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Research Article |
1 Dipartimento di Scienze della Terra, Università di Napoli Federico II, 80134 Napoli, Italy
2 Department of Earth Sciences, Indian Institute of Technology, Powai 400076, Mumbai, India
3 School of Ocean and Earth Science and Technology, University of Hawaii, Honolulu, HI 96822, USA
4 Istituto di Geoscienze e Georisorse CNR, University of Firenze, 50121 Firenze, Italy
5 Present address: Department of Earth Sciences, University of Cambridge, Cambridge CB2 3EQ, UK
6 QUAD-Lab, Roskilde University Centre, Roskilde, 4000 Denmark
*Corresponding author (e-mail: melluso{at}unina.it)
Supplementary material: Mineral analyses, mass-balance calculations and locality information are available at http://www.geolsoc.org.uk/SUP18332.
The St Mary's Islands (southwestern India) expose silicic volcanic and sub-volcanic rocks (rhyolites and granophyric dacites) emplaced contemporaneously with the Cretaceous igneous province of Madagascar, roughly 88–90 Ma ago. The St Mary's Islands rocks have phenocrysts of plagioclase, clinopyroxene, orthopyroxene and opaque oxide, moderate enrichment in the incompatible elements (e.g. Zr = 580–720 ppm, Nb = 43–53 ppm, La/Ybn = 6.9–7.2), relatively low initial 87Sr/86Sr (0.7052–0.7055) and near-chondritic initial 143Nd/144Nd (0.51248–0.51249). They have mineral chemical, whole-rock chemical and isotopic compositions very close to those of rhyolites exposed between Vatomandry–Ilaka and Mananjary in eastern Madagascar, and are distinctly different from rhyolites from other sectors of the Madagascan province. We therefore postulate that the St Mary's and the Vatomandry–Ilaka–Mananjary silicic rock outcrops were adjacent before the Late Cretaceous rifting that split Madagascar from India. If so, they provide a valuable tool to check and aid traditional Cretaceous India–Madagascar reconstructions based on palaeomagnetism, matching Precambrian geological features, and geometric fitting of continental shelves.
