Geochronology of scapolite pegmatites from the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway: Protracted crystal-melt reaction during Scandian exhumation

Abstract

Titanite and zircon from a suite of scapolite pegmatites in the Nordøyane ultra-high-pressure domain, Western Gneiss Region, Norway, were analysed by laser ablation – inductively coupled plasma – mass spectrometry (LA-ICP-MS). Titanite yielded U Pb ages ranging from 420 to 390 Ma, with dates from prismatic crystals overlapping at 400–390 Ma. Complex zoning and resorption of older titanite grains (>415 Ma), interpreted as inherited, suggests that protracted crystal-melt reaction led to variable REE enrichment. Zircon rim zones yielded U Pb ages of 392–385 Ma, with inherited cores yielding 207 Pb/ 206 Pb ages of 1774–1575 Ma. In combination, the zircon and titanite U Pb data suggest that the pegmatites crystallised at ca. 400–385 Ma, consistent with the results of earlier studies. Trace element data are compatible with crystallisation at amphibolite facies conditions from melts in which fluid activity may have increased with time. Based on the ages of inherited zircon cores and titanite εNd 400 values of −8 to −11, the melts that gave rise to the scapolite pegmatites are interpreted to have been derived from Proterozoic Baltican crust. Partial melting initiated during subduction of Baltican crust in response to an influx of mantle fluid, with melt production greatly enhanced during exhumation and decompression. Complex zoning and age signatures in both titanite and zircon point to crystal-melt-fluid interaction over a protracted interval of melt production, migration, and crystallisation. • Nordøyane scapolite pegmatites intruded during amphibolite-facies retrogression. • The pegmatites crystallised at 400–385 Ma from melts derived from Baltican crust. • Inherited titanite grains, ca. 420 Ma, survived interaction with melt at T ≥ 750 °C. • Textures and age ranges of dated grains suggest protracted crystal-melt reaction.