The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)

Abstract

This study investigates the behaviour of the Zr-in-rutile and Ti-in-zircon thermometers in granulite facies metapelites from the Ivrea-Verbano Zone lower crustal section. U-Pb ages of zircon constrain the timing of regional amphibolite-granulite facies metamorphism to 316 ± 3 Ma and record zircon recrystallisation and resetting of U-Pb ages at 276 ± 4 Ma and 258 ± 3 Ma. Zr-in-rutile thermometry records peak contact metamorphic temperatures related to intrusion of mafic magmatic rocks and gives peak temperatures between 900-930 °C and 1,000-1,020 °C that are consistent with the geological settings of the samples. Ti-in-zircon temperatures of 700-800 °C and 810-870 °C record growth or re-equilibration of zircon after cooling from peak temperatures. Ti-in-quartz thermometry for one sample records both peak and retrograde temperatures. Some rutiles in all samples record resetting of Zr-in-rutile temperatures at ~750-800 °C. Electron microprobe profiles across individual rutiles demonstrate that Zr expulsion occurred by recrystallisation rather than by diffusive exchange. Exsolution of small needles of baddelyite or zircon from rutile is an important method of Zr redistribution, but results in no net Zr loss from the grain. The demonstration that Zr-in-rutile thermometry can robustly record peak temperatures that are not recorded by any other thermometer emphasises the relevance of this technique to investigating the evolution of high-grade metamorphic terranes, such as those that characterise the lower crust.