The timing of mineral growth across a regional metamorphic sequence

Abstract

IF regional metamorphism occurs in response to crustal thickening, then low-grade rocks are likely to record earlier thermal conditions than will high-grade rocks in the same exhumed terrain^1–4. At Sulitjelma, North Norway, regional metamorphism during the Caledonian orogeny created a type sequence of progressive metamorphic zones^5–7.Garnet and coexisting minerals from the high-grade part of this terrain yield precise and concordant Sm–Nd, U–Pb and Rb–Sr ages which relate directly to the pressure-temperature conditions of mineral growth⁸. Here we present pressure-temperature-time (P-T-t) information for lower-grade rocks occurring at the garnet isograd. Taken together, these data provide a precise estimate of the timing of mineral equilibration across a prograde metamorphic terrain and indicate that 'peak' metamorphic conditions at the garnet isograd (at 458 ± 20 °C) were attained 8.3±3.0Myr (Δt, Sm-Nd) or 8.7±3.2Myr (Δt, U-Pb) before the 'peak' in the high-grade sample (at 544 ± 16 °C). These results clearly illustrate that the exhumed 'P-T array' recorded in such terrains cannot be used to deduce any single geotherm that existed during metamorphism.