Using Detrital Zircon, Rutile and White Mica Chronometry to Constrain Exhumation and Provenance of the Brahmaputra River in the Eastern Himalaya
Abstract
While geo- and thermo-chronology of detrital minerals from sedimentary basins are routinely applied to constrain sedimentary provenance and hinterland evolution, the importance of a multi-technique approach is not always recognized. Isotopic dating methods sensitive to different temperature ranges can be successfully applied to detrital mineral grains from the same sample in order to obtain a robust dataset capable of providing information on the various thermal events that affected the source terrains. We use three detrital minerals in this study (zircon, rutile and white mica) that are stable and widely distributed in igneous, metamorphic and sedimentary rocks and which together retain source crystallisation and cooling information over the range down to ~200°C and thus record complex metamorphic histories. Similarly to zircon and other U-bearing minerals, rutile can be dated by the U-Pb method, however it has so far received less attention than zircon because of: lower U content which limits measurement quality by in situ methods, often a higher proportion of common (non radiogenic) lead, and a lack of widely available good quality reference materials. We have recently characterized (by high precision ID-TIMS, isotope dilution mass spectrometry, and by LA-MC-ICP-MS two natural rutiles (Sugluk-4 and PCA-S207) and used these as reference materials for LA U-Pb dating of detrital samples, Parrish et al., this meeting, and [1]). Compared to zircon, rutile is characterized by a lower closure T for Pb diffusion (~500°C), hence rutile U-Pb dates primarily indicate the time since the last significant cooling. As it adds an important lower temperature complement to zircon and allows a much more unique isotopic fingerprint of the source region, rutile has the potential to become a key provenance tracer. In order to boost the strength of the double U-Pb detrital chronometer, we apply 40Ar/39Ar dating and zircon fission track dating to detrital grains from the same sample. These two techniques, with a closure temperature of 350-420°C for Ar diffusion in white mica and of ~240°C for the zircon fission track system, are especially suitable to date cooling from after the last stage of low-grade metamorphism to the recent exhumation and/or uplift history of the source terrain. This powerful multi-chronometer has been applied to detrital samples from modern rivers draining the eastern Himalayan orogen, and examples will be presented, with particular emphasis on the Yarlung Tsangpo-Brahmaputra river drainage where the lower temperature thermochronometers reveal the Plio-Pleistocene very rapid exhumation of the Namche Barwa syntaxis. [1] Bracciali L., Parrish R.R., Condon D., Horstwood M.S.A., Najman,Y., Two new rutile reference materials for in situ U-Pb LA-MC-ICP-MS dating and applications to sedimentary provenance, submitted to Chemical Geology.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.T33C2671B
- Keywords:
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- 1140 GEOCHRONOLOGY / Thermochronology;
- 1165 GEOCHRONOLOGY / Sedimentary geochronology;
- 1194 GEOCHRONOLOGY / Instruments and techniques;
- 8175 TECTONOPHYSICS / Tectonics and landscape evolution