Nature of Tibetan basement rocks and utility of titanite-rutile barometry: Insight from unique exposures near Amdo

Direct constraints on the nature of the deeper Tibetan crust are lacking, in large part due to the scarcity of basement exposures within the interior of the plateau. A unique exception is a ~100 km long by ~50 km wide exposure of Cambrian and older orthogneisses and tectonically interlayered metamafic and metasedimentary rocks near Amdo, ~270 km north of the city of Lhasa. These basement rocks provide an opportunity to directly investigate the composition, origin, age and structural-metamorphic evolution of Tibetan crust that is otherwise buried by low-grade supracrustal assemblages. Furthermore, our preliminary studies indicate that some Amdo gneisses and metabasites contain the assemblage: amphibole + plagioclase + quartz + epidote + titanite + rutile. These rocks provide an opportunity to explore the utility of integrated titanite-rutile barometry and U-Pb geochronology for elucidating pressure-temperature-time (P-T-t) histories by comparison with results from established thermobarometers and dating methods.

This project, which forms the basis of Jerome Guynn's PhD research involves: (1) Field mapping and detailed petrologic analysis to characterize lithologic make-up, metamorphic zonation, mineralogy, textural relations and structural fabric; (2) U-Pb igneous, metamorphic, and detrital zircon geochronology; (3) Oxygen isotope, radiogenic isotope (Sr, Nd, Pb), and major and trace element analyses to constrain petrogenesis; (4) Application of established thermobarometers, phase equilibria, and dating methods (monazite inclusions in garnet; 40Ar/39Ar hornblende) to constrain P-T-t evolution; (5) Comparison of pressures calculated using titanite-rutile equilibria and U-Pb ages of titanite to those determined from established methods; (6) Theoretical calculation of densities, seismic properties, and partial melting behavior as a function of P-T conditions given constraints on modal mineralogy and preferred mineral orientation.

These studies will constrain parameters which are fundamental to assessing (1) measured geophysical properties and the extent to which the Tibetan middle crust is partially molten through numerical experiments using MELTS, (2) the utility of titanite-rutile barometry + geochronology for constraining P-T-t histories; an approach which may be extended to epidote-bearing but garnet-absent blueschists and amphibolites which are ubiquitous in oceanic subduction-accretion settings and (3) Proterozoic-Cambrian crustal evolution and paleogeography of Tibetan terranes.