Supplementary data Appendix 2: Beaumont Glacier tonalite petrography and classification

At R.7629 and R.7630 (Fig. 3a) the granitoid sheet cuts folded Hjort Formation volcanic rocks. In thin section, it has a coarse-grained and hypidiomorphic texture with grains ranging from 0.1–0.7 mm maximum dimension.  The modal mineralogy comprises 40% plagioclase; 25% quartz; 15% hornblende; 10% biotite; 2-3% sphene/titanite; and 2% opaques.  K-feldspar is absent.  Plagioclase is An45–55 andesine–labradorite and grains show Carlsbad and lamellar twinning with abundant growth zonation and evidence of some resorption during crystallisation.  Some sericitic alteration is evident in plagioclase, often restricted to grain cores or particular growth lamellae.  Quartz grains have an irregular habit and show sweeping extinction.  Hornblende grains show Carlsbad twinning and have an idiomorphic to sub-idiomorphic habit.  They show strong grass-green to yellow-green pleiochroism in plane polarised light. Concentric zoning is evident in some grains.  Biotite grains are irregular with fox-red to brown pleiochroism in plane polarised light and show sweeping extinction under crossed-polars.  Petrographically the rock is a quartz tonalite.