METAMORPHIC HEATING RATES IN CONTACT AUREOLES: CONSEQUENCES OF INTRUSION DEPTH AND SIZE, GEOTHERMAL GRADIENT AND HOST ROCK PERMEABILITY

Heating rates associated with metamorphic thermal events coupled with the time in a specific mineral zone, are fundamental controls on the textural development of metamorphic rocks. Computational modeling permits a range of thermal histories, and consequent heating rates in a contact aureole, to be explored by varying host rock permeability (conductive and convective regimes), geothermal gradients (28C,32C,36C), and intrusion thickness (3km, 6km) and depth (9km,12km,15km).

Studies indicate that for the convective thermal regimes at all depths, aureole rocks at 0.25km from the contact with a tabular intrusion experience rapid heating (~20,000 yrs - Tmax1), followed by a period of cooling (to ~200,000 yrs), and a second long-lived heating cycle (to ~600,000 yrs - Tmax2). With increasing depth, Tmax1 increases, cooling rates are lower between Tmax1 and Tmax2 while heating rates are also lower. At 1km from the contact, rocks heat slowly, experience a single Tmax and do not begin cooling until ~650,000 yrs. Rocks above the intrusion follow similar dT/dt patterns. Higher geothermal gradients initially cause higher cooling rates but maintain higher temperatures with less cooling prior to reheating. Lower gradients cause the second heating cycle to shift to later times (~50,000 yrs).

If garnet zone conditions are assumed to be 500-550C, staurolite zone 550-600C and sillimanite zone 600-650C, rocks near the contact (12km depth, 32C/km) remain in these mineral zones for about 483,000, 735,000 and 0 yrs, respectively. Lower initial gradients result in rocks at the same position to heat longer at lower temperatures, but shorter time periods at higher temperatures. Slower heating rates with more time in the interval result in larger domains of equilibrium, less reaction overstepping and reaction mechanisms that are more stable than in rapidly heated regimes. The spatial and temporal distribution of variations in heating times and rates results in distinct patterns of crystal size and mineralogy. These studies, combined with kinetic textural modeling, help to unravel complicated mineral textures and provide insights into the linkage between metamorphic histories and textures preserved in the rocks.