The Pressure, Temperature, and Timing of Magmatism and Metamorphism, George Sound, Fiordland, New Zealand
Fiordland, New Zealand provides an opportunity to study the complex relationship between magmatism and metamorphism in the lower crust of a magmatic arc. The Eastern McKerr Intrusives, which are well exposed along George Sound, are a constituent of the Western Fiordland Orthogneiss magmatic suite. They they lack the extensive metamorphic recrystallization observed in neighboring Western Fiordland Orthogneiss plutons. This thesis presents pressure, temperature, and a Sm-Nd garnet age to better understand intrusion of the Eastern McKerr Intrusives, metamorphism of the adjacent George Sound Paragneiss, and the tectonic history. Results from amphibole thermometry and barometry indicate amphibole crystallization at 915-850°C and 8-11 kbar. Petrography, mineral compositions, and psuedosection models indicate that amphibole from the Eastern McKerr Intrusives are igneous and crystallized near solidus temperatures. Metamorphic garnet from the George Sound Paragneiss records pressure and temperature estimates of equilibration at ~720°C and 6 kbar for garnet cores. The garnet rims record lower temperature and higher pressure conditions of 680-600°C and 8-12 kbar. The overlap between magmatic and metamorphic pressure estimates is compatible with contact metamorphism in the George Sound Paragneiss. One new garnet Sm-Nd age of 108 ± 7 Ma is indistinguishable from the age of post magmatic garnet granulite metamorphism elsewhere in the Western Fiordland Orthogneiss. However, the imprecision of this age is likely two different age populations of garnet. The garnet Sm-Nd age indicates the Eastern McKerr Intrusives, either remained, or were reheated to amphibolite facies conditions after emplacement. The garnet age of ~108 Ma provides an estimate for the duration of amphibolite facies or the timing of reheating. Regardless, the igneous rocks did not significantly recrystallize to a metamorphic mineral assemblage. Based on the garnet age and the ca. 120 Ma pluton emplacement, the Eastern McKerr Intrusives would have been subject to amphibolite facies or higher temperature conditions. Possible interpretations for the lack of significant recrystallization in the igneous rocks include: 1); incorrect garnet age; 2) insufficient heat flow into George Sound rocks; 3) kinetic barriers to recrystallization and; 4) fault juxtaposition of amphibolite facies and lower temperature rocks.