How to Make a Continent: Thirty-five Years of TTG Research
How to Make a Continent: Thirty-five Years of TTG Research
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DOI：
10.1007/978-94-007-7615-9_7
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发表时间：
2014-01-01
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影响因子：
--
通讯作者：
Condie, Kent C.
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文献类型：
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作者：
Condie, Kent C.
经过35年多的TTG（英云闪长岩-奥长花岗岩-花岗闪长岩）研究，我们仍然面临着许多关于这些特殊岩石的成因和构造意义的问题。我们所知道的是，TTG的成分相似，无论年龄，他们有高的La/Yb，Sr/Y，Sr和Eu/Eu*，他们减少相对于太古代末的钙碱性花岗岩类的丰度，他们不是在海洋弧，浅层次的海洋高原或洋脊。此外，TTG锆石中的氧同位素需要TTG源与水圈的相互作用，并且从碎屑锆石套件推断的冥古宙大陆地壳的存在仍然存在问题。虽然我们现在认识到TTG需要角闪石和石榴石分馏和至少50公里深的来源，我们不知道的是（1）熔融与分离结晶和（2）熔融板与熔融加厚镁铁质地壳的相对作用。板块脱水机制和速率控制着俯冲带石榴子石和角闪石的稳定性。从我们对太古代早期绿岩的了解来看，它们比后来的绿岩更容易被改变，因此它们似乎将更多的水和流体流动元素带入俯冲带，至少到了太古代晚期，当板块构造变得普遍时。太古代较热的板块应有助于更高的挥发性释放速率。这可以解释我们在太古代末期TTG中看到的微量元素变化。要形成今天的大陆地壳，我们需要从大陆俯冲带开始，在那里我们产生钙碱性（CA）和TTG岩浆，并以大约3份CA对2份TTG的比例将长英质成分联合收割机结合起来。相比之下，要形成一个太古代大陆，我们需要几乎100%的TTG成分，并且可能开始，至少在大约3 Ga之前，通过熔化海洋高原的根部。
After more than 35 years of TTG (tonalite-trondhjemite-granodiorite) research, we still face many questions about the origin and tectonic significance of these peculiar rocks. What we do know is that TTGs are similar in composition regardless of age, they have high La/Yb, Sr/Y, Sr and Eu/Eu*, they decrease in abundance relative to calc-alkaline granitoids at the end of the Archean, and they are not made in oceanic arcs, shallow levels of oceanic plateaus or at ocean ridges. Furthermore, oxygen isotopes in TTG zircons require interaction of TTG sources with the hydrosphere, and the existence of Hadean continental crust inferred from detrital zircon suites remains problematic. Although we now realize that TTGs require amphibole and garnet fractionation and sources that are at least 50 km deep, what we do not know are the relative roles of (1) melting versus fractional crystallization and (2) melting of slabs versus melting of thickened mafic crust. The mechanisms and rates of slab dehydration control the stability of garnet and amphibole in subduction zones. From what we know about early Archean greenstones, they are more altered than later ones, and thus they would appear to bring more water and fluid-mobile elements into subduction zones, at least by the late Archean when plate tectonics became widespread. Hotter slabs in the Archean should contribute to higher volatile release rates. This may explain the trace element changes we see in TTGs at the end of the Archean.To make continental crust today we need to start at a continental subduction zone where we produce both calc-alkaline (CA) and TTG magmas, and combine the felsic components in a ratio of about 3 parts CA to 2 parts TTG. In contrast, to make an Archean continent, we need nearly 100 % of the TTG component, and may begin, at least before about 3 Ga, by melting the roots of oceanic plateaus.