Integrating in situ Geochronology and Metamorphic Petrology: An Example from the Gruf Complex, European Central Alps

Abstract

Understanding the thermal evolution (i.e. the timing, rate, duration, and magnitude of thermal events) within mountain belts has important implications for the geodynamic evolution of both ancient and modern orogenies. Ultra-high temperature (UHT) metamorphism requires geodynamic or tectonic processes that bring excess heat to the lower crust. Therefore, dating UHT metamorphism can shed light on the geodynamic evolution of the geological settings in which UHT rocks are exposed. In recent years, many researchers have used accessory mineral U-Pb geochronology to date (U)HT metamorphic events. However, it is not always clear to what part of the pressure-temperature (P-T) path the ages relate. Using an in situ approach, this study combines accessory mineral U-Pb geochronology with single mineral thermometry, thermobarometric modeling, and trace element geochemistry to elucidate the P-T-time (P-T-t) evolution of sapphirine-bearing granulites from the Gruf Complex in the Central Alps. Two main questions are addressed: (1) When did the Gruf Complex experience UHT metamorphism? (2) What parts of the P-T evolution of high-grade metamorphic rocks can be dated using U-Pb geochronology of different accessory phases? Equilibrium phase diagrams calculated from whole rock and microdomain compositions and Zr-in-rutile thermometry indicate that the Gruf granulites underwent UHT metamorphism at 900–1000°C and 7.0–9.5 kbar after decompressing from ca. 800°C and 9–12 kbar. This decompression-heating event resulted in the breakdown of garnet to form orthopyroxene, sapphirine, and cordierite. A lack of inherited monazite and presence of young (34–30 Ma) monazite within UHT textures is interpreted to indicate that UHT metamorphism was the last main metamorphic event the Gruf granulites experienced, thus precluding a Permian UHT event followed by a lower temperature (700–750°C) Alpine event. Textural observations and Ti-in-zircon thermometry reveal that minor zircon growth occurred in equilibrium with garnet at 34.8 ± 1.1 Ma, and zircon was not growing, but resorbing during UHT metamorphism. Therefore, the youngest zircon rims can only be used to date post-UHT melt crystallization and cooling at 32.7 ± 0.7 Ma. The U-Pb zircon ages of variable deformed felsic dikes indicate that the lower crustal UHT rocks were juxtaposed against the midcrustal migmatites between 30 and 27 Ma and contractional deformation ceased by 25.6 ± 0.3 Ma in the Gruf Complex. Finally, U-Pb rutile ages indicate that the amalgamated Gruf Complex cooled from 700–420°C over an 11 m.y. period from 30 to 19 Ma. These results indicate that different accessory minerals can be used to date different stages of the evolution of UHT rocks. However, depending on the reactions in the rock volume, dateable accessory minerals may be crystallizing, resorbing, or not reacting at a given P-T condition. Therefore, combining accessory mineral ages with textural, geochemical, and petrological information is necessary to elucidate the P-T-t evolution of a particular rock package.