Chemistry and fabric of olivine microstructures in cold, ultra-depleted spinel mantle wedge peridotite, Seve Nappe Complex (central Swedish Caledonides)

Abstract

This thesis investigates the formation conditions of olivine microstructures in the Kittelfjäll spinel peridotite (KSP), a fragment of lithospheric mantle which occurs as an isolated body within high grade metamorphic crustal rocks in the Seve Nappe Complex (SNC) of Northern Jämtland, central Sweden. The KSP is an orogenic peridotite containing a well developed penetrative compositional layering, defined by highly depleted dunites (high olivine Mg# (Mg/Mg+Fe = 0,92–0,93) and spinel Cr# (Cr/Cr+Al = 0,89-0,96)), and harzburgites (with lower Mg# (0,91–0,92) and higher whole-rock Ca and Al contents). Dunites are characterized by three contrasting olivine microstructures formed in response to different tectonometamorphic conditions: 1) Coarse-grained, highly strained olivine porphyroclasts (M1) up to 20 cm are surrounded by 2) dynamically recrystallized olivine grains (M2) defining a characteristic olivine “foam” microstructure (200-2000 um). In strongly localized shear zones 3) an olivine “mortar” structure (10-50 um). Olivine fabric analysis (microstructure + Crystallographic Preferred Orientations (CPO)), EMP and XRF analyses reveal a retrograde evolution for the KSP. Olivine porphyroclasts (M1) grew at high temperature during cooling following Archean melt extraction (<50%) and possible Proterozoic refertilization events. The onset of the Caledonian deformation is interpreted to be related to the crustal emplacement of the KSP during eduction of the SNC. This phase is characterized by the development of the olivine M2 foam microstructure, formed at 750-830°C – 2±1 Gpa by dislocation creep processes producing dominant E-type CPO’s by the operation of the [100](001) slip systems. The formation of an olivine M3 “mortar” structure in localized shear zones at 550-600°C- 0,45-0,6 Gpa represents further stage during eduction. Mineralogical, geochemical and structural evidence clearly demonstrates that the KSP peridotite represents old, possibly Archean sub-continental lithosphere mantle (partial melting >50%) that was crustally emplaced into the Caledonian tectonic edifice during the Jämtlandian orogeny (~450 Ma). The “relatively LT” olivine M2 “foam” microstructure can be proposed as a tool to easily discriminate mantle wedge peridotites (i.e. sub-continental affinity) from ophiolite (i.e. sub-oceanic affinity) in the Scandinavian Caledonides.