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Volume 32 Issue 6
Dec.  2021
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Lauro Valentim Stoll Nardi, Maria de Fátima Bitencourt, Luana Moreira Florisbal, Dionatan Ferri Padilha. 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. Journal of Earth Science, 2021, 32(6): 1359-1373. doi: 10.1007/s12583-021-1534-8
Citation: Lauro Valentim Stoll Nardi, Maria de Fátima Bitencourt, Luana Moreira Florisbal, Dionatan Ferri Padilha. 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. Journal of Earth Science, 2021, 32(6): 1359-1373. doi: 10.1007/s12583-021-1534-8

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

doi: 10.1007/s12583-021-1534-8
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  • 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 εNd(t) values are negative and vary from 0 to -24. Crustal contribution tends to increase 87Sr/86Sri and decrease εNd(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.
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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

doi: 10.1007/s12583-021-1534-8

Abstract: 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 εNd(t) values are negative and vary from 0 to -24. Crustal contribution tends to increase 87Sr/86Sri and decrease εNd(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.

Lauro Valentim Stoll Nardi, Maria de Fátima Bitencourt, Luana Moreira Florisbal, Dionatan Ferri Padilha. 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. Journal of Earth Science, 2021, 32(6): 1359-1373. doi: 10.1007/s12583-021-1534-8
Citation: Lauro Valentim Stoll Nardi, Maria de Fátima Bitencourt, Luana Moreira Florisbal, Dionatan Ferri Padilha. 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. Journal of Earth Science, 2021, 32(6): 1359-1373. doi: 10.1007/s12583-021-1534-8
  • Shoshonitic volcanic association is the term originally used by Joplin (1968) in reference to potassium-rich rocks that included leucite-bearing varieties. The concept was later changed by Morrison (1980), who named the absarokite-shoshonite- banakite volcanic series as shoshonite rock association and therefore excluded feldspathoid-bearing rocks. Jakeš and White (1972), and Gill (1970), among other authors, recognized rocks of shoshonitic affinity related to silica-saturated, mature island arc magmatism. Granitic associations included in the monzonitic or latitic series were described mostly by Russian authors (e.g., Tauson, 1983; Tauson and Kozlov, 1972). Tauson (1983) referred to the 'granitoids of monzonite series' as produced through differentiation of F- and K-rich alkalic basaltic magmas. Pagel and Leterrier (1980) proposed the shoshonitic affinity for gabbros to quartz-monzonites related to the monzonitic or latitic series that make up the Ballons Massif, France. Nardi (1984), and Nardi and Lima (1985) proposed a genetic link between granodioritic rocks and shoshonitic volcanic associations, thereby defining the Lavras do Sul Shoshonitic Association (LSSA) in southernmost Brazil. Potassic trachybasalts, potassic trachy-andesites (shoshonites), dacites, rhyolites, quartz diorites, monzodiorites, quartz monzonites, granodiorites, monzogranites, syenogranites, leucodioritic cumulates and spessartitic lamprophyres were included in the LSSA by Lima and Nardi (1998). Tauson (1983) reviewed the metallogenetic potential of granitoids from the shoshonitic or monzonitic series for Cu, Mo, Sn, W, Pb, Zn ore deposits, and their potential for epithermal and porphyry Cu-Au-(Mo) mineralizations has been emphasised by several authors (e.g., Yang et al., 2012; Zhang et al., 2001; Müller and Groves, 1993; Nardi and Lima, 1988; Nardi, 1986).

    The main chemical features, assumed by most authors and reviewed by Nardi(2016, 1986) for granitic rocks of the shoshonitic series are: (ⅰ) they belong to the silica-saturated, alkaline potassic series, as defined by Le Maitre (2002); (ⅱ) they show moderate contents of HFSE (e.g., Zr, Nb, Y, Ti); (ⅲ) they have high LILE (K, Rb, Pb), and particularly, Sr and Ba, as pointed out by Tauson (1983) and Nardi (1986) amongst other authors; (ⅳ) their K2O/Na2O ratios are mostly between 1 and 2. Acidic rocks that belong to the potassic series of Le Maitre (2002), with K2O/Na2O ratios greater than 2, are assigned to the ultrapotassic silica-saturated series (Plá Cid and Nardi, 2006; Plá Cid et al., 2000). Chondrite-normalized REE patterns are typically regular for rocks of variable differentiation degrees, and even Eu-negative anomalies are developed only for high-silica varieties (SiO2 > 73 wt.%). Therefore, the shoshonitic magmatic series is considered to be composed of rocks derived from liquids formed by mineral fractionation from a shoshonitic basic or intermediate parental magma, a process that may include assimilation and contamination by crustal melts. Monzonitic rocks, or their volcanic counterparts, latites, are the main components of the shoshonitic series, which is well illustrated on the QAP diagram displayed by Lameyre and Bowden (1982), even though under the name of 'calc-alkaline- monzonitic, high-K series'. Magmatic series and their connection to granitic rocks have been recently emphasised by Nardi (2016) following the view of granitoids as products of mantle and crust interaction, lately assumed by many authors (e.g., Arndt, 2013; Bachmann et al., 2007; Bonin, 2007; Barbarin, 1999). Nowadays, it is believed, following Le Maitre (2002), that the essential feature of shoshonitic rocks belong to the potassic (Na2O < K2O+2) silica-saturated alkaline series, as shown by that author in the TAS diagram.

    This paper aims to describe the major features of plutonic rocks from the shoshonitic series, emphasise their importance in post-collisional settings, describe their occurrence within shear zones, as syntectonic rocks, or post-tectonic associations out of deformational sites. The origin and source of shoshonitic magmatism are reviewed. We propose that the term "high Ba-Sr granitoids", which resemble shoshonitic ones, should be used for those generated by crustal melting, therefore, without relation with the shoshonitic series. This review is focused mainly on shoshonitic associations and high Ba-Sr granitoids from southernmost Brazil and Uruguay.

  • The shoshonitic series, as early recognized by some authors (e.g., Nardi, 1984; Tauson, 1983; Pagel and Leterrier, 1980; Tauson and Kozlov, 1972), includes large volumes of monzonitic to granitic rocks. Some of these plutonic varieties have been assigned to different tectonic settings of varying ages worldwide, as exemplified in the following cases.

    Eklund and Shebanov (2005) describe Paleoproterozoic post-collisional shoshonitic ring complexes situated along a NE-striking shear zone in southwestern Finland, where granites, monzonites and lamprophyres reflect the differentiation and fractional crystallization of shoshonitic magmas derived from an enriched lithospheric mantle. Eklund et al. (1998) describe intrusive shoshonitic rocks with compositions varying from 32 wt.% to 78 wt.% SiO2, situated from Finland to the Republic of Karelia in northern Europe. With ages between 1 875 and 1 770 Ma, these rock associations are post-collisional relative to the Svecofennian Orogeny and were formed by partial melting of subduction-modified mantle sources with negligible crustal contribution and subsequent differentiation dominated by fractional crystallization. Near the northern border of the Amazon Basin, Valério et al. (2017) described shoshonitic affinity monzonitic and syenitic rocks of ca. 1 872 Ma associated to post-collisional settings and produced, according to those authors based mainly on Lu-Hf isotope data, by partial melting of the crust. Goswami and Battacharyya (2014) referred to granitoids related to Proterozoic orogenic shoshonitic magmatism situated along the North and South Indian cratonic blocks. They form intrusions related to shear zones and post-collisional extensional structures and are fractional-crystallization products of mafic magmas derived from mantle sources modified by previous subduction.

    The Tismana Pluton (Duchesne et al., 1998), with crystallization age of 567±3 Ma, comprises plutonic rocks with 50 wt.% to 70 wt.% SiO2, including monzogabbros, monzonites, quartz monzonites, quartz monzodiorites, quartz syenites, monzogranites and granodiorites. This pluton is considered to have formed in a post-collisional setting. The parental magmas were produced from mantle sources affected by metasomatism related to the Pan-African Orogenic Cycle with negligible influence of crustal assimilation. Ferreira et al. (2015) described shoshonitic geochemical features in the Guarany monzodiorite-granite and diorites in a post-collisional setting from northeastern Brazil. These rocks show petrological, geochemical and isotopic characteristics that point to an origin by partial melting of lower crustal, incompatible-element enriched amphibolite source, triggered by underplating of mantle-derived mafic magma.

    The Tormes Dome discussed by López-Plaza et al. (1999) (ca. 320 Ma) is situated in the Iberian Variscan belt and comprises monzodiorite-monzonite-quartz monzonite-monzogranite rocks related to gently-dipping shear zones, described as belonging to the monzonitic series. Although the authors related them to a monzonitic series, they proposed that the parental magma of Tormes Dome rocks was produced by crustal melting at the base of the crust. The Baranadag quartz-monzonite (ca. 74 Ma) in central Anatolia, Turkey, was related by Köksal et al. (2004) to the post-collisional stages of the Alpine Orogeny and showed compositional features that indicate its shoshonitic affinity as referred by Liz et al. (2009). The parental magmas were generated by partial melting of metasomatised mantle sources and subsequently evolved through fractional crystallization with assimilation of crustal melts (Ilbeyli et al., 2004).

    The typical shoshonitic trend, as well illustrated by Lameyre and Bowden (1982) in the QAP diagram, and shown in the TAS diagram (Fig. 1), does not include syenitic rocks since the proportion of K-feldspar to plagioclase does not increase during differentiation of intermediate to acidic liquids. However, many authors have pointed out the shoshonitic affinity of plutonic associations that include syenitic rocks. Neoproterozoic syenitic rocks of shoshonitic affinity associated with granitoids were reported by Silva Filho et al. (1993), Guimarães and Silva Filho(1998, 1992), Nascimento et al. (2000), Campos et al. (2002), Fontes et al. (2018), Lisboa et al. (2019), among other authors, in the Borborema Province, Northeast Brazil. In this context, the granitoids are related to Neoproterozoic collisional to post-collisional settings. Conceição et al. (2000), Paim et al. (2002), Rios et al. (2007) described Paleoproterozoic syenitic massifs of shoshonitic to ultrapotassic affinity in Bahia, northeastern Brazil. Janasi et al. (1993) and Stabel et al. (2001) recognized the shoshonitic affiliation of syenitic rocks, sometimes associated with lamprophyres or ultrapotassic rocks in southern Brazil. A series of granitoids and less differentiated rocks of shoshonitic affinity and ages ranging from Proterozoic to Cenozoic occur in the western Kunlun orogenic belt, Xinjiang, northwestern China (Jiang et al., 2002).

    Figure 1.  Trends of shoshonitic rocks from southernmost Brazil on the TAS diagram (Le Maitre, 2002). LSSA. Lavras do Sul Shoshonitic Association.

    These rock assemblages include quartz-bearing monzo-diorites, monzonite, syenite, and granite produced mainly from shoshonitic magmas derived from different mantle sources with variable influence of subducted materials. Litvinovsky et al. (2002) concluded that the plutonic Cretaceous Oshurkovo Complex in South Siberia, including co-magmatic alkali monzodiorite, monzonite, plagioclase-bearing and alkali-feldspar syenites, was formed mainly by fractional crystallization of a tephritic magma produced by melting of subduction-modified mantle and small amounts of crustal assimilation.

    Therefore, the genetic association of syenites with shoshonitic series can be explained by two alternative models: (ⅰ) potassic and ultrapotassic magmas, including lamprophyric ones, are generated by partial melting of phlogopite- or K-rich, amphibole-bearing mantle (Carvalho et al., 2014; Weaver et al., 2013; Plá Cid et al., 2012, 2003, 2000; Prelević et al., 2012; Nardi et al., 2008, 2007; Conceição and Green, 2004, 2000; Williams et al., 2004; Conceição et al., 2000; Ferreira and Sial, 1993; Peccerillo, 1992), and (ⅱ) syenitic and monzonitic magmas represent liquids derived from lamprophyric or trachyandesitic potassic magmas by fractional crystallization (e.g., Rios et al., 2007; Paim et al., 2002; Melzer and Foley, 2000; Zhao et al., 1995; Janasi et al., 1993; Guimarães and Silva Filho, 1992; Leat et al., 1988). Mixing of mantle-derived magmas, sometimes of alkaline affinity, such as lamproite or tephrite, with crustal melts is frequently proposed as a model, particularly when acidic rocks are abundant (Ferreira et al., 2015; Silva Filho et al., 1993; Thompson and Fowler, 1986). Furthermore, the frequent presence of lamprophyres-spessartite and minette-forming coeval intrusions in shoshonitic or ultrapotassic associations suggests they share common sources and genetic processes.

  • Shoshonitic magmas emplaced along shear zones are prone to show crustal contamination to different extents since high-strain zones are suitable sites to dilute or even erase the register of least evolved members and highlight crustal assimilation. Also, in some cases, purely crustally-derived rocks that resemble some of the shoshonitic geochemical signatures, as the high Ba-Sr granitic rocks, are likely to be generated in high-strain zones. Such cases are well exemplified in southernmost Brazil by the Neoproterozoic Itapema granite (Martini et al., 2019; Bitencourt and Nardi, 2004; Rivera et al., 2004), Neoproterozoic Estaleiro granodiorite (Bitencourt and Nardi, 1993), Cruzeiro do Sul granodiorite and later dikes (Knijnik, 2018; Knijnik et al., 2012), orthogneisses from the Tinguí Unit (Martins et al., 2016), and in Uruguay (Lara et al., 2017).

    The Itapema granite, a Neoproterozoic syntectonic intrusion whose emplacement was controlled by thrusting under high-grade metamorphism, comprises hornblende-biotite monzogranites to granodiorites that host abundant country-rock xenoliths and irregularly-distributed mafic aggregates. According to Bitencourt and Nardi (2004), it was produced by partial melting of amphibolitic orthogneisses under minimum temperatures of 850 ℃ and medium to high pressure. Its lower alkali contents compared to typical shoshonitic associations, the lack of basic or intermediate associated rocks, and intimate temporal association with high-grade gneisses and migmatites indicate that it was crustally derived (Martini et al., 2019).

    The Estaleiro granitic complex (Bitencourt and Nardi, 1993), with 612 Ma (Peruchi et al., 2021), comprises a granodiorite intrusion with numerous veins of granitic composition, mafic microgranular enclaves and sheet-like bodies, syntectonic to a Neoproterozoic shear zone related to the post- collisional stage of Brasiliano/Pan-African orogenic cycle in South Brazil. The geochemical signature of these rocks, with high Sr and Ba contents, moderate HFSE and REE contents, whose patterns show no Eu-anomalies, is similar to that of shoshonitic series granitic rocks with increasing amounts of crustal contribution, mainly in the more evolved varieties.

    Knijnik et al. (2012) and Knijnik (2018) identified diorites to granites and late granodiorite to monzogranite dikes, both syntectonic to post-collisional transcurrent shear zones from the eastern part of Dom Feliciano belt, southernmost Brazil, with ages of ca. 634 and 605 Ma, respectively. They were recognized as of shoshonitic affinity and interpreted as produced through AFC processes, with mixing of mantle and crustal components. Their slightly lower contents of Sr and alkalies relative to typical shoshonitic associations are ascribed to the assimilation of crustal melts.

    The Tinguí Unit, situated in the central Ribeira belt (Martins et al., 2016), comprises orthogneisses and later dikes of shoshonitic affinity, with magmatic protoliths varying from quartz diorite to granodiorite crosscut by tonalitic to granodioritic dikes with crystallization ages of 551±3 Ma, for the orthogneiss, and of 495±4 to 488±4 Ma for the late dikes. Those authors interpreted the magmatism as pre- to post-collisional relative to the Buzios Orogeny (Schmitt et al., 2004) and formed by the interaction of mantle sources previously modified by the Buzios subduction and partial melting of lower crustal sequences.

    Lara et al. (2017) described post-collisional high Ba-Sr granitic magmatism of shoshonitic affinity in the southern section of the Dom Feliciano belt and its cratonic foreland in Uruguay, with magmatic crystallization ages varying from 634±7 to 597±4 Ma, all of them intrusive in either high- or low-strain zones of Neoproterozoic shear zones. Hornblende- and titanite- bearing quartz-monzonites, granodiorites and monzogranites are the most abundant rock types. They share geochemical features typical of shoshonitic granitoids, as K2O/Na2O ca. 1, high Ba (> 700 ppm) and Sr (> 500 ppm) contents, moderate fractionation of REE, high LREE contents, negligible Eu anomalies, and moderate contents of HFSE (e.g., Nb, Zr, Hf, Ta, Y). Initial 87Sr/86Sri values of 0.707 7 to 0.709 0, εNd(t) values of -15.8 to -19.3 at 600 Ma, with high Nd TDM (2.2–2.8 Ga) are reported by Lara et al. (2017) and interpreted as indications of recycling of ancient Paleoproterozoic to Late Archaean sources. Younger granitic intrusions, as the Solis de Mataojo granitic complex (584±13 Ma), Las Flores (586±2.7 Ma), and the quartz-monzodiorite intrusion named Sobresaliente Pluton (585±2.5 Ma) described by Oyhantçabal et al. (2012), increase the time span of shoshonitic magmatism in Uruguay (ca. 634–584 Ma). Its stratigraphic correlation with the Lavras do Sul Shoshonitic Association in southernmost Brazil is also supported by their magmatic crystallization ages.

  • The Lavras do Sul Shoshonitic Association (LSSA) (Lima and Nardi, 1998; Nardi and Lima, 1985) is a plutono-volcanic association of minor potassic trachybasalts (absarokites), followed by a thick sequence of shoshonitic lavas and pyroclastic rocks, cut by porphyritic to heterogranular monzonitic rocks and later basic to acidic dikes. Plutonic rocks vary from diorite to biotite syenogranite and are intruded into the basal portion of the volcanic sequence and older granulitic rocks, as described by Barros and Nardi (1994). In the QAPF diagram, the major lithological groups of LSSA plot along the monzonitic trend, as defined by Lameyre and Bowden (1982), also invading the granodiorite field. The emplacement and evolution of LSSA granitic rocks were discussed by Nardi (1984) and Gastal et al. (2015). Hornblende-bearing lamprophyres, spessartites, are mostly associated with the late monzonitic dikes, where several mingling features are recorded (Lima and Nardi, 1998, 1991). The LSSA granodiorites contain mafic microgranular enclaves, which attest to the coexistence of granodioritic liquids of shoshonitic affinity with basic magmas related to the sodic alkaline, silica-saturated series (Nardi and Lima, 2000). Their origin is probably related to A-type granitic magmatism that commonly follows the shoshonitic one. Monzonitic and quartz monzonitic rocks included in the LSSA were described by Liz et al. (2009) and Gastal and Lafon (2006). They form two main intrusive bodies crosscut by several monzonitic dikes with coeval lamprophyres, mostly spessartites (Müller et al., 2012; Liz et al., 2004; Lima and Nardi, 1998). The Cu-Au-sulphide mineralizations were formed by high- and low-temperature hydrothermal events (e.g., Fontana et al., 2017; Mexias et al., 2005; Nardi and Lima, 1988). Geochronological studies (e.g., Gastal et al., 2015; Liz et al., 2009; Gastal and Lafon, 2006; Remus et al., 2000) indicate that the multi-intrusive LSSA system was formed at ca. 600 Ma. Other volcanic and plutonic rocks of shoshonitic affinity have been described in southernmost Brazil and correlated to the LSSA (Lopes et al., 2014; Sommer et al., 2006, 2005; Wildner et al., 1999; Barros and Nardi, 1994).

    Most authors assume the origin of LSSA magmatism (Liz et al., 2009; Gastal and Lafon, 2006; Lima and Nardi, 1998) as involving partial melting of a mantle source modified by fluids produced during previous subduction related to the Brasiliano/ Pan-African Orogenic Cycle. The contribution of crustal materials is probably restricted to that resulting from subduction of oceanic crust or delamination. Fractional crystallization dominated by olivine-clinopyroxene-plagioclase+(Fe-Ti oxides+ apatite) has controlled the evolution from basic to intermediate liquids, whilst differentiation to acidic liquids involved fractionation of plagioclase-amphibole-biotite-Ti magnetite (Lima and Nardi, 1998). Syenitic or trachytic rocks have not been described in the LSSA.

    The post-collisional Piquiri Syenite Massif (PSM, Rivera, 2019) in southernmost Brazil is composed of four magmatic pulses, resulting in (ⅰ) ultrapotassic syenites and lamprophyres (609 Ma), (ⅱ) shoshonitic syenites (603 Ma), (ⅲ) shoshonitic quartz syenites and granites (584 Ma). Stabel et al. (2001), Plá Cid et al. (2003) and Nardi et al. (2008), amongst other authors, have admitted that the magmatic source of lamprophyric and syenitic magmas from PSM is a phlogopite-bearing, peridotitic veined mantle previously metasomatised by fluids and melts produced during lithosphere subduction. The lamprophyric source was at great depths, corresponding to pressures over 5 GPa (Plá Cid et al., 2003) and mingling occurred with ultrapotassic syenitic melts, generating the first pulse still at mantle depths. The Nd and Sr isotope data indicate a more radiogenic source for the ultrapotassic pulse, whilst the subsequent pulses show similar values for εNd(t) and 87Sr/86Sri indicative of less enriched sources.

    Monzonitic, syenitic, dioritic and granitic rocks of shoshonitic affinity, and ultrapotassic lamprophyres form the Arroio do Silva Pluton (ASP) (Padilha et al., 2019) with ca. 578 Ma age (LA-MC-ICP-MS, U-Pb zircon). The ASP shoshonitic rocks are compositionally similar to pulses (ⅱ) and (ⅲ) of the Piquiri Syenite Massif. The main differentiation process from dioritic to syenitic rocks was mixing and mingling of dioritic, syenitic and lamprophyric magmas, as proposed by Padilha et al. (2019). Magmatic sources are similar to those proposed for the Piquiri Syenite Massif by Plá Cid et al. (2003).

  • Based upon a selection of 115 representative samples from southern Brazil shoshonitic associations, including basic to acidic rocks and shoshonitic-like granitic rocks, some geochemical parameters were calculated (Table 1 and Fig. 2).

    Geochemical parameters Syntectonic granitoids Southernmost Brazil (A) ASP dioritic- monzonitic rocks (B) Monzonitic rocks, LSSA (C) Qz syenites, MSP (D) LSSA late dikes (E) LSSA lamprophyres (F)
    SiO2 (wt.%) 66.76 55.64 58.33 67.44 64.62 56.06
    K2O/Na2O 1.04 1.36 1.00 1.66 0.96 0.37
    K2O+Na2O (wt.%) 6.8 7.74 7.76 10.11 8.56 7.06
    K2O (wt.%) 3.45 4.41 3.83 6.31 4.19 1.89
    Sr (ppm) 495 1 312 889 1 356 936 528
    Ba (ppm) 932 2 700 1 508 2 516 1 555 1 209
    Nb (ppm) 12.4 20 18 35 9.10 6.7
    La/Nb 3 3.96 3.25 2.87 4.62 3.25
    LaN/YbN 20 31 32 59 42 32
    LaN 157 330 246 418 175 246
    (A) Syntectonic Estaleiro Granodiorite (Bitencourt and Nardi, 1993) and Cruzeiro do Sul Granodiorite (Knijnik et al., 2012); (B) Arroio do Silva Pluton dioritic and monzonitic rocks (Padilha et al., 2019); (C) Lavras do Sul Shoshonitic Association (LSSA) monzonitic rocks (Liz et al., 2009); (D) Piquiri Syenite Massif syenites and quartz-syenites (Rivera, 2019); (E) LSSA late latitic dikes and (F) LSSA lamprophyres (Müller et al., 2012).

    Table 1.  Representative values of diagnostic geochemical parameters of shoshonitic rocks from southernmost Brazil

    Figure 2.  Illustration of some diagnostic geochemical parameters of shoshonitic rocks from southernmost Brazil.

    It is of particular relevance the average values of K2O/Na2O ca. 1 for all samples, K2O from 1.89 wt.% in lamprophyres, 3.5 wt.% to 4.5 wt.% in granitic and monzonitic rocks, and 6.3 wt.% in syenitic ones, Sr > 500 ppm, Ba > 900 ppm, LaN > 150, Nb < 35 ppm. Samples from associations that are unrelated to shear zones are metaluminous and plot in the TAS field of silica-saturated alkaline series (Figs. 1 and 3). In the R1-R2 diagram (Fig. 4), they plot along the monzonitic or shoshonitic trend. Their REE patterns, in general, show LaN varying from 100 to 500, YbN from 2 to 10, and no Eu anomalies (Fig. 5). Trace element patterns are closer to those of OIB than to other types of sources, with enrichment of LILE and LREE and prominent negative Nb, Ti, and P anomalies. Spidergrams displayed in Fig. 6 show very coherent patterns for all samples, suggesting that they represent rocks formed through similar processes from sources of similar composition.

    Figure 3.  Distribution of shoshonitic rocks from southernmost Brazil in Shand's diagram (after Maniar and Piccoli, 1989).

    Figure 4.  Shoshonitic associations from southernmost Brazil plotted on R1-R2 diagram. 1. Monzo-gabbro; 2. monzonite; 3. monzo-diorite; 4. quartz monzonite.

    Figure 5.  REE normalised by chondritic values (Boynton 1984), for (a) Arroio do Silva Pluton and Piquiri Syenite Massif samples, (b) Lavras do Sul Shoshonitic Association rocks, and (c) Granitoids emplaced along active shear zones.

    Figure 6.  Spidergrams for trace elements normalised by OIB values (Sun and McDonough, 1989) for (a) Arroio do Silva Pluton and Piquiri Syenite Massif samples, (b) Lavras do Sul Shoshonitic Association rocks, and (c) granitoids emplaced along active shear zones.

    Syntectonic shoshonitic associations within shear zones show lower contents of alkalies. They are subalkaline according to the TAS diagram (Fig. 1), metaluminous to peraluminous (Fig. 3), and most of them plot in the R1-R2 fields of granodioritic rocks (Fig. 4). In addition, they show lower LREE, MREE, and higher LILE contents, but the HREE keeps the same values as displayed by samples from associations unrelated to shear zones (Figs. 5 and 6). These features can reveal the participation of crustal melts and, when they are predominant, the connection with the shoshonitic parental magma can be lost; in such cases, the granitoids should be classified as high Ba-Sr rocks.

    The good correlation shown by pairs of elements such as K, Nb, La, Zr in the entire set of samples suggests that all magmas emplaced either within or outside shear zones are derived from similar sources. On the other hand, crustal assimilation is more significant in syntectonic shoshonitic-like granites, as indicated by increased peraluminousity, decreased alkalies, HFSE, LREE, MREE, and some LILE, such as Sr and K.

    The Sr-Nd isotopic signatures of the shoshonitic and shoshonitic-like rocks emplaced outside and within shear zones (Fig. 7) show variable 87Sr/86Sri values from 0.704 up to 0.719, with mean concentration of 0.705 to 0.710. The LSSA rocks have the lowest 87Sr/86Sri values around 0.704, distinct from all the other associations. The εNd(t) values are all negative, with mean concentrations around -8 to -12. Again, LSSA is an exception with the least negative εNd(t) values (-9 to 0). The Estaleiro granitic complex also shows distinct εNd(t) values (-3 to -5) compared to other shoshonitic-like syntectonic granites.

    Figure 7.  87Sr/86Sri vs. εNd(t) values for the shoshonitic associations from southernmost Brazil. 1. Williams et al. (2004); 2. Hegner et al. (1998); 3. Miller et al. (1999); 4. Martins et al. (2016); 5. Duchesne et al. (1998); 6. Carvalho et al. (2014); 7. Rios et al. (2007).

  • The Sr-Nd isotope data from 141 representative samples from shoshonitic, ultrapotassic, shoshonitic-like and high Ba-Sr rocks, including basic to acidic, of varied ages were used to discuss the isotopic signature and the origin of granitoids and syenitic rocks. Most 87Sr/86Sri values vary from 0.704 to 0.710, whilst εNd(t) values show a large range from 0 to -24. No correlation of 87Sr/86Sri and εNd(t) isotope values is observed for different post-collisional magmatic associations displayed in Table 2, as it would be expected if crustal assimilation were the only cause for the variation of both parameters. When crustal melting is recognized as significant in the generation of intermediate to acidic shoshonitic-like and/or high Ba-Sr rocks, frequently when they are syntectonic associations, there is a tendency to increase 87Sr/86Sri and decrease εNd(t).

    Rock associations εNd(t) 87Sr/86Sri Age (Ma)
    1 0.5– -6 0.705–0.712 567
    2 -5– -7 0.708–0.709 67–80
    3 0.5 0.703 1–0.704 7 1 857–1 770
    4 -9– -14 0.719–0.735 18–25
    5* -2.9– -6.5 0.707–0.712 ca. 320
    6 -1.6– -6.8 0.708–0.719 ca. 330
    7a 0– -5 0.704 7–0.706 4 Tertiary
    7b 0– -9 0.705 9–0.711 5 Tertiary
    8 -4– -13 0.706–0.714 0.5–23
    9 -5 0.708 ca. 10
    10 -1.5– -6.8 0.708–0.710 ca. 330
    11* -10– -15 0.706–0.710 605–638
    12* -15.8– -19.3 0.708–0.709 598–604
    13 -0.2– -9 0.704–0.705 ca. 592
    14 -10– -14 0.701–0.714 ca. 592
    15 -2.4– -2.8 0.703–0.704 ca. 2 028
    16 -8– -10 0.707–0.708 ca. 610
    17* -4– -13 0.705–0.707 ca. 488–551
    18* -14– -16 0.710–0.715 573
    19 - 8– -12 0.709–0.711 609
    20 -8– -12 0.706–0.708 589–603
    21 -2– -3 0.707 ca. 611
    22* -3– - 5 0.709–0.711 ca. 611
    1. Tismana Pluton (Duchesne et al., 1998); 2. Baranadag (Ilbeyli et al., 2004); 3. Svecofennian post-collisional shoshonitic plutonism in the Fennoscandian Shield (Eklund et al., 1998); 4. potassic and ultrapotassic magmatism in Southwest Tibet (Miller et al., 1999); 5. Tormes Dome (López-Moro and López-Plaza, 2004); 6. Black Forest, Germany, post-collisional, Variscan potassic magmatism (Hegner et al., 1998); 7. Serbia, shoshonitic (7a) and ultrapotassic (7b) Tertiary rocks (Cvetković et al., 2004); 8. latitic rocks, Kunlun, Tibet (Williams et al., 2004); 9. northwestern Iran post-collisional minettes and spessartites; 10. post-collisional lamprophyres and rhyodacites, Germany (Hegner et al., 1998); 11. Cruzeiro do Sul granodiorite and granite dikes (Knijnik, 2018); 12. high Ba-Sr granodiorites (Lara et al., 2017); 13. Lavras do Sul shoshonitic association rocks of ca. 592 Ma (Gastal et al., 2005; Remus et al., 1997); 14. syenites and monzonites, NE Brazil (Guimarães and Silva Filho, 1998); 15. ultrapotassic syenites, Morro do Afonso, NE Brazil (Rios et al., 2007); 16. potassic and ultrapotassic syenites, São Paulo, South Brazil (Carvalho et al., 2014); 17. shoshonitic orthogneisses, Paraná, South Brazil (Martins et al., 2016); 18. crustally-derived, shoshonitic-like monzonitic-granitic rocks, Pernambuco, NE Brazil (Ferreira et al., 2015); 19. Piquiri Syenite Massif ultrapotassic syenites (Rivera, 2019); 20. Piquiri Syenite Massif shoshonitic quartz syenites (Rivera, 2019); 21. dioritic dykes, Estaleiro granitic complex, Santa Catarina, South Brazil (Chemale et al., 2012); 22. granodiorite and leucogranite dykes from Estaleiro granitic complex, Santa Catarina, South Brazil (Peruchi et al., 2021; Chemale et al., 2012); *. high Ba-Sr granitoids or shoshonitic rocks highly contaminated with crustal material.

    Table 2.  Sr-Nd isotopic parameters of collisional to post-collisional shoshonitic and ultrapotassic magmatism products and high-Ba, Sr granitoids of distinct ages (approximate values)

    The origin of shoshonitic (or potassic) and ultrapotassic plutonic intermediate to acidic rocks related to the shoshonitic and ultrapotassic magmatic series is addressed by two main genetic models, both proposing that their primary magmas represent melting products of mantle regions metasomatised by fluids and melts derived from subducted lithosphere (e.g., Miller et al., 1999; Guimarães and Silva Filho, 1998; Ferreira and Sial, 1993; Peccerillo, 1992). Syenitic or monzonitic shoshonitic and ultrapotassic magmas can be generated by direct partial melting of mantle sources. Liquids of such compositions, including granitoids, could also be derived from lamprophyric or intermediate magmas through fractional crystallization, sometimes with concurrent assimilation of crustal materials. Fractional crystallization of lamprophyric magmas leading to the generation of syenitic rocks has been admitted by many authors (e.g., Rios et al., 2007; Melzer and Foley, 2000; Janasi et al., 1993; Leat et al., 1988), whilst others assume that syenitic and monzonitic magmas would be directly extracted from metasomatised mantle regions (e.g., Carvalho et al., 2014; Weaver et al., 2013; Plá Cid et al., 2012, 2003, 2000; Nardi et al., 2008, 2007; Conceição et al., 2000; Peccerillo, 1992; Venturelli et al., 1984). The association of both ultrapotassic and shoshonitic magmatism has been recognized by several authors (e.g., Prelević et al., 2012; Guo et al., 2006; Miller et al., 1999; Silva Filho et al., 1993; Rogers, 1992; Thompson and Fowler, 1986; Venturelli et al., 1984; Civetta et al., 1981), sometimes including lamproitic rocks (Conticelli et al., 2009) when the influence of subduction is less evident (Avanzinelli et al., 2009). Ultrapotassic, shoshonitic and lamproitic magmas, therefore, may represent melts derived from subduction- affected mantle regions with different contents of phlogopite and K-amphibole (e.g., Weaver et al., 2013; Prelević et al., 2012; Maria and Luhr, 2008; Nardi et al., 2008; Plá Cid et al., 2003; Conceição et al., 2000). The experimental support for such models comes from Conceição and Green (2000), Williams et al. (2004) and Conceição and Green (2004).

    Liz et al. (2009) used a sliding normalization technique for comparing trace element signatures of post-collisional shoshonitic and ultrapotassic associations, which has led to the identification of two different trends: (ⅰ) Zr-Nb-Ti-Y-REE enrichment and (ⅱ) increasing of Rb-K2O with concomitant Nb decrease. The former was taken as indicative of an asthenospheric component, whilst the latter indicates crustal contribution through lithospheric subduction. A strong positive correlation of K and Nb indicates that the increase of both elements is related to the variation of melt fractions or with the magnitude of enrichment in the mantle source.

    The high incidence of 87Sr/86Sri ratios at ca. 0.708, particularly when a mantle origin is assumed, suggests that shoshonitic magmatism in different regions and ages comes from the same type of source and does not involve significant crustal assimilation. Some authors (e.g., Rivera, 2019; Miller et al., 1999) observed that the ultrapotassic magmatism frequently shows higher 87Sr/86Sri ratios in relation to the associated shoshonitic and high-K subalkaline ones. The ratios of 87Sr/86Sri in ultrapotassic lamprophyres and syenitic rocks may reach values greater than 0.720, and this has been explained in some cases as due to the influence of a strongly radiogenic mantle source (ITEM, Italian Enriched Mantle) with values of 87Sr/86Sri ca. 0.724 and εNd(t) ca. -13 (Stoppa et al., 2014; Bell et al., 2013). The more common values of 87Sr/86Sri ratios, ca. 0.708 and negative εNd(t) frequently reported for shoshonitic magmatism, as well as trace element patterns with high LREE/HFSE and LILE/HFSE ratios probably reflect mantle sources compositionally close to EM1-type, as observed by several authors (e.g., Nardi et al., 2008; Sommer et al., 2005). The trace element ratios proposed by Weaver (1991) for characterising mantle sources, on the other hand, indicated that the sources of the south Brazilian associations—LSSA, MSP and ASP—were more LILE-, LREE- and Th-enriched than EM1 sources approaching crustal values. Nb/Th and Zr/Nb ratios of less differentiated rocks (SiO2 < 56 wt.%) indicated their derivation from hydrated mantle, a source type characteristic of arc-related settings (Condie, 2015). The ratios of Th/Yb, Nb/Yb and TiO2/Yb proposed by Pearce (2008) for identifying magma sources showed that the associations from southernmost Brazil are derived from OIB-like sources with a strong influence of subduction, which causes an increase of Th/Yb and TiO2/Yb ratios. Shoshonitic lamprophyres plot near the EMORB field. Intermediate and acidic rocks plot approximately in the same field (Figs. 8 and 9).

    Figure 8.  Trace element ratio diagram (TiO2/Y vs. Th/Yb) for determination of sources, according to Pearce (2008): less differentiated rocks from Arroio do Silva Pluton, Piquiri Syenite Massif, and Lavras do Sul Shoshonitic Association lamprophyres.

    Figure 9.  Trace element ratio diagram (Nb/Y vs. Th/Yb) for determining sources, according to Pearce (2008): less differentiated rocks from Arroio do Silva Pluton, Piquiri Syenite Massif, and LSSA lamprophyres.

    According to data discussed in this review, the plutonic rocks, either syenitic, monzonitic or granitic related to the shoshonitic series, were produced dominantly by fractional crystallization of basic to intermediate, sometimes lamprophyric magmas, whose source is the mantle modified by previous lithosphere subduction, probably a veined peridotite with abundant pyroxene+phlogopite+K-richterite (e.g., Weaver et al., 2013; Conceição et al., 2004, 2000; Plá Cid et al., 2003; Peccerillo, 1992; Venturelli et al., 1984).

  • Volcanic rocks of shoshonitic and ultrapotassic affinity from the Tibetan Plateau show a compositional range that does not correspond to liquids produced by fractional crystallization or AFC mechanisms. As discussed by Turner et al. (1996), the variation of partial melting fractions and composition of mantle sources controlled the lithological variations. Guo et al. (2006) concluded that magmas with MgO > 6 wt.% in the Tibetan shoshonitic to ultrapotassic magmatism are products of 0.1% to 15% partial melting of mantle sources, whilst liquids with lower MgO content are produced by AFC processes. Eklund and Shebanov (2005) recognized in the Ava Ring complex, a Paleoproterozoic post- collisional shoshonitic ring complex in southwestern Finland, an association of granitic crustal magmas, shoshonitic monzonitic rocks and lamprophyres. Differentiation of shoshonitic rocks involves fractional crystallization, and the main magma types, lamprophyre, monzonite and granite, come from different sources and show diverse ages. Carvalho et al. (2014) related the compositional variation of the Pedra Branca Syenite with the interaction of peralkaline syenite magmas and basaltic melts. The lithological variations in the Piquiri Syenite Massif were related predominantly to processes of fractional crystallization (Nardi et al., 2008; Stabel et al., 2001). However, more recent interpretations, based mainly on isotope geochemistry, geochronology and geophysics (Rivera, 2019), proposed that it is composed of several pulses of different ages and compositions that share a compositionally similar source, whereby fractional crystallization may have produced some evolved liquids. Co-mingling is exhibited mainly between lamprophyre and ultrapotassic syenite magmas. The LSSA, composed of plutonic and volcanic rocks that vary from basic to acidic compositions, was also considered as produced mainly by fractional crystallization of shoshonitic parental magmas (Lima and Nardi, 1998). Liz et al. (2004) proposed that the evolution of LSSA was controlled by fractional crystallization and co-mingling of trachytic and latitic magmas. More recent geochronological and isotope data showed that it comprises several magmatic pulses of variable composition and different ages spanning from ca. 600 to 587 Ma. The Arroio do Silva Pluton, interpreted as a mixing and mingling product of dioritic and syenitic magmas (Padilha et al., 2019), is composed of at least three pulses of mantle-derived lamprophyric, dioritic, and syenitic magmas, all of them representing compositionally similar melts but from distinct sources, of different isotopic signatures.

    The previous examples suggest that ultrapotassic- shoshonitic magmatic associations reflect, in general, pulses of compositionally distinct magmas-dominantly lamprophyric, monzonitic, syenitic, dioritic and granitic—products of episodic partial melting of mantle sources where phlogopite is abundant. Variation of magma types was determined by the amount of phlogopite or K-amphibole in the mantle source or by variation of melting fractions. Fractional crystallization, assimilation and mixing/mingling can play relevant roles in the differentiation of parental magmas. Some authors have pointed out magma immiscibility as an additional process (Ferreira et al., 1997) to cause an increase of differentiation. Episodic magmatism has grown in importance with improvement of geochronological methods and data availability as well demonstrated in Schaltegger (1997), describing the evolution of orogenic magmatism in the Variscan orogenic belt, which is concentrated in four intervals 340–330, 310–307, 304–295, and 280–270 Ma, each episode with distinct geochemical signature and geodynamic setting. The episodic magmatism was said to be controlled by extensional processes in the lithosphere, such as thermal erosion and relaxation of the lithospheric mantle and decompression melting.

  • Crustal melts are generally of leucocratic, peraluminous, granitic compositions as experimentally determined by many authors (e.g., Patiño Douce, 1999; Montel and Vilzeuf, 1997; Stevens et al., 1997; Skjerlie and Johnston, 1996). Nevertheless, more mafic melts are produced at higher temperatures, under water-saturated or over-saturated conditions, or in systems with high K/Na ratios (Springer and Seck, 1997). According to Vielzeuf and Schmidt (2001), the melting of basic rocks with 1% of added water occurs at temperatures around 720 ℃ for pressures between 10 and 26 kbar. For lower pressures, temperatures are higher than 840 ℃. When the added water exceeds 1.8%, temperatures decrease to about 640 ℃ for 10 to 26 kbar pressure values. Carroll and Wyllie (1990) demonstrated that tonalitic compositions melt at pressure of 15 kbar at ca. 630 ℃ temperature, and plagioclase is a liquidus phase only when water is under ca. 7%. Therefore, during partial melting of crustal protoliths under high pressures, plagioclase—the most abundant silicate in the crust and the major concentrator of Sr—will not be a solid phase. Consequently, melts will show higher Sr contents. For the same reason, Eu will be relatively enriched in the melt. The consumption of plagioclase in the solid residue will increase when free water is present (Gardien et al., 2000). The behaviour of Ba in such melting systems will depend on the stability of biotite, its major concentrator. Experimental data (White et al., 2011; Gardien et al., 2000; Carroll and Wyllie, 1990) suggested that biotite will be a solid phase at temperatures under ca. 850 ℃, depending mainly on K-content and pressure in the system. Partial melting in metamorphic terranes when aqueous fluids are infiltrated, a process named water-fluxed melting (Weinberg and Hasalová, 2015; Sawyer et al., 2011), can produce low- temperature (< 740 ℃) magmas from protoliths of variable compositions, even from basic sequences (Vielzeuf and Schmidt, 2001), particularly in areas where major crustal-scale shear zones are present (Sawyer et al., 2011).

    Based upon the previous considerations, it is possible to suggest that high Ba-Sr granitoids, a name suggested by Tarney and Jones (1994), may represent crustal melts generated under high pressures (> 10 kbar), particularly when free-water is available and temperatures are high enough (above 850 ℃) to prevent the presence of plagioclase and biotite among the residual solid phases. As recognized by several authors, high Ba-Sr granitoids can have a mantle contribution (e.g., Lara et al., 2017; Fowler et al., 2008). However, we propose that if the mantle component belongs to the shoshonitic series, the more differentiated rocks, including the granitic ones, should be classified as shoshonitic. Alternatively, as suggested by Martini et al. (2019), high Ba-Sr melts can be produced from protoliths enriched in both elements. Therefore, the shoshonitic-like granitoids referred by Ferreira et al. (2015), Bitencourt and Nardi (2004) and Martini et al. (2019), and possibly also those referred by Bitencourt and Nardi (1993), Knijnik et al. (2012), Martins et al. (2016), Lara et al. (2017), and Knijnik (2018) related to large-scale shear zones from South Brazil and Uruguay could be considered high Ba-Sr granitoids, not necessarily belonging to the shoshonitic magmatic series, but products of crustal melting, in many cases with minor mantle contribution.

    The discrimination between granitoids produced from shoshonitic parental magmas with minor crustal contribution, and high Ba-Sr granitoids is complicated if intermediate or basic co-magmatic rocks are missing. That is because the effects of crustal participation are variable and will control the values of K2O/Na2O, Al2O3, SiO2, K2O+Na2O and other potential discriminant parameters, as discussed by Goswami and Bhattacharyya (2014) and Lara et al. (2017).

  • Magmatism of shoshonitic signature related in space or time with regional tectonic activity in collisional to post-collisional environments may show high amounts of crustal assimilation (Fig. 10). In some cases, the mantle component may be absent or weak enough to become hardly recognizable. Crustal contribution tends to reduce the amount of alkalis, Sr, LREE and HFSE contents and increase the peraluminosity, making the identification of original composition even more difficult, as discussed by Knijnik et al. (2012). When crustal protoliths such as amphibolitic gneisses are assumed (e.g., Martini et al., 2019; Lara et al., 2017; Ferreira et al., 2015; Bitencourt and Nardi, 2004; López-Moro and López-Plaza, 2004) as possible source of granitic magmas compositionally equivalent to those of the shoshonitic series, a specific designation should be used, such as high Ba-Sr granitoids. Lara et al. (2017) used this designation for shoshonitic-like granitoids in the southern part of the Dom Feliciano belt. Crustal contamination, favored by compressive tectonic associated with crustal melting and fluid circulation in shear zones (Fig. 10), may lead to high compositional modifications in mantle-derived magmas, making them similar to crustal reworking products. When the crustal contribution is strong, the shoshonitic isotope signature may be lost, 87Sr/86Sri ratios generally increase, and the variation of εNd(t) will depend on the melted and assimilated crust.

    Figure 10.  Cartoon depicting different settings and origins for shoshonitic and high Ba-Sr magmas. Notice the difference in sources for the shoshonitic magmas (veined mantle) as opposed to the high Ba-Sr ones (crust) and the emplacement of both within shear zones or away from deformation sites and at different crustal levels, with variable degrees of contamination by mantle or crustal material. AFC. Assimilation/fractional crystallization; FC. fractional crystallization; MM. magma mixing/mingling; PCM. pure crustal melts.

    The most effective parameter for identifying granitoids of shoshonitic series is their genetic relationship with basic and intermediate rocks of such affinity (Fig. 10), evidenced by the dominance of monzonitic-latitic rocks or plotting in the field of potassic silica-saturated alkaline series in the TAS diagram. Shoshonitic and high Ba-Sr granitoids are characterised by Sr and Ba contents generally higher than 500 ppm and 800 ppm, respectively. They commonly show moderate contents of HFSE and relative enrichment of LREE. The REE patterns show Eu-anomalies only for highly differentiated rocks (above ca. 73 wt.% SiO2), and their variation along the differentiation trend is negligible. The distinction between shoshonitic and high Ba-Sr granitoids is difficult in the absence of less differentiated members of a shoshonitic series. Compared to high-K calc-alkaline granitoids, which commonly show similar K2O and K2O/Na2O values, the shoshonitic granitoids are Sr and Ba-enriched, show REE patterns with no Eu-anomalies and, in general, higher contents of HFSE (Zr, Nb, Y). Low pressure and water-absent crustal melts, which are frequently mistaken for granitoids from calc-alkaline series, will generally show low Sr and Ba contents, low HFSE and REE contents and negative Eu anomalies, although the major element composition may be similar. As discussed in this paper, plutonic rocks which belong to the shoshonitic series show very regular87Sr/86Sri ca. 0.708, probably indicating the same sort of mantle source, whilst crustal granitoids, or granitoids with strong crustal contribution, will show values that reflect their protoliths or the crustally-derived melts added in AFC processes (Table 2). Shoshonitic and high Ba-Sr granitoids have sometimes been named high Sr/Y granitoids, a feature that is also typical of subalkaline granitoids related to orogenic cycles, such as the adakitic ones. Richards et al. (2012) have used this terminology for products of fractional crystallization from basic to intermediate magmas, whereas Wang et al. (2018) employ it to granitoids produced by melting of the lower crust.

    The shoshonitic plutonic associations from southernmost Brazil emplaced either within or away from deformation sites are, according to geochemical evidence, trace elements and isotope data, derived from mantle sources modified by previous, subduction-related metasomatism, whose compositional features are consistent with OIB-types with a strong influence of subduction. Therefore, as proposed by several authors, the model of a veined mantle enriched in phlogopite-K-amphibole seems adequate even for the syntectonic magmatism. However, this includes assimilation of crustal melts to variable extents and diverse compositions (Fig. 10).

  • The authors acknowledge the Brazilian Research Council (CNPq) for the productivity grants to Lauro Valentim Stoll Nardi (No. 306605/2018-0) and Maria de Fátima Bitencourt (No. 311486/2015-0) and funding through Universal Project (No. 481841/2012-1). The final publication is available at Springer via https://doi.org/10.1007/s12583-021-1534-8.

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