Bimodal tholeiitic—dacitic magmatism and the Early Precambrian crust

Precambrian Research, 1 (1974) 1--12 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

BIMODAL THOLEIITIC--DACITIC MAGMATISM AND THE EARLY PRECAMRBIAN CRUST FRED BARKER and ZELL E. PETERMAN

U.S. Geological Survey, Denver, Colo. (U.S.A.) (Accepted for publication March 19, 1973)

ABSTRACT Barker, F. and Peterman, Z.E., 1973. Bimodal tholeiitic--dacitic magmatism and the Early Precambrian crust. Precam. Res., 1: 1--12. Interlayered plagioclase-quartz gneisses and amphibolites from 2.7 to more than 3.6 b.y. old form much of the basement underlying Precambrian greenstone belts of the world; they are especially well-developed and preserved in the Transvaal and Rhodesian cratons. We postulate that these basement rocks are largely a metamorphosed, volcanic, bimodal suite of tholeiite and high-silica low-potash dacite--compositionally similar to the 1.8-b.y.old Twilight Gneiss -- and partly intrusive equivalents injected into the lower parts of such volcanic piles. We speculate that magmatism in the Early Precambrian involved higher heat flow and more hydrous conditions than in the Phanerozoic. Specifically, we suggest that the early degassing of the Earth produced a basaltic crust and pyrolitic upper mantle that contained much amphibole, serpentine, and other hydrous minerals. Dehydration of the lower parts of a downgoing slab of such hydrous crust and upper mantle would release sufficient water to prohibit formation of andesitic liquid in the upper part of the slab. Instead, a dacitic liquid and a residuum of amphibole and other silica-poor phases would form, according to Green and Ringwood's experimental results. Higher temperatures farther down the slab would cause total melting of basalt and generation of the tholeiitic member of the suite. This type of magma generation and volcanism persisted until the early hydrous lithosphere was consumed. An implication of this hypothesis is that about half the present volume of the oceans formed before about 2.6 b.y. ago. INTRODUCTION The n a t u r e a n d origin o f t h e E a r t h ' s early sialic crust have received r e n e w e d a t t e n t i o n f r o m geologists a n d g e o c h e m i s t s in t h e last few years ( A n h a e u s s e r et al., 1 9 6 9 ; Bridgwater, 1 9 7 0 ; Glikson, 1 9 7 1 ; White et al., 1 9 7 1 ) . T h e p u r p o s e o f this n o t e is t o suggest t h a t v o l c a n i s m o f t h e b i m o d a l t h o l e i i t e - d a c i t e t y p e f o r m e d t h e bulk o f t h e Early P r e c a m b r i a n ( > 3 b.y.) c r a t o n i c nuclei. Descriptions o f t h e L o w e r P r e c a m b r i a n rocks, as r e c e n t l y r e p o r t e d , are c o m p a r e d with t h o s e o f similar b u t y o u n g e r a n d b e t t e r preserved rocks, a n d a c o m m o n volcanic origin is d e d u c e d . T h e t h e o r e t i c a l results o f Green a n d R i n g w o o d ( 1 9 6 8 ) ,

T.H. Green (1972), and Lambert and Wyllie {1972), and geochemical results of other workers are used as a basis to suggest a model for the development of the early sialic crust. For a starting point of this discussion we assume that a great degassing and differentiation event occurred shortly after the Earth was formed; see the discussion by Fanale (1971). Water expelled then from the inner Earth was largely incorporated in hydrous minerals in the upper mantle and lower crust. We also assume, from an extremely sketchy geologic record, that sedimentary rocks were present in the very early Precambrian--i.e., more than about 3.1--3.4 b.y. ago--to such a minor extent that development of the early crust may be considered wholly in terms of igneous rocks and processes. J. Green (1972, table 1) has compiled an extensive bibliography of papers on the Early Precambrian. The major problem in the formation of cratons is the process by which Na20, K20, and SiO2 are concentrated from mantle sources and incorporated in rocks of the upper crust. Many workers have pointed out that the gneisses and intrusive rocks older than about 2.7 b.y. in much of the world's Precambrian show high Na/K ratios, and also that these relatively sodic rocks almost invariably are intruded by slightly younger potassic granites. In some areas, such as the Transvaal and Swaziland (Allsopp et al., 1962, 1969), the sequence of rocks formed is similar, but about 0.4 b.y. older. The sodic rocks apparently form crust that is tectonically metastable, and magmatism of calc-alkaline type invariably follows. C O M P O S I T I O N A L DATA

These sodic gneisses, or plagioclase--quartz gneisses, form the basements to Lower Precambrian greenstone belts or other supracrustal volcanic rocks. They typically are well foliated. Some are banded and contain interlayers or tectonically broken layers of m e t a b a s a l t - these may represent metamorphosed volcanic rocks. Others are not banded but are more homogeneous and contain what probably are metamorphosed basaltic dikes -- these probably represent original plutonic masses. The best-known example of the banded type is the Ancient Gneiss Complex of Swaziland (Hunter, 1970). On geologic grounds Hunter deduced that these rocks are older than the Swaziland System, which is a b o u t 3.36 b.y. old (Van Niekerk and Burger, 1969; Hurley et al., 1972). Viljoen and Viljoen (1970), however, suggested that the Ancient Gneiss Complex is stratigraphically equivalent to rocks of the Swaziland System. The bimodal association of plagioclase--quartz gneiss of tonalitic or trondhjemitic composition and of metabasalt is the most pronounced compositional feature of most of the Ancient Gneiss Complex. Hunter (1970) made special mention of the close and conformable interlayering of gneiss containing 70--77% SiO2 and amphibolite of 50% SiO2. He further stated that large amphibolite layers may be traced as much as 8 km along strike.

Other examples of the banded type include the quartzofeldspathic gneisses and amphibolite of the Morton--Sacred Heart area, Minnesota (Goldich et al., 1970; Grant and Goldich, 1972), which were formed about 3.55 b.y. ago; and the stratiform, interlayered enderbitic gneisses and two pyroxene--plagioclase gneisses of the Pikwitonei Province of Manitoba and Ontario ( Bell, 1971}, which underlie rocks of the Superior Province that are about 2.6--2.8 b.y. old. Another includes part of the pre-Sebakwian and pre-Bulawayan basement of Rhodesia (Stowe, 1968, 1971}, which are approximately 3.4 b.y. old or older. There bands of plagioclase--quartz--biotite gneiss and amphibolite form the oldest recognizable parts of a banded migmatite complex. The largely plutonic Amitsoq gneisses of southwestern Greenland, which have been described and dated by McGregor (in press} and Black et al., (1971), are predominantly tonalitic to granitic gneiss. Other rock types include amphibolite, hornblendite, ironstone, and ultramafic rocks. The Amitsoq gneisses are 3.6--3.7 b.y. old. The basement to the YeUowknife Supergroup of northwestern Canada consists largely of granodiorite and trondhjemitic intrusives (see McGlynn and Henderson, 1970). The recent discussions of Davidson and McGlynn (in Wanless and Loveridge, 1972) summarize geologic and isotopic work on these rocks. Two younger examples of the bimodal association are the 2.7-b.y.-old metavolcanic Northern Light Gneiss of Ontario and Minnesota (Hanson and Goldich, 1972; Arth and Hanson, 1973) and the 1.8-b.y.-old Twilight Gneiss of Colorado (Barker et al., 1969). The Northern Light mass consists largely of fine-grained gneiss of biotite trondhjemite composition. It contains minor amphibolite, metarhyodacite and metarhyolite. The Rb, K. and Sr abundances of the trondhjemitic gneiss and amphibolite are similar to those of Archean metavolcanic rocks (Hart and Brooks, 1970; Hanson and Goldich, 1972). The initial 67Sr/86Sr ratio of the Northern Light Gneiss is 0.7006 (Hanson and Goldich, 1972). The Twilight Gneiss is similar in several respects to the older bimodal suites in spite of its relatively youthful age. This formation consists of closely interlayered plagioclase--quartz--biotite gneiss and metatholeiite of amphibolite facies. The gneiss, compositionally very similar to the low-potash high-silica dacite of Saipan (Schmidt, 1957), contains phenocrysts of bipyramidal quartz and severely recrystallized feldspars. The bulk of the Twilight is plagioclase--quartz--biotite gneiss; about 15% of it is metabasalt, and less than 1% is metarhyodacite. SiO2 contents of three metatholeiites range from 51.1 to 52. 5%, and those of nine plagioclase--quartz--biotite gneisses range from 68.6 to 76.1% (Barker, 1969, and unpublished data}. The bands of metabasalt typically range in thickness from 15 cm to about 7 m, and less than 2% of t h e m are thinner than 15 cm. Thus the banding is markedly thicker than the secondary banding that forms by shear and metasomatic processes. Furthermore, both gneiss and metatholeiiteare so uniform in composition and their mutual contacts so regular that we cannot attribute their compositional differences to metamorphic differentiation. Both gneiss and

metatholeiite show the same low 8vSt/S6 Sr ratio of 0.7015. We interpreted (Barker et al., 1969) the Twilight Gneiss as being volcanic in origin, derived in either one or two stages from primitive mantle, and probably formed by the cyclical partial to complete melting of quartz eclogite. The hypersilicic compositions of the metadacite--i.e., compositions that lie in the quartz field of the normative Qz--Ab--Or diagram--aro due to accumulation of quartz, probably by settling in a pre-extrusive chamber. Some of the interlayering of dacite and tholeiite in the Twilight is on such a fine scale as to indicate an origin as tufts, rather than flows. Alk--F--M and Na20--CaO--K20 diagrams (Fig.l) show the compositional similarities of the Swaziland gneiss and amphibolite of the Twilight Gneiss, and of three samples of high-silica low-potash dacite from Saipan. The andesite gap shown in Fig.1 applies only to the Precambrian rocks, of course, for there is much andesite associated with the dacite of Saipan. Indeed, the dacite--tholeiite bimodal suite apparently is found only in the Precambrian, probably for reasons discussed below. CoO

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The compositional megascopic similarities of the Twilight Gneiss and the much older, but similarly handed rocks that underlie the Lower Precambrian greenstone belts suggest to us that these rocks share a similar, largely volcanic, mode of origin. Bridgwater (1970) pointed out in his comparison of the early

basement rocks of eastern Labrador and the pre-Ketilidian rocks of Greenland that "a repeated characteristic succession of ultrabasic, basic, quartzo-feldspathic and pelitic gneisses is seen in many localities and is interpreted as an original supracrustal succession comprising ultrabasic rocks (lavas and sills), basic lavas, acidic volcanic rocks and aluminous sediments". Such an origin means that processes of partial melting and/or magmatic differentiation are the method by which Na20, SiO2, and even relatively large amounts of K20 are concentrated from the mantle and oceanic crust of basaltic composition; and that volcanism, often accompanied by consanguineous plutonic rocks, is the means of building continental nuclei on basaltic crust. Coalescing volcanic domes or linear belts of interlayered dacite and tholeiite formed this early sialic crust, which apparently also contained small amounts of ultramafic and intermediate rocks, lenses of sediment, and nearly contemporaneous intrusive rocks. This-crust corresponds to that proposed by Annhaeusser et al. (1969). The very old but compositionally similar tonalitic and trondhjemitic gneisses, such as those of southwest Greenland, are assumed to be plutonic equivalents of the basaltic and dacitic supracrustal rocks. GENESIS OF THE BIMODALSUITE Our flow diagram (Fig.2) suggests paths of evolution. The genesis of the highsilica dacite--tholeiite bimodal suite is most important. First we believe that the differentiation of andesitic magma to produce dacite, effective as it may be in the circum-Pacific province (Bryan and Ewart, 1971), was unimportant in the Early Precambrian ( > 2.7 b.y.) because andesite was scarce and because this process requires a relatively dry crust. A second class of processes consists of the partial melting of amphibole-bearing basalt or amphibolite amphibole eclogite, or quartz eclogite. The end members of this model ,re shown in Fig. 2. These models are based largely upon the experimental work of Green and Ringwood (1968), T.H. Green (1972), and Lambert and Wyllie (1972), in which basaltic, andesitic, and normal dacitic (64% SiO2 ) compositions were studied. Several investigators have appealed to it (Barker et al., 1969; Glikson, 1971; Green and Baadsgaard, 1971; and Hanson and Goldich, 1972). The rare-earth studies by Condie and Lo (1971) on the 2.7-b.y.-old Louis Lake batholith of Wyoming, and by Hanson and Goldich (1972) and Arth and Hanson (1973) on the 2.7-b.y.-old Saganaga Tonalite and Northern Light Gneiss of Minnesota and Ontario show that these magmas are depleted in the heavy rare earths and thus were in contact with a garnet phase before emplacement. The stability of garnet is approximately known: it is stable in wet, partially melted andesite at pressures greater than about 9 kbar (T.H. Green, 1972), and in wet, partially melted basalt at more than about twice that pressure (Green and Ringwood, 1968; Lambert and Wyllie, 1972). Other, relatively direct models involve relatively dry partial melting of pyrolite to a basaltic liquid, and the conversion of that liquid to a dacitic liquid and a residuum of silica-deficient phases; or they require the partial melting of

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wet peridotite to andesitic liquid (O'Hara, 1965; Yoder, 1969) and conversion to dacite and a less siliceous precipitate. These t w o models will not be considered further here. The first model probably is analytically indistinguishable from the class of two-stage models mentioned above, and the second apparently is not applicable to the andesite-free terranes of the Early Precambrian. Modreski's experimental results (1972} on phlogopite~pyroxene--water mixtures indicate that partial melting of water-saturated mica peridotite, indeed, could produce silicic liquids at depths as great as 80 km. These experimentally produced liquids, however, are extremely potassic and thus are not comparable to dacite or rhyolite. We suggest that the low-pressure variation of the second class of models is the more likely, assuming that the early upper mantle and lower crust were both more hydrous and had a much steeper temperature gradient than at any later time. The hydrous condition presumably was a result of the early degassing of the Earth and involved the formation of a predominantly basaltic crust and heterogeneous upper mantle that contained much amphibole, serpentine, mica

and other hydrous minerals. The important phase in basaltic compositions, amphibole, would be present to a maximum depth of about 55 km, according to Wyllie's (1970) and Kushiro's (1970) stability curves and using a geothermal gradient that is steeper than the modern oceanic one. Many workers have postulated higher heat flow in the early mantle; neither the concentrations nor distributions of U, Th, and K are known sufficiently well to permit calculating heat flow as a function of time, but increases of perhaps 150°C over the present oceanic gradient at depths of 20--30 km would be expected. Green and Ringwood (1968) determined that andesitic liquid compositions lie in a thermal trough between quartz tholeiite and adamellite compositions at 30 kbar and anhydrous conditions, and that water suppressed this trough; T.H. Green's later results (1972) on andesitic compositions at about 8--20 kbar pressure, 5 weight percent of water, and temperatures of 8500--950 ° C show that the liquidus phases are clinopyroxene and amphibole throughout, garnet at higher pressures and plagioclase at lower pressures. In this region liquids of andesitic composition are thus unstable relative to mafic, less silicic residues and more siliceous liquids. We suggest that a downgoing slab of primitive oceanic crust and underlying lithosphere of the mantle, much more hydrous that its modern counterparts, will have its lower portions dehydrated. This water will stream upward and into the upper, largely tholeiitic parts of the slab, which first will partially melt to dacitic compositions--leaving a residue of amphibole and minor clinopyroxene, plagioclase and/or garnet--and which at greater depths and temperatures will totally melt to give a relatively wet tholeiitic magma. In this environment andesitic liquids are unstable relative to dacitic liquids and to solidus assemblages that are dominated by amphibole, and so the bimodal tholeiite--dacite suite forms. These dacitic liquids commonly have quartz as a liquidus phase, as evidenced by phenocrysts. They may differentiate by the accumulation of quartz to hypersilicic bulk compositions, as in the case of the Twilight Gneiss. The wet nature of these magmas must be responsible for their explosive extrusion, partly as thinlayers of tuff. T.H. Green's (1972) stability relations of andesite containing 5 weight per cent of H20 are shown in Fig.3, and geotherms for the upper surface and horizons at 2 and 5 km lower down in the descending slab are adapted and superposed from Fitton (1971)and Oxburgh and Turcotte (1970), These geotherms are 150 ° C higher than the modern ones. The upper 3 km of this slab accordingly brackets the stability region of amphibole. As the slab descends the tholeiite in it first encounters both a liquidus at these P--T conditions and H2 O streaming up from dehydration reactions below. Its initial partial fusion under water-unsaturated conditions would tend to give an andesitic liquid, but such liquids are not stable here, and hence low-potash, high-silica dacitic liquid forms. Much of the upper region of slab probably melts in toto as it moves downward, but the overall proportions of dacite to tholeiite liquid extruded in the Early Precambrian are still not known. Whether most of the original slab is only fractionally fused and largely converted to

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eclogite as it descends further, or whether most of it is melted and extruded remains to be learned. Our "downgoing slab" is analogous to that of a modern Benioff zone. We realize that comparison of Early Precambrian processes to modern plate tectonics is speculative at best. Yet the Precambrian processes apparently produced thick piles of volcanic rocks, as witness their extent and c o m m o n metamorphism to high rank; and accordingly we suggest that a conveyorbelt mechanism of undefined geometry is the most likely one to give such volumes. A second point is that much of the primordial, hydrous basaltic crust must be swept through such a dehydrating process as proposed here before the andesites of the Archean greenstone belts can be generated. The granitic gneisses of the Early Precambrian deserve special comment. Obviously rocks of granitic or rhyolitic composition do not belong to the tholeiite--dacite suite. Their presence in Greenland (Black et al., 1971; McGregor, in press) and in the Minnesota River Valley (Goldich et al., 1970), however, is limited to terrane of upper amphibolite facies or granulite facies, and so they may have been formed by partial melting of dacitic rocks. Differentiation of high-silica low-potash dacite to a rhyolitic liquid certainly is feasable at shallow depths, but whether this process actually occurred in the Precambrian has yet to be demonstrated. Such a process presumably would

involve separation of both plagioclase and quartz from the liquid, and migration of the liquid composition from near the Ab--Qz sideline towards the center of the normative Qz--Ab--Or diagram. This wet and warm model for genesis of the Lower Precambrian magmatic rocks also implies that about half the present oceans were formed after the primordial degassing and by the completion of the 2.8- to 2.6-b.y. magmatic event. For example, if 0.75 weight per cent of H20 were expelled from the upper 55 km of the entire Earth, about 1,750 m--a thickness not corrected for the effects of dissolved material or compression--of the present 3800 m mean depth of the oceans would have formed. In the nearly worldwide magmatic episode of 2.8 to 2.6 b.y. ago many of the Archean greenstone belts were formed. Enormous volumes of tholeiite, andesite, and silicic volcanic rocks, as well as intrusives of gabbroic to granitic compositions, were emplaced. The latter include much trondhjemite, which is nearly identical in major-element composition to the dacite of the older dacite--tholeiite bimodal suite. However, we cannot conclude that this trondhjemitic magma was the last produced by the primitive hydrous crust and upper mantle, for much trondhjemite of Mesozoic and even Tertiary age has been generated at or near postulated subduction (Benioff)zones where wet, oceanic upper crust has been involved--as in the Coast Range batholith of British Columbia and Alaska, in the western part of the Idaho batholith (Hamilton, 1963), Klamath Mountains, the western part of the Sierra Nevada batholith, and Fiji (Gill, 1970; Glikson, 1972). An additional consideration is that andesite forms an important part of many greenstone belts (see, e.g., Baragar, 1968;/ Baragar and Goodwin, 1969; Bowes et al., 1971), and we presume that its generation involved conditions rather similar to modern ones, and of a less hydrous nature than in the Early Precambrian. Therefore we conclude that most of the early hydrous crust and mantle was consumed by about 2.8 to 2.7 b.y. ago. The Twilight Gneiss may well be the product of one of the last remnants of the early crust and mantle, but the absence of the highsilica low-potash dacite--tholeiite bimodal suite in the Phanerozoic indicates that it was consumed before Cambrian time. The early crust of dacitic and tholeiitic volcanic rocks and consanguineous hypabyssal and plutonic rocks is tectonically metastable, perhaps because it is thin, and is subject to later magmatism, folding and metamorphism. The Ancient Gneiss Complex of Swaziland and the Transvaal, for instance, had the volcanic and sedimentary rocks of the Swaziland and Pongola Systems deposited on it, and intrusive rocks ranging in composition from gabbro through tonalite to rapakivitic granite were intruded into it over the interval 3.3 to about 2.6 b.y. ago (Allsopp et al., 1969; Hunter, 1971). An important fraction of the more siliceous of these intrusives may have been derived from the lower parts of the early crust.

10 ACKNOWLEDGMENTS We are i n d e b t e d t o m a n y o f o u r colleagues for discussions a b o u t P r e c a m b r i a n rocks, b u t especially t h a n k G.N. Hanson, C.E. Hedge, and D.R. Hunter. The Penrose C o n f e r e n c e at Taos, N e w Mexico (Barker, 1 9 7 1 ) , s p o n s o r e d b y the Geological S o c i e t y o f America, b r o u g h t to light m a n y o f t h e p r o b l e m s o f the b i m o d a l dacite--tholeiite suite. The p u b l i c a t i o n o f this w o r k is a u t h o r i z e d b y the Director, U.S. Geological Survey.

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