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

A compilation of published and unpublished geochronological and isotopic data from garnet-bearing orogenic peridotites in the HP/UHP Western Gneiss Region (WGR) of the Norwegian Caledonides indicate a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 M₁ 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. 0.87Ga. The age spectrum suggests continuous diffusion among M₂ minerals that formed ≥ 1.75Ga ago punctuated by partial re-equilibration during a 1.0Ga 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 400Ma 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 424Ma), 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±3.1Ma; 2σ) may date M_3NW prograde re-equilibration during earliest Scandian subduction. The CW peridotites show no evidence of prograde M₃ re-equilibration, suggesting derivation from a different part of the Laurentian mantle wedge during the exhumation of the WGR from the mantle.