O. M. Turkina and A. D. Nozhkin
Institute of Geology, Siberian Division of the Russian Academy of Sciences
Continental crust growth is believed to occur mainly through subduction-related magmatism at convergent plate margins and within-plate magmatism initiated by rising mantle plumes [Rudnick, 1995]. The latter results in volcanism in the form of continental flood basalts and oceanic plateaus, as well as underplating of mafic melts near the crust/mantle boundary [Condie, 1997a, 1999; Rudnick, 1995]. The established fact that the average composition of continental crust matches island arc and continental margin andesites [Taylor and McLennan, 1985] supplies the main argument for the leading role of subduction magmatism in crust formation with a minor role of other processes.
Two probable models for lateral growth of continental crust (through accretion of island arcs and oceanic plateaus) are discussed for the Early Precambrian [Condie, 1994]. However, it is not quite clear how much the subduction and plume mechanisms contributed to crustal growth. From theoretical considerations, at the early evolutionary stage the earth's thermal regime was such that mantle plumes were more frequent [Campbell and Griffiths, 1992], and seafloor spreading rate was higher [Abbott and Hoffman, 1984], which implies a greater rate for oceanic lithosphere subduction in the Archean. Hence, both mechanisms played an equally important role, and neither is seen as being predominant. Critical constraints can be obtained by studying Archean magmatic rocks in granite-greenstone provinces. These are mainly mafic volcanics: tholeiitic basalts and tonalite-trondhjemite-granodiorite series (TTG) rocks.
The use of geologic criteria for reconstructing original geodynamic settings of Archean and, particularly, Early Archean TTG suites (that make plagiogneiss, or gray gneiss assemblages) is strongly obstructed by the fact that these are often the oldest, repeatedly reworked rocks, far away in time from the formation of those geologic assemblages they are tectonically juxtaposed with. Under these circumstances, of prime importance may become petrologic and geochemical evidence, including TTG compositional features, P-T conditions of formation of the initial melt, and metabasite source composition. Additional information is provided by the composition of associated metabasites, everywhere represented by amphibolite enclaves among plagiogneisses. Let us see how advantageous the use of these criteria can be.
According to experimental data, Archean high-alumina TTG rocks, variably depleted in the heavy REE and Y, must have been produced in equilibrium with residual garnet and/or amphibole at P > 10-15 kbars [Rapp and Watson, 1995; Sen and Dunn, 1994]. This implies melting at the bottom of an over-thickened crust or at a subcrustal depth and is consistent with TTG origin in subduction-related and mantle plume settings. The available data show that crustal thickness in oceanic plateaus may range from over 35 km (as in the Ontong Java Plateau) to ca. 20 km (as in the Kerguelen Plateau) through to 10-15 km (as in the Caribbean Basin) [Saunders et al., 1996]. Crustal thickness estimated for Archean oceanic plateaus by [Kent et al., 1996] using the adiabatic partial melting model of McKenzie and Bickle  assuming a potential mantle temperature of 1600o C is ca. 43 km. Hence, the lower crustal sections of oceanic plateaus can be transformed to garnet-bearing granulite or eclogite, which means that conditions prerequisite for tonalite-trondhjemite melt genesis can be implemented. Therefore, the likely P-T conditions of melt generation are such that there is virtually no choice between the two options for TTG genesis here discussed.
Another criterion can be provided by the composition of the metabasite source of tonalite-trondhjemite melts and associated amphibolites. With plume derived TTG rocks, either of the two metabasites must be similar to oceanic plateau basalts. In order to test this criterion, let us address our study in the Archean TTG suite of the Onot block at the southwestern margin of the Siberian craton.
Assuming a subduction related origin for tonalite-trondhjemite melts, their source must be ocean floor basalts produced in spreading systems. Unfortunately, no reliable evidence on composition of the Archean oceanic crust is available, which is thought to be due to its rapid recycling into the mantle [Condie, 1994]. To date, two examples of mafic volcanics that are interpreted as being products of Archean oceanic crust have been reported. These are Late Archean mafic volcanics of the Irinogorsky structure, Baltic shield, considered as fragments of a suprasubduction assemblage [Shchipansky et al., 2001], and Early Archean basalts from the Cleaverville area, Pilbara craton, classed as mid-oceanic ridge derivatives [Ohta et al., 1996]. It is readily apparent that in both cases the mafic volcanics are MORB-like rocks, depleted markedly in the most incompatible elements (Th, Nb), and that they differ sharply in composition from the source inferred for the Onot TTG suite (Figure 10).
Earlier, a number of workers, including [Sen and Dunn, 1994] and [Tarney and Jones, 1994], pointed out that estimated compositions for felsic melts derived from the MORB source are depleted in Ba and Sr relative to Archean TTG suites, and, hence, the source of tonalite-trondhjemite suites must have higher contents of these components. Evidently, oceanic plateau basalts, enriched in Ba and Sr relative to MORBs, may be the most suitable candidate for the source for Archean TTG suites.
An additional criterion to estimate the character of the source of tonalite-trondhjemite melts can be provided by isotope compositions of TTG rocks. Adakites of the Cook Island and the Cerro Pampo volcanic center (Pacific margin of South America) have high positive e Nd (T) values, between 5 and 9.8, the highest of them approximating DM values, which is consistent with their genesis through melting of a MORB-like mafic source and which supports a subduction-related origin for these rocks [Kay et al., 1993; Stern and Kilian, 1996]; the entire range of measured e Nd (T) values from +9.8 to - 1.9 for adakites of the Andean volcanic zone must be due to a varying contribution of crustal material in the form of subducted sediments. The Aruba tonalites and basalts are virtually identical in terms of their e Nd (T) (6.6-6.8 and 6.7-7.8, respectively) [White et al., 1999]; however, it is important that either of the two has a less radiogenic Nd isotope composition than MORB ( e Nd (T) = 9.8). Basalts from other oceanic plateaus also have e Nd (T) values that are significantly lower than MORB; e.g., for the Ontong Java Plateau these values are between +4 and +7 [Kerr et al., 1997], which implies contribution from an undepleted lower mantle plume source.
Archean TTG rocks, judging by published data, are marked by a rather broad range of e Nd (T) values, from close to DM to negative. These variations must be due mainly to crustal contamination of initial melts produced from metabasites derivative from a depleted mantle source. On the other hand, one cannot preclude that the mafic sources of Archean TTG rocks could have originated, by analogy with modern oceanic plateau basalts, from a less depleted mantle. This is supported by isotope compositions of basalts and komatiites ( e Nd (T) = 2.8-4.4) of the Kostomuksha greenstone belt (Baltic shield), thought to be an obducted fragment of an oceanic plateau [Puchtel et al., 1998]. Adequate interpretation of Nd isotope composition data for Archean assemblages is a painstaking exercise (see [Moorbath et al., 1997]). Detailed discussion of this issue is beyond the scope of this study. It should be stressed, however, that the use of isotopic criteria to reconstruct source compositions for tonalite-trondhjemite melts in the Archean may prove highly rewarding, as evidenced by the case studies in Cenozoic rock assemblages.
Our analysis, based on a case study in Early Archean plagiogneisses and gneissic plagigoranites of the Onot block, shows that a number of characteristics of tonalite-trondhjemite suites (including their low Mg# and the fact that the inferred metabasite sources of initial TTG melts and plagiogneiss-associated amphibolites are similar to mafic volcanics from modern oceanic plateaus and their counterparts in Archean greenstone belts) are not explicable in the context of the subduction model. This model, which invokes adakite genesis through melting of the oceanic slab plunging steeply into subduction zone [Drummond and Defant, 1990; Martin, 1994, 1999], suggests a melting source that matches MORB-like basalts in terms of trace element and isotope compositions and assumes that the initial melt's Mg# increased through interaction with mantle wedge peridotites. In order to account for the lower Mg# and elevated SiO 2 content of Archean TTG rocks as compared to adakites, invoked is flat subduction of the oceanic plate or tectonic underplating, which makes it possible to do virtually without tonalite melt transport through mantle peridotite [Abbott and Hoffman, 1984; Martin and Moyen, 2002; Smithies and Champion, 2000].
Alternative to flat subduction are models for TTG production through melting at the bottom of thickened (35-40 km or more) crust, formed by accretion of oceanic plateaus [Smithies and Champion, 2000] or by stacking and obduction of oceanic crust [de Wit, 1998]. As was shown above using our case study in the Onot TTG rocks, the former option is corroborated by the similarity between trace element compositions of the tonalite melt source and metabasites at the level of melt crystallization on the one hand and plume derived basalts on the other. Underplating of mafic melts at the bottom of the crust above a rising mantle plume may provide a heat source to initiate tonalite melt generation.
Needless to say, generation of Archean TTG suites is not restricted to subduction related environments, and further studies are required to elucidate geodynamic settings in which they were generated. The analysis here presented shows that TTG suites may well have been formed at the bottom of a thick crust produced by accretion of oceanic plateaus. The most promising criteria to reconstruct geodynamic settings may primarily be provided by (i) compositional characteristics of metabasite enclaves in TTG suites and the model source for tonalite-trondhjemite melts, (ii) Mg#'s of TTG rocks and their consistency with experimental melts, and (iii) isotope parameters of TTG suites and their related amphibolites.
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