Unravelling source and tectonic environment of an Ediacaran magmatic province from southeast Brazil: Insights from geochemistry and isotopic investigation

Abstract

This work focuses on the 575–540 Ma meta-igneous units that crop out near Rio de Janeiro city, southeast Brazil. Tectonically, they are located in the southern part of the Oriental Terrane known as Costeiro Domain, near to the suture with the Cabo Frio Tectonic Domain. We present new zircon U-Pb and oxygen isotope SHRIMP along with geochemical data to investigate the possible source(s) and tectonic setting of this magmatic event related to the final stages of Western Gondwana consolidation. Five units comprise this Late Ediacaran magmatic province: Facoidal orthogneiss, Cassorotiba garnet-biotite orthogneiss, Maricá porphyritic orthogneiss, Tinguí hornblende-biotite orthogneiss, and Itacoatiara porphyritic meta-granite. These acid to intermediate plutons show similar whole-rock geochemistry, Sm-Nd data, and petrographic characteristics. The original magmas are calc-alkaline that evolved to a high-K calc-alkaline, and a shoshonitic term. Geochronological data indicate three intervals of magmatic activity: 575–565 Ma medium to high-K calc-alkaline intrusions; 555–545 Ma high-K and shoshonitic magmas; and the 545–540 Ma calc-alkaline medium-K Itacoatiara meta-granite, with Nd isotopic ratios presenting the highest εNd (3.7–5.1), and TDM ages (1.3–1.8 Ga). The TDM Nd model ages of all units range from 1.3 Ga to 1.8 Ga and the εNd(t) values vary mostly from −3.7 to −5.7 corroborating with the crustal affinity of these rocks. The oxygen isotope (δ18O) values average is 8‰ with slight differences among the units. These high δ18O ratios suggest crustal values, typical for continental crust in a collisional environment. Based on the geochemical data allied with melting experiments from literature, we envisage that the Late Ediacaran magmatic province could be formed by partial melt from a lower crust amphibole-biotite rich source. All the presented data point to magmatic activity in a transitional environment from subduction to continental collision in the Cambrian.