A study of metabasite and metagranite chemistry in the Adola region (south Ethiopia): implications for the evolution of the East African orogen

Pergamon Joumd of African Eanh Sciences, Vol. 21, No. 3, pp. 459-416. 1995

Copyright 8 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved

0899-536285 $9 SO + 0.W

0899-5362(95)00098-4

A study of metabasite and metagranite chemistry in the Adola region (south Ethiopia): implications for the evolution of the East African otiogen

B. WOLDEHAIMANOT and J. H. BEHRMANN

Institut fiir Geowissenschaften und Lithosphtirenforschung, Justus-Liebig-Universitit Giessen, Senckenbergstrasse 3, D-35390 Giessen, Germany

(Received 6 March 1995: revised version received 30 August 1995)

Abstract - The Adola belt forms the transition zone between the Neoproterozoic Arabian-Nubian shield and the Mozambique belt. In an attempt to assess its plate tectonic evolution, representative suites of magmatic rocks were analyzed for major and trace elements (including rare earth elements - REE). Based largely on REE and other trace element data, four tectonic environments are distinguished. These are:

i) continental basement (Awata gneiss); ii) island arc (Daba and Bursano meta-igneous complexes); iii) back-arc (Megado and Kenticha meta-igneous-sedimentary rocks); and iv) MORB (Reji amphibolite).

Boninitic meta-andesites mark the onset of back-arc tholeiitic volcanism in the Megado rocks. The recognition of MORB-type chemical characteristics from Reji makes the Adola region an important site in the East African orogen, in which a component of Neoproterozoic oceanic crust is preserved.

The rock units that characterize the different tectonic settings form narrow belts up to 15 km wide signifying shortening during collision between east and west Gondwanaland. The extensive shortening and late-stage’strike- slip components recognized in the Adola belt are consistent with the Adola region being geographically close to the Mozambique erogenic front during the Neoproterozoic. The results support the application of collision and tectonic escape models to the East African orogen.

RCsumC - La ceinture d’Adola constitue une zone de transition entre le bouclier arabo-nubique r&oprot&o- zdique et la chaine mozambiquienne. Dans un essai de caract&isation dans le cadre de la tectonique des plaques, des suites magmatiques repr&ntatives ont et6 prelev&s et analysees pour les 616ments majeurs et en traces (y compris les terres rares). A partir essentiellement des terres rares et d’autres elements en traces, quatre environne- ments gbtectoniques ont et& distingu6s. Ce sent:

i) un socle continental (gneiss d’Awata); ii) un arc ins&ire (complexes m&a-+% de Daba et de Bursano); iii) un arri&re arc (roches m&a-Sedimentaires et m&a-@&s de Megado et de Kenticha); et iv) un environnement de ride m&iio-oc&nique (amphibolites de Reji).

Des m&a-and&ites boninitiques marquent la naissance d’un volcanisme thol6iitique d’arrit?re-arc dans la s&e de Megado. La reconnaissance de caract&res chimiques de type MORB (Reji) rend la r6gion d’Adola trPs importante dans l’orogene est-africain, en representant un morceau p&serve de crofite oceanique.

Les unit& lithologiques qui caract6risent chaque environnement geotectonique forment des ceintures etroites de 15 km au plus de largeur indiquant un raccourcissement important lors de la collision du Gondwana E et W. Cet important racco urc&ement ainsi que la composante cisaillante tardive reconnue dans la ceinture d’Adola sont en accord avec une position de cette region B proximite du front orog6nique mozambiquien au tours du N&oprot&o- zolque. Ces r6sultats confortent l’application des mod&les de collision et d’&happ& tectonique pour l’orog&e est- afriCain.

INTRODUCTION

The Precambrian rocks of east and northeast Af- rica and Arabia have been the focus of geological in- vestigations for the last two decades. These studies show that zones of low-grade metamorphic rocks are common in northeast Africa and Arabia (the Arabian- Nubian shield), whereas high-grade rocks are concentrated farther south in east Africa (the Mozambique belt).

Burke and Sengiir (1986) suggested that the Ara-

459

bian-Nubian shield and the Mozambique belt resulted from the collision of two continents; the high- grade Mozambique belt representing the site of an extensive, Tibetan-style collision zone and the Ara- bian-Nubian shield a terrane of assembled arcs or mi- crocontinents, which avoided co&&n by processes of tectonic escape. This view was previously suggested by de Wit and Chewaka (1981). Subsequent workers (Key et al., 1989; Berhe, 19910; Bonavia and Chorowicz, 1992; Stem, 1994) have presented arguments and models that are consistent with the above

B. WOLDEHAIMANOT and J. H. BEHRMANN

m Gneiss complex

LEGEND

Reji amphiholite Post-tectonic granite

m Meta-igneous-sedimentary assembalges of Megado + + Daba meta-igneous (w sub-belt) and Kenticha m complex r- Tertiary basalt

(E sub-belt)

/ Bursano gneiss- amphibohte assembalge FI

, , , , , Awata gneiss F! Serpentinite

I z z i] Gariboro granite gneiss -A-A- Thrust

Figure 1. Simplified geological map of the Adola region. Modified from the geological map of Kozyrev et al. (1988) and Beraki et al. (1989). AD: Adadi Kotu; B: Bursano; KOT: Kotissa; LD: Lege Denbi (also written ‘Lega Dembi’); R: Reji; T: Tula; UU: Ula Ulo; W: Werseti. MEGADO, BUR- SANO, REJI, DABA, KENTICHA and AWATA are known localities in the region after which the main rock assemblages are named. MTZ: Megado thrust zone; Kl’Z Kenticha thrust zone. Inset map shows the approximate location of the Adola belt (squared) in relation to the Arabian-Nubian shield (stippled). C: Congo craton; Esc: East Sahara craton; MB: Mozambique belt; Rs: Red Sea.

A study of metabasite and metapnite chemistry in the Adola region 461

views. Based on the continuity of ophiolitic sutures from the Arabian-Nub&n shield to the Mozambique belt, the Arabian-Nubian shield and Mozambique belt were considered as along-strike correlatives (e.g. de Wit and Chewaka, 1981; Berhe, 1990; Stern, 1994); Stern (1994) named the whole structure the East Afri- can orogen. The Adola region is located at the transition zone between the Arabian-Nubian shield and the Mozambique belt.

Geological studies aimed at assessing the Au potential of the region have been underway for the past 20 years. Although these studies have improved the existing geological database, few systematic studies have been undertaken in the region. Except for a reconnaissance geochemical study by Gichile and Fy- son (1993), systematic geochemical and isotopic data on the Adola rocks are lacking. In an attempt to deci- pher the geochemical characteristics of the Adola belt, the authors present here the results of chemical analyses of metamorphosed magmatic and sedimentary rocks and discuss the implications of their data for the evolution of the East African orogen.

LITHOLOGICAL DESCRIPTION AND GEOCHRONOLOGY

The geological, metamorphic and strvctural evolution of the Adola belt has been described by several authors (Gilboy, 1970; Chater, 1971; Kazmin, 1972, 1976; de Wit and Chewaka, 1981; Beraki et al., 1989; Ghebreab, 1992; Woldehaimanot, 1992; Worku and Yifa, 1992).

The present work indicates that the Adola belt comprises four major groups of rocks (Fig. 1). These are:

i) the temporally and spatially related Megado and Kenticha meta-igneous-sedimentary rocks, Reji amphibolite and Bursano gneiss-amphibolite assemblage;

ii) the Daba meta-igneous complex; iii) the Awata gneiss; and id the gneiss complex.

Each of these groups of rocks are separated by major thrust zones. Of these, the Megado thrust zone and the Kenticha thrust zone (Fig. 1) are the most promi- nent deformation zones and have resulted in the formation of up to 500 m wide mylonite zones.

The rock assemblages of Megado and Kenticha constitute north-south trending sub-parallel litho- tectonic units. The main lithologies of these assemblages, which also suggest similarity to ophiolite se- quences, are very fine-grained (and commonly phe- nocryst-free) pillow and massive metabasalt (amphibolites), boninitic meta-andesites, ultramafic tectonites containing chromite pods (serpent&rite, metagabbro, metapyroxenite), intrusive complex (metadiorite, granodiorite gneiss, granite gneiss, to-

nalite gneiss), dyke(?) complex (Werseti area) and various metasedimentary rocks.

Strongly porphyritic, boninitic meta-andesites occur at the eastern margin of the Megado meta- igneous-sedimentary rocks. The consitituent minerals are actinolite and tremolite (>65% vol.), porphyro- blasts of actinolite and hornblende (CP 15%), plagioclase (up to 10%) and up to 101% of chlorite, zoisite/ clinozoisite and quartz.

The metasedimentary rocks are represented by dark-gray coloured and commonly graded metagraywacke, poorly-sorted metaconglomerate, graphitic schist, quartz-mica schist, Fe-Mn quarpite, metachert and marble. The metagraywacke and metaconglomerate lie unconformably on top of the other Megado rocks. Their composition and textural features are analogous to flysch sediments deposited in deep sea basins. They form the youngest meta$edimentary sequence in the Adola region.

The Reji amphibolite is a laterally continuous oc- currence of amphibolite that contains thin horizons of Fe-Mn quartzites at its structurally lower parts.

The Bursano gneiss-amphibolite assemblage is a distinct rock assemblage mainly made up of myloni- tic gneiss, minor amphibolite and lenses of pelitic schist.

The Daba me&igneous complex constitutes part of the Aflata Formation of Kozyrev el al. (1988). It is composed of granitic gneiss, hornblende-biotite gneiss and intercalations of amphibolite and associ- ated elastic sediments.

The Awata gneiss comprises the Zembaba, Bora and Buluka Formations of Kozyrev et ?Z. (1988), or the middle and lower groups of Gilboy (1970). Lithologi- tally, it is represented by qua&o-feldspathic gneiss, granitic gneiss, biotite-amphibole gneiss and subordinate metasandstone, amphibolite and mica schist. In addition, occurrences of staurolijte-garnet-biotite schist, marble and talc-silicate granulite have been reported by Gilboy (1970). Miginatitic biotite- hornblende-plagioclase gneiss and biotite-plagioclase gneiss occur at the bottom of the litho-structural sequence.

Except for marked, large scale, structural differ- ences, the gneiss complex (Fig. 1) shows lithological affinity to the Daba meta-igneous complex. The gneiss complex is intruded by a number of Pan- African granitic plutons with talc-alkaline affinity (Ayalew and Gichile, 1990). Detailed studies have not been done on the gneiss complex.

The available geochronological data indicate that many samples of gneisses and gneissic granites yield a whole-rock Rb-Sr age of 680 Ma; other samples of similar rock type gave ages of around 630 Ma (Gilboy, 1970). The age of intrusion of a metamorphosed tonalite, located southwest of the gneiss do- main (outside the area of Fig. 1), has been dated at

462 8. WOLDEHAIMANOT and J. H. BEHRMANN

75Ok2 Ma using U-Pb technique on zircons (Ayalew and Gichile, 1990). Pb-Pb zircon ages of metamorphosed felsic igneous rocks from the Awata gneiss range from 557 Ma to 884 Ma; zircon xenocrysts in these rocks are dated at 1125*10 Ma and 1657klO Ma (Teklay ef al., 1993). The authors suggest that the xenocrysts were scavenged from older continental crust. By inference, the older Pb-Pb ages (around 884 Ma) may have also involved zircon xenocrysts, but this should await further detailed studies. In general, the available data suggest that the Awata gneiss is older than all other rock units in the region by at least 130 Ma. The emplacement age of the post-tectonic granites range from 515 to 530 Ma (Gilboy, 1970).

GEOCHEMISTRY

Analytical Methods

Chemical analysis of representative rock samples was carried out at the geochemical laboratory of the University of Giessen. Fifty five samples were analyzed for major and trace elements using a Philips 1450 X-ray fluorescence spectrometer. Fused glass discs were used for the major element analyses, except for HzO+, COZ, and FeO; whereas pressed pow- der pellets were used for the trace elements. H20+ and CO2 were analyzed coulometrically. Fe0 was determined by manganometrical titration. Analytical uncertainties for the major and trace elements are better than 3% and 5%, respectively.

Of the 55 samples analyzed for major elements, 30 samples were selected for analysis of rare earth elements (REE) by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICI’-AES) using a Perkin- Elmer ICI’ model 5500. Complete dissolution of the powdered rock samples was achieved following a NazOz-sinter dissolution procedure. The REEs were separated and concentrated chromatographically using an ion-exchange method. The details of the analytical procedure can be found in Zuleger and Erzinger (1988). Analytical precision (with the excep- tion of Pr at concentrations ~2 ppm) is better than 5%. Representative major and trace element data are given in Tables 1 and 2.

Alteration Effects

The effects of chemical alteration due to regional metamorphism and sea water-rock interaction on the primary chemistry of rocks have been recognized for some time (e.g. Humphris and Thompson, 1978; Gillis ef al., 1992). In some cases the effect is so pronounced that chemical data from altered rocks become useless in deciphering the geochemical and petrogenetic as- pects of the rocks. Although the samples selected for this study are those with minimum observable signs

of alteration and have well-preserved primary tex- tures, it is not appropriate to assume the chemical composition of the rocks is the same as that of the protoliths. The effects of chemical alteration on the major, trace and REE contents of the Adola rocks are discussed later.

Following the observation that Zr varies system- atically with igneous processes, such as fractional crystallization and partial melting, and that it remains relatively immobile during greenschist facies metamorphism (e.g. Pearce and Cann, 1973), plots of Zr against major and other trace elements have been employed to investigate chemical alteration.

The concentrations of Ti02, Y and to a lesser extent R205 in the mafic volcanic rocks from the Adola region correlate positively with Zr. Moreover, the concentrations of FeO’, MgO, Rb and Sr of Reji and the concentrations of Ba in samples from Daba correlate strongly with Zr. These features suggest that these elements have not been detectably affected by post-crystallization alteration processes. On the other hand, NazO, K20, Rb, Sr, Ba and Nb from the different rock assemblages do not show any systematic correlation with Zr and the deviation from a well defined trend possibly indicates alteration after crystallization. The authors will closely examine later whether these deviations are due to secondary alteration processes, or some other processes such as crystal fractionation.

Representative, chondrite-normalized REE patterns are shown in Fig. 2. Except for low concentrations of the light rare earth elements (LREE) in samples ET1 and ET8 (Fig. 2a), which produce LREE depletion, no erratic enrichment or depletion patterns are observed. The lack of erratic values in the REE patterns suggests that no major selective depletion or enrichment has taken place.

Valsami and Cann (1992) suggest that intense hydrothermal alterations result in positive Eu anomalies. Although positive Eu anomalies are present in the mafic samples (average Eu/Eu*=1.2), the magni- tude of the anomalies are small and cannot be as- cribed solely to hydrothermal alteration. In addition, lack of significant La and/or Ce anomalies suggest that no dramatic chemical changes have resulted due to hydrothermal or sea-floor alteration (Menzies et al., 1977; Valsami and Cann, 1992).

Another test of alteration of the REE was per- formed by plotting whole-rock data on a Ti02 versus (La+Sm+Yb) diagram (Fig. 3) as described by Coish ef al. (1982). Sympathetic variation of the REE with the relatively immobile TiO2 implies that the REE contents of these rocks reflect igneous processes.

Comparison of REE patterns of the Megado metagraywackes with the Australian Post-Archean Aver- age Sediments (PAAS; Name and Taylor, 1976) shows excellent correlation (Fig. 2~). Such similarities

Tabl

e 1.

Maj

or a

nd t

race

ele

men

t da

ta o

f re

pres

enta

tive

sam

ples

fro

m t

he A

dola

reg

ion

hmpl

e no

LA

O.

Sior

Ti

02

Fe0

F&3

Mno

MgO

C

aO

Na2

O

W

Pzos

C

o1

H20

’ To

tal

Cr

Ni

cu

Zn

Ga

Rb Sr

Y Zr

Nb Pb

Th

Ba

Maj

or e

ET1

E

T2

ET3

E

T4

ET5

B

as

Bas

B

as

BaS

B

as

47.4

46

.6

50.3

49

.3

50.7

ET6

E

T7

Bas

B

as

49.6

50

.9

ME

GA

DO

E

T8

ET9

E

T10

ET1

1 E

T12

Gab

G

ab

Bon

B

on

Bon

49

.6

50.0

53

.4

57.7

53

.8

ET1

3 B

And

53

.9

ET1

4 E

T15

ET1

6 E

T17

ET1

8 E

T19

Dio

D

io

Tgn

Serp

Se

rp

Gtg

n 62

.8

56.1

66

.1

41.8

40

.2

74.3

0.

10

0.22

0.

37

1.15

0.

35

0.40

0.

34

0.12

0.

69

0.11

0.

24

0.23

1.

11

0.57

0.

34

0.41

0.

03

0.08

0.

33

16.5

18

.2

14.9

13

.9

15.3

15

.7

15.2

15

.1

12.8

12

.0

11.7

14

.4

13.5

13

.2

16.7

14

.8

0.75

2.

54

11.9

3.

22

9.72

7.

2 8.

36

6.82

7.

43

7.61

5.

77

10.3

8.

88

6.28

6.

54

8.81

7.

63

8.48

5.

36

4.4

1.19

3.

49

2.30

3.

46

1.99

1.

89

2.38

3.

46

1.39

1.

26

2.45

3.

23

2.04

2.

88

1.19

3.

38

2.59

1.

76

12.0

4.

44

2.02

0.

11

0.23

0.

46

0.18

0.

17

0.2

0.17

0.

16

0.22

0.

29

0.17

0.

23

0.34

0.

23

0.21

0.

19

0.08

0.

01

0.13

10

.7

6.37

10

.0

9.15

9.

15

7.85

8.

62

11.5

6.

89

8.87

9.

36

6.71

9.

92

1.97

3.

77

1.39

29

.7

39.1

0.

86

17.3

12

.6

11.9

12

.9

12.6

12

.1

12.3

14

.2

10.8

11

.4

10.6

13

.4

5.93

8.

68

9.88

8.

18

4.35

0.

01

4.89

0.

35

1.04

0.

95

1.74

1.

48

1.95

2.

10

0.92

2.

04

0.69

0.

74

0.48

1.

97

0.7l

1.

41

1.24

bd

bd

2.

27

0.02

0.

07

0.08

0.

11

0.06

0.

04

0.09

0.

02

0.05

0.

07

0.10

0.

28

1.43

0.

05

0.19

0.

28

bd

bd

0.06

0.

02

0.04

0.

06

0.12

0.

05

0.05

0.

05

0.02

0.

06

0.02

0.

10

0.03

0.

14

0.31

0.

05

0.11

0.

01

0.01

0.

12

0.07

0.

03

0.29

0.

10

0.06

0.

05

0.07

0.

06

2.30

0.

03

0.03

0.

05

0.14

0.

02

0.05

0.

10

0.08

0.

06

0.04

2.

00

1.58

1.

35

1.53

1.

52

1.50

1.

46

1.73

2.

09

1.70

1.

58

1.48

1.

98

0.96

1.

16

0.80

6.

95

11.9

0.

52

100.

1 10

0.2

99.9

10

0.4

100.

6 10

0.3

100.

3 10

0.5

100.

7 10

0.7

100.

6 10

0.5

100.

4 10

0.5

100.

9 10

0.7

100.

2 99

.5

100.

9 26

0 38

69

2 29

4 32

8 25

3 27

5 30

3 87

30

3 65

8 70

9 38

9 20

33

20

21

52

1242

13

17

l 23

18

9 12

8 98

61

79

12

4 67

55

63

10

7 27

7 1

15

5 13

44

2026

2

37

41

43

45

84

25

19

164

7 29

10

43

11

2 10

7

1 8

bd

5 32

88

13

0 69

64

78

63

41

%

13

7 71

76

93

82

10

0 80

35

21

54

8

15

12

16

10

12

11

9 11

9

10

12

17

13

12

15

bd

1 12

1

2 2

3 3

2 3

2 2

2 2

4 39

2

1 3

2 1

3 11

6 16

5 81

23

5 65

16

5 72

44

43

59

83

13

7 13

2 14

9 63

80

8

1 12

6 3

3 11

17

8

7 8

4 24

14

14

4

13

4 7

27

bd

1 4

8 8

23

65

23

22

25

8 33

30

23

22

66

10

33

75

4

4 10

3

4 3

5 4

4 4

3 2

3 2

3 10

2

4 3

3 2

3 2

4 2

4 3

2 1

2 2

bd

1 1

4 1

2 2

bd

bd

1 bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

bd

17

23

21

34

22

24

23

18

10

18

16

12

0 24

1 22

17

78

16

bd

36

ne

nt o

xide

s in

wt.%

; tr

ace

elem

ents

in

ppm

; bd

: be

low

det

ecti

on.

1

Lith

olog

ical

abb

revi

atio

ns:

BA

nd-m

etab

asal

tic

an-d

esit

e; B

as-m

etab

asal

t/am

phib

olit

e;

Bon

-met

abon

init

e; L

>io-

met

adio

rite

; Gab

-met

agab

bro;

G

ran-

post

-t&m

k gr

anite

; Gtg

n_gr

anite

gn&

s;

Gw

-met

agra

ywac

ke;

Serp

-ser

pent

init

e; T

gn-t

onal

ite

gnei

ss;

Tabl

e 1.

con

tinu

ed.

r ampl

e no

L

ithO

.

SiO

z Ti

Q

Fe0

FSq

Mno

MgO

C

aO

Na2

0 w

P2

05

Co2

w

+ To

tal

Ci Ni

cu

Zn

Ga

Rb Sf

Y Zl

Nb Pb

Tkl

Ba

l- . ..

--.

mm

E

lm

wT2

2 lx

23

ET2

4 E

r2S

FD’2

6 E

27

Gtg

n G

tgn

Gtg

n G

&n

Gtg

n G

w

Gw

G

w

74.0

76

.0

77.4

75

.1

76.8

71

.8

65.8

69

.5

0.27

0.

17

0.17

0.

28

0.18

0.

59

0.85

0.

94

0.64

0.

24

1.89

0.

40

0.57

0.

95

0.04

0.

34

0.29

0.

25

1.26

11

.7

12.3

11

.8

11.4

11

.9

12.1

14

.6

12.9

14

.0

20.5

16

.0

16.3

15

.0

23.0

0.

91

12.6

14

.4

13.1

15

.0

3.88

2.

04

1.43

4.

19

1.86

1.

48

3.41

1.

93

7.34

4.

91

10.2

7.

47

6.64

8.

17

0.68

1.

21

1.08

8.

4 6.

81

2.08

1.

37

1.77

1.

76

1.66

2.

78

2.35

2.

82

1.83

3.

33

2.52

0.

24

3.92

8.

43

5.58

1.

53

0.80

2.

47

3.53

0.

12

0.07

0.

08

0.18

0.

06

0.11

0.

09

0.13

0.

37

0.13

0.

23

0.25

0.

15

0.32

0.

08

0.04

0.

03

0.18

0.

16

0.32

0.

48

0.14

0.

51

0.41

2.

87

2.81

1.

95

5.98

9.

15

5.48

4.

78

4.33

7.

67

37.0

0.

47

0.74

18

.4

8.65

3.

00

2.15

2.

7l

2.58

2.

33

1.80

2.

30

3.25

21

.7

13.4

17

.7

16.0

10

.1

11.4

0.

61

1.49

0.

55

8.04

10

.3

4.28

4.

18

3.68

4.

07

4.06

1.

86

2.12

3.

07

0.33

1.

42

0.64

1.

24

1.09

1.

09

bd

3.61

4.

20

0.56

2.

76

0.19

0.

88

0.38

0.

26

0.62

3.

42

3.69

2.

60

0.06

0.

55

0.35

0.

20

0.13

0.

28

bd

3.68

4.

90

0.06

0.

22

0.08

0.

04

0.05

0.

06

0.05

0.

16

0.23

0.

15

0.06

0.

04

0.20

0.

09

0.06

0.

04

0.02

0.

05

0.10

0.

05

0.17

0.

22

0.07

O

.lI3

0.03

0.

03

0.04

0.

08

0.11

0.

04

0.07

0.

04

0.25

0.

04

0.03

0.

93

0.13

0.

07

0.04

0.

06

0.51

0.

53

0.40

0.

51

0.36

1.

36

1.59

0.

85

100.

7 10

0.3

100.

0 lo

o.9

loo.

3 lo

o.4

99.9

10

0.2

11

m

10

10

9 88

95

91

2

2 2

2 2

63

44

28

194

86

113

174

19

27

1949

3

9 29

9 99

bd

13

bd

bd

bd

72

13

9

19

26

99

130

14

199

bd

1 26

30

91

39

44

26

74

36

48

64

55

86

29

95

38

33

24

0 21

17

42

93

63

12

12

12

12

11

14

17

14

20

15

23

14

14

20

1.

12

22

8

15

4 17

7

5 13

91

97

66

3

15

6 9

3 5

2 57

17

0 1

3 78

69

73

68

66

42

6 37

6 42

8 27

8 56

7 93

4 43

6 11

4 15

86

6 80

13

4 22

20

8 28

40

33

16

27

17

25

27

16

4

2s

9 12

18

bd

42

8

4 27

61

97

91

49

87

15

1 21

2 20

4 30

11

12

1 21

29

25

4

204

134

15

54

3 5

3 3

4 9

10

12

4 3

13

5 2

4 2

3 10

2

5 2

1 1

1 4

14

15

15

3 6

10

10

2 15

bd

10

41

bd

7

bd

bd

bd

bd

bd

4 5

10

bd

bd

bd

bd

bd

bd

bd

2 10

bd

bd

69

25

6 10

4 86

12

6 12

31

755

858

33

123

16

86

68

50

5 63

7 49

2 10

72

ET2

8 E

T29

Bas

B

as

47.0

44

.0

ET3

0 B

as

43.5

ET3

1 B

as

52.0

KE

NTI

CH

A

FIT3

2 E

X33

ET3

4 B

And

G

ab

Serp

57

.0

35.7

41

.3

Er3

5 E

T36

ET3

7 E

T38

Gtg

n G

tgn

Bas

B

as

74.5

72

.3

42.5

49

.7

0.57

2.

25

1.28

0.

54

1.23

3.

21

12.1

0.

32

0.66

5.

01

1.65

99

.9

100.

0 10

0.0

99.8

10

0.3

100.

3 99

.3

100.

0 10

0.1

99.1

lo

o.3

693

67

207

806

44

197

2005

10

m

93

m

i

Tabl

e 1.

con

tinu

ed

r empl

enc

Lith

O.

SiQ

Ti

a

Fe0

Fe20

3 M

nO

Mgo

C

aO

Naz

O

w

PZO

5 C

Q

HzO

+ To

tal

CI

Ni

cu

Zn

Ga

Rb Sr

Y Zr

N

b Pb

Th

Ba

0.72

1.

01

0.58

0.

81

0.29

0.

22

0.49

19

.7

18.3

15

.5

16.0

14

.4

15.4

13

.8

6.56

2.

30

6.24

1.

37

1.12

0.

45

15.5

3.

62

8.74

4.

37

7.02

1.

29

0.51

1.

77

0.16

0.

22

0.11

0.

27

0.06

0.

02

0.11

7.

25

2.46

4.

10

3.02

0.

78

0.38

1.

25

14.1

15

.6

8.82

9.

23

0.47

1.

79

3.81

1.

37

1.12

2.

65

5.65

5.

25

5.11

3.

33

0.15

0.

24

0.22

0.

30

2.77

1.

85

1.77

0.

05

0.26

0.

05

0.20

0.

08

0.07

0.

11

0.04

0.

03

0.04

1.

68

0.06

0.

03

0.83

I. E

T39

ET4

0 G

ab

Gab

45

.0

48.2

KFN

TIC

HA

E

T41

ET4

2 E

T43

m-4

4 E

T45

BA

nd

Dio

G

&n

Gtg

n G

tgn

56.0

53

.8

72.8

74

.0

70.3

1.43

1.

54

1.45

0.

48

0.91

0.

58

1.16

lo

o.2

100.

0 lo

o.1

99.8

10

0.3

100.

4 10

0.3

144

32

42

224

12

9 28

19

19

74

a3

2

4 17

8

8 5

25

6 bd

24

74

81

19

34

2 40

29

46

18

23

14

18

14

18

14

4

7 4

2 82

43

34

60

0 74

1 12

0 91

9 16

8 66

6 22

7 6

22

13

20

17

3 25

21

43

30

70

10

9 85

10

9 3

3 3

4 5

5 4

10

19

2 24

2 9

18

12

bd

1 bd

4

5 3

3 22

55

39

11

8 73

3 48

3 63

0

T B

UR

SAN

O

ET4

6 E

l.47

ET4

8 E

T49

Bas

G

tgn

Gtg

n G

ran

49.5

77

.3

78.3

69

.3

0.29

0.

13

0.13

0.

13

14.2

12

.1

12.2

16

.0

6.96

0.

49

0.20

0.

51

3.60

0.

85

0.55

0.

81

0.32

0.

01

bd

0.04

9.

93

0.12

0.

04

0.28

11

.9

0.65

0.

27

2.34

1.

18

3.41

4.

08

6.30

0.

12

4.70

4.

28

2.10

0.

02

0.02

0.

02

0.06

0.

05

0.03

0.

03

1.46

1.

46

0.23

0.

07

0.66

99

.5

loo.

0 10

0.2

100.

0 15

00

7 23

30

22

9 2

11.

14

179

bd

bd

1 12

2 11

4

19

9 13

13

15

3

140

122

47

93

58

40

398

8 18

25

15

18

97

11

6 16

1 3

5 6

6 1

8 9

14

bd

12

7 8

93

361

659

577

AW

ATA

E

T53

ET5

4 E

T55

Gtg

n G

tgn

Gtg

n 71

.3

72.3

73

.7

0.57

0.

23

0.07

13

.1

14.6

14

.5

0.74

0.

93

0.35

2.

22

0.55

0.

40

0.03

0.

03

0.02

0.

74

0.73

0.

16

0.66

1.

53

1.11

3.

11

4.17

4.

67

6.37

4.

38

4.31

0.

16

0.07

0.

01

0.13

0.

04

0.06

ET5

0 Ek

lS

49.9

RE

JI

J!?r

51

ET5

2 Ba

S B

as

50.1

50

.3

1.52

1.

18

0.70

14

.6

14.5

15

.2

9.40

7.

90

7.38

1.

95

2.62

1.

56

0.20

0.

20

0.17

6.

15

7.80

8.

48

13.0

11

.5

12.9

2.

12

2.85

1.

79

0.19

0.

23

0.11

0.

14

0.11

0.

07

0.04

0.

04

0.03

0.

34

0.31

0.

19

1.18

1.

32

1.39

99

.5

99.9

99

.6

100.

4 10

0.4

100.

1 26

19

7

169

169

318

10

13

3 52

62

86

bd

4

bd

24

10

88

24

34

20

92

94

68

14

20

29

17

15

14

173

128

172

3 4

2 13

3 60

6 20

0 16

2 18

1 93

30

5

21

38

29

17

206

98

50

98

72

39

8 8

19

2 2

2 28

45

46

4

1 2

4 7

4 bd

bd

bd

11

95

1037

42

6 31

36

16

8. WOLDEHAIMANOT and J. H. BEHRMANN

Table 2. Rare earth element data (in ppm) of representative samples from the Adola region

Sample no. ET1 ET2 ET4 ET5 E?I’6 ET7 EX8 ET9 El-l0 ET11 ET12 La 0.3 1.3 3.8 1.4 2.7 1.5 0.3 1.4 2.6 2.1 1.7 Ce 0.6 2.5 12 3.6 6.2 3.8 0.4 3.7 6.8 5.5 4.1 Pr bd 1 2 1 1 1 1 1 1.0 1.0 1 Nd 0 1.4 8.2 2.1 3.5 2.3 0.3 2.8 4.7 4.6 2.0 Sm 0.3 0.4 2.6 0.6 0.9 0.6 0.2 1.0 1.4 1.5 0.6 ELI 0.14 0.39 1.03 0.35 0.42 0.27 0.15 0.58 0.36 0.38 0.26 Gd 0.4 0.5 3.2 1.1 1.2 1.0 0.5 2.3 1.9 2.1 0.6

DY 0.5 0.7 3.6 1.5 1.4 1.5 0.7 3.7 2.8 2.9 0.7 Ho 0.11 0.14 0.74 0.31 0.30 0.35 0.15 0.86 0.57 0.60 0.16 Er 0.3 0.5 2.0 1.0 1.0 1.0 0.5 2.7 1.8 1.9 0.5 Yb 0.3 0.5 1.8 1.1 1.0 1.1 0.5 2.8 1.9 1.8 0.5 LU 0.05 0.09 0.27 0.19 0.16 0.17 0.08 0.43 0.30 0.27 0.09

hple no. ET15 ET21 ET24 ET25 ET26 ET27 ET46 ET47 ET50 ET51 ET52 La 3.3 12 8.4 28 29 47 1.0 19 3.2 2.3 1.7 Ce 4.2 25 20 57 67 92 2.8 46 13 9.4 5.4 Pr 1 4 3 7.6 8.4 12 bd 4.3 3 2 0.9 Nd 3.6 15 11 24 28 37 1.6 11 10 7.6 4.2 Sm 0.9 4.2 3.0 5.1 6.4 7.8 0.5 2.6 3.4 2.7 1.5 EU 0.52 0.85 0.73 1.14 1.60 1.69 0.23 0.27 1.43 1.12 0.62 Gd 1.1 5.5 3.8 3.7 4.9 5.5 0.8 2.4 4.9 3.8 2.2

DY 1.3 6.3 4.6 3.3 4.5 5.0 1.4 3.2 6.5 5.0 2.9 Ho 0.24 1.38 1.03 0.59 0.83 0.92 0.27 0.61 1.41 1.08 0.61 Er 0.8 4.1 3.2 1.8 2.5 2.7 1.0 2.0 4.1 3.1 1.9 Yb 0.8 4.3 3.4 1.5 2.1 2.3 1.0 2.1 3.8 2.9 1.9 LU 0.12 0.66 0.54 0.26 0.36 0.40 0.17 0.34 0.57 0.43 0.29

Sample no. La Ce Pr Nd Sm EU Gd

DY Ho Er Yb LU

ET39 ET41 ET38 ET43 ET49 ET28 ET33 ET30 ET53 ET54 ET55 2.1 2.8 5.1 17 25 1.3 2.9 11 34 12 3.1 5.1 6.9 14 34 54 4.2 7.0 29 56 29 7.7 1 1.1 2.1 4.4 6.0 1.0 1.0 4.0 9.0 2.9 0.7

3.5 4.3 9.3 13 18 3.6 4.5 16 29 9.2 2.5 1.1 1.5 3.1 2.8 3.5 1.2 1.4 4.5 6.2 1.6 0.6

0.51 0.56 1.23 0.63 0.94 0.64 0.80 1.62 1.44 0.54 0.17 1.2 1.7 3.8 2.1 2.7 2.5 2.3 4.9 5.1 1.2 1.1 1.2 1.7 3.8 2.1 2.7 3.0 2.7 5.1 5.1 1.0 2.5

0.25 0.47 0.92 0.51 0.5 0.65 0.61 0.97 0.96 0.18 0.57 0.8 1.6 2.8 1.6 1.6 1.9 1.9 2.8 2.9 0.6 1.8 0.7 1.5 2.4 1.6 1.4 1.8 2.0 2.4 2.2 0.5 1.7

0.11 0.25 0.38 0.28 0.24 0.27 0.31 0.37 0.38 0.12 0.26

A study of metabasite and metagranite chemistry in the Adola region 467

Megado amphibolites

_. La Ce Pr Nd Sm Eu Gd Tb Dy Ha Er Tm Yb Lu

1001 ” * ” ” ” ” ” -4

Megado metaboninites ]

+ ET10 --a-- ET11 --&- ET12 ---& $;40 b i 0.1-l ’ , * ’ 7 ’ ’ ’ ’

La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

100: Megado metagraywackes

0) .2 &i g 10: s Y 7


Kenticha amphibolites 1

+ ET28 * ET30 --+-- ET33 d

0.1-1 ’ . . . 1 La Ce Pr Nd Sm Eu Gd Tb Dy Ha Er Tm Yb Lu

100 . ’ 1 ’ ’ ’ . .

Daba amphibolites

3 $ 10: 0

6 3

“.i 3J-td. . ,~ . ,e 1 Sm Eu Gd Tb Dy He Er Tm Yb Lu

100 . ’ 1 . 1 . I j

Bursano amphibolite i

- S ET46

0.1, . . . , .f, La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

100 ’ . . .


Figure 2. Chondrite-normalized REE diagrams for metabasites (Figs 2a, d-g), metaboninites (Fig. Zb) and metagraywackes (Fig. 2~). In Fig. 2b, sample D51-8 is a transitional boninite from the Mariana trench (data from Bloomer and Hawkins, 1987), and sample 331/540 is a boninite from Cape Vogel (data from Sun and Nesbitt, 1978). PAAS data are from Nance and Taylor (1976). Normal&&ion constants from Evensen et al. (1978).

B. WOLDEHAIMANOT and J. H. BEHRMANN 468

20-

$16-

V P12- ? E '3 8- s

4-

* Megado -$_ Kenticha --a- Daba --A- Bursano -+- Reji

0

0

0

p

8’ I I I 1

.0 0.5 1.0 1.5 2.0 -_ - TiOz (wt.% )

Figure 3. Variation of TiOz with REE (La+Sm+Yb) in the Adola metabasites.

with the effectively unmetamorphosed PAAS pro- vides indirect evidence for the absence of significant REE fractionation during diagenesis and metamorphism of the Adola rocks. Moreover, the absence of Ce anomalies in the metagraywackes indicate that equilibration with sea water, which has a pronounced negative Ce anomaly, did not occur.

In summary, the above results indicate that most of the major and some of the trace elements have been mobile, consistent with the absence of original igneous mineralogy and the abundance of greenschist and amphibolite facies mineral assemblages. On the other hand, the trace elements Zr, Y, Ti, P and most of the REE have remained immobile and hence their contents reflect igneous values. In this paper only the immobile elements have been utilized to constrain the tectonic setting and petrogenesis of the Adola rocks.

TECTONIC SETTING

All of the analyzed rocks from the Adola region have low concentrations of the high field strength elements (HFSE) and REEs relative to similar rocks of comparable SiO2 content. Because they show consistent inter-element relationships, the authors compared them with established ratios and patterns for known geotectonic environments.

The geochemical data indicate that the metavolcanic rocks of the Adola belt are mainly mafic and subordinate intermediate compositions. Rocks with SiO2 content >57% are absent. On the other hand, the meta-intrusive rocks span a range of felsic to mafic compositions. All the metavolcanic rocks are tholeiitic. In the Ti-Y-Zr diagram (Fig. 4) the metavolcanic rocks plot entirely within the ocean floor basalt (OFB) and low-K tholeiite (LKT) fields, thus demonstrating

Ti/lOO

- CAB

Zr Y*3

Figure 4. Ti/lOO-Zr-Yx3 tectonomagmatic discrimination diagram for the Adola metavolcanics. Fields aher Pearce and Cann (1973). WPB: within plate basalt; LKT: low-K thokiite; OFB: ocean floor basalt; CAB: talc-alkali basalt. Symbols as in Fig. 3.

their oceanic affinity.

Megado meta-igneous-sedimentary rocks

Representative REE patterns of metabasites are shown in Fig. 2a. Two general features are evident:

i) a LREE depleted pattern similar to normal (N-type) MORB 1 enrichment factor, (La/Sm)cn=0.6- 0.91 shown by samples ETl, ET8 and ET9; and

ii) a LREE enriched pattern resembling enriched (E-type) MORB [(La/Sm)cn=1.4-1.81 shown by samples ET4, ET5, ET6 and ET7. It is to be noted that such chemical characteristics are also found in back-arc basin basal&

Distinction between MORB and arc basalts is made using MORB-normalized multi-element plots. In such plots, N-MORB are expected to show flat patterns, whereas arc basalts show distinctive patterns with selective enrichment in certain elements such as Sr, K, Rb, Ba, Th, iCe+Sm*tP and a relative lack of enrichment in Ta, Nb, Hf, Zr, Ti, Y and Yb. These features are attributed to the modification of the MORB source by a subduction component (Saunders and Tarney, 1979; Pearce, 1983). In Fig. 5a the plots show humped patterns for the large ion lithophile elements (LILE) and Nb. Obviously, the patterns of the immobile elements differ significantly from the generally flat N-MORB patterns. These patterns of element distribution reflect magma contamination through fluids related to a subducted slab (e.g. Pearce, 1983). LILE enrichment due to metamorphism of these rocks is considered here as an improbable mechanism because there is a clear lack of LILE enrichment in the similarly metamorphosed Reji amphibolite (see Fig. 5e). Thus, although some effects of secondary alteration processes can not be ruled out,


Sr K Rb Em ThTa Nb Co P Zr HI Sm n Y n,

+ ET39 -a- ET40 + ET41

C Sr K Rb Ba ThTa Nb CeP Zr Hf Sm’ll Y yb

Gi, . . . . . . . .d 6 SrKRbB8TbTaNbCePZrHfSmTiYYb

8 . 1 * ‘. ‘. . ...*..,

Reji amphibolites

a; Sr K Rb 8a Thh Nb Ce P Zr HI SmTi Y Yb

Figure 5. MOREnormalized trace element variation diagrams for the Adola metabasites. Order of elemerits and normalizing values from Pearce (1983).

LILE patterns for the Megado rocks reflect domi- nantly the effects of contamination in the magma

field. Moreover, the ocean ridge granite (ORG)-

source, thereby suggesting that they belong to a back- normalized trace element plot (Fig. 7a) shows very

arc setting. low contents of incompatible elements of high ionic

In the Rb versus Y+Nb diagram (Fig. 6), the gran- potential (compared with normal&g values), but

ite gneisses plot in the volcanic arc granite (VAG) high concentrations of Ba (and to some extent Rb). The patterns resemble that of the supra-subduction

470 8. WOLDEHAIMANOT and J. H. BEHRMANN

P CI: - VAG

Q_

-1 10 Y+Id p8pm) 7 1000

Figure 6. Rb versus Y+Nb discrimination diagram for the Adola granitoids. Fields after Pearce et al. (1984a). Syn-COLG: syn- collision granite; WPG: within plate granite; ORG: ocean ridge granite; VAG: volcanic arc granite. Symbols: open stars-Awata gneiss; filled stars-post-tectonic granites from Bursano, Adadi Kotu and Kotissa. Data for Adadi Kotu and Kotissa are from Gil- boy (1970). Other symbols as in Fig. 3.

ridge granite from the Troodos massif (cf: Pearce et al., 1984a).

The back-arc chemical features and the lithological association of’ Megado, comprised of serpentinite, gabbro, pillow basalt, granitoids and Fe-Mn quartz- ite, thus define a mid ocean ridge ophiolitic succession above a subduction zone, rendering this rock assemblage an excellent analogue for supra-subduction zone ophiolites (Pearce et al., 1984b).

The boninitic meta-andesites. The SiO2 contents of the analyzed samples (ETlO, ET11 and ET12) vary between 53.4 wt.% and 57.7 wt.% (54.3-58.6 wt.% on an anhydrous basis) and therefore chemically represent andesites. This is coupled with high MgO (6.7- 9.4 wt.%), Cr (303-709 ppm), Ni (55-107 ppm) and Mg# (0.6-0.7). These values are within the limits of primary magmas in equilibrium with mantle perido- tite (Hickey and Frey, 1982).

A very low TiO2 content (0.11-0.24 wt.%), characteristic of modern boninites, is a typical feature of the Adola boninitic samples (see Table 1). Moreover, the samples have Ti/Zr ratios ranging from 22 to 63, similar to those obtained from modern boninites (e.g. Hickey and Frey, 1982). On the other hand, the asso- ciated back-arc tholeiites have an average Ti/Zr ratio of 105, very similar to the Ti/Zr ratio of 110 obtained for MORB and tholeiitic island arc basalts (Sun et al., 1979). A further comparison with representative boninites, arc tholeiites and N-MORB has been done by plotting TiO2 against Zr (Flower and Levine, 1987), where the boninite data fall entirely within the fields

defined by the Cape Vogel boninites (diagram not given). In the TiOz versus Zr diagram, the Megado back-arc ophiolitic rocks, which are intimately asso- ciated with the boninites, are clustered in the back-arc tholeiites field.

Chondrite-normalized REE abundance are plotted in Fig. 2b. Typical patterns of boninites from Cape Vogel and the Mariana trench are also presented for comparison. Sample ET12 has low a HREE abundance of 2 to 4 times that of chondrites and a relatively high LREE concentration at 4 to 7 times that of chondrites [(La/Yb)cn=2.2]. The open, pseudo- concave (U-shaped) pattern of ET12 has been widely observed in many boninites and is evidence of meta- somatic enrichment of the source in LREE (e.g. Bonin islands, Hickey and Frey, 1982). For ET10 and ET11 the patterns are generally flat with a slight LREE depletion [(La/Yb)cn=0.9 and 0.8, respectively], but the HREE concentrations are much higher than those of ET12.

The strong LREE enriched pattern for sample ET12 and a relatively depleted LREE pattern for samples ET10 and ET11 (Fig. 2b) is also supple- mented by a strong enrichment in Sr, K, Rb, Ba and Zr for sample ET12 relative to samples ET10 and ETll. Samples ET10 and ET11 show only marginal enrichment in the above elements. Therefore, because the patterns for samples ET10 and ET11 are more akin to MORB-type patterns and many of the geochemical features, such as higher SiO2, Cr, Ni, Mg# and lower Ti02 and Ti/Zr, are typical of boninites and/or cumulate tholeiitic andesites, it is proposed that sample ET12 is a typical metaboninite whereas samples ET10 and ET11 are transitional metaboninites that mark the onset of back-arc tholeiite volcanism.

Kenticha meta-igneous-sedimentary rocks

In the chondrite-normalized REE plots (Fig. 2d) sample ET28 shows a slightly LREE depleted pattern [(La/Sm)cn=O.‘/] suggestive of N-MORB character. On the other hand, samples ET33 and ET30 have a strongly LREE enriched pattern [(La/Sm)cn=1.3 and 1.5, respectively] compared to E-MORB. These REE features are similar to those of Megado.

In an attempt to examine the effects of source contamination by subducted crust, a MORB-normalized diagram is plotted in Fig. 5b. Identical enrichment patterns of the LILE as those of Megado are to be seen. These results suggest that the Kenticha rocks formed in a back-arc setting.

In the ORG-normalized diagram (Fig. 7b) the granite gneiss sample ET35 shows enrichment in the elements K20, Rb, Ba and Th typical of VAG. How- ever, the peraluminous granitic gneiss sample ET36 has a steep pattern similar to syn-collision granites


-ft- ET20 - ET21 --+-- ET22 -+- ET23 --m-- ET24 a

W! , , , , , . , * , , , ,

S; KpO Rb Ba Th Ta NbCe Hf Zr Sm Y Yb

- -+--ET35 q-;;:; _ --O--ET47 '--m---ET48

b- F;, , , , , , , , , , * , , S; KS0 Rb Ba Th Ta Nb Ce Hf Zr Sm Y Yb

1 + ET36 - -=--ET44

J=:F:f , , , . , , y ci 6 K20 Rb Ba Th Ta Nb Ce Hf Zr Sm Y Yb

Figure 7. Ocean ridge granite (ORG)-normalized geochemical patterns for granite gneisses from (a) Megado; (b) Bursano (ET47, ET48); Daba (ET43, ET45); and Kenticha (ET35); (c) Awata (ET53, ET54) and a syn-tectonic granite (Gariboro pluton, ET55); Daba (ET44); Kenticha (ET36) and a post-tectonic granite within Bursano terrane (ET49). Normalizing values and order of elements from Pearce et al. (1984a).

(Fig. 7~). In the Rb versus Y+Nb diagram (Fig. 6) the Kenticha rocks plot as VAG with one of the samples (ET36) plotting in the syn-collision granite field.

The ophiolitic suite of Kenticha represented by serpentinite, metabasalt, gabbro and deep sea/shelf metasediments is a typical oceanic floor succession. The geochemical features of this suite such as (1) high contents of MgO, Cr, and Ni, (2) very low values of TiO2, Zr, and Y for the serpentinite sample (ET34) and (3) the clear supra-subduction zone affinity indi- cated on a Cr versus TiOz plot of Pearce et al. (1984b) (diagram not shown) demonstrate that the Kenticha rock units represent supra-subduction zone ophiolites.

On the basis of gross lithological similarities and structural compatibilities between the Kenticha and Megado rocks, earlier workers (e.g. Kazmin, 1976; de Wit and Chewaka, 1981; Beraki et al., 1989) postulated that the Kenticha ophiolite is an eastward abducted nappe related to the Megado ophiolite. This view is supported by the striking similarities in the geochemical data of both suites (Tables 1 and 2 and Figs 2 and 5).

Bursano gneiss-amphibolite assemblage

The only mafic volcanic sample from Bursano (ET46) has a tholeiitic character and plots in the OFB

472 B. WOLDEHAIMANOT and J. H. BEHRMANN

field in the Ti-Y-Zr diagram (Fig. 4). The chondrite- normalized plot of the sample shows an almost hori- zontal REE pattern (Fig. 2f). These patterns and the low HFSE content suggest an island arc setting. The clearly humped pattern on the MORB-normalized diagram (Fig. 5d) also supports this conclusion.

Among the analyzed intrusives, the granite gneisses (ET47 and ET48) show high K20 and follow a talc-alkaline trend on an AFM diagram. In the Rb versus (Y+Nb) diagram (Fig. 6) these granitoids plot as VAG. Moreover, the characteristic enrichment in K20, Rb, Ba and Th and the high Ce content (relative to adjacent elements) in the ORG-normalized diagram (Fig. 7b) makes the Bursano granite gneiss comparable to known VAGs elsewhere (c$ Pearce et al., 1984a). The close association of the talc-alkaline granites with island-arc tholeiite amphibolites suggests that the former represent the plutonic root to the latter.

Reji amphibolite

The OFB affinity of the Reji amphibolite is con- firmed by the chondrite-normalized REE plot (Fig. 2g), which shows slightly depleted LREE and flat patterns of the medium and heavy REE typical of N- MORB. Moreover, chondrite-normalized La/Sm ratios range from 0.52 to 0.69 and Ti/Zr ratios from 93 to 108, which are similar to the average La/Sm ratio of 0.55 and Ti/Zr ratio of 110 for modern N-MORB (Sun and Nesbitt, 1977).

The MORB chemical character of the Reji amphibolite is also reflected in the MORB-normalized trace element variation diagram (Fig. 5e). Except for minor irregularities, patterns for the Reji rocks are essen- tially flat and have normalized values close to unity. These features rule out contamination due to a subducted slab. The flat patterns for these rocks strengthen the view that the Reji volcanic rocks are genuinely MORB-type and their genesis and mantle source are thus similar to those of modern N-MORB. The position of the Reji amphibolite suggests that it marks a suture zone that separates the older(?) Daba magmatic arc from the younger ophiolitic assemblages to the west.

Daba meta-igneous complex

The slightly LREE enriched, nearly flat REE pattern of the metabasites can’be interpreted either as E- MORB or an island arc tholeiite (IAT; Fig. 2e). Using the Cr-Y discrimination diagram of Pearce (1982), the Daba rocks plot in an arc setting consistent with the humped, MORB-normalized multi-element pattern (Fig. 5~). Moreover, the granitoids show a volcanic arc setting in the Rb versus Y+Nb diagram (Fig. 6). It should be noted, however, that several variations in

trace element content and in the multi-element plots suggest that a polyphase rather than a single-phase magmatic process might have been involved in the evolution of the Daba meta-igneous complex.

The granitoids (ET43, ET45) show K20, Rb, Ba, Th and Ce enriched patterns analogous to VAG (Fig, 7b). Only the peraluminous sample ET44 (Fig. 7c) shows a steep REE pattern comparable to syn-collision granites. It appears that this sample is highly fractionated and probably represents an evolved volcanic arc granite. Most of the geochemical data thus indicate that the Daba meta-igneous complex has an island arc heritage.

Awata gneiss

The rarity of mafic volcanic rocks in the region covered by the Awata gneiss compels the authors to restrict their tectonic interpretation to the felsic intrusives. The analyzed samples are alkalic and distinctly peraluminous. They show a within-plate setting on the Rb versus (Y+Nb) diagram (Fig. 6). This is also manifested in the ORG-normalized elemental plot (Fig. 7c) where all the samples are characterized by high concentrations of Rb typical of syn-collision granites. Moreover, low concentrations of Ce, Zr and Sm for sample ET55 (the Gariboro granite gneiss) are similar to concentrations in syn-collision granites. Thus, the authors infer that the emplacement of the Gariboro pluton was synchronous with the abduction of the ophiolite complexes.

The geochemical features described above suggest that the Awata gneiss represents, in contrast to the oceanic affinity of all the other rock assemblages of the region, the only rock with continental affinity. Taking into account the older age, more extensive deformation and metamorphism and the consistent plutonic signature of S-type granites, the authors speculate that the Awata gneiss represents remobilized continental crust, possibly during the Pan- African tectonic event in southern Ethiopia.

Post-tectonic intrusives

Only one representative sample (ET49) of the post-tectonic granite has been analyzed. It shows alkalic affinity, is metaluminous’ and has low abundances of the incompatible elements Ti, Y, Zr and the REEs. In the ORG-normalized diagram, the granite exhibits enrichment in K20, Rb, Ba and Th and depletion in Nb and Yb (Fig. 7c) typical of post-tectonic granites (ct Pearce et al., 1984a).

DISCUSSION AND CONCLUSIONS

The Adola belt occupies a transitional position between the Arabian-Nubian shield and the Mo-


MEG RIFTING AND DRIFTING

AW \ -“ebM.” YCH* BURSANO REI OEM

ARC COLLlSloN AND OWXJCTDN

SUx4E

BACK ARC CLOSURE AND CONTlNENTU COLLISION

POSTCOLLlSlGNAL UPLIFT AND EXHUYATION

Figure 8. Plate tectonic model for the evolution of the Adola belt.

zambique belt. It contains upper greenschist to lower amphibolite grade meta-igneous-sedimentary units similar to those of the Arabian-Nubian shield. The meta-igneous-sedimentary rocks are bounded on the east by older gneissic domains, some of which con- tain upper amphibolite metamorphic assemblages comparable to those of the Mozambique belt. The sequence and geometry of structural elements observed in the Mozambique belt of Kenya (Key et al., 1989) are also similar to those observed in the Adola region.

Despite the prevalence of polyphase deformation, metamorphism and the absence of reliable radiomet- ric age data, a reconstruction of the Neoproterozoic tectonic evolution of the Adola belt, based on geochemical and structural data, is summarized in Fig. 8.

An important aspect of the evolution of the Adola belt is the recognition of MORB-type chemical characteristics for the Reji amphibohte. The present, very limited extent of the Reji rocks suggests that the oce-

anic crust has been overridden by east verging, low angle thrusts (Beraki et aZ., 1989), or has been nearly completely subducted leaving the minor Reji rocks as a vestige.

In the Arabian-Nubian shield, where more detailed and systematic studies have been carried out, MORB-type suites comparable to those of Reji have not been recognized (Stem, 1981, 1994; Kroner et nl., 1987; Berhe, 1990). Based on this fact and on paleo- magnetic data (Piper et al., 1973), which suggest that Africa has been one crustal entity for the last 2200 Ma (i.e. without any intervening crust within it), ocean basins of limited size, such as those of the present day southwest Pacific (Gass, 1977) or &Red Sea (Stern, 1981), have been proposed for most’of the Arabian- Nubian shield terranes. From the present study, the Reji MORB character implies the existence of a mature ocean in the evohitionary history of the East Af- rican orogen.

474 B. WOLDEHAIMANOT and J. H. BEHRMANN

The probable continuation of MORB-type volcanic suites to the south is complicated by the Tibetan-style collision in the Mozambique belt, which might have consumed much of this suite, or by deeper exhuma- tion in the Mozambique belt as suggested by Key ef ~2. (1989). However, it is suggested that the Reji ocean might be part of a larger ocean that separated the continental blocks east and west of the East African orogen.

Although the Daba meta-igneous complex shows chemical characteristics of a magmatic arc, it does not fit with the geometry of subduction and accreting plate boundary-related magmatic products of Me- gado, Kenticha, Bursano and Reji. It thus appears that the Daba magmatic arc may belong to a different subduction environment that existed before the formation of the oceanic suites discussed above.

The Awata gneiss shows consistent geochemical tendencies indicative of a continental within-plate setting. This continental affinity and the old age reported for the gneiss suggests that it represents a pre- Pan-African crust that was remobilized during subsequent tectono-magmatic events in the Neoprotero- zoic. Similar continental fragments are also documented in eastern Ethiopia, northern Somalia and southern Yemen (Kroner, 1993). These features dis- tinguish the Awata continental basement from the rest of the Adola oceanic rocks.

The oceanic suites clearly overly the continental rocks (the Awata gneiss) and their contacts are ex- tensively tectonized. These characteristics strongly suggest, as predicted earlier by de Wit and Chewaka (1981), that the whole package of rocks with oceanic affinity represents an eastward transported al- lochthonous mass. This implies that all of the ophiolitic and related rocks of the region were displaced from their original sites of emplacement in a thick- skinned tectonic regime (Fig. 8). This conclusion is in agreement with observations in northeast Sudan (Stern ef al., 1990), northern Saudi Arabia (e.g. Shanti and Roobol, 1979) and Kenya (Mosley, 1993) where most of the major ophiolite occurrences represent far- traveled nappe complexes,

A final point to consider is that, in the Arabian- Nubian shield arc, terranes constitute lithological units that are more than 100 km wide. In the Adola region, the various rock types which represent different tectonic settings are characterized by highly de- formed, narrow belts, generally 1 to 2 km wide with a maximum length of about i5 km. This indicates that an extensive east-west shortening of the rocks oc- curred during continental collision between east and west Gondwanaland. The presence of consistent W- plunging stretching lineations and top-to-east move- ment shown by various shear criteria (Beraki ef al., 1989; Woldehaimanot, 1992) suggest that subduction/obduction of the oceanic crust was approxi-

mately orthogonal to the plate margins. An east directed ophiolite abduction in the Adola region, however, contradicts the mainly westerly (northwest, southwest, west) directed transport documented for the majority of the ophiolite belts in the Arabian- Nubian shield (Kroner, 1985; Vail ef al., 1986) and in the Mozambique belt of Kenya (Mosley, 1993) and points to the need for a more careful and detailed structural study and/or re-examination of earlier in- terpretations. On the other hand, late stage north- south strike-slip translations and transpressive shears conforming to components of tectonic escape are also well documented in the Adola region (Beraki et al., 1989; Worku and Yifa, 1992). This stage is clearly observable in the rest of the East African orogen for which an oblique subduction and collision has been suggested (e.g. de Wit and Chewaka, 1981; Kroner, 1985; Key et al., 1989; Berhe, 1990).

All of the above arguments suggest that the Adola belt experienced a complete Wilson cycle, including the opening of a mature oceanic crust and its closure and abduction, that was followed by late-stage strike- slip or transpressive shearing. Extensive shortening and relatively little strike-slip components are consistent with the Adola region being geographically close to the Mozambique erogenic front. The present work thus supports the application of collision and tectonic escape processes in the evolution of the East African orogen.

Acknowledgments

The authors wish to thank the Ethiopian Institute of Geological Surveys for logistical help during the field work. Mrs. M. Griinhauser and Mrs. S. Schmidt are thanked for their help during the chemical analyses. Dr. Seife M. Berhe and W. Bach are thanked for reviewing an earlier version of this paper which helped improve the manuscript. Dr. C. Chalokwu and Dr. N. Eby are gratefully acknowledged for their thorough reviews, which were useful in preparing the final version of the paper. The research was supported by the Deutscher Akademischer Austauschdi- enst (DAAD).

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A study of metabasite and metagranite chemistry in the Adola region (south Ethiopia): implications for the evolution of the East African orogen