The Origins Of Modern Style Plate Tectonics
Evidence from the Rock Record
Archean Crust and TTGs
Modern continental crust typically forms as a result of hydration melting within subduction zone environments, triggering melt formation. This melt then rises and forms a volcanic arc, usually resulting in calc-alkaline volcanos and plutons (Rollinson, 2007); however, this was not always the case. During the Archean, the mantle had a potential temperature ca. 200 ˚C degrees greater than present (Gibson, 2002; Herzberg et al, 2007) and the addition of water was likely unnecessary to generate melt. From this it was implied that during the Archean slabs themselves melted to form granitoids, as a result of elevated temperatures, rather than from dehydration (Rollinson, 2007).
Jean Bédard (2006), proposed a non-subduction model where tectonics are vertical, as opposed to horizonal, giving rise to the theory that mantle upwelling is responsible for the production of granitoid magmas, in which residual eclogite may be found. This forms by ‘piling up’ at the base of the crust, which subsequently melts and adds additional melt to the system. The granitoid component then rises to the surface whilst the eclogitic component returns to the mantle, due to density contrasts, triggering further melting and thus a cyclical process is established. If this model were true, then it challenges theories of horizontal Archaean tectonics along with making the theory of subduction being a necessary component for granitic melt production redundant and potentially inferring that estimates for older ages of modern-day tectonic onset are impractical.
To counter this, Rollinson proposed two main problems with this non-subduction model. Firstly, to generate melt of the correct composition for the observed rocks, melting of a wet basaltic source is required. This is problematic as without subduction there is no reasonable explanation for how a wet melt may be initiated and sustained at the base of the crust. The second argument regards different trace element ratios found in arc and plume rocks, particularly for La/Nb. Comparing these ratios in rocks from both tectonic setting concludes that dominant compositions throughout geological time are from arc-related magmatism (Rollinson, 2007), suggesting that plume or upwelling processes are subordinate to this and therefore not the primary mechanism for the generation of crust within the Archaean.
A second model based on ‘hot subduction’ has also been theorised and requires the burial of a hydrated slab which is then partially melted (Rollinson, 2007) in the garnet stability field (>12 kbar) (Martin and Moyen, 2002; Moyen, 2011). However, are either of these models wholly realistic and do they suggest an age to the initiation to modern-day tectonics? The tonalite-trondhjemite-granodiorite (TTG) group of Archean continental rocks have been studied extensively (Moyen and Stevens, 2006; Foley, 2008; Halla et al, 2009; Moyen and Martin, 2012) to try to answer these questions.
TTGs are sodic rich and depleted in large-ion lithophile elements (LILEs), showing very similar trace element patterns to ‘arc’ rocks except that they show highly fractionated REE traces, with La/Yb up to 150, without a pronounced EU anomaly (Moyen, 2011). They group is sub-divided into three pressure regimes, low (10–12 kbar), medium (15 kbar) and high pressure (20+ kbar), each of which has a unique range of geothermal gradient associated with it; these are 20–30 °C/km, 12–20 °C/km and 10 °C/km respectively. These gradients reflect the depth of melting and the residual assemblages, which are the distinguishing factors as all groups are thought to derive from sources of similar, basic compositions (Moyen, 2011). Moyen modelled the groups and found the high-pressure group forms an eclogitic residue that is rich in garnet while the low-pressure group forms a residue enhanced in amphiboles, pyroxenes and plagioclase but with garnet depletion. While the first appearance of TTGs is thought to be around 3.8 Ga, the actual age is debated to be between 4.0 Ga to 3.6 Ga (Foley, Tiepolo and Vannucci, 2002; Hoffmann et al, 2011; Hastie et al, 2016).
Analysis of La/Yb and Sm/Yb ratios, by Liu, Sun and Deng (2019), found an increase ca. 3.0 Ga and related it to the crystallisation of residual garnet as the geotherm entered the minerals stability field resulting from mantle cooling. Before 3.0 Ga, the ratios varied and presented no obvious pattern, likely due to a higher geotherm. The increase in the geochemical ratios, ca. 3.0 Ga, suggest a change in mantle dynamics, possibly indicating the onset of plate tectonics. Lui et al. (2019) argued that the presence of the high geotherm means the non-subduction model could not support the formation of high and medium pressure TTGs and thus the subduction model must have been globally operational by ca. 3.8 Ga. Saying this, the validity of the study may be brought into question as all arc rocks were excluded due to ‘geochemical complexity’ despite the majority of TTGs matching the chemical signatures of such rocks. The use of continental basalts may provide unreliable results, especially in the attempt to determine the age at which subduction, and the generation of arc rocks, initiated.
Despite this, the study supports the work of Moyen (2011), who explained the geotherm required to produce the melt for high-pressure TTGs is only slightly higher than that of present-day and so would be easily achieved by hot subduction, based on the elevated mantle temperatures although would not be sufficient to initiate melting of the low and medium pressure groups. Whilst formation of the low-pressure group could be explained by the non-subduction model, as this reflects rocks similar to those found at the bases of plume related oceanic plateaus, there is still no reasonable suggestion into the mantle dynamics which aided formation of the medium-pressure group; ‘very hot’ subduction is possible albeit unlikely. Based on the pressures and temperatures required it is highly probable the burial of cold crustal matter was required to form these rocks, suggesting that plate tectonics may have been operational since 3.8 Ga (Foley, 2008; Liu et al., 2019), although the Late Archean (2.5 Ga) is more accepted (Foley, 2008; Tang, Chen and Rudnick, 2016; Bédard, 2018).
Tang, Chen and Rudnick (2016) agreed that the subduction model was the most likely in explaining the formation of TTGs. This was based upon their findings that the amount of TTGs within the upper continental crust increased fivefold, with the biggest increase during the Neoarchean (2.8 to 2.5 Ga) and disregards the non-subduction model as a viable mechanism to produce sufficient melt in order to account for this increase. They also noted that the lower crust tends to be depleted in water, which is an especially important factor in TTG formation, further disregarding the viability of the non-subduction model. Based on these constraints it is implied that water was continuously supplied to the mantle by 3.0 Ga, allowing for the large increase in TTG generation, which is most easily explained by the onset of subduction. Tang et al. also found that the upper continental crust evolved from highly mafic to felsic between 3.0 and 2.5 Ga, with the composition staying remarkable similar ever since, indicating that a global, modern style tectonic regime was operational by 2.5 Ga. While this study is consistent with multiple others (Condie, 1998; Condie and Aster, 2010; Shirey and Richardson, 2011; Dhuime et al., 2012) its usefulness may be questioned as their findings were based on using Cr/Zn and Ni/Co as proxies for MgO in ancient, eroded continental crust. Magnesium cannot be directly used given its high tendency to be liberated through weathering (Albarède, 2009) so instead proxies are traced. Given the study’s correspondence with others, and the justification of the use of Cr/Zn and Ni/Co as proxies to trace MgO, it can be regarded as reliable.
Rubidium and Strontium isotope ratios were studied by Dhuime, Wuestefeld and Hawkesworth (2015) for over 13,000 volcanic and plutonic samples. The Rb/Sr ratios were also found to increase from ca. 3.0 Ga, consistent with findings of Liu, Sun and Deng (2019). Dhuime converted these values to reflect SiO2 wt% and found that the crust has become increasingly silica rich and experienced thickening with time, until ca. 1 Ga. The increase in crustal thickness was linked to the onset of subduction based on the work or Shirey and Richardson (2011), although the data from Dhuime’s study also supports this theory. The decrease in crustal thickness ca. 1 Ga implies that the crust has reached its maximum thickness and that since then the rate of destruction has exceeded the rate of generation (Dhuime et al., 2015).
Ophiolites are believed to represent tectonic processes by the obduction of oceanic crust onto continental crust, likely through collisional tectonics. There is still debate as to the direct origins of this crust, they were previously thought to be fragments of an ocean ridge but are now believed to have formed as a result of subduction in a back-arc setting (Pearce, 2003). Regardless of their origin they are still indicative of tectonic processes (Dilek, 2003; Stern, 2005). These features become prevalent in the geological record after 1 Ga and thus indicate that tectonic processes also initiated around this time. One of the oldest known example of a well-preserved ophiolite is the Jormua-Outokumpu Ophiolite in Finland (Rollinson, 2007; Condie and Kröner, 2008), dated to ca. 2.0 Ga (Lahtinen et al, 2011; Rasilainen et al, 2016), and is inconsistent with the prevalence of these structures and interpretation of the onset of plate tectonics.
However, Furnes, de Wit, Stajdigel, Rosing and Muehlenbachs (2007) believe that the Isua Supracrustal Belt (ISB) in southwest Greenland is the earliest example of obduction, implying that these processes have been occurring since ca. 3.7 Ga (Komiya et at., Furnes et al., 2007). If this is the case, Stern’s argument for the use of ophiolites to determine the origin of plate tectonics falls apart. If obduction has been operational since 3.7 Ga this would justify the presence of the Jormua-Outokumpu Ophiolite, and others dated before 1 Ga. While Stern acknowledges the presence of pre-1 Ga ophiolites he does regard them as an indicator for sustained tectonics due to their rarity and characteristic differences; younger ophiolites are thicker (Sleep, 2000). Nonetheless, ophiolites are a relatively rare feature, albeit becoming more common after 1 Ga, and their lack of evidence before 1 Ga may simply be due to destruction of the evidence, as opposed to them never existing at all. Condie and Kröner (2008) proposed that the lack of ophiolites before 1 Ga may be due to them not being preserved as well as those of modern times.
Based on the controversy still surrounding the nature of ophiolites and their potential lack of preservation they should be somewhat discounted as a viable exhibition for the origins of modern-style plate tectonics. (SHOULD THIS BE IN CONCLUSIONS INSTEAD OF HERE? OR IN BOTH?)
Blueschist has also been used as an indicator for the onset of plate tectonics (Stern, 2005; Brown, 2006). These Na-rich amphibole metamorphic rocks require a low temperature-high pressure regime suggesting an environment where a cool slab has been subducted up to 70 km (Peacock, 1993; Penniston-Dorland, Kohn and Manning, 2015) below the surface.
The oldest known blueschists are dated to ca. 940 Ma and located in Southern China (Shu and Charvet, 1996; Jahn, Caby and Monie, 2001), although most are in the ranges of 700 to 800 Ma (Maruyama, Liou and Terabayashi, 1996; Stern, 2005; Palin and White, 2016). As a ‘cool’ mantle environment did not exist before this time the conditions required to induce this metamorphic grade, around 400 ˚C/GPa (Maruyama et al., 1996), were unattainable. This implies that at ca. 1 Ga the mantle had sufficiently cooled to allow deep burial at colder temperatures possibly signifying the onset of modern style tectonics. This is a view supported by Brown (2006), who believes that the modern plate tectonic regime was established in the Neoproterozoic (1 Ga), following a transition from an unsustained Proterozoic regime.
However, similarly to the aforementioned ophiolites, the rocks are rare due to the specific conditions needed for their formation. Based on Condie and Kröner’s (2008) suggestion that ophiolites were poorly preserved in ancient crust, it is reasonable to assume that blueschist preservation was also limited. Condie and Kröner (2008), also proposed that the lack of evidence may be due to elevated mantle temperatures which hindered the exhumation of such rocks.
Whilst evidence for ancient blueschists may be lacking, it is certainly not entirely absent. Ganne et al. (2011) proposed evidence for early blueschists in the West Africa metamorphic province, dated to within 2.2 Ga and 2.0 Ga. Metamorphic minerals, chlorite and phengite, were identified and predicted to have formed under a geothermal gradient of 10-12 kbar (Ganne et al., 2011), consistent with that which formed low-pressure TTGs. Study of the 3.5 Ga Barberton greenstone terrane, South Africa (Moyen and Stevens, 2006), provides even earlier evidence for Archean blueschists (Condie and Kröner, 2008).
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