大陸地殼演化 Genesis and evolution of the

continental crust

I. Introduction - Principal topics to be covered:

1. Characteristics, ages and compositions of the continental crust (CC).

2. Mechanism of the formation of CC - in modern times and in the Archean.

3. Recycling of CC - evidence and processes. 4. Periods and manner of continental growth -

continuous or episodic?

The earth’s continental crust in unique in the solar system

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.

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

(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.

II. Characteristics, chemical compositions, age and mechanism of 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.

ReferencesArmstrong, 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: 613-

630.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 meta-sedimentary

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, Qinling-Dabie.

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/ccDominated 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?