Tectonic development of the Samail ophiolite: High-precision U-Pb zircon geochronology and Sm-Nd isotopic constraints on crustal growth and emplacement

摘要

New high-precision single grain U-Pb zircon geochronology and whole rock Nd isotopic data provide insight into the magmatic and tectonic development of the Samail ophiolite. The analyzed rocks can be broadly divided into two groups based on their structural position, dates, and isotopic composition: an older group related to on-axis magmatism and a younger group of post-ridge dikes, sills, and stocks. On-axis gabbros, tonalites and trondhjemites yielded Th-corrected ~(206)Pb/~(238)U dates from 96.441±0.062 to 95.478±0.056 Ma. These dates, combined with dates from Rioux et al. (2012), suggest that most of the ophiolite crust formed at an oceanic spreading center in <1 Ma. The post-ridge intrusions come from all depths in the crust, the upper mantle, and the metamorphic sole. Post-ridge gabbros, tonalites, and trondhjemites from the crust and mantle yielded Th-corrected ~(206)Pb/~(238)U dates of 95.405 ± 0.062 to 95.077± 0.062 Ma. A small trondhjemitic pod from the metamorphic sole yielded younger Th-corrected ~(206)Pb/~(238)U dates of 94.90± 0.38 to 94.69±0.12 Ma. Isotopic data suggest two distinct sources for the post-ridge magmas: five of the gabbros and tonalites from the crust have ε_(Nd)(96 Ma) = 6.90±0.12 to 7.88 ± 0.16, and two trondhjemites from the upper mantle and metamorphic sole have ε_(Nd)(96 Ma) = -7.77 ± 0.08 and -7.01 ± 0.16. The negative ε_(Nd)(t) and U-Pb dates from the mantle dike require that subduction or thrusting was established below the ophiolite ≤0.25-0.5 Ma after formation of the crust. The bimodal isotopic composition of post-ridge magmas may reflect coeval decompression and/or fluid fluxed melting of the mantle and melting, dehydration, or assimilation of sediment in the down going plate at this time. The new data place temporal constraints on mid-ocean ridge and supra-subduction zone models for ophiolite formation.