Geological history of Adélie Land and King George V Land, Antarctica: Evidence for a polycyclic metamorphic evolution

Abstract

Abstract Metamorphic and structural studies in Adelie Land and King George V Land, East Antarctica, show that this part of the East Antarctic Shield can be divided into four different sections. The areas east of Commonwealth Bay show evidence for a polycyclic metamorphic evolution in which low-pressure granulite facies M 1 fabrics (4–4.5 kbar; ∼ 750°C) are overprinted by retrograde hydrated decompression textures of amphibolite facies during M 2 and by late, high-pressure, low-temperature coronas during M 3 (5.7–7.0 kbar, 500–600°C). The regional structure and relative timing of events in the other three areas is consistent with the recognition of these three metamorphic events. The Cape Denison orthogneiss in Commonwealth Bay was intruded after the pervasive low-pressure M 1 event but before the major ductile deformation phases and M 2 . The Cape Denison orthogneiss therefore only suffered the M 2 amphibolite facies metamorphic event and subsequent overprinting by M 3 mid-greenschist facies assemblages involving lawsonite. The Cape Hunter phyllites were deposited after M 2 but before M 3 and consequently were only affected by the mid-greenschist metamorphic event, which also overprinted the amphibolite facies fabrics at Cape Denison. The areas west of Commonwealth Bay underwent the low-pressure granulite facies metamorphism during M 1 , decompression during M 2 but no late M 3 overprint. The variation of the grade of M 3 in the different areas from granulite facies pressures east of Commonwealth Bay to no effect west of Cape Hunter is interpreted as a consequence of differential uplift along shear zones with a north-south strike, which exposed successively deeper levels towards North Victoria Land to the east. This spatial association and the age of M 3 may suggest that there is a connection between M 3 in King George V Land and Adelie Land and the Ross orogeny in North Victoria Land. The recognition of three metamorphic events can be correlated with three radiometric ages previously established for the Cape Denison orthogneiss.