December 2015 LIP of the Month

Printer-friendly versionPrinter-friendly versionSend by emailSend by email

Evolution of a ~2.7 Ga large igneous province: insights from the Agnew Greenstone Belt and Kalgoorlie Terrane (Yilgarn Craton, Western Australia)

P.C. Hayman

Queensland University of Technology, Earth Environment and Biological Sciences, GPO Box 2434, Brisbane, QLD 4001, Australia, patrick.hayman@qut.edu.au, ph +61 7 3138 2261, fax +61 7 3138 1535

Email: patrick.hayman@qut.edu.au

The following is a shortened version of Hayman et al. 2015 Precambrian Research 270, 334-368.

Introduction:

Greenstone belts are relatively narrow and elongate successions typically consisting of a lower sequence of mafic to ultramafic rocks that are overlain by felsic volcanics and sediments. Greenstone terranes, in turn, are made up of numerous individual belts separated from one another by granitic bodies and/or regionally extensive strike-slip faults. Greenstones are Archean to Proterozoic and most have been metamorphosed to greenschist facies. And although greenstones generally fail the criteria used to define large igneous provinces (Bryan, S.E., Ernst, R.E., 2008. Revised definition of large igneous provinces (LIPs). Earth-Science Reviews 86, 175-202.e.g., in terms of area/volume; Bryan and Ernst, 2008), this is probably an effect of preservation (Ernst, R.E. 2014. Large Igneous Provinces. Cambridge University Press, 653 p.e.g. Ernst, 2014). Many greenstone belts, in fact, consist of extremely thick successions of volcanic rocks (the Agnew greenstone belt is ~3-7 km thick) that were emplaced in seemingly short periods of time (<30 Ma?).

There are still many unresolved questions about the origin of greenstone belts and associated terranes. Two of the major questions that we considered are:

  • Does each belt represent an accreted block (allochtonous origin), or do their current positions reflect, more or less, original sites of deposition (autochtonous origin)?
  • What generates melting to produce such voluminous mafic and ultramafic magmas? Plumes and/or subduction? Something else?

We examined the Agnew Greenstone Belt (AWB) in the northern part of the Kalgoorlie Terrane (Yilgarn Craton) to address these questions by developing a well constrained volcano-stratigraphy of the mafic and ultramafic sequence. The methodology employed involved firstly, detailed field work examining the succession to develop a relative chronology of events. Particular attention was focussed on distinguishing intrusive from extrusive lithofacies. Secondly, samples were analysed for major and trace elements to fingerprint units and to track the geochemical evolution. Lastly, interflow sediments and granophyric zones of mafic intrusions were dated using SHRIMP and TIMS techniques to constrain absolute ages. The new volcanic stratigraphy was then compared with data for other greenstone belts across the Kalgoorlie Terrane to develop a regional stratigraphy.   

Background Geology:

The Agnew Greenstone Belt (Fig. 1) is located in the Kalgoorlie Terrane of the Eastern Goldfields Superterrane (Yilgarn Craton). The Kalgoorile Terrane is divided into a number of domains based on geological, geophysical, geochemical, isotopic, and geochronological data (Fig. 1a; Cassidy et al., 2006). The Kalgoorlie Terrane is dominated by large calc-alkaline monzogranites to granodiorites (Smithies, R.H., Witt, W.K., 1997. Distinct basement terranes identified from granite geochemistry in late Archaean granite-greenstones, Yilgarn Craton, Western Australia. Precambrian Research 83, 185-201.Smithies and Witt, 1997) with minor tonalites and potassic granitoids encircled by predominantly young (2.71 – 2.66 Ga), and minor older (>2.73 Ga), greenstone successions.
We mainly use the scheme of Cassidy, K.F., Champion, D.C., Krapež, B., Barley, M.E., Brown, S.J.A., Blewett, R.S., Groenewald, P.B., Tyler, I.M., 2006. A revised geological framework for the Yilgarn Craton, Western Australia, Geological Survey of Western Australia, p. 8.Cassidy et al. (2006), which identifies 10 domains within the Kalgoorlie Terrane.


Figure 1. Location map of the Agnew Greenstone Belt. (A) Map of the Kalgoorlie Terrane with subdomains and faults indicated; 1 = after Cassidy et al. (2006). (B) Local map of Agnew geology. Grid reference in MGA. Location of AGB within Australia (upper inset) and the Yilgarn Craton (lower inset). NT, Naryeer Terrane; SWT, Southwest Terrane; YT, Youanmi Terrane; KT, Kalgoorlie Terrane, KuT, Kurnalpi Terrane; BT, Burtville Terrane and YmT, Yamarna Terrane.  C) Legend for (B).

The Agnew Greenstone Belt, which is ~70x25 km, occurs within the northern part of the Ora Banda Domain. In the Agnew district, the supracrustal rocks can be divided into a lower interlayered greenstone pile that consists of fine-grained tholeiitic basalt, high-Mg basalt, komatiite, gabbro and gabbro-pyroxenite-peridotite sills, with minor interbedded sedimentary layers, and an upper clastic-dominated sequence (Beardsmore, T.J., 2002. The geology, tectonic evolution and gold mineralisation of the Lawlers Region: a synopsis of present knowledge. Barrick Gold of Australia, p. 279.Beardsmore, 2002; Platt, J.P., Allchurch, P.D., Rutland, R.W.R., 1978. Archaean tectonics in the Agnew supracrustal belt, Western Australia. Precambrian Research 7, 3-30.Platt et al., 1978).

Results:

The results of our mapping, geochemistry and geochronology are summarised in Figures 2 and 3.  Key outcomes include the:

  • recognition of nine conformable lavas (each with associated co-magmatic intrusions) as well as two layered mafic sills without associated lavas. The sequence can be grouped into two cycles (I and II), each of which begins with a spinifex-textured komatiite and is overlain by several different basaltic units. A third cycle (0) is tentatively included for the poorly exposed lowermost basalt;
  • demonstration, using major and trace element geochemistry, that each cycle begins with primitive komatiitic melts that becomes progressively more evolved (fractionated) and contaminated through time;
  • timing constraints provided by new U-Pb ages and existing data. The onset of Cycle I magmatism is poorly constrained at ~2720 Ma by the detrital zircon population from the overlying sedimentary packages. The termination of Cycle I and onset of Cycle II magmatism is constrained by the youngest Cycle I date (<2711±4 Ma), the oldest Cycle II date (>2692±3 Ma) and from dates of regional correlates, at ~2705 Ma.  Termination of Cycle II is tightly constrained by a TIMS age from a granophyric interval in a mafic sill at 2690.7±1.2 Ma. Thus, each cycle lasted ~15 m.y., for a total of ~30 m.y. of volcanism without any major time breaks.


Figure 2. Summary stratigraphic column for the Agnew Greenstone Belt based mainly on lithofacies characteristics and geochemistry. The stratigraphy is subdivided into two main cycles, each of which begins with a komatiites (Cycles I and II).


Figure 3. Data grouped by different stratigraphic unit and arranged by relative time. In some cases data are subdivided into upper, lower and co-magmatic intrusions. LCC = lower continental crust; BCC= bulk continental crust. T&M’85 = Taylor, S.R., Mclennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford Taylor and Mclennan (1985).

Regional Correlations:

For our correlations we compared the Agnew data with all available published geochemical, geochronological and stratigraphic data for the Kalgoorlie Terrane (Fig. 4). Well constrained stratigraphy is restricted to the Ora Banda and Kambalda Domains in the southern Kalgoorlie Terrane, and less well defined stratigraphic columns exist for the Coolgardie, Norseman and Boorara Domains.

The most significant outcome of our regional study is the demonstration that the mafic-ultramafic stratigraphy can be confidently traced for hundreds of kilometres across the domain boundaries proposed by Cassidy et al (2006) and others. This observation is consistent with the mafic-ultramafic succession being the Archean equivalent of flood basalts. Furthermore, we have shown that the differences in the stratigraphy below the regional komatiite are not the result of variable preservation or exposure. Instead, two distinct stratigraphic groups can be recognized below the regional komatiite, one restricted to the west of the Bardoc Fault System and the other to the east. Thus, the Bardoc Fault System represents a tectonically important bounding structure. Cycle II volcanism caps these lower packages as a regional event.


Figure 4. Stratigraphic relationships of the principal mafic–ultramafic units of the Kalgoorlie Terrane based on comparing geochronology as well as the order and geochemistry of stratigraphic units in this study with published geochronology, stratigraphy and geochemistry. Vertical positions are an approximation of their relative stratigraphic positions. See Hayman, P.C., Thébaud, N., Pawley, M.J., Barnes, S.J., Cas, R.A.F., Amelin, Y., Sapkota, J., Squire, R.J., Campbell, I.H., Pegg, I., 2015. Evolution of a ~2.7 Ga large igneous province: A volcanological, geochemical and geochronological study of the Agnew Greenstone Belt, and new regional correlations for the Kalgoorlie Terrane (Yilgarn Craton, Western Australia). Precambrian Research 270, 334-368.Hayman et al. (2015) for references in Fig. 4.

Plumes vs subduction melts:

Magma generation is best explained by derivation from a plume. Komatiites are widely considered products of melting in a plume owing to the high temperatures needed to generate such Mg-rich melts (Campbell, I.H., Griffiths, R.W., Hill, R.I., 1989. Melting in an Archaean mantle plume: heads it's basalts, tails it's komatiites. Nature 339, 697-699.Campbell et al., 1989; Herzberg, C., 1995. Generation of plume magmas through time — an experimental perspective. Chemical Geology 126, 1-16.Herzberg, 1995). The origin of the basalts, however, is more contentious. Our demonstration that the overlying basalts show a progression from mildly fractionated and contaminated komatiites to highly evolved and fractioned basalts, is best explained if all overlying basalt are considered variably contaminated and fractionated komatiites. That is, all units in the succession were originally sourced from plume-derived melts. We propose a model whereby komatiitic melts stalled in the crust at relatively high levels where they fractionated and became crustally contaminated. Episodic eruptions sourced from these magma chambers reflect the progressive evolution of the melt (Figure 5).   


Figure 5. Spatial distribution of 2720–2690 Ma mafic-ultramafic rocks in the Kalgoorlie Terrane, grouped into Cycles I and II.

The manuscript in full can be found here: http://www.sciencedirect.com/science/article/pii/S0301926815003113

Click to open/close ReferencesReferences

Beardsmore, T.J., 2002. The geology, tectonic evolution and gold mineralisation of the Lawlers Region: a synopsis of present knowledge. Barrick Gold of Australia, p. 279.

Bryan, S.E., Ernst, R.E., 2008. Revised definition of large igneous provinces (LIPs). Earth-Science Reviews 86, 175-202.

Campbell, I.H., Griffiths, R.W., Hill, R.I., 1989. Melting in an Archaean mantle plume: heads it's basalts, tails it's komatiites. Nature 339, 697-699.

Cassidy, K.F., Champion, D.C., Krapež, B., Barley, M.E., Brown, S.J.A., Blewett, R.S., Groenewald, P.B., Tyler, I.M., 2006. A revised geological framework for the Yilgarn Craton, Western Australia, Geological Survey of Western Australia, p. 8.

Ernst, R.E.  2014. Large Igneous Provinces. Cambridge University Press,  653 p.

Hayman, P.C., Thébaud, N., Pawley, M.J., Barnes, S.J., Cas, R.A.F., Amelin, Y., Sapkota, J., Squire, R.J., Campbell, I.H., Pegg, I., 2015. Evolution of a ~2.7 Ga large igneous province: A volcanological, geochemical and geochronological study of the Agnew Greenstone Belt, and new regional correlations for the Kalgoorlie Terrane (Yilgarn Craton, Western Australia). Precambrian Research 270, 334-368.

Herzberg, C., 1995. Generation of plume magmas through time — an experimental perspective. Chemical Geology 126, 1-16.

Platt, J.P., Allchurch, P.D., Rutland, R.W.R., 1978. Archaean tectonics in the Agnew supracrustal belt, Western Australia. Precambrian Research 7, 3-30.

Smithies, R.H., Witt, W.K., 1997. Distinct basement terranes identified from granite geochemistry in late Archaean granite-greenstones, Yilgarn Craton, Western Australia. Precambrian Research 83, 185-201.

Taylor, S.R., Mclennan, S.M., 1985. The Continental Crust: Its Composition and Evolution. Blackwell, Oxford