Ge Rongfeng, Zhu Wenbin, Wilde Simon A, Wu Hailin
State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210093, P.R. China.
Department of Applied Geology, Curtin University, G.P.O. Box U1987, Perth, Western Australia 6845, Australia.
Sci Adv. 2018 Feb 14;4(2):eaao3159. doi: 10.1126/sciadv.aao3159. eCollection 2018 Feb.
Eoarchean [3.6 to 4.0 billion years ago (Ga)] tonalite-trondhjemite-granodiorite (TTG) is the major component of Earth's oldest remnant continental crust, thereby holding the key to understanding how continental crust originated and when plate tectonics started in the early Earth. TTGs are mostly generated by partial melting of hydrated mafic rocks at different depths, but whether this requires subduction remains enigmatic. Recent studies show that most Archean TTGs formed at relatively low pressures (≤1.5 GPa) and do not require subduction. We report a suite of newly discovered Eoarchean tonalitic gneisses dated at ~3.7 Ga from the Tarim Craton, northwestern China. These rocks are probably the oldest high-pressure TTGs so far documented worldwide. Thermodynamic and trace element modeling demonstrates that the parent magma may have been generated by water-fluxed partial melting of moderately enriched arc-like basalts at 1.8 to 1.9 GPa and 800° to 830°C, indicating an apparent geothermal gradient (400° to 450°C GPa) typical for hot subduction zones. They also locally record geochemical evidence for magma interaction with a mantle wedge. Accordingly, we propose that these high-pressure TTGs were generated by partial melting of a subducted proto-arc during arc accretion. Our model implies that modern-style plate tectonics was operative, at least locally, at ~3.7 Ga and was responsible for generating some of the oldest continental nuclei.
早太古代（36亿至40亿年前）的英云闪长岩-奥长花岗岩-花岗闪长岩（TTG）是地球最古老残余大陆地壳的主要组成部分，因此是理解大陆地壳如何起源以及板块构造在早期地球何时开始的关键。TTG大多由不同深度的水化镁铁质岩石部分熔融形成，但这是否需要俯冲作用仍不明确。最近的研究表明，大多数太古代TTG在相对较低压力（≤1.5吉帕）下形成，不需要俯冲作用。我们报道了一套新发现的来自中国西北部塔里木克拉通、年龄约为37亿年的英云闪长质片麻岩。这些岩石可能是目前全球记录的最古老的高压TTG。热力学和微量元素模拟表明，母岩浆可能由1.8至1.9吉帕和800°至830°C条件下中等富集的弧状玄武岩的水致部分熔融形成，表明其具有热俯冲带典型的明显地热梯度（400°至450°C/吉帕）。它们还局部记录了岩浆与地幔楔相互作用的地球化学证据。因此，我们认为这些高压TTG是在弧增生过程中由俯冲的原始弧部分熔融形成的。我们的模型意味着现代式板块构造在约37亿年前至少在局部地区已经起作用，并导致了一些最古老的大陆核的形成。
