Tanguá Magmatic Complex mantle sources and insights into its petrogenesis

The Tanguá Complex is an alkaline intrusion associated with the Poços de Caldas-Frio Alignment (PCCFA), in the Rio de Janeiro State. This massif is mainly formed by nepheline syenites and syenites, which show a concentric arrangement. In addition to the main lithologies, phonolite and trachyte dikes crosscutting the massif, monzonite, and magmatic breccias are identified, the last one especially in the eastern and southern zones, which are in contact with the country rock. The complex's rocks were formed in two distinct periods, one older with approximately 95 Ma, and another more recent with widespread ages between 60 Ma and 70 Ma. Sr-Nd and Lu-Hf methods were acquired using Thermal Ionization Mass Spectrometry (TIMS) multicollector and Laser ablation inductively coupled plasma mass spectrometry (LA–ICP-MS) equipment respectively. Six samples were studied, four syenites, one nepheline-syenite, and one phonolite. Related to the Sr-Nd isotopes, all lithotypes present negative εNd results. Syenite shows εNd between - 3.17 and - 2.62, with model ages (TDM) between 772 Ma and 862 Ma. The nepheline-syenite presents εNd data of - 2.31 and a TDM age of 677 Ma, and the phonolite has εNd value of - 2.77 and a TDM age of 725 Ma. From the ¹⁴³Nd/¹⁴⁴Nd versus ⁸⁷Sr/⁸⁶Sr diagram, it is noted that the nepheline syenite and syenite samples plot into the Enriched Mantle I (EMI) and Depleted Mantle (DMM) array, while the phonolite plots in the Enriched Mantle II (EMI) and DMM array. Lu-Hf geochronological analyses were carried out on zircons from eight samples (one nepheline syenite, five as syenite, one monzonite, and one phonolite), four of which coincided with the previous method. Nepheline syenite has εHf values between - 14.01 and - 2.36, syenites between - 14.26 and + 11.49, monzonite between - 17.78 and + 1.51 and phonolite between - 6.76 and + 7.30. The presence of negative values in all lithotypes also indicates the association with enriched sources, corroborated by the ¹⁷⁶Hf/¹⁷⁷Hf versus ¹⁴³Nd/¹⁴⁴Nd correlation diagram. However, the positive εHf values in all lithotypes, except for nepheline syenite, suggest the presence of depleted mantle sources, which considering the negative values, indicate an influence of crustal assimilation during melt generation. Based on the isotopic results, it is possible to propose that the syenitic rocks, and possibly monzonite, were formed from the same mantle source, in which liquid was generated from a mixture between DMM and EMI, while the phonolite has a distinct source and is linked to a mixture from the components DMM and EMII. Additionally, the data indicates that all lithotypes present a crustal influence and an association with sediment recycled in subduction zones. Furthermore, nepheline syenites show only negative εHf values while syenites have both negative and positive values. Thus, it is suggested that the first rocks' group, relatively more undersaturated in silica, presenting only normative nepheline, has less crustal assimilation when compared to the second group of rocks, the syenites, which are divided into three subgroups: (1) presenting nepheline normative, (2) with quartz normative, and (3) another with none of these normative minerals, suggesting different proportions of crustal assimilation. Chemical and mineralogical results, as well as different works carried out in this massif and others from the PCCFA, corroborate the presence of crustal assimilation and fractionation (AFC) process during the massif emplacement, as well as the arrangement of the rocks in the massif.