Multistage metamorphic events in granulitized eclogites from the North Dabie complex zone, central China: Evidence from zircon U–Pb age, trace element and mineral inclusion
Multistage metamorphic events in granulitized eclogites from the North Dabie complex zone, central China: Evidence from zircon U–Pb age, trace element and mineral inclusion
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DOI：
10.1016/j.lithos.2010.12.005
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发表时间：
2011-02
期刊：
影响因子：
3.5
通讯作者：
Yican Liu;Xiao-feng Gu;Shuguang Li;Z. Hou;B. Song
中科院分区：
文献类型：
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作者：
Yican Liu;Xiao-feng Gu;Shuguang Li;Z. Hou;B. Song
所研究的超高压榴辉岩位于中国中部北大别杂岩带（NDZ）西南部，代表了华南地块深俯冲的基性下陆壳，表现出多重变质演化。然而，对NDZ超高压变质作用的确切时间知之甚少，从而阻碍了我们对该地区构造演化的理解。为了限定大陆俯冲和发掘过程中超高压变质峰及其后的退变年龄，采用岩石学、微量元素和U-Pb同位素研究相结合的方法对NDZ榴辉岩中的锆石进行了研究。结合岩石学资料，目前锆石SHRIMP定年和LA-ICPMS U-Pb定年为该区榴辉岩多期变质事件的时间提供了精确的约束。榴辉岩锆石U-Pb同位素记录了新元古代238±2Ma、222±4-227±2Ma、210±4-215±2Ma、199±2Ma和176±2-188±2Ma等多个离散事件的时代。遗迹火成岩岩心确定的新元古代年龄代表其原岩时间，其他年龄组则通过CL图像、低Th/U比值和矿物包裹体反映其变质记录。根据锆石的U-Pb年龄、微量元素和矿物包裹体特征，确定了至少两期榴辉岩相变质作用，分别为226±3 ma和214±3Ma。较年轻的年龄（199±2）和（176 ~ 188ma）可能记录了发掘过程中发生的麻粒岩相复印和角闪岩相退变，而年龄（238±2Ma）可能反映了超高压变质作用之前的进变质时间。因此，新元古代基性下大陆地壳俯冲深度大于120km（对应于金刚石形成的最低压力），并在226±3Ma发生超高压变质作用。这些超高压变质岩在214±3Ma深度约60km处被发现，经历了高压石英榴辉岩相逆行变质作用，随后分别被麻粒岩相和角闪岩相变质叠加到地壳下部和上部。
The studied ultrahigh-pressure (UHP) eclogites, located in the southwestern part of the North Dabie complex zone (NDZ) in central China, represent deeply subducted mafic lower continental crust of the South China Block and display a multiple metamorphic evolution. However, the exact timing of the UHP metamorphism in the NDZ is poorly constrained, and thus impedes our understanding of the tectonic evolution of this area. In order to constrain the ages of peak UHP metamorphism and subsequent retrogression during continental subduction and exhumation, zircon from the eclogites in the NDZ has been investigated by a combined petrological, trace element and U–Pb isotopic study. In combination with petrological data, the present zircon SHRIMP and LA-ICPMS U–Pb dating provides precise constraints on the timing of multistage metamorphic events on the eclogites in the region. The U–Pb isotope in zircon from the eclogites records the times of multiple discrete events in the history of the rock such as Neoproterozoic, 238±2Ma, 222±4–227±2Ma, 210±4–215±2Ma, 199±2Ma and 176±2–188±2Ma. Neoproterozoic ages defined by relic igneous cores represent their protolith time and the other age-groups reflect their metamorphic records by CL images, low Th/U ratios and mineral inclusions. By U–Pb age, trace element and mineral inclusion of zircon, at least two episodes of eclogite-facies metamorphism have been identified from the eclogites, best estimated at 226±3 and 214±3Ma, respectively. The younger ages of 199±2 and 176–188Ma most likely record the granulite-facies overprinting and amphibolite-facies retrogression occurring during exhumation whereas the age of 238±2Ma probably reflects the prograde metamorphic timing prior to the UHP metamorphism. Thus, Neoproterozoic mafic lower continental crust was subducted to depths greater than 120km (corresponding to the lowest pressure for diamond formation) and suffered from UHP metamorphism at 226±3Ma. Then these UHP metamorphic rocks were exhumed to about 60km depth at 214±3Ma and experienced high-pressure quartz eclogite-facies retrograde metamorphism, and subsequently to lower- and upper-crustal levels and overprinted by granulite- and amphibolite-facies metamorphism, respectively.