| Citation: |
Tian Qiu, Fa-hui Xiong, David G. Gee, Yuan Li, Jing-sui Yang, 2024. Multi-stage formation of the Feragen ophiolite, Norway: Implication from petrology and geochemistry of peridotites and chromitites and its potential for prospecting, China Geology, 7, 686-701. doi: 10.31035/cg2023017
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Multi-stage formation of the Feragen ophiolite, Norway: Implication from petrology and geochemistry of peridotites and chromitites and its potential for prospecting
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Abstract
The ultramafic massif of Feragen, which belongs to the eastern ophiolitic belt of Norway, has abundant amounts of chromite ores. Recent studies have revealed a complex melt evolution in a supra-subduction zone (SSZ) environment. This study presents new whole-rock major element, trace element, and platinum-group element chemistry to evaluate their petrogenesis and tectonic evolution. Harzburgites have high CaO, Al2O3, TiO2, MgO, and REE contents corresponding to abyssal peridotites, whereas dunites have low CaO, Al2O3, TiO2, MgO, and REE contents corresponding to SSZ peridotites. The Cr# and TiO2 of chromian spinels in the harzburgites suggest as much as about 15%–20% melting and the dunites are more depleted with > 40% melting. The harzburgites and the dunites and high-Cr chromitites represent, respectively, the products of low-degree partial melting in a back-arc setting, and the products of melt-rock interaction in a SSZ environment. The calculated ƒO2 values for dunites and high-Cr chromitites (−0.17 – +0.23 and +2.78 – +5.65, respectively and generally above the FMQ buffer) are also consistent with the interaction between back-arc ophiolites with oxidized boninitic melts in a SSZ setting.
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Keywords:
- Geochemistry /
- ƒO2 /
- Platinum-group elements /
- High-Cr podiform chromitites /
- SSZ peridotite /
- Harzburgites /
- Feragen ophiolite /
- Norway /
- Dunite /
- Abyssal peridotite
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References
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Tian Qiu, Fa-hui Xiong, David G. Gee, Yuan Li, Jing-sui Yang, 2024. Multi-stage formation of the Feragen ophiolite, Norway: Implication from petrology and geochemistry of peridotites and chromitites and its potential for prospecting, China Geology, 7, 686-701. doi: 10.31035/cg2023017
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Figure 1.
Simplified tectonostratigraphic map of the Feragen ophiolite, Norway (modified from Moore AC and Hultin I, 1980).
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Figure 2.
Field photographs showing the various lithologies in the Feragen ophiolite. a–The Feragen ultramafic rocks; b and c–Dunite veins in harzburgite; d–Disseminated chromite ore.
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Figure 3.
Photomicrographs of peridotites and chromitites in the Feragen ophiolite. a–Harzburgite; coarse-grained olivine coexisting with orthopyroxene, granular texture (cross-polarized light); b–Dunite; olivine altered by serpentine along cracks and boundaries (cross-polarized light); c–Disseminated chromitites; olivine has been mostly altered by serpentine (plane-polarized light); d–Chromian spinels altered to ferrit-chromite or magnetite along boundaries and fractures in the disseminated chromitites (reflected light). Ol–olivine; Opx–orthopyroxene; Spl–chromian spinel; Cpx–clinopyroxene; Srp–serpentine; Ferrit-chr–Ferrit-chromite; Mgt–magnetite.
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Figure 4.
NiO (a) and MnO (b) vs. Fo contents of olivine in the different lithologies of the Feragen ophiolite. ABP–abyssal peridotite and FAP–fore-arc peridotite from Pagé P et al. (2008), partial melting trends from Ozawa K (1994), and fractionation trends from Ozawa K (1994) and Nakamura M (1995).
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Figure 5.
Compositional range of orthopyroxene (a–b) and clinopyroxene (c–d) in harzburgites of the Feragen ophiolite. ABP-abyssal peridotite and FAP-fore-arc peridotite from Pagé P et al. (2008). The melting trend from Smith SE and Elthon D (1988).
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Figure 6.
Harker diagrams of the whole-rock compositional range of harzburgites and dunites in the Feragen ophiolite. Abyssal and SSZ peridotite fields from Niu Y et al. (1997), Parkinson IJ and Pearce JA (1998) and Uysal I et al. (2009). The residual compositions from melting (at 1×109 Pa and 2×109 Pa) of Primitive Mantle (Palme H and O’Neill HSC, 2004) calculated using pMELTS program of maximum 40% melting degree (Ghiorso MS et al., 2002) are also shown in the diagram.
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Figure 7.
Variation diagrams of MgO vs. Y (a), Sc (b), V (c), and Yb (d) in harzburgite and dunite samples of the Feragen ophiolite. Abyssal and SSZ peridotite fields from Niu Y et al. (1997) and Parkinson IJ and Pearce JA (1998), respectively.
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Figure 8.
a–Chondrite-normalized REE patterns of dunites and harzburgites in the Feragen ophiolite (normalized after McDonough WF and Sun SS, 1995). b–Primitive mantle-normalized trace element spider diagrams of dunites and harzburgites in the Feragen mantle ophiolite (normalized after McDonough WF and Sun SS, 1995). Fields of abyssal peridotites (afterBodinier JL and Godard M, 2003; Niu Y, 2004) and forearc peridotites (Parkinson IJ and Pearce JA, 1998) are shown for comparison. The compositions of harzburgites sampled in the Semail (Oman) ophiolite from Godard M et al. (2000).
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Figure 9.
a–Primitive mantle-normalized PGE patterns of the Feragen peridotite and chromitites (normalized after Barnes SJ et al. (1988); b–Iridium vs. palladium of the peridotites and chromitites in the Feragen ophiolite. Chondritic ratio and mantle values from Chou CL et al. (1983). MORB and low-Ti lava fields from Hamlyn PR et al. (1985); c–Platinum/iridium vs. palladium/platinum for peridotites and chromitites in the Feragen ophiolite. Primitive mantle values from Barnes SJ et al. (1988).
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Figure 10.
a–Plot of ΔlogƒO2 (FMQ) vs. Cr# of spinels from the Feragen chromitite, harzburgites and dunites. We have calculated ΔlogƒO2 (Ballhaus C et al., 1991) from geothermometric data derived from olivine - spinel equilibria following the approach of Wan Z et al. (2008); the data are provided in Table 6. MOR–SSZ discrimination boundaries for dunites (solid line) and harzburgites (dashed line) are shown. MOR–mid-ocean ridge, SSZ–supra-subduction zone, BAB–back arc basin, BON–boninite, IAT–island arc tholeiite (Parkinson IJ and Pearce JA, 1998; Dare SAS et al., 2009); b–(FeO/MgO) melt vs (Al2O3) melt (%) calculated on the basis of the chemical composition of the Feragen dunites and chromitites. Tectonic discrimination fields from Barnes SJ and Roeder PL (2001).
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Figure 11.
a–Compositional range of chromian spinel in different lithologies of the Feragen ophiolite after Pearce JA et al. (2000). Data of spinels of MORB, forearc peridotites and boninites are form Izu-Bonin-Mariana system (Pearce JA et al., 2000, and references there in); b–Compositional relationship between Cr# and TiO2 content of spinel in peridotite and chromitite samples.