Can fractional crystallization, mixing and assimilation processes be responsible for Jamaican-type adakites? Implications for generating Eoarchaean continental crust
Can fractional crystallization, mixing and assimilation processes be responsible for Jamaican-type adakites? Implications for generating Eoarchaean continental crust
复制标题
DOI：
10.1093/petrology/egv029
复制
发表时间：
2015-07
影响因子：
3.9
通讯作者：
A. Hastie;J. Fitton;S. Mitchell;I. Neill;G. Nowell;I. Millar
中科院分区：
文献类型：
--
作者：
A. Hastie;J. Fitton;S. Mitchell;I. Neill;G. Nowell;I. Millar
了解地球第一批大陆陆块是如何形成的非常重要，因为负责的过程直接影响了地球原始硅酸盐内部的演化，也影响了地球的大气层和水圈。太古代陆壳以奥长花岗岩-英云闪长岩-花岗闪长岩(TTG)系列岩石为主。它们可划分为：(1)低SiO_2、高∼、高过渡元素含量的中晚太古代(GT；3·5~2.5Ga)系列；(2)高SiO_2、低MgO、Sr、过渡元素含量的始太古代(>；3·5Ga)系列。新生代埃达克岩在成分上与太古代中晚期(∼3·5~2·5Ga)TTGS相似，但不是最古老的TTG岩。相反，牙买加早始新世一套类似埃达克岩的流纹岩(牙买加型埃达克岩：JTA)被证明在地球化学上与始太古代的TTGs相似。与日本新发现的类JTA岩石(Ryozen Low Sr/Y)不同，新的微量元素和NdHf放射成因同位素数据证实，牙买加JTA不是由复杂的混合、同化和分离结晶作用形成的。新的部分熔融模式探索了几种不同的来源成分(大洋中脊玄武岩、洋岛玄武岩和大洋高原)、矿物模式、熔融模式和分配系数。这些模式的研究结果清楚地表明，含斜长石和石榴石的角闪岩源区可部分熔融，形成大洋高原状成分的JTA和古太古代TTg。进一步的模拟表明，JTA和太古代TTG的低镁丰度和过渡元素丰度可以来自两个主要的过程：(1)俯冲洋壳(成分类似于中生代大洋高原玄武岩)的相对较浅的部分熔融，板片熔体上升而不与地幔楔体相互作用；(2)大洋高原状俯冲洋壳的部分熔融，随后板片熔体与薄的和/或不连续的(布丁纳？)地幔楔体，其中预期增加的板片熔体中的镁、镍和铬被铁镁矿物(主要是角闪石)的分离结晶所消除。因此，利用JTA作为古太古代TTG生产的现代类似物，我们提出了古太古时代存在消耗大洋高原状洋壳的俯冲带。
Understanding how the Earth’s first continental land masses were generated is important because the processes responsible directly affected the evolution of the planet’s primordial silicate interior, and also its atmosphere and hydrosphere. Archaean continental crust is dominated by rocks of the trondhjemite–tonalite–granodiorite (TTG) suite. These can be divided into (1) a mid- to late Archaean (∼3·5–2·5 Ga) suite with low SiO2 and high MgO, Sr and transition element contents, and (2) an Eoarchaean (>3·5 Ga) suite with higher SiO2 and lower MgO, Sr and transition element concentrations. Cenozoic adakites are considered to be compositionally similar to mid- to late Archaean (∼3·5–2·5 Ga) TTGs, but not the oldest TTG rocks. Conversely, a suite of Early Eocene adakite-like rhyodacites (Jamaican-type adakites: JTA) from Jamaica are shown to be geochemically similar to the Eoarchaean TTGs. In contrast to newly discovered JTA-like rocks (Ryozen low Sr/Y) in Japan, new trace element and Nd–Hf radiogenic isotope data in this study confirm that the Jamaican JTA cannot be formed by complex mixing, assimilation and fractional crystallization processes. New partial melt models here explore several different source compositions (mid-ocean ridge basalt, ocean island basalt and oceanic plateau), mineral modes, melt modes and partition coefficients. The results of these models clearly demonstrate that the JTA and the Eoarchaean TTG can be generated by partial melting of plagioclase- and garnet-bearing amphibolite source regions with oceanic plateau-like compositions. Further modelling shows that the JTA and Eoarchaean TTG low MgO and transition element abundances can be derived from two dominant processes: (1) relatively shallow partial melting of subducting oceanic crust (compositionally similar to Mesozoic oceanic plateau basalt) whereby the slab melts ascend without interacting with a mantle wedge; (2) partial melting of oceanic plateau-like subducting oceanic crust followed by interaction of the slab melts with a thin and/or discontinuous (boudinage-like?) mantle wedge whereby the expected increase of MgO, Ni, and Cr in the slab melts is obliterated by fractional crystallization of ferromagnesian minerals (mostly amphibole). Consequently, using the JTA as a modern analogue for Eoarchaean TTG production, we propose the existence of subduction zones consuming oceanic plateau-like oceanic crust in Eoarchaean times.