Journal of the Geological Society, L ondon, Vol. 155, 1998, pp. 1009–1024. Printed in Great Britain. Sedimentology, geochronology and provenance of the Proterozoic Itremo Group, central Madagascar, and implications for pre-Gondwana palaeogeography RÓNADH COX 1 , RICHARD A. ARMSTRONG 2 & LEWIS D. ASHWAL 3 1 Department of Geosciences, W illiams College, W illiamstown M A 01267, USA ( e-mail: rcox@williams.edu) 2 Research School of Earth Sciences, Australian National University, Canberra ACT 0200, Australia 3 Department of Geology, Rand Afrikaans University, Auckland Park, Johannesburg 2006, South Africa Abstract: Proterozoic metasediments of the Itremo Group in central Madagascar probably represent a passive margin sequence predating Gondwana assembly. The quartzites are well-sorted quartz arenites that contain flat lamination, wave ripples, current ripple cross-lamination, and dune cross bedding. The carbonate rocks preserve abundant stromatolites and algal laminates. A continental source is indicated by mudrock major and trace element chemistry. The combination of lithologic association, sediment architecture and mudrock chemistry indicates that the sequence was deposited on a continental shelf or platform. SHRIMP data from detrital zircons indicate that the source area included early Proterozoic and late Archean rocks with ages between 1.85 and 2.69Ga, and that the depositional age of the Itremo Group must be less than 1855 11 Ma. The sequence has been deformed into a series of large-scale folds separated by ductile shear zones. SHRIMP data indicate both massive lead loss from detrital zircons and new zircon growth in the metasediments at 833 112 Ma, which we interpret as the age of metamorphism of the sequence. Comparison of detrital grain ages with basement ages in East Africa and in India indicates that the source area for the Itremo Group probably lay on the present African mainland. Keywords: Madagascar, Gondwana, Proterozoic, sedimentology, geochemistry. The Madagascar microcontinent exposes approximately 400 000 km 2 of strongly deformed and metamorphosed Pre- cambrian basement, the geology of which is extremely poorly understood. Current knowledge comes mainly from work done prior to 1970, summarized by Besairie (1967, 1968, 1969a, 1969b, 1970). Most existing maps are lithologically detailed, but include little or no structural data and are essentially non-interpretive. Although there has been a recent increase in U–Pb data acquisition (Paquette et al. 1994; Ashwal et al. 1996; Tucker et al. 1996), the geochronological database is small and dominated by Rb–Sr data (Caen-Vachette 1979; Cahen & Snelling 1984). Most of the Precambrian rocks of Madagascar are deformed and metamorphosed, commonly to upper amphibolite or granulite grade (Bazot et al. 1971; Nicollet 1990). The scarcity of U–Pb ages, coupled with the complex tectonic history in most areas, has confused or obscured the distinction between ancient cratonic nuclei and subsequent orogenic crustal additions. Some recent papers have been published on large- scale aspects of the Precambrian geology of Madagascar (Ackermand et al. 1989; Nicollet 1990; Windley et al. 1994), but most of their conclusions are poorly constrained because many questions remain about structural and stratigraphic relationships and regional correlation of lithologies and tectonothermal events. There is, therefore, a strong need for detailed local and regional field and geochronological studies such as those of Andriamarofahatra et al. (1990), Paquette et al. (1994) Ashwal et al. (1995) and Morel et al. (1995). This paper reports the results of initial work in the outcrop belt of the Itremo Group, a sequence of low- to medium-grade Proterozoic metasediments in central Madagascar (Figs 1 & 2). The overall aims of this contribution are to evaluate the stratigraphy, environment of deposition and provenance of the sedimentary sequence; to establish a geochronological frame- work for the geology of the Itremo Group; and to relate this to possible correlative units in Africa and India. Geological setting The Itremo Group consists of quartzite, pelite and dolomitic carbonate rocks representing a continental setting (Moine 1967). It outcrops over an area of approximately 110 120 km in the central highlands of Madagascar, south of Antananarivo (Fig. 1). The metasedimentary sequence has been deformed into a series of upright to recumbent folds, separated by mylonitic shear zones (Fig. 2). Metamorphic grade increases from east to west. In the east, the rocks are dominated by greenschist or sub-greenschist assemblages. To the centre and west of the belt these give way to kyanite-and sillimanite-bearing rocks (Raoelison 1997). The sequence is intruded by granitic rocks and gabbros that are undeformed or weakly deformed, and which give U–Pb zircon ages of 770–800 Ma (R.D. Tucker, unpublished data). The Itremo Group represents a Proterozoic depocentre which was closed and deformed during the amalgamation of Gondwana in late Precambrian time. It bears lithological similarities to Proterozoic sequences in Africa and India, with which it has been tentatively correlated (e.g. Pinna et al. 1993). Analytical procedures XRF major and trace element analyses were carried out at Rocklabs in Pretoria, South Africa, using an ARL-8420 spectrometer with Rh-target, end-window tube. Samples were prepared as fused disks with a lithium metaborate and lithium tetraborate flux mixture and as pressed pellets using a dilute solution of water-soluble PVA. 1009