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

Late Cretaceous basaltic volcanics in the Morondava Basin (SW Madagascar) p ossess high-quality and pre-fold palaeomagnetic data (declination 353.5 deg rees, inclination -54.8 degrees, alpha(95) = 2.4 degrees). The palaeomagnet ic data are all of normal magnetic polarity, and remanence acquisition is l inked to the terminal stages of the Cretaceous Normal Superchron (greater t han or equal to 83 Ma). This is sustained by an Ar-40/Ar-39 age of 83.6 +/- 1.6 Ma from one of the tested basaltic flows. A precise U/Pb zircon-baddel eyite 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 palaeomag netic data obtained from volcanics and dolerites all over Madagascar are di rectionally concordant, and the combined palaeomagnetic pole (latitude 68.5 degrees N, longitude 230.3 degrees E, A(95) = 5.5 degrees, 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 dat a yield a palaeolatitude of 45.3 degrees S--4.7(+5.3) for the proposed foca l point of the Marion plume (Volcan de l'Androy, southeast Madagascar) duri ng the Late Cretaceous. This is in perfect agreement with hotspot-controlle d reconstructions that place the Marion hotspot (46.0 degrees S) beneath so utheast Madagascar during the Late Cretaceous. Setting true polar wander as ide, 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 pow er of palaeomagnetic data. (C) 1998 Elsevier Science B.V. All rights reserv ed.