A mid-crustal tipping point between silica-undersaturated and silica-oversaturated magmas

Alkaline–silicate igneous complexes contain a huge diversity of rock types, ranging from silica-undersaturated (feldspathoid-normative) to silica-oversaturated (quartz-normative) compositions. At present, the controls on the formation of such compositional diversity are poorly quantified. Here we apply thermodynamic models to investigate these controls using a case study of the Blatchford Lake Igneous Complex (Canada), which is compositionally representative of worldwide alkaline–silicate systems. By modelling fractionation of a primitive mafic melt across crustal pressures, we identify a narrow (~0.5 kbar) ‘tipping point’ across which residual melts become silica-rich or alkali-rich when shallower or deeper, respectively. This tipping point is consistently present at mid-crustal pressures (3–5 kbar; ~10–15 km depth) for a range of viable primitive melts, moving to higher pressures within this range for more hydrous and more oxidized melts. Crystallization at these pressures (within barometric estimates for the complex) can therefore generate and explain the vast diversity of observed alkali-rich and silica-rich compositions. A similar tipping point is also present in other modelled mafic igneous systems at mid-crustal conditions, indicating it is a widespread phenomenon. This result implies a key role for mid-crustal mafic staging chambers in generating compositional diversity in alkaline–silicate complexes worldwide.