Metamorphic rates in collisional orogeny from in situ allanite and monazite dating

The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560-580 C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430-450 C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of allanite (31.5 1.3 and 29.2 1.0 Ma) and monazite (18.0 0.3 and 19.1 0.3 Ma) constrains the time elapsed between 430-450 C and 560-580 C, which implies an average heating rate of 8-15 C/m.y. Combined with new fi ssion track ages (zircon, 10-9 Ma; apatite, 7.5-6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to fi nal cooling, can be reconstructed.