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

• Main Author: Gaskill, Owen
• Format: Thesis
• Language: English
• Published: Memorial University of Newfoundland 2005
• Subjects: Barth Island; Canada; Nain
• Online Access: https://research.library.mun.ca/6663/; https://research.library.mun.ca/6663/1/OwenGaskill.pdf; https://research.library.mun.ca/6663/3/OwenGaskill.pdf

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. ...