中国大陆科学钻探(CCSD)HP-UHP变质岩中石英脉流体包裹体δD-δ^(18)O同位素组成及其意义

δD-δ^(18)O compositions of fluid inclusions in quartz veins of HP-UHP metamorphic rocks from the Chinese Continental Scientific Drilling (CCSD) Project and its geological significances.

流体包裹体δD-δ18O同位素测定显示CCSD石英脉具有较稳定的氢同位素(δD=-97‰--69‰)和相对较低且变化较大的氧同位素组成,其矿物δ18O为-1．9‰～9．6‰,相应流体的δ18O为-11．66‰-0．93‰,说明其变质岩围岩在板块俯冲前曾在地表与大气降水发生过程度不同的水／岩反应,而石英脉继承了其各自寄主变质岩的δ18O组成;在CCSD纵向上,石英脉的δ18O同位素组成出现“∑”型变化,分别在900m-1．500m和2700m出现极低值,而在1770m和4000m出现高正值,说明CCSD变质岩原岩在俯冲前与大气降水间的水／岩反应受到局部侵入的岩浆岩带来的高温和构造空间的控制; CCSD中石英脉δ18O在纵向上的变化基本同步于其寄主围岩变质矿物的δ18O组成变化,说明石英脉与其他变质矿物一样,也经历了HP,甚至UHP变质,但其主体应形成于板块折返过程中HP-UHP岩石的减压重结晶及退变质;CCSD石英脉、东海地表石英脉或水晶矿的δD-δ18O同位素值分布的不均一性,说明HP-UHP岩石在板块折返及其后退变质中释放出的流体活动范围有限,没有经历大规模的流动或迁移。东海水晶的流体包裹体δD-δ18O组成与CCSD石英脉相似,显示它们的成因基本一致,主要形成于晚三叠世板块折返过程。

δD-δ^18O isotopic compositions of fluid inclusion in quartz veins of HP-UHP metamorphic rocks in the Chinese Continental Scientific Drilling （CCSD） Project were analysed, and the results show that the CCSD quartz veins possess relative stable hydrogen isotopic compositions（ δD = - 97‰ - 69‰） but quite low and greatly varied oxygen isotopes, and the δ^18O of the quartz are - 1.9‰-9. 6‰ and the corresponding δ^18O of the fluid inclusions are - 11.66‰ to 0. 93‰, suggesting that the protoliths of the CCSD HP-UHP metamorphic rocks were reacted with meteoric water at surface before being subducted, and the quartz veins inherited the δ^18O composition of their respective hosting rocks. In the CCSD profile, with increasing depth, the δ^18O of the quartz veins vary greatly and are in the form of ＂∑＂, which show very low values at depth of 900m to 1500m and 2700m, and high values at depth of 1770m and 4000m, implying that the water/rock interactions between the protoliths and the meteoric water before subduction were controlled by heats and contact metamorphism associated with the local magmatic intrusions. The vertical variation of the CCSD quartz veins＇s δ^18O are similar to those of the rock-forming minerals of the HP-UHP metamorphic rocks, suggesting that the quartz veins probably underwent HP, even UHP metamorphism, but most of them were formed by decompression-recrystallization and retrograde metamorphism of the CCSD HP-UHP metamorphic rocks. The great inhomogeneous distribution of the δ^18O of quartz veins in the CCSD and in the outcropped HP-UHP metamorphic rocks, as well as the big crystals in the Donghai County, suggests that the metamorphic fluids expelled from the HP-UHP metamorphic rocks during the exhumation were locally migrated at the grain scale. Besides, the similarities of δD-δ^18O compositions between the Donghai crystals and the CCSD quartz veins imply that they possess similar genesis and mainly formed during exhumation of the subducted plate in the Late Triassic Epoch.