中亚造山带东段晚三叠世伸展构造及岩浆事件:来自^40Ar/^39Ar和锆石U-Pb同位素年代学的制约

Late Triassic extensional deformation and magmatism in the eastern part of the Central Asian Orogenic Belt:Constraint from ^40Ar/^39Ar and zircon U-Pb geochronology

通过对中亚造山带东段南缘发育的解放营子韧性剪切带的构造学研究,揭示出该地区岩石圈减薄后发生了一期强烈的伸展变形事件。野外观测和岩相学分析显示该韧性剪切带呈北东-南西走向,变形带内发育有大量的A型褶皱,矿物和砾石拉伸线理以及同构造花岗质岩墙。S-C组构、σ型角闪石残斑、压力影构造、斜长石书斜构造以及云母鱼等显微构造,指示该韧性剪切带为右旋剪切。多晶石英的波状消光、晶粒边界迁移重结晶、多晶石英条带等显微变形组构表现出中温(300~500℃)的变形条件。动态重结晶颗粒的粒径统计分析和岩石有限应变分析显示该韧性剪切带形成于一个地壳中等层次的伸展变形环境。剪切带内同构造白云母^40Ar/^39Ar阶段加热同位素年代学分析以及同构造花岗岩的锆石U-Pb同位素年代学测试显示,该韧性剪切带的伸展变形时代为晚三叠世(219~227Ma)。该期伸展变形事件在华北板块北缘和中亚造山带形成了低硅型和高硅型两种花岗质岩浆的侵入。其中中亚造山带内发育的低硅型岩石的岩浆源区为亏损型地幔岩石圈,并进一步演化出高硅型岩石;而华北板块北缘发育的低硅型岩浆起源于富集型岩石圈地幔,同时混入了亏损的软流圈地幔组分。

Structural studies of the Jiefangyingzi ductile shear zone in the southern part of eastern Central Asian Orogenic Belt(CAOB)reveal distinct extensional deformation event postdating Late Triassic lithospheric thinning.Field observation and petrographic analyses indicate that the ductile shear zone is a NE-SW trending belt within which A-type folds,stretching lineations and synkinematic granitic dikes are developed.The S-C fabric,sigma-type amphibole porphyroclasts,pressure shadow structures,bookshelf structure of plagioclase and mica fish structures are also developed in this shear zone,which suggest a dextral shearing.From microstructure studying,the undulatrory extinction,grain boundary migration recrystallization,banded structures of polycrystalline quartz of the shear zone reveal it was formed under a medium temperature(300~500℃)deformation condition.Analyses of dynamic recrystallization grains and strain ellipsoid show that the ductile shear zone was formed in the shallow-middle crust.^40 Ar/^39 Ar step-heating analyses of synkinematic muscovite samples and zircon U-Pb isotopic data of synkinematic granite reveal that the timing of deformation happened in Late Triassic(219~227 Ma).The Late Triassic extensional event generated the low-Si type and high-Si type granitic rocks in the CAOB and North China Crtaon(NCC).The low-Si type rocks from the CAOB were derived from the depleted mantle,and further evolved high-Si type rocks;The low-Si type rocks from the NCC were derived from the enriched lithospheric mantle,with mixture of depleted asthenospheric mantle components.