550–490 Ma pre-to post-collisional shoshonitic rocks in the Ribeira Belt (SE Brazil) and their tectonic significance

Abstract

This work presents a detailed study in a new occurrence of shoshonite rocks in the central Ribeira Belt, SE Brazil. It is the Tinguí Unit, a (hornblende) biotite orthogneiss with shoshonitic affinity, and undeformed dykes of similar composition. They crop out along the contact between Costeiro (part of Oriental Terrane, Ribeira Belt) and Cabo Frio Tectonic Domains. The Tinguí Unit is represented by mesocratic orthogneiss, ranging from quartz-diorite to granodiorite in composition. Plagioclase, biotite, quartz, hornblende, microcline, titanite, apatite, allanite, zircon, and opaque minerals are the main minerals. Tonalitic to granodioritic dykes, with similar composition, cross cut the Tinguí orthogneiss. Besides mineralogical similarities, analyzed samples from both the orthogneiss and the dykes have shoshonitic signature with crustal and mantle Nd-Sr isotope nature. They are basic to intermediate rocks with low Fe contents and high alkalis, Ba, Rb, Sr, Zr, and LREE values. High K2O/Na2O (up to 2.06) and LREE/HREE ratios along with others geochemical parameters reinforce the shoshonitic signature. Sm-Nd data point to εNd values of −13.2 to −4.38, and 1.7 to 1.2Ga TDM Nd model ages. Although the 143Nd/144Nd ratios indicate a predominance of lower continental crust heritage, the 87Sr/86Sr ratios ranging from ca. 0.705 to 0.707 suggest an enriched mantle contribution. Nevertheless, geochronological data from Tingui orthogneiss and tonalitic dykes reveal a difference of ca. 50m.y. in their crystallization ages: concordia ages of 551±3Ma, for the orthogneiss; 495±4Ma and 488±4Ma for the mesocratic and leucocratic tonalitic dykes, respectively. Comparing our new data and the tectonomagmatic events recorded in the Oriental Terrane, we propose a tectonic model emphasizing the role of this shoshonitic magma, interpreted as pre- and post-collisional intrusions regarding to the Buzios Orogeny, and possibly derived from the similar source. This magmatic activity is attributed to a long-term maintenance of high temperatures and low cooling rates in the Ribeira Belt, considered as a hot orogen. The 50m.y. crystallization age gap between the studied rocks could be assigned to two periods of melting of a similar or even same source at different tectonic stages of the orogen, pre- and post-collisional.