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Abstract of thesis entitled
THE NEELAXIONGBO METAMORPHIC CORE COMPLEX AND ITS ASSOCIATED GRANITES, IN SOUTHERN TIBET
Ho Hoi To, Lucas
for the degree of Master of Philosophy
at The University of Hong Kong
in April 2002
Continental extension and magmatism play significant roles in the formation of the Melaxiongbo Metamorphic Core Complex (NMCC), in southern Tibet. Most features of classic Cordilleran core complexes are displayed by the NMCC including: high-grade metamorphic rocks separated from markedly lower-grade rocks by detachment faults, and syntectonic granitic intrusions. However, one major difference from the Cordilleran occurrences is that the ocean-continent plate boundary which typifies the North America Cordillera is replaced by an intra-continental environment in the Himalayas. Core complexes consist of a multiply-deformed metamorphic-plutonic core (sub-decollement zone) separated from low-grade metasedimentary rocks (supra-decollement zone) by a recognizable decollement zone, which is characterized by shallow-dipping extensional faults dipping outwards everywhere from the centre of the complex.
In the NMCC, the metamorphic grade increases toward the basement core and is varied concentrically around the core. There is a marked jump in metamorphic grade across the decollement zone. The central portion of the core complex is intruded by Miocene two-mica and tourmaline-bearing leucogranite bodies. They form discordant, lens-shaped intrusions and are the result of crustal melting during continental collision. Field, petrographic, geochemical and geochronologic studies of
the metamorphic-granitic phiton of the NMCC were used to investigate the processes that formed the domal structure and the granitic intrusions based primarily on field and petrographic characteristics, the granitic rocks have been divided into two main groups. These are: the two-mica leucogranite group composed of quartz, feldspar, muscovite, biotite and in some places, garnet; and the tourmaline granite group containing mainly quartz, feldspar, muscovite with minor amounts of biotite and tourmaline.
Magmatism and extension appear strongly correlated with each other. The presence of syntectonic granitic bodies supports models in which magmatism is intimately associated with core complex formation. Crustal thickening and increasing topographic elevation resulting from continued intra-continental collision produced lateral pressure gradients within the crust. Extensional faulting occurred within the upper part of the crust and resulted in gravitational collapse of the Himalayan topographic front.
The NMCC leucogranites exhibit similar characteristics to the High Himalayan leucogranites both mineralogically and geochemically. Geochemical and field studies show that metasediments within the High Himalayan belt are the most likely source for those intrusions. It is likely that the mechanism of magma generation was the same for the NMCC, and that the granites simply differ in their respective localities and mode of emplacement (e.g. emplacement depth).
Ar-Ar studies provide ages of 13-13.5Ma and suggest rapid cooling and uplift attributed to tectonic denudation of the granite pluton by movement on structurally higher extensional faults, essentially at the same time as granite intrusion. The NMCC is one of a number of similar complexes allocated with the Miocene extensional
evolution of southern Tibet, formed by processes of decompression melting in the crust.