Carlos Eduardo de Mesquita Barros

Geochemistry of the Estrela Granite Complex, Carajás region, Brazil: an example of an Archaean A-type granitoid

Abstract The Estrela Granite Complex is an Archaean granite batholith which outcrops to the east of the Serra dos Carajas, northern Brazil. This complex, surrounded by supracrustal sequences metamorphosed under medium- to high-grade conditions, was affected by heterogeneous ductile deformation. Petrographically, this body exhibits a relatively wide facies variation where hornblende ± pyroxene monzogranites and hornblende ± biotite monzogranites dominate. The high alkali contents, the extremely high FeO 1 (FeO 1 + MgO) ratios and the elevated values of some high field strength elements (HFSE) demonstrate the alkaline affinities of the studied rocks. Al 2 O 3 -poor metaluminous monzogranites and Al 2 O 3 -rich weakly peraluminous monzogranites have been distinguished. Partial melting of meta-igneous or undepleted granulitic rocks could generate such granitic magmas. On the basis of its geochemical signature, the Estrela Granite Complex is thought to be an unequivocal example of A-type granite magmatism, being thus very different from granitoids reported southward in the Rio Maria Archaean granite-greenstone terrain, where the calc-alkaline and trondhjemitic suites are predominant.

Protracted deformation during cooling of the Paleoproterozoic arc system as constrained by 40Ar/39Ar ages of muscovite from brittle faults: the Transamazonan Bacajá Terrane, Brazil

In the Paleoproterozoic Transamazonas Province, synkinematic granitogenesis has taken place synchronously with compressive tectonic stress. The synkinematic character of the granites is marked by their WNW elongate shape, and by the presence of pervasive and concordant synmagmatic foliation. Ductile shear zones are concordant to the previous regional WNW structures, and tend to be accommodated along contacts between Rhyacian synkinematic granitoids and both Archean orthogneisses and Siderian metabasites. Locally phyllonitic shear zones and brittle-ductile shear zones with cataclasites are oriented subparallel to the preexisting ductile foliation. Late orogenic brittle faults N30E-trending strike-slip faults are either sinistral or destral. 40Ar/39Ar dating of muscovite developed on fault planes gave ages of 1977 ± 8 Ma and 1968 ± 11 Ma. Structural and geochronological data from rocks of the Transamazonas Province permit to conclude that most mylonites and brittle structures were controlled by preexisting structures such as geological contacts and petrographic facies boundaries. Compressive tectonic stress would have initiated at ca. 2100 Ma, since the former magmatic arc (Bacajai complex), still present at 2070 Ma when syntectonic granites were emplaced and remained until 1975 Ma after granite plutonism and regional cooling.

Petrologia e geoquímica da soleira de Medicilândia, diabásio Penatecaua, PA

A soleira de Medicilândia, localizada em cidade homonima no estado do Para, ocorre intrudida em rochas sedimentares da borda sul da Bacia do Amazonas e faz parte do evento magmatico Penatecaua de idade triassico-jurassica. Tendo como objetivo geral definir a evolucao do magmatismo responsavel pela geracao dessas rochas, foram realizados trabalhos de campo, coleta de amostras, analises petrograficas e estudos geoquimicos a partir de elementos maiores e elementos traco. A area mapeada abrange cerca de 300 km 2 e e composta por diabasios e gabros cuja assembleia mineral primaria compoe-se por augita, plagioclasio (andesina/labradorita), minerais opacos, apatita e quartzo. Quimicamente, as rochas sao supersaturadas em SiO 2 , classificadas como basaltos e andesito basaltos da serie toleitica, e sao divididas em quatro grupos geoquimicos, de acordo com a analise de mg* (MgO/(MgO + FeO) – grupo 1: mg* entre 0,5 e 0,6; grupos 2 e 3 com valores de mg* entre 0,4 e 0,5 e 0,3 e 0,4 respectivamente; e grupo 4 com mg* entre 0,1 e 0,3. O grupo 4 apresenta teor de TiO 2 entre 2 e 3%, enquanto os demais possuem TiO 2 < 2%. Os diagramas de variacao indicam que a evolucao do magma se deu por processo de cristalizacao fracionada com mudanca de assembleia fracionante. O mapa faciologico mostra zoneamento na soleira onde as facies primitivas localizam-se nas bordas, enquanto as evoluidas concentram-se no centro da intrusao. A hipotese mais provavel de evolucao da soleira indica uma origem relacionada com diferentes taxas de fusao parcial de uma mesma fonte, seguida por processo de cristalizacao fracionada.

Identification of mafic-ultramafic rocks using fuzzy logic classification of airborne geophysical data in the Anapu-Tuerê region, Brazilian Amazon

The Anapu-Tuerê region, in the Brazilian Amazon, is known for the occurrence of mineral deposits associated with mafic-ultramafic rocks. The spatial distribution of these rocks was investigated by integrating airborne magnetic and gamma ray spectrometry data using Geographic Information System (GIS) and fuzzy logic classification. Highest values of relevance fuzzy were associated with mafic-ultramafic rocks mapped regionally by the Brazilian Geological Survey (CPRM). Furthermore, mineral occurrences mapped by the CPRM showed good spatial correlation with the higher values of relevance fuzzy presented in this paper.