Highly depleted isotopic compositions evident in Iapetus and Rheic Ocean basalts: implications for crustal generation and preservation |
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Authors: | J Brendan Murphy John W F Waldron David I Schofield Tiffany L Barry Adrian R Band |
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Institution: | 1. Department of Earth Sciences, St. Francis Xavier University, PO Box 5000, Antigonish, NS, B2G 2W5, Canada 2. Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, T6G 2E3, Canada 3. British Geological Survey, Tongwynlais, Cardiff, CF15 7NE, UK 4. Department of Geology, University of Leicester, University Road, Leicester, LE1 7RH, UK
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Abstract: | Subduction of both the Iapetus and Rheic oceans began relatively soon after their opening. Vestiges of both the Iapetan and Rheic oceanic lithospheres are preserved as supra-subduction ophiolites and related mafic complexes in the Appalachian–Caledonian and Variscan orogens. However, available Sm–Nd isotopic data indicate that the mantle source of these complexes was highly depleted as a result of an earlier history of magmatism that occurred prior to initiation of the Iapetus and Rheic oceans. We propose two alternative models for this feature: either the highly depleted mantle was preserved in a long-lived oceanic plateau within the Paleopacific realm or the source for the basalt crust was been recycled from a previously depleted mantle and was brought to an ocean spreading centre during return flow, without significant re-enrichment en-route. Data from present-day oceans suggest that such return flow was more likely to have occurred in the Paleopacific than in new mid-ocean ridges produced in the opening of the Iapetus and Rheic oceans. Variation in crustal density produced by Fe partitioning rendered the lithosphere derived from previously depleted mantle more buoyant than the surrounding asthenosphere, facilitating its preservation. The buoyant oceanic lithosphere was captured from the adjacent Paleopacific, in a manner analogous to the Mesozoic–Cenozoic “capture” in the Atlantic realm of the Caribbean plate. This mechanism of “plate capture” may explain the premature closing of the oceans, and the distribution of collisional events and peri-Gondwanan terranes in the Appalachian–Caledonian and Variscan orogens. |
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