Journal of Petrology, Volume 40, Issue 3: March 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

DAVID W. PEATE¹, CHRIS J. HAWKESWORTH², MARTA M. S. MANTOVANI³, NICK W. ROGERS² AND SIMON P. TURNER²

¹DANISH LITHOSPHERE CENTRE, ØSTER VOLDGADE 10, L, DK-1350 COPENHAGEN K, DENMARK
²DEPARTMENT OF EARTH SCIENCES, THE OPEN UNIVERSITY, WALTON HALL, MILTON KEYNES MK7 6AA, UK
³DEPARTAMENTO DE GEOFSICO, INSTITUTO ASTRONÔO MICO E GEOFSICO, UNIVERSIDADE DE SÃO PAULO, RUA DO MATÃO 1226, CEP 05508-900, SÃO PAULO SP, BRAZIL

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/⁸⁶Sr_{i ~0·7050; [epsilon]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.

Keywords: Paraná flood basalts;lava stratigraphy;Dupal mantle anomaly; lithospheric mantle; crustal assimilation

Pages 451-473