The interaction between melt, deformation and recrystallization within northern Fiordland, New Zealand
thesisposted on 28.03.2022, 21:48 authored by James Robert Smith
Well exposed granulite facies gabbroic rocks from the Pembroke Valley, Fiordland, southern New Zealand provide a unique opportunity to study lower crustal processes. Their microstructural and rheological evolution have been studies to understand factors resulting in strain localisation. Granulite gacies assemblage of two-pyroxene-hornblende transitions to two-pyroxine garnet either side of felsic dyke intrusions resulting in garnet reaction zones. These garnet reaction zpnes can be further deformed by mylonitic shear zones which are sometimes localised to the outer region of these garnet reaction zones. As such they are termed " paired shear zones". Distinct microstructural changes are essential to result in a rheological contrast promoting strain localisation that results in the formation of a paired shear zone. Dykes consist of a similar mineralogy with slight variations of plagioclase, quartz and K-feldspar. Major and trace element analysis indicates subtle variation between dykes. Therefore the degree of garnet reaction zone recrystallisation is affected by the dyke influence, ie as a result of melt flowing through an open dyke. Consequently, garnet reaction zones can be can be partially recrystallised due to short dyke influence, or extensively recrystallised due to a longer influence. SEM-EBSD analysis provides confirmation that these replacement reactions have occurred under static conditions and garnet mimics the orientation of the preexisting phase. Therefore, lower degrees of static recrystallisation will resuklt in partially recrystallised garnet reaction zones which display varied anorthite content due to the presence of the relict (igneous) plagioclase grains. Extensively recrytallised garnet reaction zones display homogeneous anorthite content and an increase in garnet abundance close to the dyke. This recrystallisatin results in the replacement of preexisting minerals, observed in the trace element patterns. Extensive recrytallisation also results in an increased garnet abundance and a harder bulk rock. Later mylonitic shear zones are only localised at the border between extensively recrystallised garnet reaction zones and the surrounding country rock. At these borders a maximum rheological contrast is present resulting in strain localisation responsible for the formation of paired shear zones.