Shoshonitic Magmatic Series and the High Ba-Sr Granitoids: A Review with Emphasis on Examples from the Neoproterozoic Dom Feliciano Belt of Southern Brazil and Uruguay.

Fractional crystallization of parental magmas of shoshonitic or silica-saturated, ultrapotassic affinity, with variable amount of concurrent crustal assimilation, may result in granitic and syenitic rocks. Typical plutonic members of the shoshonitic series are monzonites and quartz monzonites, whilst syenites and quartz syenites are the dominant plutonic products of the ultrapotassic series. Lamprophyric magmas are commonly found in association with both series and are frequently part of coeval mingling/mixing systems. Ultrapotassic and shoshonitic primary magmas, including lamprophyric ones, are derived from amphibole-phlogopite-bearing mantle sources produced by previous, subduction-related metasomatism. Acidic and intermediate rocks can be derived from such parental magmas, generally through AFC processes. Shoshonitic-like granitoids, which have not clear relation with intermediate or basic shoshonitic rocks, or are produced dominantly by crustal melting, should be named high-Ba-Sr granitoids. This study focuses mainly on Neoproterozoic shoshonitic and silica-saturated ultrapotassic rock associations formed in post-collisional settings from southern Brazil and Uruguay. The source of magmas, their evolution, the role played by crustal contamination in modifying pristine geochemical signatures and their tectonic control are discussed based on elemental and Sr-Nd isotope geochemistry. The main features of plutonic rocks related to the shoshonitic series are their potassic, silica-saturated alkaline character, predominance of monzonitic to syenitic compositions, high Sr and Ba contents, monotonous, light REE-enriched patterns, and moderate HFSE contents. Syntectonic shoshonitic and high Ba-Sr granitoids within shear zones show lower alkali, LREE, HFSE, and Sr contents than those formed away from the main deformation sites. Plutonic rocks related to the extended silica-saturated ultrapotassic series are mostly syenites, alkali-feldspar granites and lamprophyres with K₂O/Na₂O ratios above 2. The typical values of ⁸⁷Sr/⁸⁶Sri for shoshonitic plutonic rocks are 0.706–0.708, ranging from 0.704 to 0.710. The ε_Nd(t) values are negative and vary from 0 to −24. Crustal contribution tends to increase ⁸⁷Sr/⁸⁶Sri and decrease ε_Nd(t) values, depending on protolith isotope signature, melting conditions and volume of assimilated material. Ultrapotassic rocks, on the other hand, show higher ⁸⁷Sr/⁸⁶Sri ratios, from 0.709–0.711 up to 0.720. Geochemical evidence, including Sr-Nd isotope data, indicates that the shoshonitic and ultrapotassic rocks discussed in this study were formed from OIB-like sources with strong influence of previous subduction, probably a phlogopite, K-amphibole bearing veined mantle. Lithological variability in ultrapotassic-shoshonitic associations is interpreted to result from (i) variation of source composition, (ii) different melt fractions from similar sources, (iii) mixing-mingling, fractional crystallization, and assimilation processes. [ABSTRACT FROM AUTHOR]