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

Sectors of orogenic belts that have undergone extension present opportuniti es for the study of contemporaneous styles of magmatism at contrasting stru ctural levels. The Caledonian (Ordovician-Devonian) orogenic belt of northe ast Greenland experienced widespread extensional collapse either synchronou s with or immediately following continental collision and crustal thickenin g. Extension resulted in juxtaposition of structurally simple upper plate r ocks of mainly low metamorphic grade with underlying lower plate rocks that record intense deformation and high-grade metamorphism. Caledonian intracr ustal granitic rocks occur as migmatite complexes, networks of sheets and v eins, and as major gently dipping sheeted complexes in the lower plate and as steep-sided, discordant plutons in the upper plate. Published geochemica l 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 decomp ression and extensional collapse. In this paper we test this hypothesis by the isotopic dating of two granite sheets from the lower plate and three pl utons from the upper plate. These yield U-Pb zircon or monazite ages in the range similar to 431-428 Ma that are indistinguishable within error, thus demonstrating that granitic melt emplacement was contemporaneous at the con trasting structural levels. We conclude that at midcrustal levels, processe s of magma segregation and transport were controlled by low-angle, noncoaxi al extensional shear. Batches of magma were segregated tectonically and lik ely migrated updip as a result of tectonic pumping and buoyancy. These amal gamated to form synkinematic sheets in the lower plate that we interpret as the channelways by which magma was transported to higher crustal levels ac ross zones of detachment to construct the plutons in the upper plate. The m ajor features of this model may be applicable to many orogenic belts that r ecord core complex development during extensional collapse and intracrustal granite magmatism. Copyright 2001 by the American Geophysical Union.