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The Role of Manganese in the Paragenesis of Magmatic Garnet: A Discussion

Among the minerals characteristic of peraluminous granitoid rocks the origin of garnet has probably stimulated the most controversy. Miller and Stoddard (1981) (hereinafter referred to as "M and S") conclude, based on studies in the Old WomanPiute Range, California, that most of the garnets in granitoids have crystallized from Mn-rich magmas and that Mn-poor garnets are rare and represent unusual conditions of magma generation or crystallization. We agree with M and S that Mn-enrichment in peraluminous magmas serves to enhance the stability of garnet and broaden its crystallization field; points long appreciated by petrologists (e.g., Michel-Levy 1951; Miyashiro 1953; Oliver 1956; Hall 1965) and confirmed by Green's (1977) experimental studies. However, our own studies lead us to less sweeping statements regarding the role of Mn in garnet paragenesis. Mn-poor garnets are widespread as minor phases in many peraluminous granitoids (e.g., Brammall and Bracewell 1933; Clarke 1981 and references therein; Phillips et al. 1981; Speer 1981) and similar silicic volcanics (e.g., Birch and Gleadow 1974; Oliver 1956; Clemens et al. 1979; cf. M and S, p. 233). Their host rocks are commonly neither Mnrich nor do they show other evidence of highly fractionated character. For example, in the central Victorian examples described by Clemens et al. (1979) and Phillips et al.