华北克拉通基底花岗质片麻岩变形和流变学研究——以辽西寺儿堡地区为例

辽西寺儿堡镇新太古代花岗质片麻岩内发育的宏观、微观构造变形特征表明该地区曾遭受了强烈的韧性变形改造。花岗质岩石变形程度在初糜棱岩–糜棱岩之间,岩石经历了SWW向左行剪切作用改造。岩石中石英有限应变测量判别结果表明,构造岩类型为L-S型,为平面应变。岩石的剪应变平均值为1.43,运动学涡度值为0.788~0.829,指示岩石形成于以简单剪切为主的一般剪切变形中。此外,石英颗粒以亚颗粒旋转重结晶和颗粒边界迁移重结晶作用为主,长石颗粒塑性拉长,部分发生膨凸式重结晶作用;石英组构特征(EBSD)揭示石英以中–高温柱面滑移为主;石英颗粒边界具有明显的分形特征,分形维数值为1.151~1.201,指示了中高温变形条件。综合石英、长石的变形行为、石英组构特征以及分形法Kruhl温度计的判别结果,推断辽西寺儿堡镇新太古代花岗质片麻岩经历过480~600℃的中高温变形,其同构造变质相为高绿片岩相-低角闪岩相。花岗质岩石的古差异应力为10.62~12.21 MPa,估算的应变速率为10~(–11.67)~10~(–13.34) s~(–1),即缓慢的变形,可能记录早期中高温、低应变速率的韧性变形过程,反映华北克拉通基底中下部地壳变形特征。

Structural Deformation and Rheology of Granitic Gneiss from the North China Craton Basement-An Example from the Sierbao Area in Western Liaoning Province

Macro- and micro-deformation structural characteristics of the Neoarchean granitic gneiss in the Sierbao area in western Liaoning province (Liaoxi) demonstrate that the region underwent a strong ductile deformation event. Due to the left-lateral ductile shearing with SWW trending, the granitic rocks were mainly deformed to be proto-mylonites and mylonites with gneissic structures. Systematic measurements of finite strain of quartz in the granitic gneiss indicate the strain type is plane strain in L-S tectonites. Calculation of micro-structures suggests that the average shearing strain is 1.43 and the kinematic vorticities are 0.788 to 0.829, indicating a shearing dominated deformation. Petrographically, the quartz grains in the granitic rocks mostly occur as dynamic recrystallization including subgrain rotation and high temperature grain boundary migration recrystallization. Meanwhile, most of feldspar porphyroclasts are enlongated and some are fully bulging recrystallized. Quartz C-axis fabrics (EBSD) indicates that quartz fabrics are mainly middle-high temperature prismatic slip system and less low-middle temperature rhomb slip system. Fractal analysis shows that boundaries of the recrystallized quartz grains have statistically self-similarities with numbers of fractal dimension from 1.151 to 1.201, indicative of middle-high temperature deformation condition. According to deformation behaviors of quartz and feldspar, the quartz C-axis fabrics and the Kruhl thermometer demonstrate that the ductile deformation was developed under a condition of high greenschist facies to low amphibolite facies, with the deformation temperature ranging from 480℃ to 600℃. The differential stress and strain rate of granitic rocks are estimated at 10.62 to 12.21 MPa, 10–11.67 s–1 to 10–13.34s–1, respectively, which suggest that the ductile deformation was a slow process. The characteristics of the early ductile deformation phase with middle-high temperature and slow strain rate may represent the deformation features of the middle-lower crust of the North China Craton basement.