Contrasts between i- and s-type granitoids of the kosciusko batholith

Two distinct groups of granitoids occur on the eastern side of the Kosciusko Batholith. Those considered to be derivatives of sedimentary source rocks (S-types) are usually foliated and either contain cordierite or white-mica secondary after cordierite. The granitoids produced from igneous source material (I-types) are generally massive and frequently contain hornblende. Geochemical parameters provide the best discriminant between the two groups, I-types have higher Ca, Al, Na₂O/K₂O, and Fe₂O₃/FeO, and lower Fe, Mg, Sc, V, Cr, Co, Ni, Cu, Zn, Ba, Rb, Th, La, Ce, and Y than S-types of comparable SiO₂values. The differences between the two groups are not the result of differences in the melt-forming process but reflect differences in the nature of the source material. Thus the geochemical features of the S-type granitoids are indicative of their source rocks having been through a process of chemical weathering in a sedimentary cycle. Conversely, the I-type granitoids were derived from fractionated rocks that had not been involved in weathering processes.