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IV CHARACTERIZATION OF CONTINENTAL CRUST

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Seismic Exploration of the Continental Basement: Trends for the 1980's 10 INTRODUCTION JACK E. OLIVER Cornell University In scientific research it is helpful to view one's activities occasionally from a fresh and different perspective. Imagine, for example, an astronaut-scientist visiting the earth from an advanced civilization on another planet and having the task of reporting the sate of earthly science to his leaders. I think his report would be mixed in tone. He would report favorably about some of our efforts to inves- tigate our surroundings. On one scale, for example, space- craft leave the earth to explore the solar system and beyond in an effort that strains our technology. At another extreme of scale, huge sophisticated devices cause tiny subatomic particles to collide at high velocity in an effort to learn more and more about smaller and smaller entities. Once again our technology is strained. The astronaut would probably be favorably impressed by progress in certain methods for exploring the earth sophisticated laboratory devices, the techniques of the petroleum industry, perhaps deep-sea drilling vessels. But I think he would be surprised and dismayed to find that a society of four billion people confined to the surface ofthe earth has been content to know so liKle about the rocks a few ~7 hundreds or thousands of meters below it, and from which it derives much of its livelihood the rocks of the conti- nental basement. In making this statement, I do not mean to be critical of those scientists who work very capably and professionally on this topic but rather of the magni- tude ofthe total effort directed toward study ofthis part of the earth, an effort that seems too small in relation to the need for an inventory of its resources by an expanding and ravenous society. I hope new advances within the next decade will ease the mind of the astronaut on this point. It is not that methods and tools for exploration of the continental basement are unavailable. We have them; many of them will be discussed in this paper. The seismo- logical methods that I shall discuss are but a part of our overall capability. Others include further mapping, im- proved and extended field and laboratory studies of sur- face rocks, drilling for informational purposes, study of crustal xenoliths, and a variety of geophysical techniques. The problem is one of focusing scientific interest on the topic and of devoting an appropriate portion of our efforts to study of this region, for sound economic as well as scientific reasons.

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118 Seismology has been, of course' a major producer of information on the earth's interior~the major producer of certain kinds of information including structure and cer- tain mechanical properties. Therefore, ~ wish to consider the potential of further seismic exploration of the conti- nental basement. By continental basement I mean the entire continental crust below the sediments and Me uppermost mantle. From the broadest perspective and for several reasons, it is clear that the potential of the seismic method for exploration of the earn, and particularly the continental basement, is by no means exhausted. First, to do so would require sources and receivers scattered over and through the earth at the Nyquist spacing for the shortest waves that can be detected after propagation through the deep region of interest. We are certainly far from achieving such a level of observations at this time and from predicting what we would observe if we did. Second, we lag in analysis; we are not able to understand and to make useful conclu- sions from all the information we obtain now. Third, the current rate of discovery of new features continues to be high. From these three points, one can deduce conf~- dently that a much improved understanding of the earth's interior remains to be revealed by seismology. The chal- lenge of seismology is to approach this ideal in an opti- mum manner given economic and other practical con- straints. In a sense, the various kinds of seismological studies represent venous routes toward this goal. Let us consider some ofthem, more or less in order of decreasing wavelength. SPECTRUM OF SEISMIC WAVELENGTHS Since 1960, seismologists have been observing and stucly- ing waves of very long periods (up to almost an hour) and hence very long wavelengths that may be thought of as corresponding to free oscillations of the earth. Most studies of free oscillations treat the earth as almost spheri- cally symmetrical (they include flattening and rotations. The resolution of differences between continents and oceans is very limited and the differences are averaged out in most cases. If some of the very-high-mode, i.e., sho~t-waveleng~, Dee oscillations can be observed, identified, and resolved in sufficient detail, information on gross structural differences between continents and oceans including associated mantle structure may be provided. Many such higher-mocle oscillations are alcin to their traveling counterparts, seismic surface waves. Traveling surface waves offer the opportunity for detennin~ng earn structure based on one pass of the waves, as opposed to many passes with consequent averaging with other effects in the case of free oscillations. Surface waves have been used regularly in recent years to.detennine cn~staI and upper-mantle structure, including regional variations of such structure. Many of the measurements of depth to the top of the low-velocity zone in the mantle, which is pre- sumably though not necessarily near or related to the base JACK E. OLIVER of the lithosphere. are derived from surface waves. Al- though the results are sometimes ambiguous and very dependent on the validity of certain assumptions, there is probably a good deal more information about the earth including the continental basement, to be obtained from surface waves. To utilize surface-wave data for conti- nental structure will require, as a minimum, more complete and more closely spaced observations of the phenomenon, farther attention to focusing and multi- pathing, integration of surface-wave observations with those of Other phenomena, and further development of techniques for using models that depart from flat-lying layered structures. In fact, the inadequacy of spherical or flat-layered models that portray the earth as lacking in lateral hetero- geneity is growing in importance and has probably reached the crisis stage in the case of the continental basement. Our level of understanding has reached the point where refinement of such models may be more mis- leading than informative. As a first approximation, geo- physicists have utilized simplified layered models, not only in the case of seismic surface waves but also in many other kinds of seismic studies and in other branches of geophysics as well. This approach is not without good reason. Gravity is an important factor in geology, and hence there is indeed a strong tendency for spherical layering. Furthermore, layered models or one-dimen- sional models are relatively easy to handle Mom the theo- retical point ofview. Such simple models are no longer an adequate representation ol the continental crust, where a bewildering variety of structures and rock types is found in almost any large outcrop (Figure 10.~. Figure 10.1, taken from a paper by Smithson et al. (1977), but in turn borrowed from Berthelsen (1960), shows pyroxene granu- lite layers surrounded by granitic gneiss. The important point in the present context has to do with the complex three-dimensional structure of this feature. Resolution of such complex buried contortions by seismic methods, or any methods, will be difficult, but on the other hand to apply flat-lying layered approximations to such structures is nonsense. We must develop models and observational and analytical techniques that will provide information an structures of complexity greater than that of simple layers and to the level of complexity shown in Figure 10.1, if possible. Such structures must be probe c] at depth within the crust, and perhaps the upper mantle. Seismologists have been moving away gradually from simple-layered models for some time. Plate tectonics was a step in this direction. Perhaps more than anything, study of lateral variations and complex structures will charac- terize the application of seismology to the study of the continental basement during the 1980's. Continuing through the spectrum to shorter wave- lengths, consider the body waves generated by earth- quakes, typically in the range of a few seconds per cycle to a few cycles per second. The body-wave travel-time method, which has produced so much of our knowledge of the earth's deep interior, has been applied widely on a crustal scale for study of the continents. However, the

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Seismic Exploration of the Continental Basement FIGURE 10.1 A three-dimensional structure in the continental basement. To approximate such structures by [lat- layered models is of very limited value. From Smithson et al. ( 1977) (taken from Berthelsen, 1960). IBM 1~ traditional, so-called near-earthquake studies have not produced much new information recently, partly because superior precision and flexibility may be had by use of artificial sources and partly because of the limitations of the flat-layered models usually used in interpretation of near-earthquake studies. In recent years there have been some significant depar- tures from the well-wom path, however. Tightly spaced detection networks have produced more precise locations of sources in time and space and more reliable travel-time data. Models involving lateral variations of structure such as faults can be considered using powerful computer- based methods. Techniques involving differential travel times at networks of stations encompassing a particular feature have led to investigation of features of unusual shape' the work, for example, of Aki et al. (1977) and the group at the U.S. Geological Survey (USGS) flyer, 1973; Ellsworth and Koyangi, 1977~. On the basis of the clear signs of life in current study of body waves, and the renewed emphasis that is likely to result from new efforts to predict earthquakes and reduce the earthquake hazard and to detect and identify nuclear explosions, one can foresee new results, particularly those emphasizing lateral variations, over the next decade at least. The keys for application ofthree-dimensional inver- sion of travel times are tightly spaced, probably movable, networks and developments in methods for using not only travel-time differences but also wave character resulting from attenuation, focusing, conversion, or other effects. NATURAL SEISMIC SOURCES In the case of natural seismic sources, perhaps as much information can be obtained from the sources as from the wave propagation. Historically, with each increase in pre- cision of location of hypocenters of earthquakes, our un- ~9 ~,~ 1.1 . ~ ~ it' _ _ derstanding of tectonics has grown. For example, in the 1950's and earlier, hypocenters of earthquakes in the southern hemisphere were frequently mislocated by more than 100 km. At that stage, an earthquake could at best be associated with a major regional feature such as an island arc. In the 1960's, with the advent of the World Wide Standardized Seismic Network (WVItSSN) and other advances, it became possible to locate hypocenters with a precision of less than 10 or 20 km. Earthquakes could then be associated with tectonic features of smaller scale- a rift valley, a deep earthquake zone, the outer wall of a trench. Now, with the closely spaced observing networks that are available in a few areas, earthquake hypocenters can be located with a precision of less than a kilometer, much less in some cases. Hence we can now confidently asso- ciate earthquakes with particular faults, and also associate properties ofthe quake with properties ofthat fault. Fault- plane solutions tell us of the orientation of the fault and the movement; other focal mechanism data tell us of the change in stress and scale of the movement. Further improvements in precision of hypocentral loca- tion may tell us about movements along a particular part of a fault, progression in fault activity, and complexities such as asperities and slices. Figure 10.2 shows how an interpretation of tectonics may be strongly dependent on the precision of hypocentral data. This figure is taken from a paper by Barazangi and Isacks (in press). A cross section of the seismicity through central and northern Peru is shown twice. In the upper half of the figure only hypocenters of very high precision are plotted. In the lower half of the figure other hypocenters located during the same time period but with lower precision are shown. There is clearly a great contrast between the structure defined by the well-located hypocenters and what might be deduced from the less well-Iocated hypocenters. A similar effect may be anticipated at smaller scale. Other modem techniques are telling us of fault move-

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120 meets that are slower than the abrupt displacements of typical earthquakes. Such slow movements may fail to generate short-period seismic waves, may generate only very long-period seismic waves, or may generate no detectable seismic wave and may be thought of as epi- sodes of fault creep. Such studies enhance greatly our understanding of fault motion. The study of seismicity and sources is an area of vast potential and one in which integration of seismology and geology is likely to be crucial. One may confidently state that interaction of seismol- ogy, or geophysics in general, and geology is likely to increase markedly in the next decade. A recent report by the NRc Committee on Seismology (1976) notes that much of the progress in unclerstanding the problem of earth- quake hazards in recent years has come from geological, not purely seismological, evidence. Plans to drill deeply into the San Andreas Fault are afoot. We can anticipate growing interaction between these disciplines, with mutual benefit as our understanding of the continental basement grows. ARTIFICIAL SEISMIC SOURCES In general, artificial sources have the a~lvantage of precise timing, simplicity and control of source Unction, and flexibility of location and the disadvantage of lower en- ergy except in the special case of nuclear explosions. Thus, nonnuclear artificial sources are currently of no value for study ofthe deep parts ofthe earth's interior, but for the continental crust ant! the upper mantle they can indeed provide information, ant! in fact the infonnation with the best resolution at the level of detail that we now are seeking. Seismic studies using artificial sources fall loosely and somewhat arbitrarily into three classes: (1) refraction, (2) deep seismic sounding (DSS) or refraction and wide-angle reflection, and (3) reflection profiling. The refraction rnetho~ is responsible for much of what is known about the structure of the deep crust; it provides the depth to the Mohorovicic discontinuity and typically a simple model of crustal structure consisting of layers of different velocities. Seismic refiaction studies have been carried out in the United States by venous university and private groups and the USGS. However, almost all of this work was clone in the 1960's and before. Figure 10.3, from Prodehl (1977), shows a summary of crustal models deduced from refraction data for venous parts of the United States. Nearly flat-lying layered models prevail for each area, and the differences from one area to another are illustrated at the bottom of the figure. There is considera- ble similarity in all the crustal models, and, in fact, the rather uniform depth of the mantle throughout the conti- nent is a rather striking feature. There are also substantial and measurable variations from one province to another on this gross scale. This figure is a summary of current knowledge of U.S. crustal structure based on refraction JACK E. OLIVER data. Surprisingly, for about the last decade there has been relatively little activity of this type directed toward study of the continental basement In the United States. Abroad, however, the stow Is different. The Soviet Union has operated a program of crustal exploration at a very high level of activity since World War II and has developed the DSS method, in essence the use of re- fracted and wide-angle reflected waves observed with closely spaced stations, to determine crystal structure with resolution better than that obtained by refraction measurements alone. Furthennore and after a somewhat slower start, groups of European seismologists have been using similar techniques and obtaining detailed and abundant results. Figure 10.4 shows a typical set of seis- mic refraction data from Europe arid a crustal model deduced Tom the data. Detailed velocity{lepth function and a great deal of other seismic information are obtained that is not explained by the simple model. The work in the Soviet Union and in Europe is typically characterized by much more thorough observation through very closely spaced detecting stations than is the case for the older studies of this type in the United States. The papers in Giese et al. (1976) provide a comprehensive survey of explosion seismology in central Europe. Further seismic retraction studies in the United States will surely provide new information on the crust The extent to which the new information will be coupled with other geological and geophysical information to provide enhanced under- standing of the continents will depend on He further cle~relopment of methods of interpretation to produce more realistic crustal models than the present simple layered configurations. The models should present a more realistic geological picture. The newcomer on the scene of seismic exploration of the continental basement is seismic reflection profiling. In venous countries, within the last decade, this tech- nique has been applied with some modification to study of basement to depths as much as 50 or so kilometers, primarily by the Consortium for Continental Reflection Profiling (COCORP) project in the United Sates (see Oliver et al., 1976, for a review). The method uses closely spaced vibratory or explosive sources, listening arrays of thou- sands of geophones, and sophisticated computer analysis of the data. Typically, tens of millions of rays penetrating the earth are sampled and utilized at each site. The potential resolution of structural features ofthe deep crust through reflection profiling is greater than that of any other method, but it is no small operation. Figure 10 ~ shows a field party in action. Of particular interest is that the sources of the seismic waves are not explosives but large truck-mounted vibrators. The V=ROESIS method (registered trademark of the Continental Oil Company) has been used in all the cOcORP studies to date. The re- sults Tom seismic reflection profiling so far clearly indi- cate vast potential for this method in fixture studies of the continental basement. Figure 10.6 shows a seismic reflection profile across the Rio Grande nR. The abscissa is distance in decameters;

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Seismic Exploration of the Continental Basement 5 TRENCH COAST ANDES ~ AL 7~ ' 300 ' 'U: su 5 it, I ~ , , ~ IT ~ , ~ - 70C 100: 200L SECTIONS 2,3 AND 4 | CLASS a AND B I o o Loo deco - 7 OC o ~ o ~ ~ cat i ~~ 8 ~ 0 ~ ~ ~` 'en ~ C ~ C ~ of oC ~ ~ ~ ' - - ., ! 1 BLOC 2, 3 4 200 400 600 ~ 00 ~ ~ ~ ~ o _ =~ `~ A i ~ ~ - ~ ~ A ~ ~ ~~ I ~ ~ - ~ 00 ~~ ~ s~ - V amp ~ - ^ tOC 200 c SECTIONS 2 ,3 AND 4 o | CLASS C AND D I ~ I ! 200 300 ~ -- 00 121 ~ of NO 3 ~ o o o I o 400 DISTANCE, KM. 1100 [20C 6 1 - - 1 - 1300 FIGURE 10.2 A generalized section through Peru showing by hypocentral locations of high precision in upper view and other hypocentral locations of lower precision in lower view. Combining these two sets of data can result in an interpretation different from one based only on the better data. From Barazangi and Isacks (in press). FIGURE 10.3 A summary by region of seismic refraction studies of the U.S. continental crust. Note (1) overall simi- larity, (2) variations from one province to another, (3) use of layered models. From Prodehl (1977, copyrighted ' American Geophysical Union). - ~ ,/' T +~ - ~ interior Plains / ~ r Atlantic Plain 04~' O. ~~ 0 ~ 2 3 ~ 5 _ ~~ _ lo Velocity tams) 6 ? 8 9 to

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122 - -2 FIGURE 10.4 Typical record sec- tion, travel-time curves, and flat- layered model based on refraction study. From Mueller ( 1977, copy- righted by American Geophysical Union). 45 ~ ~ 5 B 7 the ordinate is two-way travel time in seconds. To convert time in seconds to depths in kilometers approximately, multiply by 3. In a gross sense such a section may be thought of as a section of the eat, but a detailed interpre- tation of the data requires knowledge of wave propagation and the data processing involved, so that it is invalid to assume that the details of a time section such as the one shown are necessarily or precisely features of the deep earn. This section, for example, has not yet been sum jected to migration to position the reflections in their proper spatial locations. Even so, certain gross features can be seen in the data that are illustrative of the capabil- ity of the method. The sedimentary sections and the sedi- mentary basement boundary are clear near the top of the section. There is an intragraben horst of substantial size and other more subtle evidence for faulting of the sedi- ments. At the time of about 7~ see there is a rather strong reflector that corresponds to the top of the magma body first proposed for this area by Sanford et al. (1977) on the FIGURE 105 Seismic reflection pro filing park in the field in Wyoming. JACK E. OLIVER ._ ~ .~ _ ' 101) tSO 200 250 ~(km) STEINBRUNN-SW 8 ~ Vpiltm/ - c) basis of microearthquake data. Although this particular section does not show much information at a time corre- sponding to the crust~nantle boundary, other sections do. Figure 10.7 shows a line drawing of the data from the section in Figure 10~6 and another section to the east. The hvo sections together span the rift valley in the vicinity of Belen, New Mexico. The top of the basement and various sedimentary features are clear, as are certain other fea- tures within the basement itself. The magma body shows as a strong reflector in parts of both sections. At a time of about 12 see in the easternmost section there is an arrival that may be associated with the crust~nantle boundary, although it is not continuous for a very long distance. Other profiles in the Rio Grande no area show a stratified pattern to the reflectors in the vicinity of this boundary. Much more detail can be found throughout the section in the original data. Figure 10.8 shows a line drawing of a short section talcen as a test of the method in the vicinity of the San am; ~,= - , ~ . _ ... , .. _, ., ... 1 .% .-g -;~ _ - ILL ~ _

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Seismic Exploration of the Continental Basement VP NO. 400 3SO 1 1 FIGURE 10.6 COCORP seismic reflec- tion profile (unmigrated) across west- em part of Rio Grande no north of Socorro. Abscissa Is vibration point number or distance in decameters; or- dinate is two-way travel time in sec- onds. Velocity structure from a nearby refraction study shown in column on left. Courtesy Larry Brown, Comell University. Andreas Fault. There is great contrast in the data from one side of the fault to the other. In the fault zone, to a depth of about 10 or 12 km, diffraction hyperbolas, associated with discontinuities marking the fault zone, are seen. Below Mat zone, however, there is a region of little infor- mation, suggesting that the zone was not penetrated by the seismic waves, which would be surprising in view of SOCORRO Ll N E lA w WHO SOCORRO Ll N E I A 123 2s0 200 1 ~ 50 100 1 1 so 10 1 1 o ~5 We information from much greater depths on both sides, or that the zone is so distorted through flow that no coherent reflected energy was obtained. The latter seems more probable. At a depth near the crust-mantle boundary, particularly in the western part of the section, there are many reflectors, suggesting a complex feature for this boundary. ABO PASS Ll NE I 400 5.S ~ - - A,-_ ~ ~ Ad. 5 _ 6. 7.5 _~ . N~ ^ 200 150 100 so 10 10 5c '~,=~ ' -` ~ _ . ~ - f VP ~ 00 150 200 250 ~-~` Am- - . ,. ~ 10 IS FIGURE 10.7 Line drawing based on data of Figure 10.6. See text for discussion. Column on leR shows velocity structure determined from a nearby seismic refraction study.

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124 JACK E. OLIvER Sw 10 0.0- 2 n- 2 - wAr 3 0- TI~E 4.0- 5.0- 6.+ 7.0- ARKFIEL3 CAL~F SAF SAF 1 1 HE ~ so ~ ~ 60 ~ 8 ~ `= 110 i~ '= ." i~ '" '70 ' - t~ 2= 2~0 220 2= 24c 2x 2" I ~ ! ' I I I I ~ I I I ~ ~ I ~ I ~ ~ I ~ 1 1 ~ 10.0- 1 1.0- 12.0- FIGURE 10.8 Line drawing of short test section crossing San Andreas Fault near Parkfield. Distance in decameters. Tw~way travel time in seconds. S.A.F. indicates boundary of San Andreas Fault zone. See text for discussion. Figure 10.9 shows a map of the southeasternmost por- tion of the Wind River range and indicates the line of a seismic reflection profile made in this area. Line drawings of parts ofthis section are indicated in Figure 10.10. There is a great clear of over information of considerable variety. The most prominent feature is the eastward extension of We thrust fault on We western boundary of the Wind Rivers Mountains. This Cult extends to a depth of at least 25 km and more likely to 40 km without much change in dip. These data, then, seem to resolve the long-standing geological problem of the origin of the Wind Rivers in We sense that compressional forces seem to predominate as opposed to those producing vertical uplift. Substantial shortening of the crust is clear. CONCLUS ION With We possible exception of study of the lower modes of free oscillations, all the seismic methods described above have something substantial to contribute to under- standing of the continental basement. Furthermore, there are other useful methods and techniques that I will not discuss here, and in any case the dividing lines between the various methods are somewhat arbitrary. My purposes are (l ~ to draw attention to the large unrealized potential for understanding the continental basement through application of present-day seismic methods and 2J to point to means for improving that potential through con- tinuing evolution and improvement of seismological methods. Suppose, for example, that a method could be found for artificially generating shear waves of sufficient amplitude so that the deep crust and upper mantle could be explored using such waves, or that movable, tightly spaced seismic arrays could be deployed so as eventually to cover the continent. Suppose the dee~sea floor, the continental shelves, and venous remote areas were no longerareas of essentially no seismic data on earthquakes. Suppose that seismic profiling were conducted along long, closely spaced lines spanning the continents. A1- though this might seem like an ambitious undertaking to some, one should remember that it is only within about the last 30 years that there has been significant seismic exploration of the deposed floor and only about 20 years since reflection profiling ofthe seafloor began. The ocean basins, which, of course, occupy a much greater portion of the earths surface than do Me continents, have in that short time been crisscrossed innumerable times by seis- mic reflection profiling. A comparable achievement in the form of seismic profiling of the continents is within the grasp of the present generation of geophysicists. Suppose microprocessors and other electronic developments result in more sensitive and selective seismographs and new ways of managing large complex sets of data. Suppose the seismic reflection method is generalized so that three- dimensional images rather than tw~dimensional sections are produced. Suppose comparable advances are made concurrently in related branches of geophysics and geol- ogy. Every one of these suppositions is technically within our reach today. If even some of these suppositions are fulfilled, I think we can look forward to a new understanding of Me crust, one in which deep subsurface structural features become as familiar to continental scientists as the midocean edges, the trenches, the seamounts, and the submarine canyons have become to ocean scientists over the last few decades. When they are, surely our understanding of the evolution and the genesis of continents will become more

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Seismic Exploration of the Continental Basement 30' /5' _ I0900' 45' W Do' 15' 10800' 45' . .~ ~ ~ / ]4300' .' .,'.' ~ .. ~ \ . 1 ~ ,. . to,; got Jon Toteraccle Butte \ f / Aikoli Otter i5 elf P~ p,450 l -~ ~ ~ =~ ~ ~ ::: :- : I; 'T' "I ~ Who r'~ ' "''2""."~ \~N \ ~~\~' :/~250 /~00 ~50 6' :~OC~,>~ Oregon Buttes OContlnenla Peal _ ,45' N 30' _ /5' 42-00' 0 10 FIGURE 10.9 Clap of southeastern Wind River Mountains in Wyoming showing location of COCORP seismic reflection line. COCORP wrOM`NG I iNE ~ wtOMiNG L'NE iA G~_ R`~ ~~.e S. sr~rro~ ~vu-ffeS 50 `00 Is0 20o 250 100 ...a Q,~e, 84,n, sour~ PaSS C/TY | sr~rloa au_tes 20 m 125 WYOM`NG LINE 2 a.~ ei~.,. ~ c,'. ~E s0 It00 150 2nt~ ~~ ~ ~ ,~ ~ ~ ,~ 5rA7~ ~RS / 1 \ _' ~ r? -_~ ~_-- ~ -___ '~ \ _ \ \ \,,,- 1s 0 1 . i 20.0 FIGURE 10.10 Line drawing of section along line of Figure 10.9. Note especially major thrust fault bounding the Wind River uplift on west. See text for discussion. Courtesy Jonathan Brewer and Scott Smithson, University of Wyoming.

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148 Imm llJ cleft, 100 IOkb Ikb 1 3 lOOb lOb lOmm ~ , I ~ 1 ' ~ _ 101` l ll + !+ m.g.d. "C ~-1.6 - 1 1, 1, 1, 1, 1, 1, 10 9 8 7 6 ~ 0 10 10 10 10 10 JO cm~ FIGURE 12.9 Mean grain size versus dislocation density and differential stress for olivine. Analogous relations can be defined for plagioclase and clinopyroxene and should be useful in defin- ing differential stress for defonned granulites. Figure is from Koldstedt et al. (1976). An important question that follows from study of the retrograde hydration in some xenoli~s is the source ofthe water. Stable-isotope studies (hydrogen and oxygen) may be able to distinguish alternatives such as hydrothermal circulation or dehydration of underlying subducted oce- anic lithosphere. Microcrack studies may also be impor- tant in defining mechanisms for migration of fluid phases in the deep crust (Simmons and Richter, 1978~. Substantial progress on the dynamics of upper-mantle flow has been made by studies of deformation mech- anisms of olivine in the laboratory and application to peridotite xenoli~s (e.g., Kohlstedt et al., 1976~. A paral- led study using lower crustal granulites deserves atten- tion. The density of unannealed defects can be examined using the transmission electron microscope. Experi- mental work needs to be done to understand the deforrna- tion mechanisms of feldspar and pyroxene, probably the most abundant lower crustal minerals. In addition, the simple relationship between grain size and stress found in olivine (Figure 12.9) encourages We search for a simi- lar relationship for feldspars and pyroxenes. One ob- serves Mat grain size in basic granulites is variable from locality to locality; could this reflect a variable stress in ROBERT W. I[AY and SUZANNE MAHLBURG KAY Me lower crust? If stress and temperature can be deter- mined for a xenolith, Den strain rate can be calculated, and questions of lower crustal dynamics can be addressed. SUMMARY AND CONCLUSIONS Xenoliths and magmas can help to answer such questions as (a) what is the parentage of the lower crust, i.e., per- centage of original igneous versus sedimentary material, percentage derived directly from the mantle versus ma- terial derived from crustal processes; (b) is the lower crust hydrous or anhydrous; (c) what are the temperature- pressure regimes in the lower crust; (d) are rocks in the lower crust at equilibrium with present or past tempera- ture~ressure conditions; (e) what are the age relations between various units of the lower crust; and (f) what is the deformation history and what is the present state of stress in the lower crust? When knowledge derived from xenoliths and magmas is combined with drill holes and geophysical information, a three~imensional picture of the crust can be constructed. Crustal studies have barely begun: a great expansion of our ability to answer all these questions can be expected in the near future. ACKNOWLE DGM E NTS We thank D. Smith and E. Padovani for reviews and members of the Continental Tectonics Panel for discus- sion and acknowledge Me National Science Foundation for providing financial support under Grant EAR 77- 13656. BE FERENCE S Ahrens, T., and G. Schubert (1975). Gabbro~clogite reaction rate and its geophysical significance, Rev. Geophys. Space Phys. 13, 383~00. Armstrong, R. L., W. Taubeneck, and P. Hales (1977). Rb~r and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington and Idaho, Geol. Soc. Am. Bull. 88, 397411. Arth, J. G., arid G. N. Hanson (1975). Geochemistry and origin of the early Precambrian crust of northeastern Minnesota, Geo- ch~m. Cosmochim. Acta 39, 325~62. Ben Othman, D., A. Juery, and C. J. Allegre (1978). Limitations on granite connation inferred by Nd, Sr and Pb isotope sys- tematics (abstr.), Eos Trans. Am. Geophys. Union 59, 392. Berckhemer, H. (1969). Direct evidence for the composition of the lower crust and the Moho, Tectonophysics 8, 97-105. Bilal, A. (1976). Les fluides carboniques dans les enclaves char- nockitiques de Boumac (Masif Central firangais). Implications sur les structures de Ia croute continentals, These Be cycle, Nancy. Bloomer, A. G., and P. J. Nixon (1973). The geology of the Letseng-la-terae kimberlite pipes, in Lesotho K:mberl~tes, P. H. Nixon, ea., Lesotho National Development Corporation, Masern, Lesotho, pp. 20~2.

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Chemistry of the Louver Crust Brookins. D. G., and H. O. A. Meyer (1974). Crustal and upper mantle stratigraphy beneath eastern Kansas, Geophys. Res. Lett. 1, 969-272. Brookins, D. G., and M. J. Woods (1970). Rb~r geochronologic investigation of basic and ultrabasic xenoliths from the Stock- dale kimberlite, Riley County, Kansas, Kansas Geol. Sure. Bull. 199, 3-12. Brooks, C., D. E. James, and S. R. Hart (1976). Ancient litho- sphere: its role in young continental volcanism, Science 193, 1086-1094. Buckley, G. (1973). The effect of diffusion on garnet zoning, Ph.D. thesis, U. of Illinois, 112 pp. Cook, F. A., E. R. Decker, and S. B. Smithson (1978). Preliminary transient heat flow model of the Rio Grande rift in southern New Mexico, Earth Planet. Sci. Lett. 40, 316~26. Dalmayrac, B., J. Lancelot, and A. Leyreloup (1977). Two billion year granulites in the late Precambrian metamorphic basement along the southern Peruvian coast, Science 198, 49~1. Decker, E. R., and S. B. Smithson (197~). Heat flow and gravity interpretation across the Rio Grande rip in southern New Mex- ico and west Texas, J. Geophys. Res. 80, 2S42-2552. DeLong, S. E. (1974). Distribution of Rb, Sr, and Ni in igneous rocks, central and western Aleutian Islands, Alaska, Geochzm. Cosmochzm. Acta 38, 245-266. Francis, D. M. (1976). Corona-bearing pyroxene granulite xeno- liths and the lower crust beneath Nunivak Island, Alaska, Can. Mineral. 41, 291-298. Frazer, D., and P. Lawless (1978). Paleogeotherms: implications of disequilibrium in games Iherzolite xenoliths, Nature 273, 220-222. Goldsmith, J. R. (1976). Scapolites, granulites, and volatiles in the lower crust, Geol. Soc. Am. Bull. 87, 161-168. Green, T. H. (1976). Experimental generation of cordierite or games-bearing granitic liquids from a politic composition, Geology 4, 85~8. Griffin, W. L., D. A. Carswell, and P. H. Nixon (1979). Lower crustal granulites and eclogites from Lesotho, Southem Africa, in The Mantle Sample: Inclusions in Kimberlites and Other Volcanics, F. R. Boyd and H. O. A. Meyer, eds., American Geophysical Union, Washington, O.C., pp. 09-86. Gromet, L. P., and L. T. Silver (1978). Implications of rare earth distribution among minerals in a granodiorite. Peninsular Ranges Batholith, Southern California (abstr.), Eos Trans. Am. Geophys. Union 59, 399~4C)O. Hamet, J., and C. J. Allegre (1976). Hercynian orogeny in the Montagne Noire (France): application of 87Rb-~7Sr systematics, Geol. Soc. Am. Bull. 87, 1429-1442. Hamet, J., A. Vitrac, and C. J. Allegre (1978). 87Rb-~7Sr and U-Pb systematics in granulitic xenoliths in volcanic rocks and limita- tions on the formation of the lower crust (abstr.), Eos Trans. Am. Geophys. Union 59, 392. Hargraves, R. (1976). Precambrian geologic history, Science 193, 363~71. Heier, K. S. (1973). Geochemistry of granulite f:acies and prob- lems of their origin, Phil. Trans. Roy. Soc. London S273, 429Js42. Helmstacdt, H., and D. J. Schulze ( 1977). Type A type C eclogite transition in a xenolith from the Moses Rock diatreme. Further evidence for the presence of metamorphosed ophiolites beneath the Colorado Plateau, in Extended Abstracts of the Second International K~mberl~te Conference, 1977. Herzberg, C., and N. Chapman ( 1976). Clinopyroxene geo- thermometry of spinel-lherzolites, Am. Mineral. 61, 696~37. i49 Hofmann, A. W., and S. R. Hart (1978). An assessment of local and regional equilibrium in the mantle, Earth Planet. Sci. butt. 38. 4442. Holdaway, PI. J. (1971). Stability of andalusite and the aluminum silicate phase diagram, Am. J. Sci. 271. 97-131. Irving, A. J. (1974). Geochemical and high pressure experimental studies of games pyroxenite and pyroxene granulite xenoIiths Mom the Delegate basaltic pipes, Australia,l. Petrol. 1~. 1~0. Katsui, Y. K. Niida, M. Yamonoto, and S. Nemoto (1978). Genesis of calc-alkalic andesite from Oshima-Oshima and Ichinome- gata volcanoes, North Japan (abstr.), in International Geody- namics Conference "Western Pacific" awl "magma Genesis," Tokyo, March 13-17, 1978, Inter-Union Commission on Geodynamics, and Science Council of Japan, p. 966. Kay, S. NJ., R. W. Kay, J. Hangas, and T. Snedden (1978). Crustal xenoliths from potassic lavas, Leucite Hills, Wyoming, Geol. Soc. Am. Abstr. Programs 10, 432. Kistler, R. W., and Z. Peterman (1973). Variations of Sr, Rb, K, Na and initial 87Sr/86Sr in Mesozoic granitic rocks and intruded wall rocks in central California, Geol. Soc. Am. Bull. 84, 3489~512. Kohlstedt, D., C. Goetze, and W. Durham (1976). Experimental deformation of olivine single crystals with applications to flow in the mantle, in Petrophys~cs: The Physics and Chemistry of Minerals and Rocks, R. Strens, ea., John Wiley, New York, pp. 35 Js9. Kroner, A. (1977). The Precambrian geotectonic evolution of Africa: plate accretion versus plate destruction, Precambrian Res. 4, 163-213. Lachenbruch, A., and J. Sass (1977). Heat flow in the United States and the thermal regime of the crust, in The Earth's Crust, J. Heacock, ea., Am. Geophys. Union Geophys. Monogr. 20, pp. 626~75. Leyreloup, A., C. Dupoy, and R. Andriambololona (1977). Cata- zonal xenoliths in French Neogene volcanic rocks: constitution of the lower crust, Contrib. Mineral. Petrol. 62, 283~00. Lipman, P., 13. Doe, C. Hedge, and T. Steven (1978). Petrologic evolution of the San Juan volcanic field, southwestern Colo- rado: Pb and Sr isotope evidence, Geol. Soc. Am. Bull. 89, 59~2. Loomis, T. P. ( 1976). Kinetics of a gannet granulite reaction, Con- tnb. Mineral. Petrol. 62, 1-22. Lovering, J. F., and A. J. R. White (19t;8). Granulitic and eclogitic inclusions from basic pipes at Delegate, Australia, Contrib. Mineral. Petrol. 21, 9~. McGetchin, T. R., and L. T. Silver (1972). A crustal - upper mantle model for the Colorado Plateau based on observations of crystalline rock fragments in the Loses Rock Dike,J. Geophys. Res. 77, 7029-7037 Mehnert, K. R. (1975). lithe Inea Zone a model of the deep crust, Neues Jahrb. Mineral. Abh. 125. part 2, 156-199. Meyer, H. O. A., and D. G. Brookins (1976). Sapphirine, silli- manite and garnet in granulite xenoliths from Stockdale kim- berlite, Kansas, Am. Mineral. 61, 1194-1202. Necut, A., J. Connerney, and A. F. Kuckes (1977). Deep crustal electrical conductivity: evidence of water in the lower crust, Geophys. Res. Lett. 4. _39-~42. Ore ille, P. ~\I. ( 1963). Alkali ion exchange between ~ apor and feldspar phases, Am. J. Sci. 261, 678~83. Padovani, E., and J. Carter (1977). Aspects of the deep crystal evolution beneath south central New Mexico, in TI2e Earth's Crust, J. Heacock, ea., Am. Geophys. Union Geophys. Nlonogr. 20, pp. 19~.

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150 Papike, J. J., K. L. Cameron, and K. Baldwin (1974). Amphiboles and pyroxenes: characterization of other than quadrilateral components and estimates of femc iron from microprobe data, Geol. Soc. Am. Abstr. Programs 6, 1053-1054. Phinney, D., I. Tennyson, and U. Trick (1978). Xenon in CO2 well gas revisited, J. Geophys. Res. 83, 2319-2319. Reiter, M., C. Shearer, and C. L. Edwards (1978). Geothermal anomalies along the Rio Grande riPc in New Mexico, Geology 6, 85-105. Rogers, N. W (1977). Granulite xenoliths from Lesotho kimber- l~tes and the composition of the lower crust, Nature 270, 681~84. Schmid, R., and B. J. Wood (1976). Phase relationships in granu- litic metapelites from the Irvea-Verbano zone (northern Italy), Contnb. Mineral. Petrol. 54, 255-279. Shieh, Y.-N, and H. P. Schwarcz (1974). Oxygen isotope studies of granite and migmatite, Grenville province of Ontario, Canada, Geochim. Cosmoch~m. Acta 38, 21~5. Shimazu, M., T. Yano, and M. Tazima (1978). Gabbroic inclu- sions in calc-alkalic rocks of the Fossa Magna, central Japan (abstr.), in International Geodynamics Conference "Western Pacific" and "Magma Genesis," ToJcgo, March 13-17, 1978, Inter-Union Commission on Geodynamics, and Science Coun- cil of Japan, pp. 324~25. Simmons, G., and D. Richter (1978). Microcracks in rocks, in The Physics and Chemistry of Minerals and Rocks, R. Strens, ea., Wiley-Interscience, New York, pp. 105-137. Smith, D. (1977). Hydrous minerals and carbonates in peridotite inclusions from the Green Knobs and Buell Park kimberlitic diatremes on the Colorado Plateau, in Extended Abstracts of the Second International Kimberlite Conference, 1977. Smith, D., and S. Levy (1976). Petrology of the Green Knobs diatreme and impIications for the upper mantle below the con- tinental plateau, Earth Planet. Sc:. Lett. 29, 107-125. Smithson, S., and S. Brown (1977). A model for lower continental crust, Earth Planet. Sci. Lett. 35, 132-144. Smithson, S., J. Brewer, S. Kauf~nan, J. Oliver, and C. Hunch (1978). Nature ofthe Wind River thrust, Wyoming, from COCORP deep-reflection data and from gravity data, Geology 6, 648~52. Stephenson, P. J., and T. J. Griffin (1976). Cainozoic Volcanicity North Queensland, Guidebook Excursion No. 7A, 25th Inter- national Geologic Congress, 39 pp. Stoesser, D. B. (1973). Mafic and ultramafic xenoIiths of cumulus origin, San Francisco volcanic f~eld, Arizona, Ph.D. disserta- tion, U. of Oregon. Takahashi, E. (1978). Petrological model of the upper mantle and the lower crust of the island arc: petrology of mafic and ul~a- mafic xenoliths in Cenozoic all~ali basalts of the Oki-Dogo Is- land in the Japan Sea (abstr.), in International Geodynamics Conference "Western Pacific" and "Magma Genesis," Tokyo, March 13-17, 1978, Inter-Union Commission on Geodyna- mics, and Science Council of Japan, pp. 334~35. ROBERT W. KAY and SUZANNE MAHLBURG KAY Tarney, J. (1976). Geochemistry of Archean high-grade gneisses, with implications as to the origin and evol~tion of the Precam- brian crust' in The Early History of the Earth, B. Windley, ea., Wiley-Interscience, New York' pp. 405~17. Taylor, S. R. (1977). Island arc models and the composition of the continental crust, in Deep Drilling Results in the Atlantic Ocean, Am. Geophys. Union Maurice Ewing Senes, Vol. 1, pp. 229-242. Taylor, H. P., and B. Ttlri (1976). FIigh ~xo igneous rocks from the Tuscan magrnatic province, Italy, Contrzb. Mineral. Petrol. 55, 33~5. Turi, B., and H. P. Taylor (1976). Oxygen isotope studies of potassic voIcanic rocks of the Roman province, central Italy, Cont~b. Mineral. Petrol. 55, 1~1. Vogel, D. E., and G. D. Garlick (1970). Oxygen-isotope ratios in metamorphic eclogites, Cont~b. Mineral. Petrol. 28, 188-191. Wass, S., and A. J. Irving (1976). Xenmeg: A Catalogue of Occur- rences of Xenoliths and Megacrysts in Basic Volcanic Rocks of Eastern Australia, Australian Museum, Sydney, 441 pp. White, A. J. R. (1964). Clinopyroxenes from eclogites and basic granulites, Am. Mineral. 49, 883~89. White, A. J. R., and B. W. Chappell (1976). Ultrametamorphism and granitoid genesis, 25th Int. Geol. Congr. Abstr. 3, 674~75. Whitford, D. J., W. Compston, I. Nicholls, and M. Abbott (1977). Geochemistry of late Cenozoic lavas firom eastern Indonesia: role of subducted sediments in petrogenesis, Geology 5. 571~75. Whitney, J. \, and J. C. Stonner, Jr. (1977). Geothennometry and geobarometry of epizonal granitic int~usions: a comparison of iron~itanium oxides and coexisting feldspars, Am. Mineral. 61, 751-761. Wyllie, P. J. (1971). Experimental limits for melting in the earth's crust and upper mantle, in The Structure and Physical Proper- ties of the Earth's Crust, J. Heacock, ea., Am. Geophys. Union Geophys. Monogr. 14, pp. 279~01. Wyllie, P. J., W. -L. Huang, C. R. Stem, and S. Maal~e (1976). Granitic magmas: possible and impossible sources, water con- tents and crystallization sequences, Can. 1. Earth Sci. 13, 1007-1019. Yoder, H. S., and C. E. Tilley (1'-~2). Origin of basalt magrnas: an experimental study of natural and synthetic rock systems, J. Petrol. 3, 342~32. Zartman, R. (1974). Lead isotopic provinces in the Cordillera of the westem United States and their geologic signif~cance, Econ. Geol. 69, 792~05. Zielinski, R. A., and P. W. Lipman (1976). Trace-element varia- tions at Summer Coon volcano, San Juan Mountains, Colorado. and ~e origin of continental-interior andesites, Geol. Soc. Am. Bull. 87, 1477-1485.

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Geochemical Evolution of the Continental Crust l2 INTRODUCTION GILBERT N. HANSON State University of New York at Stony Brook To place limits on possible origins of the earth's conti- nental crust it is necessary to understand how the earth's crust has evolved and how the various processes acting have modified the geochemistry of the pre-existing crust. Prior to about 3900 million years (m.y.) ago the earth as well as the moon must have undergone significant infall of very large extraterrestrial bodies (Smith, 1976~. This bombardment must have played a significant role in crustal evolution. However, on the earth the record of this event has yet to be found. Thus speculations on the geo- chemical evolution of the continental crust based on the lithological record must start from 3800 m.y. ago, the age of the oldest terrestrial rocks found so far. The main purpose of this chapter is to suggest isotopic and trace-element approaches useful for studies leading to a better understanding of the geochemical evolution of the earth's continental crust. There are a number of recent papers pertinent to this topic, for example, Lowman (1976~; Tugarinov and Bilikova (19161; Smithson and Decker (19741; Smithson and Brown (1977~; Hargraves 151 (1976~; Taylor (in press); Tarney and Windley (1977~; Armstrong and Hein (19731; Jahn and Nyquist (19761; Moorbath (1977~; Heier (1973~; Tarney (1976~; Collerson and Fryer (1978~; Green (1972~; Pankhurst (19771; Brooks et al. (1976b); O'Nions and Pankhurst (19781; Oversby (19781; Engel et al. (19741; Shaw (1976), and O'Nions et al. (1979). Models for the evolution of the crust can be placed between two extreme schools ofthought (also see Chapter 15). One is that the continental crust formed early in the history of the earth (during the Archean) and that only small fractions of material have been added since then. The other model is that the continental culst has grown substantially since the Archean. Both models acknowl- edge the more or less continuous addition of igneous rocks into or upon the upper continental crust. There are, however, two possible sources for this material, the man- tle or the lower crust. Material added from the mantle will, of course, increase the mass of the continents, whereas material derived from the lower cn~st will not change the mass of the continental crust but only redis- tribute matter within it.

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152 The continental crust makes up only 0.3 percent of the mass of the earth, but it is strongly enriched in elements such as K, U. Th, Rb, Ba, and Sr (Gast, 19601. Based on heat-flow data and the abundances of K, U. and Th, Heier (1973) suggests that the lower crust has lower abundances of these elements than the upper crust and that granulite- grade rocks of intermediate composition are reasonable candidates for the lower-crust composition. A~ more exten- sive argument for this model is presented by Smithson and Brown (1977~. One of the most important factors in any interpretation of crustal evolution is how mantle convection has changed with time. In the plate tectonic model, the crust is a pas- sive feature riding on lithospheric plates, the motions of which are determined by convection within the astheno- sphere. The igneous as well as tectonic activity within the crust is directly or indirectly related to activity in the asthenosphere. Thus, to understand crustal evolution it is essential also to understand the present convection re- gimes ofthe mantle, how these regimes may have evolved with time, and the possible interactions that various parts of the mantle may have had with the continental crust. As a first approximation, the upper mantle may be divided into two parts: the suboceanic mantle and the subcontinental mantle. Based on isotope and trace- element ratios for basalts, there are two principal sources of magma in the suboceanic mantle: one is the source of ocean-ridge basalts, the other the source of the ocean- island basalts. Radiogenic isotope data would suggest that the sources are separate and have been isolated for some 2000 m.y. (Church and Tatsumoto, 1975; Brooks et al., 1976a; Sun and Hanson, 1975~. The ocean-ridge types of basalts appear to be restricted to zones of spreading either at ocean edges or in marginal basins; basalts ofthe ocean- island type occur in nearly every tectonic environment in the oceans and continents (Schwarzer and Rogers, 1974~. Where the ocean-island-type basalts occur on continents, there may be little reaction with the continental crust (e.g., Ross Island, Sun and Hanson, 1976~. The mantle source for continental basalts (a large and geochemicaIly variable group of basalts), however, may in some cases have a history associated with the continents (Peterman et al., 1970; Leeman, 197~;; Brooks et al., 1976b), and the source may have interacted or mixed with crustal compo- nents (Faure et al., 1972; 1974~. RADIOGENIC ISOTOPES Some of the key data for understanding the evolutionary history of sources for igneous rocks are the initial isotope ratios of Pb, Sr, and Nd. It must be emphasized that the initial ratios alone cannot be used to tell whether the immediate source of a rock is the mantle or the crust. The isotopic ratios only allow an estimation of the U/204Pb, Rb/Sr, and Sm/Nd ratios of the source and a determination of the time these ratios existed. If continental evolution involves input of significant quantities of igneous rocks | BUSHVELD In ID ran Or =~ G PEAT D I K E ~ =:= _ 4.6 4.0 3.5 3.0 2.5 2.0 I.S 1.0 0.5 0.0 109 YEAR S GILBERT N. HANSON 40 707 0.706 0.705 ,ISLAND O704 0.703 BRIDGE 0702 . 0701 . 0.700 0699 FIGURE 13.1 Strontium evolution diagram for mantle with data for basic and ultrarnaf~c rocks modified from Jahn and Ny- quist ( 1976), with data for the Great Dyke and Bushveld Complex from Hamilton (1977). "Island" designates field for ocean-island basalts. "Ridge" designates field for ocean-ridge basalts. derived from the mantle, it is important to understand how the subcontinental and suboceanic mantle regimes have evolved. Figure 13.1 shows some schematic mantle evolution curves for Sr. The large variation in 87Sr/86Sr in modern oceanic basalts indicates that the suboceanic mantle has considerable heterogeneity. This heterogeneity may have also existed in the Precambrian, but the limited number of basaltic rocks analyzed may not adequately sample the Precambrian mantle. Hamilton (1977) suggests that the initial 87Sr/86Sr ratios for the 2100-m.y.-old Bushveld Com- plex, which vary Tom 0.7056 to 0.7086, may reflect a het- erogeneous mantle source variably enriched in Rb/Sr and is not a result of mixing with crustal components. If he is correct, prior to 2100 m.y. ago the subcontinental mantle in the vicinity of the Bushveld Complex had been vari- ably enriched in Rb/Sr for a significant period of time. Veizer and Compston (1976) have determined initial Sr isotope ratios on sedimentary carbonates throughout the geological record. If these values represent carbonates from oceanic environments, they should indicate the average Sr isotope ratios of the rocks supplying Sr to the oceanic environment. It can be seen in Figure 13.2 that the Sr isotope ratios in the Archean are low, typical of values assumed for the mantle. This may indicate that if the continents were extensive in the Archean, either they had low 87Sr/86Sr ratios and low Rb/Sr ratios or, if the continents had higher 87Sr/86Sr ratios, the strontium in the oceanic environment was predominantly derived from volcanic regimes and thus reflected a mantle source. After the Archean, the 87Sr/86Sr ratio of the carbonates in- creases significantly. This would suggest that the conti- nental source is more exposed and volcanics are less of a source or that there is significant growth ofthe continental crust at the end of the Archean. The same evolutionary

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Geochemical Evolution of the Continental Crust 41~ K2O N a2o 0.71OI 87s r 86sr _ SEDIMENTS I GNEOU S ROC K S SEA WATER _ . 0.700 _ 0 1 109 YEARS FIGURE 13.2 87Sr/86Sr ratios in sedunenta~y carbonates (Veizer and Compston, 1976) and K2O/Na2O in sediments and volcanics (Engel et al., 1974) as a function of We age of the rocks. relationship can be seen in the K/Na ratio of sedimentary and volcanic rocks (Engel et al., 1974) and in the rare- ear~ elements content of sediments (Taylor, in press). Figure 13.3 is a single-stage mantle growth curve for Pb on a 207pbl204pb versus 206Pb/204Pb plot. The data from mod- ern oceanic basalts indicate that there is not a simple growth curve for the recent mantle. The oceanic basalt data lie along lines with slopes the ages of which are approximately 2000 m.y., suggesting that some 2000 m.y. ago their sources were variably enriched in the 238UI204Pb ratio relative to the growth curve. Some basalts from pre- sumed subcontinental mantle show a quite different rela- tionship. For example, the Pb isotope data for Absaroka volcanics from Wyoming (Peterman et al., 1970) lie about a line with a slope of 2800 m.y. These rocks, whether derived from the mantle or the lower crust, indicate a source that has had a low 238U/204Pb ratio with respect to the mantle growth curve since 2800 m.y. ago. This age is approximately that of the basement rock in this region. Leeman (1975) found similar results for basalts from the Snake River Plain as well as from Yellowstone National Park. He suggests that the trace- and major-element com- position of the basalts require their derivation from the mantle. In both studies, the lead and strontium isotopes are not correlated and cannot be explained by a simple mixing relation between crust- and mantle-derived end 153 members. These studies suggest that in these regions the subcontinental mantle has been attached to the conti- nental crust as a mantle keel since at least 2700 m.y. ago. The volcanics from these areas have 87Sr/86Sr ratios of 0.704~.709, on the higher end of the oceanic basalts, suggesting that although their source was depleted in U relative to Pb it was not depleted in Rb relative to Sr. If anything, it was enriched. Based on initial Sr, Pb, and Nd ratios, many granitic and basaltic rocks would appear to have either a source in the mantle or a source with only a short history in the crust (e.g., Moorbath, 1977; McCullogh and Wasserburg, 1978; DePaolo and Wasserburg, 1976; and O'Nions et al., 19791. This suggests an episodic and continuous addition of ma- terial to the crust through time. Further geochemical study of rocks derived from crustal sources, but with es- sentially mantle ratios, may make it possible to place limits on how the crust evolved and the times involved. Likewise, further geochemical study of rocks derived from the mantle may allow a characterization of the scale of heterogeneities in the mantle, show how they are evolving through time, and help to distinguish parts ofthe mantle interacting with the continental crust at a given time. As convection models for the mantle improve, this inforTnation should allow a direct correlation between an- cient tectonic regimes and convection in the mantle. PETROGENESIS OF IGNEOUS ROCKS Petrogenetic studies emphasizing isotope and trace- element analyses of a suite of igneous rocks are particu- larly pertinent for placing limits on the geodynamic fac- tors in the mantle responsible for tectonic activity in an area at the time of formation of the suite of rocks. The purpose of a petrogenetic study of an igneous suite is to determine the chemical and mineralogical composition of the source rocks at the time of melting; the history of the sources prior to melting; the extent of melting; the tem- perature (T) and pressure (P) or depth conditions during ~6 t5 - 1.0~ U OCEAN R ~ DO E A, 14 lo Cal D 1 3 Cal 1 2 11 ABSAROKA VOLC. _~ _ , , ~ _e~CEAN ISLAND BASALT S ~ ~ BASALTS TOO GNEISS. ISUA /4.o 0/ , ty 4.5,7 xl~y 10 9 10 11 12 13 14 15 t6 17 IS 19 20 2! 206 pb/20 P b FIGURE 13.3 Mantle growth curve for Pb with selected rock types plotted (modified from Tatsumato, 1978). Data for Absaroka volcanics are from Petennan et al. (1970). Data for Amitsoq gneisses, Isua, W. Greenland, are from Moorbath et al. (1975).

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154 melting; and modification of the primary melts by dif- ferentiation, assimilation, metasomatism, or late-stage fluids. Although a petrogenetic study relies heavily on major-j minor-, and trace-element analysis and isotopic ratios, it must be based on rocks for which the field, geochronological, and petrologic relations are well un- derstood. The major-element analyses when combined with modal mineral data allow a comparison with experi- mental studies for estimating T. P. and volatiles during melting or differentiation. The isotopic data for Sr, Pb, and Nd allow an estimate to be made of the history of the U/Pb, Rb/Sr, and Sm/Nd ratios of the source. Modeling of Pace elements allows an estimate of the trace-element composition of the source, the mineral composition of the residue at the time of removal of the melt, sequences of fractional crystallization, and the extent of these pro- cesses. Along with the initial radiogenic isotope ratios, the trace elements allow an estimate ofthe extent of mixing or reactions with other melts or rocks (Vollmer, 1976; Lang- muir et al., 1978~. To obtain the maximum information, each of the dif- ferent types of analyses must be made on the same sam- ples. There are few places where a complete study can be made in one laboratory, and it may be questioned as to how many suites of rocks require such detailed analysis. The extensive data, however, are warranted for selected suites, because they can lead to a more quantitative in- sight into crustal evolution. Once the data are available, the best petrogenetic interpretation may not be immedi- ately obvious but will probably lead to new approaches and models. As an example of tectonic application, the petrogenesis of granitic rocks in two tectonic settings will be compared. The two settings are (1) an intrusive granite~reenstone belt in northeastern Minnesota in which all the rocks dated give ages of 2700 m.y. (Arth and Hanson, 1975) and (2) a high-grade Weiss terrane in southwestern Minnesota with ages as old as 3600 m.y. (G. N. Hanson, State Uni- versity of New York at Stony Brook, in preparation). In the northeastern Minnesota greenstone belt the initial 87Sr/86Sr ratios of basic, as well as granitic, rock are all between 0.700 and 0.701, suggesting that they were derived from a mantle source or sources with high Rb/Sr ratios that existed for only a short period of time prior to melting. Dacitic and tonalitic rocks have KlRb and Rb/Sr ratios similar to those of Archean tholeiite and strongly depleted heavy rare-earth element patterns. The model Mat best fits the data is that the dacites and tonalites are derived by partial melting of a tholeiitic parent, probably derived from an oceanic mantle, leaving a residue of gar- net and clinopyroxene. The quark monzonites *om the greenstone belt have lower K/Rb ratios and higher Rb/Sr ratios than the to- nalites and dacites, and rare-ear element patterns simi- lar to that of the tonalites and dacites but with higher abundances and negative Eu anomalies. The best model for the origin of the quartz monzonites is partial melting (upper amphibolite grade) of short-lived (<~;0 m.y.) grey- wacke. The greywacke consists of dacitic and tholeiitic GILBERT N. HANSON detritus derived from within the greenstone belt that has been enriched in K and Rb by sedimentary processes. In this greenstone belt all the components are thought to be derived from either the mantle or from rocks with short histories outside the mantle. The belt probably developed on an oceanic crust. If there were a continental crust underlying the greenstone belt, it was apparently not a major source for the volcanic or intrusive rocks analyzed. In the high-grade gneiss terrane in southwestern Min- nesota, the 3600-m.y.-old Morton and Montevideo gneisses were intruded by granitic rocks at 3100, 2600, and 1800 m.y. (Goldich et al., 1970; S. S. Goldich, North- ern Illinois University, and J. Wooden, Lockheed E:lec- tronics Company, in preparation; S. S. Goldich and C. E. Hedge, U.S. Geological Survey, in preparation). The gneisses vary from quartz diorite through quartz mon- zonite, and the intruding granitic rocks are granodiorite to quartz monzonite. The rare-earth element patterns for the gneisses and the later intruding granites are all very simi- lar to one another, suggesting that they have similar sources. These patterns are quite distinctive from those of the tonalites but similar to those of the quartz monzonites from northeastern Minnesota. Based ore the trace-element abundances and the geological relations, the best model is that the gneisses and the later granites are derived from melting of similar sources, presumably the lower conti- nental crust. This model is supported by Pb isotope data (Doe and Delevaux, in press), which suggest that the later granites are derived from related sources with a signifi- cantly long history in the crust. The K content of the gneisses, mainly tonalites, is generally lower than that of the later granites, mainly granodiorites to quartz mon- zonites. Lower K content for high-grade metamorphic rocks as compared with those of lower grade is not unusual (Heier, 1973~. This might imply that the gneisses originally formed under conditions that led to melts of lower K content or that the gneisses have lost K since the time of their origin. These two examples of petrogenesis would indicate that although the major-element compositions of quartz monzonite and quartz diorite are similar in both the intru- sive granite~reenstone belt and the ancient gneiss ter- rane, a more careful study oftheir chemistry and relations shows that the similarity is superficial and that the origins are probably quite different. The greenstone belt devel- oped in a short period of time and consists of rocks derived principally from the mantle or from rocks with a short history outside the mantle; whereas the gneiss ter- rane developed over a longer period of time, and the prin- cipal source for the granite rocks appears to be the melt- ing of pre-existing crustal sources. CHEMISTRY OF THE LOWER CRUST Based on heat-flow data, Heier (1973) suggested that the lower crust has lower abundances of K, U. and Th than the upper crust. If the lower crust is made up of granulite-

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Geochemical Evolution of the Continental Crust grade rocks of intermediate composition, this could fit a model of a depleted lower crust and an enriched upper crust, because most granulite rocks are relatively de- pleted in K, U. and Th with normal abundances of Sr and Ba compared with similar rocks of lower grade. This re- sults in higher K/Rb (commonly 500 or greater) and lower Sr/Ba (~10), Rb/Sr (~0.02), and U/Pb ratios in granulite rocks (Tarney and \Vindley, 19771. The depletion in U is reflected in the low U/Pb ratios found in some granulite- grade rocks, leading to whole-rock leads that plot along isochrons below the mantle growth curve and to the left of the geochron in Figure 13.3. Is the relative depletion of these elements inherent in the origin of the types of rocks found in a granulite ter- rane, or have the rocks in a granulite tenant preferentially lost some of these elements? If the granulites have lost these elements, there are two means of transport: as melts or in aqueous or other solutions. An important difference between granulite and lower grades of metamorphism is the lower water content in the granulite facies rocks. Some of the loss of elements may thus be associated with the loss of water. One of the more surprising discoveries was that whereas fluid inclusions in rocks of amphibolite grade are rich in H2O, fluid inclusion in granulite-grade rocks have high proportions of CO2 (Touret, 1974), sug- gesting that the fluids with which they were in contact during high-grade metamorphism were CO2 rich. Gold- smith (1976) reminded us that a very important mineral in the lower crust is scapolite and that scapolite is a mineral into which substantial fractions of C03, S04, and C1 can be placed. He suggests that much of the carbonate is em- placed in the granulite terrane directly from the mantle. Lloyd and Bailey (1975) in studying peridotite nodules from the subcontinental mantle have found metasomatic textures, suggesting that normal lherzolite has been meta- somatized, resulting in the growth of titaniferous phlogo- pite, amphibole, diopside-salite, ferroaugite, titanomag- netite, sphere, perovskite, apatite, and calcite in what was originally lherzolite. It thus appears that many elements may be mobile in the mantle and are being added to the subcontinental mantle in carbonic or aqueous solutions. Wendlandt and Harrison (1978), for example, found that under mantle conditions CO2 vapor is 3 orders of magni- tude more enriched in rare-earth elements than is aqueous vapor and is also enriched in rare-earth elements relative to silicate melts. Shieh and Schwarcz (1974) have shown that the oxygen isotope ratios in rocks of the amphibolite grade in the Grenville province are characteristic ofthe~r unmetamor- phosed equivalents, whereas rocks of the highest meta- morphic grade have oxygen isotope ratios more indicative of the mantle. A similar relation has been found for Ar- chean rocks in the Superior province (Longstaffe and Schwarcz, 1977~. Although transporting material in the form of siliceous melts from the mantle to the crust or from the lower crust to the upper crust is undoubtedly important in terms of quantities of material moved, the effects of aqueous or carbonic vapors or fluids in transporting material within 155 the mantle, from the mantle to the crust, or within the crust may be significant. Particularly, they may play an important role in separating elements that behave simi- larly during magmatic processes. The solubilities of ele- ments in these fluids and mineral-fluid distribution coef- ficients must be determined experimentally under a variety of conditions so that a proper evaluation of these processes may be made. MANTLE-CRUST INTERACTION To understand the evolution of the continental crust it is necessary to understand how the mantle interacts with the continental crust. This requires characterizing variations within the mantle, determining their dimensions, and as- sessing how the variations are affected by mantle convec- tion. Sun and FIanson (1975) suggested that Rb~r and Pb-Pb isochron ages for ocean-island basalts of about 9000 m.y. reflect a real time of separation and isolation of the mantle sources for ocean-ridge and ocean-island ba- salts and that they are not the result of simple mixing between a large-ion-lithophile-element- (~IL) depleted ocean-ridge source and a -enriched ocean-island source. This is best shown in a plot of 87Sr/86Sr versus 206Pb/204Pb, in which the ocearl-ridge basalt plots away from the main trend of the data for the ocean islands and not at either end of a potential mixing curve. Although ocean-ridge basalts are only known to occur in spreading centers, whether at ocean ridges or in marginal basins, these environments encircle the globe. The ocean-island basalts are found in continental, island-arc, and oceanic terranes seemingly unrestricted in their geographic oc- currence. Thus both sources appear to be ubiquitous but separated. Until we have better information regarding convection in the mantle, the simplest model to explain these observations is a stratified mantle in which the source for the ocean-ridge basalts is a convecting mantle, below which is the source for the ocean-island basalts. This lower source may also be convecting id. Richter, University of Chicago, personal communication, 1978~. Applying this model to a continental environment, there may be a continental mantle keel attached to the conti- nental crust for hundreds to thousands of millions of years (Figure 13.41. In this model, starting from the leR side of Me figure and using the numbers in Figure 13.4: (1) Per- turbations in the convecting mantle produce upwellir~g, rifting, and melting of the continental mantle with the formation of continental basalts. The wide variety ofthese melts may or may not be a result of reaction with or melt- ing of the continental crust. (2) Carbonatites or lcimber- lites may result from melting or instability a: ;r the low- velocity zone. (3) Ocean-island-type basalts found on the continents are associated with deep-mantle plumes. (4) The addition of CON to the lower crust may be a result of continued production of CO2 over wide areas in the man- tle that reacts with the granulite-grade rocks in the lower crust, or it may be episodic, associated with tectonic dis- turbance.

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156 FIGURE 13.4 Diagrammatic repre- sentation of present day mantle. Figure 13.4 also depicts a subduction zone (far right) on the continental margin in which there is extensive tec- tonic activity, (5) the connation of a marginal basin, and (6) in the arc, volcanism and the intrusion of gabbroic through granitic plutons. Below the arc there may be melting of: the subducted plate to produce tonalities; the subcontinental mantle or the ocean-ridge-type mantle to produce basalts; magic rocks near the base of the crust to produce a`nor~ositic or gabbroic plutons and possibly an- desites; and the intennediate-composit~on continental crust to produce granitic intrusions. The melting is prob- ably enhanced by the dehydration of the subducting plate. In the marginal basin the first volcanics would be derived by melting of the subcontinental mantle. As rift- ing proceeds and the marginal basin widens, ocean-ridge- type mantle becomes the dominant source of basalt. Detailed petrogenetic studies of suites of modern igne- ous rocks should allow testing of this and other models. Similar studies on other geological time spans should al- low an evaluation of the evolution of mantle regimes, mantle convection, and the interaction of the mantle with the continental crust. ACKNOWLE DGM E NTS S. R. Hart and I. Wooden reviewed tl~e manuscript. This report was supported by NSF Grant No. EAR 76-13354 AO1 (Geochemistry). REFERENCES Annstrong, R. L., and S. M. Hein (1973). Computer simulation of Ph and Sr isotope evaluation of the earth's crust and upper mantle, Geochim. Cosmochim. Acta 37, 1-18. GILBERT N. HANSON 1 2 3 4 5 6 ~ : ~ ~ ~~ ~3~ ~ ~ = ~~;~ 1 {T^HOSPHERE~ .~ .~. . ~~ ~ +~N ~ 0. . ~ ~~ ~ ~ ~ ~ U ^, ~ ~ ~ U ~~ '4 ~ t~ ~ ~ * ,, ~ ~t- ^ _ 4 . - ~ ~ ~ n ~ ,~ .-, i a/ ,,. ~ At; At, ,,-,,, OCEAN RIDGE SOU RCE ~ ~ ~ ~ .~;~.*f r~1 B~> ~ / '1 ~ Iv ~ ~ A 7>1 1 ~1 V ~ ~ ~ L ~ ~ ~ < ~ ~ ~ ~ ~ ~ L ~1 ~ ~ A i, ~ ^~= ~ hi 1~1 ^~^ - ~1 - Arth, J. G., and G. N. Hanson (1975). Geochemistry and origin of the early Precambrian crust of northern Minnesota, Geochim. Cosmochim. Acta 39, 325~62. Brooks, C. R., S. R. Hart, A. Hoffman, and O. E. James (1976a). Rb~r mantle isochrons from oceanic regions, Earth Planet. Sci. Lett. 32, 51~1. Brooks, C., D. E. James, and S. R. Hart (1976b). Ancient lithosphere: its role in young continental volcanism, Science 193, 1086-1094. Church, S. E., and M. Tatsumoto (1975). Lead isotopic relations in oceanic ridge basalts from the Juan de Fuca~orda Ridge area N.~:. Pacific Ocean, Contrib. Mineral. Petrol. 53, 253-279. Collerson, K. D., and B. J. Fryer (1978). The role of fluids in the formation and subsequent development of the early con- tinental crust, Comets. Mineral. Petrol. 67, 151-167. DePaolo, D. J., and G. J. Wasserburg (1976). Nd isotope variations and petrogenetic models. Geophys. Res. Lett. 3, 249-252. Doe, B. R., and M. H. Delevaux (in press). Lead isotope investigations in the Minnesota River Valley I. Late- and post tectonic granites, Geol. Soc. Am. Afem. Engel, A. E. J., S. P. Itson, C. G. Engel, O. M. Stickney, and E. J. Gray, Jr. (1974). Crustal evolution and global tectonics: a petrogenetic view, Geol. Soc. Am. Bull. 85, 843-858. Faure, G., B. L. Hill, L. M. Jones, and D. H. Elliot (1972). Isotope composition of strontium and silica content of mesozoic basalt and dolerite from Antarctica, in Antarctica Geology and Geophysics, R. J. Adie, ea., Universitetaforloget, Oslo, pp. 413~18. Faure, G., J. Bowman, D. Elliot, and L. Jones (1974). Strontium isotope composition and petrogenesis ofthe Kirlcpatrick basalt, Queen Alexandria Range, Antarctica, Contrib. Mineral. Petrol. 48, 153-169. Cast, P. W. (1960). Limitations on the composition of the upper mantle,J. Geophys. Res. 65, 4. Goldich, S. S., C. E. Hedge, and T. W. Stern (1970). Age of the Morton and Montevideo gneisses and related rocks, southwestern Minnesota, Geol. Soc. Am. Bull. 81, 3671~696. Goldsmith, J. R. (1976). Scapolites, granulites and volatiles in the lower crust, Geol. Soc. Am. Bull. 87, 161-168.

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