Mid-crustal emplacement of the northern Kunene Complex massif-type anorthosite revealed by monazite petrochronology and phase equilibria modelling

Massif-type anorthosites are one of the few rock-types on Earth that are restricted in time, occurring exclusively in the Proterozoic Eon. Understanding their unique quasi-monomineralic composition and temporal restrictions challenges current petrological and geodynamic models. It is generally accepted that mechanical accumulation of plagioclase from basaltic precursors begins at the Moho, but details regarding the crystallinity of ascending plagioclase-rich magmas and final level of emplacement remain uncertain. A major challenge is estimating the pressure conditions at which anorthosite is emplaced due to the absence of necessary phases. This uncertainty is particularly evident in the Kunene Complex (KC), the largest Proterozoic massif-type anorthosite on Earth (1.50-1.36 Ga), where pyroxene and amphibole thermobarometry suggest that the southern segment was emplaced at 7-9 kbar and the northern segment at 3-5 kbar. It is unclear if these differences are geological and/or methodological.

An alternative approach for determining emplacement depth of magmatic bodies involves examining the metamorphic conditions of magma-host rock interaction in the contact metamorphic aureoles. In this study, we focus on Paleoproterozoic (1.88-1.82 Ga) nebulite and stromatite migmatitic supracrustal rocks located at the margin of the northern KC anorthosite pluton (1.384-1.375 Ga). The stromatic foliation dips towards the pluton and is characterised by leucosomes of quartz, K-feldspar, plagioclase, cordierite, garnet, and sillimanite, while the mesosome is richer in cordierite, garnet and biotite. The presence of common, transgressive, and discontinuous nebulitic migmatite of similar mineralogy and in gradational contact with the stromatite attests to the segregation of partial melt outlasting the formation of the stromatic foliation. Mineral equilibria modelling indicates that the migmatite formed at P-T conditions of 4 kbar and 730°C.

In-situ monazite ages in nebulite as well as leucosome and mesosome of the stromatite span 500 million years, from 1.8 to 1.3 Ga. Two combined textural and chemical domains have been identified in monazite, helping to categorise the age data. Domain 1 shows embayed and cuspate BSE-dark grey areas and is Y-rich, has variable Eu/Eu* and Sr content, low Th/U and is typically found in monazite cores. Domain 2 is more common and shows BSE-bright rims or convex-inward mantles around Domain 1 or can encompass the entire grain, and has consistently low Y, Eu/Eu* and Sr content, and higher Th/U. U-Pb ages for Domain 1 cluster at 1.80-1.77 Ga, interpreted as the age of a prograde event, preceding the growth of peritectic garnet and cordierite. U-Pb ages for Domain 2 cluster at 1.41-1.36 Ga and correspond to the co-crystallisation of monazite, feldspar and garnet during the migmatitic event constrained at 4 kbar and 730°C. The age of Domain 2 monazite coincides with the emplacement of the northern KC anorthosite pluton.

Altogether, the fabric analysis, migmatitic metamorphic assemblage, and coeval age of Domain 2 monazite and KC anorthosite indicate that the emplacement of anorthosite caused significant heating, melting and hypersolidus ductile flow of the contact aureole at metamorphic pressures equivalent to a mid-crustal depth of ~15 km. Consequently, this study offers new insights into the length scale of ascent and emplacement levels of massif-type anorthosite magmas.