Chapter 6.

Age and Growth of Continental Crust from Radiogenic Isot

opes

pp. 321-348

What is the crust? Continental crust?What is the crust? Continental crust?

1. Total mass = 0.6% silicate earth

2. Main reservoir for K-U-Th (heat) and mineral resources

3. Primary archive of the earth history

study of the continental crust is critical to our understanding of the origin & differentiation of the Earth.

•When did continental crust form? How to get the age?

•How did it form? What mechanism of its formation?

•What is its geodynamics?

1.Age of the continental crust

2. Mechanism of continental growth 2. Mechanism of continental growth and formation and formation

3. Geodynamics for continental 3. Geodynamics for continental growth: supercontinent and growth: supercontinent and continental growth continental growth

Three major topics

1. Age of the continental crust

The age of the continental crust is the amount of The age of the continental crust is the amount of time the crustal rocks has been isolated from the time the crustal rocks has been isolated from the mantle sources (DePaolo et al., 1991).mantle sources (DePaolo et al., 1991).

40Ar/39Ar

Rb–Sr Dating

Sm–Nd Dating

U–Pb Dating

TIMS(Thermal Ionization Mass Spectrometry)—The Isotope Dilution MethodSHRIMP (Sensitive High Mass Resolution Ion MicroProbe)

1.1 How to determine age of crust------ Isotopic dating

0.7

0.8

0.9

1.0

0 20 40 60 80 100 120

87Rb/86Sr

87S

r/8

6S

r

Age = 154.5 ± 7.6 Ma

Initial 87Sr/86Sr =0.7086 ±0.0064

MSWD = 10.0

水泉沟石榴石花岗岩全岩-黑云母 Bi

AGES OF METAMORPHISM

40Ar/39ArRb–Sr

U–PbTIMs

SHRIMP

How to determine initial age of crust?

How to constrain on age of lower crust? Rock probe

40Ar/39Ar Rb–Sr Dating Sm–Nd Dating

U–Pb Dating

Metamorphic events

Igneous eventsInherited or xenocrystzircon

?Age of crust

Lower crust

MC

Upper crust 200 Ma

1000 Ma 300 Ma

Neodymium Model AGE143Nd is produced by α-decay of 147Sm. Sm/Nd ratio of the mantle> the crust a

nd hence143Nd/144Nd ratio >crust.

Sm and Nd are not mobile, so ages and initial ratios are relatively insensitive to weathering and metamorphism.

εNd is simply the relative deviation of the 143Nd/144Nd ratio from the chondritic ratio:

where all 143Nd/144Nd values are specified at the age of interest (t). Preset-day (146Nd/144Nd)CHUR=0.512638.

εNd-- variations in143Nd/144Nd relative to the primitive mantle (CHUR).

εNd<0----crustεNd>0-----mantle

)(144

147

)(144

147

)(144

143

)(144

143

1ln1

ODMOSA

ODMOSADM

NdSm

NdSm

NdNd

NdNd

T

=0.00654/Ga ，（ O ）代表現在值。下標 SA 和 DM 分別表示樣品和虧損地幔(DePaolo, 1988), (143Nd/144Nd)DM(O) = 0.51325, (147Sm/144Nd)DM(O) = 0.2168 。

地殼樣品從虧損地幔中分離後，无 Sm/Nd變化， TDM值爲殼幔分異年齡或地殼形成年齡。若有 Sm/Nd值變化，它代表在不同的化學體系中演化時間的加權平均年齡。

Nd同位素模式年齡 TDM

 TDM定義爲由樣品現在的 147Sm/144Nd和 143Nd/144Nd值反演到其 143Nd/144Nd值與虧損地幔源區（ DM）的該比值一致的時間 :

-6

-4

-2

0

2

4

6

8

10

0 200 400 600 800 1000 1200

Age, Ma

E N

d

DM

CHUR

T C = TDM

Continental crust

TC

T CF

200 Ma1000 Ma

εNd = -40 Ma

εNd = -2200 Ma

εNd=DM1000 Ma

Model age can not be used if parental felsic melts Model age can not be used if parental felsic melts was generated by partial melting of mixed sources was generated by partial melting of mixed sources

of various ages and compositionsof various ages and compositions..

-6

-4

-2

0

2

4

6

8

10

0 200 400 600 800 1000 1200

Age, Ma

E N

d

DM

CHUR

Continental crust

T CF

Mixing ofvarious sources ?

V.I. Kovalenko et al. (2004)

Condie 1998

Tectonic agesTectonic ages

Patchett (2005)

Patchett (2005)

1.2 Growth events of continental crust

大陆的幕式生

长

时间

大陆生长

量

Condie 1998

K.C. CondieTectonophysics 322 (2000)

大陆生长模

式

大陆

体

积

时间

Models for continental growth 大陆模式生长

1.3 Phanerozoic continental growth1.3 Phanerozoic continental growth

It is widely agreed that the production of the contineIt is widely agreed that the production of the continental crust was essentially completed in the Precambntal crust was essentially completed in the Precambrian, and was minor in the Phanerozoic. rian, and was minor in the Phanerozoic.

However, recent isotope investigations in the westerHowever, recent isotope investigations in the western North America (Sierra Nevada, Peninsular Range, an North America (Sierra Nevada, Peninsular Range, and Canadian Cordillera) and eastern Australia (Lachlnd Canadian Cordillera) and eastern Australia (Lachlan and New England Foldbelts) have revealed that a an and New England Foldbelts) have revealed that a substantial proportion of the Phanerozoic crust is jusubstantial proportion of the Phanerozoic crust is juvenile.venile.

IGCP420 Phanerozoic continental growth: evidenIGCP420 Phanerozoic continental growth: evidence from central Asia (1998-2003, Bor-ming Jahn) ce from central Asia (1998-2003, Bor-ming Jahn) Two issues: Two issues:

Central Asian Orogenic Belt is the largest and typiCentral Asian Orogenic Belt is the largest and typical Phanerozoic juvenile crustcal Phanerozoic juvenile crust

Jahn (2004)

1.3 Phanerozoic continental growth1.3 Phanerozoic continental growth

NCCCOBC

Qinling belt

Dabei belt

CAOB

SCB(diamond)

Pacific

- 30

- 20

- 10

0

10

0. 7 0. 705 0. 71 0. 715 0. 72

EMI

DM CAOB

Pre-Mesozoic

εNd(t)

(87Sr / 86 Sr)i

NCB andDabei Blet

Dabei belt

Intermediate-acid rocks

Production of the continental cruProduction of the continental crust not only mainly occurred in the st not only mainly occurred in the Precambrian, but also in the PhanPrecambrian, but also in the Phanerozoic. erozoic.

Central Asian Orogenic Belt is the Central Asian Orogenic Belt is the largest Phanerozoic juvenile crustlargest Phanerozoic juvenile crust

2 How did continental crust form? 2 How did continental crust form?

Formation mechanism and Formation mechanism and processesprocesses

Juvenile continental crust is produced at two tectonic settings and ways:

Subduction upper crust

mantle plumes lower crust

Two major mechanism:Two major mechanism:

Horizontal growthHorizontal growth Vertical growth

Mantel oceanic crust continental crust

2.1 Horizontal growth2.1 Horizontal growth

Melting in subduction zoneMelting in subduction zone

Mantel oceanic crust continental crust Chemical process

Barr et al., 1999, Lithos

Physical process

Chemical process

Horizontal growthHorizontal growth

Arc magmatism and lateral accretion of arcs

2.2 Vertical growth from below2.2 Vertical growth from below

The plumes give rise to juvenile crust:by partial melting as they arrive at the base of the lithosphere,or by heating the upper mantle. Oceanic ridge subduction

Identify new juvenile compositions from beIdentify new juvenile compositions from belowlow

Probable sources for post-orogenic granitesProbable sources for post-orogenic granites

(a) Recycle from young crustal (including ocean cru(a) Recycle from young crustal (including ocean crust) by horizontal accretion----- horizontal growthst) by horizontal accretion----- horizontal growth

(b) New underplating mantle-derived magma---- po(b) New underplating mantle-derived magma---- post-orogenic vertical growthst-orogenic vertical growth

(c) pre-orogenic underplating mantle-derived magm(c) pre-orogenic underplating mantle-derived magm

a----- pre-orogenic vertical growtha----- pre-orogenic vertical growth

How to iHow to identify new juvenile compositions from belowdentify new juvenile compositions from below

εNd(t)>0

?

?

Jahn (2004)

Vertical evidence from Eastern Tianshan, NW ChinaVertical evidence from Eastern Tianshan, NW China

Vertical growthVertical growth

Vertical growthVertical growth

Vertical evidence from Eastern Tianshan, NW ChinaVertical evidence from Eastern Tianshan, NW China

What is the geodynamics for contiWhat is the geodynamics for continental growth? nental growth?

Why did continental grow?Why did continental grow?

Continental crustal growth Supercontinental cycle and are two of the most important subjects in the earth sciences. These two problems have long been studied separately.

Crustal growth: 3.6–3.5, 2.7–2.6, 2.0–1.8, 1.2–1.0, and 0.5–0.3 Ga.The supercontinental accumulation: 2.6, 1.8, 1.0, 0.65, and 0.25 Ga.

Continental crustal growth

Supercontinental cycle

Mantle convection

The Hercynian collision of Gondwanaland and Laurasia marked the initial coalescence of Pangea in the mid-Carboniferous (320 Ma), which finally became a uniformly emergent supercontinent in the Triassic. Starting in the mid-Jurassic (160 Ma), Pangea broke up intofragments (Veevers, 1994).

Pangea 450-250 Ma

Laurasia

Gondwanaland

CAOB

Condie 1998

Summary Summary 1. Age of the continental crust could be determined by N

d model age, zircon U-Pb dating. Formation of continental crust mainly took place before Precambrian (2.7, 1.9, 1,2Ga), but also during Phanerozoic (o.45-0.25Ga).

2. Mechanisms of continental growth are mainly horizont2. Mechanisms of continental growth are mainly horizontal (subduction) and vertical (underplating, plum) procal (subduction) and vertical (underplating, plum) processes. esses.

3. Continental growth may be related to supercontinental 3. Continental growth may be related to supercontinental

recycles (assemblage)recycles (assemblage)..

三、主要成果（在新疆）在新疆） 4 提出中亚造山带成矿物质来源与地幔源有关，与华南成矿背景不同，

显示了重要而特殊的成矿域，值得进一步研究，对找矿勘探有实际意义。