The Santanapolis Syenite: Genesis and evolution of Paleoproterozoic shoshonitic syenites in northeastern Brazil

The Paleoproterozoic terrane of the state of Bahia, Brazil, is characterize d by an important magmatic event that resulted in the generation of large a nd numerous syenite complexes; the terrane constitutes one of the largest s yenite provinces in the world. It includes the Itiuba (1,800km(2)), Santana polis (180 km(2)), Sao Felix (32 km(2)), and Anuri (70 km(2)) complexes. Th ese syenitic bodies of shoshonitic: affiliation are silica saturated, metal uminous, and exhibit: (1) moderate to high K2O/Na2O ratios, (2) relatively high MgO contents (up to 3.0%); (3) high incompatible-element contents, par ticularly LILE and LREE; (4) high HREE/LREE fractionations; and (5) small E u negative anomalies. Among these syenites, Santanapolis exhibits the great est lithological diversity, with a porphyritic and a heterogranular facies, and diverse lithotypes that vary from leucosyenites to melanocratic syenit es of cumulate origin, as well as enclaves and dikes. It intruded syntecton ically at 2.1 Ga and shows magmatic flow structures and subsolidus recrysta llization of quartz and alkali feldspars. The mineralogical assemblage is p erthitic feldspar, diopside, hornblende, phlogopite-biotite, apatite, zirco n, titanite, calcite, and monazite. Differentiation of the Santanapolis Sye nite was controlled by now segregation processes in an intermediate trachyt ic magma, with fractionation of early crystallized diopside, apatite, and p lagioclase. The parental intermediate shoshonitic magma was extracted direc tly from an enriched, subduction-modified mantle characterized by anomalous concentrations of LREE and LILE elements, and isotopic parameters similar to the EM1-type.