The synchronism of crustal thickening and low-pressure facies metamorphism in the Mount Isa Inlier, Australia 2. Fast convective thinning of mantle lithosphere during crustal thickening

Abstract

The Mount Isa Inlier is, like some other northern Australian Early to Middle Proterozoic fold belts, characterized by an anti-clockwise P- T- t path: i.e. regional, prograde low- P facies metamorphic assemblages grew during crustal thickening (+ ΔP, + ΔT), and were overprinted by a phase of essentially isobaric cooling ( ΔP ≈ 0, − ΔT). In this study, two possible models for the synchronism of regional, low- P facies metamorphism and crustal thickening are quantitatively evaluated using a one-dimensional, finite-difference technique: (1) extreme elevation of isotherms immediately before the thickening event, due to e.g. lithospheric extension, magmatic activity, or crust-mantle delamination, and (2) crustal thickening accompanied by (convective) thinning of the mantle lithosphere. Only the latter results in prograde low- P facies metamorphism during crustal thickening. It is the preferred model for the Mount Isa Inlier.