PORTRAIT OF A PLUTON: AN INTEGRATED OBSERVATIONAL, ANALYTICAL AND MODELING STUDY OF THE MID-CRUSTAL BERGELL INTRUSION, CENTRAL ALPS

Insights into the processes and timescales of arc magmatism are best achieved through a combination of field observations, accessory phase geochronology+geochemistry, and numerical modeling. We adopt such an approach in reconstructing the emplacement, differentiation and thermal history of the Bergell Intrusion, a normally-zoned Alpine pluton spanning a ~10 km transect through the middle crust. U-Pb zircon, titanite and allanite geo-/thermochronology provide crucial analytical constraints for simulations of pulse-wise, incremental assembly. Protracted autocrystic zircon crystallization histories (300–700 kyr) are interpreted as recording durations between magma zircon saturation and cooling to the solidus. Such records provide a direct petrologic link to forward models of magma emplacement; moreover, both geochronological data and thermal models can be used to derive quantitative magmatic cooling rates. U-Pb TIMS-TEA zircon data support in situ (emplacement-level) and ex situ (pre-emplacement) differentiation during Bergell assembly, with titanite and allanite dates providing complementary constraints on the timing of solidification. Geochronology and modeling are performed in the context of geologic mapping and structural observations. Our work demonstrates that pulsed assembly can lead to the formation of substantial mid-crustal melt reservoirs, a finding largely in contrast to models of shallow crustal plutons. This result highlights the sensitivity of variables such as ambient country rock temperature in dictating melt residence timescales, and emphasizes the importance of applying coupled data+modeling studies to intrusive rocks across a range of observed emplacement levels and tectonic settings. Understanding magmatism in the deep crust is important as such systems may act as intermediate reservoirs that feed magmas to more ephemeral systems in the shallow crust.