Appendix 2 - Analytical Methods
Geochronology
Samples were examined in thin section and then crushed in an effort to recover feldspar (particularly sanidine)
crystals for analysis. Individual feldspar crystals were then handpicked, avoiding altered or cloudy grains.
The whole rock samples were crushed, sieved (250– 500 μm fractions) and washed in dilute nitric acid, deionised water and acetone, before final handpicking to exclude altered and xenocrystic material. The feldspar
and whole rock samples were packaged in separate irradiation batches (UM#16; UM#24/25) and irradiated in
position 5C (cadmium-lined) of the McMaster University reactor, Hamilton, Ontario, Canada.
40
Ar/39Ar
analyses were conducted at the University of Melbourne Noble Gas Geochronology and Geochemistry
Laboratory, using analytical procedures analogous to those described by Matchan and Phillips (2011) and
Phillips et al. (2007). Step-heating analyses of feldspar and whole rock samples were carried out using a
tantalum furnace connected to a VG3600 mass spectrometer equipped with a Daly detector. The neutron
fluence monitors used were GA1550 biotite (UM#16; 98.8 ± 0.5 Ma, 1σ; Renne et al., 1998), which yielded Jvalues (±1σ) ranging from 0.000478 ± 0.000002 (0.465%) to 0.000482 ± 0.000003 (0.6%) and Fish Canyon
Tuff (FCT) sanidine (UM#24/25; 28.02 ± 0.14 Ma, 1σ; Renne et al., 1998), which gave J-values of 0.000703 ±
0.000001 (0.17%) to 0.000721 ± 0.000003 (0.4%). Mass discrimination (UM#16: 1.0053 ± 0.11%; UM#24:
1.0075 ± 0.20%; UM#25: 1.0081 ± 0.05%; per atomic mass unit) was monitored by measuring air aliquots
from a Doerflinger pipette. Correction factors (±1σ) for interfering isotopes were (39Ar/37Ar)Ca = 0.000679 ±
0.000005 (0.7%), (36Ar/37Ar)Ca = 0.000270 ± 0.000005 (1.7%), (40Ar/39Ar)K = 0.0005 ± 0.0002 (39%) and
(38Ar/39Ar)K = 0.01232 ± 0.00002 (0.2%) for UM#16; and (39Ar/37Ar)Ca = 0.000680 ± 0.000020 (2.9%),
(36Ar/37Ar)Ca = 0.000289 ± 0.000005 (1.7%), (40Ar/39Ar)K = 0.0004 ± 0.0004 (100%) and (38Ar/39Ar)K = 0.013
± 0.005 (39%) for UM#24/25. Plateau ages are defined as including at least 50% of 39Ar, distributed over a
minimum of 3 consecutive steps, coincident at the 95% confidence level (Fig. 9). Plateau ages are given at the
95% confidence level and calculated using the mean of all plateau steps, each weighted by the inverse variance
of their individual analytical error. Calculated uncertainties associated with mean and plateau ages include
errors in the J-values, but exclude errors associated with the age of the monitor and the decay constant (internal
errors only; see discussion by Min et al., 2000). Decay constants and isotopic ratios are those of Steiger and
Jager (1977). Note that new 40Ar/39Ar decay constants and standard ages have recently been suggested by
Renne et al. (2010). However, as these values have yet to be sanctioned by the Subcommission of
Geochronology, the values of Steiger and Jager (1977) are retained in this study. Note however, that use of the
Renne et al. (2010) values increases the calculated ages by only 1%, which is well within the quoted
uncertainties for the current analyses.
Whole-Rock Geochemistry
Rock samples were carefully selected, cleaned in distilled water and all enclaves were removed during
preliminary crushing operations. Rock chips were then powdered in an agate mill and analysed at Activation
Laboratories Ontario, for major, minor, trace and REE elements (code WRA+TRACE 4Lithoresearch) by
ICP-OES and ICP-MS methodologies. For major elements the precision is estimated better than 2% for values
higher than 5 wt% and better than 5% in the range 0.1 – 5 wt% (Table 2). Mineral phases were analysed using
polarized microscopy on a Nikon Eclipse E600 POL instrument and by combined WDS EDS techniques using
a CAMECA SX50 electron microprobe at the “Centro di Studi per il Quaternario e l’Evoluzione ambientale –
CNR” (Rome). Minerals were analysed using a 15 KeV accelerating voltage, 20nA beam current, 5µm beam
size (up to 8µm for albitic feldspars) and counting times of 100s. Data were reduced using the ZAF4/FLS
software by Link Analytical. Representative analyses are shown in Table 3a-e. Structural formulae for
feldspars, clinopyroxenes, and olivines were calculated using CALCMIN software (Brandelik, 2009). For
amphiboles the structural formulae and a preliminary IMA-04 classification were determined using
WINAMPHCAL software (Yavuz, 2007).