As rochas alcalinas de Cananéia, litoral sul do Estado de São Paulo: estudos mineralógicos, geoquímicos e isotópicos / Alkaline rocks of Cananéia, south coast of the state of São Paulo: mineralogy, geochemistcal and isotopical studies

AUTOR(ES)
DATA DE PUBLICAÇÃO

2007

RESUMO

The alkaline occurrence of Cananéia, SE Brazil, lying in the southern part of the homonymous island, is a subcircular intrusion 1.8 km² in area (Morro de São João) and 137 m high. A small and petrographically similar satellite body (Morrete) is found in the adjacent Ilha Comprida. Geomorphologically, it belongs to the Cananéia-Iguape Coast Plain. It is covered by Quaternary sediments and is enclosed into the crystalline basement (Grupo Açungui), which outcrops some kilometers away, in the continental area. Regarding tectonical control, together the neighboring alkaline-carbonatitic complexes of Jacupiranga, Juquiá and Pariquera-Açu, the intrusion is related to the Guapiara Lineament - one of the major structural features associated with the Ponta Grossa Arch. The occurrence is constituted exclusively by syenitic rocks, with alkali feldspar syenites and quartz-alkali feldspar syenites representing the main lithologies, besides microsyenites as small dikes and veins. From the mineralogical point of view, these rocks have alkaline feldspar (mesoperthite) as the major abundant phase. The mesoperthite belongs to the hypersolvus type, being in general present in the more evolved rock types. Plagioclase is scarce and quartz corresponds to the second felsic mineral in importance. Clinopyroxene and amphibole widely variable in composition are the most common mafic phases whereas biotite and olivine occur subordinately. Main accessory minerals include apatite, titanite, opaques and zircon. The crystallization conditions point to formation temperatures around 950°C for the alkali-feldspar syenites and 800°C for the quartz-alkali feldspar syenites, associated with a slightly oxidized environment and a less oxidized one under low pressure (<1 kbar), respectively. Geochemical data confirm the saturated to oversaturated in SiO2 nature of the Cananéia rocks, the potassic affinity and their high degree of evolution. The multielemental diagrams relating major and trace element contents against silica concentration indicate that fractional crystallization processes have played an important role on its formation. The normalized distribution of the incompatible elements show pronounced negative anomalies in Sr, P and Ti and subordinate in Ba. Zr and Nd display visible positive spikes, which are less evident in La, Y and Ba. In general, the behavior is quite comparable to that recognized for other Late Cretaceous Brazilian alkaline occurrences, mainly consisting of evolved rocks. The normalized distribution of the REE patterns point to their high concentration and to the enrichment of LREE in relation to HREE. The strong Eu negative anomaly could be due to feldspar removal process or to a possible assimilation of granitic material from the country rocks. Ar/Ar radiometric determinations carried out on biotite separates provide an age of 83.6±0.9 Ma for the syenitic rocks, which is similar to that given by an internal Rb-Sr isochronous, showing a value of 83.5±2.2 Ma and 87Sr/86Sr initial ratio of 0.70686. The 87Sr/86Sr(83,6 Ma) initial ratios (R0) vary between 0.7065 and 0.70700 for the less differentiated lithotypes and between 0.70538 e 0.70777 for the most evolved ones. The high values of the Cananéia rocks, in general over 0.706, clearly indicate that assimilation and crustal contamination processes have participated of its formation, as it is suggested in the R0 versus SiO2 graph, where the highest values correspond to the varieties having the largest amounts of silica, and in the normative nepheline-quartz-kalsilite diagram (petrogenetical residual system). Isotopical studies (Sr-Nd) and comparison with other alkaline occurrences of SE Brazil suggest Fernando Pelegrini Spinelli (2007) 7 that the Cananéia rocks probably developed from a parental magma of basanitic nature. Also, it is believed that its formation has involved metasomatic enrichment processes of the mantle occurred in Proterozoic times as well as the contribution of HIMU and EMI source types. Model age (TDM) calculations give values higher than those available for other alkaline occurrences of the southern region, except for the two Santa Catarina complexes (Anitápolis and Lages). The average age (1400±200 Ma) is only comparable to that shown by the alkaline potassic occurrences from the western side of the Paraná Basin, in Paraguayan territory (1500±200 Ma). Comparative study with other Brazilian alkaline occurrences almost exclusively formed of syenitic lithologies allows recognize similarities between the Cananéia rocks and those of some massives of the Serra do Mar (Serra da Mantiqueira: Itatiaia, Passa Quatro e Morro Redondo; oceanic islands: Vitória, Monte de Trigo, Búzios e São Sebastião) and Ponta Grossa Arch (Tunas) Provinces and of the Ponta do Morro intrusion in the Mato Grosso State, especially regarding geochemical characteristics (behavior of incompatible and Rare Earth elements) and age (Late Cretaceous).

ASSUNTO(S)

petrology petrologia alkaline rock geoquímica cananéia mineralogia mineralogy geochemistry cananéia rocha alcalina

ACESSO AO ARTIGO

Documentos Relacionados