The Banhad?o Alkaline Complex, Southeastern Brazil: source and evolution of potassic SiO2-undersaturated high-Ca and low-Ca magmatic series

The Cretaceous Banhad?o alkaline complex in southeastern Brazil presents two potassic SiO₂-undersaturated series. The high-Ca magmatic series consist of initially fractionated olivine (Fo_92-91) + diopside (Wo_48-43En_49-35Ae_0-7), as evidenced by the presence of xenocrysts and xenoliths. In that sequence, diopside (Wo_47-38En_46-37Ae_0-8) + phlogopite + apatite + perovskite (Prv_>92) crystallized to form the phlogopite melteigite and led to the Ca enrichment of the magma. Diopside (Wo_47-41En_32-24 Ae_3-14) continued to crystallize as an early mafic mineral, followed by nepheline (Ne_74.8-70.1Ks_26.3-21.2Qz_7.6-0.9) and leucite (Lc_65-56) and subsequently by melanite and potassic feldspar (Or_85-99Ab_1-7) to form melanite ijolites, wollastonite-melanite urtites and melanite-nepheline syenites. Melanite-pseudoleucite-nepheline syenites are interpreted to be a leucite accumulation. Melanite nephelinite dykes are believed to represent some of the magmatic differentiation steps. The low-Ca magmatic series is representative of a typical fractionation of aegirine-augite (Wo_36-29En_25-4Ae_39-18) + alkali feldspar (Or_57-96Ab_3-43) + nepheline (Ne_76.5-69.0Ks_19.9-14.4Qz_15.1-7.7) + titanite from phonolite magma. The evolution of this series from potassic nepheline syenites to sodic sodalite syenites and sodalitolites is attributed to an extensive fractionation of potassic feldspar, which led to an increase of the NaCl activity in the melt during the final stages forming sodalite-rich rocks. Phonolite dykes followed a similar evolutionary process and also registered some crustal assimilation. The mesocratic nepheline syenites showed interactions with phlogopite melteigites, such as compatible trace element enrichments and the presence of diopside xenocrysts, which were interpreted to be due to a mixing/mingling process of phonolite and nephelinite magmas. The geochemical data show higher TiO₂ and P₂O₅ contents and lower SiO₂ contents for the high-Ca series and different LILE evolution trends and REE chondrite-normalized patterns as compared to the low-Ca series. The ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb initial ratios for the high-Ca series (0.70407–0.70526, 0.51242–0.51251, 17.782–19.266 and 38.051–39.521, respectively) were slightly different from those of the low-Ca series (0.70542–0.70583, 0.51232–0.51240, 17.758–17.772 and 38.021–38.061, respectively). For both series, a CO₂-rich potassic metasomatized lithospheric mantle enriched the source with rutile-bearing phlogopite clinopyroxenite veins. Kamafugite-like parental magma is attributed to the high-Ca series with major contributions from the melting of the veins. Potassic nephelinite-like parental magma is assigned to the low-Ca series, where the metasomatized wall-rock played a more significant role in the melting process.     

Magmatism and fenitization in the Cretaceous potassium-alkaline-carbonatitic complex of Ipanema S?o Paulo State, Brazil

The Ipanema alkaline-carbonatitic complex is part of the Meso-Cenozoic alkaline magmatism located within the southeastern part of the Brazilian Platform. Drill-core and field sampling have indicated the occurrence of glimmerites, with subordinate shonkinites (mela-syenites), clinopyroxene-bearing glimmerites, diorites and syenites. The glimmerites are cross-cut by lamprophyric dykes and calciocarbonatites. Fenitization has deeply affected the country rocks, originating dioritic and syenitic rocks. The Ipanema rocks show a distinct potassic affinity. The initial Sr-Nd- isotopic composition of the Ipanema rocks (⁸⁷Sr/⁸⁶Sr?=?0.70661–0.70754 and ¹⁴³Nd/¹⁴⁴Nd?=?0.51169–0.51181) is similar to that of tholeiitic and potassium-rich-alkaline rocks of the Eastern Paraguay. Stable isotope data for the Ipanema calciocarbonatite suggest interaction with fluids at temperatures typical of hydrothermal stages, as hypothesized for other carbonatite complexes from southeastern Brazil. The chemical differences between the lamprophyre, glimmerites, carbonatites, apatitites and magnetitites, and the absence of marked REE enrichment in the evolved lithologies, all indicate that fractional crystallization and accumulus of liquidus phases in a magma reservoir, likely coupled with liquid immiscibility processes, may have played an important role in the genesis of the Ipanema rocks.     

U–Pb ages,geochemistry, C–O–Nd–Sr–Hf isotopes and petrogenesis of the Catalão II carbonatitic complex (Alto Paranaíba Igneous Province,Brazil): implications for regional-scale heterogeneities in the Brazilian carbonatite associations

International Journal of Earth Sciences - The Catalão II carbonatitic complex is part of the Alto Paranaíba Igneous Province (APIP), central Brazil, close to the Catalão I complex....     

Petrogenesis of the early Cretaceous Valle Chico igneous complex (SE Uruguay): Relationships with Paraná–Etendeka magmatism

The early Cretaceous (130 Ma) igneous complex of Valle Chico (SE Uruguay) is made up of felsic plutonic and subordinate volcanic rocks and dykes cropping out over an area of about 250 km². This complex is strictly linked with the formation of the Paraná–Etendeka Igneous Province and the first stages of the South Atlantic Ocean rifting. The plutonic rocks range from quartz-monzonite to syenite, quartz-syenite and granite. The volcanic rocks and the dykes range from quartz-latite to trachyte and rhyolite; no substantial differences in term of chemical composition have been found between plutonic and volcanic rocks. Only a sample of basaltic composition (with tholeiitic affinity) has been sampled associated with the felsic rocks. The Agpaitic Index of the Valle Chico felsic rocks range from 0.72 to 1.34, with the peralkaline terms confined in the most evolved samples (SiO₂>65 wt.%). Initial ⁸⁷Sr/⁸⁶Sr₍₁₃₀₎ of the felsic rocks range from 0.7046 to 0.7201, but the range of ⁸⁷Sr/⁸⁶Sr of low-Rb/Sr samples cluster at 0.7083; ¹⁴³Nd/¹⁴⁴Nd₍₁₃₀₎ ratios range from 0.5121 (syenite) to 0.5117 (granite). The tholeiitic basalt show more depleted isotopic compositions (⁸⁷Sr/⁸⁶Sr₍₁₃₀₎=0.7061; ¹⁴³Nd/¹⁴⁴Nd₍₁₃₀₎=0.5122), and plots in the field of other early Cretaceous low-Ti basaltic rocks of SE Uruguay. The radiogenic Sr and unradiogenic Nd of the Valle Chico felsic rocks require involvement of lower crustal material in their genesis either as melt contaminant or as protolith (crustal anatexis). In particular, most of the Valle Chico (VC) felsic rocks define a near-vertical array in Sr–Nd isotopic spaces, pointing toward classical EMI-type composition; this feature is considered to reflect a lower crust involvement as observed for other mafic and felsic rocks of the Paraná–Etendeka Igneous Province. Decompression melting of the lower crust related to Gondwana continental rifting before the opening of the South Atlantic Ocean or the presence of thermal anomalies related to the Tristan plume may have induced the lower crust to partially melt. Alternative hypothesis considers contamination of upper mantle by a mafic/ultramafic keel composed of lower crust and uppermost mantle after delamination and detachment processes. This interaction may have occurred after the continent–continent collision during the last stages of the Panafrican Orogeny. This “lower crust” model does not exclude active involvement of upper crust as contaminant, necessary to explain the strongly radiogenic ⁸⁷Sr/⁸⁶Sr₍₁₃₀₎ isotopic composition of some VC SiO₂-rich rocks. Mineralogical (sporadic presence of pigeonite, Ca–Na and Na clinopyroxene, calcic- and calco-sodic amphibole) and geochemical evidences (major and trace element as well as Sr–Nd isotopic similarities with the felsic early Cretaceous volcanic rocks of the Arequita Formation in SE Uruguay) allow us to propose for the VC rocks a transitional rock series (the most abundant rock types are of syenitic/trachytic composition) preferentially evolving towards SiO₂-oversaturated compositions (granite/rhyolite) also with a strong upper crustal contribution as melt contaminant. This conclusion is in contrast with previous studies according which the VC complex had clear alkaline affinity. Many similarities between VC and the coeval Paresis granitoids (Etendeka, Namibia) are evidenced in this paper. The genetic similarities between VC and the rhyolites (s.l.) of SE Uruguay may find counterparts with the genetic link existing between the early Cretaceous tholeiitic-alkaline Messum complex and the quartz latites (s.l.) of the Awahab Formation (Etendeka region, Namibia).     

Petrology of potassic alkaline ultramafic and carbonatitic rocks from Planalto da Serra (Mato Grosso State), Brazil

The Planalto da Serra igneous rocks form plugs, necks and dykes of carbonate-rich ultramafic lamprophyres (aillikites and glimmerites with kamafugitic affinity) and carbonatites (alvikites and beforsites). Phlogopite and/or tetraphlogopite, diopside and melanitic garnet are restricted to aillikitic rock-types, whereas pyroclore occurs only in carbonatites. Aillikites and carbonatites are altered to hydrotermalites, having chlorite and serpentine as dominant minerals. Planalto da Serra igneous rock association has kamafugitic affinity (i.e. effusive, ultrapotassic. High LREE/HREE fractionation, incompatible elements data and Sr-Nd isotopes, suggest that the K-ultramafic alkaline and carbonatite rocks originated from a variably metasomatized mantle source enriched in radiogenic Sr. Crustal contamination is negligible or absent. Age values of 600 Ma rule out the geochronological relationship between the investigated intrusions and the Mesozoic alkaline bodies from the Azimuth 125° lineament. The TDM model ages allow to conclude that Planalto da Serra magma is derived from the partial melting of a mantle source metasomatised by K-rich carbonatated melt during the Early to Late Neoproterozoic. On the basis of alkaline magmatism repetitions at 600 Ma and 90–80 Ma we question the subsistence of a stationary mantle plume for so long time.