Geochemistry and Geochronology of Horseshoe Island (Antarctic Peninsula) Igneous Rocks

The geotectonic evolution of the Antarctic Peninsula (AP), one of the most important parts of the West Antarctic mosaic, began before the breakup of the Gondwana continent during the Triassic. The geotectonic events that formed the AP continued after the breakup. Horseshoe Island, located near the center of the AP, contains crystalline records of these geological events. This study presents the magmatic stratigraphy of Horseshoe Island was reconstructed using the LA-ICP-MS U-Pb ages of zircon crystals from plutonic rocks of the island, collected during the Turkish Antarctic Science Expedition-VII (2023). The obtained ages are matched to the plutonic units mapped thoroughly by Matthews (1983) as follows. Old to young: U-Pb zircon analyses of rocks from the Antarctic Peninsula Metamorphic Complex, which consists of metamorphic rocks ranging in grade from meta-granites to gneisses, yielded mean ages ranging from 211.5 ± 1 Ma to 175.1 ± 0.5 Ma (MSWD=0.1-13). The light-colored, medium-grained Homing Head Granites, which surface south of Homing Head along the northern slopes of Mount Searle, yielded ages ranging from 108.83 ± 0.47 Ma to 106.19 ± 0.46 Ma (MSWD=0.3-1.1). The age of the myrmecitic textured brick-red coloured Gaul Cove Granites within the Andean Plutonic Series is between 106.9 ± 0.4 Ma and 103.4 ± 0.7 Ma. The coarse-grained Mite Diorite observed in a narrow area south of Lystad Bay yielded an age of 91.0 ± 0.2 My. The Sally Cove gabbro, observed on the southern slopes of Sally Cove, the northernmost of the rocky islands in Lystad Bay and in a narrow area on the beach where the Turkish Scientific Research Station is located on the island gave ages ranging from 73.4 ± 0.5 Ma to 72.0 ± 0.4 Ma. In the west of the island, U-Pb ages ranging from 74.6 ± 1.1 Ma to 72.78 ± 0.99 Ma were obtained from zircons from the coarse-grained pink-coloured Beacon Point Granites, which cut the gabbros and surface along the northern slopes of Lystad Bay. Jurassic and Cretaceous plutons of island have I-type geochemical character. The isotopic compositions of I_Sr, ε_Nd(t) and ε_Hf(t) indicate that the Homing Head Granodiorite, Gaul Cove Granite, and the Upper Beacon Head Quartz Monzonite were formed by partial melting of crustal sedimentary rocks. The Mite Diorite and Sally Cove Gabbro are hybrid magmas formed by amphibolitic mafic lower crust dominated and lithospheric mantle contribution. When geochemical data are evaluated in tectonic discrimination diagrams, Sally Cove Gabbro and Beacon Head Quartz Monzonite have the characteristics of rocks formed in intraplate environments, while Homing Head Granodiorite, Gaul Cove Granite and Mite Diorite have the characteristics of rocks formed in subduction environments.

This study was carried out under the auspices of the Presidency of the Republic of Turkey, Ministry of Industry and Technology of the Republic of Turkey and in coordination with TÜBİTAK MAM Polar Research Institute (KARE) and supported by TÜBİTAK grants 122Y192 and 122G261.