贺兰山北段古元古代结晶基底变形特征及其区域构造意义

贺兰山北段结晶基底中保留有不同程度的韧性变形剪切带.通过详细的野外考察和室内显微构造研究,明确贺兰山北段的古元古代基底经历了4期韧性剪切变形:(1)早期顺层剪切带表现出中下部地壳层次的变形样式,运动学特征一致反映了近南北向的伸展；(2)麻粒岩相变质的糜棱片麻岩剪切带为南北向挤压的产物,导致经历高温高压变质的孔兹岩系从下地壳向中部地壳抬升；(3)高级糜棱岩(低角闪岩相-高绿片岩相)剪切带涉及的2次伸展运动(北西-南东向伸展和北东-南西向伸展)使得基底进一步向中部地壳抬升,可能发生在形成孔兹岩系的同一造山运动的晚期伸展垮塌过程中；(4)北东-近东西向左行逆冲绿片岩相糜棱岩剪切带则将结晶基底抬升到中上部地壳层次,其运动学特征与高级糜棱岩剪切带明显不同,可能是另一造山运动的产物.贺兰山北段与大青山-乌拉山地区有相似的韧性剪切带和构造变形,表明华北克拉通西部北缘存在一致的近东西走向的古元古代碰撞造山运动以及随后另一造山运动的改造.

Structural deformation characteristics of the Paleoproterozoic crystaline basement in the northern segment of Helan Mountain and its regional tectonic implications

Ductile deformation shear zones in varied degrees are preserved in the crystalline basement in the northern segment of Helan Mountain. Four stages of ductile shearing are identified by detailed outcrop research and microstructure study from the Paleoproterozoic crystalline basement in the northern segment of Helan Mountain. The earliest one is a kind of layer-paralleled shear zone composed by banded khondalite and granitic gneisses along with lots of parallellying folds, boudinages and sheathe folds, showing north to south extension with deforming characteristics of mid-lower crust level. The second one is mylonitic gneiss shear zone in high amphibolite to granulite facies resulted from a compressional tectonic movement in N-S direction to cause fast uplift of the high temperature and high pressured metamorphosed khondalite from lower crust to middle crust. The third kind of shear zone metamorphosed in low amphibolite to high greenshist facies maybe happened in the late extensional collapse stage in the same orogenic cycle with the khondalite, lifting the crystalline basement more up into the middle crust. They are composed of two sets of molynitic shear zones caused by a NW-SE extension followed another NE-SW extension. The latest kind of ductile shear zones are NE to EW trending greenshist-faced mylonitic shear zones in left-lateral thrusting sense, bring the crystalline basement up into the upper crust through the brittle-ductile transition zone. They could be resulted in another orogeny different from the khondalite because owning an obviously different kinematic character from the extensional shear zones. Similar structural deformation characters and ductile shear zones occurred in the Wula Mountains and Daqing Mountains as the northern segment of Helan Mountain indicate the existence of a EW-trending Paleoproterozoic collision orogenic belt in west part of the north edge of the North China Craton and perhaps followed by another orogeny in late-Paleoproterozoic.