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Sm-Nd isotope and trace element study of Late Proterozoic metabasalts (“spilites”) from the Central Barrandian domain (Bohemian Massif, Czech Republic)

By
Christian Pin
Christian Pin
1
Département de Géologie, Centre National de la Recherche Scientifique and Université Blaise Pascal, 5 rue Kessler, 63038 Clermont-Ferrand Cedex, France
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Jarmila Waldhausrová
Jarmila Waldhausrová
2
Nad Hercovkow 422, 18200 Prague, Czech Republic
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Published:
January 01, 2007

On the basis of immobile trace elements and Nd isotope signatures, the Barrandian meta-basalts may be ascribed to two major groups, extracted from contrasting mantle sources:

  1. A depleted group, with strong light rare earth element depletion, elevated Zr/Nb ratios (>30), and highly radiogenic Nd isotopes (ϵNd600 from +7.8 to + 9.3). Multi-element patterns normalized to normal mid-ocean ridge basalt all show negative anomalies of Nb, and to a lesser degree, Zr and Ti. Eight samples may define a 605 ± 39-Ma whole-rock isochron with ϵNdi of +8.8 ± 0.2.

  2. An enriched group, comprising both mildly enriched (Zr/Nb 12–18) and strongly enriched (Zr/Nb 4–7) samples, with ϵNd600 ranging from +8.2 to +3.8.

The depleted group is interpreted to reflect generation from depleted mantle sources fluxed by subduction-related components, probably in an intraoceanic back-arc basin. In contrast, the younger enriched group is typical of the within-plate style of mantle enrichment and documents the extinction of the subduction-related component. The switch from suprasubduction zone to within-plate magmatism suggests that new mantle material flowed into the former arc and back-arc system sources. This flow might have occurred simply as a result of ocean-ward migration of the subduction zone. Alternatively, the subduction fluxing might have stopped as a result of impingement of a spreading ridge with the intraoceanic trench, leading to mutual annihilation, a switch to a transform plate boundary, and opening of a slab window that allowed the inflow of new mantle and the generation of late-stage, within-plate enriched basalts. In terms of modern analogues, the Neoproterozoic of the Barrandian and other Cadomian regions of western Europe resemble arc and back-arc systems from the western Pacific region, where large intraoceanic subduction systems fringe major continental masses with a complex mosaic of microplates and magmatic arcs, including intervening basins floored either by oceanic crust or attenuated continental crust.

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GSA Special Papers

The Evolution of the Rheic Ocean: From Avalonian-Cadomian Active Margin to Alleghenian-Variscan Collision

Ulf Linnemann
Ulf Linnemann
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R. Damian Nance
R. Damian Nance
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Petr Kraft
Petr Kraft
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Gernold Zulauf
Gernold Zulauf
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Geological Society of America
Volume
423
ISBN print:
9780813724232
Publication date:
January 01, 2007

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