TIMESCALES OF BATHOLITH ASSEMBLY: THE ADAMELLO BATHOLITH, ITALY

BRODERICK, Cindy¹, SKOPELITIS, Alexandra¹, SCHALTEGGER, Urs², BAUMGARTNER, Lukas³, ULIANOV, Alexey⁴ and BRACK, Peter⁵, (1)Earth Sciences, University of Geneva, Rue des Maraîchers 13, Geneva, CH-1205, Switzerland, (2)Section of Earth and Environmental Sciences, University of Geneva, rue des Maraîchers 13, Geneva, 1205, Switzerland, (3)Institute of Earth Sciences, University of Lausanne, Lausanne, CH-1015, Switzerland, (4)Earth Sciences, University of Lausanne, Lausanne, CH-1015, Switzerland, (5)Earth Sciences, ETH, Zürich, CH-8092, Switzerland, Cindy.Broderick@unige.ch

Large silicic batholiths were constructed from multiple injections at 100 ka to Ma timescales (Coleman et al., 2004, Schaltegger et al., 2009). Knowing the timescales of batholith assembly allows for more accurate reconstruction of the processes of magma generation, transport and emplacement. With recent advances in U-Pb zircon geochronology, we can look at batholith assembly in great detail by dating zircons from individual magma pulses.

For this case study in the Adamello batholith (N. Italy), both chemical abrasion ID-TIMS and LA-ICP-MS methods were used for dating zircons by U-Pb. The southern Adamello displays complex relationships among mafic melts that were injected into solidifying felsic magmas. Single zircon crystals were dated using CA-ID-TIMS employing the well calibrated ET2535 tracer solution for maximum precision and accuracy. Permil uncertainties on single analysis allow us to resolve autocrystic, antecrystic and inherited zircons within all dated lithologies in the southern Adamello. ²⁰⁶Pb/²³⁸U dates of autocrystic zircons from the southern Adamello are used to approximate the emplacement age and time-lapse between pulses. Zircons from rocks of more evolved tonalitic rocks from the central and northern Adamello units were dated by U-Pb using LA-ICP-MS techniques. The in situ laser ablation method combined with cathodoluminescence imaging allows for rapid dating of single zircons, avoiding xenocrystic cores, with ~2% uncertainties. We use the weighted mean ²⁰⁶Pb/²³⁸U dates as the approximate emplacement ages, well aware that auto- and antecrystic zircon is averaged.

Current results indicate that the Adamello batholith was constructed incrementally from individual pulses crystallizing over periods of few 100ka, with possible quiescence periods of several 100 ka, beginning with a complex history in the south and becoming more evolved, more homogenous and younger towards the north. The combination of two U-Pb zircon techniques shows that the whole Adamello batholith was constructed over a minimum period of 4.5 Ma. Geochemical and isotope tracers in combination with these methods will be needed to understand the architecture of the Adamello batholith.

Coleman et al., 2004, Geology 32: 433-436

Schaltegger et al., 2009, EPSL 286: 208-218

Session No. 263--Booth# 236

T68. EARTHTIME: In Honor of the Contributions of James Mattinson to High-Precision U-Pb Geochronology (Posters)

Wednesday, 3 November 2010: 8:00 AM-6:00 PM

Hall D (Colorado Convention Center)

Geological Society of America Abstracts with Programs. Vol. 42, No. 5, p.618

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