Timescales and rates of intrusive and metamorphic processes determined from zircon and garnet in migmatitic granulite, Fiordland, New Zealand

Abstract

Zircon U-Pb, and garnet Sm-Nd and Lu-Hf dates provide important constraints on local and orogenic scale processes in lower-crustal rocks. However, in high-temperature metamorphic rocks these isotopic systems typically yield significant ranges reflecting both igneous and metamorphic processes. Therefore, linking dates to specific aspects of rock history can be problematic. In Fiordland New Zealand, granulite-facies orthogneiss is cut by leucosomes that are bordered by garnet clinopyroxene reaction zones (garnet reaction zones). In both host orthogneiss and garnet reaction zones, zircon are typically anhedral with U-Pb dates ranging from 118.30±0.13 to 115.70±0.18 Ma (CA-ID-TIMS) and 121.4±2.0 to 109.8±1.8 Ma This is the peer-reviewed, final accepted version for American Mineralogist, published by the Mineralogical Society of America.The published version is subject to change. Cite as Authors (Year) Title. American Mineralogist, in press.

The Initiation and Growth of Transpressional Shear Zones Through Continental Arc Lithosphere, Southwest New Zealand

“…High U/Th ratios (>10) helped us distinguish zircon that either grew in the presence of water during amphibolite‐facies metamorphism or crystallized in a fluid‐rich magma system (e.g., samples 19MP31C, 19MP34; Figure S10 in Supporting Information S1). The dating of metamorphic mineral assemblages using Sm‐Nd garnet geochronology and their comparison with zircon ages (Stowell et al., 2010, 2014, 2017, 2021) also assisted interpretations. For example, some Sm‐Nd garnet dates appear to record cooling after peak metamorphism due to differences in the diffusion rates of Nd in garnet compared to Pb in zircon (Stowell et al., 2021).…”

“…The dating of metamorphic mineral assemblages using Sm‐Nd garnet geochronology and their comparison with zircon ages (Stowell et al., 2010, 2014, 2017, 2021) also assisted interpretations. For example, some Sm‐Nd garnet dates appear to record cooling after peak metamorphism due to differences in the diffusion rates of Nd in garnet compared to Pb in zircon (Stowell et al., 2021). Last, a comparison of zircon ages from metamorphic aureoles with igneous crystallization ages from the adjacent plutons also helped us distinguish igneous and metamorphic ages (Figure 8a).…”

“…For example, some Sm-Nd garnet dates appear to record cooling after peak metamorphism due to differences in the diffusion rates of Nd in garnet compared to Pb in zircon (Stowell et al, 2021). Last, a comparison of zircon ages from metamorphic aureoles with igneous crystallization ages from the adjacent plutons also helped us distinguish igneous and metamorphic ages (Figure 8a).…”

“…High U/ Th ratios (>10) helped us distinguish zircon that either grew in the presence of water during amphibolite-facies metamorphism or crystallized in a fluid-rich magma system (e.g., samples 19MP31C, 19MP34; Figure S10 in Supporting Information S1). The dating of metamorphic mineral assemblages using Sm-Nd garnet geochronology and their comparison with zircon ages (Stowell et al, 2010(Stowell et al, , 2014(Stowell et al, , 2017(Stowell et al, , 2021) also assisted interpretations.…”