Geochemistry of hornblende gabbros from Sonidzuoqi, Inner Mongolia, North China: Implications for magmatism during the final stage of suprasubduction-zone ophiolite formation
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Allochthonous hornblende-rich gabbroic rocks at Sonidzuoqi constitute important components of the early to middle Palaeozoic orogen, which forms the southeastern part of the Central Asian orogenic belt in Inner Mongolia. Limited hornblende K-Ar and SHRIMP U-Pb zircon ages document the Late Silurian to Early Devonian gabbroic emplacement. The rocks are tholeiitic and are characterized by moderate large-ion-lithophile-element (e.g. Th, U) abundances, high-field-strength-element (e.g. Nb, Ta, Zr, Ti) depletions, high Ti/V ratios, and MORB-like isotopic signatures [(87Sr/86Sr)i0.7030 to 0.7042; sNd(t)+4.35 to +7.80, (206Pb/ 204Pb)i=17.46 to 17.61]. These features argue for a hydrous basaltic parental magma. We postulate that the melt formed through the coupling of MORB-type mantle upwelling with aqueous fluid influx derived from slab devolatilization. This petrogenetic scenario suggests that an active spreading centre entered the trench during ridge subduction, bringing to a close an episode of suprasubduction-zone ophiolite formation. The Siluro-Devonian hornblende gabbros, together with a pre-490 Ma ophiolitic melange of MORB-OIB affinity, ~483-471 Ma arc intrusions, ~498-461 Ma trondhjemite-tonalite- granodiorite plutons, and ~427-423 Ma calc-alkaline granites from the same area, provide documentation of multistage crustal generation processes during the life cycle of this suprasubduction-zone ophiolite.
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