HR: 15:25h
AN: V22E-08 INVITED     [PDF]
TI: ID-TIMS Geochronology of ca 1 Ma leucogranites from the core of Nanga Parbat
AU: * Bowring, S A
EM: sbowring@mit.edu
AF: Department of EAPS, MIT, Cambridge, MA 02139
AU: Searle, M P
EM: Mike.Searle@earth.ox.ac.u
AF: Oxford University, Department of Earth Sciences,Parks road, Oxford,, UK OX1 3PR
AU: Waters, D J
EM: dave.waters@earth.ox.ac.uk
AF: Oxford University, Department of Earth Sciences,Parks road, Oxford,, UK OX1 3PR
AU: Hodges, K V
EM: kvhodges@mit.edu
AF: Department of EAPS, MIT, Cambridge, MA 02139
AU: Schmitz, M D
EM: schmitz@dtm.ciw.edu
AF: Department of Terrestrial Magnetism, Carnegie Institution of Washington, 5241 Broad Branch Road NW, Washington, DC 20015
AU: Crowley, J
EM: jlcrowle@mit.edu
AF: Department of EAPS, MIT, Cambridge, MA 02139
AB: Although isotope-dilution, themal-ioniaztion mass spectrometry (ID-TIMS) is the most precise method available for U-Th-Pb geochronology of accessory minerals, the technique is traditionally not applied to samples youger than ca. 10 Ma due to low concentrations of radiogenic Pb. However, recent analytical advances allow the routine measurement of less than 5-10 picograms of radiogenic Pb, and the reduction of laboratory blanks to extremely low levels. As a consequence, it is now possible to use ID-TIMS for U-Pb geochronology of very young accessory mineral suites to develop new insights regarding the time scales and patterns of inheritance, melt production, and segregation in collisional orogens. The Nanga Parbat syntaxis of the northwestern Himalaya has had a long and complex thermal history, but it is famous for the evidence it provides for extremely young ($<$5 Ma) metamorphism and anatexis. Some of the youngest anatexites in the core of the syntaxis are cordierite-bearing granites which intruded as dikes and sills along extensional shear zones. High-precision ID-TIMS U-Pb geochronology of these granites using single crystals and several grains of monazite, zircon, xenotime, and uraninite indicate a melt crystallization age of $<$1 Ma. However, mineral dates within a single sample show a dispersion of several hundred thousand years that is greater than analytical imprecision and likely to represent the timescales of melt production and segregation. These are the youngest U-Pb ages known from any Himalayan leucogranite and, combined with P-T data, show that 15-17 km has been eroded off the summit area of Nanga Parbat in less than 1 million years.
DE: 1035 Geochronology
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting