Angelo De Min1, Enzo M. Piccirillo1, Andrea Marzoli 2, Giuliano Bellieni 3, Paul R. Renne 4, Marcia Emesto 5, and Leila S. Marques 5
1 Dipartimento di Scienze della Terra, University of Trieste, Trieste,
2 Departement de Mineralogie, Universite de Geneve, Geneve, Switzerland.
3 Dipartimento di Mineralog:a e Petrologia, University of Padova, Padova, Italy.
4 Berkeley Geochronology Center, Berkeley, California State, USA
5 Departamento de Geofisica; Instituto Astronomico e Geofisico, University of Sao Paulo (USP), Sao Paulo, SP, Brazil.
The CAMP tholeiitic magmatism in Brazil (mean 40Ar/39Ar
age of 199.0±2.4 Ma) occurs on the continental margin to ca. 2,000
km into the South American platform, near the boundary between the ancient
terrains of the Amazonia craton and Proterozoic/Brazilian-cycle related
mobile belts. Geological evidence indicates that this magmatism was
preceded, in Permo-Triassic times, by continental sedimentation indicating
a possible regional uplift. The Brazilian CAMP tholeiites are generally
evolved and characterized by a low TiO2 concentration (less than 2 wt %).
The Cassipore dykes, which are usually high in TiO2 (more than 2 wt %)
are an exception. The latter low- and high-TiO2 basalts are characterized
by positive Nb anomaly and Sr-Nd isotopes parallel to "typical" mantle
arrays, spanning from the depleted to the enriched quadrant. Except
for one sample, all the other Brazilian CAMP tholeiites, low in TiO2, show
Sr-Nd isotopes trending tonvards crustal components. The latter isotopic
characteristics could be related to "crustal recycling" ancient (Middle-Late
Proterozoic) subductions and/or low-pressure crustal interaction.
All the Brazilian CAMP tholeiites show a decoupling between their Sr-Nd
isotopic composition and Rb/Sr and Sm/Nd values, suggesting "mantle metasomatism",
and/or subduction-related crustal interaction before mantle melting.
Notably, the chemical data show that tholeiites from specific Brazilian
regions are related to mantle sources that reflect compositional mantle
heterogeneity, including the lower mantle of the lithospheric thermal boundary
layer. In general, paleomagnetic poles for CAMP rocks from South
America, Africa and North America match an age of ca. 200 Ma, but also
show a distribution pattern trending to younger ages (e.g. 190 Ma), especially
for the South American poles relative to the CAMP magmatism of the continental
edge. The Brazilian CAMP magmatism cannot be easily explained through
"plume head" (active) models, being instead consistent with mantle geodynamic
processes where the unstable buoyancy of the Pangea "supercontinent" played
an essential role to approach isostatic stabilization. Therefore,
it is proposed that the Brazilian CAMP magmatism was related to hot "upper"
mantle incubation under thick continental lithosphere and to edge-driven
convection between lithospheric domains with different thickness.