ACCURATELY LINKING DIKES TO DEFORMATION AND METAMORPHISM IN DEEP CONTINENTAL CRUST

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.”