Problems with Barrovian metamorphism. Pattison, D.R.M.

Problems with Barrovian metamorphism
Pattison, D.R.M.
Barrovian metamorphism is the classic type of regional crustal metamorphism in collisional mountain belts away from subduction zones. One would think that the pressure(depth)-temperature conditions of Barrovian metamorphism are well established. One would be mistaken in thinking so. The issue turns on interpreting the formation of mineral assemblages containing staurolite(St) and kyanite(Ky), the classic Barrovian indicator association. If thermodynamically-calculated equilibrium assemblage diagrams are interpreted literally, St+Ky-bearing mineral assemblages in the dozens of Barrovian settings worldwide – from Barrow’s zones in Scotland to the Himalaya – must have developed in a restrictive, high-pressure window greater than ~6.5 kbar and ~650 °C. Conversely, the large (in terms of pressure) predicted range for St+sillimanite (Sil) sequences (~4 to 6.5 kbar), in which neither kyanite nor andalusite is developed in the sequence, is represented by very few settings worldwide. Exacerbating this conundrum is the prediction that, for all but unusually aluminous bulk compositions, the incoming of kyanite is due to the consumption of staurolite, whereas in very few natural samples do textures suggest this relationship; rather, the two minerals most commonly appear to have developed independently at about the same time. In the absence of anything obviously wrong with the experiments on which the thermodynamic modelling is based, kinetic explanations need to be entertained.