Latypov, and 5 co-authors, reported in 2020 that the formation of some Earth’s monomineralic igneous rocks appears to be prohibited by constraints imposed by liquidus phase-equilibria on evolution of mantle-derived magmas. Yet, these rocks exist as stratiform layers in many mafic-ultramafic intrusions. One conspicuous example is monomineralic anorthosites in the Bushveld Complex that occur as stratiform layers up to hundreds of kilometres in length. Such monomineralic anorthosites appear to require parental melts saturated in plagioclase only but where and how to produce these melts remains a contentious issue. Here the authors posit that anothosites are likely sourced from deep-seated magma reservoirs. In response to pressure reduction, these ascending melts become first superheated and then saturated in plagioclase after stalling and cooling in shallow-level chambers. Adcumulus growth of plagioclase from such melts at the chamber floor results in the formation of monomineralic anorthosites.
The authors propose that stratiform layers of monomineralic anorthosites in layered intrusions are products of the chamber replenishment by melts whose saturation in plagioclase as a single liquidus phase is triggered by their transcrustal ascent towards the Earth’s surface. Te transgressive relationships of stratiform anorthosites with their foot-wall rocks in the UCZ of the Bushveld Complex are best explained by the replenishment of the chamber by relatively dry, alumina-rich basaltic/ basaltic andesitic melts that were derived from deep-seated magma reservoirs. Some of these melts became superheated in response to pressure reduction during their ascent. Upon emplacement into the chamber as basal flows, these melts caused thermo-chemical erosion of pre-existing footwall rocks in the chamber.
The erosion was mostly due to chemical disequilibrium between the new melts and floor cumulates, with the superheat assisting in keeping the melt away from the onset of its crystallization. On cooling, the melts reached saturation in plagioclase only and its subsequent adcumulus growth at the chamber floor resulted in the formation of monomineralic anorthosite. Te process that is likely responsible for the melt saturation in plagioclase is thought to be the expansion of the plagioclase stability volume due to lithostatic pressure reduction. Te expansion takes place when these melts – after some differentiation and crustal contamination in deep-seated magma reservoirs – are transferred upwards into the shallow-level Bushveld chamber. We propose that many stratiform layers of monomineralic anorthosites in mafc-ultramafc layered intrusions may be associated with a lithostatic pressure-induced shift of multi-saturated melts into the stability volume of plagioclase alone during their journey through the Earth’s crust.
It should be emphasized that the authors model primarily applies to monomineralic anorthosites that occur as thick and continuous stratiform anorthosites in mafc-ultramafc intrusions and may not be extended to non-stratiform cases of monomineralic cumulates, especially in other types of intrusive complexes (e.g. massif-type anorthosites). For instance, a few cm thick rims of monomineralic anorthosite that are locally developed below the Merensky Reef and massive superheat in the Bushveld Complex are better explained by re-melting/selective dissolution of footwall noritic cumulates.
Prominent examples of discordant bodies of monomineralic anorthosite in the Skaergaard intrusion or Stillwater Complex are most likely produced by metasomatic replacement of pre-existing cumulates by late migrating fluids. In a similar manner, fine-scale mineral-graded layering involving anorthosites in both layered intrusions and mid-ocean ridge crust require some alternative mechanisms, perhaps, involving oscillatory crystallization at crystal-liquid interface or crystal sorting in density magmatic currents.
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