Zircon and titanite recording 1.5 million years of magma accretion, crystallization and initial cooling in a composite pluton (southern Adamello batholith, northern Italy)

The southern part of the Adamello batholith (the so-called "Re di Castello unit") is an example of a composite pluton, ranging from gabbro to granodiorite in composition. U-Pb dating of single-zircon crystals from four tonalitic to granodioritic lithologies reveals that zircon crystallization is protracted in all studied lithologies, showing apparent durations of growth between 90 and 700 ka. The youngest zircons crystallized near the solidus and yield identical or slightly older ages than titanite. The formation of these autocrystic zircons is considered to approximate the age of emplacement of the melt and its final crystallization, in contrast to antecrystic zircons present in the same sample, which had formed earlier in the magmatic column or were derived from re-mobilized earlier magma. The autocryst-derived "emplacement" ages range from 42.43 ± 0.09 Ma to 40.90 ± 0.05 Ma, recording 1.5 Ma of intrusion and crystallization history. We anticipate that extended periods of zircon crystallization may be common in silicic rocks, whereas the zircons from residual melts from initially undersaturated mafic liquids should yield far more precise emplacement ages within our present analytical uncertainties of 0.1-0.2% in ²⁰⁶Pb/²³⁸U age. Decreasing Th/U ratios of dated zircons within one melt batch document the depletion of the residual melt portion in Th due to the contemporaneous crystallization of titanite. Preliminary Hf isotopic compositions of the dated zircon grains suggest that the early stage melts of the southern Re di Castello unit represent hybrid melts with an important crustal component (ε_Hf between - 2.8 and +3.0). Subsequently emplaced melts are more juvenile at ε_Hf values at +6.4 to +8.9 and may thus reflect the addition of large volumes of mafic melt to the magmatic system.