The role of crustal contamination in magma evolution through geological time

Earth and Planetary Science Letters, 78 (1986) 211-223

211

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [3]

The role of crustal contamination in magma evolution through geological time R.S.J. Sparks

Department of Earth Sciences, Universityof Cambridge, Cambridge CB2 3EQ (England) Received October 23, 1985; revised version received March 7, 1986 The temperatures of the mantle-derived magmas that have ascended through an intruded continental crust have decreased through geological time. A possible consequence of this change is that crustal contamination could have been a more significant factor in magma genesis in the Precambrian than in the modern Earth. Calculations suggest that a peridotitic komatiite magma (28% MgO) has the potential to assimilate more than three times the amount of crust that can be assimilated by basalt. The greater potential for assimilation by komatiite is a consequence of the large crystallization interval ( - 4 0 0 ° C ) and the high heat of fusion of olivine. In favourable circumstances thermal constraints allow basaltic and andesitic magmas containing more than 50% crust to be derived by an assimilation and fractional crystallization (AFC) process from a komatiite parent. AFC in komatiites can generate derivative magmas that would not be expected in the modern Earth. Substantial contamination of komatiite with average Archean upper crust or tonalite-trondjhemite gneiss can generate silica-rich Mg-rich basaltic komatiites and Mg-rich andesites. Contamination may explain why many basaltic komatiite lavas cannot be generated by olivine fractionation of the komatiites with which they are closely associated. Highly contaminated magmas crystallize in the order ol-opx-plag-cpx and are appropriate parental magmas for the ultramafic zones of large Precambrian layered intrusions such as the Bushveld and Stillwater Complexes. Whereas modern basalts are unlikely to be substantially contaminated by mafic or ultramafic rocks, komatiites are also capable of assimilating large amounts of gabbro, mafic granulite and some ultramafic rocks such as pyroxenite. If the Archean lower crust were made of such rocks AFC processes with a komatiite parental magma can generate iron-rich and alumina-rich basaltic magmas. Such magma compositions would crystallize in the order ol-plag-cpx/opx at low pressure and bear some resemblance to the magmas required to form the anorthositic parts of intrusions such as Bushveld and Stillwater. A similar process could also have had a role in generating the parental magmas to the massive Proterozoic anorthosite massifs, by assimilation of lower crust into picritic parent magmas.

1. Introduction The maximum temperature of lavas erupted on the Earth's surface has diminished through geological time. In the Archean komatiite lavas with temperatures u p to 1 6 5 0 ° C w e r e f r e q u e n t l y e r u p t e d [1]. I n t h e P r o t e r o z o i c s o m e w h a t less m a g nesia-rich and therefore cooler komatiitic basalts a n d p i c r i t e s c a n b e f o u n d [2,3], a l t h o u g h t r u e peridotitic komatiites appear to be confined to the Archean. Today lavas with eruption temperatures greater than 1250°C are rare, although many petrologists [4-6] suspect that higher-temperature m a g m a s a r e g e n e r a t e d i n t h e m a n t l e b u t fail t o reach the surface. These changes in the abundance 0012-821x/86/$03.50

© 1986 Elsevier Science Publishers B.V.

and composition of primitive mantle-derived magm a s a r e g e n e r a l l y t h o u g h t to r e f l e c t a d e c r e a s e i n mantle temperature with time and/or changing tectonic processes. Whatever the reason the s u g g e s t i o n is t h a t t h e t e m p e r a t u r e s o f m a n t l e - d e rived magmas available for intrusion into and ascent through continental crust have substantially decreased with time. A possibly important c o n s e q u e n c e o f t h i s c h a n g e is t h a t c r u s t a l c o n tamination was more important in the Archean a n d P r o t e r o z o i c t h a n it is t o d a y . T h i s p a p e r exp l o r e s t h i s i d e a a n d it is s u g g e s t e d t h a t s o m e o f the distinctive characteristics of Precambrian magmatism can be explained by substantial contamination of komatiitic or picritic magmas.

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2. Basalt magmatism and time 2.1. Modern times

The record of basaltic rocks preserved within continental masses today is both biased and complicated. Globally, the most important basalt is that formed at mid-ocean ridges yet virtually all this material is subducted, Even ophiolite complexes, which represent preserved oceanic crust tectonically emplaced onto the continents, are widely viewed as being anomalous in character compared to normal oceanic crust. Continental basalts are preserved in several different situations. Orogenic basalts are ubiquitous in association with subduction zones along continental margins, but are often subsidiary in volume to more evolved intermediate and silicic magmas. Many workers suspect that much of the basic magma is intruded at depth [5,7]. The most important production of basalt in continents is associated with rifting and thinning of the continental lithosphere. This activity ranges from provinces where lava volumes and magma production rates are small to others characterised by vast outpouring of flood basalts. In general, the former kind of activity tends to produce undersaturated and alkaline magma types; the latter kind, tholeiitic basalts. There is, of course, a continuum between these extremes and detailed studies of individual provinces usually reveal complex spatial and temporal variations, reflecting the roles of polybaric fractionation, variable degrees of partial fusion, heterogeneous mantle compositions and crustal contamination. Most continental basalts have chemical compositions that can be interpreted as reflecting fractionation of gabbroic assemblages in crustal magma chambers [5,6]. Such magmas commonly erupt at temperatures of 1200 +_+_50°C. Various lines of evidence suggest that hotter Mg-rich magmas are parental to these basalts, but fail to reach the surface as liquids. They are either trapped within crustal magma chambers [8], filtered out because of their high density [9] or erupt in a porphyritic condition [4]. Because of the indirect nature of the evidence, there is some doubt about the compositions of the most primitive magmas. Various estimates in different geological environments suggest magmas with MgO contents be-

tween 12 and 18% and temperatures up to 1400°C [5,101. There is widespread petrographic and geochemical evidence for crustal contamination of some modern continental basalts [11-15], Crustal contamination has been recognised from enrichments in incompatible trace elements and from departures in radiogenic and stable isotopic ratios from those expected in basalts derived from the depleted mantle source of MORB. Although some of these differences can be attributed to partial fusion of enriched sub-continental mantle, detailed geochemical studies, combined with petrographical and geological evidence, are now enabling the relative influence of crust and mantle to be determined [13,14]. Contamination has also been recognised within Cenozoic layered intrusions by variations in radiogenic isotope ratios [16]. Quantitatively, the total amount of crustal contamination in any given lava or intrusion is difficult to estimate because there are so many uncertainties in inverting geochemical data on igneous rocks that have been formed by partial melting of heterogeneous mantle and that were then extensively fractionated and sometimes contaminated. In the few cases where quantitative evaluations have been made, total contamination does not exceed 10% and is probably much less in most examples [11,13,17]. 2.2. Archean times

The Archean greenstone belts preserve a range of ultrabasic and basic volcanic rocks as well as intermediate and silicic rocks. There is always a question of whether these rocks were erupted onto continental crust or were erupted in an oceanic environment and then emplaced by tectonic processes onto continental crust. In at least two cases, the Belingwe belt and parts of the Abitibi belt, geological evidence shows that the volcanic sequence developed on continental crust in subsiding basins [18,19], Three major kinds of mantle-derived magma have been identified, which are commonly found in close stratigraphic association. Komatiites are ultrabasic lavas with MgO contents greater than 18% of probable eruption temperatures of up to 1650°C [1]. Basaltic komatiites are MgO-rich basalts (up to 18% MgO) in which clinopyroxene

213 often displays spinifex texture as well as olivine. Cameron et al. [20] have drawn attention to their similarities to the rather rare Cenozoic boninite magma type. There is certainly a tendency for silica-rich and magnesia-rich basalts and andesites to occur in Archean terrains [20-23]. Although intimately associated with komatiites trace element data often show that basaltic komatiites cannot be derived from komatiites by fractional crystallization [24]. The third kind of mafic lava is tholeiitic basalt. These lavas are not noticeably different from modern tholeiitic basalts. In the Munro Township area of the Abitibi greenstone belt, Ontario, the komatiites, basaltic komatiites and basalts show wide variations in trace element contents, and these variations have been attributed to both mantle heterogeneity [24] and to crustal contamination [25,26]. Similar wide variations have also been observed in the Kambalda area of Western Australia, where the discovery of zircon xenocrysts in komatiitic basalts [23] has highlighted the role of crustal contamination. Another source of information on Archean magmatism is the study of large layered intrusions such as the Stillwater complex and Great Dyke. These intrusions have the advantage that the original igneous minerals are preserved although the magma compositions often have to be deduced by indirect methods. From the order of appearance of cumulus minerals, McCallum et al. [27] and Irvine et al. [28] have deduced that two different magma types were involved in the formation of the Stillwater. One magma type would have been MgO-rich and somewhat SiO2-rich, crystallizing in the order ol-opx-pl-cpx. Longhi et al. [29] have documented high-Mg dykes with such characteristics in the Beartooth Mountains adjacent to and cutting across the Stillwater Complex. The other magma type was alumina-rich basalt in which the main minerals crystallize in the order pl-cpx-olopx. The two magma types cannot be derived from a common parent by fractional crystallization. It is possible that the two magmas have a common origin, but have been modified to different degrees through crustal contamination. Williams and Hallberg [30] and Anhaeusser [31] have-described Archean ultramafic sills and layered intrusions that they interpret as having been derived from komatiitic magma. These intrusions feature the crystallization order ol-opx-

cpx/pl. Williams and Hallberg [30] estimate bulk compositions for the Australian intrusions which are silica-rich (49.1-52.2%) komatiitic basalt compositions with MgO ranging from 14.1 to 18.7%. Another distinctive feature of the Archean is the occurrence of intermediate intrusive rocks rich in silica, LIL elements and REE, and also high in Ni, Cr and MgO contents [21]. These rocks have been compared with the rare Cenozoic high-Mg andesites of Japan (sanukites and boninites). These intrusive Archean rocks also have geochemical similarities to siliceous basaltic komatiites and high-Mg andesites found in Archean terrains [21]. 2.3. Proterozoic times

Komatiitic and picritic magmas have been identified in the Proterozoic, although the MgO contents deduced for the erupted liquids appear generally to be lower than in the Archean [2]. Layered intrusions provide the clearest evidence for distinctive types of magmatism. Large layered intrusions such as the Bushveld are characterised by the crystallization order ol-opx-pl/cpx. Satellite dykes and sills [32] preserve poorly phyric rocks with high MgO and high silica. Such magmas have sometimes been termed continental boninites because they range to magma types that are high-MgO andesites. Sharpe [32] estimates a parental magma with 55% SiO 2 and 12% MgO for the Bushveld Complex. This Bushveld magma also had high K20 and radiogenic Sr contents and crustal contamination is strongly indicated [32]. Large anorogenic anorthosite intrusions are a prominent feature of Proterozoic times [33] and are thought to have formed during rifting of a stable continental craton [33-35]. The basaltic magmas responsible for anorthosites are required to be alumina-rich (16-20%) and iron-rich. The magmas also are characterised by exceedingly low K and Rb contents. Explanations for anorthosites include high-pressure fractionation of tholeiitic basalt derived from the mantle [33], remelting of plagioclase phenocrysts in porphyritic basalt magmas by injections of primitive picritic magma to form cryptocumulate high-alumina basaltic liquids [35] and partial fusion of lower crustal granulite rocks [36]. There is good evidence for the involvement of MgO-rich picritic magma in the generation of some anorthosites [37].

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3. Thermal constraints

TABLE 1

Assimilation of solid rock into magmas is limited by thermal constraints [38,39]. The heat required to raise the temperature of country rock to the magma temperature and to dissolve crystals must be provided by crystallization and cooling of the magma. In this section the potential for contamination of basalt and of komatiite is compared by considering the thermal requirements for assimilating granite and gabbro. There now exists a considerable amount of data on the thermodynamic properties of crystalline solids and silicate liquids. In the calculations presented below the data compiled in Nicholls and Stout [39] and Carmichael et al. [40] have been used to estimate heat capacities of liquids, enthalpy changes on heating or cooling crystalline phases and the heats of fusion. The procedure described by Nicholls and Stout [39] has been adopted based on Hess's Law of Heat Summation. However, the heat of unmixing of solid solutions and the heat of mixing melts together have been neglected since detailed calculations [39] indicate that both heats only represent a small fraction (typically << 0.02) of the total enthalpy budget during assimilation. Furthermore these heats are of opposite sign and of similar magnitude and thus tend to cancel each other out. There are undoubtedly much greater uncertainties in the values of the heat of fusion of some substances, notably forsterite. The proportions of substances for which thermodynamic data are available are listed for gabbro and granite in Table 1. For each rock type two calculations were performed to illustrate the extreme cases in assimilation of crustal rocks. In one calculation the rock was assumed to be initially at 0°C. The total enthalpy change in heating the rock to its fusion temperature, Tf, can be calculated by integrating the polynomial expressions for heat capacity as a function of temperature given in Nicholls and Stout [39]. Thus:

AH =273fr'cp dT c~, = a + h r -

CT

(1) ~

(2)

where a, b and c are empirical constants. In the second calculation the rocks were assumed to be

Proportions of mineral phases in gabbro and granite used to calculate enthalpy requirements for assimilation Gabbro Anorthite Albite Diopside Orthopyroxene Fosterite Fayalite

Granite 48% 12% 15% 15% 7% 3%

Quartz Albite Orthoclase Anorthite

32% 33% 28% 7%

at their fusion temperatures, 850°C for granite and 1200°C for gabbro. The enthalpy of change on melting the gabbro and the granite were calculated as described in Nicholls and Stout [39] using the information in Table 1. Heats of fusion were calculated as 122 cal g i for gabbro and 49 cal g 1 for granite. The enthalpy change on raising the granite melt to the m a g m a temperature was estimated assuming a liquid heat capacity of 0.324 cal g i K - 1 calculated by the method of Carmichael et al. [40]. For the basalt and komatiite magmas a constant heat capacity of 0.332 cal g-1 K I was assumed. During crystallization the bulk heat capacity of magma must vary, because the heat capacity of the crystallizing solid is temperature dependent and the residual liquid changes composition with temperature. However, calculations of heat capacities of basalt and komatiites using the method of Carmichael et al. [40] showed that the values varied little in these compositional ranges (0.33-0.35 cal g ~ K 1). Furthermore heat capacities of the crystalline solids which do not vary greatly. Calculations of the enthalpy required to convert solid granite at 0°C and 850°C and to convert solid gabbro at 0°C and 1200°C to melts at a temperature of 1200°C are given in Table 2a. The enthalpy required to assimilate granite initially at its fusion temperature (850°C) into a 1200°C magma is in fact greater than the enthalpy required to assimilate gabbro. Although the heat of fusion of the gabbro (121.8 cal g - l ) is greater than the granite (49.1 cal g 1) the enthalpy required to raise granite melt from 850°C to 1200°C is substantial (113.4 cal g 1). The enthalpies released per gram of magma to convert basalt at 1250°C and komatiite at 1600°C

215

1600 to a mixture of 50% crystals and residual liquid at 1200°C are given in Table 2b. The calculation for komatiite was made assuming a relationship between temperature and crystal content for a komatiite schematically shown in Fig. 1. The heats of fusion were calculated as 121.8 cal g-1 for basalt and 220 cal g-1 for komatiite. The calculations show that for the same amount of crystallization a komatiite releases more than three times the thermal energy of a basalt. This large difference can be attributed to two main factors. Furst, the heat of fusion of olivine is substantially greater than a gabbroic assemblage. Second, the crystallization interval of a komatiite is much greater than a basalt. About a 400°C decrease in temperature is required to crystallize 50% olivine and the enthalpy change due to this temperature change is comparable to the heat of crystallization. Basaltic magma, being typically a multiply saturated liquid, undergoes a large amount of crystallization over temperature intervals of only a few tens of degrees. Heat of crystallization dominates over heat capacity as a source of thermal energy. For example in the calculations (Table 2) heat of fusion contributed 60.9 cal g - ] and the enthalpy change due to cooling contributes

TABLE 2 (a) Enthalpy required per gram to convert solid rock to melt at 1200°C Granite Granite Gabbro Gabbro

0°C 850°C 0°C 1200°C

379.2 162.5 434.4 121.8

calories calories calories calories

(b) Enthalpy released per gram to convert basalt at 1250 ° and komatiite at 1600°C to mixture of 50% crystals and residual liquid at 1200°C Basalt Komatiite

76.5 calories 243.6 calories

(c) Mass of rock melted by one gram of magma assuming all enthalpy released is used. Calculation assumed the rock melt is raised to a temperature of 1200°C

Granite Granite Gabbro Gabbro

0°C 850°C 0°C 1200°C

Basalt

Komatiite

0.20 g 0.47 g -

0.64 1.50 0.56 2.00

g g g g

1500 1400 ,,=,

1300

13. ~ 11100200

j'~~~~,,gpL%,~ ~

1000 0

I I I I I I 10 20 30 40 50 60 70 80 90 100 % CRYSTALS Fig. 1. The percentage of crystals is plotted against temperature for a peridotitic komatiite with composition 1 listed in Table 3.

15.6 cal g - ] to the total energy released by basalt. Table 2c lists the total mass of gabbro and granite that can be melted by one gram of basalt or komatiite, based on the assumption that all the heat released by cooling and crystallization is used. These calculations provide an upper limit on the amount of rock that can be assimilated in a simple situation [41]. They show that a komatiite has much greater potential for being contaminated than basalt. It should be emphasised that these notions are illustrated by comparing extremes. Magmas intermediate between the peridotitic komatiite and basalt will show intermediate capabilities of assimilating crust. Thus a picritic melt with 20% MgO would be able to assimilate about twice as much crust as basalt. Another major difference between basalt and peridotitic komatiite is the kind of rocks that can be assimilated. Basalt can only assimilate crustal rocks with low fusion temperatures. Rocks such as gabbro or basic gneisses can often be refractory with respect to basalt. Komatites, however, have temperatures greatly in excess of the fusion temperatures of almost all crustal rocks, including gabbros, basic gneisses and pyroxenites. For example, basalts would have little tendency to be contaminated by lower crustal rocks (assuming that these rocks are gabbros or mafic granulites), except for those with unusually low fusion temperatures. Komatiite and less extreme picrites could potentially dissolve large amounts of a mafic lower crust.

216

4. Dynamic constraints The amount of crustal material a given m a g m a will actually assimilate is controlled by a great many factors. Several different mechanisms can be envisaged for contamination [15,25,42]. The same m a g m a can clearly undergo any amount of contamination from none at all to the maximum permitted by thermal arguments. Since few of the possible mechanisms have been studied in any detail, quantitative assessment is premature. However, some qualitative remarks can be made based on the known physical properties of mantle-derived magmas. Primitive magmas with the highest temperatures have the lowest viscosity since this property is strongly temperature and composition dependent. The crustal rocks heated up by the high-temperature magmas such as komatiite would also be raised to higher temperatures than by basalt thereby decreasing the viscosity of the melted rock. Some contamination processes are undoubtedly facilitated by low viscosities, in as much as mixing of melts becomes easier as viscosity decreases. Turbulent flow is also more probable as m a g m a temperature increases. For example in ascent through the crust, komatiitic liquids are certain to be turbulent, whereas basalt must flow at very high rates for the flow to be turbulent [25]. Under turbulent conditions the rate of wall-rock melting is greatly enhanced, whereas in laminar flow the m a g m a is protected from wall-rock assimilation by a congealed skin. In flow through dykes therefore the fluid dynamic conditions are much more favorable to contamination in komatiites and picrites. The same remarks are probably true of contamination in m a g m a chambers by processes such as sidewall and floor melting. If xenoliths of crustal rock are incorporated into magma, rates of dissolution and selective diffusion of components between magma and xenolith will be greatly enhanced in higher-temperature magmas since chemical diffusion coefficients will increase [42].

5. Magmas produced by contamination of ultramafic magma The discussions above suggest that contamination of komatiitic or picritic magmas by substan-

tial amounts of crustal rocks is feasible. For example, Table 2 shows that one gram of komatiite in cooling to 1200°C and crystallizing 50% olivine can melt 0.64 g of cold granite. If the 0.5 g of residual komatiite melt were then to mix with the rhyolite melt a highly contaminated m a g m a would result consisting of more than 50% crustal material. Magmas produced by assimilation and fractional crystallization processes, in which the rock melt mixes progressively with residual melt [38], can also generate magmas with over 50% crustal contaminant. Longhi et al. [29] have also concluded that massive amounts of contamination are possible when a komatiite magma is involved. They estimate that a residual melt containing over 80% crust can be formed using equation (12) of DePaolo [38]. Ultramafic magmas are also capable of assimilating not only upper crustal rocks, but also lower crustal rocks such as mafic granulites and gabbros and even some ultramafic rocks such as pyroxenites and garnet pyroxenites. In this section some calculations are presented on the kinds of magmas that could be generated by contaminating komatiite or picrite with various kinds of rock which were common in the Precambrian. These hypothetical compositions will then be compared with the various distinctive magmas than have been identified from the Precambrian. Table 3 lists a peridotitic komatiite composition chosen so that the composition plotted close to the three-phase 1 atm cotectic in the Di-PI-(Hy + 4Qz) plane projected from olivine (Fig. 2). The projection scheme is that or Irvine [43]. The composition is somewhat arbitrary, but has the merit of clearly illustrating trends that can result from contamination. Table 3 also lists the composition of a residual liquid caused by 35% fractional crystallization of olivine. In addition analyses of compositions that might be typical of various parts of the Archean crust are presented. The path followed by residual liquids during fractional crystallization and assimilation is governed by the phase boundaries, which can vary with pressure, and the composition of the contaminant. The liquid line of descent also depends on the mass ratio of contaminant to crystals fractionation [38] which is unlikely to remain a constant. Given the infinite number of possible permutations of assimilation and crystallization con-

217

ditions a simple procedure was adopted to illustrate the kinds of trend that can result. 35% olivine fractionation of the peridotitic komatiite (Table 3) was assumed followed by assimilation of each crustal composition. In physical terms this corresponds to assuming that the crystallizing magma and rock melts remain separate and that mixing of the melts occurs after removal of olivine. More elaborate numerical calculations have been presented by Longhi et al. [29] involving incremental crystallization and assimilation. Although the results differ quantitatively, the qualitative results are very similar. The calculations were terminated when compositions reached a cotectic surface between olivine and a second phase in the 1 atm phase diagram (Fig. 2). Further assimilation could occur but liquids would be constrained to follow the cotectic boundaries. The major point to emphasise is that in ultramafic magmas large amounts of crustal rocks can be assimilated while olivine is the only major liquidus phase. Although the olivine liquidus volume contracts with pressure, even at pressures corresponding to the base of the crust (say 10-15 kbars) komatiite and picrite melts can still crystallize substantial amounts of olivine before other phases join in. While only olivine is crystallizing large changes in bulk composition and in the eventual crystallization order are possible by contamination. Table 4 lists some examples of the theoretical compositions.

5.1. Upper crustal contamination An estimate of the Archean upper crust after Taylor and McLennan [[44] and a typical granite from Cox et al. [45] were used to generate trends I and II in Fig. 2. The effect of contamination is to produce liquid compositions with the crystallization order ol-opx-pl-cpx. This effect has previously been recognised by Longhi et al. [29] who carried out similar calculations involving combined crystallization of olivine and assimilation of granodiorite in komatiite. At low pressure the ol-opx boundary can be a reaction boundary, so highly contaminated magmas may crystallize orthopyroxene alone after olivine has completely reacted out. The resulting magmas would be enriched in silica, K 2O, LIL elements, light REE, Zr and other elements of upper crustal affinity. If the crust was considerably older than the mantle-derived magmas enrichments in radiogenic Sr and crustal Pb would be anticipated. A notable feature of these contaminated compositions is that the F e / M g ratio is not significantly changed because upper crustal rocks have low abundances of FeO and MgO. Thus the highly contaminated magmas would still have high Mg numbers and would precipitate olivines that would still be regarded as consistent with equilibrium between magma and plausible upper mantle compositions. For example contaminated magmas that are in the olivine volume are calculated to be in equilibrium with F ° 9 0 - 8 9 - Likewise the magmas would still be quite

TABLE 3 Compositions of magmas and rocks used in hypothetical calculations

SiO 2 TiO 2 A1203 FeO MgO CaO Na 2° K 20

1

2

3

4

5

6

7

45.8 0.3 8.0 10.3 28.0 7.1 0.4 0.1

48.2 0.5 12.3 11.7 15.7 10.9 0.6 0.2

71.3 0.3 14.3 2.7 0.8 1.9 3.7 5.0

66.0 0.6 16.0 4.5 2.3 3.5 3.8 3.3

63.1 0.5 16.1 5.5 3.5 5.8 4.5 1.0

50.1 1.1 19.8 8.0 5.8 9.4 4.6 1.2

51.6 0.4 6.6 6.3 12.5 19.9 1.9 0.02

1 = Peridotitic komatiite, close to an A 1 zone spinifex composition reported by Arndt et al. [51]. 2 = composition (1) due to 35% fractional crystallization of olivine. Olivine incrementally changed from Fo89 in equilibrium with (2). 3 = Typical granite [45]. 4 = Average Archean upper crust according 5 = Average Archean lower crust according to Weaver and Tarney [46]. 6 = Average of 21 granulite [49]. 7 = Clinopyroxenite xenolith from Chino Valley, Arizona [48].

Calculated residual liquid from Fo94 in equilibrium with (l) to to Taylor and McLennan [44]. xenoliths from Fidra, Scotland

218

ORTHOPYROXENE

/

l"r

PI /

"r

l'-"*f v=~ 25

(b~

]

I. GRANITE II. ARCHEAN UPPERCRUST Ill. ARCNEANGRANULITE/ ~

OI

ICpx

/4 /

~,

(b)

| CLINOPYROXENE

/

/-:o\\%0,, ° I ~ "~°I Tcpx

pi / >

\ \Hy (c)

"O"

Ol-/

......

i ENE

/ORTHOPYROXI~NE Opx

Otz

Fig. 2. Projections of hypothetical magma compositions in the system olivine-plagioclase-clinopyroxene-quartz using the scheme of lrvine [43]. Projections are: (a) from clinopyroxene onto the plane Ol-PI-Qtz; (b) from olivine onto the plane Pl-Cpx-(Hy+4Qtz); (c) from plagioclase onto the plane Cpx-Ol-Qtz. Large solid dot represents a peridotitic komatiite and small solid dots represent contaminated compositions. The percentage of the contaminant is shown alongside each dot. Trends I to V represent the effects of different contamination compositions as indicated in the inset. Phase boundaries are at 1 atmosphere,

rich in highly compatible trace elements such as Ni and Cr, although there would be a significant dilution effect for large amounts of contamination. The characteristics of the hypothetical contaminated magmas (Table 4) can be compared with some of the Archean and Proterozoic magmas discussed before. The Mg-rich andesites, some siliceous basaltic komatiites and Archean sanu-

kitoid intrusives [20,21] are comparable in general characteristics. The discovery of SiO2-rich komatiitic basalts containing xenocrystal zircon at Kambalda [23] has demonstrated that crustal contamination of komatiitic magmas had a role in at least one clear case. There is also a close resemblance to one of the magma types thought to be parental to or genetically related to the Stillwater, Bushveld and Great Dyke intrusions [28,29,32]

219 TABLE 4 Calculated compositions of contaminated derivative liquids after 35% olivine fractionation of compositions (1) in Table 2

SiO 2 TiO 2 A1203 FeO MgO CaO Na20 K20

1

2

3

4

5

6

7

8

52.8 0.4 12.7 9.9 12.7 9.1 1.2 1.2

51.8 0.5 13.0 10.3 13.0 9.4 1.2 0.8

53.9 0.5 13.5 9.4 11.4 8.5 1.6 1.2

51.3 0.5 13.0 10.5 13.3 9.9 1.4 0.3

54.1 0.5 13.8 9.2 10.8 8.9 2.2 0.5

48.6 0.6 13.8 11.0 13.7 10.6 1.4 0.4

49.0 0.7 15.3 10.2 11.7 10.3 2.2 0.6

48.9 0.5 11.1 10.6 15.1 12.6 0.9 0.1

1 = 20% contamination by granite. 2 = 20% contamination by Archean upper crust. 3 = 32% contamination by Archean upper crust. 4 = 20% contamination by Archean granulite. 5 = 40% contamination by Archean granulite. 6 = 20% contamination by mafic metagabbro. 7 = 40% contamination by mafic metagabbro. 8 = 20% contamination by pyroxenite.

and to smaller ultramafic complexes i30,31]. In the case of the Bushveld massive crustal contamination is already strongly imputed [32]. 5.2. Lower crustal contamination

The composition of the Archean lower crust is a more controversial matter. Weaver and Tarney [46,47] suggest that a composition represented by average granulite from the Lewisian of northwest Scotland is appropriate. Such exposed granulite terrains (composition 5 in Table 3) are frequently dominated by tonalitic and trondhjemitic gneisses. Contamination of komatiite by such material also produces compositions that crystallize in the order ol-opx-pl-cpx (Fig. 2). The effects are less pronounced compared to more silicic gneisses and the contaminated magmas would not be as enriched in SiO z and incompatible trace elements. In particular such intermediate granulites might already be depleted in elements such as Rb and K. Taylor and McLennan [44] propose a lower crustal composition which is close to a mafic andesite. There are, however, arguments that suggest that Lewisian granulite is not a good model for the lower crust. Xenoliths of presumed lower crustal origin obtained from modern volcanics reveal a wide range of ages and are invariably more mafic than either the Weaver and Tarney model or the Taylor and McLennan model [44] would imply [48,49]. Common compositions include metagabbros, garnet pyroxenites and pyroxenites. A more basic composition for the Archean lower crust is equally plausible to those proposed by Weaver

and Tarney [46] or Taylor and McLennan [44]. R.H. Hunter (personal communication) has found that compositions of lower crustal xenoliths plot over a broad zone in Fig. 2. Many plot between the four phase cotectic and the Di-P1-O1 face and a large number are rich in normative plagioclase. Indeed a significant number are slightly silica undersaturated and plot, somewhat artificially, along the Di-P1 join. Contamination by such materials produces very different trends from the Weaver and Tarney lower crustal composition. An average lower crust is not yet possible to estimate, but the effects of contamination of a gabbroic/mafic lower crust can be illustrated using the average of 21 mafic granulites at Fidra, Scotland reported by Hunger et al. [49]. The trend in Fig. 2 (IV) produces liquid compositions rich in normative plagioclase, that crystallize in the order ol-pl-cpx-opx at low pressure. There is such a wide scatter in the composition of potential contaminants from the lower crust that magmas crystallizing in the order ol-pl-opx-cpx at low pressure are also possible. The basaltic derivatives of such magmas would be aluminous and might also be expected to be iron- and sodium-rich. Anorthositic rock types in large layered intrusions such as Stillwater and Bushveld could represent magmas contaminated with lower crustal rocks. One way of explaining the two parental magma types recognised in these intrusions [28] is simply to postulate that there is only one ultrabasic parent magma and that the two lineages are the consequence of varying proportions of upper

220

and lower crustal contamination. The anorthositic end member magma, estimated for the Bushveld Complex by Sharpe [32], is similar in most respects to the hypothetical compositions 7 and 8 in Table 4. The Bushveld magma is also characterised by very low Rb, K 2 0 and high unsupported radiogenic Sr. Mafic or intermediate granulites, which had formed by the removal of K and Rb rich granite in an earlier melting episode would provide a suitable contaminant as suggested by Taylor et al. [36]. The origin of massif anorthositic magmas by large degrees of contamination of lower crust combines the two main alternative schools of thought on their genesis. There is a consensus that massive anorthosites developed during rifting within thickened continental crust that stabilised in the Proterozoic [33,34]. One view of their origin is by complex magma chamber processes of essentially mantle-derived magmas [35]. However, geochemical evidence [36] implies a crustal component, which must be strongly depleted in K and Rb. Taylor et al. [36] propose that the magmas are entirely the consequence of wholesale melting of the lower crust. However, such a mechanism requires a heat source in the mantle and, thus, inevitably suggests large scale melting of the mantle too. All the models [33-36] can be rationalised by the proposal that melting of the lower crust was accomplished by high temperature primitive magmas. Although there is no evidence for true peridotitic komatiites in the Proterozoic, the picritic m a g m a identified by Berg [37] has over 20% MgO and a 1 atm liquidus temperature of about 1400°C. Such a magma is still capable of assimilating substantial amounts of mafic lower crust. The parental magmas to anorthosites could thus be hybrids of lower crustal and mantle melts similar to the hypothetical compositions in Table 4. However, the extreme aluminous basalts (A1203 > 20%) proposed as parents by Wibe [33] could not be generated without additional open-system m a g m a chamber processes such as those discussed by Flower [35]. The combination of slow rifting in thick continental crust and m a g m a chamber processes appear to be important additional factors in the formation of anorthosite massifs from such magmas. In the model of Huppert and Sparks [25] for contamination on ascent, komatiitic liquids might

dissolve more lower than upper crustal material for a given flow rate. It should not be assumed that upper crustal rocks are more likely to be a contaminant than lower crustal ultramafic rocks. Komatiitic liquids might even be contaminated by ultramafic rocks. Garnet pyroxenites and pyroxenites are common as xenoliths and might originate as layers within more feldspathic lower crustal rocks, as part of a cumulate ultramafic keel to continents or veins within the mantle. Fig. 2 shows a trend (V) to a clinopyroxene to illustrate the effects. Magmas crystallizing in the order ol-cpxpl-opx would result. A prominent feature of contamination with such material would be a substantial increase in the CaO/A1203 ratio. Variation of this ratio has been noted in many komatiite suites [50] and contamination provides yet another possible explanation for variation in this parameter. 6. Conclusions

Magmas generated in the mantle appear to have decreased in temperature through geological time. Consequently it is proposed that crustal contamination had a much more important role in the genesis of basalts and derivative magmas in the early history of the Earth. The following conclusions can be drawn from the discussions and arguments presented in this paper. ( 1 ) The total amount of crust that can potentially be assimilated increases as magma becomes more Mg-rich, primitive and hotter. An Archean peridotitic komatiite can assimilate more than three times the amount of crust that can be assimilated by a modern primitive basalt. In an assimilation fractional crystallization process, derivative mafic magmas can contain as much as 50% contaminant binded with residual melt from a komatiite, as previously recognised by Longhi et al. [29]. (2) Viscosity decreases and chemical diffusivities increase as magmas become hotter and more olivine-rich. In most mechanisms of contamination the rates of assimilation of crustal rocks would be expected to increase and the fluid dynamic conditions that favor contamination are enhanced. Magmas would be less likely to reach the Earth's surface without being contaminated. (3) High-temperature Mg-rich magmas can be

221 c o n t a m i n a t e d by a m u c h wider range of crustal rock types than low temperature magmas. M o d e m primitive basalts are only capable of assimilating crustal rocks with low fusion temperatures. However, peridotitic komatiites could have assimilated almost any crustal rock type including lower crustal material such as gabbros, mafic granulites and even pyroxenites. (4) The m a g m a s formed by extensive contamination or by A F C of komatiite are no longer komatiite, but mafic magmas, some of which would not be regarded as conventional basalt. While the m a g m a crystallizes only olivine as a major phase large changes in bulk composition and eventual crystallization order can result from contamination. C o n t a m i n a t i o n of komatiite or picrite by upper crust or granite produces siliceous Mg-rich basalt and Mg-rich andesite compositions with the low-pressure crystallization order ol-opx-pl-cpx. M a g m a s with the appropriate characteristics have been recognised as important c o m p o n e n t s of Archean volcanic successions and seem to be appropriate parents for some Precambrian layered intrusions [15,29]. C o n t a m i n a t i o n of komatiite or picrite by lower crust could generate basalt magmas with wide ranges of characteristics. If the lower crust consists of cumulate gabbroic material then the c o n t a m i n a t e d m a g m a s would be rich in FeO, and A1203 and might be appropriate for forming anorthositic rocks, crystallizing in the order ol-pl-opx-cpx or ol-pl-cpx-opx. Such magmas would be appropriate parents for the parts of large Precambrian layered intrusions that crystallize in the order o l / p l - c p x - o p x . Thus the two parental m a g m a s i d e n t i f e d in the Bushveld and Stillwater complexes [27,28] m a y be produced by crustal c o n t a m i n a t i o n of a c o m m o n ultramafic p a r e n t m a g m a . The crystallization order is determined by the relative amounts of upper and lower crust in the contaminant. (5) Proterozoic anorthosite massifs could also be related to substantial assimilation of mafic lower crust into ultramafic magmas, producing alumina-rich basalt derivatives with the required geochemical features. (6) Some of the peculiarities of Precambrian m a g m a t i s m have been attributed to heterogeneities or processes in the mantle. These conjectures m a y well be correct, but a convincing case must be m a d e to eliminate crustal contamination since it is

apparent that a wide range of m a g m a types can also be generated b y this process.

Acknowledgements Bob H u n t e r is especially thanked for help and m a n y illuminating discussions on the problems of deducing the composition of the lower crust. Paul Browning also provided help in sorting out the projection scheme used. Herber H u p p e r t as always is acknowledged for comments. Discussions with M.J. O ' H a r a , M.J. Bickle, N.T. Arndt, E.G. Nisbe and T.J.B. Holland are acknowledged as is Sandra Last who typed the drafts. The manuscript was improved by helpful reviews by I.H. C a m p bell, T.N. Irvine, J. Longhi and S.A. Morse. The author is supported by the BP Venture Research Fund.

References 1 M.J. Bickle, The magnesium contents of komatiite liquids, in: Komatiites, N.T. Arndt and E.G. Nisbet, eds., pp. 479-494, George Allen and Unwin, 1982. 2 N.T. Arndt, Proterozoic-textured basalts of Gilmour Island, Hudson Bay, in: Current Research Part A. Geol. Surv. Can. Pap. 82-1a, 137-142, 1982. 3 A. Hynes and D. Francis, Komatiitic basalts of the Cape Smith foldbelt, New Quebec, Canada, in: Komatiites, N.T. Arndt and E.G. Nisbet, eds., pp. 159-170, George Allen and Unwin, 1982. 4 K.G. Cox, Komatiites and other high-magnesia lavas: some problems, Philos. Trans. R. Soc. London, Ser. A 288, 599-609, 1978. 5 K.G. Cox, A model for flood basalt vulcanism, J. Petrol. 21, 629-650, 1980. 6 M.J. O'Hara, The bearing of phase equilibria studies in synthetic and natural systems on the origin and evolution of basic and ultrabasic rocks, Earth Sci. Rev. 4, 69-133, 1968. 7 W.S. Hildreth, Gradients in silicic magma chambers: implications for lithospheric magmatism, J. Geophys. Res. 86, 10153-10192, 1981. 8 H.E. Huppert and R.S.J. Sparks, The fluid dynamics of a basaltic magma chamber replenished by influx of hot, dense ultrabasic magma, Contrib. Mineral. Petrol. 75, 279-289, 1980. 9 E. Stolper and D. Walker, Melt density and the average composition of basalt, Contrib. Mineral. Petrol. 74, 7-12, 1980. 10 D.B. Clarke and M.J. O'Hara, Nickel, and the existence of high-MgO liquids in nature, Earth Planet. Sci. Lett. 44, 153-158, 1979. 11 A.P. Dickin, Isotope geochemistry of Tertiary igneous rocks from the Isle of Skye, Scotland, J. Petrol. 22, 155-189, 1981.

222 12 M.A. Menzies, W. Leeman and C.J. Hawkesworth, Geochemical and isotopic evidence for the origin of continental flood basalts with particular reference of the Snake River Plain and Columbia River, U.S.A., Philos. Trans. R. Soc. London, Ser. A 310, 643-660, 1984. 13 K.G. Cox and C.J. Hawkesworth, Relative contribution of crust and mantle to flood basalt magmatism, Mahabaleshwar area, Deccan Traps, Philos. Trans. R. Soc. London~ Ser. A 310, 627-641, 1984. 14 R.W. Carlson, Isotopic constraints on Columbia River flood basalt genesis and the nature of the subcontinental mantle, Geochim. Cosmochim. Acta 48, 2357-2372, 1984. 15 I.H. Campbell, The difference between oceanic and continental tholeiites: a fluid dynamic explanation, Contrib. Mineral. Petrol. 91, 37-43, 1985. 16 Z.A. Palacz, Isotopic and geochemical evidence for the evolution of a cyclic unit in the R h u m intrusion, north-west Scotland, Nature 307, 618-620, 1984. 17 S.R. Carter, N.M. Evensen, P.J. Hamilton and R.K. O'Nions, Nd- and Sr-isotopic evidence for crustal contamination of continental volcanics, Science 202, 743-747, 1978. 18 M.J. Bickle, A. Marting and E.G. Nisbet, Basaltic and peridotitic komatiites and stromatolites above a basal unconformity in the Belingwe Greenstone Belt, Rhodesia, Earth Planet. Sci. Lett. 27, 155-162, 1975. 19 L.S. Jensen and D.R. Pyke, Komatiites in the Ontario portion of the Abitibi belt, in: Komatiites, N.T. Arndt and E.G. Nisbet, eds., pp. 147-157, George Allen and Unwin, 1982. 20 W.E. Cameron, E.G. Nisbet and V.J. Dietrich, Boninites, komatiites and ophiolitic basalts, Nature 280, 550-553, 1979. 21 S.B, Shirey and G.N. Hanson, Mantle-derived Archean Monzodiorites and trachyandesites, Nature 310, 222-224, 1984. 22 J.A. Hallberg, C. Johnson and S.M. Bye, The Archean Marda igneous complex, Western Australia, Precambrian Res. 3, 111-136, 1976. 23 W. Compston, I.S. Williams, I.H. Campbell and J.J. Gresham, Zircon xenocrysts from the Kambalda volcanics: age constraints and direct evidence for older continental crust below the Kambalda-Norseman Greenstones, Earth Planet. Sci. Lett. 76, 299-311, 1985/86. 24 N.T. Arndt and R.W. Nesbitt, Geochemistry of Munro Township basalts, in: Komatiites, N.T. Arndt and E.G. Nisbet, eds., pp. 309-329, George Allen and Unwin, 1982. 25 H.E. Huppert and R.S.J. Sparks, Cooling and contamination of mafic and ultramafic magmas during ascent through continental crust, Earth Planet. Sci. Lett. 74, 371-386, 1985. 26 H.E. Huppert and R.S.J. Sparks, Komatiites, 1. Eruption and Flow, J. Petrol. 26, 694-725, 1985. 27 I.S. McCallum, L.D. Raedeke and E.A. Mathez, Investigations of the Stillwater Complex, Part I. Stratigraphy and structure of the banded zone, Am. J. Sci. 280, 59-87, 1980. 28 T.N. Irvine, D.W. Keith and S.G. Todd, The J-M Platinum-Palladium reef of the Stillwater Complex, Montana, If. Origin by double-diffusive convective m a g m a

29

30

31

32

33 34 35 36

37

38

39

40

41

42

43

44

mixing and implications for the Bushveld Complex, Econ. Geol. 78, 1287-1334, 1983. J. Longhi, J.L. Wooden and K.D. Coppinger, The petrology of high-Mg dikes drom the Beartooth Mountains, Montana: a search for the parent m a g m a of the Stillwater Complex, J. Geophys. Res. 88 (Suppl.), B53-B69, 1983. D.A.C. Williams and J.A. Hallberg, Archean layered intrusions of the Eastern Goldfields Region, Western Australia, Contrib. Mineral. Petrol. 38, 45-70, 1973. C.R. Anhaeusser, Archean layered ultramafic complexes in the Barberton Mountain Land, South Africa, in: Evolution of Archean Supracrustal Sequences, L.D. Ayres, P.C. Thurston, K.D. Card and W. Weber, eds., Geol. Assoc. Can. Spec. Pap. 28, 1985. M.R. Sharpe, Strontium isotope evidence for preserved density stratification in the main zone of the Bushveld Complex, South Africa, Nature 316, 119-126, 1985. R.A. Wiebe, Anorthositic magmas and the origin of Proterozoic anorthosite massifs, Nature 286, 564-567, 1980. S.A. Morse, A partisan review of Proterozoic anorthosites, Am. Mineral. 67, 1087-1100, 1982. M.F.J. Flower, Anorthosite genesis: the mid-ocean ridge analogue, Geology 12, 651-654, 1984. S.R. Taylor, I.H. Campbell, M.T. McCulloch and S.M. McLennan, A lower crustal origin for massif-type anorthosites, Nature 311, 372-374, 1984. J.H. Berg, Snowflake troctolite in the Hettasch intrusion: evidence for m a g m a mixing and supercooling in a plutonic environment, Contrib. Mineral. Petrol. 72, 339-351, 1980. D.J. DePaolo, Trace element and isotopic effects of combined wall rock assimilation and fractional crystallization, Earth Planet. Sci. Lett. 53, 189-202, 1981. J. Nicholls and M.Z. Stout, Heat effects of assimilation, crystallization, and vesiculation in magmas, Contrib. Mineral. Petrol. 81, 328-339, 1982. I.S.E. Carmichael, J. Nicholls, F.J. Spera, B.J. Wood and S.A. Nelson, High-temperature properties of silicate liquids: applications to the equilibration and ascent of basic magmas, Philos. Trans. R. Soc. London, Ser. A 286, 373-431. 1977. The reader may note that geologically plausible situations can be envisaged where there is no limit to the amount that can be assimilated. For example, m a g m a at the top of a zoned m a g m a chamber could assimilate any amount of wallrock if extra heat is being provided by inputs of hot primitive m a g m a at the base of the chamber. In the main text the simplest case of an homogeneous m a g m a assimilating wallrock is considered. E.B. Watson, Basalt contamination by continental crust: some experiments and models, Contrib. Mineral. Petrol. 80, 73-87, 1982. T.N. Irvine, Crystallization sequences in the Muskox Intrusion and other layered intrusions, I. Olivine-pyroxeneplagioclase relations, Geol. Soc. S. Afr. Spec. Publ. 1, 441-476, 1970. S.R. Taylor and S.M. McLennan, The Continental Crust, its composition and Evolution, 312 pp., Blackwell Scientific Publications, 1985.

223 45 K.G. Cox, J.D. Bell and R.J. Pankhurst, The Interpretation of Igneous Rocks, 450 pp., George Allen and Unwin, 1981. 46 B.L. Weaver and J. Tarney, Continental crust composition and nature of the lower crust: constraints from mantle Nd-Sr isotope correlation, Nature 286, 342-346, 1980. 47 B.L. Weaver and J. Tarney, Empirical approach to estimating the composition of the continental crust, Nature 310, 575-577, 1984. 48 R.W. Kay and S.M. Kay, The nature of the lower continental crust: inferences from geophysics, surface geology and crustal xenoliths, Rev. Geophys. Space Phys. 19, 271-297, 1981.

49 R.H. Hunter, B.G,J. Upton and P. Aspen, Meta-igneous granulite and ultramafic xenoliths from basalts of the Midland Valley of Scotland: petrology and mineralogy of the lower crust and upper mantle, Trans. R. Soc. Edinburgh: Earth Sci, 75, 75-84, 1984. 50 H.S. Smith and A.J, Erlank, Geochemistry and petrogenesis of komatiites from the Barberton greenstone belt, South Africa, in: Komatiites, N.T. Arndt and E.G. Nisbet, eds.. pp. 347-398, George Allen and Unwin, 1982. 51 N.T. Arndt, A.J. Naldrett and D.R. Dyke, Komatiitic and iron-rich tholeiite lavas of Munro Township, north-east Ontario, J. Petrol. 18, 319-369, 1977.