Petrogênese do complexo alcalino Tanguá (RJ): novo mapa litofaciológico e estudos geocronológicos e isotópicos

Abstract

The Tanguá Alkaline Complex is in the Rio de Janeiro state and is of the Poços de Caldas – Cabo Frio Alignment (PCCFA) context, intruding rocks from the Ribeira Belt. The newly proposed lithofacies map for the Complex suggests that the inner portion is predominantly composed of nepheline syenites, which are further divided into central (with higher analcime content) and intermediate (with higher albite content) types. The outermost portion comprises indivisible syenites, without nepheline, and magmatic breccias on the southern and eastern edges. Additionally, phonolite and trachyte occur as dikes cutting through the other rock types. The rocks of the massif are classified as metaluminous and, to a lesser extent, as peraluminous. According to the TAS diagram, the rocks are mainly classified as syenites and phonolites, and according to the R1-R2 diagram, as phonolite, trachy-phonolite, trachyte, and quartz-trachyte. indicate that the syenites plot between a Depleted Mantle Source (DMM) and Enriched Mantle 1 (EM1), while the phonolite plots between DMM and Enriched Mantle 2 (EM2). These trends are typical of the Alignment and may suggest a mixing of sources for these rocks. The negative εNd parameters (between - 3.17 and - 2.31) and εHf, which vary between positive and negative (from – 17.78 to + 11.49), further support the participation of these sources.. The variation observed for the NdTDM model ages (670 - 860 Ma) is similar to other PCCFA complexes and is close to the Brasiliano collisional events. On the other hand, the HfTDM model ages present a more significant variation (280 - 1570 Ma), which may suggest an association with geological events before the formation of the Ribeira Belt. The εNd and εHf results, combined with geochemical data (Zr/TiO2 versus SSI, and SiO2 and ETR versus εNd), indicate significant crustal contamination during the massif's formation. This process is evidenced by the spatial distribution of the rocks within the Complex: nepheline syenites at the center, associated with the magma's original composition, and syenites at the margins, formed due to contamination by crustal rocks. Furthermore, the distinct behavior of isotopic parameters (Nd and Hf) indicates the presence of metasomatized magmas and sediments associated with oceanic slabs subducted during the massif formation, which is reinforced by different geochemical parameters, such as Th/Yb versus Ba/La and U/Th versus Th. Geochronological studies by U-Pb suggest two main pulses during the formation of the complex, a Cenomanian (ca. 95 Ma) and a Danian-Maastrichtian (ca. 60-70 Ma). The latter is supported by a larger dataset, consistent with previous studies, and indicates the primary emplacement period. The intrusion of phonolites occurred syn-genetically with the syenites of the most recent period, a relationship confirmed by field observations. Furthermore, the age of 94.8 Ma shows that the conventional plume model may not be the most suitable for the genesis of the PCCFA, as it is an older massif further east. Furthermore, the dikes and massifs following regional trends underscore the significance of pre-existing structures in the alignment's formation process.