Chapter 3. Archean Crustal Provinces - Petrological,

Geochemical and Structural Characteristics

Refs: K.C. Condie (ed.,1994). Archean crustal evolution. Elsevier, 420 pp.B. Windley (1995). The evolving continents. 3rd ed., Wiley, N.Y., 385 pp.A. M. Goodwin (1996). Principles of Precambrian geology, Academic

Press, London. 327 pp.P.G. Eriksson et al. (eds., 2004) The Precambrian Earth: tempos and

events. Elsevier, 941 pp.

IntroductionPrecambrian (4.56 - 0.54 Ga) - 88% of the Earth’s history.Our knowledge about this period remains elusive.

Diverse opinions on many important issues, such as,

1. Cratering history of the early Earth.2. Functioning of the plate tectonics in the Precambrian, especially in

the Archean.3. Nature of the primitive crust; and the age of the earliest CC.4. Evolution of the atmosphere and origin of water.5. Prevalent mechanism(s) for the formation of greenstone belts.6. Relation between greenstone belts and surrounding granitic gneiss

complexes.7. Evolution of sedimentary patterns? can sequence stratigraphy be

applied to Archean sequences?8. Particular features about structural patterns and deformation

processes in the Archean?

A new book

The very early history of the earth -

the Hadaen period

A major geological problem constantly raised: Did the plate tectonics function in the Precambrian, particularly, in the Archean?

Two opposing schools of thought:

(1) Uniformitarian - The plate tectonics functions all the time on the Earth; there is no exception for the Precambrian.

(2) Non- uniformitarian - The early lithosphere was too warm and too soft to subduct, the “plates” were likely small and thin; they were pushed and turned over like blocks of ice in the Arctic oceans. The difference in tectonic style was related to the thermal regime and the cooling history of the Earth.

About the plate tectonics

Heat production in the Archean

Heat production in the Archean is about 3 - 5 times that of the present. (make a calculation by yourselves)

More vigorous convection; zone of partial melting deeper (in the mantle); tectonic plates thinner.

To efficiently dissipate the high heat flow in the Archean, the simplest way is to increase the rate of creation (and subduction) of the oceanic lithosphere.

Implication: faster sea-floor spreading, longer mid-ocean ridges, or both. ( A more recent concept involves a more vigorous plume activity, hence significant heat dissipation via formation of oceanic plateaux.)

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Radioactive heat

production through

time

Archean plate geometry

Present observation - Young lithosphere subducts slower than old lithosphere (Abott and Hoffman, 1984).

In the Archean, the lithosphere was likely to be warmer, hence it would subduct slower. Consequently, we expect that the length of mid-ocean ridge, but not the rate of spreading, had played the more important role in the dissipation of terrestrial heat.

According to Hargraves (1986),

heat loss from the ridges ∞ cubic root of ridge length,=> for a 2-3x higher of heat flow, length of ridge ≈ 8-27x.=> many more small plates moved around at a smaller rate in the Archean.

So, what are the crustal characteristics that we have observed in the Archean terrains?

II. Archean crustal provincesCrustal province = a segment of continental crust that has recorded a similar crustal evolution (magmatism, metamorphism, deformation, etc.)

Dimension ≈ 100 to 1000 km wide and >1000 km long. Ex: Canadian Shield, Kaapvaal province, Pilbara and Yilgarn Blocks, N. China Craton.

Archean crustal provinces (ACP) comprise rocks with ages from 4.0 to 2.5 Ga.Fig. 4.26 - Superior Province (showing also very complicated structural trends)

Most ACP’s are composed of the following terranes:(1) granite-greenstone, or (2) high-grade terranes, or (3) both.

(In all the granite- greenstone and high-grade terranes, granitoids of “TTG” composition and gneiss-magmatites dominate).

Kaapvaal (S. Africa)

Craton

III. Granite-greenstone

terranes

Pilbara (Australia)

Craton

(A) Greenstone belts (GB)

Principal occurrences: Superior, Slave, Zimbabwe, Kaapvaal, Pilbara et Yilgarn.

General characteristics: GB discontinuously engulfed by the “sea” of granites and gneisses.

GB = volcanisedimentary terranes with linear or irregular shapes (20-100 km wide and several km long), commonly metamorphosed to the greenschist or amphibolite facies, and occasionally, to a grade lower than the greenschist facies. Primay textures and structures (pillowed lavas, spinifex texture) are often observed.

Generalized GB stratigraphic sections

(Mafic-plain assemblages)

Chateristics of greenstone belts

In terms of the bulk composition, the Archean volcanic rocks are quite similar to that of modern volcanic rocks. However, ther are also some remarkable differences:

(1) importance of komatiites in the Archean GB.(2) bimodality of Archean volcanic rocks.(3) scarcity of alkaline or shoshonitic volcanic rocks in the Archean.

Most GB volcanic rocks show a geochemical signature of subduction zone (ex., negative anomalies in Nb, Ta, et Ti). This is probably in favor of the model of subduction and plate tectonics in the Archean.

Condie’s (2001) summary:According to Condie (2001), most greenstones appear to represent

“arc assemblages”, but two others may represent remnants of LIPs and thus reflect mantle plume sources. Both are mostly or exclusively submarine:

1. Mafic plain assemblages: pillow basalt, komatiite + small amounts of chemical sediments (chert, BIF); felsic volcanic rocks rare.

2. Platform assemblages: commonly overlying a granitc gneiss basement. Also comprisemainly basalt and komatiite, + BIF, carbonate, and minor felsic volcanic rocks.

Most Archean greenstones are terranes that contain several to many greenstone blocks amalgamated to make one greenstone “belt”.

From a total of 51 Archean greenstones, 65% have arc affinities and 35% oceanic plateau or MORB affinities.

Out of 96 post-Archean greenstones, 90% have arc affinities and only 10% oceanic plateau or MORB affinities.

On komatiiteskomatiite = ultrabasic lava, often shows “spinifex” texture.- rich in Mg, Ni, Cr; - poor in incompatible elements.

Major interest - A very important source of information for determination of the composition of the upper mantle.

(1982)

komatiite flow units

Spinifex texture 1

Spinifex textures

Figure captions

Textures and cooling rates

Classification of Archean volcanic rocks

Jensen’s (1976) cation diagram

Komatiite chemistry

Komatiite Tm

Geochemical interest of komatiites

Olivine(Mg, Fe)

+ trace elements

Opx(Mg, Fe)

+ trace elements

Cpx(Mg, Fe, Ca, t.e.)

Grt or Sp(Mg, Fe, Al, t.e.)

Ca & Al are necessary ingredients for basalt genesis

Komatiite = a large degree of mantle melting => chemical composition mimics that of UM

Comment on the concept of “incompatibility”

Phases in the upper mantle

REE typology of komatiites

PK (peridotitic komatiite): ol (Fo90-94) ± cpx ± glass ± accessories (no feldspars!); MgO = 18 - 28%.

BK (basaltic komatiite): ol (Fo88) + cpx + glass; Cpx spinifex common. (no feldspars!); MgO ≈ 10 - 18%.

Typology of HREE = basis for classification des komatiites; and some connotation of age of formation

Anti-correlation between (Gd/Yb)N & Al2O3/TiO2

Correlation between (Gd/Yb)N & CaO/Al2O3

Rôle of garnet (majorite) fractionation - where? when? how?LREE: Existence of LREE depletion et LREE enrichment.LREE depletion: Separation of CC?LREE enrichment + negative anomaly in Nb-Ta: Recycling of CC?

Original paper of the komatiite REE typology

REE Onverwacht komatiites

(Gd/Yb)N vs CaO/Al2O3 or

Al2O3/TiO2

Komatiite REE

typology

and age connotation

Implications1. Chemical composition => high degrees of mantle melting (up to

50%).2. Textures => rapid cooling.3. REE typology => role of garnet fractionation in source. Presence of 3

types suggests different depths of origin.4. Eruption temperature ≈ 1600ºC if magma is dry (but can komatiitic

magma be wet in a high degree melting?).5. Because grt is stabilized in the mantle relative to px, the solubility of grt

in silicate melt is reduced, resulting in magmas with low Al2O3 (Al-depleted komatiite). Because CaO varies less with residual mineralogy, the CaO/Al2O3 ratio is strongly P-dependent, and a Al2O3 vs CaO/Al2O3 plot can be used to estimate the depth of mantle melting (Fig. 7.11, Condie 2001).

6. Depth of magma segregation depends on the temperature of the source. Late and early Archean komatiites have source temperatures of 1800-1900ºC and 2000ºC, respectively. (Fig. 7.12, Condie 2001).

Fig. 7.11

Mantle solidus as a function of increasing pressure. Pressure given in GPa. Also shown is the distribution of komatiite and picrite as a function of age (Condie 2001 after Herzberg, 1995).

P-T diagram showing the mantle liquidus and solidus relative to melting depths of high-MgO magmas (after Herzberg, 1995).

(B) Granitoids

3 types of granitoides are found in association with GB (Fig. 9.1):

(1) gneiss complex: composition TTG(2) diapiric plutons : composition TTG(3) discordant plutons (late granite intrusions):

composition granite.

Question: The relative ages between the greenstone belts and the surrounding granitic gneisses?

III. High-grade terranes

Principal occurrences : SW Greenland, Baltica, Ukrane, Anabar, Aldan Shield, Indian and Sino-Korean cratons.

Principal lithology: granitic gneisses, migmatites, enclaves ("remnants")

Metamorphic grade: granulite facies (650-900°C; 6-10 kb); metamorphism commonly took place soon after (≤200 Ma) the formation of the gneiss-migmatite complexes. Enclaves = supracrustal rocks and rocks of layered intrusions, including amphibolites, ultrabasic rocks, metasédimentary rocks, etc.; They could represent older GB rocks "infolded" in gneiss and subject to high-grade métamorphism. Rocks of layered intrusions include anorthosite, gabbro, and a variety of ultrabasic rocks.

Structure: extremely complicated; ductile deformation; without general direction. The close association between the deformation (folding) and formation of granitic plutons and migmatites suggests that the deformation was accompanied by partial melting.

IV. Relation between the high-grade and low-grade (granite-GB) terranes?

Two categories of models:

(1) High-grade terranes = exhumed "root-zone" of the granite-GB terranes.

(2) The two types of terranes reflect different tectonic settings.

See Fig. 9.8