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Did the character of subduction change at the end of the Archean?
Constraints from convergent-margin granitoids
Kent C. Condie
Department of Earth and Environmental Science, New Mexico Institute of Mining and Technology, Socorro, New Mexico 87801, USA
ABSTRACT
Large ion lithophile and high fi eld strength element distributions in juvenile upper
continental crust are controlled chiefl y by the abundance of tonalite-trondhjemite-
granodiorite (TTG) in the Archean shifting to a combination of TTG, calc-alkaline
granitoid, and graywacke control thereafter. Geochemical differences between TTG and
high-silica adakites do not require production of most TTG magmas in descending slabs.
Changes in the ratio of TTG to calc-alkaline granitoids after 2.5 Ga indicate that Archean
subduction zones must have differed from younger subduction zones in two very impor-
tant ways: (1) a deep mafi c crust served as a TTG magma source (either as thickened
crust or in descending slabs), and (2) they did not give rise to signifi cant volumes of calc-
alkaline magma. Thickened mafi c crust in the Late Archean may have resulted from plate
jams in subduction zones caused by thicker oceanic crust and oceanic plateaus produced
during Late Archean mantle thermal events.
Keywords
: continental crust, tonalite-trondhjemite-granodiorite, TTG, subduction, late Archean.
INTRODUCTION
Although chemical changes in the composition of juvenile con-
tinental crust were proposed in the 1970s, it was not until the 1980s
or later that most of these changes became widely recognized (Ronov,
1972; Engel et al., 1974; Taylor and McLennan, 1981, 1985; Condie,
1993). Compositional differences between Archean and later fi ne-grained
detrital sediments are also well documented, although their interpreta-
tion is still controversial because of possible tectonic setting biases and
recycling effects (McLennan et al., 2006). Increases in large ion litho-
phile elements (LILE) and changes in related element ratios (such as
K
2
O/Na
2
O, La/Yb, and Sr/Y) in juvenile post-Archean upper continen-
tal crust are now reasonably well established, although their signifi cance
in terms evolving tectonic regimes on Earth is a matter of discussion
(Taylor and McLennan, 1985; Gibbs et al., 1986; Condie, 1993; Rudnick
and Fountain, 1995; McLennan et al., 2006). Because the composition
of new upper continental crust is largely controlled by the composition of
convergent-margin granitoids (Condie, 1993), the relative volume
and origin of the tonalite-trondhjemite-granodiorite (TTG) suite are
critical to understanding changing crustal composition at the Archean-
Proterozoic boundary. Although the importance of TTG production dur-
ing the Archean is well known, it is now recognized that TTGs have been
produced at subduction zones from the Archean onward (Condie, 1993,
2005; Smithies, 2000; Kay et al., 2005). However, the question of whether
Archean TTGs were produced in a different tectonic setting (such as
melting of the roots of oceanic plateaus) from later TTGs is still debated
(Kamber et al., 2002; Martin et al., 2005; Condie, 2005; Nair and Chacko,
2005; Bedard, 2006; Foley, 2008).
Results of this study suggest that geochemical differences between
TTG and high-silica adakites do not require that all TTG magmas be pro-
duced in descending slabs, as suggested by Martin (1999). Increases in
LILE and high fi eld strength elements (HFSE) in post-Archean juvenile
upper continental crust refl ect chiefl y an increase in the ratio of calc-
alkaline to TTG magma production. This, in turn, implies that Archean
subduction zones differed from later subduction zones by producing thick
mafi c lower crust as a source for many TTG magmas, and by not produc-
ing signifi cant volumes of calc-alkaline magma.
TABLE 1. ESTIMATES OF TONALITE-TRONDHJEMITE-GRANODIORITE/
CALC-ALKALINE RATIO WITH TIME
>2800
Age
(Ma)
2800–2500 2500–1000
<1000
Map TTG/CA
8 (4–10)
15
0.10
0.37
By location
TTG
28
25
12
18
CA
5
2
13
26
TTG/CA
5.6
12.5
0.92
0.70
By number of samples
TTG
386
328
138
171
CA
169
20
242
374
TTG/CA
2.7
9.4
0.54
0.65
TTG/CAassumed
5
14
0.5
0.6
Note
: TTG—tonalite-trondhjemite-granodiorite suite; CA— calc-alkaline.
NEW AVERAGES
Table 1 shows three estimates of the ratio of TTG to calc-alkaline
granitoids (TTG/CA ratio) in four time windows. The map-scaling
estimate was described in Condie (1993), where the calc-alkaline suite
was not distinguished from the TTG suite. TTG/CA ratios in Table 1
are also given by number of locations in which each is reported and by
the total number of samples chemically analyzed. Although the three
estimates for a given age window can vary by as much as a factor
of 10, most are within a factor of 2. TTG/CA ratios used in weight-
ing each rock type for the new average upper crustal compositions are
given in Table 1.
Average chemical compositions of TTG and calc-alkaline plutonic
suites are given in GSA Data Repository Tables DR1 and DR2
1
, together
with median values and one standard deviation of the mean. The distinc-
tion between calc-alkaline and TTG suites is based on major element
trends such as those on the K-Na-Ca graph (Martin, 1994), various silica
variation diagrams, Sr content, Eu anomaly (Eu/Eu*) distribution, and low
1
GSA Data Repository item 2008148, Tables DR1–DR4 and Figures
DR1–DR4, geochemical data and additional fi gures, is available online at www.
geosociety.org/pubs/ft2008.htm, or on request from editing@geosociety.org or
Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA.
© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or editing@geosociety.org.
Geology
, August 2008; v. 36; no. 8; p. 611–614; doi: 10.1130/G24793A.1; 2 fi gures; 1 table; Data Repository item 2008148.
611
GEOLOGY, August 2008
2
A
TTG
Calc-Alkaline
Average UC
contents of heavy rare earth elements and Y. Because of restite plagioclase
or plagioclase fractional crystallization, calc-alkaline granitoids have
distinctly low Sr contents and signifi cant negative Eu anomalies, very dis-
tinct from TTGs (Martin, 1994; Martin et al., 2005). Sanukitoids are not
included in the database; they were eliminated using the geochemical fi lter
suggested by Martin et al. (2005).
Revised estimates of upper continental crustal composition (from
Condie, 1993) are given in Tables DR3 and DR4. Also given are rock
weighting factors and average compositions of each rock type. Other
than the new TTG and calc-alkaline categories, most of the weight-
ing factors and average rock compositions are similar to those given
in Condie (1993). Methods and error estimates were also discussed in
Condie (1993).
1.5
1
0.5
0
0
500 1000 1500 2000 2500 3000 3500 4000
RESULTS
Although there are minor within group secular changes in the aver-
age composition of TTGs and calc-alkaline granitoids (Tables DR1 and
DR2), almost all of these are within one standard deviation of the grand
mean values. Such changes do not contribute signifi cantly to the changes
in the composition of upper continental crust at the end of the Archean. As
shown in Tables DR3 and DR4, LILE and HFSE (Nb, Ta, Zr, Hf) distribu-
tions in the upper continental crust are controlled chiefl y by TTG in the
Archean, shifting to a combination of TTG, calc-alkaline granitoid, and
graywacke thereafter. Except for Sr and trace metals, average graywacke
is similar to average TTG for most elements. Signifi cant increases in
LILE and HFSE and a decrease in Sr between the 2.8–2.5 Ga and the
2.5–1.0 Ga time windows refl ect, in large part, a shift from TTG control
to calc-alkaline control. This is also evident in key element ratios such
as K
2
O/Na
2
O, (La/Yb)
n
, Sr/Y, Th/U, Eu/Eu*, and Nb/Ta (Fig. 1; Tables
DR3 and DR4; Figs. DR1–DR4). An increase in K
2
O/Na
2
O ratio in upper
crust at the end of the Archean is clearly controlled by the distribution of
average compositions of TTG and calc-alkaline granitoids in Figure 1A
(K
2
O/Na
2
O > 0.5 for calc-alkaline suite and <1 for most TTGs). Likewise,
decreases in (La/Yb)
n
and Sr/Y ratios in early Proterozoic upper crust
refl ect an increasing calc-alkaline component (Figs. 1B, 1C). Although
both La and Yb contents control the La/Yb ratio, Yb increases faster than
La after 2.5 Ga (due to a drop in garnet content of TTG sources), thus
the La/Yb ratio decreases. Similarly, the large drop in Sr and Eu/Eu* in
the calc-alkaline component (due to restite plagioclase and/or fractional
crystallization) controls the decreases in upper crustal Sr/Y and Eu/Eu*
ratios with time (Fig. 1C; Tables DR1 and DR2; Fig. DR1). Th and U
both increase in the upper crust at the end of the Archean, but the Th/U
ratio remains constant or drops slightly (Fig. DR2). The Nb/Ta ratio in
upper continental crust remains roughly constant from the Early Archean
onward (Tables DR3 and DR4; Fig. DR3).
Age (Ma)
100
B
TTG
Calc-Alkaline
Average UC
10
0
500 1000 1500 2000 2500 3000 3500 4000
Age (Ma)
C
TTG
Calc-Alkaline
Average UC
100
10
0
500 1000 1500 2000 2500 3000 3500 4000
DISCUSSION
A major unresolved issue regarding compositional changes near the
end of the Archean is that of whether Archean TTGs are really high-silica
adakites, and thus require a magma source in descending slabs (Kamber
et al., 2002; Martin et al., 2005; Condie, 2005). Although Martin and
colleagues made a case for a TTG-adakite connection based on various
element distributions (Martin, 1999; Martin et al., 2005), key element
indicators such as Mg#, Ni, Cr, Nb/Ta, Pb/Nd, and B/Be do not favor
such a connection for many (or most) TTGs (Smithies, 2000; Kamber
et al., 2002; Condie, 2005; Foley, 2008). Although the database of Martin
and Moyen (2002) (with no rocks younger than 2.5 Ga) suggests MgO
contents of TTGs similar to high-silica adakites, my extensive database
(Tables DR1 and DR2) clearly shows that most TTGs have Mg numbers
(<50) less than high-silica adakites (also summarized in Condie, 2005,
Fig. 5 therein). As pointed out by Kamber et al. (2002), the Nb/Ta ratio of
adakites is ≥16, but only a few of the 38 TTG averages have ratios above
Age (Ma)
Figure 1. A: Secular distribution of K
2
O/Na
2
O in average convergent-
margin granitoids and in average upper continental crust (UC). Data
are averages from Tables DR1–DR4 (see footnote 1). Vertical bars are
uncertainty estimates expressed as one standard deviation of mean
values. B: Secular distribution of (La/Yb)
n
in average convergent-
margin granitoids and in average UC. Data are averages from Tables
DR1–DR4. Vertical bars are uncertainty estimates expressed as one
standard deviation of mean values; n—normalized to primitive mantle
(Sun and McDonough, 1989). C: Secular distribution of Sr/Y in aver-
age convergent-margin granitoids and in average upper continental
crust (UC). Data are averages from Tables DR1–DR4. Vertical bars are
uncertainty estimates expressed as one standard deviation of mean
values. TTG—tonalite-trondhjemite-granodiorite.
612
GEOLOGY, August 2008
16 (Tables DR1 and DR2; Figs. DR3 and DR4). Considering these key
element indicators, it appears that only a relatively small number of TTGs
of any age have similarities to high-silica adakites.
The results of this and previous studies clearly agree that Late
Archean TTGs must be derived from wet mafi c sources in which
amphibole ± garnet remains in the restite (Martin, 1994; Rapp, 1997;
Moyen and Stevens, 2006; Bedard, 2006). This means that the mafi c
source must be deeper than post-Archean mafi c sources, and the cur-
rent debate centers around what caused this deeper Archean source.
Martin (1986, 1994, 1999) has long championed descending slabs as
this source by analogy with adakites. However, as pointed out above,
key geochemical indicators do not favor an adakite connection for many
TTGs, regardless of age. An equally viable possibility is that the thick-
ened mafi c source is produced by “plate jams” in Archean subduction
zones, during which mafi c crust is stacked up and thickened and the root
zones undergo partial melting to produce TTG magmas (Davies, 1992;
van Hunen et al., 2000; Nair and Chacko, 2005; Clemens et al., 2006).
Because this occurred during one or two Late Archean global mantle
thermal events (mantle plume events?), the upper mantle is hotter and
oceanic plates are more buoyant and tend to resist deep subduction. Parts
of oceanic plateaus may also be added to the accretionary stacks at con-
vergent margins, thickening the mafi c crust even more. In fact, Bedard
(2006) proposed a model whereby the thickened root zones of oceanic
plateaus may give rise to TTG magmas without subduction. Locally
preserved eclogite–high-pressure granulite metamorphic assemblages in
Late Archean greenstones (Brown, 2007) support the existence of thick
mafi c crust (>50 km) in Late Archean arc systems.
Although not part of this study, sanukitoids (and Closepet-type
granites) become increasingly abundant in the latest Archean (2.6–2.5 Ga).
As pointed out by Martin et al. (2005), this may result from cooling of the
mantle such that slab-derived felsic melts were completely consumed by
reactions in the mantle wedge, followed by partial melting of the wedge
to produce sanukitoid magmas. This suggests that the change from TTG
to calc-alkaline magmatism after 2.5 Ga may have involved a transition
period during which sanukitoid magma production was widespread.
The next question is why the depth of mafi c sources at convergent
margins decreased after the Archean. The model of Martin (1994, 1999) is
still most effective, i.e., decreasing thermal gradients in subduction zones
leads to steeper subduction and thus to a greater volume of mantle wedge
(Peacock, 2003). Descending plates devolatilize and melting shifts to the
mantle wedge where calc-alkaline magmas are produced. Felsic calc-
alkaline components may form by either fractional crystallization or par-
tial melting of mafi c components underplated in the lower crust at depths
<50 km. Consistent with this idea is the fact that post-Archean continental
crust typically has a seismically high velocity layer in the lower crust,
which could represent underplated mafi c crust (Durrheim and Mooney,
1991). A major unresolved question with this model is how to cool sub-
duction zones so rapidly after 2.5 Ga.
Another important constraint is that mantle wedges must be
enriched in LILE before they can yield calc-alkaline magmas. Experi-
mental studies clearly indicate that calc-alkaline basaltic magmas can-
not be produced from depleted mantle sources (Sisson et al., 2005).
Two factors may contribute to enriching post-Archean mantle wedges
in LILE. Devolatilization of downgoing slabs alone may be suffi cient
to carry LILE from descending slabs into the mantle wedge. However,
subduction of sediments derived from newly formed cratons in the Late
Archean also may contribute to the LILE inventory that eventually ends
up in mantle wedges.
Using the constraints discussed in this study, a diagrammatic sum-
mary of a possible tectonic-thermal history of Earth between 3 and 2 Ga is
given in Figure 2. The model assumes that subduction-related granitoids
during the Late Archean were derived from oceanic crustal sources (includ-
LILE enrichment
in continental crust
Depth of granitoid
sources
Continental crust
production rate
Subduction
distribution
Late Archean
mantle thermal
events
2000
2500
2700
3000
Age (Ma)
Figure 2. Schematic representation of possible tectonic-thermal his-
tory on Earth between 3 and 2 Ga (based on data in Davies, 1995;
Condie, 1998; Condie and Benn, 2006; Condie and Kroner, 2008).
LILE—large ion lithophile elements.
ing oceanic plateaus), whereas most post-Archean examples come from
mafi c underplates, which, in turn, come from mantle wedges. Although
plate tectonics began at least locally by 3 Ga and became widespread in
the Late Archean (Abbott et al., 1994; Condie and Benn, 2006; Condie
and Kroner, 2008), modern-style subduction zones did not become wide-
spread until ca. 2.2 Ga. Two major pulses of thermal activity in the upper
mantle ca. 2.7 and 2.5 Ga enhanced subduction rates and rates of produc-
tion of both continental and oceanic crust. Because of extensive plate jams
and accretion of oceanic plateaus in Late Archean subduction zones, mafi c
crust was greatly thickened and underwent partial melting to produce
TTG magma suites. Although LILE enrichment began at this time due to
delamination and devolatilization of mafi c components, widespread LILE
enrichment did not begin until the Early Proterozoic, when modern style
mantle wedges came into existence.
CONCLUSIONS
1. Compared to high-silica adakites, many Late Archean TTGs are
not enriched in Mg, Cr, or Ni, and have low Nb/Ta ratios, suggesting that
they are not derived from descending slabs.
2. Increases in LILE and HFSE, a decrease in Sr, and the appearance
of large negative Eu anomalies in juvenile upper continental crust after
the Archean refl ect chiefl y a decrease in TTG magma production and a
corresponding increase in calc-alkaline magma production at convergent
plate margins.
3. To accommodate these geochemical changes, Late Archean sub-
duction zones must have differed from younger subduction zones in two
very important ways: (1) a deep mafi c crust served as a TTG magma
source (either as thickened crust or in descending slabs), and (2) they did
not give rise to signifi cant volumes of calc-alkaline magma.
4. One way to thicken mafi c crust in the Late Archean is by plate jams
in subduction zones caused by thicker oceanic crust and oceanic plateaus
produced during Late Archean global thermal events. Modern subduction
zones became widespread in the Early Proterozoic as magma production
shifted to mantle wedges, where the calc-alkaline suite is produced. LILE
enrichment in mantle wedges refl ects some combination of devolatiliza-
tion of descending slabs and sediment subduction.
GEOLOGY, August 2008
613
ACKNOWLEDGMENTS
I appreciate in-depth reviews by Herve Martin, Bor-ming Jahn, and Ali Polat.
Martin, H., 1994, The Archean grey gneisses and the genesis of the continental
crust,
in
Condie, K.C., ed., Archean crustal evolution: Amsterdam, Elsevier,
p. 205–259.
Martin, H., 1999, Adakitic magmas: Modern analogues of Archean granitoids:
Lithos, v. 46, p. 411–429, doi: 10.1016/S0024–4937(98)00076–0.
Martin, H., and Moyen, J.-F., 2002, Secular changes in tonalite-trondhjemite-
granodiorite composition as marker of the progressive cooling of Earth:
Geology, v. 30, p. 319–322.
Martin, H., Smithies, R.H., Rapp, R., Moyen, J.-F., and Champion, D., 2005, An
overview of adakite, tonalite-trondhjemite-granodiorite (TTG), and sanuki-
toid: Relationships and some implications for crustal evolution: Lithos,
v. 79, p. 1–24, doi: 10.1016/j.lithos.2004.04.048.
McLennan, S.M., Taylor, S.R., and Hemming, S.R., 2006, Composition, differ-
entiation and evolution of continental crust: Constraints from sedimentary
rocks and heat fl ow,
in
Brown, M., and Rushmer, T., eds., Evolution and dif-
ferentiation of the continental crust: Cambridge, UK, Cambridge University
Press, p. 92–134.
Moyen, J.-F., and Stevens, G., 2006, Experimental constraints of TTG petro-
genesis: Implications for Archean geodynamics,
in
Benn, K., et al., eds.,
Archean geodynamics and environments: American Geophysical Union
Geophysical Monograph 164, p. 149–175.
Nair, R.K., and Chacko, T., 2005, Oceanic plateaus: Nuclei for Archean cratons:
Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 494.
Peacock, S.M., 2003, Thermal structure and metamorphic evolution of subduct-
ing slabs,
in
Eiler, J., ed., Inside the subduction factory: American Geo-
physical Union Geophysical Monograph 138, p. 7–22.
Rapp, R.P., 1997, Heterogeneous source regions for Archean granitoids: Experi-
mental and geochemical evidence,
in
de Wit, M.J., and Ashwal, L.D., eds.,
Greenstone belts: Oxford, Clarendon Press, p. 267–279.
Ronov, A.B., 1972, Evolution of rock composition and geochemical processes in
the sedimentary shell of the Earth: Sedimentology, v. 19, p. 157–172, doi:
10.1111/j.1365–3091.1972.tb00019.x.
Rudnick, R.L., and Fountain, D.M., 1995, Nature and composition of the con-
tinental crust: A lower crustal perspective: Reviews of Geophysics, v. 33,
p. 267–309, doi: 10.1029/95RG01302.
Sisson, T.W., Ratajeski, K., Hankins, W.B., and Glazner, A.F., 2005, Voluminous
granitic magmas from common basaltic sources: Contributions to Mineral-
ogy and Petrology, v. 148, p. 635–661, doi: 10.1007/s00410–004–0632–9.
Smithies, R.H., 2000, The Archean tonalite-trondhjemite-granodiorite (TTG)
series is not an analogue of Cenozoic adakite: Earth and Planetary Science
Letters, v. 182, p. 115–125, doi: 10.1016/S0012–821X(00)00236–3.
Sun, S.S., and McDonough, W.F., 1989, Chemical and isotopic systematics of
oceanic basalts: Implications for mantle composition and processes,
in
Saunders, A.S., and Norry, M.J., eds., Magmatism in ocean basins: Geo-
logical Society [London] Special Publication 42, p. 313–345.
Taylor, S.R., and McLennan, S.M., 1981, The composition and evolution of the
continental crust: Rare earth element evidence from sedimentary rocks:
Royal Society of London Philosophical Transactions, ser. A, v. 301,
p. 381–399.
Taylor, S.R., and McLennan, S.M., 1985, The continental crust: Its composition
and evolution: Oxford, Blackwell, 312 p.
van Hunen, J., van den Berg, A., and Vlaar, N.J., 2000, A thermal-mechanical
model of horizontal subduction below an overriding plate: Earth and
Planetary Science Letters, v. 182, p. 157–169, doi: 10.1016/S0012–821X
(00)00240–5.
REFERENCES CITED
Abbott, D., Drury, R., and Smith, W.H.F., 1994, Flat to steep transition in subduc-
tion style: Geology, v. 22, p. 937–940, doi: 10.1130/0091–7613(1994)022
<0937:FTSTIS>2.3.CO;2.
Bedard, J.H., 2006, A catalytic delamination-driven model for coupled genesis of
Archean crust and sub-continental lithospheric mantle: Geochimica et Cos-
mochimica Acta, v. 70, p. 1188–1214, doi: 10.1016/j.gca.2005.11.008.
Brown, M., 2007, Metamorphic conditions in orogenic belts: A record of secular
change: International Geology Review, v. 49, p. 193–234.
Clemens, J.D., Yearron, L.M., and Stevens, G., 2006, Barberton TTG magmas:
Geochemical and experimental constraints on source-rock petrology, pres-
sure of formation and tectonic setting: Precambrian Research, v. 151,
p. 53–78, doi: 10.1016/j.precamres.2006.08.001.
Condie, K.C., 1993, Chemical composition and evolution of the upper continen-
tal crust: Contrasting results from surface samples and shales: Chemical
Geology, v. 104, p. 1–37, doi: 10.1016/0009–2541(93)90140-E.
Condie, K.C., 1998, Episodic continental growth and supercontinents: A man-
tle avalanche connection?: Earth and Planetary Science Letters, v. 163,
p. 97–108.
Condie, K.C., 2005, TTGs and adakites: Are they both slab melts?: Lithos, v. 80,
p. 33–44, doi: 10.1016/j.lithos.2003.11.001.
Condie, K.C., and Benn, K., 2006, Archean geodynamics: Similar to or different
from modern geodynamics?,
in
Benn, K., et al., eds., Archean geodynamics
and environments: American Geophysical Union Geophysical Monograph
164, p. 47–59.
Condie, K.C., and Kroner, A., 2008, When did plate tectonics begin? Evidence
from the geologic record,
in
Condie, K.C., and Pease, V., eds., When did
plate tectonics begin on planet Earth?: Geological Society of America Spe-
cial Paper 440 (in press).
Davies, G.F., 1992, On the emergence of plate tectonics: Geology, v. 20,
p. 963–966, doi: 10.1130/0091–7613(1992)020<0963:OTEOPT>2.3.CO;2.
Davies, G.F., 1995, Punctuated tectonic evolution of the earth: Earth and Plan-
etary Science Letters, v. 136, p. 363–379,
Durrheim, R.J., and Mooney, W.D., 1991, Archean and Proterozoic crustal evolu-
tion: Evidence from crustal seismology: Geology, v. 19, p. 606–609, doi:
10.1130/0091–7613(1991)019<0606:AAPCEE>2.3.CO;2.
Engel, A.E.J., Itson, S.P., Engel, C.G., Stickney, D.M., and Cray, E.J., 1974,
Crustal evolution and global tectonics: A petrogenetic view: Geological
Society of America Bulletin, v. 85, p. 843–858, doi: 10.1130/0016–7606
(1974)85<843:CEAGTA>2.0.CO;2.
Foley, S., 2008, A trace element perspective on Archean crust formation and on
the presence or absence of Archean subduction,
in
Condie, K.C., and Pease,
V., eds., When did plate tectonics begin on planet Earth?: Geological Soci-
ety of America Special Paper 440 (in press).
Gibbs, A.K., Montgomery, C.W., O’Day, P.A., and Erslev, E.A., 1986, The Archean-
Proterozoic transition: Evidence from the geochemistry of metasedimen-
tary rocks of Guyana and Montana: Geochimica et Cosmochimica Acta,
v. 50, p. 2125–2141, doi: 10.1016/0016–7037(86)90067–0.
Kamber, B.S., Ewart, A., Collerson, K.D., Bruce, M.C., and McDonald, G.D.,
2002, Fluid-mobile trace element constraints on the role of slab melting and
implications for Archean crustal growth models: Contributions to Mineral-
ogy and Petrology, v. 144, p. 38–56.
Kay, S.M., Godoy, E., and Kurtz, A., 2005, Episodic arc migration, crustal thick-
ening, subduction erosion and magmatism in the south-central Andes: Geo-
logical Society of America Bulletin, v. 117, p. 67–88.
Martin, H., 1986, Effect of steeper Archean geothermal gradient on geochemis-
try of subduction-zone magmas: Geology, v. 14, p. 753–756, doi: 10.1130/
0091–7613(1986)14<753:EOSAGG>2.0.CO;2.
Manuscript received 18 January 2008
Revised manuscript received 9 April 2008
Manuscript accepted 11 April 2008
Printed in USA
614
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Literatura Dodatkowa.rar
(46104 KB)
VI Subduction.pdf
(5918 KB)
Tph09v468 Litosfera style extens.pdf
(4001 KB)
Thyrrenian.pdf
(5272 KB)
Th08v448 trzes model.pdf
(1699 KB)
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Literatura II blok
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