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冀东太平寨-娄子山太古宙麻粒岩相变质峰期流体研究

卢良兆, 董永胜   

  1. 吉林大学 地球科学学院, 吉林 长春 130026
  • 收稿日期:2000-12-20 修回日期:2000-12-20 出版日期:2000-12-20 发布日期:2000-12-20

Studies on Fluids of the Metamorphic Peak Time for the Archaean Granulite Series in the Taipingzhai-Lozishan District, Eastern Hebei Province

LU Liang-zhao, DONG Yong-sheng   

  1. College of Earth Sciences, jilin University, Changchun 130026, China
  • Received:2000-12-20 Revised:2000-12-20 Online:2000-12-20 Published:2000-12-20

摘要: 冀东太平寨-娄子山地区太古宙麻粒岩系变质峰期流体包裹体首次见于夕线石中,按形 态 和产状可分三类,均形成于峰期至其稍后阶段。它们成分相似。特征是:①富碳质,XC O2+C H4=0.64~0.93; ②含少量H2O,XH 2O=0~0.22,且与峰期矿物 组合平衡热力学计算的水活度(aH2O=0.04~0.31)和含量( XH2O=0.03~0.22)完全吻合; ③有时含一定量H2S和SO2; ④盐水溶液 中阴离子总量极低(0.12~0.61mol/L)。石英和石榴石中也有特征相同的这期包裹体。峰 期包裹体中C O 2均一温度多数 为-28℃~-22℃,密度为0.96~1.06g/cm3,包裹体整体密度应稍高,相应的压力为0.60 ~0.70G Pa。流体的氧逸度lgfO2在-17~-15之间。峰期流体上述各种特征在空间上较 均匀,相邻的不同岩石类型中aH2O值相似,结合地质和原岩特 征分析,认为流体中C O2可能为深部地幔来源,其成因与基性岩浆底侵及其结晶作 用有关。

Abstract: The fluid inclusions of the metamorphic peak time fo r the Archaean granulite series in the Taipingzhai-Lozishan district, eastern H ebei Province were first found in sillimanite. According to their shapes and occurrences, these inclusions ca n be subdivided into three types. The earliest type is the isolated inclus i ons and inclusions in clusters with negative hexogonal, prismatic crystal forms . They are very rare and characterized by parallelism of their elongated direct ion with c-axis of sillimanite; The second type is the inclusions with irregula r shapes. They have no spatial relation to cleavage and (00 1) parting of host mineral; The third type is most common. The inclusions are usually square or rectangular. They occur along healed (001) parting perpendicular to c-axis of sillimanite, and were entraped in the later stage of the metamorphic peak time. As a whole , the compositions of fluids in above-mentioned three types are very similer. All they are chara cterized by:①rich in carbonic materials, with XCO2+CH4≈0.64~0.93; ② low in wate r , with XH2O≈0~0.22, which coincides well with activity of H2O ( 0.04~0.31) and mole percent of water (XH2O≈0.03~0.22) based on thermodyna mic calcula tion from equilibrium assemblage of peak metamorphism; ③with XH2S,SO 2 <0.10~0.21 in some cases; ④very low content of anions in halogenic solution (mostly 0.12~0.61mol/L). These inclusions have also been found in garnet an d quartz, but in the latter case, the inclusious are characterized by abundant two-phase H2O-C O2 chemistry, with XH2O >0.25~0.33. They were probably formed in the further late regressive stag e under the low PT condition.   The homogenization temperature of CO2 in fluid inclusions of peak stage ranges from -28℃ to -22℃ in most samples. The corresponding density is 0.96~1.06g /c m3, and total fluid pressure (Pf) is 0.60~0. 70GPa. This value is about 0.1~0.3GPa lower than the confining pressure of r ocks ( Pl) estimated by various geobarometers. The temperature of this stage is es timated at 780℃~790℃. Based on the XH2O, XCO2 and XCH4 relationship of fluid inclusions, together with diagnoti c assemblage of peak stage, the oxygen fugacity is calculated as log fO2=-17~-15.   All features of peak-stage fluids are rather homogeneous in space within a give n a rea, and values of aH2O are similar in associated rocks of differe n t lithology. These evidences together with regional geology and characteristics of protolith rocks suggest that predominant amout of CO2 in fluids might be o f de ep source and related to underplating of mafic magmas from the mantle and their cry stallization. The proposed mechanism is analogous to concept of carbonic metamo rphism.