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The Tasman Fold Belt System in Victoria - Geology and Mineralisation of Proterozoic to Carboniferous Rocks

Geological Survey of Victoria Special Publication

Released: November 2000
Purchase / Download: This report is available in the Online Store

Reviewer's Note

The Palaeozoic and late Precambrian history of the interactions between the ancient Pacific Ocean and the Gondwana supercontinent are partly shown by the geology of the Tasman Fold Belt System in eastern Australia. A continuing theme of these interactions has been cratonic/orogenic detrital input into a convergent margin setting resulting in the development of several major orogenic belts. In southeastern Australia these orogenic belts (subsets of the Tasman Fold Belt System) include the Delamerian Fold Belt of South Australia, western Victoria and western New South Wales and the Lachlan Fold Belt of Victoria, New South Wales and Tasmania.

This volume is concerned with an updated account of the geology and mineralisation of the Delamerian and Lachlan fold belts in Victoria with important inferences for the extensions of these belts into other states of Australia and also for dispersed fragments of Gondwana. Among the more interesting issues canvassed is the fate of the Late Precambrian and Cambrian passive margin of Gondwana. This margin, which is more widely exposed in South Australia, was subjected to intense deformation, metamorphism and plutonism as a result of the Delamerian Orogeny when the Gondwana passive margin was converted to a new orogenic belt. Another highlight is a definitive account of the western boundary of the Lachlan Fold Belt with descriptions and interpretations of the Moyston Fault and the Grampians Allochthon. The main topic of this volume, however, is a detailed description of the complex set of what many regard as exceedingly cryptic events that heralded the formation of the Lachlan Fold Belt in the Middle Cambrian to Carboniferous. This part of the book illustrates how the Lachlan Fold Belt formed by the deformation of oceanic rocks as well as the incorporation of a northern extension of Tasmania ("Selwyn Block"). This represents a compromise of sorts to the long-standing debate between those who supported either an oceanic or a continental basement to the Lachlan Fold Belt.

Who should read this volume? Needless to say anyone with interests in the geology of Victoria will find this volume an essential and well-used addition to their bookshelf. Those with interest in the geology of adjoining states (South Australia, New South Wales and Tasmania) will also find this volume essential reading. The apparent complexity and enormity of the literature has discouraged geologists interested in the Palaeozoic history of the Pacific Ocean from tackling an understanding of southeastern Australia. At last these people will find in this volume a safe haven for establishing the factual basis and interpretation of what the Delamerian and Lachlan fold belts mean for the ancient Pacific Ocean and Gondwana. The book will also be a reliable geological guide for lay people interested in the new developments in the geology of Victoria.

I congratulate the members of the Geological Survey of Victoria for the production of this excellent book and commend it to all.

Chris Fergusson
School of Geosciences
University of Wollongong
October 1999

Book Layout

Map: Eastern Australian fold belts & Victorian structural zones

The book presents the Palaeozoic geology of Victoria through time and space. This perspective is summarised in the time-space figure (foldout inside back cover). Time is shown on the vertical axis and the Victorian structural zones on the horizontal axis. The book focuses on the relationship between time and space and how this resulted in the geological and mineralisation history of Victoria. The time-space figure therefore represents the key to understanding Victoria's first 200 million years of history, and is repeatedly referred to throughout the book. The book also contains three 1:1 000 000 maps as enclosures: Geology, Pre-Permian geology, and Mineralisation.

Boxes are used to highlight ideas and key information throughout the book. These boxes contain general information that is not easily fitted into the remainder of the text and is relevant to more than one section of the book. New ideas and definitions, or discussion of alternative or contentious ideas are also described in boxes.

Chapter 1 - Introduction - provides background historical information and a summary of the geology and mineralisation.

Chapters 2 to 4 are descriptive, for the most part dealing with geological observations. The chapters are clearly related - what is happening at any particular time must be happening at some place and vice versa. As a result there is some duplication of material between these chapters but it has been kept to a minimum. Much of the detail in these chapters is given in numerous tables.

Chapter 2 - Rock-forming Events through Time - considers the vertical axis of the time - space figure. It describes the stratigraphy of the Tasman Fold Belt System in Victoria up to the Permian, including the various rock units and deformation, metamorphic and mineralisation events. An understanding of the rock units, their inter-relationships, generalised distribution, age and depositional or tectonic setting can be gained from this chapter. Description and discussion of mineralisation chiefly linked to rock forming events is given under rock unit headings, while the genesis of orogenic gold mineralisation - underpinned by crustal heating linked to orogeny - is discussed under Silurian-Early Devonian and Middle Devonian epochs. Much of the lithological detail of rock units is given in numerous tables. Stratigraphic ordering in these is with the youngest unit at the top.

Chapter 3 - Structural Zones - concentrates on the structures. It lists the rock units found in each structural zone, the structural evolution and styles, geophysical characteristics and major faults. It describes the effects of the various deformations (horizontal axis of time-space figure) and structurally controlled goldfields for each zone. The zones are described from west (Glenelg Zone) to east (Mallacoota Zone).

Chapter 4 - Intrusive History - considers the abundant Palaeozoic plutonic rocks of Victoria, and mineralisation formed by magmatic-hydrothermal processes. Plutonic rocks have intruded at various times throughout the early geological history of Victoria. They are described in this chapter rather than in chapters 2 or 3 because the constraints on their timing are usually poor and the plutons cross structural zone boundaries. Dykes are quite common, and are also briefly described in chapter 4. This chapter is accompanied by an appendix which lists information on all currently recognised pre-Permian intrusions.

Chapter 5 - Tectonic Evolution of Victoria - is more interpretive. It integrates the temporal and spatial aspects of the rock units, their deformation histories, distribution and mineralisation as described in the previous chapters. It is divided into two sections. "Concepts and Models" examines some of the major geological issues and concepts regarding the tectonic setting of Victoria. "Tectonic evolution - a Synthesis" describes the chronological development of Victoria from the Proterozoic to Carboniferous times.


Map: Simplified pre-Permian geology. Click on image for larger version
Click on the image to view a larger version of Simplified pre-Permian geology of Victoria

The surface geology map (Encl. 1) includes the Permian, Mesozoic and Cainozoic rock units not covered in this book. It has been compiled from geological mapping carried out by the GSV and tertiary institutions. This map departs from previous large-scale maps of the State in that the geology is subdivided into geological units rather than by time.

The pre-Permian geology map (Encl. 2) shows the interpreted geology of Victoria, with the Permian and younger units stripped off. It was compiled from the surface geology map and interpretation of the extensive new magnetic, radiometric and gravity data, along with seismic profiles where available. We have attempted to reconcile the mapped geology with the geophysical data sets, to produce a map that is both geologically and geophysically reasonable. The magnetic image that forms the main map background, and the subsidiary images elsewhere on the map face help support the new interpretation presented for the covered areas. The background image also gives extra information that could not be adequately presented on the map, such as sedimentary or contact-metamorphic changes in the magnetic properties of rock packages, dyke swarms, and thick basalt cover. The cross-section was not modelled, but we believe it integrates the geological evidence with a possible geophysical solution.

The legend subdivides the rocks by time and palaeoenvironment or geochemistry, which gives a complementary view of the geology to that of the surface map. Granites were subdivided using White and Chappell's (1983) S, I and A classification, as this is a subdivision that is both generally understood and works well with the magnetic data. Consequently, the map of Lachlan Fold Belt granites by Chappell et al. (1991) has been integrated into the interpretation. The time subdivisions are at the natural breaks in the Victorian geological record, usually the ends of orogenies. This has meant that all the intrusions associated with a particular event are grouped in the same time slice, even when some could perhaps be allocated to a younger time slice.

The mineralisation map of Victoria (Encl. 3) shows the location of pre-Permian metallic and major industrial mineral resources in the context of the pre-Permian geology. It includes mines and important undeveloped resources and builds on the compilations of McHaffie and Buckley (1995b) and Weston and Nott (1993). The classification scheme was chosen to emphasise the genesis and setting of mineralisation, and to highlight the relationship between mineralisation and geology.

Bibliographic Reference

VandenBerg, A.H.M., Willman, C.E., Maher, S., Simons, B.A., Cayley, R.A., Taylor, D.H., Morand, V.J., Moore, D.H. & Radojkovic, A., 2000. The Tasman Fold Belt System in Victoria. Geological Survey of Victoria Special Publication.