LEIDSE GEOLOGISCHE MEDEDELINGEN, Deel 51, Aflevering 1, pp. 57-61, 16-6-1977

Geochronological, geochemical and geophysical investigations in the

high-grade mafic-ultramafic complex at Cabo and other pre-existing

elements in the Hercynian basement of (NW )

BY

P.W.C. van Calsteren

Summary

in the field and in the Mafic and ultramafic rocks from a high-gradecomplex at Cabo Ortegal in NW Spain were investigated laboratoryby

meansofpetrological, isotope-geochronologicaland geochemical methods. Isotope-geochronologicalmethods were applied toorthogneisses

from a high-grade complexnear Mellid and from the blastomylonitic grabenbetween Malpica and Tuy, a mica-bearingeclogite lens from the

A same graben, and to a two-mica granitefrom the area near La Guardia. gravity survey was carried out in part of the belt of sub-circular

complexes surrounding the Ordenes basin.

The revealed of about 500 Ma 320 Ma for older elements in the basement of Galicia. geochronologicalinvestigations ages to Hercynian

Geochemical in the Cabo showed that the and form that of investigations Ortegal complex granulites eclogites a group was part an older,

continental The ultramafic rocks of the Cabo have been derived in from pre-existing crust. Ortegal complex may two melting episodes a

is that the in the in the second The proposed mantle-plume. It possible metagabbros complex originated melting episode. gravity survey

showed the existence of three positive anomalies, situated below high-grade complexesnear de Compostela, near Mellid and near

Sobrado, that can be interpreted as ultramafic diapirs.

A model involving continental rifting, mantle-plumediapirism, rejuvenation ofthe lower crust and initial seafloor-spreadingis proposed for

the evolution of the Early Palaeozoic continental lithosphereof the northwestern Iberian Peninsula.

Samenvatting

Mafische ultramafische uit in in in het en gesteenten het hooggradige complex van Cabo Ortegal NW Spanje zijn onderzocht het veld en

laboratorium met behulp van petrologische, isotopen-geochronologische en geochemische methoden. Isotopen-geochronologische

methoden Mellid uit werden toegepast op orthogneizen uit het complex en de blastomylonitische slenk tussen Malpica en Tuy, op een glimmerhoudendeeklogietlens uit deze slenk enop eentwee-glimmergraniet uit het gebied rond La Guardia. Een gravimetrisch onderzoek

werd uitgevoerd in een gedeelte van de gordel van sub-circulaire complexen rond het Ordenes bekken.

Het geochronologischeonderzoek heeft ouderdommen opgeleverd van ongeveer 500 Ma tot 320 Ma vooroude elementen in het Hercynische grondgebergtevanGalicië. Het geochemische onderzoek aangesteenten uit het Cabo Ortegal complexheeft aangetoonddat de voorkomende granulieten eneklogieten een groep vormen die deel uitgemaakt heeft van een oude continentale korst. De ultramafische gesteenten uit het

Cabo Ortegal complex kunnen in twee smeltepisodes zijn ontstaan uit een gepostuleerde mantelpluim. Mogelijk zijn de in het complex voorkomende metagabbrostijdens de tweede smeltepisodeontstaan. Het gravimetrische onderzoek heeft het bestaan aangetoondvan drie zwaartekrachtsanomalieën in de ondergrond rond hooggradige complexen bij Santiago de Compostela, bij Mellid en bij Sobrado die geïnterpreteerd kunnen worden als ultramafische diapieren.

Er wordt een model voorgesteld van continentale slenkvorming, mantelpluim diapirisme,rejuvenatie van de onderste korst en initiële zeebodem-vorming voor de ontwikkeling van de vroeg-Paleozoische continentale lithosfeer in het noordwestelijke deel van het Iberisch

Schiereiland.

GEOLOGICAL SETTING are given by Koning (1967), Den Tex & Floor (1971) and

Den Tex (1977).

For the geological setting of the complexes under

discussion the reader is referred to Vogel (1967) and Engels

(1972) for the Cabo Ortegal complex, to Hubregtse (1973) GEOCHEMISTRY OF THE CABO ORTEGAL for the Mellid complex, to Den Tex & Floor (1967), Floor COMPLEX (van Calsteren, 1977a)

(1966) and van der Wegen (1977) for the blastomylonitic graben, and to Buiskool Toxopeus et al. (1977) for the La The geochemical dataof the mafic and ultramafic rocks in

the Cabo in of the Guardia area. Several related aspects to Galician geology Ortegal complex are interpreted terms 58

rocks. The mafic rocks in origin and evolution of the obtained from rocks in the Cabo Ortegal complex (see

granulite and eclogite facies appear to belong to one above). A metamorphosed and tectonized calcalkaline of geochemically homogeneous group quartz-normative granite gneiss suite from an area near Mellid revealed a

tholeiites, enriched in elements, and with mean Rb-Sr whole-rock isochron of Similar lithophile age 409 ± 24 Ma.

La/Sm and K/Rb ratios of3.25 and 284, respectively. This orthogneisses from the southernpart ofthe blastomylonitic

has similarities to continentaltholeiitessuch as group great graben yielded a Rb-Sr whole-rock isochron age of466 ± 29 Columbia River Plateau basalts and the Deccan Traps the Ma, while an orthogneiss from the northernpart ofthe same

et and (Schmitt al., 1964). Metagabbros amphibolites grabengave a Rb-Srwhole-rock age of 462 Ma. The ages of

in the western of the to the occurring extremity complex appear orthogneisses are interpreted as reflecting an episode of another of form geochemically homogeneous group granitic magmatism between 465 and 400 Ma ago,

olivine-normative tholeiites. This group of rocks is apparently contemporaneous with the intrusion of the

impoverished in lithophileelements and has mean La/Sm ultramafic rocks from the CaboOrtegal complex. A genetic ratios ofabout and and K/Rb unity 319, respectively. They relationship between these two types of magmatism seems tholeiites are comparableto oceanic island such as occur on obvious.

and Hawaii & Iceland (Schilling, 1973) (Schilling Phengites from an eclogite lens near the township of La

Winchester, 1969). Pyroxenite layers in the ultramafic Pioza in the northern part of the blastomylonitic graben

have rocks characteristics similar to the enclosing have Rb-Sr ages averaging 374 Ma while their mean K-Ar

with somewhat peridotitic rocks, although higher age is 331 Ma; paragonites from the same lens have K-Ar element and lowerLa/Sm and K/Rb mineral incompatible contents, ages averaging 352 Ma. The ages between about

ratios. This shows that the pyroxenite layers originated 370 and 330 Ma of these paragonites and phengites from rocks of local peridotitic by processes partial melting. correspond to the age deduced for the end of the

The ultramafic rocks are depleted in incompatible hornblende granulite facies metamorphism in the Cabo in elements, showing that they were the residual phase a Ortegal complex. This suggests that the conditions of which the melt fraction left the melting episode during high-grade metamorphism came to anend at aboutthe same

system (Raleigh melting); a similar behaviour is inferred, time in both high-grade complexes. of the for example, for the high-temperature peridotites In the area around La Guardia in SW Galicia, alkaline

Lizard and and & Green Tinaquillo by Frey (1969) Frey two-mica granites occur that originated before the second

However, the and ratios of (1974). high mean K/Rb La/Sm Hercynian deformation phase, and are genetically related indicate 747 and around9, respectively, that the peridotitic to a regional migmatization episode in W Galicia (Buiskool rocks themselves the melt fraction in another represent Toxopeusetal., 1977). The Rb-Srwhole-rock isochron age

melting event. ofthese granites is 318 ± 21 Ma. The timeintervalbetween

be These geochemical characteristics may explained the end of the high-grade metamorphism in the high-grade

invoking two successive episodes of partial melting, with complexes and the emplacement of these granites is thus

ultramafic rocks the melt in the and the forming phase first, comparatively short, so a genetic relationship between the

residual phase in the second episode. two episodes cannot be excluded.

ISOTOPE GEOCHRONOLOGY OF THE CABO GRAVITY SURVEY (Keasberry et al., 1976)

ORTEGAL COMPLEX (van Calsteren et al., 1977)

in of A gravity survey was carried out part a basin with The in terms of of isotopic age dataare interpreted the ages mainly sedimentary rocks around Ordenes, and in a girdle geological events. For the ultramafic rocks a Rb-Sr of high-grade complexes surrounding this basin. This isochron is obtained of487 ± 122 Ma, which is interpreted revealed the existence ofthree separate gravity anomalies. as representing the intrusion age of these rocks. An asymmetrical positive anomaly with an amplitudeof 25 Phlogopites separated from the ultramafic rocks have a mGal, compared with the regional gravity, located around Rb-Sr K-Ar age of 394 ± 10 Ma, while the ages ofdifferent Santiago de Compostela, is accompanied by two sym- minerals from various mafic and ultramafic rocks also metrical positive anomalies with amplitudes of 10 mGal cluster around 390 Ma; this age is interpreted as and 15 mGalaround Sobradoand Mellid, respectively. This approximating the time at which the granulite facies points to the presence of three deeprooted structures of metamorphism terminated. The Rb-Sr whole-rock biotite dense material, which can be interpreted as ultramafic age of354 ± 17 Ma measured on granulites, and the Rb-Sr Overmeeren diapirs. Van (1975) uses the same mineral 10 age of 344 ± Ma ofedenites from the ultramafic interpretation as one of the possible explanations for the rocks, are interpreted as reflecting the end of the conditions gravity distributionpatternin and around the Cabo Ortegal of the hornblende granulite facies metamorphism. complex.

ISOTOPE GEOCHRONOLOGY OF OTHER

PRE-EXISTING ELEMENTS IN W GALICIA (van MANTLE-PLUME MODEL (van Calsteren, 1977b)

Calsteren et al., 1977) The data summarized above, together with numerous data

Isotopic of the collected by the dating orthogneisses in Hercynian basement research group 'Galicia' from the of W Galicia of the Petrology of the yielded ages same order as the ages Department State University at Leiden 59

and many other research teams working in the western and The regional temperature increase caused by the

northernpart ofthe Iberian Peninsula, are incorporated in a mantle-plume induced anatexis in the deeper parts of the

hypothesis for the Palaeozoic evolution of W Galicia. crust, leading to the generation of calcalkaline granitic

The starting point ofthis model is a normal continental magmas. These magmas intruded higher parts ofthe crust,

crust consisting of igneous mafic and sedimentary rocks, mainly along deep reaching faults, sometimes causing

It contact aureoles. with a metamorphic grade up into the eclogite facies. is metamorphic The oldest representatives

assumed that in the Early Palaeozoic a mantle-plume of these granite suites are now orthogneisses. Someof the

developed under the crust. A short explanation of the calcalkaline magmas were enriched in juvenile material,

phenomenon'mantle-plume' seems appropriate here. derived from the mantle-plume or its diapirs, and

differentiatedto peralkaline granite magmas (Bailey, 1974)

in of from the At some level the mantle, perhaps at a depth several that also intruded higher crustal levels. Heat in of hundreds of kilometers, a comparatively thin layer mantle-plume also caused migmatization higher levels of alkaline two-mica originates with a density lower than the density of the the crust, and the generation granite overlying mantle material. Such a density inversion can be suites. The relatively low-grade metamorphism of sedimentsin the shows caused by an anomalous composition of the mantle(mantle penecontemporaneous upper crust

with the heterogeneity) or by a rising heat current from the lower a rough spatial relationship younger granites. caused deformation of the mantle. At certain places on the interfacebetween low and The mantle-plume strong

high density material, the lighter material will tend to rise crust. The regional updoming induced deepreaching faults

(Raleigh-Taylor instability). The viscosity ratio between along which mainly vertical movements took place, causing

the rising material and the wall-rock determines the style horst and graben tectonics as exemplified by the of the If between and The diapirism. the wall-rock is more viscous, blastomylonitic graben Malpica Tuy. rim-synclines will develop and the diapir becomes blastomylonitic zones, varying in thickness from a few

centimeters to concentrated the mushroom-shaped. If the material of the diapir is more 200 meters, are near

borders of the These viscous, therewill be no rim-synclines, and the diameterof high-grade complexes. zones were the diapir at its top is less than at its base. A firstorderdiapir formed during the mainly vertical movements thatbrought

to can have a diameterofabout 100 to 200 km depending on the complexes theirpresent position, mostly surrounded the thickness of the low density source layer (see: by low-grade, weakly deformed Palaeozoic sediments. The

Whitehead & Luther, 1975). horizontal schistosity planes occurring in rocks from

originally deep parts of the crust might be caused by the

In the proposed model, a first order diapir, for the sake of upward pressure of the diapirs, or the lateral spreading of convenience called 'mantle-plume', was situated in the the diapiric hats. mantle below Galicia. In this mantle-plume the diapir smaller scale. production process was duplicated on a

Second order diapirs of lherzoliticcomposition developed, CONCLUDING REMARKS

which intrudedthe lowercrust. Withinthe crust thedensity

inversion ceased to exist and the diapir lost its buoyancy. The model sketched above offers an explanation for the

Some lateral extension might have accompanied the Early Palaeozoic evolution of Galicia that is visible in the

process. high-grade complexes. Alternative models appear to be

The lherzoliteof the Cabo Ortegal complex is regardedas inadequate considering the now available data. The

of the second order and its representing one diapirs, hypothesis that the high-grade complexes are relics of a evolutionwill be discussed. The lherzolites Precambrian proposed were (Grenville) orogen (Parga-Pondal, 1956; the melt fraction of the mantle-plume at great depth as Vogel & Abdel Monem, 1971) must be rejected, based on

indicated by the high La/Sm and K/Rb ratios. During the the geochronological data. However, Precambrian crustal diapiric rise the lherzolitesolidified. At a depthofabout 100 elements might have been preserved elsewhere in the km, in the astenosphere, partial melting started again, Hercynian orogen of the northwestern Iberian Peninsula. leading to a liquid of tholeiitic composition (cf. Green & The gravity data make the hypothesis that the high-grade

Ringwood, 1963) that was able to leave the diapir, and complexes are remnants of a nappe (Ries & Shackleton, intruded the lower crust. The of and A group metagabbros 1971) very improbable. formation of the complexes

be amphibolites in the CaboOrtegal complex may regarded duringthe Caledonianorogeny is unlikely, since theeastern as representing this small of the magma. A very part margin of the known Caledonian mobile belt was situated melt-fraction was not able to leave the diapir, and some 300 to 400 km west (following a pre-Continental Drift crystallized in situ as pyroxenite layers and veins in the reconstruction). Whether the mantle-plume might also be lherzolite. The last melt-fraction in the lherzolite held for the very responsible Hercynian orogeny in Galicia is a crystallized at a temperature ofabout 1100 °C as veins and matter ofspeculation; the available data are inconclusive to lenses consisting mainly of phlogopite and pargasite or this respect. edenite (cf. Green, 1973). Heat given off by the lherzolite diapir induced granulite facies metamorphism in the One might speculate that the thermal events recorded

medium to crust. The in already high-grade metamorphic around 450 Ma ago several other Hercynian terrains in group of rocks with quartz-normative tholeiitic com- Europe are also caused by processes comparable to the Galician position may be regarded as part of this older crust. proposed mantle-plume. 60

ACKNOWLEDGEMENTS Onderzoek Centrum at Petten this study would have been

impossible. Besides those already mentioned in the in foregoing or the other papers of this dissertation my

this dissertation thanks C. W. A. M. The investigations made for were are also due to: Messrs. W. Laurijssen, proposed by Professor Dr. E. Den Tex and form part ofthe Devilé and C. van der Bliek for photographical work; M.

his J. K. A. A. research program of the working group 'Galicia' under L. Brittijn, Bult, Hakkert, B. G. Henningand H.

I benefitted much from his extensive Sion for the of C. guidance. very preparation drawings; M. Deyn, H. de knowledge ofthe geology ofthe Iberian Peninsulaand other Graaf, C. J. van Leeuwen and J. Schuilenburg for the of the of parts of the Hercynian orogen, and processes preparation of thin sections; J. Verhoeven for the X-ray crustal differentiation and evolution. Of considerable diffractionwork; F. Benavente, J. König, R. Scheveers and support to me was his interest in the progress of the B. Voorhorst for mass-spectrometric isotope analyses; Misses Hortensius investigations and the great freedom he allowed me. M. and M. Kaspers and Mr. J. C. van

carried out in Belle for the of the for The geochronological investigations were preparation samples mass- the Z.W.O. Laboratorium voor Isotopen-Geologie at spectrometric isotope analysis; Mrs. V. B. Kwee and

Amsterdam under the direction of Professor Dr. H. N. A. Messrs. R. Heijboer,K. K. H. Knies and F. Sanches for the

Priem. The resulting paper was prepared in cooperation major element analyses; Mrs. J. M. Kutova for the X-ray with the laboratory staff and Professor Den Tex. fluorescence spectrometric analyses; Mrs. E. Moes-

The reported geochemical analyses were carried out by kops-Stoeltie and Messrs. L. IJlst and J. Smeer for the

Dr. E. A. Th. Verdurmen of the Z.W.O. Laboratorium mineral separations. Mrs. M. Petit-Puts and Mrs. E. M.

Prins-van voor Isotopen-Geologie at Amsterdam and through the der Drift typed the manuscripts and Mrs. Chemisch Onderzoek Petit-Puts also them for offset 'Werkgemeenschapvoor Analytisch prepared reproduction. van Mineralen en gesteenten' (WACOM) by Mr. K. M. The manifold and ample discussions with my colleagues

Stephan of the State University at Leiden and Mr. J. in the working group 'Galicia' of Leiden University, in

Zonderhuis of the Energie Onderzoek Centrum (ECN) at particular with E. J. Keasberry, H. Koning, R. P. Kuijper

Petten. and L. D. Minnigh were fruitful and indispensable. Dr. A. carried Senior of in the of The geophysical investigations were out by a was great help correcting English my fieldparty underdirectionofMr. E. J. Keasberry. Here my manuscripts. participation consisted in some ofthe calculations involved I am greatful to 'Chartered Explorationes, S. A. and the geological interpretation. (Madrid)' for providing core drilling samples of the

I am indebted to Prof. Dr. R. D. Schuiling and Prof. Dr. lherzolites.

D. E. Vogel for critical reading and discussion of some of A grant from the 'Molengraaff Fonds' is greatfully the manuscripts. acknowledged.

Without the cooperation of the technical staff of the My special thanks go to my wife Corrie and to my

Institute at the Z.W.O. Laboratorium children Geological Leiden, Martijn and Majanka for providing my motivation voor Isotopen-Geologie at Amsterdam and the Energie and stimulation during the research for this dissertation.

REFERENCES

D. 1974. of alkaline as a result of Bailey, K., Origin magmas - & Floor, P., 1967. A blastomylonitic and polymetamorphic anatexis. Chapter VI, l,b,in:The alkaline rocks, ed. H. Sprensen. 'Graben' in western Galicia, (N.W. Spain). In: Etages

John Wiley & Sons, London, 622 Ed. pp. Tectoniques. La Baconnière, Neuchatel, Switzerland, pp.

Buiskool Toxopeus, J. M. A., Haalebos, P. E. M. & Over- 169-178.

meeren, F. A. 1977. An outline ofthe and structural - & 1971. of van, petrology -, A synopsis the geology of western Galicia. In: geology of the Hercynian complex south of (, Histoire structurele du Golfe de Gascogne. Ed. Technip, Paris.

Spain). In press. Engels, J. P., 1972. The catazonal polymetamorphic rocks of Calsteren, C. of the P. W. van, 1977a. Geochemistry poly- Cabo Ortegal (NW Spain), a structural and petrofabric study.

mafic-ultramafic at Cabo metamorphic complex Ortegal (N.W. Leidse Geol. Med., 48, pp. 83-133.

Spain). Submitted to Lithos. Floor, P., 1966. Petrology of an aegirine-riebeckite

1977b. A for the Paleozoic -, mantle-plume-modelinterpretation gneiss-bearing part of the Hesperian Massif: The Galineiro and of geology Galicia with emphasis on the Cabo Ortegal area (N.W. Leidse Geol. 36, 1-203. surroundingareas, Vigo, Spain. Med., pp.

Spain). Proc. Kon. Ned. Akad. van Wetensch., Ser. B, Phys. Sci. Frey, F. A., 1969. Rare earth abundances in a high-temperature in press. intrusion. Geoch. Cosmoch. 1429-1477. peridotite et Acta, 33, pp.

- N. A. I. E. Boelrijk, M., Hebeda, H., Priem, H. N. A., Den - & Green, D. H., 1974. The mineralogy, geochemistry and Tex, E., Verdurmen,E. A. Th. & Verschure, R. H., 1977. Isotopic origin of Iherzolite inclusions in Victorian Basanites. Geoch. et

dating of older elements the Cabo Cosmoch. (including Ortegal Acta, 38, pp. 1023-1059. mafic-ultramafic complex) in the Hercynian of orogen N.W. Green, D. H., 1973. Experimental melting studies on a model

Spain: manifestations of a Paleozoic mantle under water-saturated presumed Early upper compositionat high pressure Submitted mantle-plume. to Tectonophysics. and water-undersaturated conditions. Earth and Planet. Sci. Lett.,

Den Tex, 1977. A continental rift E., pre-Variscan system in 19, pp. 37-53. NW Spain. - Krystalinikum (in press). & Ringwood, A. E., 1963. Mineral assemblages in a model 61

937-945. 1973. Iceland mantle mantle composition. Jour. Geoph. Res., 68/3, pp. Schilling, J.-G., plume: geochemical study

Hubregtse, J. J. M. W., 1973. Petrology of the Mellid Area, a of Reykjanes ridge. Nature, 242, pp. 565-571.

Precambrian polymetamorphic rock complex, Galicia, NW Spain. - & Winchester, J. W., 1969. Rare earth contribution to the

9-31. ofHawaiian lavas. Contr. Miner, and 27-37. Leidse Geol. Med., 49, pp. origin Petrol., 23, pp.

Keasberry, E. J., Calsteren, P. W. C. van & Kuijper, R. P., 1976. Schmitt, R. A., Smith, R. H. & Olehy, D. A. 1964. Rare-earth,

Early Paleozoic mantle diapirism in Galicia. Tectonophysics, 31, yttrium and scandium abundances in meteoritic and terrestrial

T61-T65. II. Geoch. Cosmoch. 67-86. pp. matter, et Acta, 28, pp.

roches et Koning, H., 1967. Les types de basiques ultrabasiques Vogel, D. E., 1967. Petrology of an eclogite and pyrigarnite qu'onrencontre dans la part ie occidentale de la Galice (Espagne). bearing polymetamorphic rock complex at Cabo Ortegal, N.W.

Leidse Leidse Geol. Med., 36, pp. 235-242. Spain. Geol. Med., 40, pp. 121-213.

A. - Overmeeren, R. van, 1975. A gravity investigation of the & Abdel-Monem, A. A., 1971. Radiometric evidence for a catazonal rock complex at Cabo Ortegal (N.W. Spain). Precambrian metamorphic event in N.W. Spain. Geol. & Mijnb.,

293-307. 749-750. Tectonophysics, 26, pp. 50, pp.

PargaPondal, L, 1956. Nota explicativadel mapageologicodela Wegen, G. van der, 1977 in preparation. NO de la de la Coruna. Leidse Geol. Jr. & D. 1975. Dynamics of parte provincia Med., 21, Whitehead, J. A. Luther, S.,

467-484. and models. Jour. Geoph. Res., 80/5, pp. pp. laboratorydiapirs plume

Ries, A. & Shackleton, R. M., 1971. Catazonal complexes of 705-710.

remnants of northwest Spain and north Portugal, a Hercynian thrustplate. Nature, 234, pp. 65-68.