granitic perspectives on the generation and secular evolution

Granitic Perspectives on the Generation and Secular Evolution of the Continental Crust

Chapter 11

Introduction 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)

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 b

y 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, an

d 4. followed by fractional crystallization. ＝＝＞ addition of juvenile crust

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 (M

artin, 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

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 diorit

es 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

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, 19

90 etc.) Like adakites, TTG suites extend to lower Nb/Ta and higher Zr/S

m ratios than the primitive mantle and oceanic basalts. Recent experimental work: the present of residual amphibole, l

ower 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

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]

Fig. 18a modifying effects of fractional crystallization

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.

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