The U-Pb method in the UERJ MultiLab laboratory: procedures and application for rocks from the Lagos Region Complex, Rio de Janeiro, Brazil

The studied region is located in the State of Rio de Janeiro and is part of the Ribeira Belt (central portion of the Mantiqueira Province). The central segment of the Ribeira Belt can be divided into Eastern, Western, Coastal and Cabo Frio terrains. The study area is located in Cabo Frio terrane, where the basement is represented by the Região Lagos Complex, in this study divided into (i) granitic rocks (biotite-microcline-plagioclase gneiss); (ii) granodioritic rocks (biotite gneisses and (iii) tonalitic rocks (biotite-orthoclase-plagioclase gneiss). The supracrustal rocks include sillimanite-garnet gneiss with partial levels of fusion (Palmital Unit) and garnet gneiss (Cassorotiba Unit).

Geochronological studies (by LA-ICP-MS) U-Pb in zircon from Região Lagos Complex rocks aimed to identify the crystallization ages of this complex. The U-Pb ages obtained are: sample CRL-01, 1982 ± 9 Ma; sample CRL-02, 1958 ± 5 Ma; sample CRL-03, 1971 ± 16 Ma; sample CRL-07, 1964 ± 12 Ma; sample CRL-08, 1975 ±8 Ma; sample CRL-09, 1981 ± 15 Ma; sample CRL-10, 1984 ± 12 Ma; and sample CRL-12, 1957 ± 10 Ma.Complementary studies using the Lu-Hf method indicate ε_Hf values of the sample CRL-03 between -7,8 and -4,6 and T_DM values between 2,87 and 2,71Ga;ε_Hf values of the sample CRL-07 between -9,7 and -5,0 and T_DM values between 3,79 a 1,96 Ga. The ε_Hf values of the CRL-08 sample between -11 and -4,6 and T_DM values between 3,04 and 2,8; for the CRL-09 sample, the ε_Hf values are between -11,1 and -4,6 and T_DM values between 3.2 and 1,94 Ga.

These results suggest that the Região dos Lagos complex crystallizes from 1981 Ma to 1957 Ma and formed from crustal protoliths whose ages of mantle extraction are between 2.40 and 2.59 Ga. These results suggest that the Lagos Region complex crystallized from 1981 Ma to 1957 Ma and the rocks are formed from crustal protoliths whose ages of extraction of the mantle are divided into two groups, an older one with Archean age (between 3.0 and 3.5 Ga). The second group of samples indicates an isotopic evolution of Hf consistent with a source whose mantelic extraction occurred between 2.5 and 3.0 Ga and have negative ɛHf values, suggesting a mantle source with important crustal participation in the origin of these magmas. In addition, these rocks have different crystallization and mantle extraction ages than the rocks of the Juiz de Fora Complex, but they have similarities with the basement rocks of the Congo craton and may have genetic correlation with this unit in Africa. The results are important to characterize different basement units and better understand the crustal evolution of Ribeira Belt during Gondwana amalgamation.

The geological evolution proposal presented in this work is supported by U-Pb data acquired in MultiLab laboratory and information obtained in the literature, including descriptions of lithostratigraphic units, geochronological, geochemical and structural data. In this sense, the Paleoproterozoic rocks of the Juiz de Fora Complex can be related to rocks formed in a magmatic arc environment. In this way,  the Paleoproterozoic rocks of the SE of the São Francisco craton were generated in three episodes of subduction comprising distinct magmatic arcs between 2.3 and 2.05 Ga.

The T_DM ages obtained from the basement rocks (2.03 - 1.96 Ga) are lower than the ages of the Paleoproterozoic basement of the Ribeira Belt (2.2 - 2.15 Ga) and the Paleoproteozoic basement rocks of the Ribeira Belt (2 .2 - 1.9 Ga). Thus, a correlation is suggested with the rocks of the Kaoko belt, on the west coast of Namibia (Africa), which consist of type I metagranitoids whose crystallization occurred between 1.98 and 1.96 Ga.