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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。
In the Kongshe Formation of the Elephant Mountain Group in the northwestern part of Vietnam, a set of Al-rich rock blocks containing corundum, spinel, garnet and sillimanite are developed. They are lenticular inclusions in the khondalite series . The development of corundum + spinel + garnet + sillimanite assemblage in rocks indicates that the rocks underwent the transformation of ultra-high temperature metamorphism. The symbiotic assemblage of spinel and quartz indicates that the metamorphic temperature is higher than 900 ° C. However, using the biotite-garnet thermometer, biotite-plagioclase-garnet-quartz combination temperature-pressure gauge in the rock metamorphic mineral assemblages Metamorphic temperature and pressure conditions were 879 ~ 917 ℃, 0.90 ~ 0.94GPa. The presence of early corundum + sillimanite in the rocks indicates that the metamorphism of rocks has progressed from the amphibolite facies to the granulite facies. The appearance of the metamorphic mineral assemblage spinel + quartz at the peak indicates that the metamorphic temperature is higher than 900 ℃ ultra-high temperature metamorphism. On the other hand, the development of ilmenite during retrogressive metamorphism shows that the rock underwent rapid lifting and depressurization. P-T path analysis of metamorphism revealed that the rock underwent rapid warming after the early synchronous heating and pressurization reached the peak condition, and then experienced a rapid isothermal decompression process. This change of temperature and pressure conditions and plate convergence process due to the subduction of the slab off the asthenospheric upwelling cause thermal anomalies of temperature and pressure conditions are basically the same. Through the zircon SIMSU-Pb dating of ultra-high temperature metamorphic rocks, the result is greater than 58Ma. It is speculated that the ultra-high temperature metamorphism is related to the initial convergence-collision of India and Eurasia during the Himalayan movement.