The Origin of Felsic Intrusions Within the Mantle Section of the Samail Ophiolite: Geochemical Evidence for Three Distinct Mixing and Fractionation Trends

Abstract

The Samail (Oman-United Arab Emirates [UAE]) ophiolite exposes an extensive section of Late Cretaceous oceanic lithosphere. The ophiolite is dominated by ultramafic to mafic rocks, with the exception of small-volume tonalite and trondhjemite intrusions ("plagiogranites") in the upper crust (Bailey, 1981;Lippard et al., 1986) and a relatively sparse series of felsic sills, dikes, and plugs that intermittently intrude the uppermost mantle and lower crust along the length of the ophiolite (

High‐Precision U‐Pb Zircon Dating of Late Magmatism in the Samail Ophiolite: A Record of Subduction Initiation

“…This lower crustal dike yielded ε Nd (t) = 6.90 and a range of single grain dates; we interpret the youngest 206 Pb/ 238 U of 95.063 ± 0.062 Ma as the best estimate of the crystallization age. The relatively high ε Nd of this sample, relative to other mantle dikes, likely reflects a large fraction of amphibolite-derived melt, as discussed in a companion paper in this volume (Rioux et al, 2021).…”

“…Geochemical data for this sample and a more detailed discussion of its petrogenesis—and the petrogenesis of the other high ε Nd (t) dikes (Figure 6)—are provided in Rioux et al. (2021).…”

“…The geochemistry of the dikes also suggest emplacement through a thin mantle wedge (Rioux et al, 2021).…”

“…We instead attribute this dike to crystallization of a V2 magma within the upper mantle. Geochemical data for this sample and a more detailed discussion of its petrogenesis-and the petrogenesis of the other high ε Nd (t) dikes (Figure 6)-are provided in Rioux et al (2021).…”

“…The negative ε Nd (t) and trace element signatures of the dikes indicate they formed by melting of metasediments and amphibolite at the top of the subducting slab, with some contributions from mantle-derived magmas or assimilation of mantle harzburgite (Amri et al, 2007;Haase et al, 2015;Rioux et al, 2021;Rollinson, 2015;Spencer et al, 2017 Kusano et al, 2017), which is interpreted to reflect a sediment component in the younger V2 lavas. Taken together, the young dates for the felsic mantle dikes-reflecting three-component mixing between amphibolite and sediment melts on the subducted slab and a mantle component (Haase et al, 2015;Rioux et al, 2021;Rollinson, 2015)-and the enriched isotopic signature of the upper V2 lavas suggest an increasing proportion of slab melts through time, reflecting either the arrival of subducted terrigenous sediment below the ophiolite or the onset of slab melting (Maunder et al, 2020) 6. Post-95.0 Ma: The dearth of U-Pb dates younger than 95.0 Ma in the Oman section of the ophiolite, and the absence of evidence for development of a mature volcanic arc, suggest that magmatism in the ophiolite ended abruptly at this time (Rioux et al, 2012(Rioux et al, , 2013(Rioux et al, , 2016.…”