The highly magnesian dike rocks of Vestfjella (western Dronning Maud Land, Antarctica): implications for sublithospheric mantle sources and the origin of the Karoo large igneous province

Jussi S. Heinonen¹ & Arto V. Luttinen²

¹Department of Geosciences and Geography, P.O. Box 64, University of Helsinki, 00014 Finland, tel: +358-9-191-50802; jussi.s.heinonen@helsinki.fi

²Finnish Museum of Natural History, P.O. Box 17, University of Helsinki, 00014 Finland, tel: +358-9-191-28745; arto.luttinen@helsinki.fi

Introduction

The Karoo large igneous province (LIP) represents a huge (original volume ~2 x 10⁶ km³; Richards et al., 1989) manifestation of continental flood basalt (CFB) volcanism associated with the breakup of the Gondwana supercontinent at~180 Ma (Figure 1). The remnants of this province are mainly found in southern Africa, but they are also exposed in the ice-free cliffs of western Dronning Maud Land, Antarctica (Figure 1). The mantle sources of the Karoo LIP have been extensively studied, yet scientific consensus has not been reached. A mantle plume model, first suggested for Karoo by Burke & Dewey (1972) (cf. Richards et al., 1989), has recently been supported based on paleostress estimates (Curtis et al., 2008) and findings of high-T magnesian rocks (Riley et al., 2005), although structural analyses, geochemical affinities, and temporal relationships indicate a major role for continental lithosphere in Karoo magmatism in general (e.g., Hawkesworth et al., 1984; Cox, 1988; Ellam & Cox, 1989; Luttinen et al., 1998; Luttinen & Furnes, 2000; Jourdan et al., 2004, 2005, 2006, 2007a, 2007b). Volcanic rocks that show unambiguous compositional indications of sublithospheric mantle sources are rare (Duncan et al., 1990; Luttinen et al., 1998) and seem to postdate the main Karoo phase by ~10 Ma (Jourdan et al., 2007b). These observations have inspired some to suggest that sublithospheric sources are not needed at all and that the primary melts of the Karoo CFBs were invariably derived from lithospheric mantle (e.g., Scenario 1 of Jourdan et al., 2007a; Ed: see also other pages on the Karoo). The involvement of a possible mantle plume, however, has been difficult to address without compositional constraints on the sublithospheric mantle beneath Karoo LIP. Here we summarize the results on recently discovered highly magnesian dike rocks from  Antarctica and discuss the implications of our findings to Karoo magma sources (Heinonen & Luttinen, 2008, 2010; Heinonen et al., 2010).

Figure 1: Distribution of Mesozoic CFBs in reconstructed Gondwana supercontinent. In the case of the Karoo province, the known extent of intrusive equivalents (found outside CFBs) is also shown. EM=Ellsworth-Whitmore Mountains, TI=Thurston Island. See Heinonen et al. (2010) for references.

Highly magnesian dike rocks of Vestfjella

As is typical of the Karoo LIP, the CFBs of western Dronning Maud Land are characterized by significant geochemical heterogeneity that has been attributed to contamination by (or derivation from) the lithosphere and/or variably subduction-influenced upper mantle sources (Luttinen et al., 1998; Luttinen & Furnes, 2000). The crosscutting dike rocks of Vestfjella, however, include some of the most extraordinary rock types of the Karoo LIP. These are remarkably Mg- and Fe-rich ferropicrites (FeO_tot = 13–17 wt. %; MgO = 13–19 wt. %; Heinonen & Luttinen, 2008) and meimechites (FeO_tot = 13 wt. %; MgO = 19–28 wt. %; Heinonen & Luttinen, 2008, 2010).

Ferropicrites

The Vestfjella ferropicrites consist of two distinct geochemical types (Heinonen & Luttinen, 2008; Heinonen et al., 2010):

1. The depleted type exhibits geochemical MORB-affinities with depletion of the most incompatible elements, very high initial εNd of +7.2 to +8.0, and low initial ¹⁸⁷Os/¹⁸⁸Os of 0.1263–0.1277 at 180 Ma (Figures 2 and 3). In fact, the isotopic signature of the depleted type is indistinguishable from that of MORB of the SW Indian Ridge (Figure 3), the modern successor of the Jurassic Africa-Antarctica rift (cf. Figure 1).
2. The enriched type (including a Fe-Mg-rich basalt) exhibits trace element characteristics similar to those of OIB (e.g., high Nb/Y), high (Sm/Yb)_N (4.9–5.4), moderately high initial εNd of +1.8 to +3.6 and high initial ¹⁸⁷Os/¹⁸⁸Os of 0.1401–0.1425 at 180 Ma (Figures 2 and 3). Isotopic compositions and trace element characteristics indicate that both magma types have avoided significant interaction with the continental crust (Figure 3). Preliminary age data indicates that the depleted type is related to the main phase of Karoo magmatism at ~180 Ma ago (Kurhila et al., 2008;  Luttinen et al., in prep.).

Figure 2: Geochemistry of the Vestfjella ferropicrites and associated rocks shown in primitive mantle-normalized incompatible element diagram (a) and logarithmic Nb/Y vs. Zr/Y diagram (b). Compositions of N-MORB and OIB are shown for comparison. In addition, SW Indian Ridge MORB, representative lithosphere-signatured Karoo CFBs, and rare Karoo-related rocks that exhibit MORB-affinities shown in b. The Iceland Plume Array in b indicates the composition of Nb-enriched Icelandic basalts that have been interpreted to derive from a mantle plume (Fitton et al., 2003). P and F arrows denote the effect of increasing pressure and degree of melting, respectively (cf. Fitton et al. 1997). LC arrows denote the effect of lithospheric contamination with compositions that are considered to be representative of the Gondwanan lithosphere (cf. Heinonen et al., 2010). See Heinonen et al. (2010) for references. Click here or on figure for enlargement.

Figure 3: Initial εNd vs. ⁸⁷Sr/⁸⁶Sr (a), εNd vs. ¹⁸⁷Os/¹⁸⁸Os (b), εNd vs. ²⁰⁶Pb/²⁰⁴Pb (c), ⁸⁷Sr/⁸⁶Sr vs. ²⁰⁶Pb/²⁰⁴Pb (d), ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb (e), and ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb (f) of the Vestfjella samples in comparison with isotopic data for Karoo CFBs and SW Indian Ridge MORBs (N-MORB in b). DM denotes average depleted MORB mantle of Workman & Hart (2005). All data calculated at 180 Ma. In the case of MORB and DM, the Sr, Nd, and Pb isotopic compositions of the mantle sources were back-calculated at 180 Ma using mantle reservoir composition recommended by Workman & Hart (2005). Os isotopic compositions of N-MORB were back-calculated by using ¹⁸⁷Re/¹⁸⁸Os ratio of 0.06. LC denotes the effects of lithospheric contamination. Details of the contamination models and references are given in Heinonen et al. (2010). Click here or on figure for enlargement.

Meimechites

The Vestfjella meimechites are typified by highly abundant olivine phenocrysts with remarkably high forsterite contents (up to Fo₉₂) in some of the samples (Heinonen & Luttinen, 2010). We have estimated the parental melts of these meimechites to contain up to 25 wt. % of MgO and the presence of igneous amphibole implicate relatively high H₂O contents of 1–2 wt. % (Heinonen & Luttinen, 2010). The Vestfjella meimechites show similar geochemical and isotopic characteristics to the depleted type ferropicrites and are likely to be representatives of the depleted type parental melt compositions (Heinonen & Luttinen, 2010; Heinonen et al., 2010).

Sublithospheric mantle sources for Karoo magmatism

The isotopic similarity between the depleted ferropicrites, meimechites, and SW Indian Ridge MORB suggests broadly similar upper mantle sources for these magma types. The geochemical differences, e.g., the higher Sm/Yb, Nb/Y, and Zr/Y in the Vestfjella depleted types, can be largely attributed to different degrees and pressures of initial melting (Figure 2; Heinonen et al., 2010). We regard the meimechite parental melts to represent low-degree peridotite melts (≥ 3%) generated at considerably high temperatures (T_p > 1600°C) and pressures (~5–6 GPa) (Heinonen & Luttinen, 2010). In Vestfjella there are also depleted basaltic dikes that are in many respects geochemically intermediate between the meimechites and the modern SW Indian Ridge MORB (Luttinen et al., 1998; Heinonen et al., 2010). Our modeling implicates that these dikes could represent uncontaminated parental melt compositions of many Vestfjella lavas (Luttinen & Furnes, 2000; Heinonen, in prep) suggesting that the depleted  upper mantle source of the Vestfjella ferropicrites and meimechites have been involved in the petrogenesis of at least some of the Karoo CFBs. It is important to note that although this source could be generally considered as “ambient upper mantle”, minor enrichments in, e.g., LILE and ⁸⁷Sr/⁸⁶Sr in the Vestfjella depleted types and SW Indian Ridge MORB infer the presence of diluted recycled lithospheric components in the mantle source (Figure 3a; Rehkämper & Hofmann, 1997; Janney et al., 2005; Heinonen & Luttinen, 2008; Heinonen et al., 2010).

In contrast, the OIB-like compositions of the enriched type ferropicrites are suggestive of derivation from recycled or melt-metasomatized pyroxenite-bearing portions of either lithospheric or sublithospheric upper mantle (Heinonen et al., 2010). Based on just isotopic compositions, the enriched type could have been generated by lithospheric contamination of depleted type magmas (Figure 3). The overall geochemical compositions, such as higher FeO_tot, Ti/Zr, and Nb/Y of the enriched type, are not readily compatible with such a scenario, however (Figure 2b; Heinonen et al., 2010).

Implications for the origin of the Karoo CFBs

Given that the depleted type ferropicrites and meimechites are coeval with the main phase of Karoo magmatism, the high mantle temperatures (T_p > 1600°C; T_ex ≈ 200°C) required by the meimechites indicate a significant thermal anomaly in the sub-Gondwanan upper mantle at 180 Ma (Heinonen & Luttinen, 2010). Exceptionally high mantle temperatures (i.e. hotspots) have been commonly ascribed to mantle plumes. In the case of the depleted type ferropicrites and meimechites of Vestfjella, a plume model would require the source to be quite exceptional in being isotopically and also chemically akin to ambient upper mantle. Furthermore, we consider that generation of such a source by entrainment of ambient upper mantle material into a plume head is unlikely given the very high temperatures involved and the absence of other high-T Karoo magmas that would be expected to form within a plume (Figure 4a). The internal mantle heating model seems to provide a more feasible explanation for the combination of high temperatures and upper mantle geochemical signature of the depleted type ferropicrites and meimechites (Figure 4b; Gurnis, 1988; Coltice et al., 2007, 2009; Heinonen et al., 2010). In comparison, the enriched type ferropicrites are compatible with both plume and internal mantle heating models: they are compositionally similar to oceanic hotspot magmas postulated to be associated with plume activity, but they could have been derived equally well from enriched portions of heterogeneous sub-Gondwanan upper mantle (Figure 4).

Figure 4: Schematic presentations of a plume model (a) and an internal mantle heating model (b) to explain the generation of the Karoo CFBs. Circles 1 and 2 denote the sources of the enriched and depleted type magmas, respectively. T_ex denotes the approximate maximum temperature difference between the heated areas and the ambient mantle: values in a after Putirka (2008) and in b after Coltice et al. (2007). Click here or on figure for enlargement.

Further considerations

Although our findings mainly lend support to the internal mantle heating model (Coltice et al., 2009), there are some important ambiguities with regard to the implications of this result for Karoo magmatism in general.

First, the model temperatures calculated in the internal mantle heating model of Coltice et al. (2007) barely reach 1600°C in a single supercontinent model, whereas temperatures of  >1600°C seem to be required for the generation of Vestfjella meimechites (Heinonen & Luttinen, 2010). Although the model temperatures are based on a limited number of variables and thus should not be considered to be definitive, we raise the question whether an additional heat source was required, possibly related to the purported lower mantle-sourced large low-velocity province below Africa (Torsvik et al., 2006; cf. Anderson, 1982).

Second, the cause for the relative enrichment of water and V and the depletion of P (cf. Figure 2) in the sources of the Vestfjella meimechites and ferropicrites is unknown (cf. Heinonen et al., 2010). There are indications, however, that the V and P anomalies could also be related to exceptionally high-pressure conditions of initial melting (cf. Heinonen et al., 2010), but more detailed analysis is hampered by the lack of experimental data on V and P solid/liquid partition at high pressures.

Finally, although we regard the depleted type source to be responsible for many CFBs of Vestfjella, it is presently unclear whether this mantle source was significantly involved with the generation of the majority of Karoo CFBs of southern Africa. It is quite possible that the bulk of the Karoo CFBs were produced in the thick, heated SCLM (cf. Jourdan et al., 2007a) and that sublithospheric sources were preferentially sampled below the opening rift between Africa and Antarctica.

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