Spinel–cordierite symplectites replacing andalusite: evidence for melt‐assisted diapirism in the Bushveld Complex, South Africa

Abstract

Spinel-cordierite symplectites partially replacing andalusite occur in metapelitic rocks within the cores of several country rock diapirs that have ascended into the upper levels of layered mafic/ultramafic rocks in the Bushveld Complex. We investigate the petrogenesis of these symplectites in one of these diapirs, the Phepane dome. Petrographic evidence indicates that at conditions immediately below the solidus the rocks were characterized by a cordierite-, biotite-and K-feldspar-rich matrix and 5-10 mm long andalusite porphyroblasts surrounded by biotite-rich fringes. Phase relations in the MnNCKFMASHT model system constrain the near-solidus prograde path to around 3 kbar and imply that andalusite persisted metastably into the sillimanite + melt field, where the fringing relationship between biotite and andalusite provided spatially restricted equilibrium domains with silica-deficient effective bulk compositions that focused suprasolidus reaction. MnNCKFMASHT pseudosections that model these compositional domains suggest that volatile phase-absent melting reactions consuming andalusite and biotite initially produced a moat of cordierite surrounding andalusite; reaction progressed until all quartz was consumed. Spinel is predicted to grow with cordierite at around 720°C. Formation of the aluminous solid products was strongly controlled by the receding edge of andalusite grains, with symplectites forming at the andalusite-cordierite moat interface. Decompression due to melt-assisted diapiric rise of the floor rocks into the overlying mafic/ultramafic rocks occurred close to the thermal peak. Re-crossing of the solidus at P ¼ 1.5-2 kbar, T > 700°C resulted in preservation of the symplectites. Two features of the silica-deficient domains inhibited resorption of spinel. First, the cordierite moat armoured the symplectites from reaction with crystallizing melt in the outer part of the pseudomorphs. Second, an up-T step in the solidus at low-P, which may be in excess of 100°C higher than the quartz-saturated solidus, resulted in high-T crystallization of melt on decompression. Even in metapelitic rocks where melt is retained, preservation of spinel is favoured by decompression.

Arrested development – a comparative analysis of multilayer corona textures in high-grade metamorphic rocks

“…These changes are typically triggered by changing P and/or T but can also be triggered through changing component fluxes through the corona as a function of evolving local effective bulk compositions (e.g. Griffin, 1972;Griffin and Heier, 1973;Joesten, 1986;Droop, 1989;Indares, 1993;Johnson et al, 2004;Štípská et al, 2010;Baldwin et al, 2015). In contrast to the single-stage, steady-state model, the internal layer configuration of the corona reaction band evolves with time as new layers develop and old layers are resorbed.…”

“…Harley, 1989;White et al, , 2008Johnson et al, 2004), the mechanism for corona development is obscured since the final steady-state configuration of corona layers observed in a rock reflects the complex evolution of chemical potential relationships with P , T and bulk composition. These same complexities must also govern metamorphic processes on the prograde path, albeit on larger length scales.…”

“…7f;Johnson and Carlson, 1990 -Fig. 2) or melt-depleted pelitic rocks, or where the rate of change in pressure and temperature occurs sufficiently fast such that diffusion rates are exceeded. Typically, the latter scenario arises in contact aureoles characterized by rapid heating and cooling (Johnson et al, 2004 - Fig. 8g; Mcfarlane et al, 2003;Ings and Owen, 2002;Barboza and Bergantz, 2000;Wheeler et al, 2004;Daczko et al, 2002;Dasgupta et al, 1997;Joesten and Fisher, 1988), but it can also occur in shock-heated rocks within large impact structures (Gibson, 2002;Ogilvie, 2010).…”

“…The interpretation of corona textures has traditionally been a primary diagnostic tool for inferring metamorphic P -T -t paths and, hence, tectonics (Whitney and McLelland, 1973;Grew, 1980;Joesten, 1986;Droop, 1989;Clarke et al, 1989;Ashworth et al, 1992;White and Clarke, 1997;Norlander et al, 2002;Kelsey et al, 2003;Johnson et al, 2004;Tsunogae and Van Reenen, 2006;Zulbati and Harley, 2007;Hollis et al, 2006). Diffusion-constrained conditions may arise on both the prograde and retrograde paths, but, most commonly, coronas are thought to have formed during retrogression from peak P -T conditions as low-variance equilibria are crossed.…”

“…No univariant reaction is crossed. (g) Prograde complex corona comprising spinel-cordierite symplectite and leucocratic biotite, K-feldspar and plagioclase after andalusite (after Johnson et al, 2004). X Mg of cordierite decreases toward biotite (0.55-0.51) with no variation in spinel composition.…”