Rare earth elements and Sm-Nd isotope redistribution in apatite and accessory minerals in retrogressed lower crust material (Bergen Arcs, Norway)

Abstract

In the Bergen Arcs (Norway), Grenvillian granulite retrogressed under eclogite- and amphibolite-facies conditions during Caledonian subduction/collision offer a unique opportunity to investigate rare earth elements (REE) and Sm-Nd isotope redistribution in accessory minerals during fluid-assisted metamorphism. Our sampling targeted apatite-bearing REE-rich protoliths (mangerite and jotunite) that preserve distinct mineral assemblages, depending on the external fluid availability and metamorphic conditions. REE concentrations in apatite are the highest in the granulite. Two populations are present: magmatic apatite (Ap1) relics that occur as inclusions in ilmenite-hematite, and intergranular apatite (Ap2) formed under granulite-facies conditions. The presence of abundant needle-like monazite and sulphide inclusions in Ap2 indicate that granulite reactions were fluid assisted. A thin (typically <10μm) rim of REE-rich epidote (Ep1) commonly surrounds Ap2. In these accessory minerals, U and Th contents are too low, or grains are too small, for in situ U-Th-Pb dating. Sm-Nd isotope data of Ap2, monazite and Ep1 give an isochron age of 601±69Ma, which is interpreted to represent a partially reset Grenvillian age, affected by Caledonian fluid-assisted mineral growth. In amphibolitized samples, granulite Ap2 is replaced by apatite (Ap3) with lower REE contents and no monazite inclusions. The REE released by this replacement are redistributed in a corona of epidote group minerals (Ep2) surrounding Ap3. The in situ Sm-Nd isotope data for Ep2 and titanite, found in replacement of ilmenite-hematite, return an isochron age of 395±65Ma, recording the timing of amphibolite-facies mineral growth when fluids were introduced into the rock. In eclogitized samples, eclogitic apatite (Ap4) occurs as polycrystalline aggregates, suggesting for a complex replacement process during deformation. REE contents of Ap4 are low, as REE originally contained in the precursor apatite were redistributed mainly into zoisite. Apatite shielded as inclusions in ilmenite and garnet preserve the REE-rich signature of the initial magmatic (Ap1) and granulite (Ap2) apatite, indicating these grains did not undergo further re-equilibration during Caledonian metamorphism. The resistance of apatite to compositional re-equilibration in this case confirms the petrological potential of apatite inclusions shielded in chemically inert minerals to track early magmatic, or metamorphic, crystallisation stages.