Late Cretaceous magmatism in Madagascar: palaeomagnetic evidence for a stationary Marion hotspot

Abstract

Late Cretaceous basaltic volcanics in the Morondava Basin (SW Madagascar) possess high-quality and pre-fold palaeomagnetic data (declination 353.5°, inclination −54.8°, α 95 = 2.4°). The palaeomagnetic data are all of normal magnetic polarity, and remanence acquisition is linked to the terminal stages of the Cretaceous Normal Superchron (≥83 Ma). This is sustained by an 40Ar/ 39Ar age of 83.6±1.6 Ma from one of the tested basaltic flows. A precise U/Pb zircon–baddeleyite age from northeast Madagascar demonstrates magmatism at least back to 91.6±0.3 Ma; thus reliable isotope ages for the Madagascar Cretaceous igneous province span a range of 8 million years. Late Cretaceous palaeomagnetic data obtained from volcanics and dolerites all over Madagascar are directionally concordant, and the combined palaeomagnetic pole (latitude 68.5°N, longitude 230.3°E, A 95 = 5.5°, N=8 studies; sampling age range ca. 84–90 Ma) represents one of the best Late Cretaceous poles for the former Gondwanan elements. The collective palaeomagnetic data yield a palaeolatitude of 45.3°S +5.3 −4.7 for the proposed focal point of the Marion plume (Volcan de l'Androy, southeast Madagascar) during the Late Cretaceous. This is in perfect agreement with hotspot-controlled reconstructions that place the Marion hotspot (46.0°S) beneath southeast Madagascar during the Late Cretaceous. Setting true polar wander aside, hotspot movements in the Indian Ocean do not appear to exceed ca. 0.75 cm/yr, and the Marion hotspot appears stationary within the resolution power of palaeomagnetic data.