40Ar-39Ar age of mylonites along the Merens Fault, central Pyrenees

Tecronophysics,

149

129 (1986) 149-172

Elsevier Science Publishers

B.V., Amsterdam.

40Ar-39Ar AGE OF MYLONITES

- Printed

in The Netherlands

ALONG THE MERENS FAULT, CENTRAL

PYRENEES

’ and J.A. MILLER 2

A.M. McCAIG

’ Department of Earth Sciences, The University, Leeds (Great Britain) .’ Department of Earth Sciences, BullardLaboratories, Cambridge Unioersity, Cambridge (Great Britain) (Received

July 23, 1985; revised version

accepted

December

23, 1985)

ABSTRACT

McCaig,

A.M.

Pyrenees.

and

Miller,

In: E. Banda

J.A., and

1986. 40Ar-39Ar S.M. Wickham

age of mylonites (Editors),

along

The Geological

the Merens Evolution

Fault,

central

of the Pyrenees.

Tecronophysies, 129: 149-172. Seven mylonitic 40Ar-39Ar complete

of argon

excess argon

rock

specimens

of mineralogy

and texture.

released 60-73

mylonite

Ma for biotite

immediately

within the M&ens zone gave a partially together, about

compression thermal

below the major the data

indicate

50 Ma ago.

They

in the Pyrenees.

event

in this part

mid-Cretaceous

North

showed

chips

allowing

rising staircase

patterns

steps. This is believed to reflect

in plagioclase

and released

it is probable

age in the mylonites.

M&ens shear zone gave a Hercynian reset Hercynian

Deformation metamorphism

and

to have

formed

resultant

uplift

basement, (90-100

that biotite One coarse

plateau

mainly

inferred

at

mica

contains muscovite

age, while another

age.

that the shear zones were active in Alpine are believed

of the Pyrenean

Pyrenean

have been dated by the

age of 93 k 2 Ma. Other

Ma for muscovite;

to the cooling

collected Taken

Several specimens

gave a plateau

and 50-60

and that 50 Ma approximates

Pyrenees

were cut out of thin-section

from micas with excess argon contained

One biotite-quartz

collected

probably

from the central

of mylonite

Ma, with much higher ages in the highest temperature

high temperatures. ages are about

and two coarse muscovites

Whole

characterisation

in the range 50-90 mixing

samples

method.

which

during probably may

have

times

the early

terminated begun

i 100 Ma and

stages an

of Eocene important

at the time of the

Ma).

INTRGDUCTION

The Axial Zone of the Pyrenees consists of Hercynian basement rocks uplifted during Tertiary thrusting. The natm-e and extent of Alpine deformation and metamorphism in the Axial Zone has been controversial for many years; some authors consider that Alpine deformation was restricted to brittle faulting (e.g. Zwart, 1979) whilst others invoke a “supple” deformation of the basement with extensive 1973).

development

HO-1951/~6/$03.50

of Alpine

cleavage (Seguret,

@ 1986 Elsevier Science Publishers

1972; Choukroune

B.V.

and Seguret,

1 so

A useful time marker 270-300

in the Pyrenees

Ma (Vitrac-Michard

Hercynian

folding

assemblages.

thrust

dated

several

but are cut by my~onitic

shear zones containing

greenschist

Clearly,

if they are Alpine,

granodiorites

197.5). These postdate

if ductile

zones are a likely candidate, external

are late Hercynian

and Allegre,

their

deformation

It is important kinematic

belts to north

occurred

Zone

facies

the shear

to know the age of the shear zones since

development

and south,

in the Axial

at

phases of

should

be related

and the assemblages

to the more

they contain

can be

used to constrain the thermal history of the orogen. If they were Hercynian, their tectonic context would be totally different, and variations in their orientation across the chain might be useful for constraining Alpine structural rotations, from the This paper reports the results of an 40Ar- “Ar study of mylonites Merens PREVlOUS

Fault,

a major shear zone in northern

Andorra.

WORK

Steeply-dipping retrogressive ductile shear zones are present throughout the northern part of the Pyrenean Axial Zone from the Cap de Creus to the Neouvielle granodiorite (Fig. 1). No attempts have previously been made to date directly the shear zones. In the Rosas area (Fig. l), north of Barcelona, the shear zones are cut by lamprophyre dykes. About 100 km further south a lamprophyre dyke has been dated as late Triassic-early Jurassic by conventional K-Ar (Chessex et al., 1965). On this basis, Carreras (1975), Carreras et al. (1980) and Saillant (1982) consider the shear zones to be fate Hercynian in age. In contrast, in the N~ouviel~e Massif (Fig. 1) lamprophyre dykes cut the granodiorite but are mylonitised in the shear zones, and Lamouroux et al. (1979) therefore regard the shear zones as of Alpine age. In the

central-eastern

Pyrenees

Saillant

(1982)

regards

mylonites

along

the

E-W

N

i

Fig. 1. Map of the eastern fault.

Pyrenees.

after Zwart

(1979). NPF-North

F’yrenean

Fault;

MF-M&ens

151

trending M&ens Fault as wholly Hercynian, while Lamouroux et al. (1980, 1981) propose both Hercynian and Alpine mylonitic events, with associated WNW-trending shear zones being wholly Alpine in age. Previous radiometric dating in the Pyrenees presents a sparse and confusing picture. Along the North Pyrenean Fault (Fig. 1) a greenschist-amphibolite facies metamorphism in Mesozoic carbonates has been convincingly dated by the 40Ar- 39Ar method at 90-100 Ma (Albarede and Michard-Vitrac, 1978). The same method has yielded some similar plateau ages on biotites in basement rocks which must have originally crystallised in Hercynian times, particularly in the Agly Massif (Albarede et al., 1978). Other biotites give Hercynian or mixed ages, except for one which gave a plateau at 65 Ma. Micas and feldspars in the Montagne Noire to the north give purely Hercynian ages, and there is no doubt that an Alpine metamorphic event, or events, affected parts of the Axial Zone. This conclusion is supported by earlier Rb-Sr dating of biotites (Roubault et al., 1963; Jager and Zwart, I968), with a complete spectrum of ages ranging from 90 to 300 Ma from various parts of the Axial Zone. However, no study so far has related agework to detailed field or petrographic studies, and this probably accounts for the current confusing picture, The kinematics of the shear zones are also controversial, with Lamouroux et al. (1980, 1981) suggesting mainly sinistral displacements, and Carreras et al. (1980), Saillant (1982), Guitard (1970) and McCaig (1983, 1986) documenting mainly reverse or reverse + dextral movements. GEOLOGICAL

SETTING

All the samples were collected from within a 25 km2 area mapped at 1 : 10,000. This area (Figs. 1 and 2) straddles the M&ens Fault in northern Andorra where it separates the Aston and Hospitalet massifs (Zwart, 1979). The M&ens Fault is a shear zone about 500 m wide which dips N or locally NW at 60”-80”. Within the Aston Massif (Figs. 1 and 2) a number of smaller ( < 30 m) shear zones trend WNW and are asymptotic to the M&ens Fault. These zones form a conjugate set and reverse displacements of gneissic units can be mapped on both north and south-dipping zones (McCaig, 1983, 1986). Lineations plunge consistently to the NW on N-dipping zones (including the M&ens Fault) and to the S or SE on S-dipping zones, indicating a dextral component of movement on all zones (McCaig, 1983, 1986). There is no evidence in this area for sinistral movements as proposed by Lamouroux et al. (1980, 1981). The shear zones are retrogressive with respect to the amphibolite facies gneisses and schists which they cut. In the Aston Massif and in structurally higher parts of the M&ens mylonites, upper greenschist facies assemblages are found, while in the Hospitalet Massif and in structurally lower parts of the M&ens mylonites, lower greenschist facies conditions prevailed during mylonitisation. McCaig (1984) inferred temperatures of 400’ -450 o C and 250 o -400 o C during recrystallisation of

,;,

-,

+.r*;-;

+

ANDORRA

Fig. 2. Map of an area north of El Serrat. Andorra.

after McCaig (1983). showing

sample localities.

shear

Lanes cfine lines) and faults (heave linea)

higher- and lower-grade mylonites, respectively. A similar jump in metamorphic grade across the M&ens Fault is seen in pre-mylonitic assemblages (McCaig, 1986). reflecting its reverse displacement. There is no reason to separate the Merens mylonites from those in the oblique shear zones to the north on the basis of metamorphic assemblage as proposed by Lamouroux et al. (1980, 1981). SAMPLE

SELECTION

The mylonites with growth compositions

AND

METHODOLOGY

generally

show intense

recrystallisation

of new micas, epidote and feldspar. from the old ones as demonstrated

1984). Porphyroclasts

of the previous

mica or amphibole are almost invariably fine grain size of new minerals (often

metamorphic

and

neo-mineralisation,

The new minerals have different by microprobe work (McCaig, assemblage,

such as feldspar,

present, however. This, together with the < lop)), makes conventional mineral sep-

aration for dating difficult or impossible. It was decided to attempt to date the mylonites by whole rock 4”Ar-7yAr on carefully selected samples. This has several advantages over other methods: (1) Only one aliquot is required for analysis so that sample homogeneity is not a problem and very small samples can be dated ( - 0.1 g). Rock volumes containing relict pre-mylonitic prophyroblasts can therefore be avoided. (2) Stepwise degassing may allow excess of argon to be identified and can permit the identification of gas evolved from different mineral phases. (3) Complete resetting of the mineral ages at relatively low temperatures is more likely than in the Rb-Sr system.

153 TABLE 1 Sampfe details Sample number (Harker Collection, Cambridge University)

Location

Assemblage

‘142023

M&ens Zone, Port de Banyeiis, GR 37853823 M&ens Zone, Rialb Valley, GR 36123863 M&ens Zone, east of Pit de Basali, GR 3493809 Minor zone south of Etang Blaou, GR 37123946 Hospitalet Massif. just south of M&ens Zone, GR 35873814 M&ens Zone, north of Pit de Be&i, GR34553812

qz-bi-pl

142009 142080

142033

142241

ES2-98

(minor)

mu-pi-qz-cc epidote) mu-qz-ct-ab

(spbene(minor)

bi-pi-qz-ep-sph-et

mu-q2 (coarse)

ab-mu-qz

(pegmatite)

Mylonitic samples were selected on the basis of thin-section and microprobe studies using the following criteria: (1) Presence of only one major K-bearing phase. (2) Maximum degree of recrystallisation without obvious relicts of earlier K-rich minerals (3) Freshness: no staining; no leaching of CaCO, if present. (4) Maximum range of mineralogy between samples. The first restriction eli~nated all samples containing K-feldspar, and most other samples collected were rejected on the basis of the third criterion. In the end, four mylonites and two country rock samples containing coarse muscovite were selected (Table 1). Sample 142023 was collected from the margin of a mylonitised quartz vein in the hirer-grade part of the M&ens shear zone, Quartz is completely re~~stallised with a strong crystallographic preferred orientation and a shape fabric slightly oblique to seams of fine (< 10 CL)green biotite (Fig. 3a). Microprobe analyses (McCaig, 1983) show that coarser biotites are chlorine-rich and that the rare feldspar grains are about An,, and unzoned. Fine biotites invariably give mixed analyses with quartz contamination. The sample for analysis was cut out of a thin section chip avoiding areas containing plagioclase and coarse biotite. After crushing, the 60-100 mesh size fraction was magnetically concentrated to give a more biotite-rich sample.

155

Sample

142029 is a leucocratic

mylonite

of uncertain

higher grade part of the M&ens Zone. A pronounced alternations altered

of feldspar-

and

muscovite-rich

bands.

to calcite (Fig. 3c) and show a narrow

(McCaig,

1983). Fine

show a range

feldspars

in composition

marginal from An,,

protolith banding

collected

from the

(Fig. 3b) results from

Coarse

feldspars

range of composition

are often

from An,,_,,

to larger grains and in mica-rich layers to An 20. Coarse feldspars are probably

porphyroclasts relict from a previous assemblage. Muscovite is very fine-grained (< 10 p) throughout and mostly very strongly oriented (Fig. 3d). Small amounts of sphene and epidote are concentrated in mica-rich layers. Two samples of this specimen were cut out of a thin section chip and crushed

to

60-100 mesh. One was a layer rich in coarse and fine feldspar (layer 1 in Fig. 3b, spec. 142009-2) and the other was several mica-rich layers, (e.g., layer 2 in Fig. 3b, spec. 142009-1, repeated as spec. 142009-3). Sample 142080 is a shear banded phyllonite from the lower-grade M&ens Zone. A muscovite-quartz-minor albite mylonitic fabric

part of the is cut by

chlorite-quartz rich shear bands (Fig. 3e). In thin sections perpendicular to the extension direction of the shear bands a few coarser, mis-oriented muscovites are seen. A single grain of biotite was found in a powder mount thin section after crushing; no biotite has been seen in thin section. Both coarse muscovite and the biotite could be pre-mylonitic relicts, although the biotite may be contamination. The shear bands and in press).

are strongly

metasomatic

as discussed

elsewhere

(McCaig,

1983

After crushing and sieving to 60-85 mesh, a series of magnetic separates were made allowing separate analysis of chlorite-rich (spec. 142080B) and a muscovite-rich (spec. 14208OA) fraction. Sample 142033 is a relatively coarse grained (SO-150 p) mylonite derived from hornblende-diorite gneiss, and collected from a minor shear zone about 2 km north of the M&ens

Zone.

patchily

by more

veined

Fig. 3. Photomicrographs

Coarse

porphyroclasts

sodic

of analysed

feldspar.

quartz-biotite-mylonite.

lised quartz,

fine biotite.

b. Spec. 142009 (ES80-63):

leucocratic

cut out of thin section chip for dating. c. Spec. 142009 (ES80-63): d. Spec. 142009 (arrow).

strong

matrix

consists

are often

of biotite,

quartz,

Note oblique

gram

shape fabric

in well recrystal-

Scale bar 25 pm. XPL. mylonite

with plagioclase-rich

(1) and muscovite-rich

(2) bands

Scale bar 2 mm. PPL.

calcite replacing

(ES80-63):

The

(An,,_,,)

mylonites

a. Spec. 142023 (ES80-93): and extremely

of plagioclase

plagioclase.

muscovite

fabric

Scale bar 25 p, XPL. with minor

sphene.

Note

quartz

with subgrains

Scale bar 25 p. XPL.

e. Spec. 142080 (ES 80-36): chlorite-rich f. Spec. 142033 (ES80-124): bar 30 W. XPL.

shear band cutting

well recrystallised

matrix

mu-qz-ab

in dioritic

phyllonite.

mylonite

Scale bar 25 pm. PPL.

with bi, qz, ep and sph. Scale

156

plagioclase (An,,_2j), epidote, sphene and minor chlorite, with a relatively weak preferred orientation (Fig. 3f). This sample is described in detail by McCaig (1984). By cutting material out of a thin section chip areas of relict hornblende-bearing gneiss and areas containing relict brown biotite were avoided and two 60Y100 mesh samples, one rich in plagioclase porphyroclasts and quartz (spec. 142033-l) and the other rich in green matrix biotite, recrystallised piagioclase and epidote (spec. 142033-2) were obtained. Sample 142241 is a coarse muscovite-andalusite segregation collected from within andalusite-biotite schists about 100 m south of the lower boundary of the M&ens mylonites. Coarse muscovites up to 1 cm in length show only minor kinking and sericitisation and muscovite for analysis was cut out of the hand specimen with a scalpel, sieved to 60-100 mesh and cleaned ultrasonically. Sample E82-98 (no Harker collection number) is an aibite-quartz muscovite pegmatite cutting relatively undeformed si~limanite-andaiusite schists within the MCrens mylonites. The sample was collected close to several minor shear zones and the muscovite is extensively kinked and partially retrogressed to sericite. Feldspars are cut by sericite veins. Muscovite for dating was prepared as for spec. 142241. Sample preparation for irradiation and the subsequent step-heating followed closely the procedures described by Grasty and Miller (1965). Samples were irradiated at AWRE Aldermaston, the sample package being inverted halfway through irradiation. Standards used were the muscovite spec. GG2 and the biotite spec. Bi-133 (Table 2). Step-heating was performed on the Omegatron mass spectrometer at Cambridge University (Grasty and Miller, 1965). The data was processed using a computer program written by C.R. Brewitt-Taylor, which corrects for interfering isotopes using the formula and constants of Brereton (1970). Temperatures were measured by optical pyrometer accurate to about 50°C. RESULTS

Rest&s are presented in Table 2 and Fig. 4. Of the two coarse muscovite samples, spec. 142241 collected south of the M&ens Fault shows a reasonable plateau age of 280-300 Ma over 96% of the 39Ar released. Sample E82-98 collected within the M&ens Fault Zone shows a more disturbed spectrum with a “staircase” pattern rising from 220 to 270 Ma over most of the gas released. The simplest mylonite spectrum is that of spec. 142023 (biotite + quartz} which shows a good plateau with an age of 93 5 2 Ma over 95% of the s9Ar released. The integrated age for this specimen, which is equivalent to a conventional K-Ar age, is 90 + 1 Ma. The other samples all show “disturbed” spectra of one sort or another. Both fractions of spec. 142033 show similar spectra, with steadily rising ages in the range 60-90 Ma over most of the “Ar released, but with anomalously high ages up to 3277 Ma in the four highest temperature steps. The last four steps involve 5% of the

157 TABLE 2 40Ar/3pAr degassing data (Gas quantities in arbitrary units) Timesmce

Heatstep

Sample:

Irradiation (days)

14208OA

‘&nr

Sample weight:

0.114

'?A,

"Ar corrected

g

lrradret~on

JBAF

time:

4.585

1

305.52

. 0001

.OOOl

.063?

.0217

2

305.56

.oozo

.ooa5

‘2185

.a142

3

305.70

.oozz

.0002

.062V

4

310.43

.OOlb

.0006

.0234

-A*

Age(Hsf

*A,

days

Error

J = 0.03315.

. I>13 .3425

4.3500

210.9

2h.0

1.3625

131.2

2h.2

1.1620

2.8070

107.0

7.3

.Y150

1.6450

75.2

9.4

5

740

310.53

.0035

.0005

.ue7.5 .2886 *2410

.0548

I.

5.4300

67.2

2.3

6

800

310.61

.0041

.0004

.1931

.ll4S

9.0500

11.5700

66.3

1.2

8000

7

845

310.69

.0029

.0003

10.4500

13.8750

72.1

1.2

890

310.74

.0013

.OOOl

,I451 .OL84

.1280

0

.lOlO

8.0380

11.8000

81.6

1.3

9

930

310.78

.0030

.0001

.048k

. 1080

8.5200

iP.8000

94.2

i.2

10

960

311.117

.OO32

.OOOl

.I673

.1460

II*1800

23.7000

116.9

1.4

11

1000

311.52

,003s

,000)

.1229

.llPO

9.4800

27.6700

159.8

1.1

12

1090

31i.57

.a032

.OOlO

.4920

.0163

2.1750

230.9

25.8

13

IlEO

311.68

.0070

.0070

.3690

1427.8

121.2

.0358

--~ lota

gas and integrated

sampie:

142OC?O3

age:

.0341

3.2800

--

.048

Sample-night:

JOSD

.0051

0.306

.946

2.65

Irradaetion

g.

tine:

--

-

6.585

63.653

Standard

_l : 0.03979

days

.3

99.9

124.465

GG2

1

348

.0098

.0002

.2020

.041?

.1680

3.2900

163.2

2

348

.0037

.0003

.OOOJ

.3031

.2475

1.8600

105.6

20.1

720

348

.0024

.6061

.I780

a2605

46.6

40.9

4

760

352

.0033

.0006 .0007

.7653

.1800

I*5800

3.1200

95.9

6.6

5

82U

352

.a037

.0007

.x53

.0643

4.6800

6.5600

81.6

2.3

b

880

358

.002P

.0007

.6616

,0496

3.1600

5.66DO

97.3

3.0

7

‘Ia

358

.003I

*6154

.0625

4.3100

7.9300

112.7

2.4 3.9

3

.8660

59.5

6

IUZU

360

.OU25

,0005 .0005

tb402

.0565

2.6400

5.9900

137.4

Y

1100

360

.0030

.0003

,320l

.0377

1.2600

5.1000

225.3

8.0

10

I l’u

361

.0015

.0005

.6510

,023)

.0607

1.6700

1097.6

102.6

I1

1200

361

.0057

.0004

.52?_4

.0617

.a335

2119.4

85.0

4.5800

----___ Iota1

Sample:

gas and lnteqrated

102’309-1

age:

‘(141

Sample *eight:

_ .0054

.2889

6.253

Irredlatlon

i.0003

time:

46.586

19.065

a.585 days

128.4

.0118

.0032

.0423

.0120

$0660

2

128.5

.0078

.0037

.0489

.0133

3

128.5

.0071

.0032

.0423

.Ollb

.0814

,936

-1700.5

305.7

.0703

,494

-3176.5

802.5

128.6

.0042

.OoQ?

.0624

.02#

. ii342

128.7

.0044

.0191

.2s39

.0670

.I650

6

710

128.8

.1285

l.?iJll

.0765

2.2750

7

780

135.7

.0060 . 0060

.1125

1.7152

.0810

8

840

148.6

.0045

.ObZO

1.2207

9

BY5

148.7

50046

.0713

10

955

148.7

.0041

,079s

11

100s

153.6

foo66

,0456

.9?12

12

1070

153.7

‘0041

.I213

2.6389

13

1125

153.7

.a037

.a715

1.5568

14

1190

168.7

.0035

.0278

.0764

.7534

age:

GG2

-1028.9

650

gas and integrated

Standard

1.0

2.685

5

lotal

125.6

J - .03622

1

4

--

260.4

1.170

-

25.8

33.4

.94B

*

66.3

29.1

3.113

40.1

4.2

4.8300

5.580

11.2

2.0

.0440

Z.%iSCI

2.910

41.0

3.7

1.4053

.0520

2.6650

3.030

41.6

3.6

1.5685

.0650

2.2750

3.015

52.9

4.2

.0392

I2625

1.355

.0895

.1600

2.688

.0650

.0700

.0415

.0243 15.8537

.a144

-14.0639

.6224

12.4

34.2

647.1

49.8

1.563

461.2

109.1

1.640

1253.4

205.0

_31.127

38.1

.e

158

TABLE

2 (continued)

Sample :

142009-2

Sample uelght:

0.322

g

lrradvatlon

tuw:

4.505

Ftsndard GGZ

J i 0.03581

days

1

94.6

,011)

.0104

.0705

.028E

.I043

2

94.6

.0022

.0117

.0793

.0131

,084s

3.900

316.3

77.4

.618

-

22.7

114.7

-

3

710

95.5

.OOlY

.0508

.3501

.0403

.1123

,523

10.6

05.9

4

705

95.6

.OOZE

.3563

2.4602

,032s

.2870

.800

28.8

33.1

5

828

95.6

.OOM

.6317

4.1657

.OlR3

1.0100

1.770

35.6

9.4

6

865

95.1

.0043

.2550

1.7645

.0380

2.4400

3.120

50.2

3.8

7

94c

101.6

.M33

.2S40

1.9758

.0680

2.4600

3.160

59.1

3.8

G

1000

101.6

.0032

.1775

1.3ElY

.0475

1.8250

2.638

61.0

5.1

9

1065

101.7

.UO4S

.1280

1.3871

1.4950

4.000

110.9

6.1

10

1132

102.5

.0027

.lllO

.OBBO ,065O

38.9

11

1180

102.6

.005>

.2650

12

1245

114.5

.0033

.0755

Total

gas and Integrated

Sample:

142033-l

aqe:

.9264 2.1029

:577

-

-

---

.0503

2.3029

17.620

Sample wel@t:

U.234

2.825

635.8

.1450

5.225

1187.6

36.8

.0746

.0432

2.650

1613.4

92.8

_

lrradlatlon

g

.1760

.1640

.6701

time:

4.585

10,lGll

31.229

J : 0.03529

days

-__ 107.0

Standard

1.6

GG2

1

120.4

.OlOO

.0022

.0240

.OlOO

.0493

2.813

-200.6

214.3

2

120.5

.0034

.0021

.U237

.0058

.C466

,666

-536.0

272.5

3

120.7

.0073

.0055

.0624 .1093 .1901 .4504

.0114

.1373

1.254

-477.9

90.1

.0157

2940

I. 395

-173.5

35.4

.0238

.2420

1.585

- 57.1

10.0

.0745

2.0430

4.375

33.5

4.6

4

710

iZl.ir

.0073

.0095

5

763

121.5

.0070

.0165

6

807

121.6

.Olll

.0390

7

850

121.7

.Clll

.1050

1.2139

.0913

2.4375

5.600

60.5

3.G

0

895

121.7

.0106

.2355

2.7254

.0665

1.4967

4.550

65.5

L

9

938

122.4

.0085

1.9927

.0750

1.9200

4.275

64.9

I L.8

10

960

122.5

.0093

.34no .2360

2.7737

,112s

2.9400

6.050

73.2

7.1

11

1022

122.6

.C115

.2288

2.6657

.0990

2.6350

7.680

103.6

I2

1070

122.7

.0072

.1513

1.02oe

.0238

.1550

3.550

544.9

13

1110

126.5

.0005

.1320

1.6779

.0328

.0822

14

1150

126.5

.0131

.2660

3.3040

.0763

. 1000

15

1135

127.6

.0105

.a545

.7095

.0225

-___ Total

ges

Sample:

Total

and lnteqrated

142033-Z

age:

Semple weight:

.1364

0.294

Irradlatlon

g.

7.100

2008.5

39.3

17.125

3193.0

22.0

4.480

1518.4

.7408

tuw:

4.505

108.4 --

~_

21.054

1.0239

.0384

J.li 45,s

14.917

days

1

197.5

.OlOO

.0006

.0310

.0091

.0716

2

197.5

,003)

no07

.0388

.0063

3

198.6

.0030

.OOlO

.0527

.0065

.0966 .11a3 .2925

72.490

J = 0.03184 3.030

136.9

2.6

Standard GGZ 55.6

115.2

2.4

87.9

19.0

72.7

1.048

is.3

28.4

.9bO

.046

4

700

198.5

.0027

.0012

.M59

.0132

5

760

195.5

.0057

.0026

.1372

.0394

1.1000

2.910

66.3

7.5

6

820

198.6

.0070

.0070

.1416

4.1000

7.710

73.6

2.0

7

86U

203.6

.0025

.0070

.3702 .4085

8

925

203.7

.0041

.0207

1.2092

9

1000

218.5

.0042

.0272

10

lOS0

21.8.6

IO099

11

1125

218.7

12

1170

13 14 15

1350

.0638

1.9650

3.660

83.3

4.2

1.9750

4.365

90.7

4.1

2.1332

.0698 .0788

2.2075

4.925

92.7

3.6

.0518

li.0590

.2730

7.6300

15.563

93.7

1.3

.0049

.0162

1.2727

.0870

2.4500

7.340

134.1

3.3

218.7

.0051

.0305

2.1980

.0248

.I810

5.625

1016.1

28.5

1230

218.8

.0082

.0221

1.7396

.0347

.0793

10.033

2566.7

33.3

1280

219.7

.0093

.0098

,786U

.0216

.0302

7.480

3276.6

53.7

219.8

.0072

.OOlG

.I404

.oo>o

.0072

2.690

2249.5

357.0

age:

.0879

.2002

78.285

131.9

0.8

gas and integrated

-

-

16.042

.8746

-22.384

159

rABLE 2 (continued) ---

-_I.. watstep

Supls:

‘;w

Tme since irredletion (days)

JbAr

--.---

Svnpls might:

142023

37Ar

0.116

37Ar corrected

Irradiation

9.

38Ar

3Y*r

4%

Error

Ags(Hsf -__

Urn:

5.582

.I I 0.01093

days

1

275.6

.0074

.0006

.I448

.0350

*160s

2.070

2

276.5

.ooie

.oooi

.0246

.0615

.1585

.a7

3

276.5

.m22

.a002

.0092

.a742

.27e5

,661

Standard 662

-

05.6

60.6

- I8.4

52.7

1.7

29.7

94.8

2.1

4

765

276.6

#0107

.0006

.I478

I.0180

3.8~50

10 * 000

5

800

276.7

.w5e

.OOlO

.2P69

.5020

2 0900

5.2iO

90.5

3.8

6

870

276.7

.0073

.0009

,222s

.5460

2.3(100

6.220

93.8

3.4

2.5080

7

960

276.8

.0091

.0009

.2221

.6210

6.910

90.9

1.2

8

1010

217.5

.0067

.0007

.I758

.1330

1.6830

4.880

93.1

4.7

9

1080

277.6

.0067

.WlO

‘2513

‘3670

1.3500

4.875

92.4

5.9

10

1150

277.7

.OOSB

.0018

‘6410

.0277

.0773

2.075

269.2

92.8

11

1210

277.7

.0028

.0006

.1513

.0063

.0096

.879

321.7

729.6

.0683

.OOGY

2.0079

3.6517

14.5204

44.247

89.7

0.9

gas and Integrated age:

Total

Sample:

142241

Sample Might:

Irradiation time:

0.128 9.

7.063 days

.I = 0.02404

Standard 611133

.I?001

.0051

.190@

lOPE.3

1078.5

.7POS

.0013

.01?5

.26&S

162.2

451.4

.OOOl

.3906

.OOlJ

.0716

.5470

224.9

108.2

.0007

.OOOl

-4301

.003?

.0244

1.5POO

230.7

33.2

430.3

.0018

.OOOl

*so51

.OZOR

1.7063

13.0600

293.4

5.7

6

434.2

.OOlO

.0002

l.OSl5

.0197

1.6500

11.7600

279.8

6.3

1

417.1

.a001

2

417.2

.0007

.ooilz

3

417.2

.0006

4

422.1

5

7

434.2

.oooe

.OOOl

.5461

.OlOP

.9217

6.6300

279.2

9.9

e

436.2

.ooio

.OOOl

.567?

.0225

1.6450

12.sooo

296.4

6.3

9

437,2

.0010

.OOOl

.5791

-9544

1.8399

13.59QQ

288,e

5.e

IO

437.2

.OOlO

.noot

1,1591

.0054

.4005

3.2150

299.2 --

24.1

.0012

6.05

.1400

8.4917

-63.2435

287.2

2.3

Tot&l gas and integrdted age:

Sample:

E82-98

Sample Weight:

.129 g.

Irradiation time:

3.063 days

J * 0.02444

Standrrd 81-133

1

400.2

.OOI 2

.OOOl

.27R9

.0020

.0097

.3600

103.4

2

400.2

.OOlO

.0003

*R375

.0020

.0142

.273R

102.4

541.1

3

401.1

.OOlO

.OOOl

.28x?

.00?5

.066S

.5670

182.4

106.7

4

406.0

.oole

.0003

.93RR

.0097

.6700

4.0125

218.7

10.9

5

406.1

.0020

.0007

2.1942

.0175

1.1440

6.6250

223.3

6.9

6

406.2

.0018

.0007

2.1978

.0211

1.7225

11.1333

255.9

5.1

I

406.2

.0009

.OOOl

.3143

.016?

1.3050

260.5

5.P

e

407.0

.OOlO

.0004

1.2770

.0327

2.5250

16.6525

264.5

3.4

9

414.1

.0013

.OOOl

.3676

.OllO

.S%O

5.7700

266.7

9.1

10

415.1

.0009

.OOOl

.3745

.oou

.659O

5.3205

31O.P --

11.3

.0129

.0029

9.0644

A.9439

59.1P41

256.3

2.2

Total gas and integrated age:

Sample:

142009-3

Sample weight:

.3042 g

Irradiation time:

.llP6

3.063 days

J

El.5600

= 0.07454

Standwd

769.1

81-133

387.2

.0014

.ooOl

.2160

.0013

.OOQ

.3570

-254.4

1023.2

389.1

.oolZ

.ooOl

.224X

.OOM

.01?5

.3100

- PO.4

439.9

389.2

.OOZO

.0002

.4492

.0225

.1094

.8708

120.3

63.3

392.3

.OOlO

.0003

.715P

.olep

,6425

1.0267

52.1

11.3

394.0

.0006

.OOOl

.2469

.0318

1.6933

2.4500

56.7

4.3

394.1

.0007

.OOOl

a2474

.0300

1.1583

2.0333

67.9

6.3

395.0

.OOlO

.0002

-5038

.0272

.0960

1.2180

394.8

65.1

396.1

.oocz

.OcQl

.2523

.0472

90242

1.9333

1733.3

146.8

.0007

.oool

.2575

.0022

4931.P

593.6

.OlW

.0013

3.1132

.1840

396.1

------

-0036 3.753

2.2083 12.4074

--

l(19.8

3.1

160

total ‘9Ar but 33-45s of the total 40Ar. Integrated ages are 137 f 3 Ma and 132 & 1 Ma showing that conventional K-Ar dating of this rock would have been misleading. The two fractions

show significantly

different

ages over the main part of the gas

release. A very similar

pattern

is shown

spec. 142009, but with somewhat part of the two spectra. shows negative

by the muscovite-plagioclase

lower ages in the range 40-60

The reliability

of spec. 142009-l

ages for the first five heat-steps,

is questionable,

an anomalous

142023

-~__

I_

50 Ar39

cum%

lL2033-1 142033-2

cum%

Fig. 4. 40Ar-39Ar

age spectra.

c-

Ar39

1

50

bearing

1 100

mylonite

Ma over the main however;

it

age of 12 Ma for step

161

11, and an integrated

age of only 38 I!I 1 Ma. When this analysis

142009-3)

a similar

integrated

age of 110 Ma.

A different saddle only

shaped

was obtained,

type of spectrum spectra

in the last

chlorite-rich

spectrum

two steps,

sample

is shown

with minima

accounting

gives older ages than

300-

39Ar is derived

142009-1 142009-2 142009-3

mainly

by spec. 142080; for 4 and

the muscovite-rich thermal

r.SKlOl"lng

heotstepr

:: : OO

cum%

142080A 1420808 -

I I-IL:

cum% Fig. 4. (continued)

50 Ar39

100

ages of

50 Ar39

fractions

1. I I.28

remolninp

,097.?

he~trfepr

2719 : ,.. . ..j ! ;

show

high ages occur 40Ar. The

one, with integrated

release spectra

Ggesof

(spec.

ages and an

13% of the total

from mica in these samples;

= ::.:: CZl

both

of 66 and 82 Ma. Anomalously

ages of 126 and 100 Ma respectively. Figure 5 shows 39Ar and 37Ar differential samples.

was repeated

but with fewer negative

typical

for all mylonitic double

peaked

162

142241

-

I

E82-98

-

:::.:::

cum %

50 3gAr

0 Fig. 4. (continued) '

I

51

i4208OA

1

;

!

’ 5,

009-l

0808

.:09-2

1000

Fig. 5. ‘9Ar (K-derived) arbitrary

and “Ar (Ca-derived)

units normalised

to sample weight.

release spectra

for mylonitic

samples.

Gas quantities

are in

163

patterns

(cf.

temperatures released about

et al., 1980)

Roddick above

1100°C.

are seen,

Significant

with

amounts

at > 1000°C for ail samples which contain 1100°C.

(spec. 142009-2) amounts

Lower temperature or epidote

of feldspar

peaks probably

little

or no gas evolved

of 37Ar (derived plagioclase, result

from

Ca) are

with a peak often at

from degassing

(spec. 142033). Even in samples

at

containing

(spec. 142080, 142023), “‘Ar rises and becomes

of calcite only small

more important

than 39Ar in the last few temperature steps. A surprising feature of some analyses are the negative ages shown by early heat-steps. This results from 40Ar/36Ar ratios less than that of atmospheric argon: the possible origin of this problem is discussed below. DISCUSSION

Negatiue ages Negative

ages in early heat-steps

in the analysis. Dalrymple previously

Although

and Lanphere, been reported

this problem

clearly reflect a systematic has occasionally

error at some stage

been reported

in the past (cf.

1974; Fitch et al., 1978), it is not common and has not to the extent shown by some analyses here. Before

interpreting the spectra, it is necessary to establish that the negative ages do not reflect a serious error in the whole analysis. The anomalous ages are seen in the plagioclase bearing mylonites spec. 142009 and 142033 and to a lesser coarse muscovite analyses or certain samples, particuIarly not due to any failure in the The negative

extent in spec. 142023. They are not seen in the two in spec. 142080. It appears that the error is specific to fine-grained mylonites containing plagioclase, and is gas extraction or analysis procedure.

ages reflect 40Ar/36Ar ratios less than atmospheric

argon;

there are

three possible explanations for this (McCaig, 1983): (1) Inco~oration of “real” argon with a 40Ar-36Ar ratio less than atmospheric. (2) Unrecognised (3) Unrecognised

irradiation laboratory

artifacts. artifacts.

As discussed by McCaig (1983), the first possibility is extremely unlikely since all recorded measurements of atmospheric or xenolithic argon show 40Ar/36Ar > 295.5 (Ozima

and Podosek,

1984).

The second possibility was suggested account for negative ages in a hornblende

by Harrison and McDougall (1981) to analysis. 36Ar is produced in the reactor only by a 40Ca (n, ncu) reaction (Brereton, 1970). This is corrected for by analysing ?Ar produced mainly by a 4oCa (n, a) reaction. If 37Ar and “6Ar became separated in the sample due to processes such as recoil, the correction could introduce a systematic error. However, in spec. 142033-l the whole of the 36Ar calculated to be Ca-derived accounts for only 6.5% of the 36Ar evolved in the first five heat-steps, so this is unlikely to be significant. The correction coefficient used is somewhat higher

164

than that used by Cadogan and Turner enough to make a significant difference. It should prolonged

be noted, reactor

the length standard factors

however.

shut-down.

of the canister. and that either

were wrong.

(1976) for the same reactor,

that the irradiation

nuclear

reactions

The fact that the repeat analysis

The only well documented variable increases

was unusually

It may be that the irradiation

irradiation did not show so many negative ble at least in part. was described

took place immediately

and the flux gradient

unsuspected

case of isotope

bakeout procedures in 40Ar,/‘hAr initial

were far from or the correction

of spec. 1420091

with a later

that this may be responsi-

fractionation samples

after a

high (23%) along

conditions occurred

ages suggests

by Baksi (1974). He subjected

but this is not

during

of Columbia

argon

analysis

River basalt

to

prior to conventional K-Ar analysis and proved ratios of up to 11.6%. He attributed this to sealing of

atmospheric argon in cracks in the rock by smectite expansion: bakeout partially removed this weakly held gas and preferentially removed jhAr. resulting in high initial 4”Ar,/7hAr values. The methodology employed at Cambridge bakes out the line but not the samples before step-heating; if fractionation of weakly held atm~~spheric preferentially

argon occurred in the first few temperature steps, ““Ar would he evolved. Presumably the associated e’Ar would be evolved in later heat-steps leading to anomalously high ages. Calculations suggest that a maximum of about 7”2 of the ““Ar in later steps of spec. 142033-I could be from this source: this would make no essential difference to the conclusions of this study even if it appeared entirely in the last few heat-steps. In the fine-grained mylonites studied here weakly held argon may have been present

in grain boundaries

The samples porphyroclasts: diffusion density. significant

or in thin cataclastic

also contained strongly perhaps fractionation

along

subgrain

It is worth

boundaries

noting

that

650°C‘. To summarise,

or through

Harrison

gas release by undeformed

of the spectra

and

plagioclase

the origin of the negative

that the other heat-steps

zones cutting

the mylonitic

fabric.

deformed minerals, especially plagioclase of argon occurred during low temperature lattices

(1981)

at temperatures

ages is uncertain.

concerned

with a high dislocation

McDougall

demt~nstrated

between

3.50’ and

but it is very unlikely

have been seriously

affected.

The high ages in the last few heat-steps of most of the mylonitic samples are believed to reflect degassing of plagioclase containing excess argon. The steps invariably show high 17Ar,/“‘)Ar ratios indicating degassing of a mineral rich in Ca and poor in K (Fig. 5). The effect was most marked in plagioclase-rich samples and very small in plagioclase-poor samples such as spec. 142023. Other Ca-rich minerals such as epidote and calcite would be likely to degas at lower temperatures. Ages in

165

intermediate ratios

steps

are high.

Hercynian

micas would

plagioclase. Harrison excess

must

argon

in large part

If mylonitisation

the

degassing < 100 Ma

of micas ago,

(1981) made a detailed

Broken

Hill

area

since

In this

of

in recrystallising

study of plagioclases

in Australia.

39Ar/37Ar

recrystallisation

release “‘)Ar which could be incorporated

and McDougall from

reflect

occurred

area

containing granulites

metamorphosed at 1660 Ma were heated to about 350°C, 520 Ma ago. Plagioclase shows saddle shaped 40Ar/39Ar spectra with ages > 2 Ga in both low ( < 500°C) and high (> 1000°C) temperature heat-steps. Some samples gave integrated ages greater than the age of the earth. Harrison and McDougall (1981) calculated that between 4 x lo-” and 22 x 10-r’ mol gg’ (0.5 x 10e5 to 5 x lop5 cm3 gg’) of excess 40Ar were contained in plagioclase. Roddick et al. (1980) record concentrations of 40Ar ranging from 0.7 X 10e5 cm3 g-’ for a 20 Ma old biotite to 5.5 x 10e5 cm3 g- 1 for a 200 Ma biotite. Clearly in samples containing both biotite and plagioclase the latter could contribute significantly to the age spectrum as is inferred to be the case for spec. 142009

and

142033 where

up to 45% of the total

released by the rock contributes to anomalously high ages. Figure 6 compares the 40Ar release spectra of biotite and plagioclase

40Ar

using data

T ("Cl Fig. 6. 40Ar release spectra for plagioclase (solid lines) and biotite (dashed lines). Data from Harrison and McDougall, (1981) and Roddick et al. (1980). Each histogram represents the mean of five mineral analyses, with gas quantities expressed as a percentage of total gas released.

166

from Roddick et al. (1980) and Harrison and McDougall (1981). Data for muscovite is not plotted but the muscovite analysed by Harrison and McDougall (1981) shows similar

degassing

between

600°C

characteristics and

1100°C.

to biotite.

Biotite

Over this temperature

over 95% of its 4”Ar

releases interval

only about 17% of its 40Ar with only about 7% released between 63% of the 40Ar contained would

not be diluted

in these plagioclases

by any significant

was released

contribution

plagioclase 600°C

releases

and 1000°C.

at > 11OO”C, where it

from biotite.

It is clear that

degassing of plagioclase containing excess argon provides an adequate explanation for the anomalously high ages seen in the high temperature heat-steps of spec. 142009and142033. Some mixing intermediate

of gas derived

temperatures.

from mica and plagioclase

Unfortunately

must

this effect cannot

since neither the detailed degassing characteristics biotite and plagioclase in the mylonitic samples

have occurred

be quantified

at

precisely

nor the exact proportion of are known (particularly since

porphyroclast plagioclase appears to contain less argon than fine neoblast plagioclase in spec. 142033; see below). If biotite and feldspar in spec. 142033 showed the degassing characteristics of Fig. 6 and the plagioclase gave a plateau age spectrum, the ages at intermediate temperatures would be 5-10% higher than the pure biotite age. This effect would be reduced if the plagioclase spectra were saddle shaped with lower ages at intermediate temperatures. However, spectra were attributed by Harrison and McDougall

the details of saddle shaped (1981) to effective segregation

of radiogenic and excess 40Ar between different phases of the Huttenlocher exsolution in bytownite (An,,,_,,,). In addition, some segregation of excess 4”Ar and reactor-derived “Ar between cation sites with different diffusion characteristics was inferred

(Harrison and McDougall. 1981). The feldspars in the mylonites to An,,, and will contain different exsolved phases and different

albite distributions

from

likely to be different

bytownite.

The details

in the present

the lack of any sign of excess argon

of plagioclase

degassing

range from cation site

are therefore

case from those in Fig. 6: this may account derived

from plagioclase

for

at low temperatures

(except possibly in spec. 142080). To summarise, the age of each temperature step in a spectrum such as spec. 142033-l is a maximum value for the age which would be given by a separated mica at that temperature. The “staircase” profiles in spec. 142033 and 142009 may therefore reflect mixing of gas from mica showing a plateau age with increasing proportions of excess argon from plagioclase. although the precise shape of the mica spectrum cannot be assessed with certainty. Where different samples from the same rock have been dated and show different ages in intermediate temperature steps (spec. 142033) the lower ages are more likely to represent the mica age in that specimen. Using the above interpretation the following maximum ages can be assigned to the major part of gas released from mica; 60-73 Ma for biotite in spec. 142033-t and SO-60 Ma for muscovite in spec. 142009-2 and 3. These ages represent about

167

60% of the total 39Ar released in each case. The somewhat younger ages shown by spec. 142009-l are probably not reliable as discussed earlier. The spectra for spec. 142080 are more difficult to interpret. Figure 4 suggests that an “old” argon component released at high temperature is once again present. This is probably partly released from albite, which does not contribute much 37Arand so is difficult to detect, but is only present in small proportions (- 3%) on the basis of petrography. Another “old” component may be contained in relict Hercynian micas which would be expected to degas at relatively high temperatures because of their coarser grain-size. These would be unlikely to give ages > 300 Ma. The descending ages at low temperatures are probably due to chlorite since this is the only phase not present in other samples, and might be expected to degas at low temperatures. The chlorite may contain excess argon, and certainly gives higher ages than muscovite since the chlorite rich fraction is older over most of the temperature range. Albite may also be contributing to the gas released at low temperatures (cf. Fig. 6). The minimum of 66 Ma in spec. 142080-l is probably a maximum muscovite age. The other spectra are relatively easy to interpret; sample 142033 shows an excellent plateau age of 93 + 2 Ma, with only minor plagioclase contamination and no sign of resetting in low temperature steps. However this age could be too old if the biotite contains excess argon, as discussed later. The coarse muscovite spec. 142241 gives an almost undisturbed Hercynian cooling age of about 290 Ma, while spec. E82-98 shows signs of partial degassing during an event younger than 220 Ma. S~gn~~cance of the ages

New and old age data in the Aston-Hospitalet Massif can be synthesised as follows: (1) Rb-Sr ages of coarse Hercynian biotites are variably reset to < 120 Ma in the Aston Massif, but not in the Hospitalet Massif (Jager and Zwart, 1968). This implies that the closure temperatures of Rb-Sr in biotite (- 300°C Jager, 1979) was exceeded in the former Massif but not in the latter during Alpine times. Coarse muscovite Rb-Sr ages are almost unaffected by Alpine thermal events in both Massifs, suggesting maximum temperatures < 500°C. No Rb-Sr work has so far been undertaken on mylonitic samples. (2) A coarse muscovite from the Hospitalet Massif immediately below the Merens Fault gives an unreset Hercynian @Ar- 39Ar spectrum, suggesting again that temperatures did not exceed 300°C in this Massif during Alpine times. Slight resetting of a similar but more deformed muscovite from a schist enclave within the Merens shear zone suggests either slightly higher temperatures or argon loss during deformation. No coarse muscovite has so far been analysed from the Aston Massif where the mylonites show upper greenschist facies assemblages. A course pegmatite biotite from the southern part of the Aston Massif has given a plateau age of 80-90 Ma (L.G. Garwin and J.A. Miller, pers. commun., 1985) implying complete resetting

of K-Ar systematics at temperatures above 300°C biotites from the Hospitalet Massif have been dated. (3) Fine mylonitic Aston

Massif

muscovites

biotites

give ages of 93 Ma and

give maximum

(4) Zircon

from the Merens

fission

ages of 50-60

(Wagner

et al., 1977).

No

shear zone and a shear zone in the

60-73

Ma (maximum).

Fine

mylonitic

Ma and 65 Ma.

track ages in both the Aston and Hospitalet

massifs

are in the

range 40-50 Ma, implying cooling below 225°C at this time (Garwin, 1985). Taken together these data clearly indicate an Alpine thermal event in the 100-50 Ma time interval with cooling through 225°C at 40-50 Ma. Temperatures exceeded 300°C in the Aston Massif but were less than 300°C in the Hospitalet Massif; these data are consistent with the temperatures of 400”-450°C and 250”-400°C respectively estimated by McCaig (1984) on the basis of mylonitic assemblages. However \ the question of the precise age of the thermal event and whether my~onitisation occurred

at this time require

further

discussion.

Three possible scenarios can be envisaged which are consistent with known erogenic episodes in the Pyrenees: (1) Late Hercynian mylonitisation with Alpine thermal event(s) as 90-100 Ma, 50-70 Ma or both. (2) My~onitisation

at 90-100

50-70 Ma. (3) Mylonitisation

at 50-70

Several lines of evidence

Ma, with or without Ma with or without

argue against

a separate

a thermal

the first scenario.

thermal

event

event at 90-100 The rather

at Ma.

sparse data

on coarse micas from the Aston and Hospitalet massifs suggest significantly Alpine temperatures in the former. This is consistent with assemblages

higher in the

mylonites and suggests uplift of early-formed mylonites during movement 40Ar39Ar ages where recrystallised M&ens Fault. Muscovite shows Alpine

on the within

the M&ens mylonites but not outside them. Although this could represent preferential resetting of fine micas in a purely thermal event (Hunziker. 1979) it is far more likely to be the result of ~e~~stu~li~ut~o~of muscovite temperatures spectra

insufficient

to promote

general

argon

within

loss. Close examination

for spec. 142033 shows that the older ages in intermediate

are seen in the more mica-rich

sample;

the mylonites

this is surprising

at

of the

temperature

steps

in view of the mixing

model

described above until it is realised that spec. 142033-l contains mainly coarse porphyroclastic plagioclase while spec. 142033-2 contains a higher proportion of new, less calcic plagioclase grown during mylonitisation. There is therefore a correlation between incorporation of excess argon and recrystallisation of plagioclase in the mylonite. It is difficult to imagine this type of systematic redistribution occurring during a purely thermal event. The second scenario is suggested by the plateau age of 93 Ma shown by biotite in spec. 142023. However, this shows no sign of resetting towards younger ages despite the distinctly younger ages inferred for biotite in spec. 142033 and muscovite in spec. 142009 and the fact that cooling through the closure temperature for fission

169

tracks plateau

in zircon

occurred

ages for biotite

parts of the Eastern

more

detected

40 Ma later. K-Ar

High

similar

results

by incremental

(Brewer,

variable

40Ar-39Ar

are characteristic

to be present

1969; Roddick

and it appears

heating.

but

ages for muscovite

Alps where excess argon is known

greater extent than in muscovite has described

than

and younger

in biotite

et al., 1980). Foland

that excess argon in biotite

If the mylonites

formed

of to a

(1983)

cannot

be

at 90 Ma with subsequent

variable cooling histories, muscovite should give the older ages. It is therefore possible that the concordance of the 93 Ma age with the known age of metamorphism along the North Pyrenean Fault is a coincidence and that the younger ages in the 50-70 Ma range are the true age of mylonitisation. The third scenario agrees best with the fission-track

ages; movement

on the shear

zones would have initiated the uplift process although reverse movement on the M&ens Fault appears to have ceased before cooling through the zircon fission-track closure temperatures is in good agreement

(Garwin, 1985). The kinematic development with NW-SE convergence in the Pyrenees

of the shear zones inferred from plate

reconstructions for the Late Cretaceous-Palaeogene time interval (Deregnaucourt and Boillot, 1982; Olivet et al., 1983) but not with sinistral strike-slip movement inferred for mid-Cretaceous times. More data are clearly needed to establish the precise age of mylonitisation and its relationship to thermal events in the Pyrenees. However, we consider existing data are most consistent with the following sequence of events: (1) Establishment of a high-thermal gradient in the mid-Cretaceous during sinistral movement on the North Pyrenean Fault. Uplift

that

the

(90-100 Ma) of the North

Pyrenean massifs and parts of the Axial Zone (Garwin, 1985). (2) Persistence of high heat flow into the early Tertiary when reverse and dextral movement on the shear zones was induced by NW-SE directed convergence between Iberia and France. (3) Continued uplift and cooling south Pyrenean thrust belt.

during

Eocene-Oligocene

movement

on the

The tectonic implications of this sequence of events are discussed in more detail by McCaig (1986). Note that earlier (e.g., Hercynian) movements on the M&ens Fault are not ruled out by the age data.

CONCLUSION

Parts of the northern Axial Zone of the Pyrenees were affected by an important thermal event < 100 Ma ago which appears to have persisted until about 50 Ma ago. The thermal event was sufficient in the Aston Massif to completely reset biotite K-Ar and partially reset muscovite K-Ar with more complete resetting in shear zones, but the presence of excess argon makes the precise age of the event difficult to assess.

170

The

thermal

event

recrystallisation part

Alpine

is believed

to have

been

accompanied

on shear zones such as the M&ens structures.

Fault,

Ages are more completely

reset

by movement

dating

and

these as at least in

in mylonites

and excess

argon is trapped in plagioclase and probably biotite. Biotite Rb-Sr and muscovite “OAr-‘yAr ages are different across the M&ens Fault. All these features are most easily explained

if resetting

during

thermal

a purely

occurred

during

mylonitic

recrystallisation

rather

than

event.

The shear zones probably formed during the early stages of Paleogene convergence (with a dextral component) between Iberia and France. The thermal event may have begun at the time of mid-Cretaceous Pyrenean Fault, persisting into the Tertiary.

metamorphism

along

the North

ACKNOWLEDGEMENTS

Thanks are due to Marcia Amanda McCaig for typing. discussions

with

E.R.

Miller for help with the sample preparation. and to The ideas presented in this paper benefitted from

Oxburgh,

L.J. Garwin,

and

participants

Workshop held in Cambridge in May 1983. R.A. Cliff reviewers commented on various versions of this paper.

at the

and

two

Pyrenees

anonymous

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