SPATIAL AND TEMPORAL CHARACTERISTICS OF TEMPERATURE AND FLUID FLOW DURING METAMORPHISM AROUND TABULAR PLUTONS

The complex spatial and temporal interaction between fluids and temperature around tabular intrusions was investigated using a series of heat and mass transport calculations to provide thermal histories for various intrusion depths, pluton thicknesses, initial country rock geotherms, and country rock permeabilities. Tabular plutons with half-widths of 7.5 km were modeled with their upper contacts located at depths of 9, 12 and 15 km; thicknesses of 2.5 and 5 km; initial geotherms of 32 or 36 C/km; and country rock permeabilities of e-16 m^2 or e-18 m^2.

Pluton depth affects peak temperature (Tmax) in country rocks by increasing Tmax ~ 50 C in the roof and along the end of the pluton for every 3 km increase in depth. Thickening the pluton increases Tmax around the pluton and also increases the duration of the thermal pulse associated with the intrusion; changing the 12 km deep intrusion thickness from 2.5 to 5 km increases Tmax near the intrusion by about 50 C and increases the amount of time that T is within 10 C of Tmax by a factor of about 2. The geotherm influences Tmax by changing the initial country rock temperature at the onset of intrusion; higher gradients result in larger values of Tmax. For a 2.5 km thick intrusion 12 km deep, changing the initial gradient from 32 C/km to 36 C/km increases Tmax between 25 and 50 degrees C, depending on position around the pluton.

Country rock permeability determines whether advective heat transport plays an important role in the metamorphism around the pluton; K=e-18 m^2 produces a regime dominated by conductive processes, K=e-16 m^2 produces a regime where advective processes play an important role. The spatial variation of Tmax in a conductive regime is controlled by proximity to the pluton. In an advective regime, the early thermal history is controlled by conductive processes and the later history (t > ~10^5 yr for a 2.5 km thick pluton at 12 km depth) is controlled by advective processes. A conductive regime produces a single thermal event in the country rocks while an advective regime has two thermal events, a short lived one near the intrusion controlled by conduction and a later, more long lived one that is controlled by advective processes. Advective effects are most intense near the end of the pluton; rocks near the center of the pluton are more strongly influenced by conduction.