The Spacice eclogite: constraints on the P-T-t history off the Gfohl granulite terrane, MoIdanubian Zone, Bohemian Massif

摘要

In the Kutna Hora Complex, the Bestvina Formation, which is similar to Gfohl granulite, contains eclogite that has escaped widespread retrograde recrystallization. The eclogite assemblage, garnet + omphacite + quartz + rutile + - plagioclase, yields an estimate for peak metamorphic conditions of 18-20kbar and 835-935 deg C, which is comparable to that determined from felsic granulite, 14-20 kbar and 900-1000 deg C. Garnet in eclogite exhibits both prograde and retrograde compositional zoning, from which constraints on thermal history of the Gfohl terrane can be derived by diffusion modelling. At 900 deg C, a garnet grain of 800-1000 um radius would homogenize in 7.5-11.7 million years, but the existence of compositional gradients on a length scale of 100-200 Jim suggests that the duration of peak metamorphism may have been limited to ~500,000 years. Diffusion modelling of retrograde zoning in garnet yields a cooling rate of 150-100 deg C/m.y. for a radius of 800-1000 um and initial temperature of 900 deg C. The relatively brief duration of high-pressure/high-temperature metamorphism and rapid cooling and exhumation of the Gfohl terrane may be a consequence of lithospheric delamination during Early Carboniferous collision of Bohemia (Tepla-Barrandia) and Moldanubia (Franke, 2000).