An overview of geological studies of JARE in the Napier Complex, Enderby Land, East Antarctica
Hideo Ishizuka, 2008. "An overview of geological studies of JARE in the Napier Complex, Enderby Land, East Antarctica", Geodynamic Evolution of East Antarctica: A Key to the East–West Gondwana Connection, M. Satish-Kumar, Y. Motoyoshi, Y. Osanai, Y. Hiroi, K. Shiraishi
Download citation file:
Subsequent to the reconnaissance fieldwork in 1982, the Japanese Antarctic Research Expedition (JARE) carried out extensive geological studies that focused on structural and tectonic aspects, petrology, geochemistry and geochronology of the Napier Complex in Enderby Land, East Antarctica. Detailed field investigations in several key areas, including geological mapping of the Mt. Riiser-Larsen area and Tonagh Island, revealed that the Napier Complex comprises layered and massive gneiss units, of which the layered unit is composed of garnet felsic gneiss, orthopyroxene felsic gneiss, pelitic and basic gneisses, impure quartzite, and minor metamorphosed banded iron formation, whereas the massive unit consists mainly of orthopyroxene felsic gneiss. The boundary between the units is transitional in the Mt. Riiser-Larsen area, in which metamorphosed anorthosite and ultramafic rocks occur as thin layers, or blocks or pods, but on Tonagh Island the boundary is closely associated with the shear zone. Nine deformation episodes (D1–D9) were suggested for Tonagh Island. These results of fieldwork were presented in detail in two geological maps. Geochemical studies showed that (1) garnet–sillimanite gneisses and garnet-rich felsic gneisses were derived from mudstone and sandstone, respectively, both enriched in MgO, Cr and Ni; (2) orthopyroxene felsic gneisses have a close REE affinity with Archaean tonalite–trondhjemite–granodiorite (TTG); (3) basic gneisses were derived from light rare earth element (LREE)-enriched or -depleted basalts; (4) meta-ultramafic rocks are comparable with komatiite and related depleted mantle peridotite. This suite of protoliths is reminiscent of Archaean greenstone–granite belts. Precise analyses of physical conditions of metamorphism were carried out by using reliable approaches such as feldspar thermometry, alumina content of orthopyroxene, inverted pigeonite and bulk-rock compositions, and clino- and orthopyroxene compositions with different textures (porphyroblastic and neoblastic), and the results suggested that the maximum metamorphic temperature might have reached 1130 °C (i.e. ultrahigh-temperature (UHT) metamorphism). P–T evolution of the Napier UHT metamorphism was examined by analyses of reaction textures combined with fluid inclusion studies, suggesting both clockwise (Bunt Island) and counterclockwise (Mt. Riiser-Larsen and Tonagh Island) P–T–t paths. U–Pb sensitive high-resolution ion microprobe and secondary ionization mass spectrometry zircon ages from the Mt. Riiser-Larsen area and Tonagh Island indicate three stages of protolith formation at around 3.28–3.23, 3.07 and 2.68–2.63 Ga, and two contrasting ages for the timing of peak UHT metamorphism at either c. 2.55 or c. 2.51–2.45 Ga. On the basis of these results, more comprehensive studies on the Napier Complex are essential in the future for understanding (1) the role and age of TTG protolith and (2) the origin and timing of UHT metamorphism.
Figures & Tables
Geodynamic Evolution of East Antarctica: A Key to the East–West Gondwana Connection
Geological correlations of East Antarctica with adjoining continents have been puzzling geologists ever since the concept of a Gondwana supercontinent surfaced. Despite the paucity of outcrops because of ice cover, difficulty of access and extreme weather, the past 50 years of Japanese Antarctic Research Expeditions (JARE) has successfully revealed vital elements of the geology of East Antarctica. This volume presents reviews and new research from localities across East Antarctica, especially from Dronning Maud Land to Enderby Land, where the geological record preserves a history that spans the Archaean and Proterozoic. The reviews include extensive bibliographies of results obtained by geologists who participated in the JARE. Comprehensive geological, petrological and geochemical studies, form a platform for future research on the formation and dispersion of Rodinia in the Mesoproterozoic and subsequent assembly of Gondwana in the Neoproterozoic to Early Palaeozoic.