大别山碧溪岭地区超高压变质岩构造分析

STRUCTURAL ANALYSIS OF UHP METAMORPHIC ROCKS IN THE BIXILING AREA,DABIE MOUNTAINS,CHINA

大比例尺 (1∶10 0 0 0 )构造制图及构造分析表明 ,碧溪岭地区超高压变质岩石含有丰富的构造演化历史记录。同碰撞或挤压组构只保留于榴辉岩及其它超高压变质岩透镜体内部 ,表现为高角度网络状超高压剪切带与弱应变透镜体域规律组合格式。前者由面理或糜棱岩化榴辉岩组成 ,后者由块状榴辉岩及石榴橄榄岩组成。碰撞期后伸展构造表现为区域性的假单斜状 ,内部呈低缓角度的网络状强应变带及所环绕的透镜状弱应变域组合格式 ,强应变带的岩石为由榴辉岩退变成的角闪岩相高压片麻岩及部分熔融形成的含榴花岗岩 ,透镜状弱应变域的岩石为弱角闪相改造的榴辉岩及石榴橄榄岩。不同尺度上同碰撞或挤压组构及碰撞期后伸展组构所显示的这种残斑基质流变学结构样式 ,虽然与先期原岩成分、结构、流变学的不均一性有关 ,但主要是多期递进应变分解作用的结果 ,支持榴辉岩“原地”成因模式。依据构造学证据和可利用的岩石学及同位素年代学资料 ,分析了超高压变质岩石的形成及折返过程 ,指出碧溪岭地区超高压变质岩石是在 2 45～ 2 10Ma形成的 ,碰撞期后伸展作用主要发生在 2 0 0～ 170Ma。在超高压变质岩石向地壳表层折返过程中 ,张扭作用可能有重要功能 ,不支持碧溪岭地区遭受过多期超高压变质作用的推论。

The detailed structural mapping at a scale of 10 000 and structural analysis for the Bixiling area, Dabie Mountains, China have shown that it is a typical occurrence of UHP metamorphic rocks, in a structurally lower position of the UHP unit. Both the contractional deformation related to Triassic collision between the Sino—Korea and Yangtze cratons, and the deformation related to the post Triassic collisional extension have an anastomosing structural pattern with lens shaped domains of low strain separated by ductile shear zones of high strain. In the case of UHP eclogite facies deformation, the lens shaped domains consisting of the massive eclogite and garnet peridotite with a weak foliation are surrounded by UHP shear zones. Rocks in the UHP shear zones are foliated eclogites with a steeply dipping mylonitic foliation and lineation. The present main structure of the UHP unit in the Bixiling area was formed mainly by extensional processes following the collision. It is characterized by a pseudomonoclinic structure in which eclogite bodies are enclosed by an amphibolite facies matrix consisting of HP gneisses and garnet bearing granites, with a gently dipping compositional layering and pervasive foliation and lineation. The compositional layering, as seen in outcrops, no longer provides information concerning stratigraphy or gross distribution of stratigraphy horizons. This clast matrix type rheology may be observed at all scales from centimeters to kilometers. The initial protolith composition, rheology and fabrics anisotropy of the rocks would affect the behavior of the structural pattern, but multiphase and progressive strain partitioning is mainly responsible for the anastomosing structural pattern. It is argued that the preserved eclogites, which now constitute perhaps 13% of the UHP unit, are not merely isolated exotic bodies tectonically employed into the amphibolite facies gneiss but represent remnants of an once pervasive or widespread eclogite facies terrain. The petrological and geometrical relationships strongly support on in situ origin of the UHP eclogites in this area. An entire tectonic evolution process for the UHP metamorphic rocks is deduced from the structural evidence and combined with previous petrographic and chronological data, in which the age of 245～210Ma is considered as the most reliable estimate for the time of the UHP metamorphic event, and the post collisional extension or transtension is mainly dated about 200～170Ma. Moreover, structural records do not support the idea that the rocks have undergone polyphase UHP metamorphism in the Bixiling area.