大同—怀安麻粒岩地体的伸展抬升  

EXTENSION AND UPLIFT OF THE DATONG-HUAIAN GRANULITE TERRAIN

作　　者：张家声[1] 

机构地区：[1]国家地震局地质研究所

出　　处：《地质论评》1997年第5期503-514,T002,共13页Geological Review

基　　金：国家自然科学基金(编号49070133)

摘　　要：大同—怀安地区TTG片麻岩和孔兹岩系早期具有互不相同的、但又彼此相关的构造和温压历史。它们之间的低角度高应变带以正韧性剪切和非共轴变形为特征,体现了构造接触的性质。构造和岩石学数据表明,该地体至少在2.5Ga和1.8Ga前后分别受到两个麻粒岩相变质事件(M_1,M_2)的影响,其主要的构造是在M_1事件中形成的。早期构造-热事件表现为区域地壳挤压加厚和随后发生的伸展拆离。其峰期变质温度达800—900℃,压力达1.2-1.4GPa;在异地变质沉积岩中,早期构造-热事件的峰值温度大致为800℃,压力为0.8GPa左右,形成了与岩性层平行的面理,井发育大量S型花岗岩。基底和异地盖层中的早期构造代表M_1变质事件在不同地壳层次上的产物。两者沿大型拆离带的位移调整发生在M_1的晚期(D_3):拆离带上盘的盖层岩石表现为侧向位移滑脱,其中D_3期构造记录了比减压更为显著的冷却过程;相反,拆离带下盘的基底岩石则表现为与卸载有关的垂向运动,因而记录了0.4—0.6GPa的减压抬升。M_2期间发育了同时切过基底和盖层的北东向走滑带,同时伴随着某些抬升。M_2的峰期压力为0.4—0.6GPa,温度为650-700℃。上述低角度拆离带和所得到的P-T轨迹,意味着紧随地壳加厚之后的均衡调整过程发生了伸展拆离。The tectonic and P-T histories of both TTG gneiss and khondalite series in the Datong-Huaian area are different but interrelated to each other. A low-angle high strain zone lying between the two rock series is characterized by ductile shearing in a normal sense and noncoaxial deformation , showing a tectonic contact between them. Structural, textural and stratigraphic data indicate that the terrain was affected by two tectono-thermal cycles (C1 and C2) and two corresponding granulite facies events (M1 and M2) about 2500 and 1800 Ma ago. The major structures in the terrain were formed during M1, C1 is manifested by regional crustal compression and thickening in the early stage and subsequent extension and decoupling. In C1 the peak metamor-phic conditions were 1. 2-1. 4 GPa and 800-900 C ; while for the allochthonous cover sedi- ments the values were about 0. 8 GPa and 800 C and there were developed bedding-parallel foliation and large amounts of S-type granites. The earlier structures in basement and allochthonous cover are products of M1 metamorphic event at different crustal levels. Juxtaposition of basement and cover along a detachment zone occurred at the late stage of C1/M1. The cover sequence in the hanging wall of the detachment was displaced in a lateral sense and syn-D3 structure records cooling rather than decompression. In contrast, the basement in the footwall moved in a vertical sense in response to unloading, and therefore records decompression of 0. 4-0. 6 GPa and uplift. During C2/ M2, peak conditions of about 0. 4-0. 6 GPa and 650-700 C were reached. This period witnessed the development of discrete, left-lateral strike-slip zones that transect both basement and cover, accompanied by uplift. The low-angle detachment and deduced P-T paths imply that there occurred extensional collapse in the process of isostatic adjustment following crustal thickening.

关 键 词：麻粒岩 伸展塌陷 拆离带 山西省 地体 

分 类 号：P588.346[天文地球—岩石学]