Appendix 1: Detailed Ar-Ar data for actinolite infilling amygdales from sample STP-8, composite isochron calculations (runs 11475-01 and 11475-02), and Ar-Ar spectra for runs 11375-01 and 11375-02. 
Minerals were irradiated for 22 h in the 5 MW Herald-type reactor of the Chilean Nuclear Comission (Canada) in position A-9 along with Fish Canyon Sanidine (FCS) neutron fluence monitor for which an age of 28.03 ± 0.16 Ma is adopted (Renne et al., 1993). Upon return from the reactor, FCS monitors from each orifice are analysed by the total fusion 40Ar/39Ar method, and individual J factors are calculated for each grain, which represents an individual position in the disk. Due to the large neutron flux variation observed across the sample discs (10-12%), some J factor variability is observed due to random positioning of the FCs grains at the margins of the orifices. In order to reduce and account for within-orifice J factor variability, a “J-surface” is calculated across the disc by applying a 2-dimensional polynomial fit to the FCs analyses, using the x-y positions of the centres of the orifices. The average percentage difference between calculated J factors and individual factors measured on FCs grains is used as the percentage error for the disc and is applied to the calculated J factor for each orifice. Percentage errors of 0.4 to 0.7% are typically obtained. For studied samples, representative J-factor values are listed below. 
Isotope fractionation and other kinetic effects are corrected using periodic analyses of air samples, from which a correction factor (discrimination factor, D) is calculated. A representative value of discrimination factor was 1.00464 ± 0.00017. 
Correction factors for interfering nuclear reactions are determined using fluorite, synthetic kalsilite (potassium aluminium silicate) and Ca-S glasses and correspond to (40Ar/39Ar)K=0.0014057±0.0003348; (36Ar/37Ar)Ca=0.000333±0.0000012; and (39Ar/37Ar)Ca=0.000778±0.0000033, for the period during which these experiments were performed. 
Age calculations are made using the decay constants given by Steiger and Jäger (1977). The criteria to define a plateau age are defined following the approach of Fleck et al. (1977): three or more sequential steps containing at least 50% of the total liberated 39ArK, concordant at the 2 error level. Composite inverse isochron is plotted using individual steps, and eliminating those considered suspect on the basis of visual inspection, MSWD values (<2), and unreasonable 36Ar/40Ar axis intercepts.

References

Fleck, R. J., Sutter, J.F. & Elliot, D.H. 1977. Interpretation of discordant 40Ar/39Ar age-spectra of Mesozoic tholeiites from Antartica. Geochimica et Cosmochimica Acta, 41, 15-32.
Renne, P.R., Deino, A.L., Walter, R.C., Turrin, B.D., Swisher, C.C., Becker, T.A., Curtis, G.H., Sharp, W.D. & Jaouni, A. 1994. Intercalibration of astronomical and radioisotopic time. Geology, 22, 783-786.
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.