The Rayner Complex of East Antarctica: complex isotopic systematics within a Proterozoic mobile belt

• Published in: Journal of Metamorphic Geology
• Main Authors: BLACK, L. P., HARLEY, S. L., SUN, S. S., McCULLOCH, M. T.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1987
• Subjects: Antarc*; Antarctica; East Antarctica
• Online Access: http://dx.doi.org/10.1111/j.1525-1314.1987.tb00366.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1525-1314.1987.tb00366.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1525-1314.1987.tb00366.x

Abstract New isotopic (Rb–Sr, U–Pb zircon and Sm–Nd) and petrological data are presented for part of an extensive Proterozoic mobile belt (locally known as the Rayner Complex) in East Antarctica. Much of the belt is the product of Mid‐Proterozoic (∼ 1800–2000 Ma) juvenile crustal formation. Melting of this crust at about 1500 Ma ago produced the felsic magmas from which the dominant orthogneisses of this terrain were subsequently derived. Deformation and transitional granulite‐amphibolite facies conditions (which peaked at 750 ± 50°C and 7–8 kbar (0.7–0.8 GPa) produced open to tight folding about E–W axes and syn‐tectonic granitoids about 960 Ma ago. Subsequent felsic magmatism occurred at about 770 Ma and not, as has been widely advocated, at 500–550 Ma, which appears to have been a time of widespread upper greenschist facies (400–500°C) metamorphism, localized shearing and faulting. Sm‐Nd model ages of 1.65–2.18 Ga disprove a previously favoured hypothesis that the Rayner Complex mostly represents reworked Archaean rocks from the neighbouring craton (Napier Complex). Models that involve rehydration of the Napier Complex are no longer required, since the Rayner Complex was its own source of water. Two episodes of Proterozoic crustal growth are identified, the later of which occurred between about 1200 Ma and 1000 Ma, and was relatively minor. Sedimentation took place only shortly before Late Proterozoic orogenesis. The multiphase history of the Rayner Complex has resulted in complex isotopic behaviour. Three temporally discrete episodes of Pb loss from zircon have been identified, the earliest two of which are responses to the c. 960 Ma and 540 Ma tectonothermal events. Fluid leaching was operative during the later event for there is a good correlation between degree of isotopic discordance and secondary mineral growth. Pb loss during the high‐grade event was probably governed by the same process or by lattice annealing. Some zircon suites also document recent Pb loss. Most lower concordia intercepts have no ...