The Thermal History of the Lower Crust Recorded by Zircon Growth and Recrystallization: An Ion Microprobe (SHRIMP) Study from the Ivrea Zone (Southern Alps)

Abstract

The thermal history of granulite facies terrains, especially in its prograde part, is poorly dated by most mineral-based isotopic decay systems, because they become closed only after cooling from the thermal peak. The U/Pb system in nonmetamict zircon makes the only known exception. Due to the extremely low diffusivity of chemical components within such zircons, internal zoning patterns resulting from overgrowth and reerystallization are preserved even at the highest temperatures attained in the earth's crust. They are revealed by eathodoluminescence imaging of zircon grains and are U/Pb-dated by the ion microprobe. The attribution of the various domains in zircon crystals to metamorphic and anatectic processes in the host rocks is the crucial point for resolving the thermal history of the gramdites.