Geological evolution of the basement rocks in the east-central part of the Rondônia Tin Province, SW Amazonian craton, Brazil: U–Pb and Sm–Nd isotopic constraints

Abstract

On the basis of geologic, petrologic, and U–Pb geochronologic data the basement rocks in the east-central part of the Rondônia Tin Province (RTP, southwestern Amazonian craton) are grouped into five lithologic associations: (1) tonalitic gneiss (1.75 Ga); (2) enderbitic granulite (1.73 Ga); (3) paragneiss; (4) granitic and charnockitic augen gneisses (1.57–1.53 Ga); and (5) fine-grained granitic gneiss and charnockitic granulite (1.43–1.42 Ga). The first three are related to development of the Paleoproterozoic Rio Negro-Juruena Province and represent the oldest crust in the region. The tonalitic gneisses and enderbitic granulites show calc-alkaline affinities and Nd isotopic compositions (initial ε Nd=+0.1 to −1.5; T DM of 2.2–2.1 Ga) that suggest a continental arc margin setting for the original magmas. The paragneisses yield T DM values of 2.2–2.1 Ga suggesting that source material was primarily derived from the Ventuari-Tapajós and Rio Negro-Juruena crusts, but detrital zircon ages and an intrusive granitoid bracket deposition between 1.67 and 1.57 Ga. The granitic and charnockitic augen gneisses show predominantly A-type and within-plate granite affinities, but also some volcanic arc granite characteristics. The initial ε Nd values (+0.6 to +2.0) indicate mixing of magmas derived from depleted mantle and older crustal sources. These rocks are correlated to the 1.60–1.53 Ga Serra da Providência intrusive suite that reflects inboard magmatism coeval with the Cachoeirinha orogen located to the southeast. The fine-grained granitic gneiss and charnockitic granulites represent the first record of widespread magmatism at 1.43–1.42 Ga in northern Rondônia. Their geochemical signatures and the slightly positive initial ε Nd values (+0.7 to +1.2) are very similar to those of the most evolved granites of the calc-alkaline Santa Helena batholith farther southeast. U–Pb monazite and Sm–Nd whole-rock-garnet ages demonstrate that a high-grade tectonometamorphic episode occurred in this region at 1.33–1.30 Ga. This episode attained upper-amphibolite conditions and is interpreted as the peak of the Rondonian-San Ignacio orogeny. The U–Pb and Sm–Nd data presented here and data published on rapakivi granites elsewhere indicate that the east-central part of the RTP is a poly-orogenic region characterized by successive episodes of magmatism, metamorphism, and deformation between 1.75 and 0.97 Ga.