Journal of Afrtcan Earth Sctences, Vol 9, No 3/4, pp 749-757, 1989 0899-5362/89 $3 00 + 0 00 Printed m Great Britain © 1990 Pergamon Press plc

Rb-Sr ages for Archaean granitoids and tin-bearing pegmatites in Swaziland, southern Africa

*R. MAPHALALA, **A. Kaot, rER and ***J.D.K.RAMERS

*Geological Survery and Mines Department, P.O. Box 9, Mbabane, Swaziland **Institut ftlr Geowissensehaften, Universitat Mainz, Postfach 3980, 6500 Mainz, F. R. G.

***Dept. of Geology, University of Zimbabwe, P.O. Box MP 167, Harare, Zxrnbabwe

Abstract- Lain Archaean cratonization in the Kaapvaal craton of southem Africa was associated with voluminous granitoid emplacement, and we report on the Rb-Sr isotopic systemadcs of the Lochiel suite and associated tin-bearing pegmatites in northwestern Swaziland, one of the largest composite batholiths of this region.

Thirty-nine samples of the various gramtoid phases of the Lochiel batholith and related pegmatites were collected within the "tin-belt" of Swaziland that extends in a SE direction from Makwane beacon (South African border) to Nyonyane hill some 20 km SW of Mbabane. Rb-Sr whole-rock dating yielded an isochron age of 2979 + 33 M.a. for the Granodiorite phase (GD) with an initial 87Sr/~6Sr ratio of 0.7017 + 0.0005 while the Alkali-Feldspar Granite (AFG), that intrudes the GD, has an apparent age of 3000 + 125 M.a. and a STSr/~6Sr initial ratio of 0.7047 + 0.0103. Two pegmatite suites from widely separated localities have virtually identical isotopic systematics and, if regressed together, define an age of 2613 + 35 M.a.

The age of ca. 3000 M.a. for the GD and AFG is in good agreement with published ages for the Lochiel Batholith from other localities. The previously held contennon that the various phases of the Lochiel B atholith are indistinguishable with respect to their Sr isotopes is suppor tedby our new data. The low imual ratios for both gramtoid suites makes it unlikely that the Lochiel batholith was produced by remeltmg of the 3.5-3.6 G.a. old Ancient Gneiss Complex, and we favour an underplating model with a sigmficant juven i l e m a g m a co m p o n en t . Th e h i g h Sr i m u a l ratio o f the pegmat i t e pha se IS m l ine wi th repor ted ln iua l ratios of younger granitoids in Swaziland and suggests that these rocks are remelting products of older siahc crust.

I N T R O D U C T I O N A N D GEOLOGICAL S E T T I N G

T h e Loch ie l G r a n i t e , f o r m e r l y a lso k n o w n a s t h e H o m o g e n e o u s H o o d G r a n i t e (Hun te r , 1973a , 1974; Cond i e a n d H u n t e r , 1976), a n d n o w s e p a r a t e d in to d i s t i n c t b o d i e s k n o w n a s Piggs P e a k b a t h o l i t h a n d M p u l u z i b a t h o l i t h in n o r t h e r n S w a z i l a n d (Fig. I) a n d H e e r e n v e e n b a t h o l i t h in n e i g h b o u r i n g S o u t h Afr ica ( A n h a e u s s e r a n d Robb, 1983; B a r t o n et al., 1983a) , is a s c r i b e d to a w i d e s p r e a d m a g m a t i c even t in t h e K a a p v a a l c r a t o n b e t w e e n 3.2 a n d a b o u t 3 G.a. ago t h a t led to c r u s t a l t h i c k e n i n g a n d c r a t o n l z a t i o n of a t yp i ca l ea r ly A r c h a e a n g r an i t e - g r e e n s t o n e t e r r a i n ( H u n t e r , 1974; Dav ies a n d Al l sopp , 1976; A n h a e u s s e r a n d Robb, 1983). T h e K- r i ch n a t u r e of t h e s e r o c k s is in m a r k e d c o n t r a s t to t h e t o n a l i t e - t r o n d h j e m i t e c o m p o s i t i o n of t he o lde r p l u t o n s in t h i s r eg ion a n d m a y i nd i ca t e s ign i f i can t c r u s t a l m e l t i n g t h a t f ina l ly led to a n evolved, h igh ly d i f f e r e n t i a t e d u p p e r c o n t i n e n t a l c r u s t a s r e f l ec t ed in t h e f i rs t c r a t o n i c s e d i m e n t s t h a t w e r e d e p o s i t e d s o m e 2 .9 G.a. ago ( M c L e n n a n et al., 1983).

O u r i n t e r e s t in t h e Lochie l G r a n i t e w a s m a i n l y f o c u s e d o n t h e e c o n o m i c a l l y i m p o r t a n t t l n - b e a r i n g p e g m a t i t e s , a n d we i n v e s t i g a t e d t h e s e r o c k s a n d t he v a r i o u s g r a n i t o i d va r i e t i e s in o r d e r to u n d e r - s t a n d t h e i r gene t i c r e l a t i o n s h i p s a n d t h e or igin of t i n m i n e r a l i z a t i o n . T h e i n t e r e s t in t h e p e g m a t i t e s is b o m o u t by t he r ea l i z a t i on t h a t t h e s e r o c k s h a v e y i e lded c o n s i d e r a b l e t o n n a g e s of c o a r s e - g r a l n e d e luvia l a n d a l luvia l c a s s i s t e r i t e s i nce 1892 (Hun te r , 1973b) .

T h e Lochie l b a t h o l i t h is a d i s c r e t e shee t - l i ke g r a n l t o i d t h a t o c c u r s in NW S w a z i l a n d (Fig. I) a n d b e t w e e n t h e v i l lages of A m s t e r d a m a n d Lochie l in n e i g h b o u r i n g S o u t h Africa. It is a m u l t i c o m p o n e n t b o d y w i th m a r g i n a l z o n e s of K- r i ch m i g m a t i t e s t h a t ref lec t i n t e r a c t i o n b e t w e e n t he g ran i to id m a g m a s a n d o lde r c r u s t ( A n h a e u s s e r a n d Robb, 1983). P u b l i s h e d R b - S r a n d U-Pb i so top ic ages for t h e b a t h o l i t h r a n g e b e t w e e n 2 9 8 6 + 69 a n d ca. 3 0 7 5 M.a. (Allsopp etal . , 1962; O o s t h u y z e n , 1970, Davies , 1971; B a r t o n et al_, 1983b) , a n d t h i s c lose age g r o u p i n g h a s s u g g e s t e d t h a t all t h e m a g m a t i c p h a s e s a re i so top ica l ly coeva l ( A n h a e u s s e r a n d Robb, 1983).

~S s:3/4-x 749

750 R. MAr'HALALA, A. KRONER and J. D. KRAMER$

O 10 2 0 3 0 4 O k r a I I ~ t ]

Fig. 1. Simplified geological map of Swazfla_nd showing study area (solid rectangle) within the Mpuluzi Batholith (MB) and other associated Archaean rock units. Thick arrow shows pegmatite occurrences near the confluence of Usutu River [UR} and Ngwempisi River (NR). Other abbreviations are: AGC- Ancient Gneiss Complex, BGB-Barberton greenstone belt, HG-Hlal~kulu Granite, MGD-Mliba granodiorite, MP-Mbabane Pluton, NP-Ngwempisi Pluton, ORT-Other rock ty~es, PPB- Piggs Peak Batholith, SP-Sinceni Pluton, UIS-Usushwana

Intrusive Suite, US-Usutu Suite.

In NW Swaziland the Lochiel batholith intrudes older rocks of the Ancient Gneiss Complex (AGC) with ages between 3.2 and 3.64 G.a. (Barton et a/.,1980; Carlson et aL, 1983; Compston and KrOner, 1988; Kr6ner et al., 1989), the equally old Barberton Greenstone Belt (BGB) and associated schist relicts, and the Usutu Igneous Suite {US), previously known as Granodiorite Suite (GS) (Hunter 1974) of largely granodioritic composition (Rb-Sr age 3350 _+ 57 M.a., Barton etal. , 1983a). It is, in turn, in t ruded by the mafic-ultramafic Usushwana Igneous Suite (UIS) that has a Sm-Nd isochron age of 2871 + 30 M.a. (Hegner eta/., 1984) and the coarse-grained porphyritic Mbabane granite pluton (MP) with an age of 2691 + 4 (Layer et al., 1989).

The maj or and trace element geochemistry of the Lochiel batholith within the tin-belt of Swaziland (Fig. l) has led to identification of discrete phases of Granodiorite (GD), Adamellite (AD) and Alkali- feldspar granite [AFG), following the Streckeisen (1973, 1976) classification. The distribution of these phases is shown in Fig. 2 (Maphalala 1983). Major and trace element chemistry (Table l) indicate that the Granodiorite phase is character-

A l k a l i - f e l d s p a r g r a n i t e

Ad a m e l l l t e

Gr anOdlor l ie

B a r b e r Ion g r e e n a t o n l be ) t

[ ~ o n a h t l C g n e ) s s

I 1 m B 1 Luphohlo l u n n e l A s p h a l t r o a d

-- ~ Dlr l r o a d

R ~ v e r

4 ~ Old Im mine w o r k i n g

1 5 10 Km

Fig. 2. Detailed map of study area (solid rectangle in Fig. l) showing distribution of the Mpuluzi glarfltold phases based on the Streckeisen [1976) classification, sample locations and the

Luphohlo hydro-electric tunnel. Modified after Maphalala (1983).

ized by relative ennchmen t in most major elements and has rather high average concentrat ions of Ba, Sr, Ce and Zr compared to the AFG and associated pegmatltes. The AFG has relatively low abundan- ces in all major elements, except for K20 and SIO z, and is markedly enriched in Rb, Nb and Y com- pared to the GD. The pegmatites show major and trace element abundances similar to the AFG, though relatively enriched in Rb, Nb and Y and depleted in Ba, Sr, Ce and Zr. Due to this slight overlap in both major and trace elements, it was tentatively concluded that the two phases are genetically related (Maphalala, 1983).

Tin-bearlng pegmatites occur in a linear belt across NW Swaziland, about 10 km wide and extending in a SE direction from Makwane beacon (South African border) to Nyonyane Hill, some 20 km SW of Mbabane (Fig. 1). The pegmatites are generally more abundan t and larger near the strip of the AGC bordering the eastern portion of the tin belt and in xenoliths of the AGC occurring within the Lochiel batholith. Some pegmatites also occur near the contact of the BGB with the Lochiel

Rb-Sr ages for Archaean granitoids and tin-bearing pegmatites

T a b l e 1. A v e r a g e m a j o r and t r ace e l e m e n t a n a l y s e s for the G r a n o d i o r i t e , A l k a l i - F e l d s p a r G r a n i t e a n d P e gma ta t e s . D i s t r i b u t i o n a c c o r d i n g to r a n g e i n s i l i ca . F r o m M a p h a l a l a (1983) .

75

Granodiorites

Range $102 < 70 Range SzO 2 > 70 <71 Range SiOa> 71 < 72 Range

Number N = 13 N = 12 N = 9 of Samples

SlO a 67.61 65.61 - 69.94 70 45 70.08 - 70 98 71.27 71.17 - 71 72 T102 0_57 0 45 - 0 98 0 43 0.36 - 0.52 0.41 0.21 - 0 42 A120 J 15.30 14 65 - 16.28 14.82 14.37 - 15 13 14.63 14.36 - 15 02 FeOj, 3 51 2.30 - 5.27 2 43 2 08 - 3 16 2.20 1 89 - 2 43 MnO 0.06 0 04 - 0 08 0.05 0.04 - 0.07 0 04 0.04 - 0 05 M g O 0 92 0.63 - 1.33 0.57 0 45 - 0 67 0 53 0_42 - 0 68 CaO 2 22 1.43 - 2.73 1.57 1 29 - 2.26 1 50 1.22 - 2 04 NaaO 4.67 3.70 - 5.22 4 57 4.36 - 4.77 4 60 4 19 - 5.02 1(20 3.74 2.99 - 4.41 4 04 2 56 - 4 36 3 92 2.38 - 4.29 P20~ 0.28 0.20 - 0.36 0 18 0 .10 - 0.21 0 16 0 0 8 - 0.20 SO s n_d. n.d. - 0.03 n d n.d. - 0 03 n_d. n.d. - 0.03 LOI 0.46 0 .13 - 0.75 0.24 0 .13- 0 7 5 0.27 0 .00 - 0.50

T O T A L 99.34 99.35 99.53

Ba 1069 514 - 1526 1028 338 - 1267 850 0 - 1111 Rb 224 136- 324 198 125- 381 218 123- 283 Sr 623 2 9 7 - 324 717 168- 819 602 3 6 6 - 722 Ce 197 124- 294 154 6 7 - 203 135 6 9 - 183 Nb 19 1 2 - 3 9 14 9 - 48 14 9 - 20 Ta <5 <5 < 5 - 9 <5 < 5 - 9 Y 36 11- 50 26 17- 116 18 4 - 29 Z r 245 131- 452 224 125- 286 200 130- 232 K/Rb 152 106- 169 169 8 8 - 198 150 126- 171 Rb/Sr 0_26 0.17 - 1.09 0 28 0.23 - 2.27 0 38 0.29 - 0 60 Na/K 1_13 0.76 - 1.45 1 07 0.92 - 1 59 1 10 0 92 - 1 88

Pegmatites

Range SzO 2 < 74 Range $102 > 74 < 75 Range $102 > 75 Range

Number N = 12 N = 9 N = 16

of Samples

SiO 2 73.11 69.78 - 73.96 74 42 TIO 2 0.01 0 .01 - 001 0 0 2 AI203 14 13 13.34 - 15.20 13 25 FeO~, 0 84 0.69 - 1 22 1 15 MnO 0.11 0 .04 - 0_32 0 12 M g O 0.01 0.00 - 0 04 0 01 CaO 0.35 0 15 - 0_60 0 5 2 NaaO 4_84 3 40 - 6.93 4 95 I ~ O 5.27 1 81 - 8.66 3.67 PzOs 0 . 0 0 0.00 SO 3 0.11 0 0 1 - 0.23 0.13 LOI 0.71 0 59 - 0.85 0 69

74 16 - 74.71 0 0 1 - 0.06

1 2 4 7 - 14.18 0 .73 - 1.98 0 .01 - 0.32 0.00 - 0 05 0.27 0 90 2.82 7 11 1 . 7 6 7 38 0_00 0 02 0 0 4 0 2 4 0.48 0 90

76 24 75 01 - 79_71 001 0 0 0 - 0.02

12 41 10 12 - 13.55 1 11 0 7 7 - 1-55 0.15 0 .01 - 0.37 0.01 0.00 - 0 09 0.41 0_20- 071 4.86 3 .30 - 5 65 3.30 0 .93 - 4.97 0.00 0 .00 - 0.01 0.14 0.04 - 0.22 0.68 0 .43 - 0.86

T O T A L 98.76 99 19 99.28

74 474

28 19 37 14

176 82 63 32

1 6 0

Ba 16 Rb 625 Sr 31 Ce 10 Nb 31 Ta 15 Y 131 Zr 56 K/Rb 64 Rb/Sr 30 Na/K 0.85

0 160 245 649

9 84 0 43

13 80 8 23 6 368

16 297 36 123

7 65 0.86 3 61

0 75 268 1121

9 87 0 23

20 42 9 23

16 - 257 13 - 106 4 0 - 121

3 1 - 74.8 0 35 - 3.42

54 0 - 140 439 141 - 935

22 6 - 105 14 0 - 41 43 9 - 123 17 9 - 42

262 83 - 645 137 19 - 281

50 41 - 248 32 1 3 - 92

1 65 0.59 - 5 10

752

Table 1 Icont.)

R. ~ , A. KRONER and J.D. IQ~MERS

Alkali-Feldspar Granites

Range SxO= < 74

Ntanber N = 13 of Samples

Range SiO a > 74 < 75 Range $102 > 75

N = 12 N = 13

Range

$102 73,54 73.19- 73.99 TIO= 0,13 0.01- 0.20 Al203 13.88 13.21 - 14.34 FeO3, 1.53 0,94 - 2 24 MnO 0.06 0 O1 - 0.36 MgO 0 13 0.03 - 0.20 CaO 0 65 0.33 - 0.80 Na20 4.16 3 . 6 5 - 4.84 I ~ O 4.93 3 . 5 9 - 5.66 P2Os 0.03 0 .01 - 0.06 SO~ n.d. n.d. - 0 01 LOI 0.33 0 00 - 0.88

TOTAL 99.37

Ba 259 47 - 458 Rb 334 185 - 421 Sr 86 29 - 180 Ce 107 7 - 166 Nb 26 9 - 42 Ta 5 < 5 - 9 Y 41 1 7 - 94 Zr 145 56 - 239 K/Rb 127 99 - 174 Rb/Sr 3.93 1.69 - 6 38 Na/K 0.77 0.60 - 1 20

74.56 74.21 - 74,96 76.09 75 05 - 78.54 0 II 0.04 - 0.17 0.08 0.04 - 0.12 13,82 13,36- 14,30 12.89 10,99 - 13.86 1.28 0,91 - 1.97 1.31 0.56- 258 0.03 0.02 - 0.04 0 03 0 01 - 0.05 0 11 0.01 - 0.18 0.04 n.d, - 0.08 0 63 0.46 - 0.76 0,53 0.14 - 0.69 4.07 3,16- 4.80 3.83 3.19 - 4 42 4,69 3,84- 5.82 4.58 3 57 - 5 17 0.03 0.01 - 0.04 0 01 n.d - 0.03 n.d n.d. - 0 03 n.d n.d. - 0.04 0.15 0.00- 050 0.32 000- 063

99 48 99 71

231 8 - 398 122 l0 - 243 317 246 - 432 318 162 - 533

75 25 - 116 51 15 - 130 I00 17 - 148 85 8 - 250 26 14 - 43 26 5 - 66

5 < 5 - 8 6 < 5 - I I 47 8 - 99 41 6 - 74

118 62 - 180 83 25 - 348 126 92 - 196 130 77 - 188 8.85 2.12 - 10 80 8 49 1 30 -17.60 0.79 0.48 - I . I 0 0.71 0 12 -0.99

batholith. The large-scale geometry of the pegma- tite emplacement pattern has not been well esta- blished, but according to Hunter (1954} and Davies (1964) the pegmatites are characterized by a complex stockwork and are typically not wider than about one metre. It is apparent, though, from outcrops in the Mantenga area, Luphohlo Hill and in the Luphohlo Tunnel that the pegmatites vary in width from less than a metre to over three metres. Observations from these areas and outcrops near the confluence of the Usutu and Ngwempisi Rivers (Fig. 2) indicate that they display cross-cutting relationships that are partly controlled by older structures In the Lochlel batholith and in pre- Lochiel rocks and by a fracture-fault (en-echelon) system following NE-SW and NW-SE directions. The dips are variable from fiat to vertical.

Mapping in the Tunnel showed that pegmatites generally cut across all phases of the Lochiel batholith that are exposed in the Tunnel, though their abundance diminishes within the AFG. It was also observed that pegmatites generally intersect gabbroic dykes that are of slmflar composition and texture as the gabbroic phase of the U s u s h w a n a Igneous Suite and axe genetically related to this complex. The pegmatites are, in turn, intruded by

fine-grained and locally porphyritic dolerite dykes of u n k n o w n but possibly mld-Proterozoic age and related to the emplacement of the Bushveld Complex as suggested by palaeomagnetic data (Layer et al., 1989). These dykes are locally sheared and fractured, the shear zones typically being filled with talcose material and, in places, calcite. Pegmatites are displaced where they are intruded by younger dolerite dykes, indicating late tectonic movement associated with dyke emplacement.

It would appear from the above observations and general field relationships that the pegmatites: (1) post-date all phases of the Lochiel batholith and were probably also emplaced after intrusion of the UIS, i.e. after ca. 2 .87 G.a. ago, (2) are younger than the Usushwana-type gabbro dykes and are older than the pos t -Usushwana Bushveld-related dolerite dykes, (3) were emplaced in an en-echelon fracture-fault system and (4) after emplacement of the dolerite dykes, the region was subjected to mild tectonic disturbance, perhaps associated with regional uplift that caused shearing and fracturing in the gabbro and dolerite dykes.

Our isotopic study was initiated in 1984 and is

Rb-Sr ages for Archaean granitoids and tin-bearing pegmatites

based on samples of Maphalala (19831 and more recent sampling of pegmatites in the 4.5 km long Luphohlo Tunnel (thick solid line in Fig. 2) as well , oo

as additional granitoid material from the Tokwane and Mantenga quarries (Fig. 2).

LNALI ' r IC/~ PROCEDURF~

RblSr ratios and Rb and Sr concentrat ions on 24 samples were determined by XRF on duplicate pressed powder pellets at the University of Mainz, following the procedure of Pankhurs t and O~lions ( 19731, as detailed in KrOner (1982). The remaining 12 samples were analyzed by s tandard isotope dilution (ID) at the University of Zimbabwe in Harare. For both techniques the precision of the Rb/Sr ratio is 2% or better, whereas that for the Rb and Sr concentrat ions determined by XRF is about 2.5%, and better than 2% for the ID results. All isotopic analyses were carried out at the Isotope Laboratoryofthe UniversityofZimbabwe in Harare, using s tandard ion exchange techniques for sepa- ration and employing a 60 ° single focussing. 28 cm radius mass spectrometer with single collector and on-line data reduction.

All S7Sr/~Sr ratios were normalized to STSr/SaSr = 0.1194. Repeated analyses (since early 19851 of the NBS SRM 987 s tandard yielded 0.710248 _+ 87 (standard error, population of 17 runs). Age calcu- lation follows a combination of York's (19691 regression with correlated errors and Mclntyre et a t ' s (1966) statistical assessment of the variances of 87Rb/~Sr as developed by J.C. Roddick. Quoted ages are based on ~bTRb = 1.42 x 10-Ha -'. and all errors are given at the 2-sigma level. Where necessary, quoted literature data have been re- calculated using the above decay constant.

0 9(3

0 B0-

87Sr / 86Sr O r o n o d l o r l t e

Age 2979_-+33 Me_ RI = 0 7017 -+ 0 0 0 0 5 MSWD = 2 O

. . . . . . . . . 87iRb (86 Sr'-_ 10 20 30 &0 50 60 70 80

Rb-Sr ISOTOPIC DATA

The analytical data are shown in Table 2, and the resulting isochron and errorchron ages are shown in Figs. 3-5.

Granodiorite (GD) The GD is considered to represent the oldest

phase of the Lochiel Batholith on the basis of its major and trace element composition (Maphalala, 19831 and on its field relations with the adamellite (Fig. 2) that forms the main phase of the batholith. Rb-Sr whole isotopic data for twelve samples (Fig. 3) yield an isochron with an apparent age of 2979+ 33 M.a. (Mean Square of Weighted Deviates, MSWD = 2.02). This isochron age is in good agree- ment with previously published ages of the Lochiel Batholith of 3005 + 120 M.a. by Allsop et aL, (1962), 2986+_ 69 M.a. by Davies (19711, 3028 +14 by Barton et aL, (1983b) and 3005 + 6 M.a. by Kr6ner et a t , (19891. The initial STSr/Se Sr ratio of

753

Fig. 3. Whole-rock Isochron plot showing data points for the Lochlel Granodiorlte phase.

0.7017 + 0.0005 is identical, within error, to that reported by Barton et al., (1983b) of 0.7013 + 0.0002.

Alkali-feldspar granite (AFG} The AFG is probably the youngest phase of the

Lochiel Batholith. It occurs in all the other phases of the Lochiel, i.e. porphyritic adamellite, non- porphyritic adamellite and granodiorite. Chemical data indicate that it is a strongly fractionated granitoid (Maphalala, 19831.

The data for nine whole-rock samples scatter about a regression line (Fig. 4, MSWD = 11.61 corresponding to an age of 3039 + 37 M.a. (S7Sr/ SeSr initial ratio = 0.7020 + 0.0030). This scatter is in excess of analytical error, and statistical treat- ment of the data pat tern favours McIntyre et aL, (19661 model 2 (experimental and geological varia- tion proportional to the R b / S r ratio, variation of age assumed) with an age of 3000 + 125 M.a. and Sr I = 0.7047 + 0.0103.

Although sample 16A has an extremely high Rb/ Sr ratio compared to the average AFG and falls off the regression line, its omission from regression would result in an implausibly high apparent age of 3250 + 75 M.a. that contradicts the field rela- tions and all other isotopic data for the Lochiel batholith. We therefore consider all AFG samples analyzed to constitute a genetically related suite and suggest that the observed scatter is due to post-crystallization open system behaviour and / or variable contamination of the AFG with older continental crust that, by about 3 G.a. ago, had developed STSr/~Sr initial ratios higher than those now measured for the GD and AFG. The good agreement in age and Sr, between the GD and AFG supports the contention of Maphalala (1983) that these plutonic suites are genetically related.

Pegmatltes Isotopic age determinations of pegmatites within

the tln-belt (Makwane beacon-Sinceni)(Maphalala, 19831 date back to the late forties. Rb-Sr analyses

754 R. ~ , A. KaON~ and J.D.K.~M~S

Table 2. Analytical data for whole-rock samples of the Granodiorite, Alkali-Feldspar Granite and the Pegmatltes.

Sample N ° Rb Sr 87Rb/aSSr 87Sr/~Sr (ppm) (ppm)

Granodior i tePhase

6g 7f 8g

12k 13f 15h 16g 20b TQI* TQ2* TQ3* TQ4*

183 636 0.8146 0.73626+__20 126 209 1.7190 0.77740-!-__20 261 599 1.2537 0.752265:20 166 664 0.7050 0.73259+20 206 140 4.335 0.89299-+30 230 678 0.9588 0.74230!-__20 155 874 0.5030 0.72335+20 369 161 6.821 0.99323+25 254 618 1.1944 0.75428+22 190 669 0.8959 0.74144+27 219 642 0.9904 0.74581+22 331 120 8.249 1.05291+40

Alkali-Feldspar Phase

8f 9C

12a 12d 13g 16a AFGI* AFG2* AFG3* AFG4*

159 121 3.854 0.86148+20 262 32.8 24.85 1.74379-k40 186 37.0 15.58 1.40640-k30 102 29.3 10.60 1.18608+30 222 84.7 7.865 1.09338+30 523 33.6 54.41 2.83553+40 262 192 4.013 0.87818+30 256 171 4.410 0.89489+25 247 193 3.760 0.86798+30 287 174 4.870 0.91816+20

Pegmatite Phase

LP 2 LP 4 LP46 LP 51 LP 54 LP 55 LP 78 LP 79 LP 97 LP103 MAQI* MAQ3* MAQ4* MAQ5*

610 13.36 184.08 7.73420-k-__300 456 47.1 30.48 1.86560-k-__ 50 354 46.1 24.02 1.48450-k 40 257 78.9 9.558 1.10648+ 30 723 23.0 138.31 6.03537+200 358 11.0 163.98 6.94694+ 60 448 13.1 180.21 7.64000-k 60 506 25.9 72.85 3.51341+ 150 317 48.2 20.09 1.51341+ 60 485 60.2 24.85 1.63763+ 60 192 30.4 19.95 1.64976_.+. 60 281 34.7 25.62 1.66478+ 60 413 27.2 52.25 2.64595+ 70 778 22.5 155.06 6.33170-k 80

Rb and Sr concentrations in samples with" were measured by ID in Haraxe For all other samples measurements were by XRF in Mainz.

Rb-Sr ages for Archaean granitoids and tin-bearing pegmatites 75s

3 0

2 5

87Sr/86Sr ~ ' / ~ _ AIkah- Feldspar Granite

/ /

/ -

J 2 0 -

15 -

1 0 -

/

/

/ Age 3000 t125 Me

"'~" R i : 07047 t0 0103

i'o ~s 2'0 2's Yo

87Rb/86Sr

3"5 L'O 4'5 5'0 5'5

Fig. 4. Whole-rock isochron plot showing data points for Lochiel Alkali Feldspar Granite phase.

on a lepidolite pegmat l te nea r Kubu ta , s o u t h w e s t of Sinceni, were car r ied ou t by Ahrens (1947), Holmes and C a h e n (1957) and Aldrich etaL (1958). The ages ob ta ined were 2 1 0 0 Ma, 3 2 0 0 + 320 M.a., and 1920 + 145 M.a. respectively. In no r thwes te rn Swazi land U-Pb ages repor ted b y Holmes and Cahen (1957) and Kulp and E c k e l m a n n (1957) on a y t t ro tan ta l i t e -bear ing pegmat i te nea r M b a b a n e gave ages of 2 0 5 0 M.a. and 2 0 6 0 M.a. respectively. These da t a sca t t e r cons iderab ly b u t never the less s eem to favour a n early Proterozoic age.

The Rb-Sr who le - rock da ta p resen ted here con- s t i tu te an a t t e mp t to es tab l i sh a more reliable age for some of these pegmat i t e s and their age rela- t ionship with the a s soc ia ted older and younge r granitoids, especial lywith the p h a s e s of the Lochiel Batholi th. Our da ted s a m p l e s come from the Lupholo Tunne l and the Mantenga quarry. At bo th localities the s a m p l e s show no observable wea the- ring or al terat ion. Thin sec t ion inspec t ion revealed minor chlorltic a l te ra t ion of biotite, and the biotite t ended to fill smal l f r ac tu res or c racks in the feldspars .

The Rb-Sr whole - rock isotopic da ta "(Table 1) show tha t the pegmat i t e s have a wide range in SrRblSe Sr rat ios, varying from 10 to over 600. The highly radiogenic s a m p l e s LP 77 and LP 113 c a u s e large sca t t e r in the da t a dis t r ibut ion, well in excess of analyt ical error as reflected in a va lue of 35 for the MSWD if all da t a are regressed together. If t hese s a m p l e s are omitted, the remaining da ta yield a s o m e w h a t be t t e r defined regress ion line (MSWD = 17.4) co r re spond ing to an age of 2606 + 21 M.a. (Sr i = 0 .7423 + 0.0076). Stat is t ical t reat- men t f avours McIntyre et oL, (1966) model 3 with 2613 _+ 35 M.a. and Sr1= 0 .7240 _+ 0.0374. There is very little difference b e t w e e n these two ages, sup- port ing the con ten t ion tha t the s amp le s are co- genetic a n d tha t the sca t t e r is p robab ly due to the s ame p h e n o m e n a as pos tu l a t ed for the AFG.

Wha t is s ignificant in these data, however, is tha t bo th ages confi rm tha t the pegmat i t e s are signifi- cant ly y o u n g e r t h a n bo th the GD and AFG and also

70 ~ 8?Sr/86Sr Pegrnahtes

4o~

o~

10= Y . /

s?

f J

10'0

Age 2613_+ 35 Ma R~ :0 7240 ±0037Z,

87Rb / 86 Sr

Fig. 5. Whole-rock isochron plot showing data points for Pegmatite phase

y o u n g e r t h a n the U s u s h w a n a Complex, in llne with field re la t ionships . The Srl is m u c h higher t h a n tha t of the GD a n d A F G , b u t not high enough to pos tu la te der ivat ion of the pegmat i t e s from an AFG-type source . For example, the GD (mean S7Rb/8eSr = 2.37) wou ld have h ad a Sr t of 0 .714 _+ 0 .005 b y abou t 2600 M.a. ago while tha t of the AFG (mean SZRb/~Sr=- 13.43) would have evolved to 0.778 _+ 0.003.

DISCUSSION

The isotopic da ta for all three grani to ids d isplay a very good sp read of R b / S r ra t ios which gives some confidence in the regress ion ages. The ra ther wide sca t t e r of the AFG and pegmat i te da ta is p robab ly due to post -crys ta l l iza t ion open sys t em behav iou r a n d / o r n o n - h o m o g e n o u s Sr I in the magmat i c source , poss ib ly due to con tamina t ion of a mant le-der ived mel t with older cont inenta l c rus t as h a s also b e e n pos tu l a t ed for the m a g m a of the U s u s h w a n a Complex (Hegner et al., 1984). Our da ta also show conclus ively tha t the Lochiel ba tho- lith is isotopicaUy ra the r h o m o g e n o u s on a large scale with no significant age differences be tween the different granl toid phases . Al though geo- chemical da ta s eem to sugges t a genet ic relation- ship b e t w e e n the AFG and the pegmat i tes (Maphalala, 1983), the isotopic da ta rule out such a llnk.

In s u m m a r y , the available isotopic da ta clearly indicate tha t the Lochiel ba thol l th is abou t 3 G.a. in age. It is also now appa ren t from the low S7Sr/ SeSr initial rat ios tha t the m a g m a type from which the ba thol l th evolved was p robab ly maflc in com- position. Bar ton et al., (1983b) sugges ted an amphibol i t ic pa ren t b a s e d on isotopic data, w h e r e a s Maphala la (1983) specu la t ed on a quar tz gabbro or quar tz diorite, b a s e d on the I-type charac te r i s t ics of the batholi th. Magma sources have also b e e n pos tu l a t ed to lie in the lower c rus t or u p p e r mant le (e.g. Hunte r , 1973a; Condie and Hunter , 1976).

756 Con tamina t i on of the Lochlel m a g m a is indicated

by xenoli ths of older supracrus ta l rocks (e.g. Ancient Gneiss Complex, Barber ton grani te-greens tone assemblages) , and the 20% part ial melt ing of tonnll te- trondhJemite proposed b y A n h a e u s s e r and Robb (1983) sugges ts underp la t ing of dense mafic m a g m a s (KrOner, 1984), in con junc t ion with re- mel t ing of low R b / S r sialic gneissic crust .

The Lochiel Bathol i th h a s been shown to be mul t i -phase , cons is t ing of granite, adarnellite, granodiori te and mlgmat i t ic border zones (e.g. H u n t e r 1954; Condie a n d Hun te r , 1976; A n h a e u s s e r a n d Robb, 1983), or of alkali syenite, alkal i-feldspar granite, adamell i te and grano- diorite (e.g. Maphalala , 1983). The bathol i th and s u r r o u n d i n g rocks are cu t by pegmat l tes (Hunter, 1954,1973a,b; Davies, 1964; Tanka rd et a/., 1982) which are assoc ia ted with t in mineral izat ion.

CONCLUSIONS

From the foregoing cons idera t ions and discus- sion it c a n be conc luded that:

1. The Lochiel ba thol i th cons is t s of var ious phase s of which the granodiori te and alkali- fe ldspar grani te possibly represent the oldest and the younges t , respectively. These two phases have vir tual ly identical ages of about 3 G.a.

2. The pegmat i tes da ted here post -date both the Lochiel ba thol i th a n d the U s u s h w a n a Intrusive Suite with an isotopic age of 2871 + 30 M.a. (Hegner e t a t , 1984) bu t are older t h a n mos t of the dolerite dyke swa rms of probable mid-Proterozoic age. Our samples were collected from the NE-SW t rending dykes as exposed in the Luphohlo Tunnel a n d in the Man tenga qua r ry and therefore exclude early pegmat i te dykes related to the emplacement of the Lochiel bathol i th .

3. The pegmat i tes were possibly emplaced in shea r f rac tures or faul t s approximat ing an en- echelon sys tem with a s t rong NE-SW or NW-SE trend.

4. Various models have been sugges ted for the source o f A r c h a e a n bimodal gneiss and grani- toid suites. A review of some of these models is given by KrOner (I 984). On the bas is of the I-type cha rac te r of the Lochiel (e.g. Maphalala , 1983), a model suggest ing underp la t ing of dense mafic mant le-der ived m a g m a s in extensional environ- m e n t s a n d gene rat ion of tonalite- diorite -trondhj e- mlte p lu tons f rom these sources (KrOner, 1984) and from remelt ing of older sialic c rus t is favoured.

Acknowledgments - This work is part of a cooperation project between the Universities of Mainz (F.R.G.), Harare (Zimbabwe) and the Geological Survey Dept. of Swaziland, partly funded by the German Ministry of

R. ~ , A. KR0t'a~R and J.D. K~MERS Economic Cooperation (BMZ). Deutsche Forschungs- gemeinschaft (DFG) and UNESCO (laboratory work in Harare). We thank D.lq. Hunter (Pietermarltzburg) for help with literature on Swaziland geochronology.

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