大别山高压、超高压变质岩结构构造的演化规律(PTt-D轨迹)

A PTt-D Paths Study:Textural and Structural Evolution of Ultrahigh-Pressure and High-Pressure Metamorphic Rocks from Dabie Mountains,China

根据大别山高压、超高压变质岩的中尺度 -显微构造分析及PTt研究 ,建立它们的结构和构造随变质作用 (前榴辉岩相、超高压变质峰期、前角闪岩相和角闪岩相 )有序演化的PTt D轨迹。这一演化主要包括 :在岩石的矿物结构方面从石榴石静态重结晶结构到柯石英假像及放射状张裂隙构造 ;在岩石组构方面从L >S到L -S和S >L榴辉岩 ;以及在中尺度构造方面发育的D1和D2变形构造。该PTt D轨迹同时可以提供有关高压、超高压变质岩折返模式的信息。

There are abundant mineral textures and deformational structures in different scales within ultrahigh?pressure (UHP) and high?pressure (HP) metamorphic rocks in Dabie Mountains,which were developed during metamorphism. Based on textural?structural analysis and PTt research, we present the PTt?D paths of UHP and HP metamorphic rocks. They show firstly,the mineral textures change from static texture of garnet to quartz pseudomorphs after coesite and radiating fractures; secondly,the linear and planar fabrics change from L>S fabric of eclogites to L? S or S>L fabric of retrograded eclogites;lastly and most importantly,the main meso?scale deformational structures developed from original mineral foliation Se to D 1 deformation,and then superimposed D 2 deformation. The D 1 is pre?amphibolite facies deformation. It formed recumbent fold F 1e and boudinage in eclogites. The D 2 is amphibolite facies deformation. It formed amphibolite facies foliation S am , A?type lineation L am , sheath fold Sh and lineation?parallel isoclinal fold F 2am . Structural geometry of F 1e and F 2am accords with type Ⅲ interference pattern of superimposed folds of Ramsay. The microstructures of XZ plane on Sam and small?scale structures at UHP metamorphic rocks outcrops show a top?to?NW-NNW shearing sense during amphibolite facies retrogradation accompanied with D 2 deformation. The PTt?D paths of UHP and HP metamorphic rocks also reveal the exhumation history of these rocks and the evolution of collisional tectonics of Dabie Mountains. This paper gives some discussions in the following:(1) The passive continental margin of Yangtze craton subducted under Sino?Korean craton with a maximum strength at 232～221Ma. The materials of transitional crust of Yangtze passive continental margin experienced UHP metamorphism at 100 km of asthenospheric depth,and the other shallower structural layers experienced HP metamorphism. In this stage,the static texture of garnet shows that the UHP rocks stayed in their peak metamorphic region for a period of time.(2) By the mechanisms of buoyancy and high flexural rigidity of subducted passive continental margin,the UHP and HP rocks broke up to slabs and returned to the level of the boundary of crust and mantle during 220～207Ma, accompanied with D 1 deformation.(3) By the mechanisms of retro?thrust, the UHP and HP rocks returned to upper?middle crust level during 206～178Ma,accompanied with D 2 deformation.(4) Since then,the whole orogen was controlled by denudation,accompanied with granite intrusion and movement of Tan?Lu fault in Cretaceous age. The UHP and HP rocks exposed to near surface during 120～110Ma, and orogeny in Dabie Mountains completed.