Rapid crustal accretion and magma assimilation in the Oman‐U.A.E. ophiolite: High precision U‐Pb zircon geochronology of the gabbroic crust

Abstract

New high‐precision U/Pb zircon geochronology from the Oman‐United Arab Emirates (U.A.E.) ophiolite provides insight into the timing and duration of magmatism and the tectonic setting during formation of the lower crust. The new data come from a well‐preserved and exposed crustal section in the center of the Wadi Tayin massif. Single grain and grain fragment 206Pb/238U dates from upper‐level gabbros, tonalites/trondhjemites and gabbroic pegmatites, corrected for initial Th exclusion, range from 112.55 ± 0.21 to 95.50 ± 0.17 Ma, with most data clustered between 96.40 ± 0.17 to 95.50 ± 0.17 Ma. Zircon dates from upper‐level gabbros are most consistent with the ophiolite forming at a fast spreading ridge with half‐rates of 50–100 km/Ma. Dates from tonalites/trondhjemites and from a gabbroic pegmatite associated with a wehrlite intrusion overlap with dates from adjacent upper‐level gabbros, suggesting that any age differences between these three magmatic series are smaller than the analytical uncertainties or intrasample variability in the dates. Three of the dated upper‐level gabbros and a single gabbroic pegmatite from the base of the crust have >1 Ma intrasample variability in single grain dates, suggesting assimilation of older crust during the formation or crystallization of the magmas. Whole rock εNd(t) of seven samples, including the upper‐level gabbros with variable zircon dates, have tightly clustered initial values ranging from εNd(96 Ma) = 7.59 ± 0.23 to 8.28 ± 0.31. The εNd values are similar to those from other gabbros within the ophiolite, suggesting that any assimilated material had a similar isotopic composition to primitive basaltic magmas. The new dates suggest that the studied section formed at a fast spreading mid‐ocean ridge between ∼96.4–95.5 Ma. The large intrasample variability in zircon dates in some samples is unexpected in this setting, and may be related to propagation of a younger ridge into older oceanic lithosphere.