MINERAL ZONING AND EXHUMATION HISTORY IN THE MUNCHBERG ECLOGITES (BOHEMIA)

Clinopyroxene and garnet from the Munchberg eclogites (Bohemia) have b een analyzed bg electron probe, Their zoning was examined in order to obtain information on the exhumation history of these high pressure ro cks. Garnets show both growth zoning in the biggest grains and a 30 mu m diffusion boundary layer. A finite-difference formulation of a volu me diffusion model with a concentric arrangement of a garnet grain enc losed in a clinopyroxene matrix fails to reproduce the simultaneous ex istence of both zoning patterns. In contrast, exchange of Fe and Mg th rough the intergranular medium and transport by volume diffusion withi n a rock represented as an aggregate of spherical minerals account for the following general observations: (1) In most eclogites, retrograde zoning is common in garnets and absent in clinopyroxenes. (2) Retrogr ade zoning is restricted to garnets from high-temperature eclogites (T > 650 degrees C). The concentration profiles depend not only on the t emperature evolution but also on the respective proportions of garnet and pyroxene. Application of the aggregate model to the Munchberg eclo gites shows that the Fe-Mg profiles in the minerals are best explained by a multi-stage retrograde history in which the cooling rate decreas es from -10 degrees C my(-1) to -0.5 degrees C my(-1) over 17.5 my. In the same interval, the exhumation rate correlatively decreases from 3 to 0.02 mm yr(-1). Such an evolution reflects the combination of two components of exhumation: (1) the denudation of a non-deformable crust with denudation velocity decreasing with time, and (2) the internal d eformation of the crust and the existence of a vertical velocity gradi ent, The large value of exhumation rates at depth does Plot seem to be compensated by erosion which suggests a regime of intense extension i n the shallow levels of the orogenic domain.