An Archean(?) to Paleozoic Evolution for a Garnet Peridotite Lens with Sub-Baltic Shield Affinity within the Seve Nappe Complex of Jämtland, Sweden, Central Scandinavian Caledonides


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Abstract

Mineralogical, isotopic, geochemical and geochronological evidence demonstrates that the Friningen body, a garnet peridotite body containing garnet pyroxenite layers in the Seve Nappe Complex (SNC) of Northern Jämtland, Sweden, represents old, certainly Proterozoic and possibly Archean, lithosphere that became incorporated into the Caledonian tectonic edifice during crustal subduction into the mantle at c. 450 Ma. Both garnet peridotite and pyroxenite contain two (M1 and M2) generations of garnet-bearing assemblages separated by the formation of two-pyroxene, spinel symplectite around the M1 garnet and the crystallization of low-Cr spinel1C in the matrix. These textures suggest initial high-pressure (HP) crystallization of garnet peridotite and pyroxenite succeeded by decompression into the spinel stability field, followed by recompression into the garnet peridotite facies. Some pyroxenite layers appear to be characterized solely by M2 assemblages with stretched garnet as large as several centimeters. Laser ablation microprobe–inductively coupled plasma mass spectrometry Re–Os analyses of single sulfide grains generally define meaningless model ages suggesting more than one episode of Re and/or Os addition and/or loss to the body. Pentlandite grains from a single polished slab of one garnet peridotite, however, define a linear array on an Re–Os isochron diagram that, if interpreted as an errorchron, suggests an Archean melt extraction event that left behind the depleted dunite and harzburgite bodies that characterize the SNC. Refertilization of this mantle by melts associated with the development of the pyroxenite layers is indicated by enriched clinopyroxene Sr–Nd isotope ratios, and by parallel large ion lithophile-enriched trace element patterns in clinopyroxene from pyroxenite and the immediately adjacent peridotite. Clinopyroxene and whole-rock model Sm–Nd ages (TDM=1·1–2·2 Ga) indicate that fertilization took place in Proterozoic times. Sm–Nd garnet2–clinopyroxene2–whole rock ± orthopyroxene2 mineral isochrons from three pyroxenite layers define overlapping ages of 452·1 ± 7·5 and 448 ± 13 Ma and 451 ± 43 Ma (2σ). All ages are within error of Sm–Nd mineral ages from eclogite in the enclosing host gneiss, demonstrating that, whereas silicate intrusion and fertilization occurred in the mantle during the Proterozoic, the formation of at least the M2 garnet-bearing assemblages occurred in the crust, as that crust was subducted into the mantle during the Caledonian orogenic cycle. By analogy we infer a similar origin for other mantle-derived lenses in HP nappes of the SNC in Northern Jämtland.

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