LOOPING P-T PATHS DURING EXHUMATION OF PARTIALLY MOLTEN CRUST

Mineral compositions and textures used to reconstruct ‘peak’ metamorphic conditions commonly form during mass and heat transport from deep to shallow crustal levels in the later stages of orogeny. 2D Ellipsis numerical experiments offer insight into rock trajectories, strain, and rates of pressure and temperature changes during exhumation, including the timing and conditions of melting and melt crystallization. The pressure-temperature path of a rock during exhumation may be relatively smooth, but may also be convoluted if convective flow occurs. Looping paths are predicted for (1) subducted continental crust that partially melts at depth and ascends: the complexity of the exhumation path is influenced by the subduction depth and the location of a rock relative to the overlying mantle wedge; and (2) orogenic crust emplaced in gneiss domes during extension. In the latter case, 2D thermomechanical models predict that extension in the upper crust triggers convergent flow of deep crustal channels that collide beneath the extending region and flow upward along a steep, central high-strain zone, defining two compartments (subdomes) on either side of the median high-strain zone. Flow trajectories therefore rotate from horizontal to vertical and back to horizontal, in a manner akin to diapiric flow, even when buoyancy forces are negligible. When buoyancy is a significant factor, crustal materials in subdomes are progressively dragged into a convective motion, producing complex, looping flow paths, particularly under relatively slow extension (~1 mm/yr). Complex paths in both of these tectonic settings may be recorded in some minerals in the form of oscillatory zoning (e.g. garnet in high-P rocks exhumed in migmatite) and reaction textures (e.g. corona, symplectite). Some rocks may repeatedly cross the solidus, resulting in multiple crystallization events recorded in minerals such as zircon and garnet. Simple 2D modeling integrated with field analysis of the tectonic context of metamorphic rocks provides insight into complex histories of mass and heat transport and help us understand metamorphic crystallization sequences and resulting mineral compositions and textures.