Gondwana: UPb isotopic evidence from detrital zircons

37

Tectonoph_~sicx 185 (1990) 37-50 Elsevier Science Publishers

B.V.. Amsterdam

The Brabant Massif as part of Armorica/Gondwana: isotopic evidence from detrital zircons J. Von

Hoegen

‘, U. Kramm

a and R. Walter

U-Pb

’

” Zentrallahnrutorium fiir Geochronologie, institut fir Mineralogie der Universitiit, Corrensstr. 24, D-4400 Miinster, F.R.G h Geologisches institut der R WTH Aachen, Wiillnerstr. 2, D-5100 Aarhen, F. R.G. (Received

November

9. 1989: revised version accepted

April 26, 1990).

ABSTRACT Von Hoegen, J., Kramm, U. and Walter, R., 1990. The Brabant evidence from detrital zircons. Tectonophysics. 185: 37-50.

Massif

as part

of Armorica/Gondwana:

li-Pb

isotopic

U-Pb isotopic ratios of zircons are used as time-dependent geochemical tracers for the formation of the zircons and for the sum of processes and their intensities effective for the ~sturbance of the U-Pb systems during the history of the zircons. By comparison detrital zircons can thus serve as indicators for the provenance of elastic sediments. The U-Pb systematics of 57 carefully selected fractions of detrital zircons from four Cambrian sandstones of the Ardennes and the Brabant Massif have been investigated. According to the U--Pb data features of color and shape of these zircons are suited for distinguishing to major groups: (1) colourless. mainly rounded, subordinately subhedral or euhedral zircons which are characterized by U contents below 300 ppm and by apparent 207Pb/235U ages between 0.53 Ga and 1.65 Ga; (2) reddish. generally subrounded to very well rounded zircons with U contents from 240 ppm to more than 800 ppm and apparent 207Pb/2’5U ages between 1.75 Ga and 2.4 Ga. From the group of colourless zircons only one fraction yields concordant U-Pb data and documents a Cadomian/PanAfrican crystal growth at 545 Ma. U-Pb data of the other colourless zircons arrange in a fan-shaped area in the Concordia diagram below Concordia and from 530 Ma to 600 Ma. Zircons with such U-Pb characteristics are well-known from Central and Southern Europe. They suggest a strong Cadomian/Pan-African influence. The reddish zircons are dominantly derived from Archean to Early Proterozoic crystalline rocks. Based on these results and on comparison with U-Pb data of other detrital zircons the Ardennes and the Brabant Massif must be regarded to be coherent to the Gondwana continental plate before the Caledonian orogeny.

microplate ment Palaeomagnetic dence

that

Palaeozoic

and palaeofaunal

data give evi-

the palaeogeography of Middle and Western

of the Early Europe is con-

the Late

to Perroud

0040-l 951/90./$03.50

region.

and Cambrian

During

it is to be

and even during

Early

Ordovician times, when high magnetic inclinations and southern declinations indicate a global position close to the South Pole, it was located near Gondwana. At this time Armorica was separated from Laurentia by Iapetus and from Baltica by the Medio-European ocean, the “Tornquist’s Sea” after Cocks and Fortey (1982). The northward drift of Armorica which began during the Late Ordovician led to a collision with Baltica and Laurentia during Silurian and Early-Middle Devonian times. The Caledonian Mountains. includ-

et al. (1984) the Armorican 8 1990 - Elsevier Science Publishers

Precambrian

role in the develop-

European

seen as part of Gondwana

trolled by plate movements of Gondwana-ineluding Armorica--, Laurentia and Baltica (Scotese et al., 1979; Cocks and Fortey, 1982; Bond et al., 1984; Perroud et al., 1984; Ziegler, 1986, and references therein). The relative position of these plates to one another, the distances separating them during their pr~ollision history and the relationship of Middle and Western Europe with these plates are controversial matters. According

plays an important

of the Northwest

B.V.

J VON HOEGFN

3X

ing both

North-German-Polish The chronologic

after zircon

as the

episodic

secondary

process

have been discussed

by

1985, 1986).

Wetherill

(1963)

evidence

for a continuous

Pb

of datable

loss/U gain as proposed by Wasserburg (1963) has not been presented so far. In a Concordia

present-day

(Ziegler,

succession

events is different

these

tematics

in the collided

Central

and

isotope systematics plates,

which

intensities

of

therefore

should

provenance.

limb

data,

Europe.

detrital

zircons

reflect

the sum of ages and disturbance

reveal differences study

found

up The

of zircons from rocks of

effective

the

geologic

plates making

Western

processes,

related

Next to palaeomagnetic

graphic

and

limb, are interpreted

result of this collision

U-Pb

the

the Scottish-Norwegian

and zoogeo-

of isotopic in

to their

Palaeozoic

systems

of

sediments

diagram

a single

crystallization.

episodic

overprinting

sented by a linear arrangement data.

Two

turbances

or

more

Models

ET AL

is repre-

of cogenetic

geologically

of the primary

of an

U-Pb

U-Pb

induced

dis-

systematics

may

produce an uncorrelated spread of the U-Pb isotope ratios in the Concordia diagram even for cogenetic

U-Pb

systems

felder, 1979: Faure,

(Gebauer

and

Griinen-

1986).

offers thus an independant method yielding arguments for the reconstruction of plate boundaries

U-Pb systematics of detrital zircons as tracers for the provenance of elastic sediments have been

and, finally, for the macro-tectonic Central Europe.

repeatedly proposed Bowes et al., 1976;

The region

of the present-day

and the Ardennes

is located

evolution Brabant

of

Massif

close to the probable

1977; Lancelot Scharer Loske

Gondwana/Armorica.

methodical

a borderland

et al., 1985;

investigation

1982; Gariepy Drewery

difficulties,

ing of the susceptibility

The results of U-Pb dating of detrital zircons from four Cambrian sandstones of the Brabant Massif and the Ardennes are presented in this

with repeatedly

tensities

of isotopic homogenization

processes,

that

is, the age structure of the surroundings of this northwest Middle European sedimentary basin before the Caledonian

and Hercynian

orogenies

in

et al., 1981; et al., 1984;

et al., 1987).

is faced, however.

whose relationship to one of the three macroplates, however, is up to now unclear (i.e. Scotese et al., 1979; Perroud et al., 1984).

paper. The U-Pb systems of the zircons describe in a cumulative manner the various ages and in-

et al., 1974; Grtinenfelder,

et al., 1977; Gaudette

and Allegre,

position of the triple junction between the large structural provinces Laurentia, Baltica and It represents

(e.g. Grauert Gebauer and

e.g. the poor of U-Pb

Such

with a number

of

understand-

system

to post-

crystallization modifications as well as the problematic interpretation of apparent ages of zircons modified

U-Pb

systematics.

The greatest difficulty concerning age determinations on detrital zircons by the conventional method is the selection of genetically uniform zircon

fractions

(discussion

of these uncer-

tainties, e.g., by Grauert et al. (1973) and Gaudette et al. (1981). Single-grain analysis promises a way out of this difficulty

(e.g., Gaudette

Scharer

1982)

and Allegre,

et al., 1981;

but subgrain

volume

terms of the U-Pb systematics of zircons. The association of Cambrian source areas to Laurentia,

investigations of the U-Pb systematics demonstrate for numerous cases U-Pb isotopic hetero-

Baltica, and Gondwana-Armorica

geneity Kroner

can be inferred

even within single zircon crystals (e.g., and Compston, 1988; Kroner et al.. 1988a;

using these and comparable U-Pb isotopic data, which allows a more clear delineation of the plate reconstruction in modern Middle Europe.

Aleinikoff et al.. 1989). Differing U-Pb isotopic ratios between single detrital zircon grains can

Methodology

thus mean different intensity of isotopic resetting of a genetically homogeneous zircon population

Undisturbed U-Pb systems of zircons, characterized by concordant isotope ratios geologically reflect significant events. Discordant U-Pb data are only suitable for geologic dating, if just one single process modifies the primary U-Pb sys-

but does not exclude genetical heterogeneity. Age determinations of detrital zircons stay thus problematic, if multiple processes affected the U-Pb systems. In the following the U-Pb ratios of fractions of detrital zircons, which have been carefully selected

BRABANT

by

MASSIF

AS PART

independant

mineralogical

shape and inclusions, geochemical disturbing certain

way

They

summarize

systematics.

characterizes

particular

areas.

Because

zircons

(these have “seen”

the

particularity processes

which in a provenance of

or diagenesis,

is not known),

the data must be interpreted

from

zircons.

those

the influence

of magmatic

In comparisons

detrital

such as ero-

sion, transport ently

and

By this condi-

data show a pattern of

as color,

which have been effective for

the U-Pb

tion the isotopic

criteria

are used as time-dependent

parameters.

weight the processes,

39

OF ARMORICA/GONDWANA

of which differ-

or metamorphic

with corresponding

data

of other detrital zircons the results prove this method to be expressive for source area studies.

Dongelberg, Lower

Sample

No. DV-15), as well as of two

Cambrian

Cambrian

quartzites

quartzite

sif, Ardennes

Upper Mas-

(DV-1,

gated. The sampling the sampled

one

from the Stavelot-Venn DV-26,

deposits bipolar ding,

sections

according

in

various

the

turbjditic

are interpreted shallow

clay-

Cambrian

quartzite

deposits

shelf

structures:

lense and flaser

intercalated

and

siltstone

(RV-4) rep-

in the lower slope or the

upper fan region of a submarine (Von Hoegen

of

kinds of ripple bed-

in the quartzites.

cycles. The Upper resents

quartzites to intertidal

to their sedimentary

crossbedding,

bedding

and the lithology

are shown in Fig. 1.

near-shore

channels

RV-4) were investi-

locations

The Lower Cambrian as relatively

and

fan-valley

et al., 1985). All samples

system are com-

Sample description

positionally supermature, texturally submature arenites. Detrital heavy minerals do not exceed 0.2

Zircon quartzite

vol.%. Zircon and garnet

populations of one Lower Cambrian from the Brabant Massif (Assise de

dominates

over tourmaline,

.Is.2

Fig. 1. Lithology

of the sampled

units in the Ardennes

and the Brabant

massif:

sampling

locations

shown by arrows.

rutile,

40

Zircon typology

(4) completely grey-brown,

In generai

mo~hologic

tics of the zircons samples, tribution

those

The maximum While

of the Upper

According

the four

species vary

of the grain-size populations

the Lower

dis-

in the fine

Cambrian

samples

proximately size

equal

fractions.

proportion

Colourless,

zircon

types

amounts

Towards

found

within

smaller

of coloured

reddish

are

zircons

coarse

grain declines

sizes

Cambrian

China-like

crystals

hardly

occur

do not

smaller

into four major groups

(Fig. 2):

pink, red, red-brown

and red-black

crystals;

reddlrh

majority

yellowish

zircons

of the

colourless,

is transparent

the transparent netic.

types,

Pink and red coloured

which

__

reddirhde~ yellowish milky

rldalrh

RY-4

pink

__ dwk red

y.llowlsh milky

l

) 50%

Fig. 2. Combination

l

50

- 20%

in sizes

reddish

or only

0

of colour and shape features

20

- 5%

of the investigated

0

<5x

detrital

always

zircons

phlt

DY -26

types.

and

slightly

cloudy. Concerning the magnetic behav~our, the bulk of the milky zircons differs significantly from

in parts amber or orange zircons;

DY-I

the

than 80 pm.

The

the zircons of each sample

milky

grain

in favour

coloured

quartzite

and in ap-

faintty

(1) colourless to very faintly coloured crystals; (2) reddish zircons the colour spectrum of

(3) yellowish,

scarce.

milky-china-like

to

of yellowish

or very

to colour,

which includes

is very

the occurrence

of the colourless

160 pm.

can be assigned

zircons

white-grey

crystals.

up to 225 pm in diameter,

show zircons

reach beyond

within

china-like

hydrated

In all populations

characteris-

of specific

is for all zircon

range.

always

are very similar

yet the amounts

considerably. sand

and coiour

milky,

probably

zircons

are diamag-

often show inho-

BKABANT

MASSIF

mogeneous

AS PART

colour

distributions.

fuse areas

of changing

delineated.

In some reddish

is concentrated

a faintly

Leaching

experiments

colour

zircons

grain

that

to

surfaces

well

are often

core.

in 2.5 N

the majority

of the

by a secondary

rounded

Some

crystals

Sub-rounded

zircons

pre-

groups (Fig. 2). The

highly

are colour, phology,

fractions

colour

as cores or inclusions of Pupin, tion

pitted.

of each

The chemical separation ric isotope

of the well-rounded,

slightly

at

cracks.

(ZLG).

show fractures zircons

or healed

with relics of prismatic

faces represent

a less reworked

or

fraction

the

mor-

of the crystals

such

(see classification zircon

1. China-like, yellow

the investigation

zircons

elongated

transparency,

of the analyzed

in Table

recent disturbance

Globular.

features

of individual the four samples

proposals

1980, and Loske, 1985). A short descrip-

included from

within

intensity,

and internal

ratio of

markedly

can be distinguished

pyramidal

effect

sur-

for the characterization zircon

by the length/with

ovoid and occasionally

coloured

volume

uniform

well as intensely

in all the mentioned

the crystals.

can be

or colourless

ultrasonic

Criteria

dif-

the colouring

outer

are just coloured

staining. Subrounded dominate

intensities

zircons

coloured

HCl for 30 s) revealed yellowish

Frequently

in a shell-like

rounding

41

OF ARMORICA/GONDWANA

fractions

is

milky zircons

as

varieties

were excluded

due to their sensitivity

of their U-Pb decomposition

to

systems. of the zircons.

the

of U and Pb, and the mass spectrometanalysis

Central

of U and Pb were performed

Laboratory

University

for

of Miinster.

procedures (Krogh. Before decomposition

Geochronology using

standard

1973; Persson et al., 1983). with HF the surfaces of the

in the populations. Together with subhedral to euhedral crystals their part increases with decreas-

zircon crystals were etched with 2 N HCl for 20 s. Spiking for the isotope dilution analysis was per-

ing grain

size. Besides,

formed

quartzites

the amount

zircons is higher population.

in the Lower of subhedral

than

in the

Cambrian to euhedral

Upper

Cambrian

The majority of the subhedral or euhedral zircons is colourless or faintly coloured. According to their morphologic

habits,

conspicuous

longpris-

matic crystals with strongly reduced pyramidal faces, long-prismatic crystals with distinctly developed prismatic crystals with strongly

reduced

and pyramidal faces, and isometric numerous pyramidal faces and prisms can be distinguished.

The

with an enriched

initial

lead and analytical

and

*“‘Pb/ “I4 Pb

blank dicated

that the total Pb blank

pg U. For coloured ent initial

for correction, = 14.690, varieties,

procedure

An amount of 100-200 kg of each of the four quartzite samples was cleaned from weathering staining and ground to < 0.5 mm by a jaw crusher and a roll mill. The heavy minerals were enriched on a Wilfley table. From this concentrate zircons were separated using heavy liquid and magnetic separation techniques. Finally, pure zircon fractions separated into grain size classes were handpicked under a binokular microscope according to individual crystal characteristics.

The blank

lead

respectively. Repeated during the investigation in-

pg Pb while the total U blank

sizes tabular

Analytical

blank.

ratios,

determinations

for their high diamond-like may

spike.

were made on

isotopic composition was determined to be 37.5. 15.52, and 17.72, for the 208Pb/204Pb. “‘7Pb/204Pb,

latter are conspicuous

crystals

analysis

a Teledyne NBS-type 12” 90” solid source mass spectrometer. The lead data were corrected for

luster. In small rarely be found.

grain

‘08Pbp ‘35U-mixed

The mass spectrometric

did not exceed 500 was lower than 100

and colourless

lead isotopic

zircons

compositions

differ-

were used

20XPb,‘204Pb = 33.366, 2”7Pb/204Pb

and

20hPb/204Pb = 13.637 for the red which according to Stacey and Kramers

(1975) correspond to a model lead composition at 2700 Ma, and 2oXPb,‘Z”4Pb = 37.578, “‘7Pb/‘“4Pb = 15.576,

and

206Pb/204Pb ourless grains corresponding

= 17.803 for the colto a model lead com-

position

corrections

bear

at 570 Ma.

some

arbitrary

Such

component.

It

of course should

be

pointed out. however, that the measured 206Pb/2”4Pb ratio is very high in most cases. Ages were calculated with the U decay constants published by Jaffey et al. (1971). The calculation of errors and error correlations of the 206Pb/ 13’U and 207Pb/235U ratios was car-

2200

1665

> 160

63-80

1504

>I60

loo-

1463

1488

1491

90-100

1487

>I60

go-90

1475

80.-90

125-160

1455

1489

loo-125

1464

1414

>160

125

YO- 100

1476

1449

DV-15

2200

>200

1692

1660

>200

>200

1667

1675

1672

160-200

160~200

1673

>200

>200

> 200

1658

1664

:, 200

1654

1671

>2OO

> 200

2200

1655

1653

r200

1652

>200

WR < 2

WR

R-BR,

p,WR(2

R-BR,

Weight

0.90

0.55

1.25

3.00

1.30

2.50

2.80

2.70

1.75

0.90

0.80

0.55

0.90

0.65

0.15

0.50

1 .oo

0.95

1.35

0.60

1.60

0.45

0.30

0.60

0.80

(mg)

WR i 2

0.80

2.05

WR < 1.5 0.45

P, SR > 2.5

DP,WR
DP,WR
P. SR > 2.5

P, SR > 2.5

R, WR

DP,WR<2

R.WR<:2

P, SR > 2.5

C, WR

C-y, SH

C-y, WR < 2

C-p, WR < 2.5

C-y, SR < 2

C-p, WR < 2

C-y, SR i 2

C-y. SR

C, SH

C-p, SR

LLP, WR < 2

DP, WR < 1.5

P, WR <: 1.2

DR,WR<2

R-BR,

R-BL,

fraction ’

0.W

1659

Features of

LifCOll

fraction

0.118152

0.102104

0.091192

0.108448

0.138863

0.133268

0.113073

0.109461

0.098626

0.094681

0.093353

0.118390

0.544079

0.242628

0.215131

0.218991

0.269410

0.268228

0.233039

0.240285

0.175888

0.179743

0.190281

0.127426

0.219130

0.136363

0.082911

0.061627

~Ox~b,

ratios

0.164770

0.157329

0.161468

0.161906

0.157243

0.159975

0.159263

0.160428

0.164316

0.166490

0.164902

0.165148

0.077916

0.104479

0.090921

0.099018

0.110552

0.098275

0.118314

0.134587

0.146292

0.141621

0.141020

0.145940

0.158781

0.149823

0.155791

0.161549

207ph/Z%Pb

isotopic

1089.6

5111.1

3226.4

3545.7

6149.7

4413.5

7725.1

6472.4

2933.2

4206.7

8127.6

X879.6

1222.6

2042.3

2250.7

1903.5

2100.2

5041.4

3605.8

3642.1

3832.1

12361.5

4916.1

15159.3

5303.7

12796.7

16403.0

18666.8

.~ 2oh,,b,%>‘tPb

and the Brahant massif

206 P,,

Measured

data for detrital zircons of the Ardennes

Sieve

analytical

1

1669

DV-I

Sample

U-Pb

TABLE

782

406

724

341

321

242

364

340

535

563

567

300

96

89

172

158

98

174

116

147

94

179

101

361

143

694

458

861

(wm)

[,

525.6

169.1

302.6

144.7

140.9

109.4

159.5

148.7

235.9

250.5

260.6

140.6

12.6

11.4

23.9

22.3

15.6

31.8

26.7

31.6

21.9

47.1

38.8

148.8

63.2

301.4

196.3

321.3

167.6

298.5

143.0

140.4

106.4

158.5

147.9

232.4

247.6

259.1

140.0

12.6

11.4

23.8

33.0

15.4

31.6

26.7

31.5

21.7

42.1

38.8

148.3

63.0

300.7

196.0

372.0

(wm)

@pm)

372.6

Pb,,,

-.

Pb,,,

Concentrations

1257.1

649.7

1168.6

551.6

528.0

402.1

606.9

567.7

903.1

960.0

1002.8

531 .o

38.3

41.6

89.6

82.6

55.1

112.9

97.3

112.0

80.4

154.9

142.2

566.0

223.1

1136.6

768.9

1410.6

WWg)

*“‘Pb

Calculated

0.40944

0.38559

0.38397

0.38696

0.38730

0.39432

0.39809

0.39939

0.40043

0.40485

0.40076

0.42371

0.42503

0.09546

0.11176

0.12515

0.12500

0.13549

0.15541

0.20120

0.18248

0.20450

0.20770

0.33883

0.37643

0.38521

0.39284

0.40266

6.4998

7.5309

8.0908

8.0612

8.6063

9.4210

2"7pb,235"

8.1379

8.1988

8.4072

8.4569

x.4371

8.6243

8.6801

8.7502

8.9339

9.2163

9.5440

9.5958

0.8711

1.5059

1.4614

1.5805

1.9434

2.0060

3.1784

3.2950

4.0292

4.0255

ratios h

20hpb,,23XU

isotopic

0.15307

0.15486

0.15157

0.15837

0.15518

0.15712

0.15764

0.15849

0.16005

0.16353

0.16337

0.16374

0.06618

0.09772

0.08469

0.09171

0.10103

0.09347

0.11457

0.13076

0.14290

0.14057

0.13839

0.14510

0.15639

0.14883

0.15502

0.16688

207p,,,20hpb

i

z 160

> 160

>200

,200

> 160

1523

1521

1639

1648

1522

100~125

80-100

1425

1422

P = pink,

50-63

63-80

1363

1362

a Abbreviations:

ratios.

Ph

grey.

DP = dark

gr = faintley

C, SH

C, SR

C-p, SR

for blanc and common

length/width

h C‘orrected

milky,

31-50

1364

m = faintly

C. SH

80-100

1350

C, SR

C-gr, SK

loo-125

80&100

LLP. WR i 2

P, SR

DP, WR

LLP, SR

P, WR

P. SR

C, WR < 1.5

C-y, WR > 2

C, SH

C-y, WR < 2

C-m, WR c 1.5

C-m. WR > 2

C.WR
C. EN (SP)

C-y, WR i 2

C-y, SH

LLP. WR < 2

LLP, WR < 2

LLP, WR < 2

P. SR i 2.5

P. SR < 2

1349

80-100

1351

P, WR

I’, WR

1352

X0-100

63-80

1348

1421

R V-4

80~100

1424

D V-26

>200

>200

1649

>160

1502

1645

>160

>160

1490

1525

>I60

1500

>I60

100.- 125

1462

1200

63-80

1503

1640

>160

1450

1524

125-160

1454

LLP = very

0.156113

0.161589

0.178830

0.187609

0.174274

0.156617

0.160082

0.101261

0.109023

0.144395

0.136967

0.112247

0.418737

0.274161

0.371011

0.349752

0.324964

0.341786

0.704687

0.279591

0.264952

0.183284

0.229066

0.193167

0.187746

0.132892

0.106549

0.150706

0.143453

pink,

R = red.

0.102322

0.103439

0.108369

0.106391

0.121181

0.137186

0.131744

0.148124

0.152759

0.136658

0.149666

0.152413

0.074210

0.065572

0.118575

0.087326

0.094751

0.109788

0.098413

0.094631

0.114534

0.147432

0.134227

0.135665

0.138523

0.154487

0.159088

0.150153

0.155035

SK = subrounded.

light

WR = well rounded,

pink,

4.00

1.80

1.60

2.80

2.00

0.80

0.60

3.10

0.80

0.30

7.80

4.80

0.25

0.90

0.70

0.15

0.45

0.90

0.90

0.60

0.35

0.95

0.60

0.65

0.45

4.70

1.20

2.30

4.10 386X.4

red,

234

285

494

112.2

33.8

41.2

77.6

33.5

45.2

44.7

69.4

151.6

167.5

59.6

130.0

165.4

20.5

13.4

2.9

21.3

25.6

28.4

9.5

9.7

21.8

29.8

22.X

47.8

27.0

121.1

152.8

166.2

I1 1.1

53.0

40.4

76.1

32.X

44.5

44.0

6X.4

15.3

166.0

59.0

127.0

163.2

20.4

13.3

2.8

20.8

24.4

25.1

9.1

9.6

21.5

38.9

22.5

47.7

27.0

119.9

150.9

165.4

particular

refer

yellowish,

1.7602

1.7802

1.972X

2.1152

2.5711

4.0281

4.6750

6.4981

6.8983

5.4609

6.8809

7.2630

0.7159

0.7460

0.7910

0.8763

1.0149

1.0525

1.0649

1.2657

1.4947

3.8621

4.6280

5.8813

6.0979

7.9625

8.0772

8.0682

8.1041

numbers

y = faintly

0.13256

0.01325

0.14198

0.15367

0.16038

0.22149

0.26944

0.32309

0.33460

0.30354

0.34725

0.35338

0.08802

0.09058

0.08883

0.09742

0.09898

0.10425

0.10918

0.11207

0.11556

0.20064

0.26646

0.32285

0.33273

0.38010

0.37651

0.39361

0.38730

C = colorless.

129.3

157.7

292.2

124.9

168.7

161.3

260.8

587.4

643.6

227.2

486.4

632.0

68.6

48.9

9.1

73.4

87.5

91.3

25.4

34.9

79.8

144.7

Xl.8

175.6

100.2

454.7

584.0

618.7

418.4

SP = short-prismatic,

BR = brown. EH = euhedral.

BL = black,

195

252

181

232

436

461

179

336

429

186

130

27

181

212

210

56

15

166

173

74

130

72

2X7

372

377

259

SH = subhedral.

DR = dark

2360.7

2315.0

1812.7

2101.5

2749.6

2500.4

2216.1

5631.5

3934.4

2118.4

2124.9

37x1.1

948.9

2476.6

266.5

651 .O

698.3

390.2

516.6

1106.7

668.9

1670.7

1593.7

3706.7

2345.4

4942.0

3600.2

8540.2

pink. to corresponding

p = faintly

0.09645

0.09747

0.10078

0.09983

0.11627

0.13190

0.12584

0.14587

0.14953

0.13048

0.14367

0.14906

0.05895

0.05973

0.06459

0.06524

0.07437

0.07323

0.07074

0.08191

0.09381

0.13961

0.12597

0.13212

0.13292

0.15193

0.15559

0.14867

0.15176

44

I VON

ried out after Ludwig (1980) for a 95% confidence

exception

level. The errors

glossy, well-rounded

uncertainties spike,

of measured

of the

the error

isotope

U-Pb

ratio

magnification

ratios,

of the

the

of

one

error this fraction

mixed

15, Table

due to the spike/

fraction

of clear,

zircons. (analysis

F1‘ AL

HOFGFN

Within

colourless, its limits

no. 1522, sample

1) shows a concordant

U-Pb

Ma (Fig. 3). The zircons

of this fraction

ties of blank

lized during

Pan-African

event.

for detrital

zircons

and initial

and ‘06Pb/204Pb

blank

and blank

Pb are

ratio in the spike and the con-

of blank

tively. Estimated

have been

to the 207Pb/ 204Pb

ratios of initial

1%. to the U-Pb centration

lead correction

The errors assigned

correlation

factors

for initial

discordant form

Pb are 0.15% and 50%, respec-

a Cadomian/

As was to be expected U-Pb

a linear

crystal-

data of all other fractions array

in the Concordia

logic and colour characteristics.

lead are 0.7.

fan-shaped

field bounded

diagram.

indications

Fig. 3.

recent event are not observed.

600 Ma, and

Results

2100-2750

Ma. respectively. Hercynian.

data of all analyzed

in Fig.

difference

at 500-

for a Caledonian,

The U-Pb outlined

morpho-

The data plot in a

by Concordia

The resulting error ellipses are too small to be redrawn in the scale of the Concordia diagram of

3 do not

between

Clear

or a more

zircon fractions

yield

any

the age spectra

of the three Lower Cambrian

the

do not

This applies even to those of very similar

and

DV-

age of 545

sample ratio and the errors arising from uncertainconsidered.

of

significant

of the zircons

quartzites

and that

The results of the U-Pb isotope analysis on 57 fractions of detrital zircons from the studied

of the Late each sample

quartzites

tween the U-Pb ratios of all fractions of colourless zircons on the one hand (inset A in Fig. 3)

are listed in Table

1 and plotted

in the

Concordia diagram of Fig. 3. The UPb isotope ratios of both the colourless and the coloured zircons are discordant with the

Cambrian. However. there is a significant

and all fractions

of reddish

applicable separation

zircons

(inset B in Fig. 3). This and a further

to be-

on the other differentia-

r

Fig. 3. Concordia

diagram

for detrital

tion of the UpPb data according

zircons

of the Brabant

to the four samples

Massif (DV-15)

and according

A and B. For analytical

to features

and the Ardennes of colouring

data see Table 1.

(DV-1,

DV-26. RV-4). Differentia-

of the zircon fractions

arc shown in insets

BRABANT

MASSIF

AS PART

Fig. 4. Concordia coloured

zircons.

metasedimentary South

Sweden

c/h = clear/ Gebauer (Kriiner

diagram

for detrital

field B defined zircons

zircons;

and Gtinenfelder, et al.. 1988b);

zircons

by analytical

from various

(Von Hoegen

brown

45

OF ARMORlCA/GONDWANA

pre-Permian,

and Kramm,

4 = pre-Permian sandstones.

reddish

zircons.

Lower Palaeozoic

2 = Potsdam

basement,

6 = Cambro-Ordovician

felder, 1976); 7 = Ordovician

mostly

in prep.);

3 = pre-Permian 1977);

as in Fig. 3: field A (top ieft) defined data of various

basement.

sandstones Armorican

data

Cambrian,

or faintly

I = Hardeberga

detrital

or

sandstone.

New York

State (Gaudette

et al.. 1981),

Bohemian

Massif

et al., 1973:

et al., 1973);

metasediments.

of colourless

data fields of non-magnetic

for comparison,

Moldanubian.

Alps (Grauert

and low-grade

by analytical U-Pb

sediments

sandstone,

Saxo-Thuringian,

Further

5 = varied

Montagne

Massif (Vidal et al.. 1980). Additional

(Grauert

group,

Bohemian

Noir (Gebauer

explanations

Massif

and Griinen-

in the text.

and 2400 Ma, respectively). The ‘“7Pb/23’U ratios of six fractions of very light pink zircons vary

tion (pink to red and very light pink coloured zircons, respectively) leads to the definition of discrete fields in the Concordia diagram, shown in

between

the upper left part of Fig. 4.

respectively)

Field A is defined by fractions of colourless zircons with 207Pb/235U ratios between 0.7 and 4.0 corresponding to apparent 207Pb/235U ages of 530 Ma and 1650 Ma. Fractions of pink to dark red zircons define the upper part of field B with 207Pb/235U ratios between 6.5 and 9.6 (2050 Ma

The colour all but pinkish

4.6 and

6.5 (1750

Ma

and

2050

Ma,

(lower part of field B).

U content of the zircons correlates with the property. Among the transparent zircons, one of the colourless and the very light coloured varieties show U contents below

300 ppm. By this they stand out clear from the different reddish coloured zircons with distinctly

J VON HOEGEN

46

higher and

U contents

861 ppm

coloured

(all but one between

U (Table

zircons

acterized

the

by U contents

of red zircons contain red-brown contents

zircons

have the highest U

from 458 ppm to more than 860

the chemical

rivation

and

the

Early

latter

case,

granite-type

of

or

times.

For

the

but arrangement

fractions

euhedral

of the U-Pb

close to Cadomian/Pan-

ages in the Concordia

for an undisturbed

character,

to Archean

preservation

habit

data for many African

to be of inherited Proterozoic

Pb isotopic

zircon therefore.

zircons.

aging

diagram

up to Early

tions

the well-rounded

showing

type can be excluded.

We con-

although

that the consequent

For this reason,

grouping

for their genetic

in the following, colour

hand,

will

the

groups

of

they

impact

casts

requires Cambrian

growth

metamorphic grains.

severely

have

abraded.

mostly

suggests

the of

represents,

therefore.

a metamorphic

in the source area of the zircons

545 Ma or slightly

earlier.

metamorphic

affected the after deposition,

at about

of a

of

be discussed

with fraction 1522 being concordant at 545 Ma, suggests a strong Cadomian/ Pan-African in-

pro-

the rejuvenation

at 540-600

resetting

a healing

in the course

systematics

to Phanerozoic

or The

with a beginning

reorganization

the various

crystals

Bounce

disappeared.

of the crystals structural

which in part is

the U-Pb

The fan shaped discordant distribution of UPb data of the colourless zircons from the Cambrian quartzites in the Concordia diagram of Fig. 3,

host rocks

are

For this type of zircons

influences colourless

quartzitic

for the euhedral

we like to associate

Interpretation of the U-Pb systematics

Since

surface

of the surfaces

Concerning

process

a brilliant

smoothing second

there are frac-

than found

cess which

Proterozoic

fluence

zircons

and reddish

on the one hand

the other

of

to their mineralogic properties and their

of the two major

colourless

coloured on separately.

a de-

Among

more lustrous

ratios gives clear evidence data

aspects

of

from one

zircon

the detrital zircons according characteristics, their chemical difference.

point

varieties

of the different

common

U-Pb

char-

fractions

53.5 ppm to 694 ppm U; the

view. but also from morphological

U-Pb

are

age component during

times since the times of crystallization.

PPm. From

clude.

the reddish

varieties

below 461 ppm;

to red-black ranging

1)). Among pink

242 ppm

E’l AI..

Ma more likely overprinting.

reddish geologic

zircons

Late

events,

i.e.

implicated from the U-Pb ratios of the zircons, had at best verv little effect on the

U-Pb

systematics

of this

detrital

component, red zircons

if at all. The pink, reddish and dark can be derived from Archean to Early

Proterozoic

crystalline

A multiple Archean cannot be excluded.

rocks indirectly to Proterozoic

or directly. overprinting

no important Cambrian a younger

Provenance of the Cambrian detrital zircons

rejuvenation of the U-Pb systematics can be neglected. With the exception of the concordant age

from Cambrian

of fraction 1522 all fractions document then a pre-Cadomian component of Early Proterozoic to

the Ardennes describe a specific pattern of apparent ages, i.e. an individual distribution of UPb

Archean

age. This

age component

found

in all

morphologic types is not represented by either the rounded or the different euhedral crystals. The simple morphologies of the colourless. euhedral zircons suggests, according to Pupin (1980) a crystal growth in a granitic melt environment. Metamorphic overgrowth of zircons or complex zircon habits have not been detected. The crystal growth should have occurred therefore either during a Cadomian/Pan-African magmatic event suggesting the Early Proterozoic to Archean

The U-Pb

data of the analyzed detrital zircons rocks of the Brabant massif and

ratios in the Concordia clearly

defined

diagram.

expressed

by the

fields in Fig. 4. The data are the

result of the sum of all geologically effective events within the source rocks as far as can be recorded in the zircons so that in a sense this age pattern is typical of particular source area(s). By this the following conclusions must be made concerning the hinterland of the Cambrian sedimentary basin: (1) The U-Pb systematics of the colourless zircons determined by a strong magmatometamorphic event at the boundary of Late Precambrian

BRABANT

MASSIF

AS PART

to Early Cambrian must

have

times argue that the host rocks

been

Cadomian/

eroded

Pan-African

very

already

present

day northwestern

Laurentia

likely

area

Gondwana-Armorica

zircon

of

from

equivalent

sandstone

been carried

out.

orog-

1981). The most

istics, analysis

zircons

diverse

is

zircon

U-Pb ratios are known, e.g. Massif (Peucat, 1983), from

with

zircons

of southern

of these zircons

UpPb

de-

of the Cambrian Sweden

have

and the analytical

were done correspondingly on sample locality,

as described

zircon character-

data and on the interpretation

given in Von Hoegen derivation

other

of stratigraphically

For this purpose

Hardeberga

The selection

here

zircons

on detrital

to the Grenville these

presented

sediments.

above (details

for

data

data of detrital

procedure

from where

data with comparable from the Armorican

U-Pb

terminations

Europe.

neither

detrital

from where the youn-

to 700 Ma (Baer,

provenance

the from

basin

part of Middle

ages belonging

extend

Cambrian

are recorded

nor from Baltica

gest isotopic enies

events

after

are found on secondary

in the Lower

Corresponding

shortly

event. This follows

the fact that these zircons deposit

47

OF ARMORICA/C;ONDWANA

of

the

crystalline

rocks

generally

accepted

and Kramm.

Hardeberga

The

sandstone

of the Scandinavian statement

from

shield

(Ahlberg.

EUGENO-S UpPb data

1982). and from the Pan-African belt in Northwest Africa (Bertrand and Lasserre, 1976; Lance-

patterns

lot et al., 1983). (2) The U-Pb

Figure 4 demonstrates that the pattern of apparent ages of the Hardeberga zircons clearly dif-

systematics

of the various

of this detrital Proterozoic whether

source

from Archaen

rocks.

these zircons

the Early throughout whether

component

It cannot

were already

erosion

to Lower be decided

eroded

Proterozoic and repeatedly the following Precambrian the

re-

derivation

of the Archean

during

reworked times or crystalline

source rocks happened shortly before the final detritus deposition. Whereas up to now corresponding

data

with

“‘Pb/

235U

apparent

ages

reaching up to 2.4 Ga (upper part of field B in Fig. 4) are not known from rocks within the Rhenohercynian zone hints for the existence terozoic

zircons

proved by Moldanubian

in

or its northern forelands, of Archean to Early ProMiddle

Europe

several ion probe data Massif and from the

have

been

from the Montagne

Noire (Kriiner et al., 1988b; Gebauer et al., 1989). Besides source rocks with ages expressed by corresponding U-Pb ratios plates under discussion

are known from all three (for Laurentia see Frith et

Group, zircons

is a 1984:

the Moldanubian zone (Teufel. 1988) and from the Moravo-Silesian zone (Van Breemen et al.,

ddish zircons reflect a direct or indirect

Working of these

are

in prep).

representative

1988). so that the should reflect age

of (southern

parts

of) the

Baltic Shield.

fers from that yielded the detrital and

zircons

from the isotopic

analysis

from the Rhenohercynian

its northern

forelands.

tions of the Hardeberga

Nine

zircons

of

zone

different

frac-

plot in the Con-

cordia diagram very concentrated and close to the Concordia in a region which is left totally blanc by the UpPb ratios of the other Cambrian of the Brabant-Ardennes region. Because of the scarcity of publications dating of detrital son with further sediments sponding

is hardly comparison of the fact

fractions

for isotopic

according to quite grain size fractions because

on &Pb

zircons a representative comparizircon data of Lower Palaeozoic

because

finally,

zircons

possible.

Besides.

is rendered that

more

the selection

analysis

a corredifficult of zircon

has been carried

different criteria. partly have been considered.

of considerable

differences

out just and

in the

post-Cambrian history of the respective sediments. In spite of these reservations such a comparison (Fig. 4. only non-magnetic zircon fractions literature data have been taken) is indicative

of for

al., 1986; Corfu, 1988 and references therein; for Baltica see Kuovo and Tilton, 1966; Witschard 1984; and Martin, 1989; for the NW African regions of Gondwana see Bertrand and Lasserre. 1976; and Lancelot et al., 1983).

further arguments concerning the respective sedimentary provenance areas: (1) Detrital zircons of the Cambrian Potsdam sandstone derived from the Canadian Shield are

Further arguments for or against particular source areas can be obtained by comparison of the

characterized 1981). being

by U-Pb data (Gaudette et al.. similar to those of the Hardeberga

I VON HOEGEN

48

zircons

the

the one hand

on

non-magnetic

significantly

clear

from

and

of the

zircons

Cambrian

data of zircons

and

of the

et al., 1973)

et al., 1973; Gebauer

zircons

parison:

Haverkamp

et al., in prep.)

influx of detritus

Baltica into Central

Europe

of the Alps Massif

and Grtinenfelder, Massif (Vidal et al.,

the U-Pb

data

field of the

of the Brabant-Ardennes

detri-

(Gaudette

and Hurley,

age pattern. conclusion

U-Pb

1979) which fit in

too. follows

data of detrital

from

zircons

this comfrom Mid-

dle Europe plot in a certain way independently their post-Cambrian history in the Concordia agram in a typical way characterized markable lack of data points between

of di-

by a reConcordia

and a chord connecting 600 Ma and 2050 Ma (see line in Fig. 4). as already pointed to by Gebauer (1986). This even applies to the results of some conventional grain fraction analysis of zircons of

in Devonian

The U-Pb carried

isotope

analysis

out on carefully

are well suitable

for sedimentary

ies.

of detritus

This

kind

Cambrian the

sandstones

Ardennes

provenance

systems

on

of the Brabant

Massif

and

yields

important

diagram

is clearly of isotopic

tica and Laurentia, including magmatic and metamorphic and Kramm, in prep.).

confirmed

arguments

of the investigated

detrital

component

documented

by various

reddish,

source rocks, and another by

various

originating

colourless,

component partly

documented

euhedral

determinations on rocks (Haverkamp

printed by a Cadomian/Pan-African vant effect. The data

strongly

indicate

Europe

rele-

that the northwest-

of Central Europe was situated side of the Gondwana block from

on the during

Laurentia

Acknowledgements We thank D. Gebauer, Zurich, N. Arndt. Mainz, B. Grauert, Miinster, and an unknown, but very

Ahlberg,

Central

over-

zircon

South-Central

of the

Northwestern Central European crustal fragment to the Cambrian denudation areas in Northwest Scotland, the eastern parts of the Canadian Shield and/or areas of the Baltic Shield can be excluded. Recent results on detrital zircons from Lower Deof

zircons

from source rocks being strongly

References

arenites

rela-

tively U-rich zircons has been derived directly or indirectly from Archean to Early Proterozoic

ably the same as that of other Early Palaeozoic crustal sections of present-day Central and

vonian

show a

con-

data from Bal-

connections

isotope

zircons

critical reviewer, who all made helpful to improve the manuscript.

Direct

at

typical distribution pattern. This reflects the detritus consisting of two distinct constituents: one

Because of these different apparent age patterns the geologic character of the source area(s) of the Brabant-Ardennes detritus is most prob-

Europe.

for

Europe

the U-Pb

Cambrian times and separated and Baltica by oceanic distances.

great amounts

stud-

applied

sis of detrital zircons derived from Laurentia or Baltica yield U-Pb data which plot above this This age pattern

fractions

palaeogeographic coherence of Central Cambrian or even earlier times. In the Concordia

zircons

mineral

analysis

ern part northern

sidering

and

times.

of detrital

selected

Moldanubian metasediments determined by Kroner et al. (1988b). In contrast, isotopic analy-

chord.

show

from Laurentia

hand

tus (Fig. 4). Besides there are a few U-Pb data of detrital zircons of Ordovician sandstones from this apparent A striking

Mass,

considerable

Noire, France

1976)

of the Bohemian

and of the Armorican

1980) all plot within

Tunesia

Rhenish

from Lower Palaeo-

Grtinenfelder,

(Grauert

colourless

differing

on the other

of the Montagne

(Grauert 1977)

to

Conclusions

zoic metasediments (Gebauer

all this refers

fractions)

those

Brabant-Ardennes (Fig. 4). (2) U-Pb

(above

FT AL

(Ardennes,

P., 1984. The Lower

Sweden. Aleinikoff. and

Geol. Fiiren. J.N., Williams,

Worl,

component Wind

R.G.. Range,

and ion microprobe 101: 198-206.

sequence

Evidence

U-Pb

W., Stuckless.

for an

Early

to Late Archean

west-central data.

in Skane,

Fiirh.. 106: 380-381.

IS.. Compston.

1989.

in the Middle

River

Cambrian

Stockholm

suggestions

gneisses

Wyoming: Contrib.

J.S.

Archean of the

conventional

Mineral.

Petrol..

BRABANT

MASSIF

AS PART

49

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