u/pb and pb/pb zircon ages from granitoid rocks of ...€¦ · the precambrian rocks in western...

Mineralogy and Petrology (2001) 71: 251±271

U/Pb and Pb/Pb zircon ages from granitoidrocks of Wallagga area: constraints onmagmatic and tectonic evolution of Precambrianrocks of western Ethiopia

T. Kebede1;�, U. S. Kloetzli2, and C. Koeberl1

1 Institute of Geochemistry, University of Vienna, Austria2 Laboratory for Geochronology, Institute of Geology, University of Vienna, Austria

With 7 Figures

Received June 7, 2000;accepted October 29, 2000

Summary

The Precambrian rocks in western Ethiopia consist of high- and low-grade terranesintruded by granitoids with a wide compositional spectrum. The formation ages of thesegranitoid rocks are, so far, poorly understood. Single-grain zircon Pb/Pb evaporation andconventional U/Pb dating conducted on four granitoids places time constraints on theiremplacement and tectonothermal events. Three granitoid magmatic events wereidenti®ed at 815 Ma, 700±730 Ma, and 620±625 Ma, which were marked byemplacement of the calc-alkaline Ujjukka granite and granodiorite, the anatecticSuqii-Wagga two-mica granite and the Guttin K-feldspar megacrystic granite, and theanorogenic Ganjii monzogranite, respectively. We interpret the 815 Ma age to mark amajor magmatic episode in this part of Africa. A tectonothermal event at � 630 Mapreceded the emplacement of the within-plate granitoids at 620±625 Ma. The decrease ofages from the calc-alkaline to anorogenic granitoids suggests a shift of magmatic stylesand tectonic setting of the granitoids over a period of 200 million years. The Suqii-Wagga and Guttin granites, representing the granitoid population in the migmatiticterrane, formed as part of the successive evolution of the granitoid magmatism in theregion. The presence of xenocrystic zircons of Mesoproterozoic ages in both granitoidpopulations emplaced into the low-grade volcanosedimentary sequence and the high-grade, often migmatitic, gneisses suggest contribution of pre-Pan-African crust to theorigin and evolution of the granitoids. Conventional U/Pb studies of zircons from the

� Present address: Laboratory for Geochronology, Institute of Geology, University of Vienna,Austria

Guttin K-feldspar megacrystic granite and the Ganjii monzogranite yielded upperintercept ages of � 3 Ga and � 2 Ga, respectively, possibly indicating the presence ofreworked Archean-Proterozoic crustal material.

Zusammenfassung

U/Pb und Pb/Pb Zirkonalter granitoider Gesteine aus dem Gebiet von Wallagga:Hinweise zur magmatischen und tektonischen Entwicklung praÈkambrischer Gesteine inAÈ thiopien

Das PraÈkambrium im westlichen AÈ thiopien besteht aus hoch- und niedrigmetamorphenBasement Serien, die von Granitoiden unterschiedlichster Zusammensetzung intrudiertwerden. Die Bildungsalter dieser Magmatite sind bisher nur ungenuÈgend bekanntgewesen. Neue Pb/Pb-Evaporations- und konventionelle U/Pb-Datierungen an Einzel-zirkonen von vier verschiedenen Plutoniten erlauben nun RuÈckschluÈsse auf derenIntrusionsalter und die damit verbundene tektonische Entwicklung der Region. Dreizeitlich getrennte magmatische Ereignisse lassen sich unterscheiden: Intrusion der kalk-alkalischen Ujjukka Granite um 815 Ma; Bildung der anatektischen ZweiglimmerGranite der Suqii-Wagga Suite um 700±730 Ma; Intrusion der grob porphyrischen K-Feldspat Granite von Guttin und der anorogenen Ganjii Monzogranite um 620±625 Ma.Das 815 Ma Ereignis wird als wichtige magmatische Phase in diesem Teil von Afrikainterpretiert. Ein thermisches Ereignis um 630 Ma geht der Platzname von `̀ within-plate'' Granitoiden um 620±625 Ma voraus. Die beobachtete Altersabnahme von denkalk-alkalischen zu den anorogenen Granitoiden spricht fuÈr eine praÈgnante AÈ nderungdes tektonischen Regimes uÈber einen Zeitraum von ca. 200 Ma. Die Suquii-Wagga undGuttin Granite sind in das hochgradige, migmatische Basement intrudiert. Dies mag fuÈreine sukzessive tektonische Entwicklung dieser Abfolgen sprechen. Ererbte, mesopro-terozoische Zirkone deuten auf die Aufarbeitung praÈ-panafrikanischer Gesteine hin.Obere Einstichpunkte von den U/Pb Analysen im Altersbereich von ca. 3 Ga in denGuttin Graniten und von ca. 2 Ga in den Ganjii Monzograniten sprechen ebenfalls fuÈrdie Inkorporation von proterozoischen bis archaischen Krustenkomponenten.

Introduction

Granitoids constitute a signi®cant proportion of the western Ethiopian Precambrianrocks (Fig. 1). Field relationships and petrography, geochemistry, and petrogenesisof the granitoids intruded into the high-grade gneisses and low-grade metasedi-mentary and metavolcanic rocks were recently studied by Kebede et al. (1999,2000). However, the age relationships among the granites emplaced into the sameterrane or between granite populations intruding contrasting terranes are still notwell understood. In particular, systematic geochronological studies are lacking forthe western Ethiopian Precambrian areas, except the study by Ayalew et al. (1990),who reported U/Pb and Rb/Sr dating on plutonic rocks south of the area described inthe present work and attempted to put age limits on the magmatic and metamorphicevolution there. The present study, therefore, was aimed at constraining themagmatic history and establishing the sequence of events in the research area usingU/Pb and Pb/Pb single-grain zircon chronometers. The results of the study,integrated with other regional data, provide a better picture of the geologicalevolution of the western Ethiopian Precambrian rocks.

To this end, we dated samples of the Ujjukka granite and granodiorite, the Suqii-Wagga garnet-bearing two-mica leucocratic granite, the Ganjii, often porphyritic,

252 T. Kebede et al.

Fig

.1.

Gen

eral

ized

geo

logic

alm

apof

the

study

area

(modi®

edaf

ter

Keb

ede

etal

.,1999

and

refe

rence

sth

erei

n).

Tb

Ter

tiar

ybas

alt,

WP

Gw

ithin

-pla

tegra

nit

e,V

AG

volc

anic

arc

gra

nit

e,V

ST

volc

ano-s

edim

enta

ryte

rran

e,G

Tgnei

ssic

terr

ane,

SZ

sutu

rezo

ne,

GM

Gan

jii

monzo

gra

nit

e,G

KM

Gutt

inK

-fel

dsp

arm

egac

ryst

icgra

nit

e,SW

Suqii

-Wag

ga

two-m

ica

gra

nit

e,U

KU

jjukka

gra

nit

ean

dgra

nodio

rite

,G

GG

ore

-Gam

bel

ageo

trav

erse

area

.A

ges

giv

enin

the

elli

pse

sar

eder

ived

from

single

-gra

inzi

rcon

207P

b/2

06P

bev

apora

tion

and

U/P

bdat

ing

U/Pb and Pb/Pb zircon ages from granitoid rocks of Wallagga area 253

monzogranite, and the Guttin K-feldspar megacrystic granite (Fig. 1). Thesegranitoids were selected based on their ®eld relationships to represent intrusion intoboth high- and low-grade terranes so that comparison of magmatic and tectono-thermal events are possible. Accordingly, the Ujjukka and the Ganjii granitoidsrepresent plutons emplaced into the low-grade rocks, whereas the Suqii-Wagga andthe Guttin granites represent plutons in the high-grade rocks. In this study we showthat granite magmatism changed from subduction-related to anatectic to anorogenicin the time span of 815 to 620 Ma.

Sample preparation and description

Large rock samples, ranging from about 15 to 30 kg, were collected from the Ganjii,Guttin, Suqii-Wagga, and Ujjukka granitoids (see Fig. 1 for sample locations). Thesamples were crushed and panned to separate the heavy mineral fractions, whichinclude most of the zircons present. Thereafter, the samples were sieved and the sizefraction between 45mm and 200mm was used for further zircon mineral separation.Magnetic and heavy liquid separations were subsequently used to obtain zirconconcentrates. The almost pure zircon fractions were hand-picked and classi®ed intodifferent populations according to typology, colour, and translucency using a bino-cular microscope.

The zircon typologies from different granitoid bodies are summarized in Table 1and some representative crystals are shown in Figs. 2 and 3. In general, idiomorphicand translucent, seemingly magmatic, zircons were used for analysis. Twopopulations of such grains are present in samples TK117 and TK141 from theGanjii and the Guttin granitoids, respectively. Dating of samples containingcomplex zircon populations is often dif®cult, as different populations may havedifferent histories. However, Pupin (1980) was able to relate the variation in apopulation to different stages of magmatic crystallization, in which the successivestages of typological evolution of zircon were trapped in other minerals as theirgrowth proceed. He also suggested that such variations within zircon population arecaused by changes in physico-chemical conditions of the crystallization medium,which would still permit a closely related age for different populations of zircon in aparticular sample. However, samples TK117 and TK141 contain, besides themagmatic zircon population, inherited zircons, which may have resulted intypologic variations. Such population variations might complicate the dating byyielding ages that are dif®cult to interpret, or mixing ages without geologicalsigni®cance. Samples containing homogeneous zircons, for example, TK099 of theUjjukka granite and granodiorite, yielded consistent ages (Table 2).

Analytical methods

Single-grain zircon Pb/Pb evaporation and conventional U/Pb zircon datingtechniques were used. Analyses were conducted on a Finnigan MAT 262 massspectrometer, equipped with a secondary electron multiplier-ion counter system, atthe Geochronology Laboratory, Institute of Geology, University of Vienna. The ®nal207Pb/206Pb and U/Pb ages were calculated at 2� standard deviation using theIsoplot/Ex program version 2.10 of Ludwig (1999).


Tab

le1.

Typ

olo

gic

class

i®ca

tion

of

zirc

ons

and

sum

mary

of

pet

rogra

phic

and

chem

ical

chara

cter

isti

csof

sele

cted

gra

nit

oid

rock

sfr

om

Wall

agga

are

a,

wes

tern

Eth

iopia

Ro

ckn

ame

Sam

ple

num

ber

Typolo

gy

1

popula

tion

Des

crip

tio

ns

Gen

eral

pet

rog

rap

hic

feat

ure

sG

eoch

emic

alch

arac

teri

stic

s

Ujj

uk

ka

gra

nit

ean

dg

ran

od

iori

te

TK

099

2S

23

toS

25

Short

pri

smat

ic,

tran

spar

ent

gen

eral

lyco

lou

rles

s(w

ith

min

or

yel

low

vari

etie

s),

no

rmal

ly2

00

±2

50mm

lon

g,

wit

hw

idth

/len

gth

rati

os

of

1/4

to3

/5,

angula

ran

dco

nta

ins

no

incl

usi

on

san

din

her

ited

core

.T

his

po

pu

lati

on

iso

fm

agm

atic

typ

e.

Med

ium

-to

coar

se-g

rain

edh

yp

idio

mo

rph

icg

ran

ula

r,co

mp

ose

do

fK

-fel

dsp

ar,

pla

gio

clas

e,q

uar

tz,

and

bio

tite

wit

hm

ino

rsp

hen

e,al

lan

ite,

zirc

on

,�

mo

naz

ite,�

apat

ite.

Ujj

uk

ka

gra

nit

oid

sex

per

ien

ced

wea

kd

efo

rmat

ion

and

low

-gra

de

met

amo

rph

ism

.

Cal

c-al

kal

ine,

com

mo

nly

met

alu

min

ou

sw

ith

am

ino

rev

olv

edp

eral

um

ino

us

var

iety

.T

he

frac

tio

nat

edo

nes

hav

eh

igh

HF

S(h

igh

®el

dst

ren

gth

)el

emen

ts.

Ujj

uk

ka

gra

nit

oid

sg

ener

ally

hav

em

od

erat

eto

larg

en

egat

ive

Eu

ano

mal

ies,

slig

htl

yen

rich

edL

RE

Ean

d¯

atH

RE

Ed

istr

ibuti

on

pat

tern

s.

Gu

ttin

K-f

eld

spar

meg

acry

stic

gra

nit

e

TK

141

S19

and

D

2S

13

or

S8

S12

toS

18

Lon

gp

rism

atic

,cl

ear,

colo

url

ess,

euhed

ral

and

mag

mat

icty

pe.

Th

eri

ms

are

slig

htl

yco

rro

ded

.

Sho

rtp

rism

atic

,co

nta

ins

no

vis

ible

incl

usi

on

or

inh

erit

edco

re,

gen

eral

lycl

ear

and

colo

url

ess.

Th

isal

sose

ems

tobe

mag

mat

iczi

rco

n,

and

was

use

dfo

rdat

ing

.

Lar

ge

pin

kis

hg

rain

sw

ith

do

ub

lepri

sms,

that

gen

eral

lyap

pea

rcl

ou

dy.

This

po

pu

lati

on

seem

sto

be

inh

erit

edfr

om

old

erro

cks.

Th

ese

zirc

on

sw

ere

not

use

dfo

rb

oth

evap

ora

tio

nan

dU

-Pb

conv

enti

on

ald

atin

g.

Gen

eral

lyp

orp

hry

tic

wit

hK

-fel

dsp

arm

egac

ryst

sem

bed

ded

in®

ne-

tom

ediu

m-g

rain

edm

atri

xo

fq

uar

tz,

bio

tite

,fr

agm

ente

dfe

ldsp

ar(c

om

mo

nly

pla

gio

clas

e),

sph

ene,

and

oth

erm

ino

rm

iner

als

(zir

con

,al

lan

ite,

&ap

atit

e).

As

this

rock

isst

ron

gly

affe

cted

by

def

orm

atio

nan

dm

etam

orp

his

m,

anu

ltra

my

lon

itic

var

iety

isco

mm

on

.M

a®c

encl

aves

com

po

sed

of

bio

tite

and

feld

spar

sar

ep

rese

nt.

Th

isg

ran

ite

bo

dy

has

aw

ide

ran

ge

of

SiO

2co

nce

ntr

atio

ns

(66

.3±

77

.2w

t%

).It

isco

mm

on

lym

etal

um

ino

us,

char

acte

rize

db

yen

rich

edL

RE

E,

¯at

HR

EE

pat

tern

,an

dm

od

erat

eo

rn

oE

un

egat

ive

ano

mal

y.T

he

ult

ram

ylo

nit

icvar

iety

has

ad

isti

nct

cho

nd

rite

-n

orm

aliz

edR

EE

pat

tern

wit

hsi

gn

i®ca

ntl

yd

eple

ted

mid

dle

RE

E.

S24

Com

mo

nly

sho

rtp

rism

atic

,y

ello

wto

pin

kw

ith

rou

nd

eded

ges

.T

his

popu

lati

on

may

also

rep

rese

nt

inh

erit

edva

riet

ies,

and

soar

en

ot

suit

able

for

dat

ing

pu

rpo

ses.

Tab

le1

(co

nti

nu

ed)


Tab

le1

(conti

nued

)

Ro

ckn

ame

Sam

ple

num

ber

Typolo

gy

1

popula

tion

Des

crip

tio

ns

Gen

eral

pet

rog

rap

hic

feat

ure

sG

eoch

emic

alch

arac

teri

stic

s

Su

qii

-Wag

ga

two

-mic

ag

ran

ite

TK

049

2S

23

toS

25

and

P5

S24

and

D

Tra

nsp

aren

t,co

lou

rles

s,id

iom

orp

hic

and

bo

thsh

ort

and

lon

gp

rism

atic

mag

mat

icva

riet

ies.

So

me

cry

stal

sdis

pla

yap

par

entl

yh

oll

ow

incl

usi

on

sat

the

core

,w

hic

hm

ayh

ave

acte

das

nucl

eiw

hen

cry

stal

liza

tio

nb

egan

.S

ul®

de

and

apat

ite

incl

usi

on

sw

ere

also

obse

rved

.

Both

sho

rtan

dlo

ng

pri

smat

icva

riet

ies

are

pre

sen

t.T

hey

are

char

acte

rize

db

ytu

rbid

app

eara

nce

soth

atit

isd

if®

cult

tore

cog

niz

eth

ep

rese

nce

or

abse

nce

of

incl

usi

on

so

rco

res.

Th

isp

op

ula

tio

nm

ayre

pre

sen

tin

her

ited

zirc

on

s.

Med

ium

-to

coar

se-g

rain

ed,

pri

nci

pal

lyco

mp

ose

do

fp

lag

iocl

ase,

K-f

eld

spar

,q

uar

tz,

mu

scov

ite,

bio

tite

,�

gar

net

,zi

rco

n,

�al

lan

ite,�

apat

ite,�

top

az,

�ca

ssit

erit

e,�

¯u

ori

te,

Fe-

Ti

oxid

es,

and

Fe-

sul®

de.

Th

eg

ran

ite

gen

eral

lyap

pea

rsto

be

un

def

orm

ed,

bu

tb

ent

alb

ite

twin

lam

ella

e,su

gg

esti

ve

of

mec

han

ical

def

orm

atio

n,

wer

ed

ocu

men

ted

.

Ch

arac

teri

zed

by

rest

rict

edra

ng

eso

fS

iO2

con

ten

ts(7

4.4

±7

6.2

wt.

%).

Abu

nd

ance

so

fF

e 2O

3T,

CaO

,S

r,an

dB

ad

ecre

ase

wit

hin

crea

sin

gS

iO2,

wh

erea

sT

iO2,

Al 2

O3,

and

Mg

Od

on

ot

show

mu

chvar

iati

on

.It

ism

ild

lyp

eral

um

ino

us

tom

etal

um

ino

us

wit

hA

SI

ran

gin

gfr

om

0.9

6to

1.1

6.

Rel

ativ

ely

low

K/R

b;

hig

hR

b/S

r,R

b/Z

r,Y

/Zr,

and

Nb

/Zr

rati

os

dis

tin

gu

ish

this

gra

nit

efr

om

the

oth

erg

ran

ito

ids.

Ch

on

dri

te-n

orm

aliz

edR

EE

pat

tern

ssh

ow

am

od

erat

eto

stro

ng

neg

ativ

eE

uan

om

aly.

Gan

jii

mo

nzo

gra

nit

eT

K117B

2D

2S

24

Long

pri

smat

icw

ith

len

gth

reac

hin

gu

pto�

45

0mm

,w

idth

/len

gth

(1/5

to1

/4),

colo

url

ess,

freq

uen

tly

con

tain

sin

clu

sio

ns

of

apat

ite,

ang

ula

red

ges

and

oft

enco

nta

inv

isib

leco

res.

Zir

con

cryst

als

con

tain

ing

ho

llow

elo

ng

ated

centr

aln

ucl

eio

rien

ted

par

alle

lto

the

Cax

isw

ere

reco

gn

ized

.

Rel

ativ

ely

sho

rtp

rism

atic

,u

pto

�30

0mm

lon

g,

wid

th/l

eng

thra

tio

of

abo

ut

1/4

to2

/5,

colo

url

ess,

ang

ula

r,an

doft

enco

nta

ins

incl

usi

on

san

dan

inh

erit

edco

re.

Als

om

ediu

m-

toco

arse

-gra

ined

,co

mp

ose

do

fp

lag

iocl

ase,

K-

feld

spar

,q

uar

tz,

bio

tite

,an

dh

orn

ble

nd

ew

ith

sub

ord

inat

esp

hen

e,ap

atit

e,F

e-T

io

xid

es,

zirc

on

,m

on

azit

e,�

ruti

le,

and

�x

eno

tim

e.T

he

ma®

cm

iner

als

(ho

rnb

len

de,

bio

tite

,an

dsp

hen

e)co

mm

on

lyo

ccu

rin

agg

reg

ate.

Th

ese

min

eral

sar

em

ore

abu

nd

ant

inth

em

on

zod

iori

tic

mar

gin

alfa

cies

and

encl

aves

inth

isg

ran

ite.

Th

eco

nce

ntr

atio

no

fm

ajo

ro

xid

es(e

.g.,

Fe 2

O3t,

TiO

2,

Mg

O,

CaO

,an

dP

2O

5)

dec

reas

ew

ith

incr

easi

ng

SiO

2.

Itis

met

alu

min

ou

sw

ith

alu

min

asa

tura

tio

nin

dex

(AS

I)�

0.8

8to

0.9

3.

Th

eg

ran

ite

gen

eral

lyh

asch

emic

alch

arac

teri

stic

so

fsu

bal

kal

ine

mag

ma.

Ital

soh

asre

lati

vel

yh

igh

con

cen

trat

ion

so

ftr

ansi

tio

nm

etal

s,R

EE

s,Z

r,H

f,an

dT

a,an

dlo

wco

nte

nts

of

Th

and

U.

Mix

edD

and

S24

Yel

low

ish

,st

ub

by

and

lon

gp

rism

atic

zirc

on

s.T

his

gro

up

may

rep

rese

nt

not

over

gro

wn

,in

her

ited

,zi

rco

ns.

1T

yp

olo

gy

clas

si®

cati

ons

are

acco

rdin

gto

Pupin

(19

80

);2zi

rco

np

op

ula

tio

ns

use

dfo

rev

apo

rati

on

and

/or

U/P

bco

nven

tio

nal

dat

ing


Single-grain zircon Pb/Pb evaporation method

The principles of single zircon evaporation techniques are reported in Kober (1986,1987) and KloÈtzli (1997). Details of the zircon Pb evaporation procedure and Pbisotope ratio analyses are described in KloÈtzli (1997). Repeated measurements of

Fig. 2. Secondary electron images. a Short prismatic zircon (type S23±S25) from Ujjukkagranite and granodiorite (UK-gt). b and c are long prismatic (type D) and short prismatic(type S13) zircons, respectively, from Guttin K-feldspar megacrystic granite (G-gt). d and eare long prismatic (type D) and short prismatic (type S24) zircons, respectively, fromGanjii monzogranite (Ga-gt)


NIST SRM982 reference material show the mass fractionation for 207Pb/206Pb and208Pb/206Pb, for the whole duration of the analysis, to be 0.006±0.291% amu and0.012±0.583% amu, respectively. Based on the standard analyses, the measuredradiogenic Pb ratios were corrected to exclude the effects of mass fractionation.Data reduction and age evaluation were done using Isoplot/Ex (Ludwig, 1999).

Conventional U/Pb method

Zircon fractions were carefully selected from the Guttin K-feldspar megacrysticgranite and were air abraded following the procedures described by Krogh (1982).Zircons from the Ganjii monzogranite were cleaned by leaching with 8N HNO3 and6.2N HCl at 80 �C for 24 hr in Te¯on vessels. The abraded and cleaned zircons were

Fig. 3. Translucent, both long and short prismatic magmatic zircon populations used forU/Pb age determination (long dimension of zircon crystals up to 400 mm)


Tab

le2.

Sin

gle

-gra

inzi

rcon

Pb

evapora

tion

data

and

ages

of

gra

nit

oid

rock

sfr

om

Wall

agga

are

a,

wes

tern

Eth

iopia

Mea

sure

men

tE

vap

ora

tion

No.

of

207P

b/2

06P

b2�

erro

r207P

b/2

06P

bE

rro

r208P

b/2

06P

b2�

erro

rT

h/U

rati

o2�

erro

rn

um

ber

tem

p.

(�C

)S

cans

rati

oag

e(M

a)(M

a)ra

tio

atag

e

Ujj

uk

ka

gra

nit

ean

dg

ran

od

iori

te

Zir

con

11

43

A1

1400

13

0.0

66

29

0.0

00

35

81

57

0.1

36

23

0.0

08

96

0.4

04

0.0

27

Zir

con

11

43

A2

1440

27

0.0

66

37

0.0

03

13

81

85

60

.09

898

0.0

08

63

0.2

93

0.0

25

1460

90

0.0

66

65

0.0

01

33

82

71

40

.08

742

0.0

04

70

0.2

59

0.0

14

1480

18

0.0

66

48

0.0

01

96

82

26

20

.10

629

0.0

14

52

0.3

15

0.0

43

1440±1480

82

41

7Z

irco

n1

14

3B

1420

90

0.0

66

65

0.0

01

42

82

71

50

.06

973

0.0

03

60

0.2

06

0.0

11

1440

180

0.0

66

17

0.0

00

56

81

24

0.1

74

68

0.0

05

13

0.5

18

0.0

15

1460

153

0.0

65

94

0.0

00

64

80

45

0.4

20

72

0.0

06

17

0.7

14

0.0

18

1420±1460

81

24

Zir

con

11

43

D1440

20

0.0

66

07

0.0

01

07

80

93

4±

±±

±Z

irco

n1

14

3A

1,

A2,

Ban

dD

mea

n8

15

5

Gu

ttin

K-f

eld

spar

meg

acry

stic

gra

nit

e

Zir

con

11

39

A1420

117

0.0

60

86

0.0

00

27

63

43

0.1

75

37

0.0

01

82

0.5

25

0.0

06

1440

108

0.0

60

52

0.0

00

15

62

23

0.1

91

18

0.0

01

19

0.5

73

0.0

04

1420�1

440

225

62

92

1460

81

0.0

62

05

0.0

00

35

67

64

0.1

97

25

0.0

03

37

0.5

90

0.0

10

Zir

con

11

39

C1460

63

0.0

65

30

0.0

01

81

78

45

80

.18

966

0.0

04

37

0.5

63

0.0

11

Zir

con

11

39

D1460

23

0.0

63

27

0.0

00

39

71

78

0.1

65

15

0.0

13

12

0.4

92

0.0

39

Zir

con

11

39

F1460

36

0.0

97

19

0.0

00

91

15

71

90

.31

499

0.0

14

08

0.8

89

0.0

40

Su

qii

-Wag

ga

two

-mic

ag

ran

ite

Zir

con

11

42

B1400

27

0.0

62

70

0.0

01

04

69

82

70

.08

128

0.0

05

95

0.2

43

0.0

18

Gan

jii

mo

nzo

gra

nit

e

Zir

con

11

40

A1420

36

0.0

67

97

0.0

00

23

86

81

40

.11

172

0.0

01

19

0.3

30

0.0

03

Zir

con

11

40

C1420

126

0.0

85

91

0.0

03

59

12

88

28

0.2

04

38

0.0

08

31

0.5

86

0.0

21

1440

162

0.0

84

68

0.0

03

87

12

56

20

0.2

14

65

0.0

10

00

0.6

17

0.0

26

1420�1

440

288

12

68

17

Zir

con

11

41

A1400

90

0.0

60

76

0.0

01

46

63

31

70

.12

436

0.0

07

85

0.3

73

0.0

24

1440

72

0.0

67

47

0.0

00

52

85

71

20

.15

862

0.0

11

89

0.4

69

0.0

35

1460

99

0.0

74

28

0.0

00

77

10

49

12

0.1

87

14

0.0

15

16

0.5

46

0.0

44

1480

162

0.0

69

54

0.0

01

50

91

59

0.1

82

40

0.0

10

91

0.5

37

0.0

32

Zir

con

11

41

C1420

14

0.0

60

59

0.0

01

08

62

55

30

.15

018

0.0

18

58

0.4

50

0.0

55

1440

27

0.0

66

39

0.0

01

85

81

93

30

.16

268

0.0

07

43

0.4

82

0.0

21

1460

27

0.0

65

92

0.0

01

44

80

43

50

.15

032

0.0

10

13

0.4

46

0.0

29

Zir

con

11

41

D1400

180

0.0

60

59

0.0

00

49

62

54

0.0

55

64

0.0

01

59

0.1

67

0.0

05

1420

63

0.0

61

25

0.0

00

51

64

88

0.0

92

23

0.0

02

98

0.2

76

0.0

09

1440

90

0.0

61

14

0.0

00

56

64

46

0.1

13

24

0.0

03

03

0.3

39

0.0

09

1420�1

440

153

64

65

1460

90

0.0

62

16

0.0

00

83

68

09

0.1

22

25

0.0

04

06

0.3

65

0.0

12

Zir

con

11

41

E1400

39

0.0

60

30

0.0

01

53

61

43

00

.09

521

0.0

03

74

0.2

86

0.0

11

Zir

con

11

41

E�r

ims

of

323

62

27

11

41

A,

C,

and

D


vapour digested using a procedure described by Wendt and Todt (1991). A 205Pb-233U-235U mixed spike was used. The total procedural Pb-blank and U-blank were2 pg and 0.1 pg, respectively. Common Pb correction was made using the Stacey andKramers (1975) model parameters at the measured 207Pb/206Pb age.

Scanning electron microscope imaging

Backscattered electron (BSE) and cathodoluminescence (CL) images of the internalstructures and inherited cores in the zircons (sectioned by polishing) and the 3Dsecondary electron (SE) images of selected grains from different granite units andfractions were done at the Natural History Museum, Vienna, using a Jeol JSM 6400Scanning Microscope, and an Oxford instruments CL system. These images wereparticularly helpful for the interpretation of the 207Pb/206Pb evaporation ages.

Results and discussion

The single-grain zircon Pb/Pb evaporation and conventional U/Pb results, and BSEand CL images of zircons are given in Tables 2 and 3, and Figs. 4±7, respectively.

Ujjukka granite and granodiorite

The Ujjukka granitoids contain a homogeneous zircon population with shortprismatic, generally colourless (with minor yellowish varieties), angular crystalsthat commonly have no inclusion or inherited core. This zircon population isclassi®ed according to Pupin (1980) as subtypes S23±S25. Backscattered electronand CL image studies of two zircon crystals, and qualitative energy dispersiveanalyses revealed the presence of ®ne-scale internal zoning and inclusions of K-feldspar and Na, Ca, Mg, and Fe silicate. One zircon crystal fragment on whichovergrowth took place, possibly during a post-emplacement tectonothermal event,was observed (Fig. 4a). The mechanism of incorporation of supposedly late-crystallizing minerals, such as muscovite and K-feldspar found in a few zirconcrystals, and their implications, are not understood.

Evaporation analyses of four inclusion- and core-free zircon crystals from the®rst fraction yield, within error, basically identical 207Pb/206Pb ages that range from809� 34 Ma to 824� 17 Ma (Table 2 and Figs. 5a±d). The mean 207Pb/206Pbevaporation age of the four zircons is 815� 5 Ma, which is considered to representthe minimum age of the emplacement of the Ujjukka granite and granodiorite. Asall zircons have the same age, the effect of either Pb loss or overgrowth during themetamorphism and deformation that affected the Ujjukka granitoid is negligible.

Guttin K-feldspar megacrystic granite

The Guttin K-feldspar megacrystic granite is characterized by four distinct zirconpopulations (Table 1) that show inheritance and complex internal structures (e.g.,Figs. 4b and c). For example, the zircon in Fig. 4b contains two cores, one brokenand partially enclosed and the other relatively small, cryptically zoned andcompletely enclosed by a later overgrowth. The inherited cores have the same


Tab

le3.D

ata

of

zirc

on

U/P

bco

nven

tional

analy

ses

from

Ganji

im

onzo

gra

nit

eand

Gutt

inK

-fel

dsp

ar

meg

acr

ysti

cgra

nit

e,W

all

agga

are

a,w

este

rnE

thio

pia

Ages

inM

a,1S

D

Sam

ple

206P

b/2

04P

b207P

b/2

35U

1S

D2

38U

/20

6P

b1S

D2

07P

b/2

06P

b1S

D2

07P

b/2

35U

20

6P

b/2

38U

20

7P

b/2

06P

b

Gutt

inK

-fel

dsp

arm

egac

ryst

icgra

nit

e�

5A

A14423

1.0

3615

0.0

1957

8.4

38

0.1

24

0.0

6347

0.0

0120

722�

5722�

5724�

20

5A

B2468

1.2

2049

0.0

0529

8.1

98

0.0

34

0.0

6805

0.0

0029

810�

2742�

3870�

95A

C2315

1.0

4218

0.0

0421

8.3

61

0.0

32

0.0

6356

0.0

0025

725�

2728�

3727�

85A

D4008

0.8

5064

0.0

1613

10.2

54

0.1

49

0.0

6092

0.0

0115

625�

9600�

8636�

41

5B

E4143

1.0

6239

0.0

0593

8.3

16

0.0

43

0.0

6398

0.0

0033

735�

3732�

4741�

11

5B

F4627

1.2

3145

0.0

0605

8.0

01

0.0

37

0.0

7217

0.0

0034

815�

3759�

3991�

95B

G3562

1.0

7461

0.0

0502

9.7

15

0.0

43

0.0

6450

0.0

0029

741�

2632�

3758�

9

Gan

jii

monzo

gra

nit

e��

9A

-110530

0.8

4724

0.0

1433

9.8

88

0.1

05

0.0

6076

0.0

0043

623�

8621�

6631�

15

9A

-22468

0.8

4379

0.0

1307

9.9

05

0.0

82

0.0

6062

0.0

0040

621�

7620�

5626�

14

9A

-372480

0.8

4832

0.0

1586

9.8

72

0.1

05

0.0

6074

0.0

0037

624�

9622�

6630�

13

9A

-452047

0.8

5705

0.0

0998

9.7

76

0.0

75

0.0

6076

0.0

0033

629�

5628�

4631�

12

17B

-15602

2.9

7282

0.0

3611

4.8

08

0.0

31

0.1

0366

0.0

0031

1401�

91218�

71691�

617B

-25966

1.0

9233

0.0

2648

8.3

53

0.1

15

0.0

6617

0.0

0057

750�

13

729�

9812�

18

17B

-31006

0.8

6135

0.1

3802

9.7

73

1.1

72

0.0

6105

0.0

0020

631�

75

628�

72

641�

717B

-49496

0.8

5396

0.0

0930

9.7

98

0.0

69

0.0

6068

0.0

0026

627�

5626�

4628�

9

� Air

abra

ded

,��

chem

ical

lyle

ached


elongation direction, suggesting simultaneous incorporation from the melt. Thezircon crystal shown in Fig. 4c appears to contain an off-centered inherited core,relatively homogeneous central portions (around the core), and a ®nely zoned outerportion.

Six euhedral, clear and colourless zircon grains from the Guttin granite wereevaporated. Four zircons (1139A, -C, -D, and -F) produced measurable Pb ionintensities (Table 2). Zircon 1139A shows a bimodal 207Pb/206Pb evaporation agedistribution (Fig. 5e), which suggests the presence of an inherited core. The corehas a minimum age of 676� 4 Ma with a later overgrowth of 629� 2 Ma. Thehomogeneous, core-free zircon 1139D records an age at 717� 6 Ma (Table 2 andFig. 5f). The other two zircons 1139C and 1139F (Fig. 5g) yield 207Pb/206Pbevaporation ages of 784� 58 Ma and 1571� 9 Ma (cf. zircon 5AB and 5BF inTable 3), respectively.

Furthermore, the results of conventional U/Pb dating on 7 zircon grains fromthe Guttin K-feldspar megacrystic granite are shown in Fig. 6. Three zircon crystalsyield a concordia age of 730� 2 Ma (Fig. 6a). A discordia line through the twodiscordant points (5AB & 5BF) and the three concordant points (5AA, 5AC, &

Fig. 4. Cathodoluminescence (CL) images of selected zircons from UK-gt, G-gt, SW-gt(Suqii-Wagga two-mica granite), and Ga-gt. a CL image of broken zircon (whitish withgrowth zoning in CL) and later overgrowth (very dark gray) from UK-gt. b and c Longprismatic zircons, containing inherited zircon cores, from G-gt. d Short prismatic zircon,with an off-centered inherited core, from Ga-gt


5BE) yields lower- and upper-intercept ages of 729� 3 Ma and 3049� 170 Ma,respectively. The lower intercept is, within error, the same as the 730� 2 Maconcordia age mentioned above. Thus, 730 Ma is considered as the emplacementage of the Guttin K-feldspar megacrystic granite.

The 629 Ma Pb/Pb rim age of zircon 1139A is considered to represent thetectonothermal event that has strongly affected the Guttin K-feldspar megacrysticgranite. The Guttin granite shows different degrees of deformation that vary fromaugened K-feldspar megacrystic variety to a ®ne-grained mylonitic variant ±representing higher strain regime. The 717 Ma Pb/Pb evaporation age recorded inzircon 1139D probably was caused by Pb-loss during the tectonothermal event at629 Ma. Thus, the 676 Ma core age in zircon 1139A is probably a mixing agebetween the younger rim age (629 Ma) and an emplacement age of 730 Ma, or wasaffected by Pb loss during the tectonothermal event at 629 Ma. In contrast, zircons1139C and 1139F are assumed to be inherited from crustal precursor rocks orassimilated from gneissic country rocks during emplacement. The different ages ofthe inherited zircons suggest complicated magmatic and/or metamorphic historiesof the precursor rocks. The 1571 Ma xenocrystic age suggests a contribution frompre-Pan-African crust to the formation and evolution of the Guttin K-feldsparmegacrystic granite. This age may also be related to xenocrystic zircon 207Pb/206Pbevaporation ages of 1730±1820 Ma found in diorites from northern Somalia(KroÈner et al., 1989).

Zircon 5BG seems to be affected by Pb-loss, resulting in a low 238U/206Pb age(Table 3). As recent Pb-loss has little effect on 207Pb/206Pb ages, zircon 5BGprobably has the same age as the three concordant zircons (Fig. 6).

Suqii-Wagga two-mica granite

The Suqii-Wagga garnet-bearing two-mica granite has two zircon populations (seeTable 1). The crystals of the ®rst population, selected for Pb evaporation analysis,are classi®ed into subtypes S23±S25 and P5, exhibit euhedral forms, often containelongated bubble-like inclusions (interpreted as melt inclusions) in the center, andhave no visible inherited cores. Compared to the other studied granitoid bodies, theSuqii-Wagga two-mica granite has a much lower modal abundance of zircon.Despite the relatively high concentrations of U (20.5 ppm) and Th (23.3 ppm) in thebulk composition of sample TK049 of the Suqii-Wagga two-mica granite (Kebedeet al., 2000), the Pb ion beam intensities in the evaporated zircon crystals were verylow. Of the four relatively large (200±300 mm) zircons selected for Pb evaporation,only one zircon (1142B) produced measurable Pb intensity, yielding an age of698� 27 Ma (Fig. 5h). As the Suqii-Wagga two-mica granite does not show effectsof metamorphism or hydrothermal alteration, the 698� 27 Ma age is interpreted asa minimum emplacement age.

Ganjii monzogranite

The Ganjii monzogranite is characterized by a high abundance of zircon crystals.These zircons were classi®ed into two populations, namely the long prismaticsubtype D and the stubby short prismatic subtype S23±S24 (Table 1). All the


zircons are euhedral, clear, and generally contain inclusions (mostly apatite) and, inmany cases, inherited cores. The Pb/Pb evaporation data of six zircon crystals fromthe Ganjii monzogranite are given in Table 2. The age distributions in some of thezircon grains analyzed are shown in Figs. 5i±k.

The ages of the homogeneous zircons without any overgrowths or inheritedcores were used to identify different events. Accordingly, zircons 1140A, 1140C,and 1141E date different geological events. The Ganjii monzogranite was notaffected by post-intrusion tectonothermal events, therefore, the mean 207Pb/206Pbevaporation age of 622� 7 Ma, yielded by zircon 1141E and rim ages of zircons1141A, 1141C and 1141D (Table 2), are interpreted as its emplacement age.

Zircons 1140A and 1140C are dated at 868� 14 Ma and 1268� 17 Ma (e.g.,Fig. 5k), respectively, and interpreted as xenocrysts. The presence of inheritedzircons suggests the involvement of both Neoproterozoic and pre-Pan-African crustin the origin and evolutionary history of the Ganjii monzogranite. Furthermore, the

Fig. 5. Plots of 207Pb/206Pb evaporation ages vs evaporation temperature. a±d Four zirconsfrom Ujjukka granite and granodiorite giving, within error, the same age. e Zircon 1139Acontaining an inherited core (� 676 Ma). f and g Zircons 1139D and 1139F from G-gt. Theoverprint of the � 630 Ma tectonothermal event is not evident in both cases. h Fairlyhomogeneous zircon 1142B from SW-gt. i Zircon 1141A (Ga-gt) containing an off-centered inherited core, as a result of which the high temperature evaporation yielded lowerage than the previous low temperature step. j A relatively homogeneous inherited zircon1140C (Ga-gt). k Homogeneous zircon 1141E (Ga-gt) without any inheritance


1268 Ma xenocrystic zircon age indicates the probable occurrence of Mesoproter-ozoic crust underlying the low-grade volcano-sedimentary sequence in which theGanjii monzogranite was emplaced. This age can be compared with that of theMukogodo migmatites, dated at �1200 Ma, in the Mozambique Orogenic Belt (Keyet al., 1989). The zircons may have been inherited from the precursor rocks of theparent magmas or assimilated from wall rocks during ascent of the magmas.Detailed isotopic studies, which are beyond the scope of the present study, would berequired to further investigate the contribution of older crustal material to thegeneration of the Ganjii monzogranite. However, the euhedral nature and well-developed prisms, together with homogeneous Pb ratios in zircons 1140A and 1140C,suggest that the 1268 Ma and 868 Ma ages could be related to either metamorphicand/or magmatic events. The 868 Ma event is coeval with the oldest Pan-Africanrocks; for instance, a weakly foliated granodiorite-granite in the Khor Dahand areain Sudan, which gave a 207Pb/206Pb age of 870 Ma (KroÈner et al., 1991).

The age scattering observed in zircons 1141A, 1141C, and 1141D is probablydue to inheritance of older cores. Zircons 1141C and 1141D yield mixing agesyounger than the 868 Ma of zircon 1140A, suggesting the presence of xenocrystic

Fig. 6. Tera-Wasserburg diagram for zircons from the Guttin K-feldspar megacrysticgranite. a Three points (5AA, 5AC, & 5BE) yield a concordia age of 729.5� 5 Ma (2�).The dark gray ellipse represents the mean of the three concordant points. b The three pointconcordia between mean concordia (5AA, 5AC, & 5BE) and two discordant points (5AB &5BF), shown in the inset, yielded lower- and upper-intercept ages of � 729 Ma and� 3049 Ma, respectively


zircons of Neoproterozoic age in their cores. Zircon 1141D has a 208Pb/206Pb ratioof 0.05554� 0.00037 in the rim (� 625 Ma) and 0.1220� 0.0013 in the center(� 680 Ma). This suggests a higher Th/U ratio in the inherited core compared to theovergrowth. In zircon 1141A, however, the 207Pb/206Pb ages increased from 621 Mato 1049 Ma as the evaporation temperature increased from 1400 �C to 1460 �C anddecreased to 915 Ma at 1480 �C (Fig. 5i). As the evaporation front is expected tomigrate from the rim towards the center (e.g., Chapman and Roddick, 1994), Pb isexpected to be released from the core at higher evaporation temperature steps. It is,therefore, presumable that the grain contained an off-centered inherited zircon core.This is also supported by BSE and CL images of zircon crystals from the samefraction, which show the presence of an off-centered inherited zircon with complexinternal zoning features (e.g., Fig. 4d).

To further constrain the problem of Pb/Pb age scattering, we obtainedconventional U/Pb zircon ages on 8 grains from two populations of the Ganjiimonzogranite. Of these, ®ve yielded a concordant U/Pb age at 625� 2 Ma (Fig. 7a).In contrast, the other two grains show highly discordant U/Pb systematics with

Fig. 7. Tera-Wasserburg diagram for zircons from the Ganjii monzogranite. a The 5concordant zircons yield a mean age of 625� 2 Ma, which is interpreted as the bestestimate for the Ganjii monzogranite emplacement. The dark gray ellipse represents themean of the ®ve concordant points. In b, inset, the lower intercepts represent minimum ageestimates for the Ganjii monzogranite intrusion. The upper intercept age (� 1968 Ma)possibly exhibits the ranges of the xenocrystic zircons


207Pb/206Pb ages of 812� 18 Ma and 1691� 6 Ma, respectively. A discordia linedrawn through the youngest and oldest zircons gives lower- and upper-interceptages of 623� 3 Ma and 1968� 11 Ma, respectively (Fig. 7b, inset). The � 623 Malower intercept age is, within error, the same as the 207Pb/206Pb evaporation and the®ve-point concordia ages (� 625 Ma). Thus, it is concluded that the Ganjii monzo-granite was emplaced at 620±625 Ma, which was preceded by a tectonothermalevent at 630 Ma, as recorded in the Guttin K-feldspar megacrystic granite.

Correlation and sequence of events

The new zircon ages, re¯ecting the magmatic and tectonothermal events in thestudy area correlate well with various magmatic and metamorphic events elsewherein east-northeast Africa. The Ujjukka granite and granodiorite is the oldest amongthe granitoids studied, with an emplacement age of 815� 5 Ma. Contemporaneousages were reported for intrusive rocks from other parts of the Arabian-NubianShield and the Mozambique Orogenic Belt as well. South of the study area, in theGore-Gambela geotraverse area (Fig. 1), Ayalew et al. (1990) reported U/Pbemplacement ages of 814� 2 Ma and 828 �9/ÿ2 Ma for the Goma granodiorite andthe Birbir quartz diorite, respectively. Similarly, deformed granitoid rocks southeastof Tokar, close to the Sudanese and Eritrean border, were dated at 827 Ma (KroÈneret al., 1991). KroÈner et al. (1989) also reported a 207Pb/206Pb emplacement age of814� 7 Ma and 808�6 Ma for diorite and granite, respectively, that intruded intometasediments and gneisses in Somalia. Moreover, in Precambrian rocks of Kenya,the � 820 Ma age was considered to represent an upper amphibolite regionalmetamorphic event (Key et al., 1989). These authors also reported a Rb-Srerrorchron of 826 Ma for the emplacement of the Il Poloi `second generationgranite'. The widespread occurrence of 800±830 Ma granitoids suggests a majormagmatic event in east-northeast Africa, during which volcanic arc granites, asrepresented by the Ujjukka granite and granodiorite, were emplaced.

About 100 million years later, the anatectic Suqii-Wagga two-mica granite andthe Guttin K-feldspar megacrystic granite were emplaced at 698 Ma and 730 Ma,respectively, in the western Ethiopian high-grade gneiss terrane. This event wasalso observed in the form of granite emplacement, at � 713 Ma cross-cutting theOnib ophiolite in northeastern Sudan (KroÈner et al., 1992), granulite-faciesmetamorphism of the Wami River granulites at 715 Ma (Maboko et al., 1985), andmeta-anorthosite from the Uluguru Mountains, at 695�4 Ma, in the MozambiqueBelt of Tanzania (Muhongo and Lenoir, 1994), and as the magmatic crystallizationage (716 Ma) of foliated granitic gneiss from the Yavello area, southern Ethiopia(Teklay et al., 1993, 1998). As the Suqii-Wagga garnet-bearing two-mica granite isanatectic in origin (Kebede et al., 2000), the � 700 Ma emplacement age probablydates a late- to post-orogenic event in western Ethiopia.

This event was followed by about 100 million years without any majormagmatic or tectonothermal activities in the study area until the emplacement of thepost-orogenic Ganjii monzogranite and metamorphism, as well as the deformationof the Guttin K-feldspar megacrystic granite at 620±625 Ma and 629 Ma,respectively. The Ganjii monzogranite and other adjacent granitoids with A-typechemical characteristics may represent the same magmatic episode. Lenoir et al.


(1994) reported a U/Pb zircon emplacement age of 626 Ma for the post-kinematicLas Bar granodiorite from northeastern Somalia. Slightly older (646±652 Ma) post-tectonic granites were also reported from Jabal Um Achabe and Wadi Onib areas innortheastern Sudan (KroÈner et al., 1991, 1992). Ayalew et al. (1990) reportedisotopic rehomogenization in the Birbir quartz diorite and a Rb-Sr isochron age of632� 8 Ma in southwestern Ethiopia. Furthermore, Mosley (1993) reported theoccurrence of a series of N-S or NNW-SSE trending transcurrent shearing eventsbetween 630 Ma and 550 Ma in the Mozambique Belt of Kenya.

Conclusions

The western Ethiopian Precambrian rocks are characterized by emplacement ofseveral granitoid rock bodies during the Neoproterozoic. Conventional U/Pb andPb/Pb evaporation dating of zircons distinguished three different periods of granitemagmatism. These are

(a) emplacement of the calc-alkaline Ujjukka granite and granodiorite at� 815 Ma,

(b) crystallization of the Suqii-Wagga garnet-bearing two-mica granite and theGuttin K-feldspar megacrystic granite at � 700±730 Ma and,

(c) emplacement of the post-kinematic anorogenic Ganjii monzogranite at620±625 Ma.

A tectonothermal event, as recorded in the Guttin K-feldspar megacrysticgranite, also occurred shortly before the anorogenic magmatic phase at � 630 Ma.The study documented approximately 200 million years of granitic magmatism(subduction-related! anatectic! anorogenic or within-plate) in the westernEthiopian Precambrian rocks. The calc-alkaline nature of the Ujjukka granitoidsuggests the occurrence of subduction at 815 Ma in the region. This event iscontemporaneous with ubiquitous granitoid emplacements in E-NE Africa between800±830 Ma. A minimum age of arc-continent collision orogeny in the region is� 700 Ma, as represented by the anatectic Suqii-Wagga two-mica granite. Theanorogenic granite magmatism and tectonothermal episodes that took place at� 620±630 Ma may have concluded the Precambrian history in western Ethiopia.

Furthermore, the anatectic origin of the Suqii-Wagga two-mica granite suggeststhe involvement of continental crust in its parent magma generation and, possibly,the presence of older continental crust or its derivative underneath the contrastingterranes. The occurrence of inherited zircons with ages ranging from theMesoproterozoic (1268±1570 Ma) to the Archaean-Proterozoic transition (upperintercept ages) in the Guttin K-feldspar megacrystic granite and in the Ganjiimonzogranite, emplaced in high- and low-grade terranes, respectively, also suggesta considerable contribution of pre-Pan-African crust in the origin and evolution ofthe different types of granitoids in the Precambrian rocks of western Ethiopia.

Acknowledgments

We are grateful to G. Kurat (Natural History Museum, Vienna) for his interest andcooperation regarding use of the SEM laboratory, and to F. BrandstaÈtter for his help withthe operation and use of the SEM and CL instrumentation. We also thank G. Sverak for


assistance in polishing the zircons. T.K. thanks E. KloÈtzli-Chowanetz for useful discussionsand assistance during zircon hand picking and mounting. We appreciate the constructivereview comments of R.E. Zartman and B. Bonin on the manuscript. This study is part of aPh.D. research funded by Austrian Academic Exchange Service (OÈ AD) to T.K. Theanalyses were supported by the Austrian FWF, grant Y58-GEO (to C.K.).

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Authors' addresses: T. Kebede, and U. S. Kloetzli, Laboratory for Geochronology, Instituteof Geology, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, e-mail:[email protected], [email protected]; C. Koeberl (corresponding author),Institute of Geochemistry, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria,e-mail: [email protected]


u/pb and pb/pb zircon ages from granitoid rocks of ...€¦ · the precambrian rocks in western...

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