Modal Spaces for Pelitic Schists

Modal space is unconventional and still unfamiliar to most readers. Some historical remarks with reference to earlier papers are therefore necessary. Modal spaces, then called “net-transfer reaction spaces” were first introduced by myself, with Jo Laird and Alan Thompson (Thompson et al. 1982; see also Thompson 1981, 1982a, b), as an aid in understanding the metamorphism of mafic rocks over a wide range of pressures and temperatures. As viewed in retrospect, that paper was overly concerned with the accessible limits of such a space as determined by the bulk composition of a given assemblage, and was too little concerned with the fact that these spaces also contain an array of isopleths. The isopleths provide useful information on variations in modal abundance of the mineral phases involved. The orientations of such isopleths depend only on the stoichiometry of the reactions selected as basis vectors, and are thus independent of the bulk composition of a given assemblage. As a consequence of our early emphasis, the spaces we presented have been regarded as useful primarily in the study of mafic rocks lying within a highly limited range of bulk composition. In some of my later papers, including this one, I have introduced some simplifications in methodology and tried to emphasize the wider utility of the concept. The name “modal space” was introduced in a volume dedicated to the memory of Paul Niggli (Thompson 1988), and the idea was extended further, but still with emphasis on mafic rocks, in a volume dedicated to Hugh Greenwood (Thompson 1991). An extension to (mainly peraluminous) quartzo-feldspathic rocks was presented at the Korzhinskii symposium in Moscow in 1999, and has recently been published (Thompson 2000). The method presented is applicable to a wide range of metamorphosed quartzo-feldspathic rocks that may be of either igneous or …