The Finer-Grained Calcalkaline Granites of New England

Modal analyses for a collection of 145 thin sections of New England calcalkaline granites are presented and discussed. The data are used to estimate a range in which average or bulk modal compositions may be expected to fall. The internal variability of such granites is also examined in some detail. Remarkable internal homogeneity, rather small differences between granite masses, and consistent relation of data points for each mass to the thermal valley in the system Q-Or-Ab all suggest the existence of granitic (liquid) magmas as parent-materials. The origin of these magmas is not known; the most likely processes appear to be selective refusion and crystal fractionation. Critical evidence permitting a choice between these alternatives is not available, and it is curious that the difficulties confronting them are nearly identical.