西昆仑造山带中巴公路段石榴角闪岩形成的变质地质过程

The Metamorphic Process in the Formation of Garnet Amphibolite along the China-Pakistan Highway Profile,West Kunlun Orogenic Belt

西昆仑中巴公路剖面上多处产出石榴黑云角闪片岩、帘石石榴角闪岩和含榴斜长角闪片岩 ,即石榴角闪岩。不论在宏观尺度还是在微观尺度上中级变质的石榴角闪岩均可与较低级变质的黑云母 绿帘石 绿泥片岩呈条带状相间排列。通过野外岩石组合 构造变形的观察 ,结合室内岩石结构、变质组合和反应关系的分析 ,认为该区石榴角闪岩的形成与较低级变质基础上的局部变质叠加有关 ,这种叠加在构造活动的中心部位最为显著 ,石榴角闪岩的产出表明变质已达高角闪岩相。石榴子石 角闪石 斜长石的形成与一定程度上的流体活动密切相关 ,当流体挥发分组分活动强烈时 ,石榴子石不出现 ,而代之以单斜辉石 方柱石 斜长石组合。在组分活性方面 ,镁铁质组分比长英质组分的活动更为明显。石榴角闪岩的原岩既可以是基性岩 ,如玄武岩或辉长岩 ,也可以是沉积岩如杂砂岩。

Garnet amphibolite, comprising garnet-bearing biotite-amphibole schist,epidote-amphibole schist and plagioclase amphibolite, are present along the China-Pakistan highway section along the western Kunlun Mountains. The medium-grade garnet amphibolites are interleaved with the low-grade biotie-epidote-chlorite schist on both megascale and microscale. Combined with the observation of rock associations and deformation features in the field and the analysis of mineral textures, metamorphic assemblages and reaction relations, the authors have deduced that the formation of the high amphibolite facies garnet amphibolite is related to local superposition upon the relatively low grade metamorphism, and this is most prominent in the central part of the shear zone. Fluid volatiles were involved in the presence of the garnet-amphibole-plagioclase assemblage. When the components became more mobile, the clinopyroxene-scapolite-plagioclase assemblage instead of garnet will occur. As to the activity of the components, the mafic component is more active than the felsic component. The protoliths of the garnet amphibolite can be either basic magmatic rocks, such as basalt and gabbro, or sedimentary rocks, like greywacke.