Isotopic dating of older elements (including the Cabo Ortegal mafic-ultramafic complex) in the Hercynian orogen of NW Spain: Manifestations of a presumed Early Paleozoic mantle-plume

Clwmwal Gcolog'~,, 24 (1979) 3,5-I~6

3,b

~,.~Elsev=er Sc.entific Pubhshing Company, A r n a t e r d a m - Printed in The Netherlands

I S O T O P I C DATING OF OLDER ELEMENTS (INCLUDING THE CABO O R T E G A L MAFIC-LILTRAMAFIC COMPLEX) IN THE HERCYNIAN OROGEN OF NW SPAIN: MANIFESTATIONS OF A PRESUMED EARLY PALEOZOIC MANTLE-PLLIME

PW.C. V A N C A L S T E R E N ~''',N.A.I.M. BOELRIJK i,E.H H E B E D A i, H.N.A. PRIEM i, ~,E A T h V E R D U R M E N ' and R H VERSCHLIRE'

E DENTEX

Z W.C) Luboratorlum voor Isotuperl Ge~lu~w, Amsterdam I I (The Netherlands)

Geologlsch en Mmerulogtsch Illstllulrt, R(]hsurm,erslh, lt, Lc.ydcn (The NcHwrla.ds) Received Februal.'y I, 1977, accepted for publ,catu.m January 19, |97Pt)

ABSTRACT Van Calsteren, P.W C , Boelrijk, N.A I.M , Hebeda, E H , Pr=em, H.N A., Den Tax, E , Verdurmen, E.A.Th. and Verschure. R.H., 1979. Isotopic dat..ng o1' older elements (in. eluding the Cabo Ortegal m a l ' i c - u l t r a m a f i c c o m p l e x ) i n the Hercynian oroL~en of NW Spain manifestations of a presumed Early Paleozoicmantle.plume. Chem. G e o l , 2 4 35-56 K - A r and R b - S r ages are reported for high grade metamorphic rocks from pre Her cynian elements m the Hercynian orogen o1' NW Spain and I'or an Early Hercynian su=te of granites (hall'-I,l'e STRb = 5 () • l 0 ~" a, errors 2o ). Ten whole-rocks I'rom the Early Hereyn.'~n suite ul' alkaline gran.tes near La Guardia have an .sochron age of 318 ! 2l Ma with initial r~Sr/16Sr = 0.7143 .* 0.0044. I.n the b l a s t o m y l o n i t i c graben between MaJpica and Tuy five orthognelsses near Vigo reveal an ,sochron age o1' 466 * 29 Ma with in=t.al nTSr/m"Sr = 0.71(.)3 i 0 0029, wh.le stx orl, hogne=sses between Malpica ar, d Noya .ndicate an age o1' around 462 Ma. Six orthogne.sses near Mellid have an isochron age of 409 .~ 24 Ma wd.h initial °'~Sr/m6Sr = 0.71000 ; 0.00048 All orthogne.sses belong to the suite of calcalkal.ne to peralkaline granites Paragon.tes and pheng=tes rrom an eclogite lens assocmted with paragneisses near ha Pioza, m the blastomylonit,c graben, have K - A x ages averaging 352 and 331 Ma, respectively; the pheng,te R b - S r ages average 374 Ms. In the Cabo Ortegal mal'ic~ultramaftc complex, 21 mineral dat.es range 4 3 9 - 3 7 3 Ma, clusl.ering axound 390 Ma A R b - S r analysts o f six whole r~~ck Iherzolites suggests an Early PaJeozoic age o f the order of 500 ( *. 100) Ma(imtialmTSr/mbSr = 0 7 0 3 t 0.001) T w o phlogopltes and two edenites w.th the appropriate whole rocks from the Iherzolites derine R b - S r ages of 394 -~ 10 and 344 -~ l 0 Ma, respectively Mal'=c granulites .n the Cabo Ortegal complex def, n e a n i s o c h r o n ( t e n whole rocks and o n e b . o t i t e ) o f 3 5 4 .t 17 M a w . t h mit.almTSr/*~'Sr = 0 70446 *. 0 0020 It..s attempt, ed to interpret the age data m terms o1' a recently proposed model assuming an Early Paleozoic manLle-plume beneath the continental crust in ,n the NW Iberian penin sula. The motopic ages suggest, that, th.s mantle plume regime started at. around 5(.)0 Ma It induced h.gh-grade m e t a m o r p h i c conditions =n the lower crust,, folding, and a regional updoming with horsl, and graben tectonics. Early .n this regime Iherzohtm magmas were generated from the mantle-plume and intruded the lower crust, inducing granulite fat.as contact aureoles. Maf.c m ~ r n a s d,l'ferentmted from the Iherzol.t.c magmas. Processes of anat.exis in the lower crust lead I,o the generation or suites o1' calcalkaline to peraJkaline granites between ~bout 465 and 410 Ma. About. 390 Ma ago, the conditions of granul.te

36

lacie=, toni.act, metamurphism gave way to condit.iori~; ol hornblende granuhle lae=~-% which in t.hen l,urn wPre Lerrnmated at, about 350 Ma. About 32(.) Ma ago alkaline and calealkaline suit,es or granit.es were R~neral,ecl, before Lhe seetmd (mostly Ih~, mare) Hercynian I'oldmlt pha.~e.

I N'T'RODUCTION During t,he last, t,wo decennta a large amount, or dal, a Im,.,~ been gat,hered on t,he geoh:)gy or Lhe Hercynian orugen ol` wesLern Gahcta (NW Spare) by re search t,eams o[' Leyden Universit,y. The result,s or t,hls work have I~een sum rnartzed by den ,max and Floor (19'71), whde much or t,he earlier work has been included, among c)t,hers, in t,he Carte (3~,ologiqtm du nord.o,est de la pi, mnsule th~,riq,e(196'7). Pig. 1 shows a ~eologicaJ sket,t.'h map ol' west, ern Galicia. The area is underlain by a Hercyntan basemerlt, IJredominanLly consist, rag ol` mel,asediment,s and dll,t'erent, t,ypes o1' gq'antLes, tn which vartous element,s or a IJre Hercynmn cr'yst, aJline basement, have been preserved. This paper present,s t,he result,s of =sol,eft're' age det, ermtnat, ions on Early Hercynian gq'anil,es and various rocks I'rom t,he pre.Hercynian basement, in west,ern Oahcta (3 EO LO(3 IC A L S ET'T'IN(3

Rocks solely al,l,eet,ed by Hercynlan t,ecLonot,herm',d event,s in t,he NW Iberian peninsula are mainly metasediment, s and various gwanlI,ic suit, as. Tile metasediment,s are sequences or predominant, ly arenaceous t,o pelit,=c ci~mpo sltlon, t,o whtch an Early Paleozotc age is t,ent,atively assigned (van Meerbeke at,

',.tl., 1973; Engels et, .,d., 1974; van der Meer Mohr, 1975; M]nnigh, 1975; Buiskool Toxopeus et. aJ., 1978). They were l`olded, metamorphoses, migmaLtzed and invaded by granit, lc magmas d u n n g t,he Hercyman orogenesis. Three i.)hases of l`oldmg are recognized, a hrst, pimse w=t,h N -S LsoclmaJ !,old ing OLrld sub horlzont,a] axtal plane, a second phase wlt,h stmfla.r I'old axes and avert, lcaJ axial plume, aml a t,hird phase el` local chevron folding The met,a rnurphism reached t,he low t,o mt, errnedmt, e presstire anlphiholit, e !`acids, causing local migmat, izat,u.m and was followed by retrograde 6"q'eenschist, l`acles In west,ern G'aJlcla t,he sedlment,aJ.'y succession IS incomplete and constderably reduced as compared t,o slmdar sequences m east,ern Galicta and heft, hem Port, ugal ( & ' h e r m e r h o r n , 1955; Mat, t,e, 1968; R,ihelro, 197,4). Among t,he Hercyntan gwanit,es t,hree sutt,es can he dlst, mgmished, =.e., deformed cah:aJkaJine I~'anit,es, undeh~rmed caJcaJkaIine gr=lit,es, and t'olmLed t,o ma,~sive alkaline gra.ntLes (Ploor at, ',11., 1970; Capdevila at, al., 1973). Bot,h groups el` c',.dcaJ k'aJine gTa.nit,as are b lot.i t,e ul,goclase gw'amit,es Lo gra nod iorl t,~s, ytelding R h - S r whole-rock ts~chron ages* el` 316 +- 10 and 297 + 11 Ma, R e~.'alculated

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Fig I Geological sketch map or Galicia (inset' Cabo OrLega] rnafic--ulLramalic complex), shosmng t.he Iocat, ions o1" the investigated samples. A. samples COR 30, 3 l , 37, 77/78, 79/80 and 81/82, B C O R 2 3 , 4 5 , 6 l and 6 2 ; C ' C O R . 116, 118, 119, 120and 12l; D Heb 5, 50, 51, 53, 54, 55, 56 and 57; E' Heb 35, 36, 37, 58, 59, 60, 61 and 62; F: Heb 38, 39, 40, 41 and 42. The geology is based upon the work during the last two decennia by research t.eams of Leyden LlniversiLy

'

'/

38

respect, wely (Priem et, al., ]970). The alkaJine ~antLes are paJtngenet, i(' t,w(.) mica 6q'mlit,es, ran~ng l~'om migmat, tt,ic l,(.~ fairly homogeneous int,rusive rna,s se~. Elements of the pre-Hercynmn LlasemenL are mainly represent,ed by urt, hu gne.sses and several sub-circular complexes predommanLly made uf marie t,o ulLramal'tc roc ks and paragmeisses. The ort, hogneisses are sutt,es ot' caJcal kaline t,o l,~eralkahne gra==iLtc gneisses which have been preserved in the bLasLumylon =fie graben bet,ween Malpica and T u y ( Ploor, 1966; den Tex and Floor, 1967), m the Ordenes ha,.,~inaJld iLs peripheral belt,, and in the L,altn ccmiplex (Hdgen, 1971 ). In most, cases t,wo pre Hercynian del'ormaLion phases can be recognized, an older w]Lh N - S and a younger wtth varying fold dh'ecL=ons, but, h with sub-hortzont,al ax=al plane. A reconnatssmlce R,b--Sr whole.rock isoclu'on sl,udy of samples cullecl,ed all over Galicia revealed an Early PaJeo zoic age (455 t 12 Ma*, Priem el. al., 1970, 1972) for t,h=s ep.sode of granit, w magmaLism. The ort, hogneisses are ,nL.maLely a,?,soclaLed wtth marie rocks: amph]boliLic dike swarms and a few amphibolit, icst, ocks. This a,soc~at, ton indicaLes that, the gramLic inLnJsions were coeval wlt,h basalt.it magmat, lsm. Besides ort, hogneisses, the bla.sLornyloniLic graben conLains also paragnelsses, which are associaLed with amphiboht, es a.nd, near La Ploza, lenses or low t,emperal, ure eclogiLe (Lype C, according Lo Coleman el, aJ., 1965). Most. pre Hereynian rocks are polymeLamorphic, showmg the impnnts of Hercynmn and older events. The older evenLs of meLarnorph~sm are up int,o the clinopyrox ene-garnet, granuJit,e facies. EffecLs of Hercynian meLamorphism are also reflecLed in the R b - S r and K - A t mica ages (Pnem et, al., 1970, 1972) Sufl,es of pre.Hercyman real'it and ult, ramal'ic rc.'ks are the predonlinant, consLit, uents of six htgh-grade metamorphic mhers, wholly or part, ially fault, bound, throughout, the Hercynian orogen of the NW Ihenan peninsula. Wholly fault,-bound are the sub-circular complexes of Braga.nqa and Morals m nort, hern Port, ugal (Anl',honioz, 19'70)and of" (;abo Ort,egal m Lhe NW exLremlt,y of Spain (Vogel, 1967; Maaska.nt,, 19'70; Engels, 1972) Three sub circular complexes in the peripheral bell surroundmg the Ordenes hastil (near Santiago de ComposLela, Mellid and Sobrado, respectively) are at, least, partaally fault,-bound (Koning, 196'7; Warnaars, 1967; van Zuuren, 1968; Hubregt,se, 1973). The complexes also cunLain minor amounts o1' meLasedi ment~ (polymeLamorphic caJcsilicaLe rocks and garnet,-bLot, it,e bmetsses). All rocks have been only liLLle influenced by Hercynian metamorphism (up mt,o the gq'eenschtst, facies, locally the Iow-gracle amphibolit,e facies). Their maJn t,ecLonic and meLmnorphic trnprint, is due t,o pre-Hercynta.n evenLs t,hal, have reduced high-grade metamorphism and a much sLronger penet;raLive deror maLion than tmposed during Hercynian orogenesis. ClraviLy inveaLigat, ions have shown that, the C,abo Ort,egaJ complex and the complexes near Sant,mgo de ComposLela, Mellid and Sobrado are deep-root,ed massive sLrucl,ures (van Overmeeren, 19,75; Keasberr'y el, al., 19,76). These mvesl,igaLions rule out, the *RecsdculaLed

w,Lh,~mTRb

: l 39 •

10"a'

39

possibility that the complexes of high-grade rocks would represent remnants of nappes thrusted from the west, over the Paleozoic rocks, as proposed by R,ies a.nd Shacldeton (1971). The Cabo Ort,egal complex, being of special import,ante to this paper, is discussed in some detail in the next, section. THE CABO ORTEGAL COMPLEX The major components of this complex are ultramafic rocks (mainly spinel-pargasite peridot, ite, paxt,ially serpenLinized), marie rocks (tholeiitic met,agabbros: eclogites, granulites and amphibolttes), and quart, zo-feldspaUc metasediments (garnet-biotite ~,meL,~sesand minor calcsilicate rocks). All rocks have undergone high-grade metanmrphism, culminating in the eclog~te and granulite facies and followed by phases of retrogradation. Three preHercynian folding phases with N-S, E-W and N - S directions, successively, can be recogmzed, all with sub-horizontal axial plane. Hercyman orogenesis reduced two folding phases, the common N - S isoclinal folding with vertmal axtal plane and the younger phase of local chevron folding; the metamorphism reached the low-grade amphibolite facies (Engels, 1972; Engels eL al., 1974). Mctasediments. Paragneisses tn the eclogite facies are handed and intimately straUfied with mafic eclogites. Both in the eclogite and in the granulite facies the gneisses commonly have glandular and blastomyloniUc textures, while in the granulite facies rocks phenomena of anatexis can be observed. Amphibolite facies paragneisses have in some places preserved original sedimentary features such as compositional banding.

Maflc rocks. Eclogites form three roughly parallel, NN E-trending ridges in the eastern part, of the complex vJld numerous lenses within the banded paragneisses. They show steeply inclined foliation and parallel banding. Blast,omylonitic textures are common. Granulites occupy the central part of the complex. Locally, migmatization has taken place and barren pegmatoid veins were formed. The amphibol,te facies rocks are divided into two separate unit,s by a major, NNE-trending fault, zone near the western margin of the complex. To the east of this zone the amphibolites are strongly laminated and inhomogeneous, containing reUcs of mafic granulite. Amphibolites to the west, of the fault zone are homogeneous, with a nematoblastic texture parallel to the schistosity and devoid of high-grade relics. Ultrama[ic rocks. Relics show that the part, Dally serpentmized spinel-pargasite peridotite is a retrograde Iherzolite, and the rock is therefore referred to as Iherzolite. The orthop,froxene ( En 40%) and the olivine ( Fo 90%) were partially replaced by serpentine minerals, the former now being present as bastite. Most, of the clinopyroxene (diopside) has been convert,ed into pargasit, ic to edenitic hornblende. The original spinel changed its composition

4(I

towards chromlt, e or was replaced by m~meUt,e. Phlogopit, e is (~ccasionally present, both as an accessory, and in veins. O~lortt,e is a~l accessory. G a r n e t has only o n c e been observed (m a blastomylonit, tc zone of tectonic 'move

men t, ). Lherzohtes crop oul, mt, hree major masses aJld some 40 smaJler occurrences. Along the sout, hern fault boundary of the largest, lherzolite hody l,here occur lenses of completely serpent, mtzed, slickensided peridot, tt,e. The lherzolil,e (-ontains bands, veins and lenses of other ult, ramafic, rocks, which are ex l,~lained ms partial melt,,ng products of the lherzohte (Maaskant,, 1970). These p'aJ'tia] melting producL~ are, tn the probable order from ohl to young' SlJmel free websterite (most common type oi' bands), garnet-spinel pyroxerutes (in bands and mlcrofolded veins), and vetrm essenl,,ally made up ~-~fphlogupil,e and edertitie hornblende {near the roof of the lherzolite body). Veins of carbonate and serpentine minerals are post, Hercynlan phenomena. All ultramafic rocks display intense folding and deformation, IocaJly leading t.o blasl,omylonil, tc zones. EXPERIMENTAL

PROCEDLIRES AND CONSTANTS

Whole roc.k powder a.hquot,s were [,aken from t.he (~rushed and pulverized samples. Amphiboles, mlcas and the omphactLe were separated by means (.ff a modified l%'~mLz isodynamic separator (Verschure and IJlsl,, 1969) and a laboratory overflow centrifuge (Id]sl,, ]973b) employing a seL ul" sl~abillzed heavy liquids (Id]st, 1973a). In most eases the purity of the mineral eoncen trates was over 99.9%. Rb and Sr eonLents were measured emther by X-ray l'luorescence spectrom etry or by mass-spectrometric isotope dilution. Both Rb and St" were separated by means of ton-exclmnge chromal,ography. Wil, h t,he excepUon of one set oi' samples ( I.~ Guardia gramtes, see Table [I), all isotuptc measure. men{~ were made on a Varian(~) OH5 mass-spectrometer wll, h Faraday cage collector, digital output and computer control. RepeUtwe analysts of the NBS-987 SrCO~ standard gwes a value or 0.71016 ± 0.00016 (2o). The L,a Ouardia granites were analyzed on a 9,0-cm, 60" Nier.type mass-spectrometer with digital output and mulUplier detection, and adjustmenl, of the a'TSrl~Sr ratio Lo the value 0.7081 in the Eimer and Amend SrCO~ sLandard. If noL stated otherwise, all ~'TSr/8~Sr raUos were me~.~ured directly on tmsptked Sr The errors in the R b - S r data are estimated at I% ror X-ray fluorescence analysL~ of RblSr, Sr anti Rb (Verdurmen, 1977), I% for motope dilution analys.s oi" Rb and Sr, 0.05% for °TSr/S6Sr measured directly on unspiked Sr (except when obtained with l.he Nter type ma~-spectrometer, where the error is estimated at 0.2%) r and 0.2% for 87SrlS6Sr calcuJat,ed from ,sotope ddubon runs. The R b - S r isochron ages and errors were computed by means of a least-squares regression analysm according to York ( 1966, 1967). The errors quoted for the isoehron ages and initm] aTSrl°6Sr ratios are 2,o. K was analyzed by flame photomet, ry with Lt mLernaJ standard and CsAl

41

buffer. A.r was extract,ed in a bakeable-glass vacuum apparaLus and determined by isotope-dilution t,echniques in a Reynolds type glass mass-spect,rometer; all measurement,s were made by the stat,ic method. For K and Ar the analyt,ical errors are estimat,ed to be within 1 and 2%, respectively; only when the K content is less than 0.1%, does the error become larger. The age calculat, ions were made using t,he following constants: ~lRb

^ = 1.39.

IU 'lla

~

'lq~K: ,~, [] ,5.85

• 10'la'l

abundance "'K = 0 0118 atom percent total K

^~j = 4 72 • I0"'~

a ', and

Where necessary, ages quot,ed from the Iit,erature have been recalculaLed.

hwcsligatcd samples attd results Isot.opic age invest,lgations were made on samples from t.he foUowing groups of rocks: ( 1 ) Early Hercynian t,wo-mica grantt,es; (2) pre-Hercynian granlt, lc bmeisses; (3) a pre-Hercynian eclogite lens associated with paragneisses, and (4) various rocks in the mafic-ult,ramaftc Cabo Ort,egal complex. The Iocat, tons of the Investtgat,ed samples are marked on t,he maps of Fig.l. A summary of the results is listed in Table I.

Early Hercyman two nuca gramtes. Ten samples of t,wo-mica granite were cotlect,ed along the coast, near La Guardia in SW Galicia and invest,igat,ed ac cording t,o the R b - S r method (Table II). The rocks belong t,o the Hercynian pallngenetic sult,e of alkaline graniLes and display the second Hercynian scillst,osit,y (Buiskool Toxopeus eL al., 1978). A regression analysis through all data-points defines an isochron of 318 -* 21 Ma with inttlal BTSr/a"Sr = 0.7143 *- 0.0044 (Fig.2). Pre-Hercynian granttw gneisses. Granitic gneisses belonging t,o the preHercynlan SUlt,es of caJcalkaline t,o peralkaline granites were sampled near the townshll.~ of Mellld in central Galicta (ort,hognelsses wlt, h augen textures, 6 samples), in the Vigo area (planolinear ort, hogneisses, 5 samples), and in the nurt, hern part, ol' the bla,stomylonlt, tc graben beLween Malpica and Noya (planullnear ort, tlognelsses, 6 samples). All rocks were investigated according to the R b - S r method (Tables I l l - V ) . The Melhd ortho~,meBses have an L,;ochron correlation corresponding t,o all age of 409 *- 24 Ma with initial *TSr/*~Sr = 0.71000-+ 0.00048 (Fig.3). Less rigorous linear correlat,ions are shown by the sets of samples from Malpica Noya and Vigo. The best fit, bne through the Vigo data potnts has a slope corresponding to an age of 466 -+ 29 Ma with initial a~Sr/S~'Sr = 0.'7103 -+ 0.0029 (l~ig.4). The dat,a-points of the Malpica-Noya set, of samples scat, ter around a reference line w3th a slope corresponding to an age of 462 Mav,nth init,ial a'TSr/at~Sr = 0.'708 (Fig.5). Pre-Hercyman eclogite lens. Near La Pioza, likewise in the blastomylonitic

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T A B L E I[ R b - S r whole-rock data o1' the t,wo-m.iea granit,ea near La Guardia Sample No.

Rb*' (ppm wt )

Sr*' (ppm wt.)

~,Sr/Bf, Sr t ~

B,R.b/m6Sr

L?OFt CUR (.?OR COR CC)R COR COR COR COR CC)R

333 348 278 299 33r 28'7 ;]13 273 3;]1 292

52 t) 60 I 8.% 9 56 2 44 8 b8.7 78 I 59.4 90 I %5 6

0.'7968 0 7893 0/7570 0.7861 0.8088 0 7784 0/7640 0 7733 0.7601 0 7829

18 69 16.94 9 404 t5 54 21.59 14 21~ II 68 I3 ;18 I0 70 15 33

~9 89 93 96 97 98 99 100 IOI 102

*~ X ray Iluc~reseence s p e c t r o m e t r y ( d u p l i c a t e analyses)', *~ measured on the 20 cm, 60" Nier.typt= ma~,~ spectrometer ( d u p h c a t e analyses).

I

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TW(') MI('.A ('~F{ANITE':, LA (%UARI')IA

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" '

AGE 311"~'L'I MI:] ( .,r:,' .,r )~, L) 714 3 ' (.)(JC)44

l~l'lTl'~h

''''"

I II4. lal IIII ' , ,I~T

lq)II

II I ]~ ~

Rb/

Ii I

I

ll~ I

Hla I

:,r' I

Fig.2 R b - S r whol(-, rock data ploL or t w o mnca granites alonl~ =,he coast near I.a Guardia, SW Galicia.

graben, I'nve samples were Laken from an eclogit,e lens a,qsoeiat,ed wiLh para~,meisses. The rocks eont,a,n phengite (2 samples), paragoniLe (2 samples) or boLh ( 1 sample); these micas were separat,ed and analyzed according t,o t,he R,b-Sr and K - A r meLh(Jds (Tables VI and VII). The phengit,es appear t,o have younger K-A.r ages t,han the paragonites: averaging 331 Ma and 352 Ma, respect, ively. The R,b-Sr ages of Lhe phengit,es average 374 Ma (calculat,ed with an a-,~sumed init, ial *'TSr/°~'Sr rat,io of 0.705).

,44

'FABLE III R b - S r whole rock data of orthoRneisses near Melhd Sample No

R,b *l ( p p m wt, )

Sr* ' ( p p m wt )

~ 'Sr/~'Sr

~'R.b/msSr

COR 30 COR31 CO R 3'7 COR77/78 CC} R, 7 9 / 8 0 C,C)R 8 1 / 8 2

5,4 5 695 8 b. b 3 32'~ HO 6 99 O

99 3 180 I 45 658" 168 140

0 7189H 071611 0 '72006 071024 O,'71775 () '72181

1 !'~92 1 123 1 '7(19 O()146 l 397 2 064

*' X ray fluoresc.enc.e spec.trometry. (duplicate analyses), except for the figures maiked *~ which were obtained by duplic.at.= isotope dilution analysis.

T A B L E IV R b - S r whole-roc.k data of orthognetsses between MaJpica and Noya Sample No.

Rb ~ ( p p m wt. )

St* ( p p m wt )

m'Sr/m'Sr

"'R,b/"^Sr

CUR COR C,OR COR

240 206 175 131 86 9 IH3

'72.4 2'7 6 48 9 70 2 I I2 89 4

() 77067 0 84088 0 77948 0.74323 0.72231 0 74470

9 652 21 56 10.4fi !~.4t)() 2 250 ~ 936

COR

22 23 45 53 61

COR

62

~X ray fluorescence spec.trometry (duphc.ate analyses)

'FABLE V R b - S r whole-roc.k data ol' orthonneisses near ViBo Sample No

Rb* ( p p m wt.)

Sr * ( p p m wt.)

,7Sr i~,Sr

a, Rb/,,Sr

COR COR COR COR COR

134 148 183 141 268

124 47 8 55 0 71 1 60.9

0'73101 0.77066 0 77403 0.74638 0.79178

3.140 9 043 9 661 5 762 12 83

21 56 57 63 71

4 X ray fluorescence spec.trometry. (duplic.ate analyses)

I

I

L)RTH(')('~NEI']':d-'., MIELI.II")

l

:,r/

.Ir

"

,(J~

,41 -I)

/1'~

AC~I." 4l.)l.-:r~,'q~ MO (14If".',r/ Idh..,r),, q-)tIC)()(.)o t'i,i('.P(.)48

,,,"

~I=ItNIJ

I ll.lm I I I I I i i

I

~,l i ua J IT '() /(ili

Ji,9

ill=

Rb/'~ .,r

I

I=

I

I

Fig 3 R b - S r whole rock daLa plot, of orthogneis.sea near Mellld, cenl, ral Gahcia

'

'v,o'o

'

o 7B

'

~

vl.~?~

/

AGE

J

'

'

;,i"+

~

4 6 6 . * 2 9 Ma

/~87Ftl') 1 .t9, I(.I Ilo I error'., J(T

.3 7~P~,/

azRb/~6Sr >

I

I

I

I

I

I

I

i

i

I

I

I

I

FiR 4. R b - S r whole rock data plot o1" ort.hognemae~, near Vigo, SW GnilCla.

The Cabo Ortega/ma[tc---ullrama/'ic complex. In the Cabo Ort,egal complex suites oF eclogltic, granuliUc arid lherzolitic rocks were investigated, together with 24 minerals I'rom these units and associated rocks. K - A t measurements were made on 19 hornblendes, one biotite, t,hree phlogopit,es and one omphacit,e separated from marie amphibolites, ma.fic b~anulit,es, maric eclogites and Iherzolit,es (Table VIII and Fig.6). With t,he e•ception o£ two hornblendes and the omphacite, all data he between 439 and 373 Ma, clust,enng around 390 Ma. R,egardmg the deviating mineral dates, t,wo of them (hornblende and pyroxene Heb 45) appear t,o be excee,s A.r ages (see Discussion). The other hornblende (Heb 47) has a very low K content, so the K analysis is rat,her inaccurate, while this mineral is also highly sensit,ive to the presence ot" a slight, amount, of excess radiogenic A.r.

46

/ ,/'

U F#I"I-I I'l L'~N E I ", '.',I- '., HAI. F'ICA NOYA II,

,/ /'

~

,,,,,' A

,,,,'

iI I I i. il-,,,,,#l'tl I,. I r

.i~ .

I

IJ,'

,¢

i w~

F~I-I'-EF4EI~ILI- AI:,I- ,-le.~;Jl,'lq'J (l"lT",l-/Hl', -) 1"1 71.)~

,,'

'

,J 'l

t~tLitl~"

B

'

I'~ M '1, II II

I

' I ~1'~ I.'i, tl I~ ".,I ,4

II

Fig h R b - S r Claliela TABLE

IJ

',J~

?l

whqlle rock dat.a plat. o f q,t.hcl~nei.,.,ses b e L w e e n M a l p l e a and N,~'a, NW

VI

Rb-Sr

phenglt.e data o r the ecluglt, e lens near L,a Pl~.=za I

Sample

Rb*'

Sr ~'

No

(ppm wL )

(ppm wt. )

_ I

(I(.)R [ 16 COl-( 121

~ 'Sr/~"Sr ~t

~'Rb/~"Sr

Calculated age (Ma)

(I '76()3~ 0 7,.13~,-I

ll) '73 '7 2b,-I

;t'7() :1'7 Pt

I

;.i,3~ 320

91 73

I21'I

*' I~,~liol:te d l l u L I o n ( d u p l i c a t e analyses), * ~ c a l c u l a t e d f r o m the isot~.lpe d l l u t l ( m

run~,

Five samples ol' maJ'=c ecloglte were analyzed accordmg to the R b - S r method (Table IX). Four of the samples axe slicescut, from a single specimen; the fifth sample comes from the same locality. The samples do not show a linear correlation bet,ween R'~Sr/86Srand ~TRb/B6Sr. From t,he maflc granulit,es, ten samples were analyzed according to the R h - S r method (Table X). The samples axe slices cut, from three banded specimens, taken from different localities within a distaxlce of some 1200 in. Biot,ite, a scarce component m these rocl~% was separated from one sa.mple and also investigated A regression analysis through all 11 dat,a-polnt,s,in eluding the biotite, produces an isochron of 354 +- 17 M a with initialBTSr/~t'Sr = 0.70446 ± 0.00020 (Fig.7). If the biotite is omit,ted, the ten whole-rocks del'inl,an tsochrl.m of 362 -* 66 Ma with initial*'TSr/*~'Sr= 0.70444 +_ 0.00029.

4'7

TABLE VII K - A t mineral ages or I,he eclogite lens near La Pmza Sample No

Mi,eral

K*' (w't. %)

Radiogenic ~"A~* ' (ppmwt • 10')

Calculated age *=(Ma)

COR 116 COR 118

phenglt.e phenL~lle÷ paragonlle paragonil.e paragonit.e ',~hent,IIt.e

'7 94 I 29

198 35 5

324 354

(.) 392 0.880 ? P,8

10.9 2,3 7+ 205

358 347 337

(.:,OR. 119 COR 120 COR 12I

*' Duplicate analyses, e~eept, for I.he A.r analysis marked ' which represent.s a quadruplieal,e analysis The eonl.ribul, ion o1" at.muspherle '"Ar 1.o the I,ol,al 4"A.r =s aboul. 8% for the phen~lt.es. I I'7, rot the phPnglte--paragunlt.e mixl, ure, and bel.ween 20 and 40% I'or t.he paragunll.es, ~~es/..mat.Pd error 3%

The K - A r analysL,~ o f t,he biot, lt,e gives a date of 396 Ma, concordant, wit,h t,he K - A . r ages o1" five hornblendes from t,he maJ'lc granullt,e (Table V I I I ) , but, slgniflcant,ly higher than t,he R,b-Sr isochron age. Eight, samples o f Iherzollt,e rrom one core drilling (down t,o a dept,h o1" some 150 m) were invest,igated. R,b-Sr analyses were made on six wholerocks and on edenit,e and phlogopit,e sepatat,ed from t,wo samples (Table XI). There are several possibllit,les t.o int,erpret, the obt,aJned R,b-Sr dat,a (Fig.8). A regression analysis t,hrough all six whole-rocks yields an isochron corresponding t,o an age ,.11"48'7 ± 19.2 Ma wit,h init,i',.d *TSr/8"Sr = 0.7033 -+ 0.0007. If one devlat,ing point, (t,he sample wiLh t,he lowest, Rb/Sr rat,to) is omit.t,ed, t,he ot, her five dat,a pomt,s define an isochron of 541 -+ T'/ Ma wit,h init,iaJ STSr/*"Sr = 0.70282 -~ 0.000')..3. The samples Heh 50 and Heb 51 and t,he phlogopit, es separat,ed from each sample deft.ha an age ot" 394 -+ 10 Ma (inlt,laJ '~TSr/B"Sr = 0.'70425 t 0.00026). C,o m b i n l n g t,he same whole rocks wlt,h t,he separat,ed edenit,es gives ml age o f 344 -+ 10 Ma (init,tal *TSr/8~'Sr = 0.70473 -* 0.00007). The phlogopit,es and edenlt, es involved in t,he lat,t,er calculat,lons have K - A r dates bet.ween 381 and 396 Ma, in accordance wit, h K - A . r mineral dates t'rom t,hree or, her Iherzollt,e samples (Table VIII). DISCUSSION

If t,he available isot, oplc dat,es are taken at, face value, t,hey demonst,raLe t,hat, t,he cont, lne,t,al crust, In t,he NW Iberian peninsula has been aJ'fect,ed by plut,onlc and met,amorphic act.ivit,ies in t,he Early Paleozoic, before t,he onset, o f Hercyman orogenesis. However, no major unconformities are recorded irl t,he O r d o v i c . a n - S i l u n a n - D e v o n i a n sedlment,a.ry, sequences in Spain and Portugal; moreover, a pre-Cont,inental D r i f t recons[:ruct,ion o f the nort,hern At.lant, lc shows t,hat, [,he east.ern margin o f t,he Caledonian mobile belt, was

,414

'T'A B L B

VIII

K - A r mineral ares ol Iher'zohl,.es, granulil,.es, eelogit.es and amph~bnliLes in l.he/Cabo (')rl..egal c o m p l e x Sample N,~

Mineral

K *' (wl,. '3,)

Radiogenic "' A.r* ' (ppmwt.. 1(~ ~)

Calculat.ed a~÷,('Ma) ~

6 I)60 q) !,J~9 () 2 7 8 l) b i n 6 609 i) ,4h4 6 620 0 220 I,i, 402

1143 1'7 ()i,) ~ 39 ÷ + ÷ l b 45 2(16

I',3

phlogoplt.e edenlt.e edvnll,.e edenil..e phloRopil.e edenlt, e phlogoptt.u edenit.e edenlt.e

~3Pal|[llltf' Hub 27 Heb 30 Heb 3! Heb 32 Heb 33 Heh 35 Heb 3'7 H~I) 3'7

hornblende hurnblel'lde hornblende hornblende hornblende hornblende hornblende btol..it.e

I,i, 861 0 29l; | 13(I 1.231,)

2'7 bl'l ÷ 9.1~b 33.|0 3h 8b

! I,)[14

3].,6b

] 130 I 3bU 5 '7'71,'1

;14 45 39 9b I'714 +

hornblende hornblende hornblende

I,).311,) I,) 3(.)h () 3;3b

9 9.1 141.i,)i,)

41,l'7 ,ll,),q

I 1,2b

,-12,4

0 3'79 (') 29h U U2

L'2.Oh 31 61 2.2'2

431 111U 1144 t'

0 1,)14

3 1)6

4'7'7 h

L,Iterzolttv Heb Heh Heb Heb Heb Heb He'b Heb Heb

b b -19 h() b/O 51

51 52

14.LIt) +

21,)3 '7.62 12.()~

3~t3 314~ 392 3141 396 3N'7 ,~

391 4;~,.q 3144

41)'7 d 409 3'7b 3'73 396 3Pt9 3'7N 396 J

E,,I, Jgrtt, Heb 3t4 Heb 4 ( i , HetJ 41

M~,la/~;abhr,~ Heb ,43 Heb ,4b Heb ,4b

hnrnblende karint.hine omphacil,.e

.4 mphmhohle Heb 4'7

huvnblende

*' Duplieal,.e analvse~, except. I'm' Lhe figures + and ÷ ÷ ÷ whit'h reprerenl, single and I.riplicaL.e analy.,,es, re.~ppet.iw=ly The conl.wibul,.ion of al,.mu~phPric "°Ar t.,~ l,.he t.ot.al '~"Ar i~ lus~, I,han 15'?,. ror all analyse.,,, e~ccept, ro~ I.he pyroxene H~b .4b (abuuL 65%), l,.h~ hq,rnblendP Heb 4'7 (aboul. 25'T,) and I.he edenit.e HPh h2 (abouL 2L)'~,,), ¢~ ~sl.,mal,.pd ekH~r 3%, except. I',,r l,.he I'tRure~ marked a and b wh,ch have e~l,.imal,.ed Prrort~ ,,1' b% and,~vPI [L)%,respeeLiwly stt.uut,ed s o m e 3 ( ) U - 4 U 0 k m t.o Lhe west. ( P r i e m el. uJ , 19'70). 'T'he o n l y e p l s o d ~ o r ( m m u r ) I,¢,ct,onmm in t,he B a r l y P a J e o z o i t . has t,a k e n p l a c e in I,he U p p e r C a m b n m l , as r o l l o w ~ I ' r o m t,he l o c a l a n g u l a r u n e , m l ' o r m i t , ies bet, w e e n i n f r a Cambrian-C',ambrmn a n d basal O r d o v t m a n ( S c h e r m e r h o m , 1955; RtbeLro,

49

ii

Ph II .

4

t-,

np

%

hi

e np

F'h np

Ph

I

till

e II bI

I il'ill Illl II.illt= ~'d~.lrlll P hi H I iI'lh=lrll'li =

h t: 40

i~l

~ np 49

b

blillllt

h

[I I:

hul I ll'lh#l ll.:il-' ll'iui il.ililP t~l Iqi(lili+

J

tI

Ill ,:lriuh h+ ITllllllhl

Ph I1p

h t:

ITI p

FltJIllTI,Jl Oil "1

i II

h()

,'.17

.q

I.],thbll

I

I

Aqv (Ms)

I'1 7 ,'17

en b

" h "~ ,'12

h ]1 ,11

h t' 'lh

,l/L)

i :'In( i

i 1 i'l( )

b

'f

,17

h "Lm 2~h 2 7,y P

[I,1

I

,ill

h I] 41

h I.!.3 4J

h n 5;#

i

r

i

i

i

1

4{)()

41()

42(')

4,1l)

440

4==0

Pig 6 HIst,olTramof K - A r mineral dates rrom the Cabi'l OH,ella] eqlmplex Three anomaluusly high dates are not included.

'?ABLE IX

Rb-Sr whole rock data ol' eclo~,lt,es in the Cabo C)rtegal complex Sample Ni.i.

Rb * ' Sr * ' (ppm wt.) (ppm wi.)

~'Sr/°"Sr * '

~' R.b/~"Sr

Heb Heb Heb Heb

1.35 3 74 6.23 9 44 b 07

0 70591 0 7061 I 0.70595 ÷ 0 70638* 0 70582

0 0128 0.0447 0 0872 0 1502 0 2072

38 39 40 41 Heb 42

299 242 206 182 70 8

*' Isot.ope dilution (duplicate analyses), *~ duplicate aneJyses, except, ror the fil,,mres marked ' whieh represent, t,riplieat,e analyses. 1 9 7 4 ). It, IS t,h u s nut, p o s s i b l e t,o InLerpret, t,he E a r l y P a l e o z o i c e v e n t , s o r

plutonism and met,amorphism a.nd t,he recorded isot, oplc ages in t,erms or Caledonian orogenesis. On Lhe oLher hand, t,hese evenLs have convenient, ly been int,erpret,ed by van CaJsLeren ( 1 9 7 7 ) a s maniresLat, ions oi" a manUe-plume beneat, h t,he ngid cont, inent, al crust or t,he NW Iberian peninsula in Early Paleozoic Lime. This manl,le-plume is t,hought, I,o have caused a st,eepenlng of Lile regional geot, hermal gTadient, (development, or high-grade met,amorphlc condlt, lons In t,he lower crust,) and regional u p d o m i n g o1" t,he crust,. A.[so, magma of" IherzollLic composlLion was generat,ed in t,he plume, which inLruded t,he lower crust, by means o f diapLric upwelling; t,he int, rusions induced an add.it,tonal local st,eepening o f the geot,hermal gradient,, resuJt,mg in granulit,e facies conLact, aureoles (Engels eL at., 1974 ). This model explains several

b( I

TABLE

X

Rb-Sr data of granullles =n the Cabo Url.egal e~m=plex Sample No ~'

Heb H,b Heb Hub Heb Heb

31 WR 32 WR 3b WR 36 WR 3'7 WR. 3'7 Bi~* Heb 58 WR H~b b9 WE Heb 60 WR H,'b 61 WR Heb 62 WR

Rb *J (ppm wt.)

St*' (ppm wt.)

~ r / ~ " S r .4

RTRb/~"Sr

7 42 14.9 ÷ 16.2 ÷ 17.1 ÷ 24.1 ÷ 373+ 8 38 10. Sb i 1.25 8.34 7 64

192 ÷÷ 265 ÷+ 6H I ÷÷ HI.2 ÷÷ 207 +÷ 12.U4 +* 37 1 4,57 342 3.79 3.70

i,I '1(.)546 +÷÷ tl 70572 *÷÷ 0 7 0 7 7 6 +÷+ O.'7t)739 *÷+ (J 7L164.bt÷÷ 1 1656 +*÷ 0.'70462 O .70445 () 7(14.71 l).'70477 0 70443

I11116 (l 163,-I 06892 () 6090 () 3880 937 l).0660 l).(1666 0 0953 l) 0637 I.) 0606

*l WR = whole rock, BIo = blot.lie, ~ ISotope diluLion (single a.naJy=,es, except, fur l,he I'i[,lure~ marked ' which represent, duplieaLe analyses), *j X.ray guorescence spectrom etrv (duplicate analyses), et~¢epl ror the figures marked "* which were obta,ned by duplicate Isut.ope dilution analya,.% '" .single analysL% except for the figures marked 4,÷ ÷ which represent duplicate analyses Por the biotite, ~78r/6,,Sr was eaJeulated I'rom the isotupe-dilu t,on runs I"

f

1

"1"

1

"r"

r

M A F I C I G R A N U L I T E E, ~.~7~,~ C A B ( : ) O R T E G A L

•

_+.._-1ti I II i1~

I Illll

AC-~E 3 5 4 t17 M o '[

I l iJ

,I

I I.I,.], I I

J("~'S,r'/u~"Sr')c- 0 7 0 4 4 6 t C)C)C)C)2C)

uTSr/~,~:.Sr. IXsTl~b i .,u,.l,, I, o' I~r ror',,

~

U 2 i

J 17"

/:17..., 11"~1:.~

HD/

I

I.I 4 JL

~F L~ t'~

Fig 7 R b - S r data plot of whole rock (with error bars) and a biotil.e (etrch,) from marie [,Tranulites in I,he Cabo ()rtegal complex. g e o l u g w p h e n o m e n a o b s e r v e d in the E a r l y PaJeozolc h m t o r y o f G a l i c i a and un.relat,ed to a m a j o r o r o g e n i c p e r i o d , such as the o c c u r r e n c e o f d e f o r m a t , o n pha~es w i t h h o n z o n t a J axiaJ p l a n e and v a r y i n g f o l d axes, t h e h i g h - g r a d e metam o r p i ~ l s m , and the e u n L e m p o r a . n e o u s t'elsw a n d marie ma.gmatism. T h e retoon

'3i

' T A B L E XI R b - S r dat,a of' Iherzohtea in l,he Csbo Ortegal c o m p l e x Sample

Rb ~ = ( p p m wt.)

~r* a ( p p m wl,.)

, ,Sr/,~r

,, Rb/~,Sr

N u l'l

Heb Heb Heb Heb Heb Heb Heb Heb Heb Heb

101 3 97* 353* 66 I 2 65* 343* I 86* 9. b l ÷ 5.31 * 6 68*

431 677 ~ 367' 2'74 619' 344* 29 3 361 129 IU6

0.70804 0 7048(.) U.71982 t) "/0804 0 "/04t't(.) 0.'72028 l).71)4! I ().70464 *~ 0 7039(.) U 70409

(I 6809 U 017 2 865 U 6994 0 012 2 892 0.1839 U.0762 0 1190 i.).1833

50 WR 5(.) Ed bU Phi 51 WR, 51 Ed 51 Phi 54 WR 55 WR b6 WR 57 WR

*n WR = w h o l e rock; Rd = edenite; Phi = p h l o g o p i t e ; *~ ,'K ray guorescence spectrom. PI,ry ( d u p l i c a t e analyses), except for the figures marked ' which were o b t a i n e d by m u t o p e - d i l u t i o n analysis', ~' J d u p l i c a t e analysis.

,~

7,.~( I

LHERZOLITE CABO

S

ORTEGAL

',_'

L)?I% T

',P" '

AGE (Mn) '1

I.~-~7~

,,/"

.,' ....

h"

WR

I

,4/~7

t 122

(3 70:EI .o I.'lOOO7

2 WR/F'hl

~94 t

1U

q) 71.~',.'=.,' U(X:)O2('.il

., Wl~/I-d

.~4

11)

(:) 704 7 :{ I O(')(.K:~,'.)7 ~

A u~I~D

M/Rb/~:'%r

~3,~,~

'1

,, ,~'

R lq "If"/141 ii-lf

I

I 3q ~101

! I) t

P 1

2

I

I I I f I Fig.8. R b - S r daLa ploL oi" whole-rock ( w i t h error ba~), t w o edenites (cru~es, wiLh luw R.b/Sr ratios), and t w o phlogopiLes (crosses, w i t h high Rb/Sr ratios) I'rom Iherzolites in I.he Cabo Ortegal c o m p l e x L~ochron I is calculated using aJl she whole-rock points. The rererence line is o b t a i n e d when omit.Ling one slightly deviating p o i n t (the sample w i t h lowest Rb/Sr ratio)• I ~ c h r o n a 2 and 3 are calculated f r o m t w o w h o l e . r o c k s and separated phlogopit.es and edenitee, respectively In brackets the K - A . r dat.es o1" the minerals are given•

h2

',d updarnmg caused t,he horst³ and LCraben t,ecLUnlCS, a clla.ract,ensLic t'eat,ure of' the Early Paieozoic in the NW Iberian peninsula, and may also have been responsible t`or |,he condensed sequence o1" Early Paieozoic sediments. Pollowing Lhis model, iL is at,t,empLed Lo interpret, the measured Early PaJe¢.~zoic ages as reflect, rag evenLs |,hat, were manJt`est,at,u.)ns ot` the mant, le.plume re,me. All orthoL,Plelsses ore t,aken t,oget, her a..~one sult,e ot` caJcalkalme Lo per'aJ kaline L.,q'anit,es. In the hlast,omyloniLic L=Traben, near V=go a.nd beLween Maip=L:a and Noya, the ort³hogneisses ore daLed at 465 Ma. Ort, hot.,meL,~ses near Mellid have a.n age ol" 409 Ma. These dal, a encompass the whole Silurian period, con I'Lrming geolobqcaJ observat, ions suggest,lng t,hal, int,rustons ot` the ca.lcaJkaime t,o peraJkal.Jne granit,m suit,e t,ook place over a considerable Lime-span. A~.' cord,ng t,u t,he mant,le plume model, the generat,ion of` this suite o1" ~ranlt, tc mat.~mas result,ed t,rom processes of, anaLexis induced in t,he lower ~.'rusL by heat, I'rom the mant³le-,plume beluw |,he NW Iberian peninsula, so |,he oldest, groups of` orthognelsses should also denot³e the minimum age or the mantle plume. In connect,ion with t,he presumed genesL,~ of the caJcaikaJme t,opera] ka.hne sulLes ot` granites It, is sign||'|cant, t³o not, e that, they were emplaced in ch.)se assocmt,ion wit, h real'it magmas, which can plausthly be explained as dit`l'erent, tat,lon products or the ulLramai'ic magma. Within the Cabo Ort,egaJ complex t,he pat, t,ern ol" isotopic ages is rat,her ~.'umplicat,ed. The R,b-Sr whole rock isochron of" 48'7 Ma o1" the Iherzollt³e based upon all six invest.igat, ed samples has |,he very large error o1" 122 Ma, merely placing the age somewhere m Lhe Barly PaJeozoic. Omil:t,i.ng one deviat, ing sample I'rom l,he calculat, ion gives an isochron a.ge ot` 541 -+ 3'7 Ma, but, |,here IS no apparent, geolo~eai or pet.,roh)L~Cal reason t,o reject, |,his poinL. Nevertheless, the whole-rock data suggest, an inl, n.L~ion age |,or the Iherzollt,e m |,he Early Psleozoic, 500 (-* 100) Ma ago. This view is support,ed by the minimum age ot` Lhe mant,le-plume LLhat³can be inferred t'rom the age oF about 465 Ma measured on ~ e oldest, suites of" ort, ho~leisses. A.nother pos slbdll, y might, be that, |,he rot,fusion age or the Iherzolii, e Is approxlmaLLed by |,he phlogopit,e whole-rock age ot` 394 -* 10 Ma, but, it³ seems more apprqpriat, e t,o mt,erpret, t,his age as reflect, rag the closure Lime ot` the phlogopit,e m a cooling Lherzolit, e mass. The edenit, e whole rock age of, 344 -* 9 Ma may be inLerpret,ed as approxlmat,ing t,he closure t,ime ~.~t,the edemt,e R,b -$r syst,ems at, the t,ermmat, ion ot` |,he re~on'a.I hornblende granulit, e.t`acies cond.it, lons. Constdering all posslbdit, tes, |,he authors speculat,e |ha.t, |,he ma.nt,le-plume r e , m e developed around 500 Ma a~o, and thai, the generation and int, ruston of, the Iherzolite Look place at, approximat, ely the same t,ime. Around 390 Ma a~o the granuJ.it,e-t,acies tonal||,ions m the Cabo Ort.egai area gave way t,o condit, ions o1' the hornblende... ~anulit, e factes. 'This t,ransit,ton is ret'lect,ed by the phlogopit, e R,b-Sr age m the Iherzolite and by |,he concent, ral, lon ol" K - A . r minerai dales. 'The hornblende-granulite t,ac|es regime prevailed unt,il about, 350 Ma ago, when these condit,ions in t,heLr Lu.rn were t,erminat,ed, a..~reflect,ed by the closure of, the R b - S r whole rock biot,ite systems =n |,be g]ranuJite and the R,b-Sr ederziLe syst,ems m the Iherzolit,e. Possibly, the pat,t³ern of" mineral

b3

ages in the La Pioza eclogite lens (374-Ma R,b-Sr phengite, 352-Ma K - A t paragonite and 331 .Ma K-A.r phengite) may also be interpreted as reflecting dtfl'erent, cooling stages during the wanmg conditions of the regional highgrade metamorphism. The behaviour or the R,b-Sr systems in the Iherzolite differs from that in the closely relat,ed granulites. In the latter rocks the whole-rock and biotite systems remained open until about 350 Ma ago. In the Iherzolites, on the other hand, only the edemLes remained open with regard to their R,b-Sr systems untU this date, whereas the whole-rocks and phlogopttes closed much earlier. This pattern may reflect compositional differences and/or differences in PH,,O between the granulites and Iherzolites, possibly in combinat, ion with a higher blocking temperature for t,he phlogopite than for the edenlte and biotite. If three obviously suspect dat, es are omitted, no K -A.r dates exceed a value or about, 440 Ma. However, when the K - A t data are compared with the R,b-Sr data, the biotite and all the hornblendes of the ma£ic granulites appear to have K - A t dates ca. 20--50 Ma higher than the R,b-Sr whole rock blot, ire age. Also, the K-A.r dates or the edenites in the Iherzollte are some 40 Ma higher than the R,b-Sr whole.rock edenite age. Such a pattern of consistently higher K-A.r dates than the appropriate R,b-Sr dates, is rather unusual. Presence of excess radiogenic A.r seems improbable, in view of t,he concordance of most, dates at around 390 Ma, even for minerals with widely varying K contents. The concentration of K-A.r dates at, around 390 Ma (concordant with the phlogopite R,b--Sr dates) is therefore thought to approximate the time at, which the K - A r mineral systems became closed, but it, is not understood why the R b - S r systems of some minerals closed much later. One hornblende (karinthine Heb 45 from an ec,logitic lens in the Cabo Ortegal complex) has a K-A.r age as high as 1110 Ma. This sample comes from the same oub.;rop as the karinthine reported by Vogel and Abdel Monem (19'71) to have a K-A.r age of 898 +- 38 Ma. Such a. large discrepancy between two hornblendes from the same ~-Jutcrop, as well as the high K -A.r age (1144 Ma) measured on the omphacite separated from the same sample, leaves little doubt that we are dealing here with ezcess A.r ages. According to the above model and the isotopic ages, the mantle-plunle regime in the Early Paleozoic of the NW Iberian peninsula should have been i.n operation for a duration of at least, 150 Ma. During this whole period the continental crust, must have remained sta.tiona.w relative to the mantle plume. Such a long duration for the life-t, ime of a mantle-plume is not implausible. Por example, the ma.ntle-,plume presentJy situated beneath Hawaii is thought to have been active for some 80 Ma, as evidenced by the t,rall or volcanoes ranging in age from Recent (H'awali) t,o about, 80 Ma (near Kamchatka) which was carried o f f by the NW-movi.ng Pacific plate (Morgan, ].9'72). The two-mica Lcra.ntte of I.,a Guardia was emplaced before the second Her cynian deformation phase, as evidenced by the schistosity in the rocks. It, is

!'~ ,4

t,hought that, the generatmn of this granJt=c magma, like that of the whole alkaline suite, LSgenetically related to the formation of the migmatites which are wide spread in western Galicia and appear to have developed in.close associat:ion with the folding and accompanying metamorphtsm up into t,he amphibolite facies. The dale of 318 -+ 21 Ma of the La Guardia two-mica graJ1. ires may therefore be taken as approzimating both t,he age of the migmatlzatlon and the age of the folding phase. This age is confh'med by the age of 316 -* 10 Ma determined t'or the su=t,e of deformed calcalkaline granites (which have also been folded during this phase), whde a minimum age is set, by the age o1" 297 *- 11 Ma of the suite of post. tectonic calcalkalme gnmites (Pnem eL al., 1970). Earlier specular, iotas about a possible Precambrian age of the Cabo Ortega] ma.fic-ultramal'ic complex and the blastomylonttlc ~ a b e n wlt,h its orthoI~meisaes (Parga-PondaJ, 1956; Vogel and Abdel Monem, 1971 ) are chsproved by the dating results presented in this paper. However, the possibLlity that other Precambnan crustal element,s have been preserved in the Hercynian orogen of the NW Iberian penmsuta cannot be excluded. CONCLUSIONS

The multiple events of high-grade metamorphism m the basement of the NW Iberian penmsula have produced a complicated pat, tern of isotopic age relationships. Following the mantle-plume model and the interpretations of available isotopic ages sketched above, the sequence of dated events through the Paleozotc of the NW Iberian peninsula may tentatively be summarized as fogows: (1) Mantle-plume regime, start, rag at around 500 Ma (Early Paleozo=c) and mducmg conditions of high-grade metamorphism in IJle lower crust: (a) generation and intrusion of [herzolitic magma at the beginning of the mantJe plume regime, induc|ng granulite facies contact aureoles; (b) dtfferentm.tion and ascent of mafic magma from the ultramafic magma; (c) generatmn and ascent, of suites of calcalkaline to peralkahne granites between about 465 and 410 Ma. (2) Termination of condit.tons of high-grade metanmrphism at around,35(.) Ma. (3) Second (and mostly main) phase of Hercynian folding at about 320 Ma, asSOCLated wlt, h low-grade amphibohte facies metamorpbtsm, large-scale rrdgmatizaLion, and generation and Lntrusion of alkaline and calcalkaline suites of granites. (4) Termination of Hercynian orogenesis at, about 300 Ma: generation and mta'usion of post-tectonic suites of calcalkaline gramtes. ACK NOW LEDG EM ENTS

The authors are much indebted t.o members o1' the research team "Galicia"

5`5

o f t h e D e p a r t m e n t o f P e t r o l o g y , UniversiL-y ol' L e y d e n , f o r m a n y fruitful discussions, p a r t i c u l a r l y t,o Messrs. K.P. K u i j p e r a n d H. K o n i n g . T h a n k s are aJso d u e t,o Prof. D.E. Vogel, UniversiLy ol" Leuven ( B e l g i u m ) , for c o m m e n t , s a n d criticism. C h a r t e r e d E x p l o r a c i o n e s S.A. ( M a d r i d ) p r o v i d e d the c o r e driJling s a m p l e s o1" t,he Iherzolite. Most, s a m p l e s of' Lhe b~'anites a n d ~,me=sses w e r e collected by Dr. P. Floor o1" the Department, o1" Petrology, University ol" Leyden. This work rorms part o1" the research programme of t,he "Stichting voor Lsot,open-Oeologisch Onderzoek", supported by the Netherlands Organ=zat,ion for t,he Advancement, o1" Pure Research (Z.W.O.). REFERENCES

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Floor, P, K.isch, H.J. and Oen, I.S, 1970 Essai de correlation de quelques granites hercyrdene de la Galice et du nord du Portugal. Bol Geol. Min., 81: 2 4 2 - 2 4 4 . I-IJIgen, J D, 1971 The Lalin Unit: a new al:ructural element in the Hercynian orogen o1' Galicia (N.W. Spain). Proc K. Akad. Wetensch., B74 3 9 8 - 4 0 7 . Hubregtse, J.J.M.W., 1973. Petrology o1' the Mellid area, a PrecambrLan polymetamorphic rock complex, Galicia, N.W. Spain. Leidse Geol. Meded., 49: 9 - 3 1 . Mist, L., 1973a New diluents in heavy liquid mineral ~paration and an improved method I'or the recovery o1' the liquids from the washings. Am. Mineral., `58: 1084-1087. IJlst, L., 1973b. A laboratory overl'low.centril'uge for heavy liquid mineral separat,on. Am M=neraJ.,`58 1088-1093. Keasberry, E.J., Van Calsteren, P.W.C. and Kuijper, R,P., 1975. Paleozoic mantle diapirism in Galicia Tectonophysics, 31 : T61 -T6`5 Koning, H., 196'7. Lea types des roches basiques et ult,rabasiques qu'on rencontre dans la pattie occidenl, ale de la Galice (Espagne). Leidse Geol. Meded., 36' 2 3 5 - 2 4 2 . Maaskant, P., 1970. Chemical petrology o1" polymetamorphic ultrarnal'ic rocks I'rom Galicia, N.W Spain. Leidse Geol. Meded., 45 237-32`5.

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Matte, Ph , 1968. La structure de la virgation hercynienne de Gahce (F.~pagne). Geol, Alpine, 44. 157-280 Minnigh, L.D., 1975. Tectonic and petrographic aspects o1' an area S.W. of the Lahn unit (N.W Spain). Leidse Geol Meded., 49:499--504 Morgan, W J., 1972. Plate motion and deep mantle convection. Geol. Soc. Am. Mern., 132' '7-22 Parga Pondal, I., 1956 Nots explicatlva del maps getdogieo de la p~te N(.), de la Provincm de Is Coruna Leidse Geol. Meded., 21 467-484 Priem, H.N A , B o e l r i j k , N.A.I.M., Hebeda, E.H.,Verdurmen, E A.Th and Verachure, R H., 1970 Dating events of acid plutonism through the Paleozoic of the western Iberian Peninsula. Eclogae Gaol Heir., 6 3 : 2 6 5 - 2 7 4 Prtem, H N A., Boelrijk, N.A.l.M., Hebeda, E H, Verdurmen, E A . T h and Verachure, R.H , 1.972 LIpper Ordovician/Lower Siluriasl acidic magmatlsm in the pre Hercynian base ment of western Galicia, N.W. Spain Rap Invest. 197('I/19'72 Z.W.O Lab Isolop. Geol., Amsterdam, pp. 123-127 Ribeiro, A., 1974 Contribution a I't~tude tecton=que de Trls.oa Montea Oriental Serv Geol. Port.., Mem., 24 168pp. Ries, A. and Shackleton, R.M., 19'7]. Cal,azunal complexes of northweaL SpaJn and north Portugal, remnants of a Hercynian thrustplate. Nature (London), 234 65--68. Schermerhorn, L.J.G., 1955. The age o1' the Bairn schists (Portugal). Bol Soc Gaol Port., 12' 7 7 - 1 0 0 . Van Calsteren, P.W.C, 1977 A mantle plume model interpretation I'or Lhe Paleozow history o1" Galicia with emphasis on the C,abo Ortegal area (N W Spmn). Proc. K Ned Akad. Wet,. Ser. B, 80' 156-168. Van der Meer Mohr, C.G., 1975 The paleozoie .~trata near Moeehe in Galicta, N.W. Spain. Leidse Geol. Meded, 49. 487-497. Van Meerbeke, G.L.E., H.ggen, J.D. and Floor, P., 1973. Preliminary. reault~ o1' the investiga tions o1' the central Galician schist area (prov. of Orense and Pontevedra, N W Spain) Van Overmeeren, R A., 1975 A gravity investigation o1' the catazonal rock complex at. C,abo Ortegal (N.W. Spain). Tectonophyaica, 26 293-307. Van Zuttren, A., 1969. Structural petrology of an ares near Santiago de Compostela, N.W. Spain Leidse Geol. Meded., 45 1-71 VerdtLrmen, E.A.Th., 1977 Accuracy of X ray fluorescence spectronmtry determination of R.b and Sr concentrations in rock samples X. Ray Spe¢trom., 6(3)' 117-122. Verschure, R.H. and IJl~t, L , 1969. An asymetncally vibrsLing unit for the Frantz magne tic separator. Rap. Invest 1968/1.969 Z.W O Lab [sotop Geol., A.n'alterdam, p 90 Vogel, D E., 1967. Petrology of an eclogite and pyrigarnlte bearing polymetamorphic rock comples at Cabo Ortagal, N.W. Spain. Le.id=e Geol. Meded., 40' 121-213 Vogel, D.E. and Abdel Monem, A.A., 1971. Radlometric evidence for a Precambrian metamorphic event in N.W. Spain. Geol. Mijnbouw, 5 0 : 7 4 9 - 7 5 0 Warnaars, F.W., 1967. Petrography of a peridotite , amphibolite and gabbro-bearing polyorogenic terrain N.W. of Santiago de Compostels (Spain) Ph D ']'hems, State University Leyden, Leyden. York, D., 1966. LeMt-~Cluares I'ittmg ol'a straight line. C,an J Phys, 44 107,9--1086 York, D, 1967. The best iaochron. Earth PLanet. Sci. Lett., 2. 479-482.