Controversies and knowledge gaps in Geology (Part 1): Formation of ‘Archean granitoids’ (TTGs)

What are TTGs?

A suite of granitoids comprising tonalites, trondhjemite and granodiorites collectively referred to as tonalite-trondhjemite-granodiorite (TTG) granitoids are the main constituents of Archean terranes¹. TTGs were mostly emplaced in the Archean and are not generated in modern-day subduction zones¹. Thus, these rocks have invariably undergone multiple episodes of metamorphism and usually occur as strongly deformed gneisses (Fig. 1).

Figure 1. TTG specimen (Source: Wikipidea)

Geochemically, TTGs are sodic (K₂O/Na₂O<0.5), contain high contents of light rare earth elements (LREE) and low contents of high rare earth elements (HREE) resulting in strongly fractionated REE patterns (average La/Yb_N of 32.4), exhibit negative Nb-Ti anomalies, and lack pronounced Eu and Sr anomalies². The fractionated REE patterns and the Nb-Ti anomalies are interpreted to be indicative of the presence of garnet, amphibole and/or rutile in the melt residue³, whereas the Sr and Eu anomalies have been interpreted to be indicative of the absence of plagioclase as a residual or fractionating phase^4,5.

Geodynamic settings proposed for the generation of early Archean TTGs and scientific evidence

The geodynamic setting in which TTGs formed is still debated⁶. Two main models based on tectonic settings have been proposed for the generation of TTGs — the “hot” subduction and non-subduction models⁶. The “hot” subduction model proposes that the higher mantle temperatures in the Archean (compared to the present day) enabled partial melting of the subducted young and hot oceanic lithosphere (a hydrated enriched metabasaltic source) at a depth consistent with the stabilisation of garnet, amphibole and/or rutile⁷. This proposition is based on the chemical similarity between TTGs and adakites (products of lavas produced in modern arcs) such as the high contents of LREE and low contents of HREE ⁸. Adakites are proposed to have been produced by the melting of young (and therefore hot and buoyant) subducting oceanic crust (Fig. 2, left), as opposed to melting of the overlying mantle wedge as in modern-day subduction zones⁸ (Fig 2, right).

Figure 2. Geodynamic setting proposed for the production of granitoids in the early Archean (left) characterised by the melting of buoyant oceanic crust (OC) in shallow-dipping subduction zones, and late Archean (right) characterised by melting of the overlying mantle wedge. CC=continental crust (modified after⁶).

The non-subduction model proposes the generation of TTGs by partial melting of lower oceanic crust after its sagduction⁹ or heating of the base of thick oceanic crust by upwelling mantle plumes^10,11. This model is premised on the evidence pointing to the derivation of TTGs from pre-existing mafic rocks¹² in the absence of plate tectonics.

Figure 3. (a) Delamination of overthickened oceanic crust (OC), followed by (b) upwelling of mantle plume. CC=continental crust; red areas depict partially molten mafic and TTG melts (after⁶).

The proponents of this model envisage the production of TTGs by lithospheric delamination of overthickened oceanic crust (by tectonic stacking or melt production by plume activity) (Fig. 3a) followed by upwelling of asthenospheric mantle^11,13 (Fig. 3b). This would produce copious mafic and TTG melts by partial melting of mafic rocks at the base of the oceanic crust.

Evolution of TTGs in the late Archean

TTGs are common in the Archean and become less abundant in the Proterozoic and Phanerozoic¹⁴. Of note are two major changes in the composition of TTGs and other granitoids during the Archean, and around the Archean-Proterozoic boundary: (1) progressive enrichment of TTGs in Sr and K coupled with depletion in Na, Ca and HREE during the Archean forming TTGs ⁶, and (2) the progressive replacement of TTGs with two main groups of potassic granitoids (sanukitoids characterised by LILE- and LREE-rich metasomatised mantle geochemical signatures, and high-K granites characterised by crustal geochemical signatures, at   ̴2.5 ± 0.5 Ga ^15–17). The causes for these two major changes are still controversial.

The close spatial and temporal relationship of high-K granites and sanukitoids suggests the interaction between metasomatized mantle and continental crust in the genesis of such rocks, a constraint which can be satisfied by heat sources in two different geodynamic settings: (i) asthenosphere upwelling due to lithospheric delamination or slab break off in continental-collision settings, or (ii) anomalous thermal events related to rifting and plume upwelling in intraplate settings¹⁴.

References and further reading 

1.        Jahn, B.-M., Glikson, A. Y., Peucat, J. J. & Hickman, A. H. REE geochemistry and isotopic data of Archean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution. Geochim. Cosmochim. Acta 45, 1633–1652 (1981).

2.        Martin, H. The Archean Grey Gneisses and the Genesis of Continental Crust. in Archean Crustal Evolution (ed. Condie, K. C. B. T.-D. in P. G.) vol. 11 205–259 (Elsevier, 1994).

3.        Martin, H. Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology 14, 753–756 (1986).

4.        Martin, H., Smithies, R. H., Rapp, R., Moyen, J.-F. & Champion, D. An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24 (2005).

5.        Xiong, X. et al. Trace element characteristics of partial melts produced by melting of metabasalts at high pressures: Constraints on the formation condition of adakitic melts. Sci. China Ser. D Earth Sci. 49, 915–925 (2006).

6.        Moyen, J.-F. & Martin, H. Forty years of TTG research. Lithos 148, 312–336 (2012).

7.        Moyen, J.-F. & Stevens, G. Experimental constraints on TTG petrogenesis: Implications for Archean geodynamics. in Archean Geodynamics and Environments (eds. Benn, K., Mareschal, J.-C. & Condie, K. C.) vol. 164 149–175 (American Geophysical Union, 2006).

8.        Defant, M. J. & Drummond, M. S. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662–665 (1990).

9.        Bédard, J. H. A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle. Geochim. Cosmochim. Acta 70, 1188–1214 (2006).

10.      Smithies, R. H. The Archaean tonalite–trondhjemite–granodiorite (TTG) series is not an analogue of Cenozoic adakite. Earth Planet. Sci. Lett. 182, 115–125 (2000).

11.      Smithies, R. H., Champion, D. C. & Van Kranendonk, M. J. Formation of Paleoarchean continental crust through infracrustal melting of enriched basalt. Earth Planet. Sci. Lett. 281, 298–306 (2009).

12.      Kemp, A. I. S. et al. Hadean crustal evolution revisited: New constraints from Pb-Hf isotope systematics of the Jack Hill zircons. Earth Planet. Sci. Lett. 296, 45–56 (2010).

13.      Van Kranendonk, M. J., Smithies, R. H. & Champion, D. C. Paleoarchean Development of a Continental Nucleus: The East Pilbara Terrane of the Pilbara Craton, Western Australia. in Earth’s Oldest Rocks (eds. Van Kranendonk, M. J., Bennett, V. C. & Hoffmann, J. E. B. T.-E. O. R. (Second E.) 437–462 (Elsevier, 2019). doi:https://doi.org/10.1016/B978-0-444-63901-1.00019-8.

14.      Laurent, O., Martin, H., Moyen, J. F. & Doucelance, R. The diversity and evolution of late-Archean granitoids: Evidence for the onset of “modern-style” plate tectonics between 3.0 and 2.5Ga. Lithos 205, 208–235 (2014).

15.      Nebel, O. et al. When crust comes of age: on the chemical evolution of Archaean, felsic continental crust by crustal drip tectonics. Philos. Trans. A. Math. Phys. Eng. Sci. 376, 20180103 (2018).

16.      Halla, J., Whitehouse, M. J., Ahmad, T. & Bagai, Z. Archaean granitoids: an overview and significance from a tectonic perspective. Geol. Soc. London, Spec. Publ. 449, 1 LP – 18 (2017).

17.      Engel, A. E. J., Itson, S. P., Engel, C. G., Stickney, D. M. & Cray, E. J. Crustal Evolution and Global Tectonics: A Petrogenic View. GSA Bull. 85, 843–858 (1974).

1.         Jahn, B.-M., Glikson, A. Y., Peucat, J. J. & Hickman, A. H. REE geochemistry and isotopic data of Archean silicic volcanics and granitoids from the Pilbara Block, Western Australia: implications for the early crustal evolution. Geochim. Cosmochim. Acta 45, 1633–1652 (1981).

2.         Martin, H. The Archean Grey Gneisses and the Genesis of Continental Crust. in Archean Crustal Evolution (ed. Condie, K. C. B. T.-D. in P. G.) vol. 11 205–259 (Elsevier, 1994).

3.         Martin, H. Effect of steeper Archean geothermal gradient on geochemistry of subduction-zone magmas. Geology 14, 753–756 (1986).

4.         Martin, H., Smithies, R. H., Rapp, R., Moyen, J.-F. & Champion, D. An overview of adakite, tonalite–trondhjemite–granodiorite (TTG), and sanukitoid: relationships and some implications for crustal evolution. Lithos 79, 1–24 (2005).

5.         Xiong, X. et al. Trace element characteristics of partial melts produced by melting of metabasalts at high pressures: Constraints on the formation condition of adakitic melts. Sci. China Ser. D Earth Sci. 49, 915–925 (2006).

6.         Moyen, J.-F. & Martin, H. Forty years of TTG research. Lithos 148, 312–336 (2012).

7.         Moyen, J.-F. & Stevens, G. Experimental constraints on TTG petrogenesis: Implications for Archean geodynamics. in Archean Geodynamics and Environments (eds. Benn, K., Mareschal, J.-C. & Condie, K. C.) vol. 164 149–175 (American Geophysical Union, 2006).

8.         Defant, M. J. & Drummond, M. S. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662–665 (1990).

9.         Bédard, J. H. A catalytic delamination-driven model for coupled genesis of Archaean crust and sub-continental lithospheric mantle. Geochim. Cosmochim. Acta 70, 1188–1214 (2006).

10.       Smithies, R. H. The Archaean tonalite–trondhjemite–granodiorite (TTG) series is not an analogue of Cenozoic adakite. Earth Planet. Sci. Lett. 182, 115–125 (2000).

11.       Smithies, R. H., Champion, D. C. & Van Kranendonk, M. J. Formation of Paleoarchean continental crust through infracrustal melting of enriched basalt. Earth Planet. Sci. Lett. 281, 298–306 (2009).

12.       Kemp, A. I. S. et al. Hadean crustal evolution revisited: New constraints from Pb-Hf isotope systematics of the Jack Hill zircons. Earth Planet. Sci. Lett. 296, 45–56 (2010).

13.       Van Kranendonk, M. J., Smithies, R. H. & Champion, D. C. Paleoarchean Development of a Continental Nucleus: The East Pilbara Terrane of the Pilbara Craton, Western Australia. in Earth’s Oldest Rocks (eds. Van Kranendonk, M. J., Bennett, V. C. & Hoffmann, J. E. B. T.-E. O. R. (Second E.) 437–462 (Elsevier, 2019). doi:https://doi.org/10.1016/B978-0-444-63901-1.00019-8.

14.       Laurent, O., Martin, H., Moyen, J. F. & Doucelance, R. The diversity and evolution of late-Archean granitoids: Evidence for the onset of “modern-style” plate tectonics between 3.0 and 2.5Ga. Lithos 205, 208–235 (2014).

15.       Nebel, O. et al. When crust comes of age: on the chemical evolution of Archaean, felsic continental crust by crustal drip tectonics. Philos. Trans. A. Math. Phys. Eng. Sci. 376, 20180103 (2018).

16.       Halla, J., Whitehouse, M. J., Ahmad, T. & Bagai, Z. Archaean granitoids: an overview and significance from a tectonic perspective. Geol. Soc. London, Spec. Publ. 449, 1 LP – 18 (2017).

17.       Engel, A. E. J., Itson, S. P., Engel, C. G., Stickney, D. M. & Cray, E. J. Crustal Evolution and Global Tectonics: A Petrogenic View. GSA Bull. 85, 843–858 (1974).