Ttg Genesis Phase Diagram

Low = cold crust

Yes No

Metasomatized mantle wedge

Olivine + pyroxenes

Classical calc-alkaline High: (4.5 < YbN < 20)

The second part of this chapter will try to discuss the reasons why hot subduction was prominent during Archaean and what was the terrestrial dynamics during this period.

4.2 Evolution and Dynamic of the Primitive Continental Crust

4.2.1 Introduction: The Archaean Specificity

The primitive Earth is not only characterized by specific processes of continental-crust genesis, but several other features make it different from modern Earth. Among these differences the more obvious are the nature and abundance of Archaean rocks that can be summarized as follows:

- Some rocks are widespread in Archaean terrains whereas they are rare or nonexistent after 2.5Ga: komatiites, Banded Iron Formations (BIF) (Photo4.6), TTG.

- Others are abundant after 2.5Ga and rare or unknown in Archaean terrains: andesites, magmatic per-alkaline rocks and eclogites.

- Some as high-Mg granodiorites (sanukitoids) are mainly known at the Archaean-Proterozoic boundary.

These differences are classically interpreted as reflecting Archaean thermal flux greater than today. This is well demonstrated by komatiites that are ul-tramafic (Fe-Mg-rich; Si-poor) lavas that cooled rapidly, as attested by their spinifex texture where olivine or pyroxene crystals instead of being massive, form long needles (Photo 4.7). These lavas exist only in Archaean terrains. Their petrologic study showed that they were produced by a high degree of mantle melting (> 50%) and that they emplaced at temperatures ranging from 1525°C

Photo 4.6. 2.7-Ga old Sandur banded iron formation (BIF) (India). On the right part is a detail view showing the alternation of quartz (white) and magnetite (dark) layers. (Photo H. Martin)
Spinifex Texture
Photo 4.7. Spinifex textured olivine crystals in Abitibi (Canada) komatiites (2.7Ga). These textures are typical of the high-temperature (1500 °C) Archaean ultramafic lavas (Photo H. Martin)

to 1650°C (Nisbet et al., 1993; Svetov et al., 2001). By comparison, modern basalts are produced today by only 25 to 35% mantle melting at temperatures of about 1250-1350°C. The depth of the komatiite source is still actively debated and estimations range from 200 to 600km; the only certainty is that they were produced at great depth as proved by the diamond crystals that they contain (Capdevila et al., 1999). Diamond in komatiitic magmas also shows that their genesis took place below a continental lithosphere. Komatiites corroborate that the Archaean upper mantle temperature was greater during the first half of Earth history. Since its formation, the Earth has cooled such that after 2.5Ga, it could not reach high temperatures and consequently was unable to produce high degrees of mantle melting, thus accounting for the disappearance of komatiites after Archaean times.

Different estimates consider that the early Archaean mantle temperature was 100 to 200 °C greater than today. This conclusion is in perfect agreement with primitive continental crust studies that establish greater Archaean geother-mal gradients. Scarcity or absence of eclogites, andesites and per-alkaline (K-, Na-rich; Ca-Al poor) magmas are also accounted for by the same cooling process. When Earth accreted it accumulated energy, such as: residual accretion heat; heat release by exothermic core-mantle differentiation; radioactive element (mainly U, Th, K) disintegration heat, etc. After 4.55Ga this potential energetic stock is gradually consumed and consequently the Earth progressively cools (Fig. 4.21).

Fig. 4.21. Earth heat production vs. time diagram (after Brown, 1986)

4.2.2 Continental Crust and Earth Cooling

Earth progressive cooling is not only attested by komatiites, but it is also imprinted in the Archaean continental crust record. During TTG emplacement, from 4.0 to 2.5Ga, Earth heat production decreased by at least a factor of 2 (Fig. 4.21), and consequently, in subduction zones, geothermal gradients along the Benioff plane also significantly changed. Figure 4.22 is a compilation of more than 1100 TTG analyses (Martin and Moyen, 2002), that shows MgO, Ni, (Na2O + CaO) and Sr plotted against TTG emplacement age. For each period of time, TTG compositions scatter across a relatively ample range. This could be accounted for by either different degrees of partial melting of the basaltic source or subsequent fractional crystallization.

Geochemical modelling (Martin, 1994) shows that, during fractional crystallization, the main cumulate assemblage consists of hornblende and plagioclase with somewhat smaller amounts of accessory phases. During such a process, Mg and Ni remain in the cumulate (due to their very high crystal/liquid partition coefficients (Kd) for hornblende) and consequently, their content in the magma decreases during differentiation. The same conclusion can be drawn for Na2O and CaO, as well as for Sr, whose low Kd values in hornblende are offset by their high partition coefficient in plagioclase.

In summary, fractional crystallization of plagioclase + hornblende results in a decrease of MgO as well as of Ni, (Na2O + CaO) and Sr in the magma. Consequently, within each age bracket, the highest values of MgO, Ni, (Na2O + CaO) and Sr can be considered as representative of the less differentiated and more primitive TTG parental magma. Thus, the upper envelope of the data set (arrows in Fig. 4.22) reflects the compositional variation of the primitive TTG parental

Fig. 4.22. Diagrams showing time-evolution of MgO, Ni, (Na2O + CaO) and Sr content of the primitive TTG parental magmas from 4.0 to 2.5 Ga (after Martin and Moyen, 2002)

magmas with time, which can be interpreted in terms of melting conditions in the magma source as well as of possible interactions with the mantle. In all diagrams TTG parental magma show a temporal evolution: at 2.5Ga TTG are significantly MgO-, Ni-, (Na2O + CaO)- and Sr-richer than at 4.0Ga.

Figure 4.23 is a MgO vs. SiO2 diagram that not only shows the time-dependent MgO increase in TTG, but that also compares TTG composition with liquids produced by experimental melting of basalts. It clearly appears that for the same SiO2 content experimental liquids are systematically MgO-poorer than TTG. Such differences have already been reported for adakites (Maury et al., 1996; Smithies, 2000; Prouteau et al., 2001). Based on experimental work, Rapp et al. (1999) demonstrated that, during their ascent through the mantle, slab melts undergo assimilation of olivine and react with mantle peridotite leading to the crystallization of orthopyroxene and garnet. This mechanism is able to lower SiO2 and to significantly increase MgO and Ni contents of the magmas.

As high MgO and Ni contents in TTG can be regarded as reflecting slab melt/mantle interaction two main conclusions can be drawn:

1) The existence of mantle-melt interactions implies that the source of the melts is located at great depth, under a mantle slice. In other words, they are formed by melting of basalts under a significant mantle thickness, so that significant interactions can take place. This is a strong argument in favour of TTG genesis by melting of the subducted oceanic slab rather than under-

40 50 60 70 Si02%

Fig. 4.23. MgO vs. SiO2 diagram comparing composition of TTGs at different times (blue fields) with melts produced by experimental hydrous basalt melting (grey area). (after Martin and Moyen, 2002)

40 50 60 70 Si02%

Fig. 4.23. MgO vs. SiO2 diagram comparing composition of TTGs at different times (blue fields) with melts produced by experimental hydrous basalt melting (grey area). (after Martin and Moyen, 2002)

plated basalts. Indeed, unlike slab melting, the fusion of underplated basalts prevents felsic magmas from coming into contact with mantle peridotite and consequently precludes any interaction (Fig. 4.19). 2) The composition of the TTG parental magmas progressively evolves through time: it changes from low MgO and Ni at 4.0Ga toward higher values at 2.5Ga. This progressive change reflects increasing melt and mantle interactions: the efficiency of these interactions was greater at 2.5Ga than at

Both (Na2O + CaO) and Sr are contained in huge amounts in plagioclase. Consequently, (Na2O + CaO) and Sr contents of the TTG parental magma reflect the stability of plagioclase during melting: when plagioclase is stable in the residue, the magma is depleted in these elements, whereas when plagioclase is no longer stable magma is (Na2O + CaO)- and Sr-rich. The temporal increase of (Na2O + CaO) and Sr content in TTGs is thus interpreted as reflecting the declining role played by residual plagioclase in the genesis of TTG parental magmas from 4.0 to 2.5Ga. In addition, Fig. 4.24 shows that plagioclase is only stable at relatively low pressure, such that the presence or absence of plagioclase in the residue of melting can be interpreted in terms of melting depth. Consequently, it appears that from 4.0 to 2.5, the depth of basalt melting progressively increased.

Figure 4.21 shows that Earth heat production was very high at 4.0Ga such that very high geothermal gradients could be realized. Thus conditions of slab melting can be reached at shallow depth, within the stability field of plagioclase (Fig. 4.24). In the presence of residual plagioclase, Sr and (Na2O + CaO) will remain in the residue of melting and thus will not enter the generated melts. On the other hand, melting of the slab at shallow depth implies a very thin mantle wedge over the slab melting-zone (Inset A in Fig. 4.24). Depending on crustal thickness and subduction dip, no overlying mantle wedge can even be envisaged (Smithies, 2000; Martin et al., 2004). Therefore, the ascending slab melts would either cross no mantle at all or only a very thin slice, thus reducing the probability of interaction. Moreover, the temperature of the mantle crossed by the slab melts would be low, thus diminishing the efficiency of possible reactions. This absence or scarcity of mantle-melt interactions would result in low MgO and Ni in TTG parental magmas.

Number Sio2 Diagram Rapp 1991
Distance in km Distance in km Distance in km

Fig. 4.24. Pressure vs. temperature diagram and synthetic cross section of subduction zones: in the early Archaean (4.0 Ga) geothermal gradients along the Benioff plane were very high, thus the subducted slab melted at a shallow depth and plagioclase was a residual phase. Because of the small thickness and the low temperature of the wedge, mantle-melt interactions were limited or absent. At 2.5 Ga, the Earth was cooler, geothermal gradients along the Benioff plane were lower such that slab melting occurred at greater depth without residual plagioclase. The overlying mantle wedge was thick and hot and interactions can occur between mantle and slab melts. Today, modern calc-alkaline magmas are generated by metasomatized mantle-wedge melting. P-T diagram shows hydrous (5% water) and dry solidus curves for tholeiitic basalt (Wyllie, 1971; Green, 1982). Dehydration reactions correspond to destabilization of antigorite (serpentine) (A); chlorite (C) and talc (T). Also labelled are the domains where a magma generated by basalt melting coexists with a residue containing both garnet (G) and hornblende (H) without (blue field) or with (grey field) plagioclase (P) In the synthetic cross sections: O.C. = oceanic crust; C.C. = continental crust; m.s. = mantle hydrous solidus; dark blue domains correspond to the place where magma is generated and positioned; dotted grey domains represents the place where fluids pass through

Between 4.0 and 2.5Ga, the Earth's temperature decreased, leading to lower geothermal gradients along Benioff planes. Figure 4.24 indicates that, while a lower geothermal gradient would still allow hydrous slab melting, this process must occur at greater depth and outside the plagioclase stability field. In this case, (Na2O + CaO) and Sr will not be retained by residual plagioclase and will enter the resulting melt. Inset B of Fig. 4.24 shows that the mantle overlying the melting zone is thicker and also warmer than for a shallow-depth melting. Because of this, ascending slab melts must cross an even thicker and hotter mantle peridotite slice, thus increasing the efficiency of interaction and yielding MgO- and Ni-rich magmas.

Still lower geothermal gradients (Fig. 4.24, inset C) prevent slab melting and rather favour slab dehydration. Aqueous fluids released by dehydration reactions ascend into the mantle wedge, which is then metasomatized and starts melting, giving rise to the typical calc-alkaline magmas.

In conclusion, it appears that the conditions of melting significantly changed from 4.0 to 2.5Ga and that these changes are recorded in the TTG composition. As the Earth cooled, the depth of slab melting increased and as a result, slab melt and mantle-peridotite interactions were also augmented.

4.2.3 Archaean Tectonic

Today, mountain chains like the Alps or the Himalayas result in continental collision that gives rise to several tens of kilometre-sized overthrusting (Fig. 4.25). In other words, in collision environments, plate tectonics generates (and consequently is characterized by) great horizontal structures.

On the contrary, the more prominent figure in Archaean terrains consists of dome and basin structures that can be observed at all scales, from a few hundred kilometres to tens of kilometres in diameter. Generally, greenstone belts form basins stretched in between well-developed TTG domes (Fig. 4.26; Photo 4.8). Since Gorman et al. (1978), this mechanism is called sagduction and is considered as driven by gravity forces that result in vertical structures. This style of deformation that is well developed in Archaean terrains, persisted in some places during the lower Proterozoic, but is totally absent in Phanerozoic terrains. Here

Fig. 4.25. Schematic E-W cross section of French Alps (after Merle, 1994) showing the great thrusts and the mainly horizontal character of this modern mountain chain
Fig. 4.26. Map (left) and diagram bloc (right) of the Holenarsipur area (S. India) showing the dome and basin structures typical of Archaean vertical tectonics. (after Bouhallier et al., 1995)
Greenstone Belt Komatiites

Photo 4.8. Satellite view of the Archaean Pilbara block (Australia) showing the sag-duction structures: the greenstone belts (dark green colour) are located between TTG domes (white-yellow colour). (Picture width ~ 400km). (Photo landsat)

too, this main change in tectonic style (from vertical to horizontal), seems to occur at about 2.5Ga., even if the transition has been very slow.

Sagduction is gravity-driven tectonic movement that could be described as some kind of inverse diapirism. Indeed, when high density (d = 3.3) ultramafic rocks such as komatiites or even some iron-bearing sediments as BIF (Photo 4.6) are positioned over a low density (d = 2.7) continental crust made up of TTG, they generate a strong inverse density gradient. Gorman et al. (1978) calculated that a 5-7km thick komatiitic lava layer was sufficient to initiate sagduction. More recently, Chardon et al. (1996) and Rey et al. (2003) showed that crustal heating from below can enhance the development of mechanical instabilities and diapirism. Sagduction structures are not only due to the down motion of high-density greenstones into the TTG basement but also to the concomitant ascent of the surrounding low-density TTG. Sagduction can create a depression in the central part of greenstones, a depression that can be filled by sediments (Fig. 4.27).

Sagduction efficiency is strongly dependent on the inverse density gradient. On Earth, the only high-density rocks able to position at the surface are komatiites and BIF, rocks that are only known in Archaean terrains. Indeed, after

Fig. 4.27. Sagduction model as proposed by Gorman et al. (1978). High-density ul-tramafic rocks (blue) are emplaced over a low density continental crust (grey). Once started, the downward motion of ultramafic rocks creates in the centre of the structure a depression where sediments and volcanic rocks can deposit

Fig. 4.27. Sagduction model as proposed by Gorman et al. (1978). High-density ul-tramafic rocks (blue) are emplaced over a low density continental crust (grey). Once started, the downward motion of ultramafic rocks creates in the centre of the structure a depression where sediments and volcanic rocks can deposit

2.5 Ga, Earth was so cold that mantle was not able to reach a degree of melting as high as 50%, necessary to generate komatiites. Today, most basalts are generated by degrees of melting of about 25% to 30%, giving rise to rocks whose density does not exceed 2.9 or 3.0. The resulting inverse density gradient is not great enough to allow sagduction initiation. As komatiites and BIF are restricted to Archaean times, vertical tectonics (sagduction) is also restricted to the primitive Earth crustal evolution.

For a long time, vertical tectonic style, typical of Archaean, has been considered as the only one able to operate on the primitive Earth. Consequently, several authors proposed that plate tectonics did not operate during Archaean times and that, due to high heat fluxes an Archaean lithosphere (considered only from the rheological point of view) never existed.

4.2.4 Archaean Plate Tectonic?

The existence and efficiency of plate tectonic processes has been strongly debated until recently. The predominance and omnipresence of vertical tectonic patterns as well as the lack of described Archaean oceanic crust relicts, constitute the main arguments against plate tectonic operating on the primitive Earth. However, recent detailed structural studies pointed to the existence of horizontal structures in almost all Archaean terrains (Bickle et al., 1980; de Wit et al., 1992; Blais et al., 1997; Choukroune et al., 1997). Horizontal structures are generally the older ones such that they were obliterated and masked by the more recent sagduction patterns. The recognition of horizontal structures is extremely important. Indeed, the most spectacular and typical structure of modern plate tectonics is continental collision, which builds mountain chains, generates crustal-sized overthrusting, resulting in large-scale horizontal structures (Fig. 4.25). Collision-related structures, not only demonstrate the existence of rigid plates on Earth surface, but also demonstrate their relative motion. Large-scale horizontal tectonic movement appears as proof that plate tectonics operate. During the 15 last years, overthrusts and great horizontal structures have been recognized in almost all Archaean terrains: in Finland (Blais et al., 1997), Greenland (Nutman and Collerson, 1991), South Africa (de Wit et al., 1992), Australia (Bickle et al., 1980; Bickle et al., 1993), Canada (Ludden et al., 1993; Choukroune et al., 1997), (Fig. 4.28).

Figure 4.29 shows great overthrusting structures from Finland in the Baltic shield. This place is of special interest as in the thrusting plane, from place to place, crop-out kilometre-size mafic and ultramafic lenses (Lentiira group) that are interpreted as remnants of oceanic crust. This conclusion is based on 3 sets of evidence:

- Structural position: The Lentiira group is exclusively located in the thrusting plane between two continental blocs (Kivijarvi and Naavala). This position, all along the suture is exactly the position where relics of oceanic crust (ophiolites) are expected and observed in Phanerozoic mountain chains.

-ABITIBI-+-OPATICA-+ NEMISCAU

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