Metamorphism at high temperature and pressure can proceed at different rates, from several cm/year to fractions of mm/year. This talk will explore the relation between tectonic setting and rate of metamorphism during collisional orogenies.
U-Th-bearing accessory minerals are ideal for determining rates and duration of high-grade metamorphism because they record and preserve overprinting stages of metamorphism, and their age can be linked to metamorphic conditions using inclusions, thermometry and trace elements.
Extensive microbeam geochronology of zircon, titanite and allanite in high-pressure rocks of the Western Alps indicates short duration of metamorphic cycles. Relatively small slices of high-pressure rocks were exhumed fast, in the order of several cm/year. In the Sesia Zone, a double cycle of subduction-exhumation-subduction (yo-yo subduction) occurred in less than 20 m.y. and testifies to the dynamic nature of oblique convergence. These fast rates are in contrast with the slower rates determined for high-pressure metamorphism of larger crustal units, which were metamorphosed in a different tectonic setting.
Multiple zircon, allanite and monazite domains can form during regional anatexis of crustal rocks. Geochronology of samples from migmatites in the central Alps and Sikkim Himalaya indicates protracted high temperature regimes lasting over 10 m.y.. Short-lived anatexis is instead documented in the Chugach Metamorphic Complex, southern Alaska, where zircon and monazite crystallization occurred between ~55-52 Ma on a ~200 km long section. Deposition of the sediments at ~60 Ma constrain the time available for burial to a maximum of 5 m.y.. The fast Alaskan tectonic is likely the result of ridge subduction, a different setting to the thick orogenic roots of the Alpine-Himalayan system.