Age, structural setting, and exhumation of the Liverpool Land eclogite terrane, East Greenland Caledonides

The close spatial relationship between Devonian high-pressure rocks (eclogites) and Ordovician-Silurian calc-alkaline plutonic rocks, as observed in Liverpool Land, NE Greenland, is not easily explained by existing tectonic models for the Caledonide orogen. New field studies and isotope dilution-thermal ionization mass spectrometry U-Pb geochronology demonstrate, however, that the association is just coincidental, because the two rock groups are located within distinct terranes separated by a composite structure. The major element is the Gubbedalen shear zone, a N-dipping shear zone dominated by a penetrative top-up-to-the-S ductile fabric. Superimposed brittle-ductile top-down-to-the-N shear zones are typical of the structurally uppermost part of the shear zone. The contact against the hanging wall is the N-dipping, brittle Gubbedalen extensional detachment fault. A zircon age of 399.5 +/- 0.9 Ma for an eclogite body is interpreted to represent the time of high-pressure metamorphism of the footwall. The host gneiss was migmatized between ca. 388 Ma and ca. 385 Ma, as constrained by the ages of a pegmatite predating migmatization and crosscutting granites. Coeval synkinematic granites intrude along amphibolite-grade, top-to-the-S high-strain zones in the Gubbedalen shear zone. Juxtaposition of the Ordovician-Silurian plutonic terrane (hanging wall) against the Early to mid-Devonian eclogite terrane (footwall) is best explained by a tectonic model involving early mid-Devonian buoyancy-driven exhumation followed by late mid-Devonian syncontractional extension related to thrusting on the Gubbedalen shear zone in a dextral strike-slip zone. Subsequent exhumation through the brittle-ductile transition occurred by extension on early semiductile structures and the overprinting Gubbedalen extensional detachment fault, and erosion.