Paper No. 4
Presentation Time: 2:00 PM


MOTTRAM, Catherine M., Department of Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom, WARREN, Clare J., Environment, Earth and Ecosystems, The Open University, Milton Keynes, MK7 6AA, England, REGIS, Daniele, Environment, Earth and Ecosystems, The Open University, Walton Hall, Milton Keynes, MK7 6AA, United Kingdom, ROBERTS, Nick M.W., British Geological Survey, NERC Isotope Geosciences Laboratory, Keyworth, NG12 5GG, United Kingdom and PARRISH, Randall R., Dept of Geology, University of Leicester, NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, NG12 5GG, United Kingdom,

Monazite is a key mineral used to date metamorphic processes in collisional orogens. There are two major unresolved challenges inherent in interpreting monazite geochronological data in terms of the P-T evolution of a rock: (i) determining why monazite ages yielded from a single sample can spread continuously over a period of several Ma, and ii) chemically ‘fingerprinting’ the monazite formed in different reactions during prograde and retrograde metamorphism.

Pelitic samples from an inverted Barrovian metamorphic sequence in the Sikkim Himalaya were analysed by in-situ U-Th-Pb chronology (LA-HR-ICP-MS) and trace element geochemical analysis (LA-ICP-MS) to investigate linkages between monazite geochemistry, garnet geochemistry, the duration of monazite growth and the P-T history of the rock. Phase assemblages were characterised by electron microprobe and modeled using Perplex to produce pseudosections of the metamorphic assemblages. This provided the platform on which to link monazite “age” to metamorphic “stage”.

Four main methods were used to link the monazites, which yielded ages between ~24-9 Ma, to the metamorphic stage(s): i) monazite inclusion relationships; ii) the presence or absence of other accessory phases such as xenotime, apatite and allanite in determining where the REE budget of the rock resided in relation to the P-T conditions; iii) the strong correlation between REE concentrations of monazite Y and Th ‘zones’ and age; and iv) garnet-monazite partition coefficients. This is the first time monazite and garnet partitioning has been explored for rocks which equilibrated under subsolidus conditions.

The fully characterised monazite-bearing samples have been used to determine the rates of metamorphic processes in the Sikkim Himalaya and to determine how monazite and other accessory phases evolve during Barrovian metamorphism. In this location, the implications of the duration of monazite growth and the metamorphic evolution of these rocks is key for extending knowledge about the rates of deformational processes in the eastern Himalaya in particular, and mid-crustal processes in general.