Revista Brasileira de Geociências, Volume 33, 2003 339 Delia del Pilar Montecinos de Almeida et al. Revista Brasileira de Geociências 33(4):339-348, dezembro de 2003 AN IGNEOUS EVENT AT THE FILDES PENINSULA (KING GEORGE ISLAND) AND AROUND FORT POINT (GREENWICH ISLAND), SOUTH SHETLAND ISLANDS, ANTARCTICA DELIA DEL PILAR MONTECINOS DE ALMEIDA 1 , ADRIANE MACHADO 2 , MARCO ANTÔNIO FONTOURA HANSEN 3 , FARID CHEMALE JR. 2 , HENRIQUE CARLOS FENSTERSEIFER 3 , KARLA PETRY 1 & LARISSA DE LIMA 1 1 - PPGeo-Centro de Ciências Exatas e Tecnológicas – Universidade do Vale do Rio dos Sinos- UNISINOS, Av. Unisinos 950 - CEP: 93022-000 São Leopoldo (RS) Brazil. e-mail: pilar@euler.unisinos.br. 2 - Departamento de Geociências – UFRGS – Av. Bento Gonçalves, 9500 - CEP: 91500-900 Porto Alegre (RS) – Brazil 3 - Curso de Geologia, Iniversidade do Vale do Rio dos Sinos - UNISINOS Resumo EVENTOS IGNEOS NA PENINSULA FILDES (ILHA REI GEORGE) E NO PONTAL FORT (ILHA GREENWICH), ISLAS SHETLAND DO SUL – ANTARTICA As ilhas estudadas fazem parte do arquipélago Shetland do Sul, com a ilha Rei George a nordeste e a ilha Greenwich na parte central do arquipélago. As suítes vulcânicas estudadas se manifestam como derrames e de maneira sub vulcânica, de composições basáltico-andesítica e andesítica. As rochas intrusivas variam de composição diorítica a granítica. Os padrões geoquímicos indicam a similaridade geoquímica de todas as rochas e que o vulcanismo precedeu o plutonismo. O comportamento dos elementos traço incompatíveis mostra a afinidade cálcio-alcalina de todas as rochas e sua geração em zonas de subdução. Anomalias negativas de Ta, Ti, P e Nb, típicas de arcos de ilhas, também são observadas. As razões iniciais Sr87/Sr86 e os valores positivos de eNd sugerem origem mantélica do(s) magma(s), com pouca ou nenhuma assimilação de crosta continental. As razões isotópicas de Pb atestam a similaridade isotópica entre todas as rochas. Com base nos resultados petrográficos, geoquímicos e isotópicos, conclui-se que: (a) as rochas vulcânicas do Pontal Fort e Peninsula Fildes são contemporâneas; (b) as rochas intrusivas representam termos mais evoluídos; (c) estas rochas se originaram a partir de uma única fonte magmática e sofreram processos de evolução similares e; (d) considerando as idades existentes, estas rochas posicionaram-se durante o Eoceno inferior. Palavras-chave: Antártica, magmatismo, geoquímica, ambiente tectônico. Abstract King George Island is located at the northeastern and Greenwich Island at the southwestern portions of the South Shetland Islands. The studied volcanic suits were emplaced as flows and subvolcanics of basaltic-andesitic and andesitic compositions. The intrusive rocks display dioritic to granitic compositions. Geochemical patterns indicate the geochemical similarity of the rocks and that the volcanism preceded the plutonism. Incompatible trace element behavior shows a calc-alkaline affinity and that these rocks were generated in a subduction zone. Large negative Ta, Ti, P and Nb anomalies, typical of an island-arc environment, are also observed. The initial Sr87/Sr86 ratios and the positive eNd suggest derivation from a mantle source with little or no assimilation of continental crust. Pb isotopic ratios attest to the isotopic similarity of all the rocks. Based on petrographic, geochemical and isotopic data, it is concluded that: (a) the volcanic rocks from Fort Point and the Fildes Peninsula are contemporaneous; (b) the intrusive rocks represent more evolved terms; (c) these rocks were originated from one single magmatic source and have undergone similar evolution processes and; (d) considering the available ages, these rocks were emplaced during Early Eocene. Keywords: Antarctica, magmatism, geochemistry, tectonic environment. INTRODUCTION The studied areas are located in Greenwich Island (Fig. 1) and King George Island (Fig. 2), which are part of the South Shetland Island. This islands is about 550 km long, along a NE-SW trend parallel to the Antarctic Peninsula. It is located 950 km southwest of Cape Horn, the southern extremity of South America, and 150 km northwest of the Antarctic Peninsula. The South Shetland Islands are separated from South America by the Drake Sea, and from the Antarctic Peninsula by the Bransfield Strait. Although the two studied islands are not adjacent, they belong to a group of islands that supposedly share the same geological and tectonic history. The South Shetland Islands contain volcanic and plutonic rocks of Early Cretaceous to Early Miocene age, related to the subduction of the SE Pacific oceanic crust beneath the Antarctic continent (Smellie 1983, Birkenmajer et al. 1988). Subsequent volcanism from Early Miocene (Birkenmajer et al. 1990) to recent times (e.g. Fisk 1990) is associated with rifting in the Bransfield Strait. The aim of this paper is to petrographically and geochemically characterize the plutonic and volcanic rocks of the Fildes Peninsula (FP) and Fort Point (FPt), in order to determine the temporal relations between the magmatism in the two regions. The research comprised a petrographic study complemented by electron microprobe (EMP) analyses of selected minerals to determine the petrographic similarities and differences between intrusive and volcanic rocks from the two islands. The EMP analyses were carried out at Federal University of Rio Grande do Sul (UFRGS). Additionally, nine samples from the Fildes Peninsula were geochemically analyzed in the X-ray Fluorescence Laboratory at UFRGS and at the Energy, Nuclear and Neutron Activation Research at Universidade de São Paulo (USP). From Fort Point, 14