New Insights into Nd-Hf Isotopic Data and U-Pb Ages from the Tanguá Massif: Implications for the Magmatic Evolution

The Tanguá Massif (TM) is an alkaline intrusion that is part of the Poços de Caldas-Cabo Frio Alignment (PCCFA), which comprises more than 25 intrusive bodies and extends over 1000 km along a WNW-ESE trend in southeastern Brazil. This study presents new insights into the evolution and genesis of the TM based on updated geological mapping, combined with mineralogical, petrographic, lithogeochemical, geochronological, and isotopic analyses. A new lithofacies map is proposed for the Tanguá Massif, subdividing the massif into five main units: i) central nepheline syenite; ii) intermediate nepheline syenite; iii) syenite; iv) breccias; v) undivided nepheline syenite. Additionally, phonolites and trachytes occur as dikes crosscutting the massif. U-Pb geochronological data reveal two distinct age groups: an older Cenomanian phase (94.8 Ma) and a younger Danian-Maastrichtian phase (ca. 60-70 Ma). The presence of an older syenite aged than previously reported – the oldest age for the PCCFA – suggests that the conventional mantle plume model may not adequately explain the origin of this province. Sr-Nd and Lu-Hf isotopic signatures indicate that the syenites and nepheline-syenites plot in the DMM-EMI array, whereas phonolite in the DMM-EMII array, suggesting mixing of different mantle components according to other studies. Geochemical and isotopic parameters (e.g., SSI, Zr/TiO2, REE sum, and εNd) highlight the significant role of crustal assimilation during the evolution of the TM, a process also proposed for other PCCFA bodies. Furthermore, ratios such as Th/Yb and Ba/La, combined with Nd-Hf isotopic decoupling, suggest the involvement of oceanic sediments associated with subducted slabs in the genesis of the magmas. These findings provide new insights into the magmatic evolution of the Tanguá Massif and contribute to a broader understanding of the processes controlling the formation of the PCCFA alkaline province.