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Metamorphic and geochronological evolution of mid- and upper-crustal rocks in the Nepalese Himalaya

University of British Columbia

As a young, active collisional orogen, the Himalaya has been a common target for studying collisional dynamics and metamorphic processes. Despite this targeted investigation, there remain unresolved issues that hinder our understanding of the mountain belt formation. For example, while the Himalayan metamorphic core (HMC), a package of exhumed midcrustal rocks that record Cenozoic metamorphism and deformation in the Himalaya, was previously thought to be a laterally continuous unit, recent research has documented evidence of a complex structural and metamorphic evolution not accounted for in previous tectonic models. In addition, parts of the Himalaya remain understudied due to geographic isolation and/or political obstacles potentially holding back new discoveries. This study works to elucidate the metamorphic and geochronologic evolution of midand upper-crustal rocks from three different locations in the Nepalese Himalaya. The results from high-precision Lu-Hf garnet geochronology and U-Th-Pb accessory geochronology in the Kanchenjunga region outline new evidence of pre-Himalayan garnets within the HMC. It is argued that these ca. 290 - 230 Ma garnets reflect either two-stage growth during Cambro- Ordovician and Cenozoic times, or growth during the early Permian opening of the Neo- Tethys, unrelated to Himalayan metamorphism. A similar multidisciplinary approach with mineral geochemistry, monazite petrochronology and phase equilibria modelling on midcrustal rocks from the Makalu-Arun region outlines at least three late Oligocene to mid- Miocene tectonometamorphic discontinuities within the HMC, with one of these inferred as an out-of-sequence thrust. The dataset obtained is consistent with a structural model driven by thrust wedge kinematics. Finally, garnet trace element geochemistry, monazite petrochronology and classical (conventional) thermobarometry carried out on metamorphic rocks in the vicinity of a Miocene pluton in the Upper Mustang region resolved contrasting interpretations on the origin of these rocks. This work provides evidence that metamorphism in the area was driven by time-resolved heat transfer away from the pluton. The dataset and interpretations produced as part of this thesis provide critical information for our understanding of the development of the Himalayan mid-crust and the development of new tectonic models explaining orogenic processes in general.

Text

2021-01-05

Attribution-NonCommercial-NoDerivatives 4.0 International

http://hdl.handle.net/2429/76949

Earth and Environmental Sciences

University of British Columbia

UBCO

http://creativecommons.org/licenses/by-nc-nd/4.0/