What Are Greenstone Belts?

A simplified geologic map showing geological formations in the Nuvvuagittuq Greenstone Belt (CLICK TO ENLARGE).

Youknowwhoiwillbe, CC BY-SA 4.0 via Wikimedia Commons

Greenstone belts consist of large expanses of metavolcanic and metasedimentary rocks within the cratons of primarily Archean and Proterozoic terrains. They derive their color from the presence of greenish minerals like chlorite and epidote, among others1. The greenstone belts are important from a geological perspective as they are one of the key sources of information regarding the formation of the Archean Earth's early crust and its tectonic activity2. The volcanic rocks are predominantly composed of basalt and komatiites, which are ultramafic and hyper-enriched in magnesium3. Sedimentary components, when present, are made up of greywackes, mudstones, and banded iron formations4. These belts are bounded by high-grade metamorphic rocks, for example, tonalite-trondhjemite-granodiorite (TTG) complexes and are, therefore, not only of geological interest but also economically important due to their association with important mineral deposits, especially gold, nickel, and other base metals5.

Much debate and research have gone into establishing the origin of greenstone belts; however, it is generally believed that they formed in tectonically active environments6. One leading theory is that they derived from mantle plumes and hotspots7. These plumes raise buoyant roots in the lithosphere, resulting in the formation of greenstone belts through decompression melting and following volcanic activity8. The result is the main formation of thick sequences of volcanics that are later intruded by deep-seated TTG plutons; an example of this is found in the greenstone belts of the Taitao Peninsula in Chile9. Another theory's basis is that greenstone belts formed in rifting environments, where the Earth's crust was stretched and thinned sufficiently in order for magma to rise and create volcanic rocks in a similar way to what is occurring at present in island arcs and in marginal seas10. Greenstone belts can, therefore, be thought of as ancient analogues to modern tectonic environments providing information of early plate tectonic dynamics11.

Structurally, greenstone belts show complex patterns of deformation due to tectonism from more than one phase. The Agnew belt in Western Australia shows evidence of early deformation forming recumbent folds and flat-lying schistosity followed by major upright folds and steep ductile shear zones12. This structural complexity is suggestive of dynamic tectonic history that involves both compressional and extensional forces13. The deformation is often related to metamorphism under greenschist to amphibolite facies conditions, reflecting the varying pressures and temperatures these rocks have experienced through their geological history14. Hence, not all greenstone belts portray characteristics of being homogeneous. These geological belts can show great variations in their volcanic and sedimentary sequences. The Schreiber-Hemlo greenstone and the White River-Dayohessarah greenstone belts in Canada show a mosaic of oceanic plateaus, island arcs, and subduction accretion complexes15. Tholeiitic and calc-alkaline volcanic sequences and siliciclastic turbidites are common in both belts, indicating complex interactions between tectonic and sedimentary processes16.

On the basis of different lithology types present inside greenstone belts, it has been revealed that they evolved as a concatenation of different tectonic units at convergent plate margins, similar to those developed in modern accretionary prisms17. Further, trace-element geochemistry of the northern Wawa subprovince greenstone belts reveals a transition from arc-like to back-arc magmatic features, suggestive of both subduction and plume-related processes in their formation18. High magnesium content and distinctive spinifex texture in the komatiites point to high degrees of partial melting in the mantle, nuclei of a hot early Earth, which makes komatiites an extinct group of rocks (from melts of temperatures range that the Earth's mantle can no longer reach)19.

On one hand, the study of greenstone belts unravels for geologists the processes of tectonics and magmatism that presumably prevailed on early Earth; on the other hand, very many mineral deposits are connected with mineralization processes that are, for the most part, connected with the tectonic evolution of the belts20. The mineralized deposits are formed within particular structural settings, such as shear zones and fold hinges21. In a nutshell, greenstone belts are telling relics of the Early Earth, tectonically and magmatically. They have complex structure, varied lithologies, and substantial mineral resources, which make greenstone belts a focus in geological study22.

 

REFERENCES

  1. de Wit, M. J., & Ashwal, L. D. (1997). Greenstone Belts. Oxford University Press.
  2. Condie, K. C. (1994). Greenstone belts and the continental crust: an overview. Springer.
  3. Nisbet, E. G., & Fowler, C. M. R. (1983). Model for Archean plate tectonics. Nature, 303(5917), 435-440.
  4. Windley, B. F. (1995). The Evolving Continents. Wiley-Blackwell.
  5. Groves, D. I., Goldfarb, R. J., Robert, F., & Hart, C. J. R. (2003). Gold deposits in metamorphic belts: overview of current understanding, outstanding problems, future research, and exploration significance. Economic Geology, 98(1), 1-29.
  6. Abbott, D. H., Drury, R., & Mooney, W. D. (1997). Continental growth through geological time. Geological Society of America.
  7. Condie, K. C. (2001). Mantle plumes and their record in Earth history. Cambridge University Press.
  8. Sleep, N. H. (1990). Hotspots and mantle plumes: Some phenomenology. Journal of Geophysical Research: Solid Earth, 95(B5), 6715-6736.
  9. D'Orazio, M., Innocenti, F., Manetti, P., Haller, M. J., & Palma, J. (2003). The Pali-Aike volcanic field, Patagonia: slab-window magmatism near the tip of South America. Tectonophysics, 377(1-2), 89-107.
  10. Kusky, T. M., & Polat, A. (1999). Growth of granite-greenstone terranes at convergent margins, and stabilization of Archean cratons. Tectonophysics, 305(1-3), 43-73.
  11. Thurston, P. C., & Chivers, K. M. (1990). Secular variation in greenstone sequence development emphasizing Superior Province, Canada. Precambrian Research, 46(1-2), 21-41.
  12. Swager, C. P. (1997). Tectono-stratigraphic evolution of the Granite Greenstone Terrane in the Central Yilgarn Craton. Precambrian Research, 83(1-3), 11-41.
  13. Myers, J. S. (1993). Precambrian tectonic evolution of part of Gondwana, southwestern Australia. Precambrian Research, 61(1-4), 115-151.
  14. Kerrich, R., & Polat, A. (2006). Archean greenstone–tonalite duality: thermochemical mantle convection models or plate tectonics in the early Earth global dynamics?. Tectonophysics, 415(1-4), 141-165.
  15. Tomlinson, K. Y., & Condie, K. C. (2001). Archean mantle plumes: Evidence from greenstone belt geochemistry. Developments in Precambrian Geology, 11, 53-82.
  16. Percival, J. A., & Williams, H. R. (1989). Tectonic systems in the evolution of the Superior Province. Geological Society of America Special Papers, 230, 1-20.
  17. St-Onge, M. R., Hynes, A. J., & Lucas, S. B. (1992). Tectonic assembly of the southeastern Churchill Province, Canada. Canadian Journal of Earth Sciences, 29(6), 1161-1178.
  18. Wyman, D. A., & Kerrich, R. (2002). Archean greenstone belts: Geodynamic and tectonomagmatic significance. Tectonophysics, 349(1-4), 3-39.
  19. Nesbitt, R. W., & Sun, S. S. (1976). Geochemistry of Archean spinifex-textured peridotites and magnesian and low-magnesian tholeiites. Earth and Planetary Science Letters, 31(1), 433-453.
  20. Goldfarb, R. J., & Groves, D. I. (2015). Orogenic gold: Common or evolving fluid and metal sources through time. Lithos, 233, 2-26
  21. Kerrich, R., & Cassidy, K. F. (1994). Temporal relationships of lode gold mineralization to accretion, magmatism, metamorphism and deformation—Archean to present: implications from the Western Australian Shield. Earth-Science Reviews, 36(1-2), 1-58.
  22. Bierlein, F. P., & Crowe, D. E. (2000). Phanerozoic orogenic lode gold deposits. Reviews in Economic Geology, 13, 103-139.

 

Demetria Lynn

March 28, 2024, 4:15 pm

Questions or comments? 

Add comment

Comments

There are no comments yet.