新太古代晚期登封地区TTG片麻岩成因及大地构造意义

Petrogenesis and Geotectonica Significance of TTG Gneiss in Late Neoarchean Dengfeng Complex

英云闪长岩‒奥长花岗岩‒花岗闪长岩(TTG)是地球早期大陆地壳最重要的组成部分,与早期构造环境和地壳分异过程关系紧密.通过岩相学、强烈的轻重稀土元素分异等地球化学特征、锆石的Hf模式年龄和U-Pb年龄以及微量元素模拟等手段,界定了登封地区TTG片麻岩的源岩及其残留相.锆石U-Pb定年显示登封杂岩内有两期TTG岩浆作用(2.57~2.55 Ga和2.55~2.51 Ga).并且,同位素系统研究表明,登封杂岩中具有较高的εHf(t)和ε_(Nd)(t)值(1.7~9.7和0.23~3.87),接近同时代亏损地幔值.TTG片麻岩具有较高的SiO_(2)和Na_(2)O含量,较低的Mg#、Cr、Ni、Yb和Y含量,具有较高的La/Yb和Sr/Y比值.这些资料表明,登封地区的岩石代表了成熟地壳的主要岩石组成.该地区的岩性组合、构造样式和地球化学等多方面证据表明登封杂岩内部的绿岩带组合(变质火山沉积岩)是弧前杂岩和增生杂岩;以TTG为主的东部和西部登封杂岩可能是岛弧核部,它们共同组成了板块汇聚的标志性产物.微量元素模拟进一步表明登封地区TTG片麻岩是由俯冲洋壳的含水玄武岩(角闪岩)部分熔融形成,并留下了含石榴子石的角闪岩残余物.据此推断在此期间发生了年轻地壳生长,而华北克拉通南缘的新太古代晚期TTG片麻岩(2.57~2.50 Ga)见证了新陆壳的生长.

Tonalite-trondhjemite-granodiorite(TTG)is the most important component of the early continental crust of the Earth,which is closely related to the early tectonic environment and crustal differentiation process.By means of petrography,rare earth element differentiation,U-Pb age and Hf model age of zircon and trace element simulation based on the partial melting degree,this paper defines the protolith of TTG gneiss in Dengfeng area and its residual phase during partial melting.Meanwhile,zircon UPb dating shows that Dengfeng complex recorded two TTG magmatism(2.57-2.55 Ga and 2.55-2.51 Ga).TTG in Dengfeng complex has highε_(Hf)(t)andε_(Nd)(t)values(1.7-9.7 and 0.23-3.87),which are close to the depleted mantle values of the same period.In addition,TTG gneiss has high SiO_(2),Na_(2)O,La/Yb and Sr/Y ratios and low Mg#,Cr,Ni,Yb,and Y contents.These data indicate that the rocks in the Dengfeng area represent the main components of mature crust.It is suggested by various data,including rock types,structural patterns,and geochemistry,that the greenstone belt combination(metamorphic volcanic sedimentary rocks)within the Dengfeng complex is forearc and accretionary terranes.The TTG-dominated Dengfeng complexes in the eastern and western parts may represent the island arc’s core,and they collectively constitute the signature product of plate convergence.Further simulations of trace elements indicate that TTG gneisses in the Dengfeng area were partially melted from hydrated basaltic(amphibolite)rocks of the subducted oceanic crust,leaving behind garnet-bearing amphibolite residues.As such,we can infer that there was significant growth of young crust during this period,and the late Neoarchean TTG gneiss(2.57-2.50 Ga)found along the southern margin of the North China Craton provides evidence to the formation of a new continental crust.