CHANGES IN TIMING AND STYLE OF SYN-COLLISIONAL METAMORPHISM ACROSS WESTERN BHUTAN: HIGH PRECISION RECORDS FROM MONAZITE

Our understanding of crustal deformation during continental collision is underpinned by geochronologic and petrologic studies of rocks that have been deformed and transformed. U-Th-bearing accessory minerals such as monazite record and preserve geochronological evidence for overprinting stages. The rates and duration of tectonic processes may be determined by linking these ages to the contemporary metamorphic conditions. Different monazite-forming reactions appear to leave distinct ‘chemical fingerprints' in monazite and other co-existing minerals such as garnet. We are in the process of developing a geochemical fingerprinting tool set that allows monazite-forming reactions to be identified in samples of different composition and metamorphic grade. We are using these fingerprints to link the monazite-forming reaction to the PT evolution of the host rock, and hence constrain rates/timescales of key mid-lower crustal tectonic processes. The Himalayan orogen, the archetypical example of continental collision, is notable for the presence of well-exposed high-temperature monazite-rich metamorphic rocks. Two monazite-bearing metasediments samples collected in the Jomolhari Massif (NW Bhutan) were chosen for detailed investigation. They were metamorphosed at lower amphibolite (un-melted) and high amphibolite facies (melted) conditions, allowing good control on the effect of bulk composition, variation in PT conditions and variations in melt abundance in the growth, stasis and dissolution of the accessory phases. Initial results suggest that specific monazite-forming reaction ‘fingerprints' can be identified in garnet in both un-melted and partially melted metapelites. Variations in HREE, Y, Sm, Ti and Eu/Eu* can be linked to growth/dissolution of major (e.g. garnet, feldspars, biotite) and accessory phases (e.g. apatite), and therefore to the PT evolution of the samples calculated from equilibrium assemblage (pseudosection) diagrams. These results suggest that an integrated approach that involves geochronology, geochemical data and petrological models can help link multiple stages of monazite growth to a specific PT stage, thereby providing a rich and reliable record of tectonic events.