2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 7
Presentation Time: 8:00 AM-12:00 PM


JOHNSTON, Scott M.1, GEHRELS, G.E.1 and HARTZ, Ebbe H.2, (1)Department of Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th St, Tucson, AZ 85721, (2)Physics of Geological Processes, Oslo University, PO Box 1048 Blindern, Oslo, 0316, Norway, scojoh@email.arizona.edu

Recent advances in laser ablation multicollector ICP mass spectrometry (LA-MC-ICPMS) allow for U-Pb age mapping of individual zircon grains, and can efficiently determine detrital signatures and metamorphic/igneous overgrowth ages in samples that contain complex zircons with variable histories. Zircon separates are mounted in epoxy, polished one third of the way through individual grains, and imaged using cathodoluminescence to identify chemical zoning and core/rim relationships. U-Pb age maps of individual grains are then made using LA-MC-ICPMS to determine spot ages (10 microns in diameter by 4 microns deep) across the polished surface of the zircon. Total U and Pb counts integrated over a 4 second acquisition period (238U is measured using a Faraday detector while Pb isotopes are measured using channeltron multipliers) are normalized to a standard Sri Lanka zircon, and used to calculate U-Pb ages. Using total U and Pb counts eliminates fractionation problems introduced during time-integrated analyses with short analysis times; errors for unknowns are determined from the error on the standard. When plotted on a concordia diagram, spot ages from single grains yield mixing lines with upper and lower intercepts indicating core and rim ages, respectively. More precise lower-intercept ages can be achieved by identifying and averaging concordant rim analyses from multiple grains within the same sample.

In a first application of this technique, U-Pb age maps were made on zircons from two granulite-grade paragneisses and an S-type granite from Liverpool Land, East Greenland. Upper-intercept ages (accurate to 10% at 95% confidence) reveal Archean through Late Mesoproterozoic detrital cores, whereas lower-intercept ages of 430 ± 5, 422 ± 4, and 417 ± 3 Ma (95% confidence) indicate subsequent Caledonian deformation. This detrital signature and metamorphic history suggests a direct correlation of the Liverpool Land paragneisses to the Krummedal Sequence in the Greenland allochthons farther west, and contrasts sharply with the structurally lower, 400 to 390 Ma eclogite province of Liverpool Land. This study demonstrates the utility of this technique for unraveling the histories of complicated zircons, and illustrates the potential for LA-MC-ICPMS U-Pb zircon age mapping in many future applications.