Alkaline Rocks and Carbonatites of the World

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Ampasindava Area


Occurrence number: 
Longitude: 48.08, Latitude: -13.8

The Ampasindava Province was defined by Lacroix (1922a, Fig. 5) and includes several areas of igneous rocks on the Ampasindava Peninsula, a number of volcanic fields and intrusions south of Ampasindava Bay and along the eastern margin of the Bay, and the islands of Nosy-Be and Nosy Komba, which are described separately (Nos 2 and 3). These localities are shown on the 1:500,000 sheets (1) Diego-Suarez and (2) Antalaha (Besairie, 1971a and b) and the earlier 1:200,000 series geology maps. No accounts devoted to particular localities or occurrences have been located but a considerable amount of petrographic detail and numerous chemical analyses are to be found in the three volume Mineralogie de Madagascar (Lacroix, 1922a, 1922b and 1923). However, Lacroix’ descriptions are organised by rock type rather than locality, but can be traced from the very full locality index. There are also useful descriptions for some areas in Donnot (1963). There appears to have been little modern work but a dense forest cover and the steepness of much of the terrain makes access difficult. The principal igneous localities, with names from the 1:500,000 maps, apart from Ankitsika which is only so named on the 1:200,000 sheet, and their main rock types are as follows: Ambohimirahavavy (Ampasibitika)granite, syenite, trachyte; Ambohimiramatavysyenite, trachyte; Bezavonanepheline syenite, phonolite, nordmarkite, theralite; Ankitsikanepheline syenite, phonolite, tahitite; Manongarivotrachyte, syenite, granite, nepheline gabbro; Sambiranophonolite; Ankifysyenite, microsyenite, phonolites; Ambatogranite.Ambohimirahavavy is a mixed intrusive and extrusive complex about 18 km in length forming a mountainous area and including outer intrusions of granite, which may be ring-dykes (Donnot, 1963), and an inner complex of volcanic rocks. The granites grade into syenites and are highly variable in composition but strongly peralkaline. Riebeckite is ubiquitous, and in pegmatitic varieties may reach 20 cm in length, and aegirine is generally abundant as are aenigmatite and zircon; fluorite, titanite and pyrochlore are accessory (Lacroix, 1922b). There are numerous granitic dykes ranging from pegmatites to aplites all of them peralkaline and invariably containing aegirine and often riebeckite. The central area of the complex comprises, according to Donnot (1963), flows and dykes of microsyenite, trachyte and trachytic breccias with at one locality a spine of obsidian. Petrographic details have not been found. A large complex of syenite and trachyte, called Ambohimiramatavy, is shown on the 1:500,000 sheet (Besairie, 1971a) as occurring in the central northern part of the Ampasindava Peninsula northwest of Ambohimirahavavy. Few details have been traced, however. The Bezavona complex forms a line of hills the highest of which, Mont Bezavona, reaches an elevation of 700 m. There are a number of sharp ridges and several spines. Nepheline syenites form a series of intrusions and dykes of phonolite, tinguaite, nordmarkite, bostonite, aplite, monchiquite and ‘leucite phonolite’ (Lacroix, 1922a) cut the surrounding sediments. Berondrite, a melanocratic variety of theralite, was named by Lacroix from this occurrence. One of the spines comprises a rock compared with larvikite by Lacroix (1922b) and consisting of alkali feldspar, biotite, diopside, passing into aegirine-augite and sometimes aegirine, fayalite, and a brown amphibole. The rock of another spine is similar but includes a little quartz. The nepheline syenites contain sodic pyroxene and sparse rinkite (mosandrite), although some microsyenite dykes are rich in this mineral, eudialyte and blende, and one particularly coarse-grained nepheline syenite body is rich in arfvedsonite, aenigmatite, mosandrite, eudialyte and astrophyllite. An exceptional rock described by Lacroix (1922b, p. 618) and forming dykes cutting nepheline syenites is a ‘tinguaite obsidian’ in which alkali feldspar, nepheline, biotite and aegirine form spherulites set in a brown glass. The euhedral analcime crystals occuring in some phonolites were considered by Lacroix to be after leucite. Saint Ours (1960) describes a rolled block of calcite rock from the River Antsaharoro, Bezavona complex, which he discusses as a possible carbonatite. The rock comprises calcite, phlogopite, chlorite and ilmenite but he concludes that it is probably of contact metamorphic origin.The Ankitsika area is located close to the coast in the southwest corner of the province. According to Besairie (1934a) nepheline microsyenite, tinguaite and phonolite form dykes and possible flows. The phonolites comprise fine-grained orthoclase and nepheline with poikilitic aegirine-augite and aegirine. There is also a dyke, or dome, of tahitite which is described in some detail by Lacroix (1923). It contains phenocrysts of anorthoclase, up to 2 cm in length, melanite, zoned andesine, a brown amphibole, augite, hauyne and titanite set in a groundmass of orthoclase, nepheline, augite and amphibole. Lacroix (1923) also describes a ‘nepheline latite’ from this area. Manongarivo (Lacroix, 1922a, pp 90-91) consists of a central complex of syenites, granites and trachytic rocks around which are distributed at least half-a-dozen further syenite bodies. There is also a separate intrusion of nepheline-bearing gabbroic rocks. The mountain of Bekolosy (1488 m), which is surmounted by a spine, comprises a large intrusion the granite of which contains arfvedsonite and aegirine. The spine, and others in the complex, consist of syenite and microsyenite. Two other granite bodies are shown on the 1:500,000 sheet (Besairie, 1971b). Microgranite dykes are abundant and contain arfvedsonite and pyrochlore. There are also, according to Donnot (1963), sills of granosyenite, containing sodic amphibole, of orbicular rhyolite and of biotite trachyte within Liassic calcareous sedimentary rocks. The syenites are also peralkaline usually containing aegirine-augite and/or aegirine and a sodic amphibole; pyrochlore is also found in these rocks. The extensive trachytes contain orthoclase phenocrysts and aegirine-augite, which is generally altered, set in a cryptocrystalline groundmass containing a little quartz. The Sambirano occurrence consists essentially of phonolite which comprises a number of dyke-like bodies the highest point of the area in particular being formed from a giant phonolite dyke (Besairie, 1934b). The phonolites may or may not include orthoclase phenocrysts together with microphenocrysts of aegirine-augite and an amphibole in a matrix of nepheline, orthoclase, aegirine-augite, analcime and sodalite (Lacroix, 1923). There are petrographically identical intrusions of phonolite in the Ankify Peninsula, which is immediately to the north of Sambirano.The Ambato intrusion is expressed topographically as a large mountain. It is about 4 km in diameter and consists of alkaline granite (Besairie, 1934b) but petrographic details have not been found.

Traces of molybdenite are reported from the Bekolosy granite (Donnot, 1963).

BESAIRIE, H. 1934a Notice explicative sur la Feuille Nosy-Be RS-32,33 (330). Carte Geologique de Reconnaissance à l'échelle du 200.000. Gouvernement General de Madagascar et Dependances. Tananarive.BESAIRIE, H. 1934b. Notice explicative sur la Feuille Ambanja RS-34,35 (350). Carte Geologique de Reconnaissance à l'échelle du 200.000. Gouvernement General de Madagascar et Dependances. Tananarive.BESAIRIE, H. 1971a. Carte Géologique. Feuille 1. Diego-Suarez. 1:500,000. Service Géologique de Madagasikara, Tananarive. BESAIRIE, H. 1971b. Carte Géologique. Feuille 2. Antalaha. 1:500,000. Service Géologique de Madagasikara, Tananarive.DONNOT, M. 1963. Les complexes intrusifs alcalins d’Ampasindava volcanisme et cristallin de l’Ankaizina (Madagascar). Annales Géologiques de Madagascar. Comptes Rendus de la Semaine Géologique 1963, 33: 81-7.LACROIX, A. 1922a. Minéralogie de Madagascar. 1. Géologie, minéralogie descriptive. A. Challamel, Paris. 624 pp.LACROIX, A. 1922b. Minéralogie de Madagascar. 2. Minéralogie appliquée lithologie. A. Challamel, Paris. 694 pp.LACROIX, A. 1923. Minéralogie de Madagascar. 3. Lithologie Appendice - Index Geographique. Société d’Éditions Géographiques, Maritimes et Coloniales, Paris. 450 pp.SAINT OURS, J. de 1960. Etudes geologiques dans l’extreme nord de Madagascar et L’Archipel des Comores. Ministere del’Economie Nationale, Service Geologique, Tananarive, 1-272.

Fig. 3_137 The extensive Ampasindava area and the islands of Nosy-Be and Nosy Komba (after Cartes geologiques, 1:500,000, Feuille 1, Diego-Suarez, and 2, Antalaha. Besairie, 1971a and 1971b).
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