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

Schoene, Blair
Brack, Peter
Latkoczy, Christopher
Stracke, Andreas
Günther, Detlef
Published in Earth and Planetary Science Letters. 2012, vol. 355-356, p. 162-173
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 (>1 Ma) 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 15 km2 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 (∼ 105 yr) 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 10-30 ka timescales. Subsequent, more felsic pulses yield individual zircon dates spanning as much as 200 ka within single handsamples and restrict the total construction time of the LVC to <300 ka. Rim to core solidification of the LVC, as recorded by titanite U-Pb thermochronology, occurred in ≥300 ka. 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 ∼300 ka, 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.
Keywords U-Pb geochronologyId-timsPlutonic systemsMagmatic processesZirconTitanite
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Research group Isotope Geochemistry, Geochronology and Thermochronology
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SCHOENE, Blair et al. Rates of magma differentiation and emplacement in a ballooning pluton recorded by U-Pb TIMS-TEA, Adamello batholith, Italy. In: Earth and Planetary Science Letters, 2012, vol. 355-356, p. 162-173. https://archive-ouverte.unige.ch/unige:26130

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Deposited on : 2013-02-04

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