Whiteschists: Their compositions and pressure-temperature regimes based on experimental, field, and petrographic evidence

Whiteschists exhibiting the characteristic assemblage kyanite—talc are chemically almost completely contained in the model system MgO—Al ₂O ₃—SiO ₂—H ₂O. Their unusual bulk chemistry may at least partly be due to a derivation from mudstones associated with evaporites. The paragenesis talc—kyanite is stable at water pressures above 10 kbar in the temperature range of about 600-850°C. Geologically this would imply mean depths of burial near the continental Mohorovićič discontinuity. The talc—kyanite pair exhibits reaction relationships with lower-pressure assemblages such as chlorite—quartz, yoderite-quartz, and cordierite—corundum, which have also been found to occur as late metamorphic products in Whiteschists. Whiteschists and related rock types occur most widely in Central Africa, but are also known from Europe, Afghanistan, Tasmania, and Brazil. Estimated total pressures of metamorphism depend on the experimentally as yet unknown influence of water deficiency on the talc—kyanite stability field. There are indications from the variable stability of Mg-cordierite under wet and dry conditions that the talc—kyanite field may be shifted to total pressures as low as 7-8 kbar, but the influence of other gas species such as CO ₂ that can also be incorporated in the cordierite structure, may possibly counteract this shift. The three-stage metamorphic history of the Afghanistan whiteschist, with the mineral succession chlorite + quartz → talc + kyanite → cordierite + corundum all preserved in one thin section, strongly suggests, however, that compositional changes of the coexisting metamorphic fluids with time may be far more decisive for the mineralogy developed than total pressure and temperature changes. The variability of fluid compositions during whiteschist metamorphism is, at least for this locality, due to the mobilization of a former evaporite deposit. Total pressures during whiteschist formation may not always have been as extreme as initially expected.