Thermobarometry and P–T path of garnet–aluminosilicate-bearing gneisses from the KTB drill core (Continental Deep-Drilling Project, Germany)

Abstract

Garnet zoning characteristics, reaction microstructures, and thermobarometric studies document a multistage P–T evolution for the paragneisses of the KTB drill core. The prograde heating and burial path led through the sillimanite stability field before it entered the kyanite field close to the P–T peak. The uppermost paragneiss–amphibolite unit (0–560 m) reached peak conditions at 650–670 °C and 7–8 kbar. The units below (560–3574 m) show higher peak temperatures of 680–720 °C, with maximum pressures between 8 and 9 kbar. This P–T discontinuity implies a tectonic contact, but otherwise, the two sections of the drill core do not show evidence for a separate metamorphic history and probably lay on a common crustal paleogeotherm at peak conditions. Homogenization of Mg, Fe, and Mn in garnet at or near peak conditions points to a significant time interval between burial and subsequent decompression. Accordingly, the prograde metamorphic episode must be significantly older than the 380–370 Ma decompression and cooling period, possibly mid-Ordovician. Following nearisothermal decompression with partial breakdown of garnet and growth of sillimanite, a second, previously unrecognized pressure peak was reached at 5.7– 6.7 kbar and temperatures of around 590 °C. A late low-pressure imprint is documented by the local presence of andalusite. The lack of a continuous metamorphic P–T gradient in the drilled crustal section either requires strong postmetamorphic tilting of isograds and/or small-scale intraformational stacking of crustal slices of similar metamorphic grade.