Charlie J. Oldman *1, Clare J. Warren 1, Christopher J. Spencer 2, Tom W. Argles 1, Nigel B.W. Harris 1, and Sam J. Hammond 1
1 School of Environment, Earth and Ecosystem Sciences, The Open University, Milton Keynes MK7 6AA, UK Charlie.oldman@open.ac.uk
2 Department of Geological Sciences and Geological Engineering, Queen’s University, Kingston, Ontario KL7 3N6, Canada
When continental tectonic plates collide and become buried beneath one another, pressures and temperatures ~15-20 km below surface allow rocks to start melting. The presence of magma weakens the rocks and allows them to move towards the surface more easily. When the magmas solidify, it forms granite, and the abundance of granite in the Himalayas today suggests significant amounts of melt 20-30 million years ago. How quickly rocks melt, how much they melt, and how quickly melted rocks move has implications for the overall development of collisional mountain ranges.
Recent studies suggest that granite bodies formed from partial melting in collisional settings record multiple individual melting events over multiple million-year timescales. Geochemical signatures provide insight into which rocks melted, as well as where, when and how quickly. To constrain the evolution of the central Himalaya in India, we sampled granites, partially melted rocks called migmatites, and host metasediments in the Garhwal region.
We present a dataset of U-Pb, O, and trace-elements from zircon grains that constrain the source, date, and timescales of melting episodes of granite and migmatite. Two broad pulses of melting are recorded at 14-24 Ma and 26-35 Ma. Both are recorded in the migmatites while only the younger pulse is recorded in the granites. The older migmatites are also geochemically distinct. The results highlight the advantage of studying tectonic rates and timescales in young, active mountain belts, as the small uncertainty on young ages means that geological events just a few million years apart can be distinguished. .