Journal of Conference Abstracts

Migrating Cretaceous-Eocene Magmatism in the Serra do Mar Alkaline Province, SE Brazil: The Dog-Legged Track of Devious Trindade Mantle Plume

R. N. Thompson¹ (r.n.thompson@durham.ac.uk), S. A. Gibson² (sally@esc.cam.ac.uk), J. G. Mitchell³ (j.g.mitchell@ncl.ac.uk), A. P. Dickin⁴ (dickin@mcmail.cis.mcmaster.ca), O. H. Leonardos⁵ (ohljr@guarany.cpd.unb.br), J. A. Brod^1,5 (j.a.brod@durham.ac.uk) & J. C. Greenwood² (jcg24@esc.cam.ac.uk)

¹ Geological Sciences, University of Durham, U.K.

² Earth Sciences, University of Cambridge, U.K.

³ Physics, University of Newcastle Upon Tyne, U.K.

⁴ Geology, McMaster University, Ontario, Canada.

⁵ Geoquimica, Universidade de Brasilia, Brazil.

The Serra do Mar province extends for ~500 km as plutonic complexes, some with volcanic remnants, along coastal SE Brazil. Syenites and phonolites dominate most complexes but mafic-ultramafic compositions (MgO<16.4%) are also ubiquitous. The mafic rocks are alkali basalts, basanites and melanephelinites, with rare leucitites and mica-rich analogues. Elementally, all but the strongly potassic rock types are indistinguishable from ocean-island mafic magmatism (OIB). Isotopically (Sr, Nd), the Serra do Mar suite overlaps the field for OIB and aseismic ridges, especially Walvis, but also extends towards EM1. We consider the most plausible source for these magmas to be predominantly convecting mantle, with sporadic input from metasomites within the South American lithospheric plate. New radiometric dates, together with a detailed assessment of published ages, show that the initiation of Serra do Mar magmatism migrated progressively ~500 km ESE, from Poços de Caldas to Cabo Frio, between ~80 and 55 Ma. At ~85 Ma the Trindade mantle plume impacted beneath interior SE Brazil where the lithosphere was too thick, except very locally, to permit dry decompression melting and basaltic (s.l.) magmagenesis. Instead strongly alkalic, potassic, lithosphere-source magmatism broke out penecontemporaneously over a region of ~1200 km diameter. As the plume tail moved ESE beneath the Archaean Sao Francisco craton, its magmatic signal at the surface was "switched off" by the thick cratonic lithosphere. Nevertheless, hot plume mantle "leaked" progressively southward, until it encountered strongly thinned lithosphere beneath SE coastal Brazil, along the passive margin of the South American plate. Here it was able to decompress sufficiently to generate the predominant OIB-like component of the Serra do Mar magmatism. After 55 Ma this magmatism transferred NE from Cabo Frio to the Abrolhos Platform (52 Ma); the landward terminus of the seamount chain leading to Trindade and Martin Vaz in the South Atlantic.