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Montagne d'Ambre, northern Madagascar: A Cenozoic alkaline volcanic complex
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In Madagascar, volcanism is known from two distinct periods:

1) Cretaceous flood basalts, connected to the upwelling of the Marion hot spot, and
2) Tertiary to subrecent alkaline volcanism.

The latter is characterized by strongly undersaturated alkaline basalts and their derivates, sometimes in unusual voluminous sequences. The complexes (compare the sketch map) are located in the inner part of the island within continental crust as well as on a passive continental margin in the North.

Actually we concentrate our work to the complexes of Itasy (W of Antananarivo) and of Montagne d'Ambre in the North of the island. One of our aims is to establish a chronology of the volcanic history of these complexes, combining detailed field work with Ar/Ar laser ablation dating (L. Page, University of Lund). Using petrographical, geochemical and mineralogical investigations of the volcanic rocks and their mantle-derived xenoliths and megacrysts, petrologic models for the evolution of the magmatic systems through time and a characterization of the Upper Mantle sources are achieved.


map distribution volcanism

Montagne d'Ambre

The volcanic complex of Montagne d'Ambre covers an area of nearly 3000 km2, with maximum height of only 1475 m. The satellite image (Landsat image) shows the central part of the massif, covered by dense rain forest. The principal structure of the complex is evident from this picture: earlier, very long lava flows at the basis of the complex and in its periphery, overlain by younger cinder cones, craters and flows. Especially the older lava flows are often more than 20 km in length, with a thickness of 20-40 m and steep flanks with only minor erosion. The cinder cones and craters (the latter often filled up by crater lakes) are very common (compare the drawing of ROSSI 1980) and form the youngest, nearly recent volcanic manifestations.

The spectrum of the effusive rocks contains, beside alkaline dominated basalts, also limburgites, ankaratrites, some trachytes and phonolites (peninsula of Bobaomby), but also rhyolitic ignimbrites. According to BESAIRIE (1973) the major volcanic activity is of Miocene age; K/Ar age determinations (EMERICK & DUNCAN 1982, 1983) yielded ages of 9,380,45 and 9,100,57 million years for basal lava flows at Diego Suarez and 0,830,02 for a flow from the central part of the complex. Within the immediate surrounding of the Montagne d'Ambre in the E (Montagnes Francaise) und N (Windsor Castle, peninsula of Bobaomby) the only Early Tertiary volcanics of Madagascar can be found, which are intercalated with Eocene limestones. Their today's level is at 300 m, which is the same amount higher than the basic flows of the Miocene Montagne d'Ambre - an indication for young and significant uplift and collapse tectonics in this area.

Mt d'Ambre

Landsat 7 image (detail of S-39-10-2000) of Montagne d'Ambre Area
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Geological sketch map of the central part of Montagne d'Ambre massif (ROSSI 1980)


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Trace element spidergrams for Tertiary (red) to Quaternary (blue) alkaline volcanic rocks from Montagne d'Ambre
Rounded Diopside megacrysts up to 3-4 cm in size are typically characterized by a fresh, black core and a brighter rim, very porous and rich in inclusions. This rim is interpreted as a reaction product during the ascent; the megacrysts are evident for a metasomatic overprinting of the Upper Mantle. According to the Cpx geobarometer of Nimis (1995) pressures of 11 to 12 kb can be calculated for the cores, and a large range of 5 to 8 kb for the reaction rims. Analogous Diopside megacrysts are known also from olivine nephelinites of the Kaiserstuhl, southern Germany (Schleicher 1986, Schleicher et al. 1987).

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Ultramafic xenoliths from the Upper Mantle (lherzolithes, spinel bearing harzburgites and pyroxenites) are unusual common in the Montagne d'Ambre area, even in some larger lava flows with columnar structure. The olivines of many of these xenoliths often show banded textures as a result of a stress regime within the Upper Mantle.


Itasy

Landsat 7 image (detail of S-38-15-2000) central part of Itasy volcanic field


Itasy Volcanic Field
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This 750 km2 large volcanic field is situated at about 200 km W of Antananarivo, at the western lakeside of Lake Itasy. Dominant rocks are trachytes to trachyphonolites, which form well shaped domes, scoria cones and lava flows, and basanitic rocks in form of lava flows of different size. Also abundant are tuffs of trachytic/phonolitic and basanitic compositions. Xenoliths of basement rocks (granulites, gneisses) are common.

The volcanic activity is thought to cover the age range from Pliocene to about 8000 years (BRENON & BUSSIERE 1959, some recent radiocarbon dating); however, stratigraphic correlations are not possible. Few low temperature thermal springs (geysers) are indicative for persisting geothermal resources.

According to rumors in the area, in 1998 minor volcanic eruptions ("fire geysers") were observed at Kasigie cone, together with some earthquakes.

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