Mineral zoning and exhumation history in the Münchberg eclogites (Bohemia)

Abstract

Clinopyroxene and garnet from the Münchberg eclogites (Bohemia) have been analyzed by electron probe. Their zoning was examined in order to obtain information on the exhumation history of these high pressure rocks. Garnets show both growth zoning in the biggest grains and a 30 μm diffusion boundary layer. A finite-difference formulation of a volume diffusion model with a concentric arrangement of a garnet grain enclosed in a clinopyroxene matrix fails to reproduce the simultaneous existence of both zoning patterns. In contrast, exchange of Fe and Mg through the intergranular medium and transport by volume diffusion within 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) Retrograde zoning is restricted to garnets from high-temperature eclogites (T > 650°C). The concentration profiles depend not only on the temperature evolution but also on the respective proportions of garnet and pyroxene. Application of the aggregate model to the Münchberg eclogites shows that the Fe-Mg profiles in the minerals are best explained by a multi-stage retrograde history in which the cooling rate decreases from - 10°C my-1 to -0.5°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 deformation of the crust and the existence of a vertical velocity gradient The large value of exhumation rates at depth does not seem to be compensated by erosion which suggests a regime of intense extension in the shallow levels of the orogenic domain.