Deformed monazite yields high-temperature tectonic ages

The deformation of monazite in the polymetamorphic Sandmata granulite complex in India has been investigated by electron backscattered diffraction and sensitive high-resolution ion microprobe (SHRIMP). Quantitative microstructural analyses document the development of deformation twins in {100}, {001}, and {12(2)over bar} orientations; low-angle (<10 degrees) boundary development associated with dislocation creep; and the development of new grains due to dynamic recrystallization. These data represent the first quantitative evidence of crystal-plastic deformation of natural monazite. SHRIMP U-Th-Pb analysis shows that the host monazite preserves discordant ages as old as 1666 +/- 28 Ma, along a trend from ca. 1720 Ma to ca. 1000 Ma, with increasingly discordant ages recorded in zones of higher lattice distortion. Domains of recrystallized new grains within the monazite record a tightly clustered concordia age of 970 +/- 14 Ma. This age is interpreted to represent the timing of monazite dynamic recrystallization associated with deformation of the host protolith, and is consistent with partial resetting and Pb loss from domains deforming by dislocation creep. The complex, but systematic, relationship between microstructure and age data in monazite provide the first direct evidence of Pb isotope resetting during deformation. The approach illustrates a new methodology for the dating of deformation events in high-grade metamorphic rocks, which are typically difficult to constrain.