LITHOSPHERIC VERSUS ASTHENOSPHERIC SOURCE OF THE SW AMAZONIAN CRATON A-TYPES GRANITES: THE ROLE OF THE PALEO AND MESOPROTEROZOIC ACCRETIONARY BELTS FOR THEIR COEVAL AMCG SUITES

The origin of Proterozoic rapakivi granites is controversial. Most models for continental magmatism envision heat or magma transfer from the mantle asthenosphere to the base of the lithosphere leading to partial melting of the lower crust. Early models have been based on thermal doming or hot spot beneath a supercontinent. Lithospheric extension and thinning have been linked to the generation, ascent and emplacement of rapakivi magmas where an over-thickened crust underwent extensional collapse. The production of the rapakivi granite melts by fractional crystallization of I-type melts is not reliable because rapakivi complexes are anhydrous and they have contrasting chemical and isotopic signaures.

The tectonic setting and source revealed by integrated isotopic and geochemical data of the Paleo and Mesoproterozoic magmatic suites in the SW Amazonian craton allow a temporal correlation between the accretionary mobile belts and the continental magmatism (e.g. AMCG suites) within the foreland. If this hyphothesis is correct, the continental magmatism may represent a synorogenic response to high heat flow in the asthenosphere resulting from episodic ocean crust subduction, which was contemporary with the development of the magmatic arcs.