Tectonic evolution of a Caledonian-aged continental basement eclogite terrane in Liverpool Land, East Greenland

Abstract

Liverpool Land is located in an interior position within the East Greenland Caledonides. Due to the remoteness of the field locality, very little work has been published on the rocks and structures of Liverpool Land. The main focus of this thesis work, therefore, has been to aid in establishing a metamorphic and deformational framework for rocks in the area. Southern Liverpool Land is divided by the Gubbedalen Shear Zone, a greenschist-facies, ~500 m thick, N-dipping high-strain zone with a polyphase history, including tops-south (contraction) as well as tops-north (extension) displacement (discovered and documented in this study). The hanging wall to the north is characterized by the Krummedal Sequence, which is intruded by the Caledonian, Silurian Hurry Inlet Granite and Hodal-Storefjord Monzodiorite plutons. The Krummedal Sequence and Monzodiorite unit are intruded by several north-south striking lamprophyre dikes that are herein dated to 262 and 264 Ma by 40Ar/39Ar methods performed on crystals of phlogopite. The footwall block of the Gubbedalen Shear Zone is characterized by a migmatitic orthogneiss basement complex, with felsic and mafic phases. Included within the felsic orthogneiss are eclogites, garnet-pyroxenites, amphibolites, and ultramafic bodies. These mafic bodies occur as boudins and pods wrapped by crystal-plastic mylonites that overprint the gneissosity in the country rocks. Peak temperature and pressure conditions of eclogitization are herein constrained by geothermobarometry on minerals forming the eclogite-facies assemblage (i.e., garnet and omphacite), and are ~867°C and a minimum pressure of 18.2 kbars. Cutting across all lithologies in the footwall is a stockwork of granitic veins and dikes. Similarities between the orthogneiss complex of the footwall block and parts of the Western Gneiss Region in Norway imply that the footwall block may in fact be an orphaned crustal slice from the Baltic craton. This interpretation explains how the eclogites of southern Liverpool Land occur within the supposed upper plate of the Caledonian collisional event.