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Recent eruptive episodes of the Rungwe volcanic field (Tanzania) recorded in lacustrine sediments of the Northern malawi rift
Journal of African Earth Sciences, Vol. 17, No. 1, pp. 33-39, 1993. Printedin GreatBritain
0899-5362/93 $6.00+ 0.00 © 1993PergamonPresslad
Recent eruptive episodes of the Rungwe Volcanic Field (Tanzania) recorded in lacustrine sediments of the Northern Malawi Rift T. M. WILLIAMS,P. J. HENNEYand R. B. Ow~* British GeologicalSurvey,GeochemistryUnit, Keyworth,Notts, U. K. *Hong KongBaptist College,224 WaterlooRd., Kowloon,Hong Kong (First received 16th December, 1992; revisedversion received 10th May, 1993) Abstract - Discrete ash horizons in Holocenesedimentsfrom northern LakeMalawiprovide evidenceof six
eruptiveepisodeswithinthe nearbyRungweVolcanicFieldbetweenc.9000-360 BP. Rareearth element(REE) analyses show the ash layers to be stronglyenriched in La, Ce, Pr, Nd, Sm, Tb, Dy, Er, Tm, Yb and Lu, with low Eu/Eu* and high LaN/Sms values,relative to the surroundingmuds. Mixingcalculationssuggestpossible affinitiesbetweenthe Rungweash emissionsand silicicvolcanicsfromotherimportantQuaternarycentres(e.g. Naivasha) withrespect to HREE geochemistry.The LREEspectraare less comparable and may indicatea less fractionatedash assemblagefor RungweField.In the absenceof clear in situ evidenceregardingthe timingand frequencyof Holoceneeruptions at Rungwe,the LakeMalawisedimentsmay prove a valuablereconstructive tool. However,the directionand extentof ash dispersal is stronglycontrolledby wind/climaticfactorsand the retention of a completerecord at any singlelocationis unlikely.
REGIONAL
INTRODUCTION
Lake Malawi occupies the m o s t s o u t h e r l y b a s i n of the E a s t African rift s y s t e m a n d is the second largest w a t e r b o d y in Africa by volume (Williams a n d Owen, 1990). The syn-rfft s e d i m e n t s of the lake floor exceed 4 k m t h i c k n e s s (Scholz a n d R o s e n d a h l , 1988) a n d r e t a i n a record of regional tectonic, climatic, biotic a n d hydrological variat i o n s o n t i m e - s c a l e s r a n g i n g f r o m 10-108 y r (Pilskaln a n d J o h n s o n , 1990). During the p a s t decade, over 250 shallow s e d i m e n t cores have b e e n recovered from t h e Malawi Rift to a s s e s s the basin-floor m i n e r a l potential (Crossley a n d Owen, 1987; Williams a n d Owen, 1992), the surficial lithofacies d i s t r i b u t i o n (Crossley a n d Owen, 1988), r e l a t i o n s h i p s b e t w e e n rift s t r u c t u r e a n d sedimentation (Owen a n d Crossley, 1990) a n d the palaeohydrological stability of the lake (Owen et al., 1990). The identification of several a s h horizons d u r i n g lithological e x a m i n a t i o n of profiles from t h e n o r t h e r l y K a r o n g a S u b - b a s i n ( J o h n s o n etal., 1988) s u g g e s t s t h a t t h e s e cores m a y have additional v a l u e s for a p p r a i s i n g the Holocene activity of t h e R u n g w e Volcanic Field, t h e only k n o w n eruptive centre in t h e S. Tanzania-N. Malawi region (Fig. 1). Here, we describe a preliminary lithological a n d geochemical investigation of a s h layers in three cores of 1-6 m length a n d a s s e s s t h e i r significance with respect to regional v o l c a n i s m a n d palaeolimnology.
Malawi
SETTING
Rift
The S. T a n z a n i a / N . Malawi b a s e m e n t c o m p r i s e s a series of B u k o b a n clastic a n d mafic rocks, felsic i n t r u s i o n s , g n e i s s e s a n d schists. U b e n d i a n highgrade g n e i s s e s are widely exposed, locally carrying Irumide and Pan-African tectonic overprints (Dodson et al., 1975). Karroo m u d s t o n e s , coals a n d s a n d s t o n e s outcrop in several areas, n o t a b l y the R u h u h u Basin, T u k u y u a n d Rungwe. Cretac e o u s siltstones a n d m u d s t o n e s o c c u r at t h e northern e n d of Lake Malawi in t h e Songwe h e a d w a t e r s . L a c u s t r i n e d e p o s i t s of Plio-Pleistocene age (Dixey, 1926) r e a c h a t h i c k n e s s of 200 m in a restricted area close to the n o r t h - w e s t e r n lake shore. The Lake Malawi b a s i n reflects a relatively l a t e (late Miocene/Pliocene) s o u t h w a r d e x t e n s i o n of the E a s t African Rift S y s t e m (Crossley a n d Crow, 1980) a n d c o m p r i s e s a series of half-grabens, a l t e r n a t i n g a s y m m e t r i c a l l y along a single axis (Crossley, 1984; Scholz e t a l . , 1990). Half-grabens are c o m m o n l y b r o k e n by n o r m a l f a u l t s to p r o d u c e discrete s u b - b a s i n s within each block. Scholz et aL (1990) have sub-divided the Malawi Rift into four discrete u n i t s on the b a s i s of differing h a l f g r a b e n d o m a i n or dip-linking relationships. The p r e s e n t s t u d y relates exclusively to s e d i m e n t s of the n o r t h e m m o s t unit, in w h i c h s u b s i d e n c e along t h e Livingstone Border F a u l t a n d steep tilting of a
33
34
T. M. WILUAMS,P. J. ~
and R. B. Ow~
a n altitudional range of < 500-> 3 0 0 0 m. A confining rift scarp to t h e east comprises U b e n d i a n schists a n d gneisses of t h e Livingstone Mountains. A less clearly defined w e s t e r n b a s e m e n t scarp is partially overlain by s e d i m e n t s of Karoo a n d Cretaceous age. Detailed m a p p i n g a n d petrograO KM lOOO phic classification (Harkin, 1960; Ebinger et al., ! . J / H / 1989) h a s facilitated a simple division of t h e pred o m i n a n t l y trachytic, phonolitic a n d basaltic volcanics into "Older" a n d '~rounger" Extrusive is series (the former ascribed mainly to the Porotos, KIVU / VIRUNGA ¢s Katete a n d less expansive Jandjuli, Kisangasi, Wehe a n d C h a l u h a n g a vents a n d the latter closely ! / associated with Rungwe a n d Kiejo). Harkln (1960) f ! I tentatively assigned initial eruptive activity to the Pliocene, b u t K-Ar ages of up to 7.25 Ma for the I lowermost phonolitic trachyte in a stratigraphic I '~ section of Karonga Trough have s u b s e q u e n t l y confirmed activity during the Miocene (Ebinger et a/., 1989). The Ngana Tuff of the Karonga Basin / a n d felsic a s h deposits of t h e central Songwe Basin 1 constitute the y o u n g e s t dated deposits with ~4C ages of a r o u n d 11,000 BP (Crossley, 1982, Ebinger et aL, 1989). S u b s e q u e n t activity is poorly known, although the presence of u n e r o d e d cinder cones at Fig. I. Regional position of the Rungwe Volcanic Kiej o m a y indicate a very recent (< 200 yr BP) origin Province and other important volcanic centres in the for units s u c h as the Sarabwe Tephrite (Harkin, East African Rift System. 1960).
,/
single half-graben h a s p r o d u c e d the lake's deepest basinal setting (> 500 m). Rift s t r u c t u r e - l i t h o f a c i e s r e l a t i o n s h i p s have b e e n evaluated in detail t h r o u g h sediment coring (Crossley, 1984; Crossley a n d Owen, 1988; Owen a n d Crossley, 1990, Owen et al., 1991), echosounding a n d multi-channel seismic surveys (Scott, 1988; J o h n s o n a n d Davis, 1989; Scholz et oL, 1990). The superficial deposits are dominantly diatomites in t h e highly productive shallow riftsettings of t h e s o u t h - w e s t a n d s o u t h - e a s t a r m s of the lake, turbidite s a n d s along the well defined axes of b o u n d a r y faults, coarse sheet s a n d s over horst block a n d r a m p s t r u c t u r e s and varved pelagic m u d s in the deep Livingstone sub-basin. The latter result from seasonally alternating algal/ clastic s e d i m e n t a t i o n a n d the formation of a n n u a l d i a t o m - m u d couplets generally less t h a n 1 m m thick. The varves are preserved d u e to the permanently anoxic b o t t o m waters a n d c o n s e q u e n t lack ofbioturbation (Owen a n d Crossley, 1990; Williams a n d Owen, 1990; Owen et oL, 1990). Rungwe Volcanic Field The Rungwe Volcanics cover some 1500 k m 2 in the a c c o m m o d a t i o n zones of the Karonga (Lake Malawi), Songwe a n d U s a n g u t r o u g h s in s o u t h e r n Tanzania (8°45'-0°35"S a n d 33°I0'-34°0'E), with
METHODS The locations of coring stations 525, 527 a n d M86-16P are s h o w n in Fig. 2. Cores 525 a n d 527 were obtained from water d e p t h s of < 200 m on t h e western m a r g i n of the Livingstone s u b - b a s i n using a 2 m long by 5 c m diameter gravity corer weighted with a 50 kg drum. Core M86-16P w a s recovered from 450 m depth using a 6 m piston corer. Following collection, all cores were air-dried a n d logged with t h e assistance of a binocular microscope. Cores 525 a n d 527 were s u b - s a m p l e d at 1 c m resolution for geochemical analysis. Ignition loss (LOI) values were d e t e r m i n e d by weighing s u b - s a m p l e s of approximately 2 g m a s s prior to a n d following c o m b u s t i o n at 450°C in a muffle furnace. Samples of 1.0 g weight were u s e d to determine La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Er, Tm, Yd and Lu by ICP-MS following preparation by lithium metaborate fusion. Analytical precision (calculated using a series of international reference standards) was below 4 % within t h e REE concentration range 10-200 p p m , rising to 10 % at concentrations of 0.1-1 ppm. Age determinations for four stratigraphic levels of core M86-16P were obtained by c o n v e n t i o n a l ~4C m e t h o d s , at t h e SURRC Radiocarbon Laboratory, Scotland, U. K. (Fig. 2). U n s u p p o r t e d 2~°Pb assays were obtained for s u b - s a m p l e s (taken at I c m resolution) from t h e
Recent eruptive episodes of the Rungwe Volcanic Field (Tanzania) recorded in lacustrine sediments... 525
M86-16P
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527
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Fig. 2. Lithological logs and sampling locations: cores 525, 527 and M86-16P (also showing ~4C ages}. u p p e r 20 c m of c o r e s 5 2 5 a n d 527 u s i n g directc o u n t i n g g a m m a spectrometry. The a s s a y s were s u b s e q u e n t l y integrated into the C o n s t a n t Rate of S u p p l y (CRS) m o d e l (Appleby et al., 1984), to e s t a b l i s h a n average s e d i m e n t a t i o n rate of 1.2 m m / y r for t h e s e surficial strata.
RESULTS AND DISCUSSION Core l i t h o l o g y The lithological logs for c o r e s 525 a n d 527 (Fig. 2) s h o w t h e surficial s e d i m e n t s at t h e s e s t a t i o n s to b e largely h o m o g e n e o u s m u d s , with only occasional p r e s e r v a t i o n of A u l a c o s e i r a - S t e p h a n o d i s c u s diat o m laminae. In core M 8 6 - 1 6 P (Fig. 2) several l a m i n a t e d d i a t o m - m u d s e q u e n c e s are preserved d u e to the g r e a t e r w a t e r d e p t h a n d c o n s e q u e n t lack o f b i o t u r b a t i o n . The l a m i n a t e d s e q u e n c e s are s e p a r a t e d b y discrete s a n d horizons (e.g. 1.9 m, 2.8 m, 3.9 m, 4 . 6 m , 5.7 m a n d 6-6.5 m), d e p o s i t e d a s either t u r b i d i t e flows or possibly a s shalloww a t e r facies during low-lake s t a n d s a n d a s s o c i a t e d regressive cycles (Owen et aL, 1990). Two visible a s h layers are identifiable in each of the "short" c o r e s 5 2 5 a n d 527, located at 32 c m a n d (less distinctively) at 80 c m in t h e former a n d 30 c m
a n d 62 c m in t h e latter (Fig. 2). Given t h e close proximity of t h e s e coring stations, t h e 5 2 5 - 3 2 c m a n d 5 2 7 - 3 0 c m horizons are c o n s i d e r e d to be s y n c h r o n o u s deposits, c o r r e s p o n d i n g to a single eruptive event at a r o u n d 3 6 0 BP (based on downcore e x t r a p o l a t i o n of t h e 21°Pb-derived average s e d i m e n t a t i o n rate of 1.2 m m / y r for surficial sediments). Correlation of t h e lower 5 2 5 - 8 0 c m a n d 5 2 7 - 6 2 c m a s h e s is m o r e t e n u o u s , a n d w o u l d require evidence of d i s p r o p o r t i o n a t e l y high sedim e n t a t i o n during the a c c u m u l a t i o n of t h e 3 0 - 8 0 c m s e q u e n c e at s t a t i o n 525. This may, in part, b e provided b y t h e p r e s e n c e of diffuse a s h particulates from 70 c m t h r o u g h to 80 c m (Fig. 2), indicating dilution in a rapidly a c c u m u l a t i n g terrestrial clastic/biogenic matrix. Core M 8 6 - 1 6 P h o l d s clear a s h horizons at 2 0 0 cm, 2 4 0 cm, 362 c m a n d 6 4 0 c m depth. O n the b a s i s of the 14C chronology (see Fig. 2), t h e s e layers c a n be a p p r o x i m a t e l y d a t e d to 3 5 0 0 , 3 7 5 0 , 5 7 5 0 a n d 9 0 0 0 BP respectively. P u m i c e f r a g m e n t s are evident at 2 3 0 c m a n d 6 4 0 c m a n d m a y r e p r e s e n t p a r t i c u l a r l y explosive e r u p t i v e e p i s o d e s . It is notable t h a t no visible a s h o c c u r s in t h e u p p e r I m of s e d i m e n t a n d t h a t t h e r e c e n t eruptive events inferred from c o r e s 525 a n d 527 are t h u s not recorded. Fig. 3 illustrates t h a t s t a t i o n M 8 6 - 1 6 P is
T. M. WILLIAMS,P. J. HENNEYand R. B. OWEN
36
c o n s i d e r a b l y m o r e d i s t a n t from the R u n g w e s o u r c e ( I 0 0 k m SE c o m p a r e d to c. 50 k m for c o r e s 525 a n d 527). More e x t e n s i v e a t m o s p h e r i c d i s p e r s a ! , c o u p l e d with t h e g r e a t e r w a t e r depth, m a y c a u s e dilution of a s h to t h e extent that only the m o s t i n t e n s e e v e n t s are registered in t h e s t r a t i g r a p h y of core M86-16P. REE g e o c h e m i s t r y Total REE a n d LOI d a t a for a s h layers at 525-32 c m a n d 5 2 7 - 3 0 cm, a mixed a s h / p e l a g i c unit at 5 2 5 - 8 0 c m a n d "clean" m u d s from 5 2 5 - 5 2 cm, 5 2 5 - 7 0 c m a n d 5 2 7 - 2 3 c m are provided in Table 1. Total organic c o n t e n t is significantly d e p r e s s e d in the a s h layers a n d t h e LOI v a l u e s m a y t h u s provide a c o n v e n i e n t b a s i s for ascertaining the "cleanest" (i.e. m o s t a s h free) levels. The volcanogenic horiz o n s s h o w e n r i c h m e n t of b o t h light (LREE) a n d h e a v y (HREE) REEs, relative to t h e s u r r o u n d i n g matrix. O n c o m p a r i s o n of a s h s a m p l e 525-32 a n d m u d s a m p l e 525-52, REE e n r i c h m e n t factors of La 3.10, Ce 2.66, Pr 2.48, Nd 2.09, S m 1.71, Eu 1.03,
Tb 1,87, Dy 1.93, Er 2.36, Tm 3.12, Yb 2.81 a n d Lu 3.61 are e s t a b l i s h e d for the former. The distinctive lack of Eu variation b e t w e e n ashbearing a n d ash-free s a m p l e s is highlighted from their c h o n d r i t e - n o r m a l i s e d profiles (Fig. 3). This c o u p l e d with the above n o t e d e n r i c h m e n t of b o t h lighter (Sin) a n d heavier (Tb) R E E s c r e a t e s a s h a r p d e p r e s s i o n of E u / E u * v a l u e s to below 0.55 in t h e a s h (compared with c. 0 . 7 - 0 . 8 5 for "clean" sedim e n t horizons). It is also n o t a b l e that while the c h o n d r i t e - n o r m a l i s e d levels of all a n a l y s e d R E E s excluding Eu are i n c r e a s e d relative to ash-free sed i m e n t s in t h e "purest ash" s a m p l e s 5 2 5 - 3 2 a n d 527-30, only the LREEs (notably La a n d Ce) are significantly e n h a n c e d at lower a s h c o n c e n t r a tions (such as prevail in t h e matrix of 525-80). The LREEs m a y t h u s be c o n s i d e r e d the m o s t sensitive indicators of volcanic influxes in this setting. The m a r k e d steepening of LREE g r a d i e n t s a s s o c i a t e d with a s h deposition is illustrated b y the high LaN/ S m x ratios (> 9.0) of 525-32 a n d 527-30, while a r e d u c e d gradient a c r o s s the HREE s p e c t r u m is a p p a r e n t from a lowering of TbN/LU N v a l u e s (Fig. 4).
Table 1. REE and LOI data for subsamples from cores 525 and 527. (All REE analysis given in ppm after LOI) Sample 525-32 525-52 525-70 525-80 527-23 527-30
La 286.21 92.29 92.69 150.79 117.68 233.65
Ce 501.23 188.73 186.31 264.31 235.65 417.94
Pr 45.47 18.33 18.98 24.79 23.74 38.74
Nd 128.55 61.68 64.92 74.20 80.39 112.42
Sm 17.13 9.97 10.71 10.53 13.26 15.68
Eu 2.05 1.98 2.27 1.81 2.71 2.18
Tb 2.12 1.13 12 7 1.14 1.74 1.94
Dy 13.38 6.93 7.54 7.24 9.84 12.09
Er 8.31 3.51 4.00 3.96 5.18 7.36
Tin 1.25 0.40 0.49 0.50 0.74 1.08
Yb R.94 3.18 344 3.94 4.67 7.55
Lu 1.23 0.34 0.43 0.48 0.68 1.04
LOI% 9.94 17.84 16.99 12.15 16.95 12.17
Possible affinities o f ash h o r i z o n s
1ooo
The R u n g w e Volcanic Province forms one of several m a j o r volcanic c e n t r e s in the W e s t e r n Rift (e.g. Virunga, Kivu), b u t is d i s t i n g u i s h e d b y a p r e d o m i n a n c e of silicic r a t h e r t h a n basaltic acti100 vity (Harkin, 1960; K a m p u n z u a n d Lubala, 1991). Diagnostic evidence of R u n g w e a s h in Lake Malawi s e d i m e n t s s h o u l d t h u s include the d e v e l o p m e n t of o REE s i g n a t u r e s with increasing r e s e m b l a n c e to t h o s e of evolved v o l c a n i c / v o l c a n i c l a s t i c lithoo 10 ~ 525-52 ~" "l logies. While no REE data for the R u n g w e Field have b e e n p u b l i s h e d , d a t a for c o n t e m p o r a n e o u s acid volcanics of the K e n y a n Rift (Bailey a n d ,~ 527-23 / Macdonald, 1970; M a c d o n a l d et al., 1987; Davis 1 @ 527-30 , / and Macdonald, 1987) and elsewhere c a n provide i I I I I l I I l i I a c r u d e b a s i s for c o m p a r i s o n . Data for g l a s s y La Ca Pr Nd Sm Eu Tb Dy Er Tm Yb Lu o b s i d i a n s from Naivasha, Kenya (Macdonald et aL, 1987) a n d aphyric-silicic peralkaline a n d s u b a l k a Fig. 3. Chondrlte-normallsed REE profiles for line volcaniclastics from Jalisco, Mexico (Mahood, ash-horizons and non-volcanogenic 1981) have b e e n selected as "representative" sursediments: cores 525- and 527. rogates for u s e in this study.
L :;:;o
I
37
Recent eruptive episodes of the Rungwe Volcanic Field (Tanzania) recorded in lacustrine sediments...
The chondrite-normalised profiles of both the horizons. Data derived from binary mixing calculNaivasha and Jalisco volcanics (Fig. 5) are gull- ations indicate that the HREE (TbN/LuN) values of winged, with deep negative Eu anomalies and sample 525-32 could be produced by mixing of elevated LREE and HREE abundances. Both sui- 20% "non ash-bearing" Lake Malawi sediment tes are characterised by TbN/Lu Nvalues of c. 1.3, {Eu/Eu* 0.85; TbN/LU N 2.2) with 80% Naivasha/ E u / E u * values of < 0.1 and LaN/SrnNvalues of 2- Jalisco-type ash (Eu/Eu* 0.05; TbN/LUN 1.25). 6. Such values largely reflect the high degree of However, explanation of the LREE signatures is crystal fractionation of the magmas, coupled with more problematic. Samples 525-32 a n d 527-30 the effects of volatile enrichment and vapour- both display c h o n d r i t e - n o r m a l i s e d La-Ce-Nd phase separation (Mahood, 1981; Macdonald et values which are actually higher t h a n those of the a t , 1987). On comparing these signatures with the reference volcanics, and t h u s cannot be explained Lake Malawi core samples, the partial develop- by mixing of sediment and Naivasha/Jalisco-type m e n t of certain evolved volcanic "fingerprints" (e.g., ash. Whilst not discounting the possibility of increasingly negative Eu anomalies and flattening diagenetic LREE enrichment of the ash horizons of HREE profiles) is evident in the ash-bearing (though see below), such high values most probably reflect slightly less evolved assemblages in the 0.9 Rungwe ash showers (also supported by a less pronounced negative Eu anomaly). It is acknowledged that binary mixing calcula0.8 tions of the n a t u r e described are, at best, tenuous, as neither the Naivasha/Jalisco volcanics nor the "non-ash bearing" Lake Malawi sediments repre0.7sent genuine "end members". In the latter case, mineralogical studies (e.g. Yuretich, 1993) indiw cate that virtually all sediments deposited in north0.6. w ern Lake Malawi contain a minor component eroded from volcanic c a t c h m e n t rocks and are thus, O themselves, a product of dual/multiple source 0.5' mixing. O 0.4
I
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6
7
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Sediment
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REE c h a r a c t e r i s t i c s o f n o n - v o l c a n o g e n i c sediments
Shale-normalised REE for samples 525-32 and 525-52 (calculated in accordance with Piper, 1974) are presented in Fig. 6. The "non-ash" sample 525-
0.9
1000 0.8
1O0
0.7 +
LU
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0
525-32 255a
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Fig. 4. a) Eu/Eu* vs. LaN/Sm~, b) Eu/Eu* vs. TbN/LuN: sample cores 525 and 527.
210a
2.2
525-52 I
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i
La
Ce
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Sm
Eu
/
Tb
I
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i
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Lu
Fig. 5. Chondrite-normalised REE profiles for ashhorizon 525-32, sediment-horizon 525-52 and selected evolved volcanics from Naivasha, Kenya (MacDonald et al., 1987) and Jalisco, Mexico (Mahood, 1981).
38
T. M. WILLIAMS,P. J. I-IE~'Y and R. B. Ow~
52 is of particular interest as it shows disproportionate enrichment of LREE relative to global shale values (Haskin and Haskln, 1966). This trend contrasts with published d a t a for turbidites, pelagic clays, metalliferous sediments, marine clays and ferromanganese nodules (e.g. Piper, 1974), all of which display preferential upgrading of HREE relative to "average" shales. It is likely that LREE enrichment in 525-52 reflects the slow inwash of volcanogenic debris deposited over the Karonga S u b - b a s i n c a t c h m e n t area during eruptive episodes a n d transported in surface drainage alongside clays and associated terrigenous detritus. Alternative explanations involving preferential LREE enrichment during diagenesis appear inapplicable, given the absence of any characteristic Ce anomaly (Nagander et aL, 1992). The REE chemistry of the algal rain is also unlikely to be influential, as former observations suggest a tendency for HREE enrichment in diatoms (Piper, 1974).
Value o f ash h o r i z o n s for palaeolimnologlcal studies The identification ofcorrelatable core horizons is important in palaeolinmology for extrapolating radiometric d a t e s a n d calculating basin-floor s e d i m e n t a t i o n or c a t c h m e n t e r o s i o n r a t e s (Thompson, 1979; Dearing et aL, 1982). While
6-
¢I
W
tentative inter-core correlations in Lake Malawi have previously b e e n m a d e using microfossfl spectra (Owen et al., 1990), a s h layers s u c h as those identified in cores 525, 527 and M86-16P c a n provide a clear alternative. The approach h a s b e e n exemplified in tephra-bearing lake b a s i n s in northern Europe and S. E. Asia (Oldfield et a t , 1978; Thompson et aL, 1986) and is considerably less time-consuming t h a n palynological methods.
CONCLUSIONS Preliminary lithological and geochemical analyses of cores from the Karonga S u b - b a s i n indicate that ash layers in stratified s e d i m e n t s from northern Lake Malawi m a y provide a valuable record of the Holocene eruptive activity of the Rungwe Volcanicso which c o u l d u s e f u l l y s u p p l e m e n t published radiometric evidence f r o m / n situ flows (e.g., Ebinger et aL, 1989). However, the presence of ash layers of < 1000 yr age in sediments 50 km from Rungwe (cores 525, 527) and their absence at c. 100 km (core M86-16P) suggests that distance and (probably) factors s u c h as wind direction significantly influence the sedimentary ash-fall record at any individual site. The REE signatures of the ash layers confirm their silicic affinities, and provide tentative evidence that the Rungwe Field p o s s e s s e s a more LREE-rich character t h a n other evolved volcanlcs such as Naivasha and Jalisco. If u s e d with caution, the ash horizons rePorted here m a y provide a useful tool for core2correlation and radiometric date t r a n s p o s i t i o n in future palaeolimnological studies of the Lake Malawi basin.
rr
i La
•
i Ge
• i Pr
• i
• i
Nd Sm
. i Eu
- i Yb
, i
- i
- i
Dy
Er
Tm
• i
.
Yb
i I_u
LM,02 I
Diatoms
Marine clays Gironde
River
A c k n o w l e d g e m e n t s - The authors are grateful to Dr. T. Johnson and staff of Project PROBE for their kind provision of piston core M86-16P. The Malawi Government, Depertment of Fisheries placed their research vessel at our disposal during coring in the Lake Malawi basin. Funding for this work was provided by the British Government Overseas Development Administration, Malawi Government and Amoco Oil Co., for which the authors express thanks.
REFERENCES
•
La
Ce
Pr
Nd Sm
Eu
Tb
Dy
Er
Tm
Yb
1
Lu
Fig. 6. Shale normallsed profiles for Lake Malawl samples 525-32 and 525-52, plus marine clays (Piper, 1974), Lake Malawi ferromanganese nodules-LM702 (Williams, in press), algae and surface waters (Piper, 1974).
Applely, P. G., Nolan, P., Gifford, D. W., Oldfleld, F., Anderson, N. J. and Battarbee, R. W. 1984. 2~°Pb dating by low background gamma counting. Hgdrob~ol. • 141, 21-27. Bailey, D. K. and Macdonald, R. 1970. Petrochemical variations among mildly peralkaline (comendlte) obsidians from the oceans & continents. Contrtb. Mineral. Petrol 28, 340-351. Crossley, R. 1982. Late Cenozoic stratigraphy of the Karonga area in the Malawi Rift. Palaeoecol. Afr. 15, 139-144.
Recent eruptive episodes of the Rungwe Volcanic Field (Tanzania) recorded in lacustrine sediments... Crossley, R. 1984. Controls of sedimentation in the Malawi Rift. Sed/ment. Geo/. 40, 33-40. Crossley, R. a n d Crow, M. J. 1980. The Malawi Rift. In: Geodynamlc Evolution of the Afro-Arabic Rift system. Proc. Acco_dem/a del Lince/, Rome, 77-78. Crossley, IL a n d Owen, R. B. 1988. S a n d turbidites and organic-rich d i a t o m a c e o u s m u d s from Lake Malawi, Central Africa. In: Lacustr/ne Petro/eum Source Rocks (Eds. Fleet, A. J., Kelts, K. and Talbot, M, IL), Geo/. Soc. Lond. Spec. Publ. 40, 369-374. Davies, G. R. Macdonald, R. 1987. Crustal influences in the petrogenesis of the Naivasha basalt-rhyollte complex: combined trace element a n d Sr-Nd-Pb isotope constraints. J. Petrol 28, 1009-1031. Dearing, J. A., Foster, I. D. L. a n d Simpson, A. D. 1982. "l'Imescales of denudation: the lake-drainage basin approach. Recent developments in the explanation a n d prediction of erosion a n d sediment yield: Conference Proceedings, Exeter, 1982./AHS, 137, 351-360. Dixey, F. 1926. The D i n o s a u r Beds of Lake Nyasa. Trans. Roy. Soc. S. Afrlca I 0 3 , 54-67. Dodson, M. H., cavanagh, B. J., Hatcher, E. C. and Aftalion, M. 1975. Age limits for the Ubendian metamorphic episode in northern Malawi. Geol. Mag. I 1 2 , 403-410. Ebinger, C. J., Deino, A. L., Drake, E. a n d Tesha, A. L. 1989. Chronology of volcanism a n d rift basin propagation: Rungwe Volcanic Province, E a s t Africa. J. Geophys. Res. 94, 15785-15803. Harkin, D. A. 1960. The Rungwe Volcanics at the northern end of Lake Nyasa. Geol. Surv. Tanganyika Mem. 2, 172 p. Haskin, M. A. a n d Haskin, L. A. 1966. Rare earths In E u r o p e a n Shales: a redetermination. Sc/ence 154, 507-509. J o h n s o n , T. C. a n d Davies, T. W. 1989. High resolution seismic profiles from Lake Malwi, Africa. J. Aft'. Earth Sci. 8, 383-392. J o h n s o n , T. C., Davies, T. W., Halfman, 13. M. and Vaughan, N. D. 1988. Sediment core descriptions, Malawi 8 6 Project PROBE Technical Rep., Duke Univ., D u r h a m , N. C. K a m p u n z u , A. 13. a n d Lubala, R. T. (Eds.) 1991. Magmatlsm in extensional s t r u c t u r a l settings: the Phanerozic African Plate. Springer-Verlag Berlin-Heidelberg, p 637. Macdonald, R., Davis, G. R., Bliss, C., Leat, P. T., Bailey, D. K. and Smith, R. L. 1987. Geochemistry of highsilica peralkallne ryolites, Naivasha, Kenya Rift Valley. J. Petrol. 28, 979-1008. Mahood, G.A. 1981. Chemical evolution of a Pleistocene Rhyolitic center: Sierra La Primavera, Jalisco, Mexico. Contrib. Mineral. Petrol 77, 129-149. Nagander, B., Balaram, V., Sudhaker, M. a n d Pluger0 W. L. 1992. Rare e a r t h element geochemistry of ferroman-
39
ganese nodules from the Indian Ocean. Mar. Chem. 38, 185-208.
Oldfleld, F., Appleby, P. G. and Battarbee, R. W. 1978. Alternative 210Pb dating: results from the New Guinea Highlands a n d Lough Erne. Nature 271, 339-342. Owen, IL B. a n d Crossley, R. 1990. Rift s t r u c t u r e s and facies distributions in Lake Malawi. In Rifting in Africa: Karoo to Recent (Eds. Rosendahl, B. R. a n d Rodgers, J.). Spec. Pub. J. Aft. E a r t h ScL 48, I- 15. Owen, IL 13, Crossley, R., J o h n s o n , T. C., Tweddle, D., Kornfleld, I., Davison, S., Eccles, D. H., a n d Engstrom, D. E. 1990. Major low levels of Lake Malawi a n d their implications for speciation rates in cichlid fishes. Proc. R. Soc. Lond. B 240, 519-553. Owen, R. B., Crossley, R., Williams, T. M., and Sefe, F. 1991. Facies distributions associated with a s u b m e r ged fault-controlled platform in Lake Malawi, Central Africa. J. Afr. Earth ScL 13, 449-456. Pilskaln, C. H. a n d J o h n s o n , T. C. 1990. Seasonal signals in Lake Malawi sediments, E a s t Africa. L/mno/. Oceanogr. Piper, D. Z. 1974. Rare e a r t h elements in the sedimentary cycle: a s u m m a r y . Chem. Geol. 89, 179-188., 14, 285-304. Scholz, C. A. a n d Rosendahl, B. R. 1988. Low lake s t a n d s in Lake Malawi a n d Tanganyika, delineated with multifold seismic data. Sc/ence 240, 1645-1648. Scholz, C. A., Rosendahl, B. R. a n d Scott, D. L 1990. Development of coarse grained facies in lacustrine rift basins: Ewamples from E a s t Africa. Geology 18, 140144. Scott, D. L. 1988. Modern processes in a continental rift lake: a n Interpretation of 28 kHz seismic profiles from Lake Malawi, East Africa. Unpub. MSc. thesis, Duke Univ., D u r h a m , NC, USA. Thompson, R. 1979. Palaeomagnetic correlation and dating. In: Palaeohydrological changes in the temperate zone in the last 15000 years. (Ed. E. Berglund}. IGCP Project 158, Lund, Sweden. Thompson, R., Bradshaw, H. W. and Whitley, J. E. 1986. The distribution of a s h in Icelandic lake sediments a n d the relative importance of mixing and erosion processes. J. Quat. ScL I, 3-11. Williams, T. M. and Owen, R. 13. 1990. Authigenesis of ferric oolites in superficial sediments from Lake Malawi, Central Africa. Cherr~ Geol. 89, 179-188. Williams, T. M. a n d Owen, R. 13. 1992. Geochemistry and origins of lacustrine f e r r o m a n g a n e s e nodules from the Malawi Rift, CentralAfrica. Geochim. Cosmochim. Acta. 56, 2703-2712. Yuretich, R. 1993. Clay minerals in Lake Malawi: clues to diagenesis a n d environmental change./DEAL Symposium on the Limnology, Climatology and Palaeoclimatology of the East African Lakes. Jinja, Uganda, 1993.
0899-5362/93 $6.00+ 0.00 © 1993PergamonPresslad
Recent eruptive episodes of the Rungwe Volcanic Field (Tanzania) recorded in lacustrine sediments of the Northern Malawi Rift T. M. WILLIAMS,P. J. HENNEYand R. B. Ow~* British GeologicalSurvey,GeochemistryUnit, Keyworth,Notts, U. K. *Hong KongBaptist College,224 WaterlooRd., Kowloon,Hong Kong (First received 16th December, 1992; revisedversion received 10th May, 1993) Abstract - Discrete ash horizons in Holocenesedimentsfrom northern LakeMalawiprovide evidenceof six
eruptiveepisodeswithinthe nearbyRungweVolcanicFieldbetweenc.9000-360 BP. Rareearth element(REE) analyses show the ash layers to be stronglyenriched in La, Ce, Pr, Nd, Sm, Tb, Dy, Er, Tm, Yb and Lu, with low Eu/Eu* and high LaN/Sms values,relative to the surroundingmuds. Mixingcalculationssuggestpossible affinitiesbetweenthe Rungweash emissionsand silicicvolcanicsfromotherimportantQuaternarycentres(e.g. Naivasha) withrespect to HREE geochemistry.The LREEspectraare less comparable and may indicatea less fractionatedash assemblagefor RungweField.In the absenceof clear in situ evidenceregardingthe timingand frequencyof Holoceneeruptions at Rungwe,the LakeMalawisedimentsmay prove a valuablereconstructive tool. However,the directionand extentof ash dispersal is stronglycontrolledby wind/climaticfactorsand the retention of a completerecord at any singlelocationis unlikely.
REGIONAL
INTRODUCTION
Lake Malawi occupies the m o s t s o u t h e r l y b a s i n of the E a s t African rift s y s t e m a n d is the second largest w a t e r b o d y in Africa by volume (Williams a n d Owen, 1990). The syn-rfft s e d i m e n t s of the lake floor exceed 4 k m t h i c k n e s s (Scholz a n d R o s e n d a h l , 1988) a n d r e t a i n a record of regional tectonic, climatic, biotic a n d hydrological variat i o n s o n t i m e - s c a l e s r a n g i n g f r o m 10-108 y r (Pilskaln a n d J o h n s o n , 1990). During the p a s t decade, over 250 shallow s e d i m e n t cores have b e e n recovered from t h e Malawi Rift to a s s e s s the basin-floor m i n e r a l potential (Crossley a n d Owen, 1987; Williams a n d Owen, 1992), the surficial lithofacies d i s t r i b u t i o n (Crossley a n d Owen, 1988), r e l a t i o n s h i p s b e t w e e n rift s t r u c t u r e a n d sedimentation (Owen a n d Crossley, 1990) a n d the palaeohydrological stability of the lake (Owen et al., 1990). The identification of several a s h horizons d u r i n g lithological e x a m i n a t i o n of profiles from t h e n o r t h e r l y K a r o n g a S u b - b a s i n ( J o h n s o n etal., 1988) s u g g e s t s t h a t t h e s e cores m a y have additional v a l u e s for a p p r a i s i n g the Holocene activity of t h e R u n g w e Volcanic Field, t h e only k n o w n eruptive centre in t h e S. Tanzania-N. Malawi region (Fig. 1). Here, we describe a preliminary lithological a n d geochemical investigation of a s h layers in three cores of 1-6 m length a n d a s s e s s t h e i r significance with respect to regional v o l c a n i s m a n d palaeolimnology.
Malawi
SETTING
Rift
The S. T a n z a n i a / N . Malawi b a s e m e n t c o m p r i s e s a series of B u k o b a n clastic a n d mafic rocks, felsic i n t r u s i o n s , g n e i s s e s a n d schists. U b e n d i a n highgrade g n e i s s e s are widely exposed, locally carrying Irumide and Pan-African tectonic overprints (Dodson et al., 1975). Karroo m u d s t o n e s , coals a n d s a n d s t o n e s outcrop in several areas, n o t a b l y the R u h u h u Basin, T u k u y u a n d Rungwe. Cretac e o u s siltstones a n d m u d s t o n e s o c c u r at t h e northern e n d of Lake Malawi in t h e Songwe h e a d w a t e r s . L a c u s t r i n e d e p o s i t s of Plio-Pleistocene age (Dixey, 1926) r e a c h a t h i c k n e s s of 200 m in a restricted area close to the n o r t h - w e s t e r n lake shore. The Lake Malawi b a s i n reflects a relatively l a t e (late Miocene/Pliocene) s o u t h w a r d e x t e n s i o n of the E a s t African Rift S y s t e m (Crossley a n d Crow, 1980) a n d c o m p r i s e s a series of half-grabens, a l t e r n a t i n g a s y m m e t r i c a l l y along a single axis (Crossley, 1984; Scholz e t a l . , 1990). Half-grabens are c o m m o n l y b r o k e n by n o r m a l f a u l t s to p r o d u c e discrete s u b - b a s i n s within each block. Scholz et aL (1990) have sub-divided the Malawi Rift into four discrete u n i t s on the b a s i s of differing h a l f g r a b e n d o m a i n or dip-linking relationships. The p r e s e n t s t u d y relates exclusively to s e d i m e n t s of the n o r t h e m m o s t unit, in w h i c h s u b s i d e n c e along t h e Livingstone Border F a u l t a n d steep tilting of a
33
34
T. M. WILUAMS,P. J. ~
and R. B. Ow~
a n altitudional range of < 500-> 3 0 0 0 m. A confining rift scarp to t h e east comprises U b e n d i a n schists a n d gneisses of t h e Livingstone Mountains. A less clearly defined w e s t e r n b a s e m e n t scarp is partially overlain by s e d i m e n t s of Karoo a n d Cretaceous age. Detailed m a p p i n g a n d petrograO KM lOOO phic classification (Harkin, 1960; Ebinger et al., ! . J / H / 1989) h a s facilitated a simple division of t h e pred o m i n a n t l y trachytic, phonolitic a n d basaltic volcanics into "Older" a n d '~rounger" Extrusive is series (the former ascribed mainly to the Porotos, KIVU / VIRUNGA ¢s Katete a n d less expansive Jandjuli, Kisangasi, Wehe a n d C h a l u h a n g a vents a n d the latter closely ! / associated with Rungwe a n d Kiejo). Harkln (1960) f ! I tentatively assigned initial eruptive activity to the Pliocene, b u t K-Ar ages of up to 7.25 Ma for the I lowermost phonolitic trachyte in a stratigraphic I '~ section of Karonga Trough have s u b s e q u e n t l y confirmed activity during the Miocene (Ebinger et a/., 1989). The Ngana Tuff of the Karonga Basin / a n d felsic a s h deposits of t h e central Songwe Basin 1 constitute the y o u n g e s t dated deposits with ~4C ages of a r o u n d 11,000 BP (Crossley, 1982, Ebinger et aL, 1989). S u b s e q u e n t activity is poorly known, although the presence of u n e r o d e d cinder cones at Fig. I. Regional position of the Rungwe Volcanic Kiej o m a y indicate a very recent (< 200 yr BP) origin Province and other important volcanic centres in the for units s u c h as the Sarabwe Tephrite (Harkin, East African Rift System. 1960).
,/
single half-graben h a s p r o d u c e d the lake's deepest basinal setting (> 500 m). Rift s t r u c t u r e - l i t h o f a c i e s r e l a t i o n s h i p s have b e e n evaluated in detail t h r o u g h sediment coring (Crossley, 1984; Crossley a n d Owen, 1988; Owen a n d Crossley, 1990, Owen et al., 1991), echosounding a n d multi-channel seismic surveys (Scott, 1988; J o h n s o n a n d Davis, 1989; Scholz et oL, 1990). The superficial deposits are dominantly diatomites in t h e highly productive shallow riftsettings of t h e s o u t h - w e s t a n d s o u t h - e a s t a r m s of the lake, turbidite s a n d s along the well defined axes of b o u n d a r y faults, coarse sheet s a n d s over horst block a n d r a m p s t r u c t u r e s and varved pelagic m u d s in the deep Livingstone sub-basin. The latter result from seasonally alternating algal/ clastic s e d i m e n t a t i o n a n d the formation of a n n u a l d i a t o m - m u d couplets generally less t h a n 1 m m thick. The varves are preserved d u e to the permanently anoxic b o t t o m waters a n d c o n s e q u e n t lack ofbioturbation (Owen a n d Crossley, 1990; Williams a n d Owen, 1990; Owen et oL, 1990). Rungwe Volcanic Field The Rungwe Volcanics cover some 1500 k m 2 in the a c c o m m o d a t i o n zones of the Karonga (Lake Malawi), Songwe a n d U s a n g u t r o u g h s in s o u t h e r n Tanzania (8°45'-0°35"S a n d 33°I0'-34°0'E), with
METHODS The locations of coring stations 525, 527 a n d M86-16P are s h o w n in Fig. 2. Cores 525 a n d 527 were obtained from water d e p t h s of < 200 m on t h e western m a r g i n of the Livingstone s u b - b a s i n using a 2 m long by 5 c m diameter gravity corer weighted with a 50 kg drum. Core M86-16P w a s recovered from 450 m depth using a 6 m piston corer. Following collection, all cores were air-dried a n d logged with t h e assistance of a binocular microscope. Cores 525 a n d 527 were s u b - s a m p l e d at 1 c m resolution for geochemical analysis. Ignition loss (LOI) values were d e t e r m i n e d by weighing s u b - s a m p l e s of approximately 2 g m a s s prior to a n d following c o m b u s t i o n at 450°C in a muffle furnace. Samples of 1.0 g weight were u s e d to determine La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Er, Tm, Yd and Lu by ICP-MS following preparation by lithium metaborate fusion. Analytical precision (calculated using a series of international reference standards) was below 4 % within t h e REE concentration range 10-200 p p m , rising to 10 % at concentrations of 0.1-1 ppm. Age determinations for four stratigraphic levels of core M86-16P were obtained by c o n v e n t i o n a l ~4C m e t h o d s , at t h e SURRC Radiocarbon Laboratory, Scotland, U. K. (Fig. 2). U n s u p p o r t e d 2~°Pb assays were obtained for s u b - s a m p l e s (taken at I c m resolution) from t h e
Recent eruptive episodes of the Rungwe Volcanic Field (Tanzania) recorded in lacustrine sediments... 525
M86-16P
cm
35
527
c
cm
o
I
20
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20
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I
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Fig. 2. Lithological logs and sampling locations: cores 525, 527 and M86-16P (also showing ~4C ages}. u p p e r 20 c m of c o r e s 5 2 5 a n d 527 u s i n g directc o u n t i n g g a m m a spectrometry. The a s s a y s were s u b s e q u e n t l y integrated into the C o n s t a n t Rate of S u p p l y (CRS) m o d e l (Appleby et al., 1984), to e s t a b l i s h a n average s e d i m e n t a t i o n rate of 1.2 m m / y r for t h e s e surficial strata.
RESULTS AND DISCUSSION Core l i t h o l o g y The lithological logs for c o r e s 525 a n d 527 (Fig. 2) s h o w t h e surficial s e d i m e n t s at t h e s e s t a t i o n s to b e largely h o m o g e n e o u s m u d s , with only occasional p r e s e r v a t i o n of A u l a c o s e i r a - S t e p h a n o d i s c u s diat o m laminae. In core M 8 6 - 1 6 P (Fig. 2) several l a m i n a t e d d i a t o m - m u d s e q u e n c e s are preserved d u e to the g r e a t e r w a t e r d e p t h a n d c o n s e q u e n t lack o f b i o t u r b a t i o n . The l a m i n a t e d s e q u e n c e s are s e p a r a t e d b y discrete s a n d horizons (e.g. 1.9 m, 2.8 m, 3.9 m, 4 . 6 m , 5.7 m a n d 6-6.5 m), d e p o s i t e d a s either t u r b i d i t e flows or possibly a s shalloww a t e r facies during low-lake s t a n d s a n d a s s o c i a t e d regressive cycles (Owen et aL, 1990). Two visible a s h layers are identifiable in each of the "short" c o r e s 5 2 5 a n d 527, located at 32 c m a n d (less distinctively) at 80 c m in t h e former a n d 30 c m
a n d 62 c m in t h e latter (Fig. 2). Given t h e close proximity of t h e s e coring stations, t h e 5 2 5 - 3 2 c m a n d 5 2 7 - 3 0 c m horizons are c o n s i d e r e d to be s y n c h r o n o u s deposits, c o r r e s p o n d i n g to a single eruptive event at a r o u n d 3 6 0 BP (based on downcore e x t r a p o l a t i o n of t h e 21°Pb-derived average s e d i m e n t a t i o n rate of 1.2 m m / y r for surficial sediments). Correlation of t h e lower 5 2 5 - 8 0 c m a n d 5 2 7 - 6 2 c m a s h e s is m o r e t e n u o u s , a n d w o u l d require evidence of d i s p r o p o r t i o n a t e l y high sedim e n t a t i o n during the a c c u m u l a t i o n of t h e 3 0 - 8 0 c m s e q u e n c e at s t a t i o n 525. This may, in part, b e provided b y t h e p r e s e n c e of diffuse a s h particulates from 70 c m t h r o u g h to 80 c m (Fig. 2), indicating dilution in a rapidly a c c u m u l a t i n g terrestrial clastic/biogenic matrix. Core M 8 6 - 1 6 P h o l d s clear a s h horizons at 2 0 0 cm, 2 4 0 cm, 362 c m a n d 6 4 0 c m depth. O n the b a s i s of the 14C chronology (see Fig. 2), t h e s e layers c a n be a p p r o x i m a t e l y d a t e d to 3 5 0 0 , 3 7 5 0 , 5 7 5 0 a n d 9 0 0 0 BP respectively. P u m i c e f r a g m e n t s are evident at 2 3 0 c m a n d 6 4 0 c m a n d m a y r e p r e s e n t p a r t i c u l a r l y explosive e r u p t i v e e p i s o d e s . It is notable t h a t no visible a s h o c c u r s in t h e u p p e r I m of s e d i m e n t a n d t h a t t h e r e c e n t eruptive events inferred from c o r e s 525 a n d 527 are t h u s not recorded. Fig. 3 illustrates t h a t s t a t i o n M 8 6 - 1 6 P is
T. M. WILLIAMS,P. J. HENNEYand R. B. OWEN
36
c o n s i d e r a b l y m o r e d i s t a n t from the R u n g w e s o u r c e ( I 0 0 k m SE c o m p a r e d to c. 50 k m for c o r e s 525 a n d 527). More e x t e n s i v e a t m o s p h e r i c d i s p e r s a ! , c o u p l e d with t h e g r e a t e r w a t e r depth, m a y c a u s e dilution of a s h to t h e extent that only the m o s t i n t e n s e e v e n t s are registered in t h e s t r a t i g r a p h y of core M86-16P. REE g e o c h e m i s t r y Total REE a n d LOI d a t a for a s h layers at 525-32 c m a n d 5 2 7 - 3 0 cm, a mixed a s h / p e l a g i c unit at 5 2 5 - 8 0 c m a n d "clean" m u d s from 5 2 5 - 5 2 cm, 5 2 5 - 7 0 c m a n d 5 2 7 - 2 3 c m are provided in Table 1. Total organic c o n t e n t is significantly d e p r e s s e d in the a s h layers a n d t h e LOI v a l u e s m a y t h u s provide a c o n v e n i e n t b a s i s for ascertaining the "cleanest" (i.e. m o s t a s h free) levels. The volcanogenic horiz o n s s h o w e n r i c h m e n t of b o t h light (LREE) a n d h e a v y (HREE) REEs, relative to t h e s u r r o u n d i n g matrix. O n c o m p a r i s o n of a s h s a m p l e 525-32 a n d m u d s a m p l e 525-52, REE e n r i c h m e n t factors of La 3.10, Ce 2.66, Pr 2.48, Nd 2.09, S m 1.71, Eu 1.03,
Tb 1,87, Dy 1.93, Er 2.36, Tm 3.12, Yb 2.81 a n d Lu 3.61 are e s t a b l i s h e d for the former. The distinctive lack of Eu variation b e t w e e n ashbearing a n d ash-free s a m p l e s is highlighted from their c h o n d r i t e - n o r m a l i s e d profiles (Fig. 3). This c o u p l e d with the above n o t e d e n r i c h m e n t of b o t h lighter (Sin) a n d heavier (Tb) R E E s c r e a t e s a s h a r p d e p r e s s i o n of E u / E u * v a l u e s to below 0.55 in t h e a s h (compared with c. 0 . 7 - 0 . 8 5 for "clean" sedim e n t horizons). It is also n o t a b l e that while the c h o n d r i t e - n o r m a l i s e d levels of all a n a l y s e d R E E s excluding Eu are i n c r e a s e d relative to ash-free sed i m e n t s in t h e "purest ash" s a m p l e s 5 2 5 - 3 2 a n d 527-30, only the LREEs (notably La a n d Ce) are significantly e n h a n c e d at lower a s h c o n c e n t r a tions (such as prevail in t h e matrix of 525-80). The LREEs m a y t h u s be c o n s i d e r e d the m o s t sensitive indicators of volcanic influxes in this setting. The m a r k e d steepening of LREE g r a d i e n t s a s s o c i a t e d with a s h deposition is illustrated b y the high LaN/ S m x ratios (> 9.0) of 525-32 a n d 527-30, while a r e d u c e d gradient a c r o s s the HREE s p e c t r u m is a p p a r e n t from a lowering of TbN/LU N v a l u e s (Fig. 4).
Table 1. REE and LOI data for subsamples from cores 525 and 527. (All REE analysis given in ppm after LOI) Sample 525-32 525-52 525-70 525-80 527-23 527-30
La 286.21 92.29 92.69 150.79 117.68 233.65
Ce 501.23 188.73 186.31 264.31 235.65 417.94
Pr 45.47 18.33 18.98 24.79 23.74 38.74
Nd 128.55 61.68 64.92 74.20 80.39 112.42
Sm 17.13 9.97 10.71 10.53 13.26 15.68
Eu 2.05 1.98 2.27 1.81 2.71 2.18
Tb 2.12 1.13 12 7 1.14 1.74 1.94
Dy 13.38 6.93 7.54 7.24 9.84 12.09
Er 8.31 3.51 4.00 3.96 5.18 7.36
Tin 1.25 0.40 0.49 0.50 0.74 1.08
Yb R.94 3.18 344 3.94 4.67 7.55
Lu 1.23 0.34 0.43 0.48 0.68 1.04
LOI% 9.94 17.84 16.99 12.15 16.95 12.17
Possible affinities o f ash h o r i z o n s
1ooo
The R u n g w e Volcanic Province forms one of several m a j o r volcanic c e n t r e s in the W e s t e r n Rift (e.g. Virunga, Kivu), b u t is d i s t i n g u i s h e d b y a p r e d o m i n a n c e of silicic r a t h e r t h a n basaltic acti100 vity (Harkin, 1960; K a m p u n z u a n d Lubala, 1991). Diagnostic evidence of R u n g w e a s h in Lake Malawi s e d i m e n t s s h o u l d t h u s include the d e v e l o p m e n t of o REE s i g n a t u r e s with increasing r e s e m b l a n c e to t h o s e of evolved v o l c a n i c / v o l c a n i c l a s t i c lithoo 10 ~ 525-52 ~" "l logies. While no REE data for the R u n g w e Field have b e e n p u b l i s h e d , d a t a for c o n t e m p o r a n e o u s acid volcanics of the K e n y a n Rift (Bailey a n d ,~ 527-23 / Macdonald, 1970; M a c d o n a l d et al., 1987; Davis 1 @ 527-30 , / and Macdonald, 1987) and elsewhere c a n provide i I I I I l I I l i I a c r u d e b a s i s for c o m p a r i s o n . Data for g l a s s y La Ca Pr Nd Sm Eu Tb Dy Er Tm Yb Lu o b s i d i a n s from Naivasha, Kenya (Macdonald et aL, 1987) a n d aphyric-silicic peralkaline a n d s u b a l k a Fig. 3. Chondrlte-normallsed REE profiles for line volcaniclastics from Jalisco, Mexico (Mahood, ash-horizons and non-volcanogenic 1981) have b e e n selected as "representative" sursediments: cores 525- and 527. rogates for u s e in this study.
L :;:;o
I
37
Recent eruptive episodes of the Rungwe Volcanic Field (Tanzania) recorded in lacustrine sediments...
The chondrite-normalised profiles of both the horizons. Data derived from binary mixing calculNaivasha and Jalisco volcanics (Fig. 5) are gull- ations indicate that the HREE (TbN/LuN) values of winged, with deep negative Eu anomalies and sample 525-32 could be produced by mixing of elevated LREE and HREE abundances. Both sui- 20% "non ash-bearing" Lake Malawi sediment tes are characterised by TbN/Lu Nvalues of c. 1.3, {Eu/Eu* 0.85; TbN/LU N 2.2) with 80% Naivasha/ E u / E u * values of < 0.1 and LaN/SrnNvalues of 2- Jalisco-type ash (Eu/Eu* 0.05; TbN/LUN 1.25). 6. Such values largely reflect the high degree of However, explanation of the LREE signatures is crystal fractionation of the magmas, coupled with more problematic. Samples 525-32 a n d 527-30 the effects of volatile enrichment and vapour- both display c h o n d r i t e - n o r m a l i s e d La-Ce-Nd phase separation (Mahood, 1981; Macdonald et values which are actually higher t h a n those of the a t , 1987). On comparing these signatures with the reference volcanics, and t h u s cannot be explained Lake Malawi core samples, the partial develop- by mixing of sediment and Naivasha/Jalisco-type m e n t of certain evolved volcanic "fingerprints" (e.g., ash. Whilst not discounting the possibility of increasingly negative Eu anomalies and flattening diagenetic LREE enrichment of the ash horizons of HREE profiles) is evident in the ash-bearing (though see below), such high values most probably reflect slightly less evolved assemblages in the 0.9 Rungwe ash showers (also supported by a less pronounced negative Eu anomaly). It is acknowledged that binary mixing calcula0.8 tions of the n a t u r e described are, at best, tenuous, as neither the Naivasha/Jalisco volcanics nor the "non-ash bearing" Lake Malawi sediments repre0.7sent genuine "end members". In the latter case, mineralogical studies (e.g. Yuretich, 1993) indiw cate that virtually all sediments deposited in north0.6. w ern Lake Malawi contain a minor component eroded from volcanic c a t c h m e n t rocks and are thus, O themselves, a product of dual/multiple source 0.5' mixing. O 0.4
I
I
I
I
!
6
7
8
9
10
(La/Sm)n
•
Sediment
0
Ash
REE c h a r a c t e r i s t i c s o f n o n - v o l c a n o g e n i c sediments
Shale-normalised REE for samples 525-32 and 525-52 (calculated in accordance with Piper, 1974) are presented in Fig. 6. The "non-ash" sample 525-
0.9
1000 0.8
1O0
0.7 +
LU
"0 tO
uJ 0.6-
J=
0
0
10
0
M148a
o re
05'
M372 ----0---
0
525-32 255a
0.4
!
0
14
1
12
I
I
I
1.6
1.8
2.0
(Tb/Lu)n •
Sediments
0
Ash
Fig. 4. a) Eu/Eu* vs. LaN/Sm~, b) Eu/Eu* vs. TbN/LuN: sample cores 525 and 527.
210a
2.2
525-52 I
I
I
I
i
La
Ce
Nd
Sm
Eu
/
Tb
I
I
i
Tm
Yb
Lu
Fig. 5. Chondrite-normalised REE profiles for ashhorizon 525-32, sediment-horizon 525-52 and selected evolved volcanics from Naivasha, Kenya (MacDonald et al., 1987) and Jalisco, Mexico (Mahood, 1981).
38
T. M. WILLIAMS,P. J. I-IE~'Y and R. B. Ow~
52 is of particular interest as it shows disproportionate enrichment of LREE relative to global shale values (Haskin and Haskln, 1966). This trend contrasts with published d a t a for turbidites, pelagic clays, metalliferous sediments, marine clays and ferromanganese nodules (e.g. Piper, 1974), all of which display preferential upgrading of HREE relative to "average" shales. It is likely that LREE enrichment in 525-52 reflects the slow inwash of volcanogenic debris deposited over the Karonga S u b - b a s i n c a t c h m e n t area during eruptive episodes a n d transported in surface drainage alongside clays and associated terrigenous detritus. Alternative explanations involving preferential LREE enrichment during diagenesis appear inapplicable, given the absence of any characteristic Ce anomaly (Nagander et aL, 1992). The REE chemistry of the algal rain is also unlikely to be influential, as former observations suggest a tendency for HREE enrichment in diatoms (Piper, 1974).
Value o f ash h o r i z o n s for palaeolimnologlcal studies The identification ofcorrelatable core horizons is important in palaeolinmology for extrapolating radiometric d a t e s a n d calculating basin-floor s e d i m e n t a t i o n or c a t c h m e n t e r o s i o n r a t e s (Thompson, 1979; Dearing et aL, 1982). While
6-
¢I
W
tentative inter-core correlations in Lake Malawi have previously b e e n m a d e using microfossfl spectra (Owen et al., 1990), a s h layers s u c h as those identified in cores 525, 527 and M86-16P c a n provide a clear alternative. The approach h a s b e e n exemplified in tephra-bearing lake b a s i n s in northern Europe and S. E. Asia (Oldfield et a t , 1978; Thompson et aL, 1986) and is considerably less time-consuming t h a n palynological methods.
CONCLUSIONS Preliminary lithological and geochemical analyses of cores from the Karonga S u b - b a s i n indicate that ash layers in stratified s e d i m e n t s from northern Lake Malawi m a y provide a valuable record of the Holocene eruptive activity of the Rungwe Volcanicso which c o u l d u s e f u l l y s u p p l e m e n t published radiometric evidence f r o m / n situ flows (e.g., Ebinger et aL, 1989). However, the presence of ash layers of < 1000 yr age in sediments 50 km from Rungwe (cores 525, 527) and their absence at c. 100 km (core M86-16P) suggests that distance and (probably) factors s u c h as wind direction significantly influence the sedimentary ash-fall record at any individual site. The REE signatures of the ash layers confirm their silicic affinities, and provide tentative evidence that the Rungwe Field p o s s e s s e s a more LREE-rich character t h a n other evolved volcanlcs such as Naivasha and Jalisco. If u s e d with caution, the ash horizons rePorted here m a y provide a useful tool for core2correlation and radiometric date t r a n s p o s i t i o n in future palaeolimnological studies of the Lake Malawi basin.
rr
i La
•
i Ge
• i Pr
• i
• i
Nd Sm
. i Eu
- i Yb
, i
- i
- i
Dy
Er
Tm
• i
.
Yb
i I_u
LM,02 I
Diatoms
Marine clays Gironde
River
A c k n o w l e d g e m e n t s - The authors are grateful to Dr. T. Johnson and staff of Project PROBE for their kind provision of piston core M86-16P. The Malawi Government, Depertment of Fisheries placed their research vessel at our disposal during coring in the Lake Malawi basin. Funding for this work was provided by the British Government Overseas Development Administration, Malawi Government and Amoco Oil Co., for which the authors express thanks.
REFERENCES
•
La
Ce
Pr
Nd Sm
Eu
Tb
Dy
Er
Tm
Yb
1
Lu
Fig. 6. Shale normallsed profiles for Lake Malawl samples 525-32 and 525-52, plus marine clays (Piper, 1974), Lake Malawi ferromanganese nodules-LM702 (Williams, in press), algae and surface waters (Piper, 1974).
Applely, P. G., Nolan, P., Gifford, D. W., Oldfleld, F., Anderson, N. J. and Battarbee, R. W. 1984. 2~°Pb dating by low background gamma counting. Hgdrob~ol. • 141, 21-27. Bailey, D. K. and Macdonald, R. 1970. Petrochemical variations among mildly peralkaline (comendlte) obsidians from the oceans & continents. Contrtb. Mineral. Petrol 28, 340-351. Crossley, R. 1982. Late Cenozoic stratigraphy of the Karonga area in the Malawi Rift. Palaeoecol. Afr. 15, 139-144.
Recent eruptive episodes of the Rungwe Volcanic Field (Tanzania) recorded in lacustrine sediments... Crossley, R. 1984. Controls of sedimentation in the Malawi Rift. Sed/ment. Geo/. 40, 33-40. Crossley, R. a n d Crow, M. J. 1980. The Malawi Rift. In: Geodynamlc Evolution of the Afro-Arabic Rift system. Proc. Acco_dem/a del Lince/, Rome, 77-78. Crossley, IL a n d Owen, R. B. 1988. S a n d turbidites and organic-rich d i a t o m a c e o u s m u d s from Lake Malawi, Central Africa. In: Lacustr/ne Petro/eum Source Rocks (Eds. Fleet, A. J., Kelts, K. and Talbot, M, IL), Geo/. Soc. Lond. Spec. Publ. 40, 369-374. Davies, G. R. Macdonald, R. 1987. Crustal influences in the petrogenesis of the Naivasha basalt-rhyollte complex: combined trace element a n d Sr-Nd-Pb isotope constraints. J. Petrol 28, 1009-1031. Dearing, J. A., Foster, I. D. L. a n d Simpson, A. D. 1982. "l'Imescales of denudation: the lake-drainage basin approach. Recent developments in the explanation a n d prediction of erosion a n d sediment yield: Conference Proceedings, Exeter, 1982./AHS, 137, 351-360. Dixey, F. 1926. The D i n o s a u r Beds of Lake Nyasa. Trans. Roy. Soc. S. Afrlca I 0 3 , 54-67. Dodson, M. H., cavanagh, B. J., Hatcher, E. C. and Aftalion, M. 1975. Age limits for the Ubendian metamorphic episode in northern Malawi. Geol. Mag. I 1 2 , 403-410. Ebinger, C. J., Deino, A. L., Drake, E. a n d Tesha, A. L. 1989. Chronology of volcanism a n d rift basin propagation: Rungwe Volcanic Province, E a s t Africa. J. Geophys. Res. 94, 15785-15803. Harkin, D. A. 1960. The Rungwe Volcanics at the northern end of Lake Nyasa. Geol. Surv. Tanganyika Mem. 2, 172 p. Haskin, M. A. a n d Haskin, L. A. 1966. Rare earths In E u r o p e a n Shales: a redetermination. Sc/ence 154, 507-509. J o h n s o n , T. C. a n d Davies, T. W. 1989. High resolution seismic profiles from Lake Malwi, Africa. J. Aft'. Earth Sci. 8, 383-392. J o h n s o n , T. C., Davies, T. W., Halfman, 13. M. and Vaughan, N. D. 1988. Sediment core descriptions, Malawi 8 6 Project PROBE Technical Rep., Duke Univ., D u r h a m , N. C. K a m p u n z u , A. 13. a n d Lubala, R. T. (Eds.) 1991. Magmatlsm in extensional s t r u c t u r a l settings: the Phanerozic African Plate. Springer-Verlag Berlin-Heidelberg, p 637. Macdonald, R., Davis, G. R., Bliss, C., Leat, P. T., Bailey, D. K. and Smith, R. L. 1987. Geochemistry of highsilica peralkallne ryolites, Naivasha, Kenya Rift Valley. J. Petrol. 28, 979-1008. Mahood, G.A. 1981. Chemical evolution of a Pleistocene Rhyolitic center: Sierra La Primavera, Jalisco, Mexico. Contrib. Mineral. Petrol 77, 129-149. Nagander, B., Balaram, V., Sudhaker, M. a n d Pluger0 W. L. 1992. Rare e a r t h element geochemistry of ferroman-
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ganese nodules from the Indian Ocean. Mar. Chem. 38, 185-208.
Oldfleld, F., Appleby, P. G. and Battarbee, R. W. 1978. Alternative 210Pb dating: results from the New Guinea Highlands a n d Lough Erne. Nature 271, 339-342. Owen, IL B. a n d Crossley, R. 1990. Rift s t r u c t u r e s and facies distributions in Lake Malawi. In Rifting in Africa: Karoo to Recent (Eds. Rosendahl, B. R. a n d Rodgers, J.). Spec. Pub. J. Aft. E a r t h ScL 48, I- 15. Owen, IL 13, Crossley, R., J o h n s o n , T. C., Tweddle, D., Kornfleld, I., Davison, S., Eccles, D. H., a n d Engstrom, D. E. 1990. Major low levels of Lake Malawi a n d their implications for speciation rates in cichlid fishes. Proc. R. Soc. Lond. B 240, 519-553. Owen, R. B., Crossley, R., Williams, T. M., and Sefe, F. 1991. Facies distributions associated with a s u b m e r ged fault-controlled platform in Lake Malawi, Central Africa. J. Afr. Earth ScL 13, 449-456. Pilskaln, C. H. a n d J o h n s o n , T. C. 1990. Seasonal signals in Lake Malawi sediments, E a s t Africa. L/mno/. Oceanogr. Piper, D. Z. 1974. Rare e a r t h elements in the sedimentary cycle: a s u m m a r y . Chem. Geol. 89, 179-188., 14, 285-304. Scholz, C. A. a n d Rosendahl, B. R. 1988. Low lake s t a n d s in Lake Malawi a n d Tanganyika, delineated with multifold seismic data. Sc/ence 240, 1645-1648. Scholz, C. A., Rosendahl, B. R. a n d Scott, D. L 1990. Development of coarse grained facies in lacustrine rift basins: Ewamples from E a s t Africa. Geology 18, 140144. Scott, D. L. 1988. Modern processes in a continental rift lake: a n Interpretation of 28 kHz seismic profiles from Lake Malawi, East Africa. Unpub. MSc. thesis, Duke Univ., D u r h a m , NC, USA. Thompson, R. 1979. Palaeomagnetic correlation and dating. In: Palaeohydrological changes in the temperate zone in the last 15000 years. (Ed. E. Berglund}. IGCP Project 158, Lund, Sweden. Thompson, R., Bradshaw, H. W. and Whitley, J. E. 1986. The distribution of a s h in Icelandic lake sediments a n d the relative importance of mixing and erosion processes. J. Quat. ScL I, 3-11. Williams, T. M. and Owen, R. 13. 1990. Authigenesis of ferric oolites in superficial sediments from Lake Malawi, Central Africa. Cherr~ Geol. 89, 179-188. Williams, T. M. a n d Owen, R. 13. 1992. Geochemistry and origins of lacustrine f e r r o m a n g a n e s e nodules from the Malawi Rift, CentralAfrica. Geochim. Cosmochim. Acta. 56, 2703-2712. Yuretich, R. 1993. Clay minerals in Lake Malawi: clues to diagenesis a n d environmental change./DEAL Symposium on the Limnology, Climatology and Palaeoclimatology of the East African Lakes. Jinja, Uganda, 1993.