Parental melts to stratiform anorthosites in the Bushveld Complex

Our novel hypothesis implies that parental magmas for the anorthosites in the Bushveld Complex are alumina-rich basaltic to basaltic andesitic melts which were derived as residual melts from deep-seated staging chambers (Supplementary Materials). These melts are produced there by fractional crystallization and crustal contamination of primary basaltic to komatiitic magmas. Both these processes tend to increase SiO 2 , Al 2 O 3 , CaO and Na 2 O and decrease MgO in the residual melts. The residual melts are considered to be saturated in orthopyroxene at high pressure in deep-seated staging chambers but become saturated in plagioclase when transferred into and cooled within a shallow-level Bushveld magma chamber at lower pressure (Fig. 6a). It is not possible to define the exact composition of these melts from the solid products of their crystallization (i.e. anorthosite cumulates), but one can define a range of potential melts that may behave in the way predicted by our hypothesis. For illustration purposes, we have used above a basaltic andesitic melt with 20 wt.% Al 2 O 3 (Table 1) although the compositions with the lower alumina content (down to 15–16 wt.% Al 2 O 3 ) show a similar behaviour. In general, these melts can be regarded as analogues to the so-called A 0 melt (~18 wt.% Al 2 O 3 )30 – the high alumina basalt that was postulated as parental to most of the anorthosite and two-pyroxene gabbro in the Critical and Main Zones of the Bushveld Complex31,35. The composition of this A 0 melt has been estimated from samples of fine-grained two-pyroxene gabbro in marginal facies of the Critical Zone ranging in composition from ~14 to ~19 wt.% Al 2 O 3 35. The gabbro consists of plagioclase, clinopyroxene, and orthopyroxene, with many of the plagioclase crystals being arranged in weak radial growths indicating their quench crystallization31. Importantly, a low-pressure experimental study of phase relations in one of these gabbro compositions (~15.5 wt.% Al 2 O 3 ) showed plagioclase as the first liquidus phase31.

Figure 6 A physical model for the formation of stratiform anorthosites from basaltic andesitic melts saturated in plagioclase only due to reduction in lithostatic pressure. (a) Basaltic andesitic melts ascending from lower crustal storage regions experience reduction in lithostatic pressure. This results in expansion of plagioclase stability volume and, as a result, some basaltic andesitic melts saturated in orthopyroxene at high-pressure regions may become, after some cooling, saturated in plagioclase alone at lower pressure in shallow chambers. Early and almost perfect fractional crystallization of these plagioclase-saturated melts, in an open system where magma can also flow out of the chamber, will produce monomineralic layers of anorthosite. (b) Phase relations for a basaltic melt in P-T space illustrating the model that basaltic andesitic melts saturated in orthopyroxene first become slightly superheated during their ascent and then saturated in plagioclase alone after stalling and cooling in shallow-level chambers. Therefore, allowing for the development of stratiform anorthosite in shallow-level chambers. The phase diagram is simplified from Fig. 5 and is used to graphically illustrate the principle lying at the heart of our model. (c) A dense and superheated melt entered the chamber and spread across the floor of the chamber as a basal layer causing local rupturing of the chamber floor and formation of pull-apart structures. The melt caused thermochemical erosion of cumulates at the temporary floor of the chamber, widening pull-apart structures into sub-rounded potholes. (d) On cooling, the melt crystallized plagioclase in situ, i.e. directly on the irregular erosional surface, including potholes. Near perfect anorthosite adcumulates form owing to exchange between thin boundary mushy layer and overlying melt via chemical diffusion aided by convection in the chamber. Full size image

Our thermodynamic modelling using alphaMELTS shows that the capacity of alumina-rich basaltic/basaltic andesitic melts to crystallize plagioclase as a first liquidus phase is sensitive to their chemical composition. In addition, even the melts with the ‘correct’ composition have a rather small stability field of plagioclase alone crystallization (Fig. 5, yellow area). All this suggests that this type of plagioclase-saturated melts is unlikely to be prevalent among those arriving from a deep-seated magma chamber. The prediction is consistent with the observation that anorthosites in the Bushveld Complex and other layered intrusions are much less common compared to other cumulates (e.g. norite, gabbronorite, orthopyroxenite). It seems therefore that only some of the ascending melts may reach plagioclase-only saturation: those richer in MgO compared to our initial melt composition (Table 1) will still remain saturated in orthopyroxene. These orthopyroxene-only-saturated melts can be parental to orthopyroxenites that are so common in the UCZ of the Bushveld Complex. To explain the sequences with alternating orthopyroxenites and anorthosites in the UCZ, one can envisage a scenario in which the evolution of residual melt in the staging chamber by fractional crystallization and crustal assimilation (making it more evolved) was complicated by rejuvenating influxes of mantle-derived magmas (making it more primitive). In this case, the residual melt escaping from the staging chamber at different stages of its back-and-forward changes in composition, may show slightly varying SiO 2 , Al 2 O 3 and MgO contents. These small differences in the melt composition can drive the crystallization of either plagioclase or orthopyroxene as a first liquidus phase producing alternating stratiform layers of orthopyroxenites and anorthosites in the Bushveld chamber.

A deep-seated staging magma chamber to the Bushveld Complex

Our concept implies the existence of a large underlying chamber that supplies the residual melts into the overlying Bushveld chamber. It also requires the continental crust to be thick enough to ensure a large distance between the deep-seated and shallow chambers so that the residual melts would experience a substantial pressure drop when travelling from one chamber to another. The deep-seated staging chamber beneath the Bushveld Complex has long been postulated by petrological models34,36,37,38 but has only recently been imaged using seismic and gravity data39. The remnants of the staging chamber of ~10 km in thickness were identified at a depth of ~40–45 km, with its base coinciding with the Moho discontinuity39. Considering that the Bushveld chamber was emplaced at a depth of ~5 km, it leaves ~25–30 km for the melts to travel from the deep-seated staging chamber towards the shallow Bushveld chamber. This depth difference translates into a pressure drop of ~7.5–9.0 kbar (3.3 km = 1 kbar) which is in the range of a pressure decrease predicted by our model (Fig. 5). It should be noted in passing that in our model a decompression can be anywhere from 10 to 2 kbar. For instance, a melt from a pressure region of about 5 kbar having the bulk composition (liquid and crystals) as in Fig. 5 will reach the field of plagioclase alone saturation by a pressure drop of only 3 kbar.

The basal magma emplacement and thermochemical erosion of floor cumulates

The new melts from the deep-seated staging chamber are envisioned to enter the Bushveld chamber as dense basal flows (up to a few hundreds meter thick) along the chamber floor with little to no mixing with the overlying resident melt saturated in orthopyroxene. The plagioclase-saturated melts are considered to be denser than the resident orthopyroxene-saturated ones because the latter are normally richer in normative quartz30. In addition, the resident melt is expected to be lighter because of being richer in water, which dramatically decreases the density of silicate melts40. The basal emplacement of new melts is fully consistent with intensive erosion of floor cumulates which is only possible if the inflowing melts come in direct contact with the rocks of the chamber floor21,41. Some mixing of new melts with the resident melt will likely still occur upon entry into the chamber and this may increase the initial superheating of the replenishing melt30,42,43. We believe, however, that the magma mixing is unlikely to be significant because otherwise it would drive the composition of the hybrid melt outside the plagioclase alone stability field making the formation of anorthosites impossible. Alternatively, one can argue that only a small portion of the melts which were fortunate to escape the mixing with the resident melt were able to crystallize the stratiform anorthosites.

The basal flows of new replenishing melts are in thermal and, most importantly, chemical disequilibrium with the floor cumulates in a shallow-level chamber and are therefore highly reactive. Thermal disequilibrium is due to melt superheating that can be up to 90 °C for the melt rising from the deep-seated reservoir near adiabatically (Fig. 5). In reality, some cooling of rising melts will inevitably occur so that upon arrival into a shallow-level chamber they may be much less superheated (Fig. 6b). Simple calculations illustrate, however, that even at a low degree of superheating (say, 5–10 °C) a few hundreds meters’ thick melt column is capable of eroding thermochemically a couple of dozens of meters of pre-existing footwall rocks21,41,44. This is because a major agent of the erosion is not the superheat itself but rather chemical disequilibrium between the new melts and floor cumulates: the predominant erosional process is dissolution rather than melting of the floor. Melt superheating is still important because the non-superheated replenishing melts are expected to start crystallizing soon after their arrival into the chamber and will therefore form a new layer of rocks to cover the pre-existing floor cumulates and shut down the erosion of the floor cumulates. For this reason, the field relationships (Fig. 1) require the replenishing melts to have been superheated, otherwise it would not have been possible to produce potholes by erosion21,41,44.

There is yet another important aspect of the erosion process to consider. The erosion can be effective only if the melt released by dissolution of footwall rocks convects away from the surface where their melting is taking place. However, the melt produced by dissolution of orthopyroxenite is expected to be denser than the overlying melt. There is thus a danger that this dense melt will pond at the floor of the chamber and terminate further erosion of orthopyroxenites. One way in which dissolution may still operate effectively is if the floor is inclined and the melt released by dissolution flows away downslope. Some petrological studies45 do provide evidence for a sloping temporary floor to the chamber of several degrees at the time of the formation of the Critical Zone, thus eliminating the physical obstacle for thermal/chemical erosion of the pyroxenitic footwall rocks.

The transgressive relationships between anorthosites and their footwall cumulates in the studied pothole (Fig. 1) can thus be attributed to thermochemical erosion at the initial point of weakness (such as a pull-apart structure) likely produced by rupturing of the floor cumulates due to the load of replenishing magma45 (Fig. 6c). Note, however, that the erosion and contamination of the melt by footwall cumulates is not considered here as a trigger for saturation of the melt in plagioclase. Just like magma mixing, the substantial contamination of a new melt by footwall orthopyroxenite will tend to decrease its capacity to crystallize anorthosite due to driving its composition outside the plagioclase-alone stability field. A simple way to avoid this is to suggest that the melt that crystallized monomineralic anorthosite is not the same one that previously eroded the footwall. It is quite conceivable that the initial melt that caused the erosion may have been flushed away by new batches of melt entering the chamber. Yet, another possibility to avoid assimilation (not related to the studied case), is to suggest that some new melts may lose most of their superheat during the transcrustal ascent and will therefore start crystallizing anorthosites immediately after their arrival into the chamber. Some anorthosite layers with no obvious erosional relationships19,28 with their footwall rocks can be probably attributed to the formation from such non-superheated melts.

Formation of monomineralic anorthosite at the chamber floor by adcumulus growth

After some cooling, the melt started finally crystallizing plagioclase either directly along the eroded surfaces and/or within a basal layer with subsequent deposition of crystals on the chamber floor (Fig. 6d). The accumulation of plagioclase crystals on the chamber floor is not yet sufficient, however, to produce monomineralic anorthosites. It is also necessary to get rid of nearly all evolved interstitial liquid between plagioclase crystals. The interstitial liquid (or rejected solute) produced by plagioclase crystallization is expected to be denser than the original melt and therefore intrinsically stagnant on the chamber floor. Consequently, there is no way to remove the interstitial liquid from plagioclase crystals by compositional convection to produce anorthosite adcumulates if the floor is completely flat. As mentioned above, there are, however, data indicating that the temporary floor at the time of the Critical Zone formation was sloping at several degrees45 which may allow the interstitial melt released by crystallization to flow away downslope. It should be noted, however, that adcumulus growth can be effective even at the flat floor situation. Morse46 showed that in large magma chambers the characteristic transport distance for chemical diffusion alone (3–4 cm/year) is up to eight times higher than the rate of accumulation at the chamber floor (0.5–1.0 cm/year). It is therefore quite probable that anorthosite adcumulates with very low residual porosity can form directly at the crystal-liquid interface of the flat chamber floor by diffusion alone46 (Fig. 6d, insert). In addition, the high Mg-number of pyroxene oikocrysts indicates that these are not truly intercumulus phases produced from a trapped liquid within anorthosites. Rather, these are heteradcumulate minerals, i.e. cumulus phases that have grown in a thin mushy layer in direct contact with the overlying body of flowing or convecting melt47,48. Heteradcumulate minerals may only form in this way when interstitial melt can freely communicate with the overlying resident melt. In contrast, the phases that crystallize deep in the thick mushy pile will have a quite limited, if any, interaction with the overlying resident melt and therefore their final compositions will be quite evolved. This is, again, indicative of the formation of adcumulate anorthosite at a crystal-liquid interface under conditions of a very low rate of crystal accumulation46,48. The melt remaining after the formation of monomineralic anorthosites can be either mixed with a new replenishing melt that terminates the crystallization of anorthosites or, alternatively, flushed away to erupt as lavas via volcanos (now eroded away) above the Bushveld Complex (Fig. 6a). Both scenarios are in line with a current consensus among igneous petrologists that most layered intrusions are growing incrementally via numerous magma pulses that pass through the evolving magmatic chambers8.

Comparison with other existing models for the formation of stratiform anorthosites

Our novel proposal is advantageous over the earlier attempts to address the problem of the generation of plagioclase-only-saturated melts14,49,50 in that it relies on a physical process that happens in nature almost inevitably. All mantle-derived melts, including those residing and contaminated in the deep-seated staging chambers, ascend towards the Earth’s surface and none can avoid reduction in lithostatic pressure during this process. Therefore, it is quite natural that some ascending alumina-rich basaltic andesitic melts may reach plagioclase-only saturation upon their ascent and subsequent cooling. In some respects, our concept can be regarded as the development of the earlier model by Irvine and his co-workers30,31 who attributed the stratiform anorthosites in the Bushveld and Stillwater Complexes to periodical replenishment of the evolving chamber by A-type (‘A’ - anorthositic) magma which – for reasons unspecified by those authors – was also initially saturated in plagioclase only. This model implies that the new A-type magma was geochemically and even isotopically distinct from the resident U-type magma (‘U’- ultramafic) that crystallized mafic-ultramafic rocks in the chamber. This elegant concept was, however, rejected by the subsequent detailed geochemical studies of the Bushveld Complex. It turned out that anorthosites and associated norite, orthopyroxenite and dunite/harzburgite are geochemically and isotopically consanguineous rocks51,52 suggesting their formation from the same parental magma, rather than from two magmas of different lineages. This inference was further supported by similar composition of plagioclase (e.g. An-content) and pyroxenes (e.g. Mg# and Cr/Al ratio) in all these cumulate rocks38. Our model allows anorthosites to be cumulates of the externally-derived and plagioclase-only-saturated melts that have been replenishing the chamber30,31. We alleviate a major problem of the earlier proposal30,31 by suggesting that both anorthosites and associated mafic rocks in the UCZ of the Bushveld Complex are produced from derivatives of the same parental magma(s). These magma(s) were derived from the deep-seated staging chamber (Fig. 6) and crystallized cumulate rocks with different mineral assemblages (e.g. anorthosite, orthopyroxenite, norite) due to some variations in chemical composition.

Our new concept can be considered as a viable alternative to a current tendency to attribute stratiform anorthosites in layered intrusions to a mechanical segregation of plagioclase crystals from co-existing mafic phases within internally- or externally-derived crystal-rich slurries20,36,53,54. In these slurry models, the transgressive relationships of anorthosites with underlying rocks in the Bushveld Complex (Fig. 1) are attributed to sill-like emplacement of plagioclase-rich slurries (with ~50% plagioclase) into solidified footwall cumulates20,55. The slurries are derived either by large-scale slumping of the cumulate pile induced by chamber subsidence20 or partial melting of noritic rocks by syn-magmatic ultramafic sills55. The near monomineralic composition of anorthosites are explained by rapid and efficient draining of most interstitial liquid from the plagioclase-rich mush into footwall rocks prior to solidification20. However, it has been recently shown both geologically21,41,44 and texturally56 that a mushy zone at the top of the cumulate pile in the Bushveld Complex is almost non-existent (<4 m thick)56, leaving no possibility for large-scale slumping processes in a cumulate pile and for the removal of the liquid component from the postulated slurry20,54. Similarly, both field and chemical observations57,58 are not supportive of the idea that the ultramafic units of the Bushveld Complex are synmagmatic sills depriving this model of a heat source to re-melt noritic rocks to produce a plagioclase-rich mush55.