俯冲隧道是俯冲板片与上覆板块之间的剪切带，也是高压—超高压变质岩折返和深部流/熔体活动的通道。大别山超高压变质岩分布广泛，变形程度差异很大，是研究大陆俯冲隧道中岩石变质- 变形过程的理想地区。本文系统总结了前人对中大别双河地区超高压变质岩的岩石学和年代学研究成果，在双河地区开展了地质填图、应变分析和三维构造重建。通过将超高压变质岩的变形特征与P- T- t轨迹结合，识别出超高压变质岩折返过程中的三期韧性变形。在双河北部发现了一个上盘向NW剪切的千米尺度的榴辉岩相鞘褶皱，枢纽向SE倾伏，倾伏角约20°，与榴辉岩、片岩和长英质片麻岩的拉伸线理平行，表明超高压变质岩初始折返阶段的流体活动使榴辉岩的强度显著降低，榴辉岩与围岩一起发生韧性变形。该期变形被角闪岩相退变质阶段上盘向NW的剪切叠加，此时应变集中于片麻岩、片岩、大理岩等非能干层，强度较高的榴辉岩成为构造透镜体。而绿片岩相变质阶段上盘向SE方向的剪切与早白垩世北大别花岗片麻岩穹隆的形成有关。对双河南部弱变形花岗片麻岩的锆石U- Pb定年揭示了757±14 Ma的原岩年龄和 240~216 Ma的变质年龄，与双河北部含柯石英强变形花岗片麻岩类似，暗示其也经历了三叠纪超高压变质作用及随后的角闪岩相退变质作用。通过计算长英质片麻岩的有效黏度，发现无水碱长花岗片麻岩的有效黏度高于黑云斜长片麻岩，折返阶段的流体活动使超高压变质岩的强度显著降低，当局部的流体活动不足以弱化碱长花岗岩体时，应变集中于黑云斜长片麻岩。因此，大陆俯冲隧道中的应变分布受矿物组成、流体活动和岩体规模的共同影响。
As the shear zone between the subducting and overlying plates, a subduction channel is the pathway for exhumation of high- and ultrahigh- pressure (UHP) metamorphic rocks and deep fluid/melt activity. Due to the wide exposure and variable deformation degree of UHP metamorphic rocks, the Dabie orogenic belt provides an ideal area to investigate metamorphism and deformation processes in a continental subduction channel. Here we summarized previous petrological and geochronological studies in the Shuanghe area of the central Dabie orogenic belt and carried out field mapping, strain analysis and 3D structural reconstruction in this region. Combining deformation with P- T- t paths of UHP rocks, three ductile deformation phases during exhumation of UHP rocks are identified. A kilometer- scale sheath fold of eclogite facies (D1) in the northern Shuanghe area is characterized by the SE- plunging fold axis with a dipping angle of ~20° and a top- to- the- NW shear sense. This indicates that high fluid activity during the initial exhumation significantly reduced viscosity of eclogite, leading to the simultaneous ductile deformation of eclogites and surrounding rocks. The following deformation phase under amphibolite facies (D2) overprinted the D1 fabrics by the continuous top- to- the- NW shearing. Strain was localized in incompetent rocks such as gneisses, schists and marbles, whereas eclogites appeared as tectonic boudins. The top- to- the- SE detachment (D3) mainly occurred in schists under greenschist facies, which may be related with formation of the granitic gneiss dome in the northern Dabie orogenic belt during the Early Cretaceous. In the southern Shuanghe area, zircon U- Pb dating of weakly deformed granitic gneiss revealed the crystallization age of 757±14 Ma and the metamorphic age of 240~216 Ma, which are consistent with strongly sheared coesite- bearing granitic gneiss in the northern Shuanghe area. Hence the weakly deformed granitic gneiss also experienced the Triassic UHP metamorphism and the amphibolite facies retrograde metamorphism. Calculation of the effective viscosity of felsic gneiss indicates that dry alkali- feldspar granitic gneiss is stronger than biotite- plagioclase gneiss. The presence of water can significantly reduce the effective viscosity of UHP rocks. If localized fluid activity could not weaken the large- scale alkali- feldspar granite gneiss pluton, strain localization occurs in biotite- plagioclase gneiss. Therefore, strain distribution in a continental subduction channel is controlled by mineral assemblage, fluid activity and volume of the rock body.
LI Peidong, WANG Qin, WU Meiqian.2024. Heterogeneous strain distribution in a continental subduction channel: Records from ultrahigh- pressure metamorphic rocks in the Dabie Mountains[J]. Acta Geologica Sinica,98(1):50-78.