Constraints on the thermal evolution of the Adriatic margin during Jurassic continental break‑up from U–Pb dating of rutile (Ivrea–Verbano Zone, Italy)

Abstract:

U­–Pb dating of rutile is an ideal tool for exploring the cooling and exhumation history of the lower crust, given its moderate closure temperature and the occurrence of rutile in relevant lithologies. We present an example from the Ivrea–Verbano Zone (IVZ, Italy), a classic section through the Permian lower crust that records high-temperature metamorphism followed by extension and exhumation associated partly with the Jurassic opening of the Alpine Tethys ocean. Granulite facies metapelites collected across ~35 km have Zr-in-rutile temperatures that record crystallisation during Permian metamorphism and anatexis, but SHRIMP U–Pb dating of rutile records cooling through 650–550 °C in the Jurassic. Rutile age distributions are dominated by a peak at ~160 Ma, with a subordinate peak at ~175 Ma. Both age populations show excellent agreement between samples, indicating that the two distinctive cooling stages they record were synchronous on a regional scale. The ~175 Ma population is interpreted to record cooling in the footwall of rift-related faults and shear zones, for which widespread activity in the Lower Jurassic has been documented along the western margin of the Adriatic plate. The ~160 Ma age population postdates the activity of all known rift-related structures within the Adriatic margin, but coincides with extensive gabbroic magmatism and exhumation of sub-continental mantle to the floor of the Alpine Tethys, west of the IVZ. We propose that this ~160 Ma early post-rift age population records regional cooling following episodic heating of the distal Adriatic margin. The partial preservation of the ~175 Ma age cluster suggests that the post-rift (~160 Ma) heating pulse was of short duration. The regional consistency of the rutile U–Pb data is in contrast to many other thermochronometers in the IVZ, demonstrating the value of this technique for probing the thermal evolution of high-grade metamorphic terranes. The decoupling between Zr-in-rutile temperatures and rutile U–Pb ages under lower crustal conditions is exploited to constrain two different parts of the T–t evolution of this section with a single mineral.