THERMOBAROMETRY AND P-T PATH OF GARNET-ALUMINOSILICATE-BEARING GNEISSES FROM THE KTB DRILL CORE (CONTINENTAL DEEP-DRILLING PROJECT, GERMANY)

Garnet zoning characteristics, reaction microstructures, and thermobar ometric studies document a multistage P-T evolution for the paragneiss es of the KTB drill core. The prograde heating and burial path led thr ough the sillimanite stability held before it entered the kyanite fiel d close to the P-T peak. The uppermost paragneiss-amphibolite unit (0- 560 m) reached peak conditions at 650-670 degrees C and 7-8 kbar. The units below (560-3574 m) show higher peak temperatures of 680-720 degr ees C, with maximum pressures between 8 and 9 kbar. This P-T discontin uity implies a tectonic contact, but otherwise, the two sections of th e drill core do not show evidence for a separate metamorphic history a nd 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 de compression. Accordingly; the prograde metamorphic episode must be sig nificantly older than the 380-370 Ma decompression and cooling period, possibly mid-Ordovician. Following near-isothermal decompression with partial breakdown of garnet and growth of sillimanite, a second, prev iously unrecognized pressure peak was reached at 5.7-6.7 kbar and temp eratures of around 590 degrees C. A late low-pressure imprint is docum ented by the local presence of andalusite. The lack of a continuous me tamorphic P-T gradient in the drilled crustal section either requires strong postmetamorphic tilting of isograds and/or small-scale intrafor mational stacking of crustal slices of similar metamorphic grade.