Northeastern (46th Annual) and North-Central (45th Annual) Joint Meeting (20–22 March 2011)

Paper No. 3
Presentation Time: 2:15 PM


DUMOND, Gregory, Geosciences, University of Arkansas, Fayetteville, AR 72701, MCLEAN, Noah, Earth, Atomospheric, and Planetary Sciences, MIT, 77 Massachusetts Ave, Cambridge, MA 02139, BARNES, Calvin G., Department of Geosciences, Texas Tech University, Box 41053, Lubbock, TX 79409-1053, WILLIAMS, Michael L., Department of Geosciences, University of Massachusetts, Amherst, 611 North Pleasant Street, Amherst, MA 01003 and BOWRING, Samuel A., Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139,

Determining the accurate age of deformation and metamorphic events remains a first-order problem in continental tectonics. Dating the crystallization of deformed granitic dikes containing fabrics that can be linked to map-scale deformation is a historically important tool in this regard. Although in situ dating techniques utilizing ion probes, laser ablation, and/or electron probe microanalysis are continually improving, isotope dilution-thermal ionization mass spectrometry (ID-TIMS) remains the most precise tool for accessory phase geochronology across the geologic time scale. Accuracy of ID-TIMS results (perhaps combined with data from other techniques), however, is fundamentally dependent upon establishing robust links between isotopic or U-Th-total Pb dates, in situ microstructural/textural setting, and petrogenesis of the host rock. We present examples that uniquely illustrate both the advantages and disadvantages of this approach from dikes in three tectonic settings: 1) Zircon CA-ID-TIMS applied to Neoarchean UHT-HP metamorphism in the deepest exposed crust in North America, 2) Zircon CA-ID-TIMS and monazite EPMA applied to Paleoproterozoic dextral transpressive strain in the lower crust along a Tibetan-scale intracontinental shear zone, and 3) Zircon LA-ICP-MS applied to Ordovician mafic intraplating in middle to deep arc crust exposed near the top of the Norwegian Caledonian nappe stack. These examples provide insights regarding the future of “tectonic geochronology.”