GEOLOGIC HISTORY OF A METAMORPHIC CORE COMPLEX IN NW SPITSBERGEN

A Silurian-Devonian aged metamorphic core complex has recently been recognized in northwest Spitsbergen, on the northwest corner of the Barents Shelf. A major tectonic feature of interest is the associated Keisarhjelmen mylonitic detachment zone that is 200-500 meters thick. Field work in 2018, 2011 and 2009 to northwest Svalbard, revealed a significant northward continuation of the detachment along the west side of Raudfjorden, and is well exposed along the eastern base of Marstranderfjellet. Large scale corrugation of the detachment has an axis parallel to the transport direction, top to the North. This detachment zone of the core complex separates overlying faulted Silurian-Devonian aged cover rocks from underlying migmatitic rocks. The complex shows a diverse array of fault and metamorphic rocks ranging from migmatites to ultramylonites and cataclasites. Footwall granitic intrusions are the same age as Silurian deposition. Within the detachment, strong retrogression is more apparent as one moves from coarsest grained rocks near the bottom to finest near the top of the sequence. This retrogression history is associated with crustal thinning. Thin sections from previously collected samples show grain size reduction, porphyroclasts and well-developed composite fault surfaces. Relict garnet porphyroclasts exhibit internal inclusion swirls and are mantled by chlorite and sericite, indicating episodes of simple shear during retrogression. Feldspar porphyroclasts show significant sericite alteration. Quartz grains textures vary from polygonal strain free aggregates to ribbons with severe undulose extinction and deformation bands, signifying deformation through evolving temperate and pressure conditions. Various other zoning and marginal textures were observed in thin section, suggesting a strong metamorphic retrogression and a complex history. Geochronologic and pressure and temperature data from the basement rocks of the area described in the literature help constrain a complicated history. Thin sections of samples collected in summer of 2018 from the northward continuation of the detachment will be analyzed to further work out metamorphic conditions and history.