Extended Data Fig. 9: Model evolution of melt composition for primitive Caribou Lake gabbro, undergoing two-stage crystallisation: fractionation at 6 kbar, below the silica-oversaturation/-undersaturation tipping point, and shallower emplacement and final crystallisation.

Deep fractionation occurs at 6 kbar, with batches of melt (labelled according to the vol.% of original melt left in the system) progressively extracted and crystallised at 2 (dotted lines) or 4 kbar (dashed lines). In these shallow crystallisation pathways, the melts undergo fractional crystallisation to 94-95 % of the original bulk and batch crystallisation of the remaining liquid (either to complete crystallisation, or to < 1 % of the original bulk remaining if the melt does not completely crystallise in the model system). The transition between fractional and batch crystallisation is marked with a symbol as in Fig. 3, identifying the state of silica-saturation of the melt at that point (star = silica-undersaturated, cross = silica-saturated, square = silica-oversaturated). Whole-rock Geochemistry^12,14,28 for BLIC shown in coloured symbols following Fig. 1: orange shades for eastern lobe of Nechalacho Layered Suite (circle), Thor Lake syenite (diamond), Grace Lake granite (cross); blue shades for western lobe of Hearne Channel/Mad Lake granite (upwards triangle), Whiteman Lake syenite (pentagon) and Caribou Lake gabbo (inverted triangle).