Source characteristics of the adakitic lavas and intrusions in Fildes Peninsula, King George Island (South Shetland Islands, Antarctica)

Abstract

South Shetland Islands (western Antarctica) host widespread magmatism through the Meso-Cenozoic as a result of the subduction of the Phoenix plate along the South Shetland trench. With the logistical support of the Czech Antarctic Research Center and in-kind assistance from TÜBİTAK MAM Polar Research Institute, geological fieldwork was conducted on the Fildes Peninsula in order to understand the magma evolution beneath western Antarctica. This study presents preliminary results from field, petrographic and geochemical studies obtained from the volcanic and intrusive rocks in Fildes Peninsula- King George Island. Fildes Peninsula represents the southwestern parts of King George Island which is located at the northeastern tip of the South Shetland Islands. The dominant lithologies in the study area are Paleocene- Eocene volcanic and intrusive rocks, with a minor presence of sedimentary rocks. The volcanic rocks cropping out on Fildes Peninsula are referred to as the Jasper Hill, Agate Beach, Block Hill and Long Hill formations. They mostly display similar compositions changing from basalt to basaltic andesite lavas and accompanied by pyroclastic rocks (tuffs and volcanic breccias). Intrusive rocks are composed of gabbro-micro gabbro stocks and diabase dykes. Petrographic investigations show that volcanic and intrusive rocks in the area mostly display disequilibrium textures such as sieve textures and embayments in plagioclase and pyroxenes, patchy and oscillatory zoning in different generations of plagioclases. Geochemically, all volcanic rocks show similar characteristics; they are represented by basic rocks that mostly display tholeiitic affinity. Their MgO and SiO2contents range from 3.04 to 6.18 and 44.80 to 48,47wt. %, respectively. The samples are slightly enriched in large ion lithophile elements (LILE) and light rare earth elements (LREE) compared to N-MORB and they display depletions in Nb and Ti elements which are the typical indicators of subduction zone magmatism. All volcanic rocks display low Y and high Sr/Y contents which are typical for adakites.  These adakites are specifically represented by “low-silica adakites” due to their low SiO2 (<48 wt. %). They also have low Zr and high Zr/Y abundances suggesting garnet in their source. These major- trace element characteristics and published Sr-Nd-Pb isotope compositions may collectively suggest that the adakites of Fildes Peninsula were originated from the partial melting of the mantle-wedge which was metasomatized by subduction processes along the South Shetland trench.