IT’S ABOUT TIME: RATES OF HIGH-GRADE METAMORPHIC PROCESSES

The rates of metamorphic processes have been classically determined by dating a series of metamorphic minerals with presumably known, different closure temperatures. Such an approach provides insight into the cooling from amphibolite facies to near-surface conditions. However, this technique does not allow determination of exhumation rates of rocks that underwent near isothermal decompression, such as eclogites, and is not suitable to establish detailed timing of high-temperature metamorphism. We show on two examples that rates of processes at high metamorphic conditions can be determined by combining U-Pb dating of accessory minerals with petrology and geochemistry. Titanites from calcsilicates of the Dora Maira in the Western Alps formed at peak ultra-high-pressure metamorphic conditions (37 kbar, 750°C) and during two stages of retrogression at 10 kbar, 600°C and 5 kbar 550°C. The titanite generations have different inclusions and distinct chemical compositions and were analysed in-situ using a SHRIMP ion microprobe. The age of the peak titanite is 35.1±0.9 Ma whereas the second and the third titanite generations yield slightly younger ages of 32.9±0.9 Ma and 31.8±0.5 Ma, respectively. The rocks were therefore exhumed from 115 km to 35 km depth with a rate of 3.4 cm/yr. From deep to mid-crustal levels, the exhumation slowed down to 1.6 cm/yr. This provides evidence that deeply subducted rocks can be exhumed at plate tectonic speed. Granulites formed at 850°C, 10 kbar at the crust-mantle boundary (Val Malenco, N-Italy). Zircon in the granulite contains an inherited core, and three metamorphic overgrowths with ages of 281±2, 269±3 and 258±4 Ma. Using mineral inclusions in zircon and garnet and their rare earth element composition it was possible to relate the three metamorphic ages to different stages of granulite facies metamorphism. The first age is related to prograde fluid-absent partial melting of mica. The second stage represents zircon crystallisation from melts related to cooling from 850°C to ~700°C. The third metamorphic zircon overgrowth formed during a second stage of partial melting that has not been identified by conventional petrological techniques. We suggest that a Permian gabbro intrusion and a shallow asthenosphere are the two heat sources for the 25 Ma of high-temperature metamorphism.