Proterozoic anorthosite–granite Nain batholith: structure and intrusion processes in an active lithosphere-scale fault zone, northern Labrador

Abstract

We present field evidence of the structure and evolution of a typical Proterozoic massif-type anorthosite. The anorthosite forms a batholith composed of numerous plutons and ring complexes intruded episodically in two main sequences, each producing anorthositic rocks and monzonite–ferrodiorite followed by granite. The batholith is located where a lithosphere-scale fault zone, ∼50 km wide and extending over 1000 km from Greenland to Labrador, intersected an older continental suture. Intermittent episodes of mainly sinistral transtension along this Gardar – Voisey’s Bay fault zone were associated with the generation of a number of pulses of magmatism and controlled the rise of magmas to middle and upper crustal levels between ∼1363 and 1289 Ma. Both the large- and small-scale structures and magma intrusion processes are similar to those of many cordilleran batholiths, with tabular plutons associated with ring dykes emplaced by cauldron subsidence, and basin-shaped plutons associated with cone sheets. These structures are well known in subvolcanic situations but little known at the mid-crustal depth of the Nain batholith. The location of these structures within the batholith was controlled by intersecting, externally generated faults and shear zones. We conclude that this typical massif-type anorthosite is an intracratonic structural equivalent of cordilleran batholiths of continental margins.