The northern Prince Charles
The northern Prince Charles Mountains, covering an area of approximately 40,000 km2, are located roughly between longitude 64 to 69°E and latitude 70 to 72°S within MacRobertson Land, East Antarctica. Altogether, this mountainous region comprises 140 massifs and nunataks which range in area up to ~300 km2 (Fisher Massif) and extend from just above sea level (Else Platform) to 2438 m (Mount Kirkby) in height. The northern Prince Charles Mountains are bounded to the east by the Lambert Glacier - Amery Ice Shelf, to the south by the southern Prince Charles Mountains, and to the north and west by the Antarctic ice cap.
View of the west Gorman Crags from the northeast, northern Prince Charles Mountains; note large difference in the height of the glacial ice on either side of the crags (G. Nichols, 1990) © Copyright Australian Antarctic Division.
The northern Prince Charles Mountains are dominated by three east-west trending mountain ranges, the Athos, Porthos and Aramis Ranges, which are separated by the Scylla and Charybdis Glaciers. The eastern end of the southernmost Aramis Range is dominated by a number of relatively flat, ice-free areas, the Loewe, Manning and McLeod Massifs, which form the western boundary of the Amery Oasis region. The latter comprises Beaver and Radok Lakes, connected via Pagodroma Gorge and separated from the Lambert Glacier and its tributaries by Jetty Peninsula to the east and Flagstone Bench to the south. Further south, a number of relatively isolated, ice-free areas include Mount Lanyon, Mount Meredith, Fisher Massif and Mount Willing, the latter representing the southernmost exposure of the northern Prince Charles Mountains.
Map showing the location of the Prince Charles Mountains relative to other Archaean and Proterozoic exposures along the Prydz Bay coastline (after Hand et al., 1994).
Looking east towards Loewe Massif (mainly charnockites), northern Prince Charles Mountains (G. Nichols, 1988) © Copyright Australian Antarctic Division.
The basement geology of the northern Prince Charles Mountains is dominated by layered and massive felsic gneisses and metasediments, as well as minor mafic granulites. These basement ortho- and para-gneisses were formed from felsic and mafic igneous, together with minor carbonate and sedimentary, protoliths during granulite to upper amphibolite facies metamorphism at ~1000 Ma. They were subsequently intruded by late syn-tectonic orthopyroxene granitoid (charnockite) plutons, granitic stocks and pegmatite veins as well as younger mafic alkaline dykes. In the Amery Oasis region, a Permo-Triassic sedimentary sequence, the Amery Group, has been faulted against the basement metamorphic rocks. Although unmetamorphosed Mesozoic to Cenozoic mafic dykes of predominantly alkaline affinities are best exposed on the shores of Beaver and Radok Lakes, tholeiitic and alkaline mafic dyke swarms are also present in the southern Fisher Massif, Mount Collins and Mount Willing area of the northern Prince Charles Mountains. Minor olivine leucitite lava flows of Eocene age are exposed on Manning Massif. Cenozoic or younger glacigene deposits have also been identified within the Amery Oasis area, at Fisher Massif and at various locations within the Aramis Range.
Large percentage of granitoids within felsic gneisses, Lensink Peak, northern Prince Charles Mountains (G. Nichols, 1990) © Copyright Australian Antarctic Division.
The basement gneisses of the Prince Charles Mountains possess metamorphic mineral assemblages which record two main thermal events, at ~1000 Ma and ~500 Ma. Within the northern Prince Charles Mountains, the 1000 Ma thermal event (M1) is considered to have resulted in the general decrease in observed metamorphic grade from lower granulite facies in the north to upper amphibolite facies in the south, continuous with the lower amphibolite to greenschist facies metamorphism of the southern Prince Charles Mountains. In addition, moderate metamorphic pressures have been inferred from the stability of garnet-sillimanite-cordierite mineral assemblages within the metapelites and the absence of garnet within the metabasites. Within the northern Prince Charles Mountains, high-grade metamorphism at ~1000 Ma is considered to have been accompanied by at least three deformation events (D1 to D3), as well as charnockite and granite intrusion, and peak metamorphic conditions ( ~800°C, 600 to 700 MPa) are thought to have been followed by near-isobaric cooling. Later, possibly Palaeozoic, tectonism involved lower-grade, more brittle conditions and resulted in at least two more deformational overprints (D4 and D5).
The early Palaeozoic ~500 Ma thermal event (M2) is considered to be of minor importance in the northern Prince Charles Mountains, resulting in overprinting of some M1 mineral assemblages by greenschist facies assemblages, the resetting of biotite Rb-Sr ages and the development of retrograde shear zones.
Three generations of mylonites, associated with crustal thickening and subsequent thinning events, have also been identified within the northern Prince Charles Mountains. Whereas the earliest generation (MY1) predate folding and are considered to have annealed during 1000 Ma peak metamorphic conditions, the second generation (MY2) formed under conditions which post-date this peak and the third generation (MY3) are considered to have formed much later in the tectonic history of the area, possibly at ~500 Ma.
Early reconnaissance dating work in the northern Prince Charles Mountains resulted in Rb-Sr isochron ages of 550 to 1090 Ma for whole-rock samples and 400 to 600 Ma for biotite and muscovite separates. These ages were interpreted to represent the ages of major metamorphic events. Subsequent Rb-Sr isochron and conventional U-Pb zircon dating studies on rocks from Jetty Peninsula obtained ages of 1100+14/-11 Ma (U-Pb) for a granulite facies gneiss, 940+24/-17 Ma (U-Pb) and 718+/-32 Ma (Rb-Sr) for a gneissic leucogranite, and ~480 Ma (Rb-Sr mineral) for a biotite granite. Monazite U-Pb dating of the same granites resulted in ages of 530 to 540 Ma, and an age of 495 to 505 Ma (U-Pb) was obtained for a pegmatite from the same area. A separate study reported a Sm-Nd isochron age of 1233+/-160 Ma for a layered gabbronorite complex on Mount Willing as well as a zircon U-Pb age of 1400 +/-80 Ma for a granite on Mount Collins. Further studies have reported a Rb-Sr whole-rock age of 882+/-140 Ma for intrusive charnockite rocks of the Porthos Range, and Sm-Nd ages of 635 to 555 Ma for monazite inclusions within garnets in a leucogneiss sheet from Mount McCarthy. An age of 630 Ma was also obtained, via the same method, for a two pyroxene granulite from the same general area.
The most recent SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb zircon dating work undertaken on northern Prince Charles Mountains samples has revealed that two important magmatic episodes occurred at 1300 to 1280 Ma and at 1020 to 980 Ma. The first episode involved multiple intrusions of mafic to felsic magmas in the southern Fisher Massif to Mount Willing area of the northern Prince Charles Mountains, as well as volcanic extrusions on Fisher Massif. The second episode, coeval with the major ~1000 Ma tectonothermal event, resulted in extensive granitic (charnockite, granite and syenite granite) intrusion and the formation of minor leucogneiss bodies and granitoid intrusion within the northern Prince Charles Mountains. In addition, detrital zircon populations within a paragneiss on Mount Meredith have been dated at ~2800 Ma and 2100 to 1800 Ma.
Ion microprobe (232Th/208Pb) dating of monazite grains within MY2 mylonite zones from the northern Prince Charles Mountains resulted in an average age of 800+/-16 Ma.
Irregular body and dyke of garnet-bearing leucogneiss (dated at 990+/-36 Ma, Kinny et al., 1997) intruding "charnockite" at the eastern end of Mount McCarthy, Porthos Range, northern Prince Charles Mountains. Both the leucogneiss and the "charnockite" display a well developed lineation, and are within a high-strain zone (D. Thost, 1988) © Copyright Australian Antarctic Division.
Sketch map of the northern Prince Charles Mountains showing outcrops and sample localities (filled circles) with corresponding SHRIMP U-Pb zircon ages (Ma) (after Kinny et al., 1997).