Nd and Sr Isotopic, Geochemical and Petrographic Characteristics of the Morro de São João Alkaline Intrusion: Geodynamic Implications and the Composition of the Sub-lithosperical Mantle

The Morro de São João Alkaline Complex (MSJ) is located in the NE part of Rio de Janeiro State and shows pronounced topography, with the following lithology: coarse-grained felsic syenites (K-feldspar, nepheline, hornblende, titanite and pseudoleucite), and mafi c syenites (with K-feldspar, hornblende and piroxene). Petrography and lithogeochemistry data suggest a bimodal suite. Felsic and mafi c magma mixing and mingling textures are locally observed indicating thepresence of coeval magmas. Nd and Sr isotope compositions correspond to EMI (Enriched Mantle I) mantle reservoir signatures,suggesting an asthenospheric enriched mantle source with low 87Sr/86Sr values (from 0.7049 to 0.7061) and low 143Nd/144Nd (from 0.512361 to 0.512428). The εNd values range from -4.03 to -5.54, indicating an anomalous enriched mantle reservoir, confi rmed by the high TDM values between 730 - 830 Ma, very different from the 72 - 56 Ma K-Ar cooling ages. The comparison of Sr and Nd signatures with related hotspot traces (Trindade, St. Helena and Tristan da Cunha) indicates that they are similar to those of Tristan da Cunha, but somewhat different from those of Trindade and St. Helena