Number 308 in the Geological Society of London Special Publication series is concerned with the Precambrian geological evolution of Antarctica between 10°E and 60°E. Outcrops in this region comprise some 2000 km of coastal and inland mountain ranges from the Archaean Napier Complex of Enderby Land in the east, through the Proterozoic outcrops of the western Rayner Complex, the Lützow Holm Complex and Yamato-Belgica Complex of eastern Dronning Maud Land, to the Sør Rondane Mountains, and then the Schirmacher Hills and mountain ranges of central Dronning Maud Land in the west.
This book is an eclectic mix of 22 papers describing the geology, mineralogy, geochronology, petrology, geochemistry, rheology, structure and tectonics of the scattered outcrops that characterize this poorly-exposed sector of Antarctica, and which at first glance appear to be connected by little other than present-day geographic coincidence. Despite having superficially similar high-grade metamorphic mineral assemblages, these outcrops are a lithological collage of tectonic blocks preserving a range of different geological events that together span more than 3000 million years of Earth history, from early Archaean crustal relics in the Napier Complex to the widespread “Pan-African” tectonothermal event that accompanied the final amalgamation of these disparate blocks into the Gondwana supercontinent some 600–500 million years ago.
While the papers in this book focus on different locations, different problems, and different techniques, in many ways mirroring the composite geology of the region, there are also a number of recurring themes. One issue is the difficulty of establishing which outcrops are bound by faults, or major tectonic boundaries. The problem is that any significant geological boundaries will be inevitably concealed by ice, and must therefore be inferred using a range of indirect evidence. Thus Shiraishi et al. use U-Pb mineral ages and neodymium model age data to identify discrete crustal terranes in eastern Dronning Maud Land, while Toyoshima et al. use form-line analysis of regional foliations to define fault bound blocks in the Napier Complex. Hokada et al. use pressure-temperature paths recorded by metamorphic mineral assemblages to distinguish different fault-bound blocks in the Napier Complex, and similarly Baba et al. argue that different P-T paths in central Droning Maud Land reflect a major terrane boundary. Satish Kumar et al. use strontium isotopes in metacarbonate lithologies to infer the existence and age of marginal oceans in the Lützow Holm region, while Carson & Ague use petrological evidence for boron-rich fluid influx into dry granulites as an indication of a nearby subduction zone, and one possible interpretation for boron-rich mafic rocks in the Lützow Holm Complex described by Kawakami et al. is that they represent hydrothermally altered ophiolite material. Suzuki et al. use the geochemistry of Proterozoic mafic dykes to infer a possible subduction zone beneath the Napier complex about 2 billion year ago, while Owada et al. argue that the differing geochemistry of 500 Ma post-tectonic dyes in central and eastern Dronning Maud Land is consistent with an intervening oceanic suture. Finally, Suda et al. use the contrasting geochemical characteristics of metabasic rocks in the Lützow Holm Complex to suggest that this region comprises multiple amalgamated terranes, and Miyamoto et al. argue that post-tectonic ultra-potassic igneous rocks from Dronning Maud Land preserve the geochemical signature of subducted crust, consistent with a suture zone in the region.
The occurrence of extreme thermal gradients is another characteristic of the region, with both the Archaean Napier Complex and the latest Neoproterozoic “Pan-African age” rocks of Lützow Holm Bay preserving sapphirine-quartz mineral assemblages characteristic of ultra-high temperature metamorphism. Indeed, this part of Antarctica is arguably the type locality for this poorly understood phenomenon, which requires crustal temperatures in excess of 950°C and is the focus for a number of papers in this book. Previous work on UHT metamorphism in the Napier Complex is summarized by Ishizuka, while Hokada et al. present new petrological, geochronological and thermal modelling data for the Napier Complex, arguing that UHT conditions were attained across a rock package of at least 5 km thickness. Tsunogae et al. use fluid inclusions to infer that CO2 played a role both in the advection of heat into the crust to achieve UHT conditions, and in limiting wholesale melting of the crust once these conditions were attained. Papers by Kawasaki & Osanai, and by Sato et al. propose new methods for estimating metamorphic temperatures in UHT terranes, based on the titanium contents of quartz and the partitioning of iron and magnesium between orthopyroxene and spinel, respectively. Both sets of authors use Napier Complex samples to cross-check their calibrations. Yoshimura et al. describe much younger 550 Ma UHT metamorphism in the Lützow Holm Complex, and although not discussed in the book, this repeated occurrence of UHT metamorphism in a restricted geographic area raises the possibility that there is some characteristic feature of the crust in this region, or its underlying mantle, that promotes the development of extreme temperatures during tectonic events.
Another common feature of these papers is that while the authors present a range of high-quality petrological, geochemical and geochronological data, there is often insufficient evidence to make definitive geological interpretations. Many of the papers are very much “works-in-progress”. Problems include the difficulty of correlating absolute age or P-T data with geological events, the variable extent that older features are preserved through younger tectonic overprints, and the ubiquitous curse of limited outcrop. Sometimes it seems as if every outcrop is different, making it near impossible to establish any regional geologic history - a problem with which many Antarctic geoscientists are familiar. For example, Hiroi et al. describe weakly metamorphosed juvenile 1000 Ma tonalite from Cape Hinode, which have adakitic affinities and are quite unlike any of the other outcrops in the region in terms of lithology, geochemistry and age.
A final recurring element is the critical role of the Japanese Antarctic Research Expeditions in the collection of primary data from these outcrops. The Japanese Syowa Station is sited in the middle of the region covered by this book, and Japanese scientists have been central to the geologic study of these rocks since 1957, when Japan was one of 64 nations participating in Antarctic research activities through the 1957–58 International Geophysical Year. This book is in part a commemoration of 50 years of Japanese geological research in the region, and is also a formal Japanese contribution to the 2007–08 International Polar Year. Indeed, one of the strengths of this book is that it summarizes much of the prior Japanese work published in reports and journals of the Japanese National Institute of Polar Research. The papers in this volume, and in particular the reviews of the Napier complex by Ishizuka and eastern Dronning Maud Land by Shiraishi et al., provide a window into this literature, which has not always been easily accessible to other workers.
With such diverse content, including highly focused specialist papers such as a study of garnet crystal size distribution by Goto & Ikeda, and the measurement of rheological properties of Napier Complex orthogneiss by Ishikawa et al., it is not easy to identify a single target audience for this book. The most likely group to be attracted to the majority of papers is the relatively small community of researchers working on the Precambrian metamorphic and tectonic evolution of East Antarctica, but the value of this book should extend far wider than that. Its title is “Geodynamic Evolution of East Antarctica – A Key to the East-West Gondwana Connection”. While it is a stretch to argue that this book provides significant insights into the geodynamic evolution of East Antarctica - firstly there is little specific discussion of geodynamics and secondly it is concerned with only a fraction of East Antarctica - it is no exaggeration to say that that the outcrops discussed in this book hold the key to understanding the assembly of Gondwana.
Gondwana assembly was achieved by the amalgamation of multiple continental fragments along multiple suture zones during the 700–500 Ma ‘Pan-African” time period, but it is widely agreed that the most significant suture zone is the East African Orogen, a belt of deformed and metamorphosed rocks that extends for more than 7000 km along the eastern margin of the African continent. It is also preserved in Madagascar, Sri Lanka and southern India, which were adjacent to East Africa before the break-up of Gondwana in the Mesozoic. These rocks record the closure of an ocean basin followed by Himalayan-style collision between East Gondwana (India, Australia) and West Gondwana (Africa, South America), but the continuation of this collision zone into Antarctica remains controversial, as discussed by the two papers in this book to take a continent-scale viewpoint. Jacobs et al. argue that the East African Orogen continues south into Dronning Maud Land, and that Pan-African age deformation and metamorphism in this region is a consequence of a collision zone that intersects the present-day Antarctic coastline at a high angle and continues under the ice to the Shackleton Range on the palaeo-Pacific margin of East Antarctica. Conversely, Grantham et al. argue that the East African Orogen is truncated by another Pan-African age collisional orogen that runs east–west through Mozambique and Sri Lanka. In this model, Pan-African age deformation and metamorphism in Dronning Maud Land is a consequence of this later collisional orogen that extends parallel to the present-day Antarctic coastline. These conflicting viewpoints on an issue as basic as overall orogenic trend indicate how little is known about the regional tectonic setting of Pan African metamorphism in East Antarctica, with the result that outcrops in Dronning Maud Land, and closely related terranes in Sri Lanka and Mozambique, are fundamental to our understanding of Gondwana assembly - what collided with what, and in what sequence?
While this book does not provide the answer to these significant questions, it does present some of the basic data needed to make a start. As noted by Satish-Kumar et al. in their introductory paper, perhaps the most notable gap is the lack of geophysical data to provide a means of correlation across the ice. This should be a priority for future work, and perhaps the greatest value of this book is in highlighting just how much more work is needed to understand the Precambrian geological evolution of East Antarctica.
In summary, I believe that this book will become be essential reference source for anyone interested in the Precambrian geology of the Napier Complex and scattered outcrops of Dronning Maud Land, although the breadth of coverage and lack of focus will probably discourage all except the most dedicated Antarctic geologists from choosing to buy it.
