Origin and evolution of deep upper mantle rocks from Western Norway

Abstract

The growth of the first continents on Earth marks a major change in the early planetary evolution. The knowledge about that is controversial as most of the scarce accessible samples from deep cratonic lithosphere (the rigid mantle underneath the continents) hide their earliest records beneath secondary overprints during the last three billion years. This thesis investigates fragments of such old (>3 Ga) subcontinental lithospheric mantle, which occur as isolated rock bodies (orogenic peridotites) in crustal gneiss exposed on Otry in the mountain chain of the Scandinavian Caledonides in western Norway. The Otry mantle fragments are compositionally banded and include garnet peridotite, spinel peridotite, garnetite and garnet pyroxenite. Garnet is associated with tiny lamellae and comparably coarse grains of pyroxene in two mineral microstructures. The pyroxene lamellae and the host garnet have a strong crystallographic relationship (symmetry multiples of 4x3 orientations), which characterise this type of microstructure as a break down product of a former majorite precursor mineral. Majorite is a typical component of the deep mantle (>150 km depth). In-situ analyses of rare Earth elements (REE) in the majorite break down products reveal that the break down reaction occurred at minimum temperatures of 1300 C. Sm-Nd and Re-Os isotopes constrain the origin of most rock types between c. 3.3-2.9 Ga, in the Archaean era. Both temperature and age information demonstrate that the occurrence of the ultra-high pressure mineral indicator majorite in orogenic peridotites does not necessarily record conditions of the latest major geodynamic overprint, deep continental subduction, as recently proposed for other orogens. Pyroxene grains associated with garnet in the other microstructure record similar initial minimum temperatures and have similar exceptionally low concentrations of light REE as pyroxene lamellae. This implies that the Otry mantle fragments originated at much greater depth (>350 km) than the thickness of the oldest continents (c. 200 km). Mineral microstructures, mineral chemistry and melt modelling constrain that an up-welling, extremely hot (>1800 C) mantle caused melting and emplacement of the peridotites at c. 150 km depth forming an ultra-depleted cratonic root in the early Earth. Subsequent metasomatism enriched locally the melt depleted peridotites in melt-compatible elements (Fe, Ca and light REE), but also in Cr and heavy REE, elements which are generally regarded as melt-incompatible. Metasomatised peridotites preserve Archaean ages and have higher Cr content than primitive mantle. Cr and heavy REE are interpreted to be mobile during high degrees of peridotite melting (c. 50 %) at high pressures (c. 5 GPa). Sub-cratonic emplacement of the peridotites may have caused strong deformation, which explains the present compositional layering by mechanical mixing of garnet-rich and garnet-poor lithologies. Folding and recrystallization of this layering occurred during tectonic exhumation of the mantle fragments associated with the Caledonian orogeny more than 2 billion years later. Recrystallized assemblages lack pyroxene microstructures in garnet, preserve peak metamorphic conditions of 870 C and 6.5 GPa (corresponding to c. 200 km depth) and have early Scandian ages of c. 430 Ma. This shows that continental subduction during orogeny in western Norway exceeded deep into the diamond stability field. The ultra-high pressure metamorphism appears to have lasted significantly longer than generally accepted in this region. Nevertheless witnesses of lithosphere formation during early Earth evolution are preserved within the peridotite bodies, demonstrating that the Scandian tectonometamorphic overprint did not wipe out the earlier records as is generally thought by the scientific community. This recognition allows early Earth lithospheric studies to be performed in a totally new geological setting.