Chapter 6.
Age and Growth of
Continental Crust from
Radiogenic Isotopes
pp. 321-348
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?
Three major topics
1.Age of the continental crust
2. Mechanism of continental growth
and formation
3. Geodynamics for continental
growth: supercontinent and
continental growth
1. Age of the continental crust
The age of the continental crust is the amount of
time the crustal rocks has been isolated from the
mantle sources (DePaolo et al., 1991).
1.1 How to determine age of crust
------ Isotopic dating

40Ar/39Ar

Rb–Sr Dating

Sm–Nd Dating

U–Pb Dating
TIMS(Thermal Ionization Mass Spectrometry)—The Isotope Dilution Method
SHRIMP (Sensitive High Mass Resolution Ion MicroProbe)
Rb–Sr
40Ar/39Ar
1.0
水泉沟石榴石花岗岩
全岩-黑云母
Bi
87
Sr/86Sr
0.9
Age = 154.5 ± 7.6 Ma
0.8
Initial
87
86
Sr/ Sr =0.7086 ±
0.0064
MSWD = 10.0
0.7
0
20
40
60
87
80
100
120
86
Rb/ Sr
SHRIMP
U–Pb
TIMs
AGES OF METAMORPHISM
40Ar/39Ar
 Rb–Sr Dating
 Sm–Nd Dating


U–Pb Dating
How to determine
initial age of crust?
How to constrain on
age of lower crust?
Rock probe
Metamorphic events
?Age of crust
Igneous events
Inherited or xenocryst
zircon
200 Ma
Upper crust
MC
Lower crust
1000 Ma
300 Ma
Neodymium Model AGE
143Nd is produced by  -decay of 147Sm. Sm/Nd ratio of the mantle> the crust and
hence143Nd/144Nd ratio >crust.
Sm and Nd are not mobile, so ages and initial ratios are
relatively insensitive to weathering and metamorphism.
Nd 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.
Nd-- variations in
143Nd/144Nd relative to the
primitive mantle (CHUR).
Nd<0----crust
Nd>0-----mantle
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:
T DM

1 
 ln  1 
 



143
147
Nd
144

Nd

Nd
SA ( O )
Sm
144
SA ( O )



143
147
Nd
144

Nd

Nd
DM ( O )
Sm
144
DM ( O )





= 0 . 0 0 6 5 4 / 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。
10
DM
8
T CF
6
Continental crust
E Nd
4
2
CHUR
Nd=DM
1000 Ma
0
-2
-4
T C = TDM
TC
-6
0
200
400
600
800
1000
Age, Ma
Nd = -4
0 Ma
Nd = -2
200 Ma
200 Ma
1000 Ma
1200
10
DM
8
T CF
6
Continental crust
E Nd
4
2
CHUR
0
-2
Mixing of
various sources
-4
?
-6
0
200
400
600
800
1000
1200
Age, Ma
Model age can not be used if parental felsic melts
was generated by partial melting of mixed sources
of various ages and compositions.
V.I. Kovalenko et al. (2004)
Condie 1998
Tectonic ages
Patchett (2005)
Patchett (2005)
1.2 Growth events of continental crust
大
陆
生
长
量
大
陆
的
幕
式
生
长
时间
Condie 1998
K.C. Condie
Tectonophysics
322 (2000)
Models for continental growth 大陆模式生长
大
陆
体
积
时间
大
陆
生
长
模
式
1.3 Phanerozoic continental growth
It is widely agreed that the production of the
continental crust was essentially completed in the
Precambrian, and was minor in the Phanerozoic.
However, recent isotope investigations in the western
North America (Sierra Nevada, Peninsular Range, and
Canadian Cordillera) and eastern Australia (Lachlan
and New England Foldbelts) have revealed that a
substantial proportion of the Phanerozoic crust is
juvenile.
IGCP420 Phanerozoic continental growth:
evidence from central Asia (1998-2003, Bor-ming
Jahn) Two issues:
Central Asian Orogenic Belt is the largest and
typical Phanerozoic juvenile crust
1.3 Phanerozoic continental growth
Jahn (2004)
CAOB
NCC
COBC
Pacific
Qinling belt
(diamond)
SCB
Dabei belt
εNd(t)
Intermediate-acid rocks
NCB and
Dabei Blet
Dabei
belt
(87Sr / 86 Sr)i
Production
of the continental
crust not only mainly occurred in
the Precambrian, but also in the
Phanerozoic.
Central
Asian Orogenic Belt is the
largest Phanerozoic juvenile crust
2 How did continental crust form?
Formation mechanism and
processes
Juvenile continental crust is produced
at two tectonic settings and ways:
Subduction
mantle plumes
upper crust
lower crust
Two major mechanism:
Horizontal growth
Vertical growth
2.1 Horizontal growth
Mantel
oceanic crust
continental crust
Mantel
oceanic crust
Chemical process
continental crust
Melting in subduction zone
Horizontal growth
Physical process
Chemical process
Barr et al., 1999, Lithos
Arc magmatism and lateral accretion of arcs
2.2 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
below

Probable sources for post-orogenic granites
(a) Recycle from young crustal (including ocean
crust) by horizontal accretion----- horizontal
growth
(b) New underplating mantle-derived magma---post-orogenic vertical growth
(c) pre-orogenic underplating mantle-derived magma---- pre-orogenic vertical growth
How to identify new juvenile compositions from below
εNd(t)>0
?
?
Jahn (2004)
Vertical evidence from Eastern Tianshan, NW China
Vertical growth
Vertical evidence from Eastern Tianshan, NW China
Vertical growth
What is the geodynamics for
continental growth?
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
Laurasia
Pangea
CAOB
450-250 Ma
Gondwanaland
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).
Condie 1998
Summary
1. Age of the continental crust could be determined by
Nd 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
horizontal (subduction) and vertical (underplating,
plum) processes.
3. Continental growth may be related to supercontinental
recycles (assemblage).
三、主要成果（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。