GRANITOGÊNESE BRASILIANA NO SERIDÓ: O MACIÇO DE ACARI (RN)

Resumo

The Acari granitoid is a type example of the Brazilian magmatism in Seridó, NE Brazil. Its country rocks are supracrustals and orthogneisses of early proterozoic age. The oldest unit in the massif is a suite differentiated from gabbro to granodiorite, with dominant quartz díorítes and tonalites enclosed as small bodies in younger granites, specially coarse porphyritic ones, and succeeded by fined grained porphyritic and equigranular leucrogranites. The vast majority of these rocks ís syntectonic with the F₃ regional deforrnation; some leucogranites are slightly latter, while the díorites are somewhat earlier. A balooning fabric or flow structures are sometimes folder or crenulated by the F₃ event. The geometry (trajectories and tripple points of the S₃ foliation points to a process of diapirism coeval with a regime of normal folds and subhorizontal extension. Micachists surrounding the granites display parageneses such as cordierite + andaluzite or cordierite + silllmanite. The diorite suite ís akin to an expanded calc-alkaline (or type I) séries, while the porphyritic granites are closer to restrict, potassic calc-alkaline affinities, intermediate between types I and S, and the leucogranites are of S type. The diorites are thought to be derived from mantle magmas or melting of amphibolite (consumed oceanic crust?) followed by variable crustal contamination and magma mixing. The granites appear to represent crustal anatexís in progressively shallower levels. These rocks are regarded as a distal manifestation of an aclive plate margin or Tibetan type plateau.