Geochemical and isotopic constraints on the petrogenesis of the Cretaceous alkaline Morro Redondo intrusive complex, Rio de Janeiro, Brazil: implications for alkaline magma genesis

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Carlos Eduardo Mota
Fred Jourdan
Arthur de Melo Said Fabião Moreira da Silva
Mauro Cesar Geraldes
Guilherme Loriato Potratz



The Morro Redondo Complex is composed of plutonic and volcanic alkaline rocks. This igneous complex is part of the Poços de Caldas - Cabo Frio Magmatic Alignment (PCCFA). This study aims to contribute to the understanding of the petrogenesis of the rocks in the Morro Redondo complex and, thereby, contribute to the debate on the genesis of the PCCFA. In this work, we conducted analyses of whole-rock geochemistry, 40Ar/39Ar thermochronology, U-Pb geochronology on zircon grains, and Rb-Sr, Sm-Nd, and Lu-Hf isotopic analyses. Four distinct lithotypes are present: nepheline syenites, alkali feldspar syenites, trachytes, and phonolites. The rocks of the Morro Redondo complex are miaskitic, have intermediate values of SiO2, and show significant crustal assimilation, mainly observed in the relationship between the SSI and (Na+K)/Al indices, together with the Hf isotopic signature. A hornblende crystal obtained from a sample of nepheline syenite provided a plateau age of 74.35 ± 0.50 Ma. The initial ratios of 87Sr/86Sr range from 0.70565 to 0.71042, and the initial ratios of 143Nd/144Nd range from 0.51234 to 0.5124. Hf data suggest a hybrid origin for the Morro Redondo complex, with sources from both crustal and mantle origins. The data obtained indicate that at least three magmatic pulses contributed to the formation of the Morro Redondo complex, with the participation of both crustal and mantle sources, generating magmas of miaskitic affinity.

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Mota, Carlos Eduardo, Fred Jourdan, Arthur de Melo Said Fabião Moreira da Silva, Mauro Cesar Geraldes, and Guilherme Loriato Potratz. 2024. “Geochemical and Isotopic Constraints on the Petrogenesis of the Cretaceous Alkaline Morro Redondo Intrusive Complex, Rio De Janeiro, Brazil: Implications for Alkaline Magma Genesis”. Journal of the Geological Survey of Brazil 7 (2):119-34.
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