40AR/39AR MUSCOVITE THERMOCHRONOLOGY AND STRUCTURAL EVOLUTION OF SOUTHERN LIVERPOOL LAND, EAST GREENLAND

Southern Liverpool Land (LL; 70.5^oN) is located in the central East Greenland Caledonides and is divided by the Gubbedalen Shear Zone (GSZ), a ~500 m thick, N-dipping high-strain zone. The structurally lower part of the GSZ records tops-south amphibolite-facies contraction overprinted by tops-north greenschist-facies extension in the upper quarter. The hanging wall consists of high-grade Mesoproterozoic supracrustal rocks intruded by undeformed Caledonian plutons. The footwall block is a highly deformed Paleo-to-Mesoproterozoic migmatitic orthogneiss basement complex, including garnet-pyroxenite, amphibolite, ultramafics, and Caledonian (~399 Ma) eclogites. Mafic bodies occur as boudinaged pods wrapped by crystal-plastic mylonites.

Through overprinting and cross cutting fabric relationships, a five phase deformational history is established. D₀ gneissosity is overprinted by D₁ migmatitic foliation, which is retrograded by D₂ mylonitic foliations and lineations over mafic pods. The GSZ overprinted aforementioned structures during D₃ thrusting and subsequent D₄extension. Stereonets of gneissosity (S0), migmatitic foliation (S1), and mylonitic foliation (S2) indicates the GSZ is an E-W structural monocline. Plots of mineral alignments (L0), neosomal “cigars” (L1), and elongation lineations (L2) indicate a cone axis reflecting sheath fold geometries created as fabrics are swept into the GSZ.

Muscovite samples from rocks within different structural levels were dated using ⁴⁰Ar/³⁹Ar thermochronology by single-grain fusion and incremental heating. Muscovite from a hanging wall pegmatite is 387±1.5 Ma, and muscovite from a shear zone mylonite is 378.5±0.86 Ma. Muscovite from an orthogneiss in the structurally lower part of the footwall is 376.1±2.0 Ma. These dates indicate Middle Devonian regional cooling of the hanging wall prior to extension along the GSZ, which tectonically unroofed the footwall. The LL T-t cooling path is exponential, from initially very rapid rates that are followed by slow cooling during final exhumation and subsequent deposition of late Devonian sediments. We compare T-t paths with other Caledonian lower-plate eclogite terranes on conjugate sides of the orogen and explore palinspastic restorations and tectonic mechanisms that fit their exhumation histories.