Magma emplacement, differentiation and cooling in the middle crust: Integrated zircon geochronological-geochemical constraints from the Bergell Intrusion, Central Alps

- Introduction of zircon workflow, including LA-ICPMS transects, ID-TIMS geochronology, and solution ICPMS geochemistry.
- Resolution of 104-106 yr timescales of single zircon U-Pb age heterogeneity in mid-crustal plutonic rocks by ID-TIMS.
- Temporal trends in zircon geochemistry are consistent with handsample-, lithology-, and pluton-scale differentiation.
- Zircon growth histories permit determination of ~ 100-1000 °C/Myr cooling rates in the middle to lower crust.

U-Th-Pb zircon geochronology is an essential tool for quantifying the emplacement, differentiation and thermal evolution of crustal magmatic systems. However, the power of U-Pb zircon dates can be enhanced through complementary characterization of mineral texture and geochemistry, as this permits more detailed interpretations of geochronological datasets than conventionally achieved. Our approach to better relating zircon dates and geological processes consists of a multi-method analytical workflow, including cathodoluminescence imaging (CL), in situ LA-ICPMS/EPMA zircon geochemistry, U-Pb zircon ID-TIMS geochronology, and solution ICPMS zircon Trace Element Analysis (U-Pb TIMS-TEA). These methods are here applied to zircon from the Bergell Intrusion, a composite Alpine pluton preserving a ~ 10 km mid-crustal transect. Hand samples of tonalite, granodiorite and hybridized granitoid each record 250-700 kyr of autocrystic zircon growth. Bergell zircons are ubiquitously zoned with ca. 104-106 yr growth histories, as evidenced by ID-TIMS analysis of microsampled fragments from single crystals. U-Pb TIMS-TEA data exhibit compositional trends on multiple spatiotemporal scales, including the handsample-scale, representing in situ differentiation at the emplacement level (e.g., Th/U); lithology-scale, defining trajectories corresponding to the production of tonalitic versus granodioritic magmas (Lu/Hf); and pluton-scale, indicating increasingly-evolved melts over ~1.6 Myr of pluton assembly (Zr/Hf). These absolute TIMS-TEA temporal trends are corroborated by relative LA-ICPMS/EPMA core-to-rim geochemistry. We compare records of trace element evolution from TIMS-TEA, Bergell whole-rock geochemistry, and a global compilation of whole-rock geochemical data. These findings support zircon compositional evolution as a robust indicator of differentiation at local and crustal scales, and provide key empirical constraints on melt differentiation and cooling timescales in the middle crust.