EVOLUTION AND LATE STAGE DEFORMATION OF THE HIMALAYAN METAMORPHIC CORE, KANCHENJUNGA REGION, EASTERN NEPAL

New ⁴⁰Ar/³⁹Ar and fission track (FT) data help constrain the evolution and low temperature uplift history of the former Himalayan metamorphic core exposed in the Kanchenjunga region of eastern Nepal.

Biotite ⁴⁰Ar/³⁹Ar dates outline a complex history across the study area. Some biotite dates (~24-16 Ma) are older than nearby ²³²Th-²⁰⁸Pb monazite rim dates (~18-16 Ma). Previous studies in the area have attributed similar old biotite dates to excess argon. Research has demonstrated, however, that in addition to temperature, fluids, grain defects, and new crystal growth can affect retention of argon in a system. Phase equilibria models from similar structural levels to those dated using ⁴⁰Ar/³⁹Ar techniques predict biotite growth on the retrograde part of the pressure-temperature path before final melt crystallization and associated monazite rim growth. As such, the older biotite ages may be indicative of the time at which biotite crystallized rather than when it cooled through an interpreted closure temperature. Other biotite ages from the region (~13-9 Ma) are younger than nearby monazite rim ²³²Th-²⁰⁸Pb ages (~19-12 Ma) and more likely represent the timing of cooling or deformation of grains. Biotite ages in the Himalaya have often been excluded for interpreted excess argon, however, they may actually provide important information on timing constraints for the metamorphic evolution of the mid-crust.

As part of understanding midcrustal processes, detailing the recent history of the orogen not only elucidates near-surface convergence accommodation processes but also develops constraints for geometric modification of midcrustal structures. New apatite FT dates, combined with existing apatite and zircon FT dates from the region, define general younging trends towards the north - up structural section - of ~2.9 to 1.3 Ma and ~6.2 to 4.6 Ma respectively. These trends are consistent with the exhumation and uplift of these rocks associated with the growth of a duplex system developed through underplating. Additionally, there appears to be a significant jump in apatite FT dates from ~1.3 Ma to 2.4 Ma that is coincident with an abrupt change in existing muscovite ⁴⁰Ar/³⁹Ar ages from the Proterozoic to the Cenozoic. This break in ages is consistent with the mapped location of the Main Central thrust fault in the area.