Elemental and Sr–Nd isotope geochemistry of two Neoproterozoic mangerite suites in SE Brazil: implications for the origin of the mangerite–charnockite–granite series

Abstract

Neoproterozoic (∼625 Ma) mangerites are the dominant rock type in a ∼100 km long belt of deformed tabular bodies in the Socorro-Guaxupé nappe (SGN), a high-grade terrane southwest of the São Francisco craton, southeastern Brazil. The mangerites form two suites: São Pedro de Caldas shows a wide compositional variation and abundant felsic rocks (mangerite, hornblende granite, leucogranite), initial 87Sr/ 86Sr of ∼0.707 and initial ε Nd of −4 to −5; Divinolândia is dominated by more mafic mangerites with minor charnockite, granite and diorite, and has initial 87Sr/ 86Sr of ∼0.710 and initial ε Nd of −10 to −12. The probable sources of the São Pedro de Caldas mangerites were granulites depleted by previous melting events in radiogenic elements and the light rare earth element. Primary melts of São Pedro de Caldas were generated at ∼950 °C, P∼10 kbar from a biotite-free and K-feldspar-bearing source with plagioclase as an important residual phase. The amount of melting was low, as required by low mg#, enrichment in Nb and Ta relative to potential sources and high Nb/Ta, and was probably controlled by exhaustion of K-feldspar in the source. Contribution from infracrustal mafic melts cannot be ruled out and would help explain the weakly negative ε Nd and high Nb, but is not essential. The Divinolândia rocks depart from typical ‘within-plate’ mangerites towards more mafic rocks with ‘orogenic’ signatures (e.g., higher mg#, Sr, Ca; lower Zr). Radiogenic isotope data require sources with greater crustal residence time than those envisioned for São Pedro de Caldas; elemental data require that these sources were more depleted (e.g., higher K/Rb). Granulites with these characteristics are exposed in the northern portion of the SGN. These were probably melted at slightly higher temperatures than the São Pedro de Caldas suite (∼970 °C) before a sufficient amount of melt was generated and the magmas could leave their sources. In this case, K-feldspar was exhausted, and plagioclase, in amounts exceeding those of the granite minimum, was dissolved.