LEUKOGRANITES FROM THE EASTERN PART OF THE SOUTH MOUNTAIN BATHOLITH, NOVA-SCOTIA

The South Mountain Batholith is a peraluminous granitic complex rangin g in composition from biotite granodiorite to muscovite-topaz 'leucogr anite'. Leucogranitic rocks (with generally <2% biotite) form a minor part (approximately 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' formi ng generally larger bodies having no particular spatial association wi th 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 tw o types (associated, independent): SIGMAREE (34.1 ppm, 19.9 ppm); and in the degree and variability of heavy REE fractionation (Gd(N)/Yb(N) = 4.6+/-2.2, 2.0+/-0.7). In addition, associated leucogranite has REE compositions similar to those of its host rocks. Mean deltaO-18 values (associated + 11.2+/-1.2 parts per thousand, independent + 11.4+/-0.5 parts per thousand; relative to SMOW) are comparable with the mean fo r the entire South Mountain Batholith (+10.8+/-0.7 parts per thousand) . Radiometric dating (Ar-40/Ar-39 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 sampl es elsewhere in the batholith. Field relations and geochemistry sugges t that the associated leucogranite results from an open-system interac tion between a fluid and its host leucomonzogranite, whereas the indep endent leucogranite bodies are discrete intrusions of highly fractiona ted melts that underwent closed-system, late-magmatic to post-magmatic fluid alteration. Where mineralized, the associated leucogranite char acteristically hosts greisen-type or disseminated polymetallic mineral ization, whereas the independent leucogranite hosts pegmatitic or diss eminated polymetallic mineralization.