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

Abstract

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. Show more