Secular change in TTG compositions: Implications for the evolution of Archaean geodynamics
Secular change in TTG compositions: Implications for the evolution of Archaean geodynamics
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DOI：
10.1016/j.epsl.2018.10.022
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发表时间：
2019-01-01
影响因子：
5.3
通讯作者：
Santosh, M.
中科院分区：
文献类型：
--
作者：
Johnson, I. E.;Kirkland, C. L.;Santosh, M.
据估计，大约四分之三的地球第一代大陆地壳是在25亿年前的古希腊时代结束时产生的。该古陆壳主要由水化镁铁质岩部分熔融形成的英云闪长岩-奥长花岗岩-花岗闪长岩（TTG）套的变形变质岩浆岩组成。然而，TTG岩浆形成的地球动力学机制是一个持续争论的问题。使用过滤的全球地球化学数据集的563个样品的年龄范围从始新世到新太古代（4.0-2.5 Ga），我们询问的大部分岩石的主要氧化物和微量元素组成的TTG评估长期变化的证据。尽管在数据的高度分散，几个关键的主要氧化物和微量元素的浓度或比例显示出统计上显着的趋势，表明在间隔3.3-3.0 Ga的最大值，最小值和/或过渡。重要的是，K2 O/Na 2 O、Sr/Y和La-N/Yb-N的变点分析表明，在这300 Ma期间发生了统计学显著（>99%置信度）变化。这些转变可能与地球动力学制度的根本变化，在上地幔温度的峰值周围，从一个占主导地位的非均匀，变形停滞盖过程的另一个占主导地位的全球移动的盖或板块构造过程的出现，由西太平洋结束。一个显着的变化也是显而易见的，在2.8-2.7 Ga，正好与一个主要的跳跃，在生存率的变质岩与对比的热梯度，这可能涉及到更多的钾质大陆弧岩浆的出现和碰撞造山过程中的保存潜力增加。在许多情况下，TTG的化学成分显示出通过南海的扩散增加，反映了岩石圈的不可逆转的分化。(C)2018 Elsevier B. V.版权所有。
It is estimated that around three quarters of Earth's first generation continental crust had been produced by the end of the Archaean Eon, 2.5 billion years ago. This ancient continental crust is mostly composed of variably deformed and metamorphosed magmatic rocks of the tonalite-trondhjemite-granodiorite (TTG) suite that formed by partial melting of hydrated mafic rocks. However, the geodynamic regime under which TTG magmas formed is a matter of ongoing debate. Using a filtered global geochemical dataset of 563 samples with ages ranging from the Eoarchaean to Neoarchaean (4.0-2.5 Ga), we interrogate the bulk rock major oxide and trace element composition of TTGs to assess evidence for secular change. Despite a high degree of scatter in the data, the concentrations or ratios of several key major oxides and trace elements show statistically significant trends that indicate maxima, minima and/or transitions in the interval 3.3-3.0 Ga. Importantly, a change point analysis of K2O/Na2O, Sr/Y and La-N/Yb-N demonstrates a statistically significant (>99% confidence) change during this 300 Ma period. These shifts may be linked to a fundamental change in geodynamic regime around the peak in upper mantle temperatures from one dominated by non-uniformitarian, deformable stagnant lid processes to another dominated by the emergence of global mobile lid or plate tectonic processes by the end of the Archaean. A notable change is also evident at 2.8-2.7 Ga that coincides with a major jump in the rate of survival of metamorphic rocks with contrasting thermal gradients, which may relate to the emergence of more potassic continental arc magmas and an increased preservation potential during collisional orogenesis. In many cases, the chemical composition of TTGs shows an increasing spread through the Archaean, reflecting the irreversible differentiation of the lithosphere. (C) 2018 Elsevier B.V. All rights reserved.