- Email: [email protected]
Lithospheric mantle structure and the diamond potential of kimberlites in southern D.R. Congo
Lithos 112S (2009) 166–176
Contents lists available at ScienceDirect
Lithos j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s
Lithospheric mantle structure and the diamond potential of kimberlites in southern D.R. Congo J.M. Batumike a,b,⁎, W.L. Griffin a, S.Y. O'Reilly a a b
ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia Département de Géologie, Université de Lubumbashi, Route Kasapa, B.P. 1825, Lubumbashi, The Democratic Republic of the Congo
a r t i c l e
i n f o
Article history: Received 7 September 2008 Accepted 19 April 2009 Available online 3 May 2009 Keywords: Peridotitic garnets Congo kimberlites Congo lithosphere Diamond potential Mantle metasomatism Congo-Kasai Craton
a b s t r a c t Mantle-derived peridotitic garnet xenocrysts from kimberlites in the Mbuji Mayi and Kundelungu areas and from heavy-mineral concentrates collected in the Luebo area, D.R. Congo, have been analysed for major- and trace-element compositions in order to understand the structure and composition of the subcontinental lithospheric mantle (SCLM) and the diamond potential of the kimberlites. The lithosphere beneath the Kundelungu Plateau is ca 175 km thick and has been affected by pronounced melt metasomatism. Garnets from the Kundelungu Plateau indicate an initially cool geotherm (~ 35 mW/m2), which was disturbed by asthenospheric melts that penetrated the SCLM shortly before kimberlite intrusion ca 32 Ma ago. Harzburgitic garnets are very rare, but some lherzolitic garnets display compositions similar to garnets included in diamond. Garnets from the Mbuji Mayi region indicate a cool geotherm (35 mW/m2); the SCLM is ~ 210 km thick and was affected by melt-related and phlogopite-related metasomatisms. Harzburgitic garnets form about 33% of the analysed population. The garnets from the Luebo region indicate a cool lithospheric geotherm (35 mW/m2) typical of cratonic areas. The SCLM from which the garnets were derived was relatively thick (205 km), affected by melt-related and phlogopite-related metasomatisms and characterised by the presence of a ~ 80-km thick harzburgite-rich layer. In terms of peridotitic diamond potential, Mbuji Mayi and Luebo are more prospective than Kundelungu. The initially cool conductive geotherm, the presence of some garnets with compositions similar to garnets included in diamond and the presence of sporadic diamond in the Kundelungu Plateau suggest that diamond initially was present in the lithosphere and the observed paucity of diamond may be due to the melt-related metasomatism that affected the lithosphere in the region. We suggest that the lithospheric mantle beneath Kundelungu is a strongly modified Archean cratonic lithosphere that has survived beneath the area during Proterozoic tectonism. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Our knowledge of the nature and composition of the subcontinental lithospheric mantle (SCLM) is largely based on the study of xenoliths brought to the Earth's surface by volcanism (mostly basaltic to kimberlitic). The distribution of elements between different mineral phases in the xenoliths allows the estimation of the conditions of equilibration at the time of entrainment (pressure, temperature) and an understanding of chemical processes that have affected the part of the SCLM from which the xenolith was derived. However, the distribution of such xenoliths (in which most of the mineral phases are preserved) is very restricted in space and time (O'Reilly and Griffin, 2006). Minerals (xenocrysts) such as garnet and chromite represent the products of the disaggregation of the xenoliths; they commonly are
⁎ Corresponding author. ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia. E-mail addresses: [email protected] (J.M. Batumike), bill.griffi[email protected] (W.L. Griffin), [email protected] (S.Y. O'Reilly). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.04.020
well preserved and may be abundant in kimberlites, partly due to their resistance to alteration compared with other mantle minerals such as olivine and pyroxene. These minerals may thus be used for the study of the SCLM in areas where xenoliths are rare or absent. Two kimberlite fields are known in the Democratic Republic of the Congo (hereafter referred to as the Congo) at Mbuji Mayi located in the south-central part of the country (Kasai Province) and Kundelungu in the southeastern part (Kundelungu Plateau (Pivin et al., this issue)). In this study, xenocrystic peridotitic garnets from Mbuji Mayi and Kundelungu kimberlites and from heavy-mineral concentrates collected in drainages in the Luebo, Mbuji Mayi and Kundelungu regions are used to map and understand the nature of the SCLM in the southern part of the Congo and to assess its peridotitic diamond potential. This study provides an opportunity to compare the composition and structure of the lithospheric mantle and kimberlite diamond potential in two different geological settings. 2. Geological setting The Mbuji Mayi kimberlite field and the Luebo exploration area (~200 km NW of Mbuji Mayi) are located inside the Congo-Kasai
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Craton whereas the kimberlites in the Kundelungu Plateau intruded rocks belonging to the Neoproterozoic Katangan Supergroup inside the Katangan Belt (Fig. 1). The Congo-Kasai Craton consists of granulite, gneiss, granite and amphibolite, a gabbro-noritic and gabbro-charnockitic complex, and a migmatitic complex. The ages of these rocks range from 3.4 to 2.6 Ga (Delhal and Ledent, 1973; Delhal and Liégeois, 1982; Walraven and Rumvegeri, 1993; Batumike et al., 2009). The youngest Archean age in this part of the craton was found in the Malafundi granites from the Dibaya complex which gave 2648 ± 22 Ma (Rb/Sr; Delhal et al., 1976) and 2595 ± 92 Ma (Rb/Sr; Cahen et al., 1984). Paleoproterozoic, Mesoproterozoic and Neoproterozoic rocks within this craton are poorly studied, but represent parts of the Paleoproterozoic Ubendian, Mesoproterozoic Kibaran and Neoproterozoic Katangan orogenic belts. The Mbuji Mayi Supergroup, intruded by the Mbuji Mayi kimberlites, consists of 1.3–0.95 Ga sedimentary rocks, covered in places by 0.95 Ga basaltic lavas and Cretaceous sandstones (~120 Ma, Schärer et al., 1997). Kimberlites occur in two clusters in the Mbuji Mayi region: the northern Mbuji Mayi cluster is formed of elliptical bodies along an E– W-striking crustal fissure (e.g. Fieremans, 1977; Demaiffe and Fieremans, 1981; Demaiffe et al., 1990) and the southern Tshibwe cluster (Schärer et al., 1997). Because the kimberlites intrude Cretaceous sandstones with an estimated age of 120 Ma, this age has been taken as the maximum age of known kimberlite occurrences within the craton. Davis (1977) dated a megacrystic zircon from Mbuji Mayi at 71 Ma (TIMS 206Pb/238U). U–Pb analysis of zircon and baddeleyite megacrysts from Mbuji Mayi yielded discordant data with concordia intercepts at 69.8± 0.5 Ma and 2528± 452 Ma (Schärer et al., 1997), and Kinny and Meyer (1994) reported an age of 628±12 Ma for a zircon included in diamond from the same area. The Luebo region is characterised by the presence of abundant diamonds and kimberlitic indicator minerals in streams and rivers. The region has been subjected to intensive exploration for search of local sources for the diamond, but no primary sources such as kimberlite have yet been found (e.g. Bhebhe, Z., pers. comm., 2008). Our morphological examination of garnet grains from heavy-mineral concentrates suggests that these grains may have not travelled long
167
Fig. 2. Location of kimberlite pipes in the Kundelungu Plateau.
distances, and might be derived from local kimberlitic sources, although formally the source(s) for the grains studied are indeterminate. U–Pb, Hf isotope and trace-element data for zircons from the same Luebo-area heavy-mineral concentrates suggest that kimberlites were emplaced inside the Congo-Kasai Craton in Late-Archean, Neoproterozoic and Cretaceous time (Batumike et al., 2009), though the location of such sources remains to be determined. The Kundelungu Plateau is made up of sandstones, limestones and mudstones grouped as the Biano Subgroup, occurring stratigraphically at the top of the Neoproterozoic Katangan Supergroup. These rocks are subhorizontal and undeformed and were deposited in a foreland basin during the Lufilian orogeny (Kampunzu and Calteux, 1999; Batumike et al., 2007a). The kimberlites form two geographic clusters inside the Kundelungu Plateau: the eastern cluster has 16 pipes and the western has 14 pipes (Fig. 2). This division is mainly based on the presence of a thick sand cover in the central portion of the plateau that may obscure some kimberlite pipes. U–Pb analysis of perovskites indicates that there was only one magmatic kimberlite episode in the Kundelungu Plateau and this occurred at 32 ± 2 Ma, coincident with the initial stages of the East African Rift in Ethiopia and Kenya (Batumike et al., 2008). 3. Samples and methods 3.1. Sampling
Fig. 1. Geological sketch map showing the position of the Congo-Kasai, Tanzania and Kalahari Cratons in Africa (grey areas), and study areas. Small dashed lines are political boundaries. Names of countries are in small capitals.
The garnet xenocrysts studied were collected from three different locations: Kundelungu Plateau, Mbuji Mayi and Luebo. Data for garnets from Mbuji Mayi and some of the kimberlite pipes in the Kundelungu Plateau (e.g. Luanza, Golo, Mbo and Chingululu, Fig. 2) were provided by De Beers. Epoxy mounts of garnets from Luebo and their electron microprobe data were provided by Gravity Diamonds Ltd. These garnets were extracted from heavy-mineral concentrates prepared from stream samples collected during an exploration campaign. In the eastern part of the Kundelungu Plateau, stream sediment samples were collected in the Gungwania, Talala and Luanza
168
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Rivers that crosscut the kimberlite pipes of the same names (Fig. 2), and heavy-mineral concentrates prepared by hand-gravitation. The garnets were separated from the concentrates by handpicking, mounted in epoxy disks, and polished before analysis. In this study, Kundelungu Plateau data are analysed in terms of the two geographic clusters, but for some purposes the data from the two clusters are aggregated. The Mbuji Mayi data are also treated as two different clusters, the Mbuji Mayi kimberlite cluster in which 6 pipes were studied (massifs 1, 3, 4, 6, 9 and 10) and the Tshibwe kimberlite cluster. Overall the heavy-mineral concentrates from the Kundelungu Plateau are characterised by a scarcity of chromite whereas concentrates from Mbuji Mayi and Luebo contain abundant chromite. Sample distribution and representation by kimberlite cluster is summarised in Table 1.
Table 1 Summary of known kimberlites in the Democratic Republic of the Congo and data. Locality
Kundelungu East
Kundelungu West
Mbuji Mayi
Tshibwe
Luebo
Diamond (cpht) Age of kimberlites Kimberlites in cluster Kimberlites studied Total pyropes LA-ICPMS data (GEMOC) LA-ICPMS data (De Beers) Chromite abundancef
b10a 32 Mad 16
b 5a 32 Mad 14
20–260b 71 Mae 10
b100b 71 Mae –
0–200c – –
3 722 433
4 401 272
6 1236 0
356 0
– 255 255
289
129
1236
356
0
Very low
Very low
High
High
High
a b
3.2. Analytical methods
c d
Data for garnets from Mbuji Mayi and some of the Kundelungu kimberlites, including major elements (oxides) and Ni, Ga, Y and Zr contents, were supplied by De Beers and electron microprobe data for Luebo garnets were provided by Gravity Diamonds Ltd. The De Beers analyses were acquired by electron microprobe and LAM-ICPMS, using techniques similar to those used at the GEMOC ARC National Key Centre (Macquarie University). Inter-laboratory comparisons between De Beers and GEMOC show very good correspondence. Other majorelement (oxide) analyses were done at GEMOC using a CAMEBAX SX100 electron microprobe and analyses of trace elements were performed on an Agilent 7500S ICPMS attached to a Merchantek 213 nm New Wave Nd:YAG laser microprobe. The analytical procedures are similar to those described by Batumike et al. (2009 and references therein; more details at www.mq.edu.au/GEMOC). The data distribution is given in Table 1 and the analytical data are presented in online Supplementary data Tables 1, 2 and 3 respectively for the Kundelungu, Mbuji Mayi and Luebo regions. 3.3. Data processing and presentation Griffin et al. (1989) and Ryan et al. (1996) showed that Ni and Cr in pyrope garnets can be used to estimate temperature (TNi) and pressure (PCr) of equilibration. The pressure–temperature (P–T) relationships may be used to construct the geotherm, which typically follows model conductive geotherms above the geochemical lithosphere–asthenosphere boundary (O'Reilly and Griffin, 1996, 2006). Below this boundary, these authors proposed that the geotherm could be drawn parallel to the diamond–graphite curve, similar to the P–T relationships of high-temperature sheared peridotite xenoliths. The physical state of the SCLM depends strongly on the level of depletion, which is reflected in the XMg of olivine. In this study the XMg of olivine coexisting with garnet is calculated using the method of Gaul et al. (2000). Griffin et al. (1995) used xenoliths and diamond inclusions to establish the chemical signatures of garnets affected by different types of mantle processes. Using Zr, Y, Ga and Ti contents in garnets, two principal types of metasomatism can be recognised. One is defined by garnets with enrichment in Zr accompanied by increase in Y and Ti (melt metasomatism) and the second by garnets showing an increase in Zr with low to moderate Ti, Y and Ga (phlogopite metasomatism) (Griffin et al., 1992, 1995). This latter category of garnets has been observed in low-T phlogopite-bearing xenoliths (Shee et al., 1993). Griffin et al. (2002) used inter-element correlations in a large database of mantle-derived Cr-pyrope garnets to define populations that could be related to specific processes by comparison with garnets
e f
Kampata (1993). De Beers. Alluvial data, Southern Era. Batumike et al. (2008). Davis (1977). Qualitative abundance.
from well-studied xenoliths. The defined populations can be grouped into five major categories (Griffin et al., 2002): depleted harzburgites, depleted lherzolites, depleted/metasomatised lherzolites, fertile lherzolites and melt-metasomatised peridotites. Given the depth at which the garnets equilibrated and the geochemical process and rock type indicated by the composition of the garnets, the proportion of the garnets reflecting individual rock types and processes at particular depths can be calculated. This allows the construction of a section through the lithosphere showing the variation in the proportions of rock types or types of processes with depth. These sections have been described as “Chemical Tomography” (O'Reilly and Griffin, 2006 and references therein). Most peridotitic garnet inclusions in diamond are characterised by high XMg [Mg/(Mg + Fe)], reflecting coexistence with very forsteritic olivine. They also show sinuous rare-earth element (REE) patterns with enrichment in MREE and depletion in HREE. This sinusoidal pattern is shown clearly by the relationships between Sc, Nd and Y (where Sc is used as a proxy for HREE and Y as a proxy for MREE); Sc/Y measures the depletion in HREE whereas high Nd/Y measures the degree of enrichment in LREE. According to Stachel and Harris (2008), this sinusoidal REE pattern reflects re-enrichment in the most incompatible elements and represents a metasomatic overprint by a highly fractionated fluid essentially devoid of HREE. However, Malkovets et al. (2007) have suggested that these garnets crystallized directly by reaction between chromite and the metasomatic fluid. In this study, the peridotitic diamond potential is assessed based on compositional features of garnet together with P–T conditions derived from the garnet data. 4. Results 4.1. Kundelungu Plateau 4.1.1. Kundelungu East Garnets from kimberlites in the eastern cluster can be grouped compositionally into four different categories, according to the classification of Sobolev et al. (1973): lherzolitic garnets, low-Cr garnets, Ca-harzburgitic garnets and wehrlitic garnets (Supplementary data Table 1). The lherzolitic population constitutes over 88% of the whole sample. The Ca-harzburgitic and wehrlitic garnets, forming populations of 0.5% and 0.2% respectively, are restricted to the Talala pipe (Fig. 2). The trace-element patterns of most of these garnets are
Fig. 3. Conductive geotherms for Kundelungu East (a), Kundelungu West (b), Mbuji Mayi (c) and Tshibwe (d). The kink in the Kundelungu geotherm is based on the Y distribution of the lower-T data cluster, with the second cluster considered as disturbed by heating and metasomatism. Note the geotherm is 35 mW/m2 for Kundelungu East, similar to Mbuji Mayi, and 37.5 mW/m2 for Kundelungu West.
J.M. Batumike et al. / Lithos 112S (2009) 166–176
169
170
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Fig. 4. Y, Zr and Ti relationships in garnets from Kundelungu East (a, b) and Kundelungu West (c, d) indicating the type of mantle processes affecting the lithosphere.
characterised by depletion in LREE and enrichment in HREE, but some garnets of lherzolitic composition and all harzburgitic garnets are characterised by depletion in the HREE. This depletion produces the sinuous pattern commonly observed in garnets included in diamond. Most of the garnets are depleted in Sr, Co and Ni and a few grains show depletion in Ti. The conductive geotherm derived from these garnets is quite low, corresponding to a modelled surface heat flow of about 35 mW/m2 (Fig. 3a). On this plot the data show a gap in temperature distribution and this gap is also observed in the Y–TNi plot (Fig. 3a) where some high-temperature garnets have low-Y contents similar to those with low temperature. However, the high-temperature (high-Ni) population tends to have high Ti and Zr contents compared with the lowtemperature garnets. The bimodal temperature distribution of the garnets hinders estimation of the depth of the lithosphere–asthenosphere boundary (LAB), which is usually taken as the temperature above which Y-depleted garnets are rare or absent. The Kundelungu East garnet population contains abundant Ydepleted garnets with high temperatures (high-Ni contents). The Y– Zr and Zr–TiO2 plots (Fig. 4a, b) indicate that the garnets that are depleted in Y, but with high Zr and Ti show the effects of both meltrelated and phlogopite-related metasomatisms. The two styles of metasomatism occur all through the section of the Kundelungu lithospheric mantle, but the melt-related signature is dominant below depths of ca 130 km. The relationships between these three elements (Y, Zr and Ti, Fig. 5a) show two different patterns indicating that there are two different populations in the garnets, with the more meltmetasomatised population having generally higher T. These features suggest that the Kundelungu region was originally characterised by a low conductive geotherm that was disturbed by introduction of melt, which also affected the compositions of some garnets. These two garnet populations can be used to define two distinct geotherms, corresponding to a ca 35 mW/m2 model at shallow depth, and a higher one below 130 km depth. The chemical tomography section for this part of Kundelungu Plateau therefore was constructed using a conductive geotherm of 35 mW/m2 with a kink at
1000 °C (ca 130 km). Fig. 6a shows that the lithospheric section is dominantly lherzolitic (fertile, depleted or metasomatised) and also contains a high proportion of melt-metasomatised rocks. The thickness of the lithosphere is estimated at ~ 175 km. This boundary is also illustrated by a decrease in the forsterite content of coexisting olivine, which drops below XMg = 0.91 at ca 1000 °C (Fig. 6a). The lithospheric section is also characterised by the presence of minor harzburgites inside the diamond-stability field.
4.1.2. Kundelungu West Lherzolitic and low-Cr garnets are recognised in the western Kundelungu kimberlite cluster, with the lherzolitic population the most abundant (Supplementary data Table 1). There is no difference between individual pipes in this part of the Kundelungu Plateau in terms of the type of garnets. The trace-element patterns are generally similar to those in the garnets of the eastern cluster, showing enrichment in HREE and depletion in LREE, with about 7% of the lherzolitic population showing HREE depletion. The reconstructed conductive geotherm is higher than that for the eastern cluster, 37–38 mW/m2 (Fig. 3b). The garnets also show two different clusters on plots of PCr vs TNi and Y vs TNi; low-Y garnets with high temperatures also have high contents of Ti and Zr (Figs. 3b and 6b). This is interpreted as due to heating and metasomatism that affected some garnets, as in the Kundelungu East pipes, and indicates that both parts of the Kundelungu Plateau were affected by similar metasomatic fluids. The presence of garnets showing the effects of both melt- and phlogopite-related metasomatisms is illustrated in Fig. 4(c, d) by high values in Y, Ti and Zr and a scarcity of garnets derived from depleted peridotites. The chemical tomography section for this part of Kundelungu Plateau, constructed using a conductive geotherm of 37.5 mW/m2 and a kink at 1000 °C, shows a dominantly lherzolitic lithosphere which is about 160 km thick (Fig. 6b). There is no obvious difference in the lithosphere composition below the two clusters except that the lithosphere in the eastern part is slightly thicker and harzburgites
J.M. Batumike et al. / Lithos 112S (2009) 166–176
171
Fig. 5. Y and Ti relationships indicating presence of two different garnet populations in Kundelungu East (a) and West (b). Y–Zr and Zr–Ti relationships indicating the type of metasomatism affecting the source region for the garnet population with low Y/Ti ratios (c, d) and for high Y/Ti garnet population (e, f). Note that most of the garnets with high Y/Ti ratios show effect of melt-metasomatism (f).
were only observed in the eastern region. However, this may reflect a sampling bias (Table 1). The garnets from the high-temperature group plot in the meltmetasomatism field (Fig. 5e, f) whereas the garnets of the low-T group plot mostly in the depleted and phlogopite-metasomatism fields (Fig. 5c, d). This suggests that the difference in composition between the high-temperature and low-temperature garnet populations is due to Ti–Zr enriched melts affecting the deeper part of the lithospheric mantle beneath both the eastern and western clusters in the Kundelungu Plateau. 4.2. Mbuji Mayi 4.2.1. Mbuji Mayi cluster The Mbuji Mayi garnets can be grouped into five groups based on their CaO and Cr2O3 contents (Sobolev et al., 1973; Supplementary data Table 2): Ca- and low-Ca-harzburgitic, lherzolitic, wehrlitic and low-Cr garnets. Lherzolitic and Ca-harzburgitic garnets are common
in all of the studied Mbuji Mayi pipes; the Ca-harzburgitic garnets make up about 33% of the analysed population and 63% consists of lherzolitic garnets. PCr–TNi relationships in the Mbuji Mayi garnets define a relatively low conductive geotherm, about 35 mW/m2 (Fig. 3c), typical of cratonic lithosphere. A high proportion of the garnet grains plot inside the diamond-stability field in terms of P and T; most of these garnets are classified as harzburgitic or low-Ca harzburgitic. Some lherzolitic garnets also plot inside the diamond-stability field but most lherzolitic garnets are in the graphite stability field. The distribution of Y vs TNi (Fig. 3c) shows the absence of low-Y garnets at T ≥ 1100 °C, corresponding to a depth of 210 km on a conductive geotherm of 35 mW/m2. At this depth, there is also a drop in the calculated XMg of olivine coexisting with garnet (Fig. 6c), which corresponds to the geochemical lithosphere–asthenosphere boundary (O'Reilly and Griffin, 2006). The Mbuji Mayi lithospheric section (Fig. 6c) is characterised by two harzburgite-rich layers inside the diamond-stability field. The garnets
172
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Fig. 6. Subcontinental lithospheric mantle chemical tomography for Kundelungu East (a), Kundelungu West (b), Mbuji Mayi (c) and Tshibwe (d). The horizontal line marked G–D gives the depth for the graphite–diamond-stability field in the region.
have chemical compositions suggesting the effects of both melt and phlogopite metasomatisms (Fig. 7a, b), but the melt metasomatism is more pronounced in the deeper lithosphere at depths around 170 km (Fig. 6c). Unlike the Kundelungu Plateau section, there are no low-Y garnets at high temperatures (see Figs. 3a, b and 4c). 4.2.2. Tshibwe cluster Five different categories of garnets are observed in the garnets from the Tshibwe cluster, similar to the populations observed in the Mbuji Mayi cluster. The conductive geotherm derived from the garnets is ~35 mW/m2, as in the Mbuji Mayi area (Fig. 3d), entirely reasonable given the 30-km separation of the two clusters. The distribution of Y vs TNi in the Tshibwe garnets (Fig. 3d) shows the absence of low-Y garnets above 1075 °C, corresponding to a depth of 195 km. The calculated XMg in coexisting olivine ranges from 0.89 to 0.925, but in the lithospheric
section XMg decreases with depth at ~120 km and then again at 195 km (Fig. 6d). The decrease at ~120 km reflects the presence of abundant “fertile” garnets affected by phlogopite-related metasomatism, whereas the decrease at 195 km is due to garnets showing both fertile and meltmetasomatised signatures. Both the decrease in XMg of olivine and the lower depth limit of low-Y garnets characterise the transition from a depleted lithosphere to a fertile zone, which is taken as the geochemical lithosphere– asthenosphere boundary (Fig. 6d). The lithospheric section beneath the Tshibwe region contains more harzburgite inside the diamondstability field at depths of 170–220 km than observed in the upper section. The abundances of Zr, Ti and Y in the Tshibwe garnets indicate the effects of both melt and phlogopite metasomatisms (Fig. 7c, d). These metasomatic signatures are similar to those observed in the Mbuji Mayi cluster.
J.M. Batumike et al. / Lithos 112S (2009) 166–176
173
Fig. 7. Y, Zr and Ti relationships in Mbuji Mayi (a, b), Tshibwe (c, d) and Luebo (e, f) garnets indicating the type of mantle processes affecting the lithosphere in these areas.
4.3. Luebo area The Luebo garnets show four compositional groups: lherzolitic, Caand low-Ca harzburgitic, and low-Cr garnets. Lherzolitic garnets are the dominant population followed by harzburgitic garnets (Supplementary data Table 3). The trace-element data (not shown) for some garnets show sinusoidal REE patterns. The garnets define a geotherm of ~ 35 mW/m2 (Fig. 8a). There are few low-Y garnets with TNi above 1100 °C, corresponding to a depth of 205 km. The compositions calculated for coexisting olivine vary between XMg 0.895 and 0.94 and drop below 0.91 at ~205 km (Fig. 8b), corresponding to the rise in mean Y content and defining the geochemical lithosphere–asthenosphere boundary. The Luebo garnets include a population that shows enrichment in Ti, Y and Zr, due to the effects of metasomatism. Fig. 7(e, f) shows that the lithosphere represented by Luebo-area garnets was affected by both melt-related and phlogopite-related metasomatisms, though some depleted garnet compositions remain. The dominant depleted population is lherzolitic garnets but very few low-Cr and harzburgitic garnets plot inside the depleted domain (Fig. 7e, f). Assuming that the garnets collected inside the Luebo area were derived from a local kimberlite source, the lithospheric section for Luebo is also char-
acterised by the presence of a relatively thick harzburgite-rich layer at depths between 100 and 185 km (Fig. 8b). A large part of this harzburgite-rich layer lies within the diamond-stability field. 5. Discussion 5.1. Lithosphere structure, composition and evolution Garnet geochemistry indicates that the lithosphere is thicker beneath the on-craton Mbuji Mayi and Luebo areas than beneath the off-craton Kundelungu Plateau. The Mbuji Mayi and Luebo areas are characterised by abundant harzburgitic garnets compared with the almost exclusively lherzolitic garnets at Kundelungu. Kundelungu, Mbuji Mayi and Luebo are characterised by cool conductive geotherms, but at Kundelungu this initially low conductive geotherm was disturbed by a heating event, which was accompanied by metasomatism that caused an increase in Ti and Zr as observed in the high-temperature garnets. This metasomatic event affected only parts of the lithosphere, as there are still many garnets, especially at lower temperatures, that do not show this signature. We suggest that the heating was mainly around magma conduits and happened shortly before the eruption.
174
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Fig. 8. (a) Conductive geotherm for garnets from the Luebo region. The kink is based on the Y distribution with temperature. (b) Chemical tomography for garnets from Luebo. Note the presence of a thick harzburgite-rich layer largely located within the diamond-stability field (below G–D line), and pronounced melt-metasomatism below the LAB.
The kimberlite magmatism in the Kundelungu Plateau is correlated with the initial stages of the opening of the East African Rift at 32 Ma (Batumike et al., 2008), and the disturbance of the initially low conductive geotherm may reflect the upwelling of asthenospheric melts enriched in Ti and Zr. The garnet data indicate that this heating event was especially pronounced at depths below 130 km. Mbuji Mayi and Luebo (both on-craton) have garnets showing the effects of both melt-related and phlogopite-related metasomatisms as well as relatively abundant depleted garnets. Similar effects are observed in Kundelungu garnets but the phlogopite metasomatism was less pronounced at Kundelungu than at Mbuji Mayi and Luebo, whereas the melt-metasomatism was more pronounced at Kundelungu. The cool conductive geotherm observed at Kundelungu (above the heated zone) is typical of cratonic areas, and is similar to that observed at Mbuji Mayi and Luebo. This may suggest that the mantle beneath Kundelungu is strongly modified cratonic (Archean) lithospheric mantle, rather than mantle that was newly generated during the Proterozoic. Limited Re–Os data on sulfides in peridotite xenoliths from the Kundelungu kimberlites (Batumike, 2008) and zircon data (Batumike et al., 2007b) from this area also suggest the existence of a cratonic lithosphere beneath the region during Neoproterozoic time. 5.2. Peridotitic diamond potential The data presented here allow an evaluation of the relative diamond potential of the three regions, at least in terms of diamonds of the peridotitic paragenesis. The cool conductive geotherm found in the upper part of the Kundelungu lithospheric mantle is characteristic of
cratonic regions where some kimberlites contain diamond. Fig. 9(a, b) shows that some of the lherzolitic and harzburgitic garnets from Kundelungu have equilibrated within the diamond-stability field, suggesting that diamond of peridotitic paragenesis could be present in the Kundelungu Plateau kimberlites. The rare harzburgitic garnets found at Kundelungu are restricted to the Talala pipe, where Verhoogen (1938) also reported the highest diamond grade for the Kundelungu region. Kampata (1993) stated that a few grains of diamond had been found within the Kundelungu kimberlites. However, the near absence of harzburgitic garnets and the rarity of chromite in these kimberlites suggest, according to the model of Malkovets et al. (2007), that these kimberlites (at least the studied pipes) are not likely to be highly diamondiferous. This conclusion is supported by the fact that most of garnets have XMg b 0.83 and the coexisting olivines thus are characterised by low XMg (b0.91). About 40% of the analysed garnets show the effects of metasomatism, which may have been responsible for the destruction of any diamond in metasomatised domains. More sampling inside the Kundelungu Plateau may provide further constraints on the diamond potential of the Kundelungu kimberlites. The data for Mbuji Mayi (from De Beers) did not include values for Nd and Sc, making it difficult to compare them with diamond-inclusion garnets (e.g. Fig. 9). However, the Mbuji Mayi kimberlite field has been mined for diamond by the Minière de Bakwanga (MIBA) since 1961 both from alluvial and from kimberlite sources. The Mbuji Mayi area is characterised by abundant low-Ca harzburgitic, Ca-harzburgitic and lherzolitic garnets that record P–T conditions within the diamondstability field (Fig. 3c, d), coexisting with olivine with XMg (N0.91) similar to those observed in diamond inclusions (e.g. Stachel and Harris,
J.M. Batumike et al. / Lithos 112S (2009) 166–176
175
Fig. 9. Nd/Y–Sc/Y and Nd/Y–TNi plots for Kundelungu (a, b) and Luebo (c, d) garnets. Note the presence of lherzolitic and harzburgitic garnets similar to diamond-inclusion garnets, plotting within the diamond-stability field. The vertical marked G–D shows the boundary between the graphite (G) and diamond (D) stability fields.
2008). The presence of harzburgite-rich layers inside the diamondstability field (Fig. 6c, d) and the presence of abundant chromite in these kimberlites (Table 1) are consistent with the abundance of diamond in the Mbuji Mayi kimberlites. The Luebo garnet population contains many grains with sinusoidal REE patterns and Nd–Sc–Y relationships similar to those observed in diamond inclusions (Fig. 9c, d). Abundant low-Ca garnets coexisted with high XMg (N0.91) olivine. Most of these harzburgitic garnets have calculated temperatures and pressures that place them inside the diamond-stability field (Fig. 8a). These characteristics and the presence of abundant chromite in the heavy-mineral concentrates, suggest that the conditions for diamond formation were satisfied in this region (e.g. Malkovets et al., 2007). Assuming that the Luebo garnets are from local sources, this gives the Luebo region significant potential in terms of diamond exploration. 6. Conclusions The compositions of peridotitic garnets from the Kundelungu, Mbuji Mayi and Luebo areas in the southern part of the Congo provide insights into the composition and metasomatic history of the subcontinental lithospheric mantle beneath this region. The following conclusions can be drawn from this study: 1. Kundelungu, Mbuji Mayi and Luebo are characterised by low, cratonic-type conductive geotherms, but the deeper parts of the Kundelungu section have been thermally disturbed. 2. The lithospheric mantle beneath the Kundelungu area (off-craton) is relatively thin and fertile compared with that represented by garnets from Mbuji Mayi and Luebo (on-craton), and may represent refertilised Archean cratonic lithosphere.
3. The disturbance of the initially low geotherm in Kundelungu is correlated with the upwelling of melts related to the opening of the East African Rift. 4. The mantle sections beneath the Kundelungu, Mbuji Mayi and Luebo regions were affected by melt-related and phlogopiterelated metasomatisms but these effects, and especially the meltrelated metasomatism, were most pronounced in Kundelungu. 5. The Mbuji Mayi and Luebo areas have higher peridotitic diamond potential than Kundelungu. 6. The apparent scarcity of diamonds in the kimberlites of the Kundelungu Plateau is attributed to the heating and melt-related metasomatism that has affected the lithosphere in the region. Acknowledgements We thank De Beers for access to mineral-chemistry data from Mbuji Mayi and some of the pipes in Kundelungu. Gravity Diamonds is thanked for providing samples and data from Luebo. This work was supported by ARC Discovery and Linkage grants (SYO'R and WLG) and iMURS, IPRS and PGRF (Macquarie University) scholarships (JMB). JMB is grateful to Macquarie International for assistance with travel grants that allowed fieldwork in the Kundelungu Plateau and participation in the 9th International Kimberlite Conference. A travel grant from the 9th IKC is also gratefully acknowledged. We thank Herman Grütter, Ken Tainton and an anonymous referee for helpful reviews. The analytical work used instrumentation supported by ARC, DEST, Macquarie University and industry funding. GEMOC and the Department of Earth and Planetary Sciences (Macquarie University) are thanked for support during this study. The Department of Geology (University of Lubumbashi) is thanked for fieldwork logistic in Kundelungu. This is contribution 565 from the ARC National Key
176
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Centre for the Geochemical Evolution and Metallogeny of Continents (www.es.mq.edu.au/GEMOC).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.lithos.2009.04.020. References Batumike, J.M., 2008. Origin of Kimberlites from the Kundelungu Region: Lithospheric Mapping, Diamond Potential and Crustal Evolution in Southern Democratic Republic of the Congo. PhD Thesis, Dept Earth and Planetary Sciences, Macquarie University, 336 pp. Batumike, J.M., Cailteux, J.L.H., Kampunzu, A.B., 2007a. Lithostratigraphy, basin development, base metal deposits and regional correlations of the Neoproterozoic Nguba and Kundelungu rock successions, central African Copperbelt. Gondwana Research 11, 432–447. Batumike, J.M., O'Reilly, S.Y., Griffin, W.L., Belousova, E.A., 2007b. U–Pb and Hf-isotope analyses of zircon from the Kundelungu kimberlites, D.R. Congo: implications for crustal evolution. Precambrian Research 156, 195–225. Batumike, J.M., Griffin, W.L., Belousova, E.A., Pearson, N.J., O'Reilly, S.Y., Shee, S.R., 2008. LAMICPMS U–Pb dating of kimberlitic perovskite: Eocene–Oligocene kimberlites from the Kundelungu Plateau, D.R. Congo. Earth and Planetary Science Letters 267, 609–619. Batumike, J.M., Griffin, W.L., O'Reilly, S.Y., Belousova, E.A., Pawlitschek, M., 2009. Crustal evolution in the central Congo-Kasai Craton, Luebo, D.R. Congo: zircon U–Pb and Hf-isotope data. Precambrian Research 170, 107–115. Cahen, L., Snelling, N.J., Delhal, J., Vail, J.R., 1984. The Geochronology and Evolution of Africa. Clarendon Press, Oxford. 512 pp. Davis, G.L., 1977. The ages and U contents of zircons from kimberlites and associated rocks. Extended Abstract Volume, 2nd International Kimberlites Conference, Santa Fe, N.M. Demaiffe, D., Fieremans, M., 1981. Strontium isotope geochemistry of the Mbuji Mayi and Kundelungu kimberlites, Zaïre, central Africa. Chemical Geology 31, 311–323. Demaiffe, D., Fieremans, M., Fieremans, C., 1990. The kimberlites of central Africa: a review. In: Kampunzu, A.B., Lubala, R.T. (Eds.), Magmatism in Extensional Structural Setting: the Phanerozoic Africa Plate. InSpringer-Verlag, Berlin, pp. 538–557. Delhal, J., Ledent, D., 1973. Résultats de quelques mésures d'âges radiométriques par la méthode Rb/Sr dans les pegmatites de la Haute Luanyi, région du Kasai (Zaïre). Rapport Annuel, Musée Royal de l'Afrique Centrale, Tervuren, Belgique, Département de Géologie et Minéralogie (1972), pp. 102–103. Delhal, J., Liégeois, J.-P., 1982. Le socle granito-gneissique du Shaba occidental (Zaïre): Pétrographie et géochronologie. Annales de la Societé Géologique de Belgique 105, 295–301. Delhal, J., Ledent, D., Torquato, J., 1976. Nouvelles données géochronologiques relatives au complexe gabbro-noritique et charnockitique du bouclier du Kasai et son prolongement en Angola. Annales de la Societé Géologique de Belgique 99, 211–226. Fieremans, C., 1977. Mode of occurrence and tectonic control of the kimberlite bodies in East Kasai (Zaïre). Extended Abstract Volume, 2nd International Kimberlites Conference, Santa Fe, N.M.
Gaul, O.F., Griffin, W.L., O'Reilly, S.Y., Pearson, N.J., 2000. Mapping olivine composition in the lithospheric mantle. Earth and Planetary Science Letters 182, 223–235. Griffin, W.L., Ryan, C.G., Cousens, D.R., Sie, S.H., Suter, G.F., 1989. Ni in Cr-pyrope garnets: a new geothermometer. Contributions to Mineralogy and Petrology 103, 199–202. Griffin, W.L., Gurney, J.J., Ryan, C.G., 1992. Variations in trapping temperatures and trace element in peridotite-suite inclusions from African diamonds: evidence for two inclusion suites, and implications for lithosphere stratigraphy. Contributions to Mineralogy and Petrology 110, 1–15. Griffin, W.L., Ryan, C.G., Win, T.T., 1995. Mapping the Earth's mantle in 4D using the proton microprobe. Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 104, 456–463. Griffin, W.L., Fisher, N.I., Friedman, J., O'Reilly, S.Y., Ryan, C.G., 2002. Cr-pyrope garnets in the lithospheric mantle: II. Compositional populations and their distribution in time and space. Geochemistry Geophysics Geosystems 1073 (3), 12 (35 pp). Kampata, M.D., 1993, Minéralogie et géochimie des kimberlites du Haut Plateau de Kundelungu (Shaba, Zaïre). Doctorate Thesis, Catholic University of Louvain, Belgium, 248 pp. Kampunzu, A.B., Calteux, J., 1999. Tectonic evolution of the Lufilian Arc (central Africa Copperbelt) during the Neoproterozoic Pan-African orogenesis. Gondwana Research 2, 401–421. Kinny, P.D., Meyer, H.O.A., 1994. Zircon from the mantle: a new way to date old diamond. Journal of Geology 102, 475–481. Malkovets, V.G., Griffin, W.L., O'Reilly, S.Y., Wood, B.J., 2007. Diamond, subcalcic garnet, and mantle metasomatism: kimberlite sampling patterns define the link. Geology 35, 339–342. O'Reilly, S.Y., Griffin, W.L., 1996. 4-D lithosphere mapping: methodology and examples. Tectonophysics 262, 3–18. O'Reilly, S.Y., Griffin, W.L., 2006. Imaging global chemical and thermal heterogeneity in the subcontinental lithospheric mantle with garnets and xenoliths: geophysical implications. Tectonophysics 416, 289–309. Pivin, M., et al., 2009, this issue. Metasomatic mantle origin for Mbuji-Mayi and Kundelungu garnet and clinopyroxene megacrysts (Democratic Republic of Congo). Proceedings of the 9th International Kimberlite Conference. Lithos 112S, 951–960. Ryan, C.G., Griffin, W.L., Pearson, N.J., 1996. Garnet geotherms: pressure–temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. Journal of Geophysical Research 101, 5611–5626. Schärer, U., Corfu, F., Demaiffe, D., 1997. U–Pb and Lu–Hf isotopes in baddeleyite and zircon megacrysts from Mbuji-Mayi kimberlite: constraints on the subcontinental mantle. Chemical Geology 143, 1–16. Shee, S.R., Wyatt, B.A., Griffin, W.L., 1993. Major and trace element mineral chemistry of peridotite xenoliths from the Wesselton kimberlite, South Africa. International Association of Volcanology and Chemistry of the Earth's Interior General Assembly 1993, p. 98. Abstract Volume. Sobolev, N.V., Lavrent'ev, Y.G., Pokhilenko, N., 1973. Chrome-rich garnets from the kimberlites of Yakutia and their paragenesis. Contributions to Mineralogy and Petrology 40, 39–52. Stachel, T., Harris, J.W., 2008. The origin of cratonic diamonds: constraints from mineral inclusions. Ore Geology Reviews 34 (1–2), 5–32. Verhoogen, J., 1938. Les pipes de kimberlite du Katanga: Annales du Service des Mines. Comité Spécial Katanga 9, 1–49. Walraven, F., Rumvegeri, B.T., 1993. Implications of whole-rock Pb–Pb and zircon evaporation dates for the early metamorphic history of the Kasai Craton, Southern Zaïre. Journal of African Earth Sciences 16 (4), 395–404.
Contents lists available at ScienceDirect
Lithos j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / l i t h o s
Lithospheric mantle structure and the diamond potential of kimberlites in southern D.R. Congo J.M. Batumike a,b,⁎, W.L. Griffin a, S.Y. O'Reilly a a b
ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia Département de Géologie, Université de Lubumbashi, Route Kasapa, B.P. 1825, Lubumbashi, The Democratic Republic of the Congo
a r t i c l e
i n f o
Article history: Received 7 September 2008 Accepted 19 April 2009 Available online 3 May 2009 Keywords: Peridotitic garnets Congo kimberlites Congo lithosphere Diamond potential Mantle metasomatism Congo-Kasai Craton
a b s t r a c t Mantle-derived peridotitic garnet xenocrysts from kimberlites in the Mbuji Mayi and Kundelungu areas and from heavy-mineral concentrates collected in the Luebo area, D.R. Congo, have been analysed for major- and trace-element compositions in order to understand the structure and composition of the subcontinental lithospheric mantle (SCLM) and the diamond potential of the kimberlites. The lithosphere beneath the Kundelungu Plateau is ca 175 km thick and has been affected by pronounced melt metasomatism. Garnets from the Kundelungu Plateau indicate an initially cool geotherm (~ 35 mW/m2), which was disturbed by asthenospheric melts that penetrated the SCLM shortly before kimberlite intrusion ca 32 Ma ago. Harzburgitic garnets are very rare, but some lherzolitic garnets display compositions similar to garnets included in diamond. Garnets from the Mbuji Mayi region indicate a cool geotherm (35 mW/m2); the SCLM is ~ 210 km thick and was affected by melt-related and phlogopite-related metasomatisms. Harzburgitic garnets form about 33% of the analysed population. The garnets from the Luebo region indicate a cool lithospheric geotherm (35 mW/m2) typical of cratonic areas. The SCLM from which the garnets were derived was relatively thick (205 km), affected by melt-related and phlogopite-related metasomatisms and characterised by the presence of a ~ 80-km thick harzburgite-rich layer. In terms of peridotitic diamond potential, Mbuji Mayi and Luebo are more prospective than Kundelungu. The initially cool conductive geotherm, the presence of some garnets with compositions similar to garnets included in diamond and the presence of sporadic diamond in the Kundelungu Plateau suggest that diamond initially was present in the lithosphere and the observed paucity of diamond may be due to the melt-related metasomatism that affected the lithosphere in the region. We suggest that the lithospheric mantle beneath Kundelungu is a strongly modified Archean cratonic lithosphere that has survived beneath the area during Proterozoic tectonism. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Our knowledge of the nature and composition of the subcontinental lithospheric mantle (SCLM) is largely based on the study of xenoliths brought to the Earth's surface by volcanism (mostly basaltic to kimberlitic). The distribution of elements between different mineral phases in the xenoliths allows the estimation of the conditions of equilibration at the time of entrainment (pressure, temperature) and an understanding of chemical processes that have affected the part of the SCLM from which the xenolith was derived. However, the distribution of such xenoliths (in which most of the mineral phases are preserved) is very restricted in space and time (O'Reilly and Griffin, 2006). Minerals (xenocrysts) such as garnet and chromite represent the products of the disaggregation of the xenoliths; they commonly are
⁎ Corresponding author. ARC National Key Centre for Geochemical Evolution and Metallogeny of Continents, Department of Earth and Planetary Sciences, Macquarie University, Sydney, NSW 2109, Australia. E-mail addresses: [email protected] (J.M. Batumike), bill.griffi[email protected] (W.L. Griffin), [email protected] (S.Y. O'Reilly). 0024-4937/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2009.04.020
well preserved and may be abundant in kimberlites, partly due to their resistance to alteration compared with other mantle minerals such as olivine and pyroxene. These minerals may thus be used for the study of the SCLM in areas where xenoliths are rare or absent. Two kimberlite fields are known in the Democratic Republic of the Congo (hereafter referred to as the Congo) at Mbuji Mayi located in the south-central part of the country (Kasai Province) and Kundelungu in the southeastern part (Kundelungu Plateau (Pivin et al., this issue)). In this study, xenocrystic peridotitic garnets from Mbuji Mayi and Kundelungu kimberlites and from heavy-mineral concentrates collected in drainages in the Luebo, Mbuji Mayi and Kundelungu regions are used to map and understand the nature of the SCLM in the southern part of the Congo and to assess its peridotitic diamond potential. This study provides an opportunity to compare the composition and structure of the lithospheric mantle and kimberlite diamond potential in two different geological settings. 2. Geological setting The Mbuji Mayi kimberlite field and the Luebo exploration area (~200 km NW of Mbuji Mayi) are located inside the Congo-Kasai
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Craton whereas the kimberlites in the Kundelungu Plateau intruded rocks belonging to the Neoproterozoic Katangan Supergroup inside the Katangan Belt (Fig. 1). The Congo-Kasai Craton consists of granulite, gneiss, granite and amphibolite, a gabbro-noritic and gabbro-charnockitic complex, and a migmatitic complex. The ages of these rocks range from 3.4 to 2.6 Ga (Delhal and Ledent, 1973; Delhal and Liégeois, 1982; Walraven and Rumvegeri, 1993; Batumike et al., 2009). The youngest Archean age in this part of the craton was found in the Malafundi granites from the Dibaya complex which gave 2648 ± 22 Ma (Rb/Sr; Delhal et al., 1976) and 2595 ± 92 Ma (Rb/Sr; Cahen et al., 1984). Paleoproterozoic, Mesoproterozoic and Neoproterozoic rocks within this craton are poorly studied, but represent parts of the Paleoproterozoic Ubendian, Mesoproterozoic Kibaran and Neoproterozoic Katangan orogenic belts. The Mbuji Mayi Supergroup, intruded by the Mbuji Mayi kimberlites, consists of 1.3–0.95 Ga sedimentary rocks, covered in places by 0.95 Ga basaltic lavas and Cretaceous sandstones (~120 Ma, Schärer et al., 1997). Kimberlites occur in two clusters in the Mbuji Mayi region: the northern Mbuji Mayi cluster is formed of elliptical bodies along an E– W-striking crustal fissure (e.g. Fieremans, 1977; Demaiffe and Fieremans, 1981; Demaiffe et al., 1990) and the southern Tshibwe cluster (Schärer et al., 1997). Because the kimberlites intrude Cretaceous sandstones with an estimated age of 120 Ma, this age has been taken as the maximum age of known kimberlite occurrences within the craton. Davis (1977) dated a megacrystic zircon from Mbuji Mayi at 71 Ma (TIMS 206Pb/238U). U–Pb analysis of zircon and baddeleyite megacrysts from Mbuji Mayi yielded discordant data with concordia intercepts at 69.8± 0.5 Ma and 2528± 452 Ma (Schärer et al., 1997), and Kinny and Meyer (1994) reported an age of 628±12 Ma for a zircon included in diamond from the same area. The Luebo region is characterised by the presence of abundant diamonds and kimberlitic indicator minerals in streams and rivers. The region has been subjected to intensive exploration for search of local sources for the diamond, but no primary sources such as kimberlite have yet been found (e.g. Bhebhe, Z., pers. comm., 2008). Our morphological examination of garnet grains from heavy-mineral concentrates suggests that these grains may have not travelled long
167
Fig. 2. Location of kimberlite pipes in the Kundelungu Plateau.
distances, and might be derived from local kimberlitic sources, although formally the source(s) for the grains studied are indeterminate. U–Pb, Hf isotope and trace-element data for zircons from the same Luebo-area heavy-mineral concentrates suggest that kimberlites were emplaced inside the Congo-Kasai Craton in Late-Archean, Neoproterozoic and Cretaceous time (Batumike et al., 2009), though the location of such sources remains to be determined. The Kundelungu Plateau is made up of sandstones, limestones and mudstones grouped as the Biano Subgroup, occurring stratigraphically at the top of the Neoproterozoic Katangan Supergroup. These rocks are subhorizontal and undeformed and were deposited in a foreland basin during the Lufilian orogeny (Kampunzu and Calteux, 1999; Batumike et al., 2007a). The kimberlites form two geographic clusters inside the Kundelungu Plateau: the eastern cluster has 16 pipes and the western has 14 pipes (Fig. 2). This division is mainly based on the presence of a thick sand cover in the central portion of the plateau that may obscure some kimberlite pipes. U–Pb analysis of perovskites indicates that there was only one magmatic kimberlite episode in the Kundelungu Plateau and this occurred at 32 ± 2 Ma, coincident with the initial stages of the East African Rift in Ethiopia and Kenya (Batumike et al., 2008). 3. Samples and methods 3.1. Sampling
Fig. 1. Geological sketch map showing the position of the Congo-Kasai, Tanzania and Kalahari Cratons in Africa (grey areas), and study areas. Small dashed lines are political boundaries. Names of countries are in small capitals.
The garnet xenocrysts studied were collected from three different locations: Kundelungu Plateau, Mbuji Mayi and Luebo. Data for garnets from Mbuji Mayi and some of the kimberlite pipes in the Kundelungu Plateau (e.g. Luanza, Golo, Mbo and Chingululu, Fig. 2) were provided by De Beers. Epoxy mounts of garnets from Luebo and their electron microprobe data were provided by Gravity Diamonds Ltd. These garnets were extracted from heavy-mineral concentrates prepared from stream samples collected during an exploration campaign. In the eastern part of the Kundelungu Plateau, stream sediment samples were collected in the Gungwania, Talala and Luanza
168
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Rivers that crosscut the kimberlite pipes of the same names (Fig. 2), and heavy-mineral concentrates prepared by hand-gravitation. The garnets were separated from the concentrates by handpicking, mounted in epoxy disks, and polished before analysis. In this study, Kundelungu Plateau data are analysed in terms of the two geographic clusters, but for some purposes the data from the two clusters are aggregated. The Mbuji Mayi data are also treated as two different clusters, the Mbuji Mayi kimberlite cluster in which 6 pipes were studied (massifs 1, 3, 4, 6, 9 and 10) and the Tshibwe kimberlite cluster. Overall the heavy-mineral concentrates from the Kundelungu Plateau are characterised by a scarcity of chromite whereas concentrates from Mbuji Mayi and Luebo contain abundant chromite. Sample distribution and representation by kimberlite cluster is summarised in Table 1.
Table 1 Summary of known kimberlites in the Democratic Republic of the Congo and data. Locality
Kundelungu East
Kundelungu West
Mbuji Mayi
Tshibwe
Luebo
Diamond (cpht) Age of kimberlites Kimberlites in cluster Kimberlites studied Total pyropes LA-ICPMS data (GEMOC) LA-ICPMS data (De Beers) Chromite abundancef
b10a 32 Mad 16
b 5a 32 Mad 14
20–260b 71 Mae 10
b100b 71 Mae –
0–200c – –
3 722 433
4 401 272
6 1236 0
356 0
– 255 255
289
129
1236
356
0
Very low
Very low
High
High
High
a b
3.2. Analytical methods
c d
Data for garnets from Mbuji Mayi and some of the Kundelungu kimberlites, including major elements (oxides) and Ni, Ga, Y and Zr contents, were supplied by De Beers and electron microprobe data for Luebo garnets were provided by Gravity Diamonds Ltd. The De Beers analyses were acquired by electron microprobe and LAM-ICPMS, using techniques similar to those used at the GEMOC ARC National Key Centre (Macquarie University). Inter-laboratory comparisons between De Beers and GEMOC show very good correspondence. Other majorelement (oxide) analyses were done at GEMOC using a CAMEBAX SX100 electron microprobe and analyses of trace elements were performed on an Agilent 7500S ICPMS attached to a Merchantek 213 nm New Wave Nd:YAG laser microprobe. The analytical procedures are similar to those described by Batumike et al. (2009 and references therein; more details at www.mq.edu.au/GEMOC). The data distribution is given in Table 1 and the analytical data are presented in online Supplementary data Tables 1, 2 and 3 respectively for the Kundelungu, Mbuji Mayi and Luebo regions. 3.3. Data processing and presentation Griffin et al. (1989) and Ryan et al. (1996) showed that Ni and Cr in pyrope garnets can be used to estimate temperature (TNi) and pressure (PCr) of equilibration. The pressure–temperature (P–T) relationships may be used to construct the geotherm, which typically follows model conductive geotherms above the geochemical lithosphere–asthenosphere boundary (O'Reilly and Griffin, 1996, 2006). Below this boundary, these authors proposed that the geotherm could be drawn parallel to the diamond–graphite curve, similar to the P–T relationships of high-temperature sheared peridotite xenoliths. The physical state of the SCLM depends strongly on the level of depletion, which is reflected in the XMg of olivine. In this study the XMg of olivine coexisting with garnet is calculated using the method of Gaul et al. (2000). Griffin et al. (1995) used xenoliths and diamond inclusions to establish the chemical signatures of garnets affected by different types of mantle processes. Using Zr, Y, Ga and Ti contents in garnets, two principal types of metasomatism can be recognised. One is defined by garnets with enrichment in Zr accompanied by increase in Y and Ti (melt metasomatism) and the second by garnets showing an increase in Zr with low to moderate Ti, Y and Ga (phlogopite metasomatism) (Griffin et al., 1992, 1995). This latter category of garnets has been observed in low-T phlogopite-bearing xenoliths (Shee et al., 1993). Griffin et al. (2002) used inter-element correlations in a large database of mantle-derived Cr-pyrope garnets to define populations that could be related to specific processes by comparison with garnets
e f
Kampata (1993). De Beers. Alluvial data, Southern Era. Batumike et al. (2008). Davis (1977). Qualitative abundance.
from well-studied xenoliths. The defined populations can be grouped into five major categories (Griffin et al., 2002): depleted harzburgites, depleted lherzolites, depleted/metasomatised lherzolites, fertile lherzolites and melt-metasomatised peridotites. Given the depth at which the garnets equilibrated and the geochemical process and rock type indicated by the composition of the garnets, the proportion of the garnets reflecting individual rock types and processes at particular depths can be calculated. This allows the construction of a section through the lithosphere showing the variation in the proportions of rock types or types of processes with depth. These sections have been described as “Chemical Tomography” (O'Reilly and Griffin, 2006 and references therein). Most peridotitic garnet inclusions in diamond are characterised by high XMg [Mg/(Mg + Fe)], reflecting coexistence with very forsteritic olivine. They also show sinuous rare-earth element (REE) patterns with enrichment in MREE and depletion in HREE. This sinusoidal pattern is shown clearly by the relationships between Sc, Nd and Y (where Sc is used as a proxy for HREE and Y as a proxy for MREE); Sc/Y measures the depletion in HREE whereas high Nd/Y measures the degree of enrichment in LREE. According to Stachel and Harris (2008), this sinusoidal REE pattern reflects re-enrichment in the most incompatible elements and represents a metasomatic overprint by a highly fractionated fluid essentially devoid of HREE. However, Malkovets et al. (2007) have suggested that these garnets crystallized directly by reaction between chromite and the metasomatic fluid. In this study, the peridotitic diamond potential is assessed based on compositional features of garnet together with P–T conditions derived from the garnet data. 4. Results 4.1. Kundelungu Plateau 4.1.1. Kundelungu East Garnets from kimberlites in the eastern cluster can be grouped compositionally into four different categories, according to the classification of Sobolev et al. (1973): lherzolitic garnets, low-Cr garnets, Ca-harzburgitic garnets and wehrlitic garnets (Supplementary data Table 1). The lherzolitic population constitutes over 88% of the whole sample. The Ca-harzburgitic and wehrlitic garnets, forming populations of 0.5% and 0.2% respectively, are restricted to the Talala pipe (Fig. 2). The trace-element patterns of most of these garnets are
Fig. 3. Conductive geotherms for Kundelungu East (a), Kundelungu West (b), Mbuji Mayi (c) and Tshibwe (d). The kink in the Kundelungu geotherm is based on the Y distribution of the lower-T data cluster, with the second cluster considered as disturbed by heating and metasomatism. Note the geotherm is 35 mW/m2 for Kundelungu East, similar to Mbuji Mayi, and 37.5 mW/m2 for Kundelungu West.
J.M. Batumike et al. / Lithos 112S (2009) 166–176
169
170
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Fig. 4. Y, Zr and Ti relationships in garnets from Kundelungu East (a, b) and Kundelungu West (c, d) indicating the type of mantle processes affecting the lithosphere.
characterised by depletion in LREE and enrichment in HREE, but some garnets of lherzolitic composition and all harzburgitic garnets are characterised by depletion in the HREE. This depletion produces the sinuous pattern commonly observed in garnets included in diamond. Most of the garnets are depleted in Sr, Co and Ni and a few grains show depletion in Ti. The conductive geotherm derived from these garnets is quite low, corresponding to a modelled surface heat flow of about 35 mW/m2 (Fig. 3a). On this plot the data show a gap in temperature distribution and this gap is also observed in the Y–TNi plot (Fig. 3a) where some high-temperature garnets have low-Y contents similar to those with low temperature. However, the high-temperature (high-Ni) population tends to have high Ti and Zr contents compared with the lowtemperature garnets. The bimodal temperature distribution of the garnets hinders estimation of the depth of the lithosphere–asthenosphere boundary (LAB), which is usually taken as the temperature above which Y-depleted garnets are rare or absent. The Kundelungu East garnet population contains abundant Ydepleted garnets with high temperatures (high-Ni contents). The Y– Zr and Zr–TiO2 plots (Fig. 4a, b) indicate that the garnets that are depleted in Y, but with high Zr and Ti show the effects of both meltrelated and phlogopite-related metasomatisms. The two styles of metasomatism occur all through the section of the Kundelungu lithospheric mantle, but the melt-related signature is dominant below depths of ca 130 km. The relationships between these three elements (Y, Zr and Ti, Fig. 5a) show two different patterns indicating that there are two different populations in the garnets, with the more meltmetasomatised population having generally higher T. These features suggest that the Kundelungu region was originally characterised by a low conductive geotherm that was disturbed by introduction of melt, which also affected the compositions of some garnets. These two garnet populations can be used to define two distinct geotherms, corresponding to a ca 35 mW/m2 model at shallow depth, and a higher one below 130 km depth. The chemical tomography section for this part of Kundelungu Plateau therefore was constructed using a conductive geotherm of 35 mW/m2 with a kink at
1000 °C (ca 130 km). Fig. 6a shows that the lithospheric section is dominantly lherzolitic (fertile, depleted or metasomatised) and also contains a high proportion of melt-metasomatised rocks. The thickness of the lithosphere is estimated at ~ 175 km. This boundary is also illustrated by a decrease in the forsterite content of coexisting olivine, which drops below XMg = 0.91 at ca 1000 °C (Fig. 6a). The lithospheric section is also characterised by the presence of minor harzburgites inside the diamond-stability field.
4.1.2. Kundelungu West Lherzolitic and low-Cr garnets are recognised in the western Kundelungu kimberlite cluster, with the lherzolitic population the most abundant (Supplementary data Table 1). There is no difference between individual pipes in this part of the Kundelungu Plateau in terms of the type of garnets. The trace-element patterns are generally similar to those in the garnets of the eastern cluster, showing enrichment in HREE and depletion in LREE, with about 7% of the lherzolitic population showing HREE depletion. The reconstructed conductive geotherm is higher than that for the eastern cluster, 37–38 mW/m2 (Fig. 3b). The garnets also show two different clusters on plots of PCr vs TNi and Y vs TNi; low-Y garnets with high temperatures also have high contents of Ti and Zr (Figs. 3b and 6b). This is interpreted as due to heating and metasomatism that affected some garnets, as in the Kundelungu East pipes, and indicates that both parts of the Kundelungu Plateau were affected by similar metasomatic fluids. The presence of garnets showing the effects of both melt- and phlogopite-related metasomatisms is illustrated in Fig. 4(c, d) by high values in Y, Ti and Zr and a scarcity of garnets derived from depleted peridotites. The chemical tomography section for this part of Kundelungu Plateau, constructed using a conductive geotherm of 37.5 mW/m2 and a kink at 1000 °C, shows a dominantly lherzolitic lithosphere which is about 160 km thick (Fig. 6b). There is no obvious difference in the lithosphere composition below the two clusters except that the lithosphere in the eastern part is slightly thicker and harzburgites
J.M. Batumike et al. / Lithos 112S (2009) 166–176
171
Fig. 5. Y and Ti relationships indicating presence of two different garnet populations in Kundelungu East (a) and West (b). Y–Zr and Zr–Ti relationships indicating the type of metasomatism affecting the source region for the garnet population with low Y/Ti ratios (c, d) and for high Y/Ti garnet population (e, f). Note that most of the garnets with high Y/Ti ratios show effect of melt-metasomatism (f).
were only observed in the eastern region. However, this may reflect a sampling bias (Table 1). The garnets from the high-temperature group plot in the meltmetasomatism field (Fig. 5e, f) whereas the garnets of the low-T group plot mostly in the depleted and phlogopite-metasomatism fields (Fig. 5c, d). This suggests that the difference in composition between the high-temperature and low-temperature garnet populations is due to Ti–Zr enriched melts affecting the deeper part of the lithospheric mantle beneath both the eastern and western clusters in the Kundelungu Plateau. 4.2. Mbuji Mayi 4.2.1. Mbuji Mayi cluster The Mbuji Mayi garnets can be grouped into five groups based on their CaO and Cr2O3 contents (Sobolev et al., 1973; Supplementary data Table 2): Ca- and low-Ca-harzburgitic, lherzolitic, wehrlitic and low-Cr garnets. Lherzolitic and Ca-harzburgitic garnets are common
in all of the studied Mbuji Mayi pipes; the Ca-harzburgitic garnets make up about 33% of the analysed population and 63% consists of lherzolitic garnets. PCr–TNi relationships in the Mbuji Mayi garnets define a relatively low conductive geotherm, about 35 mW/m2 (Fig. 3c), typical of cratonic lithosphere. A high proportion of the garnet grains plot inside the diamond-stability field in terms of P and T; most of these garnets are classified as harzburgitic or low-Ca harzburgitic. Some lherzolitic garnets also plot inside the diamond-stability field but most lherzolitic garnets are in the graphite stability field. The distribution of Y vs TNi (Fig. 3c) shows the absence of low-Y garnets at T ≥ 1100 °C, corresponding to a depth of 210 km on a conductive geotherm of 35 mW/m2. At this depth, there is also a drop in the calculated XMg of olivine coexisting with garnet (Fig. 6c), which corresponds to the geochemical lithosphere–asthenosphere boundary (O'Reilly and Griffin, 2006). The Mbuji Mayi lithospheric section (Fig. 6c) is characterised by two harzburgite-rich layers inside the diamond-stability field. The garnets
172
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Fig. 6. Subcontinental lithospheric mantle chemical tomography for Kundelungu East (a), Kundelungu West (b), Mbuji Mayi (c) and Tshibwe (d). The horizontal line marked G–D gives the depth for the graphite–diamond-stability field in the region.
have chemical compositions suggesting the effects of both melt and phlogopite metasomatisms (Fig. 7a, b), but the melt metasomatism is more pronounced in the deeper lithosphere at depths around 170 km (Fig. 6c). Unlike the Kundelungu Plateau section, there are no low-Y garnets at high temperatures (see Figs. 3a, b and 4c). 4.2.2. Tshibwe cluster Five different categories of garnets are observed in the garnets from the Tshibwe cluster, similar to the populations observed in the Mbuji Mayi cluster. The conductive geotherm derived from the garnets is ~35 mW/m2, as in the Mbuji Mayi area (Fig. 3d), entirely reasonable given the 30-km separation of the two clusters. The distribution of Y vs TNi in the Tshibwe garnets (Fig. 3d) shows the absence of low-Y garnets above 1075 °C, corresponding to a depth of 195 km. The calculated XMg in coexisting olivine ranges from 0.89 to 0.925, but in the lithospheric
section XMg decreases with depth at ~120 km and then again at 195 km (Fig. 6d). The decrease at ~120 km reflects the presence of abundant “fertile” garnets affected by phlogopite-related metasomatism, whereas the decrease at 195 km is due to garnets showing both fertile and meltmetasomatised signatures. Both the decrease in XMg of olivine and the lower depth limit of low-Y garnets characterise the transition from a depleted lithosphere to a fertile zone, which is taken as the geochemical lithosphere– asthenosphere boundary (Fig. 6d). The lithospheric section beneath the Tshibwe region contains more harzburgite inside the diamondstability field at depths of 170–220 km than observed in the upper section. The abundances of Zr, Ti and Y in the Tshibwe garnets indicate the effects of both melt and phlogopite metasomatisms (Fig. 7c, d). These metasomatic signatures are similar to those observed in the Mbuji Mayi cluster.
J.M. Batumike et al. / Lithos 112S (2009) 166–176
173
Fig. 7. Y, Zr and Ti relationships in Mbuji Mayi (a, b), Tshibwe (c, d) and Luebo (e, f) garnets indicating the type of mantle processes affecting the lithosphere in these areas.
4.3. Luebo area The Luebo garnets show four compositional groups: lherzolitic, Caand low-Ca harzburgitic, and low-Cr garnets. Lherzolitic garnets are the dominant population followed by harzburgitic garnets (Supplementary data Table 3). The trace-element data (not shown) for some garnets show sinusoidal REE patterns. The garnets define a geotherm of ~ 35 mW/m2 (Fig. 8a). There are few low-Y garnets with TNi above 1100 °C, corresponding to a depth of 205 km. The compositions calculated for coexisting olivine vary between XMg 0.895 and 0.94 and drop below 0.91 at ~205 km (Fig. 8b), corresponding to the rise in mean Y content and defining the geochemical lithosphere–asthenosphere boundary. The Luebo garnets include a population that shows enrichment in Ti, Y and Zr, due to the effects of metasomatism. Fig. 7(e, f) shows that the lithosphere represented by Luebo-area garnets was affected by both melt-related and phlogopite-related metasomatisms, though some depleted garnet compositions remain. The dominant depleted population is lherzolitic garnets but very few low-Cr and harzburgitic garnets plot inside the depleted domain (Fig. 7e, f). Assuming that the garnets collected inside the Luebo area were derived from a local kimberlite source, the lithospheric section for Luebo is also char-
acterised by the presence of a relatively thick harzburgite-rich layer at depths between 100 and 185 km (Fig. 8b). A large part of this harzburgite-rich layer lies within the diamond-stability field. 5. Discussion 5.1. Lithosphere structure, composition and evolution Garnet geochemistry indicates that the lithosphere is thicker beneath the on-craton Mbuji Mayi and Luebo areas than beneath the off-craton Kundelungu Plateau. The Mbuji Mayi and Luebo areas are characterised by abundant harzburgitic garnets compared with the almost exclusively lherzolitic garnets at Kundelungu. Kundelungu, Mbuji Mayi and Luebo are characterised by cool conductive geotherms, but at Kundelungu this initially low conductive geotherm was disturbed by a heating event, which was accompanied by metasomatism that caused an increase in Ti and Zr as observed in the high-temperature garnets. This metasomatic event affected only parts of the lithosphere, as there are still many garnets, especially at lower temperatures, that do not show this signature. We suggest that the heating was mainly around magma conduits and happened shortly before the eruption.
174
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Fig. 8. (a) Conductive geotherm for garnets from the Luebo region. The kink is based on the Y distribution with temperature. (b) Chemical tomography for garnets from Luebo. Note the presence of a thick harzburgite-rich layer largely located within the diamond-stability field (below G–D line), and pronounced melt-metasomatism below the LAB.
The kimberlite magmatism in the Kundelungu Plateau is correlated with the initial stages of the opening of the East African Rift at 32 Ma (Batumike et al., 2008), and the disturbance of the initially low conductive geotherm may reflect the upwelling of asthenospheric melts enriched in Ti and Zr. The garnet data indicate that this heating event was especially pronounced at depths below 130 km. Mbuji Mayi and Luebo (both on-craton) have garnets showing the effects of both melt-related and phlogopite-related metasomatisms as well as relatively abundant depleted garnets. Similar effects are observed in Kundelungu garnets but the phlogopite metasomatism was less pronounced at Kundelungu than at Mbuji Mayi and Luebo, whereas the melt-metasomatism was more pronounced at Kundelungu. The cool conductive geotherm observed at Kundelungu (above the heated zone) is typical of cratonic areas, and is similar to that observed at Mbuji Mayi and Luebo. This may suggest that the mantle beneath Kundelungu is strongly modified cratonic (Archean) lithospheric mantle, rather than mantle that was newly generated during the Proterozoic. Limited Re–Os data on sulfides in peridotite xenoliths from the Kundelungu kimberlites (Batumike, 2008) and zircon data (Batumike et al., 2007b) from this area also suggest the existence of a cratonic lithosphere beneath the region during Neoproterozoic time. 5.2. Peridotitic diamond potential The data presented here allow an evaluation of the relative diamond potential of the three regions, at least in terms of diamonds of the peridotitic paragenesis. The cool conductive geotherm found in the upper part of the Kundelungu lithospheric mantle is characteristic of
cratonic regions where some kimberlites contain diamond. Fig. 9(a, b) shows that some of the lherzolitic and harzburgitic garnets from Kundelungu have equilibrated within the diamond-stability field, suggesting that diamond of peridotitic paragenesis could be present in the Kundelungu Plateau kimberlites. The rare harzburgitic garnets found at Kundelungu are restricted to the Talala pipe, where Verhoogen (1938) also reported the highest diamond grade for the Kundelungu region. Kampata (1993) stated that a few grains of diamond had been found within the Kundelungu kimberlites. However, the near absence of harzburgitic garnets and the rarity of chromite in these kimberlites suggest, according to the model of Malkovets et al. (2007), that these kimberlites (at least the studied pipes) are not likely to be highly diamondiferous. This conclusion is supported by the fact that most of garnets have XMg b 0.83 and the coexisting olivines thus are characterised by low XMg (b0.91). About 40% of the analysed garnets show the effects of metasomatism, which may have been responsible for the destruction of any diamond in metasomatised domains. More sampling inside the Kundelungu Plateau may provide further constraints on the diamond potential of the Kundelungu kimberlites. The data for Mbuji Mayi (from De Beers) did not include values for Nd and Sc, making it difficult to compare them with diamond-inclusion garnets (e.g. Fig. 9). However, the Mbuji Mayi kimberlite field has been mined for diamond by the Minière de Bakwanga (MIBA) since 1961 both from alluvial and from kimberlite sources. The Mbuji Mayi area is characterised by abundant low-Ca harzburgitic, Ca-harzburgitic and lherzolitic garnets that record P–T conditions within the diamondstability field (Fig. 3c, d), coexisting with olivine with XMg (N0.91) similar to those observed in diamond inclusions (e.g. Stachel and Harris,
J.M. Batumike et al. / Lithos 112S (2009) 166–176
175
Fig. 9. Nd/Y–Sc/Y and Nd/Y–TNi plots for Kundelungu (a, b) and Luebo (c, d) garnets. Note the presence of lherzolitic and harzburgitic garnets similar to diamond-inclusion garnets, plotting within the diamond-stability field. The vertical marked G–D shows the boundary between the graphite (G) and diamond (D) stability fields.
2008). The presence of harzburgite-rich layers inside the diamondstability field (Fig. 6c, d) and the presence of abundant chromite in these kimberlites (Table 1) are consistent with the abundance of diamond in the Mbuji Mayi kimberlites. The Luebo garnet population contains many grains with sinusoidal REE patterns and Nd–Sc–Y relationships similar to those observed in diamond inclusions (Fig. 9c, d). Abundant low-Ca garnets coexisted with high XMg (N0.91) olivine. Most of these harzburgitic garnets have calculated temperatures and pressures that place them inside the diamond-stability field (Fig. 8a). These characteristics and the presence of abundant chromite in the heavy-mineral concentrates, suggest that the conditions for diamond formation were satisfied in this region (e.g. Malkovets et al., 2007). Assuming that the Luebo garnets are from local sources, this gives the Luebo region significant potential in terms of diamond exploration. 6. Conclusions The compositions of peridotitic garnets from the Kundelungu, Mbuji Mayi and Luebo areas in the southern part of the Congo provide insights into the composition and metasomatic history of the subcontinental lithospheric mantle beneath this region. The following conclusions can be drawn from this study: 1. Kundelungu, Mbuji Mayi and Luebo are characterised by low, cratonic-type conductive geotherms, but the deeper parts of the Kundelungu section have been thermally disturbed. 2. The lithospheric mantle beneath the Kundelungu area (off-craton) is relatively thin and fertile compared with that represented by garnets from Mbuji Mayi and Luebo (on-craton), and may represent refertilised Archean cratonic lithosphere.
3. The disturbance of the initially low geotherm in Kundelungu is correlated with the upwelling of melts related to the opening of the East African Rift. 4. The mantle sections beneath the Kundelungu, Mbuji Mayi and Luebo regions were affected by melt-related and phlogopiterelated metasomatisms but these effects, and especially the meltrelated metasomatism, were most pronounced in Kundelungu. 5. The Mbuji Mayi and Luebo areas have higher peridotitic diamond potential than Kundelungu. 6. The apparent scarcity of diamonds in the kimberlites of the Kundelungu Plateau is attributed to the heating and melt-related metasomatism that has affected the lithosphere in the region. Acknowledgements We thank De Beers for access to mineral-chemistry data from Mbuji Mayi and some of the pipes in Kundelungu. Gravity Diamonds is thanked for providing samples and data from Luebo. This work was supported by ARC Discovery and Linkage grants (SYO'R and WLG) and iMURS, IPRS and PGRF (Macquarie University) scholarships (JMB). JMB is grateful to Macquarie International for assistance with travel grants that allowed fieldwork in the Kundelungu Plateau and participation in the 9th International Kimberlite Conference. A travel grant from the 9th IKC is also gratefully acknowledged. We thank Herman Grütter, Ken Tainton and an anonymous referee for helpful reviews. The analytical work used instrumentation supported by ARC, DEST, Macquarie University and industry funding. GEMOC and the Department of Earth and Planetary Sciences (Macquarie University) are thanked for support during this study. The Department of Geology (University of Lubumbashi) is thanked for fieldwork logistic in Kundelungu. This is contribution 565 from the ARC National Key
176
J.M. Batumike et al. / Lithos 112S (2009) 166–176
Centre for the Geochemical Evolution and Metallogeny of Continents (www.es.mq.edu.au/GEMOC).
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.lithos.2009.04.020. References Batumike, J.M., 2008. Origin of Kimberlites from the Kundelungu Region: Lithospheric Mapping, Diamond Potential and Crustal Evolution in Southern Democratic Republic of the Congo. PhD Thesis, Dept Earth and Planetary Sciences, Macquarie University, 336 pp. Batumike, J.M., Cailteux, J.L.H., Kampunzu, A.B., 2007a. Lithostratigraphy, basin development, base metal deposits and regional correlations of the Neoproterozoic Nguba and Kundelungu rock successions, central African Copperbelt. Gondwana Research 11, 432–447. Batumike, J.M., O'Reilly, S.Y., Griffin, W.L., Belousova, E.A., 2007b. U–Pb and Hf-isotope analyses of zircon from the Kundelungu kimberlites, D.R. Congo: implications for crustal evolution. Precambrian Research 156, 195–225. Batumike, J.M., Griffin, W.L., Belousova, E.A., Pearson, N.J., O'Reilly, S.Y., Shee, S.R., 2008. LAMICPMS U–Pb dating of kimberlitic perovskite: Eocene–Oligocene kimberlites from the Kundelungu Plateau, D.R. Congo. Earth and Planetary Science Letters 267, 609–619. Batumike, J.M., Griffin, W.L., O'Reilly, S.Y., Belousova, E.A., Pawlitschek, M., 2009. Crustal evolution in the central Congo-Kasai Craton, Luebo, D.R. Congo: zircon U–Pb and Hf-isotope data. Precambrian Research 170, 107–115. Cahen, L., Snelling, N.J., Delhal, J., Vail, J.R., 1984. The Geochronology and Evolution of Africa. Clarendon Press, Oxford. 512 pp. Davis, G.L., 1977. The ages and U contents of zircons from kimberlites and associated rocks. Extended Abstract Volume, 2nd International Kimberlites Conference, Santa Fe, N.M. Demaiffe, D., Fieremans, M., 1981. Strontium isotope geochemistry of the Mbuji Mayi and Kundelungu kimberlites, Zaïre, central Africa. Chemical Geology 31, 311–323. Demaiffe, D., Fieremans, M., Fieremans, C., 1990. The kimberlites of central Africa: a review. In: Kampunzu, A.B., Lubala, R.T. (Eds.), Magmatism in Extensional Structural Setting: the Phanerozoic Africa Plate. InSpringer-Verlag, Berlin, pp. 538–557. Delhal, J., Ledent, D., 1973. Résultats de quelques mésures d'âges radiométriques par la méthode Rb/Sr dans les pegmatites de la Haute Luanyi, région du Kasai (Zaïre). Rapport Annuel, Musée Royal de l'Afrique Centrale, Tervuren, Belgique, Département de Géologie et Minéralogie (1972), pp. 102–103. Delhal, J., Liégeois, J.-P., 1982. Le socle granito-gneissique du Shaba occidental (Zaïre): Pétrographie et géochronologie. Annales de la Societé Géologique de Belgique 105, 295–301. Delhal, J., Ledent, D., Torquato, J., 1976. Nouvelles données géochronologiques relatives au complexe gabbro-noritique et charnockitique du bouclier du Kasai et son prolongement en Angola. Annales de la Societé Géologique de Belgique 99, 211–226. Fieremans, C., 1977. Mode of occurrence and tectonic control of the kimberlite bodies in East Kasai (Zaïre). Extended Abstract Volume, 2nd International Kimberlites Conference, Santa Fe, N.M.
Gaul, O.F., Griffin, W.L., O'Reilly, S.Y., Pearson, N.J., 2000. Mapping olivine composition in the lithospheric mantle. Earth and Planetary Science Letters 182, 223–235. Griffin, W.L., Ryan, C.G., Cousens, D.R., Sie, S.H., Suter, G.F., 1989. Ni in Cr-pyrope garnets: a new geothermometer. Contributions to Mineralogy and Petrology 103, 199–202. Griffin, W.L., Gurney, J.J., Ryan, C.G., 1992. Variations in trapping temperatures and trace element in peridotite-suite inclusions from African diamonds: evidence for two inclusion suites, and implications for lithosphere stratigraphy. Contributions to Mineralogy and Petrology 110, 1–15. Griffin, W.L., Ryan, C.G., Win, T.T., 1995. Mapping the Earth's mantle in 4D using the proton microprobe. Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 104, 456–463. Griffin, W.L., Fisher, N.I., Friedman, J., O'Reilly, S.Y., Ryan, C.G., 2002. Cr-pyrope garnets in the lithospheric mantle: II. Compositional populations and their distribution in time and space. Geochemistry Geophysics Geosystems 1073 (3), 12 (35 pp). Kampata, M.D., 1993, Minéralogie et géochimie des kimberlites du Haut Plateau de Kundelungu (Shaba, Zaïre). Doctorate Thesis, Catholic University of Louvain, Belgium, 248 pp. Kampunzu, A.B., Calteux, J., 1999. Tectonic evolution of the Lufilian Arc (central Africa Copperbelt) during the Neoproterozoic Pan-African orogenesis. Gondwana Research 2, 401–421. Kinny, P.D., Meyer, H.O.A., 1994. Zircon from the mantle: a new way to date old diamond. Journal of Geology 102, 475–481. Malkovets, V.G., Griffin, W.L., O'Reilly, S.Y., Wood, B.J., 2007. Diamond, subcalcic garnet, and mantle metasomatism: kimberlite sampling patterns define the link. Geology 35, 339–342. O'Reilly, S.Y., Griffin, W.L., 1996. 4-D lithosphere mapping: methodology and examples. Tectonophysics 262, 3–18. O'Reilly, S.Y., Griffin, W.L., 2006. Imaging global chemical and thermal heterogeneity in the subcontinental lithospheric mantle with garnets and xenoliths: geophysical implications. Tectonophysics 416, 289–309. Pivin, M., et al., 2009, this issue. Metasomatic mantle origin for Mbuji-Mayi and Kundelungu garnet and clinopyroxene megacrysts (Democratic Republic of Congo). Proceedings of the 9th International Kimberlite Conference. Lithos 112S, 951–960. Ryan, C.G., Griffin, W.L., Pearson, N.J., 1996. Garnet geotherms: pressure–temperature data from Cr-pyrope garnet xenocrysts in volcanic rocks. Journal of Geophysical Research 101, 5611–5626. Schärer, U., Corfu, F., Demaiffe, D., 1997. U–Pb and Lu–Hf isotopes in baddeleyite and zircon megacrysts from Mbuji-Mayi kimberlite: constraints on the subcontinental mantle. Chemical Geology 143, 1–16. Shee, S.R., Wyatt, B.A., Griffin, W.L., 1993. Major and trace element mineral chemistry of peridotite xenoliths from the Wesselton kimberlite, South Africa. International Association of Volcanology and Chemistry of the Earth's Interior General Assembly 1993, p. 98. Abstract Volume. Sobolev, N.V., Lavrent'ev, Y.G., Pokhilenko, N., 1973. Chrome-rich garnets from the kimberlites of Yakutia and their paragenesis. Contributions to Mineralogy and Petrology 40, 39–52. Stachel, T., Harris, J.W., 2008. The origin of cratonic diamonds: constraints from mineral inclusions. Ore Geology Reviews 34 (1–2), 5–32. Verhoogen, J., 1938. Les pipes de kimberlite du Katanga: Annales du Service des Mines. Comité Spécial Katanga 9, 1–49. Walraven, F., Rumvegeri, B.T., 1993. Implications of whole-rock Pb–Pb and zircon evaporation dates for the early metamorphic history of the Kasai Craton, Southern Zaïre. Journal of African Earth Sciences 16 (4), 395–404.