P-T-TIME HISTORIES AND PARTICLE PATHS OF METAMORPHIC ROCKS IN AND AROUND GNEISS DOMES

In many orogens, high-grade metamorphic rocks exposed in the cores of gneiss domes are exhumed from deeper crustal levels by buoyancy-driven ascent of high-melt fraction migmatites. Dome rocks may therefore record higher pressures than rocks surrounding the dome. In domes that ascend and cool rapidly, rocks typically record P-T conditions of > 10 kbar and > 700 C (kyanite zone), and isothermal decompression to shallow/mid-crustal levels (3-5 kbar; cordierite-sillimanite zone), where the high-P rocks crystallize/cool rapidly in the hanging-wall of extensional structures. The highest pressures may be recorded only in refractory lithologies that occur as layers or pods within migmatitic quartzofeldspathic gneiss or pelite that record lower pressures owing to the ease of overprinting of these rock types. High-grade (amphibolite to upper amphibolite facies) metamorphic rocks mantle the cores of gneiss domes, and typically record decompression paths accompanied by cooling.

P-T-time paths can be partially reconstructed using reaction texture and phase equilibria analysis and geo/thermochronology, and simulated by numerical modeling. Results of Ellipsis numerical modeling show that deep crustal rocks will be exhumed rapidly along initially steep P-T paths in high-melt fraction domains (domes) under zones of concentrated extension, followed by rapid cooling. Particle paths of mantling rocks will result in the assembly of Barrovian-like sequences. In these ‘Barrovian' sequences, the highest-grade rocks are located closest to the dome margin. Metamorphism of the lowest grade rocks is coeval with that of the highest grade rocks, and the apparent progressive sequence results from the deformation/exhumation paths associated with coupled extension and vertical flow of structurally underlying migmatite domains.