Appendix 1. Analytical procedures.

Microchemical analyses.
The operating conditions were 15 kV accelerating voltage and beam current of 15 nA. Natural silicates were used as standards; matrix corrections were calculated with the ZAF procedure.

40Ar-39Ar analyses.
Ages are relative to the Fish Canyon Tuff sanidine (28.03 Ma; Jourdan & Renne, 2007) and were calculated using constants recommended by Steiger & Jäger (1977). Errors are given as 2σ. Details on the analytical procedures are described in Di Vincenzo et al. (2006) and Di Vincenzo & Skála (2009).  

References:

Di Vincenzo, G. & Skála, R. 2009. 40Ar–39Ar laser dating of tektites from the Cheb Basin (Czech Republic): evidence for coevality with moldavites and influence of the dating standard on the age of the Ries impact. Geochimica et Cosmochimica Acta, 73, 493–513. 
Di Vincenzo, G., Tonarini, S., Lombardo, B., Castelli, D. & Ottolini, L. 2006. Comparison of 40Ar-39Ar and Rb-Sr data on phengites from the UHP Brossasco-Isasca Unit (Dora Maira Massif, Italy): implications for dating white mica. Journal of Petrology, 47, 1439–1465. 
Jourdan, F. & Renne, P. R. 2007. Age calibration of the Fish Canyon sanidine 40Ar/39Ar dating standard using primary K-Ar standard. Geochimica et Cosmochimica Acta, 71, 387–402. 
Steiger, R. H. & Jäger, E. 1977. Subcommission on Geochronology: Convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36, 359–362.