Transcript Chapter 11 Granitic Perspectives on the Generation and Secular Evolution of the Continental Crust.

Chapter 11
Granitic Perspectives
on the Generation and Secular Evolution
of the Continental Crust
Introduction
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The present consensus is that modern Earth’s continental crust has a
buck andesitic composition (~61％ SiO2)，butit is lithologically and
chemically stratified，such that a mafic lower crust depleted in
granitic components underlies an evolved middle and upper crust.
Crust formation must therefore occur in at least two stages，first，
melting of the mantle to produce basaltic magma，and second，either
fractional crystallization or re-melting of the basalt ultimately to
produce the more evolved rocks of which continental crust is
dominantly composed.
The products of the second stage may subsequently undergo further
differentiation by one or more cycles of remelting (termed anatexis or
intracrustal melting), which is integral in the stabilization of new
continental crust, or by weathering and erosion at the Earth’s surface.
The most abundant constituents of the upper continental crust have
granitic rocks, their volcanic equivalents, and metamorphic and
sedimentary rocks of granitic composition. (Wedepohl, 1991)
Lower crust, where dry, magnesium- rich cumulates and eclogite are
expected to predominate. (Saunders et al., 1996)
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顯生宙的大陸地殼增生(以CAOB為例，Jahn, 2004)
The lateral accretion implies stacking of arc complexs, accompanied
by amalgamation of old (dispersed) microcontinental
blocks.(fragments:來自裂解的岡瓦納大陸的殘塊)＝＝＞ enlarged the size
of continent
The emplacement of large volumes of post-accretionary alkaline and
per-alkaline granites was most likely achieved by vertical accretion
through a series of processes, including 1. underplating of basaltic
magma, 2. mixing of basaltic liquid with lower- crustal rocks, 3. partial
melting of the mixed lithologies leading to generation of granitic
liquids, and 4. followed by fractional crystallization. ＝＝＞ addition of
juvenile crust
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早元古代和太古代發育在高級變質區或低級變質區的花岡岩類，並且
在形成之後由於後期地殼運動的影響，遭受變質作用，形成混合花岡
岩。如TTG suites、二長花岡岩、紫蘇花岡岩、其他花岡岩。依據地
球化學特徵劃分鉀質和鈉質類型(TTG suites)。
Phanerozoic Orogenic belts:potassium- rich granitic plutons
(Martin, 1986；Drummond and Defant, 1990)
TTG suites: tonalite- trondhjemite- granodiorite
英雲閃長岩- 奧長花岡岩- 花岡閃長岩
Granites Formed from Juvenile
Mantle-derived Materials
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1. Volumetrically minor "plagio-granites"
of mid-ocean ridge systems
＝＞ ophiolite complexes
2. alkaline granites
3. icelandic dacites and rhyolites
4. primitive "M (mantle)-type" quartz
diorites of intraoceanic island arc
Archaean
Granitic suites(Fig 5.), 平均陸殼組成(B.C.), Average TTG ＝＞
Negative Nb- Ta, Ti anomalies
How do TTG suites form？
1. formed by remelting at the base
of thickened crust
2. formed in the subducted slab
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TTG are chemically analogous to modern adakite lavas, which
are confined to arc setting and largely attributed to melting of
the downgoing oceanic lithosphere. ( Drummond and Defant,
1990 etc.)
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Like adakites, TTG suites extend to lower Nb/Ta and higher
Zr/Sm ratios than the primitive mantle and oceanic basalts.
Recent experimental work: the present of residual amphibole,
lower Mg numbers of 40-50 ＝＞需要有低鎂質的角閃岩的熔融而非
榴輝岩甚至是含角閃石的橄欖岩
TTG suites : Nb- Ta, Ti depleted in contrast to post-Archaean
counterparts
Oceanic arc granites are not similar to
upper continental crust: lower Rb, Th, U,
LREE and depleted Nb
Proterozoic to Phanerozoic
Granitic associations
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Glenelg River Complex：
syn-anatectic deformation
作用所控制下的產物，經
由多階段的熔融體(melt)
分離和萃取的地質作用。
These rocks occur in
continental collision zones,
such as Himalayas，North
American Appalachians，
and the Hercynian fold
belts of western Europe.
M=[Na+K+(2*Ca)]/[Si*Al]
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Fig. 18a modifying
effects of fractional
crystallization
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Fig. 18b 可看出兩個不
同向量顯示岩石生成過
程是操作同位素差異型
態的控治因素
The alternative is that
the variation was
inherited from a
heterogeneous source.
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Loosely defined to include rocks with
more than ~20％modal proportion of
quartz relative to feldspars, …
I-type：sourced from an igneous or
intracrustal precursor
S-type：derived by anatexis of
sedimentary or supracrustal protoliths
that have experienced a weathering cycle.
"A"-type：alkaline, anhydrous, and/or
anorogenic
"M"-type：mantle-like