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Lithos 46 Ž1999. 411–429 Adakitic magmas: modern analogues of Archaean granitoids Herve´ Martin ) Laboratoire Magmas et Volcans UMR 6524, CNRS-UniÕersite´ Blaise Pascal, Departement de Geologie, 5, rue Kessler, 63038 ´ ´ Clermont-Ferrand Cedex, France Received 5 January 1998; accepted 21 July 1998 Abstract Both geochemical and experimental petrological research indicate that Archaean continental crust was generated by partial melting of an Archaean tholeiite transformed into a garnet-bearing amphibolite or eclogite. The geodynamic context of tholeiite melting is the subject of controversy. It is assumed to be either Ž1. subduction Žmelting of a hot subducting slab., or Ž2. hot spot Žmelting of underplated basalts.. These hypotheses are considered in the light of modern adakite genesis. Adakites are intermediate to felsic volcanic rocks, andesitic to rhyolitic in composition Žbasaltic members are lacking.. They have trondhjemitic affinities Žhigh-Na 2 O contents and K 2 OrNa 2 O ; 0.5. and their Mg no. Ž0.5., Ni Ž20–40 ppm. and Cr Ž30–50 ppm. contents are higher than in typical calc-alkaline magmas. Sr contents are high Ž) 300 ppm, until 2000 ppm. and REE show strongly fractionated patterns with very low heavy REE ŽHREE. contents ŽYb F 1.8 ppm, Y F 18 ppm.. Consequently, high SrrY and LarYb ratios are typical and discriminating features of adakitic magmas, indicative of melting of a mafic source where garnet andror hornblende are residual phases. Adakitic magmas are only found in subduction zone environments, exclusively where the subduction andror the subducted slab are young Ž- 20 Ma.. This situation is well-exemplified in Southern Chile where the Chile ridge is subducted and where the adakitic character of the lavas correlates well with the young age of the subducting oceanic lithosphere. In typical subduction zones, the subducted lithosphere is older than 20 Ma, it is cool and the geothermal gradient along the Benioff plane is low such that the oceanic crust dehydrates before it reaches the solidus temperature of hydrated tholeiite. Consequently, the basaltic slab cannot melt. The released large ion lithophile element ŽLILE.-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting. Afterwards, the residue is made up of olivineq clinopyroxeneq orthopyroxene, such that the partial melts are HREE-rich Žlow LarYb and SrrY.. Contrarily, when a young Ž- 20 Ma. and hot oceanic lithosphere is subducted, the geothermal gradient along the Benioff plane is high, so the temperature of hydrated tholeiite solidus is reached before dehydration occurs. Under these conditions, garnet andror hornblende are the main residual phases giving rise to HREE-depleted magmas Žhigh LarYb.. The lack of residual plagioclase accounts for the Sr enrichment Žhigh SrrY. of the magma. Experimental petrologic data show that the liquids produced by melting of tholeiite in subduction-like P–T conditions are adakitic in composition. However, natural adakites systematically have higher Mg no., Ni and Cr contents, which are interpreted as reflecting interactions between the ascending adakitic magma generated in the subducted slab and the overlying mantle wedge. This interpretation has been recently corroborated by studies on ultramafic enclaves in Batan lavas where olivine crystals contain glass inclusions with adakitic compositions wSchiano, P., Clochiatti, R., Shimizu, N., Maury, R., Jochum, K.P., Hofmann, A.W., 1995. Hydrous, silica-rich melts in the sub-arc mantle and their relationships with erupted arc lavas. Nature 377 595–600.x. This is interpreted as demonstrating that adakitic magmas passed through the mantle wedge and interacted with it. Sajona wSajona, F.G., 1995. Fusion de la croute en contexte de subduction ˆ oceanique ´ ) Tel.: q33-4-73-34-67-40; Fax: q33-4-73-34-67-44; E-mail: [email protected] 0024-4937r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 9 8 . 0 0 0 7 6 - 0 412 H. Martinr Lithos 46 (1999) 411–429 collision: geochimie, geochronologie et petrologie du magmatisme plioquaternaire de Mindanao ŽPhilippines.. Unpublished ´ ´ ´ thesis, Brest University, France, 223 pp.x also considers that the high-Nb basalts, which are associated with adakites, reflect mantle–adakite interactions. Recent structural studies have demonstrated that plate tectonics operated during the first half of Earth history. The very strong similarities that exist between modern adakites and Archaean tonalite, trondhjemite and granodiorite ŽTTG. attest that both have the same source and petrogenesis. Consequently, when Archaean-like P–T conditions are exceptionally realised in modern subduction zones, Archaean-like magmas are generated. Contrarily, hot spots never produce TTG-like magmas, thus, strongly supporting the hypothesis of the generation of the Archaean continental crust within a subduction environment. However, Archaean TTG are poorer in Mg, Ni and Cr than adakites, indicating that mantle–magma interactions were less efficient, probably due to the shallower depth of slab melting. In this case, the slab-derived magmas rise through a thinner mantle wedge, thus, reducing the efficiency of the interactions. This is corroborated by the absence of a positive Sr anomaly in TTG, which indicates that plagioclase could have been a residual phase during their genesis. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Adakite; Archaean; Granitoid; Subduction; Geochemistry 1. Introduction Recent studies of the early continental crust have shown that Archaean juvenile continental crust has petrological and chemical compositions different from that of its present day equivalents: it is mainly TTG Žtonalite, trondhjemite and granodiorite. in composition. Both geochemical and experimental petrological research indicate that it has been generated by partial melting of Archaean tholeiite transformed into garnet bearing amphibolite or eclogite ŽBarker and Arth, 1976; Hunter et al., 1978; Barker, 1979; Tarney et al., 1979, 1982; Condie, 1981, 1986; Jahn et al., 1981; Martin et al., 1983; Sheraton and Black, 1983; Martin, 1986, 1987a, 1993; Ellam and Hawkesworth, 1988; Johnson and Wyllie, 1988; Arculus and Ruff, 1990; Nedelec ´ ´ et al., 1990; Rapp et al., 1991; Winther and Newton, 1991; Bickle et al., 1993; Neymark et al., 1993; Rapp and Watson, 1995; Garde, 1997; Rollinson, 1997; among others.. The geodynamic context of tholeiite melting is subject to controversy and dependent on whether or not plate tectonics operated during the Archaean. Today, field, theoretical and experimental researches have established that plate tectonic-like mechanisms were active during the Archaean. Detail structural analyses performed in Archaean shields have also demonstrated that two main tectonic styles coexisted during the first half of earth history: Ž1. gravitational vertical tectonics in the central part of continental plates ŽGorman et al., 1978; Martin et al., 1984; Barbey and Martin, 1987; Bouhallier et al., 1993, 1995; Choukroune et al., 1995.; Ž2. thrusting horizontal tectonics at plate boundaries ŽBickle et al., 1980; McGregor et al., 1991; De Wit et al., 1992; Treloar et al., 1992; Ludden et al., 1993; Jegouzo and Blais, ´ 1995; Blais et al., 1997.. However, if plate tectoniclike processes operated, secular compositional changes in magmatism and tectonic regimes indicate that in detail, the processes were different from modern ones. Most of the differences result from greater Archaean heat production, inducing higher geothermal gradients mainly at plate margins ŽMartin, 1986; Blais et al., 1997; Lagabrielle et al., 1997.. Recent work on arc andesites and dacites shows that modern adakites not only have the same chemical composition as Archaean TTG but also that they are exclusively generated in subduction zones where abnormally high geothermal gradients are expected along the Benioff plane Ži.e., a young subduction zone or subduction of young oceanic crust., ŽKay, 1978; Martin, 1987b, 1995; Futa and Stern, 1988; Defant and Drummond, 1990; Drummond and Defant, 1990; Defant et al., 1991, 1992; Atherton and Petford, 1993; Mahlburg Kay et al., 1993; Peacock et al., 1994; Morris, 1995; Sajona, 1995; Bourgois et al., 1996; Drummond et al., 1996; Maury et al., 1996; Sajona et al., 1996; Stern and Kilian, 1996; Samaniego, 1997; Sigmarsson et al., 1998.. Consequently, adakites could represent modern analogues of Archaean TTG, thus, providing constraints on our understanding of archaic petrogenetic processes and geodynamic environments. The purpose of this paper is: Ža. to present the main petrological and geochemical characteristics of adakitic magmas, Žb. to constrain the conditions of H. Martinr Lithos 46 (1999) 411–429 their genesis, and Žc. to use them as modern analogues of Archaean continental crust ŽTTG. in order to discuss the environment in which it could have formed. 2. Petrological and chemical characteristics of adakites Modern adakites are located in the circum-Pacific margins ŽFig. 1.: Austral Chile ŽMartin, 1987b; Futa and Stern, 1988; Mahlburg Kay et al., 1993; Bourgois et al., 1996; Drummond et al., 1996; Guivel et al., 1996; Stern and Kilian, 1996; Sigmarsson et al., 1998.; Ecuador ŽMonzier et al., 1997; Samaniego, 1997.; Panama and Costa Rica ŽDefant et al., 1991, 1992.; Mexico ŽCameron and Cameron, 1985; Rogers et al., 1985; Saunders et al., 1987.; Cascade ŽDefant and Drummond, 1993.; Aleutians ŽKay, 1978; Myers 413 and Frost, 1994.; Kamchatka ŽKepezhinskas, 1989; Honthaas et al., 1995; Kepezhinskas et al., 1995.; Japan ŽMorris, 1995.; Philippines ŽSajona, 1995; Maury et al., 1996; Sajona et al., 1996.; New Guinea ŽSmith et al., 1979.. Few adakitic plutons were reported ŽLe Moigne, 1995; Bourgois et al., 1996; Prouteau et al., 1996. and most of the adakites are phenocryst-bearing volcanic rocks. They consist of intermediate to felsic suites whose composition ranges from hornblende andesite to dacite and rhyolite. Basaltic members are systematically lacking, thus, clearly demarcating adakitic series from the classical BADR Žbasalt, andesite, dacite and rhyolite. suites, typical of subduction-related calc-alkaline magmatism. Maury et al. Ž1996. interpreted this difference in terms of distinct magma sources. Phenocrysts are mainly zoned plagioclase, hornblende and biotite. Orthopyroxene and clinopyroxene are extremely rare, they are only known in mafic andesites from the Aleu- Fig. 1. Map showing the localities where adakites have been described: Ž1. Austral Chile; Ž2. Ecuador; Ž3. Panama and Costa Rica; Ž4. Mexico; Ž5. Cascade; Ž6. Aleutians; Ž7. Kamchatka; Ž8. Japan; Ž9. Philippines; Ž10. New Guinea Žreferences are given in the text.. H. Martinr Lithos 46 (1999) 411–429 414 tians and Mexico ŽKay, 1978; Rogers et al., 1985.. Accessory phases Žapatite, zircon, sphene and titanomagnetite. are more abundant than in typical arc magmatic rocks. Adakites are associated with Au, Ag, Cu and Mo epithermal and porphyry deposits ŽThieblemont et al., 1997.. ´ Adakites typically contain more than 56% SiO 2 ŽTable 1.. In a ŽNa 2 O q K 2 O. vs. SiO 2 diagram ŽFig. 2; McDonald and Katsura, 1964. they plot in the calc-alkaline field. However, because of high Na 2 O contents Ž3.5% F Na 2 O F 7.5% ., their K 2 OrNa 2 O ratios are low Ž; 0.42., consequently, in a K–Na–Ca triangle ŽFig. 3a; Barker and Arth, 1976., they plot on the trondhjemitic differentiation trend and do not show any affinity with the typical calc-alkaline trend. Using Barker Ž1979. terminology, they have a high-Al 2 O 3 trondhjemitic composi- Fig. 2. ŽNa 2 OqK 2 O. vs. SiO 2 diagram ŽMcDonald and Katsura, 1964. showing that adakites fall in the arc calc-alkaline domain. Al salkaline; Ca sCalc-alkaline; Th s tholeiitic. tion ŽAl 2 O 3 ) 15% at SiO 2 s 70%.. Their Fe 2 O 3 q MgO q MnO q TiO 2 contents Ž; 7%. are moder- Table 1 Average chemical composition of adakites, arc magmas, Archaean TTG and liquids generated by experimental melting of basalts Žsample 1 from Rapp et al., 1991 and sample 3 from Winther and Newton, 1991. Adakites n s 81 wt.% SiO 2 Al 2 O 3 Fe 2 O 3 ) MnO MgO CaO Na 2 O K 2O TiO 2 P2 O5 Mg no. K 2 Or Na 2 O FeO q MgO q MnO q TiO 2 Adakitic pluton s ns9 s Arc dacite n s 80 Arc granodiorite s n s 250 s TTG n s 355 Experimental liquids s 1 3 64.66 16.77 4.20 0.08 2.20 5.00 4.09 1.72 0.51 0.17 0.51 0.42 3.2 1.0 1.2 0.02 1.0 1.3 0.4 0.6 0.2 0.1 0.1 0.2 67.30 15.78 3.30 0.05 1.96 3.67 4.19 2.15 0.54 0.12 0.54 0.51 1.1 0.2 0.2 0.01 0.3 0.4 0.1 0.3 0.0 0.0 0.03 0.15 68.22 14.63 4.28 0.09 1.22 2.88 4.15 3.37 0.46 0.21 0.36 0.81 5.6 1.8 1.4 0.03 1.0 1.8 0.5 1.1 0.2 0.1 0.15 0.3 68.10 15.07 4.36 0.09 1.55 3.06 3.68 3.40 0.54 0.15 0.41 0.92 6.2 1.6 2.0 0.10 1.0 0.6 0.5 1.1 0.3 0.1 0.15 0.4 69.79 15.56 3.12 0.05 1.18 3.19 4.88 1.76 0.34 0.13 0.43 0.36 4.9 1.2 1.5 0.03 0.7 1.0 0.8 0.7 0.2 0.1 0.10 0.15 69.75 16.89 2.96 0.09 1.26 3.93 4.2 1.31 0.54 69.76 15.59 3.8 0.03 0.71 3.16 4.5 1.81 0.85 0.46 0.31 0.26 0.40 6.99 1.5 5.86 0.5 6.05 2.4 6.54 3.0 4.69 1.6 4.85 5.39 8.0 15.0 150.0 9.0 0.5 5.0 14 29 454 32 0.55 7.5 39.7 60.5 10.0 30.0 200.0 20.0 0.3 4.0 ppm Ni 24 Cr 36 Sr 706 La 19 Yb 0.93 Y 10 ŽLarYb. N 14.2 SrrY 68.7 19 34 439 11 0.37 4 24 46 280 17.7 1.1 17 11.0 16.5 8.0 8.0 24.0 3.0 0.2 10.0 5 8 380 48.1 4.4 47 7.5 8.1 4.0 5.0 240.0 17.0 1.2 21.0 10 23 316 31 3.2 26 6.6 12.2 H. Martinr Lithos 46 (1999) 411–429 415 Fig. 3. K–Na–Ca triangle ŽBarker and Arth, 1976. showing adakites ŽA. and Archaean TTG ŽB. plotting on a trondhjemitic differentiation trend ŽTd. with no affinity for the classical calc-alkaline trend ŽCA.. Grey: field of Archaean TTG ŽMartin, 1995.. ately high, with high Mg no.s 0.51. From this point of view, they differ significantly from typical arc calc-alkaline volcanics ŽFe 2 O 3 q MgO q MnO q TiO 2 s 6%; Mg no.s 0.36.. Trace elements allow a better and indisputable distinction between adakitic and typical arc calc-alkaline magmas. Ni and Cr contents Ž24 and 36 ppm, respectively. are higher than in classical arc calc-alkaline dacites Ž8 and 5 ppm. ŽTable 1.. Defant and Drummond Ž1990. reported high Sr contents as typical of adakitic magmatism. Yet, in most adakites, Sr contents are high Ž; 700 ppm. and can reach 2000 ppm; however, there are several exceptions. For instance, in the Austral Volcanic Zone ŽAVZ. in southern Andes, the average Sr content of lavas is 545 ppm ŽStern and Kilian, 1996.. In the Taitao Peninsula, adakitic plutons have only 280 ppm Sr. In fact, because of the high K D of Sr between plagioclase and melt Ž K D s 4.5; Arth and Hanson, 1975., only minor amounts of plagioclase fractional crystallization in a magma chamber could drastically lower the Sr content of the magma. Nevertheless, despite Fig. 4. Chondrites normalized REE patterns for adakites ŽA. and Archaean TTG ŽB.. Typical adakites are from Costa Rica Žopen circles; Defant et al., 1992. and Ecuador Žopen squares; Samaniego, 1997.; the typical calc-alkaline dacite from the Southern Volcanic Zone from Andes Žfilled squares; Hickey-Vargas et al., 1989. and Archaean TTG from Martin Ž1995.. 416 H. Martinr Lithos 46 (1999) 411–429 these exceptions, Sr content is generally very high in adakites, thus, demonstrating that plagioclase fractionation remains an exceptional process in this kind of magma. The REE patterns are very typical of adakites ŽFig. 4A.. They are strongly fractionated ŽŽLarYb. N ) 10. with La N typically ranging between 40 and 150, and the heavy REE ŽHREE. contents are very low ŽYb F 1.8 ppm, Y F 18 ppm.. In comparison, typical calc-alkaline lavas are HREE-richer ŽHREE N G 10; Yb G 2.5 ppm; Y G 25 ppm.; this results in lower REE fractionation ŽŽLarYb. N - 10., ŽFigs. 4A and 5A.. In addition, calc-alkaline lavas often display a negative Eu anomaly. When plotted in a primitive mantle-normalized multi-element variation diagram ŽFig. 5B., all adakites display the same characteristics. In addition, they show a significant positive Sr anomaly, indicative of the absence of plagioclase fractionation. Negative Nb anomalies are important whereas negative Zr and Ti anomalies are very small or lacking. Classical calc-alkaline dacites differ not only in their higher HREE contents, but also in their negative Sr anomaly. Adakite isotopic compositions are similar to MORB Ž144 Ndr143 Nd ) 0.5129 and 87 Srr86 Sr 0.705.. With respect to U–Th disequilibria, typical calc-alkaline magmas frequently display 238 U-enrichments or radioactive equilibrium between 238 U and its product 230 Th ŽSigmarsson et al., 1990.. In contrast, adakites show uniform 230 Th-excess over 238 U but variable Th isotope ratios ŽFig. 6; Sigmarsson et al., 1998.. Fig. 6. 230 Thr232 Th vs. 238 Ur232 Th isochron diagram showing the contrasting behaviour of the adakites from the Andean Austral Volcanic Zone Žfilled circles. and the classical calc-alkaline lavas from the Andean Southern Volcanic Zone Žopen squares.. The 238 U-enrichment in calc-alkaline lavas is attributed to slab-derived fluid metasomatism and mantle melting, whereas the uniform 230 Th-enrichment in adakites reflect about 20% partial melting of the young and heterogeneous, subducted, Antarctic plate ŽSigmarsson et al., 1990, 1998.. As the main differences between adakites and calc-alkaline dacites are recorded by REE, Y and Sr behaviour, the diagrams discriminating between these two types of magmas are based on these elements ŽLarYb. N vs. Yb N ŽFig. 7A; Martin, 1986. and SrrY vs. Y ŽFig. 7B; Drummond and Defant, 1990.. 3. Petrogenesis of adakites The low HREE content of adakites is classically interpreted as reflecting the presence of garnet " hornblende in the residue of partial melting of their Fig. 5. Primitive mantle normalized multi-element diagram comparing adakite compositions with those of typical calc-alkaline dacite ŽA. and Archaean TTG ŽB.. Source of data is the same as in Fig. 4. H. Martinr Lithos 46 (1999) 411–429 417 Fig. 7. ŽLarYb. N vs. Yb N ŽA; Martin, 1986. and ŽSrrY. vs. Y ŽB; Drummond and Defant, 1990. diagrams discriminating between adakitic Žgrey fields. and classical arc calc-alkaline Žwhite fields. compositions. source, whereas these minerals are not residual phases during the genesis of typical calc-alkaline magmas ŽMartin, 1986.. The frequent positive Sr anomaly in adakites indicates the lack of important plagioclase fractionation, whereas this process is common in calc-alkaline dacites as demonstrated by their systematic negative Sr and Eu anomalies. Both adakites and calc-alkaline dacites have important negative Nb anomalies indicative of a role played by titaniferous phases andror amphibole. Geochemical modelling shows that the source of adakites cannot be ultramafic but rather basaltic in composition ŽMartin, 1986; Defant and Drummond, 1990; Drummond and Defant, 1990; Knowles, 1995; Sajona, 1995; Maury et al., 1996; Samaniego, 1997; among others.. This implies that adakitic magmas are produced by melting of a basaltic source transformed into garnet-bearing amphibolite or eclogite such that garnet " hornblende could be residual phases. Holloway and Burnham Ž1972. and Helz Ž1976. melted alkali-basalts and tholeiites under various PH 2 O conditions, and did not obtain trondhjemites but rather tonalitic liquids. Beard and Lofgren Ž1989, 1991., showed that adakitic magmas cannot be produced under water-saturated conditions, but rather form during dehydration melting. Indeed, tonalitic and trondhjemitic liquids were produced by dehydration melting of low-K tholeiite at 1–7 kbar ŽBeard and Lofgren, 1991. and at 8 kbar ŽRushmer, 1991.. Melting begins at about 800–8508C and amphibole is stable until 9508C; however, garnet was not stable during these low-pressure experiments. Vapour-absent experiments on metabasalts were conducted by Rapp et al. Ž1991. Ž8–32 kbar and 700–10008C., Winther and Newton Ž1991. on highAl basalt and an average Archaean tholeiite Ž5–30 kbar and 750–11008C., Wolf and Wyllie Ž1993. on natural low-K calcic amphibolite Ž10 kbar and 750– 10008C., Sen and Dunn Ž1994. on amphibolites Ž15– 20 kbar and 850–11508C. and Zamora Ž1996. on metabasalts Ž15 kbar, 1000–11008C.. The melts generated for 10% to 40% fusion are adakitic in composition ŽFig. 8A. and equilibrated with residues made up of plagioclase q amphibole " orthopyroxene " ilmenite at low pressure Ž8 kbar., garnet q amphibole " plagioclase " clinopyroxene " ilmenite at 16 kbar, and garnet q clinopyroxene" rutile at higher pressure. Rapp et al. Ž1991., Wolf and Wyllie Ž1993., Sen and Dunn Ž1994. and Zamora Ž1996. calculated REE patterns of liquids generated at low pressure in the garnet-absent system; the REE patterns are flat or poorly fractionated without any HREE impoverishment ŽFig. 8B.. At higher pressures, when garnet is a residual phase, REE are strongly fractionated Ž30 F ŽLarYb. N F 50. with HREE depletion. Wolf and Wyllie Ž1991. considered that the most suitable conditions for melt extraction from amphibolitic residue are achieved at 10 kbar in a temperature interval of 850–9008C. When hornblende dehydrates, the released water lowers the viscosity of the liquid. At higher temperatures, the melt fraction increases, it becomes water-undersaturated, and its viscosity increases ŽMcKenzie, 1984., thus, precluding efficient segregation. It must also be noted that, 418 H. Martinr Lithos 46 (1999) 411–429 Fig. 8. ŽA. K–Na–Ca triangle ŽBarker and Arth, 1976. showing the composition of liquids generated by experimental fusion of basalts and amphibolites. Comparison with Fig. 3 shows that they are identical to adakite compositions. ŽB. Chondrite-normalized REE patterns of liquids generated by experimental melting. FSS s tholeiite ŽRapp et al., 1991.; D15 s low-K calcic amphibolite ŽWolf and Wyllie, 1993.. At relatively low pressure Žgarnet-free residues; filled symbols., HREE contents remain almost unchanged and LREE display various degrees of enrichment. When garnet is stable in the residue Žopen symbols., adakite-like LREE-enriched and HREE-depleted patterns are obtained. in experiments, hornblende andror Fe–Ti oxides Žrutile, ilmenite. are common residual minerals, thus, being able to account for Ti–Nb–Ta negative anomalies in adakites. In Catalina Island ŽCalifornia., Sorensen and Barton Ž1987., Sorensen Ž1988., Sorensen and Grossman Ž1989. and Bebout and Barton Ž1993. described metabasalts from an obducted oceanic crust which are transformed into garnet-bearing amphibolite or hornblende eclogite, and recorded temperatures of about 650–7508C and pressures ranging from 9 to 11 kbar. Migmatitic structures and veins are described and interpreted as demonstrating that amphibolites exceeded their solidus temperature and began to melt. The liquids have high-Al 2 O 3 trondhjemitic Žadakitic. compositions. This example provides field evidence that adakites can be produced by partial melting of metamorphosed basalts. 4. Geodynamic context of genesis All active adakitic magmatism occurrences so far reported come from the circum-Pacific margins and all are related to subduction. More precisely, they are mainly situated in environments where a young ŽT 20 Ma. oceanic lithosphere is subducted, where the subducted plate is located 70 to 90 km beneath the volcanic arc ŽDefant and Drummond, 1990; Morris, 1995; Maury et al., 1996.. A well-documented ex- ample is provided by the AVZ and the Southern Volcanic Zone ŽSVZ. in Chilean Patagonia. In this region, at the triple junction between the South American, Nazca and Antarctic Plates, the active Chile ridge is being subducted under the Taitao Peninsula ŽFig. 9.. North of 448S, beneath the volcanic arc, the subducted slab is older than 20 Ma Ž50 Ma to the north of the Nazca plate; Fig. 9A.. The associated volcanism ŽSVZ. is typically calc-alkaline and evolves from olivine high-Al basalts to dacite ŽBADR suites of Drummond and Defant, 1990. whose Yb N ranges from 8 to up to 20 ŽFig. 9B.. LREE are fractionated and HREE are rather flat which are typical features of most modern subduction zones. Based on petrological and geochemical characteristics, the SVZ is considered to have been generated by partial melting of the metasomatized mantle wedge. The lack of residual garnet andror hornblende accounts for the high Yb contents of the magmas ŽLefevre, 1979; Lopez-Escobar, 1984; Stern ` et al., 1984a,b; Futa and Stern, 1988; Rogers and Hawkesworth, 1989; Sigmarsson et al., 1990.. South of 468S, the subducted lithosphere is younger than 10 Ma ŽFig. 9B.. Lavas from the AVZ are andesitic to dacitic adakites. When compared to the SVZ, the more mafic and less differentiated members are completely lacking. Yb N contents of the lavas range between 4 and 5.5 ŽFig. 9C., values which are significantly lower than those observed further to the north and indicative of garnet andror H. Martinr Lithos 46 (1999) 411–429 419 Fig. 9. Schematic map of South Chile showing the triple junction between the Nazca, Antarctic and South American plates as well as the distribution of active volcanism ŽSVZs South Volcanic Zone; AVZ s Austral Volcanic Zone.. ŽA. Age of the oceanic lithosphere under the volcanic arc vs. latitude plot. North of 458S, the age of the Nazca plate is greater than 20 Ma Ž20 to 50 Ma., whereas, south of 458S, the age of the Antarctic plate ranges from 0 to 20 Ma. ŽB. ŽYb N or Yr2.4. vs. latitude plot. To the South, where the subducted oceanic crust is young, the volcanic Žfilled circles. and plutonic Žcrosses. rocks have adakitic compositions with low Yb or Yr2.4 contents ŽYb N - 5.5., consistent with an origin by subducted slab melting. To the North, magmas are typically calc-alkaline with Yb N contents Žopen circles. ranging between 8 and 20, indicating an origin by melting of the metasomatized mantle wedge. hornblende fractionation. In the Taitao Peninsula, a 4 Ma old granodiorite also has an adakitic composition; it corresponds to the subduction of the Chile ridge under the peninsula 4 Ma ago ŽLe Moigne et al., 1993; Bourgois et al., 1994; Lagabrielle et al., 1994; Le Moigne, 1995.. Its Yb N ranges between 3.5 and 6.5, identical to those of the AVZ adakites. Detailed petrogenetic studies demonstrate that both SVZ adakites and Taitao granodiorite originate from partial melting of basalts from the subducted oceanic lithosphere, the presence of residual garnet andror hornblende accounting for the low Yb contents ŽStern and Futa, 1982; Stern et al., 1984a,b; Martin, 1987b; Bourgois et al., 1996; Stern and Kilian, 1996; Zamora, 1996; Sigmarsson et al., 1998.. Geochemical data on subduction-related magmatism were compiled by Defant and Drummond Ž1990. who showed that a world-wide correlation exists between the age of the lithosphere when it begins to be subducted and the composition of associated calc-alkaline magmas. ŽFig. 10.. These authors showed the following: Ž1. when the subducted slab is younger than 30 Ma, TTD Žtonalites, trondhjemites and dacites. magmas are produced, their Yb N Žand Yr2.4. contents are typically lower than 7.5 ŽY 18., consistent with an origin by partial melting of the slab, and Ž2. when the subducted slab is older than 30 Ma, BADR magmas are generated, these later exhibit Yb N Žand Yr2.4. contents ranging from 6.5 to 25. This feature strongly favours a metasoma- Fig. 10. Yb N Žor Yr2.4. vs. age of subducted lithosphere diagram ŽDefant and Drummond, 1990.. The horizontal scale is logarithmic when T - 30 Ma and arithmetic when T ) 30 Ma. Black lines correspond to the compositional range of the arc magmatism. There is a world-wide correlation between the age of the subducted lithosphere and its geochemical characteristics. 420 H. Martinr Lithos 46 (1999) 411–429 tized peridotitic source without garnet and hornblende in the residue of melting. Consequently, adakites seem to be related to the subduction of young oceanic lithosphere. Today, the average age of the oceanic lithosphere when it begins to be subducted is about 60 Ma ŽBickle, 1978., but may be as old as 180 Ma. Consequently, in most modern subduction zones, the oceanic lithosphere behaves as a cold slab penetrating the mantle. Energy and heat exchange result in cooling of the mantle wedge and warming of the downgoing slab. The fluids released by slab dehydration migrate upward and also tend to cool the overlying mantle ŽPeacock, 1990, 1993.. These processes should result in the progressive cooling of the mantle wedge, if hot material is not continuously brought to the wedge by mantle convection ŽToksov ¨ and Hsui, 1978; Anderson et al., 1980; Honda and Uyeda, 1983.. Consequently, in a subduction zone, the geothermal gradient along the slab–mantle interface remains low ŽF 108Crkm; Fig. 20; Hasebe et al., 1970; Toksov ¨ et al., 1971; Bird et al., 1975; Drummond and Defant, 1990; Peacock, 1990, 1993; Peacock et al., 1994.. With such thermal conditions, the subducted slab is metamorphosed and progressively dehydrates ŽFig. 11. so that the dehydration curves are intersected before reaching the solidus of hydrous tholeiite. At temperatures of about 7508C Žtemperature of the 5% hydrous-tholeiite solidus at 20 kbar, Wyllie, 1971; Green, 1982., the oceanic lithosphere should be almost completely dehydrated. Melting of such a dry tholeiite cannot occur at low temperatures but requires a temperature higher than 12008C at 20 kbar. The released large ion lithophile element ŽLILE.-rich fluids rise into the mantle wedge, inducing both its metasomatism and its partial melting. In this context, the primordial source of calc-alkaline magmas is metasomatized mantle peridotite, with a residue essentially made up of olivine q clinopyroxeneq orthopyroxene such that the magmas produced are HREE-rich and thus, display low LarYb and SrrY ratios. As already proposed for Archaean TTG ŽMartin, 1986, 1987b., with higher geothermal gradients Ž25 to 308Crkm. along the Benioff plane ŽFig. 11. and for pressures ranging from 8 to 18 kbar, the relative positions of dehydration curves and 5% hydrous Fig. 11. P – T diagram displaying the dry and 5% hydrous solidus of tholeiite ŽWyllie, 1971; Green, 1982.. The main dehydration reactions of the oceanic lithosphere are also drawn: H s hornblende-out; A santhophyllite-out; C s chlorite-out; Ta s talc-out; Tr s tremolite-out; Z s zoisite-out. The stability fields of both garnet and plagioclase are delimited by the G and P lines, respectively. The grey field is the P – T domain where a magmatic liquid generated by partial melting of an hydrated tholeiite can coexist with a hornblende- and garnet-bearing residue. Geothermal gradients along the Benioff plane in modem subduction zones are from Toksov ¨ et al. Ž1971. s TOK; Peacock et al. Ž1994. s PEA and Anderson et al. Ž1980. s AND. Archaean gradient ŽMAR. is from Martin Ž1986.. tholeiitic solidus are inverted. Except for the anthophyllite ™ forsterite q talc q H 2 O reaction, which slightly precedes the solidus temperature, a subducted slab can reach temperatures of 650 to 7008C before it dehydrates. At these temperatures, water is still present so dehydration melting can take place in the subducted slab. Under these conditions, garnet andror hornblende are the main residual phases giving rise to HREE-depleted magmas Žhigh LarYb.. The lack of residual plagioclase accounts for the Sr enrichment Žhigh SrrY. of these magmas. As noted by Maury et al. Ž1996., the scarcity of modern adakites when compared with Archaean TTG is due to the lack of high heat fluxes in modern subduction zones. Delong et al. Ž1979. made theoretical thermal simulations of ridge ŽChile ridge. subduction in an already thermally stabilised subduction zone. With the passage of the ridge, a thermal anomaly is created and isotherms rise such that the 10008C isotherm intersects the Benioff plane. However, the thermal anomaly is restricted in both time Žca. 5 Ma. and space but, as it can easily exceed the 5% hydrous-tholeiitic solidus temperature, slab dehydration melting is possible. More recently, Peacock Ž1990, 1993. and Peacock et al. Ž1994., modelled the H. Martinr Lithos 46 (1999) 411–429 pressure–temperature–time trajectories in subducted lithosphere. Three main parameters control the melting of the subducted plate. Ž1. The age of the subducted lithosphere. Fig. 12A shows the calculated geothermal gradients for a 3 cm ay1 subduction as function of the subducted slab age. It is clear that the younger the subducted oceanic lithosphere, the higher the geothermal gradients along the Benioff plane. Under these conditions, and using the tholeiite hydrous solidus proposed by Green Ž1982., slab melting is only allowed when the subducted lithosphere is younger than 30 Ma. When the subducted slab is older, large-scale dehydration occurs before the temperature of the hydrous solidus is reached. Using solidi from Sen and Dunn Ž1994. or Rapp et al. Ž1991. would reduce the window of slab melting to slab ages lower to 20 Ma and 10 Ma, Fig. 12. P – T diagram based on Peacock Ž1990, 1993. and Peacock et al. Ž1994. calculations and showing the dependence of geothermal gradients along the Benioff plane as function of the age of the subducted oceanic lithosphere ŽA. and of the amount of previously subducted oceanic crust ŽB.. Slab melting is only allowed when the subducted lithosphere is younger than 30 Ma or at the very beginning of subduction when no more than 200 km of oceanic lithosphere had been already subducted. 421 respectively. The maximum age of the subducting slab can be different with different authors, however, it remains that only very young slabs Ž- 30–20 Ma old. can melt. Ž2. Subduction consists of introducing a cold oceanic lithosphere into a hot mantle. This mechanism progressively cools the mantle wedge and thus, lowers geothermal gradients along the Benioff plane. Consequently, young subduction Ževen of an old oceanic lithosphere., would not yet have cooled the mantle such that high geothermal gradients could be expected. Fig. 12B shows the calculated geothermal gradients for a 3 cm ay1 subduction of a 50 Ma old lithosphere as function of the amount of previously subducted oceanic crust ŽPeacock et al., 1994.. In this case, it also appears that very young subduction ŽF 200 km subducted. can induce slab melting conditions. Ž3. Honda Ž1985. and Molnar and England Ž1990. showed that shear stress caused by fast subduction could result in heating the upper part of the subducted lithosphere. Peacock et al. Ž1994. calculated that a 10 cm ay1 subduction rate of a 50 Ma old oceanic lithosphere induces a shear stress of about 1 kbar that could generate geothermal gradients cutting hydrous tholeiite solidus at pressures of about 15 kbar, in the adakite genesis field. The geothermal gradient along the Benioff plane rapidly decreases as the subduction rate reduces. In conclusion, the thermodynamic conditions of adakite genesis by oceanic slab melting can be reached only when young lithosphere is subducted or in the case of a very young subduction zone; shear stress can assist in the case of high subduction rates. Today, the average age of oceanic lithosphere when it enters subduction is about 60 Ma, and the subduction of a young oceanic crust remains exceptional. Similarly, in young subduction, the conditions of slab melting would be realised only during a short period Ž4 Ma for a 50 Ma old lithosphere subducting at a 5 cm ay1 rate.. These conditions are rarely realised, thus, accounting for the scarcity of presentday adakitic magmatism. 5. Archaean continental crust The Archaean crust generated between 3.9 and 2.5 Ga is made up of TTG suites ŽJahn et al., 1981; 422 H. Martinr Lithos 46 (1999) 411–429 Martin et al., 1983.. At the end of the Archaean, between 2.8 and 2.5 Ga, high-K and high-Mg mantle-derived magmas were emplaced in most stable cratons. These rocks, referred to as sanukitoids ŽStern, 1989; Stern and Hanson, 1991; Evans and Hanson, 1992; Moyen et al., 1997., represent small volumes when compared to the whole Archaean crust Ž- 5%; Martin, 1995.. TTG follow the K 2 O-poor calc-alkaline trend of Lameyre and Bowden Ž1982. in the Q–A–P modal triangle of Streckeisen Ž1975.. They are ferromagnesian element-poor ŽFe 2 O 3 ) q MgO q MnO q TiO 2 Fs 5%. and have average Na 2 O of about 5% ŽTable 1., with low K 2 OrNa 2 O ratios Ž- 0.5., ŽBarker, 1979; Condie, 1981; Martin, 1987a, 1995.. The sodic character of the TTG can be shown in the K–Na–Ca triangle ŽFig. 3B. ŽBarker and Arth, 1976; Martin, 1995., where they plot on a trondhjemitic differentiation line far from the typical calc-alkaline trend. However, Archaean TTG appear to be slightly Ca-poorer than modern adakites ŽFig. 3A.. Another geochemical feature typical of the Archaean crust is its rare earth element pattern ŽFig. 4B.. REE are strongly fractionated and the ŽLarYb. N ratio can reach 150, whereas Yb content remains low Ž0.3 F Yb N F 8.5.. In a ŽLarYb. N vs. Yb N plot ŽFig. 13; Martin, 1986, 1995., Archaean TTG fall in the same field as adakites, except that they can Fig. 13. Compilation of ŽLarYb. N and Yb N values for Archaean TTG, two rock groups being distinguished on the basis of the age of their emplacement: T ) 3.0 Ga Žfilled circles.; 3.0 Ga-T - 2.5 Ga Žfilled squares.. TTG are characterised by low-Yb contents Ž0.3-Yb N -8.5. and correlated strongly fractionated REE patterns Ž5- ŽLarYb. N -150.. display higher LarYb ratios; in adakites, this ratio generally does not exceed 100 whereas it can commonly reach 150 in TTG. Like adakites, all Archaean TTG have negative Nb–Ta anomalies generally associated with a negative Ti anomaly ŽFig. 5B.. On the other hand, TTG do not have a Sr-positive anomaly, indicating that plagioclase could have been a residual phase during their genesis. Finally, Table 1 indicates that Mg no. as well as Ni and Cr contents are lower in TTG than in adakites. Geochemical modelling based on major and trace elements as well as isotopes shows that TTG can be generated by a three-stage mechanism ŽMartin, 1987a, 1995., which can be summarised as follows. 1. Partial melting of the mantle gave rise to large volumes of tholeiitic magma. 2. Melting of these tholeiites transformed into garnet-bearing amphibolite, or amphibole–eclogite generated the parental magma of TTG, leaving a hornblende q garnet q clinopyroxene q minor plagioclase residue. 3. Fractional crystallisation of mainly hornblende" plagioclase produced the differentiated TTG suite. This third stage of differentiation can be absent or only of very small extent. This process is very similar to the mechanisms of adakites genesis, as it has been proposed that both were derived through melting of the upper part of a subducted oceanic crust ŽMartin, 1995, 1987b.. Recent detail structural analyses performed in Archaean shields have demonstrated that large-scale Archaean horizontal structures exist in most Archaean cratons together with vertical structures Žsagduction. ŽBickle et al., 1980; McGregor et al., 1991; De Wit et al., 1992; Treloar et al., 1992; Ludden et al., 1993; Choukroune et al., 1995; Jegouzo and Blais, 1995.. ´ These studies demonstrate that plate tectonic-like processes operated during the Archaean, including the creation and destruction of oceanic lithosphere as well as collision of rigid crustal blocks. This conclusion is reinforced by the recent description of Archaean oceanic crust squeezed in a thrust plane between two continental blocks. Its position along a suture zone, which is a common feature of ophiolites in recent mountain belts, as well as both its mineralogical and chemical composition Ži.e., similar to modern oceanic crust., favour an ophiolitic charac- H. Martinr Lithos 46 (1999) 411–429 ter, thus, strongly supporting Archaean plate tectonics ŽBlais et al., 1997.. Recently, it has been proposed that Archaean continental crust could have formed by melting of a tholeiitic source in a tectonic environment which does not necessarily imply modern-like subduction, for instance, by underplating andror above a mantle plume ŽArndt and Goldstein, 1989; Kroner, 1991; ¨ Arndt, 1992; Kroner ¨ and Layer, 1992.. Present-day mantle plumes are related to mafic magmatism. In a few exceptional cases, such as in Iceland, small amounts of felsic magma are generated but they do not have TTG nor adakitic characteristics. Consequently, one of the more striking pieces of evidence in favour of the Archaean subduction model is that today, when Archaean-like thermal regimes are created in subduction environments, TTG-like magmas are generated, whereas this kind of magmatism is totally unknown in association with plume systems. 6. Discussion and conclusions Archaean TTG and modern adakites are almost identical in composition, nevertheless, they differ in some ways that can be summarized as follows: adakites have Mg no. and Ni and Cr Žand to some extent Ca. contents greater than TTG, whereas these latter never show positive Sr or Eu anomalies. Adakitic magmas are generated by melting of the subducted oceanic slab and consequently, during their transfer to the surface, they must pass through the 423 mantle wedge with which it could react andror interact. Fig. 14 compares the compositions of adakites with liquids produced by experimental melting of basalts. In the CaO vs. SiO 2 diagram, there is perfect agreement between the field of liquids obtained by partial melting of basalts and the points representing adakites. However, in the MgO vs. SiO 2 diagram, adakites systematically display MgO contents greater than those of experimental liquids. This kind of difference is interpreted as due to interaction with the mantle wedge ŽSen and Dunn, 1994; Sajona, 1995; Maury et al., 1996.. During their ascent, the adakitic magmas react with the mantle, resulting in MgO, Ni and Cr enrichment. Based on a natural adakite study, Mahlburg Kay et al. Ž1993. came to the same conclusion. This interpretation has been recently corroborated by Schiano et al. Ž1995.. They studied glassy inclusions in olivine crystals from ultramafic enclaves in typical calc-alkaline lavas from Batan Islands ŽPhilippines.. Inclusions have adakitic compositions including fractionated REE patterns waverage ŽLarYb. N s 48x with low Yb N Ž3.3. and high SrrY Ž93., thus, demonstrating that adakitic magmas passed through the mantle wedge and interacted with it. On an other hand, adakites are often associated with niobium-enriched basalts ŽNEB. ŽDefant et al., 1991; Defant and Drummond, 1993; Maury et al., 1996; Sajona et al., 1996.. All adakites are not associated with NEB, but all the NEB so far reported are associated with adakites, sometimes in the same volcano ŽDefant et al., 1991; Defant and Drummond, Fig. 14. Comparison of adakite chemical compositions Žfilled circles. with those of liquids produced by experimental melting of basalts Žgrey field.. In the CaO vs. SiO 2 plot, adakites fall perfectly in the grey field, whereas in the MgO vs. SiO 2 diagram, adakites systematically display MgO contents greater than those of experimental liquids. Such a difference is interpreted as due to the fact that adakitic magmas interact with the mantle wedge during their ascent ŽSen and Dunn, 1994; Sajona, 1995; Maury et al., 1996.. 424 H. Martinr Lithos 46 (1999) 411–429 1993.. These authors concluded that NEB and adakites are genetically linked. NEB are generated by melting of the mantle wedge; however, unlike typical calc-alkaline suites from the same source, they are high field strength element ŽHFSE.-enriched. As experimentally demonstrated by Tatsumi et al. Ž1986., aqueous fluids liberated by dehydration of the subducted slab are LILE-enriched but cannot transport significant amounts of HFSE, which remain in the subducted slab. This accounts for the classical HFSE depletion in arc magmas. Unlike aqueous fluids, adakitic magmas are able to carry Nb which can be released in the mantle metasomatic minerals, as observed in micas and amphiboles from mantle Fig. 15. P–T diagram and synthetic cross-section of subduction zones summarising the conditions of genesis of arc magmas. Ž1. The geothermal gradients along the Benioff plane are very high, thus, the subducted slab melts at shallow depth, plagioclase possibly being a residual phase. Because of the small thickness and the low temperature of the wedge, mantle–magma interactions are limited or absent. This was the normal Archaean situation which resulted in TTG genesis. Ž2. Geothermal gradients along the Benioff plane are lower but great enough to allow slab melting which occurs at greater depth without residual plagioclase. The mantle wedge is thick and hot and interactions can occur between mantle and magma. Today, this situation is realised only on rare occasions and give rise to adakitic magmatism. Ž3. Geothermal gradients along the Benioff plane are low, the subducted slab dehydrates before it could begin to melt. The aqueous fluid released by dehydration reactions ascent into the mantle wedge, which is metasomatized and begins to melt, giving rise to the typical arc calc-alkaline magmatism. This situation is more widely realised in modern subduction zones. In the P–T diagram, the symbols are the same as in Fig. 11. In the synthetic cross-sections: OC s oceanic crust; CC s continental crust; ms s solidus of an hydrated mantle; black areas s magma, and dotted areas s fluids. H. Martinr Lithos 46 (1999) 411–429 peridotitic enclaves ŽO’Reilly et al., 1991; Iovov and Hoffmann, 1995; Kepezhinskas et al., 1995; Maury et al., 1996; Sajona et al., 1996.. Consequently, the NEB adakite association provides additional proof that the adakitic magmas efficiently interact with the mantle wedge. As adakitic magmas pass through the mantle wedge, they need to have been generated at greater depth, in the subducted slab. Thus, the melting of basalts underplated at the base of the overlying continental crust ŽAtherton and Petford, 1993; Kay and Mahlburg Kay, 1993. could not account for the observed mantle–adakite interactions. Even if richer than experimental products of basalt melting, Archaean TTG do not show the same MgO, Ni and Cr enrichment as modern adakites. This could be interpreted as reflecting the lack or a lower degree of interaction between their parental magma and the overlying mantle wedge. On the other hand, NEB Žor equivalent magmatic rocks. have not been reported in Archaean terrains. In Fig. 15, it is proposed that, as a consequence of higher Archaean geothermal gradients, the melting of the subducted slab occurred at shallower depth than in the case of modern adakites. Under these conditions, the slab melts passed through a thinner and colder mantle wedge such that only very small degrees of interaction occurred. This hypothesis is corroborated by the lack of both Sr and Eu anomalies in the source of TTG, which could indicate the presence of plagioclase in the residue of melting. This is possible only if fusion occurred at shallow depth Ž P - 15 kbar.. In the case of adakites, the geothermal gradients along the Benioff plane were lower ŽFig. 15.; consequently, slab melting occurs at greater depth outside the plagioclase stability field, thus, accounting for the positive Sr anomaly. In this scenario, the magma crosscuts a thicker and hotter mantle wedge, thus, having efficient interactions with it, leading to MgO, Ni and Cr enrichment. In conclusion, if Archaean TTG and modern adakites were generated by similar processes in identical environments, the detailed petrogenetic conditions were slightly different. Three main situations can be considered. Ž1. Geothermal gradients along the Benioff plane are very high ŽFig. 15, Inset 1., the subducted slab melts before dehydration can occur, melting takes 425 place at shallow depth such that plagioclase could be a residual phase. Because of the small path length and the low temperature of the overlying mantle wedge, mantle–magma interactions are limited or absent. This situation is assumed to be the normal Archaean situation, resulting in TTG genesis. Ž2. Geothermal gradients along the Benioff plane are lower ŽFig. 15, Inset 2., but great enough to allow slab melting before dehydration takes place. Melting occurs at greater depth where plagioclase is not a residual phase. The overlying mantle is thick and hot such that interactions occur between mantle and magma. This is the case for modern adakite genesis and is realised only on rare occasions. Ž3. Geothermal gradients along the Benioff plane are low ŽFig. 15, Inset 3., and slab dehydration occurs before the beginning of melting. Thus, the subducted slab cannot melt. Aqueous fluid released by dehydration reactions rise up into the mantle wedge, which is metasomatized and begins to melt giving rise to typical arc calc-alkaline magmas. This situation is widely realised in modern subduction zones. Acknowledgements Thanks are due to D. Abbott and to an anonymous reviewer for constructive scientific comments and important language corrections. References Anderson, R.N., Delong, S.E., Schwarz, W.M., 1980. Dehydration, asthenospheric convection and seismicity in subduction zones. J. Geol. 88, 445–451. Arculus, R.J., Ruff, L.J., 1990. Genesis of continental crust: evidence from island arcs, granulites, and exospheric processes. In: Vielzeuf, D., Vidal, Ph. ŽEds.., Granulites and Crustal Evolution. Kluwer Academic Publishers, pp. 7–23. Arndt, N.T., 1992. Rate and mechanism of continent growth in the Precambrian. In: Maruyama, S. ŽEd.., Evolving Earth Symposium. Okazaki, pp. 38–41. Arndt, N.T., Goldstein, S.L., 1989. An open boundary between lower continental crust and mantle: its role in crust formation and crustal recycling. Tectonophysics 161, 201–212. Arth, J.G., Hanson, G.N., 1975. Geochemistry and origin of the Early Precambrian crust of north-eastern Minnesota. Geochim. Cosmochim. Acta 39, 325–362. Atherton, M.P., Petford, N., 1993. Generation of sodium-rich 426 H. Martinr Lithos 46 (1999) 411–429 magmas from newly underplated basaltic crust. Nature 362, 144–146. Barbey, P., Martin, H., 1987. The role of komatiites in plate tectonics. Evidence from the Archaean and early Proterozoic crust in the Eastern Baltic shield. Precambrian Res. 35, 1–14. Barker, F., 1979. Trondhjemites: definition, environment and hypotheses of origin. In: Barker, F. ŽEd.., Trondhjemites, Dacites and Related Rocks. Elsevier, Amsterdam, pp. 1–12. Barker, F., Arth, J.G., 1976. Generation of trondhjemitic–tonalitic liquids and Archaean bimodal trondhjemite–basalt suites. Geology 4, 596–600. Beard, J.S., Lofgren, G.E., 1989. Effect of water on the composition of partial melts of greenstones and amphibolites. Science 244, 195–197. Beard, J.S., Lofgren, G.E., 1991. Dehydration melting and watersaturated melting of basaltic and andesitic greenstones and amphibolites at 1, 3 and 6.9 kb. J. Petrol. 32, 465–501. Bebout, G.E., Barton, M.D., 1993. Metasomatism during subduction: products and possible paths in the Catalina schist, California. Chem. Geol. 108, 61–92. Bickle, M.J., 1978. Heat loss from the Earth: constraint on Archaean tectonics from the relationships between geothermal gradients and the rate of plate production. Earth Planet. Sci. Lett. 40, 301–315. Bickle, M.J., Battenay, L.F., Boulter, C.A., Groves, D.I., Morant, P., 1980. Horizontal tectonic interaction of the Archaean gneiss belt and greenstones Pilbara block. Contrib. Mineral. Petrol. 84, 25–35. Bickle, M.J., Battenay, L.F., Chapman, H.J., Groves, D.I., McNaughton, N.J., Campbell, I.H., deLaeter, J.R., 1993. Origin of the 3500–3300 Ma calc-alkaline rocks in the Pilbara Archaean: isotopic and geochemical constraints from the Shaw batholith. Precambrian Res. 60, 117–149. Bird, P., Toksov, ¨ M.N., Sleep, N.H., 1975. Thermal and mechanical models of continent–continent convergence zones. J. Geophys. Res. 80, 4405–4416. Blais, S., Martin, H., Jegouzo, P., 1997. Reliques d’une croute ´ ˆ oceanique archeenne en Finlande orientale. C. R. Acad. Sci. ´ ´ ŽParis. 325, 397–402. Bouhallier, H., Choukroune, P., Ballevre, M., 1993. Diapirism, ` bulk homogeneous shortening and transcurrent shearing in the Archaean Dharwar craton : the Holenarsipur area, Southern India. Precambrian Res. 63, 43–58. Bouhallier, H., Chardon, D., Choukroune, P., 1995. Strain patterns in Archaean dome-and-basin structures : the Dharwar craton ŽKarnataka, South India.. Earth Planet. Sci. Lett. 135, 57–75. Bourgois, J., Lagabrielle, Y., Le Moigne, J., Urbina, O., Janin, M.C., Beuzart, P., 1994. Preliminary results on a field study of the Taitao ophiolite ŽSouthern Chile. : implications for the evolution of the Chile Triple Junction. Ophioliti 18, 113–129. Bourgois, J., Martin, H., Lagabrielle, Y., Le Moigne, J., Frutos Jara, J., 1996. Subduction–erosion related to ridge–trench collision: Taitao Peninsula ŽChile margin triple junction area.. Geology 24, 723–726. Cameron, K.L., Cameron, M., 1985. Rare Earth element, 87 Srr 86 Sr, and 143 Ndr144 Nd compositions of Cenozoic orogenic dacites from Baja California, northwestern Mexico, and adja- cent west Texas : evidence for predominance of a subcrustal component. Contrib. Mineral. Petrol. 91, 1–11. Choukroune, P., Bouhallier, H., Arndt, N.T., 1995. Soft lithosphere during periods of Archaean crustal growth or crustal reworking. In: Coward, M.P., Ries, A.C. ŽEds.., Early Precambrian Processes. Geol. Soc. Spec. Publ., Vol. 95, pp. 67–86. Condie, K.C., 1981. Archaean Greenstone Belts. Elsevier, Amsterdam, 434 pp. Condie, K.C., 1986. Origin and early growth rate of continents. Precambrian Res. 32, 261–278. 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. Defant, M.J., Richerson, M., De Boer, J.Z., Stewart, R.H., Maury, R.C., Bellon, H., Drummond, M.S., Feigenson, M.D., Jackson, T.E., 1991. Dacite genesis via both slab melting and differentiation: petrogenesis of La Yeguada volcanic complex. Panama. J. Petrol. 32, 1101–1142. Defant, M.J., Jackson, T.E., Drummond, M.S., De Boer, J.Z., Bellon, H., Feigenson, M.D., Maury, R.C., Stewart, R.H., 1992. The geochemistry of young volcanism throughout western Panama and southeastern Costa Rica: an overview. J. Geol. Soc. ŽLondon. 149, 569–579. Delong, S.E., Schwarz, W.M., Anderson, R.N., 1979. Thermal effects of ridge subduction. Earth Planet. Sci. Lett. 44, 239– 246. De Wit, M.J., Roering, C., Hart, R.A., Armstrong, A., De Ronde, C.E.J., Green, R.W.E., Tredoux, M., Pederby, E., Hart, R.J., 1992. Formation of an Archaean continent. Nature 357, 553– 562. Drummond, M.S., Defant, M.J., 1990. A model for trondhjemite–tonalite–dacite genesis and crustal growth via slab melting: Archaean to modern comparisons. J. Geophys. Res. 95, 21503–21521. Drummond, M.S., Defant, M.J., Kepezhinskas, P.K., 1996. The petrogenesis of slab derived trondhjemite – tonalite – daciteradakite magmas. Trans. R. Soc. Edinburgh: Earth Sci. 87, 205–216. Ellam, R.M., Hawkesworth, C.J., 1988. Is average continental crust generated at subduction zones? Geology 16, 314–317. Evans, O.C., Hanson, G.H., 1992. Most late Archaean tonalites, trondhjemites and granodiorites ŽTTG. in the SW Superior Province were derived from mantle melts, not by melting of basalts. AGU V22D 3, 330, Abstr. Futa, K., Stern, C.R., 1988. Sr and Nd isotopic and trace element compositions of quaternary volcanic centres of the southern Andes. Earth Planet. Sci. Lett. 88, 253–262. Garde, A., 1997. Accretion and evolution of an Archaean highgrade grey gneiss amphibolite complex: the Fiskefjord area, southern West Greenland. Geol. Greenland Surv. Bull. 177, 115. Gorman, B.E., Pearce, T.H., Birkett, T.C., 1978. On the structure of Archaean greenstone belts. Precambrian Res. 6, 23–41. Green, D.H., 1982. Anatexis of mafic crust and high pressure H. Martinr Lithos 46 (1999) 411–429 crystallisation of andesite. In: Thorpe, R.S. ŽEd.., Andesites. Wiley, New York, pp. 465–487. Guivel, C., Lagabrielle, Y., Bourgois, J., Maury, R., Martin, H., Arnaud, N., Cotten, J., 1996. Magmatic reponses to active spreading ridge subduction: multiple magma sources in the Taitao Peninsula region Ž468–478 S, Chile triple junction.. Third International Symposium on Andean geodynamics ŽISAG 96. Saint-Malo, France. ORSTOM editeur, pp. 575– ´ 578. Hasebe, K., Fujii, N., Uyeda, S., 1970. Thermal processes under island arcs. Tectonophysics 10, 335–355. Helz, R.T., 1976. Phase relations in basalts in their melting range at PŽH 2 O. s 5 kb: II. Melt compositions. J. Petrol. 17, 139– 193. Hickey-Vargas, R., Moreno Roa, H., Lopez Escobar, L., Frey, F.A., 1989. Geochemical variations in Andean basaltic and silicic lavas from the Villarica-Lanin volcanic chain Ž39.58 S.: an evaluation of source heterogeneity, fractional crystallisation and crustal assimilation. Contrib. Mineral. Petrol. 103, 361– 386. Holloway, J.R., Burnham, C.W., 1972. Melting relations of basalt with equilibrium water pressure less than total pressure. J. Petrol. 13, 1–29. Honda, S., 1985. Thermal structure beneath Tohoku, northeast Japan. A case study for understanding the detailed thermal structure of the subduction zone. Tectonophysics 112, 69–102. Honda, S., Uyeda, S., 1983. Thermal processes in subduction zones: a review and preliminary approach on the origin of arc volcanism. In: Shimozuru, D., Yokoyama, I. ŽEds.., Arc Volcanism. pp. 117–140. Honthaas, C., Bellon, H., Kepezhinskas, P.K., Maury, R., 1995. Nouvelles datations 40 Kr40Ar du magmatisme cretace ´ ´ quaternaire du Kamchatka du Nord ŽRussie.. C. R. Acad. Sci. ŽParis. 320, 197–204. Hunter, D.R., Barker, F., Millard, H.T., 1978. The geochemical nature of the Archaean Ancient Gneiss Complex and granodioritic suite. Swaziland: a preliminary study. Precambrian Res. 7, 105–127. Iovov, D.A., Hoffmann, A.W., 1995. Nb–Ta-rich mantle amphiboles and micas; implication for subduction-related metasomatic trace element fractionations. Earth Planet. Sci. Lett. 131, 341–356. Jahn, B.M., Glikson, A.Y., Peucat, J.J., Hickman, A.H., 1981. REE geochemistry and isotopic data of Archaean silicic volcanics and granitoids from the Pilbara Block, western Australia: implications for the early crustal evolution. Geochim. Cosmochim. Acta 45, 1633–1652. Jegouzo, P., Blais, S., 1995. Structural evidence for collision ´ tectonics in the Archaean of Eastern Finland. Geodynamica Acta 8 Ž1., 1–12. Johnson, A.D., Wyllie, P.J., 1988. Constraints on the origin of Archaean trondhjemites based on phase relationships of Nuk ˆ gneiss with H 2 O at 15 kbar. Contrib. Mineral. Petrol. 100, 35–46. Kay, R.W., 1978. Aleutian magnesian andesites : melts from subducted Pacific Ocean crust. J. Volcanol. Geotherm. Res. 4, 117–132. 427 Kay, R.W., Mahlburg Kay, S., 1993. Delamination and delamination magmatism. Tectonophysics 219, 177–189. Kepezhinskas, P.K., 1989. Origin of the hornblende andesites of northern Kamchatka. Int. Geol. Rev. 31, 246–252. Kepezhinskas, P.K., Defant, M.J., Drummond, M.S., 1995. Na metasomatism in the island arc mantle by slab melt–peridotite interaction: evidence from mantle xenoliths in the north Kamchatka arc. J. Petrol. 36, 1505–1527. Knowles, J., 1995. Fusion partielle de la croute dans ˆ oceanique ´ une zone de subduction: approche par les desequilibres U–Th. ´´ Unpublished memoir, University of Clermont-Ferrand, France, 42 pp. Kroner, A., 1991. Tectonic evolution in Archaean and Protero¨ zoic. Tectonophysics 187, 393–410. Kroner, A., Layer, P.W., 1992. Crust formation and plate motion ¨ in the Early Archaean. Science 256, 1405–1411. Lagabrielle, Y., Le Moigne, J., Maury, R.C., Cotten, J., Bourgois, J., 1994. Volcanic record of the subduction of an active spreading ridge, Taitao Peninsula Žsouthern Chile.. Geology 22, 515–518. Lagabrielle, Y., Goslin, J., Martin, H., Thiriot, J.L., Auzende, J.M., 1997. Multiple active spreading centers in the hot North Fiji basin ŽSW Pacific.: a possible model for Archaean seafloor dynamics? Earth Planet. Sci. Lett. 149, 1–13. Lameyre, J., Bowden, P., 1982. Plutonic rock type series: discrimination of various granitoid series and related rocks. J. Volcanol. Geotherm. Res. 14, 169–186. Lefevre, C., 1979. Un exemple de volcanisme de marge active ` dans les Andes du Perou, du miocene ´ ` a` l’actuel: zonation et petrogenese et shoshonites. Unpublished thesis, ´ ` des andesites ´ Montpellier, France, p. 555. Le Moigne, J., 1995. Subduction d’une dorsale active; geologie ´ des ophiolites de Taitao Žpoint triple du Chili.. Unpublished thesis, University of Brest, France, p. 161. Le Moigne, J., Lagabrielle, Y., Bourgois, J., Palvadeau, E., 1993. Ophiolites en contexte de dorsale en subduction: nouvelels donnees de Taitao ŽSud Chili.. C. R. Acad. ´ sur la Peninsule ´ Sci. ŽParis. 317, 403–410. Lopez-Escobar, L., 1984. Petrology and chemistry of volcanic rocks of the Southern Andes. In: Harmon R.S., Barreiro, B.A. ŽEds.., Andean Magmatism, Chemical and Isotopic Constraints, Shiva Geology Series, Nantwich, pp. 47–71. Ludden, J., Hubert, C., Barnes, A., Mikereit, B., Sawyer, E., 1993. A three dimensional perspective on the evolution of the Earth’s largest Archaean crust. Lithoprobe seismic reflection images in the southwestern Superior Province. Lithos 30, 357–372. Mahlburg Kay, S., Ramos, V.A., Marquez, M., 1993. Evidence in Cerro Pampa volcanic rocks of slab melting prior to ridge trench collision in southern South America. J. Geol. 101, 703–714. Martin, H., 1986. Effect of steeper Archaean geothermal gradient on geochemistry of subduction-zone magmas. Geology 14, 753–756. Martin, H., 1987a. Petrogenesis of Archaean trondhjemites, tonalites and granodiorites from eastern Finland: major and trace element geochemistry. J. Petrol. 28, 921–953. 428 H. Martinr Lithos 46 (1999) 411–429 Martin, H., 1987b. Archaean and modern granitoids as indicators of changes in geodynamic processes. Rev. Bras. Geoc. 17, 360–365. Martin, H., 1993. The mechanisms of petrogenesis of the Archaean continental crust, comparison with modern processes. Lithos 30, 373–388. Martin, H., 1995. The Archaean grey gneisses and the genesis of the continental crust. In: Condie, K.C. ŽEd.., The Archaean Crustal Evolution. Elsevier, pp. 205–259. Martin, H., Chauvel, C., Jahn, B.M., 1983. Major and trace element geochemistry and crustal evolution of Archaean granodioritic rocks from eastern Finland. Precambrian Res. 21, 159–180. Martin, H., Auvray, B., Blais, S., Capdevila, R., Hameurt, J., Jahn, B.M., Piquet, D., Querre, ´ ´ G., Vidal, Ph., 1984. Origin and geodynamic evolution of the Archaean crust of Eastern Finland. Bull. Geol. Soc. ŽFinland. 56, 135–160. Maury, R.C., Sajona, F.G., Pubellier, M., Bellon, H., Defant, M.J., 1996. Fusion de la croute dans les zones de ˆ oceanique ´ subductionrcollision recentes: l’exemple de Mindanao ´ ŽPhilippines.. Bull. Soc. Geol. ŽFrance. 167 Ž5., 579–595. McDonald, G.A., Katsura, T., 1964. Chemical composition of Hawaiian lavas. J. Petrol. 5, 82–133. McGregor, V.R., Friend, C.R.L., Nutman, A.P., 1991. The lateArchaean mobile belt through Gotthabsfjord region, southern ˚ west Greenland: a continent–continent collision-zone? Bull. Geol. Soc. ŽDenmark. 39, 179–197. McKenzie, D.P., 1984. The generation and compaction of partially molten rocks. J. Petrol. 25, 713–765. Molnar, P., England, P.C., 1990. Temperatures, heat flux and frictional stress near major thrust faults. J. Geophys. Res. 95, 4833–7856. Monzier, M., Robin, C., Hall, M.L., Cotten, J., Mothes, P., Eissen, J.P., Samaniego, P., 1997. Les adakites d’Equateur: modele C. R. Acad. Sci. ŽParis. 324, 545–552. ` preliminaire. ´ Morris, P.A., 1995. Slab melting as an explanation of Quaternary volcanism and aseismicity in southwestern Japan. Geology 23, 395–398. Moyen, J.-F., Martin, H., Jayananda, M., 1997. Origine du granite fini-Archeen ´ de Closepet ŽInde du Sud.: apports de la modelisation geochimique du comportement des elements en ´ ´ ´´ traces. C. R. Acad. Sci. ŽParis. 325, 659–664. Myers, J.D., Frost, C.D., 1994. A petrologic investigation of the Adak volcanic center, central Aleutian arc, Alaska. J. Volcanol. Geotherm. Res. 60, 109–146. Nedelec, A., Nsifa, E.N., Martin, H., 1990. Major and trace ´ ´ element geochemistry of the Archaean Ntem plutonic complex ŽSouth Cameroon.: petrogenesis and crustal evolution. Precambrian Res. 47, 35–50. Neymark, L.A., Kovach, V.P., Nemchin, A.A., Morozova, I.M., Kotov, A.B., Vinogradov, D.P., Gorokhovsky, B.M., Ovchinnikova, G.V., Bogomolova, L.M., Smelov, A.P., 1993. Late Archaean intrusive complexes in the Olekma granite–greenstone terrain and Žeastern Siberia.: geochemical and isotopic study. Precambrian Res. 62, 453–472. O’Reilly, S.Z., Griffin, W.L., Ryan, C.G., 1991. Residence of trace elements in metasomatised spinel lherzolite xenolith: a proton micoprobe study. Contrib. Mineral. Petrol. 109, 98–113. Peacock, S.M., 1990. Fluid processes in subduction zones. Science 248, 329–337. Peacock, S.M., 1993. Large-scale hydration of the lithosphere above subducting slabs. Chem. Geol. 108, 43–59. Peacock, S.M., Rushmer, T., Thompson, A.B., 1994. Partial melting of subducting oceanic crust. Earth Planet. Sci. Lett. 121, 224–227. Prouteau, G., Maury, R., Rangin, C., Suparka, E., Bellon, H., Pubellier, M., Cotten, J., 1996. Les adakites miocenes ` du NW de Borneo, de la fermeture de la proto-mer de Chine. ´ temoins ´ C. R. Acad. Sci. ŽParis. 323, 925–932. Rapp, R.P., Watson, E.B., 1995. Dehydration melting of metabasalt at 8–32 kbar: implications for continental growth and crust– mantle recycling. J. Petrol. 36 Ž4., 891–931. Rapp, R.P., Watson, E.B., Miller, C.F., 1991. Partial melting of amphibolitereclogite and the origin of Archaean trondhjemites and tonalites. Precambrian Res. 51, 1–25. Rogers, G., Hawkesworth, C.J., 1989. A geochemical traverse across the North Chilean Andes: evidence for crust generation from the mantle wedge. Earth Planet. Sci. Lett. 91, 271–285. Rogers, G., Saunders, A.D., Terrell, D.J., Verma, S.P., Marriner, G.F., 1985. Geochemistry of holocene volcanic rocks associated with ridge subduction in Baja California, Mexico. Nature 315, 389–392. Rollinson, H., 1997. Eclogite xenoliths in west African kimberlites as residues from Archaean granitoid crust formation. Nature 389, 173–176. Rushmer, T., 1991. Partial melting of two amphibolites: contrasting experimental results under fluid-absent conditions. Contrib. Mineral. Petrol. 107, 41–59. Sajona, F.G., 1995. Fusion de la croute en contexte de ˆ oceanique ´ subduction collision: geochimie, geochronologie et petrologie ´ ´ ´ du magmatisme plioquaternaire de Mindanao ŽPhilippines.. Unpublished thesis, Brest University, France, 223 pp. Sajona, F.G., Bellon, H., Maury, R.C., Pubellier, M., Quebral, R.D., Cotten, J., Bayon, F.E., Pagado, E., Pamatian, P., 1996. Tertiary and Quaternary magmatism in Mindanao and Leyte ŽPhilippines: geochronology, geochemistry and tectonic setting. J. SE Asian Earth Sci., in press. Samaniego, P., 1997. Interactions entre les magmas adakitiques et calco-alcalins: geochimie des complexes volcaniques du ´ Cayambe et du Moranda–Fuya Fuya ŽEquateur.. Unpublished memoir, University of Clermont-Ferrand, France, 45 pp. Saunders, A.D., Rogers, G., Marriner, G.F., Terrell, D.J., Verma, S.P., 1987. Geochemistry of Cenozoic volcanic rocks, Baja California, Mexico: implications for the petrogenesis of postsubduction magmas. J. Volcanol. Geotherm. Res. 32, 223–245. Schiano, P., Clochiatti, R., Shimizu, N., Maury, R., Jochum, K.P., Hofmann, A.W., 1995. Hydrous, silica-rich melts in the sub-arc mantle and their relationships with erupted arc lavas. Nature 377, 595–600. Sen, C., Dunn, T., 1994. Dehydration melting of a basaltic composition amphibolite at 1.5 and 2.0 Gpa: implications for the origin of adakites. Contrib. Mineral. Petrol. 117, 394–409. H. Martinr Lithos 46 (1999) 411–429 Sheraton, J.W., Black, L.P., 1983. Geochemistry of Precambrian gneisses: relevance for the evolution of the east Antarctic shield. Lithos 16, 273–296. Sigmarsson, O., Condomines, M., Morris, J.D., Harmon, R.S., 1990. Uranium and 10 Be enrichments by fluids in Andean arc magmas. Nature 346, 163–165. Sigmarsson, O., Martin, H., Knowles, J., 1998. Melting of a subducting oceanic crust in Austral Andean lavas from U-series disequilibria. Nature 394, 566–569. Smith, T.E.M., Taylor, S.R., Johnson, R.W., 1979. REEfractionated trachytes and dacites from Papua New Guinea and their relationship to andesite petrogenesis. Contrib. Mineral. Petrol. 69, 227–233. Sorensen, S.S., 1988. Petrology of amphibolite-facies mafic and ultramafic rocks from Catalina schist, southern California: metamorphism and magmatisation in a subduction zone metamorphic setting. J. Metamorph. Geol. 6, 405–435. Sorensen, S.S., Barton, M.D., 1987. Metasomatism and partial melting in a subduction complex: Catalina schist, southern California. Geology 15, 115–118. Sorensen, S.S., Grossman, J.N., 1989. Enrichment in trace elements in garnet amphibolites from a paleo-subduction zone: Catalina schist, southern California. Geochim. Cosmochim. Acta 53, 3155–3177. Stern, R., 1989. Petrogenesis of the Archaean sanukitoid suite. Unpublished Thesis. State University of New York at Stony Brook, p. 275. Stern, C.R., Futa, K., 1982. An Andean andesite derived directly from subducted MORB or from LIL depleted subcontinental mantle. Trans. Am. Geophys. Union 63, 1148. Stern, R., Hanson, G., 1991. Archaean high-Mg granodiorites: a derivative of LREE enriched monzodiorite of mantle origin. J. Petrol. 32, 201–238. Stern, C.R., Kilian, R., 1996. Role ˆ of the subducted slab, mantle wedge and continental crust in the generation of adakites from the Austral Volcanic Zone. Contrib. Miner. Petrol. 123, 263– 281. Stern, C.R., Futa, K., Muehlenbachs, K., 1984a. Isotopic and trace element data for orogenic andesites from the austral Andes. In: Harmon R.S., Barreiro, B.A. ŽEds.., Andean Magmatism, Chemical and Isotopic Constraints, Shiva Geology Series. Nantwich, pp. 1–46. Stern, C.R., Futa, K., Muehlenbachs, K., Dobbs, M., Munoz, J., Godoy, E., Charrier, R., 1984b. Sr, Nd, Pb, O isotope composition of late Cenozoic volcanics; northernmost SVZ Ž33– 429 348S.. In: Harmon, R.S., Barreiro, B.A. ŽEds.., Andean Magmatism, Chemical and Isotopic Constraints, Shiva Geology Series. Nantwich, pp. 96–105. Streckeisen, A., 1975. To each plutonic rock its proper name. Earth Sci. Rev. 12, 1–33. Tarney, J., Weaver, B.L., Drury, S.A., 1979. Geochemistry of Archaean trondhjemitic and tonalitic gneisses from Scotland and E. Greenland. In: Barker, F. ŽEd.., Trondhjemites, Dacites and Related Rocks. Elsevier, Amsterdam, pp. 275–299. Tarney, J., Weaver, B.L., Winley, B.F., 1982. Geological and geochemical evolution of the Archaean continental crust. Rev. Bras. Geoc. 12, 53–59. Tatsumi, Y., Hamilton, D.L., Nesbitt, R.W., 1986. Chemical characteristics of fluid phase released from a subducted lithosphere and origin of arc lavas: evidence from high-pressure experiments and natural rocks. J. Volcanol. Geotherm. Res. 29, 293–309. Thieblemont, D., Stein, G., Lescuyer, J.-L., 1997. Gisements ´ epithermaux et porphyriques: la connection adakite. C. R. ´ Acad. Sci. ŽParis. 325, 103–109. Toksov, ¨ M.N., Hsui, A.T., 1978. Numerical studies of back-arc convection and the formation of marginal basins. Tectonophysics 50, 177–196. Toksov, ¨ M.N., Minear, J.W., Julian, B.R., 1971. Temperature field and geophysical effects of a downgoing slab. J. Geophys. Res. 76, 1113–1138. Treloar, P.J., Coward, M.P., Harris, N.B.W., 1992. Himalayan– Tibetan analogies for the evolution of the Zimbabwe craton and Limpopo belt. Precambrian Res. 55, 571–587. Winther, T.K., Newton, R.C., 1991. Experimental melting of an hydrous low-K tholeiite: evidence on the origin of Archaean cratons. Bull. Geol. Soc. Denmark, p. 39. Wolf, M.B., Wyllie, P.J., 1991. Dehydration-melting of solid amphibolite at 10 kbar: textural development, liquid interconnectivity and applications to the segregation of magmas. Miner. Petrol. 44, 151–179. Wolf, M.B., Wyllie, P.J., 1993. Dehydration-melting of amphibolite at 10 kbar: effect of temperature, time and texture. Contrib. Mineral. Petrol., in press. Wyllie, P.J., 1971. The role of water in magma genesis and initiation of diapiric uprise in the mantle. J. Geophys. Res. 76, 1328–1338. Zamora, D., 1996. Fusion experimentale de la croute ´ ˆ oceanique ´ subductee ´ a` 1.5 GPa, Unpublished memoir, University of Clermont-Ferrand, France, 46 pp.