Leucogranites from the Eastern Part of the South Mountain Batholith, Nova Scotia

Abstract

The South Mountain Batholith is a peraluminous granitic complex ranging in composition from biotite granodiorite to muscovite-topaz ‘leucogranite’. Leucogranitic rocks (with generally <2% biotite) form a minor part (˜1⋅5%) of the batholith, and are of two types: (1) ‘associated leucogranites’ occurring as relatively small zones in fine-grained leucomonzogranites; and (2) ‘independent leucogranites’ forming generally larger bodies having no particular spatial association with other rock types. Mean chemical compositions of these two types of leucogranite are as follows (associated, independent): Na2O(3⋅46,3⋅83),K2O(4⋅40,4⋅09),and P2O5 (0⋅26, 0⋅45)in wt.%;Li(149, 281), F(1199, 2712), Rb (393, 725), U (7⋅4, 4⋅4), Nb (12⋅8, 23⋅4), Ta (2⋅9, 7⋅1), and Zr (52, 31) in ppm. Rare earth elements also differ between the two types (associated, independent): ΣREE (34⋅1 ppm, 19⋅9 ppm); and in the degree and variability of heavy REE fractionation (GdN/YbN=4⋅6±2⋅2, 2⋅0±0⋅7). In addition, associated leucogranite has REE compositions similar to those of its host rocks. Mean δ18O values (associated +ll⋅2±1⋅2‰, independent +ll⋅4±0⋅5‰; relative to SMOW) are comparable with the mean for the entire South Mountain Batholith (+l0⋅8±0⋅7‰). Radiometric dating (40Ar/39Ar on muscovite) shows that both types of leucogranite have identical ages of 372±3 Ma, equivalent to ages determined by other techniques for granodiorite and monzogranite samples elsewhere in the batholith. Field relations and geochemistry suggest that the associated leucogranite results from an open-system interaction between a fluid and its host leucomonzogranite, whereas the independent leucogranite bodies are discrete intrusions of highly fractionated melts that underwent closed-system, late-magmatic to post-magmatic fluid alteration. Where mineralized, the associated leucogranite characteristically hosts greisen-type or disseminated polymetallic mineralization, whereas the independent leucogranite hosts pegmatitic or disseminated polymetallic mineralization.