Andrea Marzoli

The Central Atlantic Magmatic Province (CAMP) in Brazil: Petrology, Geochemistry, 40Ar/39Ar Ages, Paleomagnetism and Geodynamic Implications

The CAMP tholeiitic magmatism in Brazil (mean 40 Ar/ 39 Ar 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 TiO 2 concentration (less than 2 wt%). The Cassipore dykes, which are usually high in TiO 2 (more than 2wt%) are an exception. The Cassipore low- and high-TiO 2 basalts are characterized by a positive Nb anomaly and Sr-Nd isotopes that are parallel to typical mantle array. Except for one sample, all the other Brazilian CAMP tholeiites that are low in TiO 2 , show Sr-Nd isotopes trending towards 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.

The Northernmost CAMP: 40Ar/39Ar Age, Petrology and Sr‐Nd‐Pb Isotope Geochemistry of the Kerforne Dike, Brittany, France

The Central Atlantic Magmatic Province (CAMP) is defined by tholeiitic basaltic flows and dikes associated with the initial break-up of Pangea at 200 Ma, preceding the opening of the Atlantic Ocean. These tholeiites occur in once-contiguous parts of North America, Africa, South America and Europe over a total area of about 7 million square kilometers. The Kerforne dike, located in Brittany (NW France), represents the northernmost outcrop of this province. Due to its orientation and location, more than 1500 Km from the Early Jurassic Atlantic rift, like other CAMP dikes, it can not be considered as the magmatic expression of the Central Atlantic rifting. Despite its distal position, this dike has an Ar/Ar age (193 ± 3 Ma, obtained on plagioclase separates) similar to the CAMP tholeiites. Kerforne dolerites are characterized by augite and minor plagioclase phenocrysts. According to petrographic observation and microprobe analyses, some of these plagioclases contain resorbed high-An (An 85 ) possibly xenocrystic cores which may be evidence of interaction with a mafic lower crust. The low-TiO 2 (1.0 wt%) tholeiitic Kerforne basalts are characterized by negative Nb anomalies, by a positive correlation between . Sr and . Nd , by high radiogenic 207 Pb/ 204 Pb in comparison to relatively unradiogenic 206 Pb/ 204 Pb, and by an enrichment in LREE relative to HREE. These chemical features, along with the mineralogic observations, are indicative of a minor contamination with mafic lower crust, like that represented by granulitic xenoliths of the Massif Central, France. By contrast, contamination with the silicic upper crust (e.g., with the granitic basement) was negligible. The isotopic compositions of the little contaminated Kerforne basalts are similar to those of most other CAMP low-TiO 2 basalts, and are different from those of most oceanic basalts. It is suggested that this high 87 Sr/ 86 Sr and 207 Pb/ 204 Pb isotopic signature was inherited by interaction of primitive mantle with metasomatized portions of the continental lithospheric mantle, similar to the sources of Variscan lamproites of Brittany. Moreover, The contribution of an OIB mantle component may be ruled out as the Kerforne mantle source was isotopically different from those of the oceanic islands which have been suggested to represent the present-day expression of the hypothetic CAMP mantle plume.