The Paleoproterozoic (~2.056 Ga10, 24) RLS is the world’s largest layered mafic intrusive complex, containing ~600,000 km3 of mafic-ultramafic cumulates and extensive reserves of platinum-group elements (PGE), chromium and vanadium that dominate global resources of these elements21. The RLS intruded the 2.6–2.3 Ga sedimentary Pretoria Group and 2.061 Ga felsic lavas of the Rooiberg Group at upper crustal levels (~0.06–0.24 GPa)21, 25. In conjunction with the overlying Rashoop Granophyre and Lebowa Granite Suites, they together constitute the Bushveld Complex, comprising an enormous bimodal continental large igneous province in the Kaapvaal Craton (Fig. 2). The closest exposed analogues of Kaapvaal middle crust beneath the RLS are Archean basement amphibolite- to granulite- facies trondhjemitic-granodioritic-granitic gneisses, orthogneisses and metasedimentary rocks exposed in the Vredefort impact structure near Johannesburg26 and in the Southern Marginal Zone (SMZ) of the Limpopo belt to the north of the RLS27.
The RLS is shaped like a dinner plate about 7–9 km thick and ~400 km in diameter (Fig. 2), with moderately inward-dipping marginal zones and flatter-lying central portions. Based on lithological and geochemical investigations, the RLS is traditionally subdivided into five major and laterally continuous stratigraphic zones (Fig. 2)2, 21, 28: (1) the fine- grained, noritic to komatiitic Marginal Zone, which flanks the other zones outside the main layered series and includes a Basal Zone encountered only in drill core beneath the other zones29, (2) discontinuous trough-like bodies of ultramafic Lower Zone (LZ) comprising harzburgite, orthopyroxenite and minor dunite interlayers, (3) pyroxenitic Lower Critical Zone (CZL) and noritic Upper Critical Zone (CZU),, defined by the occurrence within both of them of prominent and laterally extensive chromitite and sulfide-bearing layers locally enriched in PGE, (4) gabbronoritic Main Zone (MZ),, (5) uppermost ferrogabbroic-noritic and dioritic Upper Zone (UZ) with abundant magnetite layers (Fig. 2). Moreover, the MZ is sub- divided into the Upper (MZU) and Lower MZ (MZL) via a prominent, 3 m-thick orthopyroxenite which marks a significant change of initial Sr isotopic ratios ((87Sr/86Sr)i) 23,28. The UZ is also sub-divided into three subzones by the first appearances of magnetite (UZa),, olivine (UZb) and apatite (UZc) (Fig. 2)30–31. It is worth noting that this broad zonal classification is oversimplified for its regional-scale utility, while the cumulate layers show numerous and complex mesoscale variations in their spatial distribution, e.g., intricate details of the lithological macrolayering in LZ, CZL and CZU (Fig. 2) which cannot be correlated regionally despite the apparent regional correlations implicit in the naming conventions used for the chromitite layers within them32. If the implicit regional correlation of the chromitites is correct, then its regional-scale uniformity is superimposed on a patchwork of locally variable layered cumulate rocks.
The overall stratigraphy of the RLS can be fitted into a two-stage pattern23 assuming that layers were deposited in sequence from bottom to top. The formation of the lower portion from LZ to MZL has been referred to as the integration stage (Fig. 2), recording multiple influxes and extreme oscillations of dramatically different magmas at the scale of individual macrolayers metres to tens of metres thick, as evidenced both by magmatic unconformities and sharp changes in lithology21, 33, and by the distribution of radiogenic isotopes, exemplified by (87Sr/86Sr)i 23. This is most clearly expressed in the CZU where anorthosite and norite locally alternate with harzburgite, pyroxenite and chromitite at scales of several metres. However, zircon within these contrasting macrolayers has been shown in some cases not to have crystallized in a sequence younging upward through the stratigraphic column, an observation that has called into question the notion of a continuously upward aggrading crystal pile at the base of a long-lived magma chamber10, 12, 33–34. The formation of the upper portion has been referred to as the differentiation stage (Fig. 2)23, recording relatively uniform parentage as shown by marked uniformity of radiogenic isotope ratios and trace element abundances against a backdrop generally considered to record a well-defined process of fractional crystallization in a large liquid-filled magma chamber with very few magma recharge events28, 31, 35–36, although some recent studies also attributed it to emplacements of several batches of magmas with constant isotopic compositions30, 37. The boundary between the integration stage and the fractionation stage is marked by the pyroxenite marker layer at the base of the MZU (Fig. 2), and the resulting composite upper body postulated to have crystallized from a large, quiescent magma body can be termed the “Upper and Upper Main Zone”, or UUMZ.
It has been presumed that the mafic sills of the Marginal Zone represent samples of the “parental” magmas that generated the RLS3, 25, 38. Cumulate rocks in the complex can be subdivided into plagioclase-rich mafic units that have been postulated to be crystallization products of tholeiitic magmas referred to as A-type magmas and ultramafic units that have been postulated to be products of high-MgO magmas referred to as U-type magmas39. The U- type magmas contained ~12 to 14 wt.% MgO, corresponding to quench-textured norites exposed in the Marginal Zone surrounding the LZ and CZL and referred to as the B1 marginal sills (Fig. 2)25, 38, 40. However, the most primitive olivine and orthopyroxene observed in LZ cannot be crystallized from the melts with the composition of the recognized B1 magma21, 41, and the newfound Basal Ultramafic Sequence beneath the Marginal Zone requires a komatiite as the true parental magma29. The Bushveld U-type magmas are compositionally similar to modern boninites formed by hydrous melting of metasomatized upper mantle38, but Barnes40 proposed a better analogue in the siliceous high magnesium basalts derived from the crustal contamination of komatiites in Archean greenstone belts.
The A-type magmas, thought to have contained ~7–8 wt.% MgO, are tentatively correlated with fine-grained gabbronorites of the Marginal Zone where it abuts the CZU and MZL, respectively termed the B2 and B3 tholeiitic magmas (Fig. 2)38–39. The origins of the A- type magmas have had less attention than that of the U-type, with most investigators apparently assuming that they are commonplace tholeiitic basaltic magmas somehow derived from the upper mantle. Since the mantle does not directly produce tholeiites containing such low MgO contents there must have been some processing of their parental magmas, though this process has not been clearly defined in the past1. B2 and B3 are also unconvincing parental magmas due to their partial cumulate characteristics and discrepant crystallization order compared to their interpreted cogenetic cumulates in the RLS21, 25. The bulk composition of the lower portion of RLS is too rich in compatible elements including Cr and the PGE to represent the composition of a liquid42 —it is necessarily regarded as being composed of cumulates deposited from larger volumes of through-going magma that are not presently exposed within the RLS.
The bulk composition of the parental magma that was injected to form the UUMZ was proposed by adding ~15–25% of a hypothetical missing segregated component into a weighted average UUMZ bulk composition, to form a basaltic andesite with ~5–6 wt.% MgO, but a modeled fractional crystallization sequence from this magma does not closely resemble the natural occurring cumulates28, 36.
Recently discovered spinifex olivine margins chilled at the base of LZ strongly argue for a komatiitic parent magma that underwent assimilation of the quartzitic floor22, 29. From this perspective, the remarkable similarities of trace-element features have led to suggestions that the B1 and B2–3 magmas were derived from komatiite via > ~40% contamination by upper and lower crust, respectively10, 22, 25. Sr- and Nd-isotopic data of cumulates were used to support the proposition that primitive melt assimilated ~15–30% partial melt of upper crust produce the LZ and CZL, whereas ~40–50% contamination with the depleted restite in staging chamber beneath RLS is required for the CZU and MZ43. Cr-enrichment and cyclic compositional reversals in the LZ have been attributed to episodic influxes of crystals+liquids slurries derived from komatiite contaminated by 20% crust at 4.5–10 kbar42. Consideration of the Cr budget during chromitite formation indicates that the parental liquids must have been komatiitic10, 20, 42. Contamination in deep-seated chambers before final crystal-slurry-type emplacement into the RLS was also proposed on the basis of stable and radiogenic isotope systems27, 44.
In contrast to the various suggestions of crustal contamination, radiogenic 187Os/186Os of sulfide45 and unradiogenic εHf of zircon from RLS46 have been used to suggest that the parental magmas were derived from ancient eclogite-bearing subcontinental lithospheric mantle (SCLM) without extensive crustal assimilation. A role for refractory SCLM was also proposed as a possible explanation for the exceptionally high Pt/Pd of Bushveld U-type magmas and mineral deposits47, but it must also be noted that a large degree of melt production is highly unlikely from relatively cool and previously melt-depleted SCLM48.
Since the alleged SCLM Os isotopic signature could equally well be derived from crustal contaminants49, and melt-rock reaction by asthenospheric melts while they pass through refractory SCLM might affect PGE distributions, in the balance we favor the idea that the massive and very short-lived injection of magma that formed the RLS resulted from rapid melting of an asthenospheric mantle plume.
Considering the need for a komatiitic parental magma for the RLS, we suggest that rather than representing samples of the magmas parental to the RLS, the sills preserved in the Marginal Zone may instead be samples of magma that had already passed through the complex, depositing layered cumulate rocks within the RLS before their eventual expulsion into the surrounding Pretoria Supergroup10. Regarding the marginal zone magmas as the complements to the cumulate rocks rather than as their parents alleviates some of the more serious mass balance concerns.
We have considered two distinct scenarios to address the possible origins of the “integration stage” cumulates beneath the base of the UUMZ and a third for the UUMZ, illustrated in Figure 3. In Scenario 1, following the one-stage ABC approach we have already successfully applied to several ultramafic suites worldwide10, 18–20, komatiite is combined with a crustal assimilant in an isenthalpic process, creating a relatively cooler equilibrated mixture of liquid and crystals which then undergoes some degree of cooling while remaining internally at equilibrium. Given the extreme low viscosity (~0.05–0.2 Pa·s), ascent rate as great as m/s and high liquidus temperature (>1550°C) of komatiite16, its emplacement into cooler host rocks (~200–300°C) approximates to forced turbulent convection, where solids are passively advected by chaotic flow and remain in suspension (Fig. 1b)15–16. This stage represents a single batch process of assimilation and cooling during transport through the lithosphere. The mixture is then separated by gravity into a cumulate comprising mostly solids and some trapped liquid, and a supernatant magma comprising mostly liquid and some entrained solids51. This next stage represents the intrusion of the mixture into a sill-like body at the level of the RLS and the resulting dumping of most of the entrained crystal load to form a macrolayer, with internal layering analogous to the stratifications of suspended sediment in water in upper plane bed flow regimes. These successive sill-like magma pulses can be vertically stacked to build up a thick layered pluton based on field observations, geophysical data and numerical models52. Hence, the model cumulate in this stage is compared with ultramafic cumulate rocks of the RLS, and the supernatant magma leaving the system is compared with B1 marginal sill compositions.
In Scenario 2, representing two successive batch steps, it is assumed that a first ABC process occurs in the mid-crust, after which the supernatant liquid rises and undergoes a second batch crystallization as it cools and is emplaced at the level of the RLS to form a mushy macrolayer (Supplementary Fig. 2). During emplacement, this new batch of crystals and melt then separates into cumulate comprising mostly solids and some trapped liquid to represent part of the RLS, and a supernatant magma comprising mostly liquid carrying some entrained crystals that can be compared with the marginal sills. Compositions of solids, liquids, cumulates and marginal sills are shown in Figure 3c, d. This second scenario is therefore a sequence of two batch equilibrium processes which allows for the separation of hidden cumulates from the bulk mixture prior to magma ascent and deposition of the cumulate layer in the RLS—it might be considered as the first step along a continuum of possible process toward AFC.
In Scenario 3, the assimilation and batch removal of crystals occur in a large number of small steps (e.g., 20 to 50 steps) that approximate to AFC as classically understood, presumably occurring within a complex lower-crustal magma reservoir that may have comprised multiple interconnected sill- and dike-like bodies largely composed of mush4–6.
Liquid that has been processed through this AFC mush zone is extracted and emplaced into a large sill-like magma chamber where it subsequently evolves by fractional crystallization, subject to some subsequent magma replenishment events during the formation of UUMZ31.
The parameters used in the models are provided in Supplementary Tables 1–4.
Compositions of endmember magmas, contaminants, solids, liquids, cumulates, and ejected magmas are all shown in Figures 3, 4 and 6. The mantle-derived melt is an Al-undepleted komatiite10. Major and trace element and Sr, Nd, and O isotopic compositions of the magmas and contaminants were estimated by comparison with upper crustal and mid-crustal rocks exposed in the Pretoria Supergroup, Vredefort impact structure, and Limpopo Belt as documented in detail in Supplementary Table 4 and Supplementary Figures 3 and 4.
Ultramafic cumulates of the LZ, CZL and B1 marginal sill compositions were modeled under Scenario 1, assuming an upper crustal assimilant at 0.2 GPa, following a previous Scenario 1 model for the UG2 pyroxenite of the CZU and complementary B1 magma10. After assimilation of 17.36% upper crust, LZ dunite could form as an adcumulate comprising 4.5% trapped liquid; LZ harzburgites require 22.48% assimilation and are modeled as mesocumulates comprising 17.8% trapped liquid, whereas LZ and CZL pyroxenites could have formed after 27 to 34% assimilation of upper crust to leave a cumulate containing ~15% trapped liquid (Supplementary Table 1 and Supplementary Fig. 1). The B1 magma is modeled as a mixture of 22% solids equivalent to CZL pyroxenite with 78% liquid. The trace- element compositions of these cumulates and B1 marginal sills coincide well with the modeled results (Figs. 4a-b). Because the B1 marginal sills that envelope the LZ and CZL of the RLS range in thickness from 100–400 m and can penetrate ~100 km into the floor rocks38, their total volume may be regarded as supernatant magmas complementary to emplacement of all of the LZ and CZ pyroxenites.
Mafic rocks of the noritic portions of the CZU and gabbronoritic MZ are modeled under Scenario 2, with the same komatiite parent melt but a mid-crustal assimilant at 0.45 GPa. The corresponding temperature of the contaminant was estimated as 390°C via the geothermal model for continental lithosphere and a higher heat flow for the Paleoproterozoic RLS (~70 mW·m–2) than its current value (51±6 mW·m–2)53. The noritic CZU has a xenolith- rich contact sequence with LZ, and has widely been regarded as an independent sill-like intrusion of progressive mixtures between B1 and B2/B3 magmas21, 25. The similar crystallization sequence of MZL also requires mixed parental magmas that intrude as crystal slurries from a deeper, staging reservoir after crustal assimilation21. We envision that their primitive komatiites experienced ABC assimilation (~21% for CZU and ~24% for MZL) and cooled to ~1240–1250ºC in the middle crust to obtain “Bulk 1” compositions (Figs. 3c, d and Supplementary Fig. 2). Retention of ~90–97% of the solids at the site of assimilation left hidden ultramafic cumulates (compositions not shown) in the middle crust with compositions very similar to the LZ pyroxenites (Fig. 3). The remaining solids and liquid (“Bulk 2” in Figs. 3c, d and Supplementary Fig. 2) were cooled by conduction during ascent and then separated at the level of the RLS into mafic cumulates and ejected supernatant magmas very similar to the B2 and B3 marginal sills. After 40% crystallization at 1181 °C adcumulate norite in the CZU contains only 5% trapped liquid; its ejected liquid complement with only 5% solids resembles the B2 magma apart from the depletion of Rb and Th in the B2 composition (Figs. 3 and 4c). After 63.9% crystallization at 1130 °C the modeled MZL magma settles to form a mesocumulate containing ~3% trapped liquid and is flanked by marginal B3 magma that is ejected at a relatively low crystallization degree (~34.2%) and contains ~42% solids (Figs. 3 and 4d). Relative slow cooling and accumulation of crystals may further work on the B3 rocks that are coarser grained than B1–2 and have the lowest trace-elements concentrations (Fig. 4).
Scenario 3 is applied to the genesis of the UUMZ. The occurrence of numerous titanomagnetite layers within UUMZ (Fig. 2) indicate that the incoming parental magma was iron-rich28, 36, and hence mafic lower crust is favored as the assimilant54. We suggest that slow rates of introduction of primitive magma into hot lower crust (assumed as 770ºC at 1 GPa due to the heat flow of ~70 mW·m–2) after passage of the vast volumes of magma that produced the Main Zone, combined with muted temperature gradients, might have permitted efficient crystal separation during ongoing crustal assimilation15 (Fig. 1a), in contrast to the vigorous forced convection that favored crystal entrainment during formation of U-type magmas at shallow depth. We modelled the genesis of the parental magma of the UUMZ (Fig. 3) by a process of 43.5% AFC contamination in the lower crust plus a further 24% fractional crystallization (FC) during slow upward ascent. After emplacement of this magma in the upper crust, the observed paragenetic sequence and mineral modes of the cumulate rocks (except possible liquid-immiscibility-induced titanomagnetite28, 55) can be reproduced via a closed-system FC model until ~21% melt remains (Fig. 5a). It is debatable whether the final residual liquid was then erupted to form the upper portions of the Rooiberg felsites36, but resolution of this controversy is not material to the success of our models because they focus on magma sources, not on their final residues. Compositional variations of major minerals throughout the UUMZ are fitted if the trapped-liquid-driven compositional shift is included (Fig. 5), but also exhibit a series of minor reversals driven by several batches of magma replenishment, which may have resulted in the formation of magnetite layers31, 37.
The largest reversals of plagioclase An% (An45 to An51, Fig. 5b), clinopyroxene Mg# (~26- 53, Fig. 5c), orthopyroxene Mg# (~27–42, Fig. 5d) and olivine Fo% (Fo6 to Fo29, Fig. 5e) across the boundary between cycles V and VI, for instance, can be modeled by a small-scale (~1.2%) magma replenishment (Fig. 5).