Quantitative temperature‐time information from retrograde diffusion zoning in garnet: constraints for the P–T–t history of the Central Black Forest, Germany

Abstract

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 derive cooling rates using recent datasets for cation diffusion in garnet. This information, together with textural relationships, thermobarometry and thermochronology, is used to constrain the pressure–temperature–time history of the high‐grade gneisses. The garnet–biotite thermometer indicates peak metamorphic temperatures for the garnet cores of 730–810 °C. The temperatures for the outer rims are 600–650 °C. Garnet–Al2SiO5–plagioclase–quartz (GASP) barometry, garnet–rutile–Al2SiO5–ilmenite (GRAIL) and garnet–rutile–ilmenite–plagioclase–quartz (GRIPS) barometry yield pressures from 6–9 kbar. U–Pb ages of monazite of 341±2 Ma date the low‐P high‐T metamorphism 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 °C Ma−1. The petrologic cooling rates, with particular consideration of the fO2 conditions for modelling retrograde diffusion profiles, agree with the geochronological cooling rate. The oldest sediments overlying the crystalline basement indicate a minimum cooling rate of 10 °C Ma−1.