Quantitative temperature-time information from retrograde diffusion zoning in garnet

Constraints for the P-T-t history of the central Black Forest, Germany

authored by

S. Weyer, J. Jarick, K. Mezger

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.

External Organisation(s)

Max Planck Institute for Chemistry (Otto Hahn Institute)
University of Münster

Type

Article

Journal

Journal of metamorphic geology

Volume

17

Pages

449-461

No. of pages

13

ISSN

0263-4929

Publication date

07.1999

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Geology, Geochemistry and Petrology

Electronic version(s)

https://doi.org/10.1046/j.1525-1314.1999.00207.x (Access: Unknown)