The contribution of charnockite magmatism to achieve near-ultrahigh temperatures in the Namaqua–Natal Metamorphic Province, South Africa

Aluminous metapelitic granulites from the central Namaqua Sector of the Namaqua–Natal Metamorphic Province, South Africa, host partly retrogressed hercynite–quartz assemblages. These assemblages record peak P–T conditions of 860–890 °C and 5–5.5 kbar, estimated via thermodynamic modelling and ternary feldspar thermometry. In-situ U–Pb geochronology and geochemistry of anatectic zircon and monazite constrains this event as part of the main Namaqua Orogeny affecting the terrane at c. 1040–1035 Ma. Detrital zircon age spectra span from about 1860 to 1100 Ma, peaking at c. 1800–1700 and 1400–1300 Ma, suggesting detritus was sourced from nearby Paleoproterozoic- and Mesoproterozoic terranes. Thin sections reveal narrow monomineralic coronae of garnet, cordierite or sillimanite separating formerly coexisting hercynite and quartz. Calculated chemical potential relationships show that coronae have developed in response to very minor, near-isobaric cooling shortly after the thermal peak, involving limited diffusion down gradients of μFeO–μMgO. Corona formation occurred in distinct compositional domains, controlled by neighbouring porphyroblasts such as garnet and/or sillimanite. Isolated hercynite–quartz pairs distal from porphyroblasts developed thin rinds of sillimanite, requiring no additional compositional gradients. Cordierite inclusions in garnet, limited melt production and extraction and corona-textures are consistent with an anticlockwise evolution, involving burial and concomitant heating to near-ultrahigh temperature conditions followed by near-isobaric cooling at mid-crustal depths. The hercynite–quartz equilibria record the highest metamorphic temperature affecting the Namaqua Sector, requiring a crucial component of advective heating via voluminous pre- to syn-metamorphic felsic-charnockitic magmatism of the Spektakel Suite. Namaqua-aged granulite facies metamorphism at the terrane scale was driven by a combination of mafic underplating and radiogenic heating. Calculated temperature–enthalpy profiles suggest that advective heating locally increased the suprasolidus enthalpy budget by a third which allowed peak temperatures to be some 50 °C higher than would otherwise have been achieved.