大陆碰撞造山带的流体成分与演化:东喜马拉雅构造结南迦巴瓦岩群高压麻粒岩的流体包裹体研究

Composition and evolution of fluids in the continental orogen:A study of fluid inclusions in high-pressure granulites from the Namche Barwa area,Tibet of southwest China.

位于喜马拉雅东构造结的南迦巴瓦岩群经历了高压麻粒岩相、中压麻粒岩相和角闪岩相三期变质作用。在高压麻粒岩中含有复杂的流体包裹体类型,按照捕获先后顺序有:H_2O-CO_2±CH_4包裹体(Ⅰ型);CO_2±CH_4±N_2包裹体(Ⅱ型);高盐度多相包裹体(Ⅲ型);中一低盐度 H_2O 包裹体(Ⅳ型)和极低密度气体包裹体或'空'包裹体(Ⅴ型)。在基性麻粒岩中,被石榴石包裹石英中孤立分布的 H_2O-CO_2±CH_4包裹体,以及部分沿石榴石晶内裂隙分布的 H_2O-CO_2±CH_4和 H_2O 包裹体轨迹未穿过围绕石榴石的辉石+斜长石后成合晶冠状体:表明它们有可能是在麻粒岩相变质阶段捕获的。然而,所有流体包沈昆等:大陆碰撞造山带的流体成分与演化:东喜马拉雅构造结南迦巴瓦岩群高压麻粒岩的流体包裹体研究 1489裹体的等容线均从麻粒岩相变质峰期 P-T 区间下方通过,说明麻粒岩相变质峰期捕获的包裹体均受到了不同程度的改造,包括部分爆裂、渗漏和流体—矿物相互作用等。现存的富 CO_2流体包襄体均具有较低密度,并且往往含有明显数量 CH_4和 N_2组分,不可能是麻粒岩相变质峰期捕获的包裹体。根据富 CO_2包裹体与具有不同相比的 H_2O-CO_2包襄体共存推测,大部分CO_2包裹体是通过 H_2O-CO_2包裹体中 H_2O 的选择性泄漏而形成的。Ⅲ型高盐度盐水包裹体很可能是角闪岩相退变质过程中捕获的,因其等容线与退变质轨迹近于平行,这些包裹体很可能保存了其在角闪岩相阶段捕获时的原生物理化学特征。沿矿物颗粒裂隙分布的大量Ⅳ型和Ⅴ型包裹体,应该是角闪岩相或更晚期形成的次生包裹体,代表了浅成(近地表)环境的循环流体。与世界许多地区麻粒岩相岩石普遍含高密度纯 CO_2流体包裹体不同,南迦巴瓦岩群高压麻粒岩以富含 H_2O-CO_2±CH_4和 H_2O 包裹体为特征,这可能与高压麻粒岩与高温麻粒岩产出于不同的构造环境和经历的退变质轨迹有关。

The Namche Barwa Complex in the eastern Himalayan syntexis has experienced high-, medium-pressure granulite-facies and amphibolite-facies metamorphism. The high-pressure granulites contain complex fluid inclusion assemblages, including, according to the trapping sequence, H2O-CO2 ± CH4 （ type Ⅰ ）, CO2 ± CH4 ±N2 （ type Ⅱ ） , saline aqueous （ type Ⅲ ） , medium-to low-salinity aqueous （type Ⅳ）, and low-density gaseous （type Ⅴ ） inclusions. In the basic granulites, isolated H2O-CO2 ± CH4 inclusions in quartz enclosed in garnet, and some H2O-CO2 ± CH4 and H2O inclusion trails along intragranular fractures in garnet do not crosscut Cpx-Pl symptectite corona surrounging the garnet. This suggests that these inclusions were probably trapped during granulite facies metamorphism. However, the isochores for all types of inclusions pass through below the P-T box of the peak granulite facies metamorphism, implying that all the fluid inclusions have been modified in various extent, including partial deerapitation, leakage and fluid-mineral interactions, etc. The present CO2 fluid inclusions have relatively low density and often contain significant amount of CH4 and N2 ; and they are unlikely trapped during the peak granulite facies metamorphism. The coexistence of CO2 fluid inclusions and H2O-CO2 inclusions with different phase ratios suggests that most CO2 inclusions were formed by selective leakage of H2O out of H2o- CO2 inclusions. The type Ⅲ saline aqueous inclusions were probably trapped during the amphibolite-facies retrometamorphism because its isochores are approximately parallel to the retrograde P-T path. These inclusions are likely to preserve its primary physico-chemical properties during their trapping in the amphibolite-facies retrogression. The types Ⅳ and Ⅴ inclusions in healed transgranular fractures of minerals may have formed in the amphibolite-facies retrogression or even later; they represent percolating fluid flows at shallow level （near surface）. In contrast to the common presence of high-density CO2 fluid inclusions in granulite terrains worldwide the highpressure granulites in the Namche Barwa group is rich in H2O-CO2 ± CH4 and H2O inclusions instead of CO2 inclusions. This may be explained as that the high-pressure granulites occur in different tectonic settings from those of high-temperature granulites.