摘要
大别 -苏鲁造山带是扬子大陆板块与华北大陆板块之间在三叠纪时期俯冲 -碰撞所形成的超高压变质带。对该带超高压变质岩的稳定同位素研究发现 ,不仅含柯石英榴辉岩出现局部氧同位素负异常 (δ18O =- 10‰ ) ,而且区域上存在氧同位素分布的不均一性 (δ18O =- 10‰～+10‰ )。前者要求榴辉岩原岩在变质前经历过大气降水热液蚀变 ,说明俯冲板块具有大陆地壳特点 ;后者反映扬子板块具有快速俯冲变质的特征 ,否则将达到同位素均一化。榴辉岩氧同位素负异常的保存显示 ,这些超高压变质岩与地幔之间没有发生显著的化学相互作用。因此 ,载有榴辉岩原岩的板块俯冲到 2 0 0多公里深的地幔内部时 ,超高压岩石形成后在地幔中的滞留时间很短(<10Ma) ,致使它们与地幔之间的氧同位素交换没有达到再平衡。榴辉岩中不同矿物对氧同位素测温不仅给出了相互一致的结果 (6 5 0～ 75 0℃ ) ,而且这些温度与阳离子交换温度计的结果 (6 0 0～80 0℃ )相一致。因此 ,在榴辉岩相变质温度下共生矿物之间的氧同位素平衡已被“冻结” ,岩石冷却过程中的氧同位素交换再平衡没有发生 ,从而证明超高压榴辉岩在变质作用后经历了快速降压/冷却过程 ,对应于板块的快速抬升。这些结果首次从地球化学角度证明了大陆板块俯冲—超高压变质—折?
The Dabie Sulu Orogen has been recognized as an ultrahigh pressure metamorphic terrane in central eastern China that was produced at Triasssic by collision between the Yangtze and North China Plates. Stable isotope studies were carried out for ultrahigh pressure metamorphic rocks from the Dabie Sulu Orogen in east central China. The results show not only local oxygen isotope anomaly with the extremely low δ 18 O of -10‰ in coesite bearing eclogite but also heterogeneous oxygen isotope ratios with δ 18 O varying from -10‰ to +10‰. The former indicates that eclogite protolith underwent meteoric hydrothermal alteration before UHP metamorphism and the subducted plate has the nature of continental crust, whereas the latter suggests rapid subduction of the Yangtze Plate otherwise oxygen isotope homogenization would be attained. Preservation of the negative δ 18 O anomaly in the eclogite indicates very limited crust mantle interaction during UHP metamorphism at mantle depths. As a result, a short period of 5 to 10Ma is reckoned when the plate containing the eclogite precursor was subducted into mantle depths greater than 200km, so that no oxygen isotope reequilibration was achieved between the 18 O depleted eclogite and the hot mantle. Oxygen isotope geothermometry yields concordant temperatures of 650 to 750℃ for different mineral pairs that have a large difference in oxygen diffusivity, and the isotope temperatures are in agreement with the petrological temperatures of 600 to 800℃ from cation exchange thermometry. Thus oxygen isotope equilibrium among the minerals has been frozen in at eclogite facies conditions, and no significant retrograde exchange took place during eclogite exhumation. This points to a process of rapid decompression or cooling subsequent to the UHP metamorphism, corresponding to rapid ascent of the UHP metamorphosed slab. These isotope results provide geochemical constraints on the geodynamics of the Yangtze continental plate, and the bulk period for this cycling is estimated to be about 10 to 20Ma. An ice frying model is therefore advanced for the rapid subduction, the short UHP metamorphism, and the rapid exhumation of the Yangtze Plate.
出处
《高校地质学报》
CAS
CSCD
2002年第4期365-379,共15页
Geological Journal of China Universities
基金
国家自然科学基金项目 (4 945 3 0 0 3和 40 0 3 3 0 10 )
中国科学院知识创新工程项目 (KZCX2 10 7)
