Flow characteristics of lower crustal rocks: comparing two polyphase rock types
thesisposted on 28.03.2022, 21:13 by Vicki A. Beecher
How do polyphase rocks deform? How do they perform to each other? Classically, deformation mechanisms which govern the rheology of a rock (i.e flow properties) are predicted from monomineralic experiments. However, how can science apply laws for one system to another? Is it valid? At the same time, when looking at deforming rocks science uses interpreted deformation mechanisms of individual phases as indicators for conditions of deformation. Again, based on monomineralic experiments. To ascertain if these predictors are valid I am considering different rock types with different modal percentages that have undergone the same PT conditions. Individual grains and their “neighbourhoods” would need to be analysed in order to ascertain different deformation mechanisms. The rocks in this study have been sampled from Fiordland, New Zealand in rare outcrop of lower crustal rocks that have been deformed and recrystallised at PT conditions with that of eclogite facies and then exhumed passively with no further metamorphic or tectonic overprinting. Observed relationships in the field of Omphacite granulite and Breaksea eclogite indicated that the rocks were co magmatic. The Breaksea eclogite was found within lenticular/elongate pods form mm, cm, 10cm scale within the Omphacite granulite. SEM EDS analysis revealed three main phases present feldspar, omphacite and garnet throughout the samples collected with varying amounts of strain. EBSD mapping of high and low strain samples of three phases’ feldspar, omphacite and garnet within the two lithologies was able to ascertain the deformation mechanism of each mineral relative to adjacent minerals. Contour pole figures indicated Lattice preferred orientation of minerals that had deformed via dislocation creep. The deformation mechanisms observed indicate that monomineralic flow laws are not a suitable tool and cannot be used as a reliable predictor of deformation mechanisms of polyphase rocks.