Campaign-style titanite U-Pb dating by ICP - Earth Science ...

100 K.J. Spencer et al. / Chemical Geology 341 (2013) 84–101distribution of all leucosome dates suggests that melting began in thecenter of the study area and ended along the outer coastline. 3) Titanitesin the gneiss have similar dates to those from the leucosomes, althoughsome of the gneiss titanites are a bit older (perhaps reflecting inheritance)and some are a bit younger, perhaps due to subsolidus recrystallization.4) Local amphibolite-facies mylonitization began at ~399 Ma and continuedthrough ~377 Ma; these titanites are among the youngest.In aggregate, these data imply a lengthy residence time at crustaldepths (cf. Walsh and Hacker, 2004) during which titanite crystallizedin leucosomes, recrystallized by reaction with other mineralsor fluids, and recrystallized in response to deformation. This conclusionstands in stark contrast to the short-lived thermal event at395 Ma inferred from the pioneering TIMS titanite work of Tuckeret al. (1990).5. ConclusionsCampaign-style, orogen scale dating in the Western Gneiss Region ofNorway shows a general gradient in U–Pb titanite dates from Precambrianto Scandian toward the core of the orogen. The titanite U–Pbdates do not show a simple relationship among grain size, peak temperature,or cooling rate, implying that thermally mediate volume diffusionwas not the principal factor controlling resetting of the U–Pb system.The preservation of Precambrian titanite in quartzofeldspathic gneiss inHP and UHP domains means that some titanite, and likely 450–750 km 2domains of gneiss, survived the entire 25 Myr long subduction–exhumation cycle; during this period the titanite was unstable, butdid not transform to (U)HP minerals and was not deformed. These observationsare further support for the recognition that equilibriumphase transformations and ductile flow of quartzofeldspathic crust attemperatures up to 750 °C cannot be assumed.AcknowledgmentsThe manuscript has been improved by comments from KlausMezger, Hannes Brueckner and an anonymous reviewer. Funded byEAR-0510453, 0649933, 0911485, and 0923552; the University ofCalifornia, Santa Barbara; and the NFR Centre of Excellence grant toPGP. Håkon Austrheim provided many years of discussion on theimportance of fluids in metamorphism and deformation. A host ofcollaborators participated in sample collection and interpretation:David Root, Emily Walsh, Dave Young, Scott Johnston, Emily Peterman,Jen Schmidt, Stacia Gordon, Steven Arauza and Adam Ginsburg. CraigStorey provided, and Daniel Condon helped analyzed the Ontario-2titanite reference material. 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