Use of nonparametric ranking statistics to characterize magmatic and post-magmatic processes in the eastern South Mountain Batholith, Nova Scotia, Canada

Abstract

The South Mountain Batholith is a large, post-tectonic, peraluminous, granitoid complex consisting of many coalesced plutons. Compositions range from biotite granodiorite, through two-mica monzogranite, to muscovite leucogranite. Application of the non-parametric Spearman rank correlation coefficient (SRCC) statistic to major- and trace-element chemical data from one large pluton (the Halifax Pluton), and to two different groups of much smaller leucogranitoid bodies, shows strongly different correlation coefficients in the three groups. The Halifax Pluton shows strong SRCCs between compatible elements (Ba, Sr, Zr, Sc, V, La) and weak SRCCs for the incompatible elements (Rb, Ta, Nb, Li, F, Sn). The leucogranites generally show correlations opposite to the Halifax Pluton, and also show some strong correlations between elements that were poorly correlated in it (Na, K, Li, P). Non-geological controls (such as closure, dissimilar compositional ranges, dissimilar groupings, and differing analytical precisions) cannot account for the observed differences between the magnitudes and signs of the correlation coefficients in the three groups. On the other hand, geological controls (principally magmatic vs. hydrothermal) do explain most of the differences in the correlation coefficients, and the transition between magmatic and hydrothermal control is apparent. Spearman rank correlation coefficients can be used with compositional data drawn from populations with non-normal distributions: to provide a comprehensive and rapid, nongraphical assessment of inter-element correlations; to quantify these inter-element relationships; and to both identify processes, and to gauge their relative contributions to the observed compositions.