Campaign-style titanite U–Pb dating by laser-ablation ICP: Implications for crustal flow, phase transformations and titanite closure

U–Pb dates of titanite from > 150 samples of chiefly quartzofeldspathic gneiss and leucosomes were measured across the Western Gneiss Region of Norway to understand deformation and metamorphism of typical crustal rocks during ultrahigh-pressure (UHP) subduction and exhumation. Titanite is unstable at these high temperatures and pressures, and, indeed, most of the titanite yielded post-UHP dates. A modest number of titanites sampled across large areas, however, have pre-UHP U–Pb dates, indicating that they survived their excursion to and return from mantle depths metastably. This has three important implications. 1. Titanite grains can remain closed to complete Pb loss during regional metamorphism at temperatures as high as 750 °C and pressures as high as 3 GPa. 2. Phase transformations in quartzofeldspathic rocks can be inhibited at the same conditions. 3. Quartz-bearing rocks can remain undeformed even at high temperature and pressure. Both of the latter were previously recognized; the present study simply presents a new method for evaluating both using titanite U–Pb dates.