CHALLENGES OF DATING ULTRAHIGH-PRESSURE METAMORPHISM

Zircon from metamorphic terranes contains information on the timing of metamorphic events and processes from prograde to retrograde conditions. Extracting this information requires the careful, thoughtful analytical approach characteristic of Jim Mattinson’s career contributions. For example, zircon from ultrahigh-pressure (UHP) terranes is commonly texturally complex and yields a range of U-Pb ages that must be interpreted in context of changing P-T conditions. Ion microprobe U-Pb ages and trace-element data from coesite-bearing zircon suites formed during UHP metamorphism in the North-East Greenland Caledonides indicate a protracted history of UHP and subsequent retrograde metamorphism. Sub-spherical zircons have well defined domains in cathodoluminescence images and contain inclusion suites indicative of growth at UHP conditions. Trace-element signatures in core domains reflect modification of igneous protolith zircon. Rim signatures show flat HREE patterns characteristic of eclogite-facies zircon. ²⁰⁷Pb-corrected ²⁰⁶Pb/²³⁸U ages give a range from 330 to 390 Ma that is subdivided by textural or compositional domains; e.g., coesite-bearing domains vary from 364 ± 8 Ma to 350 ± 4 Ma. The span of ages is inferred to record a long history of zircon growth rather than inheritance or poor precision: (1) UHP metamorphism between 365 and 350 Ma; (2) granulite to amphibolite facies metamorphism between 350 and 340 Ma; (3) continued exhumation at amphibolite facies conditions through 320 Ma. UHP zircon from the Western Gneiss region, Norway, is similarly complex and has been subjected to CA-TIMS analysis (Root et al., 2004, EPSL). Ion microprobe ages range from 435 to 395 Ma. UHP metamorphism was assigned an age of 405-400 Ma on the basis of slightly discordant CA-TIMS data and the 40 Myr age spread was interpreted to reflect variable inclusion of inherited components rather than an extended period of zircon recrystallization or growth at HP/UHP conditions. Alternatively, additional CA-TIMS experiments are needed to tease out subtle age variations in the metamorphic zircon domains. Eclogite-facies zircon, typically low in U, presents a particular geochronologic challenge, requiring a balance between the spatial resolution of ion abrasion and the high-precision of chemical abrasion techniques.