Isotopic age determinations on granitic and gneissic rocks from the Ubendian-Usagaran system in southern Tanzania

Isotopic age determinations on granitic and gneissic rocks from the Ubendian-Usagaran system in southern Tanzania

Precambrian Research, 9 ( 1 9 7 9 ) 2 2 7 - - 2 3 9 © Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m -- P r i n t e d in T h e N e t...

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Precambrian Research, 9 ( 1 9 7 9 ) 2 2 7 - - 2 3 9 © Elsevier Scientific Publishing C o m p a n y , A m s t e r d a m -- P r i n t e d in T h e N e t h e r l a n d s

227

ISOTOPIC AGE DETERMINATIONS ON GRANITIC AND GNEISSIC ROCKS FROM THE UBENDIAN-USAGARAN SYSTEM IN SOUTHERN TANZANIA

H.N.A. P R I E M , N.A.I.M. B O E L R I J K , E.H. H E B E D A , E.A. Th. V E R D U R M E N and R.H. V E R S C H U R E

Z. W.O. Laboratorium voor lsotopen-Geologie, De Boelelaan 1085, 1081 H V Amsterdam (The Netherlands) I.S. O E N

Geologisch Instituut, Universiteit van Amsterdam (The Netherlands) L. W E S T R A

Instituut voor Aardwetenschappen, Vrije Universiteit, Amsterdam (The Netherlands) ( R e c e i v e d August 15, 1 9 7 8 ; revision a c c e p t e d F e b r u a r y 2, 1 9 7 9 )

ABSTRACT Priem, H.N.A., Boelrijk, N.A.I.M., H e b e d a , E.H., V e r d u r m e n , E.A. Th., V e r s c h u r e , R.H., Oen, I.S. a n d Westra, L., 1 9 7 9 . I s o t o p i c age d e t e r m i n a t i o n s o n granitic a n d gneissic r o c k s f r o m t h e U b e n d i a n - U s a g a r a n S y s t e m in s o u t h e r n T a n z a n i a . P r e c a m b r i a n Res., 9: 2 2 7 - - 2 3 9 . I s o t o p i c age m e a s u r e m e n t s o n g r a n i t e s a n d gneisses are r e p o r t e d f r o m t h e U b e n d i a n Usagaran S y s t e m in s o u t h e r n T a n z a n i a . F o r t h e L u k u m b u r u - W i n o G r a n i t e s a R b - S r w h o l e - r o c k i s o c h r o n (9 p o i n t s ) o f 1 7 7 1 +_ 145 Ma w i t h initial s7 Sr/S6 Sr o f 0 . 7 0 4 +- 0 . 0 0 3 was o b t a i n e d (~87 Rb = 1.42 • 1 0 - " a -1 ; errors 95% c o n f i d e n c e level). T w o s a m p l e s f r o m n e i g h b o u r i n g granites a n d t h r e e gneisses f r o m t h e c o n t a c t z o n e o f t h e L u k u m b u r u - W i n o G r a n i t e s also fit to this i s o c h r o n ; i n c l u s i o n i n t o t h e i s o c h r o n c a l c u l a t i o n yields a n age o f 1747 ± 85 Ma w i t h t h e s a m e initial Sr c o m p o s i t i o n . Five p a r t s of a b a n d e d Usagaran gneiss s a m p l e p r o d u c e a n i s o c h r o n o f 589 -+ 70 Ma w i t h initial s7 Sr/86 Sr of 0 . 7 1 9 4 ± 0 . 0 0 0 5 . This i n t e r n a l i s o c h r o n is i n t e r p r e t e d as r e f l e c t i n g Sr i s o t o p i c e q u i l i b r a t i o n t h r o u g h t h e s a m p l e d u r i n g t h e main" p h a s e (or o n e o f t h e m a i n phases) o f t h e P a n - A f r i c a n t h e r m o t e c t o n i c episode. Rb-Sr a n d K-Ar ages o f 17 micas s h o w a general r e s e t t i n g b y t h e P a n - A f r i c a n t h e r m o t e c t o n i c episode. T h e R b - S r ages o f t h e m u s c o v i t e s a n d t h e biotites average 528 Ma a n d 4 6 3 Ma, respectively, and are i n t e r p r e t e d as r e f l e c t i n g s u b s e q u e n t stages in t h e cooling h i s t o r y a f t e r t h e t e r m i n a t i o n of t h e event. B i o t i t e s have K-Ar ages p e r s i s t e n t l y h i g h e r t h a n t h e c o r r e s p o n d i n g R b - S r ages, possibly r e f l e c t i n g a high partial 4o Ar pressure t h r o u g h t h e c r u s t d u r i n g t h e r e s e t t i n g o f t h e b i o t i t e K-Ar s y s t e m s .

INTRODUCTION

A geologic and mineral exploration program was c o n d u c t e d in 1 9 7 1 - - 1 9 7 3

228

cooperatively by the Netherlands Universities Geological Mapping Project in Tanzania (NUGPIT, a joint undertaking of the University of Amsterdam and the Free University of Amsterdam) and the Mineral Resources Division of Tanzania. The program was concerned with the Quarter Degree Sheets (QDS) 264, 265, 275, 276 and 277 of the geological map of Tanzania. In connection with this project, isotopic age studies of basement rocks were undertaken by the Z.W.O. Laboratorium voor Isotopen-Geologie in Amsterdam. This paper presents the results of the geochronological work. GEOLOGICAL SETTING

The investigated area (Figs. 1 and 2) is covered by QDS 264, 265, 275, 276 and 277, and comprises parts of the Districts of Njombe, Songea and Ulanga in southern Tanzania. The region consists of crystalline basement, overlain by sedimentary sequences of the Karroo System to the south and



10°

30"

35"

40 °

Fig. 1. Sketeh map showing the Ubendian-Usagaran Belt in relation to the Tanzanian Shield, the Zambia Shield and the Mozambique Belt. 1, Late Proterozoie platform deposits; 2, Proterozoic mobile belts with (horizontal shading) the approximate extension of the Early Paleozoic Pan-African thermotectonic episode; 3, shield areas; d, major faults; 5, quarter degree sheets m a p p e d by N U G P I T (Fig. 2).

229

264 ....

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,r~.:.:.:.:l LDodoma,

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277

Fig. 2. Geological sketch map of the region S and E of Njombe, southern Tanzania (QDS 264, 264,275, 276 and 277 of the geological map of Tanzania; after NUGPIT). A. Quarternary; B, Karroo System; C, granites (Lukumburu-Wino Granites in the central part and Kipengere Granite in the southwestern part of QDS 275); D, granitic gueisses; E, granulite; F, anorthosite; G, fault. H, sampling site. The figures 1--31 refer to sample numbers Tan 1 to Tan 31. Three investigated samples come from outside the map area, gneisses Tan 27 and 28 from QDS 251, north of QDS 265, and granite Tan 7 from QDS 262, north of QDS 275.

the east. A continuous gneiss belt with variable regional foliation dominates the investigated area. In the west (QDS 275) the foliation has a WNW trend, in the central part (QDS 276 and 277) an E trend, and in the northeast (QDS 264 and 265) a NE--N trend. The gneisses comprise a variety of banded rocks, probably both o f sedimentary and of igneous origin. Usually, their main components are quartz, feldspar and biotite, sometimes also muscovite and hornblende. Amphibolites are rare and calcareous rocks are absent. The central part of QDS 275 is occupied by igneous rocks (Wino granite and Lukumburu granodiorite), displaying an intrusive relationship towards the surrounding gneisses and migmatites. In the southwesternmost part of QDS 275 anorthosites occur, forming part of the Liganga anorthosite complex (Wright, 1963) known for its vanadiferous magnetite-ilmenite ores (Harris, 1961; Zakrzewski, 1977). The anorthosite complex is in part bordered b y a coarse-porphyritic granite (Kipengere granite). In the northern parts of QDS 264 and 265, granulitic gneisses are found, separated from the gneisses in the south by a major fault system. Except for this granulite terrain, the overall metamorphic grade in the investigated area is amphibolite facies, with widespread retrogradation. Originally it was thought that part of the crystalline basement in the area covered b y QDS 275 could represent the southernmost extension of the Dodoman System, forming one of the oldest orogenic belts of which vestiges

230 are ingrained in the Tanganyika Shield (Holmes, 1951). However, recent work in the region north of Njombe by the German Geological Mission in Tanzania has shown that the southern border of the Tanganyika Shield lies more to the north, in the Iringa region (Meinhold, 1970; Wendt et al., 1972; Gabert, 1973; Gabert and Wendt, 1974). The Tanganyika Shield basement was formed about 2600 Ma ago, but older elements are also included (Cahen and Snelling, 1966). In southern Tanzania the Shield is bordered by younger, highly metamorphosed rocks: in the southwest and the west by the NW-trending Ubendian-Rusizian Belt, made up of metamorphic rocks of the Ubendian System, and in the east by the predominantly N-trending Mozambique Belt. In Tanzania, the Mozambique Belt consists predominantly of metamorphic rocks belonging to the Usagaran System, but elements of the older D o d o m a n System are presumably also included. The Mozambique Belt has a width of up to 400 km and extends along the eastern coast of Africa from Mozambique to Ethiopia, a distance exceeding 1500 km. Over its whole length, the Mozambique Belt yields predominantly Late Precambrian-Early Paleozoic isotopic ages, but this metamorphic imprint has affected rocks with a complex Archean to Middle Proterozoic history (Martin and Porada, 1977a, b). South of the Tanganyika Shield, the folded metamorphic rocks of the Ubendian and Usagaran Systems merge into a zone with E-trending structures. It has been postulated by Quennel et al. (1956) that both Systems could very well represent a single one, which they tentatively refer to as the Ubendian-Usagaran System. The field investigations by the German Geological Mission in Tanzania (Gabert and Wendt, 1974) and NUGPIT (unpublished reports) have confirmed this hypothesis. The Dodoman System contains migmatitic gneisses, amphibolites, charnockites and quartzites. They are unconformably overlain by high-grade metamorphic geosynclinal sequences of the Usagaran-System (Meinhold, 1970). The rocks o f the Usagaran System comprise migmatitic gneisses, amphibolites, quartzites and marbles, discordantly overlain by metavolcanics and metasediments of the Ndembera Series. Periods of granitic magmatism accompanied the fol':ling and metamorphic phases to which the Dodoman and Usagaran rocks, including the Ndembera Series, were subjected. Immediately northwest of the area investigated by NUGPIT, elaborate isotopic age studies have been undertaken in the Dodoman System and the Ubendian-Usagaran System by the German Geological Mission in Tanzania. The results of this work have been reported by Wendt et al. (1972), Gabert (1973) and Gabert and Wendt (1974)*. They concluded an age of 2575 + 60 Ma (Rb-Sr whole-rock isochron of gneisses) for the youngest metamor* All Rb-Sr ages from these authors are recalculated here with the s7 Rb decay constant of 1.42 • 10 ~ La- ~. The errors of the ages are quoted as they have been published, i.e., at the la level, contrary to the 95% confidence level of the isochron errors reported in this paper.

231 phic episode of the Dodoman Orogeny in the Tanganyika Shield, in accordance with a few mica ages farther to the north reported by Cahen and Snelling (1966). The approximate (minimum) age of the pre-Ndembera episode of metamorphism of the Ubendian-Usagaran Orogeny is 1950 + 90 Ma (Rb-Sr whole-rock isochron of gneisses), corresponding to mineral ages reported from the Ubendian-Rusizian Belt by Cahen and Shelling (1966). In northern Malawi, Dodson et al. (1975) likewise reached (on the basis of zircon U-Pb and whole-rock Rb-Sr data) an age of about 2000 Ma for the high-grade Ubendian metamorphism. The Ndembera volcanics overlying the Usagaran high-grade metamorphic rocks were dated at 1925 -+ 33 Ma (Rb-Sr whole-rock isochron). Post-Ndembera anatectic and hybrid granites, comparable to the granitic rocks in the adjoining L u k u m b u r u area (QDS 275) investigated in this study, were dated at 1818 + 45 Ma (Rb-Sr whole-rock isochron). Micas from the Usagaran rocks yield Rb-Sr and K-Ar ages ranging from about 1950 Ma to a b o u t 420 Ma. On a regional scale, the mica ages show patterns with an alignment of equal ages parallel to the strike of the Mozambique Belt and decreasing towards the southeast. The rejuvenation of the mica ages to about 420 Ma is attributed to a metamorphic event that caused partial or complete resetting of the older isotopic age record. This metamorphic event can be recognized at many places in Africa and its imprint is everywhere recorded b y rejuvenation of mineral ages. No ages much lower than about 400 Ma have been reported anywhere. A number of terms have been proposed for the event, for example: Pan-African thermotectonic episode (Kennedy, 1964) or Damaran-Katangan orogeny (Clifford, 1968, 1970, 1974) for Africa as a whole, Mozambiquian thermal event for the Mozambique Belt (Harper et al., 1972), and Riphean orogeny for the Hoggar Area in northern Africa (Black, 1966). The Tanzanian section of the Mozambique Belt, however, consists predominantly of rocks of the Usagaran System, and no evidence has been found here for a geosynclinal stage directly preceding the Late Precambrian-Early Paleozoic metamorphic episode. Martin and Porada (1977a, b) argued that the thermotectonic episode in the Mozambique Belt cannot be explained by the subduction model. They favoured Watson's (1976) hypothesis envisaging a broad zone of mantle diapirism, causing stretching of the crust on a regional scale instead of localized rifting. Stretching of the crust would lead to tectonism of epeirogenic character and to the opening of pathways for fluid phases derived from the mantle, causing retrograde metamorphism and isotopic resetting. This geodynamic model would, according to Watson (1976), account for the variability of the tectonic and metamorphic processes, leaving older cratonized nuclei in the crust relatively undisturbed. EXPERIMENTAL PROCEDURES, ANALYTICAL ERRORS AND CONSTANTS Rb and Sr contents and Rb/Sr ratios of the whole-rocks were measured by X-ray fluorescence spectrometry, using a Philips PW 1450/AHP auto-

232 TABLE I Whole-rock Rb-Sr data of granites and gneisses in QDS 275, southern Tanzania Sample Nr. ,1

Rb *~ (ppm)

Sr .2 (ppm)

Rb/Sr*: (m/m)

87 Sr/~6 Sr,2

87 Rb/~6 Sr

Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan

103 112 101 103 123 85.9 142 146 238 184 236 220 161 126

572 536 669 473 487 373 331 294 221 227 135 192 286 315

0.1792 0.2086 0.1502 0.2177 0.2517 0.2306 0.4275 0.4972 1.075 0.8117 1.746 1.147 0.5644 0.3986

0.7164 0.7203 0.7151 0.7210 0.7225 0.7233 0.7360 0.7403 0.7805 0.7697 0.8336 0.7872 0.7456 0.7299

0.519 0.605 0.435 0.631 0.730 0.669 1.24 1.45 3.14 2.37 5.12 3.35 1.64 1.16

1A 1B 2 3A 3B 4 5A 5B 6A 6B 7 8A 8B 10

* ~The suffices A and B refer to two parts of a single rock specimen. ,2 Mean of duplicate analyses. m a t i c s p e c t r o m e t e r ( p r e s s e d - p o w d e r pellets; mass a b s o r p t i o n c o r r e c t i o n s for b o t h s a m p l e and e x t e r n a l s t a n d a r d b a s e d u p o n t h e C o m p t o n s c a t t e r i n g of t h e M o - K a p r i m a r y b e a m ; see V e r d u r m e n , 1977). F o r micas, the Rb and Sr c o n t e n t s were d e t e r m i n e d b y m a s s - s p e c t r o m e t r i c i s o t o p e dilution. Sr isot o p i c c o m p o s i t i o n s w e r e d e t e r m i n e d d i r e c t l y on u n s p i k e d Sr f o r wholerocks and c a l c u l a t e d f r o m t h e i s o t o p e d i l u t i o n runs f o r micas. T h e i s o t o p e analyses o f t h e 14 w h o l e - r o c k s T a n 1 to 10 ( T a b l e I) have b e e n m a d e on a 20 c m , 60 ° N i e r - t y p e m a s s - s p e c t r o m e t e r w i t h digital o u t p u t and m u l t i p l i e r d e t e c t i o n (s7 Sr/86 Sr ratios n o r m a l i z e d and adjusted t o t h e value 0 . 7 0 8 1 in t h e E i m e r and A m e n d Sr(COa)2 standard}. All o t h e r i s o t o p e m e a s u r e m e n t s were m a d e on a c o m p u t e r - c o n t r o l l e d Varian C H 5 m a s s - s p e c t r o m e t e r with F a r a d a y cage c o l l e c t o r and digital o u t p u t (value o f n o r m a l i z e d s7 Sr/S6 Sr o f t h e NBS 987 Sr(CO3)2 s t a n d a r d m e a s u r e d as 0 . 7 1 0 1 6 + 0.00008}. T h e K c o n t e n t s were d e t e r m i n e d b y f l a m e p h o t o m e t r y with a Li internal s t a n d a r d a n d CsA1 b u f f e r . A r g o n was e x t r a c t e d in a b a k e a b l e glass v a c u u m a p p a r a t u s and d e t e r m i n e d b y i s o t o p e d i l u t i o n t e c h n i q u e s in a R e y n o l d s t y p e glass m a s s - s p e c t r o m e t e r ; all m e a s u r e m e n t s w e r e m a d e b y the static method. T h e a n a l y t i c a l a c c u r a c y in t h e R b - S r d a t a is e s t i m a t e d at 1% for X R F R b / Sr, 1% f o r i s o t o p e d i l u t i o n Rb and Sr, 0.2% f o r s7 Sr/S6 Sr m e a s u r e d b y m e a n s o f t h e N i e r - t y p e m a s s - s p e c t r o m e t e r ( w h o l e - r o c k s T a n I to 10), and 0 . 0 5 % for all o t h e r s7 Sr/Se Sr analyses. F o r K and Ar t h e analytical a c c u r a c y is estim a t e d to b e 1% and 2%, respectively. R b - S r i s o c h r o n s w e r e c o m p u t e d b y m e a n s o f a least-squares regression

233

analysis a c c o r d i n g to Y o r k ( 1 9 6 6 , 1967). I s o c h r o n errors are q u o t e d at the 95% c o n f i d e n c e level as calculated f r o m t h e analytical data. T h e values o f t h e Mean Squares Weighted D e v i a t i o n (MSWD) were calculated a c c o r d i n g to M c I n t y r e et al. (1966). F o r t h e age calculations t h e following c o n s t a n t s were used: Xs7 Rb = 1.42 • 10 -11 a -1 ; X4°K~ = 4 . 9 6 2 • 1 0 - ' ° a -1 ; X4°Ke = 0.581 • 1 0 - 1 ° a -1 ; isotopic a b u n d a n c e 40 K = 0 . 0 1 1 6 7 a t o m % t o t a l K. Ages q u o t e d f r o m l i t e r a t u r e have been recalculated. RESULTS AND DISCUSSION

Rb-Sr w h o l e - r o c k m e a s u r e m e n t s were m a d e o n 14 samples o f gneisses and granites f r o m t h e western part o f the investigated area (QDS 275). T h e sample l o c a t i o n s are s h o w n o n the geological s k e t c h m a p (Fig. 2). T h e Rb-Sr d a t a are p r e s e n t e d in Table I and p l o t t e d in Fig. 3. Nine data-points are f r o m the L u k u m b u r u - W i n o Granite. T h e y p r o d u c e a fairly g o o d i s o c h r o n o f 1771 + 145 Ma with an initial ~7 Sr/~6 Sr ratio o f 0 . 7 0 4 -+ 0 . 0 0 3 and a MSWD value of 2.19. T w o o t h e r granites (one, T a n 7, c o l l e c t e d i m m e d i a t e l y n o r t h o f the investigated area) and t h r e e gneisses f r o m close to t h e granite c o n t a c t also fit this isochron. A regression analysis t h r o u g h all 14 samples w o u l d p r o d u c e an i s o c h r o n o f 1747 + 85 Ma with an initial s~ Sr/s6 Sr ratio o f 0 . 7 0 4 ± 0 . 0 0 2 and a MSWD value o f 2.06. Within t h e limits o f error, b o t h ages agree with the age J e t e r m i n e d b y G a b e r t and W e n d t ( 1 9 7 4 ) for the anatectic and h y b r i d granites ( 1 8 1 8 +- 45 Ma, l o error) n o r t h o f QDS 275. This age is i n t e r p r e t e d

//

0.850

GRANITES AND GNEISSES O.D.S. 275.5.TANZANIA

,..oI~ °

0.800



[~'~

~ 7 0

0.750

~/"~lA 0.7001

/ 5 B4~/I 8B

Ma

/~ 87Rb: 1.42 - 10 -11 a-1

I

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t

t

1

2

3

4

87R~B6sr~ If 5

Fig. 3. Plot of Rb-Sr data of granites and granitic gneisses from QDS 275. Circles, Lukumburu-Wino Granites; quadrangles, granitic gneisses; triangle, Kipengere Granite; asterisk, granite from QDS 262 (continuation to the north of the Lukumburu-Wino Granite). The isochron calculation is through the nine samples of the Lukumburu-Wino Granite.

a

-0.750

GNEISSES Q.D.S. 264,

S. TANZANIA

%r/%r 0.730

,2M9

0

Ma

12A~,,,.,J~I2 C 12 D

y "\0.7194

~,87~b, 1.42,10 -11 e-1

-O.710 .~6

8b/%r )

1

2

!

I

Fig. 4. Plot of Rb-Sr data of granitic gneisses from QDS 264. The isochron calculation is through five data-points from one specimen, banded gneiss Tan 12 (shown as closed circles): four slices sawn according to the compositional banding from onepart of the specimen (12A, C, D and E) and a whole-rock sample from another part (12M). *, other gneiss samples.

as reflecting a post-Ndembera episode o f granitic magmatism within the Ubendian-Usagaran orogenic cycle. The granites have intruded m et am orphi c rocks that were folded and m e t a m o r p h o s e d during the pre-Ndembera orogenic episode a b o u t 2000 Ma ago. The m e t a m o r p h i s m and the granitic magmatism of the Ubendian-Usagaran orogenic cycle thus fall within the time interval of the Eburnian Orogenic Cycle ( 1900 -+ 250 Ma ago), recognized at many places all over the African c o n t i n e n t (Clifford, 1968, 1970, 1974). Fr om the eastern part of the investigated area (QDS 264) Rb-Sr measurements were made on five samples o f gneisses. The sample locations are shown in Fig. 2. Sample Tan 12 is a specimen o f banded gneiss. From one part of this specimen f our slices ( about 20 X 20 X 5 cm each) were sawn according to the compositional banding (Tan 12 A--E), and each slice was analysed separately. The ot her part of the specimen was wholly ground to a conventional whole-rock sample (Tan 12 M). The Rb-Sr data are listed in Table II'and plotted in Fig. 4. Although the range in Rb/Sr ratios is rather small, the five data-points of gneiss sample Tan 12 display good linear correlation, corresponding to an internal isochron o f 589 + 70 Ma with an initial 87Sr/S6Sr ratio of 0.7194 -+ 0.0005 and a MSWD value o f 0.09. Of the

235 TABLE II Whole-rock Rb-Sr data of gneisses in QDS 264, southern Tanzania Sample Nr.

R b *~

Sr .1

Rb/Sr,l

(ppm)

(ppm)

(m/m)

87 Sr/8~ Sr

87 Rb/S~

Sr

B a n d e d gneiss s a m p l e Tan 12 *2

Tan Tan Tan Tan Tan

12M 12A 12C 12D 12E

102 72.8 89.0 57.4 32.7

456 466 457 404 439

0.2239 0.1564 0.1962 0.1420 0.07459

0.72489 0.72323 0.72415 0.72268 0.72126

0.650 0.454 0.569 0.412 0.216

401 251 233 404

0.3046 0.4599 0.9116 0.1169

0.72612 0.72432 0.74909 0.71018

0.884 1.33 2.65 0.339

O t h e r samples

Tan Tan Tan Tan

13 14 15 16

122 116 212 47.3

,1 Mean of duplicate analyses. ,2 Sample Tan 12 is a block of banded gneiss (about 15 kg). The samples Tan 12A--E are four individual bands sawn according to the compositional banding. Sample Tan 12M is a conventional whole-rock sample of another part of the block.

o t h e r f o u r samples, t h r e e d o n o t s h o w a n y c o r r e l a t i o n , b u t o n e fits a p p r o x i m a t e l y t o t h e 589 Ma i s o c h r o n . This i s o c h r o n is i n t e r p r e t e d as r e f l e c t i n g a p h a s e o f Sr i s o t o p e h o m o g e n i z a t i o n w i t h i n t h e s a m p l e d u r i n g t h e P a n - A f r i c a n t h e r m o t e c t o n i c episode. This w o u l d also e x p l a i n t h e high initial ~TSr/S6Sr ratio. T h e results o f R b - S r and K - A r analyses o n 14 b i o t i t e s and 3 m u s c o v i t e s are listed in T a b l e III. All m i c a ages clearly reflect t h e i m p r i n t o f t h e PanA f r i c a n t h e r m o t e c t o n i c episode, in a c c o r d a n c e w i t h t h e results o b t a i n e d b y t h e G e r m a n G e o l o g i c a l Mission. In t h e area u n d e r discussion, h o w e v e r , c o n t r a r y to t h e p a t t e r n o b s e r v e d in t h e n o r t h w e s t , n o micas h a v e b e e n f o u n d t h a t s h o w a r e c o r d o f p r e - P a n - A f r i c a n h i s t o r y in t h e i r R b - S r a n d K-Ar syst e m s . All i s o t o p e s y s t e m s w e r e t h u s t h o r o u g h l y r e s e t b y t h e P a n - A f r i c a n t h e r m o t e c t o n i c episode. T h e d a t a also s h o w a t e n d e n c y o f a d e c r e a s e in m i c a ages t o w a r d s t h e s o u t h e a s t , a c o n t i n u a t i o n o f the t r e n d o b s e r v e d n o r t h w e s t o f t h e i n v e s t i g a t e d area. O f t h e 14 b i o t i t e R b - S r ages, 12 lie w i t h i n t h e range 4 3 4 - - 4 8 4 Ma, averaging 463 Ma. T h e ages o f s a m p l e s T a n 19 ( 5 1 1 Ma) and T a n 29 ( 3 8 8 Ma) fall o u t s i d e this range. All b i o t i t e ages are significantly l o w e r t h a n t h e t w o m u s c o v i t e R b - S r ages, w i t h a m e a n value o f 528 Ma. This d i f f e r e n c e in RbSr ages b e t w e e n t h e m u s c o v i t e a n d the b i o t i t e m a y be i n t e r p r e t e d as reflecting s u b s e q u e n t stages in t h e c o o l i n g h i s t o r y o f t h e crustal b l o c k a f t e r the t e r m i n a t i o n o f t h e P a n - A f r i c a n t h e r m o t e c t o n i c episode, as t h e b l o c k i n g t e m p e r a t u r e o f m u s c o v i t e t o R b - S r is higher t h a n t h a t of b i o t i t e .

4.92 11.69 5.27 32.1 11.04 8.11 4.60 13.90 5.15 8.73 642,3 50.3 15.92 32.3*3 8.68 434,3 128 20.5

810 286 522 598 725 262

654 796 418 455 95.8 *3 229 177 599 *3 494 87.4,3 250 543

4.4131 1.9460 2.5247 1.8673 0.70884,3 0.80491 0.93138 1.0105,3 1.8889 0.71695,3 0.75552 1.2191

5.3794 1.2485 2.9748 1.0622 2.1470 1.3685

87 Sr/36 Sr

± 9 ± 9a ± 9 ± l0 b

± 9 _+ 10 ± 9 ± 9 ± 9 ± 9

~ 524 ± 10 b 484 ± 10 388± 8 9b ~ 434 ± ) 531 ± 12 b 449 ± 9

464 463 462 ) 486

472 511 456 450 467 468

Age*: (Ma)

0.344 0.327 0.213 0.284 0.326 0.268

8.50 7.20

0.312 0.285 0.284 0.286 0.318 0.304 0.319 0.317 0.273 0.296 0.305

rad.4°Ar*4 (ppm)

8.87 7.19 6.59 7.54

7.74 7.18 7.76 7.46 7.27 7.45 8.30 8.11 7.35 7.85 7.82

K .3 (%)

482 _+ 14 470 ± 14

15 17 12 14

± 15 ± 15 ± 14 +_ 14 ± 16 ± 15 ± 14 ± 15 _+ 14 ± 14 ± 15 487 ± 560 ± 414± 475 +

504 499 464 482 541 508 483 491 468 475 489

Age *s (Ma)

*~ WR, w h o l e - r o c k ; bio, b i o t i t e ; mu, m u s c o v i t e . ,2 E x c e p t for t h e figures m a r k e d a a n d b, all age calculations are based u p o n an a s s u m e d initial 87 S r / S 6 Sr ratio o f 0.705 ( R b / S r o f t h e c o r r e s p o n d i n g w h o l e - r o c k lies b e l o w 0.42). Errors based u p o n an e s t i m a t e d analytical a c c u r a c y o f 1% for Rb and Sr and 0.05% for 87 Sr/S6 Sr. a A n initial 87 Sr/S~ Sr ratio o f 0.72 is a s s u m e d ( R b / S r c o r r e s p o n d i n g w h o l e - r o c k is 2.2). b Calculated f r o m t h e pair b i o t i t e / w h o l e - r o c k o r m u s c o v i t e / w h o l e - r o c k . ,3 Mean o f d u p l i c a t e analyses. ,4 The c o n t r i b u t i o n o f a t m o s p h e r i c 4o Ar to t h e t o t a l 4o Ar lies b e l o w 5% for all analyses, e x c e p t for b i o t i t e s Tan 28 and Tan 29 w h i c h have c o n t r i b u t i o n s o f 17% and 12%, respectively. , s Errors based u p o n e s t i m a t e d analytical accuracies o f 1% for K and 2% f o r Ar,

18 bio 19 bio 20 bio 21 bio 22 bio 23 bio 23 m u 24 bio 25 bio 26 bio 27 bio 27 W R 27 m u 28 bio 29bio 30 bio 30 WR 30 m u 31 bio

Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan Tan

Sr (ppm)

Rb (ppm)

Sample Nr. .1

Rb-Sr and K - A r m i c a ages f r o m gneisses

T A B L E III b~

237 Except for biotite Tan 19 (which has an exceptionally high Rb-Sr age), the K-Ar ages of all biotites are higher than the corresponding Rb-Sr ages. For the 14 analyzed biotites, the K-Ar age/Rb-Sr age ratios average 1.14. It should be noted that this effect can also be observed to the northwest in the area investigated by the German Geological Mission, as follows from the biotite ages published by Wendt et al. (1972) and Gabert and Wendt (1974). The difference is even more pronounced in that area, but it disappears towards the Tanganyika Shield with the fading of the effects of the Pan-African thermotectonic episode. Biotites with K-Ar ages higher than the corresponding Rb-Sr ages appear to be a characteristic feature of the polymetamorphic terrain bordering the Tanganyika Shield in southern Tanzania. Possibly, the expulsion of radiogenic 4o Ar from the basement rocks during the Pan-African thermotectonic episode was incomplete, leaving a high 40 Ar partial pressure in the rocks during the recrystallization of the biotite. Some excess radiogenic 40 Ar could then have been incorporated in the biotite. One can only speculate about the physico-chemical conditions under which small rock domains become closed to Sr isotope redistribution in a cooling part of the crust. Nevertheless, it may safely be assumed t h a t on the scale o f a hand specimen this closure will take place at a higher temperature than on the scale of individual mineral grains. This bestows particulhr significance on the internal isochron of 589 + 70 Ma obtained from the banded gneiss sample Tan 12 in the eastern part of the investigated area, as it may be inferred that the isochron arrangement approximates the age of the main phase (or one of the main phases) of the Pre-African thermotectonic episode in this part of the Mozambique Belt. Finally, it should be noted that no samples were dated from the granulite terrain in the north. No conclusions can therefore be drawn regarding the age of the granulite facies metamorphism. CONCLUSIONS Geological mapping both by the German Geological Mission in Tanzania and by NUGPIT confirms the hypothesis that the Ubendian and Usagaran Belts originally formed a single orogenic belt, the Ubendian-Usagaran Belt (Orogeny), which curves around the southern border of the Archean Tanganyika Shield. A Rb-Sr whole-rock isochron investigation in the area under discussion points to an age of the order of 1800 to 1700 Ma for the main phase of post-Ndembera granitic magmatism within this Orogeny (corresponding to the Eburnian Orogenic Cycle). All Rb-Sr and K-Ar mica ages within the Usagaran System reflect a thorough resetting during the Pan-African thermotectonic episode. Regarding the Rb-Sr ages, the two muscovite ages (mean value 528 Ma) are higher than the 14 biotite ages (12 of them in the range 434--484 Ma, averaging 463 Ma), which may be interpreted as subsequent stages in the cooling history after the climax of the Pan-African thermotectonic episode. All biotite K-Ar ages are higher than the corresponding

238 R b - S r ages, possibly reflecting a high partial 40 Ar pressure in t h e r o c k s during t h e resetting o f t h e biotite K-Ar systems. O n e b a n d e d gneiss sample displays an internal Rb-Sr w h o l e - r o c k i s o c h r o n o f 5 8 9 -+ 70 Ma and a high initial s~ Sr/S~ Sr ratio o f a b o u t 0.719. This a r r a n g e m e n t is i n t e r p r e t e d as reflecting Sr i s o t o p e e q u i l i b r a t i o n within the sample, and the i s o c h r o n age m a y be c o n n e c t e d w i t h the main phase, or o n e o f the main phases, o f the Pan-African t h e r m o t e c t o n i c episode. ACKNOWLEDGEMENTS N U G P I T is p r o j e c t G U A / V U A / 1 o f the P r o g r a m m e for University Coopera t i o n in D e v e l o p m e n t W o r k o f the N e t h e r l a n d s University F o u n d a t i o n f o r I n t e r n a t i o n a l C o o p e r a t i o n ( N U F F I C ) . T h e isotopic d a t i n g w o r k was undert a k e n in c o n n e c t i o n with N U G P I T and f o r m s p a r t o f the research p r o g r a m o f the " S t i c h t i n g v o o r I s o t o p e n - G e o l o g i s c h O n d e r z o e k " , s u p p o r t e d by t h e N e t h e r l a n d s O r g a n i z a t i o n for the A d v a n c e m e n t o f Pure R e s e a r c h (Z.W.O.).

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