The Sagatu Ridge dyke swarm, Ethiopian rift margin: revised age and new Sr-isotopic data

Journal of African Earth Sciences, Vol. 11, No. 1/2, 39-42, 1990. Printed in Great Britain

0899-5362/90 $3.00 + 0.00 © 1991 PergamonPress pie

The Sagatu Ridge dyke swarm, Ethiopian rift margin: revised age and new St-isotopic data P. S. KE~AN1, J. G. MITCI-mTJ2 and P. MOI-IR3 aDept, of Geology. University College Dublin, Dublin, Ireland 2School of Physics, University of Newcastle on Tyne, U.K. 3Dept. of Geology, University College Galway, Galway, Ireland Abstract - The Sagatu Ridge dyke swarm represents a temporary locus of crustal extension and magmatic eruption outside and east of the present Ethiopian rift margin. A bimodal association of hawaiitic and comenditic dykes comprises the swarm. Previously dated as early Pleistocene, we now present a more precise K-At isochron age of 1.97 ± 0.02 M.a. Sr-isotope data reveal the effects of sialic crustal contaminationon the Sr-poor comendites. The direct correlationbetween STSr/e~Srand initial Cexcess") 4OAr suggests that this Ar is of crustal rather than mantle origin.

AGE OF THE DYKE SWARM INTRODUCTION A d o l e r i t e d y k e - s w a r m forms the core of the Sagatu Ridge, which overlooks the eastern margin of the Ethiopian rift valley, 100 k m SSE of Addis Ababa. The ridge was built up n e a r the rim of the Harar Plateau by NNE-trending fissure eruptions t h a t issued between the coeval volcanic centres of Gara Badda a n d Gala Enkwolo (Filfo) (Fig. I). About 500 k m s of flood basalts a n d high-level dykes comprise the 70 km-long ridge, which s t a n d s a n average of 1000 m above the level of the plateau. The plateau in this region is capped by a horizontal pile of Late Oligocene-Early Miocene flood basalts (Juch, 1978), u p o n which the Sagatu Ridge is founded. The Sagatu Ridge expresses a 7 km-wide zone of magmatic ascent. Major eruption was concentrated within I k m to either side of the present topographic crest of the Ridge. Because the Ridge does not continue north to intersect with the NE-trending rift margin, the relationships between e r u p t i o n a n d rift faulting r e m a i n uncertain. Mohr and Potter (1976) considered that, for a brief period during Late Pliocent-Early Pleistocene, crustal extension a n d associated magmatic ascent J u m p e d east from the rift valley some 50 k m to the Sagatu line. This line m a y have b e e n determined by a n ancient zone of crustal weakness, identified farther south as a Late Precambrian collisional s u t u r e (Kazmln, 1976). Although hawaiitic dolerites comprise the bulk of the dykes, the additional presence of comenditic dykes at the Gara Badda centre (Mohr, 1980) indicates bimodal m a g m a t i s m s u c h as is characteristic of a n active continental rifting e n v i r o n m e n t (Gibson a n d Walker, 1963; Macdonald, 1987).

39

K-Ar analyses on three Badda dykes, one dolerite and two comendite, were m a d e by Geochron Labs (Cambridge, Mass.) for Mohr a n d Potter (1976). These a u t h o r s interpreted the resulting data,and two previously published analyses falling within the same apparent age-bracket (Kunz et al., 1975) as isotopically u n p e r t u r b e d at the time of eruption and cooling. T h u s fissure volcanic activity at Sagatu was concluded to have s p a n n e d the interval, 3.6-2.1 M.a. Despite some Sagatu lava surfaces having laterite veneers, a 1.5 M.a. period of dyking h a s seemed excessive, given the small size of the fissure zone relative to the scale of the rift valley. For this reason, a new program of K-Ar isotopic dating was instigated (J.G.M.), in coordinationwith Sr-isotopic m e a s u r e m e n t s (P.S.K.). For descriptions of the analytical methods, see Wilkinson et aL (1986) and K e n n a n et al. (1987). Table I lists the new a n d previous K-At analyses; the Geochron Labs' data have b e e n recalculated using consistent decay c o n s t a n t s (Steiner and Jaeger, 1977). The new data s p a n a range of apparent ages similar to the previous data, between 3.4 a n d 2.2 M.a. However, even a 1.2 M.a. period of dyking remains excessive in the context of the volcanological evidence. Figure 2 shows a plot of radiogenic 4°Ar versus wt. percent K20 incorporating both Geochron Labs' a n d our new data. A linear relationship is apparent (correlation coefficient = 0.994). The slope of the line yields a n age of 1.97_+ 0.02 M.a. The zero-intercept of the line occurs at (5.8 + 0.2) x i 0 "s m m a. gm "I 4OAr. The non-zero intercept of a correlation diagram of this kind c a n be a consequence, either of incorporation of a uniform a m o u n t of 4°Ar at the time of dyke emplacement (Welin a n d Lundquist, 1975),

40

P.S. KENNAN,J. G. M_rrc~.LLand P. MoI-m

~45

Be o~si ~ 4 - - -

Nazret 3

o

Zikwela

/

/ "

/

/

/

/

.J

9Newcastle analyses • Geochron a n a l y s e s x

Chilalo

Baddo

/

0

I

Robi

!

!

l

,~r f co// /

/

/

/

0 Koka

Enkwolo

/ /

20km

44 i

8-

.-.

Fig. 1. Location map of the Sagatu Ridge, eastern margin of the Ethiopian rift valley and some 100 km SSE of Addls Ababa. The dyke-swarm Is shown with heavy dashed trace. Volcano calderas are shown by heavy ticked trace and dotted trace marks approximate topog r a p h i c foot of the volcanic edifice. The eastem escarpment of the rift Is Indicated ticked on the downthrown side. Lakes (thin trace), roads (thin dashed line) and towns are shown for reference.

Sample no.

I~O 4°Ar*.104 wt.% mm3.gmq

B 16" B766 B13 B764 B 14" B767 B765 B761 B17* B17

1.24 1.10 2.51 2.47 4.41 4.72 4.54 4.70 4.46 4.61

1.49 1.19 2.13 2.16 3.05 3.40 3.56 3.56 3.70 3.67

h c = 0.581 x 1(~1°al h a = 4.962 x 101°a -I = 1. 167 x 10 .2 atom%

2

3

4

5 % KzO

Pig. 2. lsochron plot of radiogenic 4°At versus percentage potash in the analysed wholeroek dyke samples. The slope of the best-fit line corresponds to an age of 1.97 + 0.02 M.a. or of p o s t - e m p l a c e m e n t e x c h a n g e of p o t a s s i u m a n d / o r r a d i o g e n i c a r g o n (Mitchell a n d Ineso n , 1988). B e c a u s e t h e r a n g e of o b s e r v e d p o t a s s i u m contents conforms with a progressive geochemical evol ut i on in t h e S a g a t u s e q u e n c e hawaiitem u g e a r i t e - a l k a l i t r a c h y t e - c o m e n d i t e (Mohr, 1980), a m odel is a d v a n c e d h e r e of initial a r g o n i n c o r p o r a t i o n to e x p l a i n t h e r e g u l a r i t i e s i n t h e c o r r e l a t i o n d i a g r a m . T h e fact t h a t all t h e d a t a , from b o t h m afi c a n d sflicic d y k e r o c k s , c o n f o r m to a single line i n d i c a t e s t h a t a si m i l ar initial a r g o n c o n t e n t of 5.8 x 10 "~ m m 3. g m -I w a s c o m m o n to t h e m all. T h l s c o m m o n level of initial a r g o n a c c o u n t s for t h e g r e a t e r p e r t u r b a t i o n of t h e a p p a r e n t age of t h e b a s a l t s , 3. 5 + 0. 3 M.a. c o m p a r e d w i t h t h a t of t h e m o r e silicic d y k e rocks, 2.4 + 0. 4 M.a. T abl e I). A 1.97 M.a. age for t h e S a g a t u Ridge e r u p t i v e activity Is c o n s i s t e n t w i t h a 1.9 - 1.2 M.a. r a n g e for

Table 1. K-Ar analyses of Sagatu dyke-rocks Age (M.a.) Excess4°Ar assuming % atmospheric + lo a 1.97 M,a. age (104mm3.gm 1) contamination 93.2 85.1 70.9 72.1 88.9 54.6 52.4 53.1 96.6 94.2

3.72 + 0.40 3.35 + 0.13 2.63 + 0.07 2.71 + 0,04 2.14 + 0.20 2.23 + 0.04 2.43 + 0.04 2.35 + 0.03 2.57 + 0.30 2.47 + 0.09

Samples suffixed * were analysed by Geochron Laboratories

~K

~'XXX X

/.

Ase e2/[]

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oo-/

~/Alutu

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0.703 0.491 0,540 0.592 0.250 0.403 0.677 0.575 0,868 0.743

41

The Sagatu Ridge dyke swarm, Ethiopian rift margin: revised age and new Sr-isotopic data t h e silicic ignimbrites of Oara Chilalo (Di Paola, personal communication). Chilalo volcano is situated only 25 k m west of Gara B a d d a a n d the two centres are considered to have c o m m e n c e d activity together (Mohr a n d Potter, 1976). 1.9 M.a. is also t h e age p r o p o s e d by Meyer et al. (1975) for t h e birth of t h e WonJi fault belt, w h e n crustal extension in t h e Ethiopian riR valley transferred from t h e rift m a r g i n to rift floor faults. However, Morton et aL (1979} prefer to assign a n age of 0.4 M.a. to this transfer. The s t a n d a r d deviation o n o u r 1.97 M.a. age limits t h e period of activity at S a g a t u to 40 000 years (at the l a confidence level). This is in keeping with recent volcanological evidence for the rapid a c c u m u l a t i o n of flood basalt piles (eg. Baksi a n d Watkins, 1973; D u n c a n a n d Pyle, 1988). Therefore, if extension a n d e r u p t i o n at S a g a t u was a transfer of t h e s e processes from t h e rift valley to a m o r e easterly locus, it w a s nevertheless extremely brief. It is p e r h a p s m o r e likely t h a t a n episode of fast crustal widening a n d related m a g m a t i s m ca. 2.0 M.a. ago i n t h e rift valleywidened out to include peripheral regions. S R - I S O T O P E DATA

The only previously p u b l i s h e d Sr-isotope data for E t h i o p i a n p l a t e a u lavas c o n c e r n six late TertiaryQ u a t e r n a r y b a s a l t s from t h e n o r t h w e s t e r n e s c a r p m e n t of t h e Harar Plateau, some 50 k m NE of Gara Badda (Barbieri et al., 1976). These s p e c i m e n s showed a range of basalt alkalinity from transitional t h r o u g h alkaline to phonolitic tephrite. All sixyielded similar aVSr/aaSr ratios, with a m e a n value of .7035 _+ .0004. Table 2 lists t h e new d a t a for Sagatu dyke rocks. SrSr/aeSr values for c o m e n d i t e s are significantly higher t h a n for basalts a n d hawaiites, the extreme values being .7105 a n d .7039 respectively. The latter value is n o t significantly different from t h a t obtained for plateau e s c a r p m e n t basalts by Barbieri et al. (1976). A linear relationship exists between arSr/~Sr Table 2. St-isotopic analyses of Sagatu dyke-rocks Sample no. Rb(ppm) Sr(ppm) STRb/s6Sr 87Srp~Sr 3766 313 3764 3767 3765 3761 317

69 49 52 24 72 98 85

783 560 561 7.3 10.8 8 4.7

.2532 .2523 .2664 9.47 19.40 35.6 52.59

.70388 + 4 .70469 +10 .70423 + 4 .70691 __+18 .70568 _+18 .70752_+10 .71046_+14

a n d I / S r (Fig. 3). This suggests t h a t t h e St-poor c o m e n d i t e s show the effects of a crustal contamin a t i o n proportionately more strongly t h a n t h e mafic rocks. Davies a n d Macdonald (1987) argue from Sr, Nd a n d Pb isotopic data t h a t comendites in the Kenya rift are derived from crustal melts a n d have not b e e n derived from basaltic m a g m a by fracUonation. We do n o t have sufficient data to decide o n this m a t t e r for Sagatu, a l t h o u g h t h e presence of olivine clots in s o m e c o m e n d i t e dykes could indicate a role for crystal fractionation. If a n age of 1.97 M.a. is a d o p t e d for all Sagatu dykes, the/nd/v/d-o_! initial argon c o n t e n t s of t h e s a m p l e s m a y be calculated from their m e a s u r e d potassium and radiogenic argon contents (Table 1). These separate v a l u e s obviously differ from the statistically averaged value obtained from the p o t a s s i u m - a r g o n correlation diagram. W h e n these initial argon values are plotted against aZSr/ aeSr (Fig. 4), it is evident t h a t the initial Sr-isotope values correlate directly with the initial argon c o n t e n t s (excepting sample B767). If crustal c o n t a m i n a t i o n is responsible for the elevated Srisotopic ratios, t h e n this suite of Sagatu dyke rocks is remarkable in displaying a n initial argon content t h a t m u s t be linked to the s a m e cause. More data are required to explore this u n e x p e c t e d possibility, t h a t selective c o n t a m i n a t i o n gradients from P r e c a m b r i a n b a s e m e n t into dyke m a g m a were similar for Sr a n d Ar. The presence or a b s e n c e of continental c r u s t b e n e a t h rifted c r u s t is considered by Tazieff a n d Varet (1969) to be indicated in Sr-isotopic values in silicic volcanics. From Q u a t e m a r y spreading zones w i t h i n n o r t h e r n Afar, t h e y o b t a i n e d ~Sr/86Sr values averaging .7035 + .0018 for b o t h basalts a n d fractionated rhyolites, w h e r e a s a centre n e a r the Afar-Plateau m a r g i n yielded values u p to.7158. The Sagatu comendites, in reflecting t h e latter situation, confirm the presence of continental crust b e n e a t h t h e Ridge.

~ '710. 708.

'706l

o

o. bs

o-lb

o.(5

I/Sr

o#_o

Fig. 3 *TSr/°6Sr v. I/Sr plot for Sagatu dykes, indicating a tendency to a direct relationship between these parameters.

42

P. S. KENNAN,J. G. Mrrc~mLLand P. MOHR

influences in the petrogenesis of the Naivasha basaltcomendite complex: combined trace element a n d SrNd-Pb isotope constraints. J. ~ 28, 1009-1031. A Di Paola, G. P. 1977. Geological m a p of the Ttfllu Moje volcanic a r e a (1:75 000). Conslglio Naz. Ricerche, Firenze. Duncan, R. A. a n d Pyle, D. G. 1988. Rapid eruption of 'I", ' 8 0 0 the Deccan flood b a s a l t s at the C r e t a c e o u s / T e r t l a r y boundary. Nature, 333, 841-843. "E Gibson, I. L. a n d Walker, G. P. L. 1963. S o m e composite x r h y o l i t e / b a s a l t lavas a n d related composite d y k e s in E eastern Iceland. Proc. GeologlstAssoc. 74, 301-318. "70. J u c h , D. 1978. Geologle des Aethiopischen Suedost< o E s c a r p m e n t s zwischen 39 ° u n d 42 ° oesflicher Laenge. Clausthaler Geol. Abh. 29, 1 - 139. Kazmin, V. 1976. Ophiolites in the Ethiopian Basement. Ethiopian Institute of Geological Surveys, Note 35, 16 p. '60Kennan, P. S., Feely, M. a n d Mohr, P. 1987. The age of the O u g h t e r a r d Granite, C o n n e m a r a , Ireland. Geol. J., 22. 273-280. Kunz, K., Kreuzer, H. a n d Mueller. P. 1975. P o t a s s i u m argon age determination of the Trap basalt of the 50s o u t h e a s t e r n p a r t of the Afar rift. In: Afar Depression of Ethiopia ~,d. by Pilger, A. and Roesler, A.) Schweizerbart, Stuttgart, 370-374. Macdonald, R. 1987. Q u a t e r n a r y peralkaline silicic rocks and caldera volcanoes of Kenya. In: Alkaline "40 7 t00 7120 Igneous Rocks ~ d . b y Fltton, J. G. a n d Upton, B. J. G.) . 64o 76+o 7+8o Geo/. Soc. Lond. Spec. Publ. 30, 313-333. ~TSr/8~Sr~ Meyer W., Pilger, A., Roesler, A. a n d Stets, J. 1975. Fig. 4 Plot of inltial ("excess") +°Arv. initial sTSr/a6Sr in Sagatu Tectonic evolution of the n o r t h e r n p a r t of the m a i n dykes (Tables I and 2}. Symbols as on Fig. 2. The aberrant E t h i o p i a n rift in s o u t h e r n Ethiopia. In: Afar point at the bottom of the graph is for comendite B767; the Depression of Ethiopia (Ed. by Pilger, A. a n d Roesler, relatively low excess 4°Ar content in this dyke is not explained. A.) Schweizerbart, Stuttgart, 352-362. Mitchell, J. G. a n d Ineson. P. R. 1988. Models of singleCONCLUSIONS stage concomitant p o t a s s i u m - a r g o n exchange: a n interpretation of discordant wholerock K- Ar d a t a from F i s s u r a l a c t i v i t y a l o n g t h e S a g a t u R i d g e is n o w hydrothermally altered igneous rocks of the South m o r e p r e c i s e l y d a t e d a t 1.97 + 0 . 0 2 M.a, T h i s Pennine Orefleld, U.K. Earth Planet. ScL Lett. 88, 69a c t i v i t y w a s brief, in c o n f o r m i t y w i t h t h e geologic 81. and volcanological evidence. Sr-isotope ratios in Mohr. P. 1980. G e o c h e m i c a l a s p e c t s of the Sagatu Ridge the comendites reveal significant contamination dike swarm, Ethiopian rift margin. Attl dei Conoegni from continental crust. A direct relationship LinceL 47, 387-406. b e t w e e n S r - i s o t o p e r a t i o a n d initial r a d i o g e n i c Mohr, P. a n d Potter, E. C. 1976. The S a g a t u Ridge dike swarm, Ethiopian rift margin. J. Volcan. Geotherm a r g o n c o n t e n t i n t h e S a g a t u d y k e r o c k s is w o r t h y Res. I, 55-71. of f u r t h e r i n v e s t i g a t i o n . Morton, W. H. Rex, D. C., Mitchell, J. G. a n d Mohr, P. 1979. Riftward younging of volcanic u n i t s in the Addis REFERENCES A b a b a region, Ethiopian rift valley. Nature, 280. 284Baksi, A. K. a n d Watkins, N. D. 1973. Volcanic 288. production rates: comparison of oceanic ridges, islands Steiger, R. H. a n d Jaeger, E. 1977. S u b c o m m i s s i o n on a n d the C o l u m b i a Plateau basalts. Scfence, 180, 493Geochronology:. convention on the u s e of d e c a y consrants in geochronology a n d cosmochronology. Earth 496. Barberi, F., Civetta, L. a n d Varet, J. 1980. Strontium Planet. ScL Lett 38, 359-362. isotopic composition of the Afar volcanics and its Tazieff, H. a n d Varet, J. 1969. Signification tectonique implications for mantle evolution. Earth Planet. ScL et m a g m a U q u e de l'Afar septentrional (Ethiopie). Rev. LetC 80, 247-259. Geogr. Phys. Dynam. (2) I I, 429-450. Barbieri, M., Brotzu, P., Morbidelli, L., Penta, A., Welln, E. a n d Lundquist. T. 1975. K-At ages of J o t n i a n Piccirillo, E. M. a n d Traversa, G. 1976. Trace elements dolerites in Vasternorrland County, Central Sweden. a n d STSr/SeSr ratios of the basic stratoid volcanism in Geol. ForerL Stockholm Forhu 97, 83-88. the s o u t h e a s t e r n Ethiopian plateau. Periodfco di Wilkinson. P., Mitchell, J. G.. Cattermole, P. J. and Mineralogla, 45, 129-145. Downie, C. 1986. Volcanic chronology of the MeruDavies, G. R. a n d Macdonald, R. 1987. Crustal Kflimanjaro region. Northern Tam,~ania. J. Geol. Soc. Lond. 143, 601-605. -90-