Paper No. 15
Presentation Time: 9:00 AM-6:00 PM
DETRITAL RUTILE U–PB GEOCHRONOLOGY BY LA–MC–ICP–MS: A NEW APPROACH AND REFERENCE MATERIALS, WITH APPLICATIONS TO SEDIMENTARY PROVENANCE IN THE BHUTAN HIMALAYAS
BRACCIALI, Laura, LEC, Lancaster University AND NIGL BGS Keyworth, Nottingham, NG12 5GG, United Kingdom, PARRISH, Randall R., Dept of Geology, University of Leicester, NERC Isotope Geosciences Laboratory, British Geological Survey, Keyworth, NG12 5GG, United Kingdom, CONDON, Daniel, NERC Isotope Geoscience Laboratory, British Geological Survey, Keyworth, Nottingham, NG12 5GG, United Kingdom, HORSTWOOD, Matthew, NERC Isotope Geosciences Laboratory, British Geological Survey, Nottingham, NG12 5GG, United Kingdom and NAJMAN, Yani, Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, United Kingdom, y.najman@lancs.ac.uk
Rutile is a widely distributed accessory mineral in metamorphic and to a lesser extent igneous rocks, and is also a stable mineral commonly found in the heavy mineral suite of detrital rocks. Although it can be dated by the U-Pb method, rutile has so far received less attention than zircon because it usually has lower U and lower relative radiogenic Pb content and because of the lack of abundant rutile U-Pb reference materials. A further complication to U–Pb dating is that rutile can contain a relatively large proportion of common (non radiogenic) Pb. Compared to zircon, it is characterized by a lower closure T for Pb diffusion (ca. 500ºC, although this is debated) and can therefore be used as a thermochronometer to constrain cooling histories of metamorphic terranes. As such, it has the potential to become a key provenance tracer as it adds an important lower temperature complement to zircon by providing more detailed information on the hinterland source.
In this study we have applied LA-MC-ICP-MS, a technique with high spatial resolution and analytical throughput, to the rapid U–Pb dating of single grains of rutile. We introduce the use of two new natural rutile materials, Sugluk-4 and PCA-S207, as primary and secondary reference materials during the analysis, for which new high precision ID-TIMS U-Pb dates have also been determined.
We have determined the age of rutile grains from modern rivers draining the Bhutan Himalaya and compared these results to detrital zircon U–Pb data from the same sample. The results show that drainages with bedrock predominantly comprised of high grade Greater Himalaya yield rutile U-Pb ages of 10–20 Ma, whereas zircons from these sample are primarily >480 Ma, with only a few grains or metamorphic rims reflecting the Himalayan Miocene metamorphism. The contrast between zircon and rutile signatures is very dramatic and as such provides important complementary information about the events within the orogen.
