- Email: [email protected]
Origin and early development of the earth's crust
Precambrian Research, 11 (1980) 183--197 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
ORIGIN AND EARLY DEVELOPMENT
183
OF THE EARTH'S CRUST*
KENT C. CONDIE Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, NM 87801 (U.S.A.) (Received and accepted December 17, 1979)
ABSTRACT Condie, K.C., 1980. Origin and early development of the earth's crust. Precambrian Res., 11: 183--197. A three-stage model for crustal origin is proposed in which Stage I (4.5--4.0 Ga) is characterized by the production of a transient ultramafic crust rapidly displaced by a widespread mafic crust formed at divergent plate boundaries, consumed at sinks, and recycled through the mantle. During Stage II (4.0--3.5 Ha) a falling geothermal gradient results in the production of isolated andesitic arc systems from partial melting of descending mafic crust. Further decreases in temperature during this stage result in decreased melting and the production of tonalitic magmas which rise, overplating the andesitic crust which is partially melted to produce high-K granites. Sialic arcs grow by tonalitic plutonism and arc collisions into two supercontinents during Stage III (3.5-2.7 Ga). During this stage, greenstone belts form in continental-rift and marginal-basin environments. INTRODUCTION T h e origin a n d early d e v e l o p m e n t o f t h e e a r t h ' s c r u s t has a t t r a c t e d considerable a t t e n t i o n as e v i d e n c e d b y t h e n u m e r o u s p a p e r s w r i t t e n o n t h e subject. T h e p r o b l e m can be b r o k e n d o w n i n t o several s u b p r o b l e m s : (1) w h e n a n d b y w h a t p r o c e s s e s did t h e first c r u s t f o r m ; (2) was this c r u s t o f local or w o r l d w i d e e x t e n t ; (3) a t w h a t r a t e a n d b y w h i c h m e c h a n i s m s did t h e early c r u s t grow; (4) w h a t was t h e c o m p o s i t i o n o f t h e early crust; a n d (5) w h e r e a n d h o w did o c e a n i c a n d c o n t i n e n t a l crustal t y p e s b e c o m e clearly established. Models f o r t h e origin o f t h e e a r t h ' s c r u s t fall i n t o t h r e e b r o a d c a t e g o r i e s (Condie, 1 9 7 6 a ; L o w m a n , 1 9 7 6 ) : i n h o m o g e n e o u s a c c r e t i o n o f t h e earth, catastrophic models, and non-catastrophic models. The inhomogeneous a c c r e t i o n m o d e l f o r e a r t h f o r m a t i o n is d e s c r i b e d b y T u r e k i a n a n d Clark {1969) a n d Clark, et al. (1972). L a t e , l o w - t e m p e r a t u r e c o n d e n s a t i o n o f alkali a n d o t h e r volatile e l e m e n t s f r o m t h e solar n e b u l a f o r m a v e n e e r o v e r t h e o u t e r p a r t o f e a r t h w h i c h m a y evolve i n t o a crust. A g r e a t deal o f evidence, h o w e v e r , does not favor inhomogeneous accretion models for the formation of the p l a n e t s ( R i n g w o o d , 1 9 7 7 ) . C a t a s t r o p h i c m o d e l s f o r t h e origin o f t h e c r u s t *Paper presented at the Archaean Geochemistry Field Conference in August 1978.
184 call upon a catastrophic, one-time event such as meteorite (or asteroid) impact, core formation, or lunar capture to provide heat which partially melts the earth and the resulting magmas rise to the surface (-+ fractionation) and form a crust (Salisbury and Ronca, 1966; Condie, 1976a; Frey, 1977). Existing data related to impact on the moon, however, indicate that the impact events which formed the lunar maria did n o t produce the basalts which filled the maria. These basalts were formed at ~> 108 years after maria basin formation (Taylor, 1975). Hence, it would appear that impact energy sources may n o t be adequate to produce the earth's early crust. Frey (1977)~ however, has suggested that a thinner lithosphere and higher thermal gradient in the earth compared to the m o o n could result in rapid, impact-triggered basaltic magmatism on the earth. Non-catastrophic models for crustal formation call u p o n more gradual heating of the earth to produce magmas which form the early crust. The problem of where the first crust formed may not be soluble in that the first crust was very likely unstable and sank into the mantle. The oldest preserved sialic crust is a b o u t 3.8 Ga, and occurs as small remnants in Archean terranes (Fig. 8). The extent of the earliest crust is dependent u p o n the model a d o p t e d for its origin. Widescale uniform melting throughout the earth would produce a worldwide crust, whereas localized melting at mantle inhomogeneities or in subduction zones would produce crust localized above the zones of melting. Strontium, lead, and n e o d y m i u m isotopic studies provide constraints on the rate of development of the early crust and existing results do not favor mantle recycling for sialic crust (Moorbath, 1977; McCulloch and Wasserburg, 1978). The early crust, once nucleated, may grow by such processes as magmatic under- and over-plating and plate collisions. The composition of the early crust and just h o w and when oceanic and continental types became differentiated are topics of considerable debate and opposing viewpoints. Some investigators have called attention to the similarity in composition between Archean anorthosites on the earth and lunar anorthosites (Windley, 1970) and have assumed that the early history of the earth (not preserved) was similar to that of the moon. Hence, the early crust of the earth m a y have been a widespread crust composed of gabbroic anorthosites, anorthosites, and high-alumina tholeiites. Many theories rely, in part, on the usual age relations between Archean greenstones and associated granites, and propose that the first terrestrial crust was of mafic or mixed mafic and ultramafic composition (Gill, 1961; Glikson, 1972, 1976). Taylor and White (1965) were among the first to point o u t that the average composition of the present continental crust is similar to andesite and that andesites are being added to the continents along marginal arc systems. These authors, Taylor (1967), Jakes (1973), and Lowman (1976) have proposed that the earliest sialic crust was andesitic in composition and Lowman has suggested that it was of worldwide extent. A primitive sialic crust has been proposed by Poldervaart (1955) Rambert (1964), Shaw (1972, 1976), Fyfe (1974) and Hargraves (1976). Some authors call upon low degrees of melting of mafic or ultramafic mantle
185
rocks to directly produce sialic crust while others form the sialic crust by fractional crystallization of basaltic magma. Although most investigators will agree that plate tectonic processes are necessary for the production of b o t h oceanic and island-arc crust, little agreem e n t exists as to when plate tectonics actually began (Burke and Dewey: 1973; Wynne-Edwards, 1976). Clear evidence of Phanerozoic-type plate tectonics (i.e., ophiolites, arc successions, etc.) does n o t appear until a b o u t 1 Ga go. The fact that the earth was hotter in the Precambrian, however, suggests that convection and plate tectonics should also have been occurring and on a more rapid time scale (Burke and Kidd, 1978). An explanation of this dilemma may be that plate tectonics has always been operative on the earth, although its manifestations in the geologic record have changed with time. Most of the models presented for crustal origin and early evolution rely on only a few constraints and this accounts, in part, for the diversity of models in the literature. In the past decade, a large a m o u n t of data have become available from experimental petrologic studies, trace-element geochemistry, Sr, Pb, and more recently Nd isotopic studies, and studies of the thermal history of the earth and moon. From these data the author has selected eight likely assumptions which must be accounted for by any model of crustal origin and early evolution. These are briefly discussed below and a model is presented which can a c c o m m o d a t e these assumptions. MODEL ASSUMPTIONS
(1) The early geothermal gradient in the earth was adiabatic. Elsasser (1963) and Ringwood (1977) have pointed out that core formation is a highly exothermic process providing enough heat, if completely retained, to largely melt the outer part of the earth. Rapid mixing in the mantle during or soon after core formation was probably adequate to produce an adiabatic gradient in the earth. An upper limit for the surface temperature at this time is a b o u t 2000°C, the evaporation temperature of silicates under reducing conditions (Ringwood, 1977). Although estimates of the energy released during core formation range from 250 to 600 cal g-i (Birch, 1965; Murthy 1976) only a b o u t 250 cal g-1 are needed to raise the surface temperature to 2000°C. Heat losses near the surface should cause a steepening of the adiabatic gradient (Elsasser, 1966) (Fig. 1). Such a temperature distribution would result in extensive melting of the earth to depths of a b o u t 500 km~ with the completely molten zone perhaps initially extending to the surface. (2) Heat production in the earth decreases with time. Both the decrease in abundance of radiogenic nuclides (chiefly U and K) in the earth with time and theoretical models of thermal evolution of the earth suggest that the earth's temperature has fallen with time (McKenzie and Weiss, 1975; Lambert, 1976). Existing data indicate that the average thermal gradient in the outer part of the earth fell from the adiabatic gradient to between 60 and 100 deg km -1
186 before 4.0 Ga. Metamorphic mineral assemblages in Archean terranes however, indicate that the average gradient beneath Archean sialic crust was not substantially greater than today {Wells, 1976; Burke and Kidd, 1978). Burke and Kidd (1978) have suggested that the additional heat in the Archean escaped from the earth by either or both more rapid creation of lithosphere at ridge systems or a greater length of ridge systems. (3) As a logical consequence of increased temperature in th e early stages of the earth's evolution, it is difficult to avoid mantle convection. Hence, as the surface region of the earth cooled and a thin crust (lithosphere) formed, plate tectonic processes must have been begun. The experimental and theoretical studies of McKenzie and Weiss (1975) suggest that two scales of convection occur in the mantle: A shallow convective flow system in which the horizontal extent of individual cells is a few hundred kilometers and a large-scale system with cells extending for thousands of kilometers. Higher temperatures in the Archean should result in more rapid convection and in thinner and smaller plates. This prediction is supported by the size and spacing of Archean greenstone belts. (4) The first stable crust was basaltic in composition. The term stable, however, is not to be equated with static; this crust is capable of being recycled through the mantle. Two lines of evidence support an early crust of mafic composition: (1) Experimental and theoretical studies indicate that mafic magmas segregate from their ultramafic residue after 20--50% melting (Arndt, 1977); and (2) REE studies support models for the production of Archean sialic and andesitic rocks which involve partial melting of garnet or/and amphibole-bearing mafic parent rocks (Arth and Hanson, 1975; Condie, 1976b). Hence, by analogy, it would appear that a mafic crust must precede sialic or andesitic crust. (5) The earth, together with the moon and other terrestrial planets, was subjected to intense surface cratering during the early stages of crustal development. Concurrent and later plate-tectonic processes on the earth have removed evidence for such cratering. The preservation of craters on the moon and other terrestrial planets results from either the absence of a plate-tectonic stage in the development of these bodies or cessation of this stage by about 4.0 Ga. (6) A diversity of igneous-rock compositions in the early crust was produced chiefly by progressively smaller amounts of melting of various source rocks in response to a cooling earth. Evidence for this comes mainly from geochemical model studies of Archean igneous rocks. Existing data suggest that although such rocks are produced by complex multistage processes involving both progressive melting and fractional crystallization, the former process has left the strongest imprint on trace-element distribution patterns (Arth and Hanson 1975; Condie, 1976b; Condie and Harrison, 1976; Condie and Hunter, 1976). If the earliest crustal rocks had compositions similar to those that have been studied, the same conclusions would apply. Employing assumed trace-element
187 distributions, Shaw (1972) evaluated fractional crystallization as the mechanism by which an early sialic crust could be produced and, in general, found poor agreement between assumed and predicted crustal compositions. Vigorous convection in the earth would also tend to minimize the effect of fractional crystallization by mixing and homogenizing crystallizing magmas in the upper mantle. (7) Continental crust, once formed, is non-destructable and is not recycled through the msntle to any appreciable extent. Isotopic studies (Sr, Pb, and Nd) of Archean rocks support this conclusion (Moorbath, 1977; Hamilton, et al., 1978; McCuUoch and Wasserburg, 1978) and indicate that the oldest preserved sialic crust must have been extracted from the mantle in no more than about 100 Ma before its radiometric age. Hence, the opposing viewpoint that most of the sialic crust was formed very early in the earth's history (Armstrong, 1968) and has subsequently been reworked is rejected. McKenzie (1969) also pointed out that the b u o y a n c y of sialic crust should prevent it from being subducted. This conclusion implies that it should be possible to find the oldest segments of sialic crust still preserved on the earth's surface. (8) Growth of the early sialic crustal fragments occurred by magmatic processes at subduction zones and by aggregation of sialic fragments from plate collisions. By 3.5 Ga several continental nuclei had formed and by 2.7 Ga, one or two Precambrian supercontinents were established (Piper, 1976). The remainder of the earth was covered by oceanic crust created at oceanic rises and consumed and partly melted in subduction zones to produce sialic crust. THE PROPOSED MODEL
Stage I, 4.5--4.0 Ga Stage I begins during or just after core formation and involves a rapid increase in the earth's temperature until an adiabatic gradient is established as shown in Fig. 1. Interpretations of Pb and Sr isotopic data suggest that core formation was completed in less than a few hundred million years (Oversby and Ringwood, 1971; Vollmer, 1977; Vidal and Dosso, 1978). Melting of the outer part of the earth may have extended to the surface as suggested by Ringw o o d (1977). Loss of heat by radiation and volatile escape cools the surface region rapidly and a very thin (few kilometers) crust c o m p o s e d chiefly of ultramafic rocks if formed. This ultramafic layer is unstable because its density is greater than that of the underlying melted mantle and it is disrupted by rapid convection in the upper mantle. As cooling continues to gradients of a b o u t 60 deg km -1 (Fig. 1), voluminous tholeiitic and picritic magmas segregate and rise to the surface along rifts in the ultramafic crust (Fig. 2). The ultramafic crust is broken-up and sinks and divergent plate boundaries are established where tholeiitic crust forms. Volatiles escaping during volcanism
188
~.
:
I.
|
i
/
F /~//II ,,:\o
oo j
cb/
50
17/; u .(#/ ///; /s,: /,/;
l ,o o
I'
i
~o
,
~~__~._._:._~ 0
400
:: I "-.,J
A~,000,
,/i,,o
) I
:/
~'~ /,
J
:: J
Eorly
~/ o\O-/.
I d J oo
A / ....... J
800 I 200 1600 T e m p e r,'~t u re (°C)
2000
2400
Fig. 1. Melting relations of hydrous peridotite (after Green, 1975). Early adiabatic gradient from Ringwood (1977). (- - - ) Contours of percent melting of peridotite.
begin to accumulate into an atmosphere and oceans. Some fractional crystallization may accompany production of the mafic crust, producing, by analogy with the moon, gabbroic anorthosites and related rocks. Melting relations are summarized in Fig. 1 for a hydrous ultramafic parent rock. As the adiabatic temperature drops, the mantle crystallizes from the base upwards. Basaltic crust {lithosphere) thickens as cooling continues at the surface. Tholeiitic magmas, which require 20--30% melting, separate from ultramafic residue at depths of 30--50 km. Partial melting at depths between 200-400 km produces plumes which rise adiabatically to the base of the crust (Fig. 1) loosing one or more batches of basaltic magma on route as dictated by experimental and theoretical studies which suggest that melts segregate from residual crystals before 50% melting is reached (Arndt, 1977). If the early tholeiites were similar in composition to depleted Archean tholeiites (DAT) which are widespread in Archean greenstone belts, trace-element model studies indicate that they were produced by about 35% melting of lherzolite in which neither garnet nor amphibole remain in the residue (Condie, 1976b; Condie and Harrison, 1976). Komatiitic and ultramafic magmas represent residual liquid-crystal mixtures in plumes after removal of tholeiitic magma as suggested by Naldrett and Turner {1977) and Arndt (1977) for later Archean examples. Ultramafic and komatiitic lavas are a minor but widespread component in the early mafic crust.
189
j
~
I-,
,-,
Tholeiitic Volcanism
~
~ I
',',.'-
'
,,, + v ; 25"
v
v
. v
v
v
v
v-
~
v
v
v
-U,,,oma,ic
C,ust,,\.,~.\q
:,,/,
v v(~,..7/~Av
\ .j.,,
v
i,/i]-
v
/v ~ Comp,etely Me,te~ " vI v v v Mantle v I
.~ " v
,,
I v 'r
" v
v
I00-
g
Partly
Melled
Mantle
£3
500 Solid
Mantle
Fig. 2. Illustration of an early ultramafic crust being disrupted by basaltic magmatism and mantle convection. Zones of melting correspond to the 100 deg km -1 geotherm in Fig. 1. The early mafic crust is p r o d u c e d along a c o m p l e x n e t w o r k of rift systems, c o n s u med at sinks, and recycled through the mantle (Fig. 3). Duffield (1972) has p r o p o s ed th at plate interactions observed in Hawaiian lava lakes are naturally occurring models of global plate tectonics. In these lava lakes, a basaltic crust (< 2 cm thick) is created at ridges and consum ed at sinks. "The crust presumably overlies magma of the same com p osi t i on but lower density. This situation may be even m or e analogous t o the tholeiitic crust f o r m e d during the early stages o f the earth's history because of the higher temperatures and greater spreading rates in the early stages of earth history. Using this model and assuming plates o f a few hundr e d kilometers on a side and velocities of meters per year (McKenzie and Weiss, 1975), the early basaltic crust could be recycled on a time scale of a few h u n d r e d t housant years. Temperatures are high enough th at such crust is c o m p l e t e l y remelted in subduction zones. By analogy with the m o o n , the earth u n d e r w e n t a period of intense bomb a r d m e n t ending bet w e e n 4.0 and 3.9 Ga (Wasserburg et al., 1977). Since the
190
Fig. 3. Diagrammatic illustration of the development of an early Archean tholeiitic crust. Tholeiitic magma is generated at rises and in the early stages of plume ascent. Crust is completely melted and recycled in sinks.
bodies that produce the lunar maria during this time interval did not cause melting (Taylor, 1975), it is unlikely that such impacts on the earth resulted in significant melting. In this sense, the early impacts on the earth were probably passive except in promoting crustal disruption and perhaps faster recycling of basaltic crust. The production and growth of the terrestrial crust must have been controlled chiefly by internal processes.
Stage II, 4.0--3.5 Ga At a b o u t 4.0 Ga, the thermal gradient beneath at least some subduction zones decreases sufficiently to allow only partial, rather than complete melting of descending mafic slabs before magma segregation (Fig. 4). This stage occurs in different areas at different times and initially only a few isolated arcs composed of andesite form on the earth's surface~ Andesitic magmas require a b o u t 25% melting of a mafic parent rock under hydrous conditions and
Fig. 4. Diagrammatic illustration of the formation of early Archean andesitic are systems at convergent plate boundaries.
]91
40
--
,
f
I
I
50
I
~'/
'
'
'
'
1115°
,o
ii
a_
I0
200
'
400
600
800 1000 Tempersture (°C)
I 200
14 O0
Fig. 5. Melting relations of hydrous tholeiite (after Wyllie, 1971) showing estimated conctou contours of percent melting. Garnet stability after Stern et al. (1975). AMP = upper stability limit of amphiobole. (- - - ) Contours of percent melting. tonalitic magmas about 10% (Fig. 5). Many Archean tonalites and andesites exhibit heavy-REE depleted patterns indicating the presence of residual garnet or/and amphibole (Arth and Hanson, 1975; Condie, 1976b). In systems with only small amounts of water present (such as shown in Fig. 5), amphibole is unlikely to form a residual phase because its upper stability limit is close to the solidus. In more water-rich systems, however, it may form a residual phase. It is clear from the figure, that for garnet to be a residual p h a s e the thermal gradient must be < 30 deg km -1. If the earliest andesitic-tonalitic crust was also heavy-REE depleted, a similar constraint can be placed on early geothermal gradients. Such gradients are consistent with those deduced from metamorphic mineral assemblages in Archean terranes as previously mentioned. As cooling continues, the rate of production of new lithosphere at ridges decreases; plume activity decreases; and the average degree of melting of descending mafic slabs decreases. Thus, tonalite magmas begin to dominate in the arc systems. Sialic arcs may locally thicken to 30--40 km as in southwest Greenland (Wells, 1976). Andesitic rocks are depressed into the lower crust (together with mafic remnants and some tonalites) and undergo partial melting. Melting relations for hydrous andesite are shown in Fig. 6. Degrees of melting ranging from 20 to 50% will produce granitic magmas ranging from granodiorite to granite in composition. Because most Archean granitic rocks of this composition are not observed to have heavy REE depletion (Condie and Hunter, 1976), geothermal gradients must n o t fall below about 25 deg km -1 in order to avoid entering the garnet stability field or garnet must be a minor constituent in the andesitic source such that it completely melts. Trace-
192 40
I
E
I
I
-~
I
J ~* II~l
I
I
I
150
30 I00 a~
/ r'1~ /
20
75
D_
5o IO zo..~/
. ~,
..t"7
;1
/ t
50 2O I0
• -.,--,~-'<"=r--l I00
i 300
k 500 Temperature
~.
~ 700 (°C)
~
c.,.j 900
j I100
Fig. 6. Melting relations of hydrous andesite (after Green, 1972; Stern et al., 1 9 7 5 ) . contours of percent melting. Garnet stability limit is also shown. AMP as in
. . . . Fig. 5.
element model studies indicate that high-K granitic magmas cannot be readily produced by the partial melting of ultramafic or mafic parents (Condie and Hunter, 1976). Siliceous granulite in the lower crust with andesitic bulk chemistry, however, can provide an adequate source for these magmas and hence is consistent with the proposed model. The granitic melts, once produced, rise diapirically into the tonalitic crust (Fig. 7). Sialic arcs grow also by arc-arc collisions (Fig. 7). By about 3.5 Ga, several
Fig. 7. Diagrammatic illustration of the growth of sialic crust by tonalitic plutonism and arc-arc collisions. High-K granitic plutons are produced by partial melting of andesitic rocks in the lower crust.
193
/
)
I
,/ J \ fJ
/
)
S
~
27
~35
5 5 -58
Ga Ga
/ g0
<:7 Fig. 8. D i s t r i b u t i o n of A r c h e a n c r u s t s h o w n o n a p a l e o m a g n e t i c r e c o n s t r u c t i o n ( a f t e r Piper, 1976) of a Proterozoic supercontinent. Minimum extent of Archean crust outlined by the h e a v y d a s h e d line is e s t i m a t e d f r o m t h e d i s t r i b u t i o n of relict A r c h e a n ages in P r o t e r o z o i c m o b i l e belts.
continental nuclei have formed as illustrated in Fig. 8. Ages of the oldest sialic crust range from about 3.8 to 3.4 Ga indicating (assuming sialic crust is essentially non-destructable) t h a t the formation of sialic crust was diachronous, occurring over a 400 Ma interval in different places at different times. Existing Sr, Pb, and Nd isotopic data indicate that such crustal nuclei cannot have been separated from their mantle sources more than 100 Ma before the ages r e c o r d e d If the existing age studies from Archean terranes are representative (Green and Baadsgaard, 1971; Arth and Hanson, 1975), it would appear that any given crustal segment evolved from andesitic -* tonalitic -~ high-K granitic components in 50--100 Ma.
Stage III, 3.5--2. 7 Ga Rocks between 3.5 and 2.7 Ga in age are well represented on the continents (Fig. 8). The model of crustal development herein proposed leads into a
194 mterorc Mqtq
'''T
Large
~*tC
Bas P
C~ceun ~rC
~ e
f
/,
L
IV
J
Fig. 9. Diagrammaticcross-sectionof proposed Archean tectonic settings between 3.5 and 2.7 Ga. mechanism for the production of both greenstone-granite and high-grade terranes characteristic of the Archean crust. In Fig. 9, a diagrammatic crosssection of an Archean continental nucleus is illustrated. It is envisioned that greenstone belts which contain major quantities of ultramafic extrusives developed in rift systems within the sialic crust (Condie, 1975). Such rift systems develop in response to spreading mantle plumes and the sequence of events recorded in typical greenstone belts can readily be accounted for by episodic plume activity (Condie, 1975; Condie and Hunter, 1976). Some greenstone belts may have developed around the edges of the growing sialic islands over subduction zones representing arcs or marginal basins as suggested by Windley and Smith (1976) and Tarney et al. (1976). Both continental-rift and marginal-basin environments provide a means of replenishing the mantle source beneath greenstone belts, a feature which is important for any model of greenstone-belt development (Condie and Baragar, 1974; Condie, 1975). Those greenstone belts which do not contain ultramafic and komatiitic rocks (or contain only minor amounts) and are rich in andesites may represent marginal-basin settings. Examples occur in the Slave Province, the western part of the Rhodesian Craton, and in Kenya (Condie, 1976b; Condie and Harrison, 1976). High-grade terranes represent down-dragged roots of sialic crust which have been partially melted and dehydrated and metamorphosed to amphibolite or granulite grades. This distribution is opposite to that proposed by Fyfe (1974) where greenstone belts form over downcurrents and high-grade areas over upcurrents. Between 3.5 and 2.7 Ga, the continental segments grew rapidly by tonalitic plutonism (occurring chiefly at 3.0 and at 2.6--2.8 Ga) and by arc-continent collisions. Paleomagnetic data, despite large errors in associating dates with magnetization times, suggest the existence of one or two super-continents by 2.7 Ga (Piper, 1976) (Fig. 8). Between 3.0 and 2.7 Ga it would appear that 40--50% of the continental crust formed. It is envisioned that basaltic magmatism continued in oceanic areas which covered more than 75% of the earth's surface.
195 REFERENCES Armstrong, R.L., 1968. A model for the evolution of strontium and lead isotopes in a dynamic earth. Rev. Geophys., 6: 175--199. Arndt, N.T., 1977. Ultrabasic magmas and high-degree melting of the mantle. Contrib. Mineral. Petrol., 64: 205--221. Arth, J.G. and Hanson, G.N., 1975. Geochemistry and origin of the early Precambrian crust of northeastern Minnesota. Geochim. Cosmochim. Acta, 39: 325--362. Birch, F., 1965. Speculations on the earth's thermal history. Geol. Soc. Am. Bull., 76: 133--154. Burke, K. and Dewey, J., 1973. An outline of Precambrian palte development. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences. Academic Press, London, pp. 1035--1046. Burke, K. and Kidd, W.S.F., 1978. Were Archean continental geothermal gradients much steeper than those of today? Nature, 272: 240--241. Clark, S.P., Jr., Turekian, K.K. and Grossman, L., 1972. Model for the early history of the earth. In: E.C. Robertson (Editor), The Nature of the Solid Earth. McGraw-Hill, New York, NY, pp. 3--18. Condie, K.C., 1975. Mantle-plume model for the origin of Archean greenstone belts based on trace element distributions. Nature, 258: 413--414. Condie, K.C., 1976a. Plate Tectonics and Crustal Evolution. Pergamon Press, New Y o r k NY, 288 pp. Condie, K.C., 1976b. Trace-element geochemistry of Archean greenstone belts. Earth-Sci. Rev., 12: 393--417. Condie, K.C. and Baragar, W.R.A., 1974. Rare-earth distributions in volcanic rocks from Archean greenstone belts. Contrib. Mineral. Petrol., 45: 237--246. Condie, K.C. and Harrison, N.M., 1976. Geochemistry of the Archean Bulawayan Group, Midlands greenstone belt, Rhodesia. Precambrian Res., 3: 253--271. Condie, K.C. and Hunter, D.R., 1976. Trace-element geochemistry of Archean granitic rocks from the Barberton region, South Africa. Earth Planet. Sci. Lett., 29: 389--400. Duffield, W.A., 1972. A naturally occurring model of global plate tectonics. J. Geophys. Res., 77: 2543--2555. Elsasser, W.M., 1963. Early history of the earth. In: J. Geiss and E.D. Goldberg (Editors), Earth Science and Meteoritics. North-Holland, Amsterdam, pp. 1--30. Elsasser, W.M., 1966. Thermal structure of the upper mantle and convection. In: P.M. Hurley (Editor), Advances in Earth Science. M.I.T. Press, Cambridge, MA, pp. 261--502. Frey, H., 1977. Origin of the earth's ocean basins. Icarus, 32: 235--250. Fyfe, W.S., 1974. Archean tectonics. Nature, 249: 338. Gill, J.E., 1961. The origin of continents. Proc. R. Soc. Can., 55: 103--113. Glikson, A.Y., 1972. Early Precambrian evidence of a primitive ocean crust and island nuclei of sodic granite. Geol. Soc. Am. Bull., 83: 3323--3344. Glikson, A.Y., 1976. Earliest Precambrian ultramafic-mafic volcanic rocks: Ancient oceanic crust or relict terrestrial maria? Geology, 4: 201--205. Green, D.C. and Baadsgaard, H., 1971. Temporal evolution and petro~enesis of an Archean crustal segment at Yellowknife, N.W.T., Canada. J. Petrol., 12: 177--217. Green, D.H., 1975. Genesis of Archean peridotitic magmas and constraints on Archean geothermal gradients and tectonics. Geology, 3: 15--18. Green, T.H., 1972. Crystallization of calc-alkaline andesite under controlled high-pressure hydrous conditions. Contrib. Mineral. Petrol., 34: 150--162. Hamilton, P.J., O'Nions, R.K., Evensen, N.M., Bridgwat~r, D. and Allaart, J.H., 1978. Sm-Nd isotopic investigations of Isua supracrustals and implications for mantle evolution. Nature, 272: 41--43. Hargraves, R.B., 1976. Precambrian geologic history. Science, 193: 363--371.
196
Jakes, P., 1973. Geochemistry of continental growth. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences. Academic Press, London, pp. 999--1010. Lambert, R.S.J., 1976. Archean thermal regimes, crustal and upper mantle temperatures, and a progressive evolutionary model for the earth. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, London, pp. 363--387. Lowman, P.D., Jr., 1976. Crustal evolution in silicate planets: Implications for the origin of continents. J. Geology, 84: 1--26. McCulloch, M.T. and Wasserburg, G.J., 1978. S m - N d and Rb-Sr chronology of continental crust formation. Science, 200 : 1003--1011. McKenzie, D.P., 1969. Speculations on the consequences and causes of plate motions. Geophys. J. R. Astron. Soc., 18: 1--32. McKenzie, D.P. and Weiss, N., 1975. Speculations on the thermal and tectonic history of the earth. Geophys. J. R. Astron. Soc., 42: 131--174. Moorbath, S., 1977. Ages, isotopes and evolution of Precambrian continental crust. Chem. Geol., 20: 151--187. Murthy, V.R., 1976. Composition of the core and the early history of the earth. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, London, pp. 21--31. Naldrett, A.J. and Turner, A.R., 1977. The geology and petrogenesis of a greenstone belt and related nickel sulfide mineralization at Yakabindie, western Australia. Precambrian Res., 5: 43--103. Oversby, V.M. and Ringwood, A.E., 1971. Time of formation of the earth's core. Nature. 234 : 463--465. Poldervaart, A., 1955. Chemistry of the earth's crust. Geol. Soc. Am. Spec. Pap., 62: 119--144. Piper, J.D.A., 1976. Paleomagnetic evidence for a Proterozoic supercontinent. Philos. Trans. R. Soc. Lond., A, 280: 469--490. Ramberg, H., 1964. A model for the evolution of continents, oceans, and orogens. Tectonophysics, 2: 159--174. Ringwood, A.E., 1977. Composition and petrology of the earth's mantle. McGraw-Hill. New York, NY, 618 pp. Salisbury, J.W. and Ronca, L.B., 1966. The origin of continents. Nature, 210: 669--690. Shaw, D.M., 1972. Development of the early continental crust. Part 1. Can. J. Earth Sci., 9: 1577--1595. Shaw, D.M., 1976. Development of the early continental crust. Part 2. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, Londnn, pp. 33--54. Stern, C.R., Huang, W. and Wyllie, P.J., 1975. Basalt-andesite-rhyolite-H20: Crystallization intervals with excess H20 and H~O-undersaturated liquidus surfaces to 35 kb, with implications to magma genesis. Earth Planet. Sci. Lett., 28: 189--196. Tarney, J., Dalziel, I.W.D. and DeWit, M.J., 1976. Marginal basin 'Rocas Verdes' complex from S. Chile: A model for Archean greenstone belt formation. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, London, pp. 131--146. Taylor, S.R. and White, A.J.R., 1965. Geochemistry of andesites and the growth of continents. Nature, 208: 271--273. Taylor, S.R., 1967. The origin and growth of continents. Tectonophyaics, 4 : 1 7 - - 3 4 . Taylor, S.R., 1975. Lunar Science: A Post-Apollo V~ew. Pergamon Press, New York, NY, 370 pp. Turekian, K.K. and Clark, S.P., Jr., 1969. Inhomogeneous accumulation of the earth from the primitive solar nebula. Earth Planet. Sci. Lett., 6: 346--348. Vidal, P. and Dosso, L., 1978. Core formation: Catastrophic or continuous? Sr and Pb isotope geochemistry constraints. Geophys. Res. Left., 5: 169--172. Vollmer, R., 1977. Terrestrial lead isotopic evolution and formation time of the earth's core. Nature, 270: 144--147.
197 Wasserburg, G.J., Papanastassiou, D.A., Tera, F. and Huneke, J.C., 1977. Outline of lunar chronology. Philos. Trans. R. Soc. Lond., A, 285: 7--22. Wells, P.R.A., 1976. Late Archean metamorphism in the Buksefjorden region, southwest Greenland. Contrib. Mineral. Petrol., 56: 229--242. Windley, B.F., 1970. Anorthosites in the early crust of the earth and moon. Nature, 226: 333--335. Windley, B.F. and Smith, J.V., 1976. Archean high grade complexes and modern continental margins. Nature, 260: 671--675. Wyllie, P.J., 1971. Experimental limits for conditions for melting in the earth's crust and upper mantle. Am. Geophys. Union Mon., 14: 279--302. Wynn-Edwards, H.R., 1976. Proterozoic ensialic orogensis: The millipede model of ductile plate tectonics. Am. J. Sci., 276: 927--953.
ORIGIN AND EARLY DEVELOPMENT
183
OF THE EARTH'S CRUST*
KENT C. CONDIE Department of Geoscience, New Mexico Institute of Mining and Technology, Socorro, NM 87801 (U.S.A.) (Received and accepted December 17, 1979)
ABSTRACT Condie, K.C., 1980. Origin and early development of the earth's crust. Precambrian Res., 11: 183--197. A three-stage model for crustal origin is proposed in which Stage I (4.5--4.0 Ga) is characterized by the production of a transient ultramafic crust rapidly displaced by a widespread mafic crust formed at divergent plate boundaries, consumed at sinks, and recycled through the mantle. During Stage II (4.0--3.5 Ha) a falling geothermal gradient results in the production of isolated andesitic arc systems from partial melting of descending mafic crust. Further decreases in temperature during this stage result in decreased melting and the production of tonalitic magmas which rise, overplating the andesitic crust which is partially melted to produce high-K granites. Sialic arcs grow by tonalitic plutonism and arc collisions into two supercontinents during Stage III (3.5-2.7 Ga). During this stage, greenstone belts form in continental-rift and marginal-basin environments. INTRODUCTION T h e origin a n d early d e v e l o p m e n t o f t h e e a r t h ' s c r u s t has a t t r a c t e d considerable a t t e n t i o n as e v i d e n c e d b y t h e n u m e r o u s p a p e r s w r i t t e n o n t h e subject. T h e p r o b l e m can be b r o k e n d o w n i n t o several s u b p r o b l e m s : (1) w h e n a n d b y w h a t p r o c e s s e s did t h e first c r u s t f o r m ; (2) was this c r u s t o f local or w o r l d w i d e e x t e n t ; (3) a t w h a t r a t e a n d b y w h i c h m e c h a n i s m s did t h e early c r u s t grow; (4) w h a t was t h e c o m p o s i t i o n o f t h e early crust; a n d (5) w h e r e a n d h o w did o c e a n i c a n d c o n t i n e n t a l crustal t y p e s b e c o m e clearly established. Models f o r t h e origin o f t h e e a r t h ' s c r u s t fall i n t o t h r e e b r o a d c a t e g o r i e s (Condie, 1 9 7 6 a ; L o w m a n , 1 9 7 6 ) : i n h o m o g e n e o u s a c c r e t i o n o f t h e earth, catastrophic models, and non-catastrophic models. The inhomogeneous a c c r e t i o n m o d e l f o r e a r t h f o r m a t i o n is d e s c r i b e d b y T u r e k i a n a n d Clark {1969) a n d Clark, et al. (1972). L a t e , l o w - t e m p e r a t u r e c o n d e n s a t i o n o f alkali a n d o t h e r volatile e l e m e n t s f r o m t h e solar n e b u l a f o r m a v e n e e r o v e r t h e o u t e r p a r t o f e a r t h w h i c h m a y evolve i n t o a crust. A g r e a t deal o f evidence, h o w e v e r , does not favor inhomogeneous accretion models for the formation of the p l a n e t s ( R i n g w o o d , 1 9 7 7 ) . C a t a s t r o p h i c m o d e l s f o r t h e origin o f t h e c r u s t *Paper presented at the Archaean Geochemistry Field Conference in August 1978.
184 call upon a catastrophic, one-time event such as meteorite (or asteroid) impact, core formation, or lunar capture to provide heat which partially melts the earth and the resulting magmas rise to the surface (-+ fractionation) and form a crust (Salisbury and Ronca, 1966; Condie, 1976a; Frey, 1977). Existing data related to impact on the moon, however, indicate that the impact events which formed the lunar maria did n o t produce the basalts which filled the maria. These basalts were formed at ~> 108 years after maria basin formation (Taylor, 1975). Hence, it would appear that impact energy sources may n o t be adequate to produce the earth's early crust. Frey (1977)~ however, has suggested that a thinner lithosphere and higher thermal gradient in the earth compared to the m o o n could result in rapid, impact-triggered basaltic magmatism on the earth. Non-catastrophic models for crustal formation call u p o n more gradual heating of the earth to produce magmas which form the early crust. The problem of where the first crust formed may not be soluble in that the first crust was very likely unstable and sank into the mantle. The oldest preserved sialic crust is a b o u t 3.8 Ga, and occurs as small remnants in Archean terranes (Fig. 8). The extent of the earliest crust is dependent u p o n the model a d o p t e d for its origin. Widescale uniform melting throughout the earth would produce a worldwide crust, whereas localized melting at mantle inhomogeneities or in subduction zones would produce crust localized above the zones of melting. Strontium, lead, and n e o d y m i u m isotopic studies provide constraints on the rate of development of the early crust and existing results do not favor mantle recycling for sialic crust (Moorbath, 1977; McCulloch and Wasserburg, 1978). The early crust, once nucleated, may grow by such processes as magmatic under- and over-plating and plate collisions. The composition of the early crust and just h o w and when oceanic and continental types became differentiated are topics of considerable debate and opposing viewpoints. Some investigators have called attention to the similarity in composition between Archean anorthosites on the earth and lunar anorthosites (Windley, 1970) and have assumed that the early history of the earth (not preserved) was similar to that of the moon. Hence, the early crust of the earth m a y have been a widespread crust composed of gabbroic anorthosites, anorthosites, and high-alumina tholeiites. Many theories rely, in part, on the usual age relations between Archean greenstones and associated granites, and propose that the first terrestrial crust was of mafic or mixed mafic and ultramafic composition (Gill, 1961; Glikson, 1972, 1976). Taylor and White (1965) were among the first to point o u t that the average composition of the present continental crust is similar to andesite and that andesites are being added to the continents along marginal arc systems. These authors, Taylor (1967), Jakes (1973), and Lowman (1976) have proposed that the earliest sialic crust was andesitic in composition and Lowman has suggested that it was of worldwide extent. A primitive sialic crust has been proposed by Poldervaart (1955) Rambert (1964), Shaw (1972, 1976), Fyfe (1974) and Hargraves (1976). Some authors call upon low degrees of melting of mafic or ultramafic mantle
185
rocks to directly produce sialic crust while others form the sialic crust by fractional crystallization of basaltic magma. Although most investigators will agree that plate tectonic processes are necessary for the production of b o t h oceanic and island-arc crust, little agreem e n t exists as to when plate tectonics actually began (Burke and Dewey: 1973; Wynne-Edwards, 1976). Clear evidence of Phanerozoic-type plate tectonics (i.e., ophiolites, arc successions, etc.) does n o t appear until a b o u t 1 Ga go. The fact that the earth was hotter in the Precambrian, however, suggests that convection and plate tectonics should also have been occurring and on a more rapid time scale (Burke and Kidd, 1978). An explanation of this dilemma may be that plate tectonics has always been operative on the earth, although its manifestations in the geologic record have changed with time. Most of the models presented for crustal origin and early evolution rely on only a few constraints and this accounts, in part, for the diversity of models in the literature. In the past decade, a large a m o u n t of data have become available from experimental petrologic studies, trace-element geochemistry, Sr, Pb, and more recently Nd isotopic studies, and studies of the thermal history of the earth and moon. From these data the author has selected eight likely assumptions which must be accounted for by any model of crustal origin and early evolution. These are briefly discussed below and a model is presented which can a c c o m m o d a t e these assumptions. MODEL ASSUMPTIONS
(1) The early geothermal gradient in the earth was adiabatic. Elsasser (1963) and Ringwood (1977) have pointed out that core formation is a highly exothermic process providing enough heat, if completely retained, to largely melt the outer part of the earth. Rapid mixing in the mantle during or soon after core formation was probably adequate to produce an adiabatic gradient in the earth. An upper limit for the surface temperature at this time is a b o u t 2000°C, the evaporation temperature of silicates under reducing conditions (Ringwood, 1977). Although estimates of the energy released during core formation range from 250 to 600 cal g-i (Birch, 1965; Murthy 1976) only a b o u t 250 cal g-1 are needed to raise the surface temperature to 2000°C. Heat losses near the surface should cause a steepening of the adiabatic gradient (Elsasser, 1966) (Fig. 1). Such a temperature distribution would result in extensive melting of the earth to depths of a b o u t 500 km~ with the completely molten zone perhaps initially extending to the surface. (2) Heat production in the earth decreases with time. Both the decrease in abundance of radiogenic nuclides (chiefly U and K) in the earth with time and theoretical models of thermal evolution of the earth suggest that the earth's temperature has fallen with time (McKenzie and Weiss, 1975; Lambert, 1976). Existing data indicate that the average thermal gradient in the outer part of the earth fell from the adiabatic gradient to between 60 and 100 deg km -1
186 before 4.0 Ga. Metamorphic mineral assemblages in Archean terranes however, indicate that the average gradient beneath Archean sialic crust was not substantially greater than today {Wells, 1976; Burke and Kidd, 1978). Burke and Kidd (1978) have suggested that the additional heat in the Archean escaped from the earth by either or both more rapid creation of lithosphere at ridge systems or a greater length of ridge systems. (3) As a logical consequence of increased temperature in th e early stages of the earth's evolution, it is difficult to avoid mantle convection. Hence, as the surface region of the earth cooled and a thin crust (lithosphere) formed, plate tectonic processes must have been begun. The experimental and theoretical studies of McKenzie and Weiss (1975) suggest that two scales of convection occur in the mantle: A shallow convective flow system in which the horizontal extent of individual cells is a few hundred kilometers and a large-scale system with cells extending for thousands of kilometers. Higher temperatures in the Archean should result in more rapid convection and in thinner and smaller plates. This prediction is supported by the size and spacing of Archean greenstone belts. (4) The first stable crust was basaltic in composition. The term stable, however, is not to be equated with static; this crust is capable of being recycled through the mantle. Two lines of evidence support an early crust of mafic composition: (1) Experimental and theoretical studies indicate that mafic magmas segregate from their ultramafic residue after 20--50% melting (Arndt, 1977); and (2) REE studies support models for the production of Archean sialic and andesitic rocks which involve partial melting of garnet or/and amphibole-bearing mafic parent rocks (Arth and Hanson, 1975; Condie, 1976b). Hence, by analogy, it would appear that a mafic crust must precede sialic or andesitic crust. (5) The earth, together with the moon and other terrestrial planets, was subjected to intense surface cratering during the early stages of crustal development. Concurrent and later plate-tectonic processes on the earth have removed evidence for such cratering. The preservation of craters on the moon and other terrestrial planets results from either the absence of a plate-tectonic stage in the development of these bodies or cessation of this stage by about 4.0 Ga. (6) A diversity of igneous-rock compositions in the early crust was produced chiefly by progressively smaller amounts of melting of various source rocks in response to a cooling earth. Evidence for this comes mainly from geochemical model studies of Archean igneous rocks. Existing data suggest that although such rocks are produced by complex multistage processes involving both progressive melting and fractional crystallization, the former process has left the strongest imprint on trace-element distribution patterns (Arth and Hanson 1975; Condie, 1976b; Condie and Harrison, 1976; Condie and Hunter, 1976). If the earliest crustal rocks had compositions similar to those that have been studied, the same conclusions would apply. Employing assumed trace-element
187 distributions, Shaw (1972) evaluated fractional crystallization as the mechanism by which an early sialic crust could be produced and, in general, found poor agreement between assumed and predicted crustal compositions. Vigorous convection in the earth would also tend to minimize the effect of fractional crystallization by mixing and homogenizing crystallizing magmas in the upper mantle. (7) Continental crust, once formed, is non-destructable and is not recycled through the msntle to any appreciable extent. Isotopic studies (Sr, Pb, and Nd) of Archean rocks support this conclusion (Moorbath, 1977; Hamilton, et al., 1978; McCuUoch and Wasserburg, 1978) and indicate that the oldest preserved sialic crust must have been extracted from the mantle in no more than about 100 Ma before its radiometric age. Hence, the opposing viewpoint that most of the sialic crust was formed very early in the earth's history (Armstrong, 1968) and has subsequently been reworked is rejected. McKenzie (1969) also pointed out that the b u o y a n c y of sialic crust should prevent it from being subducted. This conclusion implies that it should be possible to find the oldest segments of sialic crust still preserved on the earth's surface. (8) Growth of the early sialic crustal fragments occurred by magmatic processes at subduction zones and by aggregation of sialic fragments from plate collisions. By 3.5 Ga several continental nuclei had formed and by 2.7 Ga, one or two Precambrian supercontinents were established (Piper, 1976). The remainder of the earth was covered by oceanic crust created at oceanic rises and consumed and partly melted in subduction zones to produce sialic crust. THE PROPOSED MODEL
Stage I, 4.5--4.0 Ga Stage I begins during or just after core formation and involves a rapid increase in the earth's temperature until an adiabatic gradient is established as shown in Fig. 1. Interpretations of Pb and Sr isotopic data suggest that core formation was completed in less than a few hundred million years (Oversby and Ringwood, 1971; Vollmer, 1977; Vidal and Dosso, 1978). Melting of the outer part of the earth may have extended to the surface as suggested by Ringw o o d (1977). Loss of heat by radiation and volatile escape cools the surface region rapidly and a very thin (few kilometers) crust c o m p o s e d chiefly of ultramafic rocks if formed. This ultramafic layer is unstable because its density is greater than that of the underlying melted mantle and it is disrupted by rapid convection in the upper mantle. As cooling continues to gradients of a b o u t 60 deg km -1 (Fig. 1), voluminous tholeiitic and picritic magmas segregate and rise to the surface along rifts in the ultramafic crust (Fig. 2). The ultramafic crust is broken-up and sinks and divergent plate boundaries are established where tholeiitic crust forms. Volatiles escaping during volcanism
188
~.
:
I.
|
i
/
F /~//II ,,:\o
oo j
cb/
50
17/; u .(#/ ///; /s,: /,/;
l ,o o
I'
i
~o
,
~~__~._._:._~ 0
400
:: I "-.,J
A~,000,
,/i,,o
) I
:/
~'~ /,
J
:: J
Eorly
~/ o\O-/.
I d J oo
A / ....... J
800 I 200 1600 T e m p e r,'~t u re (°C)
2000
2400
Fig. 1. Melting relations of hydrous peridotite (after Green, 1975). Early adiabatic gradient from Ringwood (1977). (- - - ) Contours of percent melting of peridotite.
begin to accumulate into an atmosphere and oceans. Some fractional crystallization may accompany production of the mafic crust, producing, by analogy with the moon, gabbroic anorthosites and related rocks. Melting relations are summarized in Fig. 1 for a hydrous ultramafic parent rock. As the adiabatic temperature drops, the mantle crystallizes from the base upwards. Basaltic crust {lithosphere) thickens as cooling continues at the surface. Tholeiitic magmas, which require 20--30% melting, separate from ultramafic residue at depths of 30--50 km. Partial melting at depths between 200-400 km produces plumes which rise adiabatically to the base of the crust (Fig. 1) loosing one or more batches of basaltic magma on route as dictated by experimental and theoretical studies which suggest that melts segregate from residual crystals before 50% melting is reached (Arndt, 1977). If the early tholeiites were similar in composition to depleted Archean tholeiites (DAT) which are widespread in Archean greenstone belts, trace-element model studies indicate that they were produced by about 35% melting of lherzolite in which neither garnet nor amphibole remain in the residue (Condie, 1976b; Condie and Harrison, 1976). Komatiitic and ultramafic magmas represent residual liquid-crystal mixtures in plumes after removal of tholeiitic magma as suggested by Naldrett and Turner {1977) and Arndt (1977) for later Archean examples. Ultramafic and komatiitic lavas are a minor but widespread component in the early mafic crust.
189
j
~
I-,
,-,
Tholeiitic Volcanism
~
~ I
',',.'-
'
,,, + v ; 25"
v
v
. v
v
v
v
v-
~
v
v
v
-U,,,oma,ic
C,ust,,\.,~.\q
:,,/,
v v(~,..7/~Av
\ .j.,,
v
i,/i]-
v
/v ~ Comp,etely Me,te~ " vI v v v Mantle v I
.~ " v
,,
I v 'r
" v
v
I00-
g
Partly
Melled
Mantle
£3
500 Solid
Mantle
Fig. 2. Illustration of an early ultramafic crust being disrupted by basaltic magmatism and mantle convection. Zones of melting correspond to the 100 deg km -1 geotherm in Fig. 1. The early mafic crust is p r o d u c e d along a c o m p l e x n e t w o r k of rift systems, c o n s u med at sinks, and recycled through the mantle (Fig. 3). Duffield (1972) has p r o p o s ed th at plate interactions observed in Hawaiian lava lakes are naturally occurring models of global plate tectonics. In these lava lakes, a basaltic crust (< 2 cm thick) is created at ridges and consum ed at sinks. "The crust presumably overlies magma of the same com p osi t i on but lower density. This situation may be even m or e analogous t o the tholeiitic crust f o r m e d during the early stages o f the earth's history because of the higher temperatures and greater spreading rates in the early stages of earth history. Using this model and assuming plates o f a few hundr e d kilometers on a side and velocities of meters per year (McKenzie and Weiss, 1975), the early basaltic crust could be recycled on a time scale of a few h u n d r e d t housant years. Temperatures are high enough th at such crust is c o m p l e t e l y remelted in subduction zones. By analogy with the m o o n , the earth u n d e r w e n t a period of intense bomb a r d m e n t ending bet w e e n 4.0 and 3.9 Ga (Wasserburg et al., 1977). Since the
190
Fig. 3. Diagrammatic illustration of the development of an early Archean tholeiitic crust. Tholeiitic magma is generated at rises and in the early stages of plume ascent. Crust is completely melted and recycled in sinks.
bodies that produce the lunar maria during this time interval did not cause melting (Taylor, 1975), it is unlikely that such impacts on the earth resulted in significant melting. In this sense, the early impacts on the earth were probably passive except in promoting crustal disruption and perhaps faster recycling of basaltic crust. The production and growth of the terrestrial crust must have been controlled chiefly by internal processes.
Stage II, 4.0--3.5 Ga At a b o u t 4.0 Ga, the thermal gradient beneath at least some subduction zones decreases sufficiently to allow only partial, rather than complete melting of descending mafic slabs before magma segregation (Fig. 4). This stage occurs in different areas at different times and initially only a few isolated arcs composed of andesite form on the earth's surface~ Andesitic magmas require a b o u t 25% melting of a mafic parent rock under hydrous conditions and
Fig. 4. Diagrammatic illustration of the formation of early Archean andesitic are systems at convergent plate boundaries.
]91
40
--
,
f
I
I
50
I
~'/
'
'
'
'
1115°
,o
ii
a_
I0
200
'
400
600
800 1000 Tempersture (°C)
I 200
14 O0
Fig. 5. Melting relations of hydrous tholeiite (after Wyllie, 1971) showing estimated conctou contours of percent melting. Garnet stability after Stern et al. (1975). AMP = upper stability limit of amphiobole. (- - - ) Contours of percent melting. tonalitic magmas about 10% (Fig. 5). Many Archean tonalites and andesites exhibit heavy-REE depleted patterns indicating the presence of residual garnet or/and amphibole (Arth and Hanson, 1975; Condie, 1976b). In systems with only small amounts of water present (such as shown in Fig. 5), amphibole is unlikely to form a residual phase because its upper stability limit is close to the solidus. In more water-rich systems, however, it may form a residual phase. It is clear from the figure, that for garnet to be a residual p h a s e the thermal gradient must be < 30 deg km -1. If the earliest andesitic-tonalitic crust was also heavy-REE depleted, a similar constraint can be placed on early geothermal gradients. Such gradients are consistent with those deduced from metamorphic mineral assemblages in Archean terranes as previously mentioned. As cooling continues, the rate of production of new lithosphere at ridges decreases; plume activity decreases; and the average degree of melting of descending mafic slabs decreases. Thus, tonalite magmas begin to dominate in the arc systems. Sialic arcs may locally thicken to 30--40 km as in southwest Greenland (Wells, 1976). Andesitic rocks are depressed into the lower crust (together with mafic remnants and some tonalites) and undergo partial melting. Melting relations for hydrous andesite are shown in Fig. 6. Degrees of melting ranging from 20 to 50% will produce granitic magmas ranging from granodiorite to granite in composition. Because most Archean granitic rocks of this composition are not observed to have heavy REE depletion (Condie and Hunter, 1976), geothermal gradients must n o t fall below about 25 deg km -1 in order to avoid entering the garnet stability field or garnet must be a minor constituent in the andesitic source such that it completely melts. Trace-
192 40
I
E
I
I
-~
I
J ~* II~l
I
I
I
150
30 I00 a~
/ r'1~ /
20
75
D_
5o IO zo..~/
. ~,
..t"7
;1
/ t
50 2O I0
• -.,--,~-'<"=r--l I00
i 300
k 500 Temperature
~.
~ 700 (°C)
~
c.,.j 900
j I100
Fig. 6. Melting relations of hydrous andesite (after Green, 1972; Stern et al., 1 9 7 5 ) . contours of percent melting. Garnet stability limit is also shown. AMP as in
. . . . Fig. 5.
element model studies indicate that high-K granitic magmas cannot be readily produced by the partial melting of ultramafic or mafic parents (Condie and Hunter, 1976). Siliceous granulite in the lower crust with andesitic bulk chemistry, however, can provide an adequate source for these magmas and hence is consistent with the proposed model. The granitic melts, once produced, rise diapirically into the tonalitic crust (Fig. 7). Sialic arcs grow also by arc-arc collisions (Fig. 7). By about 3.5 Ga, several
Fig. 7. Diagrammatic illustration of the growth of sialic crust by tonalitic plutonism and arc-arc collisions. High-K granitic plutons are produced by partial melting of andesitic rocks in the lower crust.
193
/
)
I
,/ J \ fJ
/
)
S
~
27
~35
5 5 -58
Ga Ga
/ g0
<:7 Fig. 8. D i s t r i b u t i o n of A r c h e a n c r u s t s h o w n o n a p a l e o m a g n e t i c r e c o n s t r u c t i o n ( a f t e r Piper, 1976) of a Proterozoic supercontinent. Minimum extent of Archean crust outlined by the h e a v y d a s h e d line is e s t i m a t e d f r o m t h e d i s t r i b u t i o n of relict A r c h e a n ages in P r o t e r o z o i c m o b i l e belts.
continental nuclei have formed as illustrated in Fig. 8. Ages of the oldest sialic crust range from about 3.8 to 3.4 Ga indicating (assuming sialic crust is essentially non-destructable) t h a t the formation of sialic crust was diachronous, occurring over a 400 Ma interval in different places at different times. Existing Sr, Pb, and Nd isotopic data indicate that such crustal nuclei cannot have been separated from their mantle sources more than 100 Ma before the ages r e c o r d e d If the existing age studies from Archean terranes are representative (Green and Baadsgaard, 1971; Arth and Hanson, 1975), it would appear that any given crustal segment evolved from andesitic -* tonalitic -~ high-K granitic components in 50--100 Ma.
Stage III, 3.5--2. 7 Ga Rocks between 3.5 and 2.7 Ga in age are well represented on the continents (Fig. 8). The model of crustal development herein proposed leads into a
194 mterorc Mqtq
'''T
Large
~*tC
Bas P
C~ceun ~rC
~ e
f
/,
L
IV
J
Fig. 9. Diagrammaticcross-sectionof proposed Archean tectonic settings between 3.5 and 2.7 Ga. mechanism for the production of both greenstone-granite and high-grade terranes characteristic of the Archean crust. In Fig. 9, a diagrammatic crosssection of an Archean continental nucleus is illustrated. It is envisioned that greenstone belts which contain major quantities of ultramafic extrusives developed in rift systems within the sialic crust (Condie, 1975). Such rift systems develop in response to spreading mantle plumes and the sequence of events recorded in typical greenstone belts can readily be accounted for by episodic plume activity (Condie, 1975; Condie and Hunter, 1976). Some greenstone belts may have developed around the edges of the growing sialic islands over subduction zones representing arcs or marginal basins as suggested by Windley and Smith (1976) and Tarney et al. (1976). Both continental-rift and marginal-basin environments provide a means of replenishing the mantle source beneath greenstone belts, a feature which is important for any model of greenstone-belt development (Condie and Baragar, 1974; Condie, 1975). Those greenstone belts which do not contain ultramafic and komatiitic rocks (or contain only minor amounts) and are rich in andesites may represent marginal-basin settings. Examples occur in the Slave Province, the western part of the Rhodesian Craton, and in Kenya (Condie, 1976b; Condie and Harrison, 1976). High-grade terranes represent down-dragged roots of sialic crust which have been partially melted and dehydrated and metamorphosed to amphibolite or granulite grades. This distribution is opposite to that proposed by Fyfe (1974) where greenstone belts form over downcurrents and high-grade areas over upcurrents. Between 3.5 and 2.7 Ga, the continental segments grew rapidly by tonalitic plutonism (occurring chiefly at 3.0 and at 2.6--2.8 Ga) and by arc-continent collisions. Paleomagnetic data, despite large errors in associating dates with magnetization times, suggest the existence of one or two super-continents by 2.7 Ga (Piper, 1976) (Fig. 8). Between 3.0 and 2.7 Ga it would appear that 40--50% of the continental crust formed. It is envisioned that basaltic magmatism continued in oceanic areas which covered more than 75% of the earth's surface.
195 REFERENCES Armstrong, R.L., 1968. A model for the evolution of strontium and lead isotopes in a dynamic earth. Rev. Geophys., 6: 175--199. Arndt, N.T., 1977. Ultrabasic magmas and high-degree melting of the mantle. Contrib. Mineral. Petrol., 64: 205--221. Arth, J.G. and Hanson, G.N., 1975. Geochemistry and origin of the early Precambrian crust of northeastern Minnesota. Geochim. Cosmochim. Acta, 39: 325--362. Birch, F., 1965. Speculations on the earth's thermal history. Geol. Soc. Am. Bull., 76: 133--154. Burke, K. and Dewey, J., 1973. An outline of Precambrian palte development. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences. Academic Press, London, pp. 1035--1046. Burke, K. and Kidd, W.S.F., 1978. Were Archean continental geothermal gradients much steeper than those of today? Nature, 272: 240--241. Clark, S.P., Jr., Turekian, K.K. and Grossman, L., 1972. Model for the early history of the earth. In: E.C. Robertson (Editor), The Nature of the Solid Earth. McGraw-Hill, New York, NY, pp. 3--18. Condie, K.C., 1975. Mantle-plume model for the origin of Archean greenstone belts based on trace element distributions. Nature, 258: 413--414. Condie, K.C., 1976a. Plate Tectonics and Crustal Evolution. Pergamon Press, New Y o r k NY, 288 pp. Condie, K.C., 1976b. Trace-element geochemistry of Archean greenstone belts. Earth-Sci. Rev., 12: 393--417. Condie, K.C. and Baragar, W.R.A., 1974. Rare-earth distributions in volcanic rocks from Archean greenstone belts. Contrib. Mineral. Petrol., 45: 237--246. Condie, K.C. and Harrison, N.M., 1976. Geochemistry of the Archean Bulawayan Group, Midlands greenstone belt, Rhodesia. Precambrian Res., 3: 253--271. Condie, K.C. and Hunter, D.R., 1976. Trace-element geochemistry of Archean granitic rocks from the Barberton region, South Africa. Earth Planet. Sci. Lett., 29: 389--400. Duffield, W.A., 1972. A naturally occurring model of global plate tectonics. J. Geophys. Res., 77: 2543--2555. Elsasser, W.M., 1963. Early history of the earth. In: J. Geiss and E.D. Goldberg (Editors), Earth Science and Meteoritics. North-Holland, Amsterdam, pp. 1--30. Elsasser, W.M., 1966. Thermal structure of the upper mantle and convection. In: P.M. Hurley (Editor), Advances in Earth Science. M.I.T. Press, Cambridge, MA, pp. 261--502. Frey, H., 1977. Origin of the earth's ocean basins. Icarus, 32: 235--250. Fyfe, W.S., 1974. Archean tectonics. Nature, 249: 338. Gill, J.E., 1961. The origin of continents. Proc. R. Soc. Can., 55: 103--113. Glikson, A.Y., 1972. Early Precambrian evidence of a primitive ocean crust and island nuclei of sodic granite. Geol. Soc. Am. Bull., 83: 3323--3344. Glikson, A.Y., 1976. Earliest Precambrian ultramafic-mafic volcanic rocks: Ancient oceanic crust or relict terrestrial maria? Geology, 4: 201--205. Green, D.C. and Baadsgaard, H., 1971. Temporal evolution and petro~enesis of an Archean crustal segment at Yellowknife, N.W.T., Canada. J. Petrol., 12: 177--217. Green, D.H., 1975. Genesis of Archean peridotitic magmas and constraints on Archean geothermal gradients and tectonics. Geology, 3: 15--18. Green, T.H., 1972. Crystallization of calc-alkaline andesite under controlled high-pressure hydrous conditions. Contrib. Mineral. Petrol., 34: 150--162. Hamilton, P.J., O'Nions, R.K., Evensen, N.M., Bridgwat~r, D. and Allaart, J.H., 1978. Sm-Nd isotopic investigations of Isua supracrustals and implications for mantle evolution. Nature, 272: 41--43. Hargraves, R.B., 1976. Precambrian geologic history. Science, 193: 363--371.
196
Jakes, P., 1973. Geochemistry of continental growth. In: D.H. Tarling and S.K. Runcorn (Editors), Implications of Continental Drift to the Earth Sciences. Academic Press, London, pp. 999--1010. Lambert, R.S.J., 1976. Archean thermal regimes, crustal and upper mantle temperatures, and a progressive evolutionary model for the earth. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, London, pp. 363--387. Lowman, P.D., Jr., 1976. Crustal evolution in silicate planets: Implications for the origin of continents. J. Geology, 84: 1--26. McCulloch, M.T. and Wasserburg, G.J., 1978. S m - N d and Rb-Sr chronology of continental crust formation. Science, 200 : 1003--1011. McKenzie, D.P., 1969. Speculations on the consequences and causes of plate motions. Geophys. J. R. Astron. Soc., 18: 1--32. McKenzie, D.P. and Weiss, N., 1975. Speculations on the thermal and tectonic history of the earth. Geophys. J. R. Astron. Soc., 42: 131--174. Moorbath, S., 1977. Ages, isotopes and evolution of Precambrian continental crust. Chem. Geol., 20: 151--187. Murthy, V.R., 1976. Composition of the core and the early history of the earth. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, London, pp. 21--31. Naldrett, A.J. and Turner, A.R., 1977. The geology and petrogenesis of a greenstone belt and related nickel sulfide mineralization at Yakabindie, western Australia. Precambrian Res., 5: 43--103. Oversby, V.M. and Ringwood, A.E., 1971. Time of formation of the earth's core. Nature. 234 : 463--465. Poldervaart, A., 1955. Chemistry of the earth's crust. Geol. Soc. Am. Spec. Pap., 62: 119--144. Piper, J.D.A., 1976. Paleomagnetic evidence for a Proterozoic supercontinent. Philos. Trans. R. Soc. Lond., A, 280: 469--490. Ramberg, H., 1964. A model for the evolution of continents, oceans, and orogens. Tectonophysics, 2: 159--174. Ringwood, A.E., 1977. Composition and petrology of the earth's mantle. McGraw-Hill. New York, NY, 618 pp. Salisbury, J.W. and Ronca, L.B., 1966. The origin of continents. Nature, 210: 669--690. Shaw, D.M., 1972. Development of the early continental crust. Part 1. Can. J. Earth Sci., 9: 1577--1595. Shaw, D.M., 1976. Development of the early continental crust. Part 2. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, Londnn, pp. 33--54. Stern, C.R., Huang, W. and Wyllie, P.J., 1975. Basalt-andesite-rhyolite-H20: Crystallization intervals with excess H20 and H~O-undersaturated liquidus surfaces to 35 kb, with implications to magma genesis. Earth Planet. Sci. Lett., 28: 189--196. Tarney, J., Dalziel, I.W.D. and DeWit, M.J., 1976. Marginal basin 'Rocas Verdes' complex from S. Chile: A model for Archean greenstone belt formation. In: B.F. Windley (Editor), The Early History of the Earth. Wiley, London, pp. 131--146. Taylor, S.R. and White, A.J.R., 1965. Geochemistry of andesites and the growth of continents. Nature, 208: 271--273. Taylor, S.R., 1967. The origin and growth of continents. Tectonophyaics, 4 : 1 7 - - 3 4 . Taylor, S.R., 1975. Lunar Science: A Post-Apollo V~ew. Pergamon Press, New York, NY, 370 pp. Turekian, K.K. and Clark, S.P., Jr., 1969. Inhomogeneous accumulation of the earth from the primitive solar nebula. Earth Planet. Sci. Lett., 6: 346--348. Vidal, P. and Dosso, L., 1978. Core formation: Catastrophic or continuous? Sr and Pb isotope geochemistry constraints. Geophys. Res. Left., 5: 169--172. Vollmer, R., 1977. Terrestrial lead isotopic evolution and formation time of the earth's core. Nature, 270: 144--147.
197 Wasserburg, G.J., Papanastassiou, D.A., Tera, F. and Huneke, J.C., 1977. Outline of lunar chronology. Philos. Trans. R. Soc. Lond., A, 285: 7--22. Wells, P.R.A., 1976. Late Archean metamorphism in the Buksefjorden region, southwest Greenland. Contrib. Mineral. Petrol., 56: 229--242. Windley, B.F., 1970. Anorthosites in the early crust of the earth and moon. Nature, 226: 333--335. Windley, B.F. and Smith, J.V., 1976. Archean high grade complexes and modern continental margins. Nature, 260: 671--675. Wyllie, P.J., 1971. Experimental limits for conditions for melting in the earth's crust and upper mantle. Am. Geophys. Union Mon., 14: 279--302. Wynn-Edwards, H.R., 1976. Proterozoic ensialic orogensis: The millipede model of ductile plate tectonics. Am. J. Sci., 276: 927--953.