Fisher Massif

Fisher Massif, located towards the southern end of the northern Prince Charles Mountains, is a large (300 km2) isolated, uplifted highland block with a northeast-southwest orientation. It is one of only a limited number of outcrops located within the transitional zone between the Archaean cratonic block exposed in the southern Prince Charles Mountains and the Proterozoic mobile belt exposures of the northern Prince Charles Mountains.

General geology:

Although Fisher Massif represents part of the Proterozoic mobile belt exposed in the northern Prince Charles Mountains, it is geologically from the more northern exposures in that it comprises a much higher proportion of basic metavolcanic rock and underwent lower grade metamorphism and less intense deformation. Fisher Massif is considered to be Antarctica's closest approach to a greenstone belt, although it is not Archaean in age.

The layered greenstone sequence of Fisher Massif, known as the Fisher Terrane, comprises mafic and felsic to intermediate volcanics and volcaniclastics, with minor pelite, psammopelite, meta-conglomerate, banded iron formation, chert and carbonate intercalations. The metavolcanic rocks are dominated by amygdaloidal basalt flows, although basaltic andesite, andesite, dacite, rhyodacite and rhyolite are also present. Intrusive lithology's include gabbro, diorite, plagiogranite, granodiorite, S- and I-type granites and a series of minor mafic and felsic dykes. The metavolcanic rocks, which constitute at least half of the outcrop on Fisher Massif, comprise a series of NNW-NW dipping flows with an overall thickness of more than 3300m. They are best exposed on the southeastern cliffs of Fisher Massif. Deposition by lava and mass flow is considered to have occurred in a subaqueous and locally subaerial environment within an intercontinental rift.

As opposed to the higher pressure metamorphic mineral assemblages identified elsewhere in the northern Prince Charles Mountains, the metamorphic grade at Fisher Massif has only reached greenschist to lower amphibolite facies, and rock exposures show evidence for only relatively weak deformational events. Peak P-T estimates for the ~1000ma (M1) and ~500ma (M2) metamorphic events are 625 to 675°C at 4.5 to 5.3 kbar and 640 to 675°C at 3.8 to 4.5 kbar, respectively. Although M1 was accompanied by pervasive deformation (D1), followed by granite intrusion and a second deformation event (D2), M2 was purely a thermal event. Regional scale open folding, minor faulting and mylonite development occurred after M2.

Geochronology:

Preliminary U-Pb zircon data which indicated a ~1300ma age of emplacement for the felsic and intermediate metavolcanic rocks of Fisher Massif, are consistent with the recent SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb zircon dating of a metadacite (1283+/-21ma) and a granodiorite (1293+/-28 Ma).

Information sources:

Beliatsky, B.V., Laiba, A.A. and Mikhalsky, E.V. (1994) U-Pb zircon age of the metavolcanic rocks of Fisher Massif (Prince Charles Mountains, East Antarctica). Antarctic Science6, 355-358.

Crowe, W. (1994) Geology, metamorphism and petrogenesis of the Fisher Terrane, Prince Charles Mountains, East Antarctica. Unpublished MSc. thesis, Australian National University. 234 pp.

Crowe, W. (1996) Geology of the Fisher Terrane, northern Prince Charles Mountains. Abstracts, Prince Charles Mountains Workshop, Melbourne, p. 26.

Kinny, P. D., Black, L. P. and Sheraton, J. W. (1997) Zircon U-Pb ages and geochemistry of igneous and metamorphic rocks from the northern Prince Charles Mountains, Antarctica. AGSO Journal of Australian Geology and Geophysics16, 637-654.

Mikhalsky, E.V., Sheraton, J.W., Laiba, A.A. and Beliatsky, B.V. (1996) Geochemistry and origin of Mesoproterozoic metavolcanic rocks from Fisher Massif, Prince Charles Mountains, East Antarctica. Antarctic Science 8, 85-104.

This page was last modified on 2 August 2002.