Circular Structure
A large circular structure has been identified by aerial photography. By comparison with similar structures elsewhere in South America it is possibly an alkaline intrusion with carbonatite.
A large circular structure has been identified by aerial photography. By comparison with similar structures elsewhere in South America it is possibly an alkaline intrusion with carbonatite.
This occurrence comprises three separate nepheline syenite bodies covering some 200 km2 which are overlain by upper Amazonian Tertiary sedimentary rocks. The more northerly body intrudes the Araracuara Formation.
Quensel (1912, p. 79) has described a group of rocks he called 'essexites', together with 'comenditic granophyres' and syenites from the vicinity of Cerra Cagual. The essexites are gabbroic rocks with orthoclase but no nepheline. However, syenites contain aegirine-augite.
Syenites comprising orthoclase, albite-oligoclase, aegirine- augite, arfvedsonite and a little quartz, and nordmarkites including quartz, fayalite and aegirine-augite are described from the Cerro Balmaceda vicinity by Quensel (1912, p. 36).
Extrusive rocks lying close to the north shore of Seno Skyring consist of labradorite, with some albite, pyroxene, altered nepheline, serpentine and magnetite in a glass base. Little detail of field relationships is available. Described as trachydolerite (shoshonite) by Quensel (1912, p.
This is a small intrusion of trachyte.
This is a multiple intrusion covering approximately 30 km2 which is related to the nearby Tangua and Rio Bonito (Fig. 2_206) intrusions.
Rio Bonito is a zoned stock of some 28 km2 emplaced in Precambrian gneisses and granulites near to the Tangua and Saorinho intrusions (Fig. 2_206).
This approximately circular intrusion 6-7 km in diameter is cut by plugs of intrusive breccia, and emplaced in Precambrian biotite gneisses and granulites, close to Saorinho and Rio Bonito (Fig. 2_206).
A small intrusion of volcanic breccia cuts Tertiary sediments of the Barreiras Formation. It contains small alkaline rock fragments 1-10 mm in diameter in a phonolitic matrix of orthoclase, nepheline, cancrinite, pyroxene and secondary calcite.