贺兰山北段早前寒武纪孔兹岩系的变质变形演化及动力学

THE EVOLUTION OF METAMORPHISM-DEFORMATION AND ITS DYNAMICS OF EARLY PRECAMBRIAN KHONDALITE SERIES IN THE NORTHERN SEGMENT OF HELAN MOUNTAIN

通过对贺兰山北段孔兹岩系宏观及微观构造和典型变质矿物的详细研究，识别出该区曾经历了两个阶段四幕变形作用和三个阶段六幕变质作用，并结合矿物温历的估算，建立了该区变质作用演化的顺时针p-T-t-D轨迹，其反映本区孔兹岩系形成于陆陆碰撞的造山环境中，构造增厚作用是其发生的诱因。

The region studied lies in the northern segment of Helan Mountain, which is located at the western margin of the North Chlna Platform. The khondalite series is widespread in the Zong-bieli area. Four tectono-deformational episodes in two stages and six metamorphic episodes in three stages are recognized in the khondalite series. The first deformational episode in the early stage in the extentional environment was the bedding shear deformation- resulting in a series of tectonic patterns such as blasto-plastic deformational and interlayer recumbent folds- ln the later stage the second, third and fourth deformational episodes occurred in the shortening environment.ln the second deformatlonal episode were formed EW giant recumbent folds (D21 ) and NW broad folds (D22) based on the former. In the third deformatlonal episode were formed NE tight lsoclinal folds (D31 ), of which the axial planes are deeply plunglng and the NE high temperature plastic shear zone (D32 ), which brought ahout the tectonic framework of the northern segment of Helan Mountain. In the fourth deformational episode were formed the giant plastic shear helts related to the emplacement of granite bod1es- The evolution of metamorphism involved six episodes in three stages. In the early stage of prograde metamorphism (M1 - M2 ), the typical equilibrium assem-blage of M1 episode is Bi+Pl+Qz which was left in the center of garnet; M, episode is character-ized by an assemblage of Ky+(;t+Bi+Mus+Pl+Qz, which was left in the intercalation of minor two-mica schist; at the peak temperature of the metamorphism stage (M3 - M4 ), the characteristic mineral assemhlages of M, episode are Sil +Gt +Cord+Bi+Kf+Pl+Qz in Al-rich gneiss and Opx +Cpx±Bi + Pl +Qz, Opx+ Bi +Gt + Pl + Qz in minor granulite, reaching granulite facles; in the M4 episode the react1on of Bi-Sil + Melt widely occurred, and it is related to the dehydrational anatexis of khondalite. At the post-peak metamorphic and later stages (M5-M6), M5 episode was related to the formation of strongly deformed belt (D4 ), as ls evidenced hy the metamorphic reac- tions: Gt +Sil+Qz+H2O-Cord and Sil-And, equal to low-pressure amphlbolite facles; and the late low-tenlperature stage (M6) is characterized by the reaction of Sil-Mus. On the basls of the mineral paragenesis and geothermoubarometric estimates, the clockwise p-T-t-D path similar to the pattern of collislon orogenic belt is determined and it was caused by tectonic thickening.