THE EFFECT OF CONDUCTIVE VS. ADVECTIVE THERMAL REGIMES ON REACTION MECHANISMS IN PELITES NEAR TABULAR PLUTONS IN THE MID-CRUST

Processes that control reaction mechanisms in regional-contact meta-pelites are simulated using thermal models of plutons and kinetic models of metamorphic reactions. A pelite at 12 km depth in a conductive regime near the end of a 2km thick tablular pluton experiences rapid heating followed by a prolonged period of cooling (see figure). During heating, the rock spends about 1 kyr in the garnet zone, 1 kyr in the staurolite zone and 4 kyr in the sillimanite zone. During the cooling, the rock remains in the sillimanite zone for 20 kyr, followed by 20 kyr in the staurolite zone and 140 kyr in the garnet zone. The rapid heating causes substantial overstepping of the garnet, staurolite and sillimanite reactions before many nuclei form and, once a crystal has nucleated and begun to grow, the domains of local equilibrium are small due to limited diffusive transport along grain boundaries. This favors nucleation and growth of garnet, staurolite and sillimanite governed by metastable reactions that consume chlorite. Rocks close to the pluton in systems with advection have two thermal peaks, resulting in an early history similar to the conductive case followed by a second thermal event that develops more slowly and is much longer lived than the first peak (see figure). This favors early nucleation and growth of garnet, staurolite and sillimanite controlled by highly overstepped metastable reactions with small domains of local equilibrium and later growth dominated by less overstepped reactions and larger equilibration volumes.