Rutile and zircon age and geochemistry in the evolution of the juvenile São Gabriel Terrane early in the Brasiliano Orogeny

Abstract

Major issues in the tectonic evolution of continents include oceanic crust generation, the subduction of oceanic island arcs and their accretion to a craton. The understanding of geological events in the Neoproterozoic of South America and Gondwana is a challenge due to tectonic disturbance caused by the Brasiliano Orogeny. We selected key units in the South Atlantic Brasiliano Orogen to solve problems related to dating and environment of metamorphism, magmatism and metasomatism. Multiproxy methodology included aerogammaspectrometry in the Cambaizinho and Palma ophiolites. Rutile and zircon U–Pb geochronology combined with Lu-Hf-trace element analyses were conducted on metagabbro and chloritite. The aerogammaspectrometric survey delimitated the extent of ophiolite bodies. Rutile U–Pb dating in the Cambaizinho metagabbro provided a metamorphic age of 706 ± 11 Ma. Zircon U–Pb dating in the São Luís metagabbro yielded an age of 731.6 ± 0.5 Ma for the magmatic activity; chloritite yielded an 889.2 ± 2.3 Ma age for metasomatism. Zircon Hf analyses display positive εHf(t) values (metagabbro = +7.1 to +11.1; chloritite = +9.9 to +17.5). REE element diagrams show positive Ce anomalies for both metagabbro and chloritite zircons. Positive εHf(t) in zircon from metagabbro and chloritite indicates depleted mantle origin. Trace element contents of zircon (e.g., U, Yb) from the metagabbro are consistent with magma generation by partial melting of older juvenile crust; data from chloritite indicate a N-MORB magma source. We infer that the final phase of collision and thrusting of the São Gabriel arc over the Rio de La Plata Craton occurred at 706 Ma; the São Gabriel island arc had main activity at 731 Ma; mid-ocean ridge metasomatism in the ophiolite occurred at 889 Ma. Our studies provide evidence of geotectonic processes during Rodinia break-up and early Brasiliano orogeny and similar juvenile Tonian terrains within Gondwana.