集宁群和迁西群麻粒岩相变质流体的特征和成因

A comparative study on characteristics and origin of fluids during granulite-facies metamorphism of Jining Group and Qianxi Group

矿物中流体包裹体研究和脱水反应热力学计算结果表明,集宁群和迁西群麻粒岩相变质峰期流体均富碳质,x(CO2 +CH4)一般为 70 %～ 80 %,x(H2 O)≈ 10 %～ 2 0 %,密度一般为1.0～ 1.1g/cm3。集宁群中变质流体的成分和H2 O活度在空间上分布不均匀,明显受岩性控制。富铝片麻岩、黑云紫苏片麻岩和基性麻粒岩中αH2 O值分别为 0.10～ 0.2 0、0.2 0～ 0.4 0和 0.4 0～0.6 0,表明其非外源成因,未曾在岩石中均匀渗透,而可能与进变质过程原岩中普遍存在的有机质氧化和各种脱挥发分反应有关,其富碳质还与H2 O相对易于逸散有关。迁西群变质峰期流体成分在空间上分布均匀,不受岩性控制,在太平寨等地区上述 3类岩石中αH2 O值主要都在 0.13～ 0.2 5之间,XH2 O为 0.1～ 0.2,反映流体似为外源成因,并确曾在岩石中均匀渗透。本区存在幔源基性岩浆底侵的地质条件,这种富碳质流体可能与它们结晶时析出的CO2 流上升有关。

According to data of fluid inclusions within minerals and thermochemical estimation of dehydration reaction during granulite_facies metamorphism of Jining Group and Qianxi Group, the fluids existent at the peak stage must have been carbonic, with x (CO 2+CH 4)≈70%～80% and x (H 2O)≈10%～20%, the density is generally 1.0～1.1 g/cm 3. In the Jining Group, components of the metamorphic fluids and activity of H 2O seem to have been spatially inhomogeneous, and were apparently controlled by lithology. The α H 2O values of Al_rich gneisses , biotite hypersthene gneisses and basic granulites are 0.1～0.2, 0.2～0.4 and 0.4～0.6 respectively. Such inhomogeneity indicates that the fluids might have experienced internal buffering, and did not permeate pervasively through rocks. They were probably developed as a result of various kinds of devolatilization and oxidation of the organic materials (now as crystalline graphite) extensively distributed in rocks, both of which must have occurred at the progressive metamorphic stage. Moreover, H 2O could escape from the system more easily than CO 2, and this might be partly related to their carbonic affinity. On the contrary, the components and physical habits of fluids during metamorphic peak of the Qianxi Group were rather homogeneous in space, being independent to lithology. In Taipingzhai district, for example, the α H 2O values of rocks of all the three types are 0.13～0.25 with X H 2O ≈0.1～0.2. These features suggest that the fluids in this region were probably of external_buffering, and had pervasively permeated through rocks. CO 2 in fluids might mostly have come from underplating mantle_source mafic magma and its crystallization process.