Hydrothermal activity within a sedimentary succession: aragonites as indicators of Mesozoic Rifting (Iberian Basin, Spain)

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Pseudo-hexagonal aragonite crystals occur profusely at various locations within the Upper Triassic sequences in Europe, America and Africa. In the Spanish Iberian Basin, they are associated with corrensite and gypsum, which characterize saline mudflat and salt-pond facies. Similarly, shaped crystals occur as dolomite, quartz, chalcedony or even copper pseudomorphs after aragonite in several parts of North and South America, and they are currently forming in association with hydrothermal spring deposits in Kenya. In Spain, this aragonite was first described at Molina de Aragón (Spain). Its origin has previously been explained as resulting from burial diagenetic processes, although petrographical, geochemical, stable isotope, and fluid inclusion evidence suggest a formation in other conditions. The aragonite crystals occur dominantly at locations near outcrops of pre-Hettangian alkaline magmatism, along Middle-to-Late Jurassic NW-SE faults. The formation of these aragonite crystals is interpreted as the result of upwards flow of hydrothermal fluids through the Middle-to-Late Jurassic synrift faults and the interaction with infiltrated shallow marine water. This hydrothermal circulation caused also the transformation of Mg-rich clay into corrensite. The pseudomorphs after aragonite found in other areas of the world appear related to sequences of similar depositional environments and interaction of magmatic intrusions within rifting settings. Such aragonite crystals may therefore help to unravel the geological and tectonic history in their area of occurrence.