摘要
磷灰石是榴辉岩中最常见的副矿物之一,见证了高压-超高压变质岩从原岩形成、俯冲和折返所伴随的物理化学过程。为进一步揭示苏鲁超高压榴辉岩经历的物理和化学过程,我们对取自中国大陆科学钻探工程(CCSD)主孔岩心和苏北地表露头榴辉岩中的磷灰石进行了详细的岩相学分析和LA-ICP-MS原位微量元素分析。分析结果表明,在苏鲁榴辉岩中,磷灰石绝大多数是在超高压变质作用阶段重结晶生长的。未受退变质影响的磷灰石富含LREE和Sr元素,退变质作用促使磷灰石中活动性较强的LREE和Sr明显降低而HREE含量略微升高,并产生负Eu异常。磷灰石边部HREE的升高可能与折返过程中的升温作用和石榴子石分解有关,Eu负异常的产生可能还与退变质过程中发生了氧逸度f_(o_2)的降低有关;结合前人对磷灰石中"出溶"现象的研究,提出榴辉岩磷灰石中的独居石"出溶体"很可能是磷灰石与富含NaCl和硅酸盐的退变质流体发生交代反应所致,磷灰石中硫化物"出溶体"的形成除了氧逸度降低的原因外,可能也与折返过程中发生的短期升温作用有关。超高压变质岩从进变质-峰期→早期退变质→角闪岩相退变质阶段,变质流体可能经历了氧化→还原→氧化状态的复杂变化。
Apatite is a common accessory mineral in HP-UHP eclogites. It is an excellent witness of the physical and chemical process associated with the formation, subduction and exhumation of HP-UHP rocks. To further reveal the physico-chemical variations recorded in apatite grains, detailed petrography and in-situ LA-ICP-MS trace element analysis were conducted on apatites in eclogites from CCSD main hole and an outcrop sample from the Qinglongshan. Apatites were inferred to recrystallize mostly during UHP metamorphic stage, and are rich in LREE and Sr if they did not experience significant retrograde metamorphism. In contrast, LREE and Sr were evidently decreased in the rim of apatites that had seen the retrograde fluids due to more soluble and mobile of these elements in retrograde fluids. Slightly increase of HREE were associated with short heating and breakdown of garnet during retrogration, the negative anomaly of Eu could have resulted from the growth of plagioclase and decrease in oxygen fugacity during retrogration. Integrated previous studies of " exsolution lamellae" in apatites, we propose that monazite " exsolution lamellae" in eclogitic apatites were probably represented the product of metasomatic interaction between apatite grains and retrograde fluids, which may be H_2O-NaCl-and silicate-bearing solutions. Sulfide lamellae in apatites were probably caused not only by the decrease of oxygen fugacity, but also by short heating effect during retrograde reactions. The redox state of metamorphic fluids in UHP metamorphic rocks may have experienced complicated variation from oxidation →reduction →oxidation during prograde-peak →early retrograde →amphibolitic faciesstage.