L. V. S. Nardi

The Lavras do Sul Shoshonitic Association: implications for the origin and evolution of Neoproterozoic shoshonitic magmatism in southernmost Brazil

Abstract The Neoproterozoic Lavras do Sul Shoshonitic Association in southernmost Brazil is related to the last stages of Brasiliano Orogeny and comprises a wide compositional range of intrusive and extrusive basic to acid rocks. It includes potassic trachybasalts, shoshonites, dacites, rhyolites, quartz diorites and monzodiorites, quartz monzonites, granodiorites, monzogranites and syenogranites. Leucodioritic cumulates and spessartitic lamprophyres have also been identified. The whole range of plutonic and extrusive rocks are probably derived of parental shoshonitic basaltic magmas dominantly through crystal fractionation processes, as suggested by mass balance calculations for major elements and by coefficient partition studies for trace elements. Mineral fractionation was dominated by olivine, clinopyroxene and plagioclase from basic to intermediate liquids. Amphibole, biotite and Ti-magnetite, along with plagioclase, were important during the evolution of intermediate to acid liquids. Petrographic features and field relations support the same conclusion. Low initial 87 Sr Sr 86 ratios (about 0.704), slight negative ϵNd values (about −4) and a single U-Pb age population determined in zircons of shoshonitic granodiorites, suggest a negligible crustal involvement in the differentiation of this magmatism. Normalized spiderdiagrams and trace element ratios are similar to those observed in post-collisional shoshonitic associations. Geochemical data, including isotopes, are consistent with the evolution of a parental magma generated through melting of an CO2-enriched mantle source, previously metasomatised by a Brasiliano subduction.

Geochemistry of Palaeoproterozoic volcanic rocks of the Iricoumé Group, Pitinga Mining District, Amazonian craton, Brazil

The Iricoumé Group includes 1.88 Ga volcanic units of the Iricoumé–Mapuera volcano-plutonic association, part of the Uatumã magmatic series in the Guyana shield portion of the Amazonian craton. In the Pitinga Mining District, these rocks consist dominantly of felsic trachyte to rhyolite, associated with voluminous ignimbrite and minor ash-fall tuffs and surge deposits. Mafic rocks are present as basaltic clasts within volcanic breccias, and mostly as mafic microgranular enclaves in the associated Mapuera plutonic rocks. The felsic rocks have high contents of SiO2, FeOt, K2O, Rb, and alkalis; low TiO2, CaO, Sr, Ba, Nb, Ta, and Eu; and show metaluminous to weakly peraluminous bulk-rock compositions. They exhibit alkaline geochemical features, expressed by Na2O + K2O averages of 8.8 wt.%, FeOt/(FeOt + MgO) ≥ 0.8, and high Ga/Al ratios, compatible with A-type magmas. The studied samples plot in the field of within-plate or post-collisional rocks in a (Nb + Y) versus Rb diagram. Nb/Y ratios indicate that they are comparable to A2-type rocks which, allied with their high LREE/Nb ratios, suggest that they were produced from mantle sources modified by previous subduction in a post-collisional setting. Two compositional populations of Ca-amphibole, a Mg-rich (actinolite to Mg-hornblende) and a Fe-rich one (Fe-edenite to Fe-pargasite, Fe-hornblende and Fe-actinolite), characterize the Iricoumé Group volcanics. The Fe-rich amphiboles crystallized under lower fO2 and higher pressure conditions compared with the Mg-rich amphiboles, indicating different levels of crystallization or re-equilibration during ascent of the magmas. Zircons from rhyolites show trace-element compositions typical of magmatic crystals with high Th/U ratios, and REE patterns compatible with zircon-melt partition coefficients for silicic magma compositions. Their relatively lower zircon/rock partition coefficients are due to early apatite crystallization. Fractional crystallization mainly of plagioclase-hornblende and biotite-alkali feldspar with minor amounts of apatite explains the geochemical trends observed in the felsic Iricoumé volcanic rocks.

The alkaline silica-saturated ultrapotassic magmatism of the Riacho do Pontal Fold Belt, NE Brazil: an example of syenite–granite Neoproterozoic association

Abstract The Neoproterozoic of northeastern Brazil was marked by the development of collisional fold belts, mainly surrounding the Sao Francisco Craton, and an associated widespread granitic magmatism. The Casa Nova (555±10 Ma , Sri=0.7068 and Engracadinha syenites and granites, intrusive in the Riacho do Pontal Fold Belt, are related to the late stages of this collisional event. Melanocratic syenites, probably generated by magmatic-flow cumulate processes, and mesocratic and leucocratic syenites, representing magmatic liquids, are associated with granites, pegmatites, and syenite–granite dykes. Homogeneous or perthitic alkali feldspar, quartz, aegirine–augite, diopside, titanite, apatite, magnesian biotite, winchite–richterite, and magnetite are the dominant mineral phases. It is suggested that these magmas belong to an ultrapotassic silica-saturated alkaline series, defined on the basis of its alkaline, silica-saturated character and by a K2O/Na2O ratio of >3.0 — that is, intermediate. Major- and trace-element evolution is consistent with mineral fractionation processes, controlled by magmatic flow, and dominated by apatite–titanite–pyroxene in the less differentiated terms and by alkali feldspar in the more evolved. The source of primary magmas is a previously subduction-metasomatised mantle, probably with anomalous enrichment in LREE and LILE elements. Barite–ilmenite mineralisations are related to the more differentiated Engracadinha granites.

The Soca intrusion: a rapakivi granite of Uruguay

Abstract The Soca granite, emplaced in the Rio de la Plata Craton, is located 65 km east of Montevideo. It is a homogeneous porphyritic leucogranite, with almost 75 km 2 of exposed area. Petrographically, the Soca granite is characterized by the occurrence of quartz, plagioclase and alkaline feldspar in two generations, ferrous pyroxene, mica, amphibole, and relic fayalite; with zircon, allanite and apatite as abundant accessory minerals. Its main geochemical features are high K 2 O/Na 2 O, low CaO, Al 2 O 3 and P 2 O 5 , high total alkali values (average K 2 O+Na 2 O around 7.98), high FeO∗/[FeO∗+MgO] (average 0.93), and high Nb, Y, and LREE concentrations (except for Eu). The petrographic and geochemical evidence allow us to classify the Soca granite as an A-type Proterozoic granite, related to the saturated alkaline association, specifically to the rapakivi type granites. The Soca granite has features similar to the typical rapakivi granites from Scandinavia and Central Amazonia.

Mineralogy of Lamprophyres and Mafic Enclaves Associated with the Paleoproterozoic Cara Suja Syenite, Northeast Brazil

The Paleoproterozoic Cara Suja Syenite in northeastern Brazil is composed mainly of coarse-grained silica-saturated syenites with minor mafic rocks, such as syenitic autoliths and mafic micro-granular enclaves. Lamprophyric and hybrid dikes occur in this massif and crosscut the volcano-sedimentary country rocks. Syenitic autoliths are medium-grained syenites associated with mafic mineral segregations in the host syenites. Mafic microgranular enclaves are elliptic or rounded, finegrained mesocratic syenites. Lamprophyric dikes are porphyritic rocks with large biotite phenocrysts and amphibole agglomerations in a fine-grained groundmass consisting mostly of alkali-feldspar. Hybrid dikes contain abundant feldspar xenocrysts. Petrographic and field relations indicate mingling of syenitic and mafic magmas. Lamprophyres, autoliths, and mafic microgranular enclaves, as well as the host syenites, have an alkaline potassic-ultrapotassic affinity and a trace-element signature of magmas produced by the partial melting of a subduction-modified mantle. Mineral chemistry data show preserved clinopyroxene compositions only in syenitic autoliths. They are Al2O3- and TiO2-poor diopsides with slight variation in wollastonite contents (44 to 50 wt%), generally rimmed by late- or post-magmatic actinolite. The amphibole of lamprophyres and mafic microgranular enclaves is Mg-hornblende, characterized by high Si (6.95 to 7.49 atoms per formula), high F (up to 1.13 wt%) and low TiO2 (0.22 to 0.50 wt%). Micas are Mg-biotite and phlogopite with large amounts of opaque oxides and titanite inclusions. Phlogopite of lamprophyres, of mafic microgranular enclaves, and of autoliths has very similar compositions, with Si values around 5.9 atoms per formula, Mg/(Mg+Fe+2) ratio ranging from 0.65 to 0.70, and with low TiO2 (below 1.1 wt%) and high F (2.1 to 4.2 wt%) contents. The general overlap and compositional similarities of mafic minerals from lamprophyres and mafic microgranular enclaves indicate that they are co-magmatic and evolved under similar conditions. The textural features and composition of Mg-hornblende and phlogopite in these rocks suggest that they are primary phases re-equilibrated under crustal conditions during magma ascent.

Anorogenic alkaline granites from northeastern Brazil: major, trace, and rare earth elements in magmatic and metamorphic biotite and Na-mafic minerals

Abstract The anorogenic, alkaline silica-oversaturated Serra do Meio suite is located within the Riacho do Pontal fold belt, northeast Brazil. This suite, assumed to be Paleoproterozoic in age, encompasses metaluminous and peralkaline granites which have been deformed during the Neoproterozoic collisional event. Preserved late-magmatic to subsolidus amphiboles belong to the riebeckite–arfvedsonite and riebeckite–winchite solid solutions. Riebeckite–winchite is frequently rimmed by Ti–aegirine. Ti-aegirine cores are strongly enriched in Nb, Y, Hf, and REE, which significantly decrease in concentrations towards the rims. REE patterns of Ti-aegirine are strikingly similar to Ti-pyroxenes from the Ilimaussaq peralkaline intrusion. Recrystallisation of mineral assemblages was associated with deformation although some original grains are still preserved. Magmatic annite was converted into magnetite and biotite with lower Fe/(Fe+Mg) ratios. Recrystallised amphibole is pure riebeckite. Magmatic Ti–Na-bearing pyroxene was converted to low-Ti aegirine+titanite±astrophyllite/aenigmatite. The reaction riebeckite+quartz→aegirine+magnetite+quartz+fluid is also observed. Biotite and Na-mafic minerals recrystallised under metamorphic oxidising conditions corresponding to temperatures of 600°C between the NiNiO and HM buffers.

The Santanápolis Syenite: Genesis and Evolution of Paleoproterozoic Shoshonitic Syenites in Northeastern Brazil

The Paleoproterozoic terrane of the state of Bahia, Brazil, is characterized by an important mag-matic event that resulted in the generation of large and numerous syenite complexes; the terrane constitutes one of the largest syenite provinces in the world. It includes the Itiuba (1,800km2), Santanápolis (180 km2), São Felix (32 km2), and Anuri (70 km2) complexes. These syenitic bodies of shoshonitic affiliation are silica saturated, metaluminous, and exhibit: (1) moderate to high K2O/Na2O ratios, (2) relatively high MgO contents (up to 3.0%); (3) high incompatible-element contents, particularly LILE and LREE; (4) high HREE/LREE fractionations; and (5) small Eu negative anomalies. Among these syenites, Santanápolis exhibits the greatest lithological diversity, with a porphyritic and a heterogranular facies, and diverse lithotypes that vary from leucosyenites to melanocratic syenites of cumulate origin, as well as enclaves and dikes. It intruded syntectonically at 2.1 Ga and shows magmatic flow structures and subsolidus recrystallization of quartz and alkali feldspars. The mineralogical assemblage is perthitic feldspar, diopside, hornblende, phlogopite-biotite, apatite, zircon, titanite, calcite, and monazite. Differentiation of the Santanópolis Syenite was controlled by flow segregation processes in an intermediate trachytic magma, with fractionation of early crystallized diopside, apatite, and plagioclase. The parental intermediate shoshonitic magma was extracted directly from an enriched, subduction-modified mantle characterized by anomalous concentrations of LREE and LILE elements, and isotopic parameters similar to the EMI-type.

Petrogenesis of the Neoproterozoic Alkaline Ultrapotassic Suítes of Northeastern Brazil: Major- and Trace-Element Evidence from Pyroxene Chemistry and Numerical Modeling

Syenites, quartz syenites, and granites sharing ultrapotassic alkaline affinities were emplaced during the late stages of Brasiliano collision (555 ±10 Ma) along the northern border of the Sao Francisco craton, in the Riacho do Pontal fold belt, northeastern Brazil. The mafic and accessory paragenesis includes diopside evolving to aegirine-augite and eventually to aegirine, titanite, apatite, riebeckite, biotite, winchite, zircon, and magnetite, whereas barite and ilmenite are abundant in the more evolved aegirine-bearing granites. Differentiation was controlled by flow segregation and mineral fractionation. Trace-element patterns of aegirine-augite reflect the early crystallization of apatite + titanite + aegirine + augite. A mantle-enriched source affected by Paleoproterozoic subduction-related metasomatism is suggested.

Paleoproterozoic late-orogenic and anorogenic alkaline granitic magmatism from northeast Brazil

Abstract During the Transamazonian cycle (2.0±0.2 Ga), two silica-saturated alkaline granite suites were intruded along the border of the Sao Francisco craton, northeastern Brazil. The Couro de Onca intrusive suite (COIS) is late tectonic relative to major deformational events, and the Serra do Meio suite (SMS) postdates this event, being interpreted as anorogenic type, deformed during the Brasiliano cycle (0.65–0.52 Ga). The COIS encompasses alkali-feldspar granites, syenogranites, monzogranites, with dykes of similar composition and age (2.157±5 Ma). They are metaluminous rocks of alkaline affinity, whose evolution was probably controlled by mineral fractionation processes involving mainly plagioclase, amphibole, and Fe–Ti oxide. The SMS, whose probable age is 2.01 Ga, is composed of metaluminous and peralkaline granites, with associated quartz syenites, with Ti-aegirine transformed during the Brasiliano deformation to riebeckite+titanite+aenigmatite. The temperatures of two stages of metamorphic transformation, overprinted during the Brasiliano event, are estimated at about 550 and 400–450°C. Both magmatic suites are derived from enriched mantle sources. The metaluminous and peralkaline trends observed in the SMS, characterized by high concentrations of HFS and RE elements controlled mainly by F-activity, reflect the evolution of different primary magmas produced by successive melting of the same mantle sources, in an anorogenic setting.

Alkaline Ultrapotassic A-Type Granites Derived from Ultrapotassic Syenite Magmas Generated from Metasomatized Mantle

Ultrapotassic Neoproterozoic granites crop out in southern and northeastern Brazil, associated with syenitic suites. The granites occur as small circular bodies and dikes hosted by the syenitic rocks. The southern intrusions reflect post-collisional magmatism, whereas the northeastern granites were emplaced in mobile belts produced during intraplate continental collision. Systematic mineralogical differences of the granites reflect higher K2O/Na2O ratios in the northeastern assemblages. Bulk-rock compositions of the ultrapotassic granites show very high alkalis (mainly K2O) and LIL elements, with significant depletions of Nb, Y, Zr, Hf, Ga, and REE relative to anorogenic granites. These are typical characteristics of ultrapotassic, silica-saturated alkaline rocks from postcollisional settings. Trace-and rare-earth—element patterns suggest derivation by fractional crystallization from syenitic parental magmas. Geochemical and mineralogical characteristics of these granites are completely different from within-plate alkaline granites. Most compositional data are comparable to those of shoshonitic granitoids, suggesting a probable mantle source modified by subducted-slab dehydration. In spite of this, the geochemical composition of the analyzed ultrapotassic granites is also quite different from shoshonitic and calc-alkaline granitoids in collisional settings, defining a silica-saturated ultrapotassic rock series.