Rogério Guitarrari Azzone

Geologia e Geocronologia do Maciço Alcalino Máfico-Ultramáfico Ponte Nova (SP-MG)

O macico alcalino mafico-ultramafico Ponte Nova, localizado no setor norte da Provincia Serra do Mar, junto a porcao oriental da Serra da Mantiqueira, na regiao Sudeste do Brasil, encontra-se encaixado em granitoides e gnaisses migmatiticos Pre-Cambrianos. O macico apresenta duas areas distintas de exposicao, estando ambas separadas por rochas do embasamento: uma maior (~5,5 km²), de forma eliptica e com grande variedade de litotipos; outra menor (~1 km²), de forma irregular e petrograficamente menos complexa, e situada a sul da primeira. E predominantemente composto por uma associacao gabroide alcalina, gerada por sucessivos pulsos magmaticos. As rochas mais abundantes sao cumulatos maficos e ultramaficos (e.g., olivina clinopiroxenitos e melagabros com olivina) presentes nos niveis inferiores do macico, ao lado de rochas gabricas e monzogabricas porfiriticas, equigranulares e bandadas nos niveis superiores, todos representativos da evolucao magmatica dos principais pulsos. Um corpo de brecha magmatica e uma suite de diques (lamprofiros a tefrifonolitos) interceptam todas as rochas intrusivas. Analises geocronologicas por K/Ar para diferentes rochas indicam uma idade media de 87,6 Ma (Cretaceo Superior), que e similar as obtidas para as demais ocorrencias do setor norte da referida provincia. A incompatibilidade das idades torna dificil sustentar a hipotese de sua associacao genetica com a pluma mantelica de Trindade

Magmatism and fenitization in the Cretaceous potassium-alkaline-carbonatitic complex of Ipanema São 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.

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

The Cretaceous Banhadão alkaline complex in southeastern Brazil presents two potassic SiO2-undersaturated series. The high-Ca magmatic series consist of initially fractionated olivine (Fo92-91) + diopside (Wo48-43En49-35Ae0-7), as evidenced by the presence of xenocrysts and xenoliths. In that sequence, diopside (Wo47-38En46-37Ae0-8) + phlogopite + apatite + perovskite (Prv>92) crystallized to form the phlogopite melteigite and led to the Ca enrichment of the magma. Diopside (Wo47-41En32-24 Ae3-14) continued to crystallize as an early mafic mineral, followed by nepheline (Ne74.8-70.1Ks26.3-21.2Qz7.6-0.9) and leucite (Lc65-56) and subsequently by melanite and potassic feldspar (Or85-99Ab1-7) 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 (Wo36-29En25-4Ae39-18) + alkali feldspar (Or57-96Ab3-43) + nepheline (Ne76.5-69.0Ks19.9-14.4Qz15.1-7.7) + 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.

The antecryst compositional influence on Cretaceous alkaline lamprophyre dykes, SE Brazil

The question of whether the antecryst assemblage affects the bulk composition of lamprophyre dykes, and could be responsible for the compositional zonation between their centers and borders is addressed through a detailed study involving four monchiquite and camptonite dykes (basanites and tephrites) representative of the Arco de Ponta Grossa and Serra do Mar alkaline provinces. In them, antecrysts are interpreted as early-crystallized minerals that are no longer in equilibrium with their host-liquid, albeit still linked to the same magmatic system. They represent recycled crystals of earlier stages of the magmatic system at depth. The antecryst microtextures, such as zoned clinopyroxene megacrysts (augite cores and titanaugite rims) with partly corroded cores, olivine crystals with corroded rims surrounded by biotite coronas, chrome-spinel inclusions in clinopyroxene and olivine megacryst cores, and titanomagnetite crystals surrounded by biotite coronas, suggest chemical re-equilibrium with the matrix. The greatest antecryst cargo in these dykes is found in their centers. After subtracting the antecryst volume from the center analyses of each body, the calculated compositions are very similar to the border analyses. The mafic antecryst cargo of each occurrence proportionally leads to enrichment of MgO, FeO, TiO2, CaO, compatible trace elements (Cr, Ni and Co), and depletion of SiO2, K2O, Na2O, Al2O3 and incompatible trace elements (Ba, Sr and REE). The whole-rock geochemical analyses of each dyke represent the combination of accumulated crystals and melt. The compositional zonation of the studied dykes is associated with the antecryst cargo rather than different magmatic pulses.

Evolução composicional dos filossilicatos no perfil intempérico do complexo ultramáfico Alcalino-carbonatítico de Catalão I (GO)

The compositional evolution of the phyllosilicates in the lateritic profile of the Catalao I ultramafic alkaline-carbonatitic complex (GO) is proposed in this work based on petrographic and X-ray diffraction analysis, as well as, chemical composition obtained by WDS electron microprobe. The micaceous minerals found in unweathered rocks are classified as phlogopites (with different parageneses) and tetraferriphlogopites (metasomatic products). In the levels of altered rocks and isalteritic saprolite, the phyllosilicates derived from micas are vermiculite and regular interstratified phlogopite-vermiculite (which formation and characteristics are similar to hydrobiotite), characterizing a main vermiculitization process in this profile. Subordinately, in the upper portions and punctual areas of isalteritic saprolite, the alteration of the vermiculitized phlogopite generates smectitic products, which are characterized mainly by the occurrence of expansible regular interstratified mineral and even smectite. Kaolinite occurs on the top of the isalteritic saprolite and on the alloteritic level. The evolution sequence of the phyllosilicates in the weathering profile of the Catalao I complex presents the following formation order: phlogopite, interstratified phlogopite-vermiculite, vermiculite, smectitic products (expansible interstratified minerals of local occurrence), smectite and kaolinite, of which, the last two are probably associated to supergene processes. This sequence is compatible with the most recent theories for the formation of clay minerals in soils. The main differences between the phlogopites and their products of vermiculitic alteration are related to a significant decrease of K+ and oxidation of Fe2+, characteristic changes of the vermiculitization process. In the specific case of vermiculites, the interlayer site is filled mainly by Mg2+. In addition to that, the presence of interstratified minerals (intermediate alteration products) is a key indicator of relatively recent and low-altered weathering profiles.

Eudialyte-group minerals from the Monte de Trigo alkaline suite, Brazil: composition and petrological implications

The Monte de Trigo alkaline suite is a SiO2-undersaturated syenite-gabbroid association from the Serra do Mar alkaline province. Eudialyte-group minerals (EGMs) occur in one nepheline microsyenite dyke, associated with aegirine-augite, wohlerite, lavenite, magnetite, zircon, titanite, britholite, and pyrochlore. Major compositional variations include Si (25.09- 25.57 apfu ), Nb (0.31- 0.76 apfu ), Fe (1.40-2.13 apfu ), and Mn (1.36- 2.08 apfu ). The EGMs also contain relatively high contents of Ca (6.13- 7.10 apfu ), moderate enrichment of rare earth elements (0.38-0.67 apfu ), and a relatively low Na content (11.02-12.28 apfu ), which can be correlated with their transitional agpaitic assemblage. EGM compositions indicate a complex solid solution that includes eudialyte, kentbrooksite, feklichevite, zirsilite-(Ce), georgbarsanovite, and manganoeudialyte components. EGM trace element analyses show low Sr and Ba contents and a negative Eu/Eu* anomaly, which are interpreted as characteristic of the parental magma due to the previous fractionation of plagioclase and/or alkali feldspar. The EGMs from the dyke border have higher contents of Fe, Sr (2,161-2,699 ppm), Mg (1,179-3,582 ppm), and Zn (732- 852 ppm) than those at the dyke center. These differences are related to the incorporation of xenoliths and xenocrysts of melatheralitic host rock into the nepheline-syenitic magma followed by crystal-melt diffusive exchange.

New age determinations for the Banhadão and Itapirapuã complexes in the Ribeira Valley, southern Brazil

New age determinations by Ar-Ar and U-Pb SHRIMP for the alkaline complexes of Banhadao and Itapirapua that occur in the Ribeira Valley are in agreement with the previously radiometric K-Ar ages available in literature. Ar-Ar ages on biotite for Banhadao range in the 106-110 Ma interval, and suggest a minimum age of 106 Ma for the complex. Ar-Ar data on biotite for Itapirapua are more uniform, indicating an age of 102 Ma for the complex. U-Pb SHRIMP determinations on titanite from Itapirapua melanite syenitic rocks yielded an age of similar to 106 Ma. Thus, the age interval of 100 and 110 Ma does exist indeed, suggesting an intermediate magmatic pulse between the already well-defined peaks of 80-90 Ma and 130-140 Ma for the alkaline rocks in southern Brazil, confirming the periodicity of the magmatism.

In situ Sr isotope analyses by LA-MC-ICP-MS of igneous apatite and plagioclase from magmatic rocks at the CPGeo-USP

This contribution describes the successful implementation of in situ Sr isotope analyses by LA-MC-ICP-MS at the CPGeo-USP. The choice for an analytical configuration using measurements of half-masses allows the accurate assessment of lanthanide interferences, permitting the determination of Sr isotopes in important REE-rich accessory phases, such as apatite. Likewise, the on-peak-zero method effectively corrects the background contribution (both from Kr and residual Sr contributions from previous ablations) to the signals of the unknown samples. The analytical campaigns resulted in an accuracy, in respect to reference TIMS values, better than 57 ppm (~ ±0.000057 2σ SD) for a modern coral and the Batjberg clinopyroxene which impart significant quality to our data. Similarly, the majority of the stable Sr isotope ratios are close to the accepted values, which also confirms the effectiveness of the method. The achieved accuracy allows the identification and investigation of spatially-controlled isotopic heterogeneities on the micrometric scale in several Sr-rich minerals (apatite, carbonates, plagioclase, and clinopyroxene) with important implications to the understanding of relevant geochemical processes, particularly AFC, source geochemical heterogeneities and magma-mixing.