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 [superscript 206]Pb/[superscript 238]U 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 ε[subscript Nd](t) of seven samples, including the upper-level gabbros with variable zircon dates, have tightly clustered initial values ranging from ε[subscript Nd](96 Ma) = 7.59 ± 0.23 to 8.28 ± 0.31. The ε[subscript Nd] 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.