Geology of the 1.33-1.32 G Barth Island Ring Complex, near Nain, Labrador

The ca. 40 km² Barth Island Ring Complex comprises spatially associated, concentric sheets of leucotroctolite, leuconorite, ferrogabbroid, and monzonite. The ring complex is a part of the ca. 1.36 - 1.29 Ga Nain Batholith, a typical Proterozoic massif- type anorthosite complex located in northern Labrador, Canada. Unlike most anorthosites in the Grenville Province to the south, the Nain Batholith (NB) was not involved in regional ductile deformation and therefore offers an excellent opportunity to probe the problem of the formation of Proterozoic massif-type anorthosites. The Barth Island Ring Complex contains, in close proximity, some of the distinctive rock types of massif-type anorthosite complexes. The ring complex therefore provides an ideal opportunity for study of the 'anorthosite problem'. -- Previous workers had considered the ring complex to be the result of closed- system differentiation of a single pulse of magma in a single chamber. However, mapping at a scale of 1:20 000 has resulted in a reappraisal of the mechanisms of formation and magmatic history of the Barth Island Ring Complex. Field relationships demonstrate an intrusive relationship between all the major rock types, although there is abundant evidence of the contemporaneity of the ferrogabbroid and monzonite. Based on the presence of well-preserved cumulate textures and modal layering, the leucotroctolite appears to have crystallized more slowly than the other units and to have differentiated in situ to some degree. The latter interpretation is supported by an upward transition from leucotroctolite into more Fe-rich leuconorite. Monzonite is interpreted to have been emplaced before ferrogabbroid, based on the presence of features such as dyke-like bodies of ferrogabbroid within monzonite. Evidence for hybridization of the ferrogabbroid and monzonite is present in many locations. Field evidence suggests the leucotroctolite was emplaced before the ferrogabbroid and monzonite. An outer unit of leuconorite postdates all the other rock types. -- U-Pb ages were obtained by Thermal lonisation Mass Spectrometry (TIMS) from zircon in ferrogabbroid and leuconorite, and from zircon and baddeleyite in leucotroctolite. The ferrogabbroid age of ca. 1317 Ma is within error of previously published values of ca. 1320 Ma for monzonite of the Barth Island Ring Complex, and therefore supports the field evidence of contemporaneity of the ferrogabbroid and monzonite. U-Pb data obtained for zircon and some baddeleyite from the leucotroctolite, and zircon from the leuconorite, gave an age of ca. 1332 Ma. One population of baddeleyite grains gave an age of 1318 ±10 Ma, in disagreement with zircon from the same sample of leucotroctolite. The petrographic association of zircon with late-stage minerals such as magnetite and biotite strongly suggests that the zircon was not inherited, and it is therefore suggested that the U-Pb system of the ostensibly younger baddeleyite was disturbed during a heating event. This event could be associated with the intrusion of monzonite and ferrogabbroid at ca. 1318 Ma. Petrographic evidence for pervasive recrystallization at the margin of the troctolite may provide corroborating evidence, though it is possible that the observed textures are due instead to auto-annealing. -- Trace element geochemical and Nd isotopic data are presented. Good first- order evidence exists for a genetic link between the leucotroctolite and leuconorite, so the leuconorite's overall lower abundances of most incompatible trace elements must be explained by AFC processes or differences in phase proportions. The monzonite and ferrogabbroid have similar trace element abundances. A broad spectrum of negative εNd values (from -4 for one troctolite to -11.8 for one ferrogabbroid) suggests crustal influence on the major rock types of the ring complex was heterogeneous. -- The leucotroctolite and leuconorite of the Barth Island Ring Complex were emplaced at ca. 1332 Ma into massif-type anorthosite; the leucotroctolite is bounded to the west by a shear zone that was active during its emplacement. The leucotroctolite magma may have exploited the adjacent shear zone during its emplacement, because the shear zone would have been a plane of weakness in the crust. It is possible that, for the same reason, the crystal-laden monzonitic and ferrogabbroid magma utilised the same conduit ca. 13 m.y. later.

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