Multiple high-pressure events in an orogenic hangingwall, Liverpool Land, East Greenland

The East Greenland Caledonides formed in the Laurentian hangingwall leading up to and during the subduction of Baltican continental crust, and offer an excellent opportunity to study (U)HP processes in orogenic hangingwalls. In southern Liverpool Land, located in the hinterland of the Greenland Caledonides, the eclogites-facies Tvaerdal complex is characterized by protoliths with inherited Mesoproterozoic zircons and was possibly derived from a slice of subducted Baltican continental crust that was incorporated into the Laurentian hangingwall during exhumation. Despite this common Mesoproterozoic history, there are considerable east-west variations within the Tvaerdal complex. In the west, the Tvaerdal complex consists of migmatitic orthogneisses that host numerous eclogite bodies with ~399 Ma metamorphic zircon and titanite cooling ages of ~383 Ma. In contrast, the Tvaerdal complex farther east and north of the town of Scoresbysund consists of relatively homogenous orthogneisses with zircon rims of ~409 Ma and rare retrogressed eclogite boudins. Here, we present new zircon and titanite geochronology from north of Scoresbysund to further examine the variation within the Tvaerdal complex. Three samples of granodioritic orthogneiss from north of the town of Scoresbysund were selected for titanite geochronology and include titanite grains concordant to the foliation in amphibole rich layers. LA-ICPMS analyses in two samples yield discords with common Pb that indicate titanite cooling ages of 402 ± 4 and 401 ± 4 Ma, and in a third sample with a range of ages from 400-380 Ma. Rare mafic boudins included within the orthogneiss are characterized by relict garnets with rutile inclusions, and coarsened symplectites of plagioclase + clinopyroxene + amphibole interpreted to have formed after omphacite during decompression from eclogite-facies. Zircons separated from one sample of retrogressed eclogite yield rare prismatic grains with dark cores and bright rims in CL, and abundant translucent spherical zircons that are bright with little zoning in CL. LA-split stream-ICPMS analyses indicate an upper-intercept age of 1286 ± 55 Ma for prismatic zircons and inherited cores characterized by steep HREE signatures, while spherical zircons are characterized by flat HREE signatures and yield a concordant age of 418.5 ± 2 Ma. This new data suggests that the orthogneisses north of Scoresbysund represent a separate terrane in southern Liverpool Land that experienced an earlier episode of eclogite-facies burial, and that had cooled through titanite closure while the Tvaerdal complex farther west was at eclogite-facies depths. Furthermore, this data suggests that the presence of Mesoproterozoic zircon inheritance alone is not enough to correlate eclogite-facies gneiss terranes in southern Liverpool Land, and that rather than correlating with the Tvaerdal complex farther west, the orthogneisses north of Scoresbysund more closely correlate with orthogneisses and high-pressure relicts in the Marianger Fjord Dome 40 km to the north.