HIMALAYAN METAMORPHIC CHRONOLOGY COMES OF AGE – REVIEW AND PROSPECTS

Geochronologists have invested c. 30 years of research to determine the age of high-grade Himalayan metamorphism and infer tectonic processes. Early efforts broadly delineated metamorphic ages, but contradictions were common, and only recently have ages been linked convincingly with thermal and tectonic evolution. Monazite U-Pb dating was initially emphasized because monazite is common in Himalayan gneisses and has high U content and Pb retentivity. Monazite separates or whole grains from migmatitic rocks and associated leucogranites ranged from c. 15 to 35 Ma. The absence of any petrogenetic model for monazite's growth during metamorphism, however, compromised definitive interpretation. Starting ~15 years ago monazite chronology began focusing on texturally-specific (inclusion vs. matrix) and compositionally-specific (high vs. low Y) ages of individual domains. These studies demonstrated substantial age heterogeneity within and among grains – 10 to 25 Myr in a single rock – that helped explain why previous work was so contradictory. More importantly, concurrent chemical analysis linked ages petrogenetically to partial melting reactions and allowed the first direct brackets on the timing of peak temperatures. These crystal domain-specific data showed progressively younger ages structurally downward from c. 25 Ma in upper level migmatites to <5 Ma in lower level schists, freeing geologists from models that had been erroneously predicated on coeval footwall and hanging wall metamorphism. In-sequence thrusting through a lateral thermal gradient juxtaposed formerly hotter rocks in the hinterland with cooler rocks towards the foreland resulting in an inversion in metamorphic grade (i.e., higher-grade rocks upward) and not in the geotherm. However, crystal domain chemistry and ages still could not be linked to specific P-T conditions on the prograde or retrograde metamorphic paths. The latest work combines domain-scale geochronology with trace element patterns and thermometry in zircon, monazite and titanite. U-Pb geochronology combined with Ti-in-zircon and Zr-in-titanite thermometry records discrete segments of the temperature-time history. Future work must further develop direct links between chronology and petrology, e.g. through inclusion barometry and trace element thermometry.