大陸地殼演化
Genesis and evolution of the
continental crust
I. Introduction - Principal topics to be covered:
1.
2.
3.
4.
Characteristics, ages and compositions of the
continental crust (CC).
Mechanism of the formation of CC - in modern
times and in the Archean.
Recycling of CC - evidence and processes.
Periods and manner of continental growth continuous or episodic?
The earth’s continental crust in
unique in the solar system
Earth &
Moon
What’s the major
difference?
No water,
no granites;
No granites, no
continents.
Formation process (1)
Arc magmatism and lateral accretion of arcs
Formation process (2) vertical, grow from below
II. Characteristics, chemical compositions, age and
mechanism of formation of the continental crust
(1) Principal characteristics of the continental crust. Topography, tectoic
subdivision, internal structure, rheological character and subductibility.
Variation of physical properties (Vp and heat flows). Magmatism and
crustal accretion in destructive margins.
(2) Composition of the continental crust. Estimation of the composition of
UCC and LCC - geochemical models.
(3) Petrology, geochemistry and structure of the Archaean terranes. (a)
“granite-greenstone terranes” (ex., Abitibi, Finland, Pilbara, Barberton,
India); (b) “high grade gneiss-granulite terranes” (ex., Greenland, Australia,
China, India).
(4) Age and history of the continental crust. Ages of the oldest rocks and
minerals. Contribution from isotope studies (Sm-Nd, Lu-Hf) of sedimentary
rocks. Mean age of CC. Rate of growth and chemical evolution of CC.
(5) Genesis of the continental crust. petrological, experimental and
geochemical data about the formation of granitoids. Petrological and
geochemical models for the formation of the continental crust.
III. Crustal growth in a global context
(1) Differentiation of the primitive earth. Segregation of the core, the
atmosphere and the primitive crust; evolution of the depleted mantle and its
corresponding enriched reservoir(s).
(2) Formation and recycling of the continental crust. Generation of CC in
island arcs and active margins (lateral process); generation of CC by melting
of underplated mafic rocks in intra-plate settings (vertical process); relations
between magmatic, tectonic, metamorphic and sedimentary processes;
Recycling of CC. Arguments for and against sediment subduction. Other
possible mechanisms for crustal recycling (ex. delamination of LCC).
(3) Crustal formation in the Archean, proterozoic and Phanerozoic times.
Plate tectonics in the Archean? Origin of Archean granitoids: melting of
subducted mafic crust or melting of thickened crust? Crustal growth in the
Proteozoic and Phanerozoic; ex., Central Asia.
(4) Growth rate of the continental crust. Comparison between the
Armstrong model and the others. Arguments in favor of episodic growth.
References
Armstrong, R.L., 1968. A model for Sr and Pb isotope evolution in a dynamic Earth. Rev.
Geophysics, 6: 175-199.
Armstrong, R.L., 1991. The persistent myth of crustal growth. Aust. J. Earth Sci. 38: 613630.
Condie, K.C., 1989. Plate tectonics and crustal evolution. Pergamon, New York, 476 pp.
Martin, H., 1994. The Archean grey gneisses and the genesis of continental crust. In:
Archean crustal evolution (K.C. Condie, ed.), Elsevier, Amsterdam, 205-259.
Reimer, A., Schubert, G., 1984. Phanerozoic addition rates to the continental crust and
crustal growth. Tectonics, 3: 63-77.
Rudnick, R.L., 1995. Making continental crust. Nature, 378: 571-578.
Rudnick, R.L., Gountain, D.M., 1995. Nature and composition of the continental crust: a
lower crustal perspective. Rev. Geophysics, 33: 267-309.
Rudnick, R.L. 2004 (ed.) The crust. in: Treatise on Geochemistry, Elsevier,
Amsterdam, 683 pp.
Samson, S.D., Patchett, P. J., 1991. The Canadian Cordillera as a modern analogue of
Proterozoic crustal growth. Aust. J. Earth Sci., 38: 595-611.
Stein, M., Hofmann, A.W., 1991. Mantle plumes and episodic crustal growth. Nature, 372:
63-68.
Taylor, S.R., McLennan, S.M., 1985. The continental crust: its composition and evolution.
Blackwell, Oxford, 312 pp.
Taylor, S.R., McLennan, S.M., 1995. The geochemical evolution of the continental crust.
Rev. Geophysics, 33: 241-265.
Chapter 1. Physical and chemical
characteristics of the continental crust
I. Introduction
Continental crust (CC): Most accessible part; most extensively studied; but
also the most complicated among all geological units.
≈ 30% of the earth’s surface; 35-40 km thick (70 km in the Andes; 90
km in the Himalayas; <30 km in the Kenyan Rift).
Ages variable (4 Ga to 0 Ga).
Oceanic crust (OC): characterized by rather flat ocean basins and ridge
systems (≈ 2km average elevation).
≈ 70% of the surface; 5-10 km thick; Ages ≤ 200 Ma (most ≤ 100 Ma).
2 types of continental margin:
(1) Active (Pacific): presence of large trenches of 80 to 100 km.
= plate boundary.
(2) Passive (Atlantic): comprising continental plateau (50-200 km),
continental slope (45 km wide; 200 to 4000m deep).
Not the plate boundary.
Ages of the ocean basins
Major
geological
events
II. Major features of the continental crust
3 types de CC based on their surface features:
(1) Precambrian shields: crystalline rocks (magmatic and
metamorphic).
(2) Platforms: metasedimentary covers, gently folded, rest upon
Precambrian basement rocks. The shields and platforms
commonly extend right to passive margins (e.g., Atlantic).
(3) Orogenic belts: highly deformed and metamorphosed old rocks
associated with young syn-orogenic magmatic rocks.
- by subduction process (Circum-Pacific): Andes, island arcs.
- by continental collision (Tethys): Alps - Himalayas.
Tectonic provinces
Tectonic ages
Folded metasedimentary
rocks, Sequoia
Nat’l Park, CA
III. Vertical structure
Fig. 4 (Fig. 9.4 Brown and Mussett): Vp profiles of different crustal sections.
Fig. 5 (Fig. 4-31, Best): Idealized but more realistic structure of the CC.
The uppermost layer (Vp = 4.5 - 5.9 km/sec): Lithology and composition
highly variable. Unmetamorphosed or lowly metamorphosed
volcanic and sedimentary rocks (≤ greenschist facies).
Upper continental crust (UCC) (Vp = 5.9 - 6.5 km/sec; mean = 6,25):
≈ granodiorite.
Lower continental crust (LCC) (Vp = 5.9 - 7.7 km/sec): Chapter 2.
≈ granulites of intermediate compositions, metapelites and basic
granulites (the lowermost part).
Rock
cycle
IV. Crustal accretion in destructive margins
3 types de destructive margins:
(1) Island arc : ocean/ocean; andesite volcanism predominant; minor intrusives.
(2) Andean: ocean/continent; characterized by acid and andesitic volcanic rocks and
linear granitoid batholiths and clastic sediments more or less deformed. No folded
mountains. Presence of large-scale extensional zones, parallel to plate margins.
Crustal thickening in the OC/OC and OC/CC destructive margins is mainly produced by
vertical addition of juvenile components, and not by lateral compression and shortening.
Fig. 6 (Fig. 9.6 Brown and Mussett): Cross-section of the central Andes.
(3) Alpine-Himalayan: continent/continent. Less magmatism, but more deformation,
folding, faulting, thrusting, and shortening; uplift and exhumation of deep-seated rocks.
Possible progressive evolution from (2) to (3).
See Fig. 9.7 Brown and Mussett: Sequence of events from subduction to
collision.
Important point : the sites of continental accretion in active margins are always
associated with retreating oceans, such as the Pacific. The magmatic activities cease
when no more oceanic crust left for subduction - the stage of continental collision.
Ex., Australia will collide with Asia in ≤100 Ma, and the Indonesian Arc will be an
important component of the suture zone of collision.
Some well-known sutures: Urals, Caledonides, Appalachian chain, Hercynides, QinlingDabie.
OC/OC
OC/CC
CC/CC
Melting in subduction zone
V. Comparative magmatism in destructive margins
(1) Young arcs (South Sandwich, Mariana, Tonga): subduction oc/oc.
Dominated by basalts and basaltic andesites; (Extrusive/Intrusive) ratio high.
(2) Somewhat older arcs (Japan, Indonesia, New Zealand, West Indies, Central
America):
Dominated by andesites, diorites and granodiorites; (Ex/In) ratio moderate.
(3) Mature arcs (Andes, Rocky Mountains): subduction oc/cc
Dominated by intrusive rocks; (Ex/In) ratio low. Compositions of the intrusive
rocks include gabbro, diorite, granodiorite à adamellite.
(4) Collision zones (Alps, Himalayas): In general, little associated magmatism;
however, in the hercynian chain, syntectonic granitoids are abundant.
Dominated by granitic magmas (leuco-granites and rhyolites), formed by
partial melting of the CC.
Fig. 8 (Fig. 9.8 Brown and Mussett): K2O variation across the Japan arc.
Presence of a corrélation between K2O-depth (K-h).
Fig. 9 (Fig. 3.6 Tatsumi and Eggan): Geochemical distinction of arcs.
Question on the magmagenesis in subduction zones:
- melting of the subducted lithosphere?
- Melting of the mantle wedge?
- rôle and provenance of the fluids?