COLD FINGERS IN A HOT MAGMA: NUMERICAL SIMULATION OF COUNTRY-ROCK DIAPIRS IN THE BUSHVELD COMPLEX, SOUTH AFRICA

Partially molten diapirs and domes in the Earth's continental crust can be an effective means of transporting heat from lower to higher levels, often producing pronounced prograde metamorphic aureoles. Our numerical thermomechanical models (Gerya et al., 2003) show that this classical thermal scenario is violated by the diapirs of partially molten metasedimentary rocks up to 8 km in diameter that penetrate the Bushveld Complex, the world's largest layered intrusion. Here, diapirism was triggered by the emplacement of an 8-km-thick, hot, and dense mafic magma over a cold and less dense sedimentary succession. These diapirs promoted cooling of the giant magma chamber, bringing cooler material into higher crustal levels. Comparison between numerical results and geological observations indicates that diapir nucleation is crucially dependent on the presence of initial topographic disturbances between 800 and 1000 m in height in the floor of the magma chamber, and also allows critical parameters of initial geometry and temperature distribution of the Bushveld event to be outlined. Similar thermomechanical phenomena may be anticipated at subduction zones where "cold" diapirs (plumes) of hydrated, partially molten rocks generated in the proximity of the upper interface of the subducting slab at depths >100 km can form (Tamura, 1994; Gerya and Yuen, 2003). Therefore, the diapiric features observed in the Bushveld Complex provide a unique opportunity for studying "fossil evidence" of the "cold" diapir phenomenon and can lead to a broader recognition and understanding of this process. References: Gerya, T.V., Uken, R., Reinhardt, J., Watkeys, M.K., Maresch, W.V., and Clarke, B.M. (2003) Geology, (in press). Tamura, Y. (1994) J. Petrol., v. 35, p. 619-645. Gerya, T.V., and Yuen, D.A. (2003) Earth Planet. Sci. Let., v. 212, p. 47-62.