Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland
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High‐pressure (HP) and ultrahigh‐pressure (UHP) rocks have been widely recognized in the North Qinling Orogenic Belt (NQOB). The nature and age of their protoliths can provide insights into the formation and evolution of the NQOB. The Songshugou Complex, which crops out along the Shangdan Suture Zone, mainly consists of high‐grade metamorphosed mafic and ultramafic rocks, and thus, this area is an important study site for understanding the tectonic evolution of the NQOB. The metamafic rocks that enclose the ultramafic rocks are overwhelmingly composed of amphibolites that primarily exhibit amphibolite‐facies metamorphism, and minor garnet amphibolites (retrograde eclogite) and HP mafic granulites occur as discontinuous lenses or blocks within these amphibolites. Laser inductively coupled plasma mass spectrometry (LA‐ICPMS) U–Pb dating on zircons yielded ages of 499 ± 6, 507 ± 5, and 504 ± 10 Ma for the HP mafic granulite, garnet amphibolite (retrograde eclogite), and amphibolite, respectively. Those ages were interpreted to represent timing of the HP granulite or amphibolite metamorphism that occurred during later exhumation. Relict omphacites discovered in both garnet and zircon as well as in the matrix of the garnet amphibolite indicate that the rock underwent eclogite‐facies metamorphism and subsequent rapid exhumation. However, the timing of the different metamorphic stages cannot be distinguished from the zircon ages obtained using the LA‐ICPMS technique. The protolith age of the complex is constrained to be the early Neoproterozoic. Geochemical data indicate that the protoliths of the Songshugou metamafic rocks are of continental origin. These Neoproterozoic continental mafic rocks most likely formed during the breakup of the supercontinent Rodinia and were mainly derived from low‐degree partial melting of the asthenosphere with some lithospheric mantle contribution, within an extensional regime. All the available data reveal that the NQOB was one of the components of the Rodinia supercontinent and that it was likely involved in the Grenville orogenic belt and subsequently separated from the supercontinent at approximately 830–740 Ma.
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Abstract The Yacouba layered complex intrudes the Archean (3.5–2.7 Ga) Kenema-Man craton in the Samapleu-Yorodougou area, western Ivory Coast. In Samapleu area, the complex was recognized in drill holes at three locations: Samapleu Main (SM); Samapleu Extension 1 (E1) and Yorodougou (Yo). It comprises websterites, peridotites and gabbro-norites arranged symmetrically with mafic layers at the center and ultramafic layers at both margins. The complex is inclined at 70–80° to the SE. The thickness of individual layers varies from 2 to 60 m and the total thickness is 120 to 200 m. At the E1 site, the complex extends to depths > 500 m. Contacts with the country rock gneiss are characterized by a hybrid zone that is a few meters thick and composed of plagioclase-orthopyroxene bearing metabasites, and locally (E1 site) a metamorphic assemblage of sapphirine-cordierite-sillimanite-spinel ± rutile. This assemblage is attributed to contact metamorphism during intrusion of the complex in the lower crust at a depth of about 25 km. Zircons in country rock gneisses and granulites, as well as in the hybrid facies, yield Archean ages of ~ 2.78 Ga, similar to ages reported in the Man craton. Rutiles in the hybrid zone give a U-Pb age of 2.09 Ga, which is interpreted as the age of contact metamorphism and emplacement of the intrusion. The Samapleu Main and Samapleu Extension 1 sites contain Ni and Cu sulfide deposit with reserves estimated as more than 40 million tons grading 0.25% Ni and 0.22% Cu (Sama Nickel-CI, August 2013). The Ni-Cu mineralization is composed of pentlandite, chalcopyrite, pyrrhotite and rare pyrite, which is disseminated mainly in pyroxenite or occurs as subvertical and semi-massive to massive sulfide veins. The sulfide textures range from matrix ore, net-textured, droplets or breccia textures. Zones enriched in PGM, particularly Pd, are associated with the sulfides and several chromite bands are also present. These observations suggest that an immiscible sulfide liquid formed from a parental silicate liquid and percolated through the crystal pile. The parental melt composition, determined using the Chai and Naldrett [1992] method, has a SiO2-rich mafic composition with 53% SiO2 and 10% MgO. This result, the presence of the hybrid zone, and the trace-element signature determined using the Bedard [1994] method, suggest a mantle-derived basaltic parental magma that had assimilated abundant continental crust. These observations indicate that Samapleu intrusion corresponds to a magmatic conduit of the Yacouba complex as at Jinchuan (China), Voisey’s bay (Canada), Kabanga (Tanzania) or Nkomati (South Africa).
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