Growth and deformation of apatite in different metamorphic scenarios

To assess the metamorphic and deformational history preserved in apatite, we selected high-pressure granulite to lower-amphibolite facies rocks from the East Himalaya Syntaxis (EHS) in the Tibetan orogen for in situ apatite analysis. Through the cross-correlation of cathodoluminescence (CL) images, Electron Backscatter Diffraction (EBSD) analysis, and in situ trace element and U-Pb geochronology, we elucidate the evolution of apatite growth and deformation. In a high-pressure granulite facies sample, apatite has no crystallographic preferred orientation (CPO) but instead a strong shape preferred orientation (SPO) and intragranular deformation, indicating that the apatite experienced peak metamorphism/deformation. Apatite from medium-pressure granulite facies rocks exhibits obvious oscillatory zoning, grow over the main foliation, and have weak intragranular deformation, as well as SPO and CPO, suggesting crystallization after the main phase of metamorphism/deformation. Euhedral apatite grains from upper-amphibolite facies samples display strong SPO and CPO, but only weak intragranular plastic deformation, indicating apatite growth coeval with the main deformation phase. Apatite from lower-amphibolite facies samples have core-and-mantle microstructures and a CPO subparallel to the stretching lineation, with a weak SPO and intragranular deformation, suggesting multiple phases of growth induced by fluid activity during and after peak metamorphism. Almost all apatites in high-pressure granulite facies, medium-pressure granulite facies, and upper-amphibolite facies samples exhibit similar rare earth element (REE) characteristics within individual samples. Despite undergoing different growth and deformation processes, this similarity likely reflects either chemical reequilibration under high-temperature conditions or the retention of comparable initial chemical compositions during their formation. In contrast, apatites within lower-amphibolite facies samples display inconsistent REE characteristics, suggesting that chemical reequilibration did not occur after their formation. Based on microstructural analysis, in situ U-Pb ages reveal peak and retrograde ages in the EHS at ~21 Ma and <16 Ma, respectively, overlapping with published chronology results. Therefore, multiple generations of apatite can be identified through careful cross-correlation of CL, EBSD, and geochemical data, providing a robust record of a rock's P-T-t evolution.