The Karoo large igneous province: Lithosphere versus mantle plume contribution

F. Jourdan^a,b,c*, H. Bertrand^b,  U. Schärer^a, J. Blichert-Toft^b, G. Féraud^a, A.B. Kampunzu^d,§

^aUMR-CNRS 6526 Géosciences Azur, Université de Nice-Sophia Antipolis, O6108 Nice, France

^bUMR-CNRS 5570, Ecole Normale Supérieure de Lyon  et Université Claude Bernard, 69364 Lyon, France

^cBerkeley Geochronology Center; 2455 Ridge Rd., Berkeley, CA94709, USA

^dDepartment of Geology, University of Botswana, Gaborone, Botswana

^*Present address: Western Australian Argon isotope facility, Curtin university of Technology & JdL Centre, GPO Box U1987, Perth, WA 6845, Australia, f.jourdan@curtin.edu.au

^§Deceased, 2004

Introduction

The lower Jurassic is marked by the emplacement of one of the largest continental flood basalts (CFB) on Earth (≥ 3 x 10⁶ km³). The ~180 Ma Karoo magmatic province is located in southern Africa, with minor patches in Antartica, and consists of tholeiitic lava flows, sills, and giant radiating dyke swarms emplaced prior to the breakup of southern Gondwana and the opening of the SW Indian Ocean and the Southern Ocean (e.g., Cox, 1988). It was emplaced between 174 and 185 Ma (Figure 1; Le Gall et al., 2002, Jourdan et al., 2004; 2005; 2007a; 2007b; 2007c; Jourdan et al., submited). It consists, at the present time, of widespread remnants of basalts (lava flows and sills) and giant dyke swarms (Figure 1).

Figure 1: Sketch map of southern Africa showing the distribution of the Karoo magmatism and related dyke swarms (modified from Jourdan et al., 2004 and references therein). ODS: Okavango dyke swarm; SLDS: Save-Limpopo dyke swarm; SBDS: South Botswana dyke swarm; SleDS: South Lesotho dyke swarm; UDS: Underberg dyke swarm; SMDS: South Malawi dyke swarm; RRDS: Rooi Rand dyke swarm; NLDS: north Lebombo dyke swarm; GDS: Gap dyke swarm; SDZ: sill-dense zone.

All geochemical investigations concur that enriched sub-continental lithospheric mantle (SCLM) contributed substantially to the Karoo CFB. Whereas some authors maintain that only the SCLM melted (e.g., Duncan et al., 1984; Ellam & Cox, 1989), others envisage that the SCLM may have mixed with asthenospherically (plume?) derived magmas (Ellam & Cox, 1991; Sweeney et al., 1994) with an ocean island basalt (OIB) -like signature (Ellam et al., 1992). Here we attempt to bridge this gap by presenting major and trace element abundances reported for 147 samples and Sr, Nd, Hf, and Pb isotope compositions reported for a 36 sample subset of basaltic lava flows, sills, and dykes from the Karoo CFB in Botswana, Zimbabwe, and northern South Africa (see Jourdan et al., 2007a for more details).

Overview of the geochemistry of the Karoo magmatic province

Geochemical studies conducted so far have focused mainly on the lava flows and a few dykes from the eastern and southern parts of the Karoo province and provide constraints on the origin and genesis of these magmas. The rocks can be subdivided into five groups: 

Our approach

A key point at issue concerning the origin of the Karoo CFB is whether the mantle sources involved reside within the subcontinental lithospheric mantle (e.g., Hawkesworth et al., 1984), are part of a mantle plume head (Campbell & Griffiths, 1990), or both (e.g., Sweeney et al., 1994). In the following discussion, we present scenarios involving:

1. heterogeneous SCLM, and
2. progressive mixing between a mantle plume and the SCLM.

Regardless of the scenario considered, a strong SCLM contribution is required.

Evidence for a major SCLM component

• Light ion lithophile element (LILE) enrichment and the presence of high field-strength elements (HFSE - Nb in particular) negative anomalies for both the low- and high-Ti basalts and picrites;
• the Nb anomaly is particularly pronounced in the picrites (Jourdan et al., 2007a), which cannot be suspected of having undergone substantial crustal contamination (Ellam & Cox, 1989; 1991);
• a SCLM component is supported by the composition of clinopyroxenes from lherzolite xenoliths sampled by kimberlites beneath the Kaapvaal craton, which display LREE enrichment and HFSE depletion (selective LILE/HFSE fractionation, strong Nb depletion, and the absence of Zr-Hf anomalies; Grégoire et al., 2003);
• a "wet" mantle source as shown by the occurrence of (i) biotite in early peridotite cumulates from the Mount Ayliff layered intrusion (related to the Karoo sill system - H. Bertrand, unpublished data), and (ii) amphibole and/or phlogopite in South African mantle xenoliths (Grégoire et al., 2003);
• the picrites and high-Ti lava flows have in common that they are the least radiogenic in Nd and Hf, the most radiogenic in ²⁰⁷Pb and ²⁰⁸Pb, and approach the isotopic signature of some extreme alkaline magmas (Figures 2c, e, f) such as lamproites and more particularly the Karoo nephelinites. This suggests that they must have first have been strongly enriched by subduction and metasomatic processes (e.g., Mitchell & Bergman, 1991) and subsequently isolated for a long time from mantle convection (Murphy et al.,  2002).

Figure 2. Initial (179 Ma) Sr, Nd, Hf, and Pb isotopic compositions of the Botswana Karoo rocks and plagioclase separates from rocks Bot0098 and P11-2 (open squares linked to their respective whole rocks by tie-line). Click here or on figure for enlargement.

Scenario I: The Karoo mantle sources are a heterogeneous lithospheric mantle

Our data can be interpreted in terms of heterogeneous metasomatised SCLM and do not require a chemical contribution from a mantle plume.

When the Botswana - Zimbabwe Nd-Sr isotopic data are plotted alongside those previously published (Georoc database) (Figures 2a & 3), most of the high-Ti lava flows and picrites form a well-defined trend from bulk silicate Earth (BSE), as also represented by some southern Africa SCLM xenoliths (e.g., Erlank et al., 1987), towards compositions with highly negative εNd_i – down to -22), similar to lamproites and nephelinites.

Figure 3. Initial ¹⁴³Nd/¹⁴⁴Nd vs. ⁸⁷Sr/⁸⁶Sr. Mixing curve (solid black line) end-members: nephelinite and ijolite both mixed with SCLM xenoliths. Dashed grey curve represents EC-AFC-calculated upper crustal assimilation of low-Ti rocks. Large symbols = present study, small symbols = Karoo Georoc data.

Compared to this main array, the high-Ti dykes and the low-Ti lava flows form another distinct trend departing from the same BSE-like end-member, but diverging towards enriched Sr isotope ratios. Beyond the effects of secondary processes, the samples with the least radiogenic Sr probably have source characteristics (in terms of Sr-Nd isotopes) close to those of other BSE-like high-Ti rocks. Therefore, most of the Nd-Sr isotopic range of the primary magmas (the solid trends on Figure 3) can be modeled by mixing between a BSE-like and a nephelinite-like component. They therefore could reflect the degree of heterogeneity of the SCLM.

However, in εNd_i-²⁰⁶Pb/²⁰⁴Pb_i space (Figure 2b), some source differences between the low- and high-Ti groups stand out. To get a better understanding of their origin, the εHf and Ba/La vs. Th/Yb diagrams (Figure 4) are useful, as these variables are reliable indicators of potential sediment or fluid contributions to arc-like sources (Woodhead et al., 2001). Figure 4 suggests that sediments contribute to the mantle source of the high-Ti magmas, whereas low-Ti magmas more likely originated from a source enriched by fluids.

Figure 4. Hf and Ba/La vs. Th/Yb. Solid vector: slab-derived fluids; dashed vector: sediment input in the source as represented and discussed by Woodhead et al. (2001).

Lithosphere architecture and the distribution of low- and high-Ti magmas

Griffin et al. (2003) recently provided a compositional and structural map in four dimensions of the lithospheric mantle underlying the Kaapvaal craton and the surrounding mobile belts (Figure 1), highlighting the strong vertical and lateral heterogeneity of the SCLM. One of the most striking features is strong geochemical depletion in the depth interval 120 - 180 km below the Limpopo belt.

The high-Ti rocks are restricted mostly to the Limpopo-Shashe mobile belt (Figure 1). This suggests that Limpopo-Shashe mobile belt structure may have been a controlling factor in the distribution of the high-Ti magmas.

According to our calculations (Jourdan et al., 2007a), the high-Ti magmas require garnet in their source (Figure 5), implying a deep (> 80 km) melting zone below the Limpopo-Shashe belt, as opposed to the shallow melting zone of the spinel-bearing mantle source generating the low-Ti magmas elsewhere (Figure 1 & 5). The depleted harzburgitic zone identified at 120 - 180 km depth below the Limpopo belt is unsuitable for producing significant amounts of melting. However, at depths greater than 200 km, the presence of strongly metasomatised peridotites has been documented (Griffin et al., 2003) and represents a relatively fertile zone for magma supply at the garnet-bearing base of the SCLM, which is 200 - 300 km thick (James et al., 2001).

Figure 5. (La/Yb)_n vs. (Eu/Yb)_n and (Sm/Yb)_n vs. (La/Sm)_n for the Botswana Karoo rocks and non-modal batch melting modeling of a lherzolitic mantle source. See details in Jourdan et al. (2007a).

As an example, in the ¹⁴³Nd/¹⁴⁴Nd_i vs. ⁸⁷Sr/⁸⁷Sr_i plot (Figure 3), the trend between the high-Ti picrites, enriched in Nd-Hf isotope space, and the BSE-like mantle source may be interpreted as follows:

In this scenario, we emphasize the importance of the strong vertical and horizontal heterogeneity of the SCLM, consistent with the general picture inferred from mantle xenoliths (Griffin et al., 2003).

Scenario II:  Mixing between lithospheric and sub-lithospheric (plume) mantle

Although this scenario is geologically feasible, we feel that its additional complexity (compared to Scenario I) makes it less likely. However, as it cannot be disproved, and in order to stay objective, we discuss it and leave the reader to draw his/her own conclusions about it.

As explained above, Karoo nephelinites and picrites are likely to represent an SCLM-enriched end-member. We now envisage an additional sub-lithospheric component with a  slightly depleted isotopic composition. Because of the chemical contrast between this component and the extremely incompatible-trace-element-enriched veined SCLM, mixing between these two end-members would be strongly affected (and masked) by the SCLM signature (e.g., Ellam, 2006).

The rocks with the compositions closest to this sub-lithospheric component are the high-Ti Okavango dykes and the low-Ti basalts. A gradually increasing contribution from a mantle plume component as the Sr and Nd isotopic compositions of the Karoo rocks approach BSE (Figure 3) also is in accordance with the observed decrease of the size of the negative Nb anomaly (from the picrites to the high-Ti dykes). The low-Ti basalts may represent the most "plume-like" end-member of the southern Africa Karoo suite (e.g., Figure 2f).

Mixing between the SCLM and a mantle plume component. In this scenario, Karoo magmatism is hypothesized to have started by the arrival of a mantle plume at the base of the SCLM near the Mwenezi area. The plume provided sufficient heat to melt the alkaline veins (see discussion in Scenario 1) at the base of the lithospheric mantle beneath the Limpopo belt (in the garnet-bearing lherzolite facies) and gave rise to the Karoo nephelinites by low degrees of melting. The subsequent progressive ascent of the mantle plume allowed the melting of both itself and the SCLM. This process generated first the high-Ti (low degrees of melting) and then the low-Ti (slightly higher degrees of melting; Figure 5) picrites. Because of the progressive thermal and mechanical erosion of the SCLM and/or lithospheric extension due to the ongoing rifting, the plume continued to rise and expand and contributed increasingly to magma production. The resulting mixtures are reflected in the magmatic evolution from high-Ti/high-incompatible-trace-element basalts over high-Ti basaltic dykes to high-Ti/low-incompatible-trace-element basalts as the plume ascended. An increasing contribution from the mantle plume with decreasing influence of the SCLM is suggested by the progressive reduction of the negative Nb anomaly.

In this Scenario, we propose two end-members represented by

1. the metasomatically enriched alkalic veins of the SCLM, and
2. a sub-lithospheric (asthenospheric- or OIB-like?) mantle plume.

Our data are consistent with a progressive increase of a mantle plume contribution as the plume rises and expands through a laterally and vertically heterogeneous lithosphere.

Conclusions

The isotopic variations and LREE/HFSE fractionation of both low- and high-Ti magmas suggest that a sub-continental lithospheric mantle (SCLM) component predominates in the Karoo magmatism. The Karoo magmatic sequence can be explained equally well by two contrasting scenarios between which the present data cannot distinguish unambiguously. In the first, the magmas are derived from a heterogeneous SCLM, metasomatically enriched in the form of veins (sediment input?) during ancient subduction events. The Karoo magmas document the polybaric melting from a deeper "nephelinite-veined" (the most incompatible-trace-element-enriched basaltic lava flows and picrites) to a shallower BSE-like (high-Ti dykes, low-Ti and the least incompatible-trace-element-enriched high-Ti lava flows) SCLM. In the second scenario, the different magma types reflect mixing between the veined SCLM proposed in the first scenario and a (OIB- or asthenospheric-like?) mantle plume. The contribution of the mantle plume to the Karoo magmas is minimal for the nephelinites and increases progressively as the plume rises and expands through a laterally and vertically heterogeneous lithosphere (i.e. increases from high-Ti picrites to low-Ti basalts).

The distribution of the high- and low-Ti magmas reflects strong control by lithospheric architecture. The high-Ti magmas are restricted to above the thick Limpopo belt lithosphere, whereas the low-Ti magmas are located on the thinner (and vein-free) Kaapvaal and Zimbabwe cratonic lithospheres. The occurrence of a low-εNd_i, low-εHf_i, and low-²⁰⁶Pb/²⁰⁴Pb_i mantle source (thick veined SCLM?) is possibly due to higher degrees of subduction-related metasomatism (and sediment input?) beneath the Limpopo belt.

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