Contrasting structural styles during polyphase granitoid intrusion, South Victoria Land, Antarctica

Abstract

Four granitoid plutons intruded in the Walcott Bay area, South Victoria Land, during the Paleozoic Ross Orogeny, display markedly different structural styles. Bonney Pluton is a variably megacrystic hornblende-biotite granite batholith which outcrops over an area of 1000 km2. The southern end of the body is located in the Walcott Bay area, and it is the oldest of the four granitoid bodies (530–520 Ma). The pluton synkinematically intruded Koettlitz Group metasediments under mid-upper amphibolite facies conditions (700°C, 5 ± 1 bars). Bonney Pluton fabrics are highly variable, with weak magmatic fabrics in the pluton centre grading to well-developed, solid-state deformation fabrics in high strain zones at the southwestern margins. Armitage Pluton is a non-megacrystic hornblende-biotite body, which intruded central parts of Bonney Pluton while it was incompletely crystallised. The body forms randomly oriented dikes and stocks and is chemically distinct from Bonney Pluton. A weak magmatic fabric is developed through the body, subparallel to magmatic lineations in central parts of Bonney Pluton. Chancellor Orthogneiss is a strongly deformed biotite granite, which discordantly intrudes metasediments as randomly oriented dikes and stocks. Solid-state deformation fabrics overprint magmatic fabrics throughout the body, indicating post-emplacement ductile deformation. This body was emplaced into host rock close to or at the brittle-ductile transition, at least biotite zone conditions. Hidden Granite is an undeformed biotite granite which discordantly intrudes metasediments and older granitoid bodies as randomly oriented dikes and stocks. Mafic dikes coevally intrude the granite, infilling fractures with consistent northeast—southwest orientations, at a late stage of granite crystallisation. Hidden Granite was emplaced into a brittle host rock, above the brittle-ductile transition. The changing structural styles of the four plutons reflect intrusion during progressive uplift. The nearly parallel nature of the elongation direction of post-emplacement ductile fabrics in Chancellor Orthogneiss, and the extension direction indicated by brittle structures in Hidden Granite, suggests a geometric relationship exists between these fabrics and structures. The northwest—southeast elongation direction also remains relatively constant during post-emplacement deformation of the plutons. © 1997 Taylor & Francis Group, LLC.