Archaean MetaBasalts & MetaAndesites

The oldest known segments of the earth's crust are the Archaean cratons which form cores to the major continents. A major tectonic event at 2.7 byr bp has reset most radiogenic "clocks" but Pb ages from zircons in Archaean metasediments from Greenland and from West Australia give ages as high as 4.4 byr approximately for some Archaean rocks.

Two archaean cratons occur in West Australia, the Yilgarn craton extending from Norseman, south of Kalgoorlie, to about Meekatharra-Wiluna about 300 miles NNW, sometime called in mining circles the "Agnew-Wiluna Block"; and the Pilbara craton which lies further north, south of Port Hedland. Archaean rocks also occur in South Africa, in Liberia, in Scandinavia, and in much of the Canadian North from Quebec to Manitoba.

The older rocks in these cratons are usually MORB-type basalts, now isoclinally folded and metamorphosed to greenschist, occasionally amphibolite facies. They are much sheared and distorted in the Kambalda area in the southern Yilgarn block, less so in Quebec - Ontario. Renner et al, 1994, (J.Pet.35) claim that in tholeiites and komatiites of the Reliance Fm, Belingwe Greenstone belt, Zimbabwe, some flows have quite unaltered olivine up to Fo92 in composition.

Following upon the 2.7 byr year event, the Canadian archaean rocks were invaded first by a series of trondhjemitic, somewhat lineated syn-tectonic sodic diorite-granodiorites, and later by coarse grained post tectonic potassic granites, the youngest being very potassic pegmatites. A similar sequence is seen in other continents.

Archaean metabasalt-komatiite of the Chibougmau-Noranda (Superior) Region

Once, some decades ago, Gilles Duquette and I set out to investigate the compositions of the Archaean metabasalts of Northern Quebec. It was a disillusioning exercise. There were thousands of feet of greenish altered flows mainly with a massive base and 20-30 feet of pillows on the upper surface, or presumed upper surface as they had been folded in the orogeny of 2.7 byr bp and were now vertical. At least it was not hard to tell top from bottom. We recognised them immediately as the products of an oceanic environment and assumed a spreading center ortigin.
Low grade metamorphism had albitised all the feldspar and inverted the clinopyroxene to chlorite. Titanomagnetite was now represented by rods of ilmenite and there was much interstitial calcite especially between pillows, but the calcium of the feldspar had totally gone. There were many intercalated sills of clinopyroxenite, peridotite etc and some noritic layered intrusions of the Stillwater type (eg, The Dore Lake Complex, (Jean-Louis Caty, PhD Thesis, U.de M.) and a similar intrusion at Allard Lake.).

It now seems that what we thought were peridotite sills were in fact komatiites, very magnesian flows, with acicular or dendritic masses of crystal near the upper surface, now called "spinifex texture" after the prickly Australian desert grass. These may be olivine or clinopyroxene the latter being possibly of secondary origin. The body of a komatiite flow is composed of close packed olivines sometimes of 1 cm diameter and it is assumed they were formed from a very hot, very high degree melt magma. Komatiites may have 35% - 40% or more MgO and as high as 4000 ppm Ni, but this appears in some cases to be due to olivine accumulation.
The chemistry we did of the Achaean rocks did not say much other than that they were tholeiitic basalts with highly variable Na, K, Rb, Th, Ca. At that time some people in Canada were calling any Archaean basalt with normative andesine an "andesite" which was nonsense, as often up to 10% CaO had been lost during metamorphism.

Intercalated between thick belts of greenschist MORBs of Quebec-Ontario are zones of what seemed to be island-arc type andesitic sequences, in others, mature arc and high-K andesite with minor rhyolite. Lately it has been found that lamprophyres and kimberlites also exist associated with diamond pipes.
Early workers like J. Goodwin of U of Toronto, and R. Baragar of the Canadian Survey found extensive andesite-dacite-rhyolite sequences at Birch-Uchi Lake and Duparquet but their data like ours showed considerable alteration effects. We tried taking multiple samples from each flow and mashing them together but the chemistry was still unreliable with the same uncertain K, Rb, Ba, Na, Ca, Ti, especially and often P. Physically they looked just like modern MORBs and chemically they were approximately the same but compositions were always erratic, a fact not surprising when one sees how altered a marine sample a million years old can be and these were up to 3.6 byr and metamorphosed to boot.
Komatiites are found at Noranda - Chibougamau as well as in the Onverwacht series of South Africa and the Yilgarn and Pilbara Cratons of West Australia and in Finland. Their Fe/Mg slope though scattered may indicate in some cases, a very magnesian olivine, probably about Fo90-92 or more, indicating a hotter magma than usual, but rocks called komatiites are also associated with more iron rich (as in Gorgona) and LILE enriched rocks. In general, the compositions of basalt series are quite like those of LILE-enriched basalts of the present day. Of 90 tholeiites-komatiites from the Chibougamau - Noranda sector only two samples show significantly higher Ni/Mg when compared to Hawaiian picrites. No basalts comparable to modern NMORB appear to have as yet been found.
Descarreau (1977, PhD, Laval) analysed about 180 samples from the Abitibi, they appear to fall into two groups, a MORB-like group and an andesite-dacite-rhyolite group but again this involves some guesswork.
Analytical work of the last ten years by ICPMS show stable HFSE elements matching modern enriched MORBS closely but not definitely distinguishable from arc and back-arc basalts. Little can be done with the errant LILE elements in most samples however..
A great deal of geochemical work has been done on the Archaean because of the economic significance, (see below) but much that is done under the auspices of mining companies is not released to the common public. More diagrams will be added if data becomes available.

	Ternary diagram of: Noranda Komatiites (o), Chibougamau metabasalts (x), Birch-Uchi Lake (+), and Michipcoten(*) ranging from near peridotite to rhyolite. Xie, Kerrich & Fan, (1993), show three Archaean series from the Noranda region, from Tisdale, Munro and Boston Townships. All are described as komatiite-tholeites and they range from quite depleted basalt (though less depleted that the EPR), to the quite enriched Boston rocks which are more enriched that, say, Volcan Alcedo in the Galapagos. Xie et al call the Boston Township rock "alkaline" but none fall into the alkali basalt field being more akin to EMORBs. They are mainly picrites which are not found in "alkaline" rocks and are less enriched than, say, the hawaiites of Mauna Kea. We now know that tholeiitic MORBs of smaller degrees of partial melt are also alkaline in the sense they are LILE enriched, though not or only slightly, reduced in silica. These two kinds of "alkaline" rocks have lead to much confusion, not only in Archaean sphere! All three series show the parallel spacing in multi-element diagrams of typical olivine fractionation.
	Komatiitic Basalts of La/Sm <1 normalised to EMORB (=~average oceanic basalts.). From all cratons. Note complete absence of NMORBs. Occasional low K, low Rb basalts are almost certainly due to alteration. Lowest samples will be olivine cumulates. Pr and some missing elements have been interpolated.(Nov.2004)
	Basalts and "komatiites" of Munro Township. While of lower TiO2 this series is similar to Kilauean picrite series of Hawaii, with up to 30 MgO in the picrites, reaching the Mg-Ca-Al triple point at 12% CaO, ending with erratic ferrobasalts of 16 - 18% Fe2O3(t). The Mg/Ni is also the same, they appear somewhat more magnesian but there are not enough points to be definitive.(~year 2000)
	Basalts and komatiites of Boston Township. Note elevation of the LILE. These have the appearance of E-type MORBs plus picrite members, that is they are of smaller degree of partial melt than those of Munro Township.
	Tisdale Township, with no ferrobasalt members. Slightly more enriched than Munro Township.
	Zr/Nb for Noranda: (+) MORB partial Melt series. Kamenetsky et al, (2000) (O) The modern EPR, (Regelous et al, (1999) (*) Boston Township, (Xie et al, 1993) (x) Munro Township, (Xie et al, 1993) Note that the EPR rocks which are glasses, stop at the plane of partial melts. The Boston and Munro komattites continue straight through towards the olivine point at 0.0. We do not however know what happens to the partial melt series at very high degrees of melt, it may curve toward olivine. However, the Boston rocks lie well above any such possible curve. (Diagram from 1990's) Spreading rates as high as 12 cm/yr have been suggested for the EPR. Archaean spreading rates then must have been lower than this, from the few examples we have, but undepleted and hotter mantle must also have had an effect.
	La/Sm for Archaean basalt - "Komatiite" series. + = Macquarie ORB degree of partial melt series, (Kamenetsy, 2000) * = Munro Township, (Xie et al, 1993) x = Modern EPR, (Regelous, etal, 1999) Triangles, Norseman 1, Yilgarn Block. (McCuaig, et al, 1994) o = Tisdale Township, Noranda. Diamonds = Modern Kolbeinsey Ridge, (Devey et al, 1994). Note that both the EPR and Kolbeinsey are slightly more depleted than the Archaean high-mg basalts-"komatiites". Note that the Kolbeinsey samples, which are whole rocks and not glasses, also cross the plane of partial melts towards the olivine corner. The EPR glasses do not! We now (Oct.2004) would group all rocks of La/Sm < 1.25 as being parental komatiite and basalts of La/Sm>2 as being derivative partial melts.
	Pickle Lake metabasalt-komatiite, (Kerrich et al, 1999)

Archaean Basalts-komatiites of the Yilgarn-Pilbara cratons, Australia

McCuaig et al, (1994), show two series from the Norseman - Kambalda area at the southern end of the Yilgarn block. As in Noranda, one series is quite depleted, though again less so than the EPR, and includes komatiites. The other is a more enriched EMORB. Olivine accumulation is minimal at a maximum of 18% MgO and there are no real ferrobasalts, the max Fe2O3(t) being less than 14%. The enormous development in Ni mining of secondary pentlandite developed on magnesian rocks of this region has meant a great deal more data has been acquired but attempts to see it have so far failed.

	Yilgarn Craton; MgO vs Oxides. Komatiites lie between 40 and 12% MgO. There is no real explanation for the apparent variability in Al2O3 and CaO but as in all Archaean series the alumina tends to be linear from 50% down to <5% MgO.
	Yilgarn Craton; La vs Sm. The linear lower group with La/Sm approx =1 are the komatiites. Basalts of La/Sm 2 - 5 etc are probably partial melts of the komaiites. All cratons show this relationships, as do the rocks of Gorgona.
	Yilgarn Craton; REE (Ce, Pr, Nd, Sm, Eu, Gd) A typical REE dispersion for Archaean rocks, the komatiites having very low REE with La/Sm ~ 1 and Nd almost = to Ce. Derivative basalts have much higher La-Ce cf Nd.
	Yilgarn Craton; MgO vs metals. These show the common peak in chrome at 25-30% MgO in the komatiites. This MAY mean that rocks of higher MgO may be cumulates, or may indicate that Chromites does not form until olivine has reached a composition of about Fo91-2. (The most magnesian rocks have Fo93 and even Fo96.)

Pilbara Craton (NW Australia)

	Normalised diagram for Archaean rocks of the Mallina Basin, near the coast in NW Australia. Obviously the fingerprint has continental affiliations, either ARC type or Continental Flood Basalt, but Archaean rocks are considerably less modified that those of the present day, the komatiites being more chondrite-like. See Christie et al, 2004, J.Pet.45 for maps and discussion.
	Normalised REE, of enriched type. Note diversity in HREE contrasting with uniform slope of LREE.
	This fingerprint of Archaean basalts from Norseman, appears to be quite "clean" except for variable Ba, Cs, Rb, and Sr, the latter probably following albitisation of plagioclase. However the Ti remains constant so the rocks are rather similar to LILE enriched EMORB basalts, just a bit less so than Kilauea. The low Sr is almost certainly a result of the albitisation of the plagioclase and the low P may also be an artifact, while the K, Ba, Th, Cs are variable and unreliable. It seems that the elements least likely to be changed are the REE, Zr, Y, Th and Nb. (See conclusions below)
	This second diagram is of a group of N-type MORBs with less variable Sr and P but unreliable but elevated K, Ba, Rb, Cs.
	The Pilbara Craton seems enriched basalts, again close to E-type MORB though again with unreliable but generally high U, Ba, Rb.
	La vs Sm showing relationships between metabasalts and younger dioritic group.

Note that these ratios are constant in all cases between rocks of 40% MgO down to about 12% and in the case of the more dpeleted basalts may continue to ~4% However in most cases the basalts of <10% MgO have higher La/Sm (2 - 7 cf 1). lower Zr/Nb (5 - 15, cf 20-40). The range is similar to the present day but those of Boston Township would only be found on a enriched tholeiitic island, marginal seamount or failed spreading centre, in arc or back-arc basalts, or continental flood basalts.
The Zr/Hf is in the low range as expected, but some seem too low compared with more recent basalt data, see Global Dist. Of Elements Chapt., Zr/Hf. Note how the general decrease in Zr/Nb corresponds with increase in La/Lu, decrease in Zr/La and even Ce/La etc. (conclusions of about yr 2000), see below)

Archaean Craton, Baltic Shield

	Baltic Shield; La vs REE. Again a typical Archaean REE dispersion, with Komatiites showing La/Sm and Ce/Nd near to 1
	Baltic Shield; La vs Sm. Note distinct break between parental komatiites of La/sm = 1 and daughter basalts with La/Sm ~ 5.
	Baltic Shield; MgO vs oxides. The linear olivine control line extends right through komattites and basalts, but the "andesite" (=icelandite??) tend to cluster at higher Al2O3
	Baltic Shield; lead isotopes. Extreme range in lead isotopes,. Most komatiites occur in the lower group, derivative basalts in the upper, but not all samples have been analysed for ME.
	Peuramaa Metabasalts. Very similar to Hawaii, (Mauna Loa, while REE pattern is EXACTLY same as Kilauea Historic lavas demonstrating an enriched LILE origin.

Archaean Felsic rocks

	Though described as a "Lamprophyre" this rock has the negative Nb and configuration of a fairly typical andesite-dacite, though the Cs may be somewhat enhanced. Similar Nb anomalies are seen in the Marda Complex of W. Australia, (Hallberg et al, 1976 (Precamb.Res.3,111-136)
	Metandesites from the Wawa Greenstone belt, Canadian Shield. Though showing alteration effects, an undoubted Active to Mature Arc signature. It seems that for nearly three billion years, subductive margins have been producing the same kind of magmas.(approx 1yr 2000, see later interpretation below)

Archaean Craton, Zimbabwe

	Zimbabwean Archaean; MgO vs oxides. A typical Archaean pattern, with olivine control to about 12% MgO with secondary basalts forming, much as in modern ORB. The high soda group at 10-15% Mgo MAY be olivine accumulated basalts. Note the general low TiO2 in the related komatiites.
	Zimbabwean Archaean; SiO2 vs oxides.

Economic Geology of the Archaean

Archaean rocks are the source of the greater part of our heavy metals. Cu, Ag, Pb is more plentiful in the old andesitic orogens, eg the "Porphyry coppers" but Ni especially usually also accompanied by significant amounts of Cu stems almost entirely from the Archaean zones. This is also discussed in the "Global Distribution of Elements" chapter. Recoverable Ni occurs in three forms in Archaean rocks,

1. concentrated in laterites developed on the surface of old archaean peridotites, eg , as at Murrin Murrin which lies 50 km east of Leonora in West Australia in garnierite or a nickeliferous limonite.
2. as disseminated sulfides in large basic intrusions, as in Sudbury,Ont., usually as pentlandite but with many minor sulfides amd arsenides of Ni, Cu, Zn, Co and PGE.
3. As sulfides, mainly pentlandite, formed by metamorphism, after breakdown of olivine in komatiites which release Ni, which then combines into existing iron sulfides. Other theories involve intercalated shales as a source of S. This is the "Kambalda type", found south of Kalgoorlie in West Australia and becoming recognised world wide.

Metamorphosed archaean rocks are one of the main sources of gold, which separates along with quartz and sometimes calcite at metamorphic temperatures of around 250 deg C. A chain of mines stretching N-S in the Agnew-Wiluna block, including Norseman, Coolgardie, Kalgoorlie, Leonora, Laverton, Wiluna, Meekatharra, are all of this type. All centres are marked from afar by enormous spoil dumps.
Gold is also found in the Canadian Archaean , eg at Val d'Or, but I do not believe there is any in the Chibougamau region where the metamorphism is of lower grade.
Ni production from Archaean rocks in Ontario alone peaked in 1975 with an annual production of 128,000 tons Ni.

Komatiites of Gorgona, Colombia

On this Pacific offshore island, a thick layer of Cretaceous komatiite and basalt forms one of the few non Archaean examples known. They are more depleted in LILE than the Archaean examples, have higher Zr/Nb and more iron-rich olivines of about Fo85. Otherwise trends are similar with basalts forming below 10% MgO.

	Gorgona Komatiites; MgO vs Oxides. Note Al2O3 showing olivine control through to <10% MgO and the high iron olivines. Most erratic points are included gabbroic xenoliths.
	Gorgona Komatiites; La vs Sm. As in Archaean, note trend of parental komatiites at La/Sm = 1.
	Gorgona Komatiites; MgO vs metals. Gorgona rocks are less magnesian and show erratic Cr at 27% MgO, but not a definite peak. Nickel does not extend over 2000 ppm.
	Gorgona Komatiites; Zr vs AE. The basalts have greatly elevated Sr, most Ba is <=Y, and Rb<Nb.
	Gorgona Komatiites; REE normalised to EMORB. Pm and Pr as well as some missing elements have been interpolated. Data from GEOROC. Gorgona Komatiites are relatively LREE depleted. and look similar to NMORB in spite of their much higher MgO content. Again the basalts appear close general EMORB. Average Chondrite would plot near the base of diagram, slightly higher on right at about 0.2. see "Meteorites". Note also similarity to Martian Shergottites which are also ferro-peridotites.
	Komatiites of Gorgona, Pacific Coast, Columbia. Varialble but with elevated K group and low Nb

These unusual mafic flows and sills were claimed to be the only modern equivalent of the Archaean "komatiites" by L. Echevaria, (1982, Allen & Unwin) and were studied again in much more detail by Revillon,S., & Arndt.N, et al 2000, (J.Pet.(7), 1127-1153).

Their similarities to the more depleted Archaean Komatiites is undoubted, they have the same EPR-type Th/U, Ce/La, La/Sm but their Zr/Hf is oddly low at 29.9 as is their Nb/Ta at 14. Zr/Nb is 29 in the EPR but Gorgona shows a scatter of trends at 14, 30 and 57, but these low levels a difference of even 0.5 ppm in Nb make a very large difference in ratio. The same "spinifex" texture is seen marginally on Gorgona flows as seen in komatiites.

Our sincere thanks are extended to Dr Sarbas of the "GEOROC" data base who made a special effort to scan the Gorgona data for us.

Conclusions on Archaean rocks and the formation of Primary Continental Crust.

Ideas on the evolution of Archaean continental crust have progressively changed. Twenty years ago it was commonly assumed that mantle circulation occurred along with the development of spreading centres and subduction zones. The massive nature and pillowed flow tops indicated a marine, oceanic-ridge environment and the often intimate mixture of "andesitic" types was taken to mean that convection cells were smaller than today.
The compilation of all the accurate data for which we have again the thank "GEOROC" has shown the similarities of all the Archaean Cratons more plainly. We can now see that all Archaean basalts are more enriched and nothing resembing an NMORB is to be found. Very low Rb rocks analysed 20 -35 years ago and taken to be NMORBs were probably due to alteration, as the more recent REE data which is virtually unaffected by secondary alteration, does not show the presence of depleted rocks. In fact the Archaean basalt - "andesite" suite is not like anything seen in modern rocks at all.
Writers such as Kerrich and Holling(2002) have pointed out the low Nb compared to higher U.Th and suggested they are all arc or back arc basalts and also pointed out the similarities to the boninites and the arc picrites seen in the western pacific.
When we include all the K group elements in a fingerprint normalised diagram, we see several interesting things, all the rocks including komatiites appear to show continental high Cs,Rb,Ba, Th,U,K and low Nb, Ta as seen in island arc basalts and andesites. All cratons show the LILE of basalts splitting from the komatiites as though from an immediate parent. In the major elements, the secondary basalts have a most basic composition of not more than 10% MgO, though this is confused by picritic accumulation. In general the relationship between probable mantle and partial melts thereof is simlar to that seen in the ORBs, in Hawaii, and in Iceland , the Archaean basalts being often at least as LILE enriched as Hawaii. This latter MAY indicate that enriched komatiite-type mantle may underlie Hawaii in the lower mantle.
No modern rocks show the constant association of basalts with peridotitic parent though compositions of modern EMORB, back-arc basalts and some arc basalts are indistinguishable from the Archaean basalt. Arcs however have very depleted proto-arc members which, like NMORBs, are simply not seen in the Archaean sequence.
The similarity of the komatiites to average chondrite is marked, though it tends to be rather more depleted in LILE relative to HFSE. The most remarkable thing is the discovery that chondrites have a continental signature, with high Rb, Ba, U, Th, K relative to Nb-Ta. This is not hard to explain. It has been the custom to assume that an original planetary composition, usually taken as being that of a a very reduced enstatite chondrite, had lost its metallic Fe, Ni and P to the core, and it's Cs, Rb, Ba, Th, U, K to the crust.
The komatiites, if they represent original subcontinental crust-mantle, have lost their metallic Fe, Ni, P, but very little of their K-group elements. These are in the process of being concentrated in the basalts and "andesites" but can only be added to form a granitic crust by being subducted and partially melted. The main Archaean tectonic event took place at 2.7 billion years resulting in isoclinal folding. Subsequent to this, subduction would become possible, prior to this rocks might retain their original K-group enriched form. Most trondhjemitic plutonic rocks give a 2.7 byr age date.

So we can conclude that the apparent continental, "arc" type fingerprint of the Archaean basalts and higher differentiates, is still inherited from the chondritic parent. With time, the mantle becomes more depleted as almost all the K group elements are taken up by basaltic low degree partial melts which were subducted and partially melted to form granites. The modern oceanic upper mantle no longer shows the continental fingerprint, but a K-group depleted one as seen in modern spreading centres. Because of subduction we have no Palaeozoic oceanic crust remaining to compare intermediate stages.