Quantitative temperature-time information from retrograde diffusion zoningin garnet: constraints for the P-T-t history of the Central Black Forest, Germany

Garnet from a kinzigite, a high-grade gneiss from the central Black Forest (Germany), displays a prominent and regular retrograde diffusion zoning in Fe, Mn and particularly Mg. The Mg diffusion profiles are suitable to deriv e cooling rates using recent datasets for cation diffusion in garnet. This information, together with textural relationships, thermobarometry and ther mochronology, is used to constrain the pressure-temperature-time history of the high-grade gneisses. The garnet-biotite thermometer indicates peak met amorphic temperatures for the garnet cores of 730-810 degrees C. The temper atures for the outer rims are 600-650 degrees C. Garnet-Al2SiO5-plagioclase -quartz (GASP) barometry, garnet-rutile-Al2SiO5-ilmenite (GRAIL) and garnet -rutile-ilmenite-plagioclase-quart (GRIPS) barometry yield pressures from 6 -9 kbar. U-Pb ages of monazite of 341+/-2 Ma date the low-P high-T metamorp hism in the central Black Forest. A Rb/Sr biotite-whole rock pair defines a cooling age of 321 +/- 2 Ma. The two mineral ages yield a cooling rate of about 15 +/- 2 degrees C Ma(-1). The petrologic cooling rates, with particu lar consideration of the fO(2) conditions for modelling retrograde diffusio n profiles, agree with the geochronological cooling rate. The oldest sedime nts overlying the crystalline basement indicate a minimum cooling rate of 1 0 degrees C Ma(-1).