Rates of magma differentiation and emplacement in a ballooning pluton recorded by U–Pb TIMS-TEA, Adamello batholith, Italy

Abstract

Geochemical, structural, field, and geochronological data have been used to arrive at very different models for the construction of upper crustal batholiths. Models for pulsed intrusion of small magma batches over long timescales (>1Ma) versus transfer of larger magma bodies on shorter timescales predict a different thermal, metamorphic, and rheological state of the crust, highlighting the importance of robust time constraints. This study focuses on a well-characterized upper crustal intrusion, the 15km2 Lago della Vacca complex (LVC), Adamello batholith, N. Italy. Previous studies used structural and petrologic data to argue that the LVC was emplaced through pulsed magma injection and in situ expansion (or, ballooning) of a short lived (∼ 105yr) magma chamber. We test this model using a dense sampling strategy and high-precision ID-TIMS U–Pb geochronology of zircon and titanite combined with hafnium isotope and trace element analyses of the same volume of dated mineral (U–Pb TIMS-TEA). These data show that the marginal mafic pulses of magma crystallized zircon with primitive Hf isotopes and negligible Eu anomalies during fractional crystallization and ascent through the crust on 10–30ka timescales. Subsequent, more felsic pulses yield individual zircon dates spanning as much as 200ka within single handsamples and restrict the total construction time of the LVC to <300ka. Rim to core solidification of the LVC, as recorded by titanite U–Pb thermochronology, occurred in ≥300ka. U–Pb TIMS-TEA data from these complicated zircon populations limit the sources of antecrystic zircon, constrain AFC processes within host magmas, and illustrate the difficulty in interpreting zircon dates as magma emplacement ages. These data are supportive of growth of the LVC by a pulsed ballooning-type process over ∼300ka, with the restriction that the rims of the LVC had solidified prior to the center-most injection, providing important constraints for thermomechanial models and strain analysis of the LVC and other balloon-like intrusions.