ZIRCON PETROCHRONOLOGY OF UHP TERRANES: HOW DO WE INTERPRET COMPLEX DATA SETS?

Combined trace element and isotopic analysis of geochronometers has become standard practice for establishing the timing and duration of ultrahigh-pressure (UHP) metamorphism. The link between UHP conditions and an age estimate is currently best established by dating metamorphic zircon domains that contain inclusions of the UHP index minerals, coesite and/or diamond. Variations in trace element composition, temperature estimates based on Ti concentration and compositional data from inclusion suites that can be used to estimate P-T conditions based on mineral equilibria all provide additional information to link dated zircon domains to prograde, UHP, and retrograde portions of the pressure-temperature path. The resulting petrochronologic datasets can be used to decipher complex metamorphic processes associated with the formation and exhumation of long-lived UHP terranes. Petrochronology by secondary ion mass spectrometry (SIMS) of zircon from the North-East Greenland UHP terrane provides an excellent example of the complexities observed in zircon from long-lived UHP terranes. The 10-30 my range in dates from individual samples is particularly difficult to interpret with certainty, even with additional compositional data. The large age range may reflect: (1) continuous zircon growth, recrystallization or Pb-loss during metamorphism, such that the scale of growth domains or zones of recrystallization are smaller than the sample volume analyzed, or (2) a series of closely spaced events that cannot be resolved within analytical uncertainty of the dating technique. The calculated weighted mean age and associated uncertainty likely do not adequately reflect the complexity of the age variation in the zircon population for either of these scenarios because the scatter results from true natural variation. Attempts to improve precision through TIMS analysis of complex zircon grains will produce high-precision averages of multiple domains unless specific domains can be chemically or mechanically isolated. High spatial resolution petrochronology is considered optimal for attempts to link ages with the observed P-T path for UHP terranes: weighted mean ages from domains characterized by U–Pb age, CL-response, mineral inclusions and/or chemistry serve as a proxy for periods of major zircon growth.