Modeling of subduction components in the Genesis of the Meso-Cenozoic igneous rocks from the South Shetland Arc, Antarctica Adriane Machadoa,T, Farid Chemale Jr.a, Rommulo V. Conceic¸a˜oa, Koji Kawaskitaa, Diego Moratab, Orlando Oteı´zab, William R. Van Schmusc aFederal University of Rio Grande do Sul, Institute of Geosciences, Isotope Geology Laboratory, Av. Bento Gonc¸alves, 9500, Campus do Vale, Agronomia, 91501-970, Porto Alegre, RS, Brazil bDepartment of Geology, University of Chile, Casilla 13518, Correo 21, Santiago, Chile cDepartment of Geology, University of Kansas, Lawrence, KS 66045, USA Abstract Isotope data and trace elements concentrations are presented for volcanic and plutonic rocks from the Livingston, Greenwich, Robert, King George and Ardley islands (South Shetland arc, Antarctica). These islands were formed during 87 86 subduction of the Phoenix Plate under the Antarctica Plate from Cretaceous to Tertiary. Isotopically ( Sr/ Sr)o ratios vary from 143 144 0.7033 to 0.7046 and ( Nd/ Nd)o ratios from 0.5127 to 0.5129. qNd values vary from +2.71 to +7.30 that indicate asthenospheric mantle source for the analysed samples. 208Pb/204Pb ratios vary from 38.12 to 38.70, 207Pb/204Pb ratios are between 15.49 and 15.68, and 206Pb/204Pb from 18.28 to 18.81. The South Shetland rocks are thought to be derived from a depleted MORB mantle source (DMM) modified by mixtures of two enriched mantle components such as slab-derived melts and/or fluids and small fractions of oceanic sediment (EM I and EM II). The isotopic compositions of the subduction component can be explained by mixing between at least 4 wt.% of sediment and 96 wt.% of melts and/or fluids derived from altered MORB. Keywords: Subduction; Fluids; Island arc magmas; Pb isotopes; Nd isotopes; Sr isotopes 1. Introduction T Corresponding author. Tel.: +55 51 33166398; fax: +55 51 Island arcs are central to the understanding of 33166340. mantle evolution because they represent the site where E-mail address: [email protected] (A. Machado). crustal material of various types may be returned to 436 A. Machado et al. / Lithos 82 (2005) 435–453 the deep mantle. Island-arc magmatism may allow us typical island arc rocks, which constitute the bulk to sample this material that is in the process of being of the worldwide database for subduction-related recycled. compositions. White and Dupre (1986) and Ellam and Hawkes- In this paper, Sr, Nd and Pb isotopic ratios, worth (1988) proposed three component-mixing together with trace element compositions are pre- models for the island arcs mantle evolution. These sented from samples of the South Shetland Islands. involve contamination of the depleted-mantle source We use these data to discuss the South Shetland Arc of IAB with partial melts of subducted sediment and mantle source and processes of magma genesis. The large-ion lithophile element-enriched slab-derived studied samples were collected at western Livingston fluids. Evidence for such a process is seen when Island (Byers Peninsula, Fig. 2), southeastern Green- abundances of low-field-strength large-ion lithophile wich Island (Fort Point, Fig. 3), western Robert elements (LILE), such as Cs, Rb, Ba, Sr, are Island (Coppermine Peninsula, Fig. 4), southwestern normalized against high-field-strength elements King George Island (Fildes Peninsula, Fig. 5) and (HFSE), such as Nb, Zr, Hf, Y. This argument is northeastern Ardley Island (Fig. 5). The ages from based on the ratios of mobile and immobile the South Shetland samples vary from 143 Ma to 44 elements in aqueous fluids, ratios of LILE, HFSE Ma. (Grikurov et al., 1970; Gracanin, 1983; Smellie and light rare earth elements (Ishikawa and Naka- et al., 1984; Hathway, 1997; Oteiza, 1999; Pankhurst mura, 1994; Ryan et al., 1995; Scambelluri and et al., 2000). Philippot, 2001). The South Shetland Islands arc (Fig. 1) provides an opportunity to compare and contrast 2. Geological setting the effects of fluids and sediment involvement in a subduction zone. The rocks from the South Shet- The continent of Antarctica is divided into two land Islands are apparently uncontaminated by large geologic areas: East and West Antarctica. East intra-crustal components and they should have Antarctica is the large bean-shaped land mass centered largely evolved through simple fractional crystal- on 908 east longitude. West Antarctica is the area lization from a primitive island arc basalt precur- centered on 908 west longitude and includes the sor. These samples are similar in most aspects to Antarctica Peninsula. ° ° 62 W 58°W Elephant 54 W ° South 61 S Am. South N Shetland Islands Clarence Gibbs 90° Antarctica Drake Sea King George 0 2400 km 180° Nelson Robert Greenwich Snow Livingston rait Deception St Smith ld D'Urville fie ns Antarctica ra 64°S Low B Peninsula Joinville 0 100 km Dundee Fig. 1. Location map of the South Shetland Islands (modified from Machado, 1997). A. Machado et al. / Lithos 82 (2005) 435–453 437 61°S 60°W N N Drake Sea Greenwich Snow Livingston Smith Deception Low 64° W 0 10 km Start Point Byers Peninsula 128 ± 3 Ma (K-Ar - Pankhurst et al., 1979) ± 123 4 Ma Villard Point 109 ± 4 Ma (K-Ar - Pankhurst et al., 1979) (K-Ar - Pankhurst et al., 1979) 143 ± 5 Ma (Ar-Ar - Gracanin, 1983) 98012301 98012202 Chester Cone Negro Hill 98012002 President Beaches 98012501 78 ± 5 Ma (K-Ar - Oteíza, 1999) 98011501 Devils 111 ± 4 Ma Point (Rb-Sr - Smellie, 1984) 120.3 ± 2.2 Ma 119.4 ± 0.6 Ma 119.1 ± 0.8 Ma Legend (Ar-Ar - Hathway, 1987) Byers Formation Hypabyssal intrusions Agglomerates Agglomerate Member Clastic rocks Agglomerates Volcanic Member Lavas Clastic rocks Mixed marine Agglomerates Member Lavas Mainly mudstones and siltstones Mudstones Member 0 2 km Fault Sample Fig. 2. Sketch of the geological map of Byers Peninsula (Livingston Island) showing sample locations and age of the studied rocks (modified from Smellie et al., 1984). 438 A. Machado et al. / Lithos 82 (2005) 435–453 N Fort Point HA-19.1 N HA-20 Robert Drake Sea English Strait Duff Point Discovery Bay PA-8.1A Greenwi 62°30'S ch HY-4 Yankee HA-8.2A HA-1.1 Mc. FarlaneBay Strait 0 3km ° Livingston 59 45' W Outcrops Samples 0 500m Fig. 3. Sketch of the geological map of Fort Point (Greenwich Island) showing sample locations of the rocks (modified from Azevedo, 1992). East Antarctica is a large Precambrian shield; a likewise these mountains, is made of igneous stable portion of a continent composed of old rocks intrusive and volcanic as well as metamorphosed that have changed very little over a long time. The sedimentary rocks. oldest rocks found in this area are over 3 billion years The Antarctica Peninsula is bordered by a complex old. These are metamorphic rocks overlaid by system of tectonic plates, including the South Amer- younger, flat-lying ocean-deposited sediment. The ica, Scotia, Drake/Phoenix, South Orkney and Sand- rocks were recrystallized during a mountain building wich plates, which are dominated by extensional and episode caused by plate collision, in the early strike-slip tectonic limits (Fig. 6). This configuration Paleozoic Era (about 500 Ma). was formed only in the last 38 Ma with the opening of West Antarctica was built up over the last 500 Drake Passage and Scotia Sea (Barker and Burrell, million years by the addition of small continental 1977; Barker et al., 1991). Before 38 Ma the fragments (microplates) that have built up the relationships between the major plates were marked mountains of West Antarctica. Unlike East Antarctica, by destructive plate interaction generating Mesozoic– if the ice were removed in the west, the land would Cenozoic accretionary wedges, island arcs and back- have considerable relief. The area would probably arc basins recorded at the South Shetland Islands, appear as a series of island chains and mountain Antarctic Peninsula and Patagonia (Dalziel, 1984; ranges. Lawver et al., 1996). The Antarctica Peninsula and the rest of West The tectonic context of the South America-Scotia- Antarctica were the most recent additions. The Antarctic plate junction has been related to a complex Andean Orogeny of late Mesozoic and early evolution from Paleozoic–Mesozoic to the present. Cenozoic (about 60–80 million years ago) formed This evolution is marked by several tectonic episodes the peninsula. This activity coincided with the final from 250 Ma to 20 Ma, such as the Paleozoic– breakup of Gondwana as South America, Australia, Mesozoic Samfrau Orogeny, early processes of and Antarctica split apart. The peninsula is an Gondwana fragmentation, Gondwana break-up, Phoe- extension of the Andes of South America, and nix plate subduction, arc volcanism in the South A. Machado et al. / Lithos 82 (2005) 435–453 439 N Robert Drake Sea English Strait N Greenwich 62°30' S M c. Farlane Strait 0 3km 59°45' W Livingston 60 ± 1 Ma (K-Ar Smellie et al., 1984) Fort Willian 96020701 Coppermine Peninsula 42°22' S English Strait 96022302 83 ± 2 Ma 78 ± 2 Ma (K-Ar Smellie et al., 1984) Coppermine Cove Legend Coppermine Formation Tuffs, volcanic brecchas and agglomerates Olivine-basalt lavas Basaltic intrusions Ice and Snow 96022001 Samples Fault Triplet 96022005 59°44' W 0 1km Fig. 4. Sketch of the geological map of Coppermine Peninsula (Robert Island) showing sample locations and age of rocks (modified from Smellie et al., 1984). Shetland Islands, and extensional tectonism in the lands to South Sandwich Arc where four phases of Antarctica Peninsula. island arc volcanism have been identified: 130–110 The collision of a spreading centre at the Antarctica Ma; 90–70 Ma; 60–40 Ma; 30–20 Ma (Birkenmajer et Peninsula trench caused the migration of magmatism al., 1986).