Preliminary Results on the Geology and Petrography of the Burro Peaks and Cecilia Intrusions, South Shetland Islands (Antarctica): Constraints for Different Emplacement Depths

As part of the 8th Turkish Antarctic Expedition (TAE-IV) and Turkey-Ecuador Bilateral Cooperation, field studies were conducted on Dee and Cecilia Islands, two small islets situated between Robert and Greenwich Islands in the South Shetland Archipelago (Western Antarctica). This study explores the geological characteristics and petrography of gabbroic intrusions on these islands, offering insights into their emplacement depths.

The geological setting of these islands is formed from volcanic and intrusive rocks, with a minor presence of sedimentary rocks distributed throughout the landmass of the islands. Volcanic rocks primarily consist of basaltic lavas, while sedimentary rocks are represented by conglomerates. The Burro Peaks (Dee Island) and Cecilia (Cecilia Island) gabbroic intrusions share a broadly similar mineralogical composition—dominated by plagioclase, orthopyroxene, and olivine with minor opaque minerals—but exhibit different textural and structural characteristics. The Burro Peaks Intrusion is distinguished by its fine-grained, holocrystalline porphyritic texture and orthogonal cooling joints, indicative of rapid cooling at shallow crustal levels. Disequilibrium textures such as sieve-textured plagioclase, embayed crystals, and multiple generations of plagioclase are in line with this shallow-level emplacement. In contrast, the Cecilia Intrusion displays a coarser-grained holocrystalline granular texture, predominantly one-dimensional joints, and euhedral to subhedral crystals with minimal zoning or reaction rims, pointing to slower cooling at relatively deeper levels. Basaltic lavas on both islands share a similar mineralogical composition with the gabbros but are texturally distinct, exhibiting phaneritic to hemicrystalline-porphyritic textures with pilotaxitic to intersertal groundmass. On Cecilia Island, basaltic lavas overlie the intrusion in the western part, while the contact relationship between lavas and the Burro Peaks Intrusion is unclear in the field.

The field and petrographical data collectively suggest that the Burro Peaks and Cecilia intrusions were emplaced at different crustal levels. The similar compositions of the intrusions and lavas, along with their spatial relationships, support the hypothesis that they may have originated from a common magmatic system but were emplaced at varying depths rather than through separate magmatic events. However, geochronology and geothermobarometry studies are planned as future work to test this hypothesis further and elucidate the interplay with tectonic events within the framework of the South Shetland Arc.