Download Adakitic magmas: modern analogues of Archaean granitoids

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.
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