Provenance of the Ediacaran Salinas Formation (Araçuaí Orogen, Brazil): Clues from lithochemical data and zircon U-Pb (SHRIMP) ages of volcanic clasts

Abstract

Salinas Formation occurs in a large region of the Northern Araçuaí orogen, Southeastern Brazil. It includes turbiditic wackes (> 10% matrix) to arenites (< 10% matrix), pelites and clast-supported conglomerates, metamorphosed from the biotite zone of the greenschist facies to the sillimanite zone of the amphibolite facies. Salinas Formation lies unconformably on the top of or in tectonic contact with the Macaúbas Group, and hosts Cambrian granitic intrusions dated between 540 and 500 Ma. Aiming to unravel sediment provenances for the Salinas basin, we present a detailed lithochemical (45 samples) study on low-grade rocks preserved from the regional deformation, which are found in the type area of the Salinas Formation. In addition, we compare them with deformed and more metamorphic rocks of similar composition but located in other basin sectors. The lithochemical data indicate limited chemical weathering in the sediment sources, good correlations with the mineralogical compositions in respect to the variable amounts of metamorphic minerals typical of pelitic (micas, garnet, and other peralumi-nous silicates) and psammitic (feldspars, quartz) fractions. The main provenances of sedimentary protoliths are clearly related to continental magmatic arc and active continental margin environments. U-Pb (SHRIMP) analyses performed on zircon grains from clasts of intermediate to felsic volcanic rocks, extracted from a clast-supported metaconglomerate, yield concordant zircon Pb206/U238 ages from ca. 579 Ma to ca. 697 Ma, with most of the ages in the interval of 587 to 630 Ma. This indicates that the main primary sediment source is the Rio Doce magmatic arc (630-580 Ma), in very good agreement with the arc-related lithochemical signature. Eleven zircon crystals yield a Concordia age of 620 ± 10 Ma, representing an important Ediacaran volcanic episode in the tectonic evolution of the Araçuaí orogen.