GARNET GEOCHRONOLOGY: IMPROVEMENTS AND APPLICATION IN HIMALAYAN RESEARCH (Invited Presentation)

Garnet is one of the most important mineral targets for investigating tectonic and petrological processes in the lithosphere. The timing and conditions of garnet stabilization and growth mark central aspects to such investigation. Research on the tectonic evolution of the Himalaya-Tibet orogen and adjacent terranes is no different; studies on the deformed crust at all scales of research most often seek to determine this timing. Garnet chronology has great potential as a tool to unravel the complex history of this mountain belt. Nevertheless, it has not yet been widely applied. The main reasons for this are that garnet chronology is too laborious to do at large regional coverage, dissolution methods to avoid artefacts were not yet optimized, the required garnet sample size was large, and the systematics of the chronometers at high temperature were not yet well constrained. As a result, zircon and monazite chronology have for decades been the methods of choice and multi-component geochemical indicators are typically used to infer information about garnet stability.

In recent years, analytical advancements in selective garnet dissolution methods, chemical separation, and mass spectrometry have enabled high-precision garnet chronology (Lu-Hf, Sm-Nd) for increasingly smaller sample volumes. In addition, improved constraints on daughter-element diffusivity and empirical investigations into chronometric closure now enable robust age interpretation. At present, garnet chronology can be reliably done at precision levels of 0.5% or better with Lu-Hf and Sm-Nd methods, allowing precise age constraints on the timing, and even on the duration and progress, of garnet growth. When combined with modern P-T analysis, this approach yields supreme opportunities for investigating modern and ancient tectonics systems. In this contribution, we highlight some of the recent advances in garnet chronology and present newest findings from our garnet-based research in the High Himalaya and adjacent areas. Besides providing new insights into the tectonics of this complex Cenozoic mountain belt, the results yield new information on the duration of garnet growth in such systems, and the petro-chemical linkage between garnet and important chronometric minerals such zircon and monazite.