d r a t i o n and r e t r o g r a d a t i o n into hornblende-plagiopyrigarnites, a m p h i b o l i t e s and e v e n t u a l l y greenschists. This is in a c c o r d a n c e w i t h t h e m a n t l e p l u m e - - r i f t s y s t e m m o d e l advanced f o r the c a t a z o n a l c o m p l e x e s o f n o r t h w e s t Spain b y van Calsteren ( 1 9 7 7 ) , Den T e x ( 1 9 7 9 ) and Kuijper (1979).

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Hubregtse, J.J.M.W., 1973. Distribution o f elements in some basic granulite-facies rocks with special reference to coexisting minerals in a garnet-bearing metagabbro and a clinopyroxene--garnet-amphibole gneiss from the Mellid area, Galicia, NW Spain. Verh. K. Ned. Akad. Wet., Afd. Natuurkd. Reeks 1, 27 (1): 1--68. Kuijper, R.P., 1979. U--Pb systematics and petrogenetic evolution of infracrustal rocks in the Paleozoic basement of western Galicia (NW Spain). ZWO (Neth. Org. Adv. Pure Res.), Lab. Isotopengeol., Amsterdam, Verh., 5: 1--101. Kuijper, R.P., Priem, H.N.A. and Den Tex, E., 1982. Late Archaean--early Proterozoic source ages of zircons in rocks from the Palaeozoic orogen o f western Galicia, NW Spain. Precambrian Res., 19: 1--30. Maaskant, P., 1970. Chemical petrology of polymetamorphic ultramafic rocks from Galicia, NW Spain. Leidse Geol. Meded., 45: 237--325. Mysen, B.O. and Griffin, W.L., 1973. Pyroxene stoichiometry and the breakdown of omphacite. Am. Mineral., 58: 60--63. O'Hara, M.J., 1967. Mineral parageneses in ultrabasic rocks. In: P.J. Wyllie (Editor), Ultramafic and Related Rocks. Wiley, New York, N.Y., pp. 393-403. Peucat, J.J., Bernard Griffiths, J., Iglesias, M. and Cornichet, J., 1983. Preliminary U--Pb, Sm--Nd and REE data from eclogites o f the Iberian Peninsula. Terra Cognita, 3 : 1 3 9 (abstract). R~heim, A. and Green, D.H., 1974. Experimental determination of the temperature and pressure

dependence o f the Fe--Mg partition coefficient for coexisting garnet and clinopyroxene. Contrib. Mineral. Petrol., 48: 179--203. TrSger, W.E., 1962. Zur Systematik und Optik der Chloromelanit-Reihe. Tschermaks Mineral. Petrogr. Mitt., 8: 24--35. van Calsteren, P.W.C., 1977. A mantle-plume-model interpretation for the Paleozoic geology of Galicia with emphasis on the Cabo Ortegal area (NW Spain). Proc. K. Ned. Akad. Wet., Set. B, 80: 156--168. van Calsteren, P.W.C., 1978. Geochemistry of the polymetamorphic mafic--ultramafic complex at Cabo Ortegal (NW Spain). Lithos, 11: 61--72. van Calsteren, P.W.C., Boelrijk, N.A.I.M., Hebeda, E.H., Priem, H.N.A., Den Tex, E., Verdurmen, E.A.Th. and Verschure, R.H., 1979. Isotopic dating of older elements (including the Cabo Ortegal mafic--ultramafic complex) in the Hercynian orogen o f NW Spain: manifestations of a presumed early Paleozoic mantle-plume. Chem. Geol., 24: 35--56. van Roermund, H.L.M., 1982. On eclogites from the Sere Nappe, Jiimtland, Central Scandinavian Caledonides. Ph.D. Thesis, State University Utrecht, Utrecht, 99 pp. Vogel, D.E., 1967. Petrology of an eclogite and pyrigarnite-bearing polymetamorphic rock complex at Cabo Ortegal, NW Spain. Leidse Geol. Meded., 40: 121--213. Vogel, D.E. and Garlick, G.D., 1970. Oxygen-isotope ratios in metamorphic eclogites. Contrib. Mineral. Petrol., 28: 183--191.