山东仰口地区超高压变质岩混杂带的组成及其构造变形

The constituents of the Qingdao Yankou high to ultrahigh-pressure tectonic mélange and their deformations

从构造岩石角度将山东青岛仰口地区发育的超高压变质岩混杂带划分为构造整合单元和构造混杂单元两个组成部分,对不同单元的岩性组成及其超高压变质特征进行了描述。着重对不同构造单元中的构造变形期次进行了划分,总结了不同期次构造变形的宏观及微观特征。结合苏鲁超高压变质带在区域上构造变形期次的研究情况,初步提出形成仰口地区超高压变质岩中各种构造形迹的构造模式,并对它们所反映的仰口超高压带形成和折返的构造意义进行了探讨。

A high-to uhra high pressure (HP-UHP) metamorphic slab resulted from deep continental subduction and subsequent exhumation is well exposed in Yankou, northeastern Qingdao in the Sulu orogenic belt, China. It consists of two kinds of structural units, namely structurally coherent unit and non-coherent unit (UHP ductile shear melange), respectively. Similar to the ultrahighpressure metamorphic (UHPM) rocks at other localities in the Su-Lu UHPM belt, the coherent unit consists of interlayered granitic gneiss, paragneisses, retrograde eclogite and ultramafic rocks in which coesite and other UHP minerals are widely preserved, indicating that they have an UHP metamorphic history. While the later is a ductile shear complex that consists of highly sheared mylonitic matrix and weakly deformed blocks, of which the matrix is composed of interlayered granitic and eclogitic mylonites; while the blocks are granitoids and gabbro that are metamorphosed to a variable degree of coesite eclogite facies. The two units are separated by an ultramylonite zone of 20～30 cm wide, and the non-coherent unit structurally lies below the coherent one. Both units have experienced multi-phase deformation and metamorphic overprint during its long evolutionary history. In the coherent unit, three phases of foliations (S_0 to S_2), including two phases of folding F_1 and F_2, are identified in the gneisses, and their field outcroppings and structural features are described in detail. Metamorphic mineral paragenesis involved in these deformation events indicates that they took place at amphibolite to blueschist facies metamorphic conditions, while the UHP structures are preserved only in some garnet residuals of the ultra-basic rocks in this unit. In the non-coherent structural unit, deformation at HP-UHP metamorphism identified according to the metamorphic minerals involved is well preserved as foliations and lineations as well as different kinds of folds in both of the matrix and blocks, and their features are well summarized. Furthermore, three phases of mylonite are distinguished in the matrix of the unit. Kinematical indicators (the lineations and foliations preserved in this unit) suggest the top-to-the north thrust motion of the UHP slab during the exhumation. A three-phase model of tectonic evolution of the UHP rocks in Yankou is presented to explain the formtion of the multi-phase deformations in both the coherent and non-coherent units.