Download TTGs and adakites: are they both slab melts?

Lithos 80 (2005) 33 – 44
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TTGs and adakites: are they both slab melts?
Kent C. Condie*
Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, NM 87801, USA
Received 25 March 2003; accepted 9 September 2003
Available online 5 November 2004
Abstract
Although both high-Al TTG (tonalite–trondhjemite–granodiorite) and adakite show strongly fractionated REE and
incompatible element patterns, TTGs have lower Sr, Mg, Ni, Cr, and Nb/Ta than most adakites. These compositional differences
cannot be easily related by shallow fractional crystallization. While adakites are probably slab melts, TTGs may be produced by
partial melting of hydrous mafic rocks in the lower crust in arc systems or in the Archean, perhaps in the root zones of oceanic
plateaus. It is important to emphasize that geochemical data can be used to help constrain tectonic settings, but it cannot be used
alone to reconstruct ancient tectonic settings.
Depletion in heavy REE and low Nb/Ta ratios in high-Al TTGs require both garnet and low-Mg amphibole in the restite,
whereas moderate to high Sr values allow little, if any, plagioclase in the restite. To meet these requirements requires melting in
the hornblende eclogite stability field between 40- and 80-km deep and between 700 and 800 8C.
Some high-Al TTGs produced at 2.7 Ga and perhaps again at about 1.9 Ga show unusually high La/Yb, Sr, Cr, and Ni.
These TTGs may reflect catastrophic mantle overturn events at 2.7 and 1.9 Ga, during which a large number of mantle plumes
bombarded the base of the lithosphere, producing thick oceanic plateaus that partially melted at depth.
D 2004 Elsevier B.V. All rights reserved.
Keywords: TTG; Adakite; Archean tectonics; Arc systems; Mantle plume event
1. Introduction
Since the classic book edited by Fred Barker on
trondhjemites and related rocks (Barker, 1979), this
suite of rocks, now known as the TTG (tonalite–
trondhjemite–granodiorite) suite, has received considerable attention. Because they are the most volumi-
* Tel.: +1 505 835 5531; fax: +1 505 835 6436.
E-mail address: [email protected].
0024-4937/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.lithos.2003.11.001
nous rock type in the preserved Archean crust, it is
critical to understand the source and origin of TTGs to
better understand the origin and early evolution of
continents. Even in the Tertiary, TTGs are an
important juvenile component added to the continental crust in the Andes and other continental-margin
arcs. Because of geochemical similarities of TTGs to
modern adakites, they are commonly assumed to have
similar origins (Drummond and Defant, 1990; Martin,
1999), and in some cases, the two terms are used
interchangeably. For instance, on a primitive mantle
34
K.C. Condie / Lithos 80 (2005) 33–44
Fig. 1. Primitive-mantle normalized incompatible element distributions in adakites and TTGs. Data from Kay et al. (1993, 1999) and
Table 1 and Appendix A. Primitive mantle values from Sun and
McDonough (1989).
normalized element distribution diagram, both rock
types exhibit strong enrichment in the most incompatible elements, with notable negative anomalies at Nb–
Ta and Ti and strong depletion in heavy REE and Y
(Fig. 1). Hence, as with adakites, TTGs are commonly
assumed to represent melts of descending slabs, a
conclusion with important implications for tectonic
settings in the Archean. However, some investigators
have pointed to dissimilarities between adakites and
TTGs and hence to different tectonic settings
(Smithies, 2000; Kamber et al., 2002). One of the
issues in this controversy is that of how to define TTG
and whether or not TTGs are strictly an Archean
phenomena or if they occur throughout geologic time.
In this study, these questions are addressed by
comparing published chemical analyses of TTGs and
adakites of varying ages and locations. Results show
that TTGs and adakites are not the same thing, and
consequently, they may have quite different origins.
Furthermore, TTGs are not an Archean phenomenon
but have contributed to the growth of continental crust
throughout geologic time.
2. Data collection and definitions
To compare adakites and TTGs, I have compiled
chemical analyses from the literature together with
unpublished analyses from our laboratory. Included
are major elements and a group of both compatible
and incompatible trace elements (see Appendix A).
We have used a definition for TTG similar to that of
Barker (1979) and Defant and Drummond (1990):
Al2O3 contents N15% at 70% SiO2, Sr N300 ppm,
Yb20 ppm, Ybb1.8 ppm, and NbV10 ppm. Although
adakites commonly show these same chemical characteristics, they are more mafic and can be distinguished from TTGs by their relatively high Mg, Ni,
Cr, and Sr contents. TTGs also have been referred to
as blow-Mg adakitesQ (Rapp et al., 1999). For a rock
to be called an adakite, in addition to the above
geochemical characters, it should have a Mg number
greater than 50, Sr N500 ppm (often N1000 ppm), Cr
z50 ppm, and Ni z20 ppm (Figs. 2–5; Appendix A).
Also, as pointed out by Kamber et al. (2002), Archean
TTGs do not resemble adakites in terms of fluidmobile trace elements such as Ba, K, and Rb (Table
1). The relatively high contents of these elements in
TTGs compared to adakites may reflect a subduction
zone component in TTG sources, which is not present
in the descending slab source of adakites. The
relatively high Mg, Cr, and Ni contents of adakites
are generally ascribed to interaction of the slab melts
with the overlying mantle wedge, a process that has
been verified by experimental studies (Drummond
and Defant, 1990; Rapp et al., 1999). Another
relatively minor group of plutonic rocks known a
sanukitoids is also recognized in the Archean (Shirey
and Hanson, 1984). These rocks, which show the
similar distributions of incompatible elements as
TTGs, contain larger amounts of K, Rb, Th, and U
and are generally thought to be the products of
melting metasomatized mantle (Stern and Hanson,
1991). They are not included in the present
investigation.
Average values of selected element concentrations
and element ratios are give in Appendix A, and a
summary of average TTG compositions of various
ages is given in Table 1 compared with average
adakite. Although the geochemical definitional
screens given in the previous paragraph are used to
distinguish TTG from adakite, in some cases, screens
give conflicting results. In these cases, the rock is
classified according the majority of the screens. One
thing is very clear from Table 1, TTGs are not an
Archean phenomenon but occur throughout geologic
time. Some of the youngest examples are found in the
Miocene batholiths of the Andes and comprise a large
proportion of the Andean Cordillera (Petford and
Atherton, 1996; Kay et al., 1999). On average,
K.C. Condie / Lithos 80 (2005) 33–44
35
Table 1
Average chemical compositions of TTGs and adakites
TTGs
SiO2
TiO2
Al2O3
Fe2O3T
MgO
CaO
Na2O
K2O
P2O5
MnO
Th
U
Ni
Cr
Y
Zr
Nb
Hf
Ta
La
Ce
Nd
Sm
Eu
Gd
Tb
Yb
Lu
Rb
Ba
Sr
(La/Yb)n
Sr/Y
Nb/Ta
Mg#
K2O/Na2O
n
Adakites
Early Archean
Late Archean
Mean
Mean
70.4
0.31
15.2
2.79
0.96
2.74
4.71
2.22
0.1
0.06
99.49
4.1
1.2
17
45
8.5
152
6.1
3.8
0.41
22
40
16
2.9
0.82
2.2
0.31
0.82
0.14
76
500
362
25
72
12.0
40.8
0.51
212
1j
2.9
0.14
1.1
1.2
0.62
0.88
0.61
0.68
0.06
0.02
2.4
0.49
19
48
6.4
44
3.5
1.6
0.29
9.4
16
8.2
1.7
0.31
1.4
0.23
0.7
0.1
26
258
117
15
24
2.8
8.3
0.18
68.3
0.42
15.5
3.42
1.39
3.26
4.51
2.20
0.14
0.07
99.21
8.1
1.5
22
35
9.1
154
6.2
4.7
0.84
36
65
25
4.2
1.07
2.9
0.38
0.71
0.11
67
769
515
36
89
13.0
46.2
0.51
831
Proterozoic
1j
2.6
0.12
0.75
1.1
0.59
0.73
0.48
0.66
0.06
0.03
5.3
0.99
13
26
5.4
49
2.4
1.9
0.3
16
29
9.1
1.7
0.32
1.2
0.17
0.41
0.07
24
288
127
24
49
6.3
6.7
0.24
Mean
67.3
0.47
15.8
4.04
1.48
3.42
4.33
2.30
0.14
0.08
99.36
6.1
2.1
23
55
17.3
152
7.1
4.3
0.72
26
45
18
3.5
0.95
3
0.49
1.33
0.23
63
717
473
14.2
37
9.9
43.2
0.56
752
Phanerozoic
1j
3.6
0.2
0.78
1.4
0.70
1.10
0.66
0.82
0.06
0.05
3.1
1.1
12
31
8.9
63
4.6
1.6
0.54
12
26
12
2
0.44
1.9
0.27
0.86
0.14
25
239
159
8.3
30
6.7
7.5
0.25
Mean
65.9
0.47
16.5
4.11
1.67
4.36
4.00
2.14
0.12
0.09
99.36
7.6
1.9
12
32
14.5
122
6.7
3.4
0.75
17
34
16
3.1
0.84
2.8
0.40
1.16
0.18
63
716
493
11.3
56
12.2
45.4
0.68
698
1j
3.5
0.16
0.94
1.3
0.69
1.10
0.50
0.63
0.05
0.02
4.4
1.1
5.0
15
4.7
30
1.7
0.73
0.32
6.2
11
4.5
0.89
0.23
0.65
0.11
0.39
0.06
26
251
105
5.3
12
9.2
6.9
0.5
Mean
62.43
0.67
17.05
3.99
3.31
6.53
4.25
1.42
0.26
0.08
99.99
3.9
1.2
64
82
9.7
117
9.7
3.3
0.60
24
65
26
4.7
1.37
2.30
0.40
0.81
0.09
15
309
1550
18.2
160
16.1
60.4
0.33
221
Major elements as oxides in wt.%, trace elements in ppm; n, number of samples; 1j, one standard deviation of mean; Mg#=MgO/
(MgO+0.79Fe2O3T), molecular ratio; chondrite normalizing values from Haskin et al. (1968); Early Archean, z3.5 Ga; Late Archean, 3.5–2.5
Ga; Proterozoic, 2.5–0.54 Ga; Phanerozoic, b0.54 Ga. TTG references are given in Appendix A; Adakite references (and references cited
therein): Li and Li (2003), Percival et al. (2003), Kay et al. (1993), Martin (1999), Stern and Kilian (1996), Samaniego et al. (2002), Rapp et al.
(1999), Polat and Kerrich (2002), Drummond and Defant (1990), Kay and Kay (2002), Peacock et al. (1994), Smithies and Champion (2000),
Bourdon et al. (2002), Drummond et al. (1996), Defant and Drummond (1993), Kepezhinskas et al. (1997), Myers et al. (1985), Yogodzinski et
al. (1995), Xu et al. (2002), Gutscher et al. (2000).
however, Archean TTGs are more depleted in Y and
heavy REE and have higher K2O/Na2O ratios than
post-Archean TTGs (Table 1). In terms of tectonic
setting, all young TTGs share in common an arcrelated setting, often, but not always a continentalmargin arc. Archean TTGs usually are intruded into
36
K.C. Condie / Lithos 80 (2005) 33–44
greenstone belts with arc or oceanic plateau geochemical affinities. Until recently, adakites were
described only from Phanerozoic successions, but
they are now recognized in the Proterozoic and a few
rare occurrences in Archean terranes (Polat and
Kerrich, 2002). As with TTGs, adakites are found in
arc-related tectonic settings, both oceanic arcs and
continental-margin arcs. Unlike TTGs, Archean adakites are not found associated with greenstones with
oceanic plateau geochemical affinities.
Data shown on the graphs in this study are mean
values of TTGs from different geographic locations
and different ages (Appendix A). Over 150 sites and
nearly 2500 samples are represented in the database,
although not all trace elements are available for each
site. In evaluating similarities and differences, not
only mean values, but also median values and ranges
for each site are considered.
3. Adakites and TTGs
3.1. Andean examples
Perhaps the best described comparison of adakites
and TTGs is from the southern Andes (Kay et al.,
1993, 1999; Stern and Kilian, 1996; Petford and
Atherton, 1996). Well-documented adakites are associated with the Chilean triple junction where the Chile
rise is being subducted beneath the Andes in
Fig. 2. Sr vs. Na2O+CaO graph showing the distribution of TTGs in
comparison to the adakite field. Data sources given in Table 1 and
Appendix A. Andean adakites from Kay et al. (1993).
Fig. 3. Mg number vs. SiO2 showing a comparison of Andean TTGs
and adakites. Data sources given in Table 1 and Appendix A.
Experimental liquids from data published in Rapp et al. (1991),
Wylie et al. (1997) and Winther (1996). AFC, assimilation fractional
crystallization trajectory from Stern and Kilian (1996); percents are
amounts of assimilated ultramafic rock.
Patagonia (Kay et al., 1993). The Andean adakites
were erupted 12 Ma and appear to represent melts
derived from the hot descending plate. These adakitic
magmas share many geochemical features with the
type adakites from Adak Island in the Aleutians, such
as similar incompatible element distributions and
similar high Sr contents (Figs. 1 and 2). Another
feature of the Andean adakites is relatively high Mg
number (50–70) as well as high Ni (N40 ppm), Cr
(N80 ppm), and Sr (1800 ppm; Figs. 2–4), all of which
are generally interpreted to reflect reaction of slab
melts with overlying ultramafic rocks of the mantle
wedge. The AFC line in Fig. 3 is one possible
trajectory of assimilation-fractional crystallization of
ultramafic rocks in the mantle wedge.
In contrast to the Patagonia Andean adakites, a
second group of felsic volcanic and related plutonic
rocks is found in the Central Volcanic Zone (CVZ) of
the Andes. These rocks, which have similar incompatible element distributions to the Patagonian adakites (Fig. 1), have relatively low Mg numbers (b55)
and also relatively low Sr (300 ppm), Ni (5 ppm), and
Cr (50 ppm; Kay et al., 1999). Kay et al. (1999)
proposed a model for CVZ volcanic rocks involving
thickening of the crust due to tectonic shortening and
lithosphere delamination. The rapidly delaminated
lithosphere was replaced by hot asthenosphere, which
partially melted producing basaltic magmas. These
K.C. Condie / Lithos 80 (2005) 33–44
Fig. 4. Mg number vs. SiO2 showing the distribution of TTGs in
comparison to the adakite field. Data sources given in Table 1 and
Appendix A. Experimental liquid field references given in Fig. 3.
magmas, in turn, heated the thickened lower crust to
produce TTG magmas, leaving eclogitic residues in
the lower crust. Unlike the adakitic magmas, the CVZ
volcanic rocks are similar in composition to experimental melts of wet mafic sources (Fig. 3). The low
Mg numbers, Ni, and Cr in these rocks mean that
unlike the Patagonian adakites, the CVZ magmas did
not react with the mantle wedge. Employing the
definitions used in this study, the CVZ volcanics are
classic TTGs.
Hence, it would appear that, in the Andes, both
adakites and TTGs were produced during the Tertiary
depending on whether a hot descending slab or a
thickened/delaminated lower crust was melted. The
key differences in the derivative magmas are not the
incompatible element distributions but compatible
elements such Mg, Sr, Ni, and Cr.
37
reacted with the mantle wedge. Although there is
more overlap between TTGs and adakites in terms of
Cr and Ni distributions (Fig. 5), a large number of the
TTGs, regardless of age, have average Ni and Cr
contents less than most adakites (Appendix A). Sr is a
compatible element in plagioclase, and hence, the Sr
distribution in adakite and TTG magmas reflects, at
least in part, the role of plagioclase fractionation
(Ellam and Hawkesworth, 1988; Tarney and Jones,
1994; Martin and Moyen, 2002). The partitioning of
Sr into the melt is also related to the An content of the
plagioclase (Foley et al., 2002). Most adakites have
higher Sr and Na2O+CaO values than most TTGs,
although there is minor overlap in Sr contents (Fig. 2).
TTGs with b500 ppm Sr probably reflect either or
both plagioclase in the restite or plagioclase fractional
crystallization. Calcalkaline TTGs (low-Al TTGs)
contain even less Sr (b300 ppm), a feature commonly
ascribed to plagioclase fractional crystallization.
As described by numerous investigators and
especially by Martin (1993, 1999), Archean high-Al
TTGs and adakites both have steep REE patterns,
resulting in relatively high La/Yb ratios at low Yb
values (Fig. 6; Appendix A). In contrast, most postArchean TTGs belong to the calcalkaline suite and
often show a fractional crystallization sequence
extending from diorite (or gabbro) to granite. These
rocks typically have very low La/Yb ratios and exhibit
a large range of Yb values (Fig. 6). The reason that
3.2. Some geochemical contrasts
For comparative purposes, TTGs are grouped into
five age groups: z3.5, 3.5–2.5, 2.7, 2.5–0.54, and
b0.54 Ga. On a Mg#–SiO2 graph, the majority of
TTGs fall in the hydrous experimental liquid field
with only a small number plotting in the adakite field
(Fig. 4). Most TTGs have lower Mg numbers than
adakites, a feature recently emphasized by Smithies
(2000). Again, this suggests that most TTGs, although
partial melts of hydrous mafic sources, have not
Fig. 5. Ni vs. Cr graph showing the distribution of TTGs in
comparison to the adakite field. Data sources given in Table 1 and
Appendix A.
38
K.C. Condie / Lithos 80 (2005) 33–44
Fig. 6. (La/Yb)n vs. Ybn graph showing distribution of adakites and TTGs. Modified after Martin (1993). Post-Archean TTG=calcalkaline
plutonic suites. n—normalized to primitive mantle using values from Sun and McDonough (1989).
adakites and high-Al TTGs are indistinguishable on
the La/Yb–Yb plot is that reaction with the mantle
wedge does not appreciably affect REE distributions
or incompatible element ratios (Rapp et al., 1999). In
both cases, the highest La/Yb ratios require that garnet
occur in the restite (Martin, 1993).
An important incompatible element ratio that
distinguishes adakite from high-Al TTG is the Nb/
Ta ratio (Kamber et al., 2002). In making this contrast,
care must be exercised in using only high precision
analyses of Nb and Ta, and in this respect, most Nb
analyses by XRF must be rejected. Using only high
precision ICPMS Nb and Ta data, TTGs typically
have Nb/Ta ratios less than the primitive mantle value
of 16.7 and sometimes much lower (with averages
ranging down to about 5; Fig. 7; Appendix A). In
contrast, adakites have Nb/Ta ratios near the primitive
mantle value (generally between 15 and 20). In
addition, TTGs generally show Zr/Sm ratios greater
than the primitive mantle value of 25.2 while adakites
scatter on both sides of this value. Because rutile is
probably not a residual phase during melting of
eclogite (Rapp et al., 1991), eclogite-derived TTGs
will have high Nb/Ta ratios, reflecting the high ratios
in melted rutile (Foley et al., 2002; Klemme et al.,
2002). For this reason, it is unlikely that TTGs are
derived from the partial melting of rutile-bearing
eclogites.
4. Discussion
4.1. Fractional crystallization or partial melting?
Fig. 7. Nb/Ta vs. Zr/Sm graph showing the distribution of TTGs in
comparison to the adakite field. Lines represent primitive mantle
values from Sun and McDonough (1989) and melting fields from
Foley et al. (2002).
Is it possible that adakites and TTGs are both slab
melts that reacted with the mantle wedge but that
TTGs later underwent fractional crystallization, which
reduced their compatible element contents? For
K.C. Condie / Lithos 80 (2005) 33–44
instance, partial melting of Sr-rich eclogite in a
descending slab produces adakitic magmas with high
Sr contents (as there is no plagioclase in the restite)
and steep REE patterns (reflecting restite garnet or/
and amphibole). Fractional crystallization of these
magmas at shallower depths could reduce the Sr
contents by plagioclase removal and reduce the Mg
number and Cr and Ni contents by amphibole/biotite
removal without appreciably affecting the REE
patterns (Kamber et al., 2002; Samaniego et al.,
2002). Amphibole removal during fractional crystallization could also lower the Nb/Ta ratios, as observed
in TTGs relative to adakites (Fig. 7).
However, there are at least three lines of evidence
that suggest that TTGs are not simply the fractional
crystallization products of adakitic magmas. As
pointed out by several investigators, the apparent
absence of thick sequences of cumulates does not
favor a fractional crystallization connection (Smithies,
2000). To produce felsic TTGs from intermediate to
mafic adakites requires more than 50% fractional
crystallization, leaving behind large volumes of
cumulates of mafic to intermediate composition.
Where are these cumulates? This is especially a
problem in the Archean crust where large volumes
of preserved TTGs demand enormous volumes of
early cumulates. One cannot hide these in the deep
crust because there are many areas of deep Archean
crust exposed at the surface today, such as those in
southern India and Antarctica (Condie and Allen,
1984; Black et al., 1986; Sheraton et al., 1987). Even
in these cratons with Archean high-grade rocks
exposed at the surface, few cumulates of mafic to
intermediate composition are found.
Another argument against a fractional crystallization connection between adakites and TTGs is the
similarity in composition of TTGs to that of experimental melts, as revealed by numerous melting
studies of amphibolites and eclogites (Fig. 3; Rapp
et al., 1991; Winther, 1996; Wylie et al., 1997;
Prouteau et al., 2001). Is it coincidental that so many
TTGs fall in the experimental melt field if they are the
products of extensive fractional crystallization?
Also not favoring a fractional crystallization
connection between adakites and TTGs is the fact
that trace element distributions define trajectories
similar to calculated batch melting trajectories and
not to shallow fractional crystallization trajectories.
39
This is especially apparent on the (La/Yb)n–Ybn graph
(Martin, 1993; Samaniego et al., 2002). Although
amphibole fractional crystallization produces residual
melts that follow batch-melting trends, it cannot
produce the high La/Yb values characteristic of many
Archean TTGs.
4.2. P–T regimes
Are P–T regimes the same for production of
adakite and TTG magmas? Numerous experimental
studies provide a robust framework upon which the
pressures and temperatures of melting of these
magmas can be estimated (references above). Experimental results agree that both adakites and TTGs are
produced from partial melting of a wet mafic
protolith. Although experiments have covered a wide
range of water contents, only those performed at
relatively high water contents (z5%) produce felsic
magmas similar to TTG at relatively low melting
temperatures, leaving amphibole in the restite (Rapp
et al., 1991; Prouteau et al., 2001). Also, because
amphibole has a Kd for Ti (~3) greater than
neighboring elements Eu and Gd (~1.6) on an
incompatible element diagram, restite amphibole can
explain the ubiquitous negative Ti anomaly in TTGs
(Fig. 1). The high La/Yb and Sr/Y ratios in both
adakite and TTG magmas seem to require that garnet
also be present in the restite. In the case of TTG
magmas, the low Nb/Ta ratios indicate the presence of
a low-Mg amphibole in the restite, such that Nb is
retained in the restite compared to Ta (Foley et al.,
2002). In contrast, adakitic magmas, with their
comparatively high Nb/Ta ratios, cannot have lowMg amphibole in the restite (Appendix A; Fig. 7).
These results also show that rutile-bearing eclogites
cannot serve as sources for most TTG magmas, as
partial melting of such eclogites produces melts with
high Nb/Ta ratios (Foley et al., 2002).
So what P–T regimes are allowed for production
of TTG magmas? To have both low-Mg amphibole
and garnet in the restite but little, if any, plagioclase
requires melting at depths N 40 km but generally
less than about 80 km over a temperature range of
700–800 8C (Martin, 1999). This is in the stability field
of hornblende eclogite rather than garnet amphibolite,
which by definition contains significant amounts of
plagioclase.
40
K.C. Condie / Lithos 80 (2005) 33–44
4.3. Tectonic setting
If adakitic magmas have reacted with mantle
wedges, they reflect not only an arc setting but the
subduction of young relatively hot slabs such as the
Chile rise beneath the southern Andes. Martin (1993,
1999) and Foley et al. (2003) have suggested similar
tectonic scenarios for TTGs in the Archean. However,
if the Andes Central Volcanic Zone is a better
analogue for Archean TTG, perhaps it is the Archean
lower crust that is melting rather than the descending
slab (for an Archean example, see Whalen et al.,
2002). As pointed out by Andean investigators, this
crust may be thickened by magmatic underplating of
basaltic magmas coming from the mantle wedge or by
tectonic shortening in the overriding plate along a
convergent plate boundary (Petford and Atherton,
1996; Kay et al., 1999). Subduction erosion, which
brings mafic lower crust down the subducting plate to
higher pressure and temperature, can lead to partial
melting. Of course, this model requires a considerable
amount of water in Archean subduction zones to
obtain the necessary hydrous melting. If plates were
recycled faster in the Archean due to hotter mantle,
perhaps water was also recycled faster in subduction
zones, providing a rich supply at Archean convergent
margins. Regardless of the specific tectonic scenario,
it is the lower crust of the arc that is melting to
produce the Andean TTG and not the descending slab.
If the high Mg, Cr, and Ni in adakites require reaction
with the mantle wedge, the low contents of these
elements in TTGs mean a lack of such reaction and
does not favor a slab origin for TTGs (Smithies,
2000). The fact that true adakites are rare in the
Archean may be due to comparatively hot subducted
plates, which ride high displacing all or most of the
mantle wedge. Hence, there is little, if any, opportunity for Archean slab melts to react with the mantle
wedge.
Are there other acceptable tectonic settings for
TTGs and especially for Archean TTGs when plate
tectonics may not have quite the same as it is today?
Perhaps the root zones of Archean oceanic plateaus
were buried deep enough so the mafic component
underwent partial melting. Such a model was suggested as a possible origin for the Aruba batholith in
the Caribbean oceanic plateau during the Cretaceous
(White et al., 1999). Of course, these deep roots must
be hydrated to form amphibole, a requirement that
would seem probable in the Archean mantle, as
previously discussed. Hydrothermal alteration of
plateau basalts followed by deep burial would also
introduce water into the system. In fact, any tectonic
setting in which hornblende eclogite is stable and the
temperature reaches 700–800 8C could produce TTG
magmas.
At this point, it is important to emphasize a fact
that is commonly overlooked in using geochemical
data to assign tectonic settings; that is, geochemical
data can be used to help constrain tectonic settings,
but it cannot be used alone to reconstruct ancient
tectonic settings.
4.4. Secular changes in TTGs
Martin and Moyen (2002) have recently suggested
secular changes in the composition of TTGs and that
these changes, if real, can be used to constrain
evolving tectonic settings from the Early to the Late
Archean. To overcome the problem of fractional
crystallization, the authors suggested that the upper
limit in the range of variation in compatible elements
(Mg, Sr, Cr, Ni) be used as most representative of
parental magma compositions. From their study, they
concluded that, between 4 and 2.5 Ga, the Mg
number, Sr, Ni, and Cr contents of TTGs increased,
reflecting the cooling of the mantle and increased
interactions of TTG magmas (generated in descending
slabs) with the mantle wedge.
To explore this idea further, both the mean value
and the maximum values of TTGs from the database
(Appendix A) for Mg number, Ni, Sr, and (La/Yb)n
are plotted as a function of age from 4 Ga to the
present (Figs. 8–11). Unlike the conclusions of Martin
and Moyen (2002), the Mg number shows no
evidence of a trend with time either between 4 and
2.5 Ga or afterwards (Fig. 8; Appendix A). This
observation is valid for both the mean and maximum
Mg number. The TTG Ni distribution with age
suggests a maximum around 2.7 Ga with decreasing
Ni values from 2.7 Ga to the present (Fig. 9;
Appendix A). As pointed out by Martin and Moyen
(2002), there is also a suggestion of increasing Ni
between 4 and 2.7 Ga. However, this change may
reflect a rather sudden increase in Ni around 2.7 Ga
rather than a gradual increase from 4 to 2.7 Ga.
K.C. Condie / Lithos 80 (2005) 33–44
41
If these secular trends are real, they may be
important in tracking the cooling history of the Earth
and in tracking changing tectonic regimes with time.
The apparent maxima in Sr, Ni, and (La/Yb)n at 2.7
Ga and possibly at 1.9 Ga may record catastrophic
mantle overturn events, as suggested by Condie
(1998). Increased mantle plume activity at these times
could lead to increased production rates of oceanic
plateaus with thick mafic roots. These mafic roots
may have been buried deep enough so that plagioclase
was not stable and garnet was present throughout.
Partial melting of such sources would produce TTG
magmas with higher than normal La/Yb and Sr/Y
ratios due to widespread restitic garnet. Although
smaller degrees of melting could also yield higher La/
Yb ratios and lower Yb contents, this is unlikely in the
Archean when the mantle was hotter by a factor of 3
or 4. It is also possible that TTG magmas may have
been contaminated with komatiites or ultramafic
Fig. 8. Mg number in TTGs vs. age. (a) Mean values with one
standard deviation, and (b) maximum values. Data sources given in
Table 1 and Appendix A.
Although not shown, the same patterns are evident for
Cr. Sr shows an irregular pattern with time (Fig. 10;
Appendix A). In both the mean and maximum value
plots, there is a suggestion of a maximum around 2.7
Ga and perhaps again at about 1.9 Ga. Although Sr
may increase from 4 to 2.7 Ga, as pointed out by
Martin and Moyen, the data in Fig. 10 are also
consistent with little change between 4 and 3 Ga and a
relatively sudden increase around 2.7 Ga. These
results also suggest that Sr in TTGs decreases after
1.9 Ga, reaching a minimum around 400–800 Ma and
then shows an upward swing in the last 100 Ma. The
time distribution of (La/Yb)n (Fig. 11) and Sr/Y (Table
1) shows patterns much like those of Sr and Ni, with
possible maxima around 2.7 and 1.9 Ga. It is also
notable that the K2O/Na2O ratio remains approximately constant in TTGs with time (at about 0.5)
until the Phanerozoic when it increases (Table 1).
Fig. 9. Ni concentration in TTGs vs. age. (a) Mean values with one
standard deviation and (b) maximum values. Data sources given in
Table 1 and Appendix A.
42
K.C. Condie / Lithos 80 (2005) 33–44
(2)
(3)
(4)
the same rocks. Although they both show
strongly fractionated REE patterns, TTGs have
lower Sr, Mg, Ni, Cr, and Nb/Ta than most
adakites. TTGs and adakites cannot be easily
related by shallow fractional crystallization.
While adakites are probably slab melts, high-Al
TTGs may be produced by partial melting of the
lower crust in arc systems or in the root zones of
oceanic plateaus.
Depletion in heavy REE and low Nb/Ta ratios in
high-Al TTGs requires both garnet and low-Mg
amphibole in the restite. Moderate to high Sr
values allow little, if any, plagioclase in the
restite. This requires melting in the hornblende
eclogite stability field between 40- and 80-km
deep and between 700 and 800 8C.
If peaks in La/Yb, Sr, Cr, and Ni at 2.7 Ga and
perhaps also at 1.9 Ga in high-Al TTG are real,
Fig. 10. Sr concentration in TTGs vs. age. (a) Mean values with one
standard deviation and (b) maximum values. Data sources given in
Table 1 and Appendix A.
cumulates in the deep roots of oceanic plateaus,
accounting the high Ni and Cr in TTGs at 2.7 Ga (Fig.
9). Alternatively, the high Ni and Cr could reflect a
more badakiticQ character of some 2.7 Ga magmas,
and if so, this could mean more widespread melting of
descending plates at this time. A similar scenario may
be recorded at about 1.9 Ga. The fall in La/Yb, Sr, Ni,
and Cr after 2 Ga may reflect cooling of the mantle
and lower crust, decreasing the garnet/plagioclase
ratio in the restite (thus lowering TTG Sr and La/Yb
levels in derivative TTG melts). If real, an increase in
La/Yb and Sr in TTGs in the last 100 Ma is
problematic.
5. Conclusions
(1)
For the most part, high-Al TTG (tonalite–
trondhjemite–granodiorite) and adakite are not
Fig. 11. (La/Yb)n ratio in TTGs vs. age. (a) Mean values with one
standard deviation and (b) maximum values. Data sources given in
Table 1 and Appendix A. n—normalized to primitive mantle using
values from Sun and McDonough (1989).
K.C. Condie / Lithos 80 (2005) 33–44
they may reflect catastrophic mantle overturn
events at these times.
Acknowledgments
This paper was substantially improved by in-depth
reviews by Stephen Foley, Herve Martin, John Tarney,
and Hugh Rollinson. It should be pointed out,
however, that some of the reviewers do not agree
with the geochemical distinctions between TTG and
adakite as proposed in the paper. I am happy to
dedicate the paper to Professor Ilmari Haapala on the
occasion of his retirement from the University of
Helsinki.
Appendix A. Supplementary data
Supplementary data associated with this article can
be found, in the online version, at doi:10.1016/
j.lithos.2003.11.001.
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