The mantle and crustal evolution of two garnet peridotite suites from the Western Gneiss Region, Norwegian Caledonides: An isotopic investigation

Abstract

A compilation of published and unpublished geochronological and isotopic data from garnet-bearing orogenic peridotites in the HP/UHP Western Gneiss Region (WGR) of the Norwegian Caledonides indicate a common origin for all WGR peridotites, followed by different, though related, Proterozoic and Phanerozoic histories for those in the northwestern WGR (NW peridotites) compared to those in the central and western WGR (CW peridotites). All peridotites are refractory fragments of the subcontinental lithosphere generated by Archean melt extraction, which produced strongly depleted dunites and harzburgites with relict orthopyroxene and majoritic garnet megacrysts (M 1NW) within the NW peridotites. The Archean history is preserved by Re–Os sulfide and whole-rock ages from several WGR bodies and by Sm–Nd ages from the M 1NW megacrysts. Subsequently the CW peridotites were re-fertilized within the lithospheric mantle by mid-Proterozoic or older silicate melts that generated M 2CW garnet pyroxenites and adjacent garnet peridotites. Clinopyroxenes from these bodies show large variation in 143Nd/ 144Nd, but nearly constant 87Sr/ 86Sr, suggesting autometasomatism of depleted mantle by LREE-enriched, Rb-poor melts derived from equally depleted mantle. NW peridotites lack mid-Proterozoic garnet pyroxenite intrusions, but M 2NW garnet-rich assemblages that exsolved from relict M 1 megacrysts may have equilibrated at the same time as the M 2CW refertilization. Sm–Nd and Lu–Hf mineral apparent isochron ages from both suites range from 1.75 to ca. 0.87 Ga. The age spectrum suggests continuous diffusion among M 2 minerals that formed ≥ 1.75 Ga ago punctuated by partial re-equilibration during a 1.0 Ga thermal event. Much later the NW peridotites were transferred from the mantle wedge into the crust as the WGR was subducted into the mantle during the ca 400 Ma Scandian Orogeny. Further subduction heterogeneously metasomatized and recrystallized the NW peridotites to form M 3NW garnet, clinopyroxene and, where metasomatism was pervasive, new M 3NW radiogenic ( 87Sr/ 86Sr > 0.715), LIL-enriched minerals (phlogopite, amphibole) and microdiamond consistent with invasion by hydrous fluids from the enclosing Proterozoic gneisses. Nine young apparent ages (672 to 424 Ma), all from exsolved or recrystallized garnets within NW peridotites, represent mixed (M 2NW and M 3NW) apparent ages. The three youngest ages (weighted mean of 429.5 ± 3.1 Ma; 2 σ) may date M 3NW prograde re-equilibration during earliest Scandian subduction. The CW peridotites show no evidence of prograde M 3 re-equilibration, suggesting derivation from a different part of the Laurentian mantle wedge during the exhumation of the WGR from the mantle.