Evidence for contemporaneous yet contrasting styles of granite magmatism during extensional collapse of the northeast Greenland Caledonides

Abstract

Sectors of orogenic belts that have undergone extension present opportunities for the study of contemporaneous styles of magmatism at contrasting structural levels. The Caledonian (Ordovician‐Devonian) orogenic belt of northeast Greenland experienced widespread extensional collapse either synchronous with or immediately following continental collision and crustal thickening. Extension resulted in juxtaposition of structurally simple upper plate rocks of mainly low metamorphic grade with underlying lower plate rocks that record intense deformation and high‐grade metamorphism. Caledonian intracrustal granitic rocks occur as migmatite complexes, networks of sheets and veins, and as major gently dipping sheeted complexes in the lower plate and as steep‐sided, discordant plutons in the upper plate. Published geochemical and isotopic evidence supports the interpretation of these granitic rocks as a single intrusive suite formed by crustal anatexis that was initiated during crustal thickening and peak metamorphism and continued during decompression and extensional collapse. In this paper we test this hypothesis by the isotopic dating of two granite sheets from the lower plate and three plutons from the upper plate. These yield U‐Pb zircon or monazite ages in the range ∼431–428 Ma that are indistinguishable within error, thus demonstrating that granitic melt emplacement was contemporaneous at the contrasting structural levels. We conclude that at midcrustal levels, processes of magma segregation and transport were controlled by low‐angle, noncoaxial extensional shear. Batches of magma were segregated technically and likely migrated updip as a result of tectonic pumping and buoyancy. These amalgamated to form synkinematic sheets in the lower plate that we interpret as the channelways by which magma was transported to higher crustal levels across zones of detachment to construct the plutons in the upper plate. The major features of this model may be applicable to many orogenic belts that record core complex development during extensional collapse and intracrustal granite magmatism.