Ambre

The northernmost part of Madagascar is extensively covered with volcanic rocks the Massif d’Ambre being approximately circular and 85 km in diameter, extending over 2840 km2 and culminating at 1475 m in Montagne d’Ambre. The volcanic rocks, which comprise both pyroclastic rocks and lava flows, are essentially Miocene to Recent in age and overlie Cretaceous and Jurassic sedimentary rocks. Towards the centre of the Ambre massif there are numerous strato-volcanoes, scoria cones and craters, a dozen of the last varying from 600 to 1200 m in diameter with five of them occupied by lakes. Many lava flows extend along valleys radiating from the central massif. Amongst the pyroclastic debris lherzolite xenoliths are found. Outlying manifestations of the volcanicity occur around Cap d’Ambre, the most northerly part of Madagascar, on the Anorontany Peninsula and small adjacent islands to the west, and the islands of Nosy Mitsio and Nosy Lava to the southwest. The igneous rocks span a broad range including tholeiitic and alkali basalts, basanite, hawaiite, mugearite, trachyte, phonolite and rhyolite. The lowermost volcanic rocks are ignimbrites and rhyolites the former not generally being alkaline but in which blue alkali amphibole may occur as spherulitic aggregates within amygdales. There are a few domes the rhyolite of which may contain microphenocrysts or spongy aggregates of arfvedsonite, while a microgranite at one locality contains interstitial riebeckite and aenigmatite (Karche, 1973). Trachytic rocks, including dykes, are also represented amongst the earlier volcanic rocks, particularly in the northeast of the area around Mont Raynaud and the Montagne des Francais, in which aegirine-augite, usually altered, occurs (Lacroix, 1922). With decrease in olivine and increase in the proportion of Ab in the plagioclase the basalts grade into hawaiites and mugearites. The basanites, which appear to comprise a series distinct from the basalts, form flows and dykes and Karche (1973) distinguishes aphanitic and porphyritic varieties, the latter containing phenocrysts of either titaniferous augite with sodic ferrosalite cores or kaersutitic amphibole. The aphanitic rocks contain microlites of plagioclase and tiny crystals of olivine, magnetite and nepheline. Olivine (Fo75-60) is invariably present in the porphyritic rocks, and may form nodules which, together with phenocrysts of augite and rare plagioclase (An80-46), are set in a matrix of augite, amphibole, biotite, magnetite and apatite. Nepheline may be minor or form large euhedra; in some rocks analcime is present in lieu of nepheline and a little alkali feldspar may occur. Nepheline-bearing dykes having similar mineralogies to the basanites are present and these are referred to as tephrites by Karche (1973). Trachyte and phonolite are widely distributed over the area but are concentrated in the central region of the D’Ambre massif, at the western end of the Anorontony Peninsula and adjacent small islands and on the islands of Nosy Misio and Nosy Lava. They form domes, including some substantial edifices, flows and dykes as well as tuffs. The general distribution is indicated on a map in Karche (1973), who also describes briefly the rock types at many individual localities. The trachytes range from rocks with about 10% normative quartz to those with a little nepheline in the norm. The quartz trachytes contain alkali feldspar, minor interstitial quartz, sodic pyroxene, poikilitic aenigmatite, rare blue sodic amphibole (arfvedsonite?) and zeolite. Apart from nepheline the nepheline-bearing trachytes are similar but may have a little analcime and rare biotite. All the phonolites contain sanidine and sodic pyroxene (aegirine-augite to aegirine), which may form phenocrysts up to 6 mm in length, in a fluidal groundmass of the same minerals, aenigmatite and tiny crystals of nepheline, which may be partly replaced by analcime. Some varieties contain an amphibole, usually altered to iron oxides, and accessory apatite, magnetite and titanite; fayalite has been identified at some localities. Phonolites which have a non-fluidal texture have been distinguished as tinguaites and usually form dykes. Mineralogically they are essentially the same as the phonolites but one occurrence is characterised by the presence of up to 30% analcime. Karche (1973) gives numerous rock analyses and photomicrographs and describes Nosy Mitsio in some detail. There is a general geological description, some detailed petrography and rock analyses in Lacroix (1922a and 1923).