Journal of African Earth Sciences, Vol. 16, No. 4, pp. 395404.1993. Printed in Great Britain

08994362493 $6.00 + 0.00 0 1993 Pergaman press Lul

Implications of whole-rock Pb-Pb and zircon evaporation dates for the early metamorphic history of the Kasai’ craton, Southern Zaire

F. WALRA~EN and B. T. RUMJEGERI*

Isotope Laboratory, Geological Survey, private Bag X112. Pretoria, South Africa *Geology Departmens University of Lubumbashi, B. P. 1825. LubumbasK Z&e

(Fit received 13th December, 1991; revised version received 9th February, 1993)

Abstrmt -Awhole-rocltPbPbdateof3021 +4749Ma,foundforgranodioriticgranitoid occurring within biotite

gneiss in the Sandoa-Kapanga region of the Kasal Craton, southern Zaire, in reflected in single zircon evaporation data (centered around3014f 2.3 Ma) obtained from the biotite gneiss itself. other zircon evaporation dates found in the biotite gneiss range from cu. 3.0 Ga to 3.09 Ga and are interpreted as minimum ages of discordant zircon phases overprinted during one or more metamorphic events. Anomalously high Pb initial ratios characterise the granitoid and consequently its cu. 3.02 Ga age, taken together with the corresponding zircon date, is considered to reflect a metamorphic event during which anatectic melting took place to produce the granodioritic magma. During the same even& overgrowths formed on older zircon cores. The cu. 3.02 Gadate is the oldest yet recognised in the southern Kasdi Craton and a correlation is suggested with the D, metamorphic event of this region. Additionally a prehistory back to at least 3.09 Ga is indicated by the other zircon dates in the biotite gneiss.

INTRODUCTION

IGCP Project 273, entitled ‘The Archaean Rocks of the KasaI Craton” is directed at improving our understanding of the stratigraphy, structural and metamorphic history of the south-central part of Zaire (Fig. 1). The Project also includes geochrono- logical studies of the lithologies of this region. Among the units that have been dated so far during this Project is the Kakonda granitoid. a grano- diorkicbodyoccurrlngwithmtheTshambogobiotite gneiss of the Sandoa-Kapanga complex, as well as the biotite gneiss itself. The biotite gneiss forms the bulk of the KasaI Craton in the region south of Kapanga (Fig. 1). In addition to indicating an age older than previously considered for these rocks, the results obtained from the granitoid, combined with data from the surrounding biotite gneiss, provide evidence for the timing of an early meta- morphic event in the development of the KasaI Craton.

GEGLGGICAL SETTING

There are a number of contributions made to the geological knowledge of the Kasai Craton and surrounding regions. Notable among these are those of Delhal(1977, 1991); Delhal et al. (1964, 1966. 1975, 1976, 1986); Delhal and Fieremans ( 1966); Rodrlgues and Pereira ( 1973). Several major Archaean lithological units are recognised in the craton (Fig. 1): the Sandoa-Kapanga complex, the

Kasai-Lomami gabbro-norite and chamockite complex, and the Dibaya granite and migmatite complex. These are overlain by the early Proterozoic Luiza metasedimentary complex. Relationships between the various complexes are obscured by soil cover and vegetation, and are con- sequently poorly known.

The present work was undertaken in the Sandoa-Kapanga complex. Although its exact boundaries are poorly known, this complex is a predominant component of the southern part of the KasaI Craton. Its constituents - granulite. gneiss. granite, and amphibolite - are metamor- phosed to amphibolite-granulite facies. Biotite gneiss, known as the Tshambogo gneiss. is one of the dominant rock types of the Sandoa-Kapanga complex and within it granitoid bodies occur local- ly, (e.g., the Kakonda granitoid).

The earliest deformation observed in this terrain (denoted DJ is represented by: (1) general NE-SW trending axial planar foliation, S, , which has been reworked during D, folding developing a NW-SE (S,) foliation, (2) intrafolial. isoclinal, and recumbent folds, and (3) amphibolite to granulite facies metamorphism and migmatisation (Rumvegeri et al., in preparation). F, folds are generally represented by pegmatite and/or quartz veins forming tight to folds parallel to S,. Boudins and sigmoidal structures parallel to S, are frequent in the Sandoa-Kapanga complex, the sigmoidal structures indicating a dextral move- ment.

395

3% F. WALRAVEN and B. T. R~~EGERI

D, features have a NNE-SSW orientation (010” to 303’). which becomes NE-SW (040“ to 065”) further north. The S, axial planar foliation is crenulated by S, foliation (NW-SE to NNW-SSE or N-S) and is the regional foliation in the Kasai- Lomami complex.

The NW-SE foliation and folding paralleling S, of the surrounding gneiss is, although less promi- nently, also reflected in the Kakonda granodiorite, which furthermore contains similarly oriented amphibolite xenoliths. The latter are similar to amphibolite xenoliths in the gneiss. Although adequate field evidence is lacking, the granodiorite is considered to be syn-kinematic with, or pre-date the D, event, but is probably younger than D, went.

The “Haute-Luanyigneisses” of Delhal and Ledent (1973) described as the oldest component of the Kasai Craton, cover only a small area in the very northern part of the KasaT-Lomami complex. Delhal and Ledent (1973) report a 3487 + 67 Ma1 (Rb-Sr) model age (relative to bulk earth) for micro- cline from coarse-grained pegmatite in the gneiss. Neither Delhal and Ledent (1973) nor other workers have been able to replicate this date in other samples, e.g., neither the gneiss itself nor other pegmatitic microcline. This date further- more, is strongly dependent on the assumed initial %r/=Sr ratio. Anomalously high initial ratios are not uncommon in strongly differentiated rocks, e.g., Bushveld Complex granites [walmven. 1987) and lithologies in the Namaqualand region, northern Cape Province (Barton, 1983). and a ratio of 0.74 is suificient to bring the calculated age down to less than 3.0 Ga. The enclosing gneiss has an improbably low whole-rock model age of 1284 f 10 Ma (Rb-Sr. relative to bulk earth) which, in view of its large dependence on the assumed initial ratio, is not considered significant. Other pegma- tite model ages from the same region are much less dependent on the assumed initial ratio: 2271& 44 Ma (whole-rock) and 2353 f 45 Ma (microcline) (both Rb-Sr, relative to bulk earth). Delhal and Ledent (op tit) suggested that these dates represent metamorphism about that time.

Delhal and Liegeois (1982) reported a Rb-Sr whole-rock errorchron of 2852 + 46 Ma (I, = 0.70 177 + 0.00004; MSWD = 2.60 on 14 points) for gneiss of the Sandoa-Kapanga complex (referred to as Western Shaba granito-gneiss by these authors): this date corresponds to cu. 2820 Ma Musefu event, one of the multiple tectono-metamorphic events which affected the Sandoa-Kapanga com- plex (Delhal et a& 1976). This event was also responsible for the chamockite and granulite

‘where the analytical data am available, the dates have been recalculated using the procedures followed in this study.

metamorphism in the Kasai-Lomami complex further north. Delhal and Ledent ( 1973) associate the 3487 + 67 Ma pegmatite with the D, event and consider it to have been affected by the D, event.

Both fine- and coarse-grained granites intrude the Sandoa-Kapanga complex. The Kapanga granite is an example of a fine-grained, homoge- neous grey to pink biotite granite. Delhal and Liegeois (1982) reported a Rb-Sr whole-rock iso- chronof2013f44Ma (I, = 0.7051+0.0007; MSWD = 0.79 on 15 points) for various of these granites, including that at Kapanga (Fig. 1). The age iswithin error of the 2037 + 30 Ma U-Pb (not recalculated) age of the Lunge granite, located further east (Pasteels. 1971).

ISOTOPIC DATA

Analytical techniques Both the whole-rock Pb-Pb and the single zircon

evaporation analyses were carried out at the Iso- tope Laboratory of the Geological Survey, Pretoria, South Africa, using a Finnigan MAT 261 thermal ionisation mass spectrometer. The whole-rock Pb- isotopic analyses were made following the techni- que, described by Armstrong (1987). in which the Pb is separated from whole-rock powders by disso- lution in HF and HNO, followed by ion exchange separation and subsequent clean-up of the Pb by electroposition. Isotopic ratios were measured in static multicollector mode using NBS981 as ratio standard. The measured ratios have been nonnal- ised to the NBS-recommended values of NBS98 1 to correct for mass fractionation.

The single grain zircon evaporation technique involves wrapping a selected zircon grain in a rhenium filament and heating it in the source of the mass spectrometer (Kober. 1986. 1987). Ana- lysis is a series of two-step procedures repeated at increasing temperature intervals. In the first step Pb is evaporated from the zircon and condensed onto an adjacent cold filament from which it is subsequently ionised during the second data col- lection step.

The Pb signal is analysed with the SEM (second- ary electron multiplier) of the mass spectrometer, using the peak jumping mode and sequentially measuring the signals of 2c#Pb. 206Pb. 207Pb, and 208Pb. Occasionally sufficiently high Pb concentra- tions are encountered during the data collection step to permit switching to static multicollector mode using Faraday cups and measuring the various signals simultaneously. The 204Pb and 208Pb signals are recorded to monitor fractionation and the presence of common Pb while the 207pb/208pb ratio carries the age information. Data were collected only with 2olrPb/20sPb ratios smaller than 0.0001, and a common-Pb correction was applied

Implications of whole-mck Pb-Pb and zircon evaporation dates for the early metamorphic history of the KasaY . . . 397

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398 F. WALRA~~N and B. T. Rw

using the Stacey and Kramer-s (1975) Pb-isotopic Dependlng on the crystal&&ion, metamorphic, composition corresponding to the age indicated by and other history, one or more ratios may be found the 2wpb/poBpb ratio. At the 0.0001 cut-off. the cor- at thevarious temperature steps. representing one rection of a 3 Ga age amounts to 11 Ma and is quite or more domains in the zircon. In general, the insensitive to the assumed composition of the ratios obtained at the highertemperature stepsare common Pb (less than 1 Ma difference results from thought to represent the original crystaBisation using present-day instead of 3 Ga compositions age, while the lower ratios and younger dates may while a variation of 2 in the assumed initial 2sBv/ represent metamorphic events. 204Pb ratio (p-value) produces a difference of less Data regression, age and error calculations were than 0.2 Ma in the corrected ages). doneusingtheGEODATEver. 2.2software&ling-

Correction factors are applied to overcome the ton and Harmer, 1991; Harmer and Eglington, non-linearity inherent in the SEM data. The fac- 1990; Eglington and Harmer, in press). One sigma tors are obtained by analysing artificial isotopes errors of 0.01, based on 60 replicates, and an error having ratios similar to those found for the zircons correlation of 0.96 are assigned to the 2oBpb/2DIPb at various signal levels and comparison with re- and 2a’pb/20+Pb ratios for these calculations. All sults obtained with Faraday-cup analyses of the errors on the ages are reported using 95% confl- same materials. NBS98 1 and NBS982 were used to dence limits, although the Pb-isotopic composi- establish the SEM calibration factors and in the tions listed in Table 2 are shown with 1 u standard present study these corrections were never greater deviations. GEODA’IEwas used to calculate weigh- than 1.5 Ma. ted means of the data blocks. Initial Pb-isotopic

The Pb-isotopic data obtained by the evaporation compositions (expressed as model source 298U/ are directly comparable to the 207pb/208pb ratio 2crrPb (= ~1) have been calculated using the Stacey ages obtained from conventional chemical separa- and Kramers (1975) model for crustal evolution. tion and ionmicroprobe techniques. Ages obtained Where the necessary analytical results have been with this technique require the assumption that provided, results quoted from the literature have the ratios found at a particular temperature are been recalculated using GEODATE. not averages of isotopically distinct domains in the Graphical presentation of single zircon Pb-Pb zircon and that the data points lie on the concor- data is commonly done by constructing histo- dia. While it is not possible to directly determine grams of the number of ratios obtained in discrete concordance from the analytical data, internal 207pb/208Pb increments. Analytically the data are data consistency can assist in assessing single/ collected in blocks of a finite number of ratios and multiple domain problems. Comparison with data it is possible to represent the isotopic ratios as from other zircon grams and from other isotopic accumulated contributions of Gaussian distribu- systems may permit evaluation of discordancy tion curves, calculated from the means and problems.

Table 1. Pb-Pb whole-rock results for Kakonda granitoid and Tshambogo biotite gneiss

Sample 20sPb Standard 20?Pb Standard 206Pb Standard Name 204Pb Error 204Pb Error 204Pb Error

Kakonda granitoid FW89 004 48.605 0.023 17.562 0.006 25.341 0.007 FW89 005 43.908 0.003 16.648 0.001 21.220 0.001 FW89 006 41.336 0.003 16.409 0.001 20.009 0.001 FW89 007 42.226 0.012 16.604 0.003 20.971 0.003 FW89 008 43.292 0.006 16.230 0.002 19.453 0.002 FW89 009 48.941 0.012 17.329 0.004 24.059 0.006

Tshambogo biotite gneiss FW89 001 34.105 0.006 16.229 0.002 20.494 0.002 FW89 002 36.006 0.009 16.768 0.004 23.623 0.006 FW89 003 60.026 0.019 18.091 0.005 27.504 0.007 FW89 010 49.498 0.006 16.991 0.001 23.842 0.002

FW89 OlO* 37.744 0.007 16.961 0.003 22.866 0.003 Note: standard errors are 2 sigma within-run precision

Implications of whole-rock Pb-Pb and zircon evaporation dates for the early metamorphic history of the Kasdi . . . 399

18.00 - 3021+47-49 Ma

Mu = 14.59+0.58-0.60

MSWD = 12.58

(errors augmented)

Kakonda Granitoid Whole-Rock

Stacey & Kramers (1975) lead evolution curve 206Pb,‘204 Pb

16.00 19.00 22.00 25.00 28.00 31.00 34.0(

Fig. 2. Pb-Pb Isochron diagram of the Kakonda granitoid and Tshambogo biotite gneiss. Squares-Kakonda granitoid; plusses-Tshambogo biotite gneiss.

standard deviations of each of the blocks of data as as a probable crustal source for the granitoid. for ion microprobe data, (e.g.. Schiotte and Two samples of Tshambogo biotite gneiss plot Compston, 1990). Fig. 3 illustrates this represent- close to the regression line. The one is FW89 0 10’. ational technique applied to the present study. Six collected from adjacent to the Kakonda granitoid. curves are shown in Fig. 3, one for each of the and the other (FW89 003) is located north of the zircon grains studied. Where the -b/-b ratios granitoid. The remaining biotite gneiss samples, vary with temperature, the cumulative distribu- however, display greater deviations (Fig. 2, Table tion graphs have the disadvantage of exhibiting 1). Excluding the sample adjacent to the Kakonda spikes and possibly suggesting non-existent age granitoid, the Tshambogo Pb-Pb data define a components at each of the various temperature regression line of 328 1 + 2 1 l-248 Ma (whole-rock stages at which the data were collected. On the errorchron: MSWD = 505 on 4 data points: errors other hand, this representation has the advantage augmented: p = 10.0 + 11.2-14.0; Fig. 2; Table 1). that the height and width of the curves reflect the Because of the spatial separation of Tshambogo standard deviations of the data and are little samples, homogeneous isotopic compositions affected by the number of ratios collected. The cannot be assumed and there is little justification cumulative distribution graphs are considered to for accepting the date as meaningful. Neverthe- provide an improved and more direct understand- less, the data serve to confirm the likelihood of an ing of the significant peaks in the ratio distribu- older age of the biotite gneiss compared to the tion. Kakonda granitoid.

Geochronological results Whole-rockPb-Pb data obtained for the Kakonda

granitoid indicate a well-constrained errorchron of 302 1 + 47-49 Ma (MSWD = 12.58 on 6 data points: errors augmented; Fig. 2; Table 1). The associated l.i-value of 14.59 + 0.58-0.60 is abnormally high and strongly suggests a multi-stage history as well

Zircons were separated from one of the samples of biotite gneiss (FW89 003). Only a limited num- ber of zircons were obtained from the gneiss. Among these three populations can be recognisedz ( 1) a dark pink variety, containing multiple growth zones and numerous needle-like inclusions, (2) a lighter pink variety, also zoned and containing needle-like inclusions, and (3) a clear variety.

400 F. WALRA~EN and B. T. RUP~NEGERI

Table 2. Single zircon evaporation results for Tshambogo biotite gneiss (sample FW89 003). Part A - grains 1 to 3.

207Pb 206Pb

Grain 1

Std Age Error Temp Dev (Ma) (Ma) (“C)

0.22646 0.00064 3027.2 9.0 1075 0.2273 1 0.00150 3033.3 21.2 1075 0.22495 0.00015 3016.5 2.0 1075 0.22509 0.00017 3017.5 2.4 1075 0.22486 0.00013 3015.7 2.0 1075 0.22436 0.00010 3012.3 1.5 1075 0.22508 0.00011 3017.5 1.5 1075 0.22379 0.00015 3008.2 2.0 1075 0.22398 0.00012 3009.4 1.7 1075 0.22527 0.00017 3018.7 2.4 1075 0.22544 0.00011 3019.9 1.5 1075 0.22527 0.00011 3018.7 1.7 1075 0.22388 0.00009 3008.7 1.2 1075 0.23486 0.00008 3085.3 1.2 1090 0.23503 0.00003 3086.5 0.5 1090 0.23465 0.00006 3084.1 1.0 1090 0.23510 0.00005 3087.0 0.7 1090 0.23526 0.00005 3088.3 0.7 1090 0.23495 0.00008 3086.1 1.0 1090 0.23621 0.00118 3094.6 16.1 1115 0.23562 0.00 130 3090.7 17.6 1115 0.23606 0.00192 3093.6 26.1 Grain 2

0.22435 0.00011 3012.1 1.7 0.22443 0.00010 3012.8 1.5 0.22400 0.00009 3009.6 1.2 0.22224 0.00017 2996.9 2.4 0.22567 0.0003 1 3021.6 4.4

1115

1050 1050 1050 1050 1050

Grains from all three populations frequently have dark cores and their morphologies are dominated by simple, low-order crystal faces (prisms and pyramids) typical of magmatic and/or metamor- phic zircons (Pupin, 1980). The dark cores are transparent and do not appear to reflect radiation damage.

Six grains were selected for analysis: grains 1 and 2 were large pink stubby crystals (> 200 u in length) from population 1. and contained dark cores. Grains 3 and 4. from population 2, were slightly smaller (ca. 125 JJ long), with slightly rounded edges and also containing cores. Grain 4 was slightly metamict. Grain 5 (population 3) was small (< 100 I-I long) and clear, containing some bubble-like inclusions but without a visible core. Grain 6, also small and from population 3. was slightly cloudy and contained a dark core.

After removal of the common-Pb components, each of the six analysed grains yielded a pattern of increasing Pb-isotopic ratios (2wPb/20sPb) with

207% Std Age Error Temp 206%

Dev CW (Ma) ("C) 0.23229 0.00021 3068.0 2.9 1070 0.23180 0.00014 3064.6 2.0 1070 0.23332 0.00007 3075.1 1.0 1070 0.23346 0.00015 3076.0 2.2 1070 0.23323 ‘0.00008 3074.3 1.0 1070 0.23345 0.00014 3075.8 2.0 1070 0.22935 0.00023 3047.5 3.2 1090 0.23158 0.00015 3062.9 2.2 1090 0.23261 0.00012 3070.2 1.7 1090 0.23270 0.00009 3070.7 1.5 1090 0.23085 0.00014 3058.0 2.0 1090 0.23141 0.00021 3061.9 2.7 1090 Grain 3 0.21897 0.00013 2973.0 2.0 1070 0.21902 0.00015 2973.5 2.2 1070 0.21985 0.00010 2979.6 1.5 1070 0.21783 0.00017 2964.7 2.4 1070 0.21634 0.00016 2953.7 2.2 1070 0.21793 0.00016 2965.5 2.4 1070 0.21648 0.00015 2954.7 2.2 1070 0.21701 0.00020 2958.6 2.9 1070 0.21620 0.00020 2952.5 2.9 1070 0.21778 0.00044 2964.2 6.6 1070 0.22653 0.00057 3027.7 8.1 1100 0.22929 0.00042 3047.0 6.1 1100 0.22618 0.00062 3025.3 8.8 1100 0.23090 0.00023 3058.2 3.2 1100 0.23079 0.00011 3057.5 1.5 1100 0.22834 0.00047 3040.4 6.6 1100

temperature. In all cases the distribution of isoto- pic ratios is bimodal although this is most pro- nounced in grains 1. 2.3. and 6 (Fig. 3: Table 2). Grain 4 displays a bimodal distribution in which the two modes are poorly separated while grain 5 has a very large spread of ratios with two defined clusters of ratios.

Grain 1 yielded ratios corresponding to a date of 301 f 1.8 Ma at temperatures in the range of 1075°C while at higher temperatures, between 1090 and 1115°C. Pb with ratios corresponding to a date of 3086 f 0.4 Ma was evaporated. The high- temperature data form a narrow peak on the diagram while the lower temperature ratios are slightly bimodal in themselves.

At 1050°C the Pb evaporated from grain 2 had ratios corresponding to 3012 + 1.2 Ma while at higher temperatures - 1070 to 109OY! - the Pb emitted was indicative of an average date of 3070 & 0.9 Ma. A positively skewed distribution of ratios of the high-temperature Pb could indicate that a

Implications of whole-rock Pb-Pb and zircon evaporation dates for the early metamorphic history of the Masai’ . . . 401

Table 2 continued. Single zircon evaporation results for Tshambogo biotite gneiss (sample FW89 003). Part B - grams 4 to 6.

207Pb Std Age Error Temp 207~ Std Age Error Temp

206Pb Dcv (Ma) (Ma) (“C) 2oaPb Dev @W (Ma) (“C) Grain 4 0.23031 0.00010 3054.3 1.2 1100 0.22549 0.00089 3020.4 12.5 1020 0.23022 0.00007 3053.6 1.0 1100

0.22551 0.00139 3020.4 19.8 1020 Grain 5 0.22149 0.00116 2991.6 16.8 1020 0.23522 0.00081 3088.0 11.0 1060 0.22431 0.00118 3011.8 16.8 1020 0.23495 0.00092 3086.1 12.7 1060 0.22910 0.00036 3045.8 5.1 1040 0.23613 0.00098 3094.1 13.2 1060 0.22464 0.00023 3014.3 3.4 1040 0.22880 0.00088 3043.6 12.5 1080

0.22588 0.00019 3023.1 2.9 1040 Grain 6 0.22345 0.00029 3005.7 4.2 1040 0.21652 0.00298 2955.0 44.4 1040 0.22565 0.00035 3021.4 5.1 1040 0.21668 0.00099 2956.2 14.6 1040 0.22711 0.00040 3031.9 5.6 1040 0.21579 0.00058 2949.6 8.5 1040 0.22824 0.00018 3039.7 2.4 1060 0.21549 0.00074 2947.4 11.0 1040 0.22860 0.00005 3042.4 0.7 1060 0.21567 0.00023 2948.6 3.4 1040 0.22795 0.00011 3037.7 1.5 1060 0.21708 0.00004 2959.1 0.5 1040 0.22845 0.00006 3041.1 0.7 1060 0.23 154 0.00005 3062.6 0.7 1060 0.22902 0.00010 3045.3 1.2 1060 0.23120 0.00009 3060.4 1.2 1060 0.22827 0.00019 3039.9 2.1 1080 0.23144 0.00018 3062.1 2.7 1060 0.23034 0.00003 3054.3 0.5 1080 0.23285 0.00022 3071.7 3.2 1060 0.22988 0.00005 3051.1 0.7 1080 0.23260 0.00022 3069.9 2.9 1060 0.22957 0.00009 3049.0 1.2 1080 0.23119 0.00010 3060.2 1.2 1080 0.22947 0.00004 3048.5 0.5 1080 0.23247 0.00012 3069.2 1.7 1080 0.22941 0.00007 3048.0 1.0 1080 0.23320 0.00009 3074.1 1.2 1080 0.22891 0.00012 3044.3 1.7 1100 0.23326 0.00014 3074.6 2.0 1080 0.22994 0.00007 3051.6 1.0 1100 0.23336 0.00017 3075.3 2.4 1080 0.23050 0.00006 3055.5 1.0 1100

residuum of Pb was still being evaporated from the lower temperature domain in the zircon gram.

The problem of Pb evaporating simultaneously from multlple domains is also apparent in the data from gram 3. Here a variety of ratios, indicating dates from 2955 to 2979 Ma, were obtained at 1070°C while ratios with dates between 302 1 and 3060 Ma character&e the Pb evaporated at 1 100°C.

Although there is less overall spread of the Tb/ 208pb ratios in the Pb evaporated from gram 4. a bimodal distribution is stiIl apparent. At 1040°C the isotopic ratios correspond to a date of 3018 + 2 Ma while the higher temperature data-between 1060 and 1 100°C- cluster around a date of 3047 + 0.7 Ma. At 1020°C some Pb was found with a slightly lower date - co. 3 Ga.

The data from gram 5 are extremely scattered, this gram was the smallest of the six analysed and only small signals could be obtained from it, resul- ting in poor statistics and highly variable ratios. As a result the data from gram 5 cannot in themselves be used to derive dates. However, the distribution of ratios does resemble that seen in the grams analysed: a broad cluster of rat&s around 3045 Ma echoes dates indicated by grams 3 and 4. Although

smaller, grain 6 resembles gram 1 in morphology and in the bimodality of the Pb emitted at different temperatures. At 1040°C a date of CCL 2955 Ma is indicated by the Pb-isotopic ratios and at higher temperatures, from 1060 .to 1080°C. the Pb ratios point to a date of 3068 a 0.9 Ma.

DISCUSSION

The 3021 + 47 -49 Ma whole-rock Pb-Pb erro- chronobtainedfromthe Kakondagranitoidmaybe interpreted as a primary crystahisation age or as that of a metamorphic overprint. The latter could represent the event during which the foliation and folding. observed in the granitoid, were formed. In either interpretation, the date implies that the surrounding Tshambogo biotite gneiss has an excess of 3021 Ma, thereby confirming the pre- Kibaran age assignment given to this region on the 1974 geological map of Zaire. The 3021 Ma date also represents a considerable increase in the known age of the southern part of the KasaI Craton.

The abnormally high p-value found for the grani- toid argues against a primary intrusive origin and,

402 F. WALRAVTN and B. T. Rw

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* P

Tahambogo biotite gneiea Fwaaooa-5

86TPb ZEFK I I I I I I

0810 0.815 0684 0.225 0.m 0.8s5 0.240 o".-

2.95 S.00 Los 3.10 3.

& Tshambogo I biotite gneiss Fw8Qooa-8

zJoea+/-0.9 &‘

80’7Pb 808Pb

I I I I OBIO 0.215 oam 0.225 o.mo Ods6 0.2ul 0.

Fig. 3. Single g&n zircon evaporation data from sample W89 003 - Tahambogo biotite gneiaa. The data are presented as cumulative diatrlbution cures (see explanation in text) and have vertical axes of unity.

instead, supports a multi-stage history. Inherl- tance of the source U/l% signature is suggested by the data and a possible or&In for the granodioritic granitoid magma is one involving anatectic melting of the surrounding Tshambogo gneiss which itself may have a multi-stage prehistory. The proximity of the adjacent biotite gneiss sample (FW89 010.) to the granitoid regression line (Fig. 2) provides some support for this view. In such an interpreta- tion the 3021 Ma date can be taken to reflect a high-grade metamorphic event which affected the basement rocks near Sandoa (and probably else- where in the KasaI Craton). Local granodioritic melts developed during the high-grade event and

subsequently cooled and crystallised to form the Kakonda, and other, similar granitoids, in the biotite gneiss.

Support for an anatectic model may be found in the zircon evaporation data.’ A wide range of Pb- isotopic compositions was obtained from the various zircons analysed, both from individual grains at different temperatures as well as from dHerer$ grains. As such, the data are indicative of Pb-loss at some stage in the geological history of the zircons and consequent discordance ‘of the isotopic data. They thereby indicate a minimum age of 3066 f 0.4 Ma for zircon components in the Tshambogo biotite gneiss. Comparisonwith zircon

Implications of whole-rock Pb-Pb and zircon evaporation dates for the early metamorphic history of the KasaY . . . 403

morphologies. and. in particular. the presence of dark cores in the relevant zircon grains, suggests that this date may be associated with the cores. The origin of the cores, whether magmatic in the biotite gnefss precursor orxenocrystic. cannot be resolved here.

An age component near 3014 f 2.3 Ma is evident in a number of the grams ana@sed and at various temperature steps in the analyses. This average date is based on dates observed in grams 1.2. and which. in each case, were obtafned at intermediate temperature stages. Each of these zlmons grafns has, furthermore. a distinct core-mantle morpho- logy, allowing the age distribution patterns to be interpreted as resulting from an older core, which emitted Pb at hfgher temperatures. surrounded by a younger overgrowth which provided Pb at the intermediate temperatures. The overgrowths could beexplainedintennsofzirconuystallisaUon during a high-grade metamorphic event and the coinci- dence between the 3014 f 2.3 Ma date and the whole-rockPb-Pb date of 302 1 + 47- 49 Ma suggests that such zircon growth might have taken place durfng the event responsible for generating the Kakonda granitoid magma.

Since a distance of some 20 km separates the sample from which the zircons were obtained from the Kakonda granitoid, the possibility can safely be excluded that the 3014 f 2.3 Ma overgrowths resulted from contact metamorphism surrounding the Kakonda granitoid. Instead, regional metamor- phism is considered to have caused both the zircon overgrowths and anatectic melting.

The presence of NW-SE foliation and folding in the Kakonda granitoid. paralleling the S2 fabric in the surrounding biotite gneiss, permits two alternative interpretations of the relation between structural history and geochronology. In the first. the 3014 f 2.3 Ma event represents the D, event and the NW- SE foliation and folding are ascribed to the subse- quent D, event. In this case the 3487 + 67 Ma date found for the “Haute-Luanyi” by Delhal and Ledent (1973) must either be considered spurious or it must reflect a still older, pre-D, event.

In the second interpretation the 3014 f 2.3 Ma date represents the DZ event, thereby making the D, event older than cu. 3.02 Ga and possibly closer to the”Haute-Luanyl”dateofDelhalandLedent(1973). In this case the foliation in the granitoid developed during the waning stages of D, metamorphism, after crystalllsatfon of the granltoid. The latter interpretation Is favoured since (1) it agreeswith the weaker development of foliation in the Kakonda gmnitoid, compared to the surrounding Tshambo- go biotite gneiss. and (2) it does not require the introduction of a pre-D, went which no other evidence is known.

CONCLUSIONS

The coincidence between the whole-rock Pb-Pb date found for the Kakonda granitoid and one of the age components observed in the zircon from the surrounding Tshambogo biotite gneiss sug- geststhat a high-grade metamorphic event at this time affected the southern part of the Sandoa- Kapanga complex of the Kasai Craton. Anatectic melting of the biotite gneiss during this event generated the magma for the Kakonda granftoid and resulted in overgrowths on zircons in the Tshambogo biotite gnefss. Although the exfsUng data allow no conclusive assignment of the event, a correlation with the D, stage seems more likely than with D,. On this basis the early tectono- metamorphic history of the Kasai Craton can be summar&d as follows:

[ 1) formation of continental crust from granltic and/or sedimentary pre-cursors. lnvolvfng one or more stages that are no longer distinguishable,

(2) metamorphism and tectonism at or before ca. 3086 Ma-the D, event which produced the regional NE-SW fabric: this event could be as old as the ca. 3.47 Ga date suggested for the ‘Haute- Luanyl” pegmatite.

(3) metamorphism and tectonism at cu. 3014 Ma - the DZ event which caused the NW-SE fabric and locally resulted in anatectic melting to form granitoid magma.

AcknauZai~ts - The authors thank the organ- isera of IGCP Project 273, the IUGS and the University of Lubumbashi, for the opportunity to take part in the project. The ih-st author also acknowledges the Chief Director of the Geological Survey of South Africa for his support and permission to participate. Dr. J. Delhal and Paul Raucq of the Royal Museum for Central A&a, Tervuren. are grateiully acknowledged for providing background information. Danie Grobler and Stef Labuschange are thanked for their comments on earlier versions of the manuscript while the Journal ofAiiican Earth Sciences referees are acknowledged for their constructive criticisms.

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