TESES DE DOUTORADO EM GEOCIÊNCIAS SOBRE REGIÕES BRASILEIRAS
(DEFENDIDAS EM UNIVERSIDADES ESTRANGEIRAS)

Luiz Augusto Bizzi
bizzi@sopemi.com.br

VULCANISMO ALCALINO MESOZÓICO E EVOLUÇÃO DO MANTO NA PORÇÃO SUDOESTE DO CRATON DO SÃO FRANCISCO, BRASIL

Tese apresentada para o grau de DOCTOR OF PHILOSOPHY 
no Department of Geological Sciences - University of Cape Town
Junho, 1993. 

RESUMO

 PhD THESES ON EARTH SCIENCES OF BRAZILIAN REGIONS
(DEFENDED IN NON-BRAZILIAN UNIVERSITIES)

Luiz Augusto Bizzi
bizzi@sopemi.com.br

MESOZOIC ALKALINE VOLCANISM AND MANTLE EVOLUTION OF THE SOUTHWESTERN SÃO FRANCISCO CRATON, BRAZIL.

Thesis presented for the degree of DOCTOR OF PHILOSOPHY 
in the Department of Geological Sciences - University of Cape Town
June, 1993. 

ABSTRACT
This thesis explores the nature of the subcontinental lithosphere underlying the southwestern margin of the São Francisco craton and the relation of variations in the petrochemistry of Kimberlites and related alkali igneous rocks to variations in age, thickness and thermodynamic history of their continental lithospheric hosts. The São Francisco craton is a mid to late Archean basement granite-greenstone terrain flanked to the west by the Proterozoic Tocantins Province (Almeida, 1977; Almeida et al., 1981). New Rb-Sr and Sm-Nd data are presented for both on- and off-craton crustal rock sequences. The ultramafic greenstone association of the Rio das Velhas Supergroup yields 3.2 Ga Rb-Sr and Sm-Nd ages, in agreement with widespread 3.2 Ga old zircons from area. Granitic gneiss and juvenile granitoids associated with the greenstones in the Congonhas area give a Transamazonian 2128 Ma Rb-Sr age, which is in agreement with a 2124 Ma zircon age available. Further west, syntectonic granitoids and metabasalts from the Araxá Group define a 711 Ma Rb-Sr isochron. This latter age is interpreted as a Sr-isotope re-homogenization related to the development of the Brasília orogenic and foreland thrust belt. A 823 Ma Sm-Nd errorchron indicate that these rocks may be coeval to felsic volcanism of the Araxá Group which was recently dated at 794 Ma by zircon work (Pimentel et al., 1991). Further to the west still, combined samples from the Niquelândia mafic-ultramafic igneous complex and associated granitic basement rocks yield a 1.26 Ga Rb-Sr isochron, which is best interpreted as a metamorphic age. Crystallisation ages decrease and e_Nd values increase with increasing distance westward from the margin of the Archean São Francisco craton. The isotopic characteristics are consistent with a model which requires that large volumes of crust, derived in the Proterozoic from mantle reservoirs similar to the sources for modern oceanic basalts, were accreted onto the pre-existing Archean nucleus during the Brasiliano orogenic event.
The proterozoic rocks which overly and flank the São Francisco craton margin are intruded by Cretaceous Kimberlites, olivine melilitites, tuffaceous diatremes and carbonatite complexes. Eight of the freshest representatives of the alkaline magmatism are described in terms of their age and mode of emplacement, petrography and whole-rock geochemistry. Kimberlites have compositions similar to that of primary liquids derived from garnet peridotites. Their trace-element compositions indicate that melting processes occurred under the influence of the proto-Tristan hot-spot. It is suggested that the kimberlites and kimberlite-related magmas resulted from entrainment of enriched lithosphere in plume-derived small-volume melts. The source character of the kimberlitic rocks is similar to that of carbonatites and other alkalic volcanics in the area, but is dissimilar to that of kimberlites elsewhere in the world. The lower time-averaged Rb/Sr, Nd/Sm and Pb/U ratios of the kimberlites compared to the other rock types investigated might be related to a high  ²³⁵U/²⁰⁴ Pb (HIMU) component.
Major and trace elements of the alkalic rocks change systematically with petrographic character towards more evolved compositions, approximating liquid evolution paths produced by shallow-level, olivine-dominated crystal fractionation. A restricted range of isotopic signatures, and the absence of any correlation between ⁸⁷Sr/⁸⁶Sr and 1/Sr, suggest that the shallower alkalic rocks were probably derived by melting of a light-REE enriched lithospheric mantle source rather than through crustal contamination of asthenospheric melts. Compared to the kimberlites, the other alkalic rocks studied have a greater lithospheric component. The involvement of plumes in their derivation is uncertain. Isotope characteristics of rift-related magma types are probably the best candidates to date for the “Enriched Mantle I” (EMI) component. The source of the alkaline occurrences, the source of the high-Ti basalts of the northern Paraná Basin, and the source of some Ocean Island Basalts (OIB) with Dupal signatures in the South Atlantic (viz. the Walvis Ridge basalts) are closely related to this EMI-like component. The linear correlation between Platinum Group Elements (PGE) and isotopic characteristics in the studied rocks appears to follow the temperature-dominated behaviour of PGE (c.f. Tredoux et al., 1989), and suggests that a significant temperature gradient may have existed between the two recognised mixing reservoirs (i.e.the sources of the EMI- and HIMU-like components).
The Nd isotope characteristics of the EMI-like component in the Mesozoic volcanics are compatible with an origin closely related to the evolution of the Proterozoic rocks of the Tocantins Province. e_Nd values related to Archean mantle have not been found in these volcanics. It is thus indicated that large amounts of pristine Archean enriched mantle lithosphere, not affected by the Proterozoic enrichment event, were probably not incorporated at the source region. It is speculated that the low ⁸⁷Sr/⁸⁶Sr of the Mesozoic volcanics represents time-integrated Rb depletion at lower crust/upper mantle levels attained during gabbro-eclogite-granulite phase transformations (which could have been accompanied by CO₂  metasomatism) following tectonic overthickening at the end of the Brasiliano orogeny.
A tectono-thermal framework of the Paleozoic to the Cenozoic geological history of southwestern Gondwana is provided. Mantle plumes appear to have played an important role in the initial fission of Gondwana and the opening of the South Atlantic. The broad tectonic evolutionary framework and the location of the present passive continental margins of the South Atlantic, however, were highly dependent on the paleo-tectonic geometry of the Brasiliano-Pan African orogenic fold belts. The late-Mesozoic fission-related magmatism involved melts derived from both the crust (rhyolite ash-flows and related potassic granites) and the upper-mantle (kimberlites, alkaline complexes, flood basalts and related dike swarms). The compositional and isotopic characteristics of basaltic volcanism that occurred shortly before the opening of the new ocean basin are explained satisfactorily by asthenospheric plume models; but how the lithospheric and asthenospheric materials were remobilized during the melting process remain controversial, as does the original depth of plume generation. The overall plate-tectonic approach suggests it is the within-plate stress fields and fault reactivation which controls the sites of alkaline magmatism in the continental lithosphere. Alkaline magmatism along the southwestern margin of the São Francisco craton was contemporaneous with changes in the direction of plate movements which provoked reactivation of lithospheric shear zones and rifting within plates. The isotope characteristics of alkalics and HTZ Paraná basalts emplaced along the craton margin provide further evidence that discrete large scale geochemical domains existed in Southern Gondwana. It is suggested that those domains were not necessarily related to ancient lithospheric chemical heterogeneities or bounded by ancient structural features, but rather to mixing processes that can be ascribed to specific geodynamic mechanisms.