Chapter 2. Composition of
the continental crust
I. Introduction – major interest?
Interest?
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.
II. Upper continental crust (UCC)
Most accessible; but also most complicated and differentiated. About 30% of
the continental area is submerged beneath the oceans.
(a) Precambrian shields and platforms (cratons) - structure well-known, with
Z = 35 - 45 km; Vp = 5.8 - 6.4 km/sec (UCC), 6.5 - 7.2 km/sec (LCC)
(b) Conrad discontinuity - present or absent
(c) Orogenic belts - crustal structures very complicated. In some areas, the
Moho is transitional, rather than discontinuous.
A.
Methods for determining the composition of the UCC
(a) Using geological maps to obtain weighted averages (Clarke, 1889;
Clarke and Washington, 1924).
(b) Analysis of composite samples of large surface areas (Shaw et al.,
1967).
(c) Geochemical approach - analysis of fine-grained sediments (shales or
loess) and determine the composition of insoluble elements.
Estimation of other elemental abundances from a variety of
geochemical principles (Goldschmidt, 1933; Taylor and McLennan,
1985; Rudnick and Fountain, 1995).
Using geological maps to obtain weighted averages
125
135
N
145
Japan
North
American
Plate
Eurasian
Plate
Hidaka Be lt
35
Pacific
Quaternary sedimentary rocks
Plate
Neogene sedimentary rocks
Philippine
Sea Plate
Kita kam i
Mou ntai ns
25
Pre-Neogene sedimentary rokcs
Paleozoic-Jurassic-Cretaceous-Paleogene
accretionary complex rocks
Quaternary volcanic rocks
Quaternary volcano
Neogene volcanic rocks
(partly including Oligocene)
Tam ba Be lt
Neogene plutonic rocks
Ryo ke Belt
Samb agawa Belt
0
300km
Pre-Neogene volcanic rocks
(felsic dominant)
Pre-Neogene plutonic rocks
(felsic dominant)
Metamorphic rocks
Togashi et al., 2000
Major element composition - UCC
Rudnick and Gao (2004)
Taylor-McLennan (1985) approach to determine the UCC composition
Assumption - taking fine-grained sediments to represent the average UCC.
Method - analysis of a group of elements which are resistant to fractionation
by the processes of sedimentary differentiation (weathering, erosion,
transport and diagenesis). From these data, they further applied the
geochemical principles to determine the concentrations of other
elements, hence established the bulk composition of the UCC.
(1) Factors to consider: effects of alteration, erosion, solubility of
minerals, residence time (), transport physics, deposition, diagenesis
and metamorphism.
(a) Rate of physical erosion ≈ 4 x rate of chemical erosion (p. 18; Table 2.4)
Variation in rate of erosion: chemical ≈ 3 to 4; physical ≈ 25 (strongly
related to tectonic activity)
(b) Effects of sedimentary differentiation (mainly by granulometric separation)
(c) Effects of diagenesis:
Erosion and transport - generally in oxidizing conditions.
Diagenesis - in less oxidizing or reducing conditions, it may change the
geochemcial behavior (ex., solubility) of some elements.
Mass influx to the ocean
Effect of sedimentary differentiation
Al2O3
FeO
MgO
K2O
Conc.
CaO
SiO2
Grain-size decreases
Refractory element ratios (La/Yb; Eu/Eu*, La/Th, La/Sc)
unchanged (Table 2.5).
Effect of heavy minerals on REE: zircon is very important!
(Zr) zircon = 50%
(Zr) shales = 200 ± 100 ppm
(Zr) UCC ≈ 190 ppm.
Non-fractionated elements
(2) Elements not fractionated by sedimentary processes are used
to determine the UCC composition:
What are they, non-fractionated elements? In general, elements having
small KSW (<10-6) and short  (< 103 yrs):
Ti group (Ti, Zr, Hf) - good, but they are controlled by heavy minerals.
Al group (Al, Ga) - OK, but they do not vary much in igneous rocks.
Fe, Mn - maybe good, but they change solubilities with oxygen fugacity.
REE (+ Y, Sc), Th, Nb - the best
(3) REE (shale) ≈ 20% higher than REE (UCC) (p. 32) Why?
3 observations:
(a) [La]sh ≈ 20% higher than [La] of "shield composite".
(b) REE (shale) ≈ 10-30% higher than REE (greywacke or coarse
sediments)
(c) Clastic sediments = 70-80% total sediments (other 20-30% =
carbonates + evaporites) but represent about 100% of REE budget.
Residence time
vs. partition
coefficient (Ksw)
Ti group (Ti, Zr, Hf) - good, but they
are controlled by heavy minerals.
Al group (Al, Ga) - OK, but they do
not vary much in igneous rocks.
Fe, Mn - maybe good, but they
change solubilities with oxygen
fugacity.
REE (+ Y, Sc), Th, Nb - the best
C. Determination of the UCC composition
Starting from REE and Th
1. REE: fine-grained clastic sediments x 0.8
2. Th: from sediments La/Th = 2.8 => La = 30 ppm
3. From the established geochemical “rules”,
Th/U = 3,8
=> U
K/U = 10000
=> K
K/Rb = 250
=> Rb
Rb/Cs = 30
=> Cs
Nb/Th = 2.3
=> Nb
Nb/Ta = 11
=> Ta
Zr/Nb = 7.6
=> Zr
Zr/Hf = 33
=> Hf , .........etc.
REE patterns
of shales and
loess
Characteristics:
patterns extremely
uniform, almost
regardless of their
provenances
D. Conclusion
Bulk composition of UCC ≈ granodiorite
Thickness ≈ 10 km (≈ 25% of CC; constrained by
heat flow data).
UCC trace
element
abundances
Rudnick & Gao (2004)
Comparison of UCC compositions
III. Lower continental crust (LCC)
References: Rudnick and Gao, 2004; Rudnick and Fountain, 1995, Rev. Geophys., 33:
267-309; Taylor and McLennan, chapter 4, 73-95; Ringwood, 1975)
Definition:
Taylor-McLennan (1985): lower crust (LC) = below 10 km.
Rudnick-Fountain (1995): middle crust (MC) = 10-25 km.
lower crust (LC) = below 25 km.
Badly known in general. LC is probably composed of granulite facies rocks
and MC amphibolite facies rocks. Granulites and amphibolites often occur in
Precambrian terranes or as xenoliths in alkaline basalts. At present, the
composition of LC is still the principal source of uncertainty in the estimation of
the bulk composition of the continental crust for the following reasons:
(1) large difference in composition between granulites of Precambrian
terranes (mainly felsic rocks) and xenoliths (mainly mafic rocks).
(2) LC is highly heterogeneoous in lithology, as witnessed in granulite
terranes.
(3) difficult to determine the types of rocks from seismic data (Vp-Vs).
Vp data of crustal sections
A.
Seismic velocities in the crust
Vp and Vs are related to some physical parameters (T,
P, porosity, fluid, etc.) and intrinsic properties of rocks
(mineralogy, chemical composition, metamorphic
grade, oreintation of grains, etc.)
Fig. 2: Variation de Vp pour 8 sections types de la CC (Rudnick et Fountain,
1995).
1. Precambrian shields and platforms
2. Paleozoic orogenic belts (Hercynian, Caledonian, Appalachian)
3. Mesozoic-Cenozoic extensional regions (Basin and Range)
4. Continental arcs (Cascades; Honshu, Japan)
5. Forearcs (Coast of British Columbia; S. California; NE Japan)
6. Rifted margins (W. Norway; Bay of Biscay; eastern USA; W. Greenland)
7. Mesozoic-Cenozoic orogenic belts (Alps, Himalayas, Canadian Cordillera)
8. Active rifts (Rheingraben, Kenyan rift, Rio Grande rift, Dead Sea)
B. Granulite Vp’s
Granulites - Considered as the dominant rock-type in LCC. They occur in old
granulite terrains or as xenoliths in alkaline basaltic magmas.
Fig. 3: Contrast in composition between two types of granulites.
Isobaric cooling = products of "magmatic underplating"
Isothermal uplifting = supracrustal sequences (or “greenstone-granite
assemblages”).
Xenoliths = mafic lithology predominant.
Interpretation: Xenolithes of mafic granulites probably represent underplated
basalt magma crystallized directly into the granulite facies.
Critique: LCC xenoliths occur uniquely in extensional regions where alkaline
basalts are erupted. Therefore, the lithology of the xenoliths may not be
representative of the bulk of LCC.
Xenoliths
Isobaric cooling = products of
"magmatic underplating”
Isothermal uplifting = supracrustal
sequences
Isobaric
cooling
Isothermal
uplifting or
exhumation
Isobaric cooling =
products of "magmatic
underplating”
Isothermal uplifting =
supracrustal sequences
Physical sense
of P-T paths
IV. Rudnick-Fountain method (1995)
(1) Assign the types of rocks that match observed Vp data.
(2) Use the geochemical data of granulites compiled by Rudnick and Presper
(1990) to obtain the average composition of each layer and of each crustal
section of Fig. 2.
(3) Estimate the surface area of each crustal section (Fig. 2).
(4) Calculate individual volumes using the crustal thickness of each section.
(5) Calculate the mean composition of LCC, MCC and the bulk CC.
Fig. 5 - Vp vs density diagrams
Fig. 7 - Vp vs lithology diagram
Mean Vp of the LCC in all sections = 6.9 - 7.2 km/sec.
=> corresponding to basic granulite, amphibolite, anothosite, and highgrade metapelites.
Very rare rock types: basic eclogites and ultrabasic rocks.
Vp - felsic
granulites
Vp - mafic
granulites &
metapelites
Vp - eclogites & ultramafic rocks
Vp - density
correlation
Chosen models for LCC
(1) High Vp layer (≥ 6.9 km/sec):
6.9 km/sec: 40% basic granulite, 25% interm. granulite, 25% acid granulite, et 10% metapelite.
7.2 km/sec: 90% basic granulite, 10% metapelite.
(2) Intermediate Vp layer (6.5 - 6.9 km/sec):
45% interm. gneiss, 45% mixture of amphibolite + felsic gneiss, 10% metapelite.
(3) Low Vp layer (6.2 - 6.5 km/sec):
50% interm. granulite, 50% felsic granulite.
Result: Tables 9 & 10: Composition of LCC
Conclusions:
(1) Composition of LCC = basic, ≈ primitive basalt (Fig. 11)
(2) Composition of MCC = intermediate (Fig. 11).
(3) Composition of the bulk CC = intermediate.
(4) Granulite terrains = represent MCC (P = 6 - 8 kb)
(5) Xenoliths = root part of the CC (P = 10 - 15 kb)
Fig. 12: Trace element patterns of the model CC.
Fig. 13: Proportion of lithophile elements in the CC.
LCC composition & REE patterns
Middle CC
- REE
patterns
V. Model for the CC bulk composition
The bulk composition of the CC depends on the choice of LCC models and
mechanism for the generation of CC.
Approches:
(1) estimate the composition of LCC and calculate the bulk composition of CC.
(= Approch of Rudnick et Fountain)
(2) choose a crustal growth model and identify its products.
(= Approch of Taylor et McLennan, 1985, Chap. 3, 57-72).
a) lateral accretion of island arcs
b) crustal "underplating" (tonalitic or dioritic intrusion)
= a variant of the "andésite” model.
Andesite Model: Continental growth via addition of andesites and associated
rocks in orogenic zones. This is followed by intracrustal melting, leading to the
formation of UCC and LCC. (Model valid for post-Archean times)
Model of Archean bimodel magmatism:
Component 1: Basic = basalts, komatiites
Component 2: Felsic = TTG., acid volcanic rocks
Andesite ≈ 2:1 mélange of Comp 1 + Comp 2
Bulk composition of the total crust
Constraints:
1. Must satisfy the heat flow data.
2. Must have a composition capable of generation of granodiorites by
partial melting.
3. Contribution of island arc volcanisms (post-Archean): limited to 25%.
4. Principal period of crustal growth (75%) in the Archean.
Total crust = 75% Archean crust + 25% "andesite"
SiO2 = 57.3%
Cr = 185 ppm,
Ni = 105 ppm
K = 0.91%,
U = 0.91 ppm
Th = 3.5 ppm
Sm/Nd = 0.22
Rb/Sr = 0.12
(La/Yb)N = 4.9
Eu/Eu* = 1.0
K/U = 10000
Th/U = 3.8
Total crust REE &
spidergrams
LCC-MCCUCC
compositions
Comparison
of models