ADVECTIVE THERMAL PULSES AS HYDRATION-DEHYDRATION MECHANISMS IN CONTACT-REGIONAL METAMORPHIC TERRANES

In contact-regional terranes, rocks often display mineral textures suggestive of complex spatial and temporal interactions between fluids and temperature. Hydrothermal models (2D, 3D) of intrusions into the middle crust indicate that an advectively driven thermal pulse may develop after the initial conductive thermal peak when background geothermal gradients and permeability (K) are of sufficient magnitude. This second thermal event is much longer lived than the initial conductive thermal spike in rocks close to the intrusion and is preceded by a period of high fluid flux.

As an example, contact aureoles generated from the intrusion of a tabular pluton, at 875C and 12km into host rocks with a K=e-16m^2 and background temperatures (T) that varied along a linear geothermal, are examined. For a gradient of 36C/km, a rock positioned 125m horizontally from the end of the pluton experiences a rapid T increase to a maximum of 630C. After cooling to ca. 570C, thermal forces are sufficient to begin driving convection. Advective heat transfer then causes T to rise to 600C at 480,000 yrs and remain above 570 for about 60,000 yrs. Instantaneous fluid flux is maximum at ca.450,000 yrs, prior to the second thermal peak and subsequent to the initial thermal pulse. In contrast, decreasing background T's to 28C/km, causes not only a lower maximum T, 590C, but a substantially lower secondary thermal maximum, 510C. However, maximum fluid flux also precedes the second thermal event. In the 36C/km case, the second thermal pulse is of sufficient duration for reaction rates to produce mineral textures consistent with regional metamorphism. Advective circulation of heat provides a mechanism for hydration of an earlier formed assemblage prior to slow reheating. Thus, advection may be an important control of the thermal overprinting observed in metamorphic terranes.