Petrology and geochemistry of the Gore-Gambella plutonic rocks: implications for magma genesis and the tectonic setting of the Pan-African Orogenic Belt of western Ethiopia

Pergamon Journal of African Earth Sciences, Vol. 27, No. 314. pp. 397-416. 1996

0 1996 Elsevier Science Ltd All rights reserved. Printed in Great Britain

Pll:SO899-5382(98)00070-O 0699-5362196 $- see front matter

Petrology and geochemistry of the Gore-Gambella plutonic rocks: implications for magma genesis and the

tectonic setting of the Pan-African Orogenic Belt of western Ethiopia

TEKLEWOLD AYALEW’ and ANGELO PECCERILL02 ‘Department of Geology and Geophysics, Addis Ababa University, P.O. Box 1136,

Addis Ababa, Ethiopia

2Dipartimento di Scienze della Terra, Universita di Perugia, 06100 Perugia, Italy

Abstract-Major and trace element data for granitoid rocks from the Gore-Gambella

Precambrian terranes of western Ethiopia have been used to infer their petrogenetic history

and the tectonic environment in which the magmas were emplaced. The plutons are found

in the lithotectonic domains of Birbir, Baro and Geba, and were emplaced during the interval

between 830 and 540 Ma. Geochemical data show that the Neoproterozoic, pre- to syn-

kinematic metaplutonic rocks of the Birbir domain are predominantly talc-alkaline with

minor low K trondhjemitic granodiorites and tonalites. Some syn-kinematic granitoids,

occurring as sheets or lenticular bodies within high-grade metamorphic rocks of the Baro domain, are peraluminous and show a wide range of incompatible trace elements such as

Y and Zr. The post-kinematic plutons are enriched in K and incompatible elements relative

to older intrusives, suggesting a shoshonitic petrochemical affinity.

Petrological and geochemical data show that the bulk of the pre- to syn-kinematic rocks

represent mantle-derived talc-alkaline and arc-tholeiitic magmas. Major and trace element

variations suggest a multistage, probably polybaric evolution for these intrusions. The syn-

kinematic granitoid lenses of the Baro domain have petrological and geochemical

characteristics that suggest a genesis by crustal melting. The late- to post-kinematic plutons

are probably related to the melting of talc-alkaline continental crust, followed by fractional

crystallisation or AFC.

Overall, the plutonic activity reflects the geotectonic evolution of the region through time.

The pre- to syn-kinematic rocks from the Birbir domain are compositionally akin to those

generated by subduction in modern magmatic arcs; they belong in part to a low K series

and are interpreted as emplaced in an intraoceanic island-arc environment. The occurrence

of greywacke turbidites and rare carbonates of submarine deposition supports this

hypothesis. The late- to post-kinematic intrusions possess geochemical signatures that

imply a role for both subduction-related and intraplate components. o 7998 Elsevier Science

Limited. All rights reserved.

Resume-Les elements majeurs et traces avec des granitoides provenant des terrains

precambriens de Gore-Gambella en Ethiopie occidentrale on Bte utilises afin d’inferer leurs

histoires petrogenetiques et I’environnement tectonique dans lequel les magmas se sont

mis en place. Les plutons se situent dans les domains lithotectoniques de Birbir, de Baro et

de Geba, et se sont mis en place dans I’intervalle 830 a 540 Ma. Les donnees geochimiques

indiquent que les roches metaplutoniques pre- a syn-cinematiques du domaine Birbir sont

principalement calco-alcalines avec des quantites mineures de granodiorites trondhjemitiques

et de tonalites. Quelques granitoides syn-cinematiques, en feuillets ou en lentilles au sein

des roches tres metamorphiques du domaine de Baro, sont hyperalumineux et presentent

une gamme dtendue de teneurs en elements traces incompatibles tels que le endue de

teneurs en elements traces incompatibles tels que le Y et le Zr. Les plutons post-cinematiques

sont enrichis en K et en elements incompatibles par rapport aux intrusions anciennes,

suggerant une affinite shoshonitique.

Journal of African Earth Sciences 397

T. AYALEW and A. PECCERILLO

Les don&es petrologiques et geochimiques indiquent que la majeure partie des roches pre-

a syn-cinematiques represente des magmas derives du manteau a caracteres calco-alcalins

et d’art-tholeiitique. Les variations des elements majeurs et traces suggerent une evolution

a plusieurs &apes, probablement polybariques, pour ces intrusions. Les lentilles de granito’ides

syn-cinematiques du domaine de Baro ont des caracteres geochimiques qui suggerent une

genese par fusion crustale. Les plutons tardi- a post-cinematiques ont evolue par cristallisation

fractionnee et assimilation de roches crustales.

D’une facon general, I’activite plutonique reflete I’evolution geotectonique de la region

dans le temps. Les roches pre- et syn-cinematiques du domaine de Birbir ont des compositions

comparables B celles generees par subduction dans des arcs magmatiques modernes; elles appartiennent a la serie “low K” et sont interpretees comme avant et6 mises en place dans I’environnement d’une ile intraoceanique. La presence de greywackes (turbidites) et de rares carbonates appuient cette hypothese. Les intrusions tardi- & post-cinematiques presentent des signatures geochimiques qui laissent supposer un r6le pour la composante lice a la subduction et une composante intraplaque. 0 1998 Elsevier Science Limited. All

rights reserved.

(Received 24 Julv 1997: revised version received 23 July 1998)

INTRODUCTION

The Precambrian basement of Ethiopia is a mosaic of high-grade gneissic domains separated by north trending belts of lower metamorphic grade. It contains volcanic and sedimentary units, sometimes highly sheared, that are cut by several intrusive bodies of diverse composition and various ages. The coexistence of such a large variety of rock types is the effect of a complex geodynamic evolutionary history, which is still poorly understood.

The Gore-Gambella region is the area in Ethiopia in which Precambrian rocks crop out extensively and are best exposed. This has prompted geological and petrological investigations aimed at contributing to a better understanding of the evolution of the Precambrian basement in East Africa. Teferra and Berhe (1987), and Moore et a/. (1987)

recognised three major structural domains in the Gore-Gambella area. These were found to be made up of gneisses, migmatites at upper amphibolite-facies, and lower grade rocks containing abundant mafic schists. Several intrusive granitoid bodies of various ages and compositions were also recognised. Textural and field evidence indicated that intrusion took place at various stages of the evolution of the Precambrian terranes.

In this paper geochemical data for granitoid intrusions from the Gore-Gambella area are reported. Results of investigations on metamorphic rocks will be reported on separately. Integrated field, petrological and geochemical data will be discussed to place constraints on the genetic and evolutionary mechanisms of the granitoids and on their tectonic significance.

GEOLOGICAL SETTING

The Gore-Gambella area is part of the western Ethiopian Shield. It consists of several types of metamorphic and igneous rocks that belong to three lithotectonic units: the Birbir, Baro and Geba domains. A geological sketch map is given in Fig. 1.

The Birbir terranes consist of an assemblage of mafic to felsic intrusive and extrusive rocks, and of mostly volcanogenic sediments, which are metamorphosed to the low amphibolite- facies. Birbir is enclosed between the Baro and Geba domains, which are made of upper amphibolite-facies ortho- and paragneisses and migmatites. The domain boundaries exhibit low angle structural discordance on a regional scale and there is apparent transposition from east- west trends in the Geba domain towards concordance with the Birbir domain boundary. The Birbir domain is a major transcurrent shear belt, in which rock units have been mylonitised to various degrees (Moore et a/., 1987; Ayalew, 1989).

The broad division into domains is based both on metamorphic grade and on primary lithology. The Baro and Geba domains are characterised by medium- to coarse-grained quartzofeldspathic gneisses and migmatites that do not show evidence of primary sedimentary or volcanic features. For the most part they lack muscovite; sillimanite is present with potassic feldspar in aluminous rocks and clinopyroxene accompanies dark coloured hornblende in mafic rocks. These indicators are typical of upper amphibolite-facies metamorphism. In contrast, rocks of the Birbir domain are typically fine-grained schists that locally exhibit sedimentary and volcanic

398 Journal of African Earth Sciences

Petrology and geochemistry of the Gore-Gambella plutonic rocks

Q= Quaternary; TV= Tertiary volcanics

Late to post-kinematic granite

Late-to post kinematic gabbro & horneblendiie

Syn-kinematic metaleucogranite

PrelSyn-kinematic pluton

-m Meta- sedimentaryIvolcanic schist

Trace of antiform axis 0 5Km

- domain boundary

Figure 1. Geological sketch map of the Gore-Gambela area. Legend: mgb: metagabbro; mqd: metaquartz diorite; mtn: metatonalite; mgd: metagranodiorite; mg t: metagranite; gb: gabbro; gt: granite; mss: metasedimentary schists; mvs: metavolcanic schists; cgs: talc-silicate gneiss; gsg: garnet-sillimanite and garnet-cordierite-gedrite gneiss; qms: quartz- muscovite schists; msg: muscovite schists and gneiss; hgb: hornblende-biotite gneiss and amphibolite; big: biotite f hornblende gneiss.



structures. Muscovite is abundant in rocks of suitable composition; chlorite was locally stable and plagioclase is sodic even in mafic rocks. These features are all indicative of a lower regional metamorphic grade than the adjacent domains - in the low amphibolite-facies. Some more detailed information on the structural and metamorphic events in the area are discussed elsewhere (Ayalew et al., 1987, 1990; Ayalew, 1989).

META-INTRUSIVE AND INTRUSIVE ROCKS

The intrusive rocks can be grouped according to the degree of deformation and the contact relationships with the country rocks into:

i) pre- to synkinematic; and ii) late- to post-kinematic.

Pre- and syn-kinematic plutonic units The pre- and syn-kinematic plutonic units crop out in the Birbir and Baro domains. The Bure Pluton forms an elongated mass, well-exposed southwest of the Bure Village (Fig. I). The pluton consists of hornblende metagabbro and diorite with minor lenses of talc, chlorite and amphibole schists.

Along strike to the north of the Bure Gabbro there is a strongly deformed and metamorphosed body, the Haya River Pluton, made up of tonalite and granodiorite.

The Goma Pluton mostly consists of massive medium-grained granodiorite that contains narrow shear zones along its western margin. The contact, where observed, is sharp and the pluton cuts the regional foliation along its northern contact.

The rocks of the Birbir intrusive complex occupy the western half of the Birbir domain and are deformed to varying degrees. Lenticular bodies of metaquartz diorite, with subordinate gabbro and diorite, alternate with narrow, heterogeneous mylonitic units. Three strongly sheared intrusive sheets, 0.5 to 3 km wide, alternate with rock units made of predominantly mafic schist and mylonite. The intensity of shearing varies over a few metres, and strongly sheared rocks contain less deformed zones, a metre to tens of metres wide, which retain igneous textures. East of the confluence of the Baro and Birbir Rivers, the quartz diorite forms thin layers of variably sheared rocks, less than a metre to several hundred metres thick, that alternate with abundant, variably deformed, mafic and felsic dykes and sills. To the west, the unit is more homogeneous and mylonitisation

is less severe than in other parts of the intrusive complex. The quartz diorite contains elongated enclaves of microdiorite.

U-Pb zircon ages of 828 + 91-2 and 814+ 2 Ma were obtained for the pre-kinematic plutons of the Birbir Quartz Diorite and Goma Granodiorite, respectively. Younger Rb/Sr whole rock isochron ages indicate major episodes of isotopic homo- genisation, probably related to metamorphic and deformation events (see Ayalew et al., 1990).

Petrographically, the rocks of the Bure Pluton show hypidiomorphic granular texture with dominant subhedral to euhedral plagioclase

(An 60_70), 0.5 to 4 mm in length. Hornblende is the principal mafic mineral and in some cases reaches up to 60% of the mode. Both plagioclase and hornblende are commonly recrystallised and form granular aggregates, although euhedral- subhedral crystals are still preserved.

The Birbir rocks consist of metamorphosed hornblende-biotite quartz diorite. The rock is composed of andesine-oligoclase, quartz, biotite, K-feldspar and variable amounts of hornblende and epidote. Accessory minerals include euhedral apatite, titanite and zircon.

The Haya Pluton appears similar in outcrop to the Birbir Quartz Diorite, but contains more quartz and is typically less mafic, with less hornblende relative to biotite. It is mainly composed of tonalite with subordinate granodiorite and granite. It locally shows pervasive foliation and compositional layering, consisting of lamellar aggregates of biotite and muscovite alternating with granoblastic aggregates of plagioclase (An,,.,,) and quartz, minor microcline and traces of garnet.

The Goma Pluton consists of subhedral- granular, rarely blastoporphyritic granodiorite containing plagioclase (An,,.,,), K-feldspar, quartz and minor biotite. Fine-grained spheroidal mafic xenoliths, 20 cm to 0.5 m in size, with sharp margins and no tectonic fabric, and clots of mafic mineral occur locally along the eastern margin.

A particular class of syn-kinematic intrusions is made up of small bodies or large lenses of leucocratic rocks that are elongated parallel to regional trends and are enclosed in the high- grade metamorphic rocks of the Baro domain. These consist of muscovite, garnet-bearing granites and leucogranites. Scattered samples of garnet-muscovite granites from several outcrops and the Baro leucogranites have been sampled and studies in this work. U-Pb zircon age determination on the Baro leucogranites gave a value of 783 + 19/-14 Ma (Ayalew et al., 1990).



Garnet-muscovite granites are generally pale reddish-brown and medium-grained. Pegmatitic and aplitic varieties have also been observed. The rock contains quartz, microcline-perthite, oligoclase (An,,), muscovite and garnet. Muscovite forms euhedral isolated plates; garnets occur both as discrete euhedral grains up to 2 mm in diameter and as small polycrystalline aggregates scattered throughout the rock.

The Baro Leucogranite is pink to light gray, medium-grained, generally equigranular and locally contains enclaves of garnet-bearing quartz- feldspathic gneiss. It is strongly foliated along its margin, although relict subhedral-granular textures are preserved in its interior. It is composed of plagioclase (An,,), microcline perthite and quartz, with minor biotite and hornblende.

Late- to post-kinematic plutons The Mao Pluton is an oval-shaped granodiorite body that transects map-scale layering and foliation in the Birbir Shear Zone. It is mainly massive but locally exhibits well-developed shear bands along its eastern margin. It contains elongated dioritic enclaves, and is cut by aplitic dykes and quartz veins.

The Bonga Pluton is one of three undeformed, discordant granite stocks that lie along the boundary between the Birbir and Baro domains, north and south of the Baro River, east-northeast of Bonga (Fig. 1). Field evidence suggests that it comprises two nested intrusions, with quartz monzonite enclosed by hornblende-biotite granite. Intrusive relationships indicate that the quartz monzonite core is older than the enclosing granite. In the north central part of the study area, relatively undeformed gabbro and smaller bodies of hornblendite occur. These are grouped, together with more deformed mafic rocks, in unit (gb). U-Pb zircon ages of post-kinematic plutons was found to range from 571 + 1 l/-3 Ma and 541 + IO/-l 6 Ma (Ayalew et al., 1990).

The rocks of the Mao Pluton are medium- grained, buff-pink to light grey and in thin section show a porphyritic to subhedral-granular texture, in which large (5-6 mm long) subhedral to anhedral microcline and subhedral plagioclase (An ,_I crystals are surrounded by a matrix of fine-grained plagioclase, microcline, quartz, biotite and minor hornblende. The eastern margin is typically coarser-grained; it contains large feldspar augen, up to 1 cm in diameter, surrounded by anastomosing biotite lamellae.

The rocks from Bonga Pluton are pink in colour and show porphyritic texture with large K- feldspar phenocrysts set in a medium-grained

subhedral granular groundmass; the predominant mineral is microcline perthite, with quartz, plagioclase (An,,), biotite and hornblende. Accessory minerals include allanite, apatite, titanite and zircon.

PETROLOGY AND GEOCHEMISTRY

Seven plutons from the Gore-Gambella area have been considered for the present study. They are representative of:

i) pre- to syn-kinematic mafic and intermediate intrusive units;

ii. syn-kinematic leucogranite sheets; and iii) late- to post-kinematic granitic rocks. Major and some trace elements were

determined in 107 rock samples. Selected data are given in Table 1.

Analytical methods Major elements as well as Ba, Zr, Sr, Rb, Y, Nb, Ni and V were determined by XRF spectrometry on fused glass pellets using a Philips PW 141 O/ 20 AHP instrument at the University of Ottawa, Canada. Glass pellets were prepared according to the methods of Norrish and Hutton (I 969). Abundances of the rare earth elements as well as SC, Co, Sb, Cs, Hf, Ta, Th and U for selected samples were measured by instrumental neutron activation analysis in Ecole Polytechnique in Montreal, Canada. Precision is better than 10% for all the analysed elements. Analytical results are given in Table 1 where Sr isotope ratios measured by Ayalew et al. (1990) for some samples are also reported.

Classification The rocks have been classified according to both their mode and chemical composition. The plutonic rocks vary considerably in modal composition. Group (I) rocks range from diorite through quartz diorite to granodiorite; rocks of Groups (2) and (3) fall in the granite field (Streckeisen, 1976). Because rocks from Groups (I 1 and (2) are metamorphosed, a classification based on the proportions of normative minerals has been also attempted. Classification in the normative Ab-An-Or compositional data indicate that the Bure Pluton ranges from mafic gabbro to diorite, the Haya Pluton falls in the tonalite and monzotonalite fields, the Birbir data cluster predominantly in the granodiorite and quartz diorite fields and the Goma data in the granite and trondhjemite fields. Data from Groups (2) and (3) largely fall in the granite field, although a few are in the quartz monzonite field.

Journal of African Earth Sciences 40 1

Tabl

e 1.

M

ajor

an

d tra

ce

elem

ent

and

mea

sure

d (m

l S

r is

otop

e da

ta

on

sele

cted

G

ore-

Gam

bela

gr

anito

id

rock

s

Sam

ple

SiOz

TiO

,

A12

03

FeOt

M

nO

M@

Ca

O

Na20

W

p20,

BURE

BI

RBIR

1D

5D

6D

8D

7D

4D

1H

11

G 7H

8H

4H

5H

2H

3H

10

H

46.1

7 47

.27

49.5

8 51

.70

54.1

5 55

.29

55.2

2 56

.64

56.9

5 57

.02

57.4

0 58

.00

58.5

3 58

.89

59.0

8

1.06

0.

07

0.63

0.

97

0.87

1.

23

0.91

0.

69

0.89

0.

75

0.82

0.

78

0.68

0.

80

0.82

17.7

5 22

.13

10.3

4 17

.22

17.0

1 16

.91

18.9

6 16

.78

17.3

3 17

.33

17.2

5 17

.55

16.5

7 17

.23

16.9

6

11.8

7 4.

52

11.7

7 8.

56

9.79

8.

29

7.58

8.

77

8.94

8.

25

7.99

7.

38

7.56

7.

45

7.43

0.18

0.

06

0.23

0.

17

0.17

0.

15

0.14

0.

18

0.15

0.

15

0.14

0.

13

0.14

0.

13

0.13

6.60

10

.11

11.8

6 5.

61

4.70

5.

27

3.61

4.

35

3.09

3.

77

3.37

3.

56

3.42

3.

32

2.81

12.3

1 9.

90

12.3

9 11

.41

8.44

6.

54

7.10

7.

79

6.76

7.

10

6.54

6.

54

5.98

5.

61

5.91

2.23

2.

64

1.48

3.

32

3.57

3.

94

4.67

3.

54

3.57

3.

53

3.57

3.

78

3.64

3.

63

3.54

0.55

0.

37

0.43

0.

18

1.03

1.

61

1.38

1.

48

2.02

1.

83

2.14

1.

91

2.21

2.

45

2.50

0.11

0.

01

0.03

0.

16

0.17

0.

21

0.31

0.

24

0.22

0.

11

0.15

0.

18

0.09

0.

21

0.19

C

S Rb

Ba

Sr

Zr

Y Nb

Zn

Ni

V SC

co

Sb

La

Ce

Nd

Sm

Eu

Tb

Dv

Ho

Er

Yb

Lu

Hf

Ta

Th

U

3 24

6 99

9 10

2 17 3

84

25

364

0.83

1 8

3 14

7 18

0 57

3 41

2 8

41

3 22

0 74

87

6 95

23 5

90

120

158

13

432

260 80

25

45

105

372

109

28

141

114 9

254

21

453

633

124 19

5 83

69

12

9 19

.9 29

0.08

14

.7 31

15.9

3.

7 1.

15

0.55

3.

7 0.

72

1.6

1.7

0.29

2.

6 0.

6 2.

8 1.

06

0.85

28

633

959

203 23

4 10

3 35

137 18

17

.8

0.2 23

50

25

5.

4 1.

47

0.8

4.4

0.82

2.

3 1.

94

0.32

4.

1 0.

5 1.

25

0.68

27

49

44

676

574

591

898

489

454

87

154

150

17

28

30

4 5

5 64

86

49

8

6 10

18

1 18

5 17

8

52

680

480

140 27

6 86

50

54

70

506

593

775

523

500

466

152

117

132

23

25

24

5 5

8 89

83

92

40

14

34

15

5 13

1 14

3

61

677

477

3 b 16

3 is

29

&

5 2

71

s

20

2 14

5 11

3 4 I: g 6

s7Sr

PsSr

(m)

0.70

454

0.70

723

0.70

646

0.70

706

0.70

671

0.70

728

0.70

742

0.70

725

A co

mpl

ete

list

of d

ata

is a

vaila

ble

from

th

e se

nior

au

thor

on

req

uest

. -:

not

dete

rmin

ed.

Isot

opic

ra

tios

are

from

Ay

alew

er

a/.

(199

0).

I aD

re

I. co

nttn

uea

Sam

ple

7G

6G

9G

SiO

, 59

.40

60.1

2 60

.27

BIR

BIR

G

OM

A

4G

3G

111

5G

12G

8J

1J

3J

2J

25

J 6J

21

J 60

.88

61.7

5 61

.77

62.0

8 63

.89

67.2

1 67

.38

67.4

2 67

.57

67.6

9 68

.34

68.5

2

TiO

,

Al2

03

FeO

t M

nO

M9G

C

aO

Naz

O

K20

p205

cs Rb

Ba

Sr Zr

Y

Nb

Zn

Ni

V SC

co

S

b La

Ce

Nd

Sm

E

LI

Tb

BY

H

o E

r Y

b LU

H

f

Ta

Th

U

0.59

17.3

0

7.20

0.

12

2.63

6.

1 1

3.80

1.79

0.13

44

790

639

119 21

3 74

7

124

0.56

0.

67

0.65

0.

70

0.76

16.8

9 16

.63

17.0

1 17

.09

16.4

7

7.28

7.

35

7.41

7.

36

6.59

0.

13

0.13

0.

13

0.13

0.

12

2.74

2.

72

2.53

2.

55

2.51

5.

94

5.62

5.

26

5.34

5.

23

3.36

3.

58

3.56

3.

37

3.65

2.05

2.

36

2.72

2.

49

2.70

0.13

0.

17

0.13

0.

16

0.16

1.59

2.

5 42

826

574

106 23

2 71

125

51

1091

518

124 32

86

120

63

1306

53

2 17

3 29 1

73

145

59

1328

54

6 21

2 28 2

76 3

120

16.9

15

.4

0.11

19

.2

41

22

4.8

1.23

0.

75

4.5

0.79

2.

8 2.

3

0.39

4.

3 0.

34

_ I

1.16

0.

58

74

746

420

222 32

4

ai 9

99

16.3

13

.2

0.42

24

53

25

5.

1 1.

17

0.85

5.

5 0.

98

2.4

2.7

0.47

5.

4 0.

76

5.9

2.1

0.61

0.

53

16.9

4 16

.46

6.28

5.

61

0.11

O

.lC

2.06

1.

98

4.97

4.

67

3.80

3.

61

2.62

2.

79

0.14

0.14

64

1666

591

186 24

68

88

69

1068

49

7 24

1 29 1

65

12

104

0.49

0.

59

0.47

0.

49

0.45

0.

47

0.45

15.9

1 15

.30

16.0

5 15

.55

15.6

2 15

.51

15.1

5

3.55

4.

31

3.47

3.

55

3.41

3.

30

3.30

0.09

0.

11

0.09

0.

09

0.08

0.

09

0.09

0.96

1.

22

0.94

0.

99

0.91

0.

92

0.92

2.47

2.

38

2.36

2.

30

2.32

2.

29

2.19

5.50

5.

13

5.53

5.

18

5.34

5.

36

4.83

3.02

3.

13

3.05

3.

23

3.07

3.

09

3.42

0.13

0.

18

0.13

0.

13

0.12

0.

15

0.11

45

ii08 34

4 27

5 26 9

32

39

56

47

1093

11

48

304

326

348

266

33

32

14

10

79

57

25

28

47

49

1218

11

49

319

324

291

281

25

27

11

13

54

30

37

28

0.84

42

12

71

331

290 28

a 41

12

20

6.

8 4 0.

15

25

50

23

4.6

1.2

0.7 4

0.83

1.9

2.7

0.44

6.7

0.92

3.

8 1.

59

*7S

r/s6

Sr(

m)

0.70

588

0.70

608

0.70

632

0.70

694

0.70

648

0.70

7281

0.

7079

0 0.

7095

5 0.

7083

4 0.

7085

7 0.

7083

8 0.

7075

3 0.

7094

6

A c

ompl

ete

list

of d

ata

is a

vaila

ble

from

the

sen

ior

auth

or o

n re

ques

t. -:

not

det

erm

ined

. Is

otop

ic r

atio

s ar

e fr

om A

yale

w

et a

l. (1

990)

.

Tabl

e 1.

con

tinue

d

Sam

ple

GO

MA

HAYA

4J

23J

5J

8A

5A

7A

1A

61

SiOl

69

.35

69.7

1 69

.74

TiO

z 0.

52

0.35

0.

44

403

14.8

0 14

.48

15.1

1

FeOt

3.

49

2.69

3.

24

MnO

0.

09

0.07

0.

08

M9O

0.

87

0.76

0.

83

CaO

2.05

1.

98

2.05

Na,O

5.

09

4.52

4.

85

K2O

3.

49

3.47

3.

53

p2°5

0.

19

0.08

0.

12

cs

Rb

Ba

Sr

Zr

Y Nb

Zn

Ni

V

55

67

68

1125

10

87

1035

27

4 27

9 28

1 32

6 21

3 28

3 27

19

27

11

6

8 90

48

66

1

8 34

24

24

SC

co

Sb

La

Ce

Nd

Sm

EL

I Tb

Dv

Ho

Er

Yb

Lu

Hf

Ta

Th

U 87Sr

/86S

r(m)

0.70

989

0.71

070

0.71

027

69.6

5 70

.96

71.1

8 71

.40

73.5

0.17

0.

18

0.07

0.

17

0.1

16.0

4 17

.41

14.2

4 16

.35

14.9

1.76

1.

66

1.21

1.

85

1.3

0.06

0.

06

0.05

0.

06

0.0

0.70

0.

64

0.32

0.

49

0.5

4.32

4.

57

2.35

3.

76

2.8

5.07

4.

88

4.45

5.

00

4.6

0.96

0.

99

1.54

1.

06

1.6

0.06

0.

05

0.02

0.

05

0.0

13

13

443

548

634

639

83

99

6 7

35 4

56 3

12

31

1019

42

9 59 7 3

54

0.27

16

409

497 78

8 2 36

9 2.

8 2.

4 0.

1 9.

6 17

.2

7.5

1.44

0.

46

0.18

1.

4 0.

18

0.72

0.

6 0.

1 2.

3 0.

28

1.5

0.27

2 56

37

5 3

4 3 1 2 5 6 6 3 3 2 3 0 7

BAR0

LE

UCOG

RANI

TES

8F

SF

5F

IF

13M

IO

M

12M

73.0

1

0.34

13.6

1

3.53

0.

09

0.15

0.

96

4.09

5.43

0.01

0.2 69

51

5 62

726 53

20

15

5

4.1

1.11

0.

1 10

4 21

5 96

17.4

1.

68

2.4

12.3

2.

1 5.3

5.7

0.98

15

.6

1.28

12

.4

1.82

74.7

0 77

.55

75.5

6 73

.76

74.3

5 74

.61

0.24

0.

12

0.10

0.

01

0.01

0.

01

13.3

2 11

.41

12.2

5 14

.68

14.4

8 14

.68

2.63

2.

71

1.59

1

.Ol

0.92

0.

99

0.06

0.

03

0.04

0.

32

0.27

0.

32

0.09

0.

02

0.17

0.

17

0.06

0.

18

0.63

0.

12

0.45

0.

74

0.56

0.

67

4.11

2.

27

3.88

4.

11

4.87

4.

19

5.46

6.

58

5.00

4.

14

4.44

4.

14

0.01

0.

01

0.01

0.

01

0.02

0.

04

66

137

90

101

469

297

215

94

43

80

15

24

499

841

162

25

41

12

30

80

13

0 9

0 80

10

1 46

7

122

109 31

27

72

20

100 91

3 20

b

35

s

86

3

25

B P 6

8 :: g 6

A c

ompl

ete

list

of

data

is

ava

ilabl

e fro

m

the

seni

or

auth

or

on

requ

est.

-: no

t de

term

lned

. Is

otop

ic

ratio

s ar

e fro

m

Aya

lew

et

al.

(199

0).

Tabl

e 1.

co

ntin

ued

Sam

ple

SiOz

LEUC

OGRA

NITE

S BO

NGA

11M

4M

BL

6K

10

K 8K

1l

K 7K

4K

75

.41

75.5

9 68

.21

74.3

8 75

.63

76.0

5 76

.53

76.7

0 79

.10

Ti02

A12

03

FeOt

M

nO

M9O

Ca

O

Na20

K20

p2°5

0.01

14.5

4

0.72

0.

11

0.04

0.

41

4.48

4.39

0.04

0.76

0.

14

13.5

9 13

.38

6.05

2.

33

0.10

0.

03

0.78

0.

09

2.29

0.

85

3.48

3.

37

4.76

5.

78

0.35

0.12

0.11

0.10

0.

15

0.10

13.1

5 12

.51

12.6

1 13

.56

10.4

1

1.93

1.

98

1.62

2.

27

1.72

0.

03

0.03

0.

02

0.03

0.

03

0.11

0.

07

0.09

0.

08

0.09

0.

96

0.77

0.

73

0.81

0.

69

3.60

3.

38

3.29

3.

40

2.74

5.17

5.

07

5.35

5.

94

4.14

cs

Rb

Ba

Sr

Zr

Y Nb

Zn

Ni

V SC

co

Sb

La

Ce

Nd

Sm

Eu

Tb

Dv

Ho

Er

Yb

Lu

Hf

Ta

Th

U

194

112 10

30

16 6 13

94

125 63

48

35

1.32

10

9 91

4 23

0 39

5 77

26

112

0.83

14

6 11

8 11

9 26

1 24

0 17

5 37

45

32

20

5 20

7 19

3 70

53

45

17

12

12

56

58

56

6 11

7.

8 10

.3

0.32

7.

4 0.

1 0.

17

15.7

10

3 29

22

4 12

.5

100

2.4

18.5

0.

22

2.6

0.56

2.

8 4.

6 14

.7

1.13

2.

9 3.

3 7.

3 4.

3 7.

6 0.

72

1.22

2.

3 11

.1

0.43

2.

2 4.

1 13

.2

1.6

2.9

2 6

1.21

0.

69

0.09

51

12

0 57

11.4

0.

75

1.6

8.6

1.74

4 4.

4 0.

7 7.

1 1.

52

10.6

2.

6

120

275 42

19

6 37 9

27

129

92

165

294

296

928

52

38

213

239

226

359

56

52

58

10

12

28

55

49

76

1.57

17

2 96

0 22

2 38

6 62

25

58

163

184

172

904

1032

10

00

209

200

215

354

361

361

60

61

59

25

28

22

91

72

65

2 11

16

10

7.

2 4.

3 0.

08

68

142 59

11.3

1.

65

1.86

10

.7

2.16

5.

1 5.

6 0.

91

10.2

3 18

.2

2.9

3 25

27

s’Sr

PSr(m

) 0.

7854

0 0.

7640

9 0.

7764

6 0.

7657

4 0.

7466

7 0.

7565

21

0.72

220

0.72

057

0.72

181

0.72

386

0.72

110

A co

mpl

ete

list

of d

ata

is a

vaila

ble

from

th

e se

nior

au

thor

on

req

uest

. -:

not

dete

rmin

ed.

Isot

opic

ra

tios

are

from

Ay

alew

et

al.

(199

0).

MAO

60

90

50

110

70

68.9

5 69

.16

69.1

8 71

.08

70.3

3 0.

46

0.52

0.

46

0.45

0.

48

14.2

0 14

.31

14.1

2 14

.24

14.3

8 4.

20

4.24

4.

20

4.06

4.

18

0.08

0.

08

0.08

0.

08

0.08

0.

66

0.67

0.

67

0.52

0.

62

1.95

2.

14

1.90

1.

78

1.88

3.

76

3.91

3.

94

3.89

3.

73

4.97

4.

64

4.81

4.

80

4.84

0.

12

0.12

0.

16

0.10

0.

15


0 Bure X Baro leucogranite ??Bonga 0 Birbir # Garnet-muscovite granite 0 Mao A Goma

+ Haya 8 I I

X

0 A SHO

t / HKCA _mj

2 c

40 50 60 70 80

SiO 2

Figure 2. SiO, versus K20 classification diagram (Peccerillo and Taylor, 1976) for the investigated granitoids. TH: island-arc tholeiitic series; CA: c&c-alkaline series; HKCA: high K talc-alkaline series; SHO: shoshonitic series.

Fe0 total

Figure 3. AFM diagram for the investigated rocks. The field of calc- alkaline rocks is from Ringwood (19751. Symbols as in Fig. 2.

In Shand’s (1943) alumina saturation hypersthene-normative, apart from the Haya classification, based on the molecular proportions tonalites which appear to be slightly of AI,O, /(CaO + Na,O + K,O) (A/CNK), all rocks peraluminous (A/CNK about 1.04). Rocks of of Groups (I) and (3) are metaluminous and Group (2), however, are peraluminous.



I,5 I 0 I 20

0 ?? 0 - 15

l,O - OO O 0 ??0 00

0 o

TiO 2 0 0 - IO CaO 0 0

o-5 - ,B"O

B -5

25 I I I 1

0 20 0 -

0 0 ??

A',% 000 O0 ,5

0 - 0 0 w

0 0 x-2

10 - 8 0 0 o" O 0

5 I I ?I I 0

15 I I

10

FeOt

15 0' 036

"00 O

IO - 00 O,4 0 MN 0 P2O5

5- 0 00 a2 0

0 I 40 50 60 70 80

UJ 40 50 60 70 80

SiO 2 SiO,

Figure 4. Major element variation diagrams. Symbols as in Fig. 2.

The K,O versus SiO, diagram of Peccerillo and Group (3) rocks are richer in K and mostly fall Taylor (1976) (Fig. 2) shows that Bure and Haya on the limit of the field of shoshonitic series. In have low contents of K,O. In contrast, the Birbir the Haya Pluton the Na,O/K,O ratios are mostly intrusive complex and the Goma Pluton have greater than 2, suggesting trondhjemitic affinity. medium to high K,O content (2-5 wt%) and are Group (1) rocks display a typical talc-alkaline

typically talc-alkaline to high K talc-alkaline. The trend on an AFM diagram (Fig. 3). Groups (2)


T. AYALEWandA. PECCERILLO

1000 I I A

800 - X

X

600 Zr

400

200

0

Sr 600

400

200

0

40 50 60 70 80

SiO 2

0'

0

0

-l 600

500

400

300 Ni

200

100

0

400

300

200 v

100

0

0 *u - 200

@i% *

Rb

AOAW 40 50

SiO 2

Figure 5. Trace element variation diagrams. Symbols as in Fig. 2.



and (3) define a different trend that is parallel to Group (I).

Major and trace element distribution Variation diagrams of major and some trace elements are given in Figs 4 and 5 and distinct trends can be recognised. The mafic rocks of the Bure Pluton are characterised by a narrow range of silica content but show a considerable scattering of many major elements. The other intrusions mostly show smooth internal compositional variations. The Haya Pluton is characterised by low K and, to some extent, Fe and Ti, and by high Ca, Al and Na. The Goma rocks are relatively enriched in Na,O.

The Baro leucogranites and garnet-muscovite granites of Group (2) are acidic and display a rather homogeneous major element composition.

The late- to post-kinematic plutons (Group 3) have an acid composition. The Bonga Pluton has higher P, Ti and Fe than other rocks with similar silica contents.

Variation diagrams of trace elements (Fig. 5) further identify the intrusive groupings. The Bure Pluton shows a large spectrum of Sr, V and Ni. These rocks have variable, but generally low concentrations of incompatible elements, including Rb, Nb and Zr that are at a level of a few ppm in some samples. The Birbir intrusive complex shows smooth variations of several trace elements. Haya is depleted in Rb, Y, Zr and other incompatible elements, but is relatively enriched in Sr, which is unusual for acid rocks. The syn-kinematic Baro Leucogranite and garnet- muscovite granite have variable contents of several incompatible elements, such as Y, Zr and Nb. The late- to post-kinematic granitoids are enriched in Rb, Y and Nb.

Measured 87Sr/*6Sr ratios (Ayalew et a/., 1990) display a large range of values, in keeping with the variable Rb/Sr ratios (Table I). Reliable initial ratios, however, cannot be calculated for both the pre- to syn-kinematic and post-kinematic intrusive rocks. The pre- and syn-kinematic rocks have been affected by major recrystallisation and shearing events with a consequent resetting of the Rb/Sr system. On the other hand, the post- kinematic granites have very high Rb/Sr ratios, which causes calculated initial isotope ratios to change by several units on the third decimal place within the error range of measured ages. Therefore, Sr isotope ratios can only be used to gain very qualitative information on the studied granitoids.

The REE data are plotted on chondrite- normalised diagrams in Fig. 6. The patterns of

six samples from the Group (I 1 plutons (Fig. 6A) are slightly fractionated for light REE with La/ Sm, between 2.5 to 4.2 and flat to poorly fractionated for heavy REE. The Haya Pluton has low absolute REE abundances, fractionated REE (La/Lu, of about IO), poorly fractionated HREE patterns and no Eu anomaly. Rocks from different bodies of the talc-alkaline suite have extremely similar REE distributions and abundances. Some samples have a small negative Eu anomaly.

The REE data from rocks of Groups (2) and (3) differ from those of Group (I 1 in a significant negative Eu anomaly and overall higher enrichment in the LREE (Fig. 6B). The garnet- muscovite granite has lower REE abundances and a relatively flat, concave-upward pattern similar to those typically observed in some peraluminous granites (Muecke and Clarke, 1981). The Baro Granite, on the other hand, has high REE abundances and is more LREE enriched (La/Lu, = 11. I).

The incompatible element patterns (Fig. 7) of Group (I) rocks show strong fractionation and positive spikes for Ba, K and, in some cases, Sr. The Haya sample has a marked positive Sr anomaly. Negative anomalies of HFSE are observed in all the samples. The rocks from the other bodies basically have similar patterns as the Group (I ) granitoids, but show higher HFSE concentrations, negative anomalies of Sr, Ba, P, and stronger Ti depletion.

DISCUSSION

Some of the analysed rocks, especially those belonging to Group (I 1, have undergone strong recrystallisation and shearing. This may have induced elemental mobility that could ‘have modified the pristine chemical compositions of the rocks. Accordingly, the problem of possible elemental modification during metamorphism must be considered before going through discussion on petrogenesis and geodynamic significance.

Most of the intrusive bodies have shown smooth variations of several major and trace elements, which resemble typical magmatic trends. This suggests that the bulk of the rocks under study has preserved their pristine geochemical compositions. Moderate scattering of some trace elements does not conflict with this conclusion, but may simply represent the original characteristics of the magmatic bodies. These features are commonly observed in many granitoid rocks and are believed to be the effect



lllllI I I ~lllll I I I 1llllI I I 1lillI I I lllllll I )1111111 1

- m

4 10 Journal of African Earth Sciences


’ DSr= 1

k DSr= 1.5 D,= 2

100 1 I I I lllll I I IllIll I I I111111

1 10 100 1000 v

Figure 8. Log-log Sr versus V diagram for pre- and syn-kinematic intrusions. Arrows are fractional crystailisation trends. Numbers along the line represent the amount of residual liquid. Bulk partition coefficients ID) for Sr and V are reported. Rocks with Ni > 300 ppm and Sr > 800 ppm were not plotted because of possible cumulate origin. For further explanation see text. Symbols as in Fig. 2.

of several processes such as heterogeneous distribution of major and accessory phases, temperature driven diffusion processes within magmatic bodies, and occurrence of multiple intrusions of different composition (e.g. McCarty and Hasty, 1976; Hildreth, 1979; Black, 1983).

However, the Bure intrusives have shown an exceedingly large variation of several major and trace elements that calls for an alternative explanation. The high A&O, and MgO of some samples can be explained by accumulation of plagioclase or olivine. The positive correlations of MgO versus Ni and of AI,O, versus Sr (not shown) confirm this hypothesis. However, the large variations in CaO does not appear to depend on the same process, inasmuch as there is no positive correlation of Ca versus Sr or A&O,. Most probably, CaO was introduced into the rocks from outside the pluton by fluid circulation during metamorphism. The negative correlation between CaO and Na,O confirms this hypothesis, and suggests leaching of Na during metamorphism. On the other hand, there are also variable concentrations of several incompatible elements such as Y, Nb, Rb, Ba and Zr. Most of these elements are immobile during metamorphism and do not show significative correlations with Ni, Cr, Sr or CaO. There is also a well-defined positive correlation among all these elements, which all increase with increasing silica contents. Accordingly, the bulk of geochemical evidence leads to the conclusion

that the concentrations of these elements are the original for the magmatic bodies and their variations reflect evolutionary processes.

Geochemical modelling Group (I 1 rocks, with the exception of the Haya Pluton, have mafic to intermediate compositions. The mafic samples display considerable scattering for many compatible elements and moderate to very low levels of enrichment in incompatible elements. Based on interelemental variations, it has been previously suggested that some of this scattering could be the result of crystal accumulation and metamorphic mobilisation. Accordingly, those mafic rocks with high concentrations of Ca, Mg, Ni, Cr and Sr, which could represent cumulates or geochemically metasomatised rocks, will not be considered in the geochemical modelling.

A log-log diagram (Cocherie, 1986) of V versus Sr for the Group (I) granitoids is shown in Fig. 8. Geochemical models have been calculated to test possible genetic relationships between different batches of magmas, as represented by the various intrusive bodies. To avoid cumulate lithologies, samples with Sr >800 ppm and Ni > 300 ppm were not plotted. The Bure and Birbir rocks plot along a single trend, with some scattering of data. The Goma Pluton has distinctively lower Sr and V than the Birbir rocks. Similar variations are observed in plots of Sr versus Ti and all the ferromagnesian trace

Journal of African Earth Sciences 4 1 I

T. A YALEW and A. PECCERILLO

700 I I

600 - Group (1) mean composition

Sr

Dsr= 3

DRb= 0.05

Rb

Figure 9. Rb versus Sr diagram for late- to post-kinematic intrusive rocks. The Mao and part of the Bonga rocks plot along a trend of batch melting of the average composition of Group (1) rocks. Instead, the most acidic Bonga rocks can be modelled by fractional crystallisation starting from the least acidic compositions. Numbers along the lines indicate the amount of liquid. For further explanation see text. Symbols as in Fig. 2

elements. These variations can be related to fractional crystallisation processes, starting from a single type of parental magma. Geochemical modelling suggests that the transition from Bure to Birbir type magmas can be explained by assuming that Sr had an incompatible behaviour during fractionation (Dsr=O.l 1. This is possible if the mafic minerals were the major fractionating phases in place of plagioclase. On the other hand, the decrease in Sr from Birbir to Goma requires that this element had a compatible behaviour during fractionation, thus requiring that plagioclase was a main separating phase. The limited role of plagioclase in the transition from Bure to Birbir may be the result of fractionation under high confining pressure and/ or at high PHzo; both factors have a role in depressing the stability field of plagioclase in magmas (e.g. Ringwood, 1975). On the other hand, the Goma Pluton may have evolved at lower pressure relative to the other Group (1) rocks.

The Haya rocks may represent the most fractionated liquids of the high pressure trend, or may be genetically independent. The geochemical modelling reported in Fig. 8 supports a genetic link between Haya and the other pre- to syn-kinematic intrusives, if a moderately compatible behaviour for Sr ID,,=1 1 is assumed. However, the Haya granitoids have lower

incompatible elements than the other rocks with the same composition. They are also depleted in Y, Nb and Ta with respect to the bulk of mafic rocks. The low Y could be explained by garnet fractionation which has high partition coefficient for this element (e.g. Arth, 1976). However, Nb and Ta are incompatible for many major rock minerals and should increase during fractionation. The same line of reasoning excludes a genesis of Haya rocks by crustal melting since their contents in several incompatible elements (Nb, Ta, LREE) are comparable or lower than both the upper and lower continental crust (Taylor and McLennan, 1985). Accordingly, the most likely hypothesis is that Haya is the product of evolution from mafic-intermediate parents that were depleted in incompatible elements. Arc tholeiites are reasonable candidates to Haya mafic parental magmas.

In conclusion, the overall magmatic framework of Group (I) can be satisfactorily understood by assuming a polybaric fractionation of arc tholeiitic and talc-alkaline magmas. Low values of initial *‘Sr/*%r calculated from whole rock Rb/Sr isochron lines by Ayalew et al. (1990) (0.70277 j10.0001 and 0.703675 +0.00003 for Goma and Birbir, respectively) support a mantle origin for the Group (1) rocks with little or no role for crustal material. The same conclusion



Rb

syn-COLG /

VAG

I / I ,/I, I I111111 I 10 100 1000

Y+Nb

Nb VAG + \ 0

syn-COLG

Figure 10. Discriminant element diagrams of Pearce et al. 119841. ORG: erogenic granites; WPG: within-plate granites; syn-COLG: syn-collisional granites; VAG: volcanic arc granites. Symbols as in Fig. 2.

has been reached for similar rock types from the southern and eastern Ethiopian basement by Teklay et a/. (1998).

Group (2) intrusives are acidic and peraluminous with variable amounts of normative corundum. Field evidence has shown that there is no sign that these rocks are associated with mafic or intermediate lithologies, but occur as single lenses or small pockets inside metamorphic terranes. The particular mode of occurrence and the basic petrological characteristics obviously points to an origin by crustal anatexis.

Incompatible versus compatible element modelling for Group (3) rocks seems to agree with the hypothesis of generation of the Group (3) rocks by melting of crustal rocks followed by fractional crystallisation or AFC (Fig. 9). The Mao and part of the Bonga rocks may be generated by batch melting of rocks with compositions as those of Group (I 1. On the other

hand, the most acidic Bonga rocks may be the result of fractional crystallisation (or AFC) starting from the least acid anatectic liquids. Initial Sr isotope ratios, although affected by large errors, crudely agree with this hypothesis. Ayalew et a/. (1990) found that *‘Sr/‘%r intercept values of whole rock Rb/Sr isochrons of Mao and Bonga are around 0.70355 + 0.0014 and 0.70578 +O.OOl, respectively. These values are close to the Sr isotope ratios of Birbir and Goma, recalculated at 540-550 Ma, i.e. at the age of post-tectonic granitoids. Therefore, the available isotopic data are not in contrast with a derivation of Bonga and Mao by the melting of previously emplaced talc-alkaline intrusive rocks.

Tectonic implications Trace element abundances and ratios may be used to empirically infer the tectonic settings of the intrusive and meta-intrusive rocks (Pearce



et al., 1984). Some discriminant diagrams are reported in Fig. 10. The data indicate that the investigated rocks define a continuous trend from typical volcanic arc to intraplate compositions. The pre- to syn-tectonic granitoids appear to be the product of magmatic arc activity (VAG), whereas the Group (3) granites mostly fall in the field of within-plate granites (WPG). The granites of Group (2) have variable compositions and straddle the boundary between VAG and WPG.

Incompatible element patterns reported in Fig. 7 confirm the conclusions inferred from discriminant diagrams. The Group (1) rocks have fractionated patterns, negative anomalies of HFSE and positive spikes of Ba and, in most cases, Sr. All these features are typical of island- arc magmatism (Gill, 1981).

The geological environment of the low K suite is uncertain. In general terms, it could be:

i) part of an oceanic island-arc succession; 7.. the product of an incipient rift representing

transitional material along an attenuated continental margin; or

iii) the upper part of an oceanic type ophiolite suite.

Considering that the K,O content is relatively high for ophiolitic rocks and that the investigated rocks have arc geochemical signatures (i.e. high LILE/HFSE ratios), an island-arc mode of development is preferred. This setting is also supported by the nature of the associated sedimentary material. The spatial coincidence of the talc-alkaline suite with the Birbir Shear Zone, and the presence nearby of metasedimentary rocks suggestive of a continental shelf environment (quartz-muscovite schist and conglomerate-qms), imply that the eastern boundary of the Baro domain marks a continental margin. The presence of predominant wackes and pelites (mss), further east in the Birbir domain, suggest an intraoceanic environment for the low K suite.

Granites of Group (2) show discriminant element characteristics that are intermediate between VAG and WPG. However, this does not necessarily imply a role of intraplate components. As previously discussed, these rocks are likely the result of crustal anatexis. Accordingly, their contents in incompatible elements depends strongly on the composition of the source rocks and on the residual mineralogy. The interplay of these two factors can give liquids that display large variations of discriminant elements. The occurrence of the Group (2) rocks as small bodies enclosed in

metamorphic rocks suggest that at some stage of the tectonic evolution of the area the peak pressure and temperature in the garnet-sillimanite gneiss unit reached minimum melting conditions for rocks of quartzofeldspathic composition. At temperatures above the muscovite-quartz stability field, partial melting in high-grade metamorphic rocks of peraluminous composition involved sillimanite, garnet, cordierite, biotite, quartz and feldspars similar to the assemblages of map unit gsg. The presence in the Group (2) rocks of pelitic schlieren, as well as substantial amounts of muscovite and the particular mode of occurrence, suggest that they have originated by partial melting of sialic crust, perhaps from the metasedimentary schists and gneisses of the Baro domain.

Group (3) rocks possess several arc signatures, as indicated by the incompatible element patterns. This testifies to a genetic link between pre- and post-kinematic granitoids and agrees with the hypothesis of generation by anatexis of previously emplaced talc-alkaline rocks. However, Group (3) also has comparatively high abundances of HFSE and low LILE/HFSE. These compositional features could be related either to partition coefficients of the residual phases during melting, as previously discussed for Group (2), or to an interaction with an HFSE-rich magma of intraplate origin emplaced during post-tectonic plutonism. It is suggested, therefore, that post- kinematic magmatism was generated in the crust during post-collisional thermal relaxation and adiabatic decompression. Crustal sources could be represented by Group (I 1 pre-collisional rocks, which would explain the arc signatures of Group (3) as suggested for modern arcs by Atherton and Petford (I 993).

REGIONAL CORRELATION OF DOMAINS AND EVENTS

Rocks of the Birbir domain can be traced northward into the Pan-African Asir, southern Red Sea Hills and Butana terranes of northeast Africa and Arabia. The lithology is similar in all these terranes and is marked by voluminous calc- alkaline magmatic rocks typical of subduction- related arc environments.

The linear ophiolitic rocks on the eastern border of the Birbir domain north of the study area are continuous northward into ophiolitic rocks of northern Ethiopia and can also be traced into the ophiolitic rocks of the Arabian Shield. South of the studied area the Birbir domain continues into the Akobo domain (Davidson, 1983), where



lenses of ultramafic rocks are also associated with low-grade metavolcanic/sedimentary facies. These features collectively support the contention that old continental crust does not extend beneath the Birbir domain, and that its magmatic history is associated with a cycle of ocean opening and closure. The progressive narrowing of the Birbir domain southward through the study area may result from primary variation in the width of an ocean basin, and/or from the amount of crustal shortening during closure.

Rocks of the Geba domain can be traced southward into the Hamar domain of the Omo River Project area (Davidson, 1983) and then into the Mozambique Belt of northwest Kenya. The lithological features of the Baro domain are sufficiently similar to the Mozambique Belt to suggest a close correlation. Field relations and chemical data of Group (2) rocks in the Baro domain imply anatexis of a sedimentary source during folding and regional metamorphism.

The ages determined by Ayalew et al. (I 990) demonstrate that part of the plutonic and metamorphic evolution of the Baro domain is synchronous with that of the Birbir domain. At least two distinct metamorphic events, with ages of 780-760 and 635580 Ma, have affected both domains. Similar age relations were found in the rocks of the Bayuda Desert of the northeast African Shield (Pohl, 1981; Kroner et a/., 1991; Stern and Dawoud, 1991) and northwest Kenya (Shackleton, 19861, respectively.

The relatively mature metaclastic rocks in the studied area (gsg, csg) and the only unit that suggests continental derivation (mqs) are near the boundary between the Birbir and Baro domains. The eastern part of the Baro domain contains granites that appear to have been derived from a sedimentary source. The Baro domain in the area may thus have been a continental shelf that was tectonically reworked in the foreland to the Mozambique Belt. Kriiner (I 977) similarly regarded the gneissic terranes to the north in Egypt and the Sudan as having undergone transformation from a passive continental margin into a tectonically active belt that was overthrust by magmatic arc and ophiolitic rocks from the east. West of the study area, in Sudan, the gneissic rocks are relatively continuous; only small areas of low-grade volcano-sedimentary (Pan-African) belts appear to be preserved (Vail, 1983). The eastern margin of this region, trending northerly at about 34OE, appears to truncate northeast trending boundaries in the Pan-African rocks and thus is

probably the former edge of the African Craton. If so, then the Geba domain is likely to be a continental fragment.

CONCLUSIONS

The intrusive magmatism of the Gore-Gambela area covers a large time interval between 830 to 540 Ma and took place in a pre- to syn- and post-collisional tectonic environment.

Pre- to syn-kinematic intrusions consist of calc- alkaline and arc-tholeiitic rocks that have been generated by polybaric fractional crystallisation starting from mantle-derived parental magmas. Some of the syntectonic granites are melts derived by anatexis of crustal rocks during the peak of metamorphism in the Baro domain.

The late- to post-kinematic plutons are slightly more enriched in K and are characterised by incompatible trace element compositions which are intermediate between typical subduction- related and intraplate granitoids.

The evolution of magmatic activity reflects the modification of the tectonic environment through time. The pre- to syn-kinematic rocks from the Birbir domain were emplaced during subduction processes occurred both in an arc and continental margin environment.

The western part of the Birbir domain consists largely of plutonic and volcanic rocks with the chemical signature of an active, Andean-type continental margin. Easten and western boundaries of the Birbir domain are highly tectonised and exhibit relatively steep metamorphic gradients; there is evidence of both extreme flattening and transcurrent shear strain. These features are consistent with the oblique accretion of a magmatic arc complex (Birbir) to the foreland (Bare) terrane. The study shows that part of the tectonic history of the western Ethiopian Shield is consistent with the Pan- African evolution determined elsewhere in the Afro-Arabian Shield. The model that emerges is not one of ensialic orogeny but rather of plate convergence involving arc accretion and ocean closure. There are no data yet that establish the original age and source of the high-grade metamorphic rocks seen in the Baro and Geba domains and elsewhere in Ethiopia. These rocks appear to be continental crust to which the arc assemblages have been accreted.

ACKNOWLEDGEMENTS

The present work is part of a project on the Ethiopian basement rocks and on the interaction


T. A YALEW and A. PECCERILLO

between the Recent magmatism of the Ethiopian Rift and the continental crust, financed by the Italian Technical Co-operation and by the Italian Ministry of Scientific Research. The critical reading of B. Barbarin and of an anonymous referee greatly contributed to improve the manuscript. The paper was written while A.P. was a visiting scientist at the Department of Geology and Geophysics of the Addis Ababa University, in the ambit of the Technical Co- operation between the Italian Government and the Addis Ababa University. Editorial handling - R. Black

REFERENCES Arth, J. C. 1976. Behaviour of trace element during magmatic

processes. A summary of theoretical models and their applications. Journal Research US Geological Survey 4, 41-47.

Atherton, M. P. and Petford, N. 1993. Generation of sodium- rich magmas from newly underplated basaltic crust. Nature 362, 14-146.

Ayalew, T. 1989. Geology, geochemistry, age and tectonic setting of the Gore-Gambella plutonic rocks, western Ethiopia. Ph. D. Dissertation 257~. Carleton University, Ottawa, Canada.

Ayalew, T., Bell, K. and Moore, J. M. 1987. Magmatic arc intrusive complexes in the Birbir Domain, western Ethiopia. In: Current Research in African Earth Sciences (Edited by Matheis, G. and Schandelmeier, H.) ppl 13-1 16. Balkema, Rotterdam/Boston.

Ayalew, T., Bell, K., Moore, J. M. and Parrish, R. R. 1990. Geochronology of the western Ethiopian Shield; Geological Society America Bulletin 102, 1309-I 316.

Black, P. 1983. Multiple intrusion - examples from the Adamello batholith (Italy) and their significance on the mecanism of intrusion. Memorie Societa Geologica Italiana 26, 145-I 57.

Cocherie, A. 1986. Systematic use of trace element distribution patterns in log-log diagrams for plutonic suites. Geochimica Cosmochimica Acta 50. 2517-2522.

Davidson, A. (Compiler) 1983. The Omo River Project: Reconnaissance Geology and Geochemistry of parts of Illubabor, Kefa, Gemu Gofa and Sidamo, Ethiopia. Omo River Project, Canadian International Development Agency, 89p.

Gill, J. B. 1981. Orogenic andesites and plate tectonics. 270~. Springer, Berlin.

Hildreth, E. W. 1979. Gradients in silicic magma chambers: implications for lithospheric magmatism. Journal GeophysicalResearch 86, 10,153-10,192.

Kroner, A. 1977. Precambrian mobile belts of southern and eastern Africa ancient sutures or sites of ensialic mobility? A case for crustal evolution towards plate tectonics. Tectonophysics 40, 101-l 35.

Kroner, A., Linnebacher, P., Stern, R. J., Reischmann, T., Manton, W. and Hussein, I. M. 1991. Evolution of

Pan-African island arc assemblages in the southern Red Sea Hills, Sudan, and southwestern Arabia as exemplified by geochemistry and geochronology. Precambrian Research 53, 99-118.

McCarty, T. S. and Hasty, R. A. 1976. Trace element distribution patterns and their relationship to the crystallization of granitic melts. Geochimica Cosmochimica Acta 40, 1351-l 358.

Moore, J. M., Morgan, J., Teferra, M. and Teklay, M. 1987. Geology of the Gore-Gambella geotraverse, western Ethiopia. In: Current Research in African Earth Sciences (Edited by Matheis, G. and Schandelmeier, H.) pp109- 1 12. Balkema, Rotterdam/Boston.

Muecke, G. K. and Clarke, D. B. 1981. Geochemical evolution of the South Mountain Batholith, Nova Scotia: Rare Earth Element evidence. Canadian Mineralogist 19, 133-l 45.

Norrish, K. and Hutton, J. I. 1969. An accurate X-ray spectrographic method for the analysis of a wide range of geological samples. Geochimica Cosmochimica Acta 33, 431-453.

Pearce, J. A., Harris, N. B. W. and Tindle, A. G. 1984. Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. JournalPetrology 25, 956. 983.

Peccerillo, A. and Taylor, S. R. 1976. Geochemistry of Eocene talc-alkaline rocks from Kastamonu area, Northern Turkey. Contributions Mineralogy Petrology 58, 63-81.

Pohl, W. 1981. Is the Kashebib Group older basement to the Greenschist assemblage? In: Newsletter, IGCP Project 164 (Edited by Al-Shanti, A. M.) Institute Applied Geology,

Jeddah 4. 44-48. Ringwood, A. E. 1975. Composition andpetrology of the

earth’s mantle. 618~. McGraw Hill, New York. Shackleton, R. M. 1986. Precambrian collision tectonics in

Africa. In: Collision Tectonics (Edited by Coward, M. P. and Ries, A. C.) Geological Society Special Publication 19, 329-352.

Shand, S. J. 1943. Eruptive rocks. Their genesis, composition, classification, and their relations to ore deposits. 444~. Wiley, New York.

Stern, R. J. and Dawoud, A. S. 1991. Late Precambrian (740 Ma), charnokite, enderbite, and granite from Jabel Moya, Sudan: A link between the Mozambique Belt and the Arabian-Nubian Shield? Journal Geology 99, 648-659.

Streckeisen, A. 1976. To each plutonic rock its proper name. Earth Science Reviews 12, l-33.

Taylor, S. R. and McLennan, S. M. 1985. The continental crust: its composition and evolution. 323~. Blackwell, London.

Teferra, M. and Berhe, S. M. 1987. Geology of sheet NC 36-16 (Gore sheet). Ethiopian Institute of Geological Surveys. Unpublished manuscript.

Teklay, M., Kroner, A., Mezger, K. and Oberhansli, R. 1998. Geochemistry, Pb-Pb single zircon ages and Nd-Sr isotope composition of Precambrian rocks from southern and eastern Ethiopia: implications for crustal evolution in East Africa. Journal African Earth Sciences 26, 207-227.

Vail, J. R. 1983. Relationship between tectonic terrains and favourable metallogenic domains in the Central Arabian- Nubian Shield. institute Mining Metallurgy Transactions. Section B, Applied Earth Science 94, l-6.


Documents

Petrology and geochemistry of the Gore-Gambella plutonic rocks: implications for magma genesis and the tectonic setting of the Pan-African Orogenic Belt of western Ethiopia