Late Quaternary History of the Vakinankaratra Volcanic Field (Central Madagascar): Insights from Luminescence Dating of Phreatomagmatic Eruption Deposits
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Bull Volcanol (2014) 76:817 DOI 10.1007/s00445-014-0817-7 RESEARCH ARTICLE Late Quaternary history of the Vakinankaratra volcanic field (central Madagascar): insights from luminescence dating of phreatomagmatic eruption deposits Daniel Rufer & Frank Preusser & Guido Schreurs & Edwin Gnos & Alfons Berger Received: 22 November 2012 /Accepted: 12 March 2014 /Published online: 5 April 2014 # Springer-Verlag Berlin Heidelberg 2014 Abstract The Quaternary Vakinankaratra volcanic field in scoria cones and their associated lava flows. Two the central Madagascar highlands consists of scoria cones, phreatomagmatic deposits, identified near Lake Tritrivakely, lava flows, tuff rings, and maars. These volcanic landforms yield the youngest IRSL ages in the region, with respective are the result of processes triggered by intracontinental rifting ages of 32±3 and 19±2 ka. The reported K-feldspar IRSL and overlie Precambrian basement or Neogene volcanic rocks. ages are the first recorded numerical ages of phreatomagmatic Infrared-stimulated luminescence (IRSL) dating was applied eruption deposits in Madagascar, and our results confirm the to 13 samples taken from phreatomagmatic eruption deposits huge potential of this dating approach for reconstructing the in the Antsirabe–Betafo region with the aim of constraining volcanic activity of Late Pleistocene to Holocene volcanic the chronology of the volcanic activity. Establishing such a provinces. chronology is important for evaluating volcanic hazards in this densely populated area. Stratigraphic correlations of eruption Keywords Luminescence dating . Quaternary volcanism . deposits and IRSL ages suggest at least five phreatomagmatic Geochronology . Madagascar . Vakinankaratra . eruption events in Late Pleistocene times. In the Lake Phreatomagmatic deposits . Ankaratra Andraikiba region, two such eruption layers can be clearly distinguished. The older one yields ages between 109±15 and 90±11 ka and is possibly related to an eruption at the Introduction Amboniloha volcanic complex to the north. The younger one gives ages between 58±4 and 47±7 ka and is clearly In the central highlands of Madagascar, volcanic rocks of the related to the phreatomagmatic eruption that formed Lake Itasy and Ankaratra volcanic fields overlie Precambrian base- Andraikiba. IRSL ages of a similar eruption deposit directly ment rocks (Fig. 1). These two fields form part of a Late overlying basement laterite in the vicinity of the Fizinana and Cenozoic magmatic cycle that is mainly found in central and Ampasamihaiky volcanic complexes yield coherent ages of northern Madagascar, with a few small outliers in the south- 68±7 and 65±8 ka. These ages provide the upper age limit for west (Besairie 1964;Rasamimananaetal.1998;Bardintzeff the subsequently developed Iavoko, Antsifotra, and Fizinana et al. 2010). Late Cenozoic magmatism is associated with intracontinental rifting that is affecting Madagascar since Editorial responsibility: G. Giordano mid-Miocene times (Bertil and Regnoult 1998; Laville et al. D. Rufer : G. Schreurs (*) : A. Berger 1998; Piqué et al. 1999). Rifting in Madagascar has been Institute of Geological Sciences, Baltzerstrasse 1 + 3, University of related to the East African Rift system (Mougenot et al. Bern, 3012 Bern, Switzerland 1986; Laville et al. 1998), with deformation becoming more e-mail: [email protected] distributed and diffuse towards the south (Kusky et al. 2007, F. Preusser 2010). Department of Physical Geography and Quaternary Geology, Located southwest of Antananarivo, the Ankaratra volca- Stockholm University, 10691 Stockholm, Sweden nic field (Fig. 1) contains both Tertiary and Quaternary vol- canic rocks, with the latter mostly restricted to its southwest- E. Gnos Muséum d’histoire naturelle de Genève, route de Malagnou 1, CP ern margin (Mottet 1980a, b). We use the term 6434, 1211 Genève 6, Switzerland “Vakinankaratra volcanic field” for the Quaternary volcanic 817, Page 2 of 20 Bull Volcanol (2014) 76:817 Neogene and Quaternary 600 50° E 800 require closed system behavior, a suitable parent isotope-rich basin sediments 12° S MASSIF D’AMBRE Antsiranana Neogene and Quaternary constituent, and sufficient accumulation of radiogenic daugh- volcanics Cretaceous volcanics ter isotopes for accurate and precise measurements (e.g., Faure NOSY BE Carboniferous to Recent 1986). Our initial attempts to date the volcanic rocks of the sediments Vakinankaratra field using the 40Ar/39Ar method failed due to Precambrian basement AMPASINDAVA Diego Basin AMBONDRONA the young age of the material, overall low potassium concen- Brittle lineament trations in whole rock samples, and a lack of measurable 200 400 ANKAIZINA sanidine. Furthermore, the abundance of xenocrysts together with an open system behavior of uranium in phlogopite 1200 ANKASIMBELO Mahajanga prevented uranium-thorium dating, whereas the lack of organic Cap St. André material associated with the volcanic deposits impeded the 16° S application of radiocarbon dating. Here, we present an over- view of phreatomagmatic activity in the Vakinankaratra volca- 1000 nic field and present the first numerical age data of such ALOATRA eruption deposits in Madagascar using luminescence methods. Alaotra Majunga Basin Basin Toamasina Ankay General geological setting Antananarivo Basin 800 ITASY TAKARINDIONA Madagascar is a continental island with a central highland ANKARATRA plateau at a mean elevation of c. 1,200 m. The eastern two- Antsirabe Morondava 20° S thirds of the island, including the central highland plateau, consist almost exclusively of high-grade metamorphic Pre- 600 cambrian basement, whereas the remaining one third to the west comprises Late Paleozoic to Quaternary sedimentary and volcanic cover rocks. Morondava Basin ANKILIOAKA A major magmatic episode in Late Cenozoic times resulted 400 in large volcanic fields mainly in central and northern Mada- gascar (e.g., Ankaratra, Itasy, Takarindoha, or Takarindiona, Toliara see Tucker and Moine (2012) for discussion on toponymy; Massif d’Ambre) and local dyke swarms (e.g., near 24° S Ankilioaka, SW Madagascar; Bardintzeff et al. 2010). This 200 Late Cenozoic magmatism is temporally and spatially associ- 0 100 200 km ated with intracontinental rifting that is considered an exten- Tôlañaro sion of the Somalian-African diffusive plate boundary (Kusky Cap St. Marie et al. 2010). Since mid-Miocene times, the entire southwestern 200 400 600 800 edge of the Indian Ocean, including the Davie Ridge in the Fig. 1 Simplified geological map of Madagascar with location of Creta- Mozambique Channel, the Comoros Archipelago, and Mada- ceous and Neogene and Quaternary volcanic fields (after Besairie 1964). Grid shows Laborde coordinates gascar has come under an overall E-W extensional regime (Bertil and Regnoult 1998). This resulted in extensional reac- tivation of the Davie Ridge (Grimison and Chen 1988) and the rocks in the southwestern part of the Ankaratra field, in the formation of major N-S trending rift graben basins in the area between Antsirabe and Betafo (Fig. 2). The limited direct central highlands of Madagascar, such as the c. 250 km long or indirect age data available suggest that volcanic activity in Plio-Pleistocene Alaotra-Ankay graben system (Fig. 1; the Vakinankaratra field occurred during the Late Pleistocene Laville et al. 1998; Piqué et al. 1999; Kusky et al. 2010), and possibly Holocene (Gasse and Van Campo 1998). and the half-grabens near Antsirabe (Mottet 1980a, b). Obtaining accurate ages on the young volcanic rocks of the Scattered, but frequent seismicity characterizes the island of Vakinankaratra field is important not only for geomorphologic Madagascar with magnitudes up to M=5.8 (Bertil and and climate studies, but also for volcanic hazard assessment Regnoult 1998) and earthquake focal mechanisms indicating studies in this densely populated area. However, methods for mainly normal faulting (Grimison and Chen 1988). Seismicity directly dating volcanic rocks of Late Quaternary age are is particularly concentrated in the region of the Ankaratra and scarce, not always straightforward, and possess some inherent Itasy volcanic fields and in the immediate vicinity of the pitfalls. Dating methods using radioactive decay systems Ankay-Alaotra graben system (Bertil and Regnoult 1998). Bull Volcanol (2014) 76:817 Page 3 of 20, 817 47°00’ E Antananarivo Miarinarivo Lac Itasy S 19°00’ ITASY 1220 m Kasigie Soavinandriana 2643 m Ambatolampy Faratsiho 2210 m ANKARATRA 2292 m 2341m Betafo Antsirabe Quaternary volcanics Ivohitra 1649 m 2190 m Neogene volcanics Betampona fault Neogene sediments Precambrian base- VAKINANKARATRA ment rocks Brittle lineament S 20°00’ Ductile foliation 2264 m Fig. 2 Simplified geological map of the Itasy, Ankaratra, and Vakinankaratra volcanic fields in central Madagascar (after Besairie 1969). Black rectangle indicates the area shown in Fig. 3 The Ankaratra volcanic field covers approximately 2001), and has its highest elevation at 2,643 m (Fig. 2); it is 3,800 km2, between 18° 56′ to 19° 57′ S and 46° 46′ to 47° dominated by basalts and andesites (Besairie 1969). Its main 30′ E, extends over 100 km in north–south direction (Woolley crest strikes NNE-SSW and continues to the south in the 817, Page 4 of 20 Bull Volcanol (2014) 76:817 Betampona escarpment, a c. 35 km long normal fault expansion of large amounts of water (Sheridan and Wohletz delimiting a half-graben structure containing the 1981; Büttner and Zimanowski 1998). The ensuing explo- Ambohimiarivo basin;