越南西北部大象山超高温变质岩的发现及其区域构造意义

The discovery and tectonic implication of ultrahigh-temperature metamorphic rocks in the Day Nui Con Voi,northwestern Vietnam

越南西北部大象山群孔兹岩系中发育一套含刚玉+尖晶石+石榴石+夕线石组合的富铝岩石块体,它们呈透镜状包体形式赋存于孔兹岩系内。岩石中刚玉+尖晶石+石榴石+夕线石组合的发育指示岩石经历了超高温变质作用的改造。其中尖晶石和石英的共生组合表明了变质温度高于900℃,而利用岩石退变质矿物组合中的黑云母-石榴石温度计,黑云母-斜长石-石榴石-石英组合温度-压力计估算的变质温度压力条件分别为879～917℃、0.90～0.94GPa。岩石中的早期刚玉+夕线石的组合的存在说明岩石变质作用已经从高角闪岩相进入到了麻粒岩相;而峰期变质矿物组合尖晶石+石英的出现,指示了变质温度高于900℃的超高温变质作用。另一方面,退变质过程中钛铁矿的发育表明岩石经历了快速抬升降压的过程。变质作用的P-T轨迹分析揭示出岩石经历了早期同步升温增压后的快速增温达到峰期条件,后经历快速等温减压过程。这种温压条件的变化与板块会聚过程中由于俯冲板片的断离而使软流圈上涌造成热异常的温压条件变化基本一致。通过对超高温变质岩石进行锆石SIMSU-Pb测年获得的结果大于58Ma,推测这次超高温变质与喜马拉雅运动中印度与欧亚大陆的初期会聚-碰撞作用相关。

There is a pile of rock blocks which contain corundum＋spinel＋garnet＋fibrolite association in the form of lenticular enclaves in khondalite series along the Day Nui Con Voi,northwestern Vietnam.The mineral association is resulted from ultrahigh-temperature metamorphism.The coexistence of spinel and quartz indicates that the metamorphic temperature is higher than 900℃.Meanwhile,the biotite-garnet thermometer and biotite-plagioclase-garnet-quartz association thermobarometer for the mineral associations during retrogressive metamorphism gave temperature and pressure ranges of 879~917℃ and 0.90~0.94GPa,respectively.The early stage metamorphism is featured by the occurrence of corundum＋fibrolite assemblage,which marks a shift of metamorphic conditions from upper amphibolite facies to granulite facies.Meanwhile,the peak metamorphism is characterized by the spinel＋quartz assemblage,which indicates an ultrahigh-temperature metamorphic condition（temperature higher than 900℃）.On the other hand,the formation of ilmenite during retrogressive metamorphism suggests that the rocks experienced a drastic decrease in pressure after the peak metamorphic conditions.The metamorphic P-T path shows that there is an early coeval increase in temperature and pressure and a subsequent rapid increase in temperature before reaching the peak conditions.In addition,a fast isothermal decrease in pressure characterizes temperature and pressure evolution after the peak metamorphism.Such a P-T path is in accordance with the temperature and pressure evolution of anomalously heated slab due to upwelling the asthenosphere by the break-off of subducting oceanic plate during plate convergence.SIMS U-Pb dating of zircons from the ultrahigh-temperature metamorphic rocks gave a minimum age of 58Ma.Therefore,it is suggested that the ultrahigh-temperature metamorphism along the Day Nui Con Voi is related to the convergence between the Indian and the Eurasian plates.