Exhumation History of Caledonian Eclogites in Liverpool Land, East Greenland, and Comparisons with Eclogites in Norway
Type of DegreeThesis
Geology and Geography
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Lithologic information and 40Ar/39Ar cooling dates are reported from a recently discovered high-pressure (HP) Caledonian eclogite terrane in Liverpool Land, East Greenland, its overlying hanging wall block, and an extensional detachment fault separating them (Gubbedalen Shear Zone). 40Ar/39Ar analysis had not previously been reported for rocks in Liverpool Land thereby limiting our ability to understand its metamorphic and cooling history. Muscovite grains were, therefore, separated from rocks at various structural levels and analyzed using single crystal total fusion and incremental heating in the Auburn Noble Isotope Mass Analysis Laboratory (ANIMAL). Muscovite cooling ages (analyzed to 1s) from paragneiss and undeformed pegmatite in the hanging wall are 381.26± 0.41 Ma and 391.23± 0.54 Ma, respectively. Footwall orthogneiss yields a muscovite age of 378.5 ± 0.63 Ma and a mylonite from the Gubbedalen Shear Zone has an age of 379.78±0.37 Ma. Later cooling of the footwall block, after the hanging wall had already cooled is consistent with normal movement determined along the shear zone. This age relationship is also consistent with Devonian sediments that nonconformably overlie units in the hanging wall block. The argon isotopic results combine with previously reported U/Pb zircon dates of ~395 Ma for eclogitization to constrain the Liverpool Land temperature-time (T-t) path. This T-t path is distinct from paths reported for the Lofoten and Bergen Arcs eclogite terranes but compares favorably with the higher temperature parts of both the North East Greenland eclogite province (NEGEP) and the Western Gneiss Region (WGR) trajectories. Divergence of the NEGEP from the Liverpool Land T-t path likely reflects differences in the tectonic evolution between these two areas that are not yet understood. Overlapping of the Liverpool Land and WGR T-t paths from eclogite-facies temperatures at ~395 Ma to the 350°C isotherm at ~380 Ma suggests similar Devonian exhumation histories. Following ~380 Ma the WGR continued to exhume rapidly while Liverpool Land may have experienced a slight temperature rise (~75oC) for ~10 m.y. before reinitiating its similar rapid exhumation to Earth’s surface. These differences are consistent with Liverpool Land’s inferred position in an overriding plate while the WGR occupied a position in the subducting plate.