GEMOC ARC National Key Centre

RESEARCH

SCIENTIFIC CONTEXT

The focus of GEMOC's research programs is the driving role of the mantle in Earth processes and its control of element and commodity distribution in the accessible crust. This bottom-up approach involves

• Understanding the location of different types of metallogenic provinces by understanding the links between:

  * mantle processes
  * crustal generation
  * large-scale tectonics
  * heat and fluid transport
  

• An interdisciplinary and integrated approach, linking:

  * petrology and geochemistry (including fluid/vapour studies)
  * geophysics (mantle and global scales)
  * tectonics (mantle-crust links)
  * time
  

OUTLINE

The research program comprises four strands:

* Lithosphere Mapping
* Geotectonics
* Crustal Generation Processes
* Metallogenic Provinces

• These strands are conceptual units, not administrative units. They represent thematic foci required for solving the overall problem of the nature of the geochemical evolution and metallogeny of continents.

• Because they are not set up as management entities, overall integration and coordination is facilitated and there are no boundaries to inhibit interdisciplinary interfacing.

• Our research strategy is to focus on individual, clearly-defined, short- to medium-term projects that fill in pieces of the larger-scale jigsaw puzzle.

• The research program incorporates relevant funded projects existing at the time the Key Centre was awarded, as well as new initiatives.

• Program strands illustrate major directions and framework, rather than the weighting of research effort at any given time.

SUMMARY OF STRANDS

The Lithosphere Mapping strand provides the fundamental data for defining mantle domains in terms of composition and thermal state; relating these domains to a better understanding of tectonic evolution will help to define the large-scale evolution of mantle processes through time, and their influence on the development of crustal material and metallogenic provinces.

The Crustal Generation strand seeks to understand the large-scale processes that have created and modified continental crust, how these processes may have changed through time, and how crustal processes influence the concentration and localisation of economically important elements (eg Au, Cu, C).

The Geotectonics strand uses stratigraphic, tectonic, and geophysical data to study the history and causes of continental assembly and disruption, with a special focus on Australia and East Asia. It provides the fundamental framework to link the research on crustal and mantle processes with the localisation and development of metallogenic provinces.

The Metallogenic Provinces strand seeks to define the mantle and crustal reservoirs of economically important elements, and the mechanisms by which elements can be extracted from the mantle and transported into the crust. The emphasis is on understanding processes of regional scale, and relating these processes to the tectonic framework and the processes of mantle and crustal generation.

PROJECTS

Most projects are designed to link two or more strands

1. Lithosphere evolution in eastern China (O'Reilly, Griffin, Zhang, Guo). Integrates petrological data on basalts, xenoliths and xenocrysts from kimberlites and basalts, and a range of regional geophysical data, to examine the evolution of the continental lithospheric mantle from Archean to present. A large-scale collaborative project involving personnel from several Chinese institutions. Funded by ARC, DEET Targeted Institutional Links, Academia Sinica, China NSF, China Ministry of Geology, China Education Ministry, Macquarie University. Some aspects are new initiatives.

2. Lithosphere mapping in Australia (O'Reilly, Griffin, Zhang, Norman, Gaul, McCarron). Links studies of mantle petrology (using xenoliths and xenocrysts in volcanic rocks) with regional geophysical data, in collaboration with Prof. B. Kennett's tomography group at ANU-RSES. Funded by ARC, Stockdale Prospecting Ltd. (APA-I). Some aspects are new initiatives.

3. Siberia-Mongolia lithosphere traverse (Griffin, O'Reilly, Ionov). Mantle xenoliths and xenocrysts from kimberlites and basalts are being used to map the mantle along a traverse extending from the Arctic sea, south across Siberia into the Baikal Rift and further into Mongolia. Funded by ARC, Macquarie University and Almazy Rossii-Sakha (Russian diamond exploration and mining conglomerate). Some aspects are new initiatives.

4. Deep structure of the Canadian Shield (Griffin, Pearson, O'Reilly). Mantle xenoliths, xenocrysts and diamonds from kimberlites are being used to map the lithospheric mantle in selected provinces of the Canadian Shield. Funded by Macquarie University Collaborative grant with Kennecott Canada Inc. New initiative.

5. Eastern Australian and Indochinese diamonds (Davies, Win, Griffin, O'Reilly). Morphological and geochemical studies of alluvial diamonds and their inclusions, to determine the nature of the mantle source(s) and develop new models for exploration in regions without known kimberlite/lamproite source rocks. Funded by CRA Exploration and GEMOC. New initiative.

6. Geochemistry of mantle sulphides: probing the Earth's core-mantle-crust system (Guo, Griffin, O'Reilly). Geochemical and petrological studies of sulfides in mantle-derived xenoliths and ophiolites, to define the size and nature of the sulfide reservoir for chalcophile elements, and to enhance our understanding of the genesis of base-metal sulphides and platinum-group elements mineralisation in the crust. Funded by ARC and Macquarie University.

7. Palaeomagnetic study of the northern New England Orogen Queensland (Lackie) Palaeomagnetic analysis is being used to interpret the tectonic and thermal history of the region. Funded by ARC.

8. Alkali and heat-producing elements in the lithosphere of eastern Australia (Norman). The distribution of K, Rb, Cs, Th and U in the continental crust and lithospheric mantle of eastern Australia is being studied to determine the composition of the continental crust and the primitive mantle of the Earth, constrain the origin of the Earth and Moon, and measure the contribution of mantle sources to lithospheric heat production. Funded by ARC

9. Evolution of a hotspot volcano: geochemistry of glasses from Mauna Loa (Norman, Garcia (Univ. Hawaii)). This project traces the magmatic evolution of Mauna Loa, associated with the world's most active mantle plume, using laser microprobe trace element analysis of quenched melt inclusions within olivine crystals. Funded by MURG.

10. Evolution of magmas in the Peninsular Ranges Batholith, Baja California (Vernon, Johnson, Tate). 80-140 million years ago, subduction of Pacific Ocean crust beneath the North American plate caused melting of the lower continental crust, forming the Peninsular Ranges batholith. This study of the evolution of these granitic melts provides insights into the chemical evolution of magmatic arcs. Funded by MURG.

11. Microstructural processes in metamorphic rocks (Vernon). This project investigates the microstructures (shapes and arrangements of crystals) of metamorphic rocks in selected parts of Earth's deeper crust, now exposed at the surface, to determine the heating and deformation histories at the time of their formation. Funded by MURG.

12. Crustal evolution in a Mesozoic Circum-Pacific magmatic arc (Johnson, Vernon). The project investigates the ways in which large bodies of magmatic rocks (plutons) are emplaced, the timing of their intrusion relative to regional deformation and heating of the crust, and the extent to which the plutons are responsible for this deformation and heating. Funded by ARC.

13. Silurian and Early Devonian tectonic development of the Tumut-Gundagai region, southeastern NSW (Dadd). The complex history of extensional and compressional events in the region is being analysed by studies of back-arc lavas intercalated with felsic volcanic rocks and extensive units of serpentinised ultramafic rocks, intruded by Silurian(?) granodiorite. New initiative.

14. High pressure vapour-melt partitioning experiments (Nichols, Green). Models for the genesis of subduction-zone magmas usually require a hydrous fluid component. This experimental project seeks to quantify the trace element attributes of hydrous fluids equilibrated with basanitic melts, using the laser-ablation-microprobe-ICPMS technique. Funded by ARC.

15. P-T comparisons across the central Nagssugtoqidian Orogen, west Greenland (Nichols, Kriegsman (UNSW). The Nagssugtoqidian is thought to represent a Palaeoproterozoic continental collision zone. This project is aimed at defining P-T-t changes across the region in order to elucidate the tectonic processes active during collision. Funded by Danish Lithosphere Centre.

16. A metamorphic transect through the Himalayan Main Central Thrust (MCT) (Nichols, Pearson, Johnson). The MCT is a significant Himalayan structure, which experienced south-directed movement during the Tertiary. This project aims to understand the kinematics of the MCT by quantifying the associated change in P-T across the thrust zone. Funded by MURG.

17. Granite emplacement: New England Batholith (Shaw, Flood). Investigation of the emplacement mechanisms of two granite bodies in a well documented batholith. A cross-disciplinary project with structural mapping, geochemistry, geochronology, rock magnetism and geophysical modelling. Funded by MURG.

18. Magma genesis and tectonics in western Pacific island arcs (Arculus). Investigation of the impacts of lithospheric filtering on arc magmatism in the northeast Japan arc, the origin and tectonics of the Ontong Java Plume portion obducted onto Santa Isabel in the Solomon Islands (SI), and the petrogenesis of (multiply sourced) peridotites on Choiseul, Santa Isabel and San Jorge (SI). Funded by ARC.

19. Volatile, chalcophile, and noble siderophile elements in subduction zone magmas (Arculus, Eggins). Tracking the origin and distribution of the chalcophile and noble siderophile elements in subduction zone systems, with particular reference to volatile associations and episodes of volatile phase loss. Funded by ARC.

20. Evolution of continental crust: examples from the San Francisco Volcanic Field, AZ (Arculus). Investigating the evolution of the Proterozoic - Phanerozoic lower crust of the Colorado Plateau through integrated geochemical and petrological studies of volcanic rocks and crustal xenoliths. Funded by ARC.

21. Origin of granites and their role in the production and development of new crust at continental margins (Chappell). Geochemical studies of granites, mainly in eastern Australia, and their importance in crust formation processes. Funded by ARC to end 1995.

22. Magmatic and hydrothermal evolution of intrusive-related gold deposits (Chappell, Blevin). Geochemical studies of the relationships between selected major gold deposits and the granitic rocks that host them. Funded by AMIRA and ARC.

23. Geochemistry of granites formed at accreted continental margins (Chappell). This project aims to relate the composition of granitic rocks to the processes occurring at collisional margins. Funded by ARC.

24. The stability of titanite in the crust (Ellis, Allen). The aim of this project is twofold - to calibrate the Al content of titanite as a geobarometer, and to determine the PH2O, fO2 conditions under which titanite is a magmatic phase in I-type, hornblende bearing granites. Funded by ARC.

25. The stability of zirconolite in the crust (Ellis, Allen). The project will review the stability of zirconolite and undertake preliminary experiments on the stability of various polymorphs, to characterise one of the important phases in nuclear waste disposal ceramic - Synroc. Funded by ANSTO. New initiative

26. An experimental examination of eclogite partial melting: Liquids, residues and trace element partitioning (Ellis, Gust (Univ. Qld)). Experimental determination of the composition (major and trace element) of the magmas formed by partially melting natural basaltic compositions during subduction. Funded by ARC.

27. An experimental study of the diffusion of lead in zircon (Ellis, Lee (RSES)). Determination of the behaviour of lead during heating and cooling events, to improve the interpretation of U-Pb geochronology data. Funded by ARC.

28. Role of rutile in magma genesis: experimental assessment of Nb/Ta ratio variation. [Green]
    Rutile synthesized from 6 different compositions enriched in Nb and Ta (and TiO2 in 5 cases) all show Ta favoured over Nb with Nb/Ta partition coefficient ratio varying from 0.4 to 0.92. Variation as a function of bulk composition, temperature and pressure may be evaluated from the results, as well as the role of Nb and Ta as "essential structural constituents" in rutile, a feature not recognized before the current work.

29. Experimentally determined Zr and Hf partition relationships at high pressure. [Green, Fujinawa] The partitioning relationships of Zr and Hf between amphibole, clinopyroxene, garnet and basanitic and andesitic melts have been determined at high pressure and temperature. This allows evaluation of the Zr/Hf ratio in the crust/mantle system in terms of possible fractionation and/or metasomatic processes.

30. Garnet-clinopyroxene 2-mineral partition coefficients at high pressure. [Green, Blundy, Yaxley, Adam]
    Large (>40µm) crystals of coexisting garnet, clinopyroxene and basanitic matrix have been produced at 3.0 and 4.0GPa with the goal of determining trace element partition coefficients using the LAM ICP-MS at Macquarie University and the ion probe at Edinburgh University. This work is at natural element abundances or with slight (200ppm) enrichment of Hf, Nb and Ta.

31. PT phase relationships of silica-, alkali-, and alumina-rich xenolith glasses. [Draper, Green]
    An extensive study of the near-liquidus phase relationships of melts rich in silica, alkalies, and alumina, and poor in the mafic oxides, shows that such liquids can coexist with mantle harzburgite (and possibly lherzolite) under a range of conditions appropriate to the upper mantle. Their most likely origin involves a mechanism whereby harzburgitic mantle is pre- enriched in alkalies and alumina followed by low-degree partial melting. This finding suggests that glasses having these features, found in many mantle xenoliths, could bear important information about mantle melting and metasomatism.

32. Oxygen fugacity in sub-arc mantle. [Draper, Brandon]
    Contents of ferric iron in spinel separated from mantle xenoliths from the Cascade arc in North America have been determined by Msssbauer spectroscopy. The results document some of the most oxidized mantle yet reported and uphold the view that arc mantle is more oxidized than is MORB or OIB mantle. A semi- quantitative model relating the yearly flux of subducted fluids in arc settings to the observed oxidation signature shows the strong connection between slab- derived fluids, migrating through the mantle wedge, and this oxidation signature.

33. Water-undersaturated PT phase relationships of near-primary arc magmas. [Draper]
    Previous experimental work on arc magma compositions has been forced to use simpler conditions than those that prevail in arc settings. This study will control all of the most important parameters, in particular the appropriate water contents and oxygen fugacity, through use of recently-developed capsule materials and buffered cell assemblies. The results will address questions regarding the conditions of formation of primary or parental arc magmas in the mantle wedge and whether any arc magmas can be melts of subducted lithospheric slabs.

    NEW INITIATIVES

    New Projects

    As a result of the two research workshops (December 1995 and February 1996) GEMOC has formulated two large-scale umbrella projects to fulfil part of GEMOC's research and training brief. These involve significant collaboration between the nodes and with other researchers. Postgraduate programs will be set up within the research subsets.

    1. Timing and distribution of lithosphere formation and modification in the Tasmanide Belt

    This study will use the Granite Basement Terranes defined by Professor Bruce Chappell and co-workers to extend the model by studying (a) regions where different Basement Terranes defined on granites of different ages overlap (eg southern Sydney Basin region) and (b) whether the regions defined on the basis of granite geochemistry have significance in terms of basaltic mantle derived magmatism (eg New England). This two-pronged attack should show whether the lower crust changes significantly between episodes of granite magmatism and whether there is an analogous set of mantle terranes under the Tasmanides.

    This tectonic unit provides a natural laboratory to utilise and integrate GEMOC's expertise in mantle studies (through basalts and xenoliths) with granite genesis and emplacement and regional metallogeny and will interface well with the first area targeted by the Seismic Tomography Consortium, in the New England region. Macquarie University is a partner in this Consortium led by Professor Stewart Greenhalgh at Flinders University. This will promote the aim of GEMOC to integrate Geophysical and Geological datasets.

    2. Western Pacific Lithosphere Project

    Some of the world's youngest ophiolite sequences outcrop in the Solomon Islands and different tectonic units represent samples of mantle formed in different settings: beneath Cretaceous ocean ridges, in a forearc setting, and during the plume-induced eruption of the basalts of the Ontong-Java Plateau (OJP). Mantle xenoliths from volcanic rocks on Malaita and near Lihir Island provide samples of deep OJP mantle and of relatively shallow forearc mantle, respectively. The Lihir samples have been affected by fluids that may be related to gold deposits in the overlying crust. Regional gravity and seismic data are available. The region provides a natural laboratory to study the evolution of several types of "oceanic" mantle, which may be analogues to the mantle beneath accreted continental margins such as eastern Australia and China. Field, petrological and geochemical studies of the different types of mantle will be integrated with work on the overlying cumulate and volcanic rocks to understand the processes that have produced the spectrum of compositions in each mantle type. Comparisons with similar studies in eastern Australia (the Tasmanide Project) will enable us to construct a broad picture of the processes involved in the creation and modification of lithosphere at convergent margins, and to relate these to ore-forming processes.

    New Research Appointments

    Henry Tebar was appointed to a GEMOC post-doctoral fellowship. His expertise is in ore genesis and metallogeny, and his first responsibility is the development of a program of research on metallogenic provinces in eastern Australia with Dick Flood, Bruce Chappell and Phil Blevin. He will also work closely with the Hydrothermal Systems Group at CSIRO Exploration and Mining at North Ryde.

    Dr Michel Gregoire was granted a Macquarie University Research Fellowship (MURF) and will investigate the development of oceanic crust and the nature of oceanic plateaus using xenoliths in basalts from DSDP cruises. This study will incorporate acoustic velocity experimental work with Dr Ian Jackson at ANU. It is anticipated that Dr Gregoire will participate in an ODP Project cruise to the Indian Ridge to in 1997 provide further evidence on the nature of oceanic crust.

    Two other GEMOC Research Fellowships are under offer (at Macquarie and ANU).

    Context of Research Progress

    Most of the research projects listed above were initiated before GEMOC was officially formed, and are carried out largely within one of the nodes. Two planning workshops have been held, involving most of the GEMOC participants. These led to the initiation of several smaller-scale projects involving staff from two or more nodes, including cooperation on postgraduate projects and a number of research proposals have been submitted for external funding or are in preparation. Seed funding from GEMOC has been allocated for pilot studies within the Tasmanide and Solomon Island initiatives and the Burmese/Thailand diamond project.

    Postgraduate Project Initiatives in GEMOC

    These are listed in the Teaching and Training section below

    RESEARCH OUTCOMES

    The high level of activity within GEMOC's research program is illustrated by the papers published in peer-reviewed journals (appendix 3): the breadth of the activity is shown by the selection of abstracts of conference papers presented by GEMOC participants since June 1995 (appendix 5). Updated information is on the internet at : http://gemoc.es.mq.edu.au/GEMOC/. Other performance indicators include:

    • Collaboration between nodes and other participants
      
    • Interaction with CSIRO increased - joint postgraduates, postdoctoral fellows,
      technology development and collaborative research
    • Funding (see last section) Internal grants for fellowships, research and infrastructure Internal Fellowships International ARC Fellowship
    • Two new collaborative industry projects set up (in addition to 3 new collaborative PhD programs)
    • There has been no ARC round since June 1995 but new applications are pending for 1997