Journal of Petrology | Volume 40 | Number 3 | Pages 451-473 | 1999
© Oxford University Press 1999
Petrogenesis and Stratigraphy of the High-Ti/Y Urubici Magma Type in the Paraná Flood Basalt Province and Implications for the Nature of ‘Dupal’-Type Mantle in the South Atlantic Region
1 Danish Lithosphere Centre Øster Voldgade 10, L, DK-1350 Copenhagen K, Denmark
2 Department of Earth Sciences, The Open University Walton Hall, Milton Keynes MK7 6AA, Uk
3 Departamento De Geofísico, Instituto Astronômico E Geofísico, Universidade De São Paulo Rua Do MatãO 1226, CEP 05508-900, São Paulo SP, Brazil
Received January 19, 1998; Revised typescript accepted August 13, 1998
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Abstract |
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The high-Ti/Y Urubici (or Khumib) magma type of the Paraná–Etendeka large igneous province has a restricted spatial extent, near the southeast Brazilian coast and in the northern Etendeka (Namibia). Urubici flows are interbedded with low-Ti/Y Gramado flows. Flow correlations indicate that local topographic relief was important in controlling emplacement of flows, and that lavas near the coast have undergone up to 1 km of post-magmatic uplift relative to inland areas. Urubici magmas have undergone extensive fractional crystallization (MgO <5.5 wt %). Stratigraphic variations highlight complexities of mixing and minor crustal assimilation indicative of open-system magmatic plumbing. The least contaminated samples have high La/Nb (
1.5) and (Tb/Yb)N (2.5), Sr–Nd isotopes close to Bulk Earth (87Sr/86Sri 0.7050; 
Ndi –2.7), and Dupal Pb isotopes with unradiogenic 206Pb/204Pb (17.6). These features are similar to those of the Walvis Ridge DSDP (Deep Sea Drilling Project) Site 525A basalts that define the EM1 oceanic mantle component, and many are also shared with local Cretaceous alkalic magmas that are inferred to be lithospheric mantle melts. Low 206Pb/204Pb material found in the Urubici and Site 525A basalts is not seen as a mixing end-member within the modern Tristan plume system or in South Atlantic mid-ocean ridge basalt. An origin from lithospheric mantle material, delaminated and dispersed within the asthenosphere following continental break-up, is preferred. Thus the South Atlantic Dupal mantle anomaly cannot be considered as a single entity: Urubici flood basalts and Walvis Ridge Site 525A basalts have a relatively shallow origin within originally lithospheric mantle, whereas the Tristan plume is a deep mantle upwelling.
KEY WORDS: Paraná flood basalts; lava stratigraphy; Dupal mantle anomaly; lithospheric mantle; crustal assimilation
* Corresponding author. Telephone: (+45) 38 14 2664. Fax: (+45) 3311 0878. e-mail: dwp{at}dlc.ku.dk
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