The Petrology of Gabbroids of the Anagram Islands (Wilhelm Archipelago, West Antarctica)

Abstract

The Anagram Islands are located near the Graham Coast of West Antarctica not far from the Ukrainian Antarctic Station "Akademik Vernadsky". Gabbroids that are probably Cretaceous in age predominate in the geological structure of the Anagram Islands. New data on the mode of occurrence of the Anagram gabbroids and their layering were obtained during the seasonal field works of the Ukrainian Antarctic Expeditions in 2017, 2019-2020 years. Samples of the gabbroids were examined by optical microscopy, electron microscopy and microprobe analysis as well as by measuring of anisotropy of magnetic susceptibility of rocks. The goal of the research was to clarify the geological position and petrographic features of the Anagram gabbroids, to determine the typomorphic features of rock-forming minerals, and to clarify the origin of the layering in gabbroids. It was determined that the bottom part of a large layered gabbroid intrusion is fragmentarily exposed on the Anagram Islands. The continuation of the intrusion is expected in the northeastern and southeastern directions. It was established that primary magmatic textures and mineral composition are preserved in the gabbroids despite the development of metamorphic amphibolization. The gabbroids show typical cumulative textures of magmatic origin. The main cumulative phases are plagioclase (An70-95), clinopyroxene (Wo41-49En34-47), orthopyroxene (Wo2-4En49-71), olivine (Fo67-69), Ti-magnetite and ilmenite. It is proved that the fine rhythmic-gradational layering in the gabbroids was formed by gravitational precipitation of the cumulative minerals on the bottom of the magma chamber and their further sorting by specific gravity due to convection currents. Cryptic layering is firstly discovered in the Anagram gabbroids and is explained by the processes of crystallization differentiation. The latter caused the accumulation of the most calcium plagioclases and the most magnesian mafic minerals in the layered gabbroids forming the bottom part of the intrusion. The present subvertical position of layering is explained by tectonic deformation that resulted in the tilting of the investigated section of the Anagram gabbroid intrusion. This confirmed by the studying of magnetic texture of the gabbroids (by the anisotropy of magnetic susceptibility data).