Use of the lattice parameter b₀ of dioctahedral illite/muscovite for the characterization of P/T gradients of incipient metamorphism

The P/T gradient or "baric type" of low-grade metamorphic terranes can be characterized on the basis of cumulative frequency curves of the lattice parameter b₀ of dioctahedral micas (Sassi and Scolari 1974), which essentially reflects celadonite content and increases with pressure. The method is here applied to illite/muscovite of sub-greenschist ("incipient" metamorphism) and low-greenschist facies terranes from the marginal zones of the Swedish Caledonides, Swiss Alps and Venezuelan Andes, where the grade of metamorphism was determined on the basis of illite "crystallinity". Complications in the determinations of b₀ arising from (a) shifts in the spacing of the apparent 060 diffraction peaks of the illites due to the presence of inter-stratified expandable layers, and (b) persistence of more "crystalline" clastic micas are pointed out. In contrast to the earlier-reported decrease of b₀ with progressive metamorphism at higher grades, b₀ tends to increase with grade during incipient metamorphism: it is markedly lower in the "diagenetic" and low-grade anchimetamorphic grades in the Cambro-Silurian of the Caledonides of Jämtland, western Sweden (zone A), and the Lower Tertiary of the Helvetic zone of the Swiss Alps, than in the higher grade anchizone and "epizone" of the former terrane. Mean b₀ values for the intermediate and highest ("epizone") grades of the Swedish Caledonides are respectively 9.030 and 9.037 Å compared to the mean value of 9.035 Å for the medium-high pressure, Barrovian-type, terrane of Otago, N.Z. Mean b₀ of 9.005 Å for the mainly epizonal slates of the Venezuelan Andes lies between those for the low-pressure terranes of Bosost (∼ 8.992 Å), and the low-medium pressure terrane of northern New Hampshire (∼9.010 Å). These baric estimates agree well with those for the metamorphic facies series at higher grade produced by the same events. Cumulative b₀ curves are concluded to be useful for the characterization of P/T gradients of incipient metamorphism, particularly in the higher grade part of the anchizone and the "epizone".