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