PETROCHRONOLOGICAL EVOLUTION OF THE HIMALAYAN METAMORPHIC CORE IN THE MAKALU-ARUN REGION, EASTERN NEPAL

The mid- to high-grade metamorphic core of the Himalayan orogen was thought to comprise a uniform, continuous unit. Recent studies, however, have documented localized cryptic structures that divide the metamorphic core into multiple packages with distinct pressure-temperature-time histories. Such structures have been identified in various regions across the orogen and attempts are now underway to see if these structures are regional features that correlate along strike. The occurrence of widespread structural breaks may indicate that the Himalayan metamorphic core records a more complex metamorphic and structural history than previously thought.

In this study, we characterize the metamorphic history of eight specimens spanning the garnet, sillimanite, and/or kyanite bearing gneisses in the Makalu-Arun region of Nepal using monazite U-Th-Pb petrochronology. We use Gd/Yb and Y trace element data collected simultaneously with monazite age data to inform the timing of prograde and retrograde metamorphism in each specimen. Preliminary results indicate that prograde metamorphism in structurally lower specimens was coeval with retrograde metamorphism in structurally higher specimens. This ‘fingerprint’ is characteristic of rock packages separated by a thrust-sense structural discontinuity. Furthermore, additional smaller scale but distinct changes in the timing of retrograde metamorphism across the transect coincide with suspected structures in the adjacent Kanchenjunga region. Additional analysis, such as thermobarometry, garnet trace element geochemistry, and phosphate phase equilibria modelling will be carried out to further investigate these structures. The quantification of regional-scale structures will help evaluate their potential role in the overall evolution of the Himalaya, and provide information critical to develop new tectonic models.