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

摘要

Understanding the tectonic setting in which ophiolites form is necessary to determine how they can be used to study ocean spreading and subduction zone processes. Here, we present high-precision U-Pb zircon dates and Sm-Nd isotopic data from two late magmatic series in the Samail ophiolite in Oman and the United Arab Emirates (UAE), which constrain its tectonic development. Volcanic rocks in the ophiolite record a progression from MORB-like V1 lavas to subduction-related V2 lavas. Plutonic rocks related to V2 magmatism yielded Pb-206/U-238 dates of 95.557 +/- 0.063-95.289 +/- 0.067 Ma. A second late magmatic series consists of felsic dikes and sills that intermittently intrude the upper mantle and are attributed to melting of a subducting slab. These dikes in Oman range from 95.201 +/- 0.032 to 94.95 +/- 0.10 Ma, with epsilon(Nd)(t) = -5.05 to -1.63. A single dike, attributed to V2 magmatism, has a higher epsilon(Nd)(t) = 7.35 and a date of 95.478 +/- 0.032 Ma. Similar intrusions in the UAE are younger, ranging from 94.119 +/- 0.057 to 90.998 +/- 0.052 Ma. Our new and existing data indicate the following timeline of ophiolite formation during subduction initiation: (1) Initial sole metamorphism at >= 96.2 Ma; (2) Formation of the crust through primarily decompression-related V1 magmatism from 96.1 to 95.6 Ma; (3) V2 magmatism related to H2O-fluxed mantle melting from 95.6 to 95.2 Ma; and (4) Intrusion of slab-derived felsic dikes from 95.2 to 95.0 Ma. The temporal progression of magmatism is similar to the timescales of subduction initiation predicted by geodynamic models and observed in the Izu-Bonin-Mariana forearc.