Precambrian Research 172 (2009) 279–300 Contents lists available at ScienceDirect Precambrian Research journal homepage: www.elsevier.com/locate/precamres Geological evolution of the Neoproterozoic Bemarivo Belt, northern Madagascar R.J. Thomas a , B. De Waele a,1 , D.I. Schofield a , K.M. Goodenough b,∗ , M. Horstwood c , R. Tucker d,2 , W. Bauer a , R. Annells a , K. Howard d,2 , G. Walsh d,2 , M. Rabarimanana e , J.M. Rafahatelo e , A.V. Ralison e , T. Randriamananjara e a British Geological Survey, Kingsley Dunham Centre, Keyworth NG12 5GG, UK b British Geological Survey, Murchison House, Edinburgh EH9 3LA, UK c NERC Isotope Geoscience Laboratories, Kingsley Dunham Centre, Keyworth NG12 5GG, UK d United States Geological Survey, Reston, VA 20192, USA e Projet de Gouvernance des Ressources Minérales, Ampandrianomby 101, Antananarivo, Madagascar article info Article history: Received 15 December 2008 Received in revised form 31 March 2009 Accepted 27 April 2009 Keywords: Madagascar Bemarivo Neoproterozoic Gondwana Geochronology abstract The broadly east-west trending, Late Neoproterozoic Bemarivo Belt in northern Madagascar has been re-surveyed at 1:100000 scale as part of a large multi-disciplinary World Bank-sponsored project. The work included acquisition of 14 U–Pb zircon dates and whole-rock major and trace element geochemical data of representative rocks. The belt has previously been modelled as a juvenile Neoproterozoic arc and our findings broadly support that model. The integrated datasets indicate that the Bemarivo Belt is separated by a major ductile shear zone into northern and southern “terranes”, each with different lithostratigraphy and ages. However, both formed as Neoproterozoic arc/marginal basin assemblages that were translated southwards over the north-south trending domains of “cratonic” Madagascar, during the main collisional phase of the East African Orogeny at ca. 540Ma. The older, southern terrane consists of a sequence of high-grade paragneisses (Sahantaha Group), which were derived from a Palaeoproterozoic source and formed a marginal sequence to the Archaean cratons to the south. These rocks are intruded by an extensive suite of arc-generated metamorphosed plutonic rocks, known as the Antsirabe Nord Suite. Four samples from this suite yielded U–Pb SHRIMP ages at ca. 750 Ma. The northern terrane consists of three groups of metamorphosed supracrustal rocks, including a possible Archaean sequence (Betsiaka Group: maximum depositional age approximately 2477Ma) and two volcano-sedimentary sequences (high-grade Milanoa Group: maximum depositional age approximately 750 Ma; low grade Daraina Group: extrusive age = 720–740 Ma). These supracrustal rocks are intruded by another suite of arc-generated metamorphosed plutonic rocks, known as the Manambato Suite, 4 samples of which gave U–Pb SHRIMP ages between 705 and 718Ma. Whole-rock geochemical data confirm the calc-alkaline, arc-related nature of the plutonic rocks. The volcanic rocks of the Daraina and Milanoa groups also show characteristics of arc-related magmatism, but include both calc-alkaline and tholeiitic compositions. It is not certain when the two Bemarivo terranes were juxtaposed, but ages from metamorphic rims on zircon suggest that both the northern and southern terranes were accreted to the northern cratonic margin of Madagascar at about 540–530Ma. Terrane accretion included the assembly of the Archaean Antongil and Antananarivo cratons and the high-grade Neoproterozoic Anaboriana Belt. Late- to post-tectonic granitoids of the Maevarano Suite, the youngest plutons of which gave ca. 520 Ma ages, intrude all terranes in northern Madagascar showing that terrane accretion was completed by this time. © 2009 Natural Environment Research Council (NERC). Published by Elsevier B.V. All rights reserved. ∗ Corresponding author. E-mail address: kmgo@bgs.ac.uk (K.M. Goodenough). 1 Present address: SRK Consulting, Perth, Western Australia, Australia. 2 Also Regional Offices. 1. Introduction Madagascar occupies a critical, central position within the East African Orogen (EAO; Fig. 1). The Bemarivo Belt is the northernmost Precambrian terrane of Madagascar, and one of the fundamental building blocks of the island (Fig. 2). The other major Precambrian terranes that make up Madagascar include the Archaean Antongil Craton, in the east of the island, and the Antananarivo Craton which also largely dates from the Archaean, and which underlies much 0301-9268/$ – see front matter © 2009 Natural Environment Research Council (NERC). Published by Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2009.04.008