International audienceThe timing of onset of modern-style plate tectonics is debated. The apparent lack of blueschist metamorphism1--a key indicator of modern plate subduction2--in rocks aged more than about 1 billion years calls into question the existence of plate tectonics during the Archaean and Palaeoproterozoic eras3, 4. Instead, plate tectonics and subduction could have either not occurred at that time5, or could have proceeded differently6 owing to warmer conditions in the early Earth mantle7. Here we use thermodynamic models8, 9, 10 to investigate the formation conditions of metamorphic minerals in the 2.2-2.0 Gyr old West African metamorphic province. We find a record of blueschist metamorphism in these rocks. We show that minerals such as chlorite and phengite formed at high pressures of 10-12 kbar, low temperatures of 400-450 °C and under a geothermal gradient of 10-12 °C km−1. These conditions are typical of modern subduction zones. We therefore suggest that modern-style plate tectonics existed during the Palaeoproterozoic era. We conclude that ancient blueschist metamorphism may exist in other parts of the world, but the identification of these rocks has so far been hampered by methodological problems associated with deciphering their pressure and temperature evolution
Accreted terranes, comprising a wide variety of Late Jurassic and Early Cretaceous igneous and sedimentary rocks are an important feature of Cuban geology. Their characterization is helpful for understanding Caribbean paleogeography.The Guaniguanico terrane (western Cuba) is formed by upper Jurassic platform sediments intruded by microgranular dolerite dykes. The geochemical characteristics of the dolerite whole rock samples and their minerals (augitic clinopyroxene, labradorite and andesine) are consistent with a tholeiitic affinity. Major and trace element concentrations as well as Nd, Sr and Pb isotopes show that these rocks also have a continental affinity. Sample chemistry indicates that these lavas are similar to a low Ti-P 2 O 5 (LTi) variety of continental flood basalts (CFB) similar to the dolerites of Ferrar (Tasmania). They derived from mixing of a lithospheric mantle source and an asthenopheric component similar to E-MORB with minor markers of crustal contamination and sediment assimilation. However, the small quantity of Cuban magmatic rocks, similarly to Tasmania, Antarctica and Siberia differs from other volumetrically important CFB occurrences such as Parana and Deccan.These dolerites are dated as 165-150 Ma and were emplaced during the separation of the Yucatan block from South America. They could in fact be part of the Yucatan-South America margin through which the intrusive system was emplaced and which was later accreted to the Cretaceous arc of central Cuba and to the Palaeogene arc of eastern Cuba. These samples could therefore reflect the pre-rift stage between North and South America and the opening of the gulf of Mexico. L'existence de filons tardi-jurassiques de trapps continentaux dans le nord-ouest de Cuba : témoins de l'ouverture du golfe du MexiqueMots-clés. -Filons doléritiques, Jurassique supérieur, Trapps continentaux, Géochimie, Cuba.Résumé. -La géologie de l'île de Cuba est caractérisée par l'existence de terrains accrétés composés d'une grande variété de roches ignées et sédimentaires d'âge jurassique supérieur et crétacé inférieur. L'identification de la nature de ces terrains est fondamentale pour la compréhension de la paléogéographie du domaine caraïbe. La formation de Guaniguanico (N-Ouest Cuba) est constituée de sédiments de plate-forme d'âge jurassique supé-rieur recoupés par des filons doléritiques. Les caractéristiques géochimiques de ces dolérites et des minéraux qu'elles contiennent (augite et labrador) permettent de mettre en évidence l'affinité tholéiitique de ces roches. Les concentrations en éléments majeur et traces ainsi que les rapports isotopiques du Pb, Sr et Nd indiquent que ces dolérites présen-tent des signes de contamination crustale et d'assimilation des sédiments encaissants. L'étude géochimique de ces filons doléritiques montre que ces laves sont proches d'une variété de trapps continentaux pauvres en TiO 2 et P 2 O 5 (LTi), similaire aux dolérites de Ferrar (Tasmanie), elles-mêmes dérivées du mélange d'une source de type manteau lithosphérique avec u...
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