Geometry of isograds: thermodynamic constraints on inversion

Abstract

Inverted isograds have been reported from metamorphic terrains ranging in age from Precambrian to Tertiary. Most of the workers believe that metamorphic inversion was caused by an inverted temperature gradient, associated with large-scale thrusting. However, isograds are not isotherms, and the former may develop at large angles to the latter; it is shown here that the orientation of isograds depends not only on the orientation of isotherms but also on the value of ΔT/ΔP of an isograd reaction and that of the temperature gradients. Accordingly, four different sets of conditions for inversion are recognised: two for inverted temperature gradients, one for a horizontal temperature gradient and one for a normal temperature gradient. These conditions have been worked out for four different isograds: staurolite-in, staurolite-out, muscovite breakdown, and partial melting of granitic compositions. It is shown that isograds may be in normal or inverted attitudes depending on the orientation and magnitude of the temperature gradient and also on the slope ΔT/ΔP of an isograd reaction. Published accounts of metamorphic inversion do not provide all of these data but explain them in terms of only an inverted temperature gradient, which may not be correct, and may need re-investigation.