INTEGRATED APPROACHES TO UNDERSTANDING PROCESSES AND EVENT HISTORIES IN HIGH-TEMPERATURE METAMORPHIC TERRANE

Understanding the behavior of dateable accessory minerals during high-temperature metamorphism and migmatization, as well as melt extraction and transport is fundamental to the interpretation of event histories and for characterizing the transport versus retention of trace and heat producing elements within the continental crust. Central to this understanding is the ability to link growth or recrystallization of minerals such as zircon and monazite to reaction processes involving major minerals and melts. Significant advances have been made on this front through the integrated use of trace element signatures in co-existing major and accessory minerals, and the increased ability to analyze all minerals in situ using advanced microbeam techniques.

Recent work on granulite facies migmatites from Prydz Bay, east Antarctica, has been shedding more light on the local-scale controls of reaction processes involving accessory minerals during the generation and extraction of melts. Detailed textural characterization and integrated REE analysis has suggested that zircon in a number of terranes grew in equilibrium with garnet during partial melting, and not only during late-stage cooling and crystallization of melts. In a test of this hypothesis, simplified thermodynamic models of zircon behavior in melts generated through anatexis of metapelitic granulites have been generated using THERMOCALC. The models show that accessory minerals can theoretically grow during melt extraction and transport if melts pond and begin to react with wall rocks. These results have major implications for the interpretation of zircon ages in high-grade terranes because they suggest that zircon in migmatites need not date terrane-wide cooling below the solidus. Moreover, scatter in age data between and within samples may relate to local-scale differences in growth environment and date real crystallization stages throughout the thermal history of the migmatite.