Geochemical and geochronological cross section of the deep Variscan crust: The Cabo Ortegal high-pressure nappe (northwestern Spain)

Tectonoph~strs,

263

177 (1990) 263-292

Elsevier Science Publishers

B.V.. Amsterdam

- Printed

in The Netherlands

Geochemical and geochronological cross section of the deep Variscan crust: The Cabo Ortega! high-pressure nappe (northwestern Spain) J.J. PEUCAT I, J. BERNARD-GRIFFITHS R.P. MENOT 4, J. CORNICHET ’ Laboratolre

‘, J-1. GIL IBARGUCHI *, R.D. DALLMEYER ’ and M. IGLESIAS PONCE DE LEON ’

de Geochronologte-Geochrmle

’ Departamento

lsotopque

de Mneralugia-Petrologio,

’ Dep~rtme~t 4 D$artement

of Geoloq,

de Giologte, (Received

du CAESS,

CNRS.

3+

3504-7, Rennes (France)

Unrverstdad dei Pais Vasco. PO 644, Bdbao (Sparn)

Un~l)er~lt~ of Georgia, Athens, GA (U.S.A 1

UmversrtP de St Ettenne, 42023 St. Etienne Cedex (France) February

10. 1989: accepted

July 12.1989)

Abstract Peucat, J.J., Bernard-Griffiths, Leon,

J., Gil Ibargucht,

M.. 1990. Geochernicai

high-pressure

nappe

Circum-Atlantic

(northern

Paleozoic

The ~~-pressure

Spain).

Orogens.

- 380 Ma. The occurrence

of final cooling

zircon

and Rb-Sr

and

constramts

The earliest magmas matety second

Sm-Nd

on the timing

tectonothermal

of probable the same

intermediate

grant&es. assemblages

vem has yielded

before

Together,

isotope

of earlier affirnty

of the deep Vat&can Terranes

(garnet-omphactte

data

massifs

J. and Iglestas Ponce de crust:

m the Vanscan

together

at 490-480

The Cabo

Ortega1

Belt of Europe

is recorded

at

f plagioclase).

and Rb-Sr

wtth REE and

and

of syn-accretton - 420

and trace

geochemistry

of LREE-ennched

facies. Thts occurred

protholiths

grant&es

Ma in metasedimentary ultrabasic

element

to the emplacement

Thrs event IS possibly

at

of the protoliths,

in the grant&e

of MORB-like

tectonometam~rrecrystalbzatton

mtca ages of up to 350 Ma. U-I%

maJor

Ma and corresponds

metamorphism

by several

and ampbibohte-factes

events and the nature

and their metamorphism formation

are represented

overtbrusting

by 40Ar/39Ar

tectonothermal

this suggests event

and related

basm. The age and origin of the associated

an emplacement

sectton

(Edttor),

regional

is recorded

as the eclogite-facies

metamorphic

setting in a back-arc

histories

event occurred

talc-alkaline ttme

high-grade

eclogite-like

Ph, Matte

rocks of the Cabo Ortega1 nappe separate

datmg

In:

R.D.. M¬, R.P., Cormchet.

cross

Tectonoph.yssrcs, 177: 263-292.

phm units which underwent

provtde

J.I.. DalImeyer,

and geochronological

related

rocks remains

assoctated

in an acttve

margin

formattons to a colhsional unclear,

at approxi-

wrth

haste

to

settmg.

A

assoctated

wtth

plate tectomc

but a late pyroxemte

age of - 390 Ma.

Introduction

The present investigation tramafic complexes exposed

At a time when seismic reflection profiles are revealing the existence of a layered Hercynian lower crust probably composed of granulites (Matte and Him, 1988), it would appear particularly important to measure the geochronological and geochemical properties of the HP/HT metamorphic rocks which are well known at outcrop_

Ortega1 complex, perhaps the most widely studied of these complexes, is well exposed in coastal sections. Similar rocks within the Ordenes Basin (Sobrado Unit) and northern Portugal (Braganca) were also investigated. Several different geochronological methods were used, involving whole-rock and mineral analyses. The nature of the source

~1951/90/~03.50

0 1990 - Elsevier Science Publishers

B.V.

focuses on mafic-ulin Galicia. The Cabo

264

rocks for the proto~th magmas is discussed as well as the ages of igneous and metamorphic episodes. Together, these data present a clearer picture of the t~tonornet~o~~c history of a key sector of the Hercynian foldbelt representing one of the deepest-formed exposed parts of the orogen. Geologleal setting Several metamorphic complexes, including a significant component of mafic-ultramafic rocks (HP/HT gram&es and eclogites), are exposed in the northwestern part of the Iberian Peninsula. Two of these complexes (Grdenes and Cabo Ortegal) are located in Galicia (northwestern Spain), and two (Branganc;a and Morais) occur in

GA80

Tras-os-Mantes (northern Portugal) (Fig. 1). These complexes are composed of structural units separated by thrust faults. Each structural unit IS characterized by a unique lithological association and tectonometamorphic history (cf. Arenas et al.. 1986). The four complexes occupy the inner part of regional basin-like structures developed during the third deformation phase of the Hercynian orogeny (D3) which was superimposed on folds resulting from tangential tectonic phases (D, and D,). The complexes structurally overlie low-grade, Early-middle Palaeozoic metasedimentary rocks which have been interpreted as allochthonous units with respect to the materials of the Central Iberian Zone (Schistose Domain of Galicia-Tras-osMontes, Fig. 1).

ORTEGAL

COMPLEX

nh,

SOBRADO UPPER ALL5CHtl~O~ WITH WPliil GAANULIT~~ ANO ECCOC~IES OPiifOL

IT rc

ORSAL UNIT AN0 W,‘L-IT

COHI’LCX YfTll P~RALKAL~NE HETAHOtW’HISI(

5CIIISTOSC DOMAlN - TRAS-OS-MONIES CENTRAL

IDERIAN

100

OF GALE

fwwwo LONE

ROCKS

In ” CHWlON

AUTOC}~llioN

km

1 Fig. 1. Geological sketch map of the northwestern Iberian Peninsula. Modified

from Arenas et al.

(1984) and Ribero et al. (1989).

GEOCHEMISTRY

AND

GEOCHRONOLOGY

OF THE

CAB0

ORTEGAL

The tectonothermal evolution of these complexes has been controversial. “Autochtho~st” hypotheses have suggested models relating the origin of the complexes to a mantle plume active during the Palaeozoic, or have considered them as “ horst-like” fragments of Precambrian basement structurally elevated during Hercynian compression (Matte and Ribeiro. 1967; Van Overmeeren 1975; Van Calsteren, 1977; Van Calsteren et al., 1979). By constrast, “allochthonist” hypotheses have implied that the complexes were recants of one or several thrust sheets emplaced during the Hercyman (s.1.) orogeny (Bibeiro et al., 1964; Ries and Shackleton, 1971; Bayer and Matte, 1979; Iglesias et al., 1983; Matte, 1986). Most recent authors have sided with the allochthonist view and have proposed that structural imb~cation of units that compose the complexes was associated with a period of general thrust tectonism that affected the entire region (probably the Dz Hercynian deformation phase).

Geology of the Caho Ortega1 complex

Detailed studies of the petrology, ge~he~st~ and structure of different parts of the Cabo Ortegal complex have been reported by Vogel (1967), Maaskant (1970), Engels (1972), Arenas (1985) and Ben Jamaa (1988). A general description of the main units composing the complex is given below using published results as well as new data obtained during this study. The Cabo Ortega1 complex is composed of internally, imbricated nappe complexes (Fig. 2): The lower nappe complex comprises tectonic slices of retrogressive high-grade rocks (eclogites) and slightly met~o~hosed recants of an ophiolitic suite (metabasalts and peridotites) together with various types of low-grade metasediments (limestones and metapelites) and acid metavolcanites. This nappe complex (Moeche Unit) has been interpreted either as a tectonic melange with a more-or-less continuous structure, or as a wildflysch resulting from the destruction of the thrust pile of mafic-ultramafic rocks (Fem&ndez Pompa and Monteserin Lopez, 1976; Arenas, 1985).

NAPPE

265

The upper nappe complex of the Cabo Ortega1 Massif comprises amp~bolite-grade metabasites, granulites. garnet- and clinopyroxene-bearing eclogites, paragneisses and metaperidotites. Only units within the upper nappe complex were investigated in this study. The overall exposure of the Cabo Ortega1 complex is controlled by a D, Hercynian synform. The eastern margin of the complex is in tectonic contact with low-grade Silurian metasedimentary rocks and acidic metavolcanic rocks exposed along the western flank of a regional Hercynian D, antiformal structure (the 0110 de Sapo antiform). Structurally upward. the Cabo Ortega1 complex comprises the following th~st-bounded tectonic units: (1) Amphibolitic to gram&c metabusites: These rocks are mainly exposed in western and southern parts of the complex where they constitute the Purrido-Pena Escrita and Candelaria formations (Fig. 2). The Purrido-Pena Escrita Formation ts mainly composed of medium- to coarse-grained layered amphibolites. This formation underwent prograde metamo~hism under amp~bolite-facies conditions. In some places, late retrogression to greenschist-facies associations may be observed. The Purrido-Peiia Escrita amp~bolites are essentially made up of hornblende, plagioclase and epidote/clinozoisite; garnet. rutile and sphene may appear as accessory minerals. Overlying the amphibolites (separated by a shear zone) are rocks of the Candelaria Formation. These latter are characterized by a greater abundance of gabbro. dole&e and plagiogranite relicts than that occurring in the Purrido-Petia Escrita Formation. Amphibolite is most common and is composed of hornblende, plagioclase and garnet. with rutile and opaques as accessories. Metagabbros and metadolerites locally preserve relicts of chnopyroxene and orthopyroxene, or are enriched in andradite garnet and epidote. Garnet coronas in metagabbros and garnet-plagioclase-clinopyro” xene associations locally suggest a higher metamorphic grade; however P/T estimates from garnet -clinopyroxene compositions yield values of less than 700 * C and 9 kbar (Gil fbarguchi et al., 1989). (2) Medium- to high-grade gneisses: These rocks are mainly represented by the Cariiio, Banded and

7109-6769-6770

0

t--u-

y/;I

NlOECHE UNIT

pmil

CARINO GNEtSS

[rim-lBANDED @gj

BACARIZA HBASIC ~RANUliT~S m ECLOGITES

m

I

z

34

5 Km.

CANDELARIA AMPiil~OLllES

GNEISS

~IM~ARRA

GNEISS

Fig. 2. GeoiogicaJ sketch map of the Cabo OrtegaI complex (from Arenas et al., 1986; mainly after Vogel, 1967). locations.

Chimparra formations. The Chimparra gneisses are exposed in central parts of the complex, whereas the Banded and CatSo gneisses occur in eastern sectors (Fig. 2). The Chimparra gneisses are psammopelitic with local intercalations of metabasites and talc-silicates. The BaAed gneisses are more heterogeneous, and are often rich in

zoisite, amphibole and/or garnet. The Chkparra and Banded gnekes are locally migmatitic. Metabasic intercalations occur as rotated boudins and have been transformed into garnet- and omphacite-bearing rocks with or without plagio~ lase (e&gite4ike assemblages). P/T c.on&tkms at the peak of metamorphism are identical for the

GEOCHEMISTRY

gneisses

AND

and

metabasic

1989;

Gil

Chimparra

and

tive intense

ductile

titic-eclogitic in

Banded

contact

with

are psammopelitic. and

post-metamo~~c and

psammopelitic tained

ductile

not

gneisses.

The

pyroxenite

are common Amphibolitization

gneisses Banded

like

appear gneisses

the Chimparra they

are

ultramafic site-rich

Abdel

Metabasic

Kuijper

staurolite

(1985a).

minerals

of these

P/T

conditions

at-

have

been

at - 6.50 * C and 9 kbar (Vielzeuf 1988; Basterra et al., 1989).

and

Griffiths et al., 1985a). Temperature and pressure conditions were slightly higher than in the eclothe

Banded

and

rocks,

- 800” C

veins

cutting

the main

in the Herbeira

which

peridotites

Monem

deformation. Garnet,

of

and

is widespread locally

Uzal in the

develop

as parga-

and pyroxenites.

Earlier geochronological studies of the Galician complexes have been carried out by Vogel and

significant

within

at temperatures

of at least 14 kbar (Ben Jamaa.

massifs.

of met~o~~srn

assemblage

1988). Garnet

record

The

garnet

foliation

(3) Eclogites and high-pressure granulites: Eclogites are very homogen~us, and composed of garnet, omphacite, and rutile & zoisite i kyanite. They have MORB-like compositions (Bernard-

g&e-like

spine1 into

and at pressures

penetra-

However,

are characteristic

at the peak

estimated Holloway,

do

1989).

record

the

and,

are amphibolitic.

kyanite

have

267

NAPPE

after the migma-

The Cariiio

gneisses,

intercalations

ORTEGAL

and

al.,

gneisses

et al., 1989)

migmatitic

et

deformation

(Basterra not

CAB0

intercalations

Ibarguchi

episode.

tectonic

OF THE

at - 700 o C and I5 kbar (Basterra

been estimated et al.,

GEOCHRONOLOGY

(1971), Van Calsteren

et al. (1979).

et al. (1982) and Bernard-G~ffiths Van

der

some indication limestone

Meer-Mohr

(1975)

et al.

has

of the biostratigraphic

given

age of the

levels in the Moeche Unit. The complex-

ity of the tectonothe~al

events

undergone

by the

rocks of these complexes has prevented the earlier workers from establishing a clear picture of their evolution. ment

Excess

radiogenic

argon,

isotopes

and partial

of radiogenic

low enrichlate ther-

mal re-equilibration were among the main difficulties encountered in the previous studies. The main results

of the previous

detail

together

with

studies

will be discussed

the new data.

They

in

clearly

indicated, for most of rocks, a Palaeozoic history with possible involvement of 1000-1500 Ma pro-

Chimparra gneisses, i.e. 800 o C for pressures above 17 kbar (Gil Ibarguchi et al., 1989). Granulites (Bacariza Formation) are very heterogeneous.

toliths. This is in complete contrast with the granulites recently studied from the Cabo Ortega1

Some are rich in garnet and/or clinopyroxene, while others are rich in plagioclase. Most contain

Banks offshore, 100 km to the north, which correspond to an Archaean and lower Proterozoic crust

rutile, zoisite/clinozoisite, and rare scapolite and kyanite as accessories. P/T estimates reveal conditions of formation of 800” C and - 14 kb (Gil Ibarguchi et al., 1989). Amphibolitization may be

belonging to an old craton around zoic belts were developed (Guerrot, et al., 1989).

very important, especially in the granulites. Some gneissose intercalations may be found locally, especially different

in the granulites, and they are not very from some varieties of the Banded

gneisses. (4) Ultramafic rocks: These form the massifs of Limo, Herbeira and Uzal (Fig. 2) in the uppermost part of the Cabo Ortegal complex in tectonic contact upon the gram&es. The three massifs are mainly composed of harzburgites of oceanic affinity (Ben Jamaa, 1988). Pyroxenites are very abundant in the Herbeira Massif. The metamorphic evolution is best observed in these pyroxenites where syn-kinematic recrystallization is observed after the mantle stage, with transformation of

which Palaeo1989; Guerrot

GeochemicaJ data

Previous works on the Cabo Ortegal and Ordenes complexes suggest a number of tectonic settings for the mafic rocks: e.g., continental basalts (Van Calsteren, 1978) or oceanic tholeiites (Williams, 1983; Bernard-Griffiths et al., 1985a). In this paper, we present the general geochemical characteristics (in particular of the REEs and isotopes) of the mafic rocks from the northwestern Iberian Massif and discuss them in relation to their tectonic setting (analytical methods are reported in Appendix 1).

TABLE 1B

Major, trace anti rare earth ekments

Trace element data

Major and some trace analyses from amphibolites of the Candelaria

Formation

(7115),

g&e-like” bodies within the migmatitic

“eclogneisses

(7125~Braganqa),

eclogites (6767, 6768) and basic

granulites (6769,

6772,

Nb

Zr

Y

Sr

Rb

Co

V

Ni

Cr

Ba

NO.

7107

5

72

25

113

5

42

274

97

287

8

7108

6

76

26

141

I

42

269

90

301

11

8280) of the

7112

7

111

38

286 1.5 39

268

22

66

87

Bacariza Formation, as well as the basic granulites

7115

3

66

16

146 18

34

207

87

319

129

from Sobrado (6763,

6766, 7120,

7120

3

25

9

222

7

44

235

66

IS3

30

(6755)

(6774),

7121

8

142

37

229

9

36

272

42

135

88

7125

9

118

33

110 16

46

270 118

216

106

8280

12

216

66

177 14

35

279

51

191

and Bragaqa

7112, 7696,

Analysis

7121)

Me&de

are reported in Ta-

bles 1A and B. The whole-rock data emphasize the contrasting compositions of the eelogites and the

14

granulites. The former correspond, on the whole, to basaltic liquids, whereas the latter seem to be more heterogeneous:

their protoliths represent (i)

basalts with features sometimes of a slightly cumulate nature (6772, 6774, 7112, 7115), and (ii) various cumulates of gabbroie (orthopyroxenebearing ?) (6769), hornblende (6755, 6763, 6765, 7696(?)) and anorthositic (7120) composition. In Fig. 3, many of the mafic gram&es appear to be compatible with a c&c-alkaline differentiation trend, but some of the Ortega1 rocks seem to have

transitional tholeiitic affinities. The Sobrado-Mellide and 3ragaqa samples plot in the CAB and MORB fields respectively. The Or&gal eclogites display MORB compositions, but the gram&es

MORB boundaries. The REE data are presented here following the geological settings established elsewhere in this paper (see the foregoing section).

TABLE 1A Major element data Analysis No.

SiO,

-4w3

FQO~

MnO

6755

45.62

13.27

15.35

0.22

6760

52.19

16.21

9.26

0.13

CaO

Na,O

9.04

12.32

2.00

0.11

6.31

8.42

3.53

0.90

M&3

of

the same complex are scattered in the MORB and transitional fields and near the CAB-WPB-

K,O

PZOS

P.F.

1.35

0.12

0.14

99.54

1.20

0.12

1.12

99.39

TiOz

Total

6761

53.38

16.50

8.82

0.11

6.05

7.96

3.68

0.69

1.10

0.12

0.64

99.05

6762

59.51

19.03

8.03

0.09

2.70

1.28

2.33

2.70

0.99

0.08

3.17

99.91

6763

40.57

17.28

14.97

0.25

8.68

13.06

0.73

0.19

1.80

0.20

1.37

99.10

6765

44.41

13.34

14.04

0.23

7.32

14.63

2.04

0.08

1.63

0.17

1.30

99.19

6767

48.29

15.94

9.76

0.15

9.62

13.05

1.61

0.10

0.92

0.08

0.96

100.48

6768

48.98

14.92

11.55

0.17

7.82

12.72

2.11

0.00

1.26

0.07

0.11

99.71

6169

57.53

14.85

10.64

0.17

4.05

6.00

2.62

1.13

1.69

0.21

0.96

99.85

6770

49.82

14.65

10.71

0.17

7.74

10.60

3.12

0.45

1.09

0.07

0.45

98.87

6771

46.61

16.36

8.80

0.14

10.06

12.54

2.11

0.30

0.68

0.05

1.41

99.06

6772

49.88

17.63

9.96

0.15

5.97

10.24

3.12

0.43

1.13

0.13

1.00

99.62

6773

49.18

15.46

8.63

0.15

9.62

12.38

x.93

0.30

0.25

0.01

0.63

98.54

6774

49.76

17.71.

9.00

0.15

a.47

9.41

2.34

0.44

0.75

0.W

0.60

98.67

7107

48.92

15.30

10.18

0.17

9.11

13.30

1.74

0.08

0.98

0.08

0.54

100.40

7108

48.49

15.41

10.41

0.17

8.72

12.76

1.72

0.09

0.99

0.08

0.51

99.35

7112

47.17

17.90

13.44

0.25

5.91

10.07

2.06

0.63

1.29

0.18

0.29

99.19

7115

52.17

16.31

8.57

0.15

7.36

10.31

2.26

0.64

0.60

0.06

0.95

99.38

7120

41.74

19.26

8.29

0.13

9.68

16.07

0.63

0.14

0.19

0.00

3.47

99.60

7121

54.30

15.53

9.18

0.12

5.58

8.78

3.29

0.52

1.55

0.15

0.81

99.81

7125

50.81

17.11

11.06

0.18

7.87

8.88

1.95

0.39

1.47

0.16

0.68

100.56

45.78

14.39

13.89

0.24

8.34

13.05

2.35

0.33

1.40

0.13

0.58

100.48

56.17

14.30

13.37

0.21

3.42

6.87

3.13

0.41

2.07

0.28

0.38

100.61

CiEOCHEMiSTRY

AND

GEOCHRONOLOGY

OF THE

CA00

ORTEGAL

269

NAPPE

A1203 Eclogiles A

Fig. 3. Al,G,-FeO,,,

- MgO diagram. Differentiation

trends

from Besson and Fonteilles (1974). Basalt fields from Pearce et al. (1977).

0

Amphibolitic metabasites The formation investigated for REE

included

Lace Nd SmEuGd I Dy Er jYbLu Tb Tm

the Candelaria amphibolites (samples 7113-7115) and related rocks from the Ordenes Massif (sam-

Fig. 4. Rare earth element patterns; see text and Appendix 2

ples 6387, 6390, 6392) (Fig. 4 and Table 2). This group of rocks shows generally LREE-de-

a. Basic gram&es

pleted patterns, with a small degree of fractionation ((Ce/Yb), = 0.68-1.38) and nearly flat HREE

distributions

((Gd,/Yb),

from Ortegal and Sobrado umts, and eclo-

gite-like rock from Braganqa (7125). b. Amphibolites

from

Ortega1 and Ordenes units, and eclogite-like rock from the Banded gneisses (7109). c. Ultramaftc (Ortegal, 7689) and

= 0.95-1.14).

cumulate (Sobrado, 7120).

TABLE 2 REE concentrations (ppm) determined by isotopic dilution and REE ratios Sample

Bacarixa

Sobrado

No.

Band-

Brag-

ed

aw

7109

__~ 7112

7696

8280

7120

7121

UB

Candelaria

7125

7689

7113

7114

Ordenes

7115

6387

6390

6992

La tppm)

14.28

5.81

20.87

0.55

11.25

4.35

12.17

0.683

2.21

4.52

n.d.

2.39

1.09

4.12

Ce

31.12

14.51

45.31

0.40

28.28

11.06

27.97

1.02

6.11

12.27

9.27

6.70

3.84

13.10

Nd

17.52

10.50

30.25

0.763

19.33

9.27

15.88

0.373

5.12

8.60

7.12

6.46

4.15

12.06

Sm

4.63

3.11

7.86

0.319

5.35

2.76

4.02

0.116

1.82

2.47

1.99

2.26

1.58

4.09

Eu

1.65

1.09

2.03

0.216

1.33

0.92

1.29

0.046

0.787

0.92

0.68

0.89

0.723

1.41

Gd

5.05

3.80

9.08

0.72

5.98

3.59

4.61

0.20

2.72

3.22

2.46

3.21

2.24

5.42

DY Er

5.26

4.34

9.97

0.931

6.38

4.44

5.24

0.301

3.53

3.87

2.80

3.90

2.84

6.53

3.21

2.67

6.1

0.590

3.69

2.75

3.05

0.225

2.33

2.45

1.74

2.45

1.77

4.07

Yb

3.14

Lu

0.480

(Ce/Yb),

2.53

(Gd,‘Yb),

1.29

(La/Sm), Eu/Eu *

2.62

2.83

0.242

2.30

2.33

1.73

2.27

1.65

3.92

0.401

0.440

0.0399

0.374

0.356

0.270

0.33

0.260

0.578

2.21

1.08

2.53

1.08

0.68

1.34

1.38

0.76

0.59

0.86

1.47

1.10

1.38

0.67

0.95

1.11

1.14

1.14

1.09

1.11

1.05

1.28

0.98

1.84

3.62

0.74

1.11

-

0.61

0.42

0.61

1.36

0.72

0.91

0.92

0.93

1.09

1.0

0.95

1.02

1.18

0.42

5.94

0.485

0.904

0.069

1.52

2.32

0.21

1.21

1.23

1.18

1.88

1.14

1.62

1.05

0.98

0.74

UB = uftrabasrc.

2.52 n.d.

3.27 n.d.

270

Only one sample (7115) shows a slightly LREEenriched pattern. Eu anomalies are not well pronounced; they are negative for 6392 and positive for 63%. The general appearance of patterns in this group is similar to those for N-type and T-type MORBs (cf. Saunders, 1984). Thus, this set of samples shows a broad oceanic affinity.

High-grade gneisses 7109 is an eclogite-like sample which belongs to the Banded gneisses formation from Cabo Ortegal. The REE pattern (Fig. 4b) shows ~h~a~te~stics similar to those of N- and T-type MORBs, as observed in the Candelaria Formation. 7125 is also an eclogite-like rock-type, but it belongs to the Bragaqa complex (Portugal). The pattern is slightly LREE enriched (Fig. 4a), and similar to that of the basic granulites discussed below.

Ecbgites and high-pressure gram&es The Cabo Ortega1 eclogites show unfractionated HREE ((Gdlyb), = 1) and LWE depletion with respect to the HREEs (La contents = 6-10 x chondrites) and have been compared with N-type MGRBs (Bernard-Griffiths et al., 1985a). The basic gra.nulites of the Bacariza Formation were analyzed (samples 7112, 7696, 8280) with two similar rocks from the Sobrado Massif (7120, 7121), and are comparable with previously published findings (Drury, 1980). They are characterized (Fig. 4a and Table 2) by moderate LREE enrichment over a relatively wide range of differentiation ((Ce/Yb)N > 1.2-1.47), although the LREEs can be rather more fractionated ((La/ Sm)N z=1.1-1.88) than the HREEs. La abundances vary from 18 to 63 x chondrites. There is a well-marked negative Eu anomaly in samphzs 7121 and 8280, no anomaly for sample 7696, and a slight positive anomaly for 7112. Data from Drury (1980) demonstrate broadly similar REE characteristics, and these are also represented in Fig. 4a (samples Ef4, E19, E72 and E77). The basic gram&e sample from S&ado (7120) (Fig. Ire) exhibits LREE depletion; the low absolute REE concentrations and the positive Eu anomaly are evocative of a cumulate or solid residuum.

At first sight, it appears that the REE patterns of the basic granulites are clearly different from those of the eclogites in having a marked LREE enrichment. Sample 7696 (Bacariza) may be compared with the slightly REE-enriched basalt from transitional ridge segments (Saunders, 1984). Otherwise, the more fractionated patterns (8280 and E 34) are fairly similar to those found in basalts erupted in a continental setting (cf. Thompson et al., 1983; Duncan, 1987). However, the patterns of both groups also show a good resemblance with volcanic arc basalts erupted near a continental margin (e.g., Lesser Antilles, White and Patchett, 1984). Thus, it is clear that, with a limited number of samples, REE abundance patterns are insufficient criteria on their own for establishing the original tectonic environment. Nevertheless, they may reflect ~mpositional differences that exist between distinct tectonic units that originated in various different environments. Ultramafc rocks The spine1 harzburgite sample (7689) shows REE abundances around 0.6-2 x chondrites, with a U-shaped pattern reflecting HREE depletion from Lu to Gd ((Gd/Lu). = 0.67). The minimum normalized abundances around Nd-Sm contrast with the relative enrichments in Ce and La ((La/Sm). = 3.62) (see Fig. 4c). This type of REE pattern is not characteristic of any particular tectonic setting, but the relative enrichment in La and Ce has been generally attributed to me&somatic processes involving the LREEs (Saunders, 1984). Strontium and neodymium whole-rock isotopic abta The analyses are listed in Tables 3 and 4 and iflustrated in Fig. 5. Initial isotopic ratios have been calculated using the age indicated by the U-Pb results presented in a later section. The eclogites exhibit homogeneous isotopic compositions and were extracted from a time-integrated very depleted mantle ( l Nd = + 10) simifar to that of the MORB sources ~~rn~d-G~~~ et al., 1985a). The initial Sr isotope ratios range between 0.7033 and 0.7046. These values he to the right of the mantle array on the Sr-N-d diagram and suggest interaction of the oceanic crust with

GEOCHEMISTRY

AND

GEOCHRONOLOGY

OF THE

CAB0

ORTEGAL

271

NAPPE

TABLE 3 Rb-Sr

tsotope data. Whole-rock (W.R.) and minerals as indtcated 87Rb/86Sr

Sample

Rb

Sr

No

(ppm)

(ppm)

s’Sr/s%

Imtial 87Sr/86Sr

(f2o)

(or age (Ma) for WR mtca pans)

Ortegal Eclogrtes

7107

1.49

105

0.041

0.70406 f

4

0.70378

7108

1.59

129

0.036

0.70484 k 4

0.70459

7118

2.98

102

0.085

0.70390 + 3

0.70332

Basrc granulates 13.4

7109

7.73

7112 7115

16.0

156

0.248

0.70771 f

3

0.70601

247

0.090

0.70500 f

4

0.70438

136

0.341

0.70613 f

2

0.70380

7696

4.99

160

0.090

0.70543 f

8

0.70481

8280

9.14

164

0.162

0.70721 k 4

0 70610

Ultramaftes 7687-harzburgite

1.20

40.4

0.086

0.70419 f 20

0.70371

7688-hatzburgite

0.448

18.4

0.070

0.70469 f

8

0.70430

7689-harzburgite

0.302

38.0

0.023

0.70384 rt 15

0.70371

7690-dtmite

0.087

0.040

0.70505 + 7

0.70483

119

0.140

0.70466 *12

0.70388

0.70760

7692-Clinopyroxenite

(webstente)

5.78

6.27

Paragnelss CariAo 8281

134

229

1.70

0.71923 f

4

Carifio 8281 “Must.”

130

256

1.48

0.71802 f

9

Cariiio 8281 Biotite

237

Carifio 8856

113

Chimparra 8282

72.3

Chimparra 8282 MUX.

305

Chtmparra 8282 Biotite

215

Banded gneiss 8091b

92.4

24.4

28.5

0.85469 + 4

217

1.51

0.71838 + 3

269

0.777

0.71565 + 3

9.93

0.76394 f

89.3 34.2

18.4

355 + 8 0.70805 0.71034

5

371 f 8 349 + 7

0.803088 f

6

210

1.28

0.71956 f

5

0.71083

Sobrado

Granuhte 7121

7.36

219

0.097

0.70459 f

2

0.70393

Ultramafic 7120

10.4

165

0.182

0.70451 f

5

0.70326

Paragneiss 8283

92.9

216

1.24

0.71955 +

3

0.71107

Bw=Y Eclogtte-hke rock 7125

11.7

0.338

0.71110 f

3

0.70879

99.9

seawater (Jacobsen and Wasserburg, 1979; McCulloch et al., 1980) or a metamorphic alteration. The basic grant&es, in contrast to the eclogites. exhibit heterogeneous isotopic compositions. eNd ranges from + 10 to + 1.0 in the Bacariza and Sobrado rocks and initial Sr ratios vary between 0.7033 and 0.7061. The Braganqa eclogite-like sample has a negative eNd value ( - 1.7) correlated with a more radiogenic Sr ratio (0.7088).

The Sr and Nd data obtained from these basic granulites clearly indicate the interaction of primitive depleted source(s) with continental crust. This is probably not a simple mixing between two components because there is no direct correlation between the decrease in eNd values and increasing LREE enrichment. At least three sources could be involved: (1) one may be a very depleted one of oceanic character, as suggested by sample 7696

272

: i t-1: 3s‘41 I I ii.

TABLE 4 Sm-Nd isotope data. Whole-rock (W-R.) and a garnet separate. See Table 2 for Sm and Nd concentration data ‘47Sm/‘“Nd

Sample No.

‘43Nd/144Nd

Initial cNd

T &

(rt.2.a) Eciogites 7107-o~op~x~e

0.2206

0.513170 It 18

445*

7107-garnet

0.4030

0.513555 f 47

327*

7112 (W.R.)

0.1608

0.512713 + 35

+ 3.6

1258

7115 (W.R.)

0.1701

0.512669 * 22

+ 2.2

1678

7696 (W.R.)

0.1800

0.513109 f 19

f 10.2

8280 (W.R.)

0.1582

0.512672 f 13

+ 3.0

1311

0.1893

0.512837 f 39

+ 4.30

1949

Baste granuiites

189

Uhamafites 7689 (W.R.) Paragne&ses Car&o 8281 (W-R)

0.1180

0.512089 f 32

-5.9

1686

Chimparra 8282 (W.R.)

0.1211

0.5118% f 18

- 9.9

2057

Banded grwiss 809lb (W.R.)

0.1162

0.512060 f 33

- 6.4

1701

Granulites 7120 (W.R.)

0.2575

0.512884 f 38

+1.0

Negative

7121 (W.R.)

0.1685

0.512785 f 20

+ 4.6

1230

Paragneisses 8283 (W.R.)

0.1184

0.512130 f 21

-5.2

1628

0.1541

0.512421 f 29

- 1.7

1859

BrpsanFa

Eclogite-like rock 7125 (W.R.)

* 322 f 61 Ma for an orthopyroxene-garnet

pair.

(fNd = + lo), (2) another may be sl@&y depleted (CNdr 5), as suggested by ~~~~ samples located near the mantle array (this may corre-

0.705

0.710

spend to m&em volcanic arc m (F& 5X while (3) some of the mixing may have occurred with Contienti GrLWt. SU& fNd-csr VaiiationsaI% obserwd today in contixwnti--mar@ volcanic arcs (De Paolo, 1988) and probably occur too in intracoutinental environments. Four metasediments from Cabo Ortq@ and Sobrado were analyzed. The ‘47Sm/‘“Nd ratios around 0.12 are typical of sediments with LREEe at 480 MB were ~p~.~ negative (-5.2 to lie in t&e range of

0.71587Q-f*6Sr

Fig. 5. eNa-s7Sr/86Sr initial ratio diagram for 480 Ma.

both Sr and Nd isotopes. The initial s’Sr,/*‘%r

GEOCHEMISTRY

AND

GEOCHRONOLOGY

OF THE

CAB0

ORTEGAL

ratio is 0.7038, near the field of the mantle if the Rb/Sr

ratio was not disturbed.

lites, McCulloch which possible

and one phlogopite

array,

et+, is +4.3;

provided

in the ultra-

biotite

this value is lower than that observed basic rocks of ophiolites

contamination

oceanic

mantle,

with continental

388 + 10 Ma. By the Rb-Sr from the gram&e

Rb-Sr

material.

data

The samples

analytical 1 and

methods

are

the petrological

selected

reported

in Ap-

description

may be found

and 40Ar/39Ar

2.

provides

mineral ages

Table

391 3 f

dates

cannot

be

pair provides

an

8281, Table

an age of 371 f 8 Ma and a whole rock-

mica and amphibolite

concentrates

have

ages

been

from the Pur(CO-l-86) for-

If-

i 38012

II-

42

L

J

u I

300’

CO-114-86

I

0

L

1

I00 c”M2uoLm”;“%

tory) are represented

prepared

Candelaria Formation, COgranulite (Bacariza Formation.

CO-lOB-86) and two amphibolites ride (CO-24-86) and Peiia Escrita

3892?58

apparent

8282,

3).

L Hl 450

c-0

pair

CO-24-86 I

1

i

3). In the

a muscovite-whole-rock

6 6

w

Fig. 6. @Ar/j’Ar

precise

pair an age of 349 + 7 Ma (sample

(metaplagiogranite, 144-86) a basic

and eight edenites

4

pairs

from four samples collected from the Cabo Ortega1 complex. These include a felsic gneiss

age of 413 k 15 Ma (2~) six homblendes from basic granulites and one biotite provided 390 k 28

A

thus

gneisses,

Amphibole

390 Ma for K-Ar. In detail, three from eclogites provided a mean K-Ar

Ma and 396 Ma respectively,

Sr and

4”Ar/39Ar

The previously published Rb-Sr and K-Ar mineral ages are from Van Calsteren et al. (1979). The dates range between 439 and 373 Ma, clustering around homblendes

a

an age

white micas have non-

the biotite-whole-rock

Chimparra biotite

Rh-Sr

gneisses,

age of 355 f 8 Ma (sample

of the

in Appendix

method,

provides

mica ages from the paragneisses

radiogenic obtained;

pendix

complex

complex

or In the Cariiio

Geochronological

the ultramafic

yield 380 Ma.

et al., 1980) and suggests a source than

from

of 346 Ma and two whole-rock-phlogopite

( + 8 in the Semail ophio-

is less-depleted

213

NAPPE

ages and apparent

by the vertical

39:;

&&ED

K/Ca

spectra

for amphbole

width of the bars. Experimental and ages mdicated

concentrates.

temperature

Analytical

steps increase

on each spectrum.

uncertainties

from left to nght.

(20, intralaboraPlateau

increments

TABLE 5 40Ar/3gAr analytical data for incremental heating experiments on amphibole concentrates Release

40Ar/3gAr *

36Ar/JgA~ *

temp. ( *C)

“Ar

wY4r

j6ArCa

Apparent

(I&of

(non-

(%)

age(Ma)

tot.)

atmos. +)

**

Fefstc gneiss of Candeiaria Formation Sump!e CO-f fl-86: J = O.OWW 475 151.91 0.46234

9.826

0.39

10.58

0.58

241.3 + 135.2

525

211.32

0.61650

9.623

0.38

14.15

0.42

425.8 * 111.3

575

139.13

0.37109

10.097

0.84

21.76

0.74

430.5 & 68.1

610

72.20

0.14485

11.856

0.99

42.03

2.23

431.8 f

45.1

640

37.17

0.04814

12.754

4.02

64.47

7.21

349.3 f

22.1

670

34.84

0.03097

13.343

4.16

76.80

11.72

386.1 f

14.7

690

32.51

0.02866

13.416

5.43

77.25

12.73

364.7 +

7.3

705

31.27

0.01918

13.727

8.35

85.39

19.47

385.5 i

6.2

720

32.98

0.02791

14.055

4.33

78.14

13.70

379.3 f

7.3

735

30.21

0.01801

14.024

7.33

86.09

21.17

376.5 +

5.4

750

29.59

0.01453

14.056

11.79

89.28

26.30

381.9 f

3.4

765

29.80

0.01546

14.063

19.28

88.45

24.75

381.0 i

2.3

780

29.38

0.01394

14.054

20.74

89.81

27.43

381.5 +

1.8

795

34.17

0.02925

14.086

8.27

78.00

13.10

385.0 f

3.9

810

71.82

0.15275

14.050

1.21

38.72

2.50

399.9 f

23.1

61.74

0.12413

13.699

2.18

42.37

3.00

378.4 f

32.4

197.14

0.66403

15.044

0.31

1.07

0.62

0.03428

13.820

lOO.00

81.54

20.01

830 FUSiOll

Total

35.22

Total without 475”-670°C,

810°-830°C

85.51

and fusion

33.8 & 361.2 378.8 +

8.8

380.1 i

4.2

31.6

Baszc granting of La Bacariza FormatIon

Sample Co-IOE-84: J = 0.009181 120.06 500

0.29626

4.199

1.02

27.36

0.39

476.6 4

550

70.44

0.17665

6.199

0.69

26.59

0.95

287.3 f

71.0

600

94.26

0.25945

12.069

0.65

19.68

1.27

285.8 f

83.4

640

40.14

0.05072

16.342

3.02

65.92

8.76

396.0 -f 14.3

670

29.27

0.01559

16.752

15.53

88.85

29.23

390.0+

1.9

700

28.48

0.01426

16.970

18.11

89.97

32.37

384.9 +

2.4

715

28.79

0.01667

16.996

10.48

87.62

27.74

379.5 f

4.5

730

28.14

0.01286

16.390

9.82

91.16

34.68

385.1 +

6.1

740

27.13

0.00938

16.325

16.41

94.59

47.31

385.2 f

1.9

755

27.93

0.01209

16.408

7.78

91.90

36.90

385.3 i

8.1

770

27.89

0.01040

16.716

7.00

93.7%

43.73

391.9 *

6.5

785

28.52

0.01077

17.574

6.93

93.77

44.37

400.1 *

4.6

800

34.28

0.04109

17.714

1.55

68.72

11.73

356.8 f

15.1

820

44.02

0.07615

17.404

0.68

54.06

6.47

360.1 *

64.8

Fusion

68.93

0.20898

16.846

0.33

12.37

2.19

137.3 rfr149.1

Total

30.64

0.02127

16.515

100.00

88.01

34.00

385.3 f

6.1

Peiia Esctita amphiboiite Sample CO-I-86: J = ~.~lg2 475

231.30

0.62564

393872

0.75

21.45

1.73

628.0 f

63.4

525

118.15

0.31163

25.751

1.77

23.81

2.25

379.2 f

27.2

575

80.69

0.18819

21.185

2.23

33.18

3.06

361.7 f

21.3

625

51.27

0.08293

24.694

4.74

56.06

8.10

386.4 f

9.2

675

36.96

0.03588

26.417

14.57

77.04

20.03

383.5 It

3.0

700

37.73

0.04054

27.035

11.79

73.99

18.14

376.8 f

4.3

720

36.06

0.02871

26.574

5.76

82.53

25.18

391.3 f

6.3

GEOCHEMISTRY

TABLE

AND

OF THE

CAB0

ORTEGAL

NAPPE

“Ar/39Ar

’

27S

5 (continued) 40Ar/39Ar

Release temp.

GEOCHRONOLOGY

*

36Ar,’ 39Ar *

( OC)

“Ar

WmAr

36Arc,

Apparent

(F of

(non-

(%)

age (Ma) * *

atmos. + )

tot.) PeAa Escrria amphrbolrre Sample CO-i-86.

J = 0 008182

740

32.44

0.01856

26.332

15.84

89.60

38.60

390.7 +

4.0

765

31.92

0.01626

26.414

30.77

91.58

44.18

392 7 *

2.1

785

33.30

0.02095

26.695

10.98

87.83

34.66

393 0 &

3.2

805

73.43

0.19920

23.784

0.54

22.43

3.25

231.3 2 150.1

111.18

0.28561

23.086

0.26

25.75

2.20

384.7 -t_176.0

39.29

0.04207

26.383

100.00

80 88

30.55

Fusmn Total Total wtthout

94.45

475 O-575 o C. 805 o C and fusion

389 3 1

x.2

3892&

5.8

Purrrdo amphlbolrte Sample CO-,74-86. J = 0.008095 475

102.65

0.23755

4.164

2.28

3194

0.48

425.7 i

28.1

525

84.71

0.20795

2.286

4.95

27.67

0.30

313.9 i

13.8

575

103 29

0.30975

11.881

2.43

12.31

1.04

278.0 it 22.3 3730 rt 22.3

625

114.22

0.29865

25.261

1.79

24.50

2.30

675

54.81

0.09998

44.196

7.58

52.36

12.02

387.9 i

6.2

705

41 .Ol

0.05651

47 462

30.78

68.56

22.84

380.3 &

4.2

730

49.24

0.10454

44.491

3.06

53.43

1148

392.7 $: 15.9 396.7 _t 14.4

750

53.07

0.09176

44.484

3.09

55.64

13.19

715

35.07

0.03341

46.620

31.82

82.52

37.96

390 1 &

2.3

800

41.06

0.04903

46.239

10.31

73.75

25.65

406.2 5

4.5

825 Fuston Total Total without

97.56

0.22896

43.468

1.29

34.22

5.16

442.4 + 32.6

157.36

0.42406

42.980

0.66

22.56

2.76

467.0 it 72.6

48.99

0.08102

42.071

100.00

65.71

23.60

475 O-625 o C, 825OC and fusion

382.5 k

86.62

67

391.3 &y 6.6

* Measured. ‘ Corrected

for ~st-lrra~atlon

decay of 37Ar (35.1 day l/2

* ?Ar,,r -( 36Ara,m0b K295.5W40Ar,,f. * * Calculated using correctton factors of Dalrymple

Itfe).

et al. (1981);

mations. A muscovite concentrate was prepared from a migmatitic two-mica Banded gneiss (CO-l85). Sample locations are indicated in Fig. 2. The concentrates were analyzed using the “OAr,/39Ar incremental heating techniques. The analytical data are listed in Tables 5 and 6 and are shown as age spectra in Figs. 6 and 7. Isotope correlation calculation for the amphibole data are listed in Table 7. “OAr/39Ar analysis of the muscovite concentrate yields a nearly concordant age spectrum which defines a plateau age of 375.9 + 1.4 Ma. Apparent K/Ca ratios in all gas fractions are very large and display no significant or systematic variations.

20, intralaboratory

errors.

The four amphibole concentrates display discordant age spectra which are marked by considerable variation in the apparent ages calculated 5OOr

7

-0

60

3gAr Fig. from

7. NAr/“9Sr the Banded

age spectrum gneisses. Data increment

80

100

RELEASED

of a muscovtte concentrate plotted as in Fig. 6. Plateau

and age Indicated

TABLE 6 “OAr/39Ar analytical data for an incremental heating experimental on a muscovite concentrate from the Banded gneisses -_--._ 40Ar/39Ar *

Release

36Ar/ 3YAr *

temp. ( o C)

“Ar

gNAr

Apparent

(% of

(non-

age (Ma) * *

tot.)

atmos. +)

Mlgmatitic two-mica gneissof Candeleria Formation Sample CO-I-H:

J = 0.009171

475

21.63

0.01306

1.99

86.02

355.7 f 5.2

505

27.03

0.00443

4.97

95.14

381.9 f 1.3

530

26.16

0.00237

12.82

97.30

378.4 zt 0.6

550

25.99

0.00234

11.29

97.32

376.3 k 0.6

575

26.17

0.00288

12.39

96.73

376.6 f 0.6

600

25.99

0.00217

10.49

97.50

376.9 + 0.9

640

26.03

0.00272

7.53

96.88

375.3 f 1.7

680

26.08

0.00341

8.02

96.11

373.2 f 1.9

710

25.98

0.00300

12.06

96.57

373.5 + 1.5

Fusion

26.61

0.00486

3.11

94.59

374.6 + 3.6

Total

26.11

0.00296

100.00

96.64

375.5 + 1.7

Total without 475 o C and fusion

94.90

315.9 f 1.4

* Measured. + [40Ar,.-(36Ars,,,.X295.5)1/40Arlo,. ** calculated using correction factors of Dahymple et al. (1981); 2a intralaboratory errors; 37k/39Ar corrected ratio < 0.020 in all analyses.

from gas fractions evolved at both very low and very high experimental temperatures. These are matched by fluctuations in apparent K/Ca ratios. Most gas increments that are evolved at intermediate experimental temperatures are characterized by constant apparent K/Ca ratios which record plateau ages ranging from 380.1 k 4.2 Ma (CO-114-86) to 391.3 of:6.6 Ma (CO-2486). The

plateau data derived from each concentrate yield we&defined isotope correlations (MSWD < 2.0) which correspond to ages which are similar to those calculated directly from the plateau data. The intercept at the origin yields a 4oAr/39Ar ratio which is similar to that of the present-day atmosphere and does not indicate the presence of any extraneous argon components.

TABLE 7 36.4r/40Ar-s?4r/‘%r

isotope correIations using plateau analytical data from incremental heating experiments on amphibole

concentrates Sample

Isotope

%r/%r

correlation

intercept **

MSWD

Percent of

CaIcuIated

total 3pAr

%r/%r

age (Ma) *

plateau age (Ma) ***

co-114-86

376.4 f 5.3

314.6 f 11.8

1.28

85.51

380.1 f 4.2

co-lOB-86

378.3 f 4.7

339.1 f 16.3

1.36

85.12

387.4 f 3.8

Co-l-86

388.2 f 4.4

271.3 f

7.8

1.82

9445

389.2 i 5.8

CO-24-86

376.7 f 6.0

309.9 f

9.8

1.30

86.62

391.3 f 6.6

* Calculated usingthe inverse absciwa intercept (%r/39& l

* Inverse ordinate intercept.

*** Table 5.

ratio) in the age equation.

GEOCHEMISTRY

te 1. 1-3. .ramafic

AND

GEOCHRONOLOGY

Eclogite

from

unit. sample

the Cabo

7692. 9-13.

Braganqa

OF THE

Ortega&

CAB0

sample

Basic granulite

Formation,

ORTEGAL

211

NAPPE

7107. 4-7.

Basic granulite

from the Sobrado

sample 7215. 2,3,5,6,7,10

from

Unit. sample

and 12 are cathode

the Bacariza

Formation.

7112. 14 and 15. Eclogite-like luminescence

images.

sample

7112. i.

rock from the

27x TABLE 8 U-Pb

zircon analytical data

Sample fraction

U

Pb *

(ppm)

(ppm)

206Pb/ 204Pb

‘07Pb,’ 206Pb

‘08Pb/

206Pb/

“‘Pb,’

‘06Pb

238

235

U

u

207pb,

“?Pb/’

2wPb/

u”Pb,’

‘06Pb

238

235

(cor-

(Ma)

(MaI

(Ma)

U

U

l”Pb

reCt ed)

c-@ Eciogttes 7107 100-105

7.42

0.52

792

0.07497

0.1361

0~0~076

0.5534

0.05672

441

447

481

139-149

7.03

0.49

269

0.1103

0.2330

0.07044

0.5487

0.05650

439

444

472

7108 67-105 132-149 > 149

15.0 9.56 16.5

1.08

1214

0.06882

0.11732

0.07266

0.5700

0.05690

452

458

488

0.69

460

0.08824

0.17221

0.07320

0.5729

0.05676

455

450

482

1.23

947

0.07195

0.1342

0.07520

0.5875

0.05666

467

469

478

0.05681

484

Basic grant&es 7112 239

16.9

844

0.07397

0.11336

0.07285

0.5707

453

458

37-52

236

17.0

2968

0.06159

0.08329

0.07449

0.5825

463

466

480

120-132

173

12.6

1547

0*06605

0.094%

0.07495

0.5859

466

468

480

132-149

162

11.7

1641

O&6532

0.08364

0.07525

0.5862

468

468

472

164

11.8

4515

0.0602r

0.06915

0.07528

0.5917

468

472

492

53-64

408

25.9

13597

0.05801

0.09351

0.06426

0.5046

0.05694

401

415

490

120-132

370

24.6

15640

0.05777

0.10082

0.06702

0.5253

0.05684

4f8

429

485

334

23.1

9223

0.05856

0.10453

0.06949

0.5461

0.05699

433

442

491

< 37

z 149 8280

> 149 Ultramafi!es 7692

1716

94.2

1532

0.06383

0.05053

0.05908

0.4428

0.05436

370

372

386

69-80

975

55.0

2754

0.05991

0.05361

0.05995

0.4517

0.05465

37s

379

398

loo-105

702

40.6

3610

0.05868

0.06057

o.was

0.4586

0.05467

381

383

399

120-149

649

38.4

3483

0.0586s

0.07310

0.06154

0.4624

0.05449

385

386

392

120-149

747

45.0

9018

0.05607

0.08802

0.06lS6

0.4624

0.05447

385

386

391

723

42.9

2633

0.05992

0.09024

0.06fO4

0.4580

0.05441

382

383

388

-C 37

(abraded) > 149 Paragneisses 809ib 561

43.1

4124

0.07737

0.05675

0.07955

0.8110

45-55

514

42.1

9940

0.07923

0.05699

0.08423

0.9038

0.07782

69-74

449

42.1

7812

0.08890

0.07359

0.09420

1.1316

0.08712

580

769

1363

100-105

400

46.5

6622

0.10362

0.08792

0.11403

1.5969

0.10156

696

969

1653

369

52.9

11039

0.11435

0.09171

0.13864

2.1626

0.11313

837

1169

1850 1599

-c 31

> 120

493

603

1040

521

654

1142

8282 8u4

82.6

16671

0.09949

0.05552

0.10370

1.4108

O.~?

636

894

60-80

665

99.9

23782

0.12313

O.oM63

0.14656

2.4770

o.l2257

882

126s

1994

120-132

549

111

23837

0.13769

0.07817

0.19398

3.6685

0.13715

1143

1565

2192

486

119

32346

0.14380

0.08692

0.23116

4.5709

0.14340

1341

1744

2269

312

28.0

3493

0.07818

0.10012

0.08941

0.9142

0.87416

552

659

1046

326

30.2

5212

0.08145

0.10300

0.09158

0.9945

0.07876

565

701

1166

280

27.9

2600

0.09252

0.11122

0.09774

1.1750

0.08718

601

789

1365

285

26.3

2926

0.08467

0.12366

0.09032

0.9948

0.07988

557

701

1194

C 37

> 149

Grandlie 7821 <60 60-80 120-132 z 132

.-.

GI-OCHEMISTRY

AND

GEOCHRONOLOFY

OF THE CAB0

ORTEGAL

2-W

NAPPE

TABLE 8 (continued) U

Sample fraction In

tppm)

Pb * (ppm)

“‘Pb/

“‘Pb/

*OaPb

*“Pb

‘“‘Phi,’

208Pb/

“‘Pb/

z07Pb/

*“Phj

‘?S

“‘hPh/

*“Pb

238u

ZObPb

3xc,

l"( 1"'

2"hPh

(cor-

(Ma)

(Ma)

(Ma)

U

(pm)

“” Ph, ’

rect edt Braganya Eclogtie-like rock 7l?C

(1)

335

22.6

4298

0.05882

0.13792

0.06623

0.5064

0 05545

413

416

430

37-45 (2)

399

23 5

8087

0.05753

0.14545

0.05718

0 4394

0.05574

358

370

442

45-53 (1)

425

27.0

7259

0.05944

0 15532

0.06121

0.4849

0.05745

3x3

401

509

45-53 (2)

471

47.8

7449

0.05801

0.21279

0.07372

0.5705

0.05613

459

458

457

45-53 (3)

396

22.6

2505

0.06160

0.14529

0.05586

0.4299

0.05581

3.50

363

445

60-74

555

32.2

5229

0.05880

0.17388

0.05522

0.4266

0.05603

346

361

454

80-105

600

33 5

4996

0.05900

0.14870

0.05416

0.4190

0 05416

340

355

4%

937

73.6

4715

0.06010

0 17026

0.07483

0.5884

0.05703

465

470

493

37-45

> 105

-

U-Pb zmon ages

data point. Discarding this point. the upper tntercept provides an age 484 + 6 Ma. We obtain 482

The eclogites, basic granulites, ultramafic rocks and high-grade paragneisses from Cabo Ortega1 were investigated by the U-Pb zircon method. Dating of the basic granulites of Sobrado and

+ 4 Ma ( IL-2a) as a mean ‘07Pb/‘0hPb age using the four most precise points and 480 & 5 Ma using all five points: this is another possible calculation

Rraganqa

lower intercept

is also provided.

for

the

upper

intercept

age

is around

zero and that the points

are subconcordant. Eclogrtes:

Zircons

were

extracted

from

two

samples collected from the same exposure (Fig. 2). They have exactly the same composition and mineral assemblage and were treated as one aliquot. Zircons are colourless (this may be related to their very low uranium content of 7-16 ppm.

favoured age.

0.08-

Under cathode luminescence, the crystals are generally seen with unzoned cores and overgrowths, again resembling the zircons in the Vendee eclo-

O.O6-

8)

the

This last age of 480 f 5 Ma 15

as the closest to the true upper intercept

206Pb /238 U

This is the typical mo~hology observed in the Vendee eclogites (Peucat et al., 1982) for example.

(Table

that

Baw granufites: Two samples from the Racariza Formation were selected (Fig. 2). Sample 7112: The zircons are pink, with elongated crystals which are generally subrounded

Table 8). They are slightly elongated or rounded wtth visibly crystalline faces and rounded tips,

gites (cf. Plate 1.1-1.3). Five grain-size fractions

assuming

- ECLOGITES

were

analyzed. On the concordia diagram. they define (Fig. 8) an upper intercept age of 490 + 12/ - 11 Ma and a lower intercept of 90 + 12/90 Ma, with a MSWD of 2.1. The 139-149 pm fraction of sample 7107 significantly modifies the correlation; because it exhibits the lowest 206Pb/204Pb ratio (269) it is the fraction yielding the least precise

o.oz0.00 0.0

/

, 0.2

t

,:, .k_._em-m, ._ _ __ _,? ?~ I 1 I 0.6 0.4 207Pb

/ 235U

Fig. 8. U-Ph urcons ages from Cabo Ortega1 eciogrtea. (Samples 7107, 7108).

0.0

0.1

0.2

0.3

0.4

0.5 207 Pb

0.6

07

,’ 2354

Fig. 9. U-Pb zircon ages from Bacariza granulates (sample 7112).

particularly at the tips. Cathode luminescence images indicate strongly zoned cores but unzoned overgrowths (Plate 1.4-1.7). Five grain-size fractions were analyzed; the U Content is moderate to low (Table 8). Data are subconcordant on the U-Pb diagram (Fig. 9). The regression calculation yields an upper intercept age of 484 + 42/ - 6 Ma with an imprecise lower intercept close to zero Ma (MSWD = 0.74). The slight discordancy of the point enables us to consider the mean 207Pb/206Pb age of 482 + 7 Ma as close to the upper intercept. Furthermore, we note a 20 Ma spread of 207Pb/ 206Pb ages from 492 to 472 Ma, without major influence from common lead (206’204Pbratios relatively high).

Sample 8280: The zircons are comparable with those just described, with rounded tips and clear zoning in the inner parts of some crystals. “Cauliflower” zircons are present. This may be related to corrosion of the zircon or to metamorphic growths. Relatively rich in U (3~-~ ppm), they are more discordant than those of sample 7112 and provide an upper intercept age of 497 i ll/ - 13 Ma and a lower intercept of 21 + 76/ 21 Ma (MSWD = 1.4). The mean 207Pb/‘*6 age is 489 I 4 Ma (Fig. 10). Ultramafic rocks: The zircons were extracted from a clinopyroxenite vein associated with the Uzal ultramafic massif (Fig. 2, sample 7692). Crystals are subeuhedral to euhedral with easily visible zoning suggesting a magmatic origin (Plate 1.8). No overgrowths or rounded tips were observed, suggesting that the grains were not transformed during high-pressure event as previously indicated in the eclogites and basic gram&es. Five grain-size fractions and an abraded sample were analyzed (Table 8); they are rich in uranium (650-1700 ppm) and define an upper intercept age of 400 -t 16,’ - 6 Ma and a lower intercept of 77 + 157/ - 77 Ma (MSWD = 1.7) (Fig. 11). In this example too, as zircons are subconcordant and the lower intercept is statistically not different from zero, we can calculate a 257Pb/206Pb meau age (20) of 392 + 4 Ma. One fraction was abraded following the method of Gogh (3982) to check the possible existence of an older inherited phase.

t 206Pb/238U

t

0.06

0.0

0.1

0.2

0.3

0.4

0.5 207Pb

0.6

0.7

/ 23511

Fig. 10. U-Pb zircon ages fwm Bacwiza giattulitn fsm@e 8280). Inset: See Figs. S-10.

Fig. 11. U-Pb zircon ages from a garnet pyroxemte vem withiu the ultramafic unit.

GEOCHEMISTRY

AND

GEOCHRONOLOGY

Both similar

size fractions

not, provided

exactly identical

Paragneisses: great

Zircons

importance

(e.g.

Gebauer

(Table

8), abraded

and

metamorphic

Grtinenfelder,

radiogenic

whereas

the lower

to a metamorphic this study, Banded

intercepts

belt

Peucat,

indicate

the

lead comcorrespond

(Peucat,

1986a). In

we check this interpretation Chimparra

with the eclogite-like As is generally

using

paragneisses

the

boudins. the case

(Table

2.0

0.0

4.0

6.0

associated Fig. 13. Detntal

for detrital

8.0 207Pb

zircon ages from the Chlmparra

100 / 2351) gneisses.

zircons,

those of the Banded gneisses (sample 8091b, Fig. 2) are rounded. Five grain-size fractions were analyzed

0.40-

age if zircons were not recrystal-

the metamorphism

and

of

events;

1976;

existence

of an old inherited

281

NAPPE

or

are

well shown in the Variscan generally

lized during

ORTEG4L

results.

1986a). The upper intercepts ponent,

CAB0

in paragneisses

in dating

this is particularly

OF THE

8 and Fig. 12). They provide

an

upper intercept age of 2345 + 11 Ma and a lower age of 417 + 3/2 Ma (MSWD = 0.2). The

abrasion and are rounded, crystalline faces being absent. Some of them contain cores inherited from a magmatic phase before erosion, but they bear no metamorphic

overgrowths.

comprises

cannot be explained by a diffusion of lead. A model of episodic lead loss or mixing with some

metamorphic overgrowths. Rare crystals occur with the “cauliflower” structure: this may be evi-

newly formed zircon can explain this result. The age of 2345 Ma is that of inherited zircons probably derived from several sources (- 1900-2700

dence tions.

Many of the zircons from the Chlmparra gneisses (sample 8282, Fig. 2) exhibit mechanical

cal abrasion

clear crystals

population

zircons are very discordant with respect to the older age (up to 90%), and the lower intercept

Ma). The age of 417 + 3/- 2 Ma is that of the time of lead loss (and/or mixing), probably during a metamorphic event.

subhedral

Another

was more limited;

where mechani-

they do not exhibit

of overgrowth under metamorphic condiThe four size fractions analyzed define a

discordia (Table 8 and Fig. 13) which intercepts the concordia at 2479 + 6/ - 5 Ma and 422 k 4 Ma (MSWD = 0.24). The oldest intercept probably corresponds to the mean age of various families of inherited zircons. The younger age is a good indication of an episodic lead loss event (and or mixing)

during

metamorphism.

Sobrado Formation These rocks are similar to those of the Bacariza Formation (Cabo Ortegal). Locations of the studied samples are provided in Appendix 2. Basic granufites: The zircons from sample 7121 are generally elongated, clear and zoned, bearing cores; tips are generally rounded

I

0.0

I

2.0

Fig. 12. Detntal

I

I

4.0

6.0

10.0 8.0 207 Pb / 23511

zircon ages from the Banded

gnelsses.

a few (Plate

1.9-1.11). These characteristics suggest that the magmatic zircons crystallized under HP conditions. Some “cauliflower” crystals show unconcentric zoning and are probably of metamorphic origin (Plate 1.12 and 1.13). Four size fractions were analyzed (Table 8 and Fig. 14). They approximately define an upper intercept of 2763 +

0.0

4.0

8.0

16.0 12.0 207Pb / 2351)

Fig. 14. Zircon ages from Sobrado granulites. In the mset the dashed Iine corresponds to the detritat zircon discordia and the asterisk to the monazite obtained by Kuijper (1979).

269/ - 235 Ma and a lower one of 489 + 17/ - 19

Ma (MSWD = 0.98). The points are all close to the lower intercept. The discordia reflects the presence of inherited zircons (cores observed) which were mixed with the magmatic population at 489 Ma. Paragneisses: Kuijper (1979) analyzed zircons and one monazite separate from a metasediment collected from the same outcrop as our sample 8283. He obtained an upper intercept of 2272 + 83/- 79 Ma and a lower intercept of 476 f 12 Ma. The monazite is subconcordant and the 207Pb/206Pb age is 487 Ma. Kuijper interprets the data of 476 f 12 Ma, and the monazite date, as a metamorphic age, an interpretation which is consistent with the present U-Pb data for the accompanying basic granulites (Fig. 14, inset).

phism (Plate 1.14). The other family is made up of clear and facetted zircons typical of HP rocks (Plate 1.15). A total of five size fractions was analyzed (Table 8 and Fig. 15). and heterogeneous data were obtained. In particular, the duplicate and triplicate analyses are not consistent: this may be due to several factors: Analytical problems such as U loss during the chemical preparation may be a factor. This phenomenon may explain the two subconcordant size fractions: for the 37-45 pm (1) * size fraction, which shows a lower U/Pb ratio than the 45-53 pm (3) fraction for a similar Pb* content (22.6 ppm, Table 8), and for the 45-53 pm (2) fraction. which is slightly above the concordia. Nevertheless, the 45-53 pm (1) fraction exhibits the highest 207Pb/206Pb age, and cannot be explained by this hypothesis. The heterogeneous U-Pb data may also be explained by heterogeneity of zircon aliquots related to the existence of two families of crystals; the brown, elongated zircons are probably rich in U and the clear, round grains are probably poor in U and particularly well represented in the small size fractions. A complex model of mixing can be postulated linking at least the magmatic age to the oldest recorded 207Pb/206Pb age (509 Ma) assuming that

* See Table 8.

0.080

Bragan$a Formation

Basic granulites comparable to those of the Bacariza Formation and e&&e-like boudins similar to those found in the Banded and Chimparra gneisses (Cab0 Ortegal) are exposed in the area. The location of the eclogitelike sample studied is provided in Appendix 2. Eclogite-like rock: Two families of zircons were observed in sample 7125. One comprises elongated, brown, wned grains and suggests a magmatic origin; the tips are rounded and suggest morphological transformation during metarnor-

0.060

Fig. 15. U-Pb

zircon ages from the Braganpa eclogite-Iike sample. 7125.

GEOCHEMISTRY

TABLE

AND

GEOCHRONOLOGY

OF THE

CAB0

ORTEGAL

283

NAPPE

9

Whole-rock

Pb-Pb

Sample No.

analytical

data

Pb

U

206Pb,’

‘07Pb,’

“‘Pb/

(ppm)

&pm)

204Pb

204Pb

*“Pb

23sU/204Pb

“‘Pb*,’

*“Pb*/

238

235

U

U

Ortegal Edagues

7103

0.416

0.075

19.393

15.673

38.529

11.7

0.862

63.4

7104

0.304

0.090

19.226

15.642

38.169

19.0

0.521

38.7

7105

1.95

0.258

19.397

15.641

38.375

8.5

1.186

7106

0.438

0.037

18.290

15.559

38.030

5.4

1.670

86.6

7107

0.570

0.092

18.991

15.662

38.599

10.4

0.933

7108

1.07

0.085

18.566

15.643

38.376

5.1

1.825

7118

0.170

0.056

20.136

15.673

39.859

21.9

0.494

1.74

0.020

18.142

15.620

38.114

6773

0.617

0.229

19.813

15.668

39.079

24.3

0.433

30 5

6774

1.18

0.121

19.847

15.766

39.723

16.7

0.632

45.2

135 71.3 145 33.x

Sobrado Gmnui~tes 7120

0.73

12.0

1000

B=fPV Granufrtes

no inherited phases were present. The rnetamorphic age would be less than or equal to the lowest 207Pb/2~Pb age (430 Ma). Ph-Pb isotopic whole-rock

1.5 and 1.0 Ga and that the eclogitic metamorphism occurred between 1.0 and 0.5 Ga. We have no evidence for these Precambrian events from U-Pb zircon data. We have analyzed the Pb isotopes in seven eclogites from Ortega1 because they may correspond to the oldest protoliths, in one grant&e from Sobrado and in two granulites from Braganqa (Table 9 and Fig. 16). All the data fall in the range of ‘“6Pb/204Pb ratios between 18.1 and 20.1. The 2’7Pb/2WPb ratios vary between 15.60 and 15.70, except for

data

Kuijper (1979) proposed a multistage model to explain the Pb-Pb whole-rock data obtained from eclogites and basic grant&es of Ortega1 and Sobrado. This model suggested that the emplacement of the basic protoliths took place between

207Pb/204Pb 15.8 t

v Ortegol eclogltes

15.7

0

15.6

1

18 FIN. 16. Whole-rock

Pb-Pb

diagram

for eclogites

I

18.5

r_---* v v- - v--- T

I

19

and some basrc granulites. isochron.

--

v

*06PW204Pb * j I I -

19.5 The dashed

20

line corresponds

to a 500 Ma reference

one eclogite (15.56). These ratios, in particular 206Pb/‘2@4Pb, are si~ficantly lower than those obtained on similar rocks by Kuijper, which were between 19 and 28. We do not attempt to explain these differences here, but we see that our data are compatible with those from protoliths extracted from the mantle 500 Ma ago and thus there is no disagreement between the U-PI, and Pb-Pb systems. Interpretation of results and discussion

intermediate eNd value (ca. t4) is not typical oi op~olitic mafics such as those as observed in the Semail complex (McCulloch et al.. 1980) where ultramaftcs have e Nd values equivalent to the overlying basic rocks (ca. t- 8). Consequently, the Ortegai ultramafic rocks were probably derived from a slightly depleted source different from the mantle source of the eclogites (E h’d= t IO); alternatively, they may have experienced a continental influence according to the Nd system, or a metamorphic alteration.

Nature of the basic Frot~liths

The geochemical data from the high-grade rocks of Cabo Ortegal are generally consistent with igneous processes and suggest that no major alteration of primary impositions occurred during metamorphism, at least for those elements that are generally considered as “immobile”. This is well shown, for example, by the eclogites which have preserved their MORB-like REE patterns and corresponding Nd isotope ~rnp~~ons (BernardGriffiths et al., 1985a). The amphibolites of La Candelaria, and simiiar rocks from Ordenes, also exhibit REE patterns of the MORB type; indicating the oceanic affinity of the magmas. The basic gram&es of Bacariza and Sobrado were probably generated in another geodynamic

The interpretation of the geochronological data is largely dependent on knowledge of the metamorphic history. We can summarize this history m three stages: (1) an early high-grade episode, with eclogite and gram&e facies assemblages (700”800 o C), (2) an amphibolite-facies metamorphism probably related to the emplacement of the nappes (600 o -500 o C), and (3) retrogression under ~~n~~st-facies conditions { - 300.*--&Xl* C). We will attempt to interpret and correlate the obtained radiometric ages with the metamorphic and tectonic events inferred from geological studies. The mica and ~p~~~e late co&ng ages The youngest cluster of dates is around 350 Ma

setting. AlI the geochemical data indicate a continental influence in the generation of the proto-

and is recorded by the Rb-Sr

lith magmas. The enriched REE patterns, the in-

pairs from metasediments (Chimparra and Car&o

termediate

gneisses). Rb-Sr biotite ages are gene&y assumed to record cooling below 300°C (e.g., Purdy and .I&ger, 1976; tiger, 1979). These temperatures are in agreement with the supposed third, lowgrade stage of metamorphism recorded at Cabo

eNd values, as well as some high Sr

initial ratios and the talc-alkaline features, indicate mixing of several source components. Thus, a continental volcanic arc margin or continentaT rifting (attenuated crust) are favoured as settings for these magmas. The few data obtained on eclogite-like boudins included within the Bauded gneisses from Cabo Ortegal and Braganqa do not allow us to define a precise setting. If the REE patterns are compatible with transitional MORfGtype basalts, the eNd value of - 1.7 for the Bran sampIe (7125) requires an important component of continental material mixed with the primary magmas. Only one ultrabasic sample from the Or&gal complex was analyzed for REE&, Nd and Sr. The

biotite-whole-rock

Ortegal. The second group of ages is recorded by a Rb-Sr muscovite-whole-rock pair in the Chimparra gneisses, and is interpreted as dating the cooling below 500 “-450 ’ C. This data is also found in the muscovite %r/39Ar plateau age of 375.9 f 1.4 Ma, interpreted as recording the last cooling through an appropriate closure temperature. AIthough not fully calibrated experimentahy, the preliminary data of Robbins (1972) can be applied to the diffusion equations of Dodson

GEOCHEMISTRY

AND

CEOCHRONOLOGY

(1973) to indicate - 350°C.

of

estimated vite

a muscovite

These

are

by an empirical

K-Ar

ages

(e.g.,

Purdy

Wagner

et al., 1977; Jager rapid

Banded

and

results

of musco-

recorded

by

Jager,

1979). This

post-metamorphic

gneiss Formation,

ORTEGAL

to the

comparison

those

species

CAB0

closure temperature

similar

with

mineral relatively

OF THE

cooling

third

group

of cooling I(-Ar

ages

are interpreted

as dating

those

tracrystalline

temperatures

retention grains.

the last

of argon within

Harrison

in retrograde formations

for the

rocks produced

in pelitic

adjacent transformed

constituent that

rocks (Bacariza) and in pro(Purrido-Candelaria). The ages indicate that in all the forma-

of the u~tr~rn~f~c unit

The Rb-Sr whole-r~k-p~ogopite pair ages of 380 Ma are very similar to the edenite-phlogopite ages (388 + 10 Ma;

1979). All precise mafic unit cluster zircon

Van

Calsteren

et al.,

data obtained from the ultraaround 390-380 Ma, both for crystallization

event (pyroxenite

vein) and the amphibole-mica cooling ages (at 500 ’ C). but we have no direct evidence about the age of the foliated ultramafic rocks in which the 392 Ma old garnet pyroxenite veins were intruded. The ultrabasic protolith may be ancient if we consider that the HP stage recorded in these rocks is related to one of the granulitic/~lo~tic events defined in this work (480 and 420 Ma, see below). On the other hand, the oceanic origin proposed by Ben Jamaa (1988) suggests that these rocks differentiated in an extensional setting and not in a

of partial of

clinopyroxene, secondary tions

which

gneisses.

into an isotropic

development quartz

in-

gneisses

Chimparra

cooling

The Rb-Sr whole-rock age using data from Van Calsteren et al. (1979) and five new samples (Table 3) is imprecisely defined at 413 + 110 Ma.

the U-Pb

phism

for

tions considered. It was the time of nappe emplacement when rocks cooled down through a temperature around 500 o C. The emplacement

age can also be discussed

on

a contact

indicates

(1981) suggested

thrusting.

the ultramafic

The plateau

required

at around

1976,

ages is obtained

system.

- 380-390 Ma plateau cooling metamo~hism was synchronous

K-Ar

The emplacement

recorded to general

In the Uzal massif.

values of 500 &-25 o C are appropriate for the range of cooling rates likely to be encountered in most geological settings. These temperatures are in the range proposed for the second stage of metamorphism grade

one, as the event

Ma ago are related

a small degree

the hornblende

amphibole

390-380

the basis of field relationships.

faster than in the other

from

through

compressive

other

formations. The

285

NAPPE

and

a

plagioclase,

were estimated

HT “hornfels” of

garnet,

hornblende,

with

tremolite;

a possible P/T

at 800 o C and

condi-

14 kb (Gil

Ibarguc~ et al., 1989). If the pelitic gneisses are equivalent to the Chimparra from which the HP event Ma {see below), we obtain

are with

This led to the

brown

together

radial

to the

gneisses

assemblage

phlogopite,

fibrous

are similar These

melting.

new

metamor-

hornfels gneisses.

is dated at around a range of 420-390

420 Ma

for the emplacement of the ultramafic body. This span of time probably coincides with a compressive regime related

to the thrusting

a change from granulitic/eclogitic facies conditions.

of nappes

and

to amphibolite-

The origin and the age of the ultramafic rocks remain a problem. The occurrence of layers of pyroxenite forming a - 500 m thick websteritic sheet is not typical of oceanic peridotites (Ben Jamaa, 1988) and the eNd value does not suggest an oceanic origin sensu stricto. One hypothesis is that differentiation and emplacement occurred between 420 and 390 Ma; thus, the ultramafics be related

to a slice

could

of subcontinental

mantle

which was thrusted up with deep granulites. The second hypothesis, taking into account the oceanic affinities formed

of the within

ultramafics,

a back-arc

is that

system between

they

were

480 and

420 Ma and then suffered partial melting during emplacement 390 Ma ago which produced the observed garnet pyroxenite veins. Metumorphtsm in the high-grade main& metusedimentary units (Chimparra, Banded and Sobrado gnelsses) In both HP paragneiss samples from Cabo Ortegai. an event around at - 420 Ma is recorded in the U-Pb systems of detrital zircons. This is a strong argument for the existence of a metamorphic event at that time (lead loss + mixing).

286

In the Sobrado HP gneisses associated with basic granulites, similar data indicate an older event around 480 Ma which is confirmed by a monazite cooling age reported by Kuijper (1979). This event is probably related to the granulitefacies metamorphism observed in the metabasic rocks. This is major evidence for the existence of two distinct HP episodes (420 and 480 Ma) which led to the development of the granulite/eclogite assemblages. Nevertheless, old inherited zircons generally record and preserve the first event that they suffer (Peucat et al., 1982). This suggests that the 420 Ma old Cabo Ortegal paragneisses did not suffer the 480 Ma event. Furthermore, we can suspect that the 480 Ma HP Sobrado gneisses were not in high-grade conditions at 420 Ma because monarite, which records relatively HT cooling ages (Koppel et al., 1980), probably of around 600 O650 o C, would have been reset at 420 Ma if such a temperature had been reached. Metamorphic

and

magmatic

events

in basic

granulites and eclogites

The results from basic granulites and paragneisses are concordant within the Sobrado Unit and we believe that the span of 490-480 Ma (within the error ranges previously indicated) corresponds to the emplacement of the basic protoliths and their HP metamorphism. Data from Cabo Ortegal cannot be interpreted in the same way because the gram&es and paragneisses belong to different structural units. However, we can make the following observations about the U-Pb data on the eclogites and basic gram&es: (1) Zircons from the eclogites and basic granulites provide similar upper intercepts close to 480 Ma and lower intercepts close to zero Ma. (2) The internal structures of zircons in granulites are complex, with cores, sometimes zoned, which suggest a magmatic origin. The overgrowths are responsible for the rounded shape of the zircons and suggest growth under HP conditions, probably during the metamorphism. Paradoxically, the U-Pb systems are relatively simple. We cannot define a lower intercept corresponding to the metamorphism and an upper intercept corresponding to the magmatic event as usually ob-

served for mixing of two populations of zircons of very different ages. The only complication in this U-Pb system may be in the 20 Ma range from 472 to 492 Ma recorded for the ‘07Pb/ ‘06Pb ages (Table 5). A possible interpretation which could reconcile all the previous points is that the range of 207Pb/206Pb ages results from the mixing of two types of zircon. The system could have been perturbed by continuous or recent lead loss. Consequently, the magmatic protolith would be 492 Ma old, or slightly older, and the metamorphism 472 Ma old, or slightly younger. In such a model, we cannot exclude the possibility that the balance of U contents in the zircon would be largely controlled by magmatic zircons. This means that the magmatic “constituent” would be preponderant and the metamorphic age could be younger that 470 Ma. (3) In the eclogites, we observe a complex zircon structure similar to that observed in the basic gram&es. It is possible that these cores also had a magmatic origin. No magmatic zoning is discernible and their U contents are very low. In a similar context (southern Brittany) we have observed that such U-poor zircons, with or without overgrowths, yield similar results (Peucat et al., 1982) which are probably related to metamorphic recrystallizations. On the other hand, depleted MORB-like basalts probably do not contain any zircon; they have never been described in such rocks. Consequently, it is entirely possible that cores and overgrowths developed during various stages of the eclogite-facies metamorphism. We thus would have no information about the age of the protolith of the eclogite. To summa&e, the Cabo Ortegal U-Pb data strongly suggest the existence of synchronous eclogitic and gram&tic events around 480 Ma which have already been indicated within the Sobrado HP formations. The basic granulite protoliths were penecontemporaneous with these high-grade events, whereas the eclogite protoliths were probably slightly older. As the period between the crust-forming event and the granulite metamorphism was very short, an interpretation of the origin of the related basic magmatism in a continental volcanic arc margin is favoured (Peucat et al., 1989).

C;tOCHl-‘MISTRY

AND

GEOCHRONOLOGY

It is interesting from the Pb-Pb existence

OF THE

CAB0

ORTEGAL

to note that there is no evidence or U-Pb

of ancient

systems

T (“c)

for the possible

Precambrian

magmatic

proto800”

liths. Conclusion: Tectonothermal The

first

eclogites

geological

and

basic

evolution

unit

:

is made

granulites

for which

490-480

Ma ago. These rocks belong geochemical

MORB-like

the main

of

Sobrado, ferent

187

NAPPE

groups.

protoliths

involved

in high-grade

possibly

during

the

Bacariza-

evolution

The

occurred to two dif-

eclogites,

are not precisely conditions subduction

up of the

7ooo~ i\o\ T

600”-

whose

dated, were

480 Ma

ago,

of a Late

Pre-

!

I

500”~. 1

‘; ar g $ 2

\ v

\

\

-

cambrian or Early Palaeozoic oceanic crust. The magmatic protoliths of the basic granulites could have been generated in a continental volcanic arc margin rather than by intracontinental rifting because they were metamorphosed emplacement they could high-grade

(convergent regime). Alternatively, have been emplaced directly under conditions

450

a short time after

above the subduction

zone.

Consequently, these granulites do not correspond to lower continental crust but rather to accretiontype granulites directly related to the eclogitefacies event, both rock types being probably associated with the evolution of an active margin 480 Ma ago. Fast cooling to temperatures below 600 O-650 o C occurred in the Sobrado as recorded by monazite (Fig. 17).

granulites,

The second major unit is composed of hrghgrade metasediments and some basic rocks included in the Banded and Chimparra gneisses.

8 350 -r‘

400

I

Fig. 17. Temperature-time sponds

to eclogltes

and Sobrado gnelsses

and

evolutton

and basic granulites

diagram (Bacanza)

(A) Unit II to the Banded Umt

I corre-

from Ortega1

(0) and Chlmparra

III to the ultramafic

Cariiio gneisses

Unrt

tme

are indicated

formatmn

(0)

(W). The

by cl

The third unit recognized here is the Cabo Ortega1 ultramafic body. The ultrabasic rocks are not directly dated, but field relations~ps and dating of a late garnet pyroxenite vein may suggest that they were emplaced after 420 Ma, and probably at 390 Ma; they may correspond to subcontinental mantle overthrusting during the uplift of

which both appear to have suffered high-grade metamorphism 420 Ma ago. The Braganqa eclogitc-like rocks were probably also metamorphosed

the lower crust. Nevertheless, their oceanic affinities (Ben Jamaa, 1988) suggest a more complex history. They may have been developed early in

during

the back-arc only related

the same event. These younger

tion high-grade assemblages continental environment and thickening processes related ing possibly induced by the

post-accre-

system and the 390 Ma age may be to a deep stage of tectonic emplace-

were developed in a may represent crustal to major deep shearclosure of a back-arc

ment with some partial melting. From 380 Ma (Fig. 17), all the Cabo Ortega1 units exhibit a common thermal evolution. This

system. The existence of such a back-arc system is suggested by the geochemistry of the basic rocks associated with the metasediments, which is compatible with tholeiites from a crustal thinning environment. The oceanic units of Candelaria-Pena Escrita and Ordenes may be evidence for a true back-arc oceanization.

was the time of the uplift of the nappes during amphibolite-grade metamorphism, probably related to the closure of an ocean and abduction of structurally overlying H/P units. The final cooling occurred 350 Ma ago. Only the Banded gneisses show a faster cooling, around 375 Ma ago. Compared with the general evolution of the

Variscan belt in Western Europe, as proposed for example in Matte (1986), the Galician high-grade units appear to record an Early Palaeozoic metamorphic history which is still not welt documented, but which is recognized in the Gothard Massif (Gebauer et al., 1988). Further, on the basis of the existence of alkaline granites the Ordovician is characterized by an extensional regime. In northweste~ Spain, the age of this magmatism is not defined, but in Portugal a&ah granites were emplaced 480 Ma ago (Lancelot and Allegret, 1982). In the Montagne Noire, they were emplaced 530 Ma ago and may correspond to an early rifting episode (Ducrot et al., 19’79). By contrast, some talc-a.Ikahne magmas were emplaced around 480 Ma in southern Brittany, suggesting a convergent system (Jegouzo et al., 1986). During the same period, much evidence points to the existence of extension related to a back-arc system (the highly metamorphosed leptynoamphiboiitic group) (e.g., Piboule, 1979; Briand and Piboule, 1979; Pm and Lancelot, 1982; Giraud et al., 1984; Briand et al 1988). This extension probably reached a stage of true ocean-floor spreading, indicated by the Ordovician ophiolites of Belledonne (Bodinier et al., 1981; Pin and Carme, 1987; M¬ et al., 1988a, b). It may have been the closure of such a back-arc basin which produced the second event of HP metamorphism between 430 and 3’70 Ma during collision and abduction processes (e.g., Gebauer and Griinenfelder, 1976, 1979; Peucat et al., 1982; Pin and Vielzeuf, 1983; Peucat, 1986b; Paquette, 1987) and around 420-380 Ma in GaIicia. This crustdl thickening is well documented in the southern Armor&an Massif where pressures reached 18-20 kb in the Champtoceaux nappe (Ball&e et al., 1987, 1989). It was at this time that the Variscan lower continental crust was probably formed on a regional scale, followed by crustal melting to form the Hercynian granites.

The English version of this paper was improved by M.S.N. Carpenter. We thank J.L. Paquette and B. Lasnier for the cathode lumine~nce investigation and N. Morin and J. Ma& for techmeal assistance. The XRF analyses were performed in

Rennes by M. Le Coz and M. Lemome. ‘Iwo referees are to be thanked for their critical comments and useful suggestions. This study was supported by ATP Geodynamique (INSU. 514575) (JJP, JBG and JC), by UPV grants 310.08 and 130-310 and by CAYCIT (PR84-0971) (IGI), and by a USNSF grant (EAR 87-20322) (RDD). Appendii

1: Analytical ttiiues

The major element contents were determined by XRF, using a sequential Philips PW 1404 spectrometer. The analytical precisions are as folbws: SK& 1% AI,O, 1.5-3% Fe@3 2-3% MnO 108, MgO l-38, CaO 2-5% Na,O 1.5-38, K,O 2.5%, TiO, 2-5%, P,O, 5%. Trace element contents (except Sr, Rb, U, Pb and REE) were also analyzed by XRF, with an analytical precision of 10%; this precision was as good as 5% for Co, V, Cr and Ba when contents were < 30 ppm and 3% for Nb, Zr, Y and Ni when contents ranged between 30 and 150 ppm. Rb, Sr and REE contents were determined by isotope dilution mass spectrometer methods. Combined Sm-Nd spikes are used for REE and Sm/Nd ratio determinations (Gruau et al., 1987). Total blanks were: Rb = 0.1 ng, Sr = 1 ng, Sm = 0.2 ng, Nd < 0.5 ng. REE analyticd precisions in concentration determination were generally better than 28, except for La and Lu (5%). Uncertainties for R7Rb/86Sr ratios were 2%, and were 0.5% for ‘47Sm/‘44Nd. Mass analyses were performed using a Cameca TSN-206 mass spectrometer. U-Pb analyses were performed on ahquots prepared from l-4 mg of zircon separate, following the method of Krogh (1973). The exception was for one eclogite, for which we used up to 20 mg because of the low U content. Total Pb blanks were lower than 0.5 ng and common lead was assumed to have the foIIuwing isotopic composition: 206Pb/204Pb = 18.0, 207Pb/204Pb = 15.5, 208Pb/2e4Pb = 37.0. Ages were calculated using a probable error of 2a for the average 207Pb/206Pb age when zircons were subconcordant or foBowing the York (1969) regression algorithm when they defined a discordia with standard errors of 2% in the U/Pb ratios, 0.2% in the U/Pb ratios and 0.2% in the 207Pb/206Pb ratios. Pb-Pb anaIyses were performed following the procedure of Chen

GEOCHEMISTRY

AND

CEOCHRO;VOLCGY

(1977). The Pb blank The measured

OF THE

CAB0

was in the order of 7-8

value for the NBS standard

208Pb/204Pb

ORTEGAL

= 36.426,

207Pb/206Pb

ZohPb/2’)4Pb = 16.873

(mean

= 0.9130

of four

were recalculated

ng.

981 was and

runs).

The

measured

values

consistent

with

(Catanzaro

et al., 1968). The errors were typically

the

certified

0.10% for the *06Pb/204Pb

in order

NB5

to be

981

values

The

Sobrado:

7 km from

techniques

used

followed

those

Dallmeyer

and Keppie concentrates

40Ar- ‘9Ar/ 40Ar

6755, 2 km south Bragaqa: granulites

of

garnet.

from Melltde

Samples

during

the

described

4oAr/3’Ar

in detail

(1987). The analyses were

plotted

the road

Location:

of

(old quarry).

analogous

diopsrde,

hornblende.

Coxa de Braganqa

tcl

de Sobrado.

Golada

Probably

composed

rutrle + brown-green

Bun&d

by

of the

on

correlation

Roddick

36Ar/

diagrams

(1978) suggests that

an MSWD

of > ca. 2.5 indicates

correlation

line

greater

than

scatter

that

about

which

explained solely by experimental errors. ages were calculated from the corrected

to the

plagroclase.

sphene.

zorsne,

(4 km from Cotmbra

1969).

a

can

be

All the isotopic

gneisses.

mg) ts shghtly quartz.

Sample

migmattttc

plagioclase.

Chrmparra

K feldspar,

gnewses

migmatittc

gneiss

K feldspar,

garnet,

Cur&~

btottte.

Sobrado paragnetsses: chlonte

gnetss

located

2 km from

Tambre

River.

Ulirumaftc

Sobrado

Samples

of

biotite.

thnemte

futile

8282 and 6762 of muscovite collected

towards

in the quarry

Corredotras

along

the

rocks (Cabo Ortega1 cornpie.?)

~ar:burg~fes

(Herbezra Musstjj~ Sample 7689. Fohated

orthopyroxene,

secondary

mmor

serpentinite

Garnet pyrouentte

and hornblende.

kyamte,

of garnet:

thich of garnet.

plagtoclase,

ts composed

garnet,

Samples

wtth relicts

Eclog~tes (Cab0 Ortega1 complex) composed

1s blastomylomtrc quartz.

and tourmahne.

spinel:

large outcrops,

8281

plagtoclase.

40% of the mam constrtuents.

rutile If: kyamte

dat-

muscovtte,

and rutde.

8282

muscovtte,

Sample

quartz,

Appendix 2: Samples selected (Figs. 1 and 2)

rocks, formmg

m q’Ar/3”Ar

rutile and allanite.

with olivme.

zotsite. quartz,

garnet

Sample

with

Snersses:

muscovite,

8091b (CO-185

gnerss made up of btotne.

ratios using the decay constants and isotopic abondance ratios listed by Steiger and Jager (1977).

diopstde

along

ratio and 0.15% for the

isotope

correlations.

Foliated

towards

6773-6774.

Bacartza,

(cf. Anthonioz,

(Roddick et al., 1980; Radicati de Brozolo et al., 1981). The regression technique followed the method of York (1969). A mean square of the weighted deviates (MSWD) was used to evaluate

omphactte,

Teijetro.

7121 and 6761, 2 km from Corredotras

Gnersses (Caho Ortega1 complex)

amphibole

isotopic

7120 and 6763-6765,

chnozorstte.

‘07Pb/ z”4Pb ratio. analyses

289

i’iAPPE

vems cross

clinopyroxene

and chlonte

r’ems I Ural Massrf),

cutting

the foliatton

composed

of clinopyroxene,

and mmor

green spine1

substttutes Sample

rocks

and

green for lo-

7690. 10 cm

of the peridotttes,

orthopyroxene,

garnet,

and

amphtbole

wtth some secondary

6767,6768,6771,7103-7108

Metuhastes

rntercaiated w&m

meaasedrmentcq

formutlons

and 7118 Ec~oglte-~~ke rocks complex): Bucarr:a Formatron (Cab0 Ortega1 complex): 6772. 7112, 7696 (sample and 8280

Fohated

garnet,

chnopyroxene,

1s nch

m scapolite

chnozotsrte, gressive pyroxene.

6769.

in the “OAr/39Ar datmg)

rocks. forming

large outcrops,

composed

plagtoclase,

rutrle f quartz.

Sample

and

sample

granulite

CO-lOB-86

Samples

secondary

hornblende,

8280 associated with

plagioclase,

amphibole,

rutile in armonred

7112

and epidote/

with eclogites garnet,

of

is a retro-

synplectmc

chno-

rehcts in sphene.

Samples

fohated

rocks

wtthhm Banded

composed

clmopyroxene

armoured

clmopyroxene.

ruttle.

biotite.

Sample

gnetsses

7125

dtrectron

and

hornblende,

in plagioclase,

Ortega/

of

symplecttttc

garnet, located

de Douro

rock

secondary and

wrthm

plagoclase.

poorly

plagmclase,

eptdote/clmozoistte

is eclogtte-hke

rutile;

of Miranda

of garnet.

sphene.

composed

hornblende

gnersse.t (Cabo

7109 and 6710 are medtum-grained.

some

mtgmatntc

chnopyroxene.

5.5 km from

Braganqa

(cl Anthoruoz,

1969).

m the

and

some biottte. Sobrado 6765,

and ~~l~~de [Ordenes

7120,

outcrops.

7121

and

are analogous

6755.

variety

xene and

actinolite

7121

is a

plagroclase.

quartz-nch brown

rocks,

Samples

6763,

formmg

large

to those of the Bacariza

7120 IS an ultramafic secondary

complex):

Folrated

rich in zorsite, Al-rich and

retrogressrve

amphibole,

Formatton

rutrle

chlorite

are present,

granulite and

clinopyro-

sphene.

with

and garnet

Locattons:

Metabasrtes plex): garnet. without mule,

Sample

of the Cundekanu 7113 is a layered

plagroclase, garnet: and

chnozotstte.

is

tlmemte

and ruttle:

7115 is again rtch

in

Formattort (C&o OrtegaL comamphtbohte stmrlar

plagroclase.

wtth hornblende,

7114 ts tdenttcal. but wtthout sphene

and

garnet

but or

eptdote.’

Metamorphosed

plagrogramte

of the Cundelarro

Formucron

Sample CO-114-86 is composed of plagioelase, quartz, garnet, hornblende and some biotite. (Cabo Ortegal

Amphibohtes

complex):

of Pumdo-

Peiia Esertta (Cab0 Orteguf com-

plex): Samples CO-l-86 and CO-2486

are layered amphibohtes composed of hornblende, plagmclase, garnet. ilmenite and some sphene, rutile and carbonates (CO-l-86). Addttronal rock types

Metabasalt (6387) from near Espasante. east of Cabo Ortegal, amphibolites from Grdenes (6390-6392), and metagabbro from near Santiago de Compostela, towards Guntin (10 km southwest of Palas de Rey).

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