Geology and Geochronology of Abuna-Ariquemes-RO

The basement is formed by the JAMARI COMPLEX high-grade terrains, which were intensily reworked in the Rondoniano Mobile Belt related period, by 1,500 m.y. ago. In the Jamari stratigraphic unit a Rb-Sr true isochrone has been obtained in the Ariquemes granulites giving 1,495 ± 38 m.y. with IR = 0,7086 ± 0.0007. Proterozoic metasedimentary sequences are only preserved in the Guajara-Mirim (Yata) region and in the surroundings of the Formoso river, by the COMEMORACAO GROUP. This medium-grade unit consists of a aluminous sedimentary sequence with NE-SW to E-W structural trend. In the Yata settlement a Rb-Sr true isochrone has been abtained, giving 1,157 ± 40 m.y., with IR=O.7189 ± 0.0021, which suggests that it has been affected by the Sunsas Mobile Belt. The remainders of a low-grade sequence derived from psamitic-pelitic sediments are also exposed in the FORTALEZA DO ABUNA GROUP, with pronounced NW-SE structural trend. The region was lately the ground for deposition of mainly continentally derived sediments, although marine facies and volcanics are associated, owing to the rifting and deposition of the MUTUM-PARANA and PALMEIRAL FORMATIONS. These events were followed by the emplacement, by 1,300 to 950 m.y. ago. of both, plutonic and basic volcanic rocks, of the CIRIQUIQUI INTRUSIVE SUITE and NOVA FLORESTA FORMATION. Simultaneously to the basic activity, a large intensive volcanic-plutonic activity took place randing from 1,350 to 950 m.y., represented by the RONDONIA INTRUSIVE SUITE, CARIPUNAS FORMATION acid volcanics and ALCALINAS TEOTONIO related intrusives. Rb-Sr true isochrones have been obtained in the RONDONIA INTRUSIVE SUITE granites of the Samuel Dam region (1,381 ± 122 m.y., IR = 0.7126) and Periquitos (1,297 ± 170 m.y., IR = 0,7100 ± 0.0137). (author)