Transcript Chapter 3. Archean Crustal Provinces

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.
Introduction
Precambrian (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.
2.
3.
4.
5.
6.
7.
8.
Cratering history of the early Earth.
Functioning of the plate tectonics in the Precambrian, especially in
the Archean.
Nature of the primitive crust; and the age of the earliest CC.
Evolution of the atmosphere and origin of water.
Prevalent mechanism(s) for the formation of greenstone belts.
Relation between greenstone belts and surrounding granitic gneiss
complexes.
Evolution of sedimentary patterns? can sequence stratigraphy be
applied to Archean sequences?
Particular features about structural patterns and deformation
processes in the Archean?
A new
book
The very early history of the earth -
the Hadaen period
About the plate tectonics
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.
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.)
14
Heat production
Heat production per unit mass
12
Archean
40K
238U
235U
232Th
Tot al
10
8
Radioactive
heat
production
through
time
4.8
3.5
2.3
6
1.9
1.6
4
1.2
1
2
0
-1
0
1
2
Age (Ga)
3
4
5
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 midocean 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 provinces
Crustal 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).
III. Granitegreenstone
terranes
Pilbara
(Australia)
Craton
Kaapvaal
(S. Africa)
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 komatiites
komatiite = 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
Komatiite = a large
Opx
Olivine
(Mg, Fe)
+ trace elements
(Mg, Fe)
+ trace elements
Cpx
(Mg, Fe, Ca, t.e.)
Grt or Sp
(Mg, Fe, Al, t.e.)
degree of mantle melting
=> chemical composition
mimics that of UM
Comment on the
concept of
“incompatibility”
Ca & Al are
necessary
ingredients for
basalt genesis
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
Implications
1.
2.
3.
4.
5.
6.
Chemical composition => high degrees of mantle melting (up to
50%).
Textures => rapid cooling.
REE typology => role of garnet fractionation in source. Presence of 3
types suggests different depths of origin.
Eruption temperature ≈ 1600ºC if magma is dry (but can komatiitic
magma be wet in a high degree melting?).
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 (Aldepleted 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).
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 gneissmigmatite 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 highgrade 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