Speculations about crustal evolution

J. Geodynamics Vol. 16, No. 1/2, pp. 55~64, 1992

0264.-3707/92 $5.00+0.00 Pergamon Press Ltd

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Discussion SPECULATIONS

ABOUT

CRUSTAL EVOLUTION

A. L. HALES Research School of Earth Sciences, Australian National University, P.O. Box 4, Canberra ACT 2601, Australia

(Received 13 August 1992)

Abstract---The bulk of the sediments eroded from the continents end up in basins along the margins of the continents. These sediments are ultimately recycled to become once more part of the continental crust. Isotopic studies of the granites of the Lachlan Fold Belt in south-east Australia, both by conventional mass spectrometry and by ion microprobe, have demonstrated that these granites have a population of zircons with core ages greater than the ages of the granites, thus establishing that the granites are made from recycled material. Other isotopic studies show systematic variation across the batholiths, but not along the length of the batholiths. It is suggested that these granites were formed from the sediments in a back arc basin. The possible processes by which the sediments are converted to granites are discussed.

Armstrong (1968) proposed a no-growth model for the evolution of the continental crust, i.e. a model in which the volume of the continental crust had remained constant. In this paper Armstrong made the following points: (1) that the isotopic data from lead and for initial strontium could only be reconciled if continental material were recycled through the mantle; (2) that sialic material was eroded from the continents into the oceans, isotopically homogenized there and then dragged into the mantle in orogenic belts. H e acknowledged, however, that there was some difficulty in envisaging a mechanism for this process; (3) that "volumes of continental crust and ocean water have remained nearly constant for at least the last 2.5 b.y.". Armstrong's estimates of the sediments subducted ranged from 0.5 to 5-10 km 3 a -1. H e favoured the higher ranges, possibly because his estimate of present erosion rate from the continents was 60 km 3 a -1. In his most recent paper Armstrong (1991) points out that the Scholl et al. (1990) estimates of the sediment flux into the mantle as a result of subduction of sediment at the trenches and tectonic erosion, namely 1.8 km 3 a -I, is "about equal to the recorded rate of addition of material to the continental crust". 55

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The vast amount of isotopic data, and especially the S m - N d data, reported since 1968, has lead to general acceptance of the significance of recycling (Veizer and Jansen, 1979; McLennan, 1988) and the view that the growth of the volume of the continental crust was very much more rapid before 2.5 b.a. than after that time. However, the majority of the papers discussing crustal evolution in the light of the isotopic and other data have favoured moderate growth since 2.5 b.a. In fact, in view of the uncertainty about the proportion of the sediment carried to the trenches subducted as opposed to accreted, it is difficult to avoid the conclusion that the "no growth" model is only the end-point of a range of models involving moderate growth since 2.5 b.a. Certainly the age of basement data reported by Muehlberger et al. (1967), for North America and by Sclater etal. (1981),world-wide suggest that the area of the continental crust has increased since 2.5 b.a. Taylor and MeLennan (1985) list estimates of the annual sediment yield to the oceans ranging from 2.7 to 4.6 km 3 a -1. After allowing for deposition of some of this sediment on continental terraces they conclude that the amount of sediment from the continental crust deposited on the oceanic crust "is unlikely to exceed about 3 km 3 a -1''. The average height of the continental crust above sea-level is 0.623 km. Without any allowance for isostatic adjustment the continental crust would be reduced to sea-level in about 32 m.a. Allowing for isostatic rebound the reduction to sea-level would take only 160 m.a. Taylor and McLennan point out that the "major repositories of oceanic sediment are not closely related to subduction zones". Following Potter (1978) they emphasize the contribution of big rivers to sediment yield. They list the mass of sediment discharged to the oceans annually by 12 major rivers. These yields converted to volume of solids are given in Table 1. The combined yield of the 12 rivers is 1.99 km 3 a -1, three-quarters of that from four rivers. More than 1.1 km 3 a -1 originates in rivers rising in the collision created Himalayas, 0.56 km 3 a -1 being deposited in the back-arc basins off the east coast of Asia, and about the same amount in the Bengal Fan. Reading (1982) estimated that the sediment in the Bengal Fan had accumulated at a rate of 0.4 knl 3 a -1. Assuming a porosity of 30% for the sediments in the Fan this is consistent with the Taylor and McLennan estimate of the yield from the Ganges and Brahmaputra Rivers. There are considerable accumulations of sediTable 1. Annual yield of sediments to the oceans fa'om 12 major rivers (km 3) River

Yield

River

Yield

Ganges/Brahmaputra Yellow (Huangho) Amazon Yangtze Irrawaddy Magdalena

0.596 0.386 0.321 0.171 0.102 0.079

Mississippi Orinoco Huangho (Red) Mekong Indus MacKenzie

0.075 0.075 0.057 0.057 0.036 0.036

Discussion: Speculations about crustal evolution

57

ment in basins adjacent to continental margins not served by any of the 12 major rivers, for example, off the east coast of North America. It is inevitable that the bulk of the sediments in the back-arc basins along the east coast of Asia, and in the Bengal Fan will become once again part of the continental crust. The questions are: how and when? It does not seem possible that the Pacific plate should over-ride the island arcs so that ~ new subduction zone should develop close to the Asian margin. Nor in the current plate tectonic regime is there a possibility that some other major continental block should collide with these island arcs. It seems that the most likely outcome is that the back-arc basins should continue to fill and that eventually compression resulting from the westward movement of the Pacific plate would result in the development of an orogenic belt in this region. In the case of the Bengal Fan collision with a major continental block is equally unlikely but Sykes (1970) suggested the possibility that a "nascent" subduction zone was developing south of India between Ceylon and the Ninety East Ridge. Stein and his colleagues have pursued this suggestion (Stein and Okal, 1978; Gordon et al., 1990). The seismic evidence of Bergman and Solomon (1981) and Eittreim and Ewing (1972) shows that there is compression in the region. It seems to me that when compression develops in an oceanic crust the first effect will be the development of en echelon faults resulting in the creation of a number of disconnected blocks of oceanic crust. As subduction develops these blocks and the sediments will eventually become part of the continental crust. The map of ophiolites world-wide of Coleman (1977) suggests this is often the case. The upper continental crust is made up of granites, metamorphic rocks and sediments with much smaller volumes of mafic intrusions and extrusions. The question which now arises is how the oceanic sediments were converted into the rocks of the upper continental crust. Hutton conceived of the granites as having a molten origin. Tuttle and Bowen, 1958, showed that the granites could be derived by melting, or partial melting of crustal rocks. Experimental work has shown that granites can be derived by the melting of sediments (Winkler, 1957; Piwinskii and Wyllie, 1968). Chappell (1967) espoused the concept of partial melting and the occurrence of enclaves of residual material in a batholith in the New England Fold Belt. The residual material was later termed restite. Chappell and White (1974) classified granites as I-type (igneous) and S-type (sedimentary origin). Later I and S were deemed to mean derived from infracrustal and supracrustal sources respectively. The petrographic and geochemical data on the Iand S-type granites of the Lachlan Fold Belt have been reviewed by Chappell and White (1993). Evidence that some material survived the process of conversion into granite came quickly. Williams (1977) using conventional mass-spectrometry, measured the ages of some zircons from S-type granites and found that they were considerably older than the maximum possible age of the granites, some by as much as 600 m.a. H e found also that zircons from I-type granites were older than the maximum possible age of the granites, though not by as many years. Early ion

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microprobe studies at A.N.U. showed that many of the zircons from southeast Australia had cores of much greater age than the ages of the granites. In some cases the cores of the zircons were of Archean age although the ages of the granites were <450 m.a. Accumulating evidence from the ion microprobe studies of inheritance in zircons (Williams et al., 1988; Williams, 1989) has shown that almost every zircon in the S-type granites from the southern end of the Lachlan Fold Belt has an older core, there being three groups with ages from 450 to 650 m.a., 800 to 1075 m.a. and another group with ages up to 3350 m.a. Although older cores are less frequent in zircons from I-type granites the relative abundances of the three groups are much the same as those in the zircons from the S-type granites. The isotopic data from the U - P b and Rb-Sr systems also yield older source rock ages for the granites of the Lachlan Fold Belt (Compston and Chappell, 1979). Even more significantly from the point of view of understanding of the environment in which these granites developed and of the process by which they were developed, Williams et al. (1991) have shown that zircons from the Ordovician country rock sediments of these granites show the same age groups as the granites. Equally significantly the isotopic data show systematic trends from west to east. O'Neil and Chappell (1977) showed that across the Bega Batholith 8180 increased from low values, akin to those found for island arcs in the east to higher values in the west. McCulloch and Chappell (1982) found that across the Berridale batholith the 87Sr/86Sr ratios increased from about 0.704 in the east to 0.709 in the west and end decreased from +4 to - 9 east to west. The high end and low initial strontium ratios in the west suggest that the granites in the west were derived from sediments with a considerable island arc component whereas those in the east were derived from sediments from much older continental crust. The fact that the zircons in the sediments and granites show evidence of similar provenance suggests to me that the granites and sediments were derived from a large body of wet sediment as suggested by Elliston (1968, 1984, 1986). The systematic trends in the isotopic data suggest that that body of sediment lay in a back-arc basin with the sediments in the east derived mainly from the island arc and in the west mainly from the mainland. Silver and his colleagues have made a very large number of isotopic and other measurements across the Peninsula batholith in California. Silver and Chappell (1988) point out that these data show systematic west to east variations across the batholith but not along the length of the batholith. They suggest that the "Chemical and isotopic properties of the western part of the batholith indicate that it formed as the root of a primitive island arc on oceanic lithosphere at a convergent plate margin". For the eastern part of the batholith they propose derivation by partial melting of subcrustal rocks of broadly basaltic composition. However, D e Paolo (1981) interprets the neodynium and initial strontium data in terms of origin of the granites in a back arc basin. DePaolo shows that the end values decrease from 5.4 in the west to -5.9 in the east, whereas Taylor

Discussion: Speculations about crustal evolution

59

and Silver (1978) show that ~180 increases from +6.0 to +13.0 west to east, and initial strontium from 0.703 to 0.708 west to east (Early and Silver, 1973). These values are similar to those for the Lachlan Fold Belt. It seems that these values can be explained as easily by derivation from sediments eroded from the island arc in the west mixing with sediments eroded from the continent to the east as by any of the other explanations which have been offered for the origin of the granites of the Peninsular Batholith. The distribution of the isotopic and other data across the Lachlan Fold Belt and Peninsula batholiths, the evidence that the granites of the Lachlan Fold Belt and the country rock sediments have the same provenance, and the fact that most of the granites generated in the past 100 m.a. lie along continental margins suggest that Elliston's primary contention, namely that granites are developed from wet sediments in ocean basins is well founded. This conclusion is unaffected by any uncertainty whether the granites are generated in the manner suggested by Elliston, by partial melting, or by some combination of these processes and solution. It is possible that the restoration of oceanic sediments to the continents from back-arc basins is as important as the other processes in which sediments become once more part of the continental crust, namely by accretion at continental subduction zones and in collisions of continental blocks. It seems that at present the process for the development of granite most favoured is partial melting. The temperatures required for partial melting are probably achievable deep in the crust, but it is not clear how the heat needed to melt the quantities of sediment involved in producing granites would be supplied unless the temperature initially was considerably above that required for partial melting or unless there was a continuing supply of heat. Elliston (1968, 1984, 1986) offers an alternative hypothesis. Elliston's main points are: (1) that the great bulk of the products of the erosion of the continents lie in the oceans; (2) that these sediments are porous and contain >50% water to great depths; (3) that the sediments are chiefly hydrosilicates in a colloidal state; (4) that the temperature in the sediment pile increases as a result of burial and the generation of heat by radioactive elements in the sediments; (5) that gel formation occurs to produce impermeable layers and that the mechanical properties of consolidating sediments are most easily explained in terms of the properties of gels; (6) that in the processes occurring in the sediment pile surface chemistry is more important that solution chemistry: in particular that more material can be transported colloidally than in solution and that metal ions can be transported in this way--but it is not suggested that solution does not occur; (7) that granites are formed by crystallization from the colloidal state and that heat released in crystallization contributes to the increase in tem-

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perature of the residual fluid which rises through the sediment pile to continue the process. Although other processes may be involved in the evolution of ocean sediments into continental crust I think that the process proposed by Elliston plays the major role for two reasons: (i) it avoids the difficulty of providing the large amount of heat required to melt the very considerable volume of sediments needed to produce granite batholiths; (ii) seems to be the only process which would result in granites, volcanic rocks, metamorphic rocks and consolidated sediments all with the same general chemistry and provenance in the same relatively limited region. Elliston's hypothesis has been criticized because of the importance attached to the role of colloidal hydrosilicates. It is clear, however, that a significant proportion of the sediment carried by the rivers to the ocean basins is in the form of very fine particles. The names of some of the rivers attest to this; e.g. the Yellow River, the Red River, the Orange River, the Vaal River. This finegrained material settles as mud on the floor of the ocean basins. It is this mud which constitutes the colloidal mush from which Elliston (1986) argues that granites, metamorphic rocks, sediments and even some volcanic rocks are created by reactions such as Si(OH)4 ~

SIO2+2H20+49.8 kJ tool -1.

Raleigh (1965) has shown that colloidal silica in the presence of water does crystallize to quartz under pressures of 10 kbar and at temperatures from 250° to 600°C. Major problems in understanding the evolution of the continental crust are the estimation of the SiO2 abundance in the crust and explaining how it arose. The near surface rocks of the crust can be sampled and there is general agreement that the SiO2 abundance is close to 70%. Taylor (1979) bases his estimate of the SiO2 abundance in the whole Post-Archean continental crust on the abundance in the volcanics of the island arc environment. His estimate, 58%, is consistent with Ewart's, 1976, analysis of the average SiO2 abundance in "continental" island arcs, "intraoceanic" island arcs and a number of western American orogenic lavas. However, in terms of the Post-P.rchean moderate growth model favoured by Taylor, most of the SiO2 was introduced into the crust in the Archean, or earlier. Taylor (1979) argues that the similarity of R E E patterns in Archean sedimentary rocks to the R E E patterns in island arc volcanic rocks leads to the conclusion that the overall composition of the Archean crust was similar to the average composition of present day island arc volcanic rocks and points out that this view is consistent with that expressed by Rogers (1977). Weaver and Tamey (1984) discuss the composition of the crust in terms of three layers-upper, middle and lower--in the proportions 2:1:3. Taking account of a number of factors, including the composition of deep crustal xenoliths and exposed deep crustal terrains they base their estimate of lower crustal composition on the

Discussion: Speculations about crustal evolution

61

Lewisian granulite average of Weaver and Tarney (1980) of the middle crust on the Lewisian amphibolite facies of Weaver and Tarney (1981) and accept the estimate of Taylor and McLennan (1981) for the upper crust. Their estimate of the SiO2 abundance of the whole crust is 63.5%, even higher than the andesitic abundance of 58% favoured by Taylor (1979). There are difficulties with the idea that andesitic lavas are produced by melting of subducted crust and the overlying asthenospheric wedge at a depth of about 100 kin. As a result of an analysis of data from experimental petrology Wyllie (1982) concludes that andesite is not a primary magma from oceanic crust. H e does, however, point out that his conclusions would be modified if the temperature in the subducted slab were as high as 1250°C as suggested by Marsh (1979). Nevertheless the Ewart (1976) data show convincingly that lavas with an SiO2 abundance of 58% have reached the surface in considerable quantity over the past few million years. Even if the input from the mantle has an SiO2 abundance of 58% some other process is required to produce an upper crust with about 70% SiO2, and granites with about 10% quartz. Taylor (1979) favours intracrustal melting. It is probable that both requirements would be met more readily in a wet sediment mush. Mantle rocks, and rocks derived from the mantle, are typically of the form (MaOb)c(SiO2)d, o r combinations thereof. M stands for metal; a,b are usually 1,1, 2,1 sometimes 2,3; c,d are 2,1 or 1,1. Using this building block approach there are three ways in which free quartz may arise: (1) the (MaOb) and (SiO2) blocks may separate in solution; (2) the (MaOb) may combine with blocks such as CO2, or Cl, e.g. CaCO3, or NaC1, or some block involving S; (3) an (SiO2) block may be freed by some reaction such as 2 ( M O ) ( S i O 2 ) --~ ( M O ) 2 ( S i O 2 ) + (SiO2).

Some of the low abundance elements would move up with the fluid and would be trapped as the mush crystallized, or consolidated. These processes will occur more readily in wet sediment than elsewhere. During the heating of the ocean basin sediments as a result of heat generated by the radioactive elements and by burial the SiO 2 will tend to move to the top of the pile and some (MaOb) tO the bottom. If the SiO2 abundance of the upper crust is to rise to 70% the SiO 2 abundance in the lower crust must decrease. In the upper mantle also the availability of MO-SiO2 blocks is the determining factor in how much partial melt with enhanced abun dance of SiO 2 can be produced. An increased abundance of MO'SiO2 blocks in the primitive upper mantle may have been an important factor in the development of the continental crust in the early history of the Earth. The evidence presented in this paper comes from two well observed batholiths, both suggested to originate from the sediments in back arc basins, and both <500 m.a. The processes in accretionary terranes are probably more complex and the geochemistry may be more difficult to interpret. In the early stages of

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subduction the sediment delivered to the trench by the oceanic plate will be derived in the main from continental rocks. Once the volcanic arc has developed substantially the sediments deposited on the moving oceanic plate will come from the volcanic arc, from rocks accreted in the early stages of subduction or will be pelagic, resulting in more complex geochemistry. Nevertheless even in accretionary areas the source material is wet sediment. The major unresolved question is whether the undoubted growth in the area of the continents since 2500 m.a. ago is consistent with models based on the isotopic data. It may be that the processes involved in recycling in the early history of the Earth were different from those which have been obtained in the past 500 m.a. The main points in this paper can be summarized as follows: (1) More than one third of the sediments eroded from the continents is carried by rivers rising in the collision created Himalayas, and ends up in the Bengal Fan or the back-arc basins along the east coast of Asia. Thus collision zones have an important role in the recycling of continental material. (2) The ages found for the cores of zircons in granites of the Lachlan Fold Belt show that relict material is included in the granites, thus supporting the restite model of Chappell and White (1974). (3) Since the ages of the cores of the zircons in the sediments are similar to those in the cores of the zircons in the granites the sediments and the granites had the same provenance and were probably made in the same process. (4) The granites could be made either by partial melting of wet sediments in an ocean basin, or in the Elliston process, but because the consolidated sediments were probably made in the same process the latter process is favoured. (5) The systematic west to east variation of the oxygen isotopes, the initial strontium ratios, and end across the Bega batholith suggests that these rocks originated in a back arc basin as was suggested for the Peninsular Batholith by De Paolo (1981). (6) It is suggested that in the initial stages of the development of a subduction zone compression leads to en echelon faulting and the creation of a zone of broken up oceanic crust. It is conceivable that when sufficient disruption has occurred to permit the descent of the slab sediment will be trapped in front of the slab and carried down with it. (7) Introcrustal recycling plays a significant role in the evolution of the continental crust. Acknowledgements--I should express my indebtedness to Professor S. W. Carey who introduced me to the ideas of John Elliston in the mid-70s and to John Elliston for much interesting discussion of his ideas. I also thank colleagues at the Australian National University for much helpful discussion, especially in respect of the topic of this paper, to Bruce Chappell, Allan White, lan Williams, lan McDougall, Bill McDonough,

Discussion: Speculations about crustal evolution

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Roberta Rudnick, Malcolm McCulloch and Robert Hill. I hesitate to suggest that my colleagues share any, much less all, of the views expressed in this paper. I am also indebted to Professor Brian Windley for useful suggestions made in commenting on an earlier draft of this paper.

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Taylor S. R. (1979) Chemical composition of the continental crust: Rare earth element evidence from sedimentary rocks. In The Earth: Its Origin, Structure and Evolution (ed. M. W. McElhinny), pp. 353--376. Academic Press, London. Taylor S. R. and MeLennan S. H. (1981) The composition and evolution of the continental crust: rare element evidence from sedimentary rocks. Phil. Trans. R. Soc. A301, 381-399. Taylor S. R. and McLennan S. H. (1985) The Continental Crust: its Composition and Evolution. BlackweU Scientific Publications, Oxford. Tuttle O. F. and Bowen N. L. (1958) Origin of granite in the light of experimental studies in the system KAISiO-SiOH. Geol. Soc. Am. Mere- 7,g 1-153. Veizer J. and Jansen S. L. (1979) Basement and sedimentary recycling and continental evolution. J. Geol. 93, 625-643. Weaver B. L. and Tarney J. (1980) Rare-earth geochemistry of Lewisian granulite-facies gneisses, N.W. Scotland: implications for the petrogenesis of the Archean lower crust. Earth Planet. Sc~ LetL $1, 279-286. Weaver B. L. and Tarney J. (1981) Lewisian gneiss geochemistry and Archean crustal development models. Earth Planet. Sc£ Left. 55, 171-180. Weaver B. L. and Tarney J. (1984) Major and trace element composition of the continental lithosphere. In Structure and Evolution o f the Continental Lithosphere (eds H. N. Pollack and V. Rama Murthy), Physics and Chemistry of the Earth, 15. Williams I. S. (1977) Ph.D. thesis, Australian National University. Williams I. S. (1989) Inherited zircon, a unique key to granites' protoliths: an ion probe study. Geol. Soc. Amer. Progr. Abstr. 21, 361-362. Williams I. S., Chen Y. D., Chappell B. W. and Compston W. (1988) Dating the sources of Bega Batholith granites by ion microprobe. Geol. Soc Aust. Abstr. 21, p. 424. Williams I. S., Chappell B. W., Chen Y. D. and Crook K. A. W. (1991) Inherited and detrital zircons-vital clues to the granite protoliths and early igneous history of south-eastern Australia. Second Symposium on Granites and Related Rocks. Bureau of Mineral Resources Record, Geology and Geophysics, 1991/25. Winkler H. (1957) Experimentelle gesteinsmetamorphose, Pt. 1. Geochim. Cosmochim. Acta 13, 42-69. Wyllie P. J. (1982) Subduction products according to experimental prediction. Bull, Geol. So~ Am. 93, 468-476.