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Lithos 80 (2005) 33 – 44 www.elsevier.com/locate/lithos TTGs and adakites: are they both slab melts? Kent C. Condie* Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA Received 25 March 2003; accepted 9 September 2003 Available online 5 November 2004 Abstract Although both high-Al TTG (tonalite–trondhjemite–granodiorite) and adakite show strongly fractionated REE and incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. These compositional differences cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, TTGs may be produced by partial melting of hydrous mafic rocks in the lower crust in arc systems or in the Archean, perhaps in the root zones of oceanic plateaus. It is important to emphasize that geochemical data can be used to help constrain tectonic settings, but it cannot be used alone to reconstruct ancient tectonic settings. Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs require both garnet and low-Mg amphibole in the restite, whereas moderate to high Sr values allow little, if any, plagioclase in the restite. To meet these requirements requires melting in the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 8C. Some high-Al TTGs produced at 2.7 Ga and perhaps again at about 1.9 Ga show unusually high La/Yb, Sr, Cr, and Ni. These TTGs may reflect catastrophic mantle overturn events at 2.7 and 1.9 Ga, during which a large number of mantle plumes bombarded the base of the lithosphere, producing thick oceanic plateaus that partially melted at depth. D 2004 Elsevier B.V. All rights reserved. Keywords: TTG; Adakite; Archean tectonics; Arc systems; Mantle plume event 1. Introduction Since the classic book edited by Fred Barker on trondhjemites and related rocks (Barker, 1979), this suite of rocks, now known as the TTG (tonalite– trondhjemite–granodiorite) suite, has received considerable attention. Because they are the most volumi- * Tel.: +1 505 835 5531; fax: +1 505 835 6436. E-mail address: [email protected]. 0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.lithos.2003.11.001 nous rock type in the preserved Archean crust, it is critical to understand the source and origin of TTGs to better understand the origin and early evolution of continents. Even in the Tertiary, TTGs are an important juvenile component added to the continental crust in the Andes and other continental-margin arcs. Because of geochemical similarities of TTGs to modern adakites, they are commonly assumed to have similar origins (Drummond and Defant, 1990; Martin, 1999), and in some cases, the two terms are used interchangeably. For instance, on a primitive mantle 34 K.C. Condie / Lithos 80 (2005) 33–44 Fig. 1. Primitive-mantle normalized incompatible element distributions in adakites and TTGs. Data from Kay et al. (1993, 1999) and Table 1 and Appendix A. Primitive mantle values from Sun and McDonough (1989). normalized element distribution diagram, both rock types exhibit strong enrichment in the most incompatible elements, with notable negative anomalies at Nb– Ta and Ti and strong depletion in heavy REE and Y (Fig. 1). Hence, as with adakites, TTGs are commonly assumed to represent melts of descending slabs, a conclusion with important implications for tectonic settings in the Archean. However, some investigators have pointed to dissimilarities between adakites and TTGs and hence to different tectonic settings (Smithies, 2000; Kamber et al., 2002). One of the issues in this controversy is that of how to define TTG and whether or not TTGs are strictly an Archean phenomena or if they occur throughout geologic time. In this study, these questions are addressed by comparing published chemical analyses of TTGs and adakites of varying ages and locations. Results show that TTGs and adakites are not the same thing, and consequently, they may have quite different origins. Furthermore, TTGs are not an Archean phenomenon but have contributed to the growth of continental crust throughout geologic time. 2. Data collection and definitions To compare adakites and TTGs, I have compiled chemical analyses from the literature together with unpublished analyses from our laboratory. Included are major elements and a group of both compatible and incompatible trace elements (see Appendix A). We have used a definition for TTG similar to that of Barker (1979) and Defant and Drummond (1990): Al2O3 contents N15% at 70% SiO2, Sr N300 ppm, Yb20 ppm, Ybb1.8 ppm, and NbV10 ppm. Although adakites commonly show these same chemical characteristics, they are more mafic and can be distinguished from TTGs by their relatively high Mg, Ni, Cr, and Sr contents. TTGs also have been referred to as blow-Mg adakitesQ (Rapp et al., 1999). For a rock to be called an adakite, in addition to the above geochemical characters, it should have a Mg number greater than 50, Sr N500 ppm (often N1000 ppm), Cr z50 ppm, and Ni z20 ppm (Figs. 2–5; Appendix A). Also, as pointed out by Kamber et al. (2002), Archean TTGs do not resemble adakites in terms of fluidmobile trace elements such as Ba, K, and Rb (Table 1). The relatively high contents of these elements in TTGs compared to adakites may reflect a subduction zone component in TTG sources, which is not present in the descending slab source of adakites. The relatively high Mg, Cr, and Ni contents of adakites are generally ascribed to interaction of the slab melts with the overlying mantle wedge, a process that has been verified by experimental studies (Drummond and Defant, 1990; Rapp et al., 1999). Another relatively minor group of plutonic rocks known a sanukitoids is also recognized in the Archean (Shirey and Hanson, 1984). These rocks, which show the similar distributions of incompatible elements as TTGs, contain larger amounts of K, Rb, Th, and U and are generally thought to be the products of melting metasomatized mantle (Stern and Hanson, 1991). They are not included in the present investigation. Average values of selected element concentrations and element ratios are give in Appendix A, and a summary of average TTG compositions of various ages is given in Table 1 compared with average adakite. Although the geochemical definitional screens given in the previous paragraph are used to distinguish TTG from adakite, in some cases, screens give conflicting results. In these cases, the rock is classified according the majority of the screens. One thing is very clear from Table 1, TTGs are not an Archean phenomenon but occur throughout geologic time. Some of the youngest examples are found in the Miocene batholiths of the Andes and comprise a large proportion of the Andean Cordillera (Petford and Atherton, 1996; Kay et al., 1999). On average, K.C. Condie / Lithos 80 (2005) 33–44 35 Table 1 Average chemical compositions of TTGs and adakites TTGs SiO2 TiO2 Al2O3 Fe2O3T MgO CaO Na2O K2O P2O5 MnO Th U Ni Cr Y Zr Nb Hf Ta La Ce Nd Sm Eu Gd Tb Yb Lu Rb Ba Sr (La/Yb)n Sr/Y Nb/Ta Mg# K2O/Na2O n Adakites Early Archean Late Archean Mean Mean 70.4 0.31 15.2 2.79 0.96 2.74 4.71 2.22 0.1 0.06 99.49 4.1 1.2 17 45 8.5 152 6.1 3.8 0.41 22 40 16 2.9 0.82 2.2 0.31 0.82 0.14 76 500 362 25 72 12.0 40.8 0.51 212 1j 2.9 0.14 1.1 1.2 0.62 0.88 0.61 0.68 0.06 0.02 2.4 0.49 19 48 6.4 44 3.5 1.6 0.29 9.4 16 8.2 1.7 0.31 1.4 0.23 0.7 0.1 26 258 117 15 24 2.8 8.3 0.18 68.3 0.42 15.5 3.42 1.39 3.26 4.51 2.20 0.14 0.07 99.21 8.1 1.5 22 35 9.1 154 6.2 4.7 0.84 36 65 25 4.2 1.07 2.9 0.38 0.71 0.11 67 769 515 36 89 13.0 46.2 0.51 831 Proterozoic 1j 2.6 0.12 0.75 1.1 0.59 0.73 0.48 0.66 0.06 0.03 5.3 0.99 13 26 5.4 49 2.4 1.9 0.3 16 29 9.1 1.7 0.32 1.2 0.17 0.41 0.07 24 288 127 24 49 6.3 6.7 0.24 Mean 67.3 0.47 15.8 4.04 1.48 3.42 4.33 2.30 0.14 0.08 99.36 6.1 2.1 23 55 17.3 152 7.1 4.3 0.72 26 45 18 3.5 0.95 3 0.49 1.33 0.23 63 717 473 14.2 37 9.9 43.2 0.56 752 Phanerozoic 1j 3.6 0.2 0.78 1.4 0.70 1.10 0.66 0.82 0.06 0.05 3.1 1.1 12 31 8.9 63 4.6 1.6 0.54 12 26 12 2 0.44 1.9 0.27 0.86 0.14 25 239 159 8.3 30 6.7 7.5 0.25 Mean 65.9 0.47 16.5 4.11 1.67 4.36 4.00 2.14 0.12 0.09 99.36 7.6 1.9 12 32 14.5 122 6.7 3.4 0.75 17 34 16 3.1 0.84 2.8 0.40 1.16 0.18 63 716 493 11.3 56 12.2 45.4 0.68 698 1j 3.5 0.16 0.94 1.3 0.69 1.10 0.50 0.63 0.05 0.02 4.4 1.1 5.0 15 4.7 30 1.7 0.73 0.32 6.2 11 4.5 0.89 0.23 0.65 0.11 0.39 0.06 26 251 105 5.3 12 9.2 6.9 0.5 Mean 62.43 0.67 17.05 3.99 3.31 6.53 4.25 1.42 0.26 0.08 99.99 3.9 1.2 64 82 9.7 117 9.7 3.3 0.60 24 65 26 4.7 1.37 2.30 0.40 0.81 0.09 15 309 1550 18.2 160 16.1 60.4 0.33 221 Major elements as oxides in wt.%, trace elements in ppm; n, number of samples; 1j, one standard deviation of mean; Mg#=MgO/ (MgO+0.79Fe2O3T), molecular ratio; chondrite normalizing values from Haskin et al. (1968); Early Archean, z3.5 Ga; Late Archean, 3.5–2.5 Ga; Proterozoic, 2.5–0.54 Ga; Phanerozoic, b0.54 Ga. TTG references are given in Appendix A; Adakite references (and references cited therein): Li and Li (2003), Percival et al. (2003), Kay et al. (1993), Martin (1999), Stern and Kilian (1996), Samaniego et al. (2002), Rapp et al. (1999), Polat and Kerrich (2002), Drummond and Defant (1990), Kay and Kay (2002), Peacock et al. (1994), Smithies and Champion (2000), Bourdon et al. (2002), Drummond et al. (1996), Defant and Drummond (1993), Kepezhinskas et al. (1997), Myers et al. (1985), Yogodzinski et al. (1995), Xu et al. (2002), Gutscher et al. (2000). however, Archean TTGs are more depleted in Y and heavy REE and have higher K2O/Na2O ratios than post-Archean TTGs (Table 1). In terms of tectonic setting, all young TTGs share in common an arcrelated setting, often, but not always a continentalmargin arc. Archean TTGs usually are intruded into 36 K.C. Condie / Lithos 80 (2005) 33–44 greenstone belts with arc or oceanic plateau geochemical affinities. Until recently, adakites were described only from Phanerozoic successions, but they are now recognized in the Proterozoic and a few rare occurrences in Archean terranes (Polat and Kerrich, 2002). As with TTGs, adakites are found in arc-related tectonic settings, both oceanic arcs and continental-margin arcs. Unlike TTGs, Archean adakites are not found associated with greenstones with oceanic plateau geochemical affinities. Data shown on the graphs in this study are mean values of TTGs from different geographic locations and different ages (Appendix A). Over 150 sites and nearly 2500 samples are represented in the database, although not all trace elements are available for each site. In evaluating similarities and differences, not only mean values, but also median values and ranges for each site are considered. 3. Adakites and TTGs 3.1. Andean examples Perhaps the best described comparison of adakites and TTGs is from the southern Andes (Kay et al., 1993, 1999; Stern and Kilian, 1996; Petford and Atherton, 1996). Well-documented adakites are associated with the Chilean triple junction where the Chile rise is being subducted beneath the Andes in Fig. 2. Sr vs. Na2O+CaO graph showing the distribution of TTGs in comparison to the adakite field. Data sources given in Table 1 and Appendix A. Andean adakites from Kay et al. (1993). Fig. 3. Mg number vs. SiO2 showing a comparison of Andean TTGs and adakites. Data sources given in Table 1 and Appendix A. Experimental liquids from data published in Rapp et al. (1991), Wylie et al. (1997) and Winther (1996). AFC, assimilation fractional crystallization trajectory from Stern and Kilian (1996); percents are amounts of assimilated ultramafic rock. Patagonia (Kay et al., 1993). The Andean adakites were erupted 12 Ma and appear to represent melts derived from the hot descending plate. These adakitic magmas share many geochemical features with the type adakites from Adak Island in the Aleutians, such as similar incompatible element distributions and similar high Sr contents (Figs. 1 and 2). Another feature of the Andean adakites is relatively high Mg number (50–70) as well as high Ni (N40 ppm), Cr (N80 ppm), and Sr (1800 ppm; Figs. 2–4), all of which are generally interpreted to reflect reaction of slab melts with overlying ultramafic rocks of the mantle wedge. The AFC line in Fig. 3 is one possible trajectory of assimilation-fractional crystallization of ultramafic rocks in the mantle wedge. In contrast to the Patagonia Andean adakites, a second group of felsic volcanic and related plutonic rocks is found in the Central Volcanic Zone (CVZ) of the Andes. These rocks, which have similar incompatible element distributions to the Patagonian adakites (Fig. 1), have relatively low Mg numbers (b55) and also relatively low Sr (300 ppm), Ni (5 ppm), and Cr (50 ppm; Kay et al., 1999). Kay et al. (1999) proposed a model for CVZ volcanic rocks involving thickening of the crust due to tectonic shortening and lithosphere delamination. The rapidly delaminated lithosphere was replaced by hot asthenosphere, which partially melted producing basaltic magmas. These K.C. Condie / Lithos 80 (2005) 33–44 Fig. 4. Mg number vs. SiO2 showing the distribution of TTGs in comparison to the adakite field. Data sources given in Table 1 and Appendix A. Experimental liquid field references given in Fig. 3. magmas, in turn, heated the thickened lower crust to produce TTG magmas, leaving eclogitic residues in the lower crust. Unlike the adakitic magmas, the CVZ volcanic rocks are similar in composition to experimental melts of wet mafic sources (Fig. 3). The low Mg numbers, Ni, and Cr in these rocks mean that unlike the Patagonian adakites, the CVZ magmas did not react with the mantle wedge. Employing the definitions used in this study, the CVZ volcanics are classic TTGs. Hence, it would appear that, in the Andes, both adakites and TTGs were produced during the Tertiary depending on whether a hot descending slab or a thickened/delaminated lower crust was melted. The key differences in the derivative magmas are not the incompatible element distributions but compatible elements such Mg, Sr, Ni, and Cr. 37 reacted with the mantle wedge. Although there is more overlap between TTGs and adakites in terms of Cr and Ni distributions (Fig. 5), a large number of the TTGs, regardless of age, have average Ni and Cr contents less than most adakites (Appendix A). Sr is a compatible element in plagioclase, and hence, the Sr distribution in adakite and TTG magmas reflects, at least in part, the role of plagioclase fractionation (Ellam and Hawkesworth, 1988; Tarney and Jones, 1994; Martin and Moyen, 2002). The partitioning of Sr into the melt is also related to the An content of the plagioclase (Foley et al., 2002). Most adakites have higher Sr and Na2O+CaO values than most TTGs, although there is minor overlap in Sr contents (Fig. 2). TTGs with b500 ppm Sr probably reflect either or both plagioclase in the restite or plagioclase fractional crystallization. Calcalkaline TTGs (low-Al TTGs) contain even less Sr (b300 ppm), a feature commonly ascribed to plagioclase fractional crystallization. As described by numerous investigators and especially by Martin (1993, 1999), Archean high-Al TTGs and adakites both have steep REE patterns, resulting in relatively high La/Yb ratios at low Yb values (Fig. 6; Appendix A). In contrast, most postArchean TTGs belong to the calcalkaline suite and often show a fractional crystallization sequence extending from diorite (or gabbro) to granite. These rocks typically have very low La/Yb ratios and exhibit a large range of Yb values (Fig. 6). The reason that 3.2. Some geochemical contrasts For comparative purposes, TTGs are grouped into five age groups: z3.5, 3.5–2.5, 2.7, 2.5–0.54, and b0.54 Ga. On a Mg#–SiO2 graph, the majority of TTGs fall in the hydrous experimental liquid field with only a small number plotting in the adakite field (Fig. 4). Most TTGs have lower Mg numbers than adakites, a feature recently emphasized by Smithies (2000). Again, this suggests that most TTGs, although partial melts of hydrous mafic sources, have not Fig. 5. Ni vs. Cr graph showing the distribution of TTGs in comparison to the adakite field. Data sources given in Table 1 and Appendix A. 38 K.C. Condie / Lithos 80 (2005) 33–44 Fig. 6. (La/Yb)n vs. Ybn graph showing distribution of adakites and TTGs. Modified after Martin (1993). Post-Archean TTG=calcalkaline plutonic suites. n—normalized to primitive mantle using values from Sun and McDonough (1989). adakites and high-Al TTGs are indistinguishable on the La/Yb–Yb plot is that reaction with the mantle wedge does not appreciably affect REE distributions or incompatible element ratios (Rapp et al., 1999). In both cases, the highest La/Yb ratios require that garnet occur in the restite (Martin, 1993). An important incompatible element ratio that distinguishes adakite from high-Al TTG is the Nb/ Ta ratio (Kamber et al., 2002). In making this contrast, care must be exercised in using only high precision analyses of Nb and Ta, and in this respect, most Nb analyses by XRF must be rejected. Using only high precision ICPMS Nb and Ta data, TTGs typically have Nb/Ta ratios less than the primitive mantle value of 16.7 and sometimes much lower (with averages ranging down to about 5; Fig. 7; Appendix A). In contrast, adakites have Nb/Ta ratios near the primitive mantle value (generally between 15 and 20). In addition, TTGs generally show Zr/Sm ratios greater than the primitive mantle value of 25.2 while adakites scatter on both sides of this value. Because rutile is probably not a residual phase during melting of eclogite (Rapp et al., 1991), eclogite-derived TTGs will have high Nb/Ta ratios, reflecting the high ratios in melted rutile (Foley et al., 2002; Klemme et al., 2002). For this reason, it is unlikely that TTGs are derived from the partial melting of rutile-bearing eclogites. 4. Discussion 4.1. Fractional crystallization or partial melting? Fig. 7. Nb/Ta vs. Zr/Sm graph showing the distribution of TTGs in comparison to the adakite field. Lines represent primitive mantle values from Sun and McDonough (1989) and melting fields from Foley et al. (2002). Is it possible that adakites and TTGs are both slab melts that reacted with the mantle wedge but that TTGs later underwent fractional crystallization, which reduced their compatible element contents? For K.C. Condie / Lithos 80 (2005) 33–44 instance, partial melting of Sr-rich eclogite in a descending slab produces adakitic magmas with high Sr contents (as there is no plagioclase in the restite) and steep REE patterns (reflecting restite garnet or/ and amphibole). Fractional crystallization of these magmas at shallower depths could reduce the Sr contents by plagioclase removal and reduce the Mg number and Cr and Ni contents by amphibole/biotite removal without appreciably affecting the REE patterns (Kamber et al., 2002; Samaniego et al., 2002). Amphibole removal during fractional crystallization could also lower the Nb/Ta ratios, as observed in TTGs relative to adakites (Fig. 7). However, there are at least three lines of evidence that suggest that TTGs are not simply the fractional crystallization products of adakitic magmas. As pointed out by several investigators, the apparent absence of thick sequences of cumulates does not favor a fractional crystallization connection (Smithies, 2000). To produce felsic TTGs from intermediate to mafic adakites requires more than 50% fractional crystallization, leaving behind large volumes of cumulates of mafic to intermediate composition. Where are these cumulates? This is especially a problem in the Archean crust where large volumes of preserved TTGs demand enormous volumes of early cumulates. One cannot hide these in the deep crust because there are many areas of deep Archean crust exposed at the surface today, such as those in southern India and Antarctica (Condie and Allen, 1984; Black et al., 1986; Sheraton et al., 1987). Even in these cratons with Archean high-grade rocks exposed at the surface, few cumulates of mafic to intermediate composition are found. Another argument against a fractional crystallization connection between adakites and TTGs is the similarity in composition of TTGs to that of experimental melts, as revealed by numerous melting studies of amphibolites and eclogites (Fig. 3; Rapp et al., 1991; Winther, 1996; Wylie et al., 1997; Prouteau et al., 2001). Is it coincidental that so many TTGs fall in the experimental melt field if they are the products of extensive fractional crystallization? Also not favoring a fractional crystallization connection between adakites and TTGs is the fact that trace element distributions define trajectories similar to calculated batch melting trajectories and not to shallow fractional crystallization trajectories. 39 This is especially apparent on the (La/Yb)n–Ybn graph (Martin, 1993; Samaniego et al., 2002). Although amphibole fractional crystallization produces residual melts that follow batch-melting trends, it cannot produce the high La/Yb values characteristic of many Archean TTGs. 4.2. P–T regimes Are P–T regimes the same for production of adakite and TTG magmas? Numerous experimental studies provide a robust framework upon which the pressures and temperatures of melting of these magmas can be estimated (references above). Experimental results agree that both adakites and TTGs are produced from partial melting of a wet mafic protolith. Although experiments have covered a wide range of water contents, only those performed at relatively high water contents (z5%) produce felsic magmas similar to TTG at relatively low melting temperatures, leaving amphibole in the restite (Rapp et al., 1991; Prouteau et al., 2001). Also, because amphibole has a Kd for Ti (~3) greater than neighboring elements Eu and Gd (~1.6) on an incompatible element diagram, restite amphibole can explain the ubiquitous negative Ti anomaly in TTGs (Fig. 1). The high La/Yb and Sr/Y ratios in both adakite and TTG magmas seem to require that garnet also be present in the restite. In the case of TTG magmas, the low Nb/Ta ratios indicate the presence of a low-Mg amphibole in the restite, such that Nb is retained in the restite compared to Ta (Foley et al., 2002). In contrast, adakitic magmas, with their comparatively high Nb/Ta ratios, cannot have lowMg amphibole in the restite (Appendix A; Fig. 7). These results also show that rutile-bearing eclogites cannot serve as sources for most TTG magmas, as partial melting of such eclogites produces melts with high Nb/Ta ratios (Foley et al., 2002). So what P–T regimes are allowed for production of TTG magmas? To have both low-Mg amphibole and garnet in the restite but little, if any, plagioclase requires melting at depths N 40 km but generally less than about 80 km over a temperature range of 700–800 8C (Martin, 1999). This is in the stability field of hornblende eclogite rather than garnet amphibolite, which by definition contains significant amounts of plagioclase. 40 K.C. Condie / Lithos 80 (2005) 33–44 4.3. Tectonic setting If adakitic magmas have reacted with mantle wedges, they reflect not only an arc setting but the subduction of young relatively hot slabs such as the Chile rise beneath the southern Andes. Martin (1993, 1999) and Foley et al. (2003) have suggested similar tectonic scenarios for TTGs in the Archean. However, if the Andes Central Volcanic Zone is a better analogue for Archean TTG, perhaps it is the Archean lower crust that is melting rather than the descending slab (for an Archean example, see Whalen et al., 2002). As pointed out by Andean investigators, this crust may be thickened by magmatic underplating of basaltic magmas coming from the mantle wedge or by tectonic shortening in the overriding plate along a convergent plate boundary (Petford and Atherton, 1996; Kay et al., 1999). Subduction erosion, which brings mafic lower crust down the subducting plate to higher pressure and temperature, can lead to partial melting. Of course, this model requires a considerable amount of water in Archean subduction zones to obtain the necessary hydrous melting. If plates were recycled faster in the Archean due to hotter mantle, perhaps water was also recycled faster in subduction zones, providing a rich supply at Archean convergent margins. Regardless of the specific tectonic scenario, it is the lower crust of the arc that is melting to produce the Andean TTG and not the descending slab. If the high Mg, Cr, and Ni in adakites require reaction with the mantle wedge, the low contents of these elements in TTGs mean a lack of such reaction and does not favor a slab origin for TTGs (Smithies, 2000). The fact that true adakites are rare in the Archean may be due to comparatively hot subducted plates, which ride high displacing all or most of the mantle wedge. Hence, there is little, if any, opportunity for Archean slab melts to react with the mantle wedge. Are there other acceptable tectonic settings for TTGs and especially for Archean TTGs when plate tectonics may not have quite the same as it is today? Perhaps the root zones of Archean oceanic plateaus were buried deep enough so the mafic component underwent partial melting. Such a model was suggested as a possible origin for the Aruba batholith in the Caribbean oceanic plateau during the Cretaceous (White et al., 1999). Of course, these deep roots must be hydrated to form amphibole, a requirement that would seem probable in the Archean mantle, as previously discussed. Hydrothermal alteration of plateau basalts followed by deep burial would also introduce water into the system. In fact, any tectonic setting in which hornblende eclogite is stable and the temperature reaches 700–800 8C could produce TTG magmas. At this point, it is important to emphasize a fact that is commonly overlooked in using geochemical data to assign tectonic settings; that is, geochemical data can be used to help constrain tectonic settings, but it cannot be used alone to reconstruct ancient tectonic settings. 4.4. Secular changes in TTGs Martin and Moyen (2002) have recently suggested secular changes in the composition of TTGs and that these changes, if real, can be used to constrain evolving tectonic settings from the Early to the Late Archean. To overcome the problem of fractional crystallization, the authors suggested that the upper limit in the range of variation in compatible elements (Mg, Sr, Cr, Ni) be used as most representative of parental magma compositions. From their study, they concluded that, between 4 and 2.5 Ga, the Mg number, Sr, Ni, and Cr contents of TTGs increased, reflecting the cooling of the mantle and increased interactions of TTG magmas (generated in descending slabs) with the mantle wedge. To explore this idea further, both the mean value and the maximum values of TTGs from the database (Appendix A) for Mg number, Ni, Sr, and (La/Yb)n are plotted as a function of age from 4 Ga to the present (Figs. 8–11). Unlike the conclusions of Martin and Moyen (2002), the Mg number shows no evidence of a trend with time either between 4 and 2.5 Ga or afterwards (Fig. 8; Appendix A). This observation is valid for both the mean and maximum Mg number. The TTG Ni distribution with age suggests a maximum around 2.7 Ga with decreasing Ni values from 2.7 Ga to the present (Fig. 9; Appendix A). As pointed out by Martin and Moyen (2002), there is also a suggestion of increasing Ni between 4 and 2.7 Ga. However, this change may reflect a rather sudden increase in Ni around 2.7 Ga rather than a gradual increase from 4 to 2.7 Ga. K.C. Condie / Lithos 80 (2005) 33–44 41 If these secular trends are real, they may be important in tracking the cooling history of the Earth and in tracking changing tectonic regimes with time. The apparent maxima in Sr, Ni, and (La/Yb)n at 2.7 Ga and possibly at 1.9 Ga may record catastrophic mantle overturn events, as suggested by Condie (1998). Increased mantle plume activity at these times could lead to increased production rates of oceanic plateaus with thick mafic roots. These mafic roots may have been buried deep enough so that plagioclase was not stable and garnet was present throughout. Partial melting of such sources would produce TTG magmas with higher than normal La/Yb and Sr/Y ratios due to widespread restitic garnet. Although smaller degrees of melting could also yield higher La/ Yb ratios and lower Yb contents, this is unlikely in the Archean when the mantle was hotter by a factor of 3 or 4. It is also possible that TTG magmas may have been contaminated with komatiites or ultramafic Fig. 8. Mg number in TTGs vs. age. (a) Mean values with one standard deviation, and (b) maximum values. Data sources given in Table 1 and Appendix A. Although not shown, the same patterns are evident for Cr. Sr shows an irregular pattern with time (Fig. 10; Appendix A). In both the mean and maximum value plots, there is a suggestion of a maximum around 2.7 Ga and perhaps again at about 1.9 Ga. Although Sr may increase from 4 to 2.7 Ga, as pointed out by Martin and Moyen, the data in Fig. 10 are also consistent with little change between 4 and 3 Ga and a relatively sudden increase around 2.7 Ga. These results also suggest that Sr in TTGs decreases after 1.9 Ga, reaching a minimum around 400–800 Ma and then shows an upward swing in the last 100 Ma. The time distribution of (La/Yb)n (Fig. 11) and Sr/Y (Table 1) shows patterns much like those of Sr and Ni, with possible maxima around 2.7 and 1.9 Ga. It is also notable that the K2O/Na2O ratio remains approximately constant in TTGs with time (at about 0.5) until the Phanerozoic when it increases (Table 1). Fig. 9. Ni concentration in TTGs vs. age. (a) Mean values with one standard deviation and (b) maximum values. Data sources given in Table 1 and Appendix A. 42 K.C. Condie / Lithos 80 (2005) 33–44 (2) (3) (4) the same rocks. Although they both show strongly fractionated REE patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. TTGs and adakites cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, high-Al TTGs may be produced by partial melting of the lower crust in arc systems or in the root zones of oceanic plateaus. Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs requires both garnet and low-Mg amphibole in the restite. Moderate to high Sr values allow little, if any, plagioclase in the restite. This requires melting in the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 8C. If peaks in La/Yb, Sr, Cr, and Ni at 2.7 Ga and perhaps also at 1.9 Ga in high-Al TTG are real, Fig. 10. Sr concentration in TTGs vs. age. (a) Mean values with one standard deviation and (b) maximum values. Data sources given in Table 1 and Appendix A. cumulates in the deep roots of oceanic plateaus, accounting the high Ni and Cr in TTGs at 2.7 Ga (Fig. 9). Alternatively, the high Ni and Cr could reflect a more badakiticQ character of some 2.7 Ga magmas, and if so, this could mean more widespread melting of descending plates at this time. A similar scenario may be recorded at about 1.9 Ga. The fall in La/Yb, Sr, Ni, and Cr after 2 Ga may reflect cooling of the mantle and lower crust, decreasing the garnet/plagioclase ratio in the restite (thus lowering TTG Sr and La/Yb levels in derivative TTG melts). If real, an increase in La/Yb and Sr in TTGs in the last 100 Ma is problematic. 5. Conclusions (1) For the most part, high-Al TTG (tonalite– trondhjemite–granodiorite) and adakite are not Fig. 11. (La/Yb)n ratio in TTGs vs. age. (a) Mean values with one standard deviation and (b) maximum values. Data sources given in Table 1 and Appendix A. n—normalized to primitive mantle using values from Sun and McDonough (1989). K.C. Condie / Lithos 80 (2005) 33–44 they may reflect catastrophic mantle overturn events at these times. Acknowledgments This paper was substantially improved by in-depth reviews by Stephen Foley, Herve Martin, John Tarney, and Hugh Rollinson. It should be pointed out, however, that some of the reviewers do not agree with the geochemical distinctions between TTG and adakite as proposed in the paper. I am happy to dedicate the paper to Professor Ilmari Haapala on the occasion of his retirement from the University of Helsinki. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.lithos.2003.11.001. References Barker, F., 1979. Trondhjemites, Dacites and Related Rocks. Elsevier, New York. Black, L.P., Sheraton, J.W., James, P.R., 1986. Late Archean granites of the Napier complex, Enderby Land, Antarctica: a comparison of Rb–Sr, Sm–Nd and U–Pb systematics in a complex terrain. Precambrian Research 32, 343 – 368. Bourdon, E., Eissen, J.P., Monzier, M., Robin, C., Martin, H., Cotton, J., Hall, M.L., 2002. Adakite-like lavas from Antisana volcano: evidence for slab melt metasomatism beneath the Andean northern volcanic zone. Journal of Petrology 43, 199 – 217. Condie, K.C., 1998. Episodic continental growth and supercontinents: a mantle avalanche connection? Earth and Planetary Science Letters 163, 97 – 108. Condie, K.C., Allen, P., 1984. Origin of charnockites from southern India. In: Krfner, A., Hanson, G.N., Goodwin, A.M. (Eds.), Archean Geochemistry. Springer-Verlag, New York, pp. 182 – 203. Defant, M.J., Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature 347, 662 – 665. Defant, M.J., Drummond, M.S., 1993. Mount St. Helens: potential example of the partial melting of the subducted lithosphere in a volcanic arc. Geology 21, 547 – 550. Drummond, M.S., Defant, M.J., 1990. A model for trondhjemite– tonalite–dacite genesis and crustal growth via slab melting: Archean to modern comparisons. Journal of Geophysical Research 95, 21503 – 21521. 43 Drummond, M.S., Defant, M.J., Kepezhinskas, P.K., 1996. Petrogenesis of slab-derived trondhjemites–tonalite–dacite/adakite magmas. Transactions of the Royal Society of Edinburgh. Earth Sciences 87, 205 – 215. Ellam, R.M., Hawkesworth, C.J., 1988. Is average continental crust generated at subduction zones? Geology 16, 314 – 317. Foley, S., Tiepolo, M., Vannucci, R., 2002. Growth of early continental crust controlled by melting of amphibolite in subduction zones. Nature 417, 837 – 840. Foley, S., Buhre, S., Jacob, D.E., 2003. Evolution of the Archean crust by delamination and shallow subduction. Nature 421, 249 – 252. Gutscher, M.A., Maury, R., Eissen, J.P., Bourdon, E., 2000. Can slab melting be caused by flat subduction? Geology 28, 535 – 538. Haskin, L.A., Haskin, M.A., Frey, F.A., Wildeman, T.R., 1968. Relative and absolute terrestrial abundances of the REE. Origin and Distribution of the Elements. Pergamon Press, New York, pp. 889 – 912. Kamber, B.S., Ewart, A., Collerson, K.D., Bruce, M.C., McDonald, G.D., 2002. Fluid-mobile trace element constraints on the role of slab melting and implications for Archean crustal growth models. Contributions to Mineralogy and Petrology 144, 38 – 56. Kay, R.W., Kay, S.M., 2002. Andean adakites: three ways to make them. Acta Petrologica Sinica 18, 303 – 311. Kay, S.M., Ramos, V.A., Marquez, M., 1993. Evidence in Cerro Pampa volcanic rocks for slab-melting prior to ridge–trench collision in southern south America. Journal of Geology 101, 703 – 714. Kay, S.M., Mpodozis, C., Coira, A.B., 1999. Neogene magmatism, tectonism, and mineral deposits of the central Andes. In: Skinner, B.J. (Ed.), Geology and Ore Deposits of the Central Andes. Society of Economic Geology, Special Publication, vol. 7, pp. 27 – 59. Kepezhinskas, P., McDermott, F., Defant, M.J., Hochstaedter, A., Durmmond, M.S., Hawkesworth, C., Koloskov, A., Maury, R.C., Bellon, H., 1997. Trace element and Sr–Nd–Pb isotopic constraints on a three-component model of Kamchatka arc petrogenesis. Geochimica et Cosmochimica Acta 61, 577 – 600. Klemme, S., Blundy, J.D., Wood, B.J., 2002. Experimental constraints on major and trace element partitioning during partial melting of eclogite. Geochimica et Cosmochimica Acta 66, 3109 – 3123. Li, W-X., Li, X-H., 2003. Adakitic granites within the NE Jiangxi ophiolites, south China: geochemical and Nd isotopic evidence. Precambrian Research 122, 29 – 44. Martin, H., 1993. The mechanisms of petrogenesis of the Archean continental crust—comparison with modern processes. Lithos 30, 373 – 388. Martin, H., 1999. Adakitic magmas: modern analogues of Archean granitoids. Lithos 46, 411 – 429. Martin, H., Moyen, J-F., 2002. Secular changes in tonalite– trondhjemite–granodiorite composition as markers of the progressive cooling of Earth. Geology 30, 319 – 322. Myers, J.D., Marsh, B.D., Sinha, K., 1985. Srontium isotopic and selected trace element variations between two aleutian volcanic centers: implications for the development of arc volcanic 44 K.C. Condie / Lithos 80 (2005) 33–44 plumbing systems. Contributions to Mineralogy and Petrology 91, 221 – 234. Peacock, S.M., Rushmer, T., Thompson, A.B., 1994. Partial melting of subducting oceanic crust. Earth and Planetary Science Letters 121, 227 – 244. Percival, J.A., Stern, R.A., Rayner, N., 2003. Archean adakites from the Ashuanipi complex, eastern superior province, Canada: geochemistry, geochronology and tectonic significance. Contributions to Mineralogy and Petrology 145, 265 – 280. Petford, N., Atherton, M., 1996. Na-rich partial melts from newly underplated basaltic crust: the Cordillera Blanca batholith, Peru. Journal of Petrology 37, 1491 – 1521. Polat, A., Kerrich, R., 2002. Nd-isotope systematics of 2.7 Ga adakites, magnesian andesites, and arc basalts, superior province: evidence for shallow crustal recycling at Archean subduction zones. Earth and Planetary Science Letters 202, 345 – 360. Prouteau, G., Scaillet, B., Pichavant, M., Maury, R.C., 2001. Evidence for mantle metasomatism by hydrous silicic melts derived from subducted oceanic crust. Nature 410, 197 – 200. Rapp, R.P., Watson, E.B., Miller, C.F., 1991. Partial melting of amphibolite/eclogite and the origin of Archean trondhjemites and tonalites. Precambrian Research 51, 1 – 25. Rapp, R.P., Shimizu, N., Norman, M.D., Applegate, G.S., 1999. Reaction between slab-derived melts and peridotite in the mantle wedge: experimental constraints at 3.8 GPa. Chemical Geology 160, 335 – 356. Samaniego, P., Martin, H., Robin, C., Monzier, M., 2002. Transition from calc-alkalic to adakitic magmatism at Cayambe volcano, Ecuador: insights into slab melts and mantle wedge interactions. Geology 30, 967 – 970. Sheraton, J.W., Tingey, R.J., Black, L.P., Offe, L.A., Ellis, D.J., 1987. Geology of an unusual Precambrian high-grade metamorphic terrane-Enderby Land and western Kemp Land, Antarctica. Australian Bureau of Mineral Resources Geology and Geophysics Bulletin, p. 223. Shirey, S.B., Hanson, G.H., 1984. Mantle-derived Archean monzodiorites and trachyandesites. Nature 310, 222 – 224. Smithies, R.H., 2000. The Archean tonalite–tondhjemite–granodiorite (TTG) series is not an analogue of cenozoic adakite. Earth and Planetary Science Letters 182, 115 – 125. Smithies, R.H., Champion, D.C., 2000. The Archean high-Mg diorite suite: links to tonalite–trondhjemite–granodiorite mag- matism and implications for early Archean crustal growth. Journal of Petrology 41, 1653 – 1671. Stern, R.A., Hanson, G.N., 1991. Archean high-Mg granodiorites: a derivative of light REE enriched monzodiorite of mantle origin. Journal of Petrology 32, 201 – 238. Stern, C.R., Kilian, R., 1996. Role of the subducted slab, mantle wedge and continental crust in the generation of adakites from the Andean Austral volcanic zone. Contributions to Mineralogy and Petrology 123, 263 – 281. Sun, S.S., McDonough, W.F., 1989. Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A.S., Norry, M.J. (Eds.), Magmatism in Ocean Basins. Geological Society of London, Special Publication, vol. 42, pp. 313 – 345. Tarney, J., Jones, C.E., 1994. Trace element geochemistry of orogenic igneous rocks and crustal growth models. Journal of the Geological Society 151, 855 – 868. Whalen, J.B., Percival, J.A., McNicoll, V.J., Longstaffe, F.J., 2002. A mainly crustal origin for tonalitic granitoid rocks, superior province, Canada: implications for Late Archean tectonomagmatic processes. Journal of Petrology 43, 1551 – 1570. White, R.V., Tarney, J., Kerr, A.C., Saunders, A.D., Kempton, P.D., Pringle, M.S., Klaver, G.T., 1999. Modification of an oceanic plateau, Aruba, Dutch Caribbean: implications for the generation of continental crust. Lithos 46, 43 – 68. Winther, K.T., 1996. An experimentally based model for the origin of tonalitic and trondhjemitic melts. Chemical Geology 127, 43 – 59. Wylie, P.J., Wolf, M.B., van der Laan, S.R., 1997. Conditions for formation of tonalites and trondhjemites: magmatic sources and products. In: De Wit, M., Ashwal, L.D. (Eds.), Greenstone Belts. Oxford University Press, Oxford, pp. 256 – 266. Yogodzinski, G.M., Volynets, R.W., Koloskov, O.N., Kay, A.V., Kay, S.M., 1995. Magnesian andesite in the western Aleutian Komandorsky region: implications for slab melting and processes in the mantle wedge. Geological Society of America Bulletin 107, 505 – 519. Xu, J.F., Shinjo, R., Defant, M.J., Wang, Q., Rapp, R.P., 2002. Origin of Mesozoic adakitic intrusive rocks in the Ningzhen area of east China: partial melting of delaminated lower continental crust. Geology 30, 1111 – 1114.