The Early Triassic magmatism of the Alto ParaguayProvince, Central South America: Paleomagneticand ASM data

Abstract A paleomagnetic studystudywork was carriedout on the Alto Paraguay Province (APP), a belt of alkalinecomplexes that parallel the Paraguay river for morethan 40 km at the border of Brazil and Paraguay. Theprovince is well dated by 40Ar/39Ar method giving ages inthe range 240–250 Ma with a preferred age of 241 Ma. Intrusiverocks are predominant but the stocks may be toppedby lava flows and ignimbrites. Paleomagnetic work onstocks, dikes and flows of the APP identified normal andreversed magnetic components which are carried mainlyby titanomagnetites. The calculated paleomagnetic polelocated at 319ºE 78ºS (α95 = 6º; k = 23) is in agreementwith other South American poles of Permo-Triassic age.Most of the sampling sites showed large variations in rockmagnetization, but similar patterns in the variation of thewithin-site magnetizations, mainly in dikes, suggest geomagneticpolarity transition records. The magnetizationdata along with the anisotropy of magnetic susceptibilitydeterminations suggested that the South and North areasof the province have different evolution characteristics

Ilha dos Búzios, Litoral Norte do Estado de São Paulo: aspectos geológicos e petrográficos

Cretaceous syenitic rock-types intruded into Precambrian charnockites characterize the geology of Búzios island (7.5 km²) in area off the northern coast of the State of São Paulo, SE Brazil. Both lithologic units are cut by a large number of dykes, mostly NE-trending and quite variable in composition. The syenites may include charnockitic and mafic rock xenoliths and also contain quartz-filled, irregular miarolitic cavities. The alkaline rocks, corresponding to approximately 90% of the whole island, are dominantly coarse-grained and range in composition from alkali feldspar syenites to quartz-alkali feldspar syenites. Locally, fine-grained syenitic types are also found. Dykes are widespread over the island and can be placed into the felsic group, which includes lithologies varying from phonolites to trachytes to rhyolites (microgranites), or into the mafic-ultramafic group, the latter mainly consisting of diabases, microdiorites and lamprophyres. Alkali feldspar (micromesoperthite with slight dominance of albititic phases) is the most abundant mineral in syenitic rocks. Additional components are clinopyroxene (diopside-augite ranging to aegirine-augite), commonly replaced by amphibole/biotite, and opaques (magnetite, ilmenite), with apatite, sphene and zircon as accessories. The felsic dykes may contain other feldspars (sanidine and albite) either as micro- to phenocrysts or as groundmass phases; feldspathoids (nepheline, sodalite) are also found in some microsyenites. The mafic-ultramafic dykes, in particular the lamprophyres, are mainly characterized by a primary mineral assemblage consisting of olivine, clinopyroxene (titanian augite) and amphibole (kaersutite), besides a groundmass bearing glassy material and analcite. Also to be noted in the lamprophyres is the presence of small grains or aggregates of carbonate ocelli as primary phases. Dykes showing a more basaltic composition have zoned plagioclase (andesine-labradorite), interstitial alkali feldspar and, as main ferromagnesian minerals, clinopyroxene (diopside-augite), amphibole, biotite and opaques. In spite of the petrographic and chemical differences, the alkaline bodies forming the islands of Vitória, Búzios, São Sebastião and Monte de Trigo, all found off the northern coast of the State of São Paulo, seem to be part of magmatic events related to the same geologic setting. They probably represent contemporaneous intrusions derived from an anomalously hot mantle and are closely associated with the Mesozoic tectonic evolution of Southern Brazil.Rochas sieníticas cretácicas introduzidas em charnoquitos precambrianos constituem a geologia da Ilha dos Búzios (7,5 km²), situada no litoral norte do Estado de São Paulo, região sudeste do Brasil. Ambas unidades litológicas estão cortadas por grande número de diques, orientados preferencialmente para NE e variáveis em composição. Os sienitos podem incluir xenólitos de charnoquitos e de rochas máficas, além de também conter cavidades miarolíticas, de forma irregular e preenchidas ou não com quartzo. As rochas alcalinas, cobrindo aproximadamente 90% da ilha, são dominantemente de granulação grossa e variam em composição de álcali feldspato sienitos a quartzo-álcali feldspato sienitos. Localmente, podem ser também encontradas variedades sieníticas de granulação fina. Os diques, de ocorrência generalizada por toda a ilha, são enquadrados no grupo félsico, que reúne litologias variando de fonólitos a traquitos e riólitos (microgranitos), ou no máfico-ultramáfico, este consistindo principalmente em diabásios, microdioritos e lamprófiros. Feldspato alcalino (micromesopertita com pequeno predomínio das fases albíticas) é o mineral mais abundante das rochas sieníticas. Outros constituintes incluem clinopiroxênio (diopsídio-augita variando até egirina-augita), que se mostra comumente substituído por anfibólio/biotita, além de opacos (magnetita, ilmenita), apatita, titanita e zircão como principais acessórios. Os diques félsicos podem conter outros feldspatos, como sanidina e albita, quer na condição de micro a fenocristais, quer como membros da massa fundamental; feldspatóides (nefelina, sodalita) estão também presentes em alguns microssienitos. Os diques máfico-ultramáficos, em particular os de lamprófiros, são caracterizados por uma associação mineralógica primária consistindo em olivina, clinopiroxênio (augita titanífera) e anfibólio (kaersutita), além de massa fundamental contendo material vítreo e analcita. Digna também de registro nos lamprófiros é a presença de pequenos grãos ou agregados de ocelos carbonáticos de formação primária. Diques de natureza mais basáltica possuem plagioclásio (andesina-labradorita) zonado, feldspato alcalino intersticial e clinopiroxênio (diopsídio-augita), anfibólio, biotita e opacos como principais minerais ferromagnesianos. Não obstante as diferenças petrográficas e químicas, os corpos alcalinos formando as ilhas de Vitória, Búzios, São Sebastião e Monte de Trigo, todas elas situadas na costa oriental do Estado de São Paulo, parecem corresponder a eventos magmáticos apresentando o mesmo condicionamento geológico. Eles muito provavelmente representam intrusões contemporâneas associadas a anomalias térmicas no manto e estão intimamente ligados à evolução tectônica Mesozóica da porção sudeste do Brasil

Petrography and mineral chemistry of carbonatites and mica-rich rocks from the Araxá complex (Alto Paranaíba Province, Brazil)

The Araxá complex (16 km²) comprises carbonatites forming a central core and a complex network of concentric and radial dykes as well as small veins; additionally, it includes mica-rich rocks, phoscorites and lamprophyres. Fenites also occur and are represented by Proterozoic quartzites and schists of the Araxá Group. The petrographic study of 130 borehole samples indicates that the complex is basically made up by two rock-types, carbonatites and mica-rich rocks, and subordinately by a third unit of hybrid composition. Carbonatites range chemically in composition, the most abundant type being magnesiocarbonatites. Dolomite and calcite correspond to the chief constituents, but other carbonate phases, including the Ce-group RE minerals, are also recognized. Phosphates and oxides are widespread accessories whereas silicate minerals consist of olivine, clinopyroxene, mica and amphibole. Mica-rich rocks are represented by abundant glimmeritic rocks and scarce cumulitic phlogopite-, olivine- and diopside-bearing pyroxenites. Hybrid rocks mainly contain phlogopite and tetraferriphlogopite as cumulus and intercumulus phases, respectively; carbonate minerals may also be found. Chemical data indicate that the carbonatites are strongly enriched in REE and have lower contents of Nb, Zr, V, Cr, Ni and Rb compared to the mica-rich rocks. The higher K, Nb and Zr contents of the latter rocks are believed to be related to metasomatic processes (glimmeritization) of the pyroxenites. Similar REE patterns for carbonatites and mica-rich rocks seem to suggest that they are related to a single parental magma, possibly of ijolitic composition. Steep LREE/HREE fractionation and high sigmaREE content of some carbonatite samples would be explained by hydrothermal and supergenic processes

Abstracts

Resumos, em língua inglesa, dos artigos publicados nesta edição.Resumos, para a língua inglesa, dos artigos publicados nesta edição

Tropical and Boreal Forest Atmosphere Interactions : A Review

This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiala in Finland. The review is complemented by short-term observations from networks and large experiments. The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction. Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink. It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.Peer reviewe

Tropical and Boreal Forest Atmosphere Interactions : A Review

This review presents how the boreal and the tropical forests affect the atmosphere, its chemical composition, its function, and further how that affects the climate and, in return, the ecosystems through feedback processes. Observations from key tower sites standing out due to their long-term comprehensive observations: The Amazon Tall Tower Observatory in Central Amazonia, the Zotino Tall Tower Observatory in Siberia, and the Station to Measure Ecosystem-Atmosphere Relations at Hyytiala in Finland. The review is complemented by short-term observations from networks and large experiments. The review discusses atmospheric chemistry observations, aerosol formation and processing, physiochemical aerosol, and cloud condensation nuclei properties and finds surprising similarities and important differences in the two ecosystems. The aerosol concentrations and chemistry are similar, particularly concerning the main chemical components, both dominated by an organic fraction, while the boreal ecosystem has generally higher concentrations of inorganics, due to higher influence of long-range transported air pollution. The emissions of biogenic volatile organic compounds are dominated by isoprene and monoterpene in the tropical and boreal regions, respectively, being the main precursors of the organic aerosol fraction. Observations and modeling studies show that climate change and deforestation affect the ecosystems such that the carbon and hydrological cycles in Amazonia are changing to carbon neutrality and affect precipitation downwind. In Africa, the tropical forests are so far maintaining their carbon sink. It is urgent to better understand the interaction between these major ecosystems, the atmosphere, and climate, which calls for more observation sites, providing long-term data on water, carbon, and other biogeochemical cycles. This is essential in finding a sustainable balance between forest preservation and reforestation versus a potential increase in food production and biofuels, which are critical in maintaining ecosystem services and global climate stability. Reducing global warming and deforestation is vital for tropical forests.Peer reviewe