Mount Elkins - Geology & Orogeny

Geology & Orogeny

Further information: Age of the Earth, Orogeny, Radiometric dating, Thermochronology, and Ultra-high-temperature metamorphism

Much of the East Antarctic craton was formed in the Precambrian period by a series of tectonothermal orogenic events. Napier orogeny formed the cratonic nucleus approximately 4 billion years ago. Mount Elkins is a classic example of Napier orogeny. Napier orogeny is characterized by high-grade metamorphism and plate tectonics. The orogenic events which resulted in the formation of the Napier Complex (including Mount Elkins) have been dated to the Archean Eon. Radiometrically dated to as old as 3.8 billion years, some of the zircons collected from the orthogneisses of the Napier Complex are among the oldest rock specimens found on Earth. Billions of years of erosion and tectonic deformation have exposed the metamorphic rock core of these ancient mountains.

The oldest crustal components found to date in the Napier Complex appear to be of igneous derivation. This rock appears to have been overprinted by an ultra-high temperature metamorphic event (UHT) that occurred near the Archean-Proterozoic boundary. Using a lutetium-hafnium (Lu-Hf) method to examine garnet, orthopyroxene, sapphirine, osumilite and rutile from this UHT granulite belt, Choi et al determined an isochron age of 2.4 billion years for this metamorphic event. Using SHRIMPU–Pb zircon dating methodology, Belyatsky et al determined the oldest tectonothermal event in the formation of the Napier Complex to have occurred approximately 2.8 billion years ago.

Preservation of the UHT mineral assemblage in the analyzed rock suggests rapid cooling, with closure likely to have occurred for the Lu-Hf system at post-peak UHT conditions near a closure temperature of 800°C. UHT granulites appear to have evolved in a low Lu-Hf environment, probably formed when the rocks were first extracted from a mantle profoundly depleted in lithophile elements. The source materials for the magmas that formed the Napier Complex were extremely depleted relative to the chondritic uniform reservoir (CHUR). These results also suggest significant depletion of the early Archean mantle, in agreement with the early igneous differentiation of the Earth that the latest core formation models require.