CONTRASTING RETROGRADE OXYGEN-ISOTOPE EXCHANGE BEHAVIOR AND IMPLICATIONS - EXAMPLES FROM THE LANGTANG VALLEY, NEPAL

Quantitative modelling of oxygen exchange by diffusion during slow coo ling has been compared to the observed oxygen isotope distributions fr om high-grade metamorphic and granitic rocks of the High Himalayan Cry stallines, Langtang Valley, central Nepal, in order to investigate the effect of retrograde diffusional exchange on the preservation of high -temperature, oxygen isotope systematics. Modelled fractionations, usi ng water-present diffusion data reported in the literature, predict qu artz-mica fractionations to be much larger than those at peak metamorp hic and igneous conditions due to low closure temperatures for micas. Quartz-feldspar fractionations may be less than those at peak conditio ns, and in some samples may even be slightly negative. The observed ox ygen isotope fractionations in the metamorphic rocks are small and lar gely appear to record equilibrations close to peak conditions determin ed by other methods. Hence these rocks clearly do not conform to predi ctions of fluid-present diffusional retrograde exchange. It is suggest ed that their retrograde history was therefore within an anhydrous clo sed system in which diffusion was slow and hence mineral closure tempe ratures were high. The granitic rocks record rather larger quartz-biot ite fractionations, approaching those predicted by the diffusion model ling. However, quartz-feldspar fractionations are large and hence, alt hough significant retrograde exchange has clearly occurred, simple dif fusion alone is not sufficient to explain the observed data and open-s ystem exchange may be required. The presence of fluids during the retr ograde history of this part of the section is supported by petrographi c evidence.The different retrograde oxygen exchange histories recorded between the regional metamorphic and magmatic regimes of the Langtang section would appear to support the importance of water on the kineti cs of such exchange, and suggests that in its absence, diffusional exc hange may become insignificant, allowing oxygen isotope thermometry to record meaningful high-temperature data.